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    IPCS/CEC EVALUATION OF ANTIDOTES SERIES



    VOLUME 3


    ANTIDOTES FOR POISONING BY PARACETAMOL







    First drafts of the chapters, subsequently reviewed and revised
    by the Working Group, were prepared by:

    Dr. L. F. Prescott, Clinical Pharmacology Unit, Royal Infirmary,
    Edinburgh, United Kingdom (Overview)

    Dr. D. G. Spoerke and Dr. B. H. Rumack, Micromedex Inc., Denver,
    Colorado, USA (N-acetylcysteine)

    Dr. T. J. Meredith, UK Department of Health, London, United
    Kingdom, and Ms J. Tempowski, National Poisons Information 
    Service (London Centre), London, United Kingdom (Methionine)


    IPCS/CEC Evaluation of Antidotes Series

    IPCS   International Programme on Chemical Safety
    CEC    Commission of the European Communities

    Volume 1   Naloxone, flumazenil and dantrolene as antidotes
    Volume 2   Antidotes for poisoning by cyanide
    Volume 3   Antidotes for poisoning by paracetamol

    This important new series will provide definitive and authoritative
    guidance on the use of antidotes to treat poisoning.  The
    International Programme on Chemical Safety (IPCS) and the Commission
    of the European Communities (CEC) (ILO/UNEP/WHO) have jointly
    undertaken a major programme to evaluate antidotes used clinically
    in the treatment of poisoning.  The aim of this programme has been
    to identify and evaluate for the first time in a scientific and
    rigorous way the efficacy and use of a wide range of antidotes.
    This series will therefore summarise and assess, on an antidote by
    antidote basis, their clinical use, mode of action and efficacy. The
    aim has been to provide an authoritative consensus statement which
    will greatly assist in the selection and administration of an
    appropriate antidote. This scientific assessment is complemented by
    detailed clinical information on routes of administration,
    contraindications, precautions and so on.  The series will therefore
    collate a wealth of useful information which will be of immense
    practical use to clinical toxicologists and all those involved in the
    treatment and management of poisoining.


    Scientific Editors

    T.J. MEREDITH
    Department of Health, London, United Kingdom

    D. JACOBSEN
    Ulleval University Hospital, Oslo, Norway

    J.A. HAINES
    International Programme on Chemical Safety,
    World Health Organization, Geneva, Switzerland

    J-C. BERGER
    Health and Safety Directorate,
    Commission of the European Communities, Luxembourg


    EUR 15693 EN

    Published by Cambridge University Press on behalf of the World Health
    Organization and of the Commission of the European Communities

    CAMBRIDGE UNIVERSITY PRESS


    The mention of specific companies or of certain manufacturers'
    products does not imply that they are endorsed or recommended by the
    World Health Organization in preference to others of a similar
    nature that are not mentioned.

    Neither the Commission of the European Communities nor any person
    acting on behalf of the Commission is responsible for the use which
    might be made of the information contained in this report.

    (c) World Health Organization, Geneva, 1995 and
    ECSC-EEC-EAEC, Brussels-Luxembourg, 1995

    First published 1995

    Publication No. EUR 15693 EN of the Commission of the European
    Communities, Dissemination of Scientific and Technical Knowledge
    Unit, Directorate-General Information Technologies and Industries,
    and Telecommunications, Luxembourg

    ISBN 0 521 49576 8 hardback

    CONTENTS

    PREFACE

    ABBREVIATIONS

    1. OVERVIEW OF ANTIDOTAL THERAPY FOR
         ACUTE PARACETAMOL POISONING

         1.1. Introduction and historical review
         1.2. Toxicity in man
         1.3. Assessment of the severity of intoxication
         1.4. Mechanisms of toxicity and antidotal activity
         1.5. Factors influencing the toxicity of paracetamol
               1.5.1. Factors that may increase paracetamol toxicity
               1.5.2. Factors that may reduce paracetamol toxicity
         1.6. Diagnosis of paracetamol intoxication
         1.7. Management of severe paracetamol poisoning
               1.7.1. Supportive care
                       1.7.1.1   Role of  N-acetylcysteine in
                                 paracetamol-induced liver failure
                       1.7.1.2   Role of liver transplantation
               1.7.2. Specific antidotal therapy
                       1.7.2.1   Intravenous  N-acetylcysteine
                       1.7.2.2   Oral  N-acetylcysteine
                       1.7.2.3   Oral methionine
                       1.7.2.4   Intravenous methionine
                       1.7.2.5   Oral versus intravenous therapy
                       1.7.2.6   Comparative efficacy of
                                  N-acetylcysteine and methionine
               1.7.3. Summary of treatment recommendations
         1.8. Areas for future research
               1.8.1. Choice of antidote
               1.8.2. Optimum dose and route of administration
               1.8.3. Role of  N-acetylcysteine in liver failure
               1.8.4. Role of  N-acetylcysteine 24-50 h after the
                       overdose
               1.8.5. New approaches to the treatment of
                       paracetamol poisoning
               1.8.6. Treatment failure
               1.8.7. The treatment line
               1.8.8. The role of ethanol
               1.8.9. Paracetamol poisoning in pregnancy
         1.9. References

    2. METHIONINE

         2.1. Introduction
         2.2. Name and chemical formula
         2.3. Physico-chemical properties

               2.3.1. Melting point (decomposition)
               2.3.2. Solubility in vehicle of administration
               2.3.3. Optical properties
               2.3.4. pH
               2.3.5. pKa
               2.3.6. Stability in light
               2.3.7. Thermal stability/flammability
               2.3.8. Loss of weight on drying
               2.3.9. Excipients and pharmaceutical aids
               2.3.10. Pharmaceutical incompatibilities
         2.4. Pharmaceutical formulation and synthesis
         2.5. Analytical methods
               2.5.1. Quality control of antidote
               2.5.2. Methods for identification of antidote
               2.5.3. Methods for analysis of antidote in biological
                       samples
               2.5.4. Methods for analysis of toxic agent
         2.6. Shelf-life
         2.7. General properties
               2.7.1. Mode of antidotal activity
               2.7.2. Other properties
         2.8. Animal studies
               2.8.1. Pharmacodynamics
               2.8.2. Pharmacokinetics
                       2.8.2.1   Metabolism
               2.8.3. Toxicology
                       2.8.3.1   Acute toxicity
                       2.8.3.2   Subacute and chronic toxicity
                       2.8.3.3   Toxicity in experimental liver damage
               2.8.4. Effect in pregnancy
         2.9. Volunteer studies
               2.9.1. Methionine in patients with hepatic dysfunction
         2.10. Clinical studies - clinical trials
               2.10.1. Study by Prescott et al. (1976)
                       2.10.1.1  Patients and treatment
                       2.10.1.2  Investigations
                       2.10.1.3  Results of treatment
                       2.10.1.4  Toxicity of methionine
                       2.10.1.5  Likelihood of benefit due to antidote
               2.10.2. Study by Solomon et al. (1977)
                       2.10.2.1  Results of treatment
                       2.10.2.2  Toxicity of methionine
                       2.10.2.3  Likelihood of benefit due to antidote
               2.10.3. Study by Hamlyn et al. (1981)
                       2.10.3.1  Results of treatment
                       2.10.3.2  Likelihood of benefit due to antidote
         2.11. Clinical studies - case reports
               2.11.1. Study by Vale et al. (1981)
                       2.11.1.1  Patients and treatment
                       2.11.1.2  Investigations
                       2.11.1.3  Results of treatment

                       2.11.1.4  Liver damage
                       2.11.1.5  High-risk patients
                       2.11.1.6  Renal impairment
                       2.11.1.7  Deaths
                       2.11.1.8  Toxicity of methionine
                       2.11.1.9  Likelihood of benefit due to antidote
         2.12. Summary of evaluation
               2.12.1. Indications
               2.12.2. Advised route and dosage
               2.12.3. Other consequential or supportive therapy
               2.12.4. Areas of use where there is insufficient
                       information to make recommendations
               2.12.5. Proposals for further studies
               2.12.6. Adverse effects
               2.12.7. Restrictions of use
         2.13. Model information sheet
               2.13.1. Uses
               2.13.2. Dosage and route
               2.13.3. Precautions/contraindications
               2.13.4. Adverse effects
               2.13.5. Use in pregnancy and lactation
               2.13.6. Storage
         2.14. References

    3.  N-ACETYLCYSTEINE

         3.1. Introduction
         3.2. Name and chemical formula
         3.3. Physico-chemical properties
               3.3.1. Melting point
               3.3.2. Physical state
               3.3.3. Solubility
               3.3.4. Optical properties
               3.3.5. pKa
               3.3.6. pH
               3.3.7. Stability
               3.3.8. Incompatibilities
               3.3.9. Proprietary names and manufacturers
         3.4. Pharmaceutical formulation and synthesis
         3.5. Analytical methods
               3.5.1. Quality control of antidote
               3.5.2. Methods for identification of the antidote
               3.5.3. Methods for analysis of the antidote in
                       biological samples
               3.5.4. Methods for analysis of toxic agent
         3.6. Shelf-life
               3.6.1. Formulations for oral use
               3.6.2. Formulations for intravenous use

         3.7. General properties
               3.7.1. Mode of antidotal activity
               3.7.2. Effect in paracetamol-induced liver failure
               3.7.3. Other therapeutic uses
         3.8. Animal studies
               3.8.1. Pharmacodynamics
               3.8.2. Pharmacokinetics
               3.8.3. Toxicology
               3.8.4. Studies with modified cytochrome P-450 activity
         3.9. Volunteer studies
               3.9.1. Absorption and bioavailability
               3.9.2. Distribution
               3.9.3. Elimination
               3.9.4. Oral  N-acetylcysteine and interaction with
                       activated charcoal
               3.9.5. Pharmacodynamics
         3.10. Clinical studies - clinical trials
               3.10.1. Efficacy of intravenous  N-acetylcysteine
               3.10.2. Efficacy of oral  N-acetylcysteine
               3.10.3. Oral versus intravenous  N-acetylcysteine
               3.10.4. Therapeutic drug monitoring during
                        N-acetylcysteine treatment
               3.10.5.  N-Acetylcysteine in paracetamol-induced
                       liver failure
         3.11. Clinical studies - case reports
               3.11.1. Adverse effects
               3.11.2. Use in pregnancy
         3.12. Summary of evaluation
               3.12.1. Indications
               3.12.2. Advised route and dosage
                       3.12.2.1  Intravenous  N-acetylcysteine
                       3.12.2.2  Oral  N-acetylcysteine
               3.12.3. Other consequential or supportive therapy
               3.12.4. Controversial issues and areas where there is
                       insufficient information to make recommendations
               3.12.5. Proposals for further studies
               3.12.6. Adverse effects
               3.12.7. Restrictions of use
         3.13. Model information sheet
               3.13.1. Uses
                       3.13.1.1  Use in liver failure
               3.13.2. Dosage and route
                       3.13.2.1  Intravenous  N-acetylcysteine
                       3.13.2.2  Oral  N-acetylcysteine
               3.13.3. Precautions/contraindications
               3.13.4. Pharmaceutical incompatibilities and drug
                       interactions
               3.13.5. Adverse effects
               3.13.6. Use in pregnancy and lactation
               3.13.7. Storage
         3.14. References
    

    WORKING GROUP ON VOLUME 3, EVALUATION OF ANTIDOTES

     Members

    Dr D.N. Bateman*, Department of Clinical Pharmacology, University of
    Newcastle, Newcastle-upon Tyne, United Kingdom

    Professor C. Bismuth, H˘pital Fernand Widal, Paris, France

    Professor B. Fahim, The Poison Control Unit, Ains Shams University
    Hospital, Cairo, Egypt

    Dr R. Fernando, National Poisons Information Centre, Faculty of
    Medicine, Colombo, Sri Lanka

    Dr R.E. Ferner, West Midlands Poisons Unit, Dudley Road Hospital,
    Birmingham, United Kingdom

    Dr J.A. Holme, National Institute of Public Health, Oslo, Norway

    Professor A. Jaeger, Pavillon Pasteur, Hospice Civil de Strasbourg,
    Service de Reanimation Medicale et Centre Anti-poisons, Strasbourg,
    France

    Dr C.K. Maitai, College of Health Sciences, Department of
    Pharmacology, University of Nairobi, Nairobi, Kenya

    Dr T.J. Meredith*, Department of Health, London, United Kingdom

    Dr H. Persson, Poison Information Centre, Karolinska Sjukhuset,
    Stockholm, Sweden  (Joint Chairman)

    Dr J. Pimentel, Intensive Care Unit, University Hospital, Coimbra,
    Portugal

    Professor L. Prescott*, Scottish Poison Information Service, The
    Royal Infirmary, Edinburgh, Scotland  (Joint Chairman)

    Dr J. Pronczuk, C.I.A.T., Hopital de Clinicas, Montevideo, Uruguay

    Dr M.-L. Ruggerone, Ospedale Niguarda, Centro Antiveleni, Milan, Italy

    Dr B.H. Rumack*, Micromedex Inc., Denver, Colorado, USA

    Dr H. Smet, Centre Belge Anti-Poisons, Brussels, Belgium

    Dr D.G. Spoerke, Micromedex Inc., Denver, Colorado, USA

    Dr J. Szajewski, Warsaw Poison Control Centre, Szpital Praski III.
    Oddzial Chorob Wewnetrznych, Warsaw, Poland

    Dr U. Taitelman, National Poisons Information Centre, Rambam Medical
    Centre, Haifa, Israel

    Dr W. Temple, National Toxicology Group, Otago University Medical
    School, Dunedin, New Zealand  (Joint Rapporteur)

    Ms J. Tempowski, Guy's Hospital, London, United Kingdom

    Dr J.A. Vale*, West Midlands Poison Unit, Dudley Road Hospital,
    Birmingham, United Kingdom

    Professor A.N.P. van Heijst, Bosch en Duin, The Netherlands

    Dr G. Volans, Poisons Unit, New Cross Hospital, London, United Kingdom

    Dr E. Wickstrom, National Poison Centre, Oslo, Norway

    Dr T. Zilker, Toxikologische Abteilung, II. Med. Klinik rechts der
    Isar, Munich, Germany

     Secretariat

    Dr J.-C. Berger*, Health and Safety Directorate, European
    Commission, Luxembourg

    Dr J.A. Haines*, International Programme on Chemical Safety, World
    Health Organization, Geneva, Switzerland

    Dr M. ten Ham, Drug Safety Programme, World Health Organization,
    Geneva, Switzerland

    *  Members of the drafting group that worked specifically on the
         texts in this volume at the Working Group meeting.

    PREFACE

         The need for an international evaluation of the clinical efficacy
    of antidotes and other substances used in the treatment of poisoning
    was first recognized at a joint meeting of the World Federation of
    Associations of Clinical Toxicology Centres and Poisons Control
    Centresa, the International Programme on Chemical Safety (IPCS) and
    the Commission of the European Communities (CEC), held at WHO
    headquarters, Geneva, 6-9 October 1985.  At the same time, the need to
    encourage the more widespread availability of those antidotes that are
    effective was recognised.  As a result, a joint IPCS/CEC project was
    subsequently initiated to address these problems.

         In the preparatory phase of the project, an antidote was defined
    for working purposes as a therapeutic substance used to counteract the
    toxic action(s) of a specified xenobiotic.  A preliminary list of
    antidotes for review, as well as of other agents used to prevent the
    absorption of poisons, to enhance their elimination and to treat their
    effects on body functions, was established.  For the purpose of the
    review process, antidotes and other substances were classified
    according to the urgency with which treatment with the antidote was
    thought on current evidence to be required and the (currently judged)
    clinical efficacy of the antidote in practice.  Those corresponding to
    the WHO concept of an essential drug were designated as such.  Some
    have already been incorporated into the WHO list of essential
    drugsb.  Antidotes and similar substances for veterinary use were
    also listed.  A methodology on the principles for evaluation of
    antidotes and other agents used in the treatment of poisonings was
    developed and this has subsequently been used as a framework for
    drafting monographs on specific antidotes (see also the introduction
    to this series in volume I for more information on the programme).

         Among the priorities established for evaluation in this project
    were antidotes for paracetamol poisoning.  The reason for this was the
    many patients poisoned with this over-the-counter analgesic, many of
    whom suffered serious liver damage and subsequently died, and the fact
    that there were two antidotes available, namely  N-acetylcysteine and
    methionine, with apparently similar efficacy but with different
    availability and therapy costs.  Furthermore, there were significant
    disagreements between research centres concerning the route by which
    the antidotes should be administered.

                 

    a  Now World Federation of Associations of Poisons Control Centres
       and Clinical Toxicology.

    b  WHO (1992) Use of Essential Drugs.  Model list of essential drugs
       (seventh list).  Fifth Report of the WHO Expert Committee. WHO
       Technical Report Series 825, Geneva, World Health Organization.

         Another important factor for avoiding complications in
    paracetamol poisoning is early administration of the antidotes; there
    is marked loss of efficacy when they are administered more than 10 h
    after ingestion of paracetamol. It is of interest that, during the
    course of preparation of this volume, there was increasing published
    evidence of the beneficial effect of therapy with  N-acetylcysteine
    even when administered at a very late stage of poisoning.  This
    observation further underlined the need for a scientific evaluation of
    this area by leading experts in the field.

         Of the two antidotes in this volume,  N-acetylcysteine has been
    most widely studied clinically.  There are far fewer published
    clinical data on methionine and therefore a special attempt has been
    made to evaluate both the preclinical and the few clinical data
    available for this antidote.

         The review and evaluation of these antidotes was initiated at a
    joint meeting of the IPCS and the CEC, organized by the Northern
    Poisons Unit and held at the Medical School of the University of
    Newcastle-upon-Tyne, United Kingdom, 13-17 March 1989.  In preparation
    for this meeting, monographs were drafted, using the proforma, on
     N-acetylcysteine by Dr B.H. Rumack and Dr D.G. Spoerke, and on
    methionine by Dr T.J. Meredith and Ms J. Tempowski.  After
    presentation in plenary, the draft documents on  N-acetylcysteine and
    methionine were reviewed by a Working Group consisting of Dr D.N.
    Bateman (Chairman), Dr T.J. Meredith (Rapporteur), Dr L. Prescott, Dr
    B.H. Rumack and Dr J.A. Vale. Following the meeting, preliminary
    revisions of the  N-acetylcysteine monograph were undertaken by Dr
    T.J. Meredith in consultation with Dr D.N. Bateman, Dr L. Prescott and
    Dr B.H. Rumack.  Both monographs were again reviewed at a Working
    Group consisting of Dr D.N. Bateman, Dr J.-C. Berger, Dr J.A. Haines,
    Dr T.J. Meredith and Dr L. Prescott, held at the Royal Infirmary,
    Edinburgh, United Kingdom, 25-26 September 1989, after which Dr L.
    Prescott prepared an overview chapter of antidotal therapy for acute
    paracetamol poisoning.

         Following this meeting, further drafting work was undertaken by
    authors and the overview chapter was reviewed by Dr D.N. Bateman and
    Dr J.A. Holme.  Draft texts were further revised by the series editors
    (Dr T.J. Meredith, Dr D. Jacobsen, Dr J.A. Haines, and Dr J.-C.
    Berger).  The efforts of all who helped in the preparation and
    finalization of this volume are gratefully acknowledged.

    ABBREVIATIONS

    ALAT           alanine aminotransferase
    ASAT           aspartate aminotransferase
    AUC            area under the curve
    GSH            glutathione
    HPLC           high-performance liquid chromatography
    NAD            nicotinamide adenine dinucleotide
    NAPQI           N-acetyl- p-benzoquinone imine
    U              units (international)

    IPCS/EC Evaluation of Antidotes Series

                                  Volume 3



                   Antidotes for Poisoning by Paracetamol



    First drafts of the chapters, subsequently reviewed and revised by the
    Working Group, were prepared by:

    Dr L.F. Prescott, Clinical Pharmacology Unit, Royal Infirmary,
    Edinburgh, United Kingdom (Overview)

    Dr D.G. Spoerke and Dr B.H. Rumack, Micromedex Inc., Denver, Colorado,
    USA ( N-acetylcysteine)

    Dr T.J. Meredith, UK Department of Health, London, United Kingdom, and
    Ms J. Tempowski, National Poisons Information Service (London Centre),
    London, United Kingdom (Methionine)

    1.  OVERVIEW OF ANTIDOTAL THERAPY FOR ACUTE PARACETAMOL POISONING

    1.1  Introduction and historical review

         Paracetamol (acetaminophen,  N-acetyl- p-aminophenol, APAP,
    NAPA, 4-hydroxy-acetanilide) was first introduced into clinical
    medicine towards the end of the last century but it attracted little
    attention and was soon forgotten (Smith, 1958).  There was a
    resurgence of interest in paracetamol when it was found to be the
    major metabolite of acetanilide and phenacetin (Brodie & Axelrod,
    1948a,b) and it was commonly assumed to be responsible for the
    therapeutic effects of both of these drugs.  Paracetamol has since
    been used increasingly as a substitute for other analgesics such as
    aspirin and phenacetin, and in the United Kingdom its sales have
    exceeded those of aspirin for more than a decade.  As a consequence of
    the "back door" introduction of paracetamol, there were no formal
    preclinical animal toxicity studies such as would be required today,
    and its potential hepatotoxicity was not suspected until the first
    clinical reports of severe and fatal liver damage following overdosage
    (Davidson & Eastham, 1966; Thomson & Prescott, 1966).  Severe hepatic
    necrosis was first observed in cats treated with paracetamol (25 mg/kg
    and then 50 mg/kg) for 22 weeks (Eder, 1964), and it was also
    described in rats given doses in the range of the acute LD50 and the
    100-day LD50 (Boyd & Bereczky, 1966; Boyd & Hogan, 1968).  The
    ability of paracetamol to produce acute centrilobular hepatic necrosis
    in experimental animals has since been confirmed repeatedly and there
    are major species differences in susceptibility.  Mice and hamsters
    are very sensitive while rats are resistant, and these differences
    have been related to species differences in the extent of the
    metabolic activation of paracetamol (Tee et al., 1987).

         Apart from single case reports from South Africa (Pimstone & Uys,
    1968) and the USA (Boyer & Rouff, 1971), the initial clinical
    descriptions of liver damage following paracetamol overdosage came
    from the United Kingdom, and substantial numbers of patients were
    involved (MacLean et al., 1968; Proudfoot & Wright, 1970; Prescott et
    al., 1971; Farid et al., 1972; Clark et al., 1973b).  With its
    increasing use, poisoning with paracetamol has since emerged as a
    significant problem in many other countries.  In the United Kingdom,
    paracetamol is taken in overdose most frequently by young adults who
    are not being prescribed psychotropic drugs by their general
    practitioners (Prescott & Highley, 1985).  In one study of 737
    patients in Newcastle-upon-Tyne it was taken by 11% of patients aged
    more than 65 years, 25% of those aged 35-64 years and 41% of patients
    less than 35 years of age (Wynne et al., 1987).  Overall, paracetamol
    is involved in some 15 to 30% of deliberate self-poisonings in the
    United Kingdom, and there is considerable regional variation (Platt et
    al., 1988).

         Much publicity has been given to paracetamol poisoning and there
    is no doubt that the problems have often been exaggerated.  Only a
    small minority of patients is at risk of severe liver damage and the
    liver has remarkable powers of regeneration.  Recovery from even
    severe damage is usually rapid and complete, and the overall mortality
    rate is low.  In England and Wales in 1984, a total of 176 deaths was
    attributed to poisoning with paracetamol alone and a further 305 to
    paracetamol taken with other drugs, notably d-propoxyphene.  However,
    a survey of such deaths showed that half of those officially recorded
    as being due to paracetamol and a quarter of those attributed to
    paracetamol taken with d-propoxyphene could not be substantiated. 
    Furthermore, more than 90% of patients dying outside hospital had no
    evidence of hepatic necrosis at necropsy (Meredith et al., 1986).  In
    a series of 394 fatal poisonings in New Zealand from 1975 to 1982,
    only 2 deaths were related to paracetamol overdosage (Cairns et al.,
    1983), and over a period of 20 years only one death was attributed to
    paracetamol among children in the United Kingdom (Fraser, 1980).

    1.2  Toxicity in man

         The major target organ in paracetamol poisoning is the liver and
    the primary lesion is acute centrilobular hepatic necrosis.  In adults
    the single acute threshold dose for severe liver damage (which has
    been arbitrarily defined as elevation of the plasma alanine or
    aspartate aminotransferase activity above 1000 U/l) is 150 to 250
    mg/kg but there is marked individual variation in susceptibility
    (Mitchell, 1977;  Prescott, 1983).  Children under the age of about 10
    years appear to be much more resistant than adults, but in any event
    they rarely ingest enough paracetamol to cause liver damage (Rumack,
    1984).  Only a small proportion of unselected adult patients who take
    an overdose of paracetamol are at risk of severe liver damage. 
    Without specific antidotal therapy, less than 10% would suffer severe
    liver damage but 1 to 2% will develop fulminant hepatic failure and
    this is often fatal.  One to 2% of patients develop acute renal
    failure requiring dialysis (Hamlyn et al., 1978; Prescott, 1983).

         When the patient is first seen, the severity of intoxication with
    paracetamol cannot usually be determined on clinical grounds alone, as
    there are no specific symptoms or signs.  Consciousness is not
    depressed unless other drugs have also been taken or there is a very
    high plasma paracetamol concentration of the order of 6.62 mmol/l
    (1000 mg/l) with a metabolic acidosis (Gray et al., 1987).  Nausea and
    vomiting usually develop within a few hours of ingestion of a
    hepatotoxic dose of paracetamol and at this stage liver function tests
    may be normal or only slightly deranged.  From about 18 to 72 h after
    ingestion there may be hepatic tenderness and abdominal pain due to
    swelling of the liver capsule.  Unless hepatic failure develops, there
    is usually rapid improvement after the third day with eventual
    complete recovery.

         The maximum abnormality of liver function tests is usually
    delayed until the third day.  The characteristic changes include
    dramatic elevation of the plasma alanine and aspartate transaminase
    activity from normal values of less than 40 to as much as 10 000 or
    even 20 000 U/l with mild to moderate increases in the plasma
    bilirubin concentration and prothrombin time ratio.  The sudden
    dramatic increase in the activity of plasma transaminases is
    presumably caused by their release from a large mass of necrotic
    hepatocytes, and the prolongation of the prothrombin time reflects
    acute impairment of synthesis of the vitamin K-dependent clotting
    factors.  There is little or no increase in the plasma alkaline
    phosphatase activity unless liver damage is severe or the patient is
    a chronic alcoholic.  Liver biopsies show extensive centrilobular
    hepatic necrosis with little inflammatory reaction.  In patients who
    recover, liver function tests become normal within 1 to 3 weeks and
    follow-up histological examination reveals regeneration, repair and
    eventually a return to normal appearances (Portmann et al., 1975;
    Lesna et al., 1976).  Other reported complications of paracetamol
    poisoning include disturbances of coagulation with disseminated
    intravascular coagulation (Clark et al., 1973a), acute pancreatitis
    (Gilmore & Tourvas, 1977), impaired carbohydrate tolerance (Record et
    al., 1975), myocarditis (Wakeel et al., 1987) and hypophosphataemia
    (Jones et al., 1989).  In the context of massive hepatic necrosis and
    fulminant hepatic failure, it is doubtful whether these abnormalities
    can be specifically related to paracetamol toxicity  per se.  Serial
    measurements of the prothrombin time probably give the best guide to
    prognosis (Harrison et al., 1990).

         Oliguric renal failure may become apparent within 24 to 48 h
    after the overdose of paracetamol, and in this setting it is almost
    always associated with back pain, microscopic haematuria and
    proteinuria.  This early impairment of renal function can occur in the
    absence of significant hepatic injury (Cobden et al., 1982; Prescott,
    1983). Renal failure may be mild and transient or severe and prolonged
    requiring dialysis. It may also occur later, after the onset of
    hepatic encephalopathy.

         Fulminant hepatic failure may develop in severely poisoned
    patients from the third to the sixth day.  It is characterized by
    deepening jaundice, encephalopathy, increased intracranial pressure,
    grossly disordered haemostasis with disseminated intravascular
    coagulation and haemorrhage, hyperventilation, acidosis, hypoglycaemia
    and renal failure.  The prognosis is very poor (Clark et al., 1973b;
    Canalese et al., 1981).

    1.3  Assessment of the severity of intoxication

         Because of the absence of early specific symptoms and signs, the
    only reliable method of assessment of the severity of poisoning (and
    hence the need for antidotal therapy) is emergency measurement of the
    plasma paracetamol concentration in relation to the time since

    ingestion.  Patients with concentrations above a line joining plots on
    a semi-logarithmic graph of 1.32 mmol/l (200 mg/l) at 4 h and 0.20
    mmol/l (30 mg/l) at 15 h after ingestion (called the "treatment line")
    have about a 60% chance of developing severe liver damage as defined
    by elevation of the plasma transaminase activity above 1000 U/l.  In
    patients with concentrations above a parallel line joining 2 mmol/l
    (300 mg/l) at 4 h and 0.33 mmol/l (50 mg/l) at 15 h the probability
    rises to 90% (Fig. 1).

         Plasma paracetamol concentrations determined less than 4 h after
    the overdose cannot be interpreted because of the possibility of
    continuing absorption.  The "treatment line" defined above was derived
    from studies in patients admitted to the Regional Poisoning Treatment
    Centre in Edinburgh from 1969-1973 before effective treatment became
    available (Prescott et al., 1971, 1974, 1977).  Its validity for
    patients in the United Kingdom was subsequently confirmed in studies
    carried out in London (Gazzard et al., 1977) and Newcastle-upon-Tyne
    (Hamlyn et al., 1978).  The data from the original studies carried out
    in Edinburgh were later used by Rumack & Matthew (1975) to develop the
    "nomogram" which is used in the USA.  Although generally accepted as
    a good guide to management and the need for specific treatment, the
    "treatment line" is not infallible.  Patients with values above the
    line often do not develop liver damage while severe liver damage may
    rarely occur in patients with paracetamol concentrations as low as
    0.83 mmol/l (125 mg/l) at 4 h.  In the USA, patients are given
     N-acetylcysteine when concentrations are above a lower treatment
    line corresponding to 1 mmol/l (150 mg/l) at 4 h (Smilkstein et al.,
    1988, 1991) (Fig. 1).

    1.4  Mechanisms of toxicity and antidotal activity

         Until Mitchell and his colleagues elucidated the mechanisms of
    paracetamol hepatotoxicity, there was no effective treatment for
    paracetamol poisoning (Mitchell et al., 1973a,b; Potter et al., 1973;
    Jollow et al., 1973).  In a series of classical studies they showed
    that a minor route of paracetamol metabolism involved its conversion
    by cytochrome P-450-dependent mixed-function oxidase to a reactive
    arylating metabolite, now known to be  N-acetyl- p-benzoquinone
    imine (NAPQI), which may cause acute hepatic necrosis with toxic doses
    of paracetamol (Dahlin et al., 1984; Holme et al., 1984).  Initially,
    the reactive metabolite of paracetamol was believed to result from
    oxidation of the drug to  N-hydroxy-paracetamol followed by
    dehydration to NAPQI (Hinson et al., 1980; Holme et al., 1982). More
    recent studies indicate a direct two-electron oxidation of paracetamol
    to NAPQI by cytochrome P-450, or alternatively, a one-electron
    oxidation to  N-acetyl- p-benzosemiquinone imine by peroxidase,
    prostaglandin H synthetase or cytochrome P-450 (Dahlin et al., 1984;
    Potter & Hinson, 1987).  NAPQI causes a depletion of both the
    mitochondrial and cytosolic pools of reduced glutathione (GSH)
    (Tirmenstein & Nelson, 1989).  Once GSH is depleted, cellular proteins
    are directly arylated and oxidized by the reactive metabolite (Albano

    et al., 1985; Holme & Jacobsen, 1986), resulting in inhibition of
    enzyme activities.  Two of the enzymes that have been shown to be
    inhibited in paracetamol-treated animals are glutathione peroxidase
    and thiol transferase (Tirmenstein & Nelson, 1990).  Inhibition of
    these enzymes renders the cell vulnerable to endogenous activated
    oxygen species with further oxidation of protein thiols.  Decreased
    plasma membrane Ca2+-ATPase activity and impaired mitochondrial
    sequestration of Ca2+ lead to influx of extracellular Ca2+
    (Tsokos-Kuhn et al., 1988; Tirmenstein & Nelson, 1989), with
    large-scale calcium cycling by mitochondria resulting in oxidative
    stress and cell death (Thomas & Reed, 1988). Disturbed Ca2+
    homeostasis is likely to activate Ca2+-dependent catabolic processes
    such as phospholipid degradation, protein degradation, disruption of
    the cytoskeleton and DNA fragmentation (Ray et al., 1990; Orrenius et
    al., 1991). Although several lines of evidence suggest that Ca2+
    influx is an early event in the development of toxicity, results from
    a recent paper indicate that this is not always the case (Herman et
    al., 1992). Furthermore, secondary microcirculatory changes may
    exacerbate the original injury and extend the necrosis through
    ischaemic infarction of the periacinar region.  Macrophages and
    neutrophils are attracted to the damaged areas and lead to additional
    protein thiol modification by releasing oxidants (Mitchell, 1988).

         The maintenance of hepatic glutathione (GSH) concentrations by
    administration of  N-acetylcysteine was first suggested as a
    treatment for paracetamol poisoning by Prescott & Matthew (1974).  GSH
    itself, due to its inability to cross the plasma membrane, cannot be
    used as an antidote.  However, GSH precursors such as
     N-acetylcysteine have been found to be effective both in
    experimental animals and in humans (Boobis et al., 1989). 
     N-acetylcysteine may reduce the severity of liver necrosis by
    directly conjugating with and/or reducing the reactive metabolite
    NAPQI (Tee et al., 1986).  In addition,  N-acetylcysteine forms other
    nucleophiles, such as cysteine and GSH, that are also capable of
    detoxifying NAPQI (Corcoran et al., 1985; Boobis et al., 1989).

          N-acetylcysteine is effective as an antidote when given some
    time after paracetamol exposure (Devalia et al., 1982).  It appears
    that  N-acetylcysteine, either directly or through synthesis to
    cysteine and GSH, decreases the toxic effect of activated oxygen and
    reduces oxidized thiol groups on enzymes (Boobis et al., 1989).  In
    addition,  N-acetylcysteine has been shown to decrease the amount of
    paracetamol bound covalently to proteins, possibly by dissociation of
    the covalently bound paracetamol from proteins and/or enhancing
    degradation of the arylated proteins (Bruno et al., 1988; Rundgren et
    al., 1988).

    FIGURE 01

         The ability of  N-acetylcysteine to restore the function of
    enzymes after paracetamol exposure and its capacity to detoxify,
    either directly or indirectly, reactive metabolites through
    facilitation of GSH synthesis, are probably both responsible for its
    protective effect against paracetamol toxicity in humans.

         Theoretically,  N-acetylcysteine could be preferred to
    methionine for the treatment of paracetamol poisoning.  Unlike
     N-acetylcysteine and glutathione, methionine is not a thiol and
    therefore cannot form an adduct directly with the reactive metabolite
    of paracetamol. Furthermore, enzymes such as cystathione synthetase
    and cystathionase, which are necessary for the essential conversion of
    methionine to cysteine  in vivo, themselves have functional SH groups
    which might be expected to be vulnerable to inactivation by
    paracetamol. In such circumstances, it might also be expected that
    methionine would be less effective than  N-acetylcysteine in the late
    treatment of severe paracetamol poisoning.  Despite these theoretical
    arguments, clear differences in clinical efficacy have not been
    established.

    1.5  Factors influencing the toxicity of paracetamol

         Paracetamol hepatotoxicity depends on the metabolic balance
    between the rate of formation of the toxic arylating metabolite and
    the rate of glutathione conjugation. In animals, experimental
    stimulation of metabolic activation of paracetamol and glutathione
    depletion increases toxicity, while, conversely, toxicity is decreased
    by inhibition of paracetamol oxidation and stimulation of glutathione
    synthesis. In addition, inhibition of direct detoxification such as
    sulfate conjugation and glucuronidation may increase the proportion of
    the dose which is activated. One might assume that the same factors
    apply in humans but this has never been proved. Both the rate of
    formation and the total amount of NAPQI formed depend on the rate of
    absorption and environmental and genetic determinants of oxidative
    drug-metabolizing enzyme activity, as well as on the capacity of
    parallel pathways for elimination of paracetamol (glucuronide and
    sulfate conjugation).

    1.5.1  Factors that may increase paracetamol toxicity

         Of a number of purified rabbit hepatic isoenzymes of cytochrome
    P-450, P-4502E1 and P-4501A2 exhibit appreciable activity in the
    bioactivation of paracetamol (Morgan et al., 1983).  Using monoclonal
    antibodies, isoenzymes P-4502E1 and P-4501A2 have been found to be
    approximately equally responsible for paracetamol bioactivation in
    human hepatic microsomes (Raucy et al., 1989).  There are large human
    interindividual differences in the oxidative metabolism of paracetamol
    (Raucy et al., 1989).  In animals cytochrome P-4502E1 is induced by
    pretreatment with ethanol (Morgan et al., 1982), and diabetes, acetone
    or fasting (Jeffery et al., 1991).  Song et al. (1990) have been able
    to quantify cytochrome P-4502E1 in the peripheral blood lymphocytes of

    some individuals and have shown the level to be considerably enhanced
    in diabetic patients who do not respond to insulin.  The level of
    hepatic cytochrome P-4502E1 has been found to be elevated in
    alcoholics (Perrot et al., 1989; Raucy et al., 1989).

         Chronic administration of ethanol to mice, rats or hamsters can
    enhance the hepatotoxic effects of paracetamol, and there have been a
    number of anecdotal case reports of paracetamol-induced hepatic injury
    among alcoholics resulting from apparent therapeutic misadventure
    (Zimmerman, 1986; Seeff et al., 1986; Floren et al., 1987).  There is,
    however, some disagreement as to whether therapeutic doses of
    paracetamol produce liver injury in patients with chronic alcoholism
    (Prescott, 1986; Mitchell, 1988).  Of interest is the fact that acute
    intake of ethanol at the time of paracetamol overdose is protective in
    animals and humans (Zimmerman, 1986).  Taking into consideration
    animal and human studies, a reduction of the threshold for use of
     N-acetylcysteine after paracetamol overdose in patients with chronic
    alcoholism has been suggested by McClements et al. (1990).  There are,
    however, no firm data in support of this recommendation.

         Depletion of hepatic glutathione stores by feeding a low protein
    diet or by pretreatment with diethylmaleate will markedly augment
    paracetamol toxicity (Price & Jollow, 1983).  Decreased concentrations
    of glutathione may also explain any increased susceptibility to
    paracetamol in alcoholics (Lauterburg & Velez, 1988; Smilkstein et
    al., 1988).

         A possible protective effect of antioxidants and a possible
    increased toxicity of paracetamol in vitamin E-deficient mice
    (Fiarhurst et al., 1982) have no documented clinical significance.

    1.5.2  Factors that may reduce paracetamol toxicity

         Many compounds, such as  N-acetylcysteine and methionine (see
    section 1.4), have been shown to reduce paracetamol toxicity either by
    reacting directly with NAPQI or by facilitating glutathione synthesis. 
    Since the first step in paracetamol metabolism is its bioactivation to
    NAPQI, inhibition of this process is, theoretically, of clinical
    relevance.  Several experimental studies have shown a more or less
    protective effect on paracetamol toxicity, as discussed below. 
    However, the clinical relevance of these experimental results has yet
    to be established.

         Pretreatment with piperonyl butoxide or cobaltous chloride, which
    inhibit hepatic microsomal function, protects against paracetamol-
    induced hepatotoxicity in animals.  Cimetidine protects against
    hepatotoxicity of paracetamol in animals by inhibiting its metabolic
    activation (Speeg et al., 1985).  However, the effect of cimetidine in
    the prevention of liver damage in humans is uncertain (Critchley et
    al., 1983).  Concomitant exposure to ethanol appears to reduce
    activation of paracetamol to reactive metabolites in rats (Wong et

    al., 1980).   In vitro studies with liver slices, however, indicate
    that ethanol also protects after paracetamol exposure has ceased,
    which could be due to an increase in the NADH/NAD ratio (Mourelle et
    al., 1990).  Ethanol given acutely appears to reduce the metabolic
    activation of paracetamol in humans (Critchley et al., 1983).

         Calcium channel blocking agents such as nifedipine (Landan et
    al., 1985) and diltiazem (Deakin et al., 1991) have been shown to
    reduce marginally the development of paracetamol-induced liver
    necrosis in rats. Similar effects have been reported with inhibitors
    of phospholipase A2, cyclooxygenase and thromboxane synthetase
    (Horton & Wood, 1989).

         The hepatotoxic effect of paracetamol in female mice is reduced
    by feeding the animals a diet containing 0.75% butylated hydroxyanisol
    (Miranda et al., 1983), possibly by increasing the concentration of
    reduced glutathione in the liver (Miranda et al., 1985). Other
    antioxidants and inhibitors of lipid peroxidation such as
    diethyldithiocarbamate and anisyldithiolthione, may also protect
    against paracetamol-induced liver damage (Mansuy et al., 1986; Younes
    et al., 1988).

    1.6  Diagnosis of paracetamol intoxication

         Many methods have been described for the estimation of
    paracetamol in plasma.  These include procedures based on ultraviolet
    (UV) spectrophotometry (Routh et al., 1968), colorimetry, (Brodie &
    Axelrod, 1948a; Glynn & Kendal, 1975), gas liquid chromatography
    (Prescott, 1971) and high performance liquid chromatography with UV
    (Howie et al., 1977) or electrochemical detection (Riggin et al.,
    1975).  More advanced techniques for the identification and estimation
    of paracetamol and its metabolites include fast atom bombardment mass
    spectrometry (Lay et al., 1987), thermospray liquid
    chromatography/mass spectrometry (Betowski et al., 1987) and proton
    nuclear magnetic resonance (Bales et al., 1988).  At the same time, a
    number of operationally simple methods have been introduced for
    clinical use.  These depend on electrochemical or colour reactions
    after enzymatic hydrolysis of paracetamol to  p-aminophenol (Price et
    al., 1983) and immunoassay including techniques based on fluorescence
    polarisation (Hepler et al., 1984; Coxon et al., 1988).

         The ideal method for the emergency estimation of plasma
    paracetamol in poisoned patients should be inexpensive, simple, rapid
    and accurate at least over the range of 0.1-3.31 mmol/l (15 to 500
    mg/l).  It should not be subject to interference by metabolites or
    other drugs, not require the use of complex apparatus and be capable
    of being used by staff without special skills or training.  No one
    method meets all of these criteria, and the subject has been reviewed
    critically (Weiner, 1978; Stewart & Watson, 1987). Whatever method is
    used, it is particularly important to check the units used by the 
    laboratory for reporting plasma paracetamol concentrations.  Most

    clinical toxicologists still use mass units such as mg/l, while some
    laboratories report results in SI units.  This can cause confusion
    which may be dangerous (1 mmol/l is equivalent to 151 mg/l).  Serious
    problems have also arisen through the inappropriate use of non-
    specific methods which can give gross overestimates of plasma
    paracetamol concentrations because they also measure metabolites
    (Stewart et al., 1979).

    1.7  Management of severe paracetamol poisoning

         Management of the patient with severe paracetamol poisoning can
    be considered under the headings of supportive care and specific
    antidotal therapy.  The possible role of liver transplantation is also
    briefly discussed.

    1.7.1  Supportive care

         Supportive care is based on removal of unabsorbed drug,
    symptomatic treatment and the management of serious complications such
    as hepatic and renal failure.  Gastric aspiration with lavage, or
    induction of emesis with syrup of ipecac (ipecacuanha), is usually
    carried out in patients who are thought to have taken at least 100 mg
    paracetamol/kg within the previous 1-2 h. Activated charcoal has also
    been recommended. Unfortunately, paracetamol is normally absorbed very
    rapidly, and it is uncommon to obtain a good return of tablet
    material.  Provided that more than 4 h have elapsed since the time of
    ingestion, a blood sample should be taken for the emergency estimation
    of the plasma paracetamol concentration and for baseline measurements
    of liver function tests, prothrombin time ratio, and plasma urea,
    creatinine and electrolytes.  It will be found that most patients are
    not severely poisoned and so do not require specific treatment or
    further supportive care.  In patients with protracted nausea and
    vomiting, maintenance of intravenous fluids and electrolytes may be
    required and a careful watch should be kept on the fluid balance;
    hypophosphataemia has been reported (Jones et al., 1989).  Because of
    the possibility of impending liver failure with gross impairment of
    drug metabolism, other drugs (including anti-emetics) should only be
    given if really necessary.  The biochemical tests of hepatic and renal
    function should be monitored in patients at risk at least every 12 to
    24 h, depending on the severity of intoxication and clinical state.

         Acute oliguric renal failure during the first 24 to 48 h may be
    accompanied by severe back and loin pain.  Fluid and electrolyte
    balance must be monitored carefully and dialysis is often necessary. 
    The plasma urea and creatinine concentrations may rise slowly but
    progressively over a period of many days before renal function
    recovers.

         The onset of acute, possibly fatal, hepatic failure is indicated
    by a rapid rise of the prothrombin time to a ratio of more than 5.0,
    gross elevation of the plasma alanine aminotransferase (ALAT) and
    appearance of mild jaundice within 36 to 48 h.  In such circumstances
    vitamin K1 is usually given parenterally and, depending on the
    results of serial clotting screens, the intravenous administration of
    clotting factor concentrate or fresh frozen plasma may be necessary to
    keep the prothrombin time ratio within a safe range.  Careful
    attention must be given to fluid, electrolyte and acid-base balance,
    and it is important to avoid fluid overload as this will aggravate
    cerebral oedema.  Neomycin (1 g every 4 to 6 h) and lactulose
    administration by nasogastric tube should be considered, as in the
    case of acute liver failure from other causes.  Hypoglycaemia may
    occur at any time and should be prevented by intravenous
    administration of fluids containing glucose.  Established acute liver
    failure should be treated by conventional methods (Williams, 1988) but
    the prognosis is very poor, even in specialist centres using measures
    such as orthotopic liver transplantation (O'Grady et al., 1988, 1991;
    Harrison et al., 1991).

    1.7.1.1  Role of  N-acetylcysteine in paracetamol-induced liver
             failure

         The original studies of  N-acetylcysteine treatment for
    paracetamol poisoning gave no evidence of benefit when this treatment
    was delayed for more than 15 h (Prescott et al., 1977, 1979).  Later,
    the prospective studies by Smilkstein et al. (1988, 1991) suggested
    that treatment with oral  N-acetylcysteine may be effective up to 24
    h after ingestion of the paracetamol.  None of these studies were,
    however, designed for studying the effect of  N-acetylcysteine on
    established paracetamol-induced liver failure.  In patients with
    fulminant hepatic failure after paracetamol overdose (without previous
     N-acetylcysteine treatment),  N-acetylcysteine significantly
    increased the survival rate (48%, 12/25 patients) as compared to
    controls (20%, 5/25) (Keays et al., 1991). The intravenous dose
    regimen in this prospective randomised controlled study was the same
    as recommended for paracetamol overdose, and  N-acetylcysteine was
    given 53 h (range 36-80 h) after the overdose.

         The mechanism(s) for this protective effect of  N-acetylcysteine
    on established liver failure is not clear but may be related to
    increased tissue oxygen consumption and decreased oxidant stress, thus
    reducing the oxidation of important protein thiol groups (Keays et
    al., 1991).

         Earlier fears that the late administration of intravenous
     N-acetylcysteine might be hazardous have proved to be unfounded. 
    The antidote is therefore indicated both in the acute phase of
    paracetamol intoxication (section 1.7.2), provided that serum
    paracetamol concentrations fall above the so-called treatment line,
    and in established paracetamol liver failure.

         The role of  N-acetylcysteine in other types of acute liver
    failure has not been studied, nor has the effect of methionine on
    paracetamol-induced liver failure been studied.

    1.7.1.2  Role of liver transplantation

         It is very difficult to perform, at the right moment, an adequate
    triage, based on clinical and biochemical parameters, of patients at
    significant risk of dying from hepatic failure in paracetamol
    poisoning.  The correct time for doing this is early enough to provide
    the potential recipient with a donor organ at a time where he/she is
    still in an operable condition.  Many studies over the years have
    indicated that the prothrombin time is the most reliable parameter in
    evaluating the risk of dying from liver failure following paracetamol
    overdose (Harrison et al., 1990).  Patients with a continuous increase
    in prothrombin time on day 4 after overdose and a peak prothrombin
    time of > 180 seconds appear to have a less than 8% chance of
    survival (Harrison et al., 1990).

         Recently O'Grady et al. (1991) performed a prospective study of
    66 cases of severe paracetamol poisoning transferred to their Liver
    Unit in London.  Of these, 37 patients (of whom 30 survived) were
    considered to have a reasonable prognosis with intensive care.  Of 14
    out of 29 patients considered to have a very poor prognosis and
    registered for urgent liver transplantation, six received liver
    transplants, four of whom survived, while seven died and one survived
    without a transplant.  Three out of 15 patients who had poor
    prognostic indicators but were not selected for transplantation
    survived.

         These results indicate that liver transplantation may have a
    definite, but very limited role in the treatment of paracetamol
    poisoning. Among arguments against liver transplantation are the fact
    that some patients recover completely while waiting in vain for their
    donor liver, and that liver transplantation in this acute stage is not
    without complications. Even a successful transplantation implies
    life-long immunosuppressive therapy.

    1.7.2  Specific antidotal therapy

    1.7.2.1  Intravenous  N-acetylcysteine

         Treatment with intravenous  N-acetylcysteine is indicated in
    patients who present within 15 h of taking paracetamol in overdose and
    who have plasma paracetamol concentrations above the treatment line
    defined in section 1.3.  The regimen consists of intravenous
    administration of 150 mg/kg made up in 200 ml of 5% dextrose over 15
    min, followed by 50 mg/kg in 500 ml of 5% dextrose over 4 h and 100
    mg/kg in 1 litre of 5% dextrose over 16 h.  The total dose is 300
    mg/kg given over 20 h.  This regimen effectively prevents liver

    damage, renal failure and death if started within 8 h of paracetamol
    ingestion but efficacy falls off rapidly after this time.

         Later studies have suggested that treatment with oral or
    intravenous  N-acetylcysteine may be effective up to 24 h after
    ingestion of the paracetamol (Smilkstein et al., 1988, 1991).  It
    therefore appears reasonable to propose treatment with
     N-acetylcysteine as an antidote up to 24 h after ingestion. In the
    most recent study by Smilkstein et al. (1991), the intravenous dose
    regimen of  N-acetylcysteine was increased to 980 mg/kg over 48 h.
    Although this study was not scientifically comparable with that of
    Prescott et al. (1979), there are indications that less
    hepato-toxicity may occur using the 48-h treatment protocol among
    patients at "high risk" (Fig. 1) and admitted more than 10 h
    post-ingestion.

         Because of the critical ingestion-treatment interval of 8 h,
    patients who are thought to be at risk and who present at or after
    this time should be treated with intravenous  N-acetylcysteine
    immediately.  A blood sample should be taken for the emergency
    estimation of the plasma paracetamol concentration, and if this
    subsequently turns out to be below the treatment line,
     N-acetylcysteine can easily be discontinued.  The plasma paracetamol
    concentration should also be determined in patients who present
    earlier, but treatment with  N-acetylcysteine must always be started
    by 8 h if the laboratory result is not available.  Although it might
    appear simpler to give all patients  N-acetylcysteine on admission,
    this is not appropriate because a majority of patients would be
    treated unnecessarily.  Moreover, the use of  N-acetylcysteine is
    some times accompanied by adverse effects.

         "Anaphylactoid" reactions to intravenous  N-acetylcysteine have
    been reported but the overall incidence is low.  In some cases the
    doses were excessive (Mant et al., 1984), while in others the drug was
    not indicated in the first place and should never have been given (Ho
    & Beilin, 1983; Dawson et al., 1989).  The reactions have usually
    consisted of urticaria, hypotension or bronchospasm and most have been
    mild and transient.  They usually occur during the first 15 to 60 min
    of therapy at a time when plasma concentrations of  N-acetylcysteine
    are highest, and they probably represent a concentration-dependent
    pharmacological effect (Bateman et al., 1984; Prescott et al., 1989;
    Smilkstein et al., 1991).

    1.7.2.2  Oral  N-acetylcysteine

          N-acetylcysteine is given orally in the USA and there have been
    several reports of the results of a National Multicentre Study (Rumack
    & Peterson, 1978; Rumack et al., 1981; Smilkstein et al., 1988).  The
    dose was 140 mg/kg followed by 17 doses of 70 mg/kg every 5 h, and the
    total dose was 1330 mg/kg over 72 h (i.e. about 100 g in a 70 kg
    adult).  This dose is much larger than that used in any other study.

    In the most recent update, the cumulative results were described for
    2540 patients, and efficacy was assessed according to the initial
    plasma paracetamol concentration and the delay between ingestion and
    treatment.  Hepatotoxicity developed in 6.1% of patients at "probable"
    risk when treatment was started within 10 h and in 26.4% when therapy
    was commenced 10 to 24 h after ingestion.  Hepatotoxicity also
    occurred in 41% of the patients at "high risk" treated between 14 and
    16 h after ingestion.  There were 11 deaths (0.43% of 2540 patients),
    but none could clearly be attributed to paracetamol, when
     N-acetylcysteine was started within 16 h.  On the basis of the
    results obtained, the authors suggest that treatment might still be
    effective when delayed for as long as 24 h, and that this oral regimen
    might be more effective than intravenous  N-acetylcysteine,
    particularly when treatment was delayed (Smilkstein et al., 1988).
    This suggestion was, however, based on comparisons between patients
    given oral  N-acetylcysteine and patients treated with intravenous
     N-acetylcysteine and control patients seen up to 15 years previously
    in the United Kingdom.  The patients were not comparable from a
    demographic point of view and more importantly, the American patients
    were less severely poisoned than the patients with whom they were
    compared.  Smilkstein et al. (1988) presented results for a total of
    2540 patients, but only 2023 had plasma paracetamol concentrations
    above a treatment line starting at 1 mmol/l (150 mg/l) at 4 h and only
    1462 (58%) had concentrations above the treatment line accepted in the
    United Kingdom (which starts at 1.32 mmol/l (200 mg/l) at 4 h).  Thus
    almost half of the American patients were at very low risk and would
    not have been treated in the United Kingdom or included in the study. 
    It is therefore not surprising that oral  N-acetylcysteine appeared
    to be more effective when given orally than intravenously. However,
    when the patients at "high risk" admitted late (16-24 h) were studied
    separately, there was an indication in favour of prolonged
     N-acetylcysteine treatment in this group.

         Even so, oral  N-acetylcysteine may be employed in the majority
    of patients with paracetamol poisoning who are thought to be at
    significant risk of liver damage.  Treatment in this manner has been
    recommended up to 24 h after ingestion of the paracetamol.  No serious
    adverse effects have been reported, although nausea and vomiting are
    common (Rumack & Peterson, 1978).  Intravenous therapy should be
    considered in patients who are vomiting and in those who have been
    given emetics or oral activated charcoal.

    1.7.2.3  Oral methionine

         Treatment with oral methionine is indicated in patients who
    present within 15 h of taking paracetamol in overdose and who have
    plasma paracetamol concentrations above the treatment line defined in
    section 1.3.  Oral methionine is very safe, and although the plasma
    paracetamol concentration should always be measured if possible,
    treatment should never be delayed while awaiting the laboratory
    result.  The dose of methionine is 2.5 g (10 x 250 mg tablets) orally

    repeated 4 hourly to a total dose of 10 g over 12 h.  There have been
    two reports of the use of oral methionine in the treatment of
    paracetamol poisoning, and overall the results are similar to those
    obtained with  N-acetylcysteine.

         One study involved a comparison of patients in London and
    Newcastle-upon-Tyne, United Kingdom, treated within 10 h with oral
    methionine (13 patients), intravenous cysteamine (14 patients) or
    supportive therapy only (13 patients).  Both active agents gave
    significant protection against liver damage but there were no
    important differences between them (Hamlyn et al., 1981).  In the
    other study, the results of treating 132 patients in London with oral
    methionine were compared with those of similarly poisoned control
    patients who had previously received supportive therapy in Edinburgh
    (Vale et al., 1981).  As before, oral methionine was found to be very
    effective in preventing liver damage when given within 10 h.  It was
    much less effective when treatment was delayed to 10-24 h.  Oral
    methionine may therefore be used to treat patients with paracetamol
    poisoning who are at significant risk of liver damage.  There are no
    recommendations at present for the use of oral methionine more than 15
    h after ingestion of an overdose of paracetamol.  Side-effects to oral
    methionine have not been reported in patient with paracetamol
    poisoning.  Intravenous  N-acetylcysteine should be considered in
    those who are vomiting and in patients who have been given emetics or
    oral activated charcoal.

    1.7.2.4  Intravenous methionine

         In the study by Prescott et al. (1976), 3 out of 15 patients at
    risk of liver damage from paracetamol and treated within 10 h
    developed severe liver damage.  All three were given intravenous
    methionine 9-10 h following the ingestion of paracetamol (see section
    2.10 for further details).

         Centres in other countries (such as in Oslo, Norway) also have
    experience with the use of intravenous methionine (10 g over 12 h) and
    none of about 50 patients at risk of liver damage suffered such damage
    or side effects provided that methionine was given within 10 h
    following paracetamol ingestion (E. Enger, personal communication). 
    Methionine is no longer given intravenously, there being no
    pharmaceutical preparation available.

    1.7.2.5  Oral versus intravenous therapy

         There is controversy concerning the optimal route of
    administration of  N-acetyl-cysteine and methionine.  The obvious
    advantage of the oral route is that most of the absorbed dose passes
    directly to the sites of action in the liver.  Oral therapy is also
    simpler and cheaper, and can be given by non-medical health care
    workers in developing countries. Since systemic adverse effects have
    not been reported following oral therapy with either

     N-acetylcysteine or methionine, it is not so important to identify
    patients requiring treatment by prior measurement of the plasma
    paracetamol concentration.

         On the other hand, the efficacy of oral treatment may be
    compromised if absorption is delayed or incomplete as a result of
    nausea and vomiting. A substantial proportion of severely poisoned
    patients develop nausea and vomiting within a few hours and it is in
    these circumstances that effective reliable treatment is most needed. 
    Oral therapy must be given by nasogastric tube in unconscious patients
    and this route is inappropriate in patients who have been given
    emetics or oral activated charcoal.  Although one route of
    administration does not appear to have any striking advantage over the
    other, prospective comparative studies in patients admitted at the
    critical time of about 8 h after ingestion of paracetamol have not
    been carried out.

    1.7.2.6  Comparative efficacy of  N-acetylcysteine and methionine

         On the limited data available, it is not possible to state
    whether  N-acetylcysteine is superior to methionine.  The comparisons
    which have been made so far are only valid up to a point because of
    the lack of proper controls.  As discussed above, there are
    theoretical reasons why  N-acetylcysteine may be more effective than
    methionine in preventing liver damage under certain circumstances;
    there are also few data on the use of methionine in children and no
    clinical data on its use in established paracetamol-induced liver
    failure. There is also an indication of a beneficial effect of
     N-acetylcysteine in patients admitted 10-24 h after the overdose
    (Smilkstein et al., 1988). Lack of such an indication in methionine-
    treated patients may, however, be related to the fact that this
    compound has not been studied in as much detail as  N-acetylcysteine.
    This question can only be answered by careful prospective comparative
    studies in large numbers of properly matched patients with appropriate
    controls.

    1.7.3  Summary of treatment recommendations

         Paracetamol poisoning is not an immediate threat to life, and
    little can be achieved in the way of first aid outside the hospital. 
    The most that can be done is to induce vomiting by pharyngeal
    stimulation, and to arrange transport to hospital. Definitive hospital
    treatment is based on early administration of sulfhydryl donors such
    as  N-acetylcysteine and methionine, and supportive care.  The latter
    includes removal of unabsorbed drug and management of complications
    such as hepatic and renal failure.  N-acetylcysteine also has a
    documented therapeutic effect in established paracetamol-induced liver
    failure.

         Intravenous and oral  N-acetylcysteine and oral methionine are
    normally indicated in patients who are thought to have taken more than
    100 mg paracetamol/kg in the preceding 24 h, or who have plasma
    paracetamol concentrations above a treatment line joining plots on a
    semilogarithmic graph of 200 mg/l at 4 h after ingestion and 30 mg/l
    at 15 h.  Every effort must be made to start therapy within 8 h as
    their efficacy declines progressively after this time. Treatment is
    required in only a small proportion of unselected patients and
    measurement of the plasma paracetamol concentration should be
    determined first if time and circumstances allow.

         The recommended dosage regimens are given in detail in sections
    2.13.2 and 3.13.2.

    1.8  Areas for future research

    1.8.1  Choice of antidote

         Multicentre studies are the only practical way to compare the
    relative efficacy and safety of  N-acetylcysteine and methionine.
    Appropriate controls will be necessary with stratification according
    to factors such as age, sex, severity of poisoning, use of ethanol and
    other drugs, and the ingestion-to-treatment interval.  It is possible
    that for optimal results, different drugs and different routes may be
    indicated for different clinical circumstances. Since both antidotes
    are effective and safe, however, a very large number of patients will
    be necessary to document what is likely to be a marginal effect. Such
    a study may be difficult to justify when health resources globally are
    limited.

    1.8.2  Optimum dose and route of administration

         Work is also required to define optimal dosage regimens and
    routes of administration.  The regimens in current use were chosen
    arbitrarily and there seems little doubt that some could be changed
    with benefit. For example, the total dose and duration of treatment
    with oral methionine (10 g over 12 h) is much less that the total dose
    and duration of treatment with oral  N-acetylcysteine (100 g over
    72 h) yet their efficacy is comparable.  The dose of oral
     N-acetylcysteine may therefore be unnecessarily large.

         The initial rapid intravenous infusion of  N-acetylcysteine
    produces very high plasma concentrations in the range of 300 to 900
    mg/l, which may be more than is necessary.  Most adverse reactions to
    intravenous  N-acetylcysteine occur early when concentrations are
    highest, and they could probably be avoided without loss of efficacy
    by modifying the infusion rates according to predictions based on the
    kinetics of  N-acetylcysteine in patients with severe paracetamol
    poisoning (Prescott et al., 1989).  Information is also required
    concerning the bioavailability and plasma concentrations of oral

     N-acetylcysteine and methionine in patients with paracetamol
    poisoning.  The proper characterization of the action of these agents
    depends on the full definition of the dose- or concentration-response
    curves, but this would be a formidable task.  However, further useful
    information could be obtained about the concentration-time-response
    relationships of the antidotes in relation to the severity of
    poisoning and the time since ingestion.

    1.8.3  Role of  N-acetylcysteine in liver failure

         The effect of  N-acetylcysteine on paracetamol-induced fulminant
    liver failure should be studied further, if possible in a double-
    blinded manner. The mechanisms behind this effect also warrant further
    study. A similar study to investigate a possible therapeutic effect of
    methionine could be considered, perhaps in comparison with
     N-acetylcysteine, in a double-blind design.

         The possible role of  N-acetylcysteine in other types of liver
    failure might also be justified if the effect on paracetamol-induced
    liver failure was reproduced in a double-blind study.

    1.8.4  Role of  N-acetylcysteine 24-50 h after the overdose

         In the studies of Smilkstein et al. (1988, 1991), an antidotal
    effect of  N-acetyl-cysteine was demonstrated up to 24 h after the
    ingestion of paracetamol. As seen from Fig. 1 the treatment line is
    only useful up to 24 h post-ingestion.  In the study by Keays et al.
    (1991), the average time from ingestion to inclusion in the study was
    53 (36-80) h in the  N-acetylcysteine-treated group.  Thus we are
    left with a time period from 24 to 50 h after ingestion where there
    are no scientific data as to whether  N-acetylcysteine is beneficial
    or not.  Until such data become available, it may be reasonable to
    give  N-acetylcysteine to patients admitted 24-50 h after ingestion
    of paracetamol if they are considered to be at risk of developing
    liver failure.

    1.8.5  New approaches to the treatment of paracetamol poisoning

         There seems little doubt that a large number of sulfhydryl
    compounds may be effective in preventing liver damage after
    paracetamol overdosage.  Given that antidotes such as
     N-acetylcysteine and methionine act indirectly via glutathione, it
    is difficult to envisage other precursors with the same mechanisms of
    protection that would be safer and more effective.  At present, the
    greatest need is for a new approach to the prevention of severe
    hepatotoxicity in patients who present too late for effective
    treatment with existing antidotes.  Such treatment would have to be
    based on mechanisms other than inhibition of the metabolic activation
    of paracetamol or stimulation of glutathione synthesis.

         Future research may also find a role for cytochrome P-450
    inhibitors, such as ethanol, in reducing the severity of paracetamol-
    induced liver toxicity. There is experimental evidence of efficacy but
    clinical data are scarce. The effects of different agents on the
    metabolism and toxicity of paracetamol could be better predicted if
    the specific isoenzymes of cytochrome P-450 that are involved in the
    metabolic activation of paracetamol in man were characterized.

    1.8.6  Treatment failure

         Patients occasionally suffer liver damage despite apparently
    adequate treatment started well within the critical time of 8 to 10 h. 
    In such cases it is easy to assume that the patient's history is
    inaccurate, or that failure of oral therapy is due to delayed or
    incomplete absorption. However, similar problems have been encountered
    following intravenous administration of antidote, and further studies
    are needed to establish the reasons for treatment failure.

    1.8.7  The treatment line

         The line on a semilogarithmic graph joining plots of 200 mg/l at
    4 h after ingestion of paracetamol and 30 mg/l at 15 h is used to
    determine the need for antidotal therapy in most countries (Fig. 1). 
    The decision to treat only those patients with paracetamol
    concentrations above this line represents a compromise between the
    unnecessary treatment of the majority of poisoned patients on the one
    hand and failure to treat a very small minority who will suffer liver
    damage at concentrations below the line on the other.  With
     N-acetylcysteine it is important to ensure that treatment really is
    necessary, although the position of the established treatment line is
    probably about right.  It is also a useful guide for treatment with
    methionine, but, as this is so cheap and safe, unnecessary treatment
    is of less consequence and the line could probably be lowered to
    correspond to 150 mg/l at 4 h.

    1.8.8  The role of ethanol

         It is important to know whether acute or chronic heavy
    consumption of ethanol has significant effects on susceptibility to
    the hepatotoxicity of paracetamol following overdosage.  To this end,
    the outcome of poisoning should be compared in a sufficiently large
    number of chronic alcoholics and appropriate control patients matched
    for severity of poisoning and delay in treatment.  A similar approach
    might be used to determine whether early acute administration of
    ethanol influences the outcome of paracetamol poisoning.

    1.8.9  Paracetamol poisoning in pregnancy

         Limited information is available concerning the effects of an
    overdose of paracetamol at different times during the course of

    pregnancy but serious problems for mother and child seem to be
    uncommon (MacElhatton et al., 1990).  National registers of patients
    who take an overdose of paracetamol during pregnancy should be kept
    with proper follow-up, so that the outcome and effects of different
    treatments can be compared.

    1.9  References

    Albano E, Rundgren M, Harvison PS, Nelson SD, & Moldeus P (1985)
    Mechanisms of  N-acetyl-p-benzoquinone imine cytotoxicity. Mol
    Pharmacol, 28: 306-311.

    Bales JR, Bell JD, Nicholson JK, Sadler PJ, Timbrell JA, Hughes RD,
    Bennett PN, & Williams R (1988)  Metabolic profiling of body fluids by
    proton NMR: self-poisoning episodes with paracetamol (acetaminophen).
    Magn Reson Med, 6: 300-306.

    Bateman DN, Woodhouse KW, & Rawlins MD (1984) Adverse reactions to
     N-acetyl-cysteine. Human Toxicology, 3: 393-398.

    Betowski LD, Korfmacher WA, Lay JO, Potter DW, & Hinson JA (1987)
    Direct analysis of rat bile for acetaminophen and two of its
    conjugated metabolites via thermospray liquid chromatography/mass
    spectrometry. Biomed Environ Mass Spectrom, 14: 705-709.

    Boobis AR, Fawthrop DJ, & Davis DS (1989) Mechanisms of cell deaths.
    Trends Pharmacol Sci, 10: 275-280.

    Boyd EM & Bereczky GM (1966) Liver necrosis from paracetamol. Br J
    Pharmacol, 26: 606-614.

    Boyd EM & Hogan SE (1968) The chronic oral toxicity of paracetamol at
    the range of the LD50 (100 days) in albino rats. Can J Physiol
    Pharmacol, 46: 239-245.

    Boyer TD & Rouff SC (1971) Acetaminophen induced hepatic necrosis and
    renal failure. J Am Med Assoc, 218: 440-441.

    Brodie BB & Axelrod J (1948a) The estimation of acetanilide and its
    metabolic products aniline,  N-acetyl- p-aminophenol and
     p-aminophenol (free and total conjugated) in biological fluids and
    tissues. J Pharmacol Exp Ther, 94: 22-28.

    Brodie BB & Axelrod J (1948b) The fate of acetanilide in man. J
    Pharmacol Exp Ther, 94: 29-38.

    Bruno MK, Cohen SD, & Khairallah EA (1988 ) Antidotal effectiveness of
     N-acetylcysteine in reversing acetaminophen-induced hepatotoxicity.
    Enhancement of the proteolysis of arylating proteins. Biochem
    Pharmacol, 37: 4319-4325.

    Cairns FJ, Koelmeyer TD, & Smeeton WMI (1983) Deaths from drugs and
    poisons. N Z Med J, 96: 1045-1048.

    Canalese J, Gimson AES, Davis M, & Williams R (1981) Factors
    contributing to mortality in paracetamol-induced hepatic failure. Br
    Med J, 282: 199-201.

    Clark R, Borirakchanyavat V, Gazzard BG, Rake MO, Shilkin KB, Flute
    PT, & Williams R (1973a) Disordered haemostasis in liver damage from
    paracetamol overdose. Gastroenterology, 65: 788-795.

    Clark R, Thompson RPH, Borirakchanyavat V, Widdop B, Davidson AR,
    Goulding R, & Williams R (1973b) Hepatic damage and death from
    overdose of paracetamol. Lancet, 1: 66-70.

    Cobden I, Record CO, Ward MK, & Kerr DNS (1982) Paracetamol-induced
    acute renal failure in the absence of fulminant liver damage. Br Med
    J, 284: 21-22.

    Corcoran GB, Racz WJ, Smith CV, & Michell JR (1985) Effects of
     N-acetylcysteine on acetaminophen covalent binding and hepatic
    necrosis in mice. J Pharmacol Exp Ther, 232: 864-872.

    Coxon RE, Gallacher G, Landon J, & Rae C (1988) Development of a
    specific polarization fluoroimmunoassay for paracetamol in serum. Ann
    Cli Biochem, 25: 49-52.

    Critchley JAJH, Dyson EH, Scott AW, Jarvie DR, & Prescott LF (1983) Is
    there a place for cimetidine or ethanol in the treatment of
    paracetamol poisoning? Lancet, 1: 1375-1376.

    Dahlin DC, Miwa GT, Lu AYH, & Nelson SD (1984)  N-acetyl- p-
    benzoquinone imine: a cytochrome P-450 mediated oxidation product of
    acetaminophen. Proc Natl Acad Sci (USA), 81: 1327-1331.

    Davidson DGD & Eastham WN (1966) Acute liver necrosis following
    overdosage of paracetamol. Br Med J, 2: 497-499.

    Dawson AH, Henry DA, & McEwen J (1989) Adverse reactions to
     N-acetylcysteine during treatment for paracetamol poisoning. Med J
    Aust, 150: 329-331.

    Deakin CD, Gove CD, Fagan EA, Tredger JM, & Williams R (1991) Delayed
    calcium channel blockade with diltiazem reduces paracetamol
    hepatotoxicity in mice. Hum Exp Toxicol, 10: 119-123.

    Devalia JL, Ogilvie RC, & McClean AE (1982) Dissociation of cell death
    from covalent binding by flavones in a hepatocyte system. Biochem
    Pharmacol, 31: 3745-3749.

    Eder H (1964) Chronic toxicity studies on phenacetin,  N-acetyl- p-
    aminophenol (NAPA) and acetylsalicylic acid on cats. Acta Pharmacol
    Toxicol, 21: 197-204.

    Farid NR, Glynn JP, & Kerr DNS (1972) Haemodialysis in paracetamol
    self-poisoning. Lancet, 2: 396-398.

    Fiarhurst S, Barber DJ, Clark B, & Horton AA (1982) Studies on
    paracetamol induced lipid peroxidation. Toxicology, 23: 249-259.

    Floren CH, Thesleff P, & Nilsson A (1987) Severe liver damage caused
    by therapeutic doses of acetaminophen. Acta Med Scand, 222: 285-288.

    Fraser NC (1980) Accidental poisoning deaths in British children
    1958-77. Br Med J, 280: 1595-1598.

    Gazzard BG, Hughes RD, Widdop B, Goulding R, Davis M, & Williams R
    (1977) Early prediction of the outcome of a paracetamol overdose based
    on an analysis of 163 patients. Postgrad Med J, 53: 243-247.

    Gilmore IT & Tourvas E (1977) Paracetamol-induced acute pancreatitis.
    Br Med J, 1(6063): 753-754.

    Glynn JP & Kendal SE (1975) Paracetamol measurements. Lancet, 1:
    1147-1148.

    Gray TA, Buckley BM, & Vale JA (1987) Hyperlactataemia and metabolic
    acidosis following paracetamol overdose. Q J Med, 65: 811-821.

    Hamlyn AN, Douglas AP, & James O (1978) The spectrum of paracetamol
    (acetaminophen) overdose: clinical and epidemiological studies.
    Postgrad Med J, 54: 400-404.

    Hamlyn AN, Lesna M, Smith PA, & Watson AJ (1981) Methionine and
    cysteamine in paracetamol (acetaminophen) overdose, prospective
    controlled trial of early therapy. J Int Med Res, 9: 226-231.

    Harrison PM, O'Grady JG, Keays RT, Alexander GJM, & Williams R (1990)
    Serial prothrombin time as prognostic indicator in paracetamol induced
    fulminant hepatic failure. Br Med J, 301: 964-966.

    Harrison PM, Wendon JA, Gimson AES, Alexander GJM, & Williams R (1991)
    Improvement by acetylcysteine of hemodynamics and oxygen transport in
    fulminant hepatic failure. N Engl J Med, 324: 1852-1857.

    Hepler B, Weber J, Sutheimer C, & Sunshine I (1984) Homogenous enzyme
    immunoassay of acetaminophen in serum. Am J Clin Pathol, 81: 602-610.

    Herman AW, Mahar SO, Burcham PC, & Madsen BW (1992) Level of cytosolic
    free calcium during acetaminophen toxicity in mouse hepatocytes. Mol
    Pharmacol, 41: 665-670.

    Hinson JA, Pohl LR, Monks TJ, Gillette JR, & Guengerich FP (1980)
    3-Hydroxy-acetaminophen: a microsomal metabolite of acetaminophen.
    Evidence against an epoxide as the reactive metabolite of
    acetaminophen. Drug Metab Dispos, 8: 289-294.

    Ho SWC & Beilin LJ (1983) Asthma associated with  N-acetylcysteine
    infusion and paracetamol poisoning: report of two cases. Br Med J,
    287: 876-877.

    Holme JA & Jacobsen D (1986) Mechanism of paracetamol toxicity.
    Lancet, 1: 804-805.

    Holme JA, Wirth PJ, Dybing E, & Thorgeirsson SS (1982) Cytotoxic
    effects of  N-hydroxyparacetamol in suspensions of isolated rat
    hepatocytes. Acta Pharmacol Toxicol, 51: 87-95.

    Holme JA, Dahlin DC, Nelson SD, & Dybing E (1984) Cytotoxic effects of
     N-acetyl- p-benzoquinone imine, a common arylating intermediate of
    paracetamol and  N-hydroxyparacetamol. Biochem Pharmacol, 33:
    401-406.

    Horton AA & Wood JM (1989) Effects of inhibitors of phospholipase A2,
    cyclo-oxygenase and thromboxane synthetase on paracetamol
    hepatotoxicity in the rat. Eicosanoids, 2: 123-129.

    Howie D, Adriaenssens PI, & Prescott LF (1977) Paracetamol metabolism
    following overdosage. Application of high performance liquid
    chromatography. J Pharm Pharmacol, 29: 235-237.

    Jeffery EH, Arndt K, & Haschek WM (1991) The role of cytochrome
    P-4502E1 in bioactivation of acetaminophen in diabetic and acetone-
    treated mice. Adv Exp Med Biol, 283: 249-251.

    Jollow DJ, Mitchell JR, Potter WZ, Davis DC, Gillette JR, & Brodie BB
    (1973) Acetaminophen-induced hepatic necrosis. II. Role of covalent
    binding  in vivo. J Pharmacol Exp Ther, 187: 195-202.

    Jones AF, Harvey JM, & Vale JA (1989) Hypophosphataemia and
    phosphaturia in paracetamol poisoning. Lancet, 2: 608-609.

    Keays R, Harrison PM, Wendon JA, Forbes A, Gove C, Alexander GJM, &
    Williams R (1991) Intravenous acetylcysteine in paracetamol induced
    fulminant hepatic failure: a prospective controlled trial. Br Med J,
    303: 1026-1029.

    Landan EJ, Naukam RJ, Rama F, & Sastry BV (1985) Effects of calcium
    channel blocking agents on calcium and centrilobular necrosis in the
    liver of rats treated with hepatotoxic agents. Biochem Pharmacol, 35:
    697-705.

    Lauterburg BH & Velez ME (1988) Glutathione deficiency in alcoholics;
    risk factor for paracetamol hepatotoxicity. Gut, 29: 1153-1157.

    Lay JO, Potter WD, & Hinson JA (1987) Fast atom bombardment mass
    spectro-metry and fast atom bombardment mass spectrometry/mass
    spectrometry of three glutathione conjugates of acetaminophen. Biomed
    Environ Mass Spectrom, 14: 517-521.

    Lesna M, Watson AJ, Douglas AP, Hamlyn AN, & James OFW (1976)
    Evaluation of paracetamol-induced damage in liver biopsies. Virchows
    Arch Pathol Anat Histol, 370: 333-344.

    McClements BM, Hyland M, Callender MF, & Blair TL (1990) Management of
    paracetamol poisoning complicated by enzyme induction due to alcohol
    or drugs. Lancet, 335: 1526.

    MacElhatton PR, Sullivan FM, Volans GN, & Fitzpatrick R (1990)
    Paracetamol poisoning in pregnancy: an analysis of the outcomes of
    cases referred to the Teratology Information Service of the National
    Poisons Information Service. Hum Exp Toxicol, 9: 147-153.

    MacLean D, Peters TJ, Brown RAG, McCathie M, Baines GF, & Robertson
    PGC (1968) Treatment of acute paracetamol poisoning. Lancet, 2:
    849-852.

    Mansuy D, Sassi A, Dansette PM, & Plat M (1986) A new potent inhibitor
    of lipid peroxidation  in vitro and  in vivo, the hepatoprotective
    drug anisyldithiolthione. Biochem Biophys Res Commun, 135: 1015-1021.

    Mant TGK, Tempowski JH, Volans GN, & Talbot JCC (1984) Adverse
    reactions of acetylcysteine and effects of overdose. Br Med J, 298:
    217-219.

    Meredith TJ, Prescott LF, & Vale JA (1986) Why do patients still die
    from paracetamol poisoning? Br Med J, 293: 345-346.

    Miranda CL, Henderson MC, Smith JA, & Buhler DR (1983) Protective role
    of dietary butylated hydroxyanisole against chemical-induced acute
    liver damage in mice. Toxicol Appl Pharmacol, 69: 73-80.

    Miranda CL, Henderson MC, & Buhler DR (1985) Dietary butylated
    hydroxyanisole reduces covalent binding of acetaminophen to mouse
    tissue proteins  in vivo. Toxicol Lett, 25: 89-93.

    Mitchell JR (1977) Host susceptibility and acetaminophen liver injury.
    Ann Int Med, 87: 377-388.

    Mitchell JR (1988) Acetaminophen toxicity. N Engl J Med, 319:
    1601-1602.

    Mitchell JR, Jollow DJ, Potter WZ, Davis DC, Gillette JR, & Brodie BB
    (1973a) Acetaminophen-induced hepatic necrosis. I. Role of drug
    metabolism. J Pharmacol Exp Ther, 187: 185-194.

    Mitchell JR, Jollow DJ, Potter WZ, Gillette JR, & Brodie BB (1973b)
    Acetaminophen-induced hepatic necrosis. IV. Protective role of
    glutathione. J Pharmacol Exp Ther, 187: 211-217.

    Morgan ET, Koop DR, & Coon MJ (1982) Catalytic activity of cytochrome
    P-450 isozyme 3a isolated from liver microsomes of ethanol-treated
    rabbits. J Biol Chem, 257: 13951-13957.

    Morgan ET, Koop DR, & Coon MJ (1983) Comparison of six rabbit liver
    cytochrome P-450 isozymes in formation of a reactive metabolite of
    acetaminophen. Biochem Biophys Res Commun, 112: 8-13.

    Mourelle M, Beales D, & McLean AE (1990) Electron transport and
    protection of liver slices in the late stage of paracetamol injury.
    Biochem Pharmacol, 40: 2023-2028.

    O'Grady JG, Gimson AES, O'Brien CJ, Pucknell A, Hughes RD, & Williams
    R (1988) Controlled trials of charcoal hemoperfusion and prognostic
    factors in fulminant hepatic failure. Gastroenterology, 94: 1186-1192.

    O'Grady JG, Wendon J, Tan KC, Potter D, Cottam S, Cohen AT, Gimson
    AES, & Williams R (1991) Liver transplantation after paracetamol
    overdose. Br Med J, 303: 221-223.

    Orrenius S, McConkey DJ, & Nicotera P (1991) Role of calcium in toxic
    and programmed cell death. Adv Exp Med Biol, 283: 419-425.

    Perrot N, Nalpas B, Yang CS, & Beaune PH (1989) Modulation of
    cytochrome P-450 isozymes in human liver by ethanol and drug intake.
    Eur J Clin Invest, 19: 549-555.

    Pimstone BL & Uys CJ (1968) Liver necrosis and cardiomyopathy
    following paracetamol overdosage. S Afr Med J, 42: 259-262.

    Platt S, Hawton K, Kreitman N, Fagg J, & Foster J (1988) Recent
    clinical and epidemiological trends in parasuicide in Edinburgh and
    Oxford: a tale of two cities. Psychol Med, 18: 405-418.

    Portmann B, Talbot IC, Day DW, Davidson AR, Murray-Lyon IM, & Williams
    R (1975) Histopathological changes in the liver following a
    paracetamol overdose: correlation with clinical and biochemical
    parameters. J Pathol, 117: 169-181.

    Potter DW & Hinson JA (1987) Mechanisms of acetaminophen oxidation to
     N-acetyl- p-benzoquinone imine by horseradish peroxidase and
    cytochrome P-450. J Biol Chem, 262: 966-973.

    Potter WZ, Davis DC, Mitchell JR, Jollow DJ, Gillette JR, & Brodie BB
    (1973) Acetaminophen-induced hepatic necrosis. III. Cytochrome P-450-
    mediated covalent binding  in vitro. J Pharmacol Exp Ther, 187:
    203-210.

    Prescott LF (1971) Gas-liquid chromatographic estimation of
    paracetamol. J Pharm Pharmacol, 23: 807-808.

    Prescott LF (1983) Paracetamol overdosage: pharmacological
    considerations and clinical management. Drugs, 25: 290-314.

    Prescott LF (1986) Effects of non-narcotic analgesics on the liver.
    Drugs, 32(Suppl 4): 129-147.

    Prescott LF & Matthew H (1974) Cysteamine for paracetamol overdosage.
    Lancet, 1: 998.

    Prescott LF & Highley MS (1985) Drugs prescribed for self-poisoners.
    Br Med J, 290: 1633-1636.

    Prescott LF, Wright N, Roscoe P, & Brown SS (1971) Plasma-paracetamol
    half-life and hepatic necrosis in patients with paracetamol
    overdosage. Lancet, 1: 519-522.

    Prescott LF, Newton RW, Swainson CP, Wright N, Forrest ARW, & Matthew
    H (1974) Successful treatment of severe paracetamol overdosage with
    cysteamine. Lancet, 1: 588-592.

    Prescott LF, Park J, Sutherland GR, Smith IJ, & Proudfoot AT (1976)
    Cysteamine, methionine and penicillamine in the treatment of
    paracetamol poisoning. Lancet, 2: 109-114.

    Prescott LF, Park J, Ballantyne A, Adriaenssens P, & Proudfoot AT
    (1977) Treatment of paracetamol (acetaminophen) poisoning with
     N-acetylcysteine. Lancet, 2: 432-434.

    Prescott LF, Illingworth RN, Critchley JAJH, Stewart MJ, Adam RD, &
    Proudfoot AT (1979) Intravenous  N-acetylcysteine: the treatment of
    choice for paracetamol poisoning. Br Med J, 2(6198): 1097-1100.

    Prescott LF, Donovan JW, Jarvie DR, & Proudfoot AT (1989) The
    disposition and kinetics of intravenous  N-acetylcysteine in patients
    with paracetamol overdosage. Eur J Clin Pharmacol, 37: 501-506.

    Price VF & Jollow DJ (1983) Mechanism of ketone-induced protection
    from acetaminophen hepatotoxicity in the rat. Drug Metab Dispos, 11:
    451-457.

    Price CP, Hammond PM, & Scawen MD (1983) Evaluation of an enzymic
    procedure for the measurement of acetaminophen. Clin Chem, 29:
    358-361.

    Proudfoot AT & Wright N (1970) Acute paracetamol poisoning. Br Med J,
    3: 557-558.

    Raucy JL, Lasker JM, Lieber CS, & Black M (1989) Acetaminophen
    activation by human liver cytochrome P-4502E1 and P-4501A2. Arch
    Biochem Biophys, 271: 270-283.

    Ray SD, Sorge CL, Raucy JL, & Corcoran GB (1990) Early loss of large
    genomic DNA  in vivo with accumulation of calcium in the nucleus
    during acetaminophen-induced liver injury. Toxicol Appl Pharmacol,
    106: 346-351.

    Record CO, Chase RA, Alberti KGMM, & Williams R (1975) Disturbances in
    glucose metabolism in patients with liver damage due to paracetamol
    overdose. Clin Sci Mol Med, 49: 473-479.

    Riggin RM, Schmidt AL, & Kissinger PT (1975) Determination of
    acetaminophen in pharmaceutical preparations and body fluids by high-
    performance liquid chromatography with electrochemical detection. J
    Pharm Sci, 64: 680-683.

    Routh JI, Shane NA, Arredondo FG, & Paul WD (1968) Determination of
     N-acetyl- p-aminophenol in plasma. Clin Chem, 14: 882-885.

    Rumack BH (1984) Acetaminophen overdose in young children. Am J Dis
    Child, 138: 428-433.

    Rumack BH & Matthew H (1975) Acetaminophen poisoning and toxicity.
    Paediatrics, 55: 871-876.

    Rumack BH & Peterson RG (1978) Acetaminophen overdose: incidence,
    diagnosis and management in 416 patients. Paediatrics, 62(Suppl 2):
    898-903.

    Rumack BH, Peterson RC, Koch GC, & Amara IA (1981) Acetaminophen
    overdose. 662 cases with evaluation of oral acetylcysteine treatment.
    Arch Int Med, 141: 380-385.

    Rundgren M, Porubek DJ, Harvison PJ, Cotgreave IA, Moldeus P, & Nelson
    SD (1988) Comparative cytotoxic effects of  N-acetyl- p-benzoquinone
    imine and two dimethylated analogues. Mol Pharmacol, 34: 566-572.

    Seeff LB, Cuccherini BA, Zimmerman HJ, Adler E, & Benjamin SB (1986)
    Acetaminophen hepatotoxicity in alcoholics. A therapeutic
    misadventure. Ann Intern Med, 104: 399-404.

    Smilkstein MJ, Knapp GL, Kulig KW, & Rumack BH (1988) Efficacy of oral
     N-acetylcysteine in the treatment of acetaminophen over dose;
    analysis of the National Multicenter study (1976 to 1985). N Engl J
    Med, 319: 1557-1562.

    Smilkstein MJ, Bronskin AC, Linden C, Augenstein WL, Kulig KW, &
    Rumack BH (1991) Acetaminophen overdose: A 48-h intravenous
     N-acetylcysteine treatment protocol. Ann Emerg Med, 20: 1058-1063.

    Smith PK (1958) Acetophenetidin: A critical bibliographic review. New
    York, Interscience Publishers, p 3.

    Song BJ, Veech RL, & Saenger P (1990) Cytochrome P-4502E1 is elevated
    in lymphocytes from poorly controlled insulin-dependent diabetics. J
    Clin Endocrinol Methods, 71: 1036-1040.

    Speeg KV, Mitchell MC, & Maldonado L (1985) Additive protection of
    cimetidine and  N-acetylcysteine treatment against acetaminophen-
    induced hepatonecrosis in the rat. J Pharmacol Exp Ther, 234: 550-554.

    Stewart MJ & Watson ID (1987) Analytical reviews in clinical
    chemistry: methods for the estimation of salicylate and paracetamol in
    serum, plasma and urine. Ann Clin Biochem, 24: 552-565.

    Stewart MJ, Adriaenssens PI, Jarvie DR, & Prescott LF (1979)
    Inappropriate methods for the emergency determination of plasma
    paracetamol. Ann Clin Biochem, 16: 89-95.

    Tee LBG, Boobis AR, Huggett AC, & Davies DS (1986) Reversal of
    acetaminophen toxicity in isolated hamster hepatocytes by
    dithiothreitol. Toxicol Appl Pharmacol, 83: 294-314.

    Tee LBG, Davies DS, Seddon CE, & Boobis AR (1987) Species differences
    in the hepatotoxicity of paracetamol are due to differences in the
    rate of conversion to its cytotoxic metabolite. Biochem Pharmacol, 36:
    1041-1052.

    Thomas CE & Reed DJ (1988) Effect of extracellular Ca++ omission on
    isolated hepatocytes. II. Loss of mitochondrial membrane potential and
    protection by inhibitors of uniport Ca++ transduction. J Pharmacol
    Exp Ther, 245: 501-507.

    Thomson JS & Prescott LF (1966) Liver damage and impaired glucose
    tolerance after paracetamol overdosage. Br Med J, 2(5512): 506-507.

    Tirmenstein MA & Nelson SD (1990) Acetaminophen-induced oxidation of
    protein thiols: Contributions of impaired thiol-metabolising enzymes
    and the breakdown of adenosine nucleotides. J Biol Chem, 265:
    3059-3065.

    Tirmenstein MA & Nelson SD (1989) Subcellular binding and effects on
    calcium homeostasis produced by acetaminophen and a non-hepatotoxic
    regioisomer, 3-hydroxyacetoanilide in mouse liver. J Biol Chem, 264:
    9814-9819.

    Tsokos-Kuhn JO, Hughes H, Smith CV, & Mitchell JR (1988) Alkylation of
    the liver plasma membrane and inhibition of the Ca2+ ATPase by
    acetaminophen. Biochem Pharmacol, 37: 2125-2131.

    Vale JA, Meredith TJ, & Goulding R (1981) Treatment of acetaminophen
    poisoning. Arch Intern Med, 141: 394-396.

    Wakeel RA, Davies HT, & Williams JD (1987) Toxic myocarditis in
    paracetamol poisoning. Br Med J, 295: 1097.

    Weiner K (1978) A review of methods for plasma paracetamol estimation.
    Ann Clin Chem, 15: 187-196.

    Williams R (1988) Management of acute liver failure Postgrad Med J,
    64: 769-771.

    Wong LT, Whitehouse LW, Solomonraj G, & Paul CJ (1980) Effect of
    concomitant single dose of ethanol on the hepatotoxicity and
    metabolism of acetaminophen in mice. Toxicology, 17: 297-309.

    Wynne H, Bateman DN, Hassanyeh F, Rawlins MD, & Woodhouse KW (1987)
    Age and self-poisoning: the epidemiology in Newcastle-upon-Tyne. Hum
    Toxicol, 6: 511-515.

    Younes M, Sause C, Seigers C-P, & Lemoine R (1988) Effect of
    deferrioxamine and diethyldithiocarbamate on paracetamol-induced
    hepato- and nephrotoxicity. The role of lipid peroxidation. J Appl
    Toxicol, 84: 261-265.

    Zimmerman HJ (1986) Effects of alcohol on other hepatotoxins.
    Alcoholism: Clin Exp Res, 10: 3-15.

    2.  METHIONINE

    2.1  Introduction

         The amino acid methionine is indicated for the treatment of acute
    paracetamol (acetaminophen) poisoning provided that patients present
    sufficiently early to benefit from therapy (see below, and Meredith et
    al., 1978; Vale et al., 1981; Meredith, 1987).

         Methionine acts as a glutathione precursor (McLean & Day, 1975;
    Vina et al., 1978; Vina et al., 1980) and protects against
    paracetamol-induced hepatic and renal toxicity provided that it is
    administered within 8-10 h of ingestion of the overdose (Meredith et
    al., 1978; Vale et al., 1981; Meredith, 1987).  Some protection is
    afforded even when methionine is administered later than this, and the
    point at which methionine treatment becomes "ineffective" has not been
    determined with certainty (Vale et al., 1981; Meredith, 1987). 
    However, no significant benefits have been documented in cases where
    more than 10 h has elapsed after a paracetamol overdose (Prescott et
    al., 1979; Meredith et al., 1986).  The need for specific protective
    therapy with methionine in cases of paracetamol overdose may be judged
    by measurement of plasma paracetamol concentrations in relation to the
    time elapsed since ingestion (Vale et al., 1981).  However, methionine
    is usually administered orally and it is therefore unsuitable for use
    in patients who are vomiting and for those in coma.

    2.2  Name and chemical formula

         It should be noted that L-methionine is the physiologically
    active enantiomorph but the pharmaceutical preparation is usually the
    racemic mixture.

    International
    non-proprietary name:        methionine

    Synonyms:                    DL-methionine, Racemethionine, DL-2-
                                 amino-4-(methylthio)butyric acid, alpha-
                                 amino-gamma-methylmercaptobutyric acid,
                                 2-amino-4-methylthiobutanoic acid,
                                 gamma-methylthio-alpha-aminobutyric acid

    IUPAC-name:                  2-amino-4-(methylthio)butyric acid

    CAS Number:                  L-methionine 63-68-3
                                 DL-methionine 59-51-8

    EINECS Number:               L-methionine 2005629
                                 DL-methionine 2004321

    NIOSH Number:                L-methionine PD0457000
                                 DL-methionine PD0456000

    Empirical formula:           C5H11NO2S

    Chemical structure:          CH3-S-CH2-CH2-CH-COOH
                                           |
                                           NH2

    Relative molecular mass:     149.2

    Conversion table:            1 mmol = 149.2 mg   1 g = 6.7 mmol
                                 1 Ámol = 149.2 Ág   1 mg = 6.7 Ámol

    Manufacturers:

         The major manufacturers of DL-methionine worldwide are
         Rhone-Poulenc and Degussa (Goldfarb et al., 1981). Pharmaceutical
         grade methionine is produced by the following companies:

         Degussa Ltd, Earl Road, Stanley Green, Handforth, Wilmslow,
         Cheshire SK9 3RL, United Kingdom (tel: +44 (0)61-486-6211; fax:
         +44 (0)61-485-6445)

         Degussa AG, Anwendungstechnik Av, Rodenbacher Chausee 4, Postfach
         1345, D-6450 Hanaul, Germany (tel: (01049) 59-35-54; telex:
         415200-0 dw d)

         Rhone Poulenc, 217 High St, Uxbridge, Middlesex UB8 1LQ, United
         Kingdom (tel: +44 (0)895-74080; fax: +44 (0)895-39323)

         Walton Pharmaceuticals Ltd, Bowes House, 17 Bowes Road,
         Walton-on-Thames, Surrey KT12 3HS, United Kingdom (tel: +44
         (0)932-241032; fax: +44 (0)932-255461)

    2.3  Physico-chemical properties

    2.3.1  Melting point (decomposition)

         266-267 ░C (Degussa AG, 1985)

    2.3.2  Solubility in vehicle of administration

         Methionine is usually given orally in a solid dose formulation,
    although in animal studies and in trials in humans, it has been
    administered parenterally (Prescott et al., 1976; Solomon et al.,
    1977).

         The solubility in water at 20 ░C is 29.1 g/l; it is also soluble
    in dilute acids and dilute solutions of alkaline hydroxides.
    Methionine is very slightly soluble in ethanol and practically

    insoluble in ether (Degussa AG, 1985; Budavari, 1989; Martindale,
    1989).

    2.3.3  Optical properties

         DL-methionine has no significant optical properties.

    2.3.4  pH

         The pH of a 1% aqueous solution is 5.6-6.1 (Martindale, 1989).

    2.3.5  pKa

         DL-methionine has two ionizable groups and it therefore possesses
    two pKa values.  The pKa of the carboxyl group is 2.28, and that
    of the amino group, 9.21 (Weast & Astle, 1978).

    2.3.6  Stability in light

         Methionine should be stored in the dark.

    2.3.7  Thermal stability/flammability

         DL-methionine decomposes at about 267 ░C (Degussa AG, 1985),
    emitting fumes of sulfur and nitrogen oxides (Sax, 1989).

    2.3.8  Loss of weight on drying

         The loss of weight is < 0.3% on drying at 105 ░C for 3 h
    (Degussa AG, 1985).

    2.3.9  Excipients and pharmaceutical aids

         An intravenous preparation may be made by dissolving methionine
    in 5% dextrose immediately before use and sterilizing by passage
    through a biological filter (Prescott et al., 1976).

    2.3.10  Pharmaceutical incompatibilities

         Methionine has been shown to be adsorbed by activated charcoal.
    Klein-Schwartz & Oderda (1981) added 10-ml aliquots (n=4) of a
    methionine solution (25 mg/ml) to 3, 6 and 10 g samples of activated
    charcoal in 50 ml of 0.1N hydrochloric acid.  The 3 g charcoal-
    methionine mixtures were agitated for 30 seconds, 10 min, 30 min and
    60 min, and then suction filtered.  Based on the results of this
    time-course study, the 6- and 10-g samples were studied after 30
    seconds of agitation.  The binding of methionine by activated charcoal
    was found to occur rapidly (46.9% within 30 seconds).  A tendency
    towards desorption was noted over the 60-min observation period (37.2%
    at 60 min), but did not achieve statistical significance.  As the
    amount of charcoal was increased, and therefore as the ratio of

    charcoal to methionine increased, the percentage of methionine
    adsorbed increased.  The percentages adsorbed by 3, 6, and 10 g of
    charcoal were 46.9 ▒ 4.0% (mean ▒ 1.96 SE), 76.8 ▒ 1.4%, and 89.5 ▒
    1.5%, respectively.  A statistically significant difference was found
    between all three groups (P < 0.01).

    2.4  Pharmaceutical formulation and synthesis

         The raw materials for synthesis of DL-methionine are acrolein,
    methanethiol (methyl mercaptan), hydrogen cyanide, and ammonia or
    ammonium carbonate.  These compounds are utilised in a number of
    different processes to yield the amino acid.

         The Strecker process involves the addition of methanethiol to
    acrolein to form ▀-methylthiopropionaldehyde, which is reacted with
    cyanide to give alpha-hydroxy-gamma-methylthiobutyronitrile.  This
    compound is treated with ammonia to produce alpha-amino-gamma-
    methylthiobutyronitrile, which is hydrolysed to DL-methionine
    (Goldfarb et al., 1981; Ullman, 1985).

         A variation on this method involves the treatment of alpha-
    hydroxy-gamma-methyl-thiobutyronitrile with ammonia and carbon dioxide
    or ammonium carbonate to yield 5-(▀-methylthioethyl)hydantoin.  This
    product is subjected to alkaline hydrolysis at elevated temperature
    and pressure to yield sodium methionate.  DL-methionine is isolated by
    acidification of the sodium methionate solution to the isoelectric
    point of the amino acid (pH = 5.7) (Goldfarb et al., 1981; Ullman,
    1985).

         L-methionine may be produced by the acylase-catalysed hydrolysis
    of  N-acetyl-DL-methionine (Hoppe & Martens, 1984; Ullman, 1985).

         Details of contaminants, excipients and pharmaceutical aids
    remain confidential to manufacturers.

         Methionine is available as tablets of 250 mg (racemate).

         The incorporation of methionine into tablets of paracetamol has
    been suggested as a means of protecting against hepatic and renal
    toxicity following paracetamol overdosage (McLean, 1974; McLean & Day,
    1975).  A preparation is now available commercially which contains
    paracetamol 500 mg and DL-methionine 250 mg (Pameton, Sterling
    Winthrop).  Its use has been recommended in psychiatric wards in
    patients with depression who need a simple analgesic, and in families
    who are at risk (McLean, 1986).  However, the formulation costs more
    than any other brand of paracetamol and its efficacy in preventing
    liver damage in humans following intentional paracetamol poisoning has
    not yet been established.

    2.5  Analytical methods

    2.5.1  Quality control of antidote

         The preparation must contain not less than 99% and not more than
    the equivalent of 101% of DL-2-amino-4-(methylthio)butyric acid,
    calculated with reference to the dried substance (European
    Pharmacopoeia, 1989).

         The European Pharmacopoiea (1989) describes the following assay
    method:

         Dissolve 0.14 g of the substance in 3 ml of anhydrous formic
    acid. Add 30 ml of glacial acetic acid. Immediately after dissolution
    titrate with 0.1N perchloric acid, determining the end-point
    potentiometrically.

         1 ml of 0.1N perchloric acid is equivalent to 14.92 mg
    DL-methionine.

    2.5.2  Methods for identification of antidote

         The European Pharmacopoiea (1989) stipulates that the preparation
    must be tested with either infrared absorption spectrophotometry or
    thin layer chromatography and the spectrum or chromatogram obtained
    compared with that for a reference sample of DL-methionine. In
    addition, one or both of the tests described below should be carried
    out. If the sample was tested spectrophotometrically then only test a)
    need be carried out; if chromatographically, then tests a) and b)
    should be carried out.

    Additional tests:

         a)   Dissolve 2.5 g in 1N hydrochloric acid and dilute to 50 ml
              with the same acid. The angle of optical rotation is
              -0.05 ░ to +0.05 ░.

         b)   Dissolve 0.1 g of substance and 0.1 g of glycine in 4.5 ml
              of 2M sodium hydroxide. Add 1 ml of a 2.5% (w/v) solution of
              sodium nitroprusside. Heat to 40 ░C for 10 min.  Allow to
              cool and add 2 ml of a mixture of 1 volume of phosphoric
              acid and 9 volumes of hydrochloric acid.  A deep red      
              colour develops.

    2.5.3  Methods for analysis of antidote in biological samples

         Plasma methionine concentrations may be measured using ion
    exchange chromatography.  There are several automated amino acids
    analysers available which utilize this technique.  Before analysis it
    is necessary to deproteinise the plasma sample by mixing with
    sulfosalicylic acid.  An equal volume of an external standard is added

    and the mixture centrifuged.  The supernatant is injected into the
    analyser (Smolin et al., 1981).

         Finkelstein et al. (1982) describe a method for measuring
    methionine and its metabolites using ion-exchange chromatography and
    subsequent radio-enzymatic assay.

    2.5.4  Methods for analysis of toxic agent

         Section 1.6 gives details of analytical techniques available for
    measuring plasma or serum paracetamol concentrations.

    2.6  Shelf-life

         DL-methionine should be stored in closed containers in cool, dry,
    dark conditions.  Degussa AG (1985) recommended that the time limit
    for storage is two years.

    2.7  General properties

    2.7.1  Mode of antidotal activity

         Methionine acts as a glutathione precursor and replenishes
    glutathione stores depleted as a consequence of paracetamol overdose
    (McLean & Day, 1975).  Glutathione is a naturally occurring
    tripeptide, composed of glycine, glutamic acid and cysteine, which
    inactivates the reactive intermediate metabolite of paracetamol,
     N-acetyl- p-benzoquinoneimine (NAPQI), by conjugation, resulting in
    the formation of mercapturate and cysteine conjugates (Jagenburg &
    Toczko, 1964), which are then excreted in the urine (see section 1.4
    for details).  Although, methionine acts as a glutathione precursor
    (McLean & Day, 1975; Vina et al., 1978; Stramentinoli et al., 1979;
    Vina et al., 1980), it must first undergo demethylation and then
    transulfuration to produce cysteine (see section 2.8.2.1 for further
    details of methionine metabolism).

    2.7.2  Other properties

         L-methionine has been administered to patients with Parkinson's
    disease with differing results.  Pearce & Waterbury (1974) found that
    patients on levodopa or other antiparkinsonian therapy deteriorated
    when given supplementary methionine.  The patients were placed on a
    low methionine diet (0.5 g/day; 3.35 mmol/day) and were given either
    1.5 g (10.05 mmol) of L-methionine or placebo daily on a randomized,
    double-blind basis.  Clinical deterioration was noted from the fifth
    day of the trial and was reversed after discontinuation of the
    methionine.  In a longer term, open study Smythies & Halsey (1984)
    gave patients, whose parkinsonism was maximally controlled by drug
    therapy, doses of L-methionine starting at 1 g/day (6.7 mmol/day) and
    rising to 5 g/day (33.5 mmol/day).  After a total of eleven weeks,
    there was subjective improvement in 10 of 15 patients.

         Oral administration of a large amount of methionine to
    schizophrenic patients treated with a monoamine oxidase inhibitor has
    been reported to produce either an intensification of schizophrenic
    symptoms or superimposed toxic symptoms (Pollin et al., 1961; Brune &
    Himwich, 1962; Park et al., 1965; Berlet et al., 1965).  The reason
    for this observation has not been established, but Kakimoto et al.
    (1967) found evidence of amino acid imbalance (increased urinary
    excretion of serine, threonine, glutamine and histidine) in eight
    schizophrenic patients given isocarboxazid (1 mg/kg per day) and oral
    L-methionine (0.3 g/kg per day; 2 mmol/kg per day) together, but not
    when given isocarboxazid alone.

         DL-methionine has been given in doses of 200 mg three or four
    times daily to lower the pH of the urine and thus reduce odour and
    irritation due to ammoniacal urine (Martindale, 1989).  DL-methionine
    is also used as a dietary supplement, as is L-methionine which is also
    used in amino acid solutions given parenterally (Martindale, 1989).

    2.8  Animal studies

    2.8.1  Pharmacodynamics

         There is considerable species difference in susceptibility to
    paracetamol-induced liver damage, which correlates with differences in
    the activity of the oxidative pathway in these species.  Thus, while
    doses of 750 mg/kg (4.96 mmol/kg) are sufficient to cause severe
    hepatic necrosis in mice, doses of 1250-1500 mg/kg (8.3-9.9 mmol/kg)
    cause very little hepatic necrosis in rats, despite being lethal
    (Mitchell et al., 1973).  In animal experiments to test the efficacy
    of methionine, therefore, rats are usually sensitized to paracetamol
    by pretreatment with phenobarbitone or other microsomal enzyme-
    inducing compounds.

         The time-scale for the development of liver damage in laboratory
    animals is shorter than in humans.  The results of animal studies
    investigating the efficacy of methionine in relation to the time of
    administration of paracetamol overdose cannot, therefore, readily be
    extrapolated to humans.  Nonetheless, in view of the rapidity of
    glutathione depletion and the onset of covalent binding and consequent
    liver damage, preventative treatment would seem to be needed soon
    after paracetamol overdose in order to be maximally effective.  Animal
    studies investigating the efficacy of methionine in the prevention of
    liver damage have largely involved its prior or simultaneous
    administration with paracetamol.

         When methionine was given to mice 5 min before, and 20 min after,
    a toxic intraperitoneal dose (710 mg/kg; 4.7 mmol/kg) of paracetamol,
    mortality was reduced from 43 to 16.7%.  Methionine was administered
    intramuscularly at a concentration of 7.5 mg/kg (0.05 mmol/kg)
    (Stramentinoli et al., 1979).  It is not clear from the report of this

    study whether this represented the total dose of methionine given or
    if the total dose was, in fact, 15 mg/kg (0.1 mmol/kg) (Stramentinoli
    et al., 1979).  The effective dose of methionine represented either 1
    or 2% (w/w) of the dose of paracetamol.

         The development of liver damage in mice, as indicated by
    elevation of alanine aminotransferase (ALAT) activity, was prevented
    completely by the intraperitoneal administration of L-methionine (1000
    mg/kg; 6.7 mmol/kg) at the same time as oral administration of
    paracetamol (300 mg/kg; 1.98 mmol/kg) (Miners et al., 1984). In the
    control group, given no antidotal therapy, ALAT activity rose above
    16 000 U/l, whereas in the group given methionine the ALAT activity
    remained within the normal range.  The LD50 of paracetamol was
    increased more than three-fold, from 295 mg/kg (1.95 mmol/kg) to 910
    mg/kg (6 mmol/kg).  However, the co-administration of methionine did
    not completely prevent depletion of hepatic glutathione content, which
    dropped by about one third.

         That the protective effect of methionine was due to facilitation
    of glutathione synthesis was suggested by two findings in this study. 
    Firstly, the co-administration of methionine resulted in an increased
    proportion of the dose of paracetamol being excreted as glutathione-
    derived and sulfate conjugates.  There was no significant change in
    the amount excreted as glucuronide conjugates or as unchanged
    paracetamol.  Secondly, the protective effect of methionine was
    removed by pretreating the animals with buthionine sulfoximine.  This
    compound specifically inhibits glutathione synthesis without affecting
    any of the other enzyme systems pertinent to the mechanism of
    paracetamol toxicity (Miners et al., 1984).

         In another study where the toxin and protective agent were given
    simultaneously, McLean & Day (1975) compared the efficacy of different
    doses of methionine.  They used rats sensitized by pretreatment with
    phenobarbitone and gave the drugs orally.  When the test animals were
    given paracetamol (1 g/kg; 6.6 mmol/kg), 90% showed histological
    evidence of liver damage and 16.7% died.  When methionine was given at
    a concentration of 300 mg/kg (2 mmol/kg), i.e. 30% (w/w), liver damage
    was prevented completely. A 25% dose of methionine increased the
    LD50 of paracetamol more than three-fold, from 2 g/kg (13.2 mmol/kg)
    to more than 7.5 g/kg (49.6 mmol/kg). Neuvonen et al. (1985) also
    found that the simultaneous oral administration of L-methionine was
    effective in reducing the toxicity of paracetamol. In their studies on
    mice, a 20% dose of L-methionine increased the LD50 of paracetamol
    from a mean of 610 mg/kg (4 mmol/kg) to a mean of 1096 mg/kg (7.2
    mmol/kg).  Methionine has also been shown to be effective in reducing
    mortality in dogs (Maxwell et al., 1975).  When paracetamol (750
    mg/kg; 4.95 mmol/kg) and methionine (150 mg/kg; 1 mmol/kg) were given
    orally, mortality was reduced from 67% in controls to zero.

         These studies confirm the usefulness of methionine in preventing
    or reducing paracetamol-induced liver damage when it is given at the
    same time as the toxin.  Other animal work has shown that methionine
    is effective when given after paracetamol.

         Legros (1976) treated mice given an oral lethal dose of
    paracetamol (875 mg/kg; 5.78 mmol/kg) with intraperitoneal methionine
    administered 2 and 6 h later.  The lowest dose tested was 25 mg/kg
    (0.17 mmol/kg) given twice, the total representing 5.7% of the dose of
    paracetamol.  This dose significantly reduced the mortality of the
    animals from 75% in the untreated group to 20%.  Mortality decreased
    with increasing dose of methionine.  The optimal dose was found to be
    200 mg/kg (1.34 mmol/kg), the total representing 45.7% of the dose of
    paracetamol, which reduced mortality to 3%.

         In the same study an attempt was made to define the time-limit
    for the efficacy of methionine.  Using the dosage of 200 mg/kg (1.34
    mmol/kg), the optimal time was found to be within 2 h of paracetamol
    administration.  At this time interval, mortality was reduced to 10%
    compared with 65% in control animals.  As the delay before
    administering methionine increased so its efficacy decreased, so that
    by 6 h after administration of paracetamol the antidote was
    ineffective.  Even after 8 h, however, when the animals would have
    sustained some liver damage, the administration of methionine did not
    increase mortality above control levels (Legros, 1976).

    2.8.2  Pharmacokinetics

         Animal studies describing the rate of absorption or elimination
    of orally or parenterally administered methionine have not been found
    in the literature (except as below).

         When rats were given intraperitoneal L-methionine in a dose of
    149.2 mg/kg (1 mmol/kg), peak concentrations in the liver were reached
    1 h after the injection and concentrations fell to less than twice
    normal by 3 h.  In rats fed a low protein diet, the rate of decline of
    hepatic methionine was slower, so that the 3 h level was 700% of
    control (Finkelstein et al., 1982).

    2.8.2.1  Metabolism

         The liver is the main site of methionine metabolism; as much as
    48% of methionine metabolism occurs here (Zeisel & Poole, 1979).  The
    mechanism is well understood and has been reviewed extensively by
    Stipanuk (1986), Mato et al. (1990) and Pisi & Marchesini (1990).  It
    is illustrated diagrammatically in Fig. 2.

         The main pathway of methionine metabolism involves
    transmethylation and transulfuration, but there is increasing evidence
    for the importance of a transamination pathway (Benevenga, 1984).

         The first step in the metabolism of methionine involves its
    activation to the high energy sulfonium compound,  S-adenosyl-L-
    methionine.  This is achieved by the transfer of the adenosyl moiety
    of ATP to the sulfur atom of methionine. Thus one mole of ATP is
    required for each mole of methionine metabolized in this system.
     S-adenosyl-L-methionine synthetase (also called methionine adenosyl
    transferase) catalyses this reaction.

          S-adenosyl-L-methionine is then demethylated to produce
     S-adenosyl-L-homo-cysteine, which is hydrolysed in a reversible
    reaction to yield homocysteine and adenosine.

         The formation of homocysteine marks a branch point in this
    pathway of methionine metabolism.  Homocysteine may either be
    remethylated to methionine, used for the resynthesis of  S-adenosyl-
    L-homocysteine by reversal of hydrolysis, or irreversibly converted to
    cystathionine.

         The final stage in the transulfuration pathway is the cleavage of
    cystathionine to produce cysteine and alpha-ketobutyrate.  This is
    catalysed by the enzyme cystathionine-▀-synthase, which is deficient
    in individuals with homocystinuria.  Homozygotes would be unable to
    utilize DL-methionine as a glutathione precursor; obligate
    heterozygotes also demonstrate an impaired ability to form
    cystathionine (Boers et al., 1985).

         Another metabolic pathway for methionine involves decarboxylation
    of  S-adenosyl-L-methionine and synthesis of polyamines and also,
    possibly, the synthesis of methylthio compounds.

         An alternative pathway for methionine metabolism that is
    independent of the formation of  S-adenosyl-L-methionine has been
    suggested (Benevenga, 1984).  It is thought that methionine is
    transaminated to alpha-keto-gamma-methiolbutyrate which is then
    decarboxylated to 3-methylthiopropionate.   In vitro experiments
    suggest that this compound may be further metabolized to yield
    methanethiol and hydrogen sulfide.  It is possible that the toxicity
    (see below) of methionine may be due, at least in part, to the
    formation of these compounds via the transamination pathway
    (Benevenga, 1984; Finkelstein & Benevenga, 1986).

         Methionine transamination takes place in several tissues
    including heart, brain and spleen.  Mitchell & Benevenga (1978) have
    calculated, however, that 48% of transamination may occur in the
    muscle and 40% in the liver.

    The Cystathionine Pathway;V03AN02.BMP

    2.8.3  Toxicology

    2.8.3.1  Acute toxicity

         Although L-methionine is an essential amino acid, it has been
    shown to be toxic in excess.  D-methionine is thought to be less
    toxic, presumably because it is not metabolically active itself but
    only after it has been converted by deamination and reamination to the
    L-form.  The rate of conversion may limit the cellular concentration
    of L-methionine (Benevenga, 1974; Bowman & Rand, 1980).

         In the rat the oral LD50 of L-methionine is 36 g/kg (241
    mmol/kg) and the intraperitoneal LD50 is 4.3 g/kg (29 mmol/kg)
    (NIOSH, 1992).  For DL-methionine in the rat the lowest published
    intraperitoneal lethal dose is 2 g/kg (13.4 mmol/kg) (NIOSH, 1992). 
    In mice the lowest published lethal doses of DL-methionine are: oral -
    4 g/kg (26.8 mmol/kg); intraperitoneal - 1.5 g/kg (10 mmol/kg); and
    intravenous - 300 mg/kg (2 mmol/kg) (NIOSH, 1992).

         The effects of excess methionine have been investigated in a
    number of animal species and differential toxicity has been noted
    (Hardwick et al., 1970).

         Loss of appetite and death was found in guinea-pigs given large
    doses of L-methionine (Hardwick et al., 1970).  These animals were
    fasted for 18 h and then given methionine doses of 492 mg/kg (3.3
    mmol/kg) by gavage at 8-h intervals.  The animals stopped eating and
    died at an average of 42 h (maximum 65 h) after initiation of the
    experiment, i.e. after a total dose of 2.5 g/kg (16.5 mmol/kg).  There
    were no deaths among the control group of fasted animals.

         Terminally, the experimental animals were found to be
    hypothermic, immobile and opisthotonic.  They had severe hypoglycaemia
    (blood sugar averaged 207 mg/l compared with 1295 mg/l in controls)
    with significantly depressed hepatic glycogen stores and generalised
    aminoacidaemia.  On postmortem examination, the liver was found to be
    fatty; lipid was concentrated in the periportal hepatocytes with a
    little present around the central veins.  Hepatic lipid appeared as
    early as 16 h into the experiment, after a total dose of 584 mg/kg
    (6.6 mmol/kg).  No hepatic lipid was found in control animals. 
    Hepatic ATP concentrations were only one-third that of the control
    guinea-pigs.  Even after a single dose of L-methionine (492 mg/kg; 3.3
    mmol/kg), both blood sugar and hepatic glycogen were significantly
    lower than in the controls.  The administration of adenine at the same
    time as L-methionine prevented a decrease in blood sugar and hepatic
    ATP concentration (Hardwick et al., 1970).

         The fall in hepatic ATP concentration was accompanied by a
    comparable increase in the concentration of adenosyl thionium
    compounds.  This suggested that the reduction in ATP was, at least
    partly, due to its utilization in the activation of methionine to

     S-adenosyl-L-methionine (Hardwick et al., 1970).

         The same dose fed to rats had no adverse effect; none of the
    animals were ill at 21 days when the study terminated (Hardwick et
    al., 1970).

    2.8.3.2  Subacute and chronic toxicity

         Although less relevant to acute treatment with methionine in
    humans, the following observations have been made in long-term animal
    studies.

         Excessive dietary L-methionine has been shown to depress hepatic
    cytochrome oxidase activity in rats fed 3% methionine for seven days,
    compared with animals fed 0.3% (Finkelstein & Benevenga, 1986).

         Suppression of growth was shown in another, longer term feeding
    study (Stekol & Szaran, 1962).  This study examined the D- and
    L-enantiomorphs separately. D-and L-methionine supplements were fed to
    30-day-old rats in four different proportions: 0.5%, 1.0%, 2.0% and
    4.0% for periods of 1, 2, 3, 4 and 9 months.  Both the 2% and 4%
    supplements of both enantiomorphs suppressed weight gain, D-methionine
    more so than L-methionine.  After one month the rats given these
    supplements were found to have enlarged livers and spleens, while the
    pancreas was smaller and softer than normal. Some rats exhibited
    hydronephrosis.  On microscopic examination these organs showed
    moderate to severe degenerative changes, although there were also
    signs of regeneration in the liver.  At this time there was no
    increase in hepatic fat. The gonads, heart and other organs showed no
    changes.  L-methionine caused greater damage than D-methionine. Rats
    fed 1% methionine for 1 month showed milder changes and those given
    0.5% methionine showed no changes.  After 3 months of feeding the 1-4%
    supplements, the tissues were normal except for an increase in droplet
    fat in the liver with each subsequent month (Stekol & Szaran, 1962).

         In a study of the ability of rats of different ages to utilize
    L-methionine, it was found that 30-day-old animals utilized far less
    methionine in the production of phospholipids, choline and creatine
    than those older than 90 days.  This reduction in methionine
    utilization may partly account for the organ damage seen in these
    animals, as adult rats fed 4% methionine suffered no organ damage
    (Stekol & Szaran, 1962).

         Another study, in which different age groups of mice were fed
    supplementary L-methionine for 42 days, did not show such a clear
    age-related difference (Massie & Aiello, 1984).  When 42-day-old mice
    were fed methionine in their drinking-water at a concentration of 0.05
    mol/litre, they did not show any reduction in weight gain but did have
    a significantly shorter life-span than control mice. Older mice (581
    days) given the same regimen had a similar life-span to that of the
    young mice.  Their life-span was not significantly different from that

    of their control group, although the control group was considered to
    have an unusually short life-span.  The cause of death was not
    investigated.  The estimated total daily intake of L-methionine by the
    mice in this study was 1.67 g/kg (11.2 mmol/kg), i.e. 1.03% of the
    animals' total solid intake.  This concentration was found to cause
    only minor organ changes (Massie & Aiello, 1984).

    2.8.3.3  Toxicity in experimental liver damage

         Although the study by Legros (1976) (section 2.8.1) showed no
    increase in mortality in mice with paracetamol-induced liver damage
    who were given methionine, a study in rats with liver damage has shown
    that oral methionine may cause hepatic encephalopathy when
    administered in combination with ammonia.  Higashi (1982b) induced
    liver damage in rats by administering carbon tetrachloride. 
    Intragastric methionine 3.4 g/kg (23 mmol/kg) was then given, followed
    an hour later by an intraperitoneal dose of ammonium acetate
    (5 mmol/kg). While rats given methionine or ammonium acetate alone did
    not develop hepatic encephalopathy, those given the two agents in
    combination did so.  Encephalopathy did not develop when glycine or
    leucine was substituted for methionine.

         When the interval between administration of methionine and
    ammonium acetate was varied, the severity of encephalopathy was also
    found to vary. Only mild and brief encephalopathy developed if the
    ammonium compound was given either immediately or 30 min after
    methionine.  The most severe encephalopathy developed in rats given
    ammonium acetate 120 min after methionine. This suggests that
    methionine metabolites may be responsible for enhancement of the
    encephalopathic effect of ammonia in rats with liver damage (Higashi,
    1982b).

    2.8.4  Effect in pregnancy

         As described above, excessive dietary methionine reduces weight
    gain, and in pregnant rats this may adversely affect fetal
    development.  When rats were given a diet containing 4% methionine
    from day one of pregnancy, their food intake, and thus their weight
    gain compared to control animals, was reduced.  Fewer rats were able
    to maintain their pregnancy and both average fetal weight and
    placental weight were abnormally low.  It is possible that some of
    this adverse effect was due to impaired secretion of ovarian hormones
    (Viau & Leathem, 1973).

    2.9  Volunteer studies

         Volunteer studies to investigate the pharmacokinetics of
    methionine following therapeutic doses (or overdoses) of paracetamol
    have not been undertaken.  However, Stegink et al. (1986) studied the
    ability of adult volunteers to utilize L-methionine and D-methionine

    for nutritional purposes.  The data indicate that adult humans do not
    utilize D-methionine efficiently as a methionine source.

         In a study to investigate methionine metabolism in patients with
    liver disease, Higashi (1982a) measured serum methionine
    concentrations in 20 healthy control subjects; values of 19 ▒ 12
    Ámol/l (2.8 ▒ 1.8 mg/l) were obtained.  An intravenous methionine
    loading test was performed in five of the control subjects to obtain
    elimination constants. Serum methionine concentrations were 19 ▒ 4
    Ámol/l (2.8 ▒ 0.6 mg/l) prior to the administration of 10 mg
    methionine per kg body weight; the mean elimination constant was
    24.0 ▒ 6.3 x 10-3 min-1.

         Plasma concentrations achieved after an oral loading dose of
    methionine have been measured in two studies.  In both cases
    L-methionine in a dose of 0.1 g (0.7 mmol) per kg body weight was
    given to male volunteers.  In the study by Murphy-Chutorian et al.
    (1985), the subjects were a group of 138 men, aged 31 to 65 years
    (mean 53 years), referred for cardiac catheterization.  Plasma
    methionine concen-trations were measured before, and 6 h after, dosing
    with L-methionine; the concentrations in 39 subjects found to have
    normal coronary arteries were 20.25 ▒ 4.68 Ámol/l (3.02 ▒ 0.7 mg/l)
    before, and 483.3 ▒ 141.9 Ámol/l (72.1 ▒ 21.2 mg/l) after loading; the
    corresponding concentrations in the remaining 99 subjects were 20.53
    ▒ 7.53 Ámol/l (3.06 ▒ 1.12 mg/l) and 537.5 ▒ 165.0 Ámol/l (80.2 ▒ 24.6
    mg/l), respectively.  The differences in plasma methionine
    concentrations were not statistically significant.

         By comparison, Boers et al. (1985) examined a group of 20 men
    aged 22-61 years (mean 42 years) and found a considerably higher mean
    peak methionine concentration of 1063 ▒ 65 Ámol/l (158.6 ▒ 9.7 mg/l)
    between 1 and 8 h after dosing.  The fasting methionine concentration
    was slightly higher than in the study of Murphy-Chutorian et al.
    (1985), i.e. 28 ▒ 2 Ámol/l (4.2 ▒ 0.3 mg/l).  Boers et al. (1983) also
    measured methionine concentrations in pre- and post-menopausal women
    given the same dose of methionine.  In 10 pre-menopausal women (aged
    14-42, mean 25 years) the fasting concentration of methionine was 26
    ▒ 2 Ámol/l (3.9 ▒ 0.3 mg/l) and the peak concentration after the oral
    loading dose was 1033 ▒ 60 Ámol/l (154 ▒ 9 mg/l).  In 10 post-
    menopausal women (aged 45-59, mean 54 years) the corresponding figures
    were 21 ▒ 1 Ámol/l (3 ▒ 0.15 mg/l) and 1107 ▒ 53 Ámol/l (165 ▒ 8
    mg/l).

         Elimination values for methionine after the loading dose were 0.1
    ▒ 0.03 l/h per kg for men and 0.08 ▒ 0.02 l/h per kg for both groups
    of women (Boers et al., 1983).

    2.9.1  Methionine in patients with hepatic dysfunction

         Phear et al. (1956) studied the effects of DL-methionine
    administered orally to 17 patients with liver disease.  Nine patients

    suffered from cirrhosis of the liver and had previously experienced
    episodes of impending hepatic coma (chronic portal systemic
    encephalopathy).  Electroencephalograms were compatible with this
    diagnosis.  In eight of the patients, the extent of the portal venous
    collateral circulation was assessed by transplenic portal venography
    and a very extensive collateral circulation was demonstrated.  In the
    ninth patient, disturbance of blood clotting prohibited this
    investigation.  Enteric-coated tablets containing 250 mg DL-methionine
    were given in divided doses between 6 am and 9 pm; the total daily
    dose, usually 10 g, varied between 8 and 20 g.  During the
    administration of methionine, neurological deterioration occurred in
    seven patients, and in two this effect was reproduced when the drug
    was administered a second time.  This occurred from 1 to 4 days after
    commencing methionine administration and after total doses of 11 to
    46 g methionine.

         Two of the nine patients tolerated 80 and 99 g methionine without
    neurological change.  A further seven patients with portal cirrhosis
    who had never exhibited neurological complications, tolerated 50-102
    g methionine without neurological change.  Liver function was
    considered to be less severely impaired than in the patients who were
    sensitive to the drug and an extensive collateral circulation was
    demonstrated in only three.  A patient with extra-hepatic portal vein
    obstruction also showed no change.

         In four patients who deteriorated after methionine
    administration, the control blood methionine concentrations were
    normal in two (9 and 18 mg/l; normal < 20 mg/l) and elevated in two
    (26 and 39 mg/l).  The concentration was increased in two (24 and 27
    mg/l) and normal (13, 12, 20, 15 and 15 mg/l) in five of the patients
    who were unaffected by methionine feeding.  In both groups, the rise
    in blood methionine concentrations after methionine administration was
    comparable.  Moreover, neurological deterioration occurred without
    significant change in blood ammonium, blood pH or serum bilirubin
    levels.

         In summary, this study showed that oral methionine caused
    neurological deterioration in patients with large collateral channels
    between the portal and systemic venous systems with chronic portal
    systemic encephalopathy.  Clearly, this situation is very different
    from that which obtains in the early stages following acute
    paracetamol overdosage.

    2.10  Clinical studies - clinical trials

         Only one controlled trial of oral DL-methionine therapy in
    paracetamol poisoning has been undertaken (Hamlyn et al., 1980, 1981;
    Meredith, 1987).  However, there have been two studies of the
     intravenous use of methionine in paracetamol poisoning (Prescott et
    al., 1976; Solomon et al., 1977).

    2.10.1  Study by Prescott et al. (1976)

         Prescott et al. (1976) reported 60 patients with paracetamol
    poisoning that were treated with intravenous cysteamine, L-methionine,
    or D-penicillamine and in whom the incidence and severity of hepatic
    necrosis were compared with those in 70 patients who received
    supportive therapy only.

    2.10.1.1  Patients and treatment

         The admission plasma paracetamol concentration, related to the
    time after ingestion, was used as a guide to the severity of
    intoxication and the need for treatment.  Gastric aspiration and
    lavage were carried out on all patients admitted within 4 h of
    overdosage.  Patients with nausea and vomiting were given maintenance
    intravenous fluids and cyclizine intramuscularly if necessary. 
    Vitamin K and fresh-frozen plasma or clotting factor concentrates were
    given if the prothrombin time ratio exceeded 3.0.  Once specific
    treatment became available, it was given to all patients with plasma
    paracetamol concentrations above the line in Fig. 3.

         Twenty patients (mean age 27 years) received methionine, and
    three were thought to be at particular risk.  Methionine was given
    intravenously in an initial loading dose of 5 g injected over 10 min
    followed by an infusion of a further 15 g over 20 h.  The methionine
    was stored in vials, dissolved in 5% dextrose immediately before use
    and sterilized by passage through a bacterial filter.

    2.10.1.2  Investigations

         The following investigations were carried out daily for 5 days in
    most patients: haemoglobin, total and differential white blood cell
    count, platelet count, plasma urea, electrolytes, creatinine,
    bilirubin, alkaline phosphatase, aspartate and alanine
    aminotransferases (ASAT, ALAT), blood glucose level, prothrombin time
    ratio and electrocardiogram.  The plasma paracetamol concentration on
    admission was estimated by a rapid spectrophotometric method (Routh et
    al., 1968).  Additional blood samples were taken 4-8 hourly for up to
    72 h and the plasma concentrations of unchanged paracetamol in these
    and the admission sample were subsequently determined by gas-liquid
    chromatography (Prescott, 1971a).  Urine was collected for 3-5 days
    for estimation of paracetamol and its sulfate and glucuronide
    conjugates to determine the minimum amount of paracetamol absorbed
    (Prescott, 1971a,b).  Many collections were incomplete.

    FIGURE 3

    2.10.1.3  Results of treatment

         The actual or extrapolated plasma paracetamol concentration 4 h
    after ingestion was used as a guide to the risk of liver damage.  To
    assess the effects of treatment within 10 h, patients were placed into
    four groups of increasing risk according to the 4-h plasma paracetamol
    concentration (<150, 150-250, 250-300, and > 300 mg/l).  In these
    four groups the mean plasma paracetamol concentrations on admission
    and at 4 h after ingestion did not differ significantly among
    different treatment regimens (Table 1).  The severity of intoxication
    and the risk of liver damage could therefore be regarded as similar,
    allowing direct comparisons of the results of early treatment.  The
    incidence and severity of hepatic necrosis were assessed using
    standard liver function tests, together with a "liver damage score"
    which was calculated for each patient as the sum of the minimum values
    of prothrombin time ratio, plasma bilirubin, and ASAT + ALAT/1000.
    Thus with upper normal limits of 1.3, 1.0 mg/dl, 40 and 40 U/l,
    respectively, the liver damage score would be 2.38.  Liver damage was
    defined as "severe" if the ASAT or ALAT activity exceeded 1000 U/l. 
    Tests of statistical significance were carried out with Student's
    test, chi2, and Fisher's exact test as appropriate.

    a)    Supportive therapy only:  The results of laboratory
         investigations in patients receiving supportive therapy only are
         set out in Table 1.  The incidence and severity of hepatic
         necrosis rose progressively with increasing 4-h plasma   
         paracetamol concentrations.  With one exception, liver damage was
         insignificant in the patients with 4-h values less than 150 mg/l,
         whereas every patient with a paracetamol concentration greater
         than 300 mg/l developed severe liver damage.  In the latter group
         the mean maximum values for ASAT, ALAT, bilirubin, and
         prothrombin time ratio were 5293 and 3751 U/l, 7.3 mg/dl and 2.4,
         respectively.  The same pattern of increasing liver damage was
         reflected in the liver damage scores.  Three patients died in
         hepatic failure and four developed acute renal failure.

    b)    Methionine:  None of the 15 patients treated with methionine
         within 10 h of ingestion died or developed acute renal failure,
         and in 12 liver damage was absent or trivial.  However, three
         patients sustained severe damage with mean values of ASAT, ALAT,
         bilirubin, prothrombin time ratio and liver damage score of 8133
         and 6867 U/l, 3.3 mg/dl, 2.8 and 21.2, respectively.  The
         ingestion-treatment intervals were 8.8, 9.5 and 9.7 h, and the
         4-h paracetamol concentrations were 357, 252 and 353 mg/l.  Only
         one patient was thought to be at particular risk.  The inclusion
         of these three patients is largely responsible for the high mean
         values in Table 1.  The incidence of severe liver damage in the 
         methionine group with a 4-h paracetamol concentration greater
         than 300 mg/l was significantly less than in the corresponding
         group receiving supportive therapy only (p < 0.01), but in all

         other comparisons of treatment with methionine and supportive
         therapy the differences were not statistically significant.

         Liver-function tests were only mildly disturbed in five patients
    receiving methionine 10-12 h after overdosage (see Table 1).

    2.10.1.4  Toxicity of methionine

         There were no adverse effects on haemoglobin, white blood cell
    count, platelet count, or plasma urea, creatinine, electrolyte, and
    calcium and blood glucose concentrations that could be attributed to
    treatment with methionine.

    2.10.1.5  Likelihood of benefit due to antidote

         Intravenous methionine appeared to be effective in most patients. 
    However, three patients treated within 10 h suffered severe liver
    damage, including one, who was apparently not at great risk with a 4-h
    plasma paracetamol concentration of 252 mg/l.  The results of
    treatment with cysteamine and D-penicillamine in this study have not
    been presented in detail, but none of 23 patients given cysteamine
    within 10 h of ingestion suffered severe liver damage or renal
    failure, and none died.

         Cysteamine was partially effective at 10-12 h, but ineffective
    12 h or more after ingestion.  Of five patients treated with
    D-penicillamine, one developed severe liver damage with acute renal
    failure.

    2.10.2  Study by Solomon et al. (1977)

         Solomon et al. (1977) reported 12 patients with paracetamol
    poisoning, half of whom were treated with cysteamine and half with
    intravenous amino acid preparations (Aminosol 10%, Aminoplex 14)
    containing methionine and cysteine.

    2.10.2.1  Results of treatment

         In only one patient in the cysteamine-treated group was the ALAT
    activity raised, and the bilirubin concentration was not raised above
    normal in any patient.  In the amino acid-treated group, two patients
    showed a mild rise in ALAT activity, and serum bilirubin
    concentrations remained normal in all patients.

    2.10.2.2  Toxicity of methionine

         No side effects attributable to therapy occurred in the amino
    acid-treated group, but nausea, vomiting and muscle twitching variably
    occurred in all the patients treated with cysteamine.


        Table 1.  Results (mean ▒ SEM) of investigations in patients receiving either supportive therapy or methioninea
                                                                                                                                                 

    No. of     4-h Plasma      Mean 4-h       Mean minimum    Mean         No. of                              Mean maximum
    patients   paracetamol     plasma         urinary         ingestion-   patients (%)
    (n=95)     concentration   paracetamol    recovery of     treatment    with severe         ASAT         ALAT      bilirubin       prothrombin
               (mg/l)          (mg/l)         paracetamol     interval     liver damageb       (U/l)        (U/l)     concentration   time ratio
                                              (g)             (h)                                                     (mg/dl)
                                                                                                                                                 

    Supportive therapy only (n=70)

       16          < 150         92 ▒ 11        7.4 ▒ 0.7          -            1 (6)         90 ▒ 44        107 ▒ 60    0.8 ▒ 0.1    1.3 ▒ 0.02

       23        150-250         199 ▒ 5        9.5 ▒ 1.3          -           6 (26)       515 ▒ 190       591 ▒ 232    1.1 ▒ 0.1     1.6 ▒ 0.1

       15        250-300        275 ▒ 15       12.1 ▒ 1.2          -           6 (40)      1412 ▒ 704      1702 ▒ 754    2.1 ▒ 0.6     1.6 ▒ 0.1

       16          > 300        404 ▒ 26       15.6 ▒ 3.2          -        16 (100)c      5293 ▒ 798      3751 ▒ 779    7.3 ▒ 1.5     2.4 ▒ 0.2

    Methionine < 10 h (n=20)

        5        150-250         219 ▒ 4       13.5 ▒ 0.7      5.7 ▒ 0.8        0 (0)          26 ▒ 5          31 ▒ 9    1.0 ▒ 0.1    1.4 ▒ 0.02

        3        250-300        277 ▒ 13       12.6 ▒ 0.7      8.7 ▒ 0.6       1 (33)     1087 ▒ 1057     1096 ▒ 1052    1.1 ▒ 0.2     1.4 ▒ 0.2

        7          > 300        379 ▒ 27       15.7 ▒ 1.7      7.7 ▒ 0.5       2 (29)     3070 ▒ 1968     2539 ▒ 1606    2.1 ▒ 0.7     2.0 ▒ 0.4

    Methionine > 10 h (n=5)

        5                       314 ▒ 30            -         11.1 ▒ 0.2          0         251 ▒ 178      268 ▒ 182       1.1 ▒ 0.2   1.4 ▒ 0.1
                                                                                                                                                 

    a  From: Prescott et al. (1976)
    b  ASAT or ALAT > 1000 U/l
    c  3 deaths
    

    2.10.2.3  Likelihood of benefit due to antidote

         Unfortunately, it is impossible to determine whether methionine
    had any beneficial effect in these patients, for the reasons set out
    below.

         Firstly, the amino acid solutions contained cysteine as well as
    methionine.  Cysteine alone has been shown to protect against
    paracetamol toxicity in mice (Strubelt et al., 1974) and it is thought
    to act as a glutathione precursor (Reed & Beatty, 1980).  Indeed,
     N-acetylcysteine is first converted to cysteine by deacetylation
    before it acts as a glutathione precursor (Lauterberg et al., 1983). 
    Cysteine may also act as a source of sulfate for conjugation with
    unchanged paracetamol (Glazenberg et al., 1984).   Secondly, in six
    patients plasma paracetamol concentrations were measured within 4 h of
    ingestion of the overdose and are therefore uninterpretable.  Only
    five of the six remaining patients were at risk when judged by the
    criterion now applied before considering specific therapy [a plasma
    paracetamol concentration falling above a line joining 200 mg/l (1.32
    mmol/l) at 4 h and 50 mg/l (0.33 mmol/l) at 12 h after ingestion of
    the overdose when plotted on a semi-logarithmic paracetamol
    concentration time scale (Meredith et al., 1986)].  Only one of these
    five patients was treated with an amino acid solution and she
    developed minor disturbance of hepatic function with a peak ALAT
    activity of 67 U/l.

    2.10.3  Study by Hamlyn et al. (1981)

         Hamlyn et al. (1981) were stimulated by case reports (Crome et
    al., 1976) of the successful use of oral methionine in the treatment
    of paracetamol poisoning to undertake a collaborative, controlled
    trial.

    2.10.3.1  Results of treatment

         In total, 40 patients at risk from hepatic and renal toxicity due
    to serious paracetamol overdose were studied.  Hepatic necrosis, as
    judged by "blind" histological assessment, occurred significantly less
    often in actively treated patients (methionine p < 0.02, cysteamine
    p < 0.01).  Significant differences in peak serum ASAT activity were
    seen, the geometric means being 1046 U/l in the supportive group,
    96 U/l in the cysteamine group, and 139 U/l following methionine
    treatment (Wilcoxon sum of ranks p < 0.01 in favour of methionine,
    p < 0.002 in favour of cysteamine).  These differences in favour of
    cysteamine and methionine were also reflected in peak serum bilirubin
    concentrations and prothrombin time ratios.

    2.10.3.2  Likelihood of benefit due to antidote

         This trial is the only controlled study of the use of oral
    methionine in the treatment of paracetamol poisoning, although it was
    not truly prospective because four patients had been included in an
    earlier trial and had been reported previously (Douglas et al., 1976). 
    A convincing hepatoprotective effect of early intravenous cysteamine
    and oral methionine was demonstrated.  However, no clear difference in
    efficacy of the two protective agents was shown, although early
    methionine administration was associated with few, if any, undesirable
    side-effects.

    2.11  Clinical studies - case reports

         The London Centre of the National Poisons Information Service in
    the United Kingdom recommended the use of oral methionine from 1974
    (Crome et al., 1976) in patients thought to be at risk of paracetamol-
    induced liver damage.  Initially, this was as an alternative to
    intravenous cysteamine and then later to intravenous
     N-acetylcysteine (Meredith, 1987).

    2.11.1  Study by Vale et al. (1981)

         Vale et al. (1981) reported on the efficacy of oral methionine
    therapy in 132 patients at risk of paracetamol-induced hepatic and
    renal toxicity.  Some of these patients had been reported previously
    (Crome et al., 1976; Meredith et al., 1978; Vale et al., 1979);
    because of the retrospective nature of the study, it does not
    constitute a formal clinical trial.

    2.11.1.1  Patients and treatment

         This study included 132 adult patients who were initially seen
    within 24 h of their overdose, which occurred between 1974 and 1979. 
    These patients had plasma paracetamol concentrations falling above a
    line joining plots of 200 mg/l (1.32 mmol/l) at 4 h and 70 mg/l (0.46
    mmol/l) at 12 h.  More severely poisoned patients, with plasma
    paracetamol concentrations above a line joining 300 mg/l (1.98 mmol/l)
    at 4 h and 75 mg/l (0.50 mmol/l) at 12 h, were classified as a high-
    risk group.  Where necessary, the plasma paracetamol plots were
    extended to 24 h.

         Of the 132 patients, 88 patients claimed to have taken only
    paracetamol, 16 took ethanol in addition, eight took benzodiazepines,
    three took dextropropoxyphene hydrochloride in combination with
    paracetamol (Distalgesic), and 17 ingested other drugs (including
    aspirin, tricyclic antidepressants, and orphenadrine hydrochloride). 
    The mean age of the 132 patients was 26 years (range, 14 to 73 years);
    48 were male and 84 were female.

         All patients underwent gastric lavage within 6 h of ingestion of
    the overdose.  Patients with persistent nausea and vomiting were also
    given intravenous fluid replacement and anti-emetics.  Each patient
    received methionine orally in a dose of 2.5 g, which was then repeated
    on three subsequent occasions at 4-hourly intervals (i.e. 10 g during
    12 h).

    2.11.1.2  Investigations

         All patients had plasma paracetamol levels measured on admission
    and at least once subsequently.  Haematological and biochemical tests
    were performed on admission and daily thereafter for at least 7 days. 
    Severe liver damage was defined as an increase in serum ASAT activity
    to a level above 1000 U/l, and renal impairment as a serum creatinine
    concentration greater than 300 Ámol/l.

    2.11.1.3  Results of treatment

         The efficacy of methionine was assessed by comparing it with the
    results of supportive therapy alone [when given to 57 similarly
    poisoned patients treated at the Royal Infirmary, Edinburgh (Prescott
    et al., 1979)].  Chi-square statistics were used for this purpose,
    with Yates' correction for continuity.  Comparisons were also made
    with patients treated with oral (Rumack & Peterson, 1978) and
    intravenous (Prescott et al., 1979)  N-acetylcysteine.  It should be
    noted, however, that in these two other studies (Rumack & Peterson,
    1978; Prescott et al., 1979), the "treatment" lines adopted joined
    plots of 200 mg/l (1.32 mmol/l) at 4 h and 50 mg/l (0.33 mmol/l) at
    12 h, and that they cannot therefore be compared directly.

         The patients were divided into two main groups depending on
    whether oral methionine was given either within, or later than, 10 h
    after paracetamol ingestion.  These two groups of patients were
    further subdivided into those who were at moderate risk and those who
    were at high risk of severe liver damage on the basis of their plasma
    paracetamol concentrations (Table 2).

    2.11.1.4  Liver damage

    a)    Oral methionine given within 10 h:  Only seven of the 96
         patients given methionine within ten hours had severe liver
         damage, as compared with 33 (58%) of the 57 patients given
         supportive treatment (Table 3).

         Patients given methionine showed an increase in mean maximum
    serum ASAT activity to 294 U/l, compared to a mean maximum level of
    more than 2000 U/l in those treated supportively.  (Between 1969 and
    1973, the liver enzyme test results at the Royal Infirmary, Edinburgh,
    were reported as more than 850 or 1000 U/l; therefore, a precise
    figure cannot be given.)

        Table 2.  Severity of poisoning and time of methionine administration
              in 132 patients at risk of paracetamol-induced hepatorenal
              toxicity
                                                                        

                                     Commencement of treatment
                                      (hours after overdose)

                                    < 10                10-24
                                                                        

    All patientsa (n = 132)           96                   36

    High-risk patientsb (n = 74)      43                   31
                                                                       

    a  Plasma paracetamol concentration > 200 mg/l (1.32 mmol/l) at
       4 h and > 70 mg/l (0.46 mmol/l) at 12 h
    b  Plasma paracetamol concentration > 300 mg/l (1.98 mmol/l) at
       4 h and > 75 mg/l (0.50 mmol/l) at 12 h
    
         Of the seven patients in whom severe liver damage developed, six
    were severely poisoned (high-risk cases), but only one had transient
    hepatic failure; none of these patients died.  All seven patients
    underwent gastric lavage within 5┤ h of ingestion of the overdose, and
    all received methionine within 8 h, although two vomited the first
    dose.

         The incidence of severe liver damage in patients given methionine
    was significantly less than that found in those treated supportively
    (chi2 corrected for continuity, 44.8; p < 0.0001).

    b)   Oral methionine given within 10-24 h

         In 36 cases, methionine was given 10-24 h after ingestion
    (Table 3).  Severe liver damage occurred in 17 (47%), and the mean
    maximum ASAT activity was 1464 U/l.  These results were not
    statistically different from those found in patients given supportive
    treatment only.

    2.11.1.5  High-risk patients

         Severe hepatic damage occurred in 89% of high-risk patients
    treated supportively (Table 4), whereas only 14% of those treated with
    oral methionine within 10 h of ingestion had severe liver damage
    (chi2 corrected for continuity, 36.1; p < 0.0001).  In patients
    treated between 10 and 24 h after ingestion, those in whom severe
    liver damage developed numbered half those of the group receiving

    supportive treatment alone (chi2 corrected for continuity, 10.9;
    p < 0.0001).

    2.11.1.6  Renal impairment

         One patient given oral methionine within 10 h had transient renal
    failure (even though his maximum ASAT activity was only 160 U/l). 
    Renal impairment occurred in two other patients (6%) treated between
    10 and 24 h after ingestion, whereas six (11%) of 57 patients in the
    group treated supportively suffered the same complication.

    2.11.1.7  Deaths

         None of the patients given oral methionine within 10 h died of
    hepatic failure, but two high-risk patients, both treated later than
    10 h after ingestion, died.

    2.11.1.8  Toxicity of methionine

         Methionine did not produce any serious side effects.  Twenty-two
    (17%) of the 132 patients vomited before treatment with methionine,
    but only seven (5%) vomited afterwards.

    2.11.1.9  Likelihood of benefit due to antidote

         Comparative data for oral and intravenous acetylcysteine are
    shown in Tables 3-4.  When given within 10 h, both methionine and
     N-acetylcysteine protected against death caused by hepatic failure. 
    The incidence of severe liver damage was not significantly different
    in the three treatment groups; similar results were seen for those
    patients treated between 10 and 24 h.  In this retrospective study,
    oral methionine was clearly effective in protecting against severe
    liver damage, renal failure and death after paracetamol overdose when
    given within 10 h of ingestion.

    2.12  Summary of evaluation

    2.12.1  Indications

         Oral methionine is indicated for the treatment of acute
    paracetamol (acetaminophen) poisoning.  When 4 h or more have elapsed
    after ingestion of an overdose the plasma concentration of paracetamol
    should be measured.  Specific treatment with methionine is required if
    the plasma paracetamol concentration falls above a line which on a
    semilog graph joins plots of 200 mg/l (1.32 mmol/l) at 4 h and 30 mg/l
    (0.33 mmol/l) at 15 h after the overdose (see Fig. 1 in section 1.3). 
    In cases where plasma concentrations of methionine are not available,
    methionine should be given if more than 100 mg paracetamol/kg is taken
    in a single dose.


        Table 3.  Hepatic and renal damage in patients poisoned with paracetamol given oral methionine, or oral or
              intravenous  N-acetylcysteine
                                                                                                                     

    Treatment                          No. of       No. (%) with     Mean         No. (%)        Mortality
                                       patients     severe liver     maximum      with acute        (%)
                                                    damage (ASAT     ASAT         renal
                                                    > 1000 U/l)      (U/l)        failure
                                                                                                                     

    Oral methionine                       96            7 (7)         294           1 (1)            0
    given within 10 ha

    Oral  N-acetylcysteine
    given within 10 h (Rumack             49           8 (16)         210               0            0
    & Peterson, 1978)b

    Intravenous  N-acetylcysteine
    given within 10 h (Prescott           62            1 (2)         113               0            0
    et al., 1979)b

    Oral methionine given
    within 10-24 ha                       36          17 (47)        1464           2 (6)        2 (6)

    Oral  N-acetylcysteine
    given within 10-24 h (Rumack
    & Peterson, 1978)b                    51          23 (45)        2207               0            0

    Intravenous acetylcysteine
    given within 10-24 h
    (Prescott et al., 1979)b              38          20 (53)        3814          5 (13)        2 (5)

    Supportive treatment only
    (Prescott et al., 1979)b              57          33 (58)      > 2022          6 (11)        3 (5)
                                                                                                                     

    a  Plasma paracetamol concentration > 200 mg/l (1.32 mmol/l) at 4 h and > 70 mg/l (0.46 mmol/l) at 12 h
    b  Plasma paracetamol concentration > 200 mg/l (1.32 mmol/l) at 4 h and > 50 mg/l (0.33 mmol/l) at 12 h

    Table 4.  Hepatic and renal damage in high-riska patients poisoned with paracetamol given oral methionine
              or intravenous  N-acetylcysteine
                                                                                                                     

    Treatment                          No. of       No. (%) with     Mean         No. (%)        Mortality
                                       patients     severe liver     maximum      with acute        (%)
                                                    damage (ASAT     ASAT         renal
                                                    > 1000 U/l)      (U/l)        failure
                                                                                                                     

    Oral methionine given
    within 10 h                           43           6 (14)         464           1 (2)            0

    Intravenous  N-acetylcysteine
    given within 10 h
    (Prescott et al., 1979)b              33            1 (3)         185               0            0

    Oral methionine given
    within 10-24 h                        31          14 (45)        1532           2 (6)        2 (6)

    Intravenous acetylcysteine
    given within 10-24 h
    (Prescott et al., 1979)               27          18 (67)        4919          5 (19)        2 (7)

    Supportive treatment only
    (Prescott et al., 1979)               28          25 (89)      > 3186          6 (21)       3 (11)
                                                                                                                     

    a  Plasma paracetamol concentration > 300 mg/l at 4 h and > 75 mg/l at 12 h
    b  Plasma creatinine concentration > 300 Ámol/l
    

         If the patient is either vomiting or unconscious (usually the
    result of taking another drug in addition to paracetamol), then it is
    more appropriate to give  N-acetylcysteine by the intravenous route. 
    Children aged less than 6 years tend to swallow only small amounts of
    paracetamol and specific treatment is rarely indicated (Meredith et
    al., 1978).  Nevertheless, plasma paracetamol concentrations should be
    measured and interpreted as in adult patients (Meredith, 1987). 
    Adolescents, who are more likely than younger children to take a
    hepatotoxic dose of paracetamol, should be dealt with similarly.

         Certain special situations arise in adult patients.  Firstly,
    some patients take repeated doses of paracetamol over a period of
    hours or days that either individually or in total are potentially
    hepatotoxic.  The interpretation of single measurements of the plasma
    paracetamol concentration is therefore difficult or impossible in
    these circumstances, and immediate specific therapy is advised
    provided that liver dysfunction has not already developed, as shown,
    for example, by an elevated prothrombin time ratio.  Secondly, a
    paracetamol overdose is sometimes taken in combination with drugs that
    delay gastric emptying (for example, dextropropoxy-phene or tricyclic
    antidepressants) and, therefore, gastrointestinal absorption.  The
    so-called "treatment line" (which on a semilog graph joins plots of
    200 mg/l at 4 h and 30 mg/l at 15 h after ingestion) will be shifted
    to the right as a consequence.  The extent to which this will occur
    cannot easily be predicted and care must be taken to avoid
    under-treatment of this group of patients.  If there is any doubt,
    specific treatment should be given.

    2.12.2  Advised route and dosage

         Methionine should be given  orally in a dose of 2.5 g, followed
    by three further doses of 2.5 g at 4-hourly intervals.  Adolescents at
    risk may be given the same doses, but children aged less than 6 years
    should be given 1 g orally followed by three further doses of 1 g at
    4-hourly intervals.  If a patient is vomiting and is unable to
    tolerate oral methionine, intravenous  N-acetylcysteine should be
    given.  The intravenous administration of methionine cannot be
    recommended (see section 2.12.4).

    2.12.3  Other consequential or supportive therapy

         Patients who present to hospital within 1-2 h and who are thought
    to have ingested 100 mg paracetamol/kg body weight or more should
    undergo gastric lavage.  As already mentioned, children aged less than
    six years tend to swallow only small amounts of paracetamol and
    gastric lavage is probably unnecessary.  In neither adults nor
    children has the value of syrup of ipecacuanha or activated charcoal
    been established.  In addition, the former may preclude the
    administration of methionine because of resultant persistent nausea
    and vomiting, and the latter is likely to adsorb (and therefore annul
    the effect of) methionine administered either beforehand or

    afterwards.  For these reasons syrup of ipecacuanha should not be
    employed in the treatment of paracetamol overdosage, and the use of
    activated charcoal should be avoided unless intravenous therapy (with
     N-acetylcysteine) is to be employed.

    2.12.4  Areas of use where there is insufficient information to make
            recommendations

         The time after ingestion of a paracetamol overdose at which the
    oral administration of methionine ceases to provide protection against
    hepatic and renal damage has not been established with certainty.  The
    evidence shows that the degree of protection afforded declines rapidly
    once more than 10 h has elapsed after the overdose.  On present
    evidence, specific treatment should be given up to 15 h after the
    overdose provided that the International Normalized Ratio (prothrombin
    time) remains normal.  There are insufficient data to make
    recommendations about methionine therapy for the period 10-24 h after
    a paracetamol overdose or thereafter.

         There are insufficient published data on the clinical efficacy
    and safety in use of methionine administered  intravenously (and
    neither is a pharmaceutical preparation available).  For this reason,
    a recommendation concerning the use of methionine in this manner
    cannot be made.

    2.12.5  Proposals for further studies

         The dosage regimen for methionine was empirically based on its
    action as a glutathione precursor.  Further information is needed
    about the optimal dosage, kinetics of absorption and treatment period. 
    Different formulations of methionine should be studied in paracetamol
    overdose patients to define and obtain optimal absorption
    characteristics.  Childhood dosage regimens should also be
    investigated because the dose employed currently may be more than is
    necessary, and the use of intravenous preparations of
     N-acetylcysteine may sometimes cause problems with fluid overload.

         Prospective studies comparing oral methionine and intravenous
     N-acetylcysteine therapy should be performed because: (a) those
    undertaken so far have used historical controls; and (b) the use of
    methionine in developing countries may have wider applicability than
     N-acetylcysteine (because of the greater cost of the latter and
    difficulties with the cold chain in relation to transportation and
    storage).  The efficacy of treatment should be assessed during
    successive periods of time between 10 and 24 h, or even later, after
    paracetamol overdosage to determine the latest point at which it may
    be employed effectively.  (Unfortunately, the safety in use and
    established clinical efficacy of  N-acetylcysteine reduces the
    likelihood of such studies being undertaken.)

         There is no evidence as yet, from the use of methionine in
    humans, as to whether protection against paracetamol-induced toxicity
    results in retardation of fetal growth or even teratogenicity. 
    Epidemiological studies are therefore indicated.  Finally, it would be
    desirable to determine the efficacy of methionine as a protective
    agent for paracetamol poisoning in heterozygotes for homocystinuria
    (see section 2.8.2.1), but there is the obvious practical difficulty
    of being able to study a sufficient number of individuals with this
    condition who have taken a paracetamol overdose.

    2.12.6  Adverse effects

         Oral methionine therapy in the dosage used for paracetamol
    poisoning has not been associated in practice with any adverse effects
    in patients with or without paracetamol-induced liver failure.  There
    is, however, evidence of neurological deterioration associated with
    methionine administration to patients with chronic hepatic dysfunction
    from causes other than paracetamol.

         There is also evidence of increased hepatotoxicity when
    methionine is given to mice with liver failure induced experimentally
    by carbon tetrachloride, but  not if induced by paracetamol.

    2.12.7  Restrictions of use

         On theoretical grounds, methionine is likely to be ineffective in
    patients with homocystinuria and  N-acetylcysteine should be given
    instead following paracetamol overdose.

         If patients are vomiting or unconscious, intravenous
     N-acetylcysteine should be given in preference to methionine. 
     N-acetylcysteine is also the preferred agent in patients with
    chronic hepatic failure who have taken paracetamol in overdose.

    2.13  Model information sheet

    2.13.1  Uses

         Methionine is indicated for the treatment of acute paracetamol
    (acetaminophen) poisoning if a patient is at risk of developing
    hepatic or renal toxicity.  The risk may be judged by measuring the
    plasma paracetamol concentration when more than 4 h have elapsed after
    the overdose.  Oral methionine should be given if the plasma
    paracetamol concentration lies above a line which on a semilog graph
    joins plots of 200 mg/l (1.32 mmol/l) at 4 h and 30 mg/l (0.20 mmol/l)
    at 15 h after overdose (see Fig. 1 in section 1.3).

         Methionine is also indicated if more than 100 mg paracetamol/kg
    has been ingested in a single dose and plasma paracetamol
    concentrations are not available.

    2.13.2  Dosage and route

         The dose for adults and adolescents is 2.5 g orally, followed by
    a further three doses of 2.5 g at 4-hourly intervals (10 g in total). 
    Children aged less than 6 years tend to swallow only small amounts of
    paracetamol and specific treatment is rarely indicated but, if
    necessary, the dose is 1 g orally, followed by a further three doses
    of 1 g at 4-hourly intervals.

    2.13.3  Precautions/contraindications

         If a patient is vomiting or unconscious (usually the result of
    taking another drug), then alternative specific treatment should be
    administered.  Special attention should be given to the need for
    treatment in patients who present 10-24 h after the overdose and in
    patients who have taken repeated doses of paracetamol.  In patients
    who have taken other drugs that might delay gastric emptying (for
    example, opiates or tricyclic antidepressants) and who present more
    than 10 h after the overdose, an intravenous protective agent
    ( N-acetylcysteine) may be preferred because the kinetics of
    absorption of methionine are likely to be disturbed.

          N-acetylcysteine, rather than methionine, should be used as
    protective therapy in patients with homocystinuria and chronic liver
    failure.

    2.13.4  Adverse effects

         Oral methionine therapy in the dosage used for paracetamol
    poisoning has not been associated in practice with any adverse
    effects.  Early concerns that methionine might contribute to hepatic
    encephalopathy in paracetamol-induced liver failure have not been
    substantiated, and review of the data suggests that it is unlikely to
    occur.

    2.13.5  Use in pregnancy and lactation

         Animal studies, in which large amounts of methionine (up to 4% of
    the total diet) were given orally, have shown suppression of growth,
    but there is no evidence in humans that methionine is a hazard in
    pregnancy or during lactation.  If the mother is at risk of
    paracetamol-induced hepatic and renal toxicity, then treatment should
    not be withheld.  Moreover, it should be remembered that the unborn
    fetus is itself at risk of paracetamol-induced hepatotoxicity.

    2.13.6  Storage

         Methionine should be stored in closed containers in cool, dry,
    dark conditions.  The recommended time-limit for storage is 2 years.

    2.14  References

    Benevenga NJ (1974)  Toxicities of methionine and other amino acids.
    J Agric Food Chem, 22: 2-9.

    Benevenga NJ (1984)  Evidence for alternative pathways of methionine
    catabolism.  Adv Nutr Res, 6: 1-18.

    Berlet HH, Matsumoto K, Pscheidt GR, Spaide J, Bull C, & Himwich HE
    (1965)  Biochemical correlates of behavior in schizophrenic patients. 
    Schizophrenic patients receiving tryptophan and methionine or
    methionine together with a monoamine oxidase inhibitor.  Arch Gen
    Psychiatr, 13: 521-531.

    Boers GH, Smals AG, Trijbels FJ, Leermakers AI, & Kloppenborg PW
    (1983) Unique efficiency of methionine metabolism in premenopausal
    women may protect against vascular disease in the reproductive years.
    J Clin Invest, 72: 1971-1976.

    Boers GHJ, Fawler B, Smals AGH, Trijbels FJA, Leermakers AI, Kleijer
    WJ, & Kloppenborg PWC (1985)  Improved identification of heterozygotes
    for homocystinuria due to cystathionine synthase deficiency by the
    combination of methionine loading and enzyme determination in cultured
    fibroblasts. Hum Genet, 69: 164-169.

    Bowman WC & Rand MJ (1980)  Textbook of pharmacology. Oxford,
    Blackwell Scientific Publications, p 1-20.

    Brune GG & Himwich HEJ (1962)  Effects of methionine loading on the
    behaviour of schizophrenic patients. J Nerv Ment Dis, 134: 447-450.

    Budavari S ed. (1989) The Merck Index, 11th ed. Rohway, New Jersey,
    Merck & Co., Inc., p 5901.

    Crome P, Vale JA, Volans GN, Widdop B, & Goulding R (1976) Oral
    methionine in the treatment of severe paracetamol (acetaminophen)
    overdose. Lancet, 2: 829-830.

    Degussa AG (1985)  Amino acids: specifications and analytical methods.
    Hanau, Germany, Degussa AG.

    Douglas AP, Hamlyn AN, & James O (1976)  Controlled trial of
    cysteamine in treatment of acute paracetamol (acetaminophen)
    poisoning. Lancet, 1: 111-115.

    European pharmacopoiea (1989) 2nd. Part II, 13th fascicule. Sainte
    Ruffine, France, Maisonneuve SA.

    Finkelstein A & Benevenga NJ (1986)  The effect of methanethiol and
    methionine toxicity on the activities of cytochrome oxidase and
    enzymes involved in protection from peroxidative damage.  J Nutr, 116:
    204-215.

    Finkelstein JD, Kyle WE, Harris BJ, & Martin JJ (1982)  Methionine
    metabolism in mammals:  concentration of metabolites in rat tissues.
    J Nutr, 112: 1011-1018.

    Glazenberg EJ, Jekel-Halsema IMC, Baranczyk-Kuzma A, Krijgsheld KR, &
    Mulder GJ (1984)  D-Cysteine as a selective precursor for inorganic
    sulphate in the rat  in vivo. Biochem Pharmacol, 33: 625-628.

    Goldfarb AS, Goldgraben GR, Herrick EC, Ouellette RP, & Cheremissinoff
    PN (1981) Organic chemicals manufacturing hazards. Chapter 4:
    Condensation process for DL-methionine production. Ann Arbor,
    Michigan, Ann Arbor Science Publishers, Inc., pp 115-194.

    Hamlyn AN, Lesna M, Record CO, Smith PA, Watson AJ, Meredith TJ,
    Widdop B, Volans GN, & Crome P (1980)  Prevention of hepatic necrosis
    in severe paracetamol (acetaminophen) poisoning: Prospective
    controlled kind of early treatment with cysteamine or methionine. Gut,
    21: A448.

    Hamlyn AN, Lesna M, Record CO, Smith PA, Watson AJ, Meredith T, Volans
    GN, & Crome P (1981)  Methionine and cysteamine in paracetamol
    (acetaminophen) overdose. Prospective controlled trial of early
    therapy. J Int Med Res, 9: 226-231.

    Hardwick DF, Applegarth DA, Cockcroft DM, Ross PM, & Calder RJ (1970) 
    Pathogenesis of methionine-induced toxicity. Metabolism, 19: 381-391.

    Higashi T (1982a)  Impaired metabolism of methionine in severe liver
    diseases I. Clinical and pathophysiological significance of elevated
    serum methionine levels. Gastroenterol Jpn, 17: 117-124.

    Higashi T (1982b)  Impaired metabolism of methionine in severe liver
    diseases II. Clinical and experimental studies on the role of impaired
    methionine metabolism in the pathogenesis of hepatic encephalopathy.
    Gastroenterol Jpn, 17: 125-134.

    Hoppe B & Martens J (1984)  [Manufacture and extraction of amino
    acids.] Chem Zeit, 3: 73-86 (in German).

    Jagenburg OR & Toczko K (1964) The metabolism of acetophenetidine.
    Isolation and characterization of S-(1-acetamido-4-hydroxyphenyl)-
    cysteine, a metabolite of acetophenetidine. Biochem J, 92: 639-643.

    Kakimoto Y, Sanmo I, Kanazawa A, Tsujio T, & Kaneko Z (1967) 
    Metabolic effects of methionine in schizophrenic patients pretreated
    with a monoamine oxidase inhibitor. Nature (Lond), 216: 1110-1111.

    Klein-Schwartz W & Oderda GM (1981)  Adsorption of oral antidotes for
    acetaminophen poisoning (methionine and  N-acetylcysteine) by
    activated charcoal. Clin Toxicol, 18: 283-290.

    Lauterberg BH, Corcoran GB, & Mitchell JR (1983) Mechanism of action
    of  N-acetylcysteine in the protection against the hepatotoxicity of
    acetaminophen in rats  in vivo.  J Clin Invest, 71: 980-991.

    Legros J (1976)  Animal studies - a theoretical basis for treatment.
    J Int Med Res, 4 (Suppl. 4): 46-54.

    McLean AEM (1974)  Prevention of paracetamol poisoning. Lancet, 1:
    729. McLean AEM (1986)  Why do patients still die from paracetamol
    poisoning?  Br Med J, 294: 1172.

    McLean AEM & Day PA (1975)  The effect of diet on the toxicity of
    paracetamol and the safety of paracetamol-methionine mixtures. Biochem
    Pharmacol, 24: 37-42.

    Martindale (1989)  In: Reynolds JEF ed. The extra pharmacopoiea, 29th
    ed. London, Pharmaceutical Press, p 843.

    Massie HR & Aiello VR (1984)  The effect of dietary methionine on the
    copper content of tissues and survival of young and old mice. Exp
    Gerontol, 19: 393-399.

    Mato JM, Corrales F, Martin-Duce M, Ortiz P, Pajares MA, & Cabrero C
    (1990)  Metabolism and consequences of the impaired trans-sulphuration
    pathway in liver disease: Part I Biochemical implications. Drugs,
    40(Suppl. 3): 58-64.

    Maxwell LF, Cotty VF, Marcus AD, & Barnett L (1975)  Prevention of
    acetaminophen (paracetamol) poisoning. Lancet, 11: 610-611.

    Meredith TJ (1987) Paracetamol poisoning in England and Wales.
    University of Cambridge, pp 93-112 (M.D. Thesis).

    Meredith TJ, Crome P, Volans GN, & Goulding R (1978)  Treatment of
    paracetamol poisoning. Br Med J, 1: 1215-1216.

    Meredith TJ, Prescott LF, & Vale JA (1986) Why do patients still die
    from paracetamol poisoning?  Br Med J, 293: 345-346.

    Miners JO, Drew R, & Birkett DJ (1984)  Mechanism of action of
    paracetamol protective agents in mice  in vivo. Biochem Pharmacol,
    33: 2995-3000.

    Mitchell AD & Benevenga NJ (1978) The role of transamination in
    methionine oxidation in the rat. J Nutr, 108: 67-78.

    Mitchell JR, Jollow DJ, Potter WZ, Davis DC, Gillette JM, & Brodie BB
    (1973)  Acetaminophen-induced hepatic necrosis. I. Role of drug
    metabolism. J Pharmacol Exp Ther, 187: 185-194.

    Murphy-Chutorian DR, Wexman MP, Grieco AJ, Heininger JA, Glassman E,
    Gaull GE, Ng SKC, Feit F, Wexman K, & Fox AC (1985) Methionine
    intolerance:  a possible risk factor for coronary artery disease. J Am
    Coll Cardiol, 6: 725-730.

    Neuvonen PJ, Tokola O, Toivonen ML, & Simell O (1985)  Methionine in
    paracetamol tablets, a tool to reduce paracetamol toxicity. Int J Clin
    Pharmacol Ther Toxicol, 23: 497-500.

    NIOSH (1992) Registry of toxic effects of chemical substances (RTECS).
    Cincinnati, Ohio, National Institute for Occupational Safety and
    Health.

    Park LC, Baldessarini MJ, & Kety SS (1965)  Methionine effects on
    chronic schizophrenics. Patients treated with monoamine oxidase
    inhibitors. Arch Gen Psychiatr, 12: 346-351.

    Pearce LA & Waterbury LD (1974)  L-methionine: a possible levodopa
    antagonist.  Neurology, 24: 640-641.

    Phear EA, Ruebner B, Sherlock S, & Summerskill WHJ (1956)  Methionine
    toxicity in liver disease and its prevention by chlortetracycline. 
    Clin Sci, 15: 93-117.

    Pisi E & Marchesini G (1990)  Mechanisms and consequences of the
    impaired trans-sulphuration pathway in liver disease: Part II.
    Clinical consequences and potential for pharmacological intervention
    in cirrhosis. Drugs, 40(Suppl. 3): 65-72.

    Pollin W, Cardon PV, & Kety SS (1961) Effects of amino acids feedings
    in schizophrenic patients treated with iproniazid.  Science, 133:
    104-105.

    Prescott LF (1971a) Gas-liquid chromatographic estimation of
    paracetamol. J Pharm Pharmacol, 23: 807-808.

    Prescott LF (1971b)  The gas-liquid chromatographic estimation of
    phenacetin and paracetamol in plasma and urine. J Pharm Pharmacol, 23:
    111-115.

    Prescott LF, Park J, Sutherland GR, Smith IJ, & Proudfoot AT (1976) 
    Cysteamine, methionine and penicillamine in the treatment of
    paracetamol poisoning. Lancet, 2: 109-113.

    Prescott LF, Illingworth RN, Critchley JAJH, Stewart MJ, Adam RD, &
    Proudfoot AT (1979)  Intravenous  N-acetylcysteine: the treatment of
    choice for paracetamol poisoning. Br Med J, 2: 1097-1100.

    Reed DJ & Beatty PW (1980)  Biosynthesis and regulation of
    glutathione: toxicological implications. In: Hodgson S, Band JR, &
    Philpot RM Ed. Reviews in biochemical toxicology. New York, Amsterdam,
    Oxford, Elsevier/North Holland, Vol. II, pp 213-241.

    Routh JM, Shane NA, Arrendondo EG, & Paul WD (1968)  Determination of
     N-acetyl- p-aminophenol in plasma. Clin Chem, 14: 882-889.

    Rumack BH & Peterson RG (1978)  Acetaminophen overdose: incidence,
    diagnosis and management in 416 patients. Pediatrics, 62(Suppl.):
    898-903.

    Sax NI (1989)  Dangerous properties of industrial materials, 7th ed.
    New York, Van Nostrand Reinhold Company, p 2219.

    Smilkstein MJ, Knapp GL, Kulig KW, & Rumack BH (1988)  Efficacy of
    oral  N-acetylcysteine in the treatment of acetaminophen overdose.
    Analysis of the National Multicenter Study (1976-1985). N Engl J Med,
    319: 1557-1562.

    Smolin LA, Benevenga NJ, & Berlow S (1981)  The use of betaine for the
    treatment of homocystinuria. J Pediatr, 99: 467-472.

    Smythies JR, & Halsey JH (1984)  Treatment of Parkinson's disease with
    L-methionine.  South Med J, 77: 1577.

    Solomon AE, Briggs JD, Knepil J, Henry DA, Winchester JF, & Birrell R
    (1977)  Therapeutic comparison of thiol compounds in severe
    paracetamol poisoning.  Ann Clin Biochem, 14: 200-202.

    Stegink LD, Bell EF, Filer LJ, Ziegler EE, Andersen DW, & Seligson FH
    (1986)  Effects of equimolar doses of L-methionine, D-methionine and
    L-methionine-dl-sulphoxide on plasma and urinary amino acid levels in
    normal adult humans. J Nutr, 116: 1185-1192.

    Stekol JA & Szaran J (1962)  Pathological effects of excessive
    methionine in the diet of growing rats. J Nutr, 77: 81-90.

    Stipanuk M (1986)  Metabolism of sulfur-containing amino acids. Ann
    Rev Nutr, 6: 179-209.

    Stramentinoli G, Pezzoli C, & Galli-Kienle M (1979)  Protective role 
    of S-adenosyl-L-methionine against acetaminophen induced mortality and
    hepatotoxicity in mice.  Biochem Pharmacol, 28: 3587-3571.

    Strubelt O, Siegers CP, & Schutt A (1974)  The curative effects of
    cysteamine, cysteine and dithiocarb in experimental paracetamol
    poisoning. Arch Toxicol, 33: 55-64.

    Ullman F (1985)  Encyclopedia of industrial chemicals, 5th ed.
    Deerfield Beach, New York, Cambridge, Weinheim, Basel, VCH
    Verlagsgesellschaft, Vol 2A, pp 71-72.

    Vale JA, Meredith TJ, Crome P, Helliwell M, Volans GN, Widdop B, &
    Goulding R (1979) Intravenous  N-acetylcysteine: the treatment of
    choice in paracetamol poisoning?  Br Med J, 2: 1435-1436.

    Vale JA, Meredith TJ, & Goulding R (1981)  Treatment of acetaminophen
    poisoning. The use of oral methionine. Arch Intern Med, 141: 394-396.

    Viau AT & Leatham JH (1973)  Excess dietary methionine and pregnancy
    in the rat.  J Reprod Fertil, 33: 109-111.

    Vina J, Hems R, & Krebs HA (1978)  Maintenance of glutathione content
    in isolated hepatocytes. Biochem J, 170: 627-630.

    Vina J, Romero FJ, Estrela JM, & Vina JR (1980)  Effect of
    acetaminophen (paracetamol) and its antagonists on glutathione (GSH)
    content in rat liver.  Biochem Pharmacol, 29: 1968-1970.

    Weast RC & Astle MJ ed. (1978) CRC handbook of chemistry and physics,
    59th ed. Boca Raton, Florida. CRC Press Inc., p C758.

    Zeisel SH & Poole JR (1979) Dietary intake of methionine: influence in
    mammals on brain S-adenosylmethionine. In: Transmethylation.
    Amsterdam, Oxford, New York, Elsevier Science Publishers, pp 59-68.

    3.   N-ACETYLCYSTEINE

    3.1  Introduction

          N-acetylcysteine is the  N-acetyl derivative of L-cysteine, a
    naturally occurring amino acid.  It was originally introduced into
    clinical medicine as a mucolytic agent in the 1960s but is now widely
    used in the management of paracetamol poisoning, initially having been
    used as an alternative to the more toxic agent cysteamine (Prescott et
    al., 1977).

          N-acetylcysteine has been shown to protect against paracetamol-
    induced liver damage and has been used both intravenously and orally
    for this purpose.  It is given intravenously in a number of countries,
    including the United Kingdom and Australia, whereas in the USA its use
    has principally been by the oral route.  It protects against
    paracetamol-induced liver damage when given by both routes (Prescott
    et al., 1979; Smilkstein et al., 1988) if given within 8 h of
    intoxication.  Use of  N-acetylcysteine should be guided by plasma
    concentrations of paracetamol taken in conjunction with the time of
    the overdose (Prescott et al., 1979; Rumack et al., 1981).  It is
    clear that the efficacy of  N-acetylcysteine is reduced with
    increasing time after administration of paracetamol and there is no
    evidence of antidotal efficacy if given more than 24 h after
    poisoning.  The intravenous and oral regimens are essentially
    empirical and no controlled studies have been performed of the optimal
    dose or duration of therapy.

          N-acetylcysteine has also been investigated in a number of
    other clinical situations in which reactive metabolites are believed
    to be important in the toxicity of the poison.  In animals,
     N-acetylcysteine may protect against hepatotoxicity of halothane
    (Keaney & Cocking, 1981).   N-acetylcysteine has also been shown in
    animals to be protective against heavy metals, alkylating agents and
    radiation, and in man to haemorrhagic cystitis secondary to
    cyclophosphamide (Flanagan, 1987).  The only formal clinical trials in
    this situation have been conducted on cyclophosphamide- and
    isophosphamide-induced haemorrhagic cystitis.   N-acetyl-cysteine is
    effective in the management of cystine stones in cystinuria
    (Martindale, 1988a).

         Recently it has also been shown that  N-acetylcysteine reduces
    mortality in patients with acute paracetamol-induced liver failure
    (Keays et al., 1991). Thus  N-acetylcysteine may have two effects in
    paracetamol poisoning: firstly, it prevents liver damage from
    occurring and secondly, if that does occur,  N-acetylcysteine may
    significantly reduce mortality in such patients.

    3.2  Name and chemical formula

    International non-
    proprietary name:          N-acetylcysteine

    Synonyms:                 Acetylcysteine;  N-acetyl-L-cysteine; NSC-11180;
                              L-alpha-acetamido-▀-mercaptopropionic acid

    IUPAC name:                N-acetyl-3-mercaptoalanine

    CAS number:               616-91-1

    Empirical formula:        C5H9NO3S

    Chemical structure:
                                       HSCH2CHCOOH
                                            |
                                            NHCOCH3

                                        N-acetylcysteine

    Relative molecular
    mass:                     163.2

    Conversion table:         1 g = 6.1 mmol
                              1 mg =  6.1 Ámol
                              1 mmol = 163.2 mg
                              1 Ámol = 163.2 Ág

    3.3  Physico-chemical properties

    3.3.1  Melting point

            109-110 ░C

    3.3.2  Physical state

          N-acetylcysteine consists of a white crystalline powder with a
    slightly acetic odour.  In its liquid formulation for drug use, it has
    an odour common to sulfhydryl-containing compounds, i.e. an odour
    similar to that of hydrogen sulfide ("rotten eggs").

    3.3.3  Solubility

         Solubility values in water of 1 part in 8 (British Pharmacopoeia,
    1993) and 1 part in 5 (United States Pharmacopeia, 1990) and in
    ethanol of 1 part in 2 (British Pharmacopoeia, 1993) and 1 part in 4
    (United States Pharmacopeia, 1990) have been reported.

     N-acetylcysteine is practically insoluble in chloroform and ether
    (Martindale, 1988b).

    3.3.4  Optical properties

          N-acetylcysteine has no significant optical properties.

    3.3.5  pKa

         The pKa value is 9.5 (Clarke, 1986).

    3.3.6  pH

         A 1% solution has a pH of 2.0 to 2.8. Sterile solution is
    buffered with sodium hydroxide to a pH of 7.

    3.3.7  Stability

          N-acetylcysteine should be stored in airtight containers at a
    temperature below 15 ░C and protected from light (Martindale, 1988b).

    3.3.8  Incompatibilities

          N-acetylcysteine is incompatible with many metals, with rubber,
    and with oxygen and oxidizing substances.  Some antibiotics including
    amphotericin, ampicillin sodium, erythromycin lactobionate and some
    tetracyclines are either physically incompatible with, or may be
    inactivated on mixture with,  N-acetylcysteine.  A change in colour
    of solutions of  N-acetylcysteine to light purple does not indicate
    significant impairment of safety or efficacy.

    3.3.9  Proprietary names and manufacturers

         Many preparations of  N-acetylcysteine are marketed, most as
    mucolytics.  The proprietary names, manufacturers and countries
    include:

         Airbron (Allen & Hanburys, Canada; Duncan Flockhart, United
         Kingdom); Brunac (Bruschettini, Italy); Eurespiran (Nicholas,
         Germany); Exomuc (Bouchara, France); Fabrol (Ciba, Denmark;
         Inpharzam, Sweden; Zyma, United Kingdom); Fluimucil (Arsac,
         France; Inpharzam, Germany; Azmbom, Italy, Netherlands; Zambon,
         Spain; Inpharzam, Switzerland); Fluprowit (Thiemann, Germany);
         Granon (DAK, Denmark); Ilube (Duncan Flockhart, United Kingdom);
         Inspir (Sweden); L Cimexyl (Cimex, Switzerland); Lysomucil
         (Belgium); Muco Sanigen (Beecham-Wulfing, Germany); Mucocedyl
         (Kettelhack Riker, Germany); Mucofilin Sol (Japan); Mucolysin
         (Durascan, Denmark); Mucolyticum (Bristol, Germany); Mucomist
         (Bristol Italiana Sud, Italy); Mucomyst (Astra, Argentina,
         Australia, Belgium; Bristol, Canada; Astra, Denmark; Allard,
         France, Netherlands; Draco, Norway; Tika, Sweden, Switzerland;

         Mead Johnson Pharmaceutical, USA); Mucosal (Dey, USA); Mucothiol
         (Ozothine, France); Mucret (Astra, Germany); Nac (Canada);
         Parvolex (Glaxo, Australia; Allen & Hanburys, Canada; Duncan,
         Flockhart, United Kingdom); Solmucol (Ibsa, Switzerland); Tixair
         (Valpan, France).

         Preparations available for use in paracetamol poisoning include
    Parvolex (for intravenous use) and Mucomyst (for oral and intravenous
    administration).

         It should be noted that in some parts of the world a similar
    compound  S-carboxymethylcysteine is available rather than
     N-acetylcysteine.  Although its efficacy is not proven, animal data
    appear to be comparable to those for  N-acetylcysteine (Ioannides et
    al., 1983).

    3.4  Pharmaceutical formulation and synthesis

         A synthetic route has been described by Smith & Gorin (1961). 
    Cysteine methyl ester hydrochloride and ethyl acetimidate
    hydrochloride are condensed to synthesize methyl 2-methyl-2-
    thiazoline-4-carboxylate hydrochloride.  In the presence of hydro-
    chloric acid, this compound is converted to  N-acetylcysteine and
     S-acetylcysteine.

         Commercial products usually contain ethylenediaminetetraacetic
    acid (EDTA) to remove trace amounts of metals, such as copper, that
    will catalyse oxidation of  N-acetylcysteine.

         The available commercial preparations for oral use contain a
    10-20% solution of  N-acetylcysteine. This formulation needs to be
    diluted before administration to avoid gastrointestinal irritation.
    Dilution is best performed with water or a commercial carbonated or
    still flavoured drink.

    3.5  Analytical methods

          N-acetylcysteine exists in biological systems in the oxidized
    or reduced form, and also in combination with thiol groups of, for
    example, proteins.  Methods to measure these other forms have been
    described (Burgunder et al., 1989).  As the levels necessary to
    protect the liver and kidney against paracetamol damage are unknown,
    it is inappropriate at present to measure levels of  N-acetylcysteine
    in blood or serum during treatment.  The concentrations of
     N-acetylcysteine that are toxic to man have not been established but
    there are indications of a relationship between adverse effects of
     N-acetylcysteine and high plasma  N-acetylcysteine levels (> 300
    mg/l) (Prescott et al., 1989).

    3.5.1  Quality control of antidote

          N-acetylcysteine can be assayed in formulations by liquid
    chromatography (United States Pharmacopeia, 1990) or
    spectrophotometrically after reaction according to Ellman (1959) and
    Fontana & Toniolo (1974).

    3.5.2  Methods for identification of the antidote

          N-acetylcysteine is identified by its infrared spectrum
    directly when dispersed in potassium bromide or, if in solution, after
    isolation by precipitation subsequent to saturation with sodium
    chloride at pH 2-3 (United States Pharmacopeia, 1990; personal
    communication from Astra Draco, Sweden).

    3.5.3  Methods for analysis of the antidote in biological samples

         Techniques for quantifying  N-acetylcysteine in plasma and urine
    by high performance liquid chromatography have been developed and
    appear to be highly accurate (Kagedal et al., 1984; Lewis et al.,
    1984).  Other similar methods have been described (Frank et al., 1984;
    Drummer et al., 1986; Johansson & Westerlund, 1986).

    3.5.4  Methods for analysis of toxic agent

         Methods for paracetamol analysis are discussed in section 1.6.

    3.6  Shelf-life

    3.6.1  Formulations for oral use

         The shelf-life in the unopened commercial container appears to
    vary depending on the volume and concentration of the product.  The
    average quoted shelf-life is 24 months (10-60 months) when the product
    is protected from light.

         In the original opened container the shelf-life is 96 h but this
    shelf-life is reduced to one hour after dilution or preparation of
     N-acetylcysteine for administration.

    3.6.2  Formulations for intravenous use

         Duncan Flockhart state that  N-acetylcysteine for intravenous
    use has a shelf-life of 3 years when stored at 25 ░C and protected
    from light.

    3.7  General properties

    3.7.1   Mode of antidotal activity

         The toxicity of paracetamol in overdose is thought to be mediated
    by its conversion to  N-acetyl- p-benzoquinone imine (NAPQI) and the
    subsequent arylation and oxidation of critical thiol groups in the
    cell membrane (see section 1.4 for further details). 
     N-acetylcysteine could theoretically protect against paracetamol
    toxicity in a number of ways (see chapter 1). Commonly cited mechanism
    is that  N-acetylcysteine can provide necessary sulfhydryl groups
    through synthesis of glutathione (GSH).  In addition to acting as a
    conjugating agent, glutathione may have other effects including
    reduction of oxidized thiol groups such as those of the calcium
    translocases in cell membranes (Tee et al., 1986). The mode of
    antidotal action of  N-acetylcysteine in the possible management of
    other forms of intoxication is likely to be similar to that in
    paracetamol poisoning (Flanagan, 1987).

    3.7.2  Effect in paracetamol-induced liver failure

         The mechanism of action of the protective effect of
     N-acetylcysteine on paracetamol-induced liver failure in humans has
    not been established. Most probably it is related to the ability of
     N-acetylcysteine to reverse tissue hypoxia through improved tissue
    microperfusion (Harrison et al., 1990; Keays et al., 1991).

    3.7.3  Other therapeutic uses

         The most common medical use of  N-acetylcysteine is inhalation
    therapy to reduce the viscosity of pulmonary secretions by reducing
    disulfide linkages of mucoproteins (Sheffner, 1983).  It is also used
    to dissolve cystine stones in cystinuria (Martindale, 1988a).  These
    properties are not relevant to its antidotal use.

         The hypothesis that  N-acetylcysteine might prevent nitrate
    tolerance has also been investigated on the basis that this tolerance
    was due to lack of reduced sulfhydryl groups in vascular smooth
    muscle. This has not been found to be the case (Parker et al., 1987;
    Hogan et al., 1990).

         The suggestion that acetylcysteine may produce a reduction in
    lipoprotein a in patients with hyperlipidaemia and an associated risk
    of atherosclerosis (Gavish & Breslow, 1991) has been questioned by
    others (Stalenhoef et al., 1991).

    3.8  Animal studies

    3.8.1  Pharmacodynamics

         Following a suggestion for the use of  N-acetylcysteine by
    Prescott & Matthew (1974), studies in a number of animal species have
    shown that paracetamol-induced hepatic damage can be prevented. 
    Piperno et al. (1978) reported that in the mouse  N-acetylcysteine
    protected against paracetamol-induced liver damage and that this
    protective effect could be demonstrated even when administration was
    instituted 4┤ h after dosing with paracetamol.  More recent studies
    have concentrated on the mechanism of this effect.

         Lauterburg et al. (1983) studied the effects of
     N-acetylcysteine on rats  in vivo.   N-acetylcysteine was found
    not to form significant amounts of conjugate with the reactive
    intermediate, though it did result in increased glutathione synthesis. 
    It was concluded that this mechanism is likely to be the most
    important one that operates  in vivo.  Studies by Corcoran et al.
    (1985) using mice did not support the hypothesis that
     N-acetylcysteine increases the proportion of paracetamol sulfated to
    a degree that would reduce the amount of toxic intermediate produced.

         The pharmacodynamics of  N-acetylcysteine as derived from
    studies on animals are discussed further in section 1.4.

    3.8.2  Pharmacokinetics

         Studies using radiolabelled  N-acetylcysteine in rats showed
    moderately good absorption of the drug given by the oral route. The
    percentage of radioactivity recovered in the urine and faeces was 42,
    33 and 20% by the intravenous, intramuscular and oral routes,
    respectively. A similar study in dogs dosed orally resulted in 36%
    recovery in the urine and faeces (Bonanomi & Gazzaniga, 1980).    
    Distribution and elimination studies have only been performed with
    radio-labelled  N-acetylcysteine and radioactivity measurements
    (Bonanomi & Gazzaniga, 1980). The results are difficult to interpret,
    but there is an indication of rapid distribution and elimination.

    3.8.3  Toxicology

         The LD50 of  N-acetylcysteine has been examined in a number of
    species and values range from 700 mg/kg intraperitoneally in the mouse
    to between 5100 and 6000 mg/kg orally in the rat (NIOSH, 1983;
    Johnston et al., 1983).

         The oral LD50 in the mouse was found to be 7900 mg/kg compared
    to an intravenous LD50 of 3800 mg/kg (NIOSH, 1983).  The oral LD50
    in dogs was > 1000 mg/kg (Gosselin et al., 1984).

         Respiratory failure is the usual terminal event in laboratory
    animals acutely poisoned with  N-acetylcysteine (Johnston et al.,
    1983).

          N-acetylcysteine has not been shown to be teratogenic in rats
    or rabbits (Bonanomi & Gazzaniga, 1980; USPCI, 1985).  When
    administered to rabbits during the critical phase of embryogenesis, no
    malformation resulted (Johnston et al., 1983).

          N-acetylcysteine is negative in the Ames mutagenicity test, and
    also reduces the mutagenic affect of chemical carcinogens in the same
    assay (Wilpart et al., 1985).

    3.8.4  Studies with modified cytochrome P-450 activity

         The important step in paracetamol metabolism is the formation of
    the reactive intermediate NAPQI by the cytochrome P-450 system. 
    Inhibition or stimulation of activity of this enzyme group will
    therefore reduce or increase paracetamol toxicity, as discussed in
    section 1.5.

    3.9  Volunteer studies

         No studies on the efficacy of  N-acetylcysteine in paracetamol
    poisoning have been carried out in volunteers.  Studies on the
    pharmacokinetics of  N-acetylcysteine, and effects of paracetamol on
    these, have however been performed.

         From a pharmacokinetic point of view, one can look at
     N-acetylcysteine in two ways (Olsson et al., 1988).  The reduced
    form can be considered as the parent drug and all other forms as
    metabolites.  Alternatively, all  N-acetylcysteine, irrespective of
    the fraction oxidized, may be regarded as the parent drug.  The latter
    approach has been considered most logical from a clinical point of
    view (Olsson et al., 1988), although it is the reduced form that is
    the active form in paracetamol poisoning.

    3.9.1  Absorption and bioavailability

         Human studies that attempt to quantify  N-acetylcysteine
    absorption have shown great variations in plasma area under the curve
    (AUC). Difficulties in clarifying this issue relate to the complicated
    pharmacokinetics and difficulties with the assay (reduced versus total
     N-acetylcysteine), and the possible contribution of a significant
    first-pass effect.

          N-acetylcysteine appears to be absorbed rapidly when given as
    standard release preparations, the Tmax being around 40-45 min in
    the study of Borgstrom et al. (1986), and exists as the free reduced
    species in plasma for only a short time.  It also causes an increase
    in protein and non-protein sulfhydryl concentrations (Maddock, 1980).

         When given orally, as tablets or a solution, total
     N-acetylcysteine was found to have a bioavailability as low as 6-10%
    (Borgstrom et al., 1986; Olsson et al., 1988).  The bioavailability
    was lowest when a slow-release preparation was used.

         The good absorption and low bioavailability is most probably due
    to an extensive first-pass elimination by the liver.  However, the
    rather low hepatic extraction ratio (0.26) in one study argues against
    this theory (Borgstrom et al., 1986). Rapid uptake and deacetylation
    by tissues in the gut wall and liver, to form cysteine, glutathione,
    and inorganic sulfite, have been proposed to be the most probable
    explanation for the extensive first-pass effect (Borgstrom et al.,
    1986; Olsson et al., 1988). It is not clear whether this first-pass
    effect favours the administration of  N-acetylcysteine by the oral
    route, as suggested by some authors (Borgstrom et al., 1986).

    3.9.2  Distribution

         The volume of distribution of total  N-acetylcysteine in healthy
    volunteers receiving low doses (200-600 mg) was 0.33-0.47 litres/kg
    (Borgstrom et al., 1986; Olsson et al., 1988) compared to 0.54
    litres/kg in patients receiving standard  N-acetylcysteine treatment
    intravenously (Prescott et al., 1989).  The volume of distribution of
     N-acetylcysteine therefore appears to be independent of dose.  The
    volume of distribution of reduced  N-acetylcysteine was 0.59
    litres/kg in healthy volunteers (Olsson et al., 1988).

         The mechanism of plasma protein binding of  N-acetylcysteine is
    unlike that of other drugs.   N-acetylcysteine is oxidized by
    reacting with thiol groups in plasma proteins to form mixed
    disulfides.  In the study by Olsson et al. (1988), plasma proteins
    were precipitated with perchloric acid and dithiothreitol and the
    supernatant stored at -70 ░C to prevent oxidation of reduced
     N-acetylcysteine.  Reduced  N-acetylcysteine was measured by
    chromatography of deproteinized plasma. The total plasma concentration
    of  N-acetylcysteine, including protein-bound  N-acetylcysteine, was
    assayed after initial reduction of disulfide bonds in plasma. With
    this technique, covalent protein binding of  N-acetylcysteine in
    plasma increased with time after dosing to a maximum of about 50% 4 h
    after intravenous administration. This percentage fell to
    approximately 20% after 12 h.

    3.9.3  Elimination

         Borgstrom et al. (1986) studied the pharmacokinetics of
     N-acetylcysteine given intravenously and orally as three separate
    oral formulations to ten normal volunteers.  Renal clearance accounted
    for about 30% of the total body clearance.  Total body clearance was
    of the order of 0.2 litres/h per kg and the terminal elimination half-
    life of  N-acetylcysteine, measured in this study by HPLC, was
    2.27 h.

         Other workers have measured both free  N-acetylcysteine and
    total plasma sulfhydryls.  Thus Burgunder et al. (1989) noted that
    only a small proportion of the administered  N-acetylcysteine was in
    its free form, the majority being present as disulfides. 
    Administration of  N-acetylcysteine increased free cysteine, but
    total cysteine and free and total glutathione in plasma were
    unchanged.  Olsson et al. (1988) reported the terminal elimination
    half-life of total  N-acetylcysteine, measured as free and disulfide,
    to be 6.25 h after oral administration and 5.15 h (range, 3.4-13)
    after intravenous administration.

         In patients receiving the standard intravenous  N-acetylcysteine
    treatment, the half-life of total  N-acetylcysteine was 5.7 h
    (Prescott et al., 1989).

    3.9.4  Oral  N-acetylcysteine and interaction with activated charcoal

         Three  in vitro studies have indicated effective adsorption of
     N-acetylcysteine by activated charcoal (Chinouth et al., 1980;
    Klein-Schwarz & Oerda, 1981; Rybolt et al., 1986).  In studies on
    volunteers, the  in vivo effects of charcoal have been contradictory. 
    Two studies showed no statistical difference in plasma AUC between
     N-acetylcysteine alone and  N-acetylcysteine plus charcoal
    (Krenzelok et al., 1980; Renzi et al., 1985). In contrast, Ekins et
    al. (1987) reported a decrease in the mean peak plasma
     N-acetylcysteine level of 29% and a decrease in mean plasma AUC of
    39% in the activated charcoal group.

         Although the clinical implication of an interaction between
     N-acetylcysteine and activated charcoal appears unclear, it is
    generally recommended that the use of activated charcoal should be
    avoided if  N-acetylcysteine is being administered orally.

    3.9.5  Pharmacodynamics

         Burgunder et al. (1989) measured the effects of giving
    paracetamol in a dose of 2 g with 2 g  N-acetylcysteine orally. In a
    control experiment, this dose of paracetamol alone resulted in a
    decrease in both plasma cysteine and glutathione levels.  In contrast,
    administration of paracetamol with 2 g  N-acetylcysteine resulted in
    an increase in cysteine and glutathione levels.  The authors concluded
    that, in humans,  N-acetylcysteine supports glutathione synthesis
    when demand for this is increased, as is the case after paracetamol
    administration.

    3.10  Clinical studies - clinical trials

    3.10.1  Efficacy of intravenous  N-acetylcysteine

         Prescott et al. (1977) first reported the effects of
     N-acetylcysteine given in a dose of 300 mg/kg body weight over 20 h
    to 15 patients whose plasma paracetamol concentrations (range 262-369
    mg/l at 4 h) suggested the likelihood of subsequent hepatic
    dysfunction.  In this study, 11 out of 12 patients treated within 10
    h either suffered no liver dysfunction or developed only mild
    disturbance, as judged by liver function tests.  The other patient,
    and three further patients treated more than 10 h after ingestion of
    paracetamol, developed liver damage.

         In a subsequent report dealing with a hundred cases of severe
    paracetamol poisoning (Prescott et al., 1979), only 1 of 62 patients
    treated within 10 h developed severe liver damage.  This was in
    comparison with a 58% incidence of severe liver damage in a
    retrospective series of patients who had received supportive treatment
    alone (33 of 57 patients).  This study also showed that efficacy after
    10 h diminishes and that treatment after 15 h with intravenous
     N-acetylcysteine is likely to be ineffective. Furthermore
    paracetamol-induced renal damage did not occur in patients treated
    less than 10 h after overdose.  The incidence of renal impairment in
    patients treated after this time (13%, 5/38) was similar to that in a
    group treated supportively (11%, 6/57).  Only four patients with renal
    impairment required haemodialysis.

         The dosage regimen used in both the studies of Prescott consisted
    of an initial loading dose of 150 mg/kg in 200 ml of 5% dextrose over
    15 min followed by 50 mg/kg in 500 ml of 5% dextrose over 4 h and a
    final 100 mg/kg in one litre of 5% dextrose given over 16 h.  The
    total dose was thus 300 mg/kg over 20 h.  The basis for this dosage
    regimen was empirical and not based on experimental data. The
    treatment line was defined as a line joining the following plots on a
    semi-logarithmic graph of plasma paracetamol concentration: 200 mg/l
    at 4 h after ingestion and 30 mg/l at 15 h (Fig. 1).  As a result of
    this study intravenous  N-acetylcysteine was adopted as a treatment
    for paracetamol poisoning in the United Kingdom and in most countries
    worldwide.

         Smilkstein et al. (1991) reported a non-randomized open
    multicentre trial of 179 patients with acute paracetamol overdose and
    plasma concentrations above the treatment line.  All patients received
    a 48-h intravenous  N-acetylcysteine dosage regimen consisting of a
    loading dose of 140 mg/kg, followed by 12 doses of 70 mg/kg every 4 h,
    giving a total dose of 980 mg/kg over 48 h.  For patients classified
    as having a "probable risk", as defined in the Smilkstein et al.
    (1988) study, hepato-toxicity occurred in 5 out of 50 (10%) patients
    treated within 10 h of ingestion and in 23 of 85 (27%) patients
    treated 10 to 24 h after ingestion.  Of the high-risk patients treated

    16 to 24 h after ingestion, 11 of 19 (58%) developed hepatotoxicity. 
    When compared with historical controls in the 20-h intravenous
    protocol (Prescott et al., 1979), these results were equivalent for
    patients treated within 10 h post-ingestion and superior for patients
    treated after 10 h (27 versus 53%).  Smilkstein et al. (1991),
    however, used a treatment line that was 25% lower than that of
    Prescott et al. (1979), thus precluding valid comparison of data. 
    However, when comparing the high-risk patients treated 16-24 h post-
    ingestion in the Smilkstein et al. (1991) study, i.e. patients with
    paracetamol concentrations above a treatment line between 300 mg/l at
    4 h and 75 mg/l at 12 h (Fig. 1), there is some evidence for less
    hepatotoxicity resulting from use of the 48-h intravenous protocol (58
    versus 82%).

    3.10.2  Efficacy of oral  N-acetylcysteine

         Oral  N-acetylcysteine is the most widely used antidote for
    paracetamol poisoning in the USA.  Rumack and his colleagues have
    published a series of reports on their experience with
     N-acetylcysteine in cases of paracetamol poisoning.  Rumack &
    Peterson (1978) reported on 100 patients who had ingested toxic doses
    and were treated with  N-acetylcysteine.  In this series, 17% of 49
    patients beginning therapy within 10 h of paracetamol poisoning
    developed hepatotoxicity.  Treatment between 10 and 24 h was less
    successful; 45% of patients treated at this time developed severe
    hepatotoxicity.

         In a second report (Rumack et al., 1981) only 7% of 57 patients
    treated within 10 h after ingestion developed hepatotoxicity, 29% of
    52 patients treated between 10 and 16 h developed hepatotoxicity and
    63% of 39 patients beginning treatment at 16 to 24 h after ingestion
    developed hepatic dysfunction.

         Smilkstein et al. (1988) reported on their experience over a 10-
    year period (1976-1985) in a large prospective study involving 2540
    patients treated orally with a loading dose of  N-acetylcysteine of
    140 mg/kg, followed by 17 oral maintenance doses of 70 mg/kg (i.e. a
    total of 18 doses).  In this large series no deaths were directly
    related to paracetamol toxicity in patients in whom  N-acetylcysteine
    therapy began within 16 h after ingestion.  There was, however, a
    relationship between delay in treatment and the risk of
    hepatotoxicity, as judged by liver function tests in patients in whom
    plasma paracetamol concentrations indicated a likelihood of
    hepatotoxicity. In patients considered at high risk of hepatotoxicity,
    as judged by paracetamol concentrations, 8.3% (17 out of 206) of
    patients treated within 10 h developed hepatotoxicity, 34.4% (199 of
    578) patients treated 10 to 16 h after presentation developed
    hepatotoxicity, and 41% (116 of 283) of patients treated 16 to 24 h
    after presentation developed hepatotoxicity.  The 95% confidence
    intervals for the three groups were 5-13%, 31-37%, and 35-46%,
    respectively.  High risk was defined as initial plasma paracetamol

    concentrations above the line intersecting 300 mg/l at 4 h and 75 mg/l
    at 12 h (n=1067) (see Fig. 1).

    3.10.3  Oral versus intravenous  N-acetylcysteine

         In summary, the frequency of severe liver damage in patients
    treated within 10 h was 2% in patients (n=62) treated with intravenous
     N-acetylcysteine, compared to 17% (n=49), 7% (n=57) and 8% (n=206)
    in patients treated with oral  N-acetyl-cysteine (sections 3.10.1 and
    3.10.2).  However, there have been no studies published which allow a
    direct comparison of oral versus intravenous  N-acetylcysteine
    treatment, nor has the optimal duration of either the oral or
    intravenous regimen been clearly established on the basis of
    comparative clinical trials.

    3.10.4  Therapeutic drug monitoring during  N-acetylcysteine treatment

         The doses of  N-acetylcysteine given in both the oral and
    intravenous dose regimen are empirical and are not based on studies of
    the plasma concentrations of  N-acetylcysteine required for antidotal
    activity. Although there are pharmacokinetic data available for the
    standard intravenous dose regimen, there is no indication for
    measuring  N-acetylcysteine in plasma during this treatment (Prescott
    et al., 1989).

    3.10.5   N-acetylcysteine in paracetamol-induced liver failure

         When  N-acetylcysteine was introduced as an antidote in
    paracetamol poisoning, there was some concern that late administration
    (> 24 h post-ingestion) of  N-acetyl-cysteine, and thereby amino and
    sulfhydryl-groups, could be potentially dangerous in the presence of
    failing liver function.  Keays et al. (1991) performed an open
    randomized study of 50 consecutive patients with paracetamol-induced
    fulminant hepatic failure without previous  N-acetylcysteine
    treatment.  The study could not be performed blind because the
     N-acetylcysteine solution has an easily identifiable pungent aroma.
    The  N-acetylcysteine dose given was the standard 20-h intravenous
    regimen.  The rate of survival was significantly higher among the
    patients given  N-acetylcysteine (48%, 12/25) as compared to controls
    (20%, 5/25).  The  N-acetyl-cysteine-treated patients also had a
    significantly lower incidence of cerebral oedema and hypotension
    requiring inotropic support.  The average time from paracetamol
    ingestion to inclusion in the study was 53 h (range 36-80) in the
     N-acetylcysteine-treated group as compared to 56 h (33-96) among
    controls.

         The mechanism for the therapeutic effect of  N-acetylcysteine in
    paracetamol-induced liver failure is not clear but may be related to
    increased tissue oxygen consumption and decreased oxidant stress,
    thereby reducing the oxidation of important protein thiol groups
    (Keays et al., 1991).  Although this study was not placebo-controlled,

    it seems justified at present to give  N-acetylcysteine to all
    patients with paracetamol-induced fulminant liver failure.

    3.11  Clinical studies - case reports

         Clinical studies of the use of oral and intravenous
     N-acetylcysteine have clearly documented its effect and have thus
    limited the need for efficacy information from case reports.  However,
    case reports have provided valuable information about the adverse
    effects and toxicity of  N-acetylcysteine in humans.

    3.11.1  Adverse effects

         The principal toxic effects of  N-acetylcysteine when given
    intravenously consist of anaphylactoid reactions following therapeutic
    doses (Bateman et al., 1984a,b; Gervais et al., 1984; Mant et al.,
    1984; Tenenbein, 1984).  This effect could be a pseudo-allergic
    reaction, since it appears to be due to histamine release (Bateman et
    al., 1984b).  The most appropriate therapy for this adverse effect
    appears to be intravenous antihistamines.  Flushing of the chest or
    face is common and usually begins 15 to 75 min after the initiation of
    the infusion, being associated with peak  N-acetylcysteine
    concentrations of between 300 and 900 mg/l.  These concentrations are
    considerably higher than peak levels obtained following the oral
    regimen (Prescott et al., 1989).  Occasionally more severe
    anaphylactic reactions to intravenous  N-acetylcysteine have been
    reported (Walton et al., 1979; Vale & Wheeler, 1982).  Urticarial
    reactions may also follow the use of oral  N-acetylcysteine (Charley
    et al., 1987).

         Extravasation of a 20%  N-acetylcysteine solution caused pain
    and inflammation (Casola & van Sonnenburg, 1984).

         Nausea and vomiting are very common during oral
     N-acetylcysteine therapy in paracetamol overdose.  Dilution to at
    least a 9% solution prior to oral administration is therefore
    recommended (Shaw, 1969).  Granules of  N-acetylcysteine (200 mg)
    given in sachets did not result in histopathological changes in the
    gastrointestinal mucosa (Marini et al., 1980).  Diarrhoea may also
    occur following  N-acetylcysteine administration (Ferrari, 1980). 
    One report suggested that liver enzyme changes may have occurred in a
    3-year-old male child with cystic fibrosis treated with both oral and
    rectal  N-acetylcysteine for meconium ileus.  Liver enzyme levels
    were elevated on two separate occasions, but the doses of
     N-acetylcysteine delivered were considerably larger than those
    recommended for antidotal use for paracetamol poisoning in a child of
    this age.  Intrinsic hepatobiliary disease in the infant could not be
    ruled out (Bailey & Andres, 1987).

         Bronchospasm, which may be part of the anaphylactoid reaction to
    intravenous  N-acetylcysteine (Ho & Beilin, 1983; Mant et al., 1984),
    may occur after  N-acetyl-cysteine inhalation in the management of
    pulmonary disease (Dano, 1971).  Intracranial hypertension has also
    been reported following inhalational therapy (Venturelli & Tein,
    1984).

         Cardiovascular collapse and death was reported in a 4-year-old
    child who received 2.17 g  N-acetylcysteine intravenously as a
    loading dose and intravenous infusion of 0.36 g/h (Anon, 1984).  This
    dose is in excess of that recommended.

         In another case reported by Mant et al. (1984), hepatorenal
    failure secondary to paracetamol overdose was associated with
    disseminated intravascular coagulation.  This patient also received
    between 2 and 6┤ times the recommended loading dose of intravenous
     N-acetylcysteine, but a cause-and-effect relationship between
     N-acetyl-cysteine and disseminated intravascular coagulation cannot
    be established.  Similarly, haemolysis developed in a patient with
    glucose-6-phosphate dehydrogenase deficiency who had received a 6-fold
    overdose of  N-acetylcysteine, but this cannot be explained on the
    basis of oxidative haemolysis, and other mechanisms must, presumably,
    have been responsible.

         In the study by Smilkstein et al. (1991), 980 mg
     N-acetylcysteine/kg was given intravenously over 48 h to 223
    patients initially entering the study.  Adverse reactions to
     N-acetylcysteine occurred in 32 out of 223 cases (14%), consisting
    in 29 out of 32 patients (91% of reactions) of transient, patchy skin
    erythema or mild urticaria during the loading dose (140 mg/kg) that
    did not require discontinuation of therapy. One patient suffered a
    potentially life-threatening reaction as 12 g (instead of 3.6 g) was
    given as the fifth dose.  The patient developed oedema, diffuse rash,
    wheezing, throat tightness, and itching. All symptoms and signs
    responded to antihistamine administration.

    3.11.2  Use in pregnancy

         Experience in 59 pregnant patients suggested that use of
     N-acetylcysteine in pregnancy did not result in toxic effects on the
    fetus (Bronstein & Rumack, 1984).  In practice, the risk to the mother
    and baby of paracetamol-induced liver damage probably far outweighs
    any potential risk of  N-acetylcysteine, and pregnancy should not be
    considered a contraindication to the use of this agent.

    3.12  Summary of evaluation

    3.12.1  Indications

          N-acetylcysteine is indicated in the management of moderate to
    severe paracetamol poisoning.  If at all possible, plasma paracetamol
    concentrations should be used to predict the likelihood of paracetamol
    toxicity (see Fig. 1), and therefore the need for treatment.  The
    treatment line (Fig. 1) differs, in that American studies have
    generally used a line 25% below the one proposed originally (joining
    200 mg/l at 4 h and 50 mg/l at 12 h).  The treatment line used should
    therefore be identified before comparing results between studies.

         In patients presenting up to 8 h after overdose, it is reasonable
    to measure paracetamol levels to assess the need for treatment
    according to the treatment line most commonly employed (Fig. 1) before
    starting treatment.  After this time, if the history suggests an
    intake greater than 7 g (or 100 mg/kg) paracetamol in adults, therapy
    should be started immediately, even before the plasma paracetamol
    concentration has been measured.  If the concentration suggests that
    paracetamol toxicity is unlikely (i.e. it falls below a line joining
    200 mg/l at 4 h and 30 mg/l at 15 h, Fig. 1),  N-acetylcysteine can
    be discontinued.  In the case of a potentially toxic paracetamol
    concentration,  N-acetylcysteine should be continued and the full
    treatment regimen completed even if paracetamol concentrations
    subsequently fall below the treatment line.

         Treatment with  N-acetylcysteine may be instituted up to 24 h
    after a paracetamol overdose.  The efficacy of the oral and
    intravenous regimens falls when treatment is started more than 8 h
    after the ingestion of paracetamol.  The antidotal efficacy, when
    treatment is started later than 24 h, has not been established.

         Intravenous  N-acetylcysteine has been shown to increase
    significantly the survival rate in patients with paracetamol-induced
    fulminant liver failure through unknown mechanisms, and should
    therefore be given in such cases.  There are currently no data on the
    use of  N-acetylcysteine in patients admitted 24-50 h post-ingestion
    and who are at particular risk of developing liver failure.  However,
    it appears to be safe to use intravenous  N-acetylcysteine in these
    patients, and since they may benefit from this treatment, the use of
     N-acetylcysteine in this manner should be considered.

    3.12.2  Advised route and dosage

    3.12.2.1  Intravenous  N-acetylcysteine

         Intravenous  N-acetylcysteine should be given as an initial
    loading dose of 150 mg/kg body weight in 200 ml of 5% dextrose over 15
    min, followed by 50 mg/kg in 500 ml of 5% dextrose over 4 h and then

    100 mg/kg in 1 litre of 5% dextrose over the next 16 h.  The total
    dose by this route will be 300 mg/kg over 20 h.

         There are indications that a dosage regimen of 980 mg/kg over 48
    h may be more effective for patients admitted 10-24 h post-ingestion,
    especially those at "high risk". The fact that  N-acetylcysteine also
    increases survival in patients with established acute paracetamol-
    induced liver failure ( N-acetylcysteine being given in this instance
    at about 50 h post-ingestion) may support the use of a 48-h dosing
    regimen to "high risk" patients admitted late after ingestion of a
    paracetamol overdose.

    3.12.2.2  Oral  N-acetylcysteine

         A 5% solution of  N-acetylcysteine should be given as an oral
    loading dose of 140 mg/kg.  The available commercial preparations of
     N-acetylcysteine are 10 and 20% solutions, and these need to be
    diluted.  Seventeen further doses of 70 mg  N-acetylcysteine/kg
    should be given as a 5% solution in diluent every 4 h.  The total dose
    by this route will be 1330 mg/kg over 72 h.

         The intravenous regimen is preferable to the oral one because of
    the predictable occurrence of vomiting in seriously poisoned patients
    when using the oral regimen.  The relative efficacy of the two
    regimens in the prevention of paracetamol-induced hepatotoxicity
    cannot be judged on present evidence.

    3.12.3  Other consequential or supportive therapy

         Symptomatic treatment of ensuing complications should be
    according to conventional principles of intensive care.  Special
    emphasis should be given to the treatment of liver failure and acute
    renal failure, as discussed in section 1.7.  At present, liver
    transplantation has no clearly defined role in the treatment of acute
    liver failure due to paracetamol poisoning, although it may be useful
    in selected cases being treated at specialist centres (see section
    1.7.1.2).

    3.12.4  Controversial issues and areas where there is insufficient
            information to make recommendations

         Studies to determine the relative efficacy of the oral and
    intravenous regimens have not been performed.  Data exist suggesting
    that the 48-h intravenous infusion regimen and the 72-h oral regimen
    may be more effective than the 20-h intravenous regimen when the
    latter is used as the historical control.  Direct comparison of the
    results from the 48 and 72-h studies with those from the 20-h study is
    not possible.

         From a practical point of view, the shortest dosing regimen is
    desirable, provided that it is effective.  One problem when comparing

    the different  N-acetylcysteine studies is the efficacy parameter or
    end-point "severe liver damage", defined as an ALAT/ASAT activity of
    > 1000 U/l. Firstly, this is a measurement of hepatic necrosis and
    not of liver function/protein synthesis.  Secondly, clinical
    experience shows that more than 99% of patients suffering from
    paracetamol-induced hepato-toxicity where ALAT/ASAT activity is >
    1000 U/l will recover completely without long-term sequelae provided
    that appropriate therapy is given.  This parameter was arbitrarily
    chosen in early studies of paracetamol poisoning.  However, in view of
    the marginal differences in outcome when different dosage regimens are
    employed, the need for a new efficacy parameter should be considered
    by clinical toxicologists and hepatologists.

         When comparing the results obtained from use of the 20-h
    intravenous dosing regimens with those from the 48-h intravenous and
    72-h oral dosing regimens, no differences are observed when end-points
    such as mortality or permanent sequelae are considered.

         Possible benefit from the use of  N-acetylcysteine in patients
    admitted late and at risk of developing fulminant liver failure has
    not been proven and remains controversial (see section 3.12.1).

    3.12.5  Proposals for further studies

         Future studies on intravenous  N-acetylcysteine should be
    performed with the 20-and 48-h regimens in such a way as to make the
    data obtained comparable.  Such studies should concentrate on patients
    in the high-risk group admitted late (10-24 h post-ingestion).  The
    use of firmer end-points than an ALAT/ASAT activity > 1000 U/l should
    be considered.

         It would be useful to investigate whether side-effects of
     N-acetylcysteine might be avoided and antidotal efficacy maintained
    if the initial intravenous bolus dose is given more slowly.

         The possible benefit of  N-acetylcysteine treatment in patients
    admitted late (24-50 h after poisoning), and at particular risk of
    developing liver failure, needs further study.  Since the treatment
    nomogram does not extend beyond 24 h, it would be useful to study
    correlations between ingested dose and the risk of liver failure and
    whether there is a dose threshold for this risk.

    3.12.6  Adverse effects

          N-acetylcysteine appears to have a good therapeutic index and
    side-effects are few, consisting mainly of mild flushing.  These
    side-effects are more common following intravenous administration and
    are considered to be dose-dependent.  The incidence and severity of
    side-effects may therefore be reduced by giving the initial bolus dose
    more slowly.  There have been some reports of serious side-effects
    associated with the intravenous administration of  N-acetylcysteine.

    These appear to be anaphylactoid and have generally been reported when
    more than the recommended dose has been given in error; at least one
    death has resulted. Antihistamines appear to be effective.

         Oral  N-acetylcysteine may cause gastrointestinal irritation;
    dilution to a solution of 9% or less is recommended to reduce the
    incidence and severity of this side-effect.

    3.12.7  Restrictions of use

         If a patient has previously experienced a severe anaphylactoid
    reaction to  N-acetylcysteine, oral methionine should be given in
    preference.  Otherwise there are no contraindications to the use of
     N-acetylcysteine following the ingestion of paracetamol.  If the
    time of ingestion is difficult to assess, current evidence indicates
    that no harm will come if  N-acetylcysteine is given later than 24 h
    after ingestion of paracetamol.


    3.13  Model information sheet

    3.13.1  Uses

          N-acetylcysteine is indicated in the treatment of paracetamol
    poisoning if:

    a)   plasma paracetamol levels taken between 4 and 8 h post-ingestion
         fall above "the treatment line" indicated in Fig. 1;

    b)   more than 100 mg paracetamol/kg has been ingested in a single
         dose and plasma paracetamol concentrations are not available;

    c)   acute paracetamol-induced liver failure has developed or is
         likely to develop (see 3.13.1.1).

          N-acetylcysteine treatment should be started as soon as
    possible if the criteria above are fulfilled, as its antidotal
    efficacy rapidly declines once more than 8 h have elapsed from the
    time of ingestion of paracetamol.  No antidotal effect has been
    documented when  N-acetylcysteine has been given later than 24 h
    post-ingestion.  However, harm is unlikely if  N-acetylcysteine is
    given later than this, e.g., in the event of an incomplete or
    uncertain case history.

         In the event of an unreliable case history, where the time of
    ingestion and/or amount of paracetamol ingested is not known, it is
    reasonable to adopt a low threshold for the use of  N-acetylcysteine,
    i.e. if in doubt,  N-acetylcysteine should be administered.

         If a plasma sample taken later than 8 h post-ingestion shows that
    the plasma paracetamol concentration is definitely below the treatment
    line,  N-acetylcysteine treatment should be stopped.

    3.13.1.1  Use in liver failure

          N-acetylcysteine significantly increases survival in patients
    with acute paracetamol-induced fulminant liver failure when given
    according to the 20-h intravenous dosing regimen indicated in section
    3.13.2.1.  There appear to be no contraindications to this therapy,
    which should be instituted as soon as possible in patients admitted in
    this late stage of poisoning or who for some other reason have not
    received  N-acetylcysteine previously.

         In patients admitted more than 24 h post-ingestion who are not
    suffering from liver failure but who are at particular risk of
    developing it (> 150-200 mg paracetamol/kg ingested), it would seem
    reasonable to give  N-acetylcysteine intravenously according to the
    dosage regimen given in section 3.13.2.1.

    3.13.2  Dosage and route

    3.13.2.1  Intravenous  N-acetylcysteine

         Intravenous  N-acetylcysteine should be given as an initial
    loading dose of 150 mg/kg body weight in 200 ml of 5% dextrose over 15
    min, followed by 50 mg/kg in 500 ml of 5% dextrose over 4 h and then
    100 mg/kg in 1 litre of 5% dextrose over the next 16 h.  The total
    dose given by this route will be 300 mg/kg over 20 h.

         In patients admitted 10-24 h after ingestion of paracetamol, and
    especially if large amounts are ingested, one may consider using the
    48-h intravenous dosage regimen instead: an initial loading dose of
    140 mg  N-acetylcysteine/kg, followed by 12 doses of 70 mg/kg every
    4 h, i.e. a total of 980 mg/kg over 48 h (in 5% dextrose).

    3.13.2.2  Oral  N-acetylcysteine

         A 5% solution of  N-acetylcysteine should be given as an oral
    loading dose of 140 mg/kg.  The available commercial preparations of
     N-acetylcysteine are 10 and 20% solutions and need to be diluted.
    This can be done using water or a commercial carbonated or still
    flavoured drink.  Seventeen further doses of 70 mg
     N-acetylcysteine/kg should be given as a 5% solution in diluent
    every 4 h.  The total dose given by this route will be 1330 mg/kg over
    72 h.

         The intravenous treatment regimen is preferred to the oral one
    (see sections 3.12.2 and 3.12.4) unless the intravenous
     N-acetylcysteine formulation is unavailable or there is lack of
    expertise/equipment for such treatment.

    3.13.3  Precautions/contraindications

         Previous severe anaphylactoid reactions to  N-acetylcysteine
    should be considered an absolute contraindication if methionine is
    available as an alternative antidote.  But in the case of a
    potentially severe paracetamol intoxication where methionine is
    unavailable, a history of such a reaction is a relative
    contraindication.

    3.13.4  Pharmaceutical incompatibilities and drug interactions

          N-acetylcysteine is incompatible with solutions containing
    certain antibiotics, including ampicillin sodium, amphotericin,
    erythromycin lactobionate and some tetracyclines.  Activated charcoal
    should not be given if oral  N-acetylcysteine is administered.

    3.13.5  Adverse effects

         In about 15% of patients given  N-acetylcysteine intravenously,
    mild urticaria is seen during infusion of the bolus dose.  This does
    not usually require discontinuation of infusion, although a slower
    infusion rate of the bolus dose (1-2 h) may be advisable.  If more
    pronounced skin erythema or urticaria develops, an antihistamine
    should be given.

         Patients should be monitored for possible anaphylactoid reactions
    including bronchospasm, hypotension and urticaria; such reactions are
    more common with intravenous  N-acetylcysteine.  Management of these
    reactions involves the following measures:

    a)   stop the  N-acetylcysteine infusion, at least temporarily;

    b)   administer an antihistamine intravenously;

    c)   in the rare event of severe bronchospasm consider using nebulized
         salbutamol or, in severe cases, parenteral adrenaline. If the
         reaction is mild,  N-acetylcysteine may be restarted cautiously. 
         As an alternative, especially in the case of a severe reaction,
         the use of oral methionine is advisable.

         Other adverse reactions to oral  N-acetylcysteine include
    vomiting and diarrhoea.

    3.13.6  Use in pregnancy and lactation

         There is no evidence of toxic effects on the fetus arising from
    the use of standard oral doses of  N-acetylcysteine in pregnant women
    poisoned by paracetamol.  The risk to the mother and baby of
    paracetamol-induced liver damage is likely to far outweigh any
    potential risk of  N-acetylcysteine administration.

    3.13.7  Storage

         The shelf-life of the intravenous preparation (Parvolex) is
    stated by the manufacturer, Duncan Flockart, to be 3 years at room
    temperature.

         The shelf-life of oral  N-acetylcysteine is usually 2 years.

         A change in colour of solutions of  N-acetylcysteine to light
    purple does not indicate significant impairment of safety or efficacy.

    3.14  References

    Anon (1984) Death after  N-acetylcysteine. Lancet, 1: 1421.

    Bailey DJ & Andres JM (1987) Liver injury after oral and rectal
    administration of  N-acetylcysteine for meconium ileus equivalent in
    a patient with cystic fibrosis. Pediatrics, 79: 281-282.

    Bateman DN, Woodhouse KW, & Rawlins MD (1984a) Adverse reactions to
     N-acetylcysteine. Lancet 2: 228.

    Bateman DN, Woodhouse KW, & Rawlins MD (1984b) Adverse reactions to
     N-acetylcysteine. Hum Toxicol, 3: 393-398.

    Bonanomi L & Gazzaniga A (1980) Toxicological, pharmacokinetic and
    metabolic studies on acetylcysteine. Eur J Respir Dis, 61(Suppl 111):
    45-51.

    Borgstrom L, Kagedal B, & Paulsen O (1986) Pharmacokinetics of
     N-acetylcysteine in man.  Eur J Clin Pharmacol, 31: 217-222.

    British Pharmacopoeia (1993) London, Her Majesty's Stationery Office.

    Bronstein AC & Rumack BH (1984) Acute acetaminophen overdose during
    pregnancy:  review of fifty-nine cases. Vet Hum Toxicol, 26: 401.

    Burgunder JM, Varriale A, & Lauterburg BH (1989) Effect of
     N-acetylcysteine on plasma cysteine and glutathione following
    paracetamol administration. Eur J Clin Pharmacol, 36: 127-131.

    Casola G & van Sonnenberg E (1984) Skin damage from acetylcysteine
    leak during percutaneous abscess drainage. Radiology, 152: 233.

    Charley G, Dean BS, & Krenzelok EP (1987) Oral  N-acetylcysteine-
    induced urticaria: a case report. Vet Hum Toxicol, 29: 477.

    Chinouth RW, Czajka PA, & Peterson RG (1980)  N-acetylcysteine
    absorption by activated charcoal. Vet Hum Toxicol, 22: 392-394.

    Clarke ECG (1986) Clarke`s identification and isolation of drugs, 2nd
    ed. London, Pharmaceutical Press.

    Corcoran GB, Todd EL, Racz WJ, Hughes H, Smith CW, & Mitchell JR
    (1985) Effects of  N-acetylcysteine on the disposition and metabolism
    of acetaminophen in mice. J Pharmacol Exp Ther, 232: 857-863.

    Dano G (1971) Bronchospasm caused by acetylcysteine in children with
    bronchial asthma.  Acta Allergol, 26: 181-190.

    Drummer OH, Christophidis N, Horowitz JD, & Louis WJ (1986)
    Measurement of penicillamine and  N-acetylcysteine in human blood by
    high-performance liquid chromatography and electrochemical detection.
    J Chromatogr, 374: 251-257.

    Ekins BR, Ford DC, Thompson MI, Bridges RR, Rollins DE, & Jenkins RD
    (1987) The effect of activated charcoal on  N-acetylcysteine
    absorption in normal subjects. Am J Emerg Med, 5: 483-487.

    Ellman GL (1959) Tissue sulfhydryl groups. Arch Biochem Biophys, 82:
    70-77.

    Ferrari V (1980) Safety and drug interactions of oral acetylcysteine
    related to utilization data.  Eur J Respir Dis, 61(Suppl 111):
    151-157.

    Flanagan RJ (1987) The role of acetylcysteine in clinical toxicology.
    Med Toxicol, 2: 93-104.

    Fontana A & Toniolo C (1974) In: Patai S ed. The chemistry of the
    thiol group -Part I. New York, Chichester, Brisbane, Toronto, John
    Wiley & Sons, p 288.


    Frank H, Thiel D, & Langer K (1984) Determination of  N-acetyl-
    L-cysteine in biological fluids. J Chromatogr, 309: 261-267.

    Gavish D & Breslow JL (1991) Lipoprotein(a) reduction by
     N-acetylcysteine. Lancet, 337: 203-204.

    Gervais R, Lussier-Labelle F, & Beaudet P (1984) Anaphylactoid
    reaction to acetylcysteine. Clin Pharm, 3: 586-587.

    Gosselin RE, Smith RP, & Hodge HC ed. (1984) Clinical toxicology of
    commercial products, 5th ed.  Baltimore, Maryland, Williams & Wilkins.

    Harrison PM, O'Grady JG, Keays RT, Alexander GJM, & Williams R (1990)
    Serial prothrombin time as a prognostic indicator in paracetamol
    induced fulminant hepatic failure. Br Med J, 301: 964-966.

    Ho SW & Beilin LJ (1983) Asthma associated with  N-acetylcysteine
    infusion and paracetamol poisoning: report of two cases. Br Med J,
    287: 876-877.

    Hogan JC, Lewis MJ, & Henderson AH (1990) Chronic administration of
     N-acetylcysteine fails to prevent nitrate tolerance in patients with
    stable angina pectoris. Br J Clin Pharmacol, 30: 573-577.

    Ioannides C, Hall DE, Mulder DE, Steele CM, Spickett J, Delaforge M,
    & Parke DV (1983) A comparison of the protective effects of
     N-acetylcysteine and S-carboxymethylcysteine against paracetamol-
    induced hepatotoxicity. Toxicology, 28: 313-321.

    Johansson M & Westerlund D (1986) Determination of  N-acetylcysteine,
    intact and oxidized in plasma by column liquid chromatography and
    post-column derivatization. J Chromatogr, 385: 343-356.

    Johnston RE, Hawkins HC, & Weikel JH Jr (1983) The toxicity of
     N-acetylcysteine in laboratory animals. Sem Oncol, 10(Suppl 1):
    17-24.

    Kagedal B, Kallberg M, & Martensson J (1984) Determination of non-
    protein-bound  N-acetylcysteine in plasma by high-performance liquid
    chromatography. J Chromatogr, 311: 170-175.

    Keaney NP & Cocking G (1981)  N-acetylcysteine protection against
    halothane hepatotoxicity:  experiments in rats. Lancet, 2: 95.

    Keays R, Harrison PM, Wendon JA, Forbes A, Gove C, Alexander GJM, &
    Williams R (1991) Intravenous acetylcysteine in paracetamol induced
    fulminant hepatic failure: a prospective controlled trial. Br Med J,
    303: 1026-1029.

    Klein-Schwarz W & Oerda GM (1981) Absorption of oral antidotes for
    acetaminophen (methionine and  N-acetylcysteine) by activated
    charcoal. Clin Toxicol, 18: 283-290.

    Krenzelok EP, North DS, & Peterson RG (1980) The effect of activated
    charcoal administration on  N-acetylcysteine serum levels in human
    subjects. Vet Hum Toxicol, 22: 56-58.

    Lauterburg BH, Corcoran GB, & Mitchell JR (1983) Mechanism of action
    of  N-acetylcysteine in the protection against the hepatotoxicity of
    acetaminophen in rats  in vivo. J Clin Invest, 71: 980-991.

    Lewis PA, Woodward AJ, & Maddock J (1984) High-performance liquid
    chromatographic assay for  N-acetylcysteine in plasma and urine. J
    Pharm Sci, 73: 996-998.

    Maddock J (1980) Biological properties of acetylcysteine: assay
    development and pharmacokinetic studies. Eur J Respir Dis,
    61(Suppl 111): 52-58.

    Mant TGK, Tempowski JH, Volans GN, & Talbot JC (1984) Adverse
    reactions of acetylcysteine and effects of overdose. Br Med J, 289:
    217-219.

    Marini U, Visconti G, & Spotti D (1980) Controlled endoscopic study on
    gastroduodenal safety of acetylcysteine after oral administration. Eur
    J Respir Dis, 61(Suppl 111): 147-150.

    Martindale (1988a) In: Reynolds JEF ed. The extra pharmacopoeia, 29th
    ed. London, Pharmaceutical Press.

    Martindale (1988b) In: Reynolds JEF ed. The extra pharmacopoeia, 29th
    ed. Denver, Colorado, Micromedex, Inc. (CDROM version).

    NIOSH (1983) Registry of toxic effects of chemical substances
    1981-1982 (RTECS). Cincinnati, Ohio, National Institute for
    Occupational Safety and Health.

    Olsson B, Johansson M, Gabrielsson J, & Bolme P (1988)
    Pharmacokinetics and bioavailability of reduced and oxidized
     N-acetylcysteine. Eur J Clin Pharmacol, 34: 77-82.

    Parker JO, Farell B, Lahey KA, & Rose BF (1987) Nitrate tolerance: the
    lack of effect of  N-acetylcysteine. Circulation, 76: 572-576.

    Piperno E, Mosher AH, Berssenbruegge DA, Winkler JD, & Smith RB (1978)
    Pathophysiology of acetaminophen overdosage toxicity; implications for
    management. Pediatrics, 62(Part 2 Suppl): 880-889.

    Prescott LF & Mattew H (1974) Cysteamine for paracetamol overdosage.
    Lancet, 1: 998.

    Prescott LF, Park J, Vallantyre A, Adrianssen P, & Proudfoot AT (1977)
    Treatment of paracetamol (acetaminophen) poisoning with
     N-acetylcysteine. Lancet, 2: 432-434.

    Prescott LF, Illingworth RN, Critchley JA, Stewart MJ, Adam RD, &
    Proudfoot AT (1979) Intravenous  N-acetylcysteine:  the treatment of
    choice for paracetamol poisoning. Br Med J, 2: 1097-1100.

    Prescott LF, Donovan JW, Jarvie DR, & Proudfoot AT (1989) The
    disposition and kinetics of intravenous  N-acetylcysteine in patients
    with paracetamol overdosage. Eur J Clin Pharmacol, 37: 501-506.

    Renzi FD, Donovan JW, & Martin TG (1985) Concomitant use of activated
    charcoal and  N-acetylcysteine. Ann Emerg Med, 14: 568-572.

    Rumack BH & Peterson RG (1978) Acetaminophen overdose: Incidence,
    diagnosis and management in 416 patients. Pediatrics, 62: 898-903.

    Rumack BH, Peterson RC, Koch GG, & Amara IA (1981) Acetaminophen
    overdose: 662 cases with evaluation of oral acetylcysteine treatment.
    Arch Intern Med, 141: 380-385.

    Rybolt TR, Burell DE, & Schultz JM (1986)  In vitro coabsorption of
    acetaminophen and  N-acetylcysteine onto activated carbon powder. J
    Pharm Sci, 75: 904-906.

    Shaw A (1969) Safety of  N-acetylcysteine in treatment of meconium
    obstruction of the newborn.  J Pediatr Surg, 4: 119-125.

    Sheffner AL (1983) The reduction  in vitro in viscosity of
    mucoprotein solutions by a new mucolytic agent  N-acetyl-L-cysteine.
    Ann NY Acad Sci, 106: 298-310.

    Smilkstein MJ, Knapp GL, Kulig KW, & Rumack BH (1988) Efficacy of oral
     N-acetylcysteine in the treatment of acetaminophen overdose:
    Analysis of the National Multicenter study (1976 to 1985). N Engl J
    Med, 319: 1557-1562. Smilkstein MJ, Bronstein AC, Linden C, Augenstein
    WL, Kulig KW, & Rumack BH (1991) Acetaminophen overdose: A 48-hour
    intravenous  N-acetylcysteine treatment protocol. Ann Emerg Med, 20:
    1058-1063.

    Smith HA & Gorin G (1961) S-methyl-S-thiazdine-4-carboxylic acid:
    Formation from  N-acetylcysteine and hydrolysis. J Org Chem, 26:
    820-823.

    Stalenhoef AFH, Kroon AA, & Demacker PNM (1991)  N-acetylcysteine and
    lipoprotein. Lancet, 337: 491.

    Tee LBG, Boobis AR, Huggett AC, & Davies DS (1986) Reversal of
    acetaminophen toxicity in isolated hamster hepatocytes by
    dithiothreitol. Toxicol Appl Pharmacol, 83: 294-314.

    Tenenbein M (1984) Hypersensitivity-like reactions to
     N-acetylcysteine. Vet Hum Toxicol, 26 (Suppl 2): 3-5.

    United States Pharmacopeia (1990) 22nd ed. Rockville, Maryland, United
    States Pharmacopeial Convention, Inc.

    USPCI (1985) Acetylcysteine (Inhalation). Rockville, Maryland, United
    States Pharmacopeial Convention, Inc., pp 10-11.

    Vale JA & Wheeler DC (1982) Anaphylactoid reactions to acetylcysteine
    (letter). Lancet, 2: 988.

    Venturelli J & Tein I (1984) Increased intracranial pressure
    associated with  N-acetylcysteine inhalation therapy (letter). Crit
    Care Med, 12: 926-927.

    Walton NG, Mann TAN, & Shaw KM (1979) Anaphylactoid reaction to
     N-acetyl-cysteine (letter). Lancet, 2: 1298.

    Wilpart M, Mainguet P, Geeroms D, & Roberfroid M (1985) Desmutagenic
    effects of  N-acetylcysteine on direct and indirect mutagens. Mutat
    Res, 142: 169-177.
    See Also:
       Toxicological Abbreviations