WHO/FOOD ADD./69.35



    Issued jointly by FAO and WHO

    The content of this document is the result of the deliberations of the
    Joint Meeting of the FAO Working Party of Experts and the WHO Expert
    Committee on Pesticide Residues, which met in Geneva, 9-16 December,



    Geneva, 1969


    This pesticide was evaluated toxicologically by the 1965 Joint Meeting
    of the FAO Committee on Pesticides in Agriculture and the WHO Expert
    Committee on Pesticide Residues (FAO/WHO, 1965). Additional
    toxicological data was presented and discussed at the 1966 Joint
    Meeting (FAO/WHO, 1967). Since the previous evaluations, additional
    data have become available and are summarized and discussed in the
    following monograph addendum.


    Additional information on identity and properties

    Phosphamidon is a mixture of approximately 30 per cent alpha-isomer
    (trans-phosphamidon) and 70 per cent beta-isomer (cis-phosphamidon),
    the latter form being the more active biologically. Technical
    phosphamidon contains: cis- and trans-phosphamidon, 89 per cent;
    gamma-chlorophosphamidon, 2 per cent; dechlorophosphamidon, 1 per
    cent; and inert by-products, 8 per cent (Chevron Chemical Co., 1968;
    Ciba Ltd, 1968).


    Biochemical aspects

    In vitro gamma-chlorophosphamidon, forming two to four per cent of
    technical phosphamidon, is about 50 times as potent an inhibitor of
    human plasma cholinesterase as pure phosphamidon (Voss, 1967).
    In vivo, however, there was no difference in inhibitory effect on
    plasma between the technical and pure phosphamidon when fed to rabbits
    in their diet (Rose, 1968a). These findings may be explained by the
    effect of rabbit liver homogonates which metabolized
    gamma-chlorophosphamidon extremely rapidly: five minutes incubation
    was sufficient to degrade 95 per cent, while only 60 per cent of
    phosphamidon was decomposed in one hour (Rose, 1968b).
    gamma-chlorophosphamidon is degraded in plants at least as rapidly as
    phosphamidon (Ciba Ltd, 1968), Dechlorophosphamidon has a lower
    mammalian toxicity and lower anti-cholinesterase activity than
    phosphamidon (Ciba Ltd, 1968).

    The metabolism of phosphamidon by rats and a goat has been recently
    investigated. In addition to the metabolites previously reported, two
    additional ones were identified, namely "phosphamidon amide"
    (N,N-bisdesethylphosphamidon) and "dechlorophosphamidon amide" (the
    dimethyl phosphate ester with 3-hydroxycrotonamide). Although these
    two compounds and N-desethylphosphamidon are more toxic than
    phosphamidon, they do not persist in urine and milk. After the oral
    administration of 3 mg/kg of a mixture of 32P-labelled and
    14C-labelled phosphamidon to a goat, its milk was fractionated and
    the chloroform-soluble component (containing the toxic residues)
    reached a maximum after four hours of 1.2 ppm equivalents of 32P but
    this level had dropped to 0.1 ppm after 32 hours and had disappeared

    after 64 hours. One major metabolite, although transitory, remains to
    be identified (Clemons and Menzer, 1968).

    Additional data on the metabolites of phosphamidon are included in the
    section entitled "Residues in food and their evaluation, Fate of

    Acute toxicity

    The following provides additional data on the acute toxicity of
    phosphamidon to mice and rats.

                           LD50 (mg/kg         Reference
    Animal        Route    body-weight)

    Mouse         oral         10         May and Baker Ltd, 1959

                  s.c.          8         May and Baker Ltd, 1959

    Rat (M)       oral         19         May and Baker Ltd, 1959

        (F)       oral         12         May and Baker Ltd, 1959

        (M)       s.c.          7         May and Baker Ltd, 1959

        (F)       s.c.          4         May and Baker Ltd, 1959

    The acute oral toxicity to rats of the more common metabolites of
    phosphamidon is given below:

                                          LD50 (mg/kg
          Metabolite                     body-weight)    Reference

    cis-N-desethylphosphamidon                  8.5      Ciba Ltd, 1968

    trans-N-desethylphosphamidon              250        Ciba Ltd, 1968

    alpha-chloro-N,N-diethylacetoacetamide  3 000        May and Baker Ltd, 1959

    alpha-chloro-N-ethylacetoacetamide        735        May and Baker Ltd, 1959
    Two groups, each containing 10 mice, inhaled an aerosol containing
    five per cent or 10 per cent of phosphamidon for 90 minutes. All the
    mice inhaling the 10 per cent aerosol died but there were no deaths
    from the five per cent aerosol, although serious signs of
    intoxication were evident (May and Baker Ltd, 1959).

    In acute toxicity studies in mice, there was no potentiation of the
    toxicity of phosphamidon when it was administered in equal quantities
    with each of the following organo-phosphorus insecticides: dimethoate,
    endothion, ethion, malathion mevinphos, oxydemeton-methyl, parathion
    and phenkapton (May and Baker Ltd, 1959).

    Special studies

    Studies on the metabolite, N-desethylphosphamidon

    Rat. N-desethylphosphamidon was applied to groups of 10 male and 10
    female rats at daily doses of 1.5 and 10 mg/kg (stomach tube) over a
    90-day period. Growth was retarded at the 10 mg/kg level only.
    Differences in food consumption did not occur. Blood and urine were
    found to be normal and no pathological changes were observed
    (Industrial Bio-Test Laboratories Inc., 1964a).

    Dog. N-desethylphosphamidon was given orally in capsules to groups
    of three male and three female dogs over 98 days at daily doses of
    0.2, 1 and 5 mg/kg. No deaths occurred at the two lower dose levels.
    Pathological changes in succumbed or sacrificed animals were not
    detected. In particular no degenerative changes occurred in peripheral
    nerves, spinal cord or brain. Urine and blood were normal (Industrial
    Bio-Test Laboratories Inc., 1964b).


    Additional data from 90-day experiments on rats and dogs did not
    reveal any new results permitting a change in the acceptable daily

    Further studies are desirable on the metabolites of phosphamidon and
    their toxicity.


    Remains the same as in 1966, i.e. acceptable daily intake, 0-0.001
    mg/kg body-weight.


    Use pattern

    Pre-harvest treatments

    Phosphamidon is a systemic insecticide used to control a wide variety
    of plant-feeding arthropods. It is employed as a pre-harvest treatment
    for many agricultural and horticultural crops as well as in forestry.

    The Joint Meeting had no information on post-harvest treatments or
    other uses.

    Residues resulting from supervised trials


                                                    Pre-harvest      Residue
                       Per cent.        No. of        interval     at harvest
    Crop             concentration    treatments       (days)         (ppm)

    Apples               0.03             4              20           0.3*

    Pears                0.04             3              31           0.2*

    and lemon            0.15             4              15           0.1*

    Oranges              0.4              4              15           0.3*

    Strawberries         0.12             1              20           0.2*

    Grapes               0.06             3              21           0.2*

    Broad beans          0.04             1              10           0.1**

    Cucumbers            1.1              1              14           0.1*

    Water-melons         0.06             3               3           0.1*

    Spinach              0.25             1              16           0.1*

    Carrots              0.03             1               0           0.1*

    Wheat                0.06             1              60           0.1*

    Rice                 0.55             1               7           0.05*

    *  Chevron Chemical Co., 1968.
    ** Ciba Ltd, 1968.
    Fate of residues

    Phosphamidon and its most important toxic intermediate are rapidly
    degraded in plants, the rate being influenced by age and environmental
    factors, particularly temperature. The major pathways of metabolism in
    animals are qualitatively the same with rapid degradation to non-toxic

    The more important metabolites detected in beans were
    N-desethylphosphamidon, alpha-chloro-N,N-diethylacetoacetamide and
    alpha-chloro-N-ethylacetoacetamide (May and Baker Ltd, 1959).

    The main toxic metabolite, N-desethylphosphamidon (Anliker et al.,
    1961) is more rapidly degraded in the bean plant than the parent
    compound, while its mammalian toxicity and plasma cholinesterase
    activity are similar to phosphamidon. This observation has been
    confirmed for other fruit and vegetables, except apples where the rate
    of degradation is of the same order of magnitude (Chevron Chemical
    Co., 1968),

    The comparative fate of the two geometrical isomers of phosphamidon in
    plants and insects was studied (Bull et al., 1967). Although the
    cis- is much less toxic than the trans-isomer the difference could
    not be entirely accounted for by the more extensive oxidative
    N-dealkylation of the cis-isomer. The main metabolites of both
    isomers were O-demethyl and N-desethyl phosphamidon and dimethyl
    phosphate, of which only the N-desethyl derivative is toxic.

    From petiole injections of cotton plants with cis- and
    trans-isomers of phosphamidon the biological half-life for the toxic
    compounds was slightly less than one day.

    The metabolism of a cis-trans mixture (30:70) by excised cotton
    leaves and alfalfa sprigs followed a similar pattern.

    No information is available on losses of phosphamidon during storage
    and processing, although it is known to be readily degraded on

                                            Per cent. of applied dose recovered after
                                                     various time intervals

        Product                           cis-isomer                         trans-isomer
                                             Day                                 Day

                                1        2        4         8         1         2         4        8

    Dimethyl phosphate         17.2     23.0     27.8      24.0       7.4      10.8      14.3     16.3

    O-demethylphosphamidon     26.4     11.8      4.9       2.9       9.2       4.5       4.2      2.3

    N-desethylphosphamidon     18.4      8.6      5.8       2.5       0         1.3       3.0      3.2

    Phosphamidon               15.6      7.2      0.7       3.2      38.6      13.9       3.8      4.0

                                            Per cent. of applied dose recovered after
                                                     various time intervals

        Product                            Cotton                                Alfalfa
                                             Day                                    Day

                                1        2        4         8         1         2         4        8

    Dimethyl phosphate          5.6     13.3     20.9      20.6      12.6      27.3      29.8     32.2

    O-demethylphosphamidon      9.7      6.9      6.3       6.8       4.9       3.0       1.6      1.1

    N-desethylphosphamidon     16.0      9.1      4.5       1.7      10.6       9.5       4.6      2.1

    Phosphamidon-cis           10.4      1.5      0.4       0.4      13.4       4.4       2.6      0.9

    Phosphamidon-trans         28.3      6.7      1.5       1.4      39.3      14.5       6.5      2.1

    Methods of residue analysis

    Residues have been determined by measuring the extent of inhibition of
    human plasma cholinesterase (Chevron Chemical Co., 1963). The original
    method did not distinguish between active impurities and metabolites
    but subsequent refinements allow the separation and estimation of
    gamma-chlorophosphamidon and the active metabolite,
    desethylphosphamidon. The original method detects 0.1 ppm with a
    certain loss of precision at 0.05 ppm.

    Another enzymatic method is based on the hydrolysis of
    acetylthiocholine followed by coupling with dithiobisnitrobenzoic acid
    and measurement of the resulting colour. By introduction of thin-layer
    chromatography the individual components can be measured separately
    with a sensitivity between 0.05 and 0.1 ppm (Voss and Geissbühler,
    1967). A double-paper chromatography method is highly specific but is
    too time consuming for serial analyses (Pack et al., 1964).
    Phosphamidon may also be analysed by a gas chromatographic method
    developed for related enolphosphates (Bowman and Beroza, 1967).

    National tolerances

         Country              Crop             Tolerance (ppm)
         Germany              Apples           0.5
                              Vegetables       0.1

         Italy                General          0.5

         Switzerland          General          0.5

         United States of     Apples           1.0
         America              Citrus           0.75
                              vegetables       0.5
                              water-melons     0.25
                              Walnuts          0.1



    There is a wide range of use in vegetable, fruit and cereal crops. No
    quantitative information was available to the Joint Meeting on the
    amounts used annually for stated purposes.

    There is good evidence that phosphamidon, which consists of a mixture
    of two geometrical isomers, undergoes relatively rapid metabolism in
    plant tissues. The most significant anti-cholinesterase metabolite
    appears to be the result of oxidative N-de-ethylation. Other
    anti-cholinesterase metabolites have been identified but are rapidly
    degraded. On the basis of acute toxicity and cholinesterase inhibition

    data, this metabolite has a comparable toxicity to that of the parent
    compound. Residue assays should, therefore, take both phosphamidon,
    the N-desethyl derivative and the minor anti-cholinesterase
    metabolites into account. This has been done by the use of the
    cholinesterase inhibition technique for the residue measurements made
    in the supervised trials.

    Many data on the residues resulting from supervised trials were made
    available by the manufacturers; mainly from Switzerland and the United
    States of America. As a result of the residue and metabolism studies
    it is possible to recommend the following temporary tolerances.


    The following temporary tolerances, to be in effect until 1972, are to
    apply to raw agricultural products moving in commerce unless otherwise
    indicated. In the case of fruit and vegetables the tolerances should
    be applied as soon as practicable after harvest and in any event prior
    to actual retail to the public. In the case of commodities entering
    international trade, the tolerances should be applied by the importing
    country at the point of entry or as soon as practicable thereafter.

    Temporary tolerances

         Raw cereals                           0.1 ppm
         Apples, pears                         0.5 ppm
         Citrus fruit                          0.4 ppm
         Other fruit                           0.2 ppm
         Cucumbers, lettuce, tomatoes,
         water-melons                          0.1 ppm
         Cole crops, other vegetables
         except root vegetables                0.2 ppm
         Root vegetables                       none required

    Residues to be determined by cholinesterase inhibition technique and
    results to be expressed as phosphamidon.

    Further work or information

    Required before 30 June 1972

    1.   Data on the required rates and frequencies of application,
         pre-harvest intervals, and the resultant residues from different

    2.   Confirmatory studies of the nature and persistence of the
         residues in fruits and vegetables, fresh and processed.

    3.   Development of an analytical method that is specific for
         phosphamidon and its N-de-ethyl derivative.


    1.   Biochemical and metabolic fate of phosphamidon in man following
         different types of exposure.

    2.   Reproduction studies in at least one species other than the rat.


    Anliker, R., Beriger, E., Geiger, M. and Schmid. K. (1961) Über die
    synthese von phosphamidon und seinen abbau in pflanzen. Helv. Chim.
    Acta., 44: 1622-1645

    Bowman, M. C. and Beroza, M. (1967) Gas chromatographic analysis of
    3-hydroxy-N-methyl-cis-crotonamide dimethylphosphate (Azodrin) and
    3-hydroxy-N,N-dimethyl-cis-crotonamide methylphosphate. J. Agric. Food
    Chem., 15: 465-468

    Bull, D. L., Lindquist, D. A. and Grabbe, R. R. (1967) Comparative
    fate of the geometric isomers phosphamidon in plants and animals. J.
    Econ, Entomol., 60: 332-341

    Chevron Chemical Co. (1963) Analysis of phosphamidon residues. Method
    RM-4. Internal report

    Chevron Chemical Co. (1968) Unpublished data from Company submission
    to Codex Committee on Pesticide Residues

    Ciba Ltd. (1968) Unpublished data from Company submission to Codex
    Committee on Pesticide Residues

    Clemons, G. P. and Menzer, R. E. (1968) Oxidative metabolism of
    phosphamidon in rats and a goat. J. Agric. Food Chem., 16: 312-318

    FAO/WHO. (1965) Evaluation of the toxicity of pesticide residues in
    food. FAO Mtg. Rept. PL/1965/10/1; WHO/Food Add./27.65

    FAO/WHO. (1967) Evaluation of some pesticide residues in food. (FAO,
    PL:CP/15; WHO/Food Add./67.32)

    Industrial Bio-Test Laboratories Inc. (1964a) 90 day subacute oral
    toxicity of desethylphosphamidon - albino rat. Unpublished report

    Industrial Bio-Test Laboratories Inc. (1964b) 14-week subacute oral
    toxicity of desethylphosphamidon - beagle dogs. Unpublished report

    May and Baker Ltd. (1959) Phosphamidon. Unpublished data submitted to
    the Ministry of Health, London, United Kingdom

    Pack, D. E., Ospenson, J. N. and Kohn, G. K. (1965) In: Analytical
    methods for pesticides, plant growth regulators and food additives,
    2: 375-392 Academic Press

    Rose, J. A. (1968a) A preliminary study of factors influencing the
    toxicity of pure phosphamidon and technical phosphamidon. Ciba Ltd,
    unpublished report

    Rose, J. A. (1968b) Degradation of phosphamidon and related vinyl
    phosphates by rabbit liver homogenates. Ciba Ltd, unpublished report

    Voss, G. (1967) Determination of gamma-chlorophosphamidon in samples
    of technical phosphamidon by an automated cholinesterase inhibition
    procedure. Ciba Ltd, unpublished report

    Voss, G. and Geissbühler, H. (1967) Automated residue determination of
    insecticidal enolphosphates. (19th Internat. Sympos. Plant
    Protection.) Med. Rijksfaculteit Landbouwwetenschappen, Ghent,
    23: 877-889

    See Also:
       Toxicological Abbreviations
       Phosphamidon (ICSC)
       Phosphamidon (PIM 454)
       Phosphamidon (FAO Meeting Report PL/1965/10/1)
       Phosphamidon (FAO/PL:CP/15)
       Phosphamidon (FAO/PL:1969/M/17/1)
       Phosphamidon (WHO Pesticide Residues Series 2)
       Phosphamidon (WHO Pesticide Residues Series 4)
       Phosphamidon (Pesticide residues in food: 1982 evaluations)
       Phosphamidon (Pesticide residues in food: 1986 evaluations Part II Toxicology)