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    FENBUTATIN OXIDE

    First draft prepared by M.A. Clevenger
    Office of Pesticide Programs
    US Environmental Protection Agency
    Washington DC USA

    EXPLANATION

          Fenbutatin oxide was previously evaluated by the JMPR in 1977
    (Annex I, reference 28) when an ADI of 0-0.03 mg/kg bw was
    allocated. The data reviewed in 1977 consisted of studies on
    pharmacokinetics, short-term toxicity in mice, rats, and dogs, long-
    term toxicity in mice and rats, reproductive toxicity in rats,
    neurotoxicity in rats, developmental toxicity in rabbits, and
    genotoxicity. The present review evaluates studies conducted since
    the 1977 review. Relevant portions of the previous monograph have
    been incorporated into this toxicological monograph.

    EVALUATION FOR ACCEPTABLE DAILY INTAKE

    BIOLOGICAL DATA

    Biochemical aspects

    Absorption, distribution, and excretion

    Rats

          Groups of 2 male and 2 female rats were fed an average of 19.5
    ppm 119Sn-labelled fenbutatin oxide (with or without a 3 day
    withdrawal period before killing following 3 or 6 days exposure) for
    1, 3 or 6 days to study the absorption, distribution and excretion
    of fenbutatin. Most of the administered 119Sn was rapidly excreted
    in the faeces following multiple dosing (80-90% of the administered
    119Sn). Essentially no 119Sn was excreted via the urine. The GI
    tract contained approximately 30, 15 and 5% of the administered
    119Sn in animals sacrificed after 1, 3 or 6 days of dosing,
    respectively. The highest tissue concentrations of 119Sn were
    found in liver (0.015-0.09 mg/kg), kidneys (0.015-0.055 mg/kg) and
    fat (< 0.01-0.065 mg/kg) . No 119Sn was detected in brain or bone
    tissue (Loeffler, 1972).

          The pharmacokinetics of fenbutatin oxide following oral
    administration was studied using groups (5/sex) of Crl:CD BR rats.
    One group received a single low dose of 10 mg 119mSn-labelled
    fenbutatin oxide/kg bw. A second group received a single high dose
    of 500 mg/kg bw of radiolabelled compound. A third group received
    multiple doses (14 daily doses) of unlabelled compound followed by a
    single dose of radiolabelled compound. Urine and faeces were
    collected over 7 days at which time rats were sacrificed and tissues
    collected.

          Over a 5-7 day period, 83%-100% of the administered dose was
    excreted in the faeces with less than 1% of the radiolabel detected
    in the urine. Excretion was somewhat slower following the single
    high dose compared to the low-dose exposure. For all dosage
    regimens, excretion was almost complete within 72 h. The excretion
    pattern was similar for all three dosage regimens and for males and
    females. Tissue residues were low 5-7 days after treatment. Highest
    radioactivity was found in liver, kidney, and heart. Concentrations
    ranged from 0.2 to 3 ppm following the low dose (single or repeated)
    and high dose, respectively. The elimination half-life was estimated
    to be 24 h following the low dose (single or repeated) and 40 h
    following the high dose (Reynolds, 1989).

    Biotransformation

    Rats

          Analysis of the faeces from rats dosed with 119Sn-labelled
    fenbutatin oxide indicated that ,-dimethylphenethylstannoic acid
    and 1,3-dihydroxy-1,1,3,3-tetrakis(2-methyl-2-phenylpropyl)-
    distannoxane are the major metabolites (Loeffler, 1972)

          The metabolism of fenbutatin oxide was studied in rats by
    analyzing faeces obtained in the pharmacokinetics study described
    above (Reynolds, 1989). Metabolites were identified by co-
    chromatography with reference compounds using TLC. The majority (86-
    96%) of the extracted radiolabel co-chromatographed with unchanged
    fenbutatin oxide. Two minor metabolites were found in faecal
    extracts, one of which was unidentified. The other metabolite was
    tentatively identified as 1,3-dihydroxy-1,1,3,3-tetrakis(2-methyl-2-
    phenylpropyl)-distannoxane (IN-CG200). Traces of IN-CG200 were
    detected as an impurity of the test material prior to dosing
    (Reynolds, 1989).

    Effects on enzymes and other biochemical parameters

          In starch gel electrophoretic studies on sera from rats fed
    either 0 or 600 ppm fenbutatin oxide in the diet for 6 months (2
    buffer systems) control and test animal sera were identical. The
    effect of 1-phenylanatine or heat was similar on both sera.
    Comparisons with intestinal, bone, liver and kidney alkaline
    phosphatase indicated that the serum alkaline phosphatase was
    similar to intestinal alkaline phosphatase. Further studies on rat
    sera derived from animals fed 0 or 600 ppm fenbutatin oxide in the
    diet for one year, utilizing heat inactivation, selective substrates
    and inhibitors, confirmed the absence of any difference between
    control and test animal sera, as well as confirming the similarity
    of the serum alkaline phosphatase to intestinal alkaline phosphatase
    (Pickering, 1973a).

          In a further series of studies, the elevation of serum alkaline
    phosphatase observed in rats after administration of 300 ppm
    fenbutatin oxide in the diet for 90 to 120 days was shown to be
    completely reversible following one month's withdrawal from
    exposure. Limited data indicate that the reversal of elevated serum
    alkaline phosphatase in the test rats may be completed within 7 days
    of withdrawal of the fenbutatin oxide (Pickering, 1973b).

          Fenbutatin oxide and other tri-substituted tin compounds were
    shown to inhibit uptake and stimulate release of 5-hydroxytryptamine
    (5-HT) in rat platelets  in vitro (at 10 M) and to a lesser extent
    in platelets isolated after intraperitoneal injection (2.5 mg/kg
    bw). Interference with membrane ATP was suggested as the site of
    inhibition of 5-HT uptake by organotins (Johnson & Knowles, 1983).

          Fenbutatin oxide (10 M) did not affect aggregation of rat
    platelets  in vitro (Knowles & Johnson, 1986).

    Toxicological studies

    Acute toxicity studies

          Acute toxicity studies are summarized in Table 1. Fenbutatin
    oxide has low toxicity by the oral route but is highly toxic by the
    inhalation route. Inhalation exposure produced necrosis of
    bronchiolar epithelium, lung congestion and edema, and lesions in
    renal tubule epithelium (Parker, 1981).

        Table 1 Acute toxicity of fenbutatin oxide
                                                                                         
                                                  LD50       LC50
    Species   Strain           Sex   Route        mg/kg bw   mg/l       Reference
                                                                                         

    Rat       Crl:CD BR        M     oral         44001                 Sarver (1988)

              Crl:CD BR        F     inhalation              0.0721,2   Valentine (1987)
                                     (4 hr exp)

              Sprague-Dawley   M&F   inhalation              0.231      Parker (1981)
                                     (4 hr exp)              0.143

              CFE              M&F   i.p.         334                   Cassidy (1978)
                                                                                         

    1 Purity of technical fenbutatin oxide > 98%.
    2 LD50 for males could not be calculated. Lowest achievable concentration of
      0.046 mg/l produced 1/5 deaths.
    3 Concentration corrected for particles in respirable range (<4.5 m).
    4 Purity of test material unspecified.
    
    Short-term toxicity studies

    Mice

          Groups of 5-10 young or mature male Swiss-Webster mice were
    administered fenbutatin oxide (purity unspecified) at 137, 411, or
    1230 ppm (260, 780, or 2340 eq/kg diet), equal to 21, 62 or 190
    mg/kg bw/day, for 7 days. Mice were sacrificed 4 or 7 days after
    treatment. In young mice (13-14 g bw), a dose level of 137 ppm had
    no effect on body, brain, heart, liver, or spleen weight 7 days
    after treatment. In mature mice (27-29 g bw), a dose level of 411
    ppm had no effect on body-weight, haemoglobin, haematocrit, or

    erythrocyte count 4 days after treatment. Spleen weight was reduced
    20% and WBC count was reduced 11% compared to control values. At
    1230 ppm, body-weight, spleen weight, and WBC count were decreased
    and haemoglobin, haematocrit, and erythrocyte count were increased.
    A number of other triorganotin compounds were more potent than
    fenbutatin oxide producing effects on body-weight, spleen weight,
    and blood composition at lower dose levels. The NOAEL was 137 ppm
    (equal to 21 mg/kg bw/day) based on slight effects on the spleen and
    WBC count at 411 ppm. (Ishaaya et al., 1976).

    Rats

          Five groups of 5 male and 5 female rats were fed 0, 30, 100,
    300 or 1000 ppm fenbutatin oxide in the diet for 5 weeks. Body-
    weight was significantly reduced, as was food intake at 300 and 1000
    ppm. Reductions in absolute brain, heart, liver, spleen and kidney
    weights were noted in both sexes at 1000 ppm. Spleen weight was
    reduced in males at 300 ppm and brain, liver, spleen and kidney
    weights were reduced at 300 ppm in females. Serum alkaline
    phosphatase was increased in both sexes and SGPT levels were
    elevated in females at 1000 ppm (Simpson, 1972a).

          Three groups of five male rats were used in a paired feeding
    study, one group being a normal control, the second being fed 600
    ppm and the third pair-fed at the food intake levels of the 600 ppm
    group. The study ran for 5 weeks. Results (daily body-weights)
    indicated that the reduction in body-weight at 600 ppm can be
    totally accounted for by reduced food intake (Simpson, 1972b).

          Groups of 6 male and 6 female rats were fed 50, 100, 300 or 600
    npm fenbutatin oxide (97%) in the diet for 3 months. Concurrent
    controls comprised 12 male and 12 female rats. Health and behaviour
    were comparable in all groups. Body-weight gain was reduced in males
    at 600 ppm and in females at 300 and 600 ppm. Food intake was
    reduced in males at 600 ppm and in females during the first 3 or 4
    weeks of the study at 300 and 600 ppm. Absolute organ weights of
    liver and kidney were reduced in both sexes, heart in females, and
    brain and spleen in males at 600 ppm. At 300 ppm, liver, spleen and
    kidney absolute weights were reduced in males. Organ/body-weight
    ratios were elevated for brain in males at 600 ppm and in females at
    300 and 600 ppm. Heart and liver to body-weight ratios were elevated
    in females at 600 ppm and the heart to body-weight ratio was
    increased at 300 ppm. The testes to body-weight ratio was elevated
    in males at 300 and 600 ppm. Haematology parameters were comparable
    in all groups. Clinical chemistry determinations were comparable for
    total protein, SGOT and electrolytes (K+, Na+ and Cl-). BUN
    and SAP in males were elevated at 300 and 600 ppm in males, as was
    SGPT at 300 ppm. In females, BUN was elevated at 100 ppm and above.
    Serum protein fractions were comparable in all groups. No compound-
    related gross or histopathological effects were noted (Simpson &
    Thorpe, 1973).

          Groups of rats, 20/sex/control group or 10/sex/test group, were
    fed 0, 3, 10, 30, 100 or 300 ppm of 99% 1,3-dihydroxy-1,1,3,3-
    tetrakis(2-methyl-2-phenylpropyl)-distannoxane (a metabolite of
    fenbutatin oxide) for 90 days. Health and behaviour were comparable
    in all groups. Body-weights of 300 ppm males were significantly
    decreased throughout the study. This was also true for the females
    in the 30, 100 and 300 ppm groups, but the suppression was not
    significant. Slight reductions in haemoglobin, erythrocyte count and
    packed cell volume occurred in the 300 ppm males. No changes were
    noted in any of the other groups of either sex. The other clinical
    values of animals killed after 13 weeks showed no dose-related
    differences when compared to the control values. Gross and
    microscopic examination of a wide range of tissues from all control,
    100 and 300 ppm animals revealed no consistent changes associated
    with exposure to the metabolite. Organ weights of males at 13 weeks
    did not differ from control weights, while the organ body-weight
    ratios were increased for brain, heart, liver, kidneys and testes in
    this group and for the liver in the 100 ppm male group. Brain,
    spleen and kidney weights were reduced in the 300 ppm females
    (Simpson & Dix, 1972).

    Rabbits

          Groups of 5-10 male and 5-10 female New Zeeland white rabbits
    received dermal applications (6 h/day) of technical fenbutatin oxide
    (98% purity) at doses of 0.05, 0.5, or 5 mg/kg bw for 21 consecutive
    days. Control groups received dermal applications of the vehicle,
    dimethyl phthalate. Doses of 0.5 and 5 mg/kg bw produced mild to
    severe skin irritation, which was not completely reversed after a
    14-day recovery period. Slight irritation observed sporadically in
    control and 0.05 mg/kg bw groups was attributed to the wrapping
    technique and the vehicle. No systemic toxicity was observed. The
    NOAEL for systemic toxicity was greater than 5 mg/kg bw (Brock,
    1988).

    Dogs

          Five groups of 2 male and 2 female beagle dogs were orally
    dosed (gelatin capsules) with 0, 10, 30, 100 or 300 mg fenbutatin
    oxide/kg bw/day for 5 weeks. Slight vomiting and diarrhoea occurred
    at 100 and 300 mg/kg bw/day which was associated with decreased
    body-weight gain. At 30 mg/kg bw/day and above, serum alkaline
    phosphatase was elevated. No pathological changes were observed
    (Simpson, 1972a).

          Groups of dogs (8/sex/control group and 4/sex/test group) were
    administered, by capsule, 0, 2.5, 5, 15, 30 or 60 mg/kg bw/day 97%
    fenbutatin oxide for 2 years. Blood samples were taken every six
    weeks to measure BUN, glucose, plasma protein, sodium, potassium,
    chloride, SGPT, SGOT and alkaline phosphatase. Every 12 weeks
    haemoglobin, packed cell volume, RBC, WBC, differential WBC counts,

    prothrombin and kaolin-cephalin coagulation times were determined
    Urine analysis was carried out every 3 months. Emesis and diarrhoea
    occurred in most of the dosed dogs. These effects continued in some
    dogs in the 30 and 60 mg/kg bw/day groups throughout the study,
    although at a reduced frequency towards the end of the study. Lower
    dosage groups showed none of these effects after the first year.
    Convulsions occurred in 5 dogs, predominantly female, during the
    second year (control, one female; 2.5 mg/kg bw/day, one female; 15
    mg/kg bw/day, one female and one male; and 30 mg/kg bw/day, one
    female). None of the 60 mg/kg bw/day animals exhibited this effect.
    EEGs with simultaneous single lead monitoring of ECGs in one female
    each in the 0, 2.5 and 30 mg/kg bw/day groups revealed a normal EEG
    for the control dog, while the other 2 dogs showed normal waking
    rhythms but abnormal activity during light sleep. The investigators
    contended that the lack of a dose-response relationship suggests a
    diagnosis of spontaneous epilepsy not related to exposure to
    fenbutatin oxide. General health and behaviour of all other dogs
    were similar to control dogs. Significant reductions in rate of
    weight gains occurred in both sexes in the 60 mg/kg bw/day group.
    Males receiving 30 mg/kg bw/day exhibited no significant rate of
    gain. This effect was not evident in females of this group.
    Haematological and clinical chemistry and urine data showed no
    differences between control and test groups. Necropsies revealed no
    gross changes. Microscopic examination, including frozen sections of
    liver and kidney stained for lipid, revealed no compound-related
    pathological changes in a wide range of tissues. Organ weights were
    not affected by exposure to the test compound. The NOAEL was 15
    mg/kg bw/day (Granville & Dix, 1973).

    Long-term toxicity/carcinogenicity studies

    Mice

          Groups of 48 male and 48 female Carworth Farm No. 1 strain mice
    were fed 50, 100, 300 or 600 ppm fenbutatin oxide in the diet for 18
    months. A control group comprised 96 males and 96 females. All
    animals were examined grossly at death. Microscopic examination was
    undertaken on all animals dying during the study and on all 600 ppm
    mice, on 10 mice/sex at 50, 100 and 300 ppm, and on 45 controls/sex
    at 18 months. Survival rate was comparable in all groups and
    exceeded 75%. There was a high incidence of tumours in all groups.
    However there was no indication of compound or dose-related tumour
    induction (Granville  et al., 1973).

    Rats

          Groups of rats (114/sex/control group and 72/sex/test group)
    were fed 0, 50, 100, 300 or 600 ppm 97% fenbutatin oxide for two
    years. Rats were killed at 3 and 12 months (12/sex/control group and
    6/sex/test group) and at 6 months (24/sex/control group and
    12/sex/test group). Organ weights of brain, heart, liver, spleen,
    kidney and testes were recorded and a wide variety of tissues

    examined from all rats dying or killed at interim periods and from
    all rats from the control, 300, and 600 ppm groups. Tissues were
    examined from 10 animals/sex in the 50 and 100 ppm groups. Blood
    determinations were made at each kill period for haemo-globin, PCV,
    RBC, WBC and WBC differential counts; prothrombin and kaolin-
    cephalin coagulation times; BUN; total protein; potassium, sodium
    and chloride, SGPT, SGOT and alkaline phosphatase. Serum proteins
    were fractionated electrophoretically and estimates made of albumin
    and alpha,  and gamma-globulin concentrations.

          Fenbutatin oxide rendered the diet unpalatable to both sexes
    causing, in the earIy months of the study, significant reductions in
    food consumption and body-weight gains. Males were more affected
    than females. For the remainder of the study all groups exhibited
    similar rates of gain to the control group except both sexes
    receiving 300 and 600 ppm, which exhibited reduced body weiphts
    throughout the study. BUN levels were increased at the higher dose
    levels for the first 6 months but remained within normal limits for
    the remainder of the study. Serum alkaline phosphatase activity was
    increased in both sexes at 300 and 600 ppm throughout the study, in
    the 100 ppm males at 6 months, and in the 100 ppm females at 3
    months and 2 years. All other haematological and clinical chemistry
    values remained within normal limits at all times. Survival and
    behaviour were not affected by administration of the compound. No
    compound-related tissue lesions nor any increase in tumour
    incidences were found. Organ weights revealed no compound-related
    differences except kidney-weight reduction in all treated males and
    the 600 ppm females at 2 years. No specific compound-related lesions
    were seen in the kidney of any treated group. Absolute and relative
    testes weights, unaccompained by hypertrophy, were noted in the 300
    and 600 ppm groups at 2 years. The NOAEL was 50 ppm, equivalent to
    2.5 mg/kg bw/day (Simpson  et al., 1973).

    Reproduction studies

    Rats

          In a standard 3 generation, 2 litter/generation reproduction
    study, groups of 10 male and 20 female rats were fed 0, 50, 100 or
    300 ppm fenbutatin oxide (98% purity). Rats of the F0 generation
    commenced on diet and were mated at 100 days of age, and litters
    were culled to 10 pups/litter on post-partum day 5. Necropsy was
    performed on 10 male and 10 female F3b weanlings from the 0 and
    300 ppm group, and on 5 weanlings/sex from the 50 and 100 ppm
    groups. In the 300 ppm group, parents and pups were smaller,
    hyperactive and irritable; mean litter size was slightly reduced in
    the F1b litters, and in the two F3 generations survival to
    weaning was reduced at 300 ppm and the testicular organ/body-weight
    ratio was reduced in the F3b weanlings. Necropsies of F2b adults
    did not show any compound-related changes. No consistent effects

    were observed at 50 or 100 ppm with respect to fertility, gestation,
    viability, or lactation indices, litter size, litter weight, or
    necropsy of F3b weanlings (Hine  et al., 1973).

          Technical fenbutatin oxide (99.4%) was administered
    continuously to Crl:CD BR rats over two generations at 0, 40, 75,
    250, or 500 ppm in the diet, (equal to 2.8, 5.2, 17 or 38 mg/kg
    bw/day for males and 3.2, 6.0, 20 or 44 mg/kg bw/day for females).
    The first parental (F0) animals (30/sex) were treated beginning 72
    days prior to mating through weaning of the F1 offspring. Selected
    F1 offspring (30/sex) were treated beginning 105 days before
    mating through weaning of the F2 offspring. During the pre-mating
    period, body-weight, body-weight gain, and food consumption were
    reduced in F0 and F1 males and females receiving 500 ppm and
    females receiving 250 ppm. Body-weights of F0 and F1 females
    were also reduced during lactation at 500 ppm. Reproductive
    performance of F0 and F1 rats was unaffected. During lactation,
    mean body-weights of F1 and F2 offspring were reduced at 250 and
    500 ppm. The NOAEL was 75 ppm, equal to 6.0 mg/kg bw/day, based on
    reduced body-weight of parental animals and offspring at 250 and 500
    ppm (Bentley, 1990).

    Special studies on embryo/fetotoxicity

    Rats

          The developmental toxicity of fenbutatin oxide was studied in
    Wistar rats. Fenbutatin oxide (98.7% purity) was administered by
    oral gavage to groups of 27 mated rats at doses of 0, 15, 30, or 60
    mg/kg bw/day. Controls received the vehicle, aqueous
    carboxymethylcelluose. Rats were treated on days 6-15 of gestation
    and sacrificed on day 20. The incidence of diarrhoea was increased
    at the high dose of 60 mg/kg bw/day. Body-weight was reduced (4-8%)
    at 30 and 60 mg/kg bw/day during the treatment period. Body-weight
    of the high-dose group remained lower than controls post-exposure
    (day 20). The only other finding was a slight increase in pre-
    implantation loss at 60 mg/kg bw/day (1.1 per litter) compared to
    controls (0.6 per litter). Because treatment began around the time
    of implantation, the increase in pre-implantation loss may or may
    not have been related to treatment. The NOAEL for maternal toxicity
    was 15 mg/kg bw/day, based on reduced weight gain at 30 mg/kg
    bw/day. The NOAEL for embryo-fetal toxicity was 30 mg/kg bw/day
    based on increased pre-implantation loss at 60 mg/kg bw/day (Dix,
    1981a).

    Rabbits

          Three groups of 15 pregnant rabbits were dosed by gelatin
    capsule, on days 6-18 inclusive of gestation (mating day = day 0),
    with 3 or 10 mg fenbutatin oxide/kg bw/day, or with 37.5 mg
    thalidomide/kg bw/day. Twenty-six rabbits were used as capsule-only

    treated controls. Rabbits were killed on day 28 of pregnancy. A
    repeat experiment differed only in that 20 pregnant females/dose
    level and 30 controls were used, autopsy being on day 29. Fetuses
    were incubated for 24 h following removal from the uterus, after
    which approximately 1/3 were examined for visceral abnormalities,
    and 2/3 for skeletal defects. Prior to clearing for skeletal
    examination, viscera from these animals were also examined for
    defects. Although some maternal deaths occurred, they were random
    between groups. Mean live litter size, resorption rate, fetal loss,
    survival of incubated fetuses and incidence of anomalies were not
    affected by fenbutatin oxide at either dose level. Crown-rump length
    and fetal weight were comparable to controls except at 3 mg/kg
    bw/day, in the repeat experiment, where a significant increase was
    noted. The positive control group behaved as expected (Dix & Wilson,
    1973).

          The developmental toxicity of fenbutatin oxide was studied in
    New Zeeland white rabbits. Fenbutatin oxide (98.7% purity) was
    administered orally in capsules to groups of 18-23 artificially
    inseminated rabbits at doses of 0, 1, 5, or 10 mg/kg bw/day. Rabbits
    were treated on days 6-18 of gestation and sacrificed on day 29.
    Maternal toxicity was observed at 5 and 10 mg/kg bw/day. The high
    dose was extremely toxic producing anorexia and substantial weight
    loss during and after the treatment period. The group receiving 5
    mg/kg bw/day also experienced anorexia. Anorexia may have been the
    result of gastric irritation. One-third of the does treated with 5
    or 10 mg/kg bw groups showed lesions in the gastric mucosa at
    necropsy. Twelve of 20 does (60%) receiving 10 mg/kg bw/day aborted
    compared to 3/11 (27%) controls. Another 5/20 (25%) at the high dose
    had litters with no live fetuses compared to 2/11 (18%) controls. At
    5 mg/kg bw/day, 2/17 (12%) aborted and 4/17 (24%) had litters with
    no live offspring. Mean post-implantation loss per litter was 1.0,
    1.2, 2.8, and 3.2 for the 0, 1, 5, and 10 mg/kg bw/day groups,
    respectively. Litter weights were reduced at the high dose. The
    NOAEL for maternal toxicity and embryo-fetal toxicity was 1 mg/kg
    bw/day based on clinical signs and gastric lesions in does and
    increased post-implantation loss at 5 mg/kg bw/day (Dix, 1981b).

    Special studies on genotoxicity

          The results of genotoxicity studies are summarized in Table 2.
    Fenbutatin oxide tested negative in assays for point mutation in
    bacteria and mammalian cells, chromosome aberrations  in vitro and
     in vivo, and unscheduled DNA synthesis.

    Special study on male reproduction

    Rabbits

          The effect of a single high dose of fenbutatin oxide on the
    reproductive system of male rabbits was investigated. The study was
    conducted as a follow-up to a report of a reduced spermatogenesis
    and increased multinucleate spermatid formation in an LD50 study
    using doses of 1500-3000 mg/kg bw and no control group (Simpson,
    1972). Male New Zeeland white rabbits received a single oral dose of
    100, 500, or 1500 mg/kg bw of fenbutatin oxide (98.3% purity). The
    control group consisted of pair fed groups and an unpaired group
    (i.e. fed  ad libitum). Rabbits were sacrificed and examined 21
    days after treatment. Five of 8 rabbits receiving 1500 mg/kg bw and
    one each from the 100 and 500 mg/kg bw groups died. Two controls
    paired with 1500 mg/kg bw rabbits died due to dosing errors.
    Fenbutatin oxide-treated groups became emaciated due to markedly
    reduced food intake and weight gain. Body-weight of paired controls
    were similarly reduced except the group receiving 500 mg/kg bw had
    lower weights than its paired control. Fenbutatin oxide treatment
    produced lesions of the gastric mucosa including ulceration,
    erosion, and haemorrhage. Histological examination of reproductive
    tissues and testes weight did not reveal any significant differences
    between fenbutatin oxide groups and paired and unpaired controls.
    This study using a control group did not show male reproductive
    toxicity in rabbits following acutely toxic doses of fenbutatin
    oxide (Dix, 1981c).

    Special studies on neurotoxicity

    Rats

          Following preliminary studies to determine suitable dose levels
    and to confirm the induction of brain edema by triethyltin bromide,
    a comparative edema assay of five organotin compounds (including
    fenbutatin oxide, and its metabolite) for edema formation in the
    central nervous system of rats was undertaken. Groups of 15 male
    rats were intubated with corn oil (10 mg/kg bw, 40 mg/kg bw
    triethyltin bromide, 1000 mg/kg bw fenbutatatin oxide, 100 mg/kg bw
    1,3-dihydroxy-1,1,3,3-tetrakis (2-methyl-2-phenylpropyl)-
    distannoxane and sizeable doses of two other tin compounds. The
    triethyltin bromide group showed increased brain water content after
    48 h. Seven of ten brains examined at 24 and 48 h showed spongeosis
    of white matter in the cerebellum and sometimes of the corpus
    callosum and pons. No histological changes were observed in 4 rats
    killed after 6 h. Fenbutatin oxide (24 and 48 h kills), and the
    metabolite (6, 24 and 48 h kills) did not show either increased
    water content or abnormal brain histology, although rats on both
    compounds showed signs of intoxication (Samuels & Dix, 1972).

    Dogs

          Fenbutatin oxide was reported to have no effect on the EEG
    pattern in dogs treated with oral doses of 30 mg/kg bw/day for 14
    days. Groups of 2 male and 2 female beagle dogs were used. Each dog
    served as its own control. EEG was performed prior to treatment,
    during treatment, and 7 and 14 days after treatment. During the
    treatment period, the dogs exhibited occasional soft/loose faeces
    and tremors and experienced weight loss and reduced food intake.
    These symptoms resolved post-exposure. Later treatment of the same
    dogs with nikethamide (up to 1 g intravenously), an analeptic agent,
    produced paroxysmal activity (Greenough, 1991).

    Observations in humans

          No data available.


    
    Table 2. Results of genotoxicity assays on fenbutatin oxide
                                                                                                                                        
                                                                Conc. of
    Test System           Test object                           fenbutatin oxide              Purity    Results     Reference
                                                                                                                                        

    Ames test (1)         S. typhimurium                        5-300 g/plate dissolved      98%       Negative    Arce (1987)
                          TA98, TA97, TA100,                    in acetone
                          TA1535

                          S. typhimurium                        ?                             ?         Negative    Moriya  et al. (1983)
                          TA98, TA100, TA1535,
                          TA1537, TA1538

    E. coli mutation      E. coli, WP2 hcr                      ?                             ?         Negative    Moriya  et al. (1983)
    assay (1)

    V79/HGPRT mutation    V79 Chinese hamster lung cells        0.3-27 g/ml suspended        98.8%     Negative    Davis (1988)
    assay (1)                                                   in DMSO

    CHO/HGPRT mutation    Chinese hamster ovary cells           0.025-7.5 g/ml dissolved     99.4%     Negative    Stahl (1988)
    assay (1)             (CHO-K1-BH4)                          in acetone

    Unscheduled DNA       Male rat (Crl:CD BR) primary          0.001-50 g/ml dissolved      99.4%     Negative    Bentley (1988)
    synthesis             culture hepatocytes                   in acetone

    In vitro              Human lymphocytes, healthy male and   0.7-5 g/ml dissolved         99.4%     Negative    Vlachos (1988a)
    cytogenetics (1)      female volunteers                     in acetone

    In vivo cytogenetics  Male and female Crl:(ICR)BR mice,     500-5000 mg/kg bw orally X 1  99.4%     Negative    Vlachos (1988b)
                          bone marrow
                                                                                                                                        

    (1) Both with and without metabolic activation.


    
    COMMENTS

          Fenbutatin oxide is poorly absorbed from the gastrointestinal
    tract. Most (more than 90%) was excreted unchanged in faeces. Less
    than 1% was excreted in urine.

          Fenbutatin oxide has low acute oral toxicity. The World Health
    Organization has classified fenbutatin oxide as unlikely to present
    acute hazard in normal use. It is highly irritating to the skin,
    lungs, and gastrointestinal tract. By the oral route, bolus
    administration was particularly irritating. Oral administration to
    dogs resulted in diarrhoea and vomiting. Following gavage
    administration, rabbits exhibited anorexia and developed gastric
    mucosal lesions. An increase in SAP in a two-year study in rats may
    also have been related to gastrointestinal tract injury.

          A seven-day feeding study in mice gave some indication that
    fenbutatin oxide possessed less immunotoxicity potential than other
    organotin compounds. However, the data were inadequate to evaluate
    immunotoxicity.

          In a two-generation reproduction study in rats using dietary
    concentrations of 0, 40, 75, 250, or 500 ppm, the NOAEL was 75 ppm,
    equal to 6.0 mg/kg bw/day, based on reduced weight of adults and
    offspring at 250 ppm. Reproductive performance was unaffected.

          In a teratology study in rats at doses of 0, 15, 30, or 60
    mg/kg bw/day, the NOAEL for maternal toxicity was 15 mg/kg bw/day
    based on reduced body-weight gain at 30 mg/kg bw/day. The NOAEL for
    embryo-fetal toxicity was 30 mg/kg bw/day based on an increase in
    pre-implantation loss at 60 mg/kg bw/day. In a study in rabbits at
    doses of 0, 1, 5, or 10 mg/kg bw/day, the NOAEL for maternal and
    embryofetal toxicity was 1 mg/kg bw/day based on clinical signs of
    toxicity and gastric lesions in does and an increase in post-
    implantation loss at 5 mg/kg bw/day. No teratogenic effects were
    found in rats or rabbits.

          After reviewing the available genotoxicity data the Meeting
    concluded that fenbutatin oxide was not genotoxic.

          The 1977 Joint Meeting reviewed two-year studies in rats and
    dogs in which NOAELS of 2.5 mg/kg bw/day and 15 mg/kg bw/day,
    respectively, were observed. A multi-generation reproduction study
    in rats, in which the NOAEL was 100 ppm, equivalent to 5 mg/kg
    bw/day, was also reviewed at that time.

          The ADI of 0-0.03 mg/kg bw previously allocated in 1977 (which
    was based on the NOAEL of 2.5 mg/kg bw/day (50 ppm) observed in a
    two-year dietary study in rats in which an increase in SAP was
    observed at higher doses) was retained. A lower NOAEL of 1 mg/kg
    bw/day from a teratology study in rabbits, in which GIT irritation

    was observed, was considered less reflective of human exposure
    because of the high sensitivity of the GIT of the rabbit and the
    particular physiological characteristics of this species. Therefore,
    this study was not used as the basis of the ADI.

    TOXICOLOGICAL EVALUATION

    Level causing no toxicological effect

          Rat:        50 ppm, equivalent to 2.5 mg/kg bw/day (two-year
                            study reviewed by JMPR in 1977)

                      75 ppm, equal to 6.0 mg/kg bw/day (two-generation
                            reproduction study)

                      15 mg/kg bw/day (teratology study, maternal toxicity)

          Rabbit:     1 mg/kg bw/day (teratology study, maternal and
                            embryofetal toxicity)

          Dog:        15 mg/kg bw/day (two-year study reviewed by JMPR in
                            1977).

    Estimate of acceptable daily intake for humans

          0-0.03 mg/kg bw

    Studies which will provide information valuable in the continued
    evaluation of the compound

          1.    Adequate information on the immunotoxic potential of
                fenbutatin oxide.

          2.    Observations in humans.

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    See Also:
       Toxicological Abbreviations
       Fenbutatin oxide (Pesticide residues in food: 1977 evaluations)
       Fenbutatin oxide (Pesticide residues in food: 1979 evaluations)