Methacrifos was evaluated for acceptable daily intake by the
    Joint Meeting in 1980, and reviewed in 1982 (Annex 1, FAO/WHO, 1981a
    and 1983a). A toxicological monograph was published after the 1980
    Meeting (Annex 1, FAO/WHO, 1981b) and a monograph addendum was
    published after the 1982 Meeting (Annex 1, FAO/WHO, 1983b). A
    temporary ADI was allocated in 1980, which was extended in 1982,
    pending the submission of a teratogenicity study in rats and an
    appropriate reproduction study. Information on the sensitivity of
    chinchilla rabbits to known teratogens and further observations on man
    were considered desirable. The results of these studies have been
    submitted and are summarized in this monograph addendum.



    Toxicological studies

    Special study on reproduction

         In a 2-litter, 2-generation reproduction study, groups of 28
    (F0) or 24 (F1a) Sprague-Dawley derived rats received methacrifos
    (purity 98%) admixed in the diet at 0, 75, 225, or 450 ppm. Dietary
    analyses generally indicated that exposure levels were less than
    nominal values as follows: 75 ppm (60 - 90%), 225 ppm (71 - 90%), and
    450 ppm (70 - 88%). However, analyses of the diet consumed during
    mating, gestation, and part of the weaning period of the F2a litters
    showed that exposure levels were significantly reduced from nominal
    values (75 ppm (< 8%), 225 ppm (24%), 450 ppm (< 36%)).

         Significant body-weight depression occurred in the 225 and
    450 ppm dams during lactation in the first generation, but only at the
    high dose in the second generation. Depressed body-weight gain was
    associated with decreased food consumption. The fertility indices in
    both matings were comparable. There was a statistically-insignificant
    decrease in the implantation rate of the F0 parents (combined data
    for both litters). Litter sizes of F1a litters were significantly
    reduced, but not those of F1b litters. Sex ratios were normal in
    both litters. Pup weights were reduced at 14 days post-partum in all
    litters and continued to be depressed in F1a and F1b pups at
    450 ppm. At 225 ppm, pup weights were reduced at 28 and 35 days
    post-partum in F1a offspring only. Pup mortality was increased at
    225 and 450 ppm in F1a and F1b litters. There was a dose-related
    increase in pup mortality in all generations.

         In behavioural tests, negative geotaxis was observed in F1a and
    F1b offspring, with a positive dose-response relationship at 225 and
    450 ppm; however, this attained statistical significance only in F1a
    pups at 450 ppm. In the F1a parents, female body weights were
    reduced during lactation at 225 and 450 ppm in both matings. A very
    slight body-weight decrease also occurred at 75 ppm. The body-weight
    gain of the pups was reduced at 225 and 450 ppm. There were no
    consistent effects on gross or histopathological findings, on absolute
    or relative organ weights, or on body weights of either adults or pups
    at necropsy. Although the above-mentioned dietary analysis confounds
    interpretation of the results of this study, a no-observed-effect
    level could not be established because of the observed increase in pup
    mortality in all generations (Fritz et al., 1985).

    Special studies on teratogenicity


         The teratogenic potential of methacrifos (98% purity in 0.5%
    carboxymethylcellulose aqueous solution) was investigated in Tif:
    RAI(SPF) rats by daily oral intubation from days 6 to 15 of gestation
    inclusively. As a pilot study had indicated marked maternal toxicity
    at 60 mg/kg b.w./day, daily doses of 0, 7.5, 22.5, or 45 mg/kg b.w.
    were given to groups of 24 pregnant females. The rats were sacrificed
    after 21 days and necropsied. Maternal toxicity (reduced food intake
    and body-weight gain) occurred at the high dose and to a lesser extent
    in the mid-dose group. Some high-dose dams adopted a prone position
    for up to 2 days during treatment, but no abortions occurred. The
    following parameters were not influenced by treatment in each group:
    implantation rates, embryonal or fetal deaths, the number of live
    fetuses, or sex ratios. At necropsy, 1 fetus from the control group
    exhibited partial pulmonary aplasia, but no visceral abnormalities
    were found in the fetuses of treatment groups. However, retarded
    skeletal ossification, particularly of hind-limb phalangeal nuclei and
    the fifth sternebrae, occurred in high-dose fetuses; mid-dose fetuses
    also exhibited delayed ossification of the fifth sternebrae.
    Accordingly, the results of this study indicate a no-observed-effect
    level of 7.5 mg/kg b.w./day (Giese et al., 1985).


         To ensure the susceptibility of chinchilla rabbits to a known
    teratogen, thalidomide was suspended in 2% aqueous carboxymethyl
    cellulose solution and was administered by intubation to groups of
    12 - 14 acclimatized 4- to 5-month old pregnant female rabbits at
    dose levels of 100 or 250 mg/kg b.w./day. The day of mating was
    deemed to be day 0 of gestation. Dosing was performed on days 6 -
    10 inclusive, and the dams were sacrificed on day 28. Terminal
    maternal body weights (i.e., terminal weight minus gravid uterine
    weight) were reduced in both test groups. Total abortions occurred in
    1 rabbit at 100 mg/kg b.w./day and in 2 rabbits at 250 mg/kg b.w./day.
    There were dose-related increases in embryonal and fetal resorptions
    and in terata with arthryogryposes, agenesis of the kidney and ureter,
    hydrocephaly, and microphthalmia. Several anomalies (cleft palate,
    micrognathia, cranioschesis, and renal hypoplasia) were observed at
    the top dose level only. The study indicates that the chinchilla
    rabbit shows embryo- and fetotoxicity as well as increased incidences
    of several malformations following oral exposure to thalidomide
    (Fritz, 1982).

    Observations in humans

         Nine male volunteers, aged 24 - 45 years (mean 39.9 years), took
    encapsulated methacrifos (5 mg; purity unspecified) daily with
    breakfast for 28 consecutive days. Blood was collected on 3 occasions
    in the week preceding exposure so that baseline cholinesterase
    activity could be determined. Haematological and biochemical
    parameters were monitored at 3-day intervals during the study.
    Urinalysis was conducted daily and urine microscopy was performed
    twice weekly during exposure and 4, 8, and 15 days after cessation of
    exposure. Electrocardiograms were conducted twice weekly for 5 weeks
    after initial exposure. Blood was taken 6 - 7 hours after breakfast
    for the determination of plasma and erythrocyte cholinesterase
    activities daily for the first week, twice weekly during the rest of
    the exposure period, and 3 days after cessation of exposure. The mean
    daily exposure was 0.063 mg/kg b.w. (range 0.054 - 0.075 mg/kg b.w.).
    Three of 9 subjects experienced mild indigestion, lasting up to
    2 hours, after exposure; some developed anorexia. There were no
    adverse clinical findings at 10 regular physical examinations
    conducted during and 2 weeks after the study. Plasma and erythrocyte
    cholinesterase activities were unaffected, as were standard
    haematological and biochemical parameters. Urinalysis microscopy and
    electrocardiography revealed no abnormal findings (Shephard &
    Kyffin, 1984).


         As previously found, methacrifos was not teratogenic in either
    chinchilla rabbits or Sprague-Dawley rats. The induction of several
    malformations by thalidomide in chinchilla rabbits indicated the
    validity of that animal model for teratogenicity testing. The
    consistent deaths of rat pups observed in 2-generation reproduction
    studies warrant clarification, particularly in view of the
    embryotoxicity in rats noted by the 1980 Joint Meeting.

         A further study with human volunteers, each receiving 5 mg/day,
    indicated that neither plasma nor erythrocyte cholinesterase
    activities were affected. Although 3 of the 9 volunteers experienced
    mild indigestion, sometimes with mild anorexia, there were no
    significant findings on clinical examination.

         The Meeting noted that previous Joint Meetings had accepted the
    validity of chronic toxicity and carcinogenicity studies originally
    conducted by Industrial Bio-test Laboratories (IBT). In view of the
    difficulties experienced in achieving nominal dietary concentrations
    of methacrifos in the 2-generation reproduction study, the Meeting
    concluded that the full details of these studies, including the
    details of diet preparation and dietary analyses, should be
    re-evaluated. Accordingly, the Meeting agreed to extend the temprorary



         Rat:    1 ppm in the diet, equivalent to 0.05 mg/kg b.w./day.
         Dog:    1 ppm in the diet, equivalent to 0.025 mg/kg b.w./day.


         0 - 0.0003 mg/kg b.w.


         1. Submission of all dietary preparation and analytical data for
    the chronic oral toxicity study in rats and the carcinogenicity study
    in mice conducted by IBT.


         2. An adequate multigeneration study in rats.


         Further observations in man.


    Fritz, H. Report on thalidomide teratology study - nonstandard in
    1982      rabbits (chinchilla-type). Unpublished report No. K-S.80
              from Ciba-Geigy Ltd., Basle, Switzerland. Submitted to WHO
              by Ciba-Geigy Ltd., Basle, Switzerland.

    Fritz, H., Suter, H.P., Zak, F., Krinke, G., & Hess, R. Two
    1985      generation, two litter reproduction toxicity study in rats
              with CGA 20,168. Unpublished report No. 83.0814 from
              Ciba-Geigy Ltd., Basle, Switzerland. Submitted to WHO by
              Ciba-Geigy Ltd., Basle, Switzerland.

    Giese, K., Suter, H.P., & Gfeller, W. Report on CGA 20,168 tech:
    1985      teratology study in rats. Unpublished report No. 83.0813
              from Ciba-Geigy Ltd., Basle, Switzerland. Submitted to WHO
              by Ciba-Geigy Ltd., Basle, Switzerland.

    Shephard, N.W. & Kyffin, P. A study of the tolerance to methacrifos
    1984      administered orally in a dose of between 0.06 mg/kg/day and
              0.1 mg/kg/day for a period of 28 days to normal volunteers.
              Unpublished report No. CG/METHAC/83/1 from Medical Science
              Research, Beaconsfield, Bucks., UK. Submitted to WHO by
              Ciba-Geigy Ltd., Basle, Switzerland.

    See Also:
       Toxicological Abbreviations
       Methacrifos (Pesticide residues in food: 1980 evaluations)
       Methacrifos (Pesticide residues in food: 1982 evaluations)
       Methacrifos (Pesticide residues in food: 1988 evaluations Part II Toxicology)
       Methacrifos (Pesticide residues in food: 1990 evaluations Toxicology)