METHACRIFOS EXPLANATION 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. EVALUATION FOR ACCEPTABLE INTAKE BIOLOGICAL DATA 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 Rats 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). Rabbits 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). COMMENTS 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 ADI. TOXICOLOGICAL EVALUATION LEVEL CAUSING NO TOXICOLOGICAL EFFECT 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. ESTIMATE OF TEMPORARY ACCEPTABLE DAILY INTAKE FOR MAN 0 - 0.0003 mg/kg b.w. STUDIES WITHOUT WHICH THE DETERMINATION OF A FULL ADI IS IMPRACTICABLE, TO BE SUBMITTED TO WHO BY 1987 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. TO BE SUBMITTED TO WHO BY 1989: 2. An adequate multigeneration study in rats. STUDIES WHICH WILL PROVIDE INFORMATION VALUABLE FOR THE CONTINUED EVALUATION OF THE COMPOUND Further observations in man. REFERENCES 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)