OMETHOATE EXPLANATION Omethoate was evaluated by the Joint Meeting in 1971, and reviewed in 1975, 1978, 1979, and 1981 (Annex 1, FAO/WHO, 1972a, 1976a, 1979a, 1980a, and 1982a). A toxicological monograph was prepared by the Joint Meeting in 1971 (Annex 1, FAO/WHO, 1972b) and monograph addenda were prepared in 1975, 1978, 1979, and 1981 (Annex 1, FAO/WHO, 1976b, 1979b, 1980b, and 1982b). In 1981, the Joint Meeting requested carcinogenicity studies in a rodent species at higher dose levels than were used in the study that was reviewed, while further mutagenicity studies were deemed to be desirable. A chronic toxicity study on mice, a teratogenicity study in rabbits, and additional mutagenicity studies have become available in the meantime, and are summarized in this monograph addendum. EVALUATION FOR ACCEPTABLE DAILY INTAKE BIOLOGICAL DATA Biochemical aspects Special studies on the inhibition of cholinesterase activity in mice Groups of BOR:NMRI mice (50 males and 50 females/group) were given diets containing omethoate (97.1% purity) at concentrations of 0, 1, 3, or 10 ppm for 4 weeks. Animals were observed twice daily and weighed once a week. Plasma, erythrocyte, and brain cholinesterase activity were measured on days 1, 3, 7, 14, and 29 of the treatment on 10 males and 10 females per dose group. Appearance, behaviour, and mean body weights were not significantly different between the control and the treated groups. No mortality occurred. Plasma cholinesterase activity in both sexes was significantly lower than in controls in the 10-ppm group after the first determination. Erythrocyte cholinesterase activity was depressed in females from day 3 onward at 10 ppm, and on day 14 at 3 ppm; in males it was depressed only on day 29 at 10 ppm. Brain cholinesterase activity of both sexes was significantly lower than in controls at all test times at 10 ppm and at nearly all test times at 3 ppm. In addition, males in the 1-ppm group also had significantly lower brain enzyme activity on days 1 and 3; however, an opposite trend was observed on day 7. The 1-ppm dose level was the no-effect level on cholinesterase activity in this study (Kroetlinger, 1982). Toxicological studies Special studies on teratogenicity Rabbit Groups of 14 pregnant New Zealand White rabbits were dosed daily by oral gavage with omethoate (96.8% purity), dissolved in distilled water, at dose levels of 0, 0.1, 0.3, or 1.0 mg/kg b.w. from day 6 to day 18, inclusive, of gestation. On day 29 of gestation the females were sacrificed and the uterine contents examined. The reproductive tract, complete with ovaries, was dissected out and the following recorded; weight of gravid uterus, number of corpora lutea in each ovary, number of implantation sites, number of resorption sites (classified as early and late), number and distribution of live and dead foetuses in each uterine horn, weight and sex of individual foetuses, individual placental weights, and external, internal and skeletal abnormalities of individual foetuses. Whole-blood cholinesterase activity was determined prior to dosing on day 6 of gestation and 2 hours after dosing on day 18 of gestation. The general condition of control and treated females was comparable throughout the study. Maternal mean body weights and corrected day-29 body weights were unaffected by the treatment. Mortality, the incidence of abortions and total-litter losses, and the number of pregnant females with viable young on day 29 were not altered by treatment. Whole-blood cholinesterase activity was significantly depressed only among the 1.0 mg/kg b.w. females, compared both to the pre-dosing level and to the control post-dosing level. There were no treatment-related differences between the control and the treated groups with respect to the corpora lutea count, implantations, male and female viable young, early and late resorptions, pre- and post-implantation losses, or foetal and placental weights. Examination of foetuses at necroscopy on day 29 of gestation or following skeletal investigation revealed a number of non-dose-related findings of the type and incidence previously recorded in this strain of rabbit and in the laboratory that performed the study. The 1.0 mg/kg b.w. dose level was the no-effect level for embryotoxic/teratogenic effects (Tesh et al., 1982). Special study on carcinogenicity Mouse Groups of SPF mice of the BOR:CWF1 strain (50 males and 50 females per dose level) were fed diets containing omethoate (94% purity) at levels of 0, 1, 3, or 10 ppm for 24 months. The animals were inspected daily for clinical symptoms. Food consumption was determined weekly. The body weights were determined weekly during the first 14 weeks and at 3-week intervals thereafter. Necropsy was performed on the mice that died or were sacrificed when moribund, and on all the animals sacrificed at the end of the experiment. Heart, lungs, liver, spleen, kidneys, and testes were weighed. A range of organs and all changes detected during gross examination were subjected to histopathological examination. Appearance, behaviour, and activity were not significantly different between the control and treated groups. Total and mean daily food consumption were essentially the same among all the animals. The body weights of the male mice of all the treated groups were, for the most part, higher than those of the controls during the entire experiment, whereas there were no differences from controls in the body weights of the females. Mortality and the frequency-distribution of mortality by death- date-intervals were comparable among all the groups. Mortality at 18 months was between 12% and 27% for the males and between 14% and 31% for the females. Absolute and relative organ weights showed no dose- related and/or significant differences between control and treated groups. Gross anatomical and histopathological examinations revealed a range of non-neoplastic changes commonly observed in old mice. Comparison of these changes by type, site, and frequency distribution by sex and dose level gave no indications of any treatment-related toxic effects. Neoplastic changes were found primarily in the lungs, liver, adrenal cortex, and haemopoietic system. Neither the data concerning the type, site, or frequency-distribution of tumours by sex and dose level, nor the data regarding the number of tumour-bearing mice, mice with benign tumours, mice with malignant tumours, or mice with both benign and malignant tumours provided any indication of treatment effects. The somatic no-effect level in this study was 10 ppm, equal to 2.1 mg/kg b.w./day and 3.1 mg/kg b.w./day, for male and female mice, respectively (Kroetlinger & Loeser, 1982). Special studies on mutagenicity Several mutagenicity assays have been carried out on omethoate (Table 1); omethoate was found to be mutagenic only in gene-conversion and mitotic-recombination tests in S. cerevisiae D7. All other tests were negative. Special studies on delayed neuropathy Hens (2-4/dose group) were dosed orally with omethoate dissolved in corn oil at levels of 20 to 300 mg/kg b.w. (ca. 4-8 times the unprotected LD50) under eserine and atropine protection. Omethoate used was a sample that caused a fatal human poisoning accident. AChE and neurotoxic esterase (NTE) acitivites of brain homogenates were assayed 24 hours after dosing. Pair-dosed birds which survived were observed for signs of ataxia for 3-4 weeks after dosing. Hens dosed at 4 times the LD50 did not show any inhibition of NTE at 24 hours, nor signs of ataxia for 3-4 weeks after dosing. Hens dosed at 8 times the LD50 did not survive, despite treatment with high doses of atropine; however, the NTE activity in the brain of those animals who died within 36 hours was measured immediately after death and was found to be normal. In all the birds acute cholinergic symptoms were correlated with high inhibition of AChE in the brain. An inhibition of 70% of brain AChE in a bird dosed with 20 mg/kg b.w. of omethoate was not enough to produce detectable signs of acute poisoning. The in vitro inhibition activity of pure omethoate and of the incriminated sample of omethoate were measured for NTE and AChE activities using both hen and human brain enzymes. For both hen and human tissue the I50 for AChE was in the range of 0.08-0.15 mM, so that it would be virtually totally inhibited at 5 mM, the concentration which caused no detectable inhibiton of NTE. NTE and AChE activities were measured on samples of brain tissue (cortex) taken during the post-mortem examination performed 24 hours after death on a 30-year-old male farmer who was acutely poisoned by a commercial formulation of omethoate. The NTE acitivity was within the normal range, while AChE was strongly inhibited. The conclusion was that omethoate is extremely unlikely to cause delayed neuropathy in man (Lotti et al., 1981). Table 1. Results of mutagenicity assays of omethoate Test Test Range of doses or Result Reference organism substance concentration tested S. cerevisiae Omethoate 0.03-66.67 µl/ml* No significant Hoorn, 1982 S138, & (96.9% differences in S211 alfa purity) revertant frequencies compared to control S. cerevisiae Omethoate 0.03-66.67 µl/ml* Dose-related Hoorn, 1983 D7 (96.0% increases of purity) frequencies of tryptophan convertants and mitotic recombinants Mouse lymphoma Omethoate 500-2000 µg/ml No significant Bootman & L5178Y (96.9% without S-9 mix, increase in Rees, 1982 (TK +/-) purity) 500-5000 µg/ml mutation cells - with S-9 mix frequency at the in vitro TK gene locus* Micronucleus Omethoate 2×6 mg/kg b.w., No significant Herbold, test on (97.1 2×12 mg/kg b.w. differences 1981 mouse purity) between control & treated groups with respect to incidence of micronucleated polychromatic erythrocytes Table 1. (Con't) Test Test Range of doses or Result Reference organism substance concentration tested E. coli (K12) Omethoate 625-10,000 µl/ No indication of Herbold, p 3478 with (96.0% plate* potential for 1983 DNA repair purity) DNA damage deficiency & E. coli W3110 with intact repair system * With and without S-9 mix Acute toxicity After exposure to omethoate (96.9% purity) for 4 hours, a primary irritant effect on the sheared uninjured skin of albino rabbits was not observed. The test compound caused a slight conjunctival reaction that was completely reversible within 7 days (Pauluhn, 1982). Short-term studies Dog Groups of Beagle dogs (6 males and 6 females/group) received omethoate (97.1% purity; dissolved in acidulated water) daily for 12 months by stomach tube at dose levels of 0, 0.025, 0.125, or 0.625 mg/kg b.w. All the animals were observed several times daily for appearance and behaviour. Food and water consumption were recorded daily. Body weights were noted weekly. Reflex tests, ophthalmoscopic examinations, body temperatures, pulse-rate measurements, haematology, clinical chemistry, and urinalyses were carried out before the start of the study and during weeks 6, 13, 26, 40, and 52 of treatment. Cholinesterase activity in the plasma and erythrocytes was determined 1 week before the start and during weeks 1, 3, 6, 9, 13, 26, 40, and 52 of treatment. On autopsy, after 52 weeks of treatment, cholinesterase activies in the brains of all the animals were determined. All the animals were normal in appearance and behaviour. WP signs attributable to treatment were not observed. All the animals survived the treatment. There were no significant differences between the control and the treated groups with respect to reflex tests, ophthalmoscopic examinations, body temperatures, pulse rates, food and water consumption, mean body weights, haematology, clinical chemistry (except for cholinesterase activity) or urinalysis. A clear depression of plasma cholinesterase activity was observed only in the 0.625 mg/kg b.w. dose group, ranging between 25% and 32% in the males, and 16% and 29% in the females, in comparison to the controls. The depression remained essentially constant over the entire period of study. A marked depression of erythrocyte cholestinerase activity was measured in the males (17% to 40%) and females (22% to 40%) at the 0.625 mg/kg b.w. dose level, which varied only slightly during the study. At the 0.125 mg/kg b.w. dose level, only the males in the first third of the study showed slight (less than 28%) depression of erythrocyte cholinesterase activity. Brain cholinesterase activity was depressed in the males of the 0.125 mg/kg b.w. group (20%) and of the 0.625 mg/kg b.w. group (39%), and in the females of the 0.625 mg/kg b.w. group (30%). On autopsy, absolute and relative organ weights were not significantly different between the control and treated groups. The gross pathological and histopathological examinations did not reveal dose-related findings. The dose level of 0.625 mg/kg b.w. was the no-effect level for somatic effects, while 0.025 mg/kg b.w. was the no-effect level for erythrocyte cholinesterase activity (Hoffmann & Schilde, 1984). COMMENTS The results of an adequate chronic toxicity/oncogenicity study in mice, in which neither oncogenic effects nor other somatic damage were observed, were provided to the meeting. A no-effect level of 1.0 mg/kg b.w. was determined for embryotoxic/teratogenic effects in the rabbit. Delayed neuropathy was not observed in hens. No potential for delayed neuropathy was observed in man. Omethoate was found mutagenic only in gene-conversion and mitotic-recombination tests in S. cerevisiae D7, whereas other tests were negative. The Meeting estimated an ADI for omethoate. TOXICOLOGICAL EVALUATION LEVEL CAUSING NO TOXICOLOGICAL EFFECT Rat: 1 ppm in the diet, equivalent to 0.05 mg/kg b.w. Dog: 0.025 mg/kg b.w./day. ESTIMATE OF ACCEPTABLE DAILY INTAKE FOR MAN 0 - 0.0003 mg/kg b.w. FURTHER WORK OR INFORMATION DESIRED Observations in man. REFERENCES Bootman, J. & Rees, R. S 6876: Investigation of mutagenic activity in (1982) the TK +/- mouse lymphoma cell mutation system. Unpublished report No. 82/BAG 027/488 from Life Science Research, England. Submitted to WHO by Bayer F.R.G. Herbold, B. S 6876 (Omethoate, Folimat(R) active ingredient): (1981) Micronucleus test on mouse to evaluate S 6876 for mutagenic potential. Unpublished report No. 10,021 from Institute of Toxicology, Bayer AG. Submitted to WHO by Bayer F.R.G. Herbold, B. S 6876 (Omethoate, Folimat(R) active ingredient): Pol test (1983) on E. coli to evaluate for DNA damage. Unpublished report No. 12,126 from Institute of Toxicology, Bayer AG. Submitted to WHO by Bayer F.R.G. Hoffman, K. & Schilde, B. S 6876 (Omethoate): Chronic toxicity to dogs (1984) on oral administration (twelve-month stomach tube study). Unpublished report No. 12,561 from Institute of Toxicology, Bayer AG. Submitted to WHO by Bayer F.R.G. Hoorn, A.J.W. Mutagenic evaluation of S 6876 (omethoate) in the (1982) reverse mutation induction assay with Saccharomyces cerevisiae strains S138/S211-alfa. Unpublished final report No. E-9030 from Litton Bionetics, The Netherlands. Submitted to WHO by Bayer F.R.G. Hoorn, A.J.W. Evaluation of S 6876 (c.n. omethoate; a.i. of Folimat(R)) (1983) in the induced mitotic crossing over and gene conversion assay in Saccharomyces cerevisiae strain D7. Unpublished final report No. E-9127 from Litton Bionetics, The Netherlands. Submitted to WHO by Bayer F.R.G. Kroetlinger, F. S 6876 (Omethoate, the active ingredient of Folimat(R)) (1982) Study of the inhibition of cholinesterase activity in mice. (4-week feeding experiment). Unpublished report No. 11,235 from Institute of Toxicology, Bayer AG. Submitted to WHO by Bayer F.R.G. Kroetlinger, F. & Loeser, E. S 6876 (Omethoate, the active ingredient (1982) of Folimat(R)): Chronic toxicity study on mice. (2-year feeding experiment). Unpublished report No. 11,161 from Institute of Toxicology, Bayer AG. Submitted to WHO by Bayer F.R.G. Lotti, M., Ferrara, S.D., Caroldi, S., & Sinigaglia, F. Enzyme studies (1981) with human and hen autopsy tissue suggest omethoate does not cause delayed neuropathy in man. Arch. Toxicol., 48, 265-270. Pauluhn, J. S 6876 (Omethoate, the active ingredient of Folimat(R)): (1982) Study of the irritant/corrosive effect. Unpublished report No. 11,977 from Institute of Toxicology, Bayer AG. Submitted to WHO by Bayer F.R.G. Tesh, J.M., Ross, F.W., Wightman, T.J., & Wilby, O.K. S 6876: Effect (1982) of oral administration upon pregnancy in the rabbit. 2. Main study. Unpublished report No. 82/BAG 023/111 from Life Science Research, England. Submitted to WHO by Bayer F.R.G.
See Also: Toxicological Abbreviations Omethoate (WHO Pesticide Residues Series 1) Omethoate (WHO Pesticide Residues Series 5) Omethoate (Pesticide residues in food: 1978 evaluations) Omethoate (Pesticide residues in food: 1979 evaluations) Omethoate (Pesticide residues in food: 1980 evaluations) Omethoate (Pesticide residues in food: 1981 evaluations) Omethoate (Pesticide residues in food: 1984 evaluations)