PESTICIDE RESIDUES IN FOOD - 1997 Sponsored jointly by FAO and WHO with the support of the International Programme on Chemical Safety (IPCS) TOXICOLOGICAL AND ENVIRONMENTAL EVALUATIONS 1994 Joint meeting of the FAO Panel of Experts on Pesticide Residues in Food and the Environment and the WHO Core Assessment Group Lyon 22 September - 1 October 1997 The summaries and evaluations contained in this book are, in most cases, based on unpublished proprietary data submitted for the purpose of the JMPR assessment. A registration authority should not grant a registration on the basis of an evaluation unless it has first received authorization for such use from the owner who submitted the data for JMPR review or has received the data on which the summaries are based, either from the owner of the data or from a second party that has obtained permission from the owner of the data for this purpose. METHIDATHION (addendum) First draft prepared by A. Moretto Istituto di medicine del Lavoro Universita degli Studi di Padova Padua, Italy Explanation Evaluation for acute reference dose Acute toxicity Observations in humans Comments Toxicological evaluation relevant for establishing an acute reference dose References Explanation Methidathion was last evaluated toxicologically by the 1992 JMPR (Annex 1, reference 65)when an ADI of 0-0.001 mg/kg bw was established on the basis of an NOAEL of 0.14 mg/kg bw per day in three-month, one-year, and two-year studies in dogs. The ADI was based on studies in which effects on the liver were observed. The CCPR requested the JMPR to establish an acute reference dose for methidathion in view of its high acute toxicity. New studies submitted for review were evaluated by the present Meeting. Evaluation for acute reference dose 1. Acute toxicity Rats Groups of three to six male and female Crl:CD(SD)BR rats were given technical-grade methidathion (purity, 93.2%) as single doses of 0, 4, 8, 16, 20, 25, or 30 mg/kg bw by gavage in corn oil. The acute oral LD50 was about 25 mg/kg bw. An abbreviated functional observational battery of tests, including observations of mortality, signs in the home cage, during handling, and in the open field, and righting reflexes in air, were conducted. Signs consistent with cholinergic poisoning (e.g. lachrymation, salivation, diarrhoea, tremors, ataxia, and muscle fasciculations) were observed in males at doses > 8 mg/kg bw and in females at doses > 16 mg/kg bw. Two of three females at 30, one of five males and three of six females at 25, and one of six males at 20 mg/kg bw died. The NOAEL for signs of cholinergic overstimulation was 4 mg/kg bw, and the highest non-lethal dose was 20 mg/kg bw, when the estimated time to peak effect for toxic signs was 2 h. Males recovered faster than females, and many of the signs had abated 4-6 h after treatment (Leahy, 1993). Groups of five male and five female Crl:CD BR VAF/Plus rats were given technical-grade methidathion (purity, 93.2%) as single doses of 0 or 0.5-35 mg/kg bw in corn oil by gavage. Body weights, clinical signs, and liver and brain weights were recorded. Serum, erythrocyte, and brain chlinesterase activities were measured 4 h after treatment. All males at 35 mg/kg were dead by day 3, and four females at 20 mg/kg died within one day. The signs of cholinergic overstimulation included decreased activity, tremors, absence of pain reflex, excessive salivation, dyspnoea, lachrymation, red-stained face, discoloured urogenital area, soft stools, few faeces, meiosis, and fasciculations and were observed in animals treated with doses > 5 mg/kg bw. The peak effect was seen 2 h after treatment. There was no meaningful effect on body-weight gain or on liver or brain weights. The NOAEL for cholinergic signs was 3 mg/kg bw. In a subsequent phase, the doses administered were 0, 0.5, 1, 2.5, 5, or 10 mg/kg bw, and cholinesterase activity was measured in plasma, erythrocytes, and brain about 4 h after treatment. The results are shown in Table 1. Thus, single oral doses of methidathion caused acute signs and cholinesterase inhibition. The NOAEL for cholinesterase inhibition was 1 mg/kg bw (Glaza, 1994). Crl:CD-BR VAF/Plus rats were given single doses of technical-grade methidathion (purity, 93.2%) at 0, 1, 4, 8, or 16 mg/kg bw in corn oil by gavage. As a positive control, carbaryl was administered at 30 mg/kg intraperitoneally. Body weights, food consumption, and clinical signs were recorded. A complete functional observational battery of tests and figure-eight maze activity counts were performed before treatment, at the time of peak effect (1.5 h after treatment with methidathion and 0.5 h after carbaryl), and one and two weeks after treatment. Serum and erythrocyte cholinesterase activities were measured before treatment, at the time of peak effect, and 14 days after treatment. Acetylcholinesterase activity in regions of the brain (cerebellum, striatum, and cerebral cortex or hippocampus) were measured at the time of peak effect and one and two weeks after treatment. Necropsies were performed, and selected nervous tissues examined histologically. All animals survived. In comparison with controls, the body-weight gain of males at 16 mg/kg bw was decreased. Food consumption was decreased in animals of each sex at this dose during the first week. Changes in clinical signs and in the results of functional observational tests and maze activity were seen only at the time of peak effect at doses > 8 mg/kg bw in males and > 4 mg/kg bw in females. The changes consisted of typical cholinergic signs and decreased body temperature. Similar changes were seen in carbaryl-treated animals. The cholinesterase activities at the time of peak effect and two weeks after treatment are reported in Table 2. Acetylcholinesterase activity in the cerebral cortex or hippocampus was inhibited in males at 1 mg/kg bw; however, this effect was not considered to be of toxicological significance because no inhibition was seen in acetylcholinesterase activity in erythrocytes or in other regions of the brain at this dose, the activity in controls was Table 1. Mean cholinesterase activity (mU/ml, standard deviation in parentheses) in rats 4 h after treatment with methidathion Dose Males Females (mg/kg bw) Plasma Erythrocytes Brain Plasma Erythrocytes Brain 0 316 (53) 1411 (145) 1593 (260) 821 (128) 944 (126) 1659 (63) 0.5 332 (139) 1329 (268) 1715 (152) 857 (181) 1107 (170) 1549 (56) 1.0 300 (41) 1282 (386) 1687 (178) 800 (186) 979 (126) 1626 457) 2.5 282 (26) 1177 (328) 1676 (423) 857 (206) 769 (48) 1016 (102)* 5.0 186 (19)* 1119 (238) 1138 (249)* 668 (109) 559 (97)* 605 (98)* 10.0 169 (26) 1037 (190) 744 (91)* 477 (77)* 618 (t3)* 477 (135)* * Statistically significant in comparison with the control group, Dunnett's t test, p < 0.05 Table 2. Mean cholinesterase activity (µg/ml) after administration of methidathion to rats, at the time of peak effect (1.5 h after treatment) and at week 2 Dose Serum Erythrocytes Cerebellum Cortex Striatum (mg/kg bw) 1.5 h 2 weeks 1.5 h 2 weeks 1.5 h 2 weeks 1.5 h 2 weeks 1.5 h 2 weeks Males 0 390 359 1530 1488 3.13 3.31 8.22 6.24 25.32 27.95 1 392 415 1482 1516 2.74 3.20 4.81** 8.07 27.00 30.80 4 316 398 646** 968* 1.48** 3.26 2.60** 8.09 7.08** 37.60 8 272** 462 394** 1224 0.99** 3.09 1.01** 6.50 4.12** 19.12 16 231** 394 248** 1084 0.69** 2.79 0.51** 5.94 2.32** 17.88 Females 0 1128 1225 1852 1548 3.10 3.10 7.78 7.26 28.96 15.36 1 874 1128 1690 1456 2.69 2.95 6.74 7.11 26.52 35.84 4 858 1237 610** 1440 1.22** 2.95 2.26** 7.41 7.76** 17.92 8 863 1370 336** 1208 0.77** 2.84 0.82** 6.63 2.32** 28.68 16 800 1320 260** 1252 0.53** 2.81 0.50** 6.16 1.52** 21.52 10 animals at 1.5 h; 5 animals at 2 weeks Statistically significant in comparison with the control group, Dunnett's t test: * p < 0.05, ** p < 0.01 relatively high; the measurements were variable, and there were no detectable clinical signs of cholinergic dysfunction. There were no treatment-related findings at necropsy or histopathologically in the central or peripheral nervous system. It is concluded that single oral doses of methidathion caused acute neurobehavioural effects and cholinesterase inhibition but no histopathological lesions. The overall NOAEL was 1 mg/kg bw (Chang & Richter, 1994). 2. Observations in humans No inhibition of plasma or erythrocyte cholinesterase activity or any other effect was observed in groups of eight men who took daily oral doses of 0.04 or 0.11 mg/kg bw methidathion in capsules for six weeks. No low-observed-adverse-effect level (LOAEL) was determined because cholinesterase was not inhibited (Coulston, 1970; reviewed in Annex 1, reference 67). In addition to two cases of massive poisoning (Teitelmann et al., 1975; Zoppellari et al., 1990) reviewed by the 1992 JMPR (Annex 1, reference 67), a third clinical case of a massive overdose was described in the literature. A 52-year-old man ingested an estimated 31 g of a commercial product containing methidathion. He was found comatose and was treated with gastric lavage, pralidoxime, atropine, and prolonged mechanical ventilation. Six hours after ingestion, his plasma cholinesterase activity was 545 IU/ml (normal range, 5500-13 500). The signs had disappeared by day 8, and treatment was discontinued; however, the patient showed relapse of cholinergic symptoms, and the treatment was resumed. The patient recovered after 22 days. An electrophysiological study performed on day 15 showed some evidence of axonal polyneuropathy (no details reported). On day 30, the plasma cholinesterase activity was still low, at 3390 IU/ml. Three months after the incident, slight indication of peripheral neuropathy persisted, as evidenced by hyporeflexia and distal hypoaesthesia. The plasma cholinesterase activity had returned to 6150 IU/ml by day 90 (Cantais et al., 1993). In the case reported by Zoppellari et al. (1990, reviewed in Annex 1, reference 67), there was slight liver involvement, with transient jaundice (4.3 mg% of total bilirubinas compared with normal values up to 1.2 mg%) but no other biochemical indication of liver damage. Comments The NOAEL for behavioural changes in rats after a single oral dose was 3 mg/kg bw. The NOAEL for the inhibition of brain acetylcholinesterase activity measured 4 h after treatment was 1 mg/kg bw in males and 2.5 mg/kg bw in females. In another study of acute neurotoxicity in rats, changes in clinical signs, the results of a battery of functional observational tests, and maze activity were observed at the time of peak effect (about 2 h after treatment) at doses of 8 mg/kg bw and above in males and 4 mg/kg bw and above in females. Inhibition of acetylcholinesterase activity in various regions of the brain was found at doses of 4 mg/kg bw and above. Reduced acetylcholinesterase activity in the cortex and hyppocampus of a male treated with 1 mg/kg bw was not considered to be relevant. The overall NOAEL in this study was 1 mg/kg bw. In a human case of methidathion poisoning,, the estimated dose was more than 10 times the LD50 in rats. The patient recovered from the cholinergic toxicity and developed mild neuropathy; however, no details were given. The hepatic changes observed in short-term studies in dogs were not considered relevant for establishing an acute reference dose for humans, as no such changes were observed in the volunteer studies after repeated dosing for up to six weeks. Furthermore, no indication of hepatic toxicity (except slight, transient jaundice in one case) was seen in three cases of massive oral overdose which required atropine and oxime administration and assisted ventilation. The Meeting established an acute reference dose of 0.01 mg/kg bw, on the basis of an NOAEL in humans for inhibition of erythrocyte acetylcholinesterase activity of 0.11 mg/kg bw (highest dose tested) in a study reviewed by the 1992 JMPR (Annex I, reference 65), and a safety factor of 10. This value is supported by the NOAEL of 1 mg/kg bw in rats for the inhibition of brain acetylcholinesterase activity. Toxicological evaluation relevant for establishing an acute reference dose Levels that cause no toxic effect Rat: 1 mg/kg bw (single oral administration, inhibition of brain acetylcholinesterase activity) Human: 0.11 mg /kg bw per day (six-week study in volunteers, highest dose tested) Estimate of acute reference dose for humans 0.01 mg/kg bw References Cantais, I., Escarment, J., Ledantec, P., Deloffre, I., Stephannazi, J. & Quinot, J.-F. (1993) Poisoning by an organophosphate: Methidathion. JEUR, 6, 100-102 (in French). Chang, J.C.F. & Richter, A.G. (1994) Acute neurotoxicity study with methidathion technical in rats. Unpublished report No. F-00178, dated 15 February 1994, amended 22 February 1994, from Ciba-Geigy Corp., Farmington, Connecticut, USA. Submitted to WHO by Novartis Crop Protection AG, Basel, Switzerland. Glaza, S.M. (1994) Acute oral toxicity study of methidathion technical in rats. Unpublished report No. HWI 6117-235, dated 1 April 1994, amended 4 May 1994, from Hazleton Wisconsin, Madison, Wisconsin, USA. Submitted to WHO by Novartis Crop Protection AG, Basel, Switzerland. Leahy, C.L. (1993) Acute range finding neurotoxicity study with methidathion technical in rats. Unpublished report No. F-00177, dated 7 December 1993, amended 22 February 1994, from Ciba-Geigy Corp., Farmington, Connecticut, USA. Submitted to WHO by Novartis Crop Protection AG, Basel, Switzerland. Teitelman, U., Adler, M., Levy, I. & Dikstein, S. (1975) Treatment of massive poisoning by the organophosphate methidathion. Clin. Toxicol., 8, 277-282. Zoppellari, R., Targa, L., Tonini, P. & Zatelli, R. (1990) Acute poisoning with methidathion: A case. Hum. Exp. Toxicol., 9, 415-419.
See Also: Toxicological Abbreviations Methidathion (ICSC) Methidathion (WHO Pesticide Residues Series 2) Methidathion (WHO Pesticide Residues Series 5) Methidathion (Pesticide residues in food: 1979 evaluations) Methidathion (Pesticide residues in food: 1992 evaluations Part II Toxicology)