DIQUAT JMPR 1977 Explanation Diquat has been evaluated previously at the Joint Meeting in 1970, 1972, and 1976 (FAO/WHO, 1971, 1973, 1977). New data have become available on metabolism and excretion following diquat administration and a preliminary assessment of the toxicological significance of detected metabolites has been made. A new "no effect" level based on the oataractopenic activity of chronically administered diquat to rats is also presented. Details of additional toxicological studies on the cause of death following oral diquat administration to rats and data an the toxic effects of this compound to humans following ingestion are also included, in the following monograph addendum. Questions concerning residues in food and their evaluation were also considered and are discussed below. EVALUATION FOR ACCEPTABLE DAILY INTAKE BIOCHEMICAL ASPECTS Further studies on metabolism and excretion Following oral and parenteral administration of diquat to various laboratory species, the majority of the administered dose is excreted unchanged (Hughes et al., 1973; Mills, 1975). Excretion of metabolites in the urine after intraperitoneal injection was greater in the rabbit and guinea pig than in the rat (18%, 5% and < 1% of the dose respectively). These metabolites were not identified but were similar for the three species studies (Hughes et al., 1973). After oral administration of 14C-diquat (45 mg/kg b.w.) to rats most of the radioactivity was excreted in 4 days (6% of dose in urine, 89% of dose in faeces). Chromatography indicated that unchanged diquat was the major radioactive component of both the urine (5% of the dose) and faeces (at least 57% of the dose). The two metabolites identified were diquat monopyridone, (5% of the dose, mainly in the faeces) and diquat dipyridone as a minor urinary metabolite (approx. 0.1% of the dose) (Mills, 1975). Following subcutaneous injection of 14C-diquat (10 mg/kg b.w.) in rats, 87% of the administered dose was excreted in the urine and 5% in the faeces within 4 days. The urine contained mainly unchanged diquat (75% of the dose) together with diquat monopyridone (about 3% of the dose) and diquat dipyridone (about 6% of the dose) (Mills, 1975). Intestinal bacteria from rat caecal contents caused metabolism of about 10% of added diquat in a 24 hr incubation period, with the formation of some diquat monopyridone (Mills, 1975). TOXICOLOGICAL STUDIES Preliminary toxicological study with two metabolites Diquat monopyridone was the major metabolite following oral closing and could, therefore, be the toxicologically active species following administration by this route. However, the oral LD50 of this compound to rats was in excess of 4000 mg/kg, (maximum dose administered) and oral administration of 1000 mg/kg five days a week for two weeks to a group of 10 males and 10 females showed no clinical, haematological, biochemical or histopathological abnormalities, except for a decreased number of lymphocytes in both males and females (Parkinson, 1974). Preliminary data also suggest that diquat monopyridone is less toxic than diquat when administered subcutaneously. When diquat monopyridone and diquat were administered at equivalent doses of 90 µmoles/kg (i.e. 16 mg active ingredient/kg) to groups of 10 rats, the animals in the monopyridone group showed no adverse effects for the entire 14 day period studied with no deaths occurring. In the diquat group, however, all animals appeared subdued by day 3 and many animals developed abdominal distention. Nine animals of the original group of 10 were dead by the 14th day after dosing with the remaining animal exhibiting abdominal distention. When diquat dipyridone was administered at a dose of 90 µmoles/kg (i.e. 16 mg active ingredient/kg) to 10 rats, the animals showed no adverse effects for the entire 14 day period studied with no deaths occurring. For diquat the equivalent dose is approximately the subcutaneous LD50 (Crabtree, 1976), Special study on cataractogenesis A study was carried out with diquat dibromide monohydrate in which special attention was paid to the incidence of eye opacity. Groups of 35 male and 35 female rats were maintained for up to 2 years on diets containing 0, 15, 25 and 75 ppm diquat ion. Body weights, food consumption and deaths did not vary significantly in the four groups, but a significant increase in the number of cataracts was observed in the 75 ppm group when compared with the control group. There was no significant difference between the number of cataracts in rats fed at dietary levels of 15 and 25 ppm and those on control diets. The first eye lesions in animals fed 75 ppm diquat appeared within 9 months. In the period 9-18 months there was a further large increase in the number of cataracts in this group. The cataracts in the other experimental groups, especially in the 25 ppm group, appeared somewhat earlier than in the control group (see Table 1) (Rogerson and Broad, 1976). TABLE 1. No. of cataracts observed at 6, 9, 18 and 24 months. I(male) I(female) II(male) II(female) III(male) III(female) IV(male) IV(female) 6 months 0 0 0 0 0 0 2 0 9 months 0 0 0 1 0 1 12 14 18 months 3 2 4 3(1)* 4 8 24 19 2 years 5 6 4 2 5 8 25 21 * One annual with doubtful cataract at 18 months dead at 2 years. Special studies to determine the cause of death following oral administration of diquat to rats Histological changes in liver, myocardium, brain, kidneys and lung have been reported following oral dosing (Verbetskii and Pushkar, 1968) but none of the changes seen are severe enough to account for death (Clark and Hurst, 1970). However, recent work indicates that early deaths following administration of an oral LD50 are associated with a considerable redistribution of body water from the rest of the body into the lumen of the gastrointestinal tract. Dehydration of blood and other organs and tissues appears to be sufficiently rapid and severe to be consistent with development of shock and death due to peripheral circulatory failure. The cause of death following subcutaneous injection, however, appears to be different. The delayed deaths are associated with an increased water retention in the gastrointestinal tract, but also in many other tissues. Water accumulation in the gastroinstestinal tract was also observed after oral administration of paraquat (Crabtree at al., 1977). OBSERVATIONS IN HUMANS A few cases of human poisoning have been reported following accidental and suicidal ingestion of diquat in the form of the concentrate, RegloneR (Weirich, 1969; Oreopoulos and McEvoy, 1969; Schönborn et al., 1971; Okonek and Hofmann, 1975). Toxic symptoms vary but have included vomiting, diarrhoea and impaired kidney function. Changes in the brain, liver, kidney, lung and gastrointestinal tract have been noted post-mortem and signs of peripheral circulatory failure and incidence of shock have also been reported in fatal cases (Schönborn et al., 1971; Okonek and Hofmann, 1975). It appeared that haemodialysis is not an efficient technique for removing toxicologically relevant amounts of diquat from the organism, (Okonek and Hofmann, 1975). COMMENTS Two metabolites, diquat monopyridone and diquat dipyridone, have been identified after oral and subcutaneous administration of diquat to rats. The same type of metabolites were found after i.p. injection in rabbits, guinea pigs and rats. These metabolites are produced only in small amounts and are less toxic than diquat itself. It seems therefore that the parent compound is the toxicologically active substance. In a new long-term study with rats special attention was paid to the cataractogenic action of diquat. A significant increase in the number of cataracts was observed in the 75 ppm group. There was no significant difference, between the number of cataracts at dietary levels of 15 and 25 ppm and those on control diets. In this respect 25 ppm of diquat ion was found to be the no-effect level. However, the cataracts in the experimental groups, especially in the 25 PPM group, appeared somewhat earlier than in the control group. Cataracts have never been observed in humans following occupational exposure to diquat. TOXICOLOGICAL EVALUATION Level causing no toxicological effect Rat: 15 mg/kg diquat ion in the diet (fed as dibromide), equivalent to 0.75 diquat ion/kg bw Dog: 68 mg/kg diquat dichloride in the diet, corresponding to 1.22 mg diquat ion/kg bw ESTIMATE OF ACCEPTABLE DAILY INTAKE FOR HUMANS 0-0.008 mg/diquat ion/kg bw RESIDUES IN FOOD AND THEIR EVALUATION Some questions concerning diquat were raised at the 1977 Meeting of the Codex Committee on Pesticide Residues (ALINORM 78/24, para. 91-93). The use pattern and residue information on wheat and barley previously considered by the Joint Meeting in 1972 referred only to the desiccation of lodged crops leading to the harvesting, of grain which would be suitable only for animal feed. The meeting was informed that experimental work concerning residues of diquat in wheat and barley possibly useful for human consumption was still in progress. It was hoped that this work would be completed and reported by the middle of 1978 for consideration by the 1978 Joint Meeting. Data on residues in brown rice were not available to the Meeting. REFERENCES Clark, D.G. and Hurst, E.W. (1970) The toxicity of diquat. Br. J. Ind. Med., 27, 51-55 Crabtree, H.C. (1976) Comparison of the sub-cutaneous toxicity of diquat, diquat monopyridone and diquat dipyridone. ICI Central Toxicology Laboratory Data. (Unpublished report). Crabtree, H.C., Lock, E.A. and Rose, M.S., (1977) Effects of diquat on the gastrointestinal tract of rats. Toxicol. Appl. Pharmacol., 41, 585-595. Hughes, R.D., Millburn, P. and Williams, R.T. (1973) Biliary excretion of some diquaternary ammonium cations in the rat, guinea pig and rabbit. Biochem. J., 136, 979-984. Mills, I.H. (1975) Diquat: disposition and metabolism in the rat. ICI Ltd Central Toxicology, Report No. CTL/P/214 (Unpublished report). Okonek, S. and Hofmann, A. (1975) On the question of extracorporeal hemodialysis in diquat intoxication. Arch. Toxicol., 33. 251-257. Oreopoulos, D.G. and McEvoy, K. (1969) Diquat poisoning. Postgrad, Med. J., 45, 635-637. Parkinson, G.P. (1974) Diquat monopyridone: acute and subacute oral toxicity. ICI Ltd Central Toxicology Laboratory, Report No.: HO/CTL/P/122B. (Unpublished report). Rogerson, A. and Broad, R.D. (1976) Diquat dibromide: 2 year feeding study in rats. ICI Ltd. Pharmaceuticals Division, Report No. OTL/P/253. (Unpublished report). Schönborn H., Schuster, H.P. and Kössling, F.K. (1971) Klinik und Morphologie der akuten peroralen diquat Intoxikation (Reglone). Arch. Toxikol., 27, 204-216. Verbetskii, V.E. and Pushkar, M.S. (1968) Pathological changes in the organs of animals on acute poisoning with the herbicide diquat (Reglone). In "Some questions concerning human and animal morphology". Odessa, 51-52. Weirich, J. (1969) Intoxikation mit Diquat (Reglone). Deuts Gesundh., 24, 1986-1988. FAO/WHO (1971) 1970 evaluations of some pesticide residues in food. AGP:1970/M/12/1; WHO Food Add./71.42. FAO/WHO (1973) 1972 evaluations of some pesticide residues in food. AGP:1972/M/9/1; WHO Pesticide Residues Series, No. 2. FAO/WHO (1977) 1976 evaluations of some pesticide residues in food. AGP:1976/M/14.
See Also: Toxicological Abbreviations Diquat (HSG 52, 1991) Diquat (PIM 580F, French) Diquat (AGP:1970/M/12/1) Diquat (WHO Pesticide Residues Series 2) Diquat (Pesticide residues in food: 1976 evaluations) Diquat (Pesticide residues in food: 1978 evaluations) Diquat (Pesticide residues in food: 1993 evaluations Part II Toxicology)