FEBANTEL, FENBENDAZOLE AND OXFENDAZOLE First draft prepared by Dr R. Fuchs Department of Experimental Toxicology and Ecotoxicology Institute for Medical Research and Occupational Health Zagreb, Croatia Explanation Biological data Toxicological studies Febantel Fenbendazole Oxfendazole Comments Evaluation References 1. EXPLANATION Febantel, fenbendazole and oxfendazole were previously evaluated at the thirty-eighth meeting of the Committee (Annex 1, reference 97). A temporary ADI of 0-25 µg/kg bw was established for fenbendazole at that time, based on a NOEL of 5 mg/kg bw/day in a long-term toxicity/ carcinogenicity study in rats and a safety factor of 200. Additional information on the mechanism of the observed increased incidence of liver tumours in female rats at high doses, including the results of an in vivo DNA binding study, was requested. A temporary ADI of 0-10 µg/kg bw was established for febantel at the thirty-eighth meeting, based on a NOEL of 2 mg/kg bw/day in a 2-generation reproductive toxicity study in rats and a safety factor of 200. Additional information on the genotoxic and carcinogenic potential of febantel, including the results of an in vivo DNA binding assay in rats, was requested. A temporary ADI of 0-4 µg/kg bw was established for oxfendazole at the thirty-eighth meeting, based on a NOEL of 0.7 mg/kg bw/day in a carcinogenicity study in rats and a safety factor of 200. Additional data from genotoxicity and teratogenicity studies were requested, including the results of an in vivo DNA binding assay in rats. At its present meeting, the Committee considered the data available at the thirty-eighth Committee meeting (Annex 1, reference 97), together with the results of additional genotoxicity study on fenbendazole, which are summarized in this monograph addendum. 2. BIOLOGICAL DATA 2.1 Toxicological studies 2.1.1 Febantel Combined long-term toxicity/carcinogenicity studies on febantel in rats and mice did not reveal increased incidences of any tumour types (Bomhard & Kaliner, 1985; Bombard & Mager, 1987; Annex 1, reference 98). No evidence of a distinct mutagenic potential of febantel could be derived from a variety of mutagenicity tests. Febantel was positive in the dominant lethal test in male mice only at the extremely high dose of 2000 mg/kg bw/day given twice daily at 24 h intervals, but was negative at the lower test dose of 500 mg/kg bw/day. This level was far in excess of the therapeutic dose (Machemer & Dycka, 1976; Annex 1, reference 98). As febantel had no point mutagenic effect, a DNA binding assay was not deemed necessary. 2.1.2 Fenbendazole 2.1.2.1 Long-term toxicity/carcinogenicity studies There were no increase in tumour incidence in a 2-year carcinogenicity study in mice with doses of fenbendazole up to 405 mg/kg bw/day (Goldenthal, 1980; Annex 1, reference 98). A lifetime toxicity/carcinogenicity study (including an in utero phase) of fenbendazole in Charles River CD rats used the F1 generation of pups that had been exposed to the same dose levels in utero. Fenbendazole was fed in the diet at dose levels of 0, 5, 15, 45 or 135 mg/kg bw/day. In utero exposure to 45 mg/kg bw/day and 135 mg/kg bw/day caused severe toxicity in the pups characterized by decreased body weights, diarrhoea, bloated stomachs, icterus and alopecia. F1 pups exposed to 135 mg/kg bw/day had 33% decreased body weights as compared to controls, while pups exposed to 45 mg/kg bw/day were 15% below controls. There was also increased mortality, 18% of pups exposed to 135 mg/kg bw/day and 14% of the pups exposed to 45 mg/kg bw/day died by day 21 of age as compared to 6% in the controls. These data suggest that pups at 45 and 135 mg/kg bw/day had been dosed above the MTD prior to the start of the lifetime study. This toxicity and increased neonatal mortality severely limit the conclusions that can be drawn from this study. The histopathological changes observed in the liver of treated animals were reported as hepatocellular hypertrophy, vacuolation and bile duct proliferation in the 15, 45 and 135 mg/kg bw/day groups, hepatocellular hyperplasia and biliary cysts in the 45 and 135 mg/kg bw/day groups, and hepatocellular adenomas and carcinomas in the 135 mg/kg bw/day group. A group of independent pathologists reviewed the histopathologic findings and concluded that fenbendazole treatment did not result in an increased incidence of hepatocellular neoplasms (Goldenthal, 1981; Lewis, 1982; Brown, 1982; Annex 1, reference 98). 2.1.2.2 Special studies on genotoxicity The results of additional genotoxicity studies on fenbendazole are given in Table 1. Table 1. Results of genotoxicity studies on fenbendazole Test system Test object Concentration Results Reference Forward mutation Mouse up to 100 µg/ml Negative Den Boer & assay1 lymphoma Horn, 1986 1 Both with and without rat liver S9. Fenbendazole was assayed in L5178Y TK+/- mouse lymphoma cells using doses ranging from 2.5 to 10 µg/ml without activation and from 4.0 to 10 µg/ml with metabolic activation. The observed toxicity ranged from low to moderate. The substance showed steep toxicity so that treatment in the range of 10 to 20% relative growth was not possible. All of the treatment, both with and without metabolic activation, induced mutation frequencies that were below the minimum criterion of mutagenesis. The authors concluded that fenbendazole was not mutagenic in the mouse lymphoma assay (Den Boer & Horn, 1986). However, the test concentrations used in this study were considerably lower than those used in the previous study when concentrations up to 62 µg/ml were tested (Cifone & Myhr, 1983). In the opinion of independent experts, a DNA binding study would not result in further clarification of the clastogenic effects of fenbendazole in vitro on V79 cells, but would merely show whether there might be an additional potential for point mutations (letter from professor C. Schlatter, Institute for Toxicology, Zurich, Switzerland, to Dr Müller, Hoechst AG, 1991; submitted to WHO by Hoechst AG, Frankfurt, Germany). Genotoxicity studies on all essential genetic endpoints gave negative results. Clastogenic effects of benzimidazoles in vitro are related to the inhibition of tubulin formation. In vivo assays for binding to DNA in liver following the oral administration of fenbendazole have not been performed. In an expert review on the toxicology of fenbendazole, the authors concluded that it showed no mutagenic, genotoxic or carcinogenic potential and that sufficient data were available to establish an ADI for fenbendazole residues in animal-derived food (Bolt & Gansevendt, 1991). 2.1.3 Oxfendazole Oxfendazole has been tested only in a bacterial mutation assay that gave negative results (Mourot, 1990; Annex 1, reference 98). The doses used in the oral rat carcinogenicity study were selected on the basis of results of a 3-month oral dosing study in rats. Based on increased mortality at 600 mg/kg (equal to 48 mg/kg bw/day for males and 50 mg/kg bw/day for females), and elevated ALP levels at 200 mg/kg (equal to 17 mg/kg bw/day for males and 18 mg/kg bw/day for females), a level of 100 mg/kg (equal to 7.4 mg/kg bw/day for males and 7.8 mg/kg bw/day for females) was determined to be the maximum tolerated dose that could be administered over a 2-year lifespan of rats without compromising survival (these doses have been corrected and do not correspond exactly to those given in Food Additives Series 29 (Annex 1, reference 98). In addition, in the oral carcinogenicity study in rats, histopathologic findings of hepatocellular vacuolation in the liver of rats fed 30 mg/kg of feed (equal to 2.0 mg/kg bw/day for males and 2.4 mg/kg bw/day for females), and 100 mg/kg of feed (equal to 6.6 mg/kg bw/day for males and 7.8 mg/kg bw/day for females) further suggested that there was sufficient systemic exposure to oxfendazole to show that it had been tested adequately. There was no carcinogenic effect of oxfendazole at any dose level (De Pass & Bidlack, 1987; Annex 1, reference 98). The thirty-eighth Committee (Annex 1, reference 97) requested a teratogenicity study in rabbits using oxfendazole at sufficiently high doses to explore its teratogenic potential. This study was not performed. 3. COMMENTS Febantel and fenbendazole have been tested in a range of genotoxicity assays, while oxfendazole has been tested only in an Ames test in which four strains of Salmonella typhimurium were used. The compounds consistently gave negative results in the Ames test, a test for DNA repair and in vivo cytogenetic assays. Febantel, fenbendazole and a metabolite (2-amino-5-phenylsulfinyl-2- benzimidazole) were weakly positive in the mouse lymphoma forward mutation assay in the presence of a metabolic activation system. Febantel, when tested at a sufficiently high dose to reduce fertility, was found to induce dominant lethal mutations in mice. The present Committee noted that available data on the mode of action of benzimidazoles as inhibitors of the polymerization of tubulin to microtubulin, with consequent disruption of mitosis, together with the results of genotoxicity tests, do not provide any evidence of a direct interaction of these compounds with DNA. It therefore concluded that the positive results in some genotoxicity assays are likely to have been caused by an indirect mechanism, and that further studies on the binding of febantel, fenbendazole and oxfendazole to DNA are not required. The present Committee re-evaluated the long-term toxicity/ carcinogenicity studies on all three compounds that were reviewed at the thirty-eighth meeting, and reached the conclusions given below. Febantel. Combined long-term toxicity/carcinogenicity studies on febantel in rats and mice did not reveal increased incidences of any types of tumour. At its thirty-eighth meeting (Annex 1, reference 97), the Committee was of the opinion that higher doses could have been used in the carcinogenicity study in rats. However, in the group of rats given febantel at 500 mg/kg in the diet (equal to 40 mg/kg bw/day), decreases in body-weight gain, slight anaemia (in females only), increased liver weight, hepatocellular enlargement and vacuolization, and significant increases in serum ALP activity were observed. These changes were not seen at the lower doses or in the control groups. The present Committee considered that these changes indicated that the dose levels used were sufficient for the carcinogenic potential of febantel to be assessed, and concluded that there was no evidence that it was carcinogenic in mice or rats. Fenbendazole. In a lifetime study with an initial in utero phase, rats were dosed with fenbendazole at doses of 0, 5, 15, 45 or 135 mg of fenbendazole/kg bw/day. There were no increases in tumour incidence at 5, 15 or 45 mg/kg bw/day. At 135 mg/kg bw/day only a marginal increase in the incidence of liver tumours was observed. At the beginning of the lifetime study in rats, the mean body weights of the highest-dose group (84 g for males and 79 g for females) were more than 40% lower than those of the controls (142 g for males and 123 g for females). Furthermore, the rats in the highest-dose group showed signs of toxicity at that time, including alopecia, icterus, diarrhoea, and lethargy. The Committee considered that the results for this group could not be used for the assessment of the carcinogenicity of fenbendazole because the maximum tolerated dose had been exceeded in utero. However, proliferative liver lesions in the group receiving 45 mg/kg bw/day, described as hepatocellular hyperplasia and biliary cysts, which were not observed at lower doses, indicated that this dose level was high enough to permit an assessment of the carcinogenic potential of fenbendazole. On the basis of this study in rats and the negative results in the carcinogenicity study in mice reviewed at the thirty-eighth meeting (Annex 1, reference 97), the Committee concluded that there was no evidence that fenbendazole possessed carcinogenic potential. Oxfendazole. The highest dose used in the rat carcinogenicity study was selected in the light of the results of a 3-month oral toxicity study in rats. It was based on increased mortality at a dose level of 600 mg/kg in the diet (equal to 48 mg/kg bw/day for males and 50 mg/kg bw/day for females) and elevated ALP levels at a dose level of 200 mg/kg in the diet (equal to 17 mg/kg bw/day for males and 18 mg/kg bw/day for females). A dose level of 100 mg/kg in the diet (equal to 7.4 mg/kg bw/day for males and 7.8 mg/kg bw/day for females) was selected as the highest dose in the carcinogenicity study. No carcinogenic effects were seen at any dose level in this study. The present Committee considered that histopathological findings of hepatocellular vacuolation in the liver of rats fed more than 2 mg/kg bw/day provided evidence of sufficient systemic exposure to oxfendazole, and concluded that the study was adequate for assessing the carcinogenic potential of the compound. On the basis of this study and the negative results in the carcinogenicity study in mice reviewed at the thirty-eighth meeting (Annex 1, reference 97), the present Committee concluded that there was no evidence that oxfendazole possessed carcinogenic potential. At its thirty-eighth meeting (Annex 1, reference 97), the Committee had requested an additional teratogenicity study in rabbits using oxfendazole. This was not provided at the present meeting. Instead, the sponsor suggested that the available reproductive data in sheep were sufficient. The Committee concluded, however, that the study in sheep did not adequately explore the teratogenic potential of oxfendazole, because it was designed to assess the safety of the drug in the target species, not to study its teratogenicity. 4. EVALUATION The available data from reproductive and teratogenicity studies with each of the three drugs suggest that oxfendazole is the most potent in producing embryotoxic effects. The data on fenbendazole suggest that the rabbit is the species most sensitive to the embryotoxic and teratogenic effects of these three compounds. The Committee therefore decided that an adequate teratogenicity study in rabbits with oxfendazole was still required. The Committee noted that, on the basis of the available toxicological data, the NOELs identified at the thirty-eighth meeting and a safety factor of 100, ADIs of 0-20 and 0-50 µg/kg bw could have been established for febantel and fenbendazole, respectively. However, it also noted that febantel, fenbendazole and oxfendazole have common metabolic routes, and that the residue data demonstrate that oxfendazole is the major residue in food-producing animals following administration of these three compounds. Therefore, the Committee assigned a group temporary ADI of 0-4 µg/kg bw to febantel, fenbendazole and oxfendazole based on the NOEL of 0.7 mg/kg bw/day for oxfendazole identified at the thirty-eighth meeting and a safety factor of 200 (Annex 1, reference 97). The results of a teratogenicity study in rabbits, in which oxfendazole is administered at sufficiently high doses for its teratogenic potential to be adequately explored, are required by the Committee for evaluation in 1998. 5. REFERENCES Bolt HM & Gansevendt B (1991). Evaluation of the toxicology of fenbendazole. Unpublished report from the Institut fur Arbeitsphysiologie, Universitat Dortmund, Dortmund, Germany. Submitted to WHO, by Hoechst AC, Frankfurt, Germany. Bomhard, E & Kaliner G (1985). BAY Vh 5757 chronic toxicity and carcinogenicity studies in mice. 21-month feeding study. Unpublished report No. 13961 from Bayer AG. Submitted to WHO by Bayer AG, Leverkusen, Germany. Bomhard E & Mager H (1987). BAY Vh 5757 combined chronic toxicity and carcinogenicity studies in Wistar rats. Unpublished report No. 15844 from Bayer AG. Submitted to WHO by Bayer AG, Leverkusen, Germany. Brown WR (1982). Lifetime oral toxicity study of fenbendazole in rats, histopathology - liver. Research pathology Serices Inc., New Britain, PA, USA. Unpublished report. Submitted to WHO by Hoechst AG, Frankfurt am Main, Germany. Cifone MA & Myhr BC (1983). Mutagenicity evaluation of BAY L5156 in the mouse lymphoma forward mutation assay. Unpublished project No. 10989 from Litton Bionetics Inc. Kensington, MD, USA. Submitted to WHO by Bayer AG, Leverkusen, Germany De Pass LR & Bidlack DE (1987) Carcinogenicity study in rats with RS-8858 (oxfendazole) mixed in the feed. Unpublished report No. 53-R-83-8858-PO-CA from Syntex Inc. Palo Alto, CA, USA. Submitted to WHO by Syntex Inc. Palo Alto, CA, USA. Den Boer WC & Horn AJW (986) Mutagenic evaluation of S711881 in the L 5178 Y TK +/- mouse lymphoma forward mutation assay. Unpublished report from Hazleton Biotechnologies, Veenendaal, Netherlands No. 1003E. Submitted to WHO by Hoechst AG, Frankfurt, Germany. Goldenthal EJ (1980) 24-month oral carcinogenicity study in mice. Unpublished report, International Research and Development Corporation, Mattawan, NJ, USA. Submitted to WHO by Hoechst AG, Frankfurt am Main, Germany. Goldenthal EI (1981) Lifetime oral toxicity study in rats. Unpublished report. International Research and Development Corporation, Mattawan, MI, USA. Submitted to WHO by Hoechst AG, Frankfurt am Main, Germany. Lewis JC (1982) Review and conclusions of the histopathology data of "Lifetime oral toxicity study of fenbendazole in rats". Unpublished report No. 102E from Hoechst AG, Frankfurt, Germany. Submitted to WHO by Hoechst AG. Machemer L & Dycka G (1976) BAY Vh 5757 dominant lethal test on male mice to test for mutagenic effects. Unpublished report No. 6031 from Bayer A.G. Submitted to WHO by Bayer AG Leverkusen, Germany. Mourot D (1990) Oxfendazole Ames test. Unpublished report from Laboratoire des médicaments vétérinaires, La Haute-Manche-Jarene, Fougères. Submitted to WHO by Centre National d'études vétérinaires et alimentaires, Javene, Fougères, France.
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