The European Agency for the Evaluation of Medicinal Products Veterinary Medicines Evaluation Unit EMEA/MRL/625/99-FINAL July 1999 COMMITTEE FOR VETERINARY MEDICINAL PRODUCTS MEBENDAZOLE SUMMARY REPORT (1) 1. Mebendazole is a benzimidazole anthelmintic which is used in both human and veterinary medicine. In veterinary medicine, it is administered orally to horses, at a target dose of 8.8 mg/kg bw and to sheep and goats at a target dose of 15 mg/kg bw. Mebendazole has also been used in game birds, pigs, deer, poultry and cattle, including lactating animals and laying birds, but these uses were not supported with regard to the establishment of MRLs. Mebendazole is authorised in a range of mono-preparations including premixes for medicated feed, pastes, tablets, liquids, granules, drenches and suspensions for oral administration. Mebendazole is also used in combination products additionally containing either metrifonate, closantel or minerals (selenium, cobalt). 2. The pharmacokinetics of mebendazole was studied in rats, mice, dogs, humans and several target species. In rats given oral doses in the range of 0.06 to 10 mg/kg bw 14C-mebendazole, most of the radioactivity was recovered from the organs of the gastrointestinal tract and consisted mostly of unmetabolised mebendazole. Less than 1% of the administered radioactivity was detected in blood. Excretion was predominantly via the faeces, with 70 to 90% of the faecal radioactivity consisting of unmetabolised mebendazole. In rat liver, 1 hour after administration, 15% of the radioactivity consisted of unmetabolised mebendazole. Four hours after administration, the percentage of mebendazole had declined to 1%. Absorption in humans was increased when the same dose was given with a meal. Plasma concentrations, 2 to 4 hours later, were in the range of 27 to 42 µg/l. Significantly higher concentrations were found in a patient with cholestasis. In humans, around 9% of an oral dose of 14C-mebendazole was excreted in urine and the remainder in faeces; the dose level was not stated in this study. Following intravenous and oral administration of 1.7 µg of 3H-mebendazole to an adult human volunteer, the oral bioavailability was estimated to be 17%. In a further 5 volunteers, the oral bioavailability was 22%. At higher, therapeutic dose levels, oral bioavailability was only around 2%. 3. In rats, dogs and humans, the major metabolite in urine was identified as 2-amino-1H- benzimidazol-5-yl)phenylmethanone, formed by carbamate hydrolysis of mebendazole. The major metabolite found in rat bile was methyl[5-(1-hydroxy-1-phenyl)methyl-1H-benzimidazol- 2-yl] carbamate, formed by ketoreduction of mebendazole. In in vitro studies using preparations derived from pig, rat and dog liver, the major metabolite was methyl[5-(1-hydroxy-1- phenyl)methyl-1H-benzimidazol-2-yl] carbamate which accounted for 50%, 58% and 93% of the total radioactivity in the extracts. The metabolite (2-amino-1H-benzimidazol-5- yl)phenylmethanone was present in pig liver extracts. Another in vitro study was carried out in hepatocytes and liver subcellular fractions of rat, dog, goat, sheep, horse, cattle and human. Two major metabolic pathways were identified: ketoreduction, leading to methyl[5-(1-hydroxy-1- phenyl)methyl-1H-benzimidazol-2-yl] carbamate, the major metabolite in rat, dog, goat, sheep, horse and cattle subcellular liver fractions, and carbamate hydrolysis to (2-amino-1H- benzimidazol-5-yl)phenylmethanone, an important metabolite following incubations with horse and human hepatocytes. 7 Westferry Circus, Canary Wharf, London, E14 4HB, UK Tel. (44-20) 74 18 84 00 Fax (44-20) 74 18 84 47 E-mail: [email protected] http://www.emea.eu.int EMEA 2001 Reproduction and/or distribution of this document is authorised for non commercial purposes only provided the EMEA is acknowledged 4. The metabolism of mebendazole was similar to that of its fluoro-derivative, flubendazole and involved ketoreduction, carbamate hydrolysis and conjugation as the main metabolic pathways. 5. The oral LD50 values of mebendazole were 714 and 1434 mg/kg bw in female and male Wistar rats, respectively. The acute oral LD50 values in Swiss Albino mice, guinea pigs and New Zealand White rabbits were all greater than 1280 mg/kg bw. Diarrhoea and emesis were the most frequent signs of toxicity in the oral studies. After intramuscular injection, there were no deaths in Wistar rats given doses up to 350 mg/kg bw nor in Beagles given up to 2000 mg/kg bw; the overt signs of toxicity in these studies included head-twitching, scratching, exophthalmos and piloerection. 6. In a 13-week study, groups of Wistar rats were fed diets equivalent to 0, 7.8, 32.1 or 127.3 mg/kg bw/day in males and 8.4, 33.7 or 151.6 mg/kg bw/day in females, respectively. At the top dose level, mortality was increased, body weight gain was reduced and changes indicative of anaemia were observed in haematology values. Serum alkaline phosphatase concentrations were significantly increased at the top dose. At termination, mean gonad weights were significantly reduced in both the mid- and top-dose group. Histopathological changes found in the testes of the top dose group included tubular degeneration and desquamation and the absence of spermatogenesis. Relative liver weights were significantly increased in the mid- and top-dose groups. Histopathological examination revealed centrilobular swelling, hepatocellular vacuolation and bile duct proliferation. The NOELs were 7.8 and 8.4 mg/kg bw/day in males and females, respectively. Beagle dogs were given oral doses of 0, 2.5 or 10 mg/kg bw/day of mebendazole, in gelatin capsules, 6 days per week, for 13 weeks. Another group of dogs was given 0.63 mg/kg bw/day from weeks 1 to 7 followed by 40 mg/kg bw/day from weeks 8 to 13. In the group given 10 mg/kg bw/day, haemoglobin and erythrocyte counts were reduced, serum alkaline phosphatase, bilirubin, cholesterol and total protein concentrations were significantly increased and relative liver weight was significantly increased. Similar though less marked effects were observed in the group given 0.63 and then 40 mg/kg bw/day. There were no pathological findings attributable to the test substance. The NOEL was 2.5 mg/kg bw/day. In another study, Beagle dogs received oral doses of 0, 2.5, 10 or 40 mg mebendazole/kg bw/day in gelatin capsules, 6 days per week, for 24 months. One female dog died during the first week; the death was attributed to enteritis and did not seem to be substance-related. There were no overt signs of toxicity and no substance-related effects on body weight, electrocardiogram, heart rate or blood pressure. There were considerable fluctuations in haematology and clinical chemistry values but no consistent dose-related trends. Liver weights were significantly increased in the 10 mg/kg bw group but not at 40 mg/kg bw. There were no pathological findings attributable to treatment. Drug blood concentrations were not monitored in these studies so there is no explanation for the inconsistent results. 7. In a fertility study, groups of rats were fed diets corresponding to 0, 5 10, 20 or 40 mg/kg bw/day of mebendazole. Treated males were mated with untreated females and vice-versa. Treatment of the females continued up to day 22 of gestation when the uterine contents were examined. There were no substance-related effects on fertility and no evidence of embryolethality. 8. In a 3-generation study in rats, the F0 and F1 dams were fed diets corresponding to 0, 2.5 or 10 mg/kg bw/day of mebendazole from day 6 to 15 of gestation. There was no evidence of maternal toxicity, foetotoxicity or teratogenicity. However the study was inadequate as a multigeneration study because of the restricted treatment regimen. 9. In a peri/post-natal study, pregnant female rats were fed diets corresponding to 0, 5, 10, 20 or 40 mg/kg bw/day of mebendazole from day 16 of gestation up to the end of lactation. The dose of 40 mg/kg bw was maternally toxic causing reduced body weight gain. The NOEL for maternal toxicity was 20 mg/kg bw. The dose of 20 mg/kg bw was foetotoxic causing reduced litter size, an increased incidence of stillbirths and reduced pup weights. The NOEL based on foetotoxicity was 10 mg/kg bw/day. 2/9 EMEA 2001 10. Mated Wistar rats were given mebendazole in the feed at doses of 0, 2.5, 10, 40 or 160 mg/kg bw/day from day 6 to 15 of gestation. Other groups were given 0.63, 2.5, 5, 10 or 40 mg/kg bw by gavage on days 7, 8, 9 or 10 of gestation. When mebendazole was given in the feed, 10 mg/kg bw/day was a NOEL for foetotoxicity and teratogenicity. However, there was clear evidence of teratogenicity, when 10 mg/kg bw was given by gavage with 5 and 17 malformed foetuses following treatment on days 9 and 10, respectively. The malformations included exencephaly, tail malformations, coelosomy, malformed ribs and legs, and scoliosis. Teratogenicity was most marked after treatment on day 10 of gestation. The numbers of malformations in the 2.5 and 5 mg/kg bw groups were not significantly different from controls. In a second rat teratology study, the dams received 0, 2.5, 10 or 40 mg/kg bw/day from day 6 to 15 of gestation. Maternal toxicity was observed in all treated groups. There were no live foetuses in the 40 mg/kg bw group. Litter size and pup weight were reduced at 10 mg/kg bw. Thirty one out of 133 foetuses in the 10 mg/kg bw group were malformed.