The oxidative metabolism of fenbendazole: a comparative study.

The oxidative metabolism of fenbendazole (FBZ) was studied in hepatic fractions prepared from livers of cattle, sheep, goats, chickens, ducks, turkeys, rats, rabbits and catfish. All species produced the sulfoxide metabolite (oxfendazole; FBZ-SO), and p-hydroxyfenbendazole (FBZ-OH) was produced by all species except sheep. The product of demethoxycarbonylation, fenbendazole amine (FBZ-NH2), was not produced by liver preparations of any species. A fourth metabolite, resulting from the further oxidation of oxfendazole, fenbendazole sulfone (FBZ-SO2), was formed in all species but at highly varying rates. The chicken exhibited the highest overall rate of FBZ metabolism, followed by the duck, goat, sheep, steer, catfish, rat, rabbit, and turkey. Considerable variation was evident among avian species, the duck and turkey produced substantially less of the FBZ-OH and FBZ-SO2 metabolites than the chicken. Catfish liver preparations formed equivalent amounts of metabolite at 25 degrees C and 37 degrees C incubation temperatures. The formation of the sulfone metabolite (FBZ-SO2), however, was practically nonexistent in catfish.