Comparative drug metabolism and disposition in minor species

The disposition of fenbendazole (FBZ) was studied in vivo in cattle (steers), goats, chickens, ducks, turkeys and rabbits, and in vitro in hepatic enzyme preparations from cattle, sheep, goats, rabbits, rats, chickens, ducks, turkeys and catfish. The major excretory metabolite when FBZ was administered either iv or po (5 mg/kg) was p-hydroxyfenbendazole (FBZ-OH). The sulfoxide (FBZ-SO) and sulfone (FBZ-SO2) appeared in plasma but were recovered in only trace amounts in urine or feces, and the amine (FBZ-NH2) was a minor metabolite appearing only occasionally in plasma. The greatest species differences were seen among the avian species, and differences in metabolite excretion correlated well with the ability of the species to metabolize the drug (especially to FBZ-OH) in vitro. Other in vitro studies measured the rate of oxidative, hydrolytic, and conjugative (glucuronide, acetate, sulfate) pathways in liver preparations.