Potentiation of the anthelmintic action of benzimidazole carbamates, such as fenbendazole [methyl 5(6)-(phenylthio)-1H-benzimidazol-2-ylcarbamate], has been noted during concurrent administration of benzimidazoles that possess no intrinsic anthelmintic activity. This study investigated the possibility that inhibition of P450 enzymes by fenbendazole and its metabolites could play a role in the potentiation phenomenon. Fenbendazole underwent P450-mediated oxidation in microsomes from untreated rat liver to the sulfoxide and (4'-hydroxyphenyl)thio metabolites [2.92 and 2.87 nmol/(mg of protein.h)]. Pretreatment of rats with phenobarbital or dexamethasone enhanced sulfoxidation by 1.9- and 2.9-fold, respectively. 4'-Hydroxylation was increased slightly (by 28%) by phenobarbital and decreased slightly (by 41%) by dexamethasone. Induction also promoted further metabolism of the sulfoxide to fenbendazole sulfone. Immunoinhibition and chemical inhibition studies suggested that P450 3A proteins and the flavin-containing monooxygenase are involved in sulfoxide and sulfone formation whereas 4'-hydroxylation involved the P450s 2C11, 2C6, and 2B1, depending on the type of induction. In untreated rat liver, the sulfoxide and (4'-hydroxyphenyl)thio metabolites of fenbendazole were relatively potent inhibitors of P450-mediated androstenedione 16 alpha-, 16 beta-, and 6 beta-hydroxylation (IC50 values of 42, 36, and 74 microM, respectively); 7 alpha-hydroxylase activity was uninhibited. In contrast, fenbendazole and its sulfone metabolite were not inhibitors of these reactions. Mixed-function oxidase activities in phenobarbital-induced rat hepatic microsomes were refractory to inhibition by most compounds, but P450 1A1 mediated activities in microsomes from beta-naphthoflavone-induced rat liver were quite susceptible to inhibition by fenbendazole sulfoxide. Studies with two analogous sulfoxides yielded similar findings.(ABSTRACT TRUNCATED AT 250 WORDS)