The metabolism of clozapine by human liver has been investigated in vitro. Irreversible protein-binding and conjunction with model nucleophiles have been used as markers for bioactivation of clozapine, while stable metabolite formation has been assessed using radiometric HPLC. In all nine liver microsomal preparations investigated, clozapine was extensively metabolized to the stable products desmethylclozapine (range 19%-27.2%), N-oxide (1.5-20.5%) and three polar metabolites (0-20.8%), and was bioactivated to a protein-reactive metabolite (0.6-2.1%). The CYP2D6 genotype did not influence the capacity of the livers to form these metabolites. All metabolic pathways were inhibited by ketoconazole, indicating the involvement of the cytochrome P450 enzymes. Isozyme-selective inhibitor studies demonstrated that whereas demethylation was performed by CYP1A2, N-oxidation and chemically reactive metabolite formation were dependent upon multiple forms of P450. The N-oxide was readily reduced back to clozapine in the presence of NADPH, this conversion being inhibited by ascorbic acid. Glutathione (1 mM) decreased covalent binding by 70%. The amount of putative adduct formed in the presence of glutathione (13.4 +/- 0.9%) was much greater than the covalent binding (mean 1.1 +/- 0.2%). The bioactivation of clozapine was, like the N-oxidation of clozapine, a reversible process. In summary, our results indicate clozapine undergoes extensive metabolism by human liver to both stable and chemically reactive metabolites, the formation of which is catalyzed by the cytochrome P450 enzymes. The role of the reactive metabolite, which may be a free radical, in the pathogenesis of clozapine agranulocytosis and hepatotoxicity requires further study.