Identification of cytochrome P450 enzymes involved in the metabolism of zotepine, an antipsychotic drug, in human liver microsomes

1. Studies using human liver microsomes and recombinant human cytochrome P4 50 (P450) enzymes and flavin-containing monooxygenase (FMO) were performed to identify the enzymes responsible for the formation of zotepine metabolit es in man. 2. Human liver microsomes produced four metabolites and a tentative order o f importance was: norzotepine, 3-hydroxyzotepine, zotepine S-oxide and 2-hy droxyzotepine. Zotepine N-oxide was also detected, but it could not bt quan tified. 3. The rates of formation of the major metabolite, norzotepine, and zotepin e S-oxide (at a substrate concentration of 20 mu M) were significantly corr elated with the testosterone 6 beta-hydroxylase activities and CYP3A4 conte nts of the 12 different human liver microsomal samples. Inhibition studies with P450 enzyme selective inhibitors and anti-rat CYP3A2 antibodies also i ndicated a predominant role of CYP3A4 in the formation of norzotepine and z otepine S-oxide. Furafylline and sulphaphenazole inhibited the N-demethylat ion of zotepine by up to similar to 30%. 4. Correlation and inhibition data for the 2- and 3-hydroxylation of zotepi ne were consistent with the predominant role of CYP1A2 and 2D6 in the forma tion of these metabolites, respectively. 5. Recombinant CYP1A1, 1A2, 2B6, 2C19, 3A4 and 3A5 efficiently catalysed N- demethylation of zotepine. CYP1A1, 1A2, 2B6 and 3A4 were also active for S- oxidation. CYP1A2 and 2D6*1-Val374 efficiently produced 2-hydroxyzotepine a nd 3-hyroxyzotepine, respectively. Recombinant human FMO3 did not catalyse zotepine S-oxidation. 6. These results suggest that both the N-demethylation and S-oxidation of z otepine are mediated mainly by CYP3A4, and that CYP1A2 and 2D6 play an impo rtant role in the 2- and 3-hydroxylation of zotepine, respectively.