Nefazodone, meta-chlorophenylpiperazine, and their metabolites in vitro: cytochromes mediating transformation, and P450-3A4 inhibitory actions

Rationale: Understanding of the mechanisms of biotransformation of antidepr essant drugs, and of their capacity to interact with other medications, is of direct relevance to rational clinical psychopharmacology. Objectives: To determine the human cytochromes P450 mediating the metabolism of nefazodon e, and the inhibitory activity of nefazodone and metabolites versus human P 450-3A. Methods: Biotransformation of nefazodone to its metabolic products, and of meta-chlorophenylpiperazine (mCPP) to para-hydroxy-mCPP, was studie d in vitro using human liver microsomes and heterologously expressed human cytochromes. Nefazodone and metabolites were also tested as inhibitors of a lprazolam hydroxylation, reflecting activity of cytochrome P450-3A isoforms . Results: mCPP and two hydroxylated derivatives were the principal metabol ites formed from nefazodone by liver microsomes. Metabolite production was strongly inhibited by ketoconazole or troleandomycin (relatively specific P 450-3A inhibitors), and by an anti-P450-3A antibody. Only heterologously ex pressed human P450-3A4 mediated formation of nefazodone metabolites from th e parent compound. Nefazodone, hydroxy-nefazodone, and para-hydroxy-nefazod one were strong 3A inhibitors, being more potent than norfluoxetine and flu voxamine, but less potent than ketoconazole. The triazoledione metabolite a nd mCPP had weak or negligible 3A-inhibiting activity. Formation of para-hy droxy-mCPP from mCPP was mediated by heterologously expressed P450-2D6; in liver microsomes, the reaction was strongly inhibitable by quinidine, a rel atively specific 2D6 inhibitor. Conclusion. The complex parallel biotransfo rmation pathways of nefazodone are mediated mainly by human cytochrome P450 -3A, whereas clearance of mCPP is mediated by P450-2D6. Nefazodone and two of its hydroxylated metabolites are potent 3A inhibitors, accounting for ph armacokinetic drug interactions of nefazodone with 3A substrate drugs such as triazolam and alprazolam.