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

RATIONALE: Understanding of the mechanisms of biotransformation of antidepressant 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 nefazodone, and the inhibitory activity of nefazodone and metabolites versus human P450-3A.

METHODS

Biotransformation of nefazodone to its metabolic products, and of meta-chlorophenylpiperazine (mCPP) to para-hydroxy-mCPP, was studied in vitro using human liver microsomes and heterologously expressed human cytochromes. Nefazodone and metabolites were also tested as inhibitors of alprazolam hydroxylation, reflecting activity of cytochrome P450-3A isoforms.

RESULTS

mCPP and two hydroxylated derivatives were the principal metabolites formed from nefazodone by liver microsomes. Metabolite production was strongly inhibited by ketoconazole or troleandomycin (relatively specific P450-3A inhibitors), and by an anti-P450-3A antibody. Only heterologously expressed human P450-3A4 mediated formation of nefazodone metabolites from the parent compound. Nefazodone, hydroxy-nefazodone, and para-hydroxy-nefazodone were strong 3A inhibitors, being more potent than norfluoxetine and fluvoxamine, but less potent than ketoconazole. The triazoledione metabolite and mCPP had weak or negligible 3A-inhibiting activity. Formation of parahydroxy-mCPP from mCPP was mediated by heterologously expressed P450-2D6; in liver microsomes, the reaction was strongly inhibitable by quinidine, a relatively specific 2D6 inhibitor.

CONCLUSION

The complex parallel biotransformation 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 pharmacokinetic drug interactions of nefazodone with 3A substrate drugs such as triazolam and alprazolam.