Abstract

Finasteride, a prescription drug for the treatment of benign prostatic hypertrophy and alleviation of symptoms associated with benign prostatic hypertrophy and alleviation of symptoms associated with benign prostatic hypertrophy, has been shown to be metabolized in rat hepatic microsomes by hydroxylation at the t-butyl group (omega-OH finasteride), followed by further oxidation to the corresponding acid (omega-oic acid finasteride), with omega-aldehyde finasteride as an intermediate. In this study, we identified specific human cytochrome P450 (CYP) isozyme(s) involved in the in vitro metabolism of [14C]finasteride using CYP isozyme-selective inhibitors and microsomes containing specific recombinant human CYP isozymes (expressed in human AHH-1 TK+/-cells). Each of the three steps of the oxidative pathway was examined separately by using [14C]finasteride and its consecutive metabolites (omega-OH finasteride and omega-aldehyde finasteride) as substrates, and human liver microsomes or expressed recombinant CYP isozymes as the enzyme source. Gestodene, a mechanism-based inhibitor of CYP3A isozymes, showed a concentration-dependent inhibition of the oxidative metabolism of [14C]finasteride. In addition, the respective omega-OH finasteride and omega-oic acid finasteride metabolites were generated only by microsomes containing recombinant CYP3A4, but not the other isozymes (CYP1A1, CYP2B6, CYP2C8, CYP2C9, CYP2D6, and CYP2E1). Similar results were obtained for the oxidation of omega-OH finasteride to omega-aldehyde finasteride, suggesting that human CYP3A isozymes were involved in the oxidation of omega-OH finasteride. When omega-aldehyde finasteride was incubated with human liver microsomes in the presence of an NADPH regenerating system, both the omega-oic acid finasteride and the omega-OH finasteride were detected, suggesting that oxidative and reductive reactions were occurring simultaneously and that they were NADPH- or NADP-dependent. Inhibitors of CYP3A isozymes inhibited the oxidation of omega-aldehyde finasteride in a concentration-dependent manner; an increase in the reduction was also observed, presumably caused by inhibition of the competitive oxidative reaction. Other selective CYP inhibitors for CYP1A1/2 (alpha-naphthoflavone), CYP2C8-10 (sulfaphenazole), CYP2D6 (quinidine), and CYP2E1 (diallylsulfone) showed minor or no effects on both reactions. Consistent with these results, only microsomes containing human recombinant CYP3A4 catalyzed the oxidation of omega-aldehyde finasteride to omega-oic acid finasteride. These results indicate that the oxidation of omega-aldehyde finasteride was NADPH-dependent and was mediated at least in part by CYP3A4. In addition, NAD-dependent enzymes in cytosolic, microsomal, and mitochondrial fractions were capable of oxidizing omega-aldehyde finasteride to omega-oic acid finasteride. Other cellular fractions, particularly mitochondria, were shown to convert finasteride to omega-oic acid finasteride in a similar fashion.