In vitro biotransformation of finasteride in rat hepatic microsomes. Isolation and characterization of metabolites.

Metabolism of finasteride ([N-(1,1-dimethylethyl)-3-oxo-4-aza-5 alpha-androst-1-ene-17 beta- carboxamide]; MK-906), a new type of specific inhibitor of testosterone 5 alpha-reductase, was investigated using rat hepatic microsomes. The metabolism of finasteride by rat hepatic microsomes was oxygen- and NADPH-dependent, and addition of metyrapone, 7,8-benzoflavone, and cytochrome c to the incubation mixture inhibited the metabolism of finasteride. It is suggested that the metabolic reaction of finasteride was mediated by a mixed function oxidase involving P-450. Four major metabolites were detected in vitro on incubating finasteride with hepatic microsomes of rats treated with phenobarbital (PB-Ms), whereas two major metabolites were found in the incubation mixture with microsomes of untreated rats (UT-Ms). These metabolites were isolated and purified by solvent extraction and semi-preparative HPLC, and identified by MS spectrometry and NMR spectroscopy. The metabolites consisted of omega-hydroxy finasteride (M-1), finasteride-omega-al (M-2), finasteride-omega-oic acid (M-3), and 6 alpha-OH finasteride (M-4). M-1 and M-4 are the major metabolites in UT-Ms, and M-1 and M-3 in PB-Ms. These studies revealed that hydroxylation of the t-butyl group and ring hydroxylation at the 6-position were key steps in the metabolism of finasteride in the rat hepatic microsomes. Further, the major metabolite M-4 was hydroxylated at the 6 alpha-position, but not at the 6 beta-position of the drug. This finding suggests the existence of a novel enzyme that catalyzes the 6 alpha-hydroxylation of the 4-azasteroid.