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 i nhibitor of testosterone 5 alpha-reductase, was investigated using rat hepatic microsomes. The metabolism of finasteride by rat hepatic micr osomes was oxygen and NADPH-dependent, and addition of metyrapone, 7,8 -benzoflavone, and cytochrome c to the incubation mixture inhibited th e metabolism of finasteride. It is suggested that the metabolic reacti on of finasteride was mediated by a mixed function oxidase involving P -450. Four major metabolites were detected in vitro on incubating fina steride with hepatic microsomes of rats treated with phenobarbital (PB Ms), whereas two major metabolites were found in the incubation mixtu re with microsomes of untreated rats (UT-Ms). These metabolites were i solated and purified by solvent extraction and semi-preparative HPLC, and identified by MS spectrometry and NMR spectroscopy. The metabolite s consisted of omega-hydroxy finasteride (M-1), finasteride-omega-al ( M-2), finasteride-omega-oic acid (M3), 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 metabol ism of finasteride in the rat hepatic microsomes. Further, the major m etabolite 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 Ga-hydroxylation of the 4-azasteroid.