Flavin-containing monooxygenase-mediated metabolism of N-deacetyl ketoconazole by rat hepatic microsomes

Although ketoconazole is extensively metabolized by hepatic microsomal enzy mes, the route of formation and toxicity of suspected metabolites are large ly unknown. Reports indicate that N-deacetyl ketoconazole (DAK) is a major initial metabolite in mice. DAK may be susceptible to successive oxidative attacks on the N-1 position by flavin-containing monooxygenases (FMO) produ cing potentially toxic metabolites. Previous laboratory findings have demon strated that postnatal rat hepatic microsomes metabolize DAK by NADPH-depen dent monooxygenases to two metabolites as determined by HPLC, Our current i nvestigation evaluated DAK's metabolism in adult male and female rats and i dentified metabolites that may be responsible for ketoconazole's hepatotoxi city. DAK was extensively metabolized by rat liver microsomal monooxygenase s at pH 8.8 in pyrophosphate buffer containing the glucose 8-phosphate NADP H-generating system to three metabolites as determined by HPLC, The initial metabolite of DAK was a secondary hydroxylamine, N-deacetyl-N-hydroxyketoc onazole, which was confirmed by liquid chromatography/mass spectrometry and NMR spectroscopy. Extensive metabolism of DAK occurred at pH 8.8 in pyroph osphate buffer (female 29% and male 53% at 0.25 h; female 55% and male 57% at 0.5 h; and female 62% and male 66% at 1.0 h), Significantly less metabol ism of DAK occurred at pH 7.4 in phosphate buffer (female 11%, male 17% at 0.25 h; female 20%, male 31% at 0.5 h; and female 27%, male 37% at 1 h), He at inactivation of microsomal-FMO abolished the formation of these metaboli tes from DAK, SKF-525A did not inhibit this reaction. These results suggest that DAK appears to be extensively metabolized by adult FMO-mediated monoo xygenation.