THE METABOLISM OF ZIDOVUDINE BY HUMAN LIVER-MICROSOMES IN-VITRO - FORMATION OF 3'-AMINO-3'-DEOXYTHYMIDINE

The characterization of the enzymatic step(s) involved in the reductio n of 3'-azido-3'-deoxythymidine (zidovudine)(ZDV) to 3'-amino-3'-deoxy thymidine (AMT) was pursued. AMT formation by human liver microsomes w as NADPH dependent, enhanced under anaerobic conditions, and increased by flavin adenine dinucleotide (FAD) and FMN. Carbon monoxide inhibit ed AMT formation by up to 80%. The effect of theophylline (CYP1A subst rate), tolbutamide (CYP2C substrate), chlorzoxazone, thiobenzamide, p- nitrophenol, mercaptoethanol, isoniazid (CYP2E substrates), cortisol ( CYP3A substrate), ketoconazole, itraconazole, fluconazole, cimetidine, miconazole (CYP inhibitors), methimazole (flavin-containing mono-oxyg enase inhibitor), chloramphenicol (undergoes nitroreduction), allopuri nol (xanthine oxidase inhibitor) and dicoumarol (DT-diaphorase inhibit or) on AMT formation were studied to see if the reduction reaction was mediated by a particular isozyme. The greatest inhibition was observe d with ketoconazole (concentration producing 50% inhibition = 78.0 mu M). At this concentration ketoconazole acted as a non-selective inhibi tor of several CYP isozymes. Overall, these data suggested that ZDV re duction was probably mediated by both cytochrome P450 isozymes and NAD PH-cytochrorne P450 reductase. Formation of AMT, as measured by intrin sic clearance (CI,,), was significantly increased in microsomes from r ats pre-treated with phenobarbitone, dexamethasone and clofibrate (ind ucers of CYP2B, CYP3A and CYP4A, respectively). Pre-treatment of rats with beta-naphthoflavone and ethanol (CYP1A and CYP2E1 inducers, respe ctively) had no effect on AMT formation.