MICROSOMAL METABOLISM OF DELAVIRDINE - EVIDENCE FOR MECHANISM-BASED INACTIVATION OF HUMAN CYTOCHROME-P450 3A

Administration of delavirdine, an HIV-1 reverse transcriptase inhibito r, to rats or monkeys resulted in apparent loss of hepatic microsomal CYP3A and delavirdine desalkylation activity. Human CYP3A catalyzes th e formation of desalkyl delavirdine and 6'-hydroxy delavirdine, an uns table metabolite, while CYP2D6 catalyzes only desalkyl delavirdine. CY P2D6 catalyzed desalkyl delavirdine formation was linear with time (up to 30 min) but when catalyzed by cDNA expressed CYP3A4 or human liver microsomes the reaction rate declined progressively with time. Coincu bation with triazolam showed that delavirdine caused a time- and NADPH -dependent loss of CYP3A4 activity in human liver microsomes as measur ed by triazolam 1'-hydroxylation. The catalytic activity loss was satu rable and was characterized by a K-i of 21.6 +/- 8.9 mu M and a k(inac t) of 0.59 +/- 0.08 min(-1). An apparent partition ratio of 41 was det ermined with cDNA expressed CYP3A4, based on the substrate depletion m ethod. Incubation of [C-14]delavirdine with microsomes from several sp ecies resulted in irreversible association with an approximately 50 kD a protein, as demonstrated by SDS-PAGE/autoradiography. Binding to the protein was NADPH dependent, glutathione insensitive, proportional to the level of CYP3A expression and was inhibited by ketoconazole, a sp ecific CYP3A inhibitor. NADPH-dependent irreversible binding to human and rat total microsomal protein was demonstrated following exhaustive extraction of microsomal protein. Binding was decreased in the presen ce of glutathione and appeared to be related to expression level of CY P3A. These results suggest that delavirdine can inactivate CYP3A and h as the potential to slow the metabolism of coadministered CYP3A substr ates.