In Vitro Studies on the Metabolic Activation of the Furanopyridine L-754,394, a Highly Potent and Selective Mechanism-Based Inhibitor of Cytochrome P450 3A4

L-754,394, a furanopyridine derivative, is an experimental anti-HIV agent which has been shown to be an unusually potent and selective inhibitor of cytochrome P450 3A enzymes in a number of mammalian species. In the present studies, L-754,394 was demonstrated to undergo NADPH-dependent metabolic activation in hepatic microsomal preparations from rats, dogs, rhesus monkeys, and humans to electrophilic intermediates which became bound covalently to cellular proteins. The extent of binding was species-dependent, the highest levels being observed with liver microsomes from rhesus monkeys. Inclusion in incubation media of the nucleophilic trapping agents glutathione, cysteine, or methoxyamine led to a modest (15−25%) decrease in the covalent binding, while trichloropropylene oxide, an inhibitor of epoxide hydrolase, had no effect. When L-754,394 was incubated with monkey liver microsomes, the corresponding dihydrofurandiol was identified as a metabolite by liquid chromatography−tandem mass spectrometry. In contrast, when incubations were carried out in the presence of methoxyamine, the O-methyloxime derivative of the ring-opened dihydrodiol tautomer was formed, while inclusion of glutathione or N-acetylcysteine led to the formation of S-linked conjugates of a putative furan epoxide. Collectively, these results are taken to indicate that L-754,394 undergoes cytochrome P450-dependent oxidation of the fused furan ring system, leading to the formation of chemically-reactive intermediates. One or more of these electrophilic species may be responsible for the autocatalytic destruction of cytochrome P450 enzymes which accompanies L-754,394 metabolism in vitro and in vivo.