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ABSORPTION, DISTRIBUTION, METABOLISM, AND EXCRETION

Mechanism-Based Inactivation of CYP3A by HIV Protease Inhibitors

Division of Clinical Pharmacology, Indiana University, Indianapolis, Indiana

Human immunodeficiency virus (HIV) protease inhibitors (PIs) are inhibitors of CYP3A enzymes, but the mechanism is poorly defined. In this study, time- and concentration-dependent decreases in activity as defined by maximum rate of inactivation (k_inact) and inhibitor concentration that gives 50% maximal inactivation (K_I) of CYP3A by amprenavir, indinavir, lopinavir, nelfinavir, ritonavir, and saquinavir were quantified using testosterone 6-hydroxylation as a marker for CYP3A activity with recombinant CYP3A4(+b₅), recombinant CYP3A5, and pooled human liver microsomes (HLMs). All the PIs, except indinavir, displayed inactivation with CYP3A4(+b₅) and HLMs. Ritonavir was the most potent (K_I = 0.10 and 0.17 µM) and demonstrated high k_inact values (0.32 and 0.40 min^-1) with both CYP3A4(+b₅) and HLMs. Ritonavir was not significantly depleted by high-affinity binding with CYP3A4(+b₅) and confirmed that estimation of reversible inhibition was confounded with irreversible inhibition. For CYP3A5, nelfinavir exhibited the highest k_inact (0.47 min^-1), but ritonavir was the most potent (K_I = 0.12 µM). Saquinavir and indinavir did not show time- and concentration-dependent decreases in activity with CYP3A5. Spectrophototmetrically determined metabolic intermediate complex formation was observed for all of the PIs with CYP3A4(+b₅), except for lopinavir and saquinavir. The addition of nucleophilic and free aldehyde trapping agents and free iron and reactive oxygen species scavengers did not prevent inactivation of CYP3A4(+b₅) by ritonavir, amprenavir, or nelfinavir, but glutathione decreased the inactivation by saquinavir (17%) and catalase decreased the inactivation by lopinavir (39%). In conclusion, all the PIs exhibited mechanism-based inactivation, and predictions of the extent and time course of drug interactions with PIs could be underestimated if based solely on reversible inhibition.

Address correspondence to: Dr. David R. Jones, Division of Clinical Pharmacology, Wishard Memorial Hospital, 1001 W. 10th St., WD Myers Bldg., W7123, Indianapolis, IN 46202. E-mail: drjones1{at}iupui.edu

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