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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 6β-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 …