Comparison of kinetic parameters for drug oxidation rates and substrate inhibition potential mediated by cytochrome P450 3A4 and 3A5.

Cytochrome P450 (P450 or CYP) 3A is one of the most important P450 subfamilies in terms of its broad substrate specificity and relatively high abundance in humans. The substrate specificities of CYP3A4 and CYP3A5 are generally overlapped, but sometimes could differ from each other. It is still important to understand drug interactions more precisely in individual subjects. However, there are few review articles regarding comparative drug oxidation rates catalyzed by CYP3A4 and CYP3A5 and/or substrate inhibition potential towards CYP3A4 and CYP3A5. In this article, we summarize 1) Michaelis-Menten constants (Km), maximal velocities (Vmax), and intrinsic clearance (Vmax/Km) values for 63 substrates (94 reactions) mediated by CYP3A4 and/or CYP3A5, 2) inhibition constants (Ki) and 50% inhibitory concentrations (IC50) of 18 substrates, and 3) maximum inactivation rate constants (kinact) of 14 inhibitors from the literature. The relative contribution of polymorphic CYP3A5 compared with inducible CYP3A4 varies with the substrates and the reaction positions of the substrates. Inhibitory effects of azole antifungal agents and macrolide antibiotics, with low Ki and/or IC50 values for CYP3A4, are likely to be determinant factors for predominant drug interactions in humans, although Asian subjects with relatively high frequency of genetic CYP3A5 expressers should be carefully treated with CYP3A substrates. The collective findings in our present survey provide fundamental and useful information for drug oxidations catalyzed by CYP3A4 and CYP3A5, in spite of some contradictive kinetic parameters for the same reactions reported from many laboratories in different conditions. To understand causal factor(s) and mechanism(s) for such different reports summarized here is still one of the hot research topics to be solved in current drug metabolism.