Impact of CYP3A5 Expression on the Inhibition of CYP3A-Catalyzed Drug Metabolism: Considerations for Modeling CYP3A-Mediated Drug-Drug Interactions

This article has not been copyedited and formatted. The final version may differ from this version. Abstract The purpose of this study was to determine the impact of CYP3A5 expression on inhibitory potency (K i or IC 50 values) of CYP3A inhibitors, using recombinant CYP3A4 and CYP3A5 (rCYP3A4 and rCYP3A5) and CYP3A5 genotyped human liver microsomes (HLMs). IC 50 ratios between rCYP3A4-and rCYP3A5 (rCYP3A5/rCYP3A4) of ketoconazole (KTZ) and itraconazole (ITZ) were 8.5 and 8.8 for midazolam, 4.7 and 9.1 for testosterone, 1.3 and 2.8 for terfenadine, and 0.6 and 1.7 for vincristine, respectively, suggesting substrate and inhibitor-dependent selectivity of the two azoles. Due to the difference in the IC 50 values for CYP3A4 and CYP3A5, non-concordant expression of CYP3A4 and CYP3A5 protein can significantly affect the observed magnitude of CYP3A-mediated drug-drug interactions in humans. Indeed, the IC 50 values of KTZ and ITZ for CYP3A-catalyzed MDZ and TST metabolism were significantly higher in HLMs with CYP3A5*1/*1 and CYP3A5*1/*3 genotypes than in HLMs with CYP3A5*3/*3 genotype, showing CYP3A5 expression-dependent IC 50 values. Moreover, when IC 50 values of KTZ and ITZ for different HLMs were kinetically simulated based on CYP3A5 expression level and enzyme-specific IC 50 values, a good correlation between the simulated and the experimentally measured IC 50 values was observed. Further simulation analysis revealed that both the K i ratio (for inhibitors) and V max /K m ratio (for substrates) between CYP3A4 and CYP3A5 were major factors for CYP3A5 expression-dependent IC 50 values. In conclusion, the present study demonstrates that CYP3A5 genotype and expression level have a significant impact on inhibitory potency for CYP3A-catalyzed drug metabolism, but that the magnitude of its effect is inhibitor-substrate pair specific. This article has not been copyedited and formatted. The final version may differ from this version.