Effect of CYP3A5 expression on the inhibition of CYP3A-catalyzed drug metabolism: impact on modeling CYP3A-mediated drug-drug interactions

Abstract

The purpose of this study was to determine the impact of CYP3A5 expression on inhibitory potency (Ki or IC50 values) of CYP3A inhibitors, using recombinant CYP3A4 and CYP3A5 (rCYP3A4 and rCYP3A5) and CYP3A5 genotyped human liver microsomes (HLMs). IC50 ratios between rCYP3A4 and rCYP3A5 (rCYP3A5/rCYP3A4) of ketoconazole (KTZ) and itraconazole (ITZ) were 8.5 and 8.8 for midazolam (MDZ), 4.7 and 9.1 for testosterone (TST), 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 IC50 values for CYP3A4 and CYP3A5, nonconcordant expression of CYP3A4 and CYP3A5 protein can significantly affect the observed magnitude of CYP3A-mediated drug-drug interactions in humans. Indeed, the IC50 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 the CYP3A5*3/*3 genotype, showing CYP3A5 expression-dependent IC50 values. Moreover, when IC50 values of KTZ and ITZ for different HLMs were kinetically simulated based on CYP3A5 expression level and enzyme-specific IC50 values, a good correlation between the simulated and the experimentally measured IC50 values was observed. Further simulation analysis revealed that both the Ki ratio (for inhibitors) and Vmax/Km ratio (for substrates) between CYP3A4 and CYP3A5 were major factors for CYP3A5 expression-dependent IC50 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.

Fig. 1.

Relationship between CYP3A5 expression and IC₅₀ values of KTZ and ITZ for the CYP3A-catalyzed metabolism of MDZ and TST in HLM. Relationship between CYP3A5 expression and IC₅₀ values of KTZ (A) and ITZ (B) for MDZ and KTZ (C) and ITZ (D) for TST metabolism. CYP3A5 expression represents CYP3A5 protein expression (pmol/mg) as a percentage of a total CYP3A protein expression in HLM (CYP3A4 pmol/mg + CYP3A5 pmol/mg). Solid and dotted lines represent simulated data and their 2-fold error ranges, respectively (see Materials and Methods); ●, ▲, and ■ represent observed data using HLMs with CYP3A5*1/*1, CYP3A5*1/*3, and CYP3A5*3/*3 genotypes, respectively. Data are presented as the mean ± S.D. (n = 3 determinations).

Fig. 2.

Simulation analysis of effect of CYP3A5 expression on inhibition of CYP3A-catalyzed drug metabolism. (A) Impact of K_i ratio (CYP3A5/CYP3A4) on CYP3A5 expression–dependent IC₅₀ values. Kinetic parameters for a virtual CYP3A substrate and inhibitor used in current simulations are set as follows: substrate concentration = 100 nM, V_max/K_m ratio = 1, and K_i,CYP3A4 = 100 nM. (B) Impact of V_max/K_m ratio (CYP3A5/CYP3A4) on CYP3A5 expression–dependent IC₅₀ values. Kinetic parameters for a virtual CYP3A substrate and inhibitor used in current simulations are set as follows: substrate concentration = 100 nM (< < K_m) and K_i ratio = 10 (K_i,CYP3A4 = 100 nM, K_i,CYP3A5 = 1000 nM). CYP3A5 expression represents the percentage of total CYP3A (CYP3A4 + CYP3A5) expression (see Materials and Methods).