• Guoping Yang, Zhimin Fu, Xiaoping Chen, Hong Yuan, Heng Yang, Yuanyuan Huang, Dongsheng Ouyang, Zhirong Tan, Hongyi Tan, Zhijun Huang, and Honghao Zhou.
• Center of Clinical Pharmacology, the Third Xiangya Hospital, Central South University, Changsha, China.
• Clin Ther. 2011 Dec 1; 33 (12): 2060-70.

BackgroundCytochrome P450 (CYP) oxidoreductase (POR) is the electron donor for microsomal CYP enzymes. The POR Ala503Val (POR*28 C > T) polymorphism has been reported to influence CYP3A activity in vivo in a white population. The influence of this polymorphism on CYP3A activity in vivo in the Chinese population currently unknown.ObjectiveThis study was designed to assess the influence of the POR*28 polymorphism on the CYP3A activity in vivo in healthy Chinese men using midazolam (MID) as a probe drug.MethodsThe POR*28 polymorphism was genotyped in healthy Chinese men. A randomized, 2-phase, open-label, crossover study was performed to assess in vivo CYP3A activity after both oral and intravenous MID administration, which reflect both intestinal and hepatic CYP3A or only hepatic CYP3A activity, respectively. The plasma concentrations of MID and 1-hydroxy-midazolam (1-OH-MID) were determined by liquid chromatography-tandem mass spectrometry.ResultsA total of 73 healthy Chinese men were enrolled (CC genotype, 21 subjects; TT genotype, 11; CT genotype, 41), 22 of whom were selected for additional phenotyping of the CYP3A5*3 polymorphism (CC, 7; TT, 8; CT, 7). The mean (range) age, weight, height, and body mass index of the 22 subjects were 23 (20-28) years, 65.0 (57-75) kg, 1.74 (1.63-1.80) m, and 22.01 (19.27-24.46) kg/m(2), respectively. The frequency of the POR*28 T (503V) allele was 43.2%. No significant differences in the demographic characteristics of the subjects were observed between the POR*28 genotype groups. All of the POR*28 CC and TT homozygotes and 2 of the POR*28 CT heterozygotes carried the CYP3A5*3/*3 genotype (CYP3A5 low expressors); 6 CT heterozygotes carried the CYP3A5*1 allele (CYP3A5 expressors). The mean (SD) 1-OH-MID AUC(0-8) was significantly greater in the TT homozygotes compared with the CT heterozygotes after intravenous (86.15 [24.34] vs 53.21 [31.36] ng/mL/h; P = 0.026) but not oral (126.36 [31.60] vs 103.09 [31.00] ng/mL/h; P = 0.159) MID administration. Mean 1-OH-MID C(max) was significantly greater in the TT homozygotes (51.40 [10.72] ng/mL) compared with the CC homozygotes (31.47 [11.54] ng/mL; P = 0.002) and CT heterozygotes (30.12 [9.21] ng/mL; P = 0.001) after intravenous MID administration. After intravenous MID injection, the MID metabolic ratio was significantly greater in the TT homozygotes compared with carriers of the C allele (P = 0.031). Based on these findings, no significantly differences in overall (hepatic plus intestinal) CYP3A in vivo activity were observed between the POR*28 genotypes.ConclusionThese findings suggest that individuals with the POR*28 C > T polymorphism underwent an increase in 1-hydroxylation of MID after intravenous MID administration, and that the polymorphism was associated with increased hepatic, but not intestinal, CYP3A activity in these healthy Chinese volunteers.Crown Copyright © 2011. Published by EM Inc USA. All rights reserved.

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