Effect of CYP2C9 Genetic Polymorphism in a Chinese Population on the Metabolism of Mestranol in vitro

Background: Mestranol is a widely used estrogen, which is converted into its active metabolite ethinyl estradiol by cytochrome P450 (CYP) 2C9. To comprehensively examine the enzymatic activity of reported CYP2C9 variants in Chinese individuals in response to mestranol, wild-type CYP2C9*1 and 35 allelic variants were highly expressed in Sf21 insect cell microsomes and used for the detection of their enzymatic values in vitro. These results showed that the majority of tested variants exhibited decreased clearance values compared to wild type, except for CYP2C9*40 and *36. Method: Insect microsomes expressing the 36 CYP2C9 variants were incubated with 0.25-8 μmol/l mestranol for 30 min at 37°C. Then, the production of the metabolite of mestranol, ethinyl estradiol, was analyzed using high-performance liquid chromatography. Results: Most CYP-catalyzed reactions were sufficiently described by classical Michaelis-Menten kinetic parameters (e.g., Km and Vmax), while 9 variants exhibited atypical or non-Michaelis-Menten kinetic values, which were largely due to the self-inhibitory effect in response to mestranol. Conclusion: This is the first report of these rare alleles for mestranol metabolism, which provides fundamental data for further clinical studies on CYP2C9 alleles for mestranol metabolism.