Biotransformation of Mestranol to Ethinyl Estradiol In Vitro: The Role of Cytochrome P-450 2C9 and Metabolic Inhibitors

Mestranol, the estrogen component of some oral contraceptive formulations, must be demethylated to its active metabolite, 17α‐ethinyl estradiol, to produce estrogenic activity. To investigate the transformation of mestranol to ethinyl estradiol, an in vitro assay was used with human liver microsomes from four different donors. Incubation of a fixed concentration of mestranol (3 μmol/L) with varying concentrations of CYP inhibitors revealed strong inhibition of ethinyl estradiol formation by sulfaphenazole, a specific CYP2C9 inhibitor, with an average inhibitor concentration at one half of Emax (IC50) of 3.6 μmol/L (range, 1.8–8.3 μmol/L) and an average maximal inhibitory capacity (Emax) of 75% (range, 60–91%). Troleandomycin (a CYP3A3/4 inhibitor) and quinidine (a CYP2D6 inhibitor), however, produced no substantial inhibitory activity. α‐Naphthoflavone (a CYP1A1/2 inhibitor only at concentrations