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 formulati ons, must be demethylated to its active metabolite, 17 alpha-ethinyl e stradiol, to produce estrogenic activity To investigate the transforma tion of mestranol to ethinyl estradiol, an in vitro assay was used wit h human liver microsomes from four different donors. Incubation of a f ixed concentration of mestranol (3 mu mol/L) with varying concentratio ns of CYP inhibitors revealed strong inhibition of ethinyl estradiol f ormation by sulfaphenazole, a specific CYP2C9 inhibitor, with an avera ge inhibitor concentration at one half of E(max) (IC50) of 3.6 mu mol/ L (range, 1.8-8.3 mu mol/L) and an average maximal inhibitory capacity (E(max)) of 75% (range, 60-91%). Troleandomycin (a CYP3A3/4 inhibitor ) and quinidine (a CYP2D6 inhibitor), however, produced no substantial inhibitory activity. alpha-Naphthoflavone (a CYP1A1/2 inhibitor only at concentrations <2 mu mol/L and a CYP2C9 inhibitor at higher concent rations) had a weak inhibitory effect on ethinyl estradiol formation ( <20% decrease in mestranol demethylation activity). Of the three antif ungal azoles tested, miconazole strongly inhibited mestranol demethyla tion, with an average IC50 of 1.5 mu mol/L (range, 0.7-3.2 mu mol/L) a nd an average E(max) of 90% (range, 77-100%), whereas fluconazole disp layed relatively weak inhibition only at the highest concentration of 50 mu mol/L (mean reduction in demethylation activity was 29%). Itraco nazole produced no meaningful inhibition. Strong inhibition of ethinyl estradiol formation by sulfaphenazole suggests a major contribution o f CYP2C9 to this reaction.