Effects of cytochrome P450 inducers on 17 alpha-ethinyloestradiol (EE2) conjugation by primary human hepatocytes

Aims Our objective was to elucidate further the underlying mechanism respon sible for therapeutic failures observed with concomitant administration of the oral contraceptive 17 alpha-ethinyloestradiol (EE2) and rifampicin. Methods We investigated both oxidative and direct conjugative [H-3]-EE2 met abolism by human Liver S9 fraction and the effect of known enzyme-inducing drugs using a human hepatocyte induction model in vitro. Results Cofactor dependent [H-3]-EE2 metabolism by human liver S9 fraction produced 2-hydroxy-[H-3]-EE2, 2-methoxy-[H-3]-EE2, and direct [H-3]-EE2 sul phate and glucuronide conjugates. Only two detectable metabolites of [H-3]- EE2 were produced by the S9 fraction in the presence of all cofactors: [H-3 ]-EE2-3-sulphate (75.7+/-7.6% s.d.) and 2-methoxy-H-3-EE2 (2.6% +/- 0.5% s. d.). Human hepatocytes extensively metabolized [H-3]-EE2 to its glucuronide and sulphate conjugates. Small amounts of a 2-methoxy-[H-3]-EE2 3-conjugat e, less than or equal to 10%, was observed but no. 2-hydroxy-[H-3]-EE2 was detected. An unexpected finding in our study was increased [H-3]-EE2-3-sulp hate production (1.5-3.3 fold, n = 3 donor livers) by hepatocytes pretreate d with rifampicin compared to control hepatocytes. No statistically signifi cant increase in [H-3]-EE2-3-sulphation was observed in hepatocytes pretrea ted with 3-methylcholanthrene, phenobarbitone, dexamethasone, or omeprazole over nontreated hepatocytes. To our knowledge, this is the first observati on of sulphotransferase induction by rifampicin in human hepatocytes in vit ro resulting in increased [H-3]-EE2 sulphation. Conclusions Our data indicate that the major EE2 metabolic products formed by human hepatocytes in vitro are direct EE2 conjugates with EE2 oxidation representing minor pathways. Further studies are required to establish the mechanism of sulphotransferase induction and the clinical relevance of our findings.