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