Ethynylestradiol-mediated induction of hepatic CYP3A9 in female rats: Implication for cyclosporine metabolism

Repeated treatment of female rats with the synthetic estrogen ethynylestrad iol (EE2) increases the formation of the cyclosporine A (CyA) metabolites A M1c and AM9 by 3-fold, whereas the formation of AM1 and AM4N is not signifi cantly enhanced. The formation of all four CyA metabolites was inhibited by greater than 80% by the CYP3A-selective substrate midazolam or polyclonal anti-rat CYP3A IgGs in liver microsomes of untreated and EE2-induced rats. In contrast, anti-rat CYP2C6 IgGs had little effect, indicating the involve ment of a CYP3A but not 2C6 in this EE2-stimulated CyA metabolism. Semiquan titative reverse-transcriptase polymerase chain reaction was used to determ ine the mRNA content for four CYP3A genes (CYP3A2, CYP3A9, CYP3A18, and CYP 3A23) in livers of control and EE2-treated female rats. EE2 selectively ind uced CYP3A9 by 3.3-fold whereas the expression of CYP3A18 and CYP3A23 was s lightly decreased; neither CYP3A2 mRNA nor CYP3A1 mRNA was detectable in th ese EE2-treated livers. To determine whether rat liver microsomal CYP3A9 wa s indeed responsible for the EE2-stimulated CyA metabolism, a recombinant C YP3A9 was heterologously expressed in Escherichia coli. When functionally r econstituted, this enzyme was active in metabolizing CyA preferentially to its AM9 and AM1c metabolites as compared with CYP3A4. These findings thus s upport the notion that the increased CyA-metabolizing capacity of EE2-treat ed female rat liver microsomes is due to the induction of the CYP3A9 enzyme .