In Vitro P450-Mediated Metabolism of Exemestane

Exemestane is a potent and irreversible steroidal aromatase inhibitor drug used for the treatment of estrogen receptor-positive breast cancer. Our aim was to identify and assess the contribution of the specific P450s responsible for exemestane primary in vitro metabolism. Using HPLC and LC-MS/MS analytical techniques, 17-hydroexemestane (MI) and 6-hydroxymethylexemestane (MII) formation were found to be the predominant exemestane metabolic pathways. In a bank of 15 well characterized human liver microsomes with known cytochrome P450 (CYP) isoformspecific activities, MI formation rate significantly correlated with CYP1A2 (spearman r = 0.60, p =0.02) and CYP4A11 (spearman r = 0.67, p = 0.01) isoform specific activities. Whereas, MII production rate significantly correlated with CYP2B6 (spearman r = 0.57, p = 0.03) and CYP3A (spearman r = 0.76, p = 0.005) isoform-specific activities. Recombinant CYP1A1 metabolized exemestane to MI with a catalytic efficiency (CLint) of 150 nl/pmol P450 x min that was at least 3.5-fold higher than those of other investigated P450s. Recombinant CYP3A4 catalyzed MII formation from exemestane with a catalytic efficiency of 840 nl/pmol P450 x min that was at least 4-fold higher than those of other investigated P450s. Among a panel of 10 chemical inhibitors tested, only ketoconazole and troleandomycin (CYP3A specific chemical inhibitors) inhibited significantly the formation of MII by 45% and 95%, respectively. None of them remarkably inhibited the formation of MI. In summary, exemestane appears to be metabolized to MI by multiple CYPs that include CYP4A11 and CYP1A1/2, whereas its oxidation to MII is primarily mediated by CYP3A. This article has not been copyedited and formatted. The final version may differ from this version. DMD Fast Forward. Published on September 28, 2010 as DOI: 10.1124/dmd.110.032276 at A PE T Jornals on N ovem er 6, 2017 dm d.aspurnals.org D ow nladed from