title: In vitro cytochrome P450-mediated metabolism of exemestane.
abstract: 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 cytochromes P450 (P450s) responsible for exemestane primary in vitro metabolism. With the use of high-performance liquid chromatography and liquid chromatography-tandem mass spectrometry analytical techniques, 17-hydroexemestane (MI) formation 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 P450 isoform-specific activities, the MI formation rate correlated significantly with CYP1A2 (Spearman r = 0.60, p = 0.02) and CYP4A11 (Spearman r = 0.67, p = 0.01) isoform-specific activities, whereas the 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 (Cl(int)) of 150 nl/pmol P450 × min that was at least 3.5-fold higher than those of other P450s investigated. Recombinant CYP3A4 catalyzed MII formation from exemestane with a catalytic efficiency of 840 nl/pmol P450 × min that was at least 4-fold higher than those of other P450s investigated. Among a panel of 10 chemical inhibitors tested, only ketoconazole and troleandomycin (CYP3A-specific chemical inhibitors) significantly inhibited the formation of MII by 45 and 95%, respectively. None of them markedly inhibited the formation of MI. In summary, exemestane seems to be metabolized to MI by multiple P450s that include CYP4A11 and CYP1A1/2, whereas its oxidation to MII is primarily mediated by CYP3A.
authors.affiliation: Division of Clinical Pharmacology, Department of Medicine, Indiana University School of Medicine, Indianapolis, Indiana, USA. lkamdem@harding.edu
authors.forename: Landry K
authors.initials: LK
authors.lastname: Kamdem
authors.forename: David A
authors.initials: DA
authors.lastname: Flockhart
authors.forename: Zeruesenay
authors.initials: Z
authors.lastname: Desta
mesh_terms: D000730:Androstadienes / Q000737:chemistry / Q000378:metabolism*; D047072:Aromatase Inhibitors / Q000737:chemistry / Q000378:metabolism*; D019363:Cytochrome P-450 CYP1A1 / Q000378:metabolism; D019388:Cytochrome P-450 CYP1A2 / Q000378:metabolism; D051544:Cytochrome P-450 CYP3A / Q000378:metabolism; D065692:Cytochrome P-450 CYP3A Inhibitors; D042926:Cytochrome P-450 CYP4A; D003577:Cytochrome P-450 Enzyme System / Q000378:metabolism*; D005260:Female; D006801:Humans; D008862:Microsomes, Liver / Q000378:metabolism
chemical_list: D000730:Androstadienes; D047072:Aromatase Inhibitors; D065692:Cytochrome P-450 CYP3A Inhibitors; D003577:Cytochrome P-450 Enzyme System; C504573:CYP1A2 protein, human; D019363:Cytochrome P-450 CYP1A1; D019388:Cytochrome P-450 CYP1A2; D051544:Cytochrome P-450 CYP3A; C465478:CYP4A11 protein, human; D042926:Cytochrome P-450 CYP4A; C056516:exemestane