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 (Clint) 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.
Footnotes
- Received January 16, 2010.
- Accepted September 28, 2010.
This work was supported by the National Institutes of Health National Institute of General Medical Sciences [Grant 5U01GM061373-09].
Article, publication date, and citation information can be found at http://dmd.aspetjournals.org.
doi:10.1124/dmd.110.032276.
ABBREVIATIONS:
- P450
- cytochrome P450
- thioTEPA
- 1,1-phosphinothioylidynetrisaziridine
- HPLC
- high-performance liquid chromatography
- HLM
- human liver microsome
- LC
- liquid chromatography
- MS/MS
- tandem mass spectrometry
- MRM
- multiple reaction monitoring
- amu
- atomic mass units.
- Copyright © 2011 by The American Society for Pharmacology and Experimental Therapeutics
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