Open AccessIn vitro metabolism of exemestane by hepatic cytochrome P450s: impact of nonsynonymous polymorphisms on formation of the active metabolite 17 β ‐dihydroexemestane
Author(s) -
Peterson Amity,
Xia Zuping,
Chen Gang,
Lazarus Philip
Publication year - 2017
Publication title -
pharmacology research and perspectives
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.975
H-Index - 27
ISSN - 2052-1707
DOI - 10.1002/prp2.314
Subject(s) - metabolite , cytochrome p450 , biochemistry , drug metabolism , microsome , active metabolite , enzyme , biology , pharmacology , chemistry
Exemestane ( EXE ) is an endocrine therapy commonly used by postmenopausal women with hormone‐responsive breast cancer due to its potency in inhibiting aromatase‐catalyzed estrogen synthesis. Preliminary in vitro studies sought to identify phase I EXE metabolites and hepatic cytochrome P450s ( CYP 450s) that participate in EXE biotransformation. Phase I metabolites were identified by incubating EXE with HEK 293‐overexpressed CYP 450s. CYP 450s 1A2, 2C8, 2C9, 2C19, 2D6, 3A4, and 3A5 produce 17 β ‐dihydroexemestane (17 β ‐ DHE ), an active major metabolite, as well as two inactive metabolites. 17 β ‐ DHE formation in pooled human liver microsomes subjected to isoform‐specific CYP 450 inhibition was also monitored using tandem mass spectrometry. 17 β ‐ DHE production in human liver microsomes was unaffected by isoform‐specific inhibition of CYP 450s 2A6, 2B6, and 2E1 but decreased 12–39% following inhibition of drug‐metabolizing enzymes from CYP 450 subfamilies 1A, 2C, 2D, and 3A. These results suggest that redundancy exists in the EXE metabolic pathway with multiple hepatic CYP 450s catalyzing 17 β ‐ DHE formation in vitro. To further expand the knowledge of phase I EXE metabolism, the impact of CYP 450 genetic variation on 17 β ‐ DHE formation was assessed via enzyme kinetic parameters. Affinity for EXE substrate and enzyme catalytic velocity were calculated for hepatic wild‐type CYP 450s and their common nonsynonymous variants by monitoring the reduction of EXE to 17 β ‐ DHE . Several functional polymorphisms in xenobiotic‐metabolizing CYP 450s 1A2, 2C8, 2C9, and 2D6 resulted in deviant enzymatic activity relative to wild‐type enzyme. Thus, it is possible that functional polymorphisms in EXE ‐metabolizing CYP 450s contribute to inter‐individual variability in patient outcomes by mediating overall exposure to the drug and its active metabolite, 17 β ‐ DHE .