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In vitro metabolism of exemestane by hepatic cytochrome P450s: impact of nonsynonymous polymorphisms on formation of the active metabolite 17?-dihydroexemestane.

ABSTRACT: 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 (CYP450s) that participate in EXE biotransformation. Phase I metabolites were identified by incubating EXE with HEK293-overexpressed CYP450s. CYP450s 1A2, 2C8, 2C9, 2C19, 2D6, 3A4, and 3A5 produce 17?-dihydroexemestane (17?-DHE), an active major metabolite, as well as two inactive metabolites. 17?-DHE formation in pooled human liver microsomes subjected to isoform-specific CYP450 inhibition was also monitored using tandem mass spectrometry. 17?-DHE production in human liver microsomes was unaffected by isoform-specific inhibition of CYP450s 2A6, 2B6, and 2E1 but decreased 12-39% following inhibition of drug-metabolizing enzymes from CYP450 subfamilies 1A, 2C, 2D, and 3A. These results suggest that redundancy exists in the EXE metabolic pathway with multiple hepatic CYP450s catalyzing 17?-DHE formation in vitro. To further expand the knowledge of phase I EXE metabolism, the impact of CYP450 genetic variation on 17?-DHE formation was assessed via enzyme kinetic parameters. Affinity for EXE substrate and enzyme catalytic velocity were calculated for hepatic wild-type CYP450s and their common nonsynonymous variants by monitoring the reduction of EXE to 17?-DHE. Several functional polymorphisms in xenobiotic-metabolizing CYP450s 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 CYP450s contribute to inter-individual variability in patient outcomes by mediating overall exposure to the drug and its active metabolite, 17?-DHE.

SUBMITTER: Peterson A

PROVIDER: S-EPMC5464343 | biostudies-literature | 2017 Jun

REPOSITORIES: biostudies-literature

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In vitro metabolism of exemestane by hepatic cytochrome P450s: impact of nonsynonymous polymorphisms on formation of the active metabolite 17<i>β</i>-dihydroexemestane.

Peterson Amity A Xia Zuping Z Chen Gang G Lazarus Philip P

Pharmacology research & perspectives 20170427 3

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 (CYP450s) that participate in EXE biotransformation. Phase I metabolites were identified by incubating EXE with HEK293-overexpressed CYP450s. CYP450s 1A2, 2C8, 2C9, 2C19, 2D6, 3A4, and 3A5 produce 17<i>β</i>-d ...[more]

PMID: 28603633

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Project description:Using oxygen and NADPH, the redox enzymes cytochrome P450 (CYP) and its reductase (CPR) work in tandem to carry out the phase I metabolism of a vast majority of drugs and xenobiotics. As per the erstwhile understanding of the catalytic cycle, binding of the substrate to CYP's heme distal pocket allows CPR to pump electrons through a CPR-CYP complex. In turn, this trigger (a thermodynamic push of electrons) leads to the activation of oxygen at CYP's heme-center, to give Compound I, a two-electron deficient enzyme reactive intermediate. The formation of diffusible radicals and reactive oxygen species (DROS, hitherto considered an undesired facet of the system) was attributed to the heme-center. Recently, we had challenged these perceptions and proposed the murburn ("mured burning" or "mild unrestricted burning") concept to explain heme enzymes' catalytic mechanism, electron-transfer phenomena and the regulation of redox equivalents' consumption. Murburn concept incorporates a one-electron paradigm, advocating obligatory roles for DROS. The new understanding does not call for high-affinity substrate-binding at the heme distal pocket of the CYP (the first and the most crucial step of the erstwhile paradigm) or CYP-CPR protein-protein complexations (the operational backbone of the erstwhile cycle). Herein, the dynamics of reduced nicotinamide nucleotides' consumption, peroxide formation and depletion, product(s) formation, etc. was investigated with various controls, by altering reaction variables, environments and through the incorporation of diverse molecular probes. In several CYP systems, control reactions lacking the specific substrate showed comparable or higher peroxide in milieu, thereby discrediting the foundations of the erstwhile hypothesis. The profiles obtained by altering CYP:CPR ratios and the profound inhibitions observed upon the incorporation of catalytic amounts of horseradish peroxidase confirm the obligatory roles of DROS in milieu, ratifying murburn as the operative concept. The mechanism of uncoupling (peroxide/water formation) was found to be dependent on multiple one and two electron equilibriums amongst the reaction components. The investigation explains the evolutionary implications of xenobiotic metabolism, confirms the obligatory role of diffusible reactive species in routine redox metabolism within liver microsomes and establishes that a redox enzyme like CYP enhances reaction rates (achieves catalysis) via a novel (hitherto unknown) modality.

Dataset Information

In vitro metabolism of exemestane by hepatic cytochrome P450s: impact of nonsynonymous polymorphisms on formation of the active metabolite 17?-dihydroexemestane.

Publications

In vitro metabolism of exemestane by hepatic cytochrome P450s: impact of nonsynonymous polymorphisms on formation of the active metabolite 17<i>β</i>-dihydroexemestane.

Similar Datasets

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