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METABOLISM, TRANSPORT, AND PHARMACOGENOMICS

Metabolic Activation of Mifepristone [RU486; 17β-Hydroxy-11β-(4-dimethylaminophenyl)-17-(1-propynyl)-estra-4,9-dien-3-one] by Mammalian Cytochromes P450 and the Mechanism-Based Inactivation of Human CYP2B6

Department of Pharmacology, University of Michigan, Ann Arbor, Michigan

Mifepristone [RU486; 17β-hydroxy-11β-(4-dimethylaminophenyl)-17-(1-propynyl)-estra-4,9-dien-3-one] inactivates CYP2B6 in the reconstituted system in a mechanism-based manner. The loss of 7-ethoxy-4-(trifluoromethyl)-coumarin deethylation activity of CYP2B6 is concentration- and time-dependent. The inactivation requires NADPH and is irreversible. The concentration of inactivator required to give the half-maximal rate of inactivation is 2.8 µM, and the maximal rate constant for inactivation at a saturating concentration of the inactivator is 0.07 min^-1. Incubation of CYP2B6 with 20 µM RU486 for 15 min resulted in 61% loss of catalytic activity, 60% loss of the reduced cytochrome P450 (P450)-CO complex, and a 40% loss of native heme. The partition ratio is 5, and the stoichiometry of binding is 0.6 mol RU486/mol P450 inactivated. SDS-polyacrylamide gel electrophoresis and high-pressure liquid chromatography analysis showed that [³H]RU486 was irreversibly bound to CYP2B6 apoprotein. RU486 is metabolized to form three major metabolites and bioactivated to give reactive intermediates by purified P450s in the reconstituted system. After incubation of RU486 with the purified P450s and liver microsomes from rats and humans in the presence of glutathione (GSH) and NADPH, GSH conjugates with MH⁺ ions at m/z 769, 753, and 751 were detected by liquid chromatography-tandem mass spectrometry. Two GSH conjugates with MH⁺ ions at m/z 753 are formed from the reaction of GSH with RU486. The adducts are formed after addition of an activated oxygen to the carbon-carbon triple bond of the propynyl moiety. This suggests that oxirene intermediates may be involved in the mechanism of inactivation. It seems that the potential for drug-drug interactions of RU486 may not be limited only to CYP3A4 and should also be evaluated for drugs metabolized primarily by CYP2B6, such as bupropion and efavirenz.

Address correspondence to: Paul F. Hollenberg, Department of Pharmacology, 2301 MSRB III, 1150 West Medical Center Drive, Ann Arbor, MI 48109-5632. E-mail: phollen{at}umich.edu