title: Glucuronidation converts gemfibrozil to a potent, metabolism-dependent inhibitor of CYP2C8: implications for drug-drug interactions.
abstract: Gemfibrozil more potently inhibits CYP2C9 than CYP2C8 in vitro, and yet the opposite inhibitory potency is observed in the clinic. To investigate this apparent paradox, we evaluated both gemfibrozil and its major metabolite, an acyl-glucuronide (gemfibrozil 1-O-beta-glucuronide) as direct-acting and metabolism-dependent inhibitors of the major drug-metabolizing cytochrome P450 enzymes (CYP1A2, 2B6, 2C8, 2C9, 2C19, 2D6, and 3A4) in human liver microsomes. Gemfibrozil most potently inhibited CYP2C9 (IC50 of 30 microM), whereas gemfibrozil glucuronide most potently inhibited CYP2C8 (IC50 of 24 microM). Unexpectedly, gemfibrozil glucuronide, but not gemfibrozil, was found to be a metabolism-dependent inhibitor of CYP2C8 only. The IC50 for inhibition of CYP2C8 by gemfibrozil glucuronide decreased from 24 microM to 1.8 microM after a 30-min incubation with human liver microsomes and NADPH. Inactivation of CYP2C8 by gemfibrozil glucuronide required NADPH, and proceeded with a K(I) (inhibitor concentration that supports half the maximal rate of enzyme inactivation) of 20 to 52 microM and a k(inact) (maximal rate of inactivation) of 0.21 min(-1). Potent inhibition of CYP2C8 was also achieved by first incubating gemfibrozil with alamethicin-activated human liver microsomes and UDP-glucuronic acid (to form gemfibrozil glucuronide), followed by a second incubation with NADPH. Liquid chromatography-tandem mass spectrometry analysis established that human liver microsomes and recombinant CYP2C8 both convert gemfibrozil glucuronide to a hydroxylated metabolite, with oxidative metabolism occurring on the dimethylphenoxy moiety (the group furthest from the glucuronide moiety). The results described have important implications for the mechanism of the clinical interaction reported between gemfibrozil and CYP2C8 substrates such as cerivastatin, repaglinide, rosiglitazone, and pioglitazone.
authors.affiliation: XenoTech, LLC, 16825 West 116 Street, Lenexa, KS 66219, USA.
authors.forename: Brian W
authors.initials: BW
authors.lastname: Ogilvie
authors.forename: Donglu
authors.initials: D
authors.lastname: Zhang
authors.forename: Wenying
authors.initials: W
authors.lastname: Li
authors.forename: A David
authors.initials: AD
authors.lastname: Rodrigues
authors.forename: Amy E
authors.initials: AE
authors.lastname: Gipson
authors.forename: Jeff
authors.initials: J
authors.lastname: Holsapple
authors.forename: Paul
authors.initials: P
authors.lastname: Toren
authors.forename: Andrew
authors.initials: A
authors.lastname: Parkinson
mesh_terms: D000408:Alamethicin / Q000378:metabolism / Q000494:pharmacology; D001189:Aryl Hydrocarbon Hydroxylases / Q000037:antagonists & inhibitors* / Q000378:metabolism; D002851:Chromatography, High Pressure Liquid / Q000379:methods; D065727:Cytochrome P-450 CYP2C8; D004305:Dose-Response Relationship, Drug; D004347:Drug Interactions; D015248:Gemfibrozil / Q000031:analogs & derivatives / Q000378:metabolism / Q000494:pharmacology*; D005965:Glucuronates / Q000378:metabolism / Q000494:pharmacology; D020719:Glucuronides / Q000378:metabolism*; D006801:Humans; D000960:Hypolipidemic Agents / Q000378:metabolism / Q000494:pharmacology; D013058:Mass Spectrometry / Q000379:methods; D008862:Microsomes, Liver / Q000187:drug effects / Q000378:metabolism; D014535:Uridine Diphosphate Glucuronic Acid / Q000378:metabolism / Q000494:pharmacology
chemical_list: D005965:Glucuronates; D020719:Glucuronides; D000960:Hypolipidemic Agents; D014535:Uridine Diphosphate Glucuronic Acid; D000408:Alamethicin; C094370:gemfibrozil 1-O-acylglucuronide; D001189:Aryl Hydrocarbon Hydroxylases; C450259:CYP2C8 protein, human; D065727:Cytochrome P-450 CYP2C8; D015248:Gemfibrozil