Identification of Gemfibrozil Metabolites, Produced as Positional Isomers in Human Liver Microsomes, by On-line Analyses Using Liquid Chromatography/Mass Spectrometry and Liquid Chromatography/Nuclear Magnetic Resonance Spectroscopy

Liquid chromatography/mass spectrometry (LC/MS) and liquid chromatography/nuclear magnetic resonance spectroscopy (LC/NMR) were applied to the identification of the positional isomers of gemfibrozil metabolites produced in vitro by human liver microsomes. Gemfibrozil was metabolized to two major oxidative metabolites (M1 and M2) by human liver microsomes in the presence of an nicotinamide adenine dinucleotide phosphate, reduced form (NADPH)-generating system. M1 and M2 were characterized as metabolites formed by the oxidation of the 2′,5′-dimethylphenoxy moiety, which has potentially 5 different positions for hydroxylation, based on the results of negative-ion tandem mass spectrometry in combination with those of accurate mass measurements by quadrupole time-of-flight mass spectrometry. Separation and identification of M1 and M2 were conducted by LC/NMR analyses at 500 MHz. An on-flow 1H NMR analysis demonstrated that M1 was produced by hydroxylation at either the 2′-methyl group or the 5′-methyl group and M2 by hydroxylation at either the 3′-C or the 4′-C position of the aromatic ring. A stop-flow nuclear Overhauser effect spectroscopy (NOESY) analysis demonstrated that M1 and M2 were metabolites hydroxylated at the 5′-methyl group and the 4′-C position of the aromatic ring, respectively.