Simultaneous determination of omeprazole and 5′-hydroxyomeprazole in human plasma by liquid chromatography–tandem mass spectrometry

A method for the simultaneous determination of omeprazole and 5′-hydroxyomeprazole in human plasma is described. Isolation of the analytes from plasma was achieved via solid-phase extraction using a polymeric sorbent based cartridge. The analytes were chromatographed under reversed-phase conditions on a Zorbax XDB-C 8 column (50×4.6 mm). The HPLC mobile phase consisted of a mixture of acetonitrile–water (21:79, v/v) containing 10 m M ammonium hydroxide. The apparent pH of the mobile phase was adjusted to 8.5 with formic acid prior to use. A Sciex API III+ tandem mass spectrometer equipped with a heated nebulizer atmospheric pressure chemical ionization interface was used as a detector and was operated in the positive ion mode. Multiple reaction monitoring using the precursor→product ion combinations of m/z 362→214, 346→198 and 316→147 was used to detect 5′-hydroxyomeprazole, omeprazole and internal standard, respectively. The method was validated in the concentration range of 10–500 ng/ml plasma with adequate assay precision and accuracy. The assay was used to determine the cytochrome P450 2C19 phenotype of subjects participating in clinical trials of compounds under development.