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 x 4.6 mm), The HPLC mobile phase consisted of a mixture of acetonitrile-water (21:79, v/v) containing 10 mM 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. (C) 1998 Published by Elsevier Science B.V. All rights reserved.