Sensitive determination of omeprazole and its two main metabolites in human plasma by column-switching high-performance liquid chromatography: Application to pharmacokinetic study in relation to CYP2C19 genotypes

A simple and sensitive column-switching high-performance liquid chromatographic method was developed for the simultaneous determination of omeprazole and its two main metabolites, 5-hydroxyomeprazole and omeprazole sulfone, in human plasma. Omeprazole, its two metabolites and lansoprazol as an internal standard were extracted from 1 ml of alkalinized plasma sample using diethyl ether–dichloromethane (45:55, v/v). The extract was injected into a column I (TSK-PW precolumn, 10 μm, 35 mm × 4.6 mm i.d.) for clean-up and column II (Inertsil ODS-80A column, 5 μm, 150 mm × 4.6 mm i.d.) for separation. The mobile phase consisted of phosphate buffer–acetonitrile (92:8 v/v, pH 7.0) for clean-up and phosphate buffer–acetonitrile–methanol (65:30:5 v/v/v, pH 6.5) for separation, respectively. The peak was detected with an ultraviolet detector set at a wavelength of 302 nm, and total time for chromatographic separation was ∼25 min. The validated concentration ranges of this method were 3–2000 ng/ml for omeprazole, 3–500 ng/ml for 5-hydroxyomeprazole and 3–1000 ng/ml for omeprazole sulfone. Mean recoveries were 84.3% for omeprazole, 64.3% for 5-hydroxyomeprazole and 86.1% for omeprazole sulfone. Intra- and inter-day coefficient variations were less than 5.1 and 6.6% for omeprazole, 4.6 and 5.0% for 5-hydroxyomeprazole and 4.6 and 4.9% for omeprazole sulfone at the different concentrations. The limits of quantification were 3 ng/ml for omeprazole and its metabolites. This method was suitable for use in pharmacokinetic studies in human volunteers, and provides a useful tool for measuring CYP2C19 activity.