Determination of oseltamivir carboxylic acid in human serum by solid phase extraction and high performance liquid chromatography with UV detection

This study was aimed at developing a fast and sensitive method for determination of oseltamivir carboxylic acid (OCA), the active moiety of antiinfluenza agent, oseltamivir phosphate, in human serum by high performance liquid chromatography and UV detection. The analyte and an internal standard (vanillin) were extracted from human serum by a solid phase extraction (SPE) procedure. Chromatographic separation was achieved using a reverse phase C 18 column with a mobile phase consisting of 0.05 M phosphate buffer containing triethylamine (1 mL/L; pH 3.0) and acetonitrile (70:30, v/v). The detection wavelength was set at 215 nm. The average recoveries of the drug and internal standard were 98 and 85%, respectively. The calibration curve was linear over a concentration range of 15-6400 ng/mL of OCA in human serum. The lower limits of detection and quantification were 5 and 15 ng/mL, respectively. The coefficient variation values of both inter- and intra-day analysis were less than 12% whereas the percentage error was less than 4.5. The stability of the drug at the serum samples maintained at -40 degrees C for 60 days was found to be 100% from the initial value and no interferences were found from either endogenous components in serum or commonly co-administrated antiviral drugs. The validated method was applied to a randomized cross-over bioequivalence study of two different oseltamivir phosphate preparations in 24 healthy volunteers. (C) 2008 Elsevier B.V. All rights reserved.