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 anti-influenza 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 C18 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 °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.