Determination of simvastatin in human plasma by column-switching HPLC with UV detection

The purpose of this study was to develop and validate a sensitive and specific analytical method for determination of simvastatin in human plasma by the column-switching high performance liquid chromatography (HPLC) system with UV detection. Simvastatin was extracted in diethyl ether from plasma. The residue was dissolved in mobile phase I [acetonitrile-20 mM potassium phosphate buffer (45 : 55, v/v, pH 5.6)] and the solution was injected into a pre-column. The analytes fractionated from the pre-column by a valve switching step were concentrated on the top of an intermediate C-18 column. Then, the concentrated simvastatin was separated to the analytical column with a mobile phase II [acetonitrile-20 mM potassium phosphate buffer (65 : 35, v/v, pH 5.6)], using a UV detector at the wavelength of 238 nm. Simvastatin was eluted at 28.7 min without interference of endogenous material in plasma. The limit of quantification (LOQ) was 0.5 ng/mL of simvastatin. The calibration curve was linear in the concentration range of 0.5-20 ng/mL (r(2) = 0.9986). Moreover, the inter- and intra-day precisions of this method were less than 15%. The average recovery of extraction was 89.7% over the concentration. The assay was successful in measuring plasma concentrations of simvastatin in three volunteers after oral administration (80 mg simvastatin).