Determination of olanzapine in human plasma by reversed-phase high-performance liquid chromatography with ultraviolet detection

A sensitive high-performance liquid chromatographic method with ultraviolet absorbance detection for the determination of olanzapine in human plasma is described. Clozapine was used as internal standard. Analytes were concentrated from alkaline plasma by liquid-liquid extraction with n-hexaneisoamyl alcohol (90:10, vol/vol). The organic phase was back-extracted with 150 mu L phosphate buffer (KH2PO4 with 25% H3PO4) 0.1 mol/L pH 2.2. The chromatographic separation required 6 minutes at a flow-rate of 1.0 mL/min and was carried out on a Water Spherisorb S5 C-6 analytical column (250 mm x 4.6 min ID) with a mobile phase water-acetonitrile 55:45 vol/vol containing 0.009 moL/L eptansulfonic acid sodium salt and 0.06mol/L potassium phosphate monobasic pH 2.7. The peaks were detected at 254nm. Mean recoveries for olanzapine and internal standard were 89.4 +/- 3.3% and 90.4 +/- 1.0%, respectively. Coefficient of variation value for intraday was 5.0% at concentrations of 10, 50, and 100 ng/mL and for interday was 4.0% at concentrations of 5 and 25 ng/mL. Accuracy, expressed as percent error, ranged from -8.00% to 1.24%. Limit of detection and limit of quantification for olanzapine were 2 and 5 ng/mL, respectively. The method is suitable for pharmacokinetic studies and therapeutic drug monitoring.