SIMPLE AND EFFICIENT HPLC-UV QUANTITATION OF ETOPOSIDE AND ITS CIS-ISOMER IN RAT MICRO-VOLUME PLASMA AND TISSUE SAMPLES: APPLICATION TO PHARMACOKINETIC AND BIODISTRIBUTION STUDIES

The present study was aimed at developing a validated HPLC method to quantify the sufficiently low concentrations of etoposide and its cis-isomer in plasma and tissue samples with emphasize on (1) employing a conventional UV detector, (2) using a low biological sample size and a minimized organic solvent volume, and (3) providing precise information about the conversion of etoposide to its cis-isomer in biological samples. The analysis was carried out on a monolithic C-18 column using a mixture of methanol, acetonitrile, and phosphate buffer (18: 19:63, v/v/v) containing 0.007% triethylamine as the mobile phase. The analytes were detected using UV absorption at 285 nm. The sample preparation procedure involved the liquid-liquid extraction of analytes and IS from 100 mu L plasma or 200 mu L of tissue homogenates in 0.5-1 mL mixture of chloroform and n-hexane (80: 20, v/v). The limit of detection of etoposide and its cis-isomer in plasma and tissue samples, with considering a signal-to-noise ratio of 3: 1, was 20 ng/mL. Stability studies at room temperature indicated that etoposide in plasma samples was stable for only two hr, after which the trans-isomer was converted to the cis-isomer. The described method was successfully applied toward etoposide pharmacokinetic and tissue distribution studies in rats.