Simultaneous Determination of Resibufogenin and Its Major Metabolite 3-epi-Resibufogenin in Rat Plasma by HPLC Coupled with Tandem Mass Spectrometry

A rapid, sensitive and specific method for the simultaneous quantification of resibufogenin (RBG) and 3-epi-resibufogenin (3-ERBG) in rat plasma was developed by using a liquid–liquid extraction procedure and liquid chromatography–electrospray ionization/tandem mass spectrometric (LC–ESI–MS/MS) analysis. The separation was performed by HPLC on a reversed phase C18 HPLC column (150 × 2.1 mm, 3.5 μm) using a mobile phase of acetonitrilel-0.1% formic acid aqueous solution (45:55, v/v). The determination was performed by a triple-quadrupole mass spectrometer in the multiple reaction monitoring using positive mode of electrospray ionization (ESI). The calibration curves were both linear (R > 0.995) over the concentration range of 3.0–5,000 ng mL−1, and the lower limits of quantification were 3.0 ng mL−1 for both RBG and 3-ERBG. The intra-day and inter-day precisions (% RSD) were all less than 15%, and the accuracies (%RE) were within the range of ±15%. The mean recoveries of RBG, 3-ERBG and IS were over 82.7, 84.8 and 90.0% (n = 6), respectively. The method was proved to be rapid, sensitive and specific, and has been successfully applied to determine RBG and its major metabolite 3-ERBG in rat plasma after oral administration of RBG for pharmacokinetic study. Comparison of pharmacokinetic data with anti-tumor activities of RBG and ERBG suggested that 3-ERBG, as a major metabolite of RBG in rats, was perhaps also a bioactive form of RBG in vivo.