Simultaneous quantification of oxysophocarpine and its active metabolite sophocarpine in rat plasma by liquid chromatography/mass spectrometry for a pharmacokinetic study

A sensitive, rapid and specific method for the simultaneous quantification of oxysophocarpine (OSC) and its active metabolite sophocarpine (SC) in rat plasma was developed and validated, using a liquid-liquid extraction procedure followed by liquid chromatography/ electrospray ionization mass spectrometric (LC/ESI-MS) analysis. The separation was performed on a Zorbax Extend-C₁₈ column (2.1 mm i.d. × 50 mm, 5 μm) with a C₁₈ guard column using methanol-water containing 5 mM ammonium acetate (15:85, v/v) as mobile phase. Analysis was performed in selected ion monitoring (SIM) mode with an electrospray ionization (ESI) interface. [M + H]⁺ at m/z 263 for OSC, [M + H]⁺ at m/ z 247 for SC and [M + H]⁺ at m/z 249 for matrine (internal standard) were selected as detecting ions, respectively. The method was linear in the concentration ranges 10-1000 ng/mL for OSC and 5-500 ng/mL for SC. The intra- and inter-day precisions (coefficient of variation) were within 7% for both analytes. Their accuracy (relative error) ranged from -6.4 to 1.5%. The limits of detection for OSC and SC were 3 and 1.5 ng/ mL, respectively. The limits of quantitation for OSC and SC were 10 and 5 ng /mL, respectively. Recoveries of both analytes were greater than 85% at the low, medium and high concentrations. Both analytes were stable during all sample storage, preparation and analytic procedures. The method was successfully applied to a pharmacokinetic study after an oral administration of OSC to rats with a dose of 15 mg/kg.