Simultaneous analysis of oxybutynin and its active metabolite N-desethyl oxybutynin in human plasma by stable isotope dilution LC-MS/MS to support a bioequivalence study.

An isotope dilution high performance liquid chromatography-tandem mass spectrometry (LC-MS/MS) method has been developed for the simultaneous determination of oxybutynin and its pharmacologically active metabolite N-desethyl oxybutynin in human plasma. Extraction of oxybutynin, its metabolite and their deuterated analogs as internal standards (ISs) from 300 μL human plasma was carried out by liquid-liquid extraction with methyl tert-butyl ether-ethyl acetate solvent mixture. Chromatographic separation of analytes was performed on Cosmosil C18 (150 mm × 4.6 mm, 5 μm) column under isocratic conditions with acetonitrile-1.0mM ammonium acetate (90:10, v/v) as the mobile phase. Six endogenous plasma phospholipids (496.3/184.0, 524.3/184.0, 758.5/184.0, 786.5/184.0, 806.5/184.0 and 810.5/184.0) were monitored to determine the extraction efficiency under different extraction conditions. The precursor→product ion transition for both the analytes and ISs were monitored on a triple quadrupole mass spectrometer, operating in the multiple reaction monitoring and positive ionization mode. The method was validated over a concentration range of 0.050-10.0 ng/mL for oxybutynin and 0.500-100 ng/mL for N-desethyl oxybutynin. The mean extraction recovery for analytes (80.4%) and ISs (76.9%) was consistent across five QC levels. Bench top, wet and dry extract, freeze-thaw and long term stability was evaluated for both the analytes. The method was applied to support a bioequivalence study of 5mg tablet formulation in 74 healthy Indian subjects. Assay reproducibility was demonstrated by reanalysis of 344 incurred samples.