Simultaneous quantification of oxybutynin and its active metabolite N‐desethyl oxybutynin in rat plasma by ultra high performance liquid chromatography–tandem mass spectrometry and its application in a pharmacokinetic study of oxybutynin transdermal patch

: A rapid, selective and sensitive ultra-high-performance liquid chromatography-tandem mass spectrometry method was developed to simultaneously determine oxybutynin and its active metabolite N-desethyl oxybutynin in rat plasma. A 0.1 mL sample of plasma was extracted with n-hexane. Chromatographic separation was performed on a UPLC BEH C18 column (2.1 × 100 mm i.d.,1.7 μm) with mobile phase of methanol-water (containing 2 mmol/L ammonium acetate and 0.1% formic acid; 90:10, v/v). The detection was performed in positive selected reaction monitoring mode. Each plasma sample was chromatographed within 3 min. The linear calibration curves were obtained in the concentration range of 0.0944-189 ng/mL (r ≥ 0.99) for oxybutynin and 0.226-18.0 ng/mL (r ≥ 0.99) for N-desethyl oxybutynin. The intra- and inter-day precision (relative standard deviation) values were not more than 14% and the accuracy (relative error) was within ±7.6%. The method described was superior to previous methods for the quantitation of oxybutynin with three product ions and was successfully applied to a pharmacokinetic study of oxybutynin and its active metabolite N-desethyl oxybutynin in rat plasma after transdermal administration.