Simultaneous determination of sildenafil and N-desmethyl sildenafil in human plasma by high-performance liquid chromatography method using electrochemical detection with application to a pharmacokinetic study

A method which employed high-performance liquid chromatography coupled with electrochemical detection was developed for the simultaneous determination of sildenafil and its metabolite, N-desmethyl sildenafil, in human plasma has. The method was developed and validated for purposes of its application to a pharmacokinetic study in healthy volunteers after an oral dose of 50 mg/tablet under fasting conditions. High precision and accuracy were demonstrated. A one-step liquid–liquid extraction further provides a simple and practical way to process plasma samples containing sildenafil with good quantitative recovery. Sampling lasted for 24 h after dosing; consequently a limit of quantitation (LOQ) of 7.858 ng/mL was achieved for sildenafil whereas a LOQ of 8.675 ng/mL was obtained for N-desmethyl sildenafil. The mobile phase consisted of acetonitrile, methanol and phosphate buffer (0.05 M) (18.5:34.5:47.0, v/v/v) pH 7.68. The stationary phase was a C8 (150 mm × 4.6 mm), 5 μm particle size operated at 27 °C. All analytes were stable at the pH of the supernatant, and during the analytical time window. At the applied potential of +1.20 V versus Ag/AgCl, no interferences from endogenous plasma compounds were recorded at the retention times of sildenafil, N-desmethyl sildenafil. High resolution was obtained between the analytes and the employed internal standards.