Liquid chromatographic-tandem mass spectrometric method for the quantitation of sildenafil in human plasma

A method to determine sildenafil in human plasma involving liquid chromatography–tandem mass spectrometry (LC–MS/MS) has been developed. Sildenafil and the internal standard (I.S.), diazepam, are extracted from human plasma with ether–dichloromethane (3:2, v/v) at basic pH and analyzed by reversed-phase high-performance liquid chromatography (HPLC) using methanol–10 mM ammonium acetate pH 7.0 (85:15, v/v) as the mobile phase. Detection by electrospray positive ionization mass spectrometry in the multiple-reaction monitoring mode was linear over the concentration range 0.125–40.0 ng/ml. Intra- and inter-day precision of the assay at four concentrations within this range were 2.5–8.0%. The method was used to evaluate plasma concentration–time profiles in healthy volunteers given an oral dose of 20 mg sildenafil as a combination tablet also containing apomorphine.