Development and validation of a gas chromatography-mass spectrometric method for the determination of sildenafil and desmethyl-sildenafil in whole blood.

Sildenafil (SDL) is a phosphodiesterase type 5 inhibitor and it is approved for the treatment of erectile dysfunction and pulmonary hypertension. SDL is extensively metabolized to its pharmacologically active metabolite, desmethyl-sildenafil (DSDL). A sensitive and specific GC/MS method for the determination of SDL and DSDL in whole blood was developed and validated to support therapeutic drug monitoring of SDL patients. The combination of solid-phase extraction with derivatization using BSTFA with 1% TMCS in acetonitrile efficiently reduced matrix effect and improved sensitivity of the method. In this assay, protriptyline was used as internal standard for both analytes. The LODs were 1.50 and 5.00 ng/mL for SDL and DSDL, respectively, whereas the respective LOQs were 5.00 and 15.0 ng/mL. The calibration curves were linear up to 500.0 ng/mL (SDL: R(2) 0.992, DSDL: R(2) 0.990). Absolute recovery values for both analytes ranged from 83.1 to 93.2%. Within- and between-batch accuracy was less than 11.8 and 10.2%, respectively, whereas within- and between-batch precision was less than 8.1 and 10.8%, correspondingly. The developed method is suitable for the determination of SDL and DSDL concentrations in blood samples obtained from patients under Viagra(®) treatment, for pharmacokinetic studies or for the investigation of related forensic cases.