HPLC determination of sildenafil and its demethylated metabolite, UK-103,320, in cat plasma with solid-phase extraction for the pharmacokinetic study

A simple and reliable high performance liquid chromatographic (HPLC) assay method was developed for the simultaneous determination of sildenafil and its demethylated metabolite (UK-103,320) in cat plasma. Solid-phase extraction with a C₁₈SepPak cartridge after deproteinization was used, and the extraction recoveries were over 85% in both sildenafil and UK-103,320. The coefficient of variation was less than 15% in the 50-1000 ng/ml range for both sildenafil and UK-103,320, indicating that this assay has adequate reproducibility. This method was used to perform pharmacokinetic studies following oral administration of sildenafil citrate. Moment analysis revealed that the elimination half-life (7.4 ± 2.1 hr) was similar to those reported in studies using [¹⁴C]-sildenafil and the AUC percentage of UK-103,320 to sildenafil was also similar to the results of a previous report. These results indicate that this convenient analytical method is readily applicable to pharmacokinetic studies of sildenafil and UK-103,320.