Solid phase extraction and liquid chromatographic determination of sildenafil and N-demethylsildenafil in rat serum with basic mobile phase.

HPLC method for the determination of sildenafil and its metabolite (N-demethylsildenafil) in rat serum has been developed. The technique included a solid phase extraction of the serum samples on a [poly(divinylbenzene-co-N-vinylpyrrolidone)] solid phase extraction sorbent. After conditioning, the cartridge was loaded with 0.5 mL of buffered serum containing internal standard. Elution was made with 1 mL of acetonitrile. After evaporation of the eluates to dryness and reconstitution with methanol, the samples were analyzed on Kromasil C18 column phase with phosphate buffer 0.05 M/acetonitrile: 54/46, pH 8. Detection was carried out using a photodiode array detector. For sildenafil and demethylsildenafil, full validation of the proposed method was provided (linearity range, calibration curves, average extraction efficiency; average intra-day and interday variabilities, limit of detection, limit of quantification, specificity). The proposed method was successfully utilised to quantify sildenafil and N-demethylsidenafil in rat serum for a pharmacokinetic study.