Effect of hepatic CYP inhibitors on the metabolism of sildenafil and formation of its metabolite,N‐desmethylsildenafil,in rats in vitro and in vivo

Objectives It has been reported that hepatic cytochrome P450 (CYP)2C9 and CYP3A4 are responsible for the metabolism of sildenafil and formation of its metabolite, N‐desmethylsildenafil, in humans. However, in‐vivo studies in rats have not been reported. Methods Sildenafil (20 mg/kg) was administered intravenously to rats pretreated with sulfaphenazole, cimetidine, quinine hydrochloride or troleandomycin, inhibitors of CYP2C6, CYP2C11, CYP2D subfamily and CYP3A1/2, respectively. In‐vitro studies using rat liver microsomes were also performed. Key findings The area under the plasma‐concentration time curve (AUC) was increased and clearance of sildenafil decreased in rats pretreated with cimetidine or troleandomycin. The AUC ratio for N‐desmethylsildenafil (0–4 h): sildenafil (0–∞) was significantly decreased only in rats pretreated with cimetidine. Similar results were obtained in the in‐vitro study using rat liver microsomes. Conclusions Sildenafil is metabolised via hepatic CYP2C11 and 3A1/2, and N‐desmethylsildenafil is mainly formed via hepatic CYP2C11 in rats. Thus, rats could be a good model for pharmacokinetic studies of sildenafil and N‐desmethylsildenafil in humans.