IC50: 3.5 nM Sildenafil mesylate is an inhibitor of cyclic guanosine monophosphate (cGMP)-specific phosphodiesterase type 5 (PDE5). PDE5 is the predominant phosphodiesterase in the corpus cavernosum. The catalytic site of PDE5 degrades cGMP, and PDE5 inhibitors can potentiate endogenous increases in cGMP. In vitro: Sildenafil mesylate had been identified as a potent PDE5 reversible and selective inhibitor. Sildenafil enhanced sodium nitroprusside- or transmural electrical stimulation-induced relaxation of precontracted corpus cavernosum muscle strips in organ baths. Sildenafil also increased intracellular cGMP concentrations in cultured smooth muscle cells treated with sodium nitroprusside . In vivo: In anesthetized dogs, sildenafil could enhance the erectile function following pelvic nerve stimulation as measured by increased intracavernosal pressure . Clinical trial: Ten previous randomised controlled trials had been conducted. Dose optimisation resulted in >60% of attempts at sexual intercourse, which was successful in 49% of men. Sildenafil treatment related adverse events occurred in 30% of men compared with 11% on placebo. Dose optimisation of sildenafil showed efficacy equivalent to the highest fixed doses, and adverse events equivalent to the lowest fixed doses .