Sildenafil Citrate (Synonyms: Apodefil, Tonafil, UK 92480)

Sildenafil Citrate is a potent inhibitor of cGMP-specific phosphodiesterase including PDE5 with IC50 values of 3.6 nM.[1]
PDE5 (cGMP-specific phosphodiesterase type 5) is an enzyme from the phosphodiesterase class. The function of PDE5 is to accept cGMP and breaks it down. cGMP (cyclic guanosine monophosphate) is one of second messenger which is the production of guanylate cyclase. cGMP is a regulator of cellular apoptosis glycogenolysis, conductance , and ion channel. It can relaxe smooth muscle tissues. In blood vessels, It also can relaxe vascular smooth muscles, then lead to increased blood flow and vasodilation. The inhibitors of PDE5 can inhibit the degradation of cGMP, then leads to increase bloodflow to the penis. Sildenafil Citrate is used treatment of erectile dysfunction.
Sildenafil is a potent cGMP-specific phosphodiesterase. It is highly selective for PDE5. It inhibits the activity of PDE5 with an IC50 value of 3 nM in human corpus cavernosum (3.6nM in isolated rabbit platelets). At the same condition, It inhibits the activity of PDE1 and PDE3 activities with IC50s = 0.26 and 65 μM, respectively.[1] Sildenafil potently relaxed anococcygeus muscle strip swith maximum response of 100 ± 2% and a pEC50 value of 6.44 ± 0.03 in rat. Sildenafil also increased by approximately 55% the duration of nitrergic relaxations.[1, 2] Compared to the base salt, the citrate salt has pharmacokinetics and superior water solubility.[3]
References:
[1].    Teixeira CE, Priviero FB, Claudino MA, Baracat JS, De Nucci G, Webb RC, Antunes E: Stimulation of soluble guanylyl cyclase by BAY 41-2272 relaxes anococcygeus muscle: interaction with nitric oxide. Eur J Pharmacol 2006, 530(1-2):157-165.
[2].    Richalet JP, Gratadour P, Robach P, Pham I, Dechaux M, Joncquiert-Latarjet A, Mollard P, Brugniaux J, Cornolo J: Sildenafil inhibits altitude-induced hypoxemia and pulmonary hypertension. Am J Respir Crit Care Med 2005, 171(3):275-281.
[3].    Jung SY, Seo YG, Kim GK, Woo JS, Yong CS, Choi HG: Comparison of the solubility and pharmacokinetics of sildenafil salts. Arch Pharm Res 2011, 34(3):451-454.