Sildenafil Citrate

Potent and selective phosphodiesterase (PDE5) inhibitor, IC50=4 nM) [1-3]. Clinically useful agent for male erectile disfunction [4] and pulmonary hypertension [5]. 

References:

1) Terrett et al. (1996), sildenafil (VIAGRATM), a potent and selective inhibitor of type 5 cGMP phosphodiesterase with utility for the treatment of male erectile dysfunction;  Bioorg. Med. Chem. Lett., 6 1819

2) Ballard et al. (1998), Effects of sildenafil on the relaxation of human corpus cavernosum tissue in vitro and on the activities of cyclic nucleotide phosphodiesterase isozymes;  J. Urol., 159 2164

3) Turko et al. (1999), Inhibition of cyclic GMP-binding cyclic GMP-specific phosphodiesterase (Type 5) by sildenafil and related compounds; Mol. Pharmacol., 56 124

4) Teixeira et al. (2006), differential effects of the phosphodiesterase type 5 inhibitors sildenafil, vardenafil, and tadalafil in rat aorta;  J. Pharmacol. Exp. Therap., 316 654

5) Ghofrani et al. (2006), Sildenafil: from angina to erectile dysfunction to pulmonary hypertension and beyond;  Nat. Rev. Drug Discov., 5 689