The pharmacology of sildenafil, a novel and selective inhibitor of phosphodiesterase (PDE) type 5
Abstract
Sildenafil (1-[4-ethoxy-3-(6,7-dihydro-1-methyl-7-oxo-3-propyl-1H-pyrazolo [4,3-d]pyrimidin-5-yl) phenylsulphonyl]-4-methylpiperazine) has been shown to be an effective oral treatment for male erectile dysfunction. Sildenafil is a potent competitive inhibitor of PDE5 (IC50 3.5 nM) and is selective over PDE1 to 4 (80 to 19,000-fold) and retinal PDE6 (10-fold). Sildenafil enhanced cGMP accumulation driven with sodium nitroprusside in the corpus cavernosum of rabbits without affecting cAMP formulation. In the absence of nitric oxide drive, sildenafil had no functional effect on the human and rabbit isolated corpus cavernosum, but potently potentiated the relaxant effects of nitric oxide on these tissues. In the anaesthetised dog, sildenafil (ED50: 12 to 16 micrograms/kg i.v.) enhanced the increase in intracavernosal pressure induced by electrical stimulation of the pelvic nerve or intracavernosal injection of sodium nitroprusside in the absence of meaningful effects on blood pressure. Consistent with its mode of action, sildenafil potentiated the vasorelaxant effects of glyceryl trinitrate on rabbit isolated aortic rings. However, unlike milrinone, sildenafil had no inotropic effects on the dog isolated trabeculae carneae. Thus it is unlikely to have the deleterious effects on cardiac function associated with PDE3 inhibitors. As a consequence of inhibition of PDE6 in the retina, sildenafil (1 to 100 microM) altered the kinetics of the light response of the dog isolated retina.…
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