Sildenafil (Item No. 10008671) is a potent cGMP-specific phosphodiesterase (PDE) inhibitor that is highly selective for PDE5 (IC50 = 3.6 nM).{21636} Inhibition of PDE5 prolongs nitric oxide-mediated relaxation of smooth muscle in a range of vascular tissues. N-Desmethyl sildenafil is a major metabolite of sildenafil.{29612} The conversion of sildenafil to this metabolite is affected by age and by renal and hepatic impairment.{29611}