Inhibition of cyclic GMP hydrolysis in human corpus cavernosum smooth muscle cells by vardenafil, a novel, selective phosphodiesterase type 5 inhibitor

One of the key mediators of penile erectile function is nitric oxide (NO), which activates soluble guanylyl cyclase within the smooth muscle of erecti le tissue and stimulates the production of cGMP. In addition to synthesis b y cyclases, intracellular cGMP concentrations are tightly regulated by phos phodiesterases, which hydrolyze and inactivate cyclic nucleotides. In this study, we compared the inhibition of cGMP hydrolysis by vardenafil and sild enafil; two inhibitors selective for phosphodiesterase type 5 (PDE5). Varde nafil is a novel, high affinity PDE5 inhibitor currently under clinical dev elopment. In soluble extracts of human corpus cavernosum smooth muscle cell s, vardenafil and sildenafil effectively inhibited cGMP hydrolysis at subst rate concentrations of 1, 5 and 10 muM cGMP. The IC50 values for vardenafil were approximately 5-fold lower than for sildenafil at the substrate conce ntrations tested. Dixon plot analyses of the inhibition data demonstrated t hat vardenafil had a smaller inhibition constant (K-i = 4.5 nM) than silden afil (K-i = 14.7 nM) in the same cellular extracts. In intact cells, 10 muM of the nitric oxide donor sodium nitroprusside resulted in a minimal (17%) increase in cGMP, relative to basal levels (321 +/- 65 fmol/mg prot). Trea tment of cells with 10, 50 or 100 nM vardenafil, in the presence of 10 muM sodium nitroprusside, elevated cGMP levels in a dose dependent fashion, fro m 63% to 137% of basal levels. Equimolar concentrations of sildenafil also caused dose dependent increases in intracellular cGMP, but to a lesser exte nt (27-60%). These observations suggest that vardenafil is a more potent PD E5 inhibitor, than sildenafil in vitro. The more pronounced increase of cGM P in the presence of NO in intact cells suggests that vardenafil will be ef fective at lower doses than sildenafil under clinical conditions. (C) 2001 Elsevier Science Inc. All rights reserved.