New Approaches For The Treatment Of Erectile Dysfunction In Conditions Of Low Nitric Oxide Formation Or Bioavailability: Investigation Of Rho-kinase Inhibitors And Soluble Guanylate Cyclase-targeted Therapies.
Nitric oxide (NO) is the principal mediator of erectile function. NO is released from the nerves and endothelium of small arteries in the penis and diffuses into surrounding smooth muscle to vasodilate through activation of soluble guanylate cyclase (sGC). Erectile dysfunction (ED) occurs in 50% of men between the ages of 40 and 70. It is likely that the pathology of ED results from impairment of NO formation or bioavailability in penile tissue. Iatrogenic nerve damage occurring during prostatectomy can attenuate neurotransmission and release of vasodilators from cavernosal nerves. Oxidative stress from chronic conditions such as diabetes and cardiovascular disease generates reactive oxygen species that can oxidize NO and decrease the molecule's bioavailability. The "gold standard" treatment for ED involves use of oral PDE-5 inhibitors that rely on an intact NO-signaling mechanism for efficacy. Although these therapies are easy to use, they are not effective in many patients suffering from ED associated with pathological conditions of decreased NO bioavailability. Rho-kinase inhibitors, sGC stimulators and sGC activators offer three new interventions that may demonstrate efficacy in treating ED associated with low NO bioavailability. Our results suggest that erectile responses to Rho-kinase inhibitors are not modulated by muscarinic receptor blockade, soluble guanylate cyclase inhibition or cavernosal nerve injury in the rat and that Rho-kinase inhibitors are additive and do not potentiate the endogenous NO-mediated erectile response. Our results with BAY 41-8543 show that this sGC stimulator has significant erectile activity and can potentiate erectile responses to low levels of exogenous and endogenously released NO. These results suggest that BAY 41-8543 would be useful in the treatment of ED occurring following nerve damage from prostatectomy. The sGC activator BAY 60-2770 has very potent erectile activity that is enhanced significantly in conditions of oxidative stress when erectile responses to endogenous NO or sGC stimulators are severely diminished. In oxidizing conditions erectile activity of sGC activators may be enhanced further with concomitant PDE-5 inhibitor therapy, providing evidence that sGC activators may be used alone and in combination with existing treatments to improve erectile function in patients who are non-responsive to standard therapeutic options for ED.