Concept
A phosphodiesterase type 5 inhibitor (PDE5 inhibitor) is a vasodilating drug that works by blocking the degradative action of cGMP-specific phosphodiesterase type 5 (PDE5) on cyclic GMP in the smooth muscle cells lining the blood vessels supplying various tissues. These drugs dilate the corpora cavernosa of the penis, facilitating erection with sexual stimulation, and are used in the treatment of erectile dysfunction (ED). Sildenafil was the first effective oral treatment available for ED. Because PDE5 is also present in the smooth muscle of the walls of the arterioles within the lungs, two PDE5 inhibitors, sildenafil and tadalafil, are FDA-approved for the treatment of pulmonary hypertension. As of 2019, the wider cardiovascular benefits of PDE5 inhibitors are being appreciated. Phosphodiesterase-5 (PDE5) inhibitors such as sildenafil (Viagra), tadalafil (Cialis), and vardenafil (Levitra) are clinically indicated for the treatment of erectile dysfunction. Sildenafil and tadalafil are also indicated for the treatment of some subtypes of pulmonary hypertension, while tadalafil is also licensed for the treatment of benign prostatic hyperplasia. PDE5 inhibitors have been used as a second line therapy in severe cases of Raynaud phenomenon when it is related to systemic sclerosis per The European Society for Vascular Medicine guidelines. Sildenafil, the prototypical PDE5 inhibitor, was originally discovered during the search of a novel treatment for angina. Studies in 2002 explored its potential for increasing neurogenesis after stroke, but clinical evidence for benefit in cerebrovascular diseases is currently lacking. PDE5 inhibitors are contraindicated within 24 hours (or 48 hours with tadalafil) of taking alpha-blockers, soluble guanylate cyclase stimulators, or nitrate medications such as isosorbide mononitrate or isosorbide dinitrate. Concurrent use of these medications can lead to life-threatening low blood pressure. PDE5 inhibitors are also contraindicated in patients with previous nonarteritic anterior ischaemic optic neuropathy and hereditary eye diseases.
