Pharmacology and Drug Interaction Effects of the Phosphodiesterase 5 Inhibitors: Focus on α-Blocker Interactions

Currently, 3 phosphodiesterase 5 (PDE5) inhibitor agents are available worldwide for the treatment of erectile dysfunction (ED): sildenafil, vardenafil, and tadalafil. Each of these agents is effective across a broad range of etiologies, including vasculogenic ED in men. Because PDE5 enzyme is found within the vascular smooth muscle cells in the walls of systemic arteries and veins, PDE5 inhibitors are mild vasodilators associated with small (and in general, clinically insignificant) decreases in blood pressure. However, because of the synergistic decrease in blood pressure (both systolic and diastolic) in the presence of organic nitrates, these 3 agents are contraindicated in patients receiving organic nitrates. The duration of interaction between a PDE5 inhibitor and nitrate administration depends on the specific drug being studied. The interaction between sildenafil or vardenafil and nitroglycerin is no longer observed by 24 hours. A preliminary study with sildenafil and sublingual nitroglycerin suggested the interaction is no longer observable by 4 hours. The interaction between tadalafil and nitroglycerin has dissipated by 48 hours after tadalafil administration. This is consistent with the longer elimination half-life of the drug. When PDE5 inhibitors are administered to patients with hypertension who are taking most antihypertensive agents (eg, beta-blockers, angiotensin-converting enzyme inhibitors, angiotensin receptor blockers, calcium antagonists, diuretics), there are usually small additive decreases in blood pressure without a significant increase of adverse events. Some patients develop orthostatic hypotension when PDE5 inhibitors are used in conjunction with an alpha-blocker (typically for hypertension or for urologic conditions, such as benign prostatic hypertrophy). Precautions are necessary for all 3 of the PDE5 inhibitors regarding this potential interaction. Some studies suggest that the interaction is less relevant clinically if the patient has been undergoing long-term alpha-blocker therapy. Several analyses have suggested that PDE5 inhibitors do not increase myocardial infarction rates or death rates compared with placebo controls or expected rates from age-matched populations. In contrast, recent studies have shown that PDE5 inhibitors may have therapeutic potential for a host of cardiovascular diseases. In general, these agents, when used appropriately, are highly safe and effective.