PDE5

Class

Phosphodiesterase-5 inhibitor

Phosphodiesterase type 5 (PDE5) is an enzyme that breaks down cyclic guanosine monophosphate (cGMP), a small signaling molecule that relaxes smooth muscle, the involuntary muscle lining blood vessels and certain other tissues. cGMP is generated when nitric oxide (NO), released from the lining of blood vessels, activates an enzyme called guanylate cyclase. By degrading cGMP, PDE5 limits how long and how strongly this relaxation signal persists. Inhibiting PDE5 slows that breakdown, so cGMP accumulates and smooth muscle relaxation is prolonged and amplified wherever NO signaling is already active. This mechanism is central to erectile function, where NO release during sexual stimulation triggers vessel relaxation and increased blood flow, and to pulmonary vascular tone, where excessive constriction contributes to pulmonary hypertension. Because PDE5 is expressed in vascular smooth muscle across multiple organ systems, its inhibitors have also been investigated for cardiovascular and neurological conditions where blood flow or cGMP signaling is implicated, including cognitive decline. The overall rationale is to selectively reinforce an existing, localized signaling pathway rather than to introduce a new stimulus, which is thought to limit the mechanism's effects to tissues where NO release is already occurring.