piperazine sulfonamide pyrazolopyrimidine sildenafil http://id.loc.gov/authorities/subjects/sh98000937 http://purl.bioontology.org/ontology/RXNORM/136411 http://purl.bioontology.org/ontology/SNOMEDCT/372572000 http://purl.obolibrary.org/obo/CHEBI_9139 http://purl.obolibrary.org/obo/NCIT_C61940 https://en.wikipedia.org/wiki/Sildenafil https://www.wikidata.org/wiki/Q191521 A pyrazolo[4,3-d]pyrimidin-7-one having a methyl substituent at the 1-position, a propyl substituent at the 3-position and a 2-ethoxy-5-[(4-methylpiperazin-1-yl)sulfonyl]phenyl group at the 5-position. G04BE03 An orally bioavailable pyrazolopyrimidinone derivative structurally related to zaprinast, with vasodilating and potential anti-inflammatory activities. Upon oral administration, sildenafil selectively targets and inhibits cyclic guanosine monophosphate (cGMP)-specific phosphodiesterase type 5 (PDE5), thereby inhibiting the PDE5-mediated degradation of cGMP found in smooth muscle and increasing cGMP availability. This results in prolonged smooth muscle relaxation in the corpus cavernosum of the penis, thereby causing vasodilation, blood engorgement and a prolonged penile erection. In the smooth muscle of the pulmonary vasculature, the increase in cGMP results in smooth muscle relaxation, vasodilation of the pulmonary vascular bed, relieving pulmonary hypertension and increasing blood flow in the lungs. In addition, sildenafil may reduce airway inflammation and mucus production.