Res-Erection of Viagra as a Heart Drug

Abstract

In 1998, the world was formally introduced to Viagra (sildenafil), the first oral agent to be approved in the United States for the treatment of erectile dysfunction. Developed by Pfizer chemists, sildenafil inhibited phosphodiesterase type 5 (PDE5), one of the 11-member superfamily of enzymes that hydrolyze cyclic nucleotide monophosphates and among the first discovered that was selective for cGMP. PDE5 had been identified more than 2 decades earlier, first as a protein that bound cGMP and later as one that hydrolyzed it (the binding function turning out to be a mechanism to regulate activity).1 Sildenafil induces vasorelaxation by blocking PDE5-cGMP hydrolysis, raising cGMP levels in smooth muscle to activate protein kinase G (also known as cGK). In platelets, this blunts thrombosis; together, the effects suggested a potential use for coronary vascular disease and hypertension. Pfizer initiated trials targeting coronary ischemia, but, although antianginal effects were disappointing, the study gave a whole new meaning to the term “side effect,” effectively hijacking sildenafil from cardiovascular development to a rather different indication. Article see p 8 There were still some cardiovascular studies performed, but essentially all were limited assessments of volunteer populations, mostly normal and at rest and some with cardiovascular disease; the results were interpreted to support safety for patients taking the drug. In particular, blood pressure changes were slight if present at all, and cardiac function appeared unaltered. Well into the mid 2000s, PDE5 was thought to be expressed in selective vascular beds, lung and corpus cavernosum being dominant, and to have a minor cardiovascular profile elsewhere, with a negligible role in the heart. Sildenafil reemergence as …