Mezzion Pharma and the National Heart, Lung and Blood Institute (NHLBI) announced the initiation of enrollment in a pivotal Phase 3 efficacy and safety trial for the treatment of adolescents with congenital single ventricle heart disease. This randomized, double-blind, placebo-controlled clinical trial will evaluate the safety and efficacy of orally administered udenafil in approximately 400 adolescents with a single functional ventricle who had previously undergone Fontan surgical palliation. The study is enrolling male and female adolescent subjects at 24 Pediatric Heart Network (PHN) and auxiliary sites throughout the US, Canada and Korea. The clinical program for this orphan drug indication is using protocols developed by PHN which is funded by NHLBI, an Institute of the National Institutes of Health (NIH). The Phase 3 protocols recently received special protocol assessment (SPA) review by the FDA and were found sufficient to support a regulatory submission for the orphan drug indication. Having such an indication would be a first for patients with congenital heart disease. The primary efficacy outcome will be improvement in exercise capacity as measured by the change in maximal VO2 after 6 months of daily therapy compared to placebo. People who have had the Fontan procedure often find that their ability to exercise decreases significantly during their teenage years, so preventing this decline would be an important clinical outcome. There are also a number of the secondary endpoints, including safety assessments. The Fontan procedure is the final surgery in a series of palliative surgical operations in children born with single ventricle congenital heart disease. Fontan patients have a significantly shortened life span, often not surviving past the third or fourth decade of life, due to deteriorating ventricular performance, increasing pulmonary vascular resistance and associated secondary pathologies. Mezzion Pharma calculates that there are approximately 29,000 persons alive in the US with this disease. Udenafil is a relatively long-acting PDE5 inhibitor. PDE5 inhibitors have demonstrated utility in reducing pulmonary vascular resistance and in improving ventricular performance in other types of patients. Those characteristics make PDE5 inhibitors, especially a long-acting compound such as Udenafil, potentially attractive as a long-term interventional therapy in that segment of the Fontan population that is still relatively unimpaired by the disease.