PDE5A and congenital heart disease: Age 12–70 years with symptomatic PAH (idiopathic, familial, or associated with congenital heart disease, connective tissue disease, or HIV), Mean pulmonary arterial pressure >25 mmHg, pulmonary capillary wedge pressure ≤15 mmHg, PVR > 3 Wood units, Baseline 6MWD: 100–450 m, Stable background ERA and/or PDE-5 inhibitor therapy for ≥90 days (stable dose for ≥30 days).