Study to Evaluate Efficacy and Safety of JTT-251 in Participants With Pulmonary Arterial Hypertension

Inclusion Criteria: * Diagnosis of PAH as classified by idiopathic, heritable, drug and toxin- induced, congenital heart disease or associated with connective tissue disease (i.e., WHO Group 1) * Clinical diagnosis of PAH confirmed by RHC at any time prior to Visit 1 * WHO functional status of Class II-IV at Visit 1 * Two 6MWD test measurements between 100 and 450 meters with a relative difference of ≤15%. The baseline 6MWD test must be performed at Visit 2 before randomization. * Have a qualifying RHC performed between Visit 1 and Visit 2 * On stable dose(s) of guideline-directed medical therapy for PAH (endothelin receptor antagonists, phosphodiesterase type-5 (PDE-5) inhibitors, soluble guanylate cyclase stimulators and prostacyclin pathway analogs) for at least 90 days prior to the qualifying RHC Exclusion Criteria: * PAH associated with portal hypertension, human immunodeficiency virus (HIV), schistosomiasis or sickle cell disease as well as participants with pulmonary parenchymal disease or thromboembolic disease * Known significant left heart disease including: left ventricular dysfunction (i.e., left ventricular ejection fraction \<35%); hemodynamically compromising, symptomatic, or severe aortic stenosis, aortic regurgitation, mitral stenosis, mitral regurgitation * Pulmonary hypertension belonging to WHO groups 2 to 5 * Moderate to severe obstructive lung disease defined as forced expiratory volume in 1 second (FEV1) \<55% of predicted value * Moderate to severe restrictive lung disease defined as total lung capacity (TLC) \<60% of predicted value * Acute decompensated heart failure or hospital admission for worsening PAH symptoms within 30 days prior to the qualifying RHC