A Study of HS135 for the Treatment of Pulmonary Arterial Hypertension in Adults

Inclusion Criteria: 1. Male and female patients, 18 years of age and older. 2. Body Mass Index (BMI) 18.5 and 40 kg/m2 and body weight at or below 120 kg 3. Documented diagnostic RHC at any time prior to screening. 4. Diagnosis of WHO PAH Group 1. 5. Symptomatic PAH classified as WHO FC II to IV. 6. Baseline RHC performed during the Screening Period documenting a minimum PVR of ≥ 5 Wood units (WU) or ≥ 400 dyn・sec・cm-5 and a pulmonary artery wedge pressure (PAWP) of ≤ 15 mmHg. 7. On stable doses of at least 2 background PAH therapies. Exclusion Criteria: 1. Left ventricular ejection fraction \< 50% at screening. 2. Any symptomatic coronary disease events within 6 months of the screening visit. 3. Uncontrolled systemic hypertension. 4. History of restrictive, constrictive or congestive cardiomyopathy. 5. History of atrial septostomy. 6. Patients who have an abnormality in the Echocardiogram or in the 12-lead ECG that, in the opinion of the Investigator, increases the risk of participating in the study. 7. Pulmonary function test (PFT) values of forced vital capacity (FVC) and or FEV1 \< 60% predicted at the screening visit or within 6 months prior to the screening visit.