The purpose of this study is to assess the safety and efficacy of Rodatristat Ethyl in pulmonary arterial hypertension (PAH) patients.

Rodatristat Ethyl is a peripherally restricted TPH inhibitor being studied as a potential treatment for PAH. This dose-ranging, randomized, double-blind, placebo-controlled, multicenter study will evaluate the effect of Rodatristat Ethyl from baseline on pulmonary vascular resistance as measured at right heart catheterization.

Patients will be enrolled into a main study with an option to enroll into an open label extension.

The study is expected to enroll patients in the USA, Canada and Europe.

Inclusion Criteria:

1. Male and female 18 years or older 2. Body Mass Index (BMI) >18kg/m2 to <=40kg/m2 3. Symptomatic PAH belonging to one of the following 2018 WHO Clinical Group 1 subtypes:

a. Idiopathic PAH b. Heritable PAH c. Drug- or toxin-induced d. PAH associated with:

1. Connective tissue disease

2. Congenital systemic to pulmonary shunt (atrial septal defect, ventricular septal defect, patent ductus arteriosus) repaired at least one year prior to Screening

3. Human immunodeficiency virus (HIV) infection - if diagnosed with HIV, must have stable disease status defined as follows:

   1. stable treatment with HIV medications for at least 8 weeks prior to Screening
   2. no active opportunistic infection during the Screening Period
   3. no hospitalizations due to HIV for at least 4 weeks prior to Screening

4. WHO FC II or III

5. Confirmed diagnosis of PAH and meet all the following hemodynamic criteria by means of a screening RHC completed prior to randomization:

   1. mPAP of >20 mmHg
   2. PVR ≥ 350 dyne•sec/cm5
   3. Pulmonary capillary wedge pressure (PCWP) or left ventricular end diastolic pressure (LVEDP) of ≤ 12 mmHg if PVR ≥ 350 and < 500 dyne•sec/cm5, or PCWP/LVEDP ≤ 15 mmHg if PVR ≥ 500 dyne•sec/cm5

6. 6MWD of 100 to 550 meters at Screening

7. Currently on a stable treatment regimen with one or more treatments approved for PAH. Stable therapy is defined as receiving the same medication(s) for ≥ 12 weeks prior to the screening RHC and at a stable dose level for each for ≥ 8 weeks prior to the screening RHC (see Protocol Section 6.6.2 for approved PAH medications). Any instances where doses of a medication have been missed prior to RHC must be discussed with the Medical Monitor prior to performing the RHC.

8. Meet all of the following criteria determined by pulmonary function tests completed no more than 24 weeks prior to Screening (performed with or without bronchodilation):

   1. Forced expiratory volume in one second (FEV1) ≥ 60% of predicted normal, and
   2. Total lung capacity (TLC) ≥ 70% of predicted normal or FVC ≥ 70% predicted if TLC is not available; For subjects with CTD associated PAH, if TLC is ≥ 60% of predicted but < 70% of predicted of if FVC ≥ 60% or predicted but < 70% of predicted, high resolution computed tomography [HRCT] obtained within 6 months of screening may be utilized to demonstrate limited interstitial lung disease

9. If participating in an exercise program for pulmonary rehabilitation, the program must have been initiated ≥ 12 weeks prior to Screening, and patient must agree to maintain the current level of rehabilitation for the first 24 weeks of receiving IP. If not participating in an exercise training program for pulmonary rehabilitation, patient must agree not to enroll in an exercise training program for pulmonary rehabilitation during the Screening Period and the first 24 weeks of receiving IP.

Exclusion Criteria:

1. Women of childbearing potential who are pregnant, planning to become pregnant, or lactating or female/male patients unwilling to use effective contraception

2. WHO pulmonary hypertension (PH) Group 1 PAH associated with portal hypertension or schistosomiasis; PH due to left heart disease (WHO PH Group 2), lung diseases and/or hypoxia (WHO PH Group 3), chronic thromboembolic PH (WHO PH Group 4), or PH with unclear multifactorial mechanisms (WHO PH Group 5)

3. PH associated with significant venous or capillary involvement (PCWP > 15 mmHg), pulmonary capillary hemangiomatosis, portal hypertension, or unrepaired congenital heart defects (CHD)

4. Three or more of the following risk factors for left ventricular disease:

   1. BMI > 30 kg/m2
   2. Diagnosis of essential hypertension that is actively treated
   3. Diabetes mellitus
   4. History of significant coronary artery disease (e.g., chronic stable angina, history of coronary intervention within the last 3 months, or a stenosis > 70% at coronary angiography)
   5. Atrial fibrillation
   6. Left atrial volume index > 41 mL/m2 [or left atrial diameter (LA) > 4 cm if LAVi unavailable]

5. Known genetic hypertrophic cardiomyopathy

6. Known cardiac sarcoidosis or amyloidosis

7. The patient has a history of, or currently has, a constrictive cardiomyopathy.

8. Known history of any left ventricular ejection fraction (LVEF) < 40% by echocardiogram within 3 years of randomization (Note: a transient decline in LVEF below 40% that occurred and recovered more than 6 months before the start of Screening and was associated with an acute intercurrent condition [e.g., atrial fibrillation] is allowed).

9. Hemodynamically significant valvular heart disease as determined by the Investigator, including:

   1. greater than mild aortic and/or mitral stenosis and/or
   2. severe mitral and/or aortic regurgitation (> Grade 3)

10. Severe arthritis, musculoskeletal problems, or morbid obesity that, in the opinion of the Investigator, is the cause of the patient's functional limitation and would affect the patient's ability to perform or complete the 6MWT.