• Must be age 18 or older at the time of signing the informed consent form (ICF). The participant must understand and voluntarily sign an ICF prior to any study-related procedures

• Documented findings on a right heart catheterization (RHC) consistent with a diagnosis World Health Organization (WHO) Group 1 PAH or WHO GROUP 3 PAH

• Symptomatic Pulmonary Hypertension (PH) classified as WHO Functional Class II or III symptoms

• PAH participants: Pulmonary Vascular Resistance (PVR) of ≥ 5 Wood units, Pulmonary Capillary Wedge Pressure (PCWP) ≤ 15 mmHg and Mean Pulmonary Arterial Pressure (mPAP) \> 20 mm Hg and ILD-PH participants: PVR of ≥ 3 Wood units, PCWP ≤ 15 mmHg and mPAP \> 20 mm Hg

• Participants on stable background therapy for PAH or ILD-PH.

• Females of childbearing potential (as defined in protocol) must agree to use highly effective contraception (as defined in the protocol)

• Male participants must follow protocol-specified contraception guidance.

• Participants must be able to communicate well with Investigators, understand the study procedures in the ICF and are agreeable to complete the study in accordance with the protocol.

• Must be able to swallow tablets.

⁃ Pulmonary function tests (PFT):

⁃ PAH participants at Screening as follows:

∙ Forced Vital Capacity (FVC) \> 70% predicted; or if between 60% to 70% predicted, or if not possible to be determined, confirmatory High-Resolution Computed Tomography (HRCT) indicating no more than mild (\<10% fibrosis) ILD; and

∙ The ratio of FEV1 (first second)/FVC \> 0.70 of predicted.

⁃ ILD-PH participants at Screening as follows:

∙ PFTs consistent with their ILD diagnosis and showing FEV1/ FVC ratio \> 65% and HRCT \> 10% fibrosis, based on the proportion of lung parenchyma affected by fibrotic changes.

• and,

∙ Minimum FVC of 50% and diffusing capacity for carbon monoxide (DLCO) (corrected for Hb g/dl) \>25%

⁃ In PAH participants, i.e., Cohorts 1 and 2 only, ventilation-perfusion (VQ) scan (or, if unavailable, a negative CT pulmonary angiogram \[CTPA\] or pulmonary angiography result), any time prior to Screening or conducted during the Screening Period, with a normal or low probability result that is not clinically significant

⁃ Acceptable Electrocardiogram (ECG) findings as assessed by the Investigator or qualified designee at the Screening Visit and at the Baseline Visit (Day 1), including each criterion as listed below:

∙ Normal sinus rhythm (HR) between 40 and 100 beats per minute, inclusive);

‣ Corrected QT Interval (QTcF) interval ≤ 450 msec (males) and ≤ 460 msec (females);

‣ QRS interval ≤ 120 msec; if \> 120 msec, result will be confirmed by a manual over read

⁃ Body weight at the Screening visit and at Baseline (Day 1) is greater than 50.0 kg and the body mass index (BMI) is in the range of 19.00 to 36.00 kg/m2, inclusive

⁃ 6MWD ≥ 100 and ≤ 550 meters repeated twice, once during Screening Period and once at the Baseline Visit (Day 1) and both values within 15% of each other, allowing for a third repeat if \> 15% difference, calculated from the higher/highest value

• Participants must complete the main study period (defined as completion of assessments through the Week 24 visit)

• Women of child-bearing potential (WOCBP) must have negative pregnancy test

• All participants must comply with contraceptive guidance until 28 days after last dose of study drug for WOCBP and 90 days after the last dose of study drug for males