This is an international, multicenter, randomized, double-blind, placebo-controlled, event driven study in subjects with pulmonary arterial hypertension.

Study TDE-PH-310 is an international, multicenter, randomized (1:1 oral treprostinil (UT-15C): placebo), double-blind, placebo-controlled study in subjects with pulmonary arterial hypertension (PAH) who are receiving background oral monotherapy for PAH for at least 30 days at randomization. Subjects are randomly allocated to receive oral treprostinil extended-release tablets or placebo by a stratified randomization by type of background therapy (Strata 1: phosphodiesterase type 5 inhibitor [PDE5-I] or soluble guanylate cyclase [sGC] stimulator; Strata 2: endothelin receptor antagonist [ERA]. Subjects are also stratified by baseline 6-minute walk distance (6MWD) less than or equal to 350 m or greater than 350 m. Subjects receive their first dose of study drug (0.125 mg) or matching placebo on the day of randomization. Oral dosing of study drug is continued at 0.125 mg 3 times daily (TID; every 6 to 8 hours) with food. The dose (or matching placebo) is titrated throughout the study up to a maximum dose of 12 mg TID to reach and maintain a tolerated dosing regimen that provided optimal clinical benefit. Once randomized, subjects return for study visits every 4 weeks for the first 12 weeks, then every 12 weeks for the duration of the study. Subjects continue in the study until experiencing clinical worsening, the number of adjudicated events necessary for study closure occurr, or prematurely discontinue participation in the study for any reason other than protocol-specified clinical worsening. At each scheduled visit, subjects undergo efficacy assessments for clinical worsening, exercise capacity (6MWD and Borg dyspnea score), WHO functional class (FC), and plasma N-Terminal pro-brain natriuretic peptide (NT-proBNP). Subjects could participate in an optional hemodynamic sub-study (assessed by RHC). Safety assessments consist of adverse events (AEs), physical examinations, vital signs, 12-lead electrocardiograms (ECGs), and clinical laboratory parameters. Patients who complete all required assessments are eligible to enter a long-term, open-label, extension study (TDE-PH-311).

Subject Inclusion Criteria:

1. Voluntarily gave informed consent to participate in the study.

2. Are 18 to 75 years of age (inclusive) at Screening.

3. Women of childbearing potential must practice abstinence from intercourse when in line with their preferred and usual lifestyle, or use 2 different forms of highly effective contraception for the duration of the study, and for at least 30 days after discontinuing study medication. A negative urine pregnancy test is required at Screening and Baseline prior to initiating study medication.

4. Male subjects must consent to use a condom during intercourse for the duration of the study, and for at least 48 hours after discontinuing study medication.

5. Have a diagnosis of symptomatic idiopathic or heritable PAH, PAH associated with connective tissue disease (CTD), PAH associated with HIV infection, PAH associated with repaired congenital systemic-to-pulmonary shunt, or PAH associated with appetite suppressant or toxin use.

6. If known to be positive for HIV infection, have a CD4 lymphocyte count of at least 200 cells/mm^3 assessed at Screening and are receiving current standard of care anti retroviral or other effective medication for the treatment of HIV infection.

7. Have a baseline 6MWD greater than or equal to 150 m in the absence of a concurrent injury, illness, or other confounding factor including, but not limited to, use of an aid for ambulation or connection to a nonportable machine, that would have prevented the accurate assessment of the subject's exercise capacity.

8. Are optimally treated with conventional pulmonary hypertension therapy with no additions, discontinuations, or dose changes for a minimum of 10 days prior to randomization. The exceptions are the discontinuation or dose changes of anticoagulants and/or dose change of diuretics.

9. Are receiving a PAH-approved oral monotherapy at a minimum dose that complies with the approved prescribing information for the product for at least 30 days prior to randomization and are receiving a stable dose for at least 10 days prior to randomization.

10. Have had previously undergone a cardiac catheterization within 3 years prior to the start of Screening or during the Screening Period, and the most recent assessment documented a pulmonary artery pressure mean of at least 25 mmHg, a pulmonary capillary wedge pressure (PCWP) (or in the event a PCWP could not be reliably obtained, a left ventricular end diastolic pressure [LVEDP]) less than or equal to 15 mmHg, and absence of unrepaired congenital heart disease (other than patent foramen ovale). If a reliable PCWP or LVEDP are unable to be obtained during cardiac catheterization, subjects with clinically normal left heart function and absence of clinically relevant mitral valve disease on echocardiography are eligible for enrollment.

11. Undergo echocardiography with evidence of clinically normal systolic and diastolic left ventricular function and absence of any clinically significant left sided heart disease (eg, mitral valve disease). Subjects with clinically insignificant left ventricular diastolic dysfunction due to the effects of right ventricular overload (ie, right ventricular hypertrophy and/or dilatation) are eligible.

12. Have a previous ventilation perfusion lung scan, high-resolution computerized tomography scan of the chest, and/or pulmonary angiography that are consistent with the diagnosis of PAH.

13. Have pulmonary function tests conducted within 6 months before Screening or during the Screening Period to confirm the following:

    1. Total lung capacity is at least 60%
    2. Forced expiratory volume at 1 second is at least 50%

14. In the opinion of the Principal Investigator, is able to communicate effectively with study personnel and is considered reliable, willing, and likely to cooperate with protocol requirements, including attending all study visits.

Subject Exclusion Criteria:

1. Is pregnant or lactating.

2. Have previously received oral treprostinil.

3. Have received a PGI2 (except if used during acute vasoreactivity testing) within 30 days prior to randomization or have previous intolerance or significant lack of efficacy to any PGI2 or PGI2 analogue that resulted in discontinuation or inability to titrate that therapy effectively.

4. Have any background conventional therapies for PAH added, removed, or dose-adjusted within 10 days prior to randomization. The exceptions are removal or dose adjustments of anticoagulants and/or dose adjustments of diuretics.

5. Receive their first dose of a PAH-approved oral monotherapy less than 30 days prior to randomization, or have their PAH-approved oral monotherapy dose changed within 10 days prior to randomization, or the subject discontinues any PAH approved therapy within 30 days prior to Screening, or the subject has previously received 2 PAH approved oral therapies at the same time (specifically, a PDE5-I, an ERA, or a sGC stimulator) concomitantly for more than 90 days cumulatively.

6. Have any disease associated with PAH other than CTD, HIV infection, repaired (for at least 1 year) congenital systemic-to-pulmonary shunt, PAH associated with appetite suppressant/toxin use, or have an atrial septostomy.

7. Have a current diagnosis of uncontrolled sleep apnea as defined by their physician.

8. Have a history of ischemic heart disease, including a previous myocardial infarction or symptomatic coronary artery disease within 6 months prior to Screening or a history of left-sided myocardial disease as evidenced by a mean PCWP (or a LVEDP) greater than 15 mmHg or left ventricular ejection fraction less than 40% as assessed by either multigated angiogram, angiography, or echocardiography.

9. Have uncontrolled systemic hypertension as evidenced by systolic blood pressure (BP) greater than 160 mmHg or diastolic BP greater than 100 mmHg.

10. Have alanine aminotransferase or aspartate aminotransferase levels at least 3 times greater than the upper limit of normal, clinically significant liver disease/dysfunction, or known Child-Pugh Class C hepatic disease at Screening.

11. Have any other disease or condition that would interfere with the interpretation of study assessments.

12. Have a musculoskeletal disorder, is using a device to assist walking, or any disease that is likely to limit ambulation, or is connected to a machine that is nonportable.

13. Have an unstable psychiatric condition or is mentally incapable of understanding the objectives, nature, or consequences of the study, or has any condition which in the Investigator's opinion would constitute an unacceptable risk to the subject's safety.

14. Is receiving an investigational drug, have an investigational device in place, or have participated in an investigational drug or device study within 30 days prior to Screening.

15. Have chronic renal insufficiency as defined by either a Screening creatinine value greater than 2.5 mg/dL or the requirement for dialysis.

16. Does not have 3 or more of the following left ventricular disease/dysfunction risk factors:

    1. Body mass index at least 30 kg/m^2
    2. History of essential hypertension
    3. Diabetes mellitus (any type)
    4. Historical evidence of significant coronary artery disease established by any 1 of the following: history of myocardial infarction, percutaneous coronary intervention, or angiographic evidence of coronary artery disease; positive stress test with imaging; previous coronary artery bypass graft; or stable angina.