Inclusion Criteria:

- A subject is eligible for inclusion in this study if all of the following criteria apply:

1. The subject is between the ages of 18 and 75 years of age at Screening.
2. The subject weighs a minimum of 40 kilograms with a body mass index less than 40 kg/m2 at Screening.
3. The subject, if female, is physiologically incapable of childbearing or practicing an acceptable method of birth control as deemed appropriate by the physician or institution (i.e., surgical sterilization, approved hormonal contraceptives, barrier methods [such as a condom or diaphragm] used with a spermicide, an intrauterine device, abstinence, or partner(s) with a vasectomy). For women of childbearing potential, a negative serum pregnancy test is required at Screening.
4. The subject has a diagnosis of symptomatic idiopathic or familial PAH, PAH associated with collagen vascular disease, PAH associated with HIV infection, or PAH associated with appetite suppressant or toxin use.
5. The subject, if HIV positive, has a CD4 lymphocyte count ≥ 200 cells/mm3 within 30 days of Baseline and is receiving current standard of care anti-retroviral or other effective medication for treatment of HIV.
6. The subject must have a Baseline 6-Minute Walk distance between 150 and 400 meters, inclusive, in the absence of a concurrent injury, illness (other than PAH or a PAH related condition), or other confounding factor that would prevent the accurate assessment of the subject's exercise capacity.
7. The subject must have received an approved PDE-5 inhibitor and/or an approved ERA for at least 90 days and at the current stable dose for 30 days prior to Baseline and is willing to remain on a PDE-5 inhibitor and/or an ERA and at the same dose for the duration of the 16-week Treatment Phase.
8. The subject must be optimally treated with conventional pulmonary hypertension therapy (oral vasodilators, oxygen, digoxin, etc) with no additions, discontinuations, or dose changes for at least 14 days prior to Baseline (excluding anticoagulants).
9. The subject has previously undergone a cardiac catheterization and been documented to have a mean pulmonary artery pressure (PAPm) > 25 mmHg, a pulmonary capillary wedge pressure (PCWP) or a left ventricular end diastolic pressure (LVEDP) < 15 mmHg, and pulmonary vascular resistance (PVR) > 3 Wood units and absence of unrepaired congenital heart disease prior to study initiation. In the event that a reliable PCWP or LVEDP are unable to be obtained during right heart catheterization, subjects with normal left heart function and absence of clinically relevant mitral valve disease on echocardiography are eligible for enrollment.
10. The subject has previously undergone echocardiography with evidence of normal left systolic and diastolic ventricular function, and absence of any clinically significant left sided heart disease (e.g. mitral valve stenosis).
11. The subject has a previous chest radiograph, ventilation perfusion scan, high resolution computerized tomography scan, or pulmonary angiography that are consistent with the diagnosis of PAH (i.e., low probability of pulmonary embolism; absence of major perfusion defects).

12. The subject has pulmonary function tests done within 6 months of Baseline with the following:

    • Total lung capacity (TLC) ≥ 60% (predicted); if the TLC is between 60% and 70% of predicted, a high resolution CT scan must be performed to rule out diffuse interstitial fibrosis or alveolitis.
    • Forced expiratory volume/forced vital capacity (FEV/FVC) ratio ≥ 50%
13. In the opinion of the Principal Investigator, the subject is able to communicate effectively with study personnel, and is considered reliable, willing and likely to be cooperative with protocol requirements, including attending all study visits.
14. The subject will voluntarily give informed consent to participate in the study.

Exclusion Criteria:

• A subject is not eligible for inclusion in this study if any of the following criteria apply:

  1. The subject is pregnant or lactating.
  2. The subject has received epoprostenol, treprostinil, iloprost, beraprost, or any other prostacyclin therapy within 30 days of Baseline (except if used during acute vasoreactivity testing).
  3. The subject has previously received UT-15C SR.
  4. The subject has had previous intolerance or significant lack of efficacy to an oral or parenteral prostacyclin or prostacyclin analogue that resulted in discontinuation or inability to effectively titrate that therapy.

  5. The subject has any disease associated with PAH other than collagen vascular disease, HIV infection, or appetite suppressant / toxin use (e.g.

     portal hypertension, chronic thromboembolic disease, pulmonary veno-occlusive disease, etc.) or has had an atrial septostomy.

  6. The subject has a current diagnosis of uncontrolled sleep apnea as defined by their physician.
  7. The subject has chronic renal insufficiency as defined by either a Screening creatinine value greater than 2.5 mg/dL (221 μmol/L) or the requirement for dialysis.
  8. The subject has liver function tests (AST or ALT) greater than three times the upper limit of normal at Screening.
  9. The subject has anemia as defined by a Screening hemoglobin value of less than 10 g/dL, active infection, or any other condition that would interfere with the interpretation of study assessments.
  10. The subject has a history or current evidence of left-sided heart disease including previous myocardial infarction, or evidence of current left-sided heart disease as defined by PCWPm or LVEDP > 15 mmHg or left ventricular ejection fraction (LVEF) < 40% as assessed by either multigated angiogram (MUGA), angiography or echocardiography, or symptomatic coronary artery disease (i.e., demonstrable ischemia either at rest or during exercise).
  11. The subject has uncontrolled systemic hypertension as evidenced by systolic blood pressure greater than 160 mmHg or diastolic blood pressure greater than 100 mmHg.
  12. The subject has a musculoskeletal disorder (e.g. hip replacement, artificial leg, etc.) or any other disease that is likely to limit ambulation, or is connected to a machine that is not portable.
  13. The subject has an unstable psychiatric condition or is mentally incapable of understanding the objectives, nature, or consequences of the trial, or has any condition which in the Investigator's opinion would constitute an unacceptable risk to the subject's safety.
  14. The subject is receiving an investigational drug, has an investigational device in place or has participated in an investigational drug or device study within 30 days prior to Screening.