For business reasons.

This is a multicenter, randomized (2:1 oral treprostinil: placebo), double-blind, placebo-controlled study in subjects with Pulmonary Arterial Hypertension (PAH) who are currently receiving background dual therapy (ambrisentan, tadalafil) for at least 30 days at randomization for their PAH. Once randomized, subjects will return for 5 study visits up to Week 28 during the blinded period. After the Week 28 Visit, eligible subjects will transition to the open-label period of the study for up to 20 weeks.

Inclusion Criteria:

1. Between 18 and 79 years of age, inclusive
2. PAH that is idiopathic / heritable, PAH associated with connective tissue disease, HIV infection, repaired congenital systemic-to-pulmonary shunts (repaired > 1 year), or appetite suppressant / toxin use
3. Receiving dual therapy (ambrisentan and tadalafil)
4. Previous testing (e.g. right heart catheterization, echocardiography) consistent with diagnosis of PAH

Exclusion Criteria:

1. Nursing or pregnant
2. PAH due to conditions other than noted in the above inclusion criteria
3. Received PAH-specific drug therapy for >2 years
4. History of uncontrolled sleep apnea, severe liver disease, severe renal impairment, left sided heart disease, uncontrolled systemic hypertension
5. Participated in an investigational drug or device study within 90 days prior to signing consent.