This study is an international, multi-center, randomized, double-blind, placebo-controlled study in subjects with PAH who are currently receiving approved therapy for their PAH (i.e., endothelin receptor antagonist and/or phosphodiesterase-5 inhibitor)or as a monotherapy treatment. Study visits will occur at 4 week intervals for 12 weeks with the key measure of efficacy being the 6-minute walk test. Study procedures include routine blood tests, medical history, physical exams, disease evaluation, and exercise tests. At the end of the first 12-weeks, the patient will be un-blinded. Patients will continue with another 12-Week open label portion with visits occuring at 4-week intervals.
Patients who complete all assessments for 24-weeks will also be eligible to enter a 36 month open-label, extension phase study (FREEDOM - EXT).
