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). Study visits will occur at 4 week intervals for 16 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. Patients who complete all assessments for 16-weeks will also be eligible to enter an open-label, extension phase study (FREEDOM - EXT).