Patients will be randomized to 1 of 3 treatment groups in a 1:1:1 ratio and will be stratified by PAH background therapy (Endothelium Receptor Antagonist (ERA), Phosphodiesterase-5 (PDE-5), and both). The treatment groups consist of one Maximum Tolerated Dose (MTD) and two Fixed Dose (FD) groups. Following randomization, patients will begin taking active drug (60µg) orally twice daily. Patients will visit their investigational site at Week 6 and Week 12 for study evaluations.