The study population will be randomized into Dose Group 1, Dose Group 2, or an Individual Maximum Tolerated Dose (iMTD) of UT-15C SR by Week 10 and maintained through Week 12. Patients may be either currently receiving an approved oral background therapy for their PH (phosphodiesterase-5 [PDE-5] inhibitor, OR endothelin receptor antagonist [ERA]) (no dual background therapy), or not currently receiving therapy for PH.