A total of approximately 36 patients will be randomized to 1 of 3 treatment groups (12 per group) in a 1:1:1 ratio and will be stratified by PAH background therapy (ERA, PDE-5, and both). The treatment groups consist of one MTD and two 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. Between visits, clinical site personnel will contact patients by phone each week to assess tolerability, provide instructions for a change in dosage, record changes in concomitant medications, and record adverse events. Patients who complete the study will be offered the opportunity to continue taking study medication in a separate open-label continuation protocol. Patients who withdraw early from the study or who otherwise do not elect to enroll into the open-label continuation protocol will be down-titrated off of BPS-MR at the discretion of the Investigator, at a maximum decrement not to exceed one tablet (60µg) b.i.d. per day and a minimum decrement of one tablet (60µg) b.i.d. per week.