An open label, non-comparative study design is appropriate for this Phase 4 study designed to assess whether a core therapy of bosentan, either as monotherapy or with the addition of sildenafil, enables patients with PAH to achieve a 6 MWD of ≥380 meters after 28 weeks of therapy This design is also appropriate to pioneer the utility of cardiac MRI in assessing improved functional capacity in PAH and exploring its correlation with other parameters.

• Drug: bosentan
• Drug: bosentan and sildenafil combination

• Active Comparator: Bosentan monotherapy
• Active Comparator: bosentan and sildenafil combination therapy

Inclusion Criteria:

1. Signed informed consent prior to initiation of any study-mandated procedures.
2. Males or females ≥ 12 years of age (females of child-bearing potential must have been surgically sterilized or use a reliable method of contraception).

3. Symptomatic patients with the following types of PAH belonging to WHO Pulmonary Hypertension Classification Group I:

   • Idiopathic (IPAH)
   • Familial (FPAH)

   • Associated with PAH (APAH):

     • Collagen vascular disease
     • Drugs and toxins

4. Patients naïve to treatment with advanced PAH therapies (i.e., ERAs, PDE-5 inhibitors or prostacyclins) with a RHC showing all of the following:

   • Mean pulmonary arterial pressure (mPAP) ≥ 25 mm Hg
   • Pulmonary capillary wedge pressure (PCWP) ≤ 15 mm Hg or left ventricular end diastolic pressure (LVEDP) ≤ 15 mm Hg when PCWP is not accurately obtained
   • Pulmonary vascular resistance ≥ 3 Wood units
5. 6-MWT distance ≥ 200 meters and < 360 meters.

Exclusion Criteria:

1. Patients with Pulmonary Hypertension (PH) belonging to WHO Classification Group II-V.
2. Patients with PAH (WHO PH Classification Group I) other than that listed in the Inclusion Criteria.
3. Pregnant and/or nursing.
4. Women of childbearing potential not using a reliable method of contraception.
5. Patients with known HIV infection.
6. Patients with significant vasoreactivity during right heart catheterization (i.e., a fall in mPAP to < 40 mm Hg with a decrease of ≥ 10 mm Hg and with a normal cardiac index ≥ 2.5 l/min.m2).
7. Patients with restrictive lung disease (i.e., total lung capacity (TLC) < 60% of normal predicted value).
8. Patients with obstructive lung disease (i.e., forced expiratory volume/ forced vital capacity (FEV1/FVC) < 0.5).
9. Patients with impaired left ventricular function (LVEF <50%) or diastolic dysfunction.
10. Patients with portal hypertension, cirrhosis, moderate to severe liver impairment (Child-Pugh Class B or C), or liver enzymes (AST and/or ALT) > 1.5 times the upper limit of normal range.
11. Treatment with glibenclamide (glyburide) and calcineurin inhibitors (cyclosporine A, tacrolimus) sirolimus, everolimus up to 1 week prior to Baseline (Day 1).
12. Patients currently receiving or predicted to require treatment, during the course of the study, with nitrates, protease inhibitors, or alpha-blockers.
13. Patients with a hemoglobin concentration < 75 % of the lower limit of the normal range or < 8.5 g/dL.
14. Patients currently receiving or predicted to require treatment with a prostanoid during the course of the study.
15. Patients with systolic blood pressure < 85 mm Hg.
16. Patients with body weight < 40 kg.
17. Patients who have received any investigational product within 90 days prior to Baseline.
18. Patients who previously received any advanced therapy for PAH (e.g., ERAs, PDE-5 inhibitors or prostacyclins).
19. Patients with hypersensitivity to sildenafil or any excipients of its formulation.
20. Patients with any contraindication to sildenafil treatment (i.e., nitrates).
21. Patients with any recent medical condition limiting the ability to comply with the study requirements (i.e., stroke, myocardial infarction).
22. Patients unable to complete a MRI scan (e.g., claustrophobia).
23. Patients with permanent cardiac pacemakers, AICD's, neurostimulators, hearing aides, and other implanted metallic devices that are contraindicated during a MRI study.
24. Patients with conditions that would interfere with proper cardiac gating during MRI, such as atrial fibrillation or multiple PVCs/PACs.
25. Patients with conditions that prevent compliance with the protocol or the ability to adhere to therapy.