The study evaluates the effect of macitentan on right ventricular and hemodynamic properties in patients with symptomatic pulmonary arterial hypertension. Patients are treated with macitentan for 1 year. Patients undergo right heart catheterization (RHC) at baseline and Week 26. They also undergo cardiac magnetic resonance imaging (MRI) at baseline, Week 26 and Week 52. Safety is monitored throughout the study. The study has three stub-studies. Each patient can participate in no sub-study or in one sub-study. The sub-studies are: (1) metabolism sub-study (with PET-MR scans); (2) biopsy sub-study (biopsies taken during the RHC); (3) Echo sub-study.