Pulmonary Artery Remodelling With Bosentan

Inclusion Criteria : · Men or women \>18 years of age.· * Symptomatic (modified NYHA class III) iPAH or PAH-SSc· * PAH confirmed by right heart catheterization performed within 3 months before enrolment mPAP \> 25 mmHg, PCWP \< 15 mmHg and PVR \> 3 mmHg/l/min. * Women of childbearing potential must have a negative pre-treatment pregnancy test and use a reliable method of contraception during study treatment and for 3 months after study treatment termination. * Bosentan naïve patients Exclusion Criteria : · PAH other than iPAH or PAH-SSc * Significant vasoreactivity during right heart catheterization defined as a fall in mPAP to \< 40 mmHg with a decrease \>= 10 mmHg and with a normal cardiac index (\>= 2.5 l/min.m2)· Severe obstructive lung disease: FEV1/FVC \< 0.5 * Severe restrictive lung disease: TLC \< 0.7 of normal predicted value * Hemoglobin \<75% of the lower limit of the normal range· Systolic blood pressure \< 85 mmHg * Body weight \< 40 kg * Pregnancy or breast-feeding * Moderate to severe hepatic impairment, i.e., Child-Pugh Class B or C. * Baseline aminotransferases, i.e., aspartate aminotransferases (AST) and/or alanine aminotransferases (ALT) \> 3 times the upper limit of the normal (ULN) range. * Treatment for iPAH or PAH-SSc within 1 month before start of study treatment, excluding warfarin and acute administration of vasodilators for vascular reactivity testing during heart catheterization. * Treatment with epoprostenol or other prostacyclin analogs for i…