Pulmonary Artery Remodelling With Bosentan

The main purpose of this study is to investigate whether bosentan (Tracleer®) affects the wall thickness of the pulmonary arteries in patients with idiopathic pulmonary arterial hypertension (iPAH) and PAH related to systemic sclerosis (PAH-SSc). The second purpose is to investigate if bosentan affects the enlargement of small vessels in the lungs in response to natural chemicals in patients with iPAH and PAH-SSc.

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 \ 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 iPAH or PAH-SSc within 1 month before start of study treatment * Treatment with glibenclamide (glyburide), fluconazole ketoconazole or ritonavir within 1 week before start of study treatment. * Current treatment with cyclosporine A or tacrolimus * Hypersensitivity to bosentan or any of the excipients of its formulation. * Patient who received an investigational drug (such as sildenafil) within 3 months before start of study treatment * Conditions that prevent compliance with the protocol or adherence to therapy.