Bosentan in the Treatment of Giant Cell Arteritis

* Inclusion Criteria : * Patients having given their written informed consent prior to participation in the study * Patients affiliated with social security or CMU (profit or being entitled) * Diagnosis of GCA, as defined by the revised GCA diagnosis criteria. Patients must satisfy criteria 1-2-3 and 4 (irrespective of time): * Age ≥50 years at disease onset * History of erythrocyte sedimentation rate (ESR) ≥ 50 mm/h or CRP ≥ 20 mg/L (not mandatory if TAB is positive: see below) * At least one of the following: * unequivocal cranial symptoms of GCA (new onset headache, scalp tenderness, jaw claudication, temporal artery abnormality, ischemia-related vision loss) * unequivocal symptoms of polymyalgia rheumatica (PMR) * At least one of the following: * Temporal artery biopsy (TAB) compatible with the diagnosis of GCA (non-necrotizing vasculitis with a predominance of mononuclear cell infiltration or granulomatous inflammation, usually with multinucleated giant cells) * Evidence of large vessel vasculitis: * angio-CT or angio-MRI: thickened and/or contrast-enhanced arteries especially aorta (≥2mm) and epiaortic arteries (≥1mm) and contrast enhanced arteries in T1-weighted sequences * or PET scan: ≥ grade 2 (from 0 to 3) tracer uptake on large arteries * At least a sign of active GCA within the 2 weeks prior to randomisation. Active GCA is defined by ESR ≥30 mm/h or CRP ≥10 mg/L and at least one of the following: * unequivocal cranial symptoms of GCA (new onset loc…