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 localized headache, scalp or temporal artery tenderness, ischemia-related vision loss, or otherwise unexplained mouth or jaw pain upon mastication) * unequivocal symptoms of PMR, defined as shoulder and/or hip girdle pain associated with inflammatory stiffness * other features judged by the clinical investigator to be consistent with GCA or PMR flares * Menopausal women (no gynaecological cycle over the past two years), or women who had a gynaecological cycle within previous 24 months (non-menopausal women) only if they have (1) an effective non hormonal contraceptive method throughout study and (2) a negative urinary beta-hCG test at inclusion. Exclusion Criteria: * Patients under maintenance of justice, wardship or legal guardianship * Patient unable to give written informed consent prior to participation in the study * Patients included in other investigational therapeutic study within the previous 3 months * Patients suspected not to be observant to the proposed treatments * Weight \<40 Kg or \> 100 Kg * Moderate to severe hepatic impairment, i.e., Child-Pugh class B or C. History of chronic alcohol abuse (consumption \> 20 g/day) * Severe chronic heart failure or severe systolic dysfunction * Recent (\< 3 months) or incoming surgery requiring a general anaesthesia * History of stem cell or organ transplantation (except corneas if performed more than 3 months prior inclusion) * Hypersensitivity to bosentan or one of its excipients * Prior treatment with any of the following: * Tocilizumab or methotrexate or secukinumab within 12 weeks preceding inclusion * Cell-depleting agents (i.e., anti-CD20) * Alkylating agents including cyclophosphamide * Hydroxychloroquine, cyclosporine A, dapsone, azathioprine, mycophenolate mofetil or janus kinase inhibitors within 4 weeks preceding inclusion * Tumor necrosis factor inhibitors within 8 weeks preceding inclusion * Anakinra within 1 week preceding inclusion * Ongoing treatment with glibenclamide, fluconazole and rifampicin. Concomitant administration of both a CYP3A4 inhibitor or a CYP2C9 inhibitor * Long-course systemic glucocorticoid therapy for other conditions than GCA or PMR * Laboratory abnormalities: AST or ALT \>3 x upper limit of normal (ULN) * Infections: * Active hepatitis B or C * HIV infection