* 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
