A Phase II Trial to Evaluate the Clinical Efficacy, Safety and Tolerability of MAS825 in Pediatric and Adult Participants With Still's Disease

Inclusion Criteria: * Age ≥ 1 with a diagnosis of Still's Disease * Active diseases defined as: * CRP or ferritin levels greater than ULN, and any of: * Fever ≥ 38°C attributed to Still's Disease activity and documented for a number of days prior to Day 1 or * Rash attributed to Still's Disease activity or * Musculoskeletal involvement: arthritis in a number of joints per ACP criteria for active joint or * Serositis or * Macrophage activation syndrome activity as defined by ferritin levels and at least one of: platelet count, a biomarker or fibrinogen levels attributed to Still's Disease activity by the investigator * Intolerance or inadequate response to available biologic therapy Exclusion Criteria: * Patients out of weight range * Ongoing or previous treatment with immunomodulatory drugs * A limited number of Still's Disease patients that have previously received MAS825 through a managed access program are permitted on the study * Glucocorticoid dose exceeding a set limit * Any conditions or significant medical problems which places the patient at unacceptable risk for MAS825 therapy * Still's disease patients with evidence of macrophage activation syndrome are permitted in the study * Still's Disease patients with evidence of interstitial lung disease including those requiring supplemental oxygen therapy are permitted in the study * History of ongoing, chronic or possibly recurrent infection (e.g. HIV, TB, HCV, HBV) and/or symptoms and signs of clini…