Ravulizumab Versus Placebo in Adult Participants With Dermatomyositis

Key Inclusion Criteria: * 18 years of age or older at the time of signing the informed consent. * Body weight ≥ 30 kilograms at the time of Screening. * Male or female. * Diagnosis: Meet 2017 American College of Rheumatology/European League Against Rheumatism classification criteria for definite or probable DM. * Participants who have an inadequate response or are intolerant to 1 or more DM treatments, including systemic corticosteroids or immunosuppressive/immunomodulatory therapies (for example, azathioprine, methotrexate, rituximab, intravenous immunoglobulin), either in combination or as monotherapy. * Vaccinated against Neisseria meningitidis within 3 years prior to initiating ravulizumab as per national and local guidelines. Participants must receive the vaccination at least 2 weeks before first study intervention. The sponsor recommends that national and local guidelines for prophylactic antibiotics should also be followed. * Female participants of childbearing potential and male participants must follow specified contraception guidance as described in the protocol. Key Exclusion Criteria: * Participants who have been diagnosed with cancer within the last 3 years need to have appropriate negative cancer screening as per local standard of care within 6 months before Screening (basal or squamous cell skin cancer or carcinoma in situ of the cervix needs to have been excised and without evidence of residual disease for at least 3 months before Screening). * Evidence of …