A Study to Investigate the Efficacy and Safety of Brepocitinib in Adults With Dermatomyositis

Inclusion Criteria: * A diagnosis of dermatomyositis according to 2017 EULAR/ACR Classification Criteria for Idiopathic Inflammatory Myopathies * Adult subjects (18-75 years old) * Active muscle and skin disease at screening and baseline * Prior therapy OR current therapy with corticosteroids, hydroxychloroquine, and/or one non-steroid immunosuppressant * Weight \> 40 kg to \< 130 kg, and with a body mass index (BMI) \< 40 kg/m2. Exclusion Criteria: * Dermatomyositis with end-stage organ involvement * Dermatomyositis with irreversible muscle involvement * History of: * Any lymphoproliferative disorder * Active malignancy; * History of cancer within 5 years prior to randomization (exceptions for basal cell carcinoma, squamous cell carcinoma, ductal carcinoma in situ of the breast, carcinoma in situ of the uterine cervix, or thyroid carcinoma.) * Cancer-associated dermatomyositis * Overlap myositis/connective tissue disease (except for overlap with Sjögren's syndrome) * Participants at a risk of thrombosis or cardiovascular disease * Participants with a high risk for herpes zoster reactivation * Participants with active or recent infections