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 Criteriafor 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/orone 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 basalcell carcinoma, squamous cell carcinoma, ductal carcinoma in situ of thebreast, carcinoma in situ of the uterine cervix, or thyroid carcinoma.)

• Cancer-associated dermatomyositis

• Overlap myositis/connective tissue disease (except for overlap with Sjögren'ssyndrome)

• Participants at a risk of thrombosis or cardiovascular disease

• Participants with a high risk for herpes zoster reactivation

• Participants with active or recent infections