This research study is being done to investigate how safe and effective the investigational drug brepocitinib is for treating dermatomyositis. If you agree to join the study, you will be asked to complete the following research procedures: • signing this informed consent • having reviews of your medical/health history and physical exams • checks of your vital signs and use of contraception • chest X-ray • CT (chest, abdomen, pelvis), if needed • ECG (check of your heart) • muscle checks • labs to test your blood and urine, the amount of study drug absorbed by your body and other specialty and safety related labs • checks of your breathing, if the study doctor determines these checks are necessary • photos of your skin lesions, if the study doctor determines these checks are necessary • questionnaires • receiving the study drug or placebo (the placebo does not contain any active ingredients) in pill form during the treatment part of the study. Your participation will last for about 64 weeks and include a Screening period up to 8 weeks.

You may be eligible for this study if you meet the following criteria:

• Conditions: Dermatomyositis
• Age: 18 years - 75 years
• Gender: All