This study is looking to understand how safe and effective a medicine called M5049 is for people with inflammatory muscle diseases, specifically dermatomyositis (DM) and polymyositis (PM). These are conditions where the immune system attacks the body's own muscle tissue. The study will last for 24 weeks, and the medicine is taken by mouth. To join, you must have a confirmed diagnosis of DM or PM, with active symptoms, and be on stable doses of certain medications. You cannot join if you have other similar muscle conditions, certain types of cancer, or other uncontrolled diseases. The study will check if M5049 helps reduce symptoms and is safe to use.

• The study involves taking medication for 24 weeks.
• Participants must have specific types of DM or PM and meet health criteria.
• Some diseases or health issues will exclude participation.