This study tests if a new medicine called anifrolumab works and is safe for adults with muscle diseases called Polymyositis (PM) and Dermatomyositis (DM). These diseases cause muscle weakness and skin rash. Anifrolumab is given as a shot under the skin. In the study, some people will get anifrolumab, while others will get a fake medicine (placebo), and both groups will continue their usual treatment.

• Participants must be 18-75 years old and weigh 40-100 kg (about 88-220 lbs).
• They need a diagnosis of PM or DM and already be on stable treatment.
• They must follow birth control guidelines and cannot have certain other health conditions.

Being part of the study means visiting the study center multiple times. You should also know there might be risks, like side effects. It's important to discuss with your doctor to see if this study is right for you.