A Study to Investigate the Efficacy and Safety of Efgartigimod PH20 SC in Adult Participants With Active Idiopathic Inflammatory Myopathy.

To join this study, you need to be an adult (18 years or older) with a confirmed diagnosis of idiopathic inflammatory myopathy (IIM). This includes specific types like dermatomyositis, polymyositis (including antisynthetase syndrome), or immune-mediated necrotizing myopathy.

It's also important that your IIM is currently active. This might mean you have certain abnormal blood test results, recent signs of muscle disease on an electromyogram or MRI scan, an active skin rash from dermatomyositis, or changes seen in a recent muscle biopsy. You should also be experiencing muscle weakness and be on a stable background treatment for your IIM.

There are also some reasons why you might not be able to join. For example, if you have an active infection, have recently tested positive for COVID-19, or have another serious autoimmune disease that might complicate the study. You also cannot have a history of cancer unless it's completely cured.

If you decide to take part in this study, you'll be assigned to either receive the new drug, efgartigimod PH20 SC, or a placebo (a dummy treatment). You won't know which one you're getting, and neither will your study doctor, to ensure the results are unbiased. The new drug is given as an injection under the skin.

Your participation will involve regular visits to the study clinic. During these visits, you'll have various assessments to check your progress and safety. These might include physical examinations, blood tests, and questionnaires about your symptoms and how you're feeling. The study will carefully monitor how your body reacts to the treatment and if your IIM symptoms are improving. The total duration of the study will be explained in detail by the study team, along with specific timelines for follow-up appointments.