This clinical trial is focused on studying a condition known as Idiopathic Inflammatory Myopathy (IIM), which is a group of diseases that cause muscle inflammation and weakness. The study is investigating a treatment called efgartigimod PH20 SC, which is given as a solution for injection. This treatment is being compared to a placebo, alongside standard care treatments that help manage the immune system.

The purpose of the study is to evaluate how well efgartigimod PH20 SC works in improving the symptoms of IIM, as well as to assess its safety and how well it is tolerated by participants. Participants in the study will receive either the investigational treatment or a placebo, and their progress will be monitored over a period of time to see if there are any improvements in their condition. The study will also look at how the body processes the treatment and whether it causes any immune reactions.

Throughout the study, participants will undergo various assessments to measure changes in muscle strength and overall disease activity. These assessments will help determine the effectiveness of the treatment in providing clinical improvement for those with active IIM. The study aims to provide valuable information on whether efgartigimod PH20 SC can be a beneficial treatment option for individuals with this condition.