This clinical trial is focused on studying the effects of a new treatment called YTB323 in people with severe inflammatory muscle diseases known as idiopathic inflammatory myopathies (IIM). These are conditions where the body’s immune system mistakenly attacks its own muscles, causing inflammation and weakness. The main treatment being tested is rapcabtagene autoleucel, which is a type of cell therapy that uses the patient’s own immune cells, modified to better fight the disease. This treatment is given as a single infusion directly into the bloodstream.

The purpose of the study is to see if rapcabtagene autoleucel can provide better results compared to other treatments chosen by doctors. Participants will receive either the new treatment or a different treatment selected by their doctor. The study will last for about a year, during which participants will be monitored to see if there is an improvement in their condition. The study will also look at the safety of the treatment and any side effects that may occur.

In addition to YTB323, other medications that may be used in the study include rituximab, mycophenolic acid, tacrolimus, tocilizumab, fludarabine phosphate, and cyclophosphamide. These medications are commonly used to manage symptoms and control the immune system in people with inflammatory muscle diseases. The study aims to provide valuable information on how effective and safe these treatments are for people with severe forms of these conditions.