Abatacept Treatment in Polymyositis and Dermatomyositis

The aim of this study is to investigate the efficacy and safety of abatacept in patients with Dermatomyositis (DM) and polymyositis (PM) refractory to conventional treatment using a randomised trial design with delayed start in one arm. Abatacept will be administered intravenously to participants at a dose based on body weight at the screening visit followed by six follow-up treatments (Active treatment arm). Abatacept will also be administered intravenously to participants at a dose based on body weight starting at 3 months followed by six follow-up treatments (Delayed-onset treatment arm). The International Myositis Assessment and Clinical Studies Group (IMACS) preliminary definition of improvement (DOI) will be used for assessment.