A Study to Assess the Long-term Safety and Efficacy of a Subcutaneous Formulation of Efgartigimod in Adults With Active Idiopathic Inflammatory Myopathy

Open to people ages 18 years and up

• Has completed trial ARGX-113-2007
• Being capable of providing signed informed consent and complying with protocol requirements
• Agrees to use contraceptive measures consistent with local regulations and women of childbearing potential must have a negative urine pregnancy test at baseline before receiving the investigational medicinal product

• Intention to have major surgery during the study period; or any other medical condition that has arisen since enrollment in study ARGX-113-2007, that in the investigator's opinion, would confound the results of the study or put the participant at undue risk
• Known hypersensitivity reaction to investigational medicinal product or 1 of its excipients Development of any malignancy, either new or recurrent, other than basal cell carcinoma of the skin, regardless of relatedness
• Permanent discontinuation of IMP in ARGX-113-2007, or met the permanent discontinuation criteria at the rollover visit
• Diagnosis with a deselected subtype of myositis based on the analysis of the phase 2 stage data in ARGX-113-2007, unless the investigator determines that the participant is benefiting from IMP as defined by a score of "much better" or "moderately better" on the 'Clinical Global Impression of Change' and 'Patient Global
• Impression of Change' assessments for at least 12 weeks, and that enrolling in the study is in the participant's best interest