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

The main purpose of this study is to measure the long-term safety and tolerability of efgartigimod PH20 SC in adult participants with Idiopathic Inflammatory Myopathy (IIM) who previously participated in ARGX-113-2007. The study consists of a treatment period where participants will receive efgartigimod PH20 SC for up to 51 months. The treatment period will be followed by a treatment-free safety follow-up period of 56 days.

Inclusion Criteria: * 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 study drug Exclusion Criteria: * 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 the study drug or 1 of its excipients * Development of any malignancy, either new or recurrent, other than basal cell carcinoma of the skin, regardless of relatedness