Participation begins after completing the previous study, ARGX-113-2007.

Eligibility requires the ability to provide informed consent and comply with study requirements.

Women of childbearing potential must have a negative pregnancy test before starting the study.

The study involves the administration of efgartigimod PH20 SC, a subcutaneous solution for injection.

The medication is designed to assess long-term safety and tolerability in adults with active idiopathic inflammatory myopathy.

Regular monitoring of treatment-emergent adverse events, including any serious adverse events, is conducted.

Vital signs, electrocardiogram (ECG), and laboratory parameters are checked for changes and abnormalities over time.

The study evaluates the reduction in prednisone dosage over time and the proportion of participants who discontinue corticosteroids by week 52.

The total improvement score (TIS) and the proportion of TIS responders are assessed at weeks 12, 24, and 52.

The percentage of participants with clinically inactive disease is measured, defined by no evidence of disease activity and normal creatine kinase values for at least 12 weeks.

The study also measures the percentage of participants achieving remission, defined as clinically inactive disease for at least 24 weeks.

The study is estimated to conclude by September 23, 2027.

Recruitment for the study began on September 21, 2023.