Upon joining the study, you will be randomly assigned to receive either empasiprubart or placebo. Empasiprubart is the medication being tested, while placebo is an inactive substance that looks like the medication but contains no active ingredient.

Neither you nor your doctor will know which treatment you are receiving during the study. This is called a double-blinded study design.

You will receive your assigned treatment through an intravenous infusion, which means the medication will be delivered directly into your vein through a needle.

The study medication is called ARGX-117 and comes as a concentrate that will be prepared as a solution for infusion.

The first part of the study lasts 24 weeks.

During this period, your functional ability will be assessed using the aINCAT score, which measures your ability to perform daily activities with your arms and legs.

Your muscle strength will be evaluated using the MRC-SS, which stands for Medical Research Council Sum Score.

Your grip strength will be measured in your dominant hand and both hands using daily measurements.

The I-RODS score will assess your ability to perform activities of daily living. This stands for Inflammatory Rasch-built Overall Disability Scale.

The TUG test will measure the time it takes you to stand up, walk, turn around, and sit back down. This stands for Timed Up and Go test.

Your quality of life will be evaluated using the EQ-5D-5L questionnaire, which assesses mobility, self-care, usual activities, pain, and anxiety.

Fatigue will be measured using the RT-FSS, which is the Rasch-built fatigue severity scale.

Any pain you experience will be assessed with the BPI-SF, which stands for Brief Pain Inventory – Short Form.

You will rate your overall condition using PGI-S (Patient Global Impression of Severity) and changes in your condition using PGI-C (Patient Global Impression of Change).

After completing the first 24 weeks, you may continue in the study for an extended period.

Part B can last up to 96 weeks in total.

During this extended period, the same assessments will continue to be performed to monitor your muscle strength, functional ability, and overall well-being over time.

Throughout the study, you will be monitored for any adverse events, which are any unwanted medical occurrences or side effects.

Blood samples will be taken to check for ADA (anti-drug antibodies) and NAb (neutralizing antibodies), which are substances your body might produce in response to the study medication.

Laboratory tests will be performed regularly to monitor your blood values and overall health.

Your heart function will be checked using ECG (electrocardiogram), which records the electrical activity of your heart.

Your vital signs will be measured, including blood pressure, heart rate, and temperature.

Blood levels of C2, which is a component of your immune system called complement, will be measured to see how the medication affects it.

The concentration of empasiprubart in your blood will be measured at various times during the study.