This study aims to find the correct dose of ARGX-119 for children with SMA. The study will also look at how safe the study drug is, how well it works, how it moves through the body, and how the immune system responds to it. The study consists of a double-blinded treatment period (DBTP) where participants will either receive ARGX-119 IV or placebo IV, in addition to disease-modifying therapy (DMT) for 24 weeks. Participants who complete the DBTP will enter the open-label active-treatment extension period (ATEP) during which all participants will receive ARGX-119 IV up to 100 weeks (approximately 2 years).
Is aged ≥5 to <18 years when completing the informed consent process, defined as providing informed assent according to local regulations and having a parent or guardian sign the ICF, and can comply with protocol
requirements.
Has documented historical genetic diagnosis of 5q-SMA.
Currently receiving a stable SMA treatment regimen (nusinersen or risdiplam) and/or have a history of onasemnogene abeparvovec treatment
Known medical condition that would interfere with an accurate assessment of SMA, confound the results of the study, or put the participant at undue risk, as assessed by the investigator
Recent major surgery, except spinal fusion, within 3 months of screening or intends to have major surgery during the study
Current or previous administration of antimyostatin therapies in the past 6 months
Severe scoliosis (defined as curvature >40°) and/or contractures at screening. o History of spinal fusion within 6 months before screening or planned during the study