This study is researching an experimental drug called ALN-SOD (called "study drug"). This study is focused on people with amyotrophic lateral sclerosis (ALS) who have a mutation in a gene called the superoxide dismutase-1 (SOD1) gene. This type of ALS is known as "SOD1-ALS". This is the first time that ALN-SOD will be given to people. The aim of the study is to see how safe and tolerable the study drug is.

The study is looking at several other research questions, including:

• The effect the study drug has on specific biomarkers, which are molecules in the blood or in the fluid that surrounds the brain and spinal cord, known as cerebrospinal fluid (CSF)

• How much study drug is in the blood and in the CSF, at different times

• Whether the body makes antibodies against the study drug (which could make the drug less effective or could lead to side effects)

• What effects the study drug has on ALS symptoms

• ClinicalTrials.gov Identifier: NCT06351592

• A Phase 2a, Double-Blinded, Randomized, Placebo-Controlled, and Active-Treatment Extension Study to Assess the Safety, Tolerability, Efficacy, Pharmacokinetics, and Immunogenicity of ARGX-119  in Participants with Amyotrophic Lateral Sclerosis

• This study aims to evaluate the safety of ARGX-119 in adults with ALS. The study will also assess the impact of ARGX-119 on ALS disease outcomes, including muscle function. The study consists of 2 periods: a treatment period when participants will receive one of three ARGX-119 doses or placebo and an extension period when all participants will receive the same dose of ARGX-119. Participation in the study will last up to approximately 100 weeks.

• ClinicalTrials.gov Identifier: NCT06441682