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.

The study was terminated early following review of interim data indicating that continuation was unlikely to demonstrate a clinically meaningful treatment effect. The decision was made to minimize unnecessary participant burden. This decision is not related to safety concerns.

End-of-study and Safety-Follow-Up visits are ongoing for the participants of this trial

Inclusion Criteria:

• The participant is at least 18 and ≤80 years of age
• The participant is diagnosed with familial or sporadic ALS according to Gold Coast criteria
• The participant has a Treatment Research Initiative to Cure ALS (TRICALS) risk profile of ≥ -6.0 to < -2.0
• Slow vital capacity (SVC) of ≥ 60% of the predicted value according to Global Lung Function Initiative 2012

Exclusion Criteria:

• Use of noninvasive ventilation more than 10 hours a day or use of a tracheostomy for ventilatory support
• Any history of or current exposure to any gene or cell therapies (off-label use or investigational) for ALS
• Pregnant or lactating state or intention to become pregnant during the study