Purpose

The ALSpire Study is a clinical trial evaluating the investigational drug BIIB105 in adults living with amyotrophic lateral sclerosis (ALS). The ALSpire Study consists of two parts: - Part 1: 6-month placebo-controlled study. During Part 1, participants are randomly assigned to receive either BIIB105 or placebo in a 3:1 or 2:1 ratio (depending on the participant's assigned Cohort). - Part 2: up to 3-year long-term open-label extension. During Part 2, all participants receive BIIB105. The objectives of the study are to evaluate: - The safety and tolerability of BIIB105 in people with ALS - What the body does to BIIB105 (also called "pharmacokinetics") - What BIIB105 does to the body (also called "pharmacodynamics") - Whether BIIB105 can slow the worsening of clinical function