An Efficacy, Safety, Tolerability, Pharmacokinetics and Pharmacodynamics Study of BIIB067 in Adults With Inherited Amyotrophic Lateral Sclerosis (ALS)

This is a 3-part study to examine the efficacy, safety, tolerability, PK and PD of BIIB067.
Part A is the single ascending dose (SAD) component of the study, Part B is the multiple
ascending dose (MAD) component of the study and Part C will be the fixed dose component of
the study. Hence, the overall phase of development of the study is 1/2/3.

Key Inclusion Criteria: Part A and B

- Weakness attributable to ALS and documented SOD1 mutation at Screening Visit 2.

- A forced vital capacity (FVC) ≥50% of predicted value as adjusted for sex, age, and
height (from the sitting position). Participants with stable FVC <50% but ≥45%, whose
FVC has not declined by more than 5% in the last 6 months may be considered for
inclusion, at the discretion of the Investigator.

- If taking riluzole, participant must be on a stable dose for ≥30 days prior to Day 1
and expected to remain at that dose until the final study visit.

- Medically able to undergo the study procedures, and to adhere to the visit schedule at
the time of study entry, as determined by the Investigator.

Key Exclusion Criteria: Part A and B

- History of or positive test result for human immunodeficiency virus.

- History of, or positive test result at Screening, for hepatitis C virus antibody.

- Current hepatitis B infection (defined as positive for hepatitis B surface antigen
[HBsAg] and/or hepatitis B core antibody [HBcAb]). Participants with immunity to
hepatitis B from previous natural infection (defined as negative HBsAg, positive
hepatitis B surface antibody immunoglobulin G, and positive HBcAb) or vaccination
(defined as positive anti-HBs) are eligible to participate in the study.

- Treatment with another investigational drug, biological agent, or device within 1
month or 5 half-lives of study agent, whichever is longer. Specifically, no prior
treatment with small interfering ribonucleic acid, stem cell therapy, or gene therapy
is allowed.

- Current enrollment in any other interventional study.

- Current or recent (within 1 month) use, or anticipated need, in the opinion of the
Investigator, of copper (II) (diacetyl-bis (N4-methylthiosemicarbazone)) or
pyrimethamine.

- Current or anticipated need, in the opinion of the Investigator, of a diaphragm pacing
system (DPS) during the study period.

Key Inclusion Criteria: Part C

- Weakness attributable to ALS and confirmed SOD1 mutation at Screening Visit.

- If taking riluzole, participant must be on a stable dose for ≥30 days prior to Day 1
and expected to remain at that dose until the final study visit.

- If taking edaravone, participant must have initiated edaravone ≥60 days (2 treatment
cycles) prior to Day 1 and expected to remain at that dose until the final study
visit, unless the Investigator determines that edaravone should be discontinued for
medical reasons, in which case it may not be restarted during the study. Edaravone may
not be administered on dosing days of this study.

- Medically able to undergo the study procedures and to adhere to the visit schedule at
the time of study entry, as determined by the Investigator.

Key Exclusion Criteria: Part C

- History of or positive test result for human immunodeficiency virus.

- History of, or positive test result at Screening, for hepatitis C virus antibody.

- Current hepatitis B infection (defined as positive for HBsAg and/or anti-HBc).
participants with immunity to hepatitis B from previous natural infection (defined as
negative HBsAg, positive IgM anti-HBc, and positive anti-HBc) or vaccination (defined
as positive anti-HBs) are eligible to participate in the study.

- Treatment with another investigational drug (including investigational drugs for ALS
through compassionate use programs), biological agent, or device within 1 month or 5
half-lives of study agent, whichever is longer. Specifically, no prior treatment with
small interfering RNA, stem cell therapy, or gene therapy is allowed.

- Current enrollment in any other interventional study.

- Current or recent (within 1 month) use, or anticipated need, in the opinion of the
Investigator, of copper (II) (diacetyl-bis(N4-methylthiosemicarbazone)) or
pyrimethamine.

- Current or anticipated need, in the opinion of the Investigator, of a DPS during the
study period.

NOTE: Other protocol defined Inclusion/Exclusion criteria may apply.