A Study to Assess the Safety, Tolerability, and Pharmacokinetics of BIIB105 in Participants With Amyotrophic Lateral Sclerosis With or Without Poly-cytosine-adenine-guanine (CAG) Expansion in the Ataxin-2 Gene

Key Inclusion Criteria:

• Ability of the participant and/or his/her legally authorized representative (e.g., parent, spouse, or legal guardian), as appropriate and applicable, to understand the purpose and risks of the study, to provide informed consent, and to authorize the use of confidential health information in accordance with national and local privacy regulations.
• All women of childbearing potential and all men must ensure that highly effective contraception is used during the study and for at least 6 months for female participants and 8 months for male participants after their last dose of study treatment.
• No known presence or family history of mutations in the superoxide dismutase 1 (SOD1) or fused in sarcoma (FUS) genes.
• Participants in Cohorts A, B, C1 and D1, must meet the laboratory-supported probable, probable, or definite criteria for diagnosing ALS according to the World Federation of Neurology El Escorial criteria (revised according to the Airlie House Conference 1998 [Brooks 2000]). Participants in Cohort C2 and D2, must meet any of the prior conditions, but may also only meet clinically possible criteria for diagnosing ALS, or exhibit weakness attributable to ALS in the presence of ataxin-2 protein (ATXN2) intermediate repeats.
• In participants in Cohorts C2 and D2, confirmed intermediate cytosine-adenine-guanine/cytosine- adenine-adenine (CAG/CAA) repeat expansion in the ataxin-2 gene or RNA (ATXN2) gene as defined by at least 1 allele carrying 30 to 33 CAG/CAA repeats.
• Slow vital capacity (SVC) criteria:

• In participants in Cohorts A, B, C1, and D1, SVC

  • 60% of predicted value as adjusted for sex, age, and height (from the sitting position).
• In participants in Cohorts C2 and D2, SVC ≥50% of predicted value as adjusted for sex, age, and height (from the sitting position).

• 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, unless the Investigator determines that it should be discontinued for medical reasons, in which case it may not be restarted during the study.
• Participants taking concomitant edaravone at study entry must be on a stable dose for ≥60 days prior to the first dose of study treatment (Day 1).
• Screening values of coagulation parameters including platelet count, international normalized ratio (INR), prothrombin time (PT), and activated partial thromboplastin time (aPTT) should be within normal ranges.
• Has an informant/caregiver who, in the Investigator's judgment, has frequent and sufficient contact with the participant as to be able to provide accurate information about the participant's cognitive and functional abilities at screening.

Key Exclusion Criteria

• History or positive test result at Screening for HIV.
• Current hepatitis C infection.
• Current hepatitis B infection.
• History of alcohol or substance abuse ≤6 months of Screening that would limit participation in the study, as determined by the Investigator.
• Current or anticipated need, in the opinion of the Investigator, of a diaphragm pacing system during the study period.
• Presence of tracheostomy.
• In participants from Cohorts A, B, C1, and D1, history of myocardial infarction, as determined by the Investigator.

• In participants from Cohorts A, B, C1, and D1, poorly controlled type 1 or 2 diabetes mellitus defined as HbA1c

  • 8% during Screening.
• Prescreening Amyotrophic Lateral Sclerosis Functional Rating Scale-Revised (ALSFRS-R) slope > -0.4 points/month, where prescreening ALSFRS-R slope is defined as: (48 - ALSFRS-R score at Screening) / (months from date of symptom onset to date of Screening) in participants from Cohorts A, B, C1, and D1.
• Treatment with another investigational drug (including investigational drugs for ALS through compassionate use programs) or biological agent within 1 month or 5 half-lives of study agent, whichever is longer, before Screening.
• Treatment with an antiplatelet or anticoagulant therapy that cannot safely be interrupted for lumbar puncture (LP) according to local standard of care and/or institutional guidelines, in the opinion of the Investigator or Prescriber.
• Female participants who are pregnant or currently breastfeeding and those intending to become pregnant during the study.

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