Affiliated study NCT03318523 did not meet its primary outcome measure of change from baseline measured by Movement Disorder Society-Unified Parkinson's Disease Rating Scale (MDS-UPDRS) and did not demonstrate efficacy on secondary outcome measures

The primary objective of this study is to evaluate the safety and tolerability of a range of single and 13 repeated doses of BIIB054, administered as intravenous (IV) infusion, in Japanese participants with Parkinson's disease (PD). The secondary objectives are to evaluate the immunogenicity, and serum pharmacokinetics (PK) profile of BIIB054 after single and multiple dose administration.

Key Inclusion Criteria:

• Diagnosed with PD within a maximum of 3 years prior to screening.
• Has not received levodopa or any other treatment for PD, herein referred to as symptomatic PD medication (including but, not limited to, dopamine agonists, amantadine, anticholinergics, monoamine oxidase type B (MAO-B) inhibitors, or safinamide) for at least 12 weeks prior to Day 1. Maximum total duration of prior PD regimens should not exceed 30 days.
• Score of less than equal to (<=) 2.5 on the Modified Hoehn and Yahr Scale.
• Screening dopamine transporter (DaT)/ single-photon emission computed tomography (SPECT) results consistent with neurodegenerative Parkinsonism (central reader).

Key Exclusion Criteria:

• Presence of freezing of gait.
• History of or positive test result at Screening for human immunodeficiency virus (HIV) or hepatitis C virus antibody (anti-HCV).
• Screening value for hemoglobin less than (<)12 gram per deciliter (g/dL) for men or <11 g/dL for women.
• Montreal Cognitive Assessment (MoCA) score <23 or other significant cognitive impairment or clinical dementia.
• History of any brain surgery for PD.
• Participation in any passive or active immunotherapy targeting alpha-synuclein or other PD-related protein.

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

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