Evaluating the Efficacy, Safety, Pharmacokinetics, and Pharmacodynamics of BIIB054 in Participants With Parkinson's Disease

Inclusion Criteria

   - Diagnosed with Parkinson's disease (PD) within a maximum of 3 years prior to
   Screening.

   - Score of ≤2.5 on the Modified Hoehn and Yahr Scale.

   - Has not received any medication for the treatment of the motor symptoms of PD for at
   least 12 weeks prior to Day 1 and, in the opinion of the Investigator, is not expected
   to require PD treatment for at least 6 months following Day 1. Maximum total duration
   of prior PD regimens should not exceed 30 days. Stable (at least 8 weeks) dosages of
   medications that are used to treat conditions other than PD tremor are allowed.
   Further guidance will be provided by the study's Medical Monitor on a case by case
   basis.

   - Screening dopamine transporter (DaT)/ single-photon emission computed tomography
   (SPECT) results consistent with neurodegenerative Parkinsonism (central reading).

   - All women of childbearing potential and all men must practice highly effective
   contraception during the study and for 6 months after their last dose of study
   treatment.

Exclusion Criteria:

   - Presence of freezing of gait.

   - Montreal cognitive assessment (MOCA) score <23 or other significant cognitive
   impairment or clinical dementia that, in the opinion of the Investigator, would
   interfere with study evaluation.

   - History of or screening brain magnetic resonance imaging (MRI) scan indicative of
   clinically significant abnormality, as read by central reader.

   - History of severe allergic or anaphylactic reactions, or history of hypersensitivity
   to BIIB054 or any of the inactive ingredients in the drug product or to radioligands
   or iodine used in the study.

   - Participation in any active immunotherapy study targeting alpha-synuclein.

   - Use of allowed medications not previously specified at doses that have not been stable
   for at least 8 weeks before Day 1, and/or that are not expected to remain stable for
   the duration of the study.

   - Clinically significant abnormal laboratory test values at Screening, as determined by
   the Investigator.

   - Blood donation (1 unit or more) within 8 weeks before Day 1 (must also refrain from
   donating blood for the duration of the study).

NOTE : Other protocol defined Inclusion/Exclusion criteria may apply