In this study, researchers will learn more about BIIB122 in participants with early-stage Parkinson's disease (PD). The study will include adults aged 30 to 80 who were diagnosed with PD within 2 years of starting the study.

The main objective of the study is to learn about the effect BIIB122 has on slowing down the worsening of PD symptoms. The main question researchers want to answer is:

- How long does it take for PD symptoms to worsen during BIIB122 treatment?

Researchers will answer this and other questions by measuring the symptoms of PD over time using a variety of scoring tools. These include the Movement Disorder Society Unified Parkinson's Disease Rating Scale (MDS-UPDRS) and the modified Schwab and England Activities of Daily Living Scale (mSE-ADL).

The MDS-UPDRS is used to measure symptoms of PD. It has 4 parts: Part I, II, III, and IV. Each part measures different aspects of motor and non-motor symptoms. The mSE-ADL measures a participant's ability to perform daily activities or personal chores.

Researchers will also learn more about the safety of BIIB122. They will check participants for adverse events. Adverse events are unwanted health problems that may or may not be caused by the study drug.

The study will be done as follows:

• Participants will be randomly assigned to take either BIIBB122 or placebo. A placebo looks like the study drug but contains no real medicine.
• Neither the researchers nor the participants will know if the participants are receiving BIIB122 or placebo.
• Participants will take BIIB122 or placebo tablets by mouth once a day.
• The treatment period for each participant will last between 48 and 144 weeks.
• There will be a safety follow-up period for 2 weeks after the last dose of BIIB122.
• In total, participants will have up to 29 study visits.
• Participants will stay in the study for at least 1 year, up to about 3 years.

BIIB122 is an investigational central nervous system-penetrant small molecule inhibitor of leucine-rich repeat kinase 2 (LRRK2). Participants who completed the early termination (ET) visit of the study 283PD302 (NCT05418673) would be eligible for screening of this study and if enrolled, these participants are not eligible for the sub studies of 283PD201.

Key Inclusion Criteria:

• Clinical diagnosis of PD meeting the Movement Disorder Society Clinical Diagnostic Criteria within 2 years of the Screening Visit, inclusive, and at least 30 years of age at the time of diagnosis
• Modified Hoehn and Yahr scale stages 1 to 2 (in OFF state), inclusive, at screening
• MDS-UPDRS Parts II and III (in OFF state) combined score less than or equal to (≤)50 at screening

Key Exclusion Criteria:

• Clinically significant neurological disorder other than PD, including but not limited to stroke, dementia, or seizure, within 5 years of screening visit, in the opinion of the Investigator
• Clinical evidence of atypical parkinsonism (e.g., multiple-system atrophy or progressive supranuclear palsy) or evidence of drug-induced parkinsonism.
• Montreal Cognitive Assessment (MoCA) score <24 at the screening visit.

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

In accordance with Biogen's Clinical Trial Transparency and Data Sharing Policy on https://www.biogentrialtransparency.com/