A Study to Assess if BIIB122 Tablets Are Safe and Can Slow Worsening of Early-Stage Parkinson's Disease in Participants With Specific LRRK2 Genetic Variants Between the Ages of 30 and 80 Using the Movement Disorder Society-Unified Parkinson's Disease Rating Scale (LIGHTHOUSE)

A Study to Assess if BIIB122 Tablets Are Safe and Can Slow Worsening of Early-Stage Parkinson's Disease in Participants With Specific LRRK2 Genetic Variants Between the Ages of 30 and 80 Using the Movement Disorder Society-Unified Parkinson's Disease Rating Scale

A Phase 3, Multicenter, Randomized, Double-Blind, Placebo-Controlled Study to Determine the Efficacy and Safety of BIIB122/DNL151 in Participants With Parkinson's Disease and Pathogenic LRRK2 Variants

In this study, researchers will learn more about a study drug called BIIB122 in participants with early-stage Parkinson's disease (PD). The study will focus on participants with a specific genetic variant in their LRRK2 gene. The main question researchers are trying to answer is if taking BIIB122 slows the worsening of PD more than placebo in the early stages of PD. To help answer this question, researchers will use a questionnaire called the Movement Disorder Society-Unified Parkinson's Disease Rating Scale, also known as the MDS-UPDRS. The MDS-UPDRS measures impairment and disability in people living with PD. It was created in the 1980s and is one of the most used rating scales for PD symptoms. The MDS-UPDRS has 4 parts, and a higher score means more severe PD symptoms. Part I assesses non-motor experiences of daily living, including but not limited to memory loss, problems sleeping, pain, depression, and anxiety. Part II measures motor experiences of daily living. Part III is the results of a motor symptoms exam by a medical professional. Part IV records PD complications caused by motor symptoms. Researchers will also learn more about the safety of BIIB122. A description of how the study will be done is given below. Participants will take BIIB122 or a placebo as tablets by mouth. A placebo looks like the study drug but contains no real medicine. Participants will be in the study for 103 weeks to 187 weeks. This includes the screening and follow-up periods. Participants will take BIIB122 or placebo 1 time a day for 96 to 180 weeks. Participants can continue to take certain medications for PD. Participants must be on the same dose of medication for at least 90 days before the study begins. Participants will visit the clinic less often as the study continues, ranging every 4 weeks to every 24 weeks.

BIIB122 is an investigational central nervous system-penetrant small molecule inhibitor of LRRK2 kinase

Time to Confirmed Worsening in Movement Disorder Society Unified Parkinson's Disease Rating Scale (MDS-UPDRS) Parts II and III Over the Treatment Period

Time to confirmed worsening is defined as a worsening event sustained over 2 consecutive assessments. MDS-UPDRS is a multimodal scale assessing impairment and disability consisting of 4 parts. Part II assesses motor experiences of daily living (range 0-52). It contains 13 questions which are to be completed by the participant. Part III assesses the motor signs of PD and is administered by the rater (range 0-132). Part III contains 33 scores based on 18 items. For each question a numeric score is assigned between 0-4, where 0 = Normal, 1 = Slight, 2 = Mild, 3 = Moderate, 4 = Severe. MDS-UPDRS Part II and III combined score equals the sum of Parts II and III (range 0-184). A higher score indicates more severe symptoms of PD.

Number of Participants With Treatment Emergent Adverse Events (TEAEs) and Serious Adverse Events (SAEs)

An adverse event (AE) is any untoward medical occurrence in a participant or clinical investigation participant administered a pharmaceutical product and that does not necessarily have a causal relationship with this treatment. An AE can therefore be any unfavorable and unintended sign (including an abnormal laboratory finding), symptom, or disease temporally associated with the use of a medicinal (investigational) product, whether or not related to the medicinal (investigational) product. An SAE is any untoward medical occurrence that at any dose results in death; in the view of the investigator, places the participant at immediate risk of death (a life-threatening event); requires inpatient hospitalization or prolongation of existing hospitalization; results in persistent or significant disability/incapacity; results in a congenital anomaly/birth defect or is a medically important event.

Time to confirmed worsening is defined as a worsening event sustained over 2 consecutive assessments. MDS-UPDRS is a multimodal scale assessing impairment and disability consisting of 4 parts. Part II assesses motor experiences of daily living (range 0-52). It contains 13 questions which are to be completed by the participant. For each question a numeric score is assigned between 0-4, where 0 = Normal, 1 = Slight, 2 = Mild, 3 = Moderate, 4 = Severe. A higher score indicates more severe symptoms of PD.

MDS-UPDRS is a multimodal scale assessing impairment and disability consisting of 4 parts. Part II assesses motor experiences of daily living (range 0-52). It contains 13 questions which are to be completed by the participant. For each question a numeric score is assigned between 0-4, where 0 = Normal, 1 = Slight, 2 = Mild, 3 = Moderate, 4 = Severe. A higher score indicates more severe symptoms of PD. Part III assesses the motor signs of PD and is administered by the rater (range 0-132). Part III contains 33 scores based on 18 items. For each question a numeric score is assigned between 0-4, where 0 = Normal, 1 = Slight, 2 = Mild, 3 = Moderate, 4 = Severe. MDS-UPDRS Parts II and III combined score equals the sum of Part II and III (range 0-184). A higher score indicates more severe symptoms of PD. Positive change from baseline indicates severe PD.

Time to Confirmed Worsening in Modified Schwab and England Activities of Daily Living Scale (mSE-ADL) Score Over the Treatment Period

Time to confirmed worsening is defined as a worsening event sustained over 2 consecutive assessments. The mSE-ADL scale reflects the speed, ease, and independence with which an individual performs daily activities or personal chores with 100% indicating total independence, falling to 0%, which indicates a state of complete dependence. The individual is asked to rate his or her function using an 11-point scale (10% increments), from 100% (completely independent; able to do all chores without slowness, difficulty, or impairment; essentially normal; unaware of any difficulty) to 0% (vegetative functions such as swallowing, bladder and bowels are not functioning; bedridden). The lower the score, the worse the functional status.

MDS-UPDRS is a multimodal scale assessing impairment and disability consisting of 4 parts. Part I assesses non-motor experiences of daily living and has 2 components (range 0-52). Part IA contains 6 questions and is assessed by the examiner (range 0-24). Part IB contains 7 questions on non-motor experiences of daily living which are to be completed by the participant (range 0-28). Part II assesses motor experiences of daily living (range 0-52). It contains 13 questions which are to be completed by the participant. Part III assesses the motor signs of PD and is administered by the rater (range 0-132). Part III contains 33 scores based on 18 items. For each question a numeric score is assigned between 0-4, where 0 = Normal, 1 = Slight, 2 = Mild, 3 = Moderate, 4 = Severe. MDS-UPDRS total score equals the sum of Parts I, II, and III (range 0-236). A higher score indicates more severe symptoms of PD. Positive change from baseline indicates severe PD.

Key Inclusion Criteria: Clinical diagnosis of PD meeting the Movement Disorder Society Clinical Diagnostic Criteria within 5 years of the Screening Visit, inclusive, and at least 30 years of age at the time of diagnosis Modified Hoehn and Yahr scale (mHY), Stages 1 to 2.5 (in OFF state), inclusive, at Screening MDS-UPDRS Parts II and III (in OFF state) combined score ≤40 at Screening Screening genetic test results verifying the presence of a pathogenic leucine-rich repeat kinase 2 (LRRK2) variant Key Exclusion Criteria: Clinically significant neurologic 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 NOTE: Other protocol defined Inclusion/Exclusion criteria may apply.

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