A Study to Learn About the Safety of BIIB122 Tablets and Whether They Can Slow the Worsening of Early-Stage Parkinson's Disease in Adults Between the Ages of 30 and 80

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

An 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 frame: Up to Week 144

Time to Confirmed Worsening in MDS-UPDRS Part II Score 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. 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.

Time frame: Up to a minimum of 48 weeks and a maximum of 144 weeks

Change From Baseline in MDS-UPDRS Parts II and III Combined Score

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.

Time frame: From Baseline up to Week 48

Time to Confirmed Worsening in Modified Schwab and England Activities of Daily Living Scale (mSE-ADL) 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.

Time frame: Up to a minimum of 48 weeks and a maximum of 144 weeks

Change From Baseline in MDS-UPDRS Parts I, II, and III Combined Score

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.

Time frame: From Baseline up to Week 48