A Study to Learn About the Safety of BIIB142 and How it is Processed in the Body of Healthy Adult Participants Aged 18 to 55 Years Old

The primary objective of this study is to evaluate the safety and tolerability of single and multiple ascending oral doses of BIIB142 in healthy adult participants.

The secondary objective of this study is to evaluate the pharmacokinetics (PK) profile of single and multiple ascending oral doses of BIIB142 in healthy adult participants.

Key Inclusion Criteria:

• Have a body mass index between 18 and 32 kilograms per square meter (kg/m^2), inclusive, at screening.
• Weight ≥ 50 kilograms (kg) at screening.
• Negative polymerase chain reaction (PCR) test for severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) at Check-in prior to randomization.
• Must be in good health as determined by the Investigator.

Key Exclusion Criteria:

• History of any clinically significant cardiac, endocrine, gastrointestinal, hematologic, hepatic, immunologic, metabolic, urologic, pulmonary, neurologic, dermatologic, psychiatric, or renal disease, or other major disease, as determined by the Investigator.
• History of severe allergic or anaphylactic reactions
• History of or ongoing malignant disease (with limited exceptions)
• Systolic blood pressure >150 millimeters of mercury (mmHg) or <90 mmHg.
• Clinically significant (as determined by the Investigator) electrocardiogram (ECG) abnormalities.
• History of or positive test for human immunodeficiency virus (HIV).
• Chronic, recurrent, or serious infection within 90 days prior to Screening.
• Symptoms of bacterial, fungal, or viral infection within 14 days prior to Screening.
• Any live or attenuated immunization within 14 days prior to Screening.
• Use of prescription medications, over-the-counter medications that alter hepatic or renal clearance, or nutraceuticals within 28 days prior to Check-in.
• MAD Cohorts only: Suicidal ideation with some intent to act within 6 months prior to the start of Screening or history of suicidal behavior within one year prior to Screening.

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