In this study, researchers will learn more about the safety of BIIB142 and how it is processed in the body. This is the first time that researchers will learn about BIIB142 and how it affects people.

The main question researchers want to answer in this study is:

• How many participants have adverse events (AEs) and serious adverse events (SAEs)?

An AE is a health problem that may or may not be caused by a drug during the study. An AE is considered serious when it results in death, is life-threatening, causes lasting problems, or requires hospital care.

Researchers will also learn more about:

• How the body processes BIIB142

This is a "dose escalation study." This is a study in which increasing amounts of the study drug are given to different groups of participants. This is done until researchers find the highest dose that does not cause harmful effects.

First, participants will be screened to check if they can join the study. The screening period will be up to 28 days. This study will be split into 2 parts - Part A and Part B.

During Part A:

• Participants will be randomly placed into 1 of 6 groups to receive a single dose of either BIIB142 or a placebo. A placebo looks like the study drug but contains no real medicine.
• Participants in Groups 1 through 5 will take either BIIB142 or the placebo without food. Participants in Group 6 will take 2 doses of their assigned treatment - once with food and once without food.
• Neither the researchers nor the participants will know if the participants will receive BIIB142 or the placebo.
• Participants will stay at their study research center for 5 days. They will return for another 4 visits. Each participant in Part A will be in the study for up to 58 days.

During Part B:

• Participants will be randomly placed into 1 of 3 groups to receive BIIB142 or the placebo. In Part B, participants will take BIIB142 or the placebo once a day for 14 days.
• Neither the researchers nor the participants will know if the participants will receive BIIB142 or the placebo.
• Participants will stay at their study research center for 16 days. They will return for another 4 visits. Each participant in Part B will be in the study for up to 58 days.

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.

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