In this study, researchers will learn how the body processes a study drug called BIIB144. This is a "first-in-human" study. This means that this study drug will be given to people for the first very time in this study. These studies are important because they help researchers learn about the safety of the study drug, how the body processes it, and what dose might be appropriate before testing it in larger groups.

The main goal of this study is to learn more about the safety of a single dose of BIIB144 compared to a placebo. A placebo is something that looks like the study drug but does not contain any medicine. A placebo is also given in the same way as the study drug.

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 the study drug.

Researchers will also learn more about:

• How the body processes BIIB144 compared to placebo.

This study will be done as follows:

• Participants will be screened to check if they can join the study. The screening period will be up to 28 days, after which eligible participants will check into their study research center.
• Participants will stay at their study research center for about 8 days. Afterwards, they will have up to 13 visits to their study research center.
• This will be a "dose escalation" study. In this kind of study, increasing amounts of study drug are given to different groups of participants. Usually, this is done until researchers find the highest dose that does not cause harmful effects. In this study, the number of dose groups and maximum dose are planned in advance. The dose escalation will stop once these planned groups are done or if safety concerns arise.
• There will be a total of 6 groups. In each group, participants will receive a single dose of either BIIB144 or the placebo through an intravenous (IV) needle.
• Throughout the study, participants will give blood and urine samples. Participants will also have physical exams and answer questions about how they are feeling.
• Each participant will be in the study for up to 44 weeks.

The primary objective of this study is to evaluate the safety and tolerability of IV administered ascending single doses of BIIB144 compared with placebo in healthy adult participants. The secondary objective is to characterize the pharmacokinetics (PK) of IV administered ascending single doses of BIIB144 in healthy adult participants.

Key Inclusion Criteria:

• Have a body mass index (BMI) between 18 and 32 kilograms per square metre (kg/m^2), inclusive, at Screening.
• Must be in good health as determined by the Investigator, based on medical history and Screening evaluations.

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, or of any allergic reactions that in the opinion of the Investigator are likely to be exacerbated by any component of the study treatment.
• History of, or ongoing, malignant disease, including solid tumors and hematologic malignancies (with the exception of basal cell carcinomas and squamous cell carcinomas that have been completely excised and considered cured at least 1 year prior to Day -1).
• History of, or positive test result at Screening for, human immunodeficiency virus (HIV).
• Any live or attenuated immunization or vaccination given within 14 days prior to Screening, between Screening and Day -1, or planned to be given during the study period.

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/