In this study, researchers will learn how the heart reacts to multiple doses of BIIB091 in healthy participants. In previous studies looking at BIIB091, researchers have found that a certain range of doses can cause changes in the electrical activity of the heart. In this study, researchers want to learn more about that activity after participants take BIIB091 at lower and higher doses for 5 days. The main objective of this study is to learn how the heart's electrical activity changes after taking BIIB091. To measure this, researchers will look at something called the QT interval corrected for heartbeat, also known as "QTc". This represents the time it takes for the heart to contract and relax. A QT interval that is too long means that the heart's electrical system is taking longer than normal to recharge between heartbeats. This could increase the risk of abnormal heartbeats that may lead to sudden death.

The main questions researchers want to answer are:

• What is the change in the participants' QTc after taking BIIB091?

Researchers will also learn more about:

• Any changes in other measures of the heart's electrical activity
• How the body processes BIIB091, as well as something called "M23", which is made in the body when BIIB091 is broken down
• Any medical problems the participants have during the study
• Any changes in the participants' overall health during the study

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 29 days, after which participants will check into their study research center.

• Participants will be randomly assigned to be in 1 of 2 groups:

  • Group 1: Participants will take a low dose of BIIB091 for 5 days followed by a high dose of BIIB091 for 5 days.
  • Group 2: Participants will take a single dose of moxifloxacin. Moxifloxacin is an antibiotic used to treat bacterial infections. Researchers are using it in this study to compare to BIIB091, because it is known to cause abnormal changes to the heart's electrical activity.
  • Participants in both Group 1 and Group 2 will also take a placebo while at the study research center. A placebo looks like the study drug or moxifloxacin, but does not have any real medicine in it.

• Participants will remain at their study research center for 15 days. Afterwards, there will be a follow-up visit 11 to 14 days after each participant's last dose. Each participant will be in the study for up to 57 days.

The primary objective of this study is to evaluate the effect of BIIB091 and its metabolite M23 following multiple therapeutic and supratherapeutic doses of BIIB091 on QTc intervals in healthy participants. The secondary objectives of this study are: to evaluate the effect of BIIB091 and its metabolite M23 following multiple therapeutic and supratherapeutic doses of BIIB091 on other electrocardiogram (ECG) parameters (heart rate [HR], pulse rate [PR] and QRS intervals, and ECG morphology); to demonstrate assay sensitivity of the study to detect a QTc effect using moxifloxacin as a positive control; to assess the pharmacokinetics (PK) of BIIB091 and its metabolite M23, following multiple oral dose administrations in healthy participants; to evaluate the safety and tolerability of BIIB091 in healthy participants.

Key Inclusion criteria:

• Have a body mass index between 18 and 32 kilograms per meter square (kg/m^2), inclusive, and a total body weight > 50 kg, at screening and Day -2.
• Must be in good health as determined by the Investigator, based on medical history and Screening evaluations.
• Negative polymerase chain reaction (PCR) test result for severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) at check-in (Day -2).

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.
• Clinically significant 12-lead ECG findings at screening and prior to first dose, including confirmed demonstration of QTcF > 430 milliseconds (ms), QRS > 120 ms, PR > 220 ms, or heart rate < 45 beats per minutes (bpm) based on the average of triplicate measurements, early repolarization, or any other clinically significant 12-lead ECG abnormalities as determined by the Investigator.
• History of torsades de pointes or additional risk factors for torsades de pointes.
• History of, or positive test result at Screening for, human immunodeficiency virus, hepatitis B virus, or hepatitis C virus infection.
• Use of any prescription or over-the-counter medication (including herbal supplements) that are known to prolong QT/QTc interval or induce torsades de pointes.
• Chronic, recurrent, or serious infection (e.g., pneumonia, septicemia), as determined by the Investigator, within 90 days prior to Screening or between Screening and Check-in.

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/