Study of BIIB091 Formulations in Healthy Participants

Key Inclusion Criteria:

• Have a body mass index (BMI) between 18.0 and 30.0 kilograms per meter square (kg/m^2), inclusive, and a body weight of at least 50 kilogram (kg), as measured at screening.
• Must be in good health as determined by the Investigator, based on medical history and screening evaluations.
• A negative test for severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) according to local guidelines, at screening and admission or prior to admission.

Key Exclusion Criteria:

• History of any clinically significant cardiac, endocrine, GI, 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 abnormalities at screening and prior to first dose, including confirmed demonstration of QT interval corrected for heart rate using Fridericia's formula (QTcF) >450 milliseconds (msec), QRS >120 msec, PR >220 msec, or heart rate <50 beats per minute (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 (e.g. heart failure, hypokalemia, family history of long QT syndrome, or any medications known to prolong QT interval administered within 5.5 half-lives prior to screening), in the opinion of the Investigator.
• Receipt of any vaccination within 30 days prior to screening, or plans to receive the same any time from screening through to 30 days after the last study visit. However, non-live coronavirus disease 2019 (COVID-19) vaccination will be permitted 21 days or more prior to the first dose of BIIB091, as per local regulation and Investigator discretion.
• Evidence of current SARS-CoV-2 infection within the past 4 weeks at screening, between screening and admission, or at admission, including but not limited to any of the following symptoms: fever (temperature more than {>} 37.5 degree Celsius {°C}), new and persistent cough, breathlessness, or loss of taste or smell as per the judgement of the Investigator.
• Contact with an individual with COVID-19 infection in the past 14 days at screening, between screening and admission, or at admission.
• Presence or history of chronic, recurrent, or serious infection, as determined by the Investigator, within 90 days prior to screening or between screening and admission.
• Clinically significant abnormal laboratory test values, as determined by the Investigator, at screening.
• Current enrollment or plan to enroll in any other drug, biological, device, or clinical study, or treatment with an investigational drug or approved therapy for investigational use within 90 days prior to Day 1, or 5 half-lives of the drug or therapy, whichever is longer.
• Use of CYP3A4 inducers or inhibitors (including hormonal contraceptives as applicable) within 14 days before the first dose of study medication. Use of organic anion transporting polypeptide 1 (OATP1) B1 and B3 substrates in the 14 days before first dose of study medication.
• Chronic use of immunosuppressive or immunomodulatory drugs within 6 months prior to admission. (Recent acute use of immunosuppressants should be discussed with Sponsor.)
• Consumption of any product containing grapefruit, pomelos, or Seville oranges within 14 days of admission and an unwillingness to refrain from such products during study participation.
• Participants who have previously been enrolled in this study.

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