A Study to Learn More About the Safety of BIIB145 and How it is Processed in the Body of Healthy Adult Participants

The primary objective of the study is to evaluate the safety and tolerability of single and multiple ascending doses of BIIB145 in healthy adult participants.

The secondary objectives of the study are to evaluate the pharmacokinetics (PK) profile of single and multiple ascending doses of BIIB145 and its enantiomers in healthy adult participants; and the effect of food on the PK profile of BIIB145.

Key Inclusion Criteria:

• Have a body mass index (BMI) between 18 and 32 kilograms per square metre (kg/m^2), inclusive, and a total body weight > 50 kilograms (kg) at Screening and Check-In.
• Must be in good health as determined by the Investigator, based on medical history and Screening evaluations. Good health is defined as no clinically relevant abnormalities identified by a detailed medical history, full physical examination, including blood pressure and pulse rate measurement, 12-lead electrocardiogram (ECG), and clinical laboratory tests.

Key Exclusion Criteria:

• History of any clinically significant blood disorders or 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, or ongoing, malignant disease, including solid tumours and hematologic malignancies (except for basal cell carcinomas and squamous cell carcinomas that have been completely excised and considered cured at least 1 year prior to Check-In).
• History of uncontrolled bleeding, or any risk of bleeding that, in the opinion of the Investigator, is clinically significant.
• History of torsades de pointes or additional risk factors for torsades de pointes (e.g., heart failure, hypokalemia, or family history of long QT syndrome), in the opinion of the Investigator.
• Any values of coagulation parameters including international normalized ratio, prothrombin time, and activated partial thromboplastin time above upper limit of normal at Screening or Check-In.
• Systolic blood pressure > 150 millimetres of mercury (mmHg) or < 90 mmHg after resting for 5 minutes in the supine position at Screening and prior to dosing. If out of range, testing may be repeated once at Screening and once prior to dosing.
• Any live or attenuated immunization or vaccination given within 30 days prior to Check-In or planned to be given during the study period.
• Prior exposure to BIIB145 or any lymphocyte-depleting therapy or exposure to any lymphocyte-targeting therapy within 3 months prior to Check-In, or at least 5 half-lives or for the anticipated duration of the product's pharmacodynamics (PD) effects, whichever is longer.
• Known lumbar spine deformity, degenerative arthritis of the lumbar spine, history of lumbar spinal surgery, spinal infection, spinal mass or trauma, and/or known evidence on magnetic resonance imaging (MRI) contraindicating lumbar puncture (LP).

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