A Study to Determine the Biodistribution, Safety, and Tolerability of a Microdose of Radiolabeled BIIB080 Co-administered With BIIB080 in Healthy Adults

Eligibility Criteria

• * Must be healthy adult and have a body mass index (BMI) 18 through 30 kilograms per square meter (kg/m\^2).
• * Males must practice highly effective contraception and females must be of no childbearing potential.
• * Conditions that influence cerebrospinal fluid (CSF) kinetics e.g., significant scoliosis or a physical disability that limits mobility.
• * Contraindication to:
• * Magnetic resonance imaging (MRI) scan: e.g., indwelling ferrous metal, implanted medical devices, or claustrophobia.
• * Radiation/PET scan: e.g., inability to lie flat or still for the duration of the scan) or intolerance to previous nuclear medicine scans.
• * Lumbar Puncture (LP, spinal tap): e.g., current use of a medication that prolongs coagulation times, including aspirin, known disorders of the coagulation cascade, platelet function, or platelet count, low white blood cell counts, chronic low back pain; prior lumbar spine surgery, anatomical factors at or near the spinal tap site, clinical signs of raised intracranial pressure (e.g., headache, or focal neurologic signs).
• * Findings from the screening MRI that may interfere with participant safety or the scientific integrity of the study e.g., Chiari malformation, high grade spinal stenosis, hydrocephalus, or neural tube defect.
• * Abnormal Laboratory Values e.g., liver functions, anemia, abnormal blood coagulation profile.
• * Cardiac, endocrinologic, hematologic, hepatic, immunologic, metabolic, urologic, pulmonary, neurologic, dermatologic, psychiatric, renal, or malignant disease.
• * Severe allergic or anaphylactic reactions/ systemic hypersensitivity to BIIB080 or any component of the study treatment.
• * Alcohol or substance abuse.
• * Plans to undergo elective procedures or surgeries during the study.
• * Enrollment in any other drug, biologic, device, or clinical study within 30 days or 5 half-lives, whichever is longer, prior to Check-in.
• * Use of any prescription medication (except hormone therapy), over-the-counter medication (excluding acetaminophen), or dietary and herbal supplements (e.g., St. John's wort) within 28 days of the of dosing and during the study.