A Study to Evaluate Safety, Tolerability, and Distribution of a Microdose of Radiolabeled BIIB067 (99mTc-MAG3-BIIB067) Co-administered With BIIB067 (Tofersen) to Healthy Adults

Key Inclusion Criteria:

• Must have a body mass index between 18 and 30 kilogram per square meter (kg/m^2), inclusive.
• All men must practice highly effective contraception during the study and for 5 months after their last dose of study treatment.
• All female participants must be of non-childbearing potential and must meet 1 of the following criteria to participate in the study: a. Postmenopausal, b. History of bilateral oophorectomy (performed at least 6 weeks prior to Screening), c. History of hysterectomy (performed at least 6 weeks prior to Screening), or d. History of female surgical sterilization (e.g., bilateral tubal ligation; performed at least 6 weeks prior to Screening).

Key Exclusion Criteria:

• History of or ongoing clinically significant cardiac, endocrinologic, hematologic, hepatic, immunologic, metabolic, urologic, pulmonary, neurologic, dermatologic, psychiatric, renal, or other major disease, as determined by the Investigator.
• Clinically significant abnormal laboratory test values, as determined by the Investigator, at Screening or Check-in.
• Use of any prescription medication, over-the-counter medication (e.g., aspirin and nonsteroidal anti-inflammatory drugs [excluding acetaminophen]), or dietary and herbal supplements (e.g., St. John's wort) within 28 days of Check-in and an unwillingness to refrain from use through end of study (i.e., through the Day 91 visit), unless specifically permitted elsewhere within the protocol.
• Current enrollment in any other drug, biologic, device, or clinical study, or treatment with an investigational drug or approved therapy for investigational use within 30 days (6 months for biologics) or 5 half-lives, whichever is longer, prior to Check-in.
• History or physical examination that could be suggestive of a medical or mechanical condition, disorder, or disease that could represent a relative contraindication to lumbar puncture (LP), including but not limited to: (a) medication use that prolongs coagulation times, including aspirin; (b) known disorders of the coagulation cascade, platelet function, or platelet count; (c) low white blood cell counts; (d) chronic low back pain; (e) prior lumbar spine surgery; (f) anatomical factors at or near the LP site; (g) clinical signs of raised intracranial pressure (e.g., headache, papilledema, or focal neurologic signs).
• Contraindications to radiation exposure for research purposes.
• Exposure to ionizing radiation within the last 12 months that would result in approaching the exposure limits for healthy volunteers.

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