Key Inclusion Criteria:  Female or infertile/vasectomised males aged 18 to 64 years (Parts A and B), 20 to 64 years (Part C), or 65 to 75 years (Part B Dose 7Cohort 9 only), inclusive, at the time of informed consent Have a body mass index between 18 and 32 kilograms per square meter (kg/m^2), inclusive, at screening Weight ≥50 kilograms (kg) at screening Negative Polymerase Chain Reaction (PCR) test for severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) at Day -1 prior to randomization  Key Exclusion Criteria:  History or positive test result at Screening for Human Immunodeficiency Virus (HIV) Chronic, recurrent, or serious infection (e.g., pneumonia, septicemia), as determined by the investigator, within 90 days prior to screening or between screening and Day -1 History of severe allergic or anaphylactic reactions, or of any allergic reactions that in the opinion of the investigator are likely to be exacerbated by any component of the study treatment History of systemic hypersensitivity reaction to BIIB113 or the excipients contained in the formulation, and if appropriate, any diagnostic agents to be administered during the study Has suicidal ideation with some intent to act within 6 months prior to the start of screening, per the investigator's clinical judgment or based on the C-SSRS, corresponding to a response of "Yes" on Item 4 (active suicidal ideation with some intent to act, without specific plan) or Item 5 (active suicidal ideation with specific plan and intent) for suicidal ideation on the C-SSRS, or a history of suicidal behaviour within one year prior to the start of screening. Any condition affecting study treatment absorption (e.g., gastrectomy) Previous exposure to an OGA inhibitor Current enrolment in any other drug, biologic, device, or clinical study, or treatment with an investigational drug or approved therapy for investigational use within 90 days (6 months for biologics) prior to Day -1, or 5 half-lives of the agent, whichever is longer For Part C only: Previously undergone PET scans for research purposes  NOTE: Other protocol defined Inclusion/Exclusion criteria may apply.