Study of ARO-MAPT-SC in Healthy Subjects and Subjects With Early Alzheimer's Disease

Inclusion Criteria (All Participants): * Body mass index between 18.0 and 35.0 kg/m\^2 at Screening * Not pregnant or breast-feeding * Able and willing to provide written informed consent prior to the performance of any study specific procedures * Participants of childbearing potential must agree to use highly effective contraception in addition to a condom during the study and for at least 90 days following the end of the study or last dose of study drug, whichever is later; participants must not donate sperm or eggs during the study and for at least 90 days following the end of the study or last dose of study drug whichever is later Inclusion Criteria (Alzheimer's Disease): * Adults aged 50 to 75 years of age with a clinical diagnosis of early AD and plasma, CSF, or imaging biomarkers consistent with the diagnosis * On stable doses of AD-related medications for at least 8 weeks prior to Screening Visit and throughout the Screening period until Day 1 Exclusion Criteria (All Participants): * Uncontrolled hypertension * Human immunodeficiency virus (HIV) infection (seropositive at Screening) * Seropositive for hepatitis B (HBV) or hepatitis C (HCV) at Screening * Intellectual disability or significant behavioral neuropsychiatric manifestation * Clinically significant cardiac, liver, or renal disease * Any contraindications to lumbar puncture Note: Additional inclusion/exclusion criteria may apply per protocol.