A Randomized, Double-blind, Placebo-controlled Single Ascending Dose and Open-label Multi-dose Study to Evaluate the Safety, Tolerability, Pharmacokinetics and Pharmacodynamics of Intrathecally Administered ALN-APP in Adult Patients With Early-onset Alzheimer's Disease (EOAD)

Inclusion Criteria: * Has mild cognitive impairment or mild dementia due to EOAD * Has Clinical Dementia Rating (CDR) global score 0.5 or 1.0 and Mini Mental State Examination (MMSE) \>20 Exclusion Criteria: * Has Non-Alzheimer's disease dementia * Has alanine aminotransferase (ALT) or aspartate aminotransferase (AST) \>2×upper limit of normal (ULN) * Has estimated glomerular filtration rate (eGFR) \<45 mL/min/1.73m\^2 at Screening * Has recently received an investigational agent * Has recent treatment with amyloid-targeting antibody Note: other protocol defined inclusion / exclusion crite…