A Study to Evaluate ALN-5288 in Patients With Alzheimer's Disease

The purpose of this study is to: * Evaluate the safety and tolerability of intrathecal (IT) ALN-5288 in patients with Alzheimer's Disease (AD) * Evaluate the pharmacodynamic (PD) and pharmacokinetic (PK) effects of ALN-5288 after dose administration

Inclusion Criteria: * Is able and willing to meet all study requirements in the opinion of the Investigator * Has a diagnosis of Alzheimer's disease (AD) based on clinical findings supported by cerebrospinal fluid (CSF) biomarkers or positive positron emission tomography (PET) amyloid imaging within 7 years prior to screening * Has mild cognitive impairment (MCI) or dementia due to AD Exclusion Criteria: * Has non-AD dementia * Has alanine aminotransferase (ALT) or aspartate aminotransferase (AST) \>2× upper limit of normal (ULN) * Has total bilirubin \>1.5×ULN * Has known human immunodeficiency virus infection * Has history of hepatitis C virus or current hepatitis B virus infection * Has systolic blood pressure \>160 mmHg and/or a diastolic blood pressure \>100 mmHg after 10 minutes of rest at screening * Has an estimated glomerular filtration (eGFR) of \<45 mL/min/1.73 m\^2 at screening * Has clinically significant ECG abnormalities at screening * Has uncontrolled psychiatric disease, including patients deemed by the Investigator to be at significant risk of suicide, major depressive episode, psychosis, confusional state, or violent behavior * Has history of bleeding diathesis or coagulopathy due to chronic conditions * Has a medical history of brain or spinal disease that would interfere with the IT injection and LP procedures * Has history of uncontrolled seizures within the last 6 months prior to Screening