A Randomized, Double-blind, Placebo-controlled Study to Evaluate the Safety, Tolerability, Pharmacokinetics, and Pharmacodynamics of Single Oral Doses of E2609 in Subjects With Mild Cognitive Impairment or Mild Dementia Due to Alzheimer's Disease

Subjects will be adults aged 50 to 85 years who have subjective memory complaints and mild cognitive impairment or mild dementia due to Alzheimer's disease (AD). Subjects taking thyroxine or thyroid supplements and subjects receiving an acetylcholinesterase inhibitor (AChEI) and/or memantine for AD must be on a stable dose for at least 12 weeks prior to Screening and remain on their stable dose throughout the trial. Subjects will receive placebo or a single oral dose of E2609. Safety assessments will be conducted. Additionally, the pharmacokinetics of E2609 and drug effects will be evaluated using cerebrospinal fluid biomarkers and cognitive and psychological measures.