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.

Inclusion criteria:

• Meets the current cognitive classification of MCI or mild dementia due to AD pathology (all subjects having a "positive" biomarker for amyloid β) as defined by the National Institute on Aging - Alzheimer's Association (NIA-AA) research criteria
• Aged 50 to 85 years, inclusive at time of consent

Exclusion criteria:

• Currently has any neurological condition other than AD (Alzheimer's disease)-related that could be contributing to the subject's cognitive impairment
• Significant pathological findings on brain MRI at Screening, including but not limited to multiple microhemorrhages
• Any psychiatric diagnosis or symptoms, e.g., hallucinations, major depression,anxiety or delusions that in the Investigator's opinion could interfere with assessment of cognition or confound the diagnosis of MCI or mild dementia due to AD in the subject.
• A lifetime history of cerebrovascular events or non-vasovagal related loss of consciousness within the last 10 years
• Any other abnormality of the ECG at Screening and/or Baseline (including QRS > 110 ms, abnormal electrical axis, PR interval > 220 ms and conduction abnormalities) considered clinically significant by the investigator
• History of cardiac arrhythmias, ischemic heart disease or cerebrovascular disease
• Lower spinal malformation on physical or lumbar spine radiography, local spinal infection, or other abnormality, including but not limited to obesity, that would prevent LP or insertion of an indwelling catheter for CSF sampling
• Any history of seizure disorder, symptomatic seizures (not including a history of simple febrile seizures in childhood) or any past or present medical condition which, in the opinion of the investigator has the potential to reduce seizure threshold (e.g., history of head trauma or concussion, previous alcohol abuse, substance abuse).