A 24-Month Study to Evaluate the Efficacy and Safety of E2609 in Subjects With Early Alzheimer's Disease_

Inclusion Criteria:

- Mild cognitive impairment due to AD or mild AD dementia including

1. Mini Mental State Examination score equal to or greater than 24

2. CDR global score of 0.5

3. CDR Memory Box score of 0.5 or greater

- Impaired episodic memory confirmed by a list learning task

- Positive biomarker for brain amyloid pathology as indicated by either amyloid PET or
CSF assessment or both

- Study partner able to support the participant for duration of the study.

- Provide written informed consent. Participants must, in the investigator's judgment,
have the capacity to consent.

Exclusion Criteria:

- Females who are breastfeeding or pregnant at Screening or Baseline. Females of
child-bearing potential must use a highly effective method of contraception throughout
the entire study period and for 28 days after study drug discontinuation

- Any condition that may be contributing to cognitive impairment above and beyond that
caused by the participant's AD

- Participants with a history of seizures within 5 years of Screening

- History of transient ischemic attacks or stroke within 12 months of Screening

- Psychiatric diagnosis or symptoms (e.g., hallucinations, major depression, delusions,
etc.)

- Suicidal ideation or any suicidal behavior within 6 months before Screening or has
been hospitalized or treated for suicidal behavior in the past 5 years

- Have any contraindications to magnetic resonance imaging (MRI) scanning or

1. Have lesions that could indicate a dementia diagnosis other than AD on brain MRI

2. Exhibit other significant pathological findings on brain MRI.

- Participants who have a history of moderate to severe hepatic impairment (eg,
Child-Pugh Class B or C)

- Results of laboratory tests conducted during Screening that are outside the following
limits:

1. Absolute lymphocyte count below the lower limit of normal (LLN)

2. Thyroid stimulating hormone above normal range

3. Abnormally low Vitamin B12 levels

- Participants at risk of increased risk of infection

- Have received any live/live attenuated vaccine in the 3 months before randomization

- Any chronic inflammatory disease that is not adequately controlled or that requires
systemic immunosuppressive or immunomodulatory therapy

- Any other clinically significant abnormalities

- Severe visual or hearing impairment

- A prolonged corrected QT (QTc) interval (QTcF greater than 450 milliseconds [ms])

- Malignant neoplasms within 5 years of Screening

- Known or suspected history of drug or alcohol abuse

- Taking prohibited medications, which must be reviewed with the Investigator

- Have participated in a recent clinical study

Note: Other protocol-defined Inclusion/Exclusion Criteria may apply.