A 24-Month Study to Evaluate the Efficacy and Safety of Elenbecestat (E2609) in Participants With Early Alzheimer's Disease

Inclusion Criteria:

Core Study

   - Mild cognitive impairment due to Alzheimer's disease (AD) or mild AD dementia
   including

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

      2. Clinical Dementia Rating (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 positron
   emission tomography or cerebrospinal fluid AD assessment or both

Extension Phase

• Participants who complete the Core Study

Exclusion Criteria:

Core Study

   - 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 (example, 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 (example,
   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 increased risk of infection

   - Have received any live vaccine/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 (QT interval with Fridericia's correction
   [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.