The APEX study is a multicenter, observational study designed to capture longitudinal follow-up of plasma biomarkers and cognitive and functional assessments on individuals who screen failed in the AHEAD study over approximately 4 years.

Approximately 1000 participants will be enrolled across three groups:

• Group A: Approximately 500 participants who are discordant on screening (plasma positive / Positron Emission Tomography (PET) negative),
• Group B: Approximately 250 participants who are concordant on screening (plasma negative / PET negative), and
• Group C: Approximately 250 participants selected from the individuals who previously screen failed prior to PET for the AHEAD study with oversampling of racial and ethnic populations underrepresented in Alzheimer's disease (AD) clinical trials.

Primary Objectives:

• Collect longitudinal cognitive and functional assessments and blood-based biomarker data
• Evaluate, characterize, and compare the longitudinal cognitive and functional data between the three groups of participants
• Compare longitudinal change across race and ethnicity, sex, and Apolipoprotein E (ApoE) status

Exploratory Objectives:

• Collect baseline amyloid PET on participants without prior amyloid PET data (Group C)

Inclusion Criteria:

1. Documentation of the participant's informed consent to study procedures and for the use of protected health information (HIPAA Authorization, if applicable). Informed consent processes and documentation must adhere to state laws/local requirements, including consent provided by the participant's legally authorized representative (LAR), responsible next of kin, surrogate consent with assent, etc.
2. Previously consented to participate in A3-45 screening.
3. Has A3-45 screening plasma biomarker results required for determining eligibility to participate in the A3-45 trial.
4. If an amyloid PET scan was conducted in A3-45, the scan was determined to be below the 20 centiloid cutpoint required for eligibility into the treatment arms of the A3 or A45 trial.
5. As assessed by the site PI, participant is likely to be able to comply with the protocol, including completion of all required procedures for the duration of the study, and has adequate vision, hearing (hearing aid permitted), and literacy in English or Spanish sufficient for compliance with required testing procedures.

Exclusion Criteria:

1. Current treatment with an FDA approved medication for Alzheimer's disease, including prior or current treatment with a prohibited medication.

2. Enrollment in another investigational study, or intake of investigational drug, within 30 days prior to screening, or five half-lives of the investigational drug, whichever is longer, unless it can be documented that the participant was in the placebo treatment arm.

   Participants enrolled in other observational studies may be permitted with Medical Monitor review and approval.

3. Screen failed from A3-45 due to not meeting basic inclusion criterion (i.e., age requirement; current diagnosis of AD dementia).

This study will enroll approximately 1000 participants who screen failed from the AHEAD study without evidence of elevated or intermediate amyloid by plasma biomarker or by amyloid PET scan at screening. Participants will have consented to participate in the AHEAD study and previously met demographic, cognitive and clinical criteria, however, had screening plasma biomarker and/or PET scan results that fell below the ABeta threshold levels required for randomization into the treatment arms for the AHEAD study.