A Study to Assess Safety, Tolerability, Pharmacodynamics, Immunogenicity, Exploratory Clinical Efficacy, and Pharmacokinetics of Intravenous Infusions of E2814 in Healthy Participants and Participants With Prodromal Alzheimer's Disease (AD) to Moderate AD With and Without Detectable Tau Pathology

Inclusion Criteria:

Healthy Volunteers

1. Nonsmoking, healthy male or female (Caucasian and Japanese), age greater than or equal
to (>=) 18 years (>=20 years for Japanese participants) and less than or equal to (<=) 50
years at the time of informed consent.

Participants with Prodromal to Moderate AD

1. Meets the National Institute of Aging-Alzheimer's Association (NIA-AA) core clinical
criteria for mild cognitive impairment due to AD (that is, prodromal AD), or have a
clinical diagnosis of probable mild to moderate AD dementia by the National Institute
of Neurological and Communicative Disorders and Stroke and the Alzheimer's Disease and
Related Disorders Association (NINCDS-ADRDA) criteria

2. For MAD: Must show evidence of amyloid by CSF determination of amyloid pathology and
preferably tau tangle pathology which may be assessed by both CSF and tau-PET imaging,
respectively

For Phase 2a: Must show evidence of both amyloid and tau tangle pathology by CSF
determination of amyloid pathology and tau-PET imaging (tau pathology detectable above
cut-point on composite standard uptake value ratios evaluation (SUVr) uptake in select
brain normalized to the cerebellar crus gray), respectively

3. Participants recruited to the first 2 MAD cohorts must have a Mini Mental State
Examination (MMSE) score >20; other subsequent cohorts must have MMSE score >16

4. Where symptomatic treatment of AD is clinically indicated, participants must be on
stable treatment (example, with an acetylcholinesterase inhibitor (AChEI), memantine,
or both) for at least 12 weeks before the Baseline Visit

Exclusion Criteria:

Healthy Volunteers

1. Clinically significant illness that requires medical treatment within 8 weeks or a
clinically significant infection that requires medical treatment within 4 weeks of
dosing

2. Evidence of disease that may influence the outcome of the study within 4 weeks before
dosing; example, psychiatric disorders and disorders of the gastrointestinal tract,
liver, kidney, respiratory system, endocrine system, hematological system,
neurological system, or cardiovascular system, or participants who have a congenital
abnormality in metabolism

Participants with Prodromal to Moderate AD

1. Any neurological condition that could be contributing to cognitive impairment above
and beyond that caused by the participant's AD

2. Any psychiatric diagnosis or symptoms, example, hallucinations, major depression, or
delusions that could interfere with assessment of cognition in the participant

3. A clinical contraindication to a lumbar puncture (LP), such as space-occupying brain
lesion if examination suggests any suspicion of such a lesion

4. Treatment with low molecular weight heparin, warfarin, or thrombolytics (treatment
with aspirin or nonsteroidal antiinflammatory agents is permitted)

5. History of transient ischemic attack (TIA), stroke, or seizures within 12 months of
Screening

6. Evidence of infection, tumor, stroke, or other clinically significant lesions that
could indicate a dementia diagnosis other than AD on brain magnetic resonance imaging
(MRI) at Screening.

7. Participation in a clinical study involving any new chemical entities for AD in the 6
months before Screening

8. Participation in any other investigational medication or device study in the 8 weeks
or 5 half-lives (whichever is longer) of the medication before randomization

9. Contraindications to MRI scanning, including cardiac pacemaker/defibrillator,
ferromagnetic metal implants, example, in-skull and cardiac devices other than those
approved as safe for use in MR scanners

10. Have a "yes" answer to Columbia Suicide Severity Rating Scale (C-SSRS) suicidal
ideation questions nos. 4 or 5, or on the suicide behavior assessment within 6 months
before Screening, at Screening, or at the Baseline Visit; has been hospitalized or
treated for suicidal behavior in the past 5 years before Screening

11. Use of any medications that, in the opinion of the investigator, may contribute to
cognitive impairment, put participants at higher risk for adverse events (AEs), or
impair the participant's ability to perform cognitive testing or complete study
procedures