A Study of E2814 With Concurrent Lecanemab Treatment in Participants With Early Alzheimer's Disease

Inclusion Criteria:

1. For participants diagnosed with mild cognitive impairment (MCI) due toAD-intermediate likelihood:

2. Meet the National Institute on Aging-Alzheimer's Association (NIA-AA) coreclinical criteria for MCI due to AD-intermediate likelihood

3. Have a global Clinical Dementia Rating Scale (CDR) score of 0.5 and a CDRMemory Box score of greater than or equal to (>=) 0.5 at Screening

4. For participants diagnosed with mild AD dementia:

5. Meet the NIA-AA core clinical criteria for probable AD dementia

6. Have a global CDR score of 0.5 to 1.0 and a CDR Memory Box score of >=0.5 atScreening

7. Mini Mental State Examination (MMSE) score >=22 at Screening and less than or equalto (<=) 30 at Screening

8. Able to have CSF lumbar puncture performed and not on, or have a medical conditionthat may require initiation of, any anticoagulant therapy at Screening

9. Male or female participants aged between >=50 years and <=80 years, at the time ofinformed consent

10. If receiving an approved AD symptomatic treatment (such as acetylcholinesteraseinhibitors (AchEIs), memantine, or both) for AD, participants must be on a stabledose for at least 12 weeks before Baseline. Treatment-naïve participants for ADmedications can be enrolled into the study. Unless otherwise stated, participantsmust have been on stable doses of all other (that is, non AD-related) permittedconcomitant medications for at least 4 weeks before Baseline. Use of memantine willnot be allowed at screening for participants in Japan

11. Have an identified study partner (defined as a person able to support theparticipant for the duration of the study and who spends at least 8 hours per weekwith the participant).

12. Provide written informed consent. If a participant lacks the capacity to consent inthe Investigator's opinion, the participants' assent should be obtained, if requiredin accordance with local laws, regulations, and customs, plus the written informedconsent of a legal representative should be obtained (capacity to consent anddefinition of legal representative should be determined in accordance withapplicable local laws and regulations)

13. Willing and able to comply with all aspects of the protocol including multiple CSFcollections

Exclusion Criteria:

1. Any neurological condition that may be contributing to cognitive impairment aboveand beyond that caused by the participant's AD

2. History of transient ischemic attacks, stroke, or seizures within 12 months ofScreening

3. Any psychiatric diagnosis or symptoms (example, hallucinations, major depression, ordelusions) that could interfere with study procedures in the participants. Psychoticdisorder(s) or unstable recurrent affective disorder(s) evident by use ofantipsychotics within 2 years before Screening

4. Contraindications to MRI scanning, including cardiac pacemaker/defibrillator,ferromagnetic metal implants (example, in skull and cardiac devices other than thoseapproved as safe for use in MRI scanners). Evidence of other clinically significantlesions on brain MRI at Screening that could indicate a dementia diagnosis otherthan AD. Other significant pathological findings on brain MRI at Screening,including but not limited to: greater than 4 microhemorrhages (defined as 10millimeter (mm) or less at the greatest diameter); a single intracerebral hemorrhagegreater than 10 mm at greatest diameter; an area of superficial siderosis; evidenceof vasogenic edema; evidence of cerebral contusion, encephalomalacia, aneurysms,vascular malformations, or infective lesions; evidence of multiple lacunar infarctsor stroke involving a major vascular territory, severe small vessel, or white matterdisease; space occupying lesions; or brain tumors (however, lesions diagnosed asmeningiomas or arachnoid cysts and less than 1 cm at their greatest diameter neednot be exclusionary). Other minor or clinically insignificant magnetic resonanceimaging (MRI) abnormalities, as agreed by the medical monitor and after discussionwith the investigator, may not be exclusionary

5. Malignant neoplasms within 3 years of Screening (except for basal or squamous cellcarcinoma in situ of the skin, or localized prostate cancer in male participants, orlocalized breast cancer in female participants). Participants who had malignantneoplasms but who have had at least 3 years of documented uninterrupted remissionbefore Screening need not be excluded

6. A clinically significant ECG abnormality, including a marked prolonged corrected QTinterval (Fridericia's Correction Formula; QTcF) interval (example, a repeateddemonstration of a QTcF interval greater than (>) 450 milliseconds [ms])

7. Participants with a bleeding disorder that is not under adequate control (includinga platelet count <50,000 or international normalized ratio [INR] >1.5). Anyparticipants who are on anticoagulant therapy are not permitted to be enrolled

8. Have thyroid-stimulating hormone above normal range. Other tests of thyroid functionwith results outside the normal range should only be exclusionary if they areconsidered clinically significant by the investigator. This applies to allparticipants whether or not they are taking thyroid supplements

9. Abnormally low serum vitamin B12 levels for the testing laboratory (if participantis taking vitamin B12 injections, level should be at or above the lower limit ofnormal for the testing laboratory). Levels of vitamin B12 may be confirmed withreflex testing to include methylmalonic acid analysis, if available in region

10. Any suicidal ideation with intent with or without a plan at Screening, Baseline, orwithin 6 months of Screening (that is, answering "Yes" to questions 4 or 5 on theSuicidal Ideation section of the Columbia-Suicide Severity Rating Scale [C-SSRS])Type 4 or 5, or any suicidal behavior assessment within 6 months before Screening,at Screening, or at the Baseline Visit, or has been hospitalized or treated forsuicidal behavior in the past 5 years before Screening)

11. Evidence of clinically significant disease (example, cardiac, respiratory,gastrointestinal, and renal disease) that in the opinion of the investigator(s)could affect the participants safety or interfere with the study assessments. Anyother medical conditions (example, cardiac, respiratory, gastrointestinal, and renaldisease) that are not stably and adequately controlled or that in the opinion of theinvestigator(s) could affect the participant's safety or interfere with the studyassessments

12. Hypersensitivity to lecanemab, E2814, or any of the excipients

13. Any immunological disease, that is not adequately controlled, or that requirestreatment with immunoglobulins, systemic monoclonal antibodies (mAbs) (orderivatives of mAbs), systemic immunosuppressants, or plasmapheresis during thestudy

14. Any history of or concomitant medical condition that in the opinion of theinvestigator(s) would compromise the participant's ability to safely complete thestudy

15. Planned surgery that requires general, spinal, or epidural anesthesia that wouldtake place during the study. Planned surgery that requires only local anesthesia andthat can be undertaken as a day case without inpatient stay postoperatively need notresult in exclusion if in the opinion of the investigator this operation does notinterfere with study procedures and participant safety

16. Known to be human immunodeficiency virus positive

17. Known or suspected history of drug or alcohol abuse or dependence within 2 yearsbefore Screening or a positive urine drug test at Screening. Participants who testpositive for benzodiazepines or opioids in urine drug testing need not be excludedif in the clinical opinion of the investigator, this is due to the participanttaking prior/concomitant medications containing benzodiazepines or opioids for amedical condition and not due to drug abuse

18. Severe visual or hearing impairment that would prevent the participant fromperforming psychometric tests accurately

19. Participation in a clinical study involving any antiamyloid plaque lowering oranti-tau therapies (including any mAb and antisense oligonucleotide therapies unlessit can be documented that the subject only received placebo)

20. Participation in a clinical study involving any therapeutic mAb, protein derivedfrom a mAb, immunoglobulin therapy, or vaccine within 6 months before Screeningunless it can be documented that the subject only received placebo