A Study to Confirm Safety and Efficacy of Lecanemab in Participants With Early Alzheimer's Disease

Core Study: Inclusion Criteria

Diagnosis: Mild Cognitive Impairment (MCI) due to Alzheimer's disease - intermediate
likelihood:

   - Meet the National Institute of Aging - Alzheimer's Association (NIA-AA) core clinical
   criteria for MCI due to Alzheimer's disease - intermediate likelihood

   - Have a global Clinical Dementia Rating (CDR) score of 0.5 and CDR Memory Box score of
   0.5 or greater at Screening and Baseline

   - Report a history of subjective memory decline with gradual onset and slow progression
   over the last 1 year before Screening; must be corroborated by an informant

Mild Alzheimer's disease dementia:

   - Meet the NIA-AA core clinical criteria for probable Alzheimer's disease dementia

   - Have a global CDR score of 0.5 to 1.0 and a CDR Memory Box score of 0.5 or greater at
   Screening and Baseline

Key Inclusion Criteria that must be met by all participants:

   - Objective impairment in episodic memory as indicated by at least 1 standard deviation
   below age-adjusted mean in the Wechsler Memory Scale IV-Logical Memory (subscale) II
   (WMS-IV LMII)

   - Positive biomarker for brain amyloid pathology

   - Male or female participants aged greater than or equal to (>=) 50 and less than or
   equal to (<=) 90 years, at the time of informed consent

   - Mini mental state examination (MMSE) score >=22 at Screening and Baseline and <=30 at
   Screening and Baseline

   - Body mass index (BMI) greater than (>)17 and less than (<) 35 at Screening

   - If receiving an approved Alzheimer's disease treatment such as acetylcholinesterase
   inhibitor (AChEIs) or memantine or both for Alzheimer's disease, must be on a stable
   dose for at least 12 weeks prior to Baseline. Treatment-naive participants for
   Alzheimer's disease can be entered into the study. Unless otherwise stated,
   participants must have been on stable doses of all other (that is, non-Alzheimer's
   disease-related) permitted concomitant medications for at least 4 weeks prior to
   Baseline. Use of memantine will not be allowed for participants in Japan

   - Have an identified study partner (defined as a person able to support the participant
   for the duration of the study and who spends at least 8 hours per week with the
   participant)

   - Provide written informed consent. If a participant lacks capacity to consent in the
   investigator's opinion, the participant's assent should be obtained, if required in
   accordance with local laws, regulations and customs, plus the written informed consent
   of a legal representative should be obtained (capacity to consent and definition of
   legal representative should be determined in accordance with applicable local laws and
   regulations). In countries where local laws, regulations, and customs do not permit
   participants who lack capacity to consent to participate in this study (example,
   Germany and Spain), they will not be enrolled

Extension Phase: Inclusion Criteria:

   - Participants who have completed the Core Study (except de novo participants)

   - Must continue to have a study partner who is willing and able to provide follow-up
   information on the participant throughout the course of the Extension Phase

   - Provide written informed consent for the Extension Phase. If a participant lacks
   capacity to consent in the investigator's opinion, the participant's assent should be
   obtained, if required and in accordance with local laws, regulations and customs, plus
   the written informed consent of a legal representative should be obtained (capacity to
   consent and definition of legal representative should be determined in accordance with
   applicable local laws and regulations). In countries where local laws, regulations,
   and customs do not permit participants who lack capacity to consent to participate in
   this study (example, Germany and Spain), they will not be enrolled

   - Participants entering the subcutaneous (vial) substudy at Extension Phase Week 1, must
   be willing to participate, or continue participating in the amyloid positron emission
   tomography (PET) substudy. All participants must have an amyloid PET scan within 4
   weeks before starting subcutaneous BAN2401.

   - Participants enrolling into the subcutaneous autoinjector substudy must have had at
   least 6 months exposure to BAN2401 10 mg/kg intravenously (IV) biweekly or BAN2401 720
   mg subcutaneously (SC) weekly.

Exclusion Criteria

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

   - History of transient ischemic attacks (TIA), stroke, or seizures within 12 months of
   Screening

   - Any psychiatric diagnosis or symptoms (example, hallucinations, major depression, or
   delusions) that could interfere with study procedures in the participant

   - Geriatric Depression Scale (GDS) score >=8 at Screening

   - 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 MRI scanners)

   - Evidence of other clinically significant lesions on brain MRI at Screening that could
   indicate a dementia diagnosis other than Alzheimer's disease

   - Other significant pathological findings on brain MRI at screening, including but not
   limited to: more than 4 microhemorrhages (defined as 10 millimeter [mm] or less at the
   greatest diameter); a single macrohemorrhage >10 mm at greatest diameter; an area of
   superficial siderosis; evidence of vasogenic edema; evidence of cerebral contusion,
   encephalomalacia, aneurysms, vascular malformations, or infective lesions; evidence of
   multiple lacunar infarcts or stroke involving a major vascular territory, severe small
   vessel, or white matter disease; space occupying lesions; or brain tumors (however,
   lesions diagnosed as meningiomas or arachnoid cysts and <1 centimeter [cm] at their
   greatest diameter need not be exclusionary)

   - Any immunological disease which is not adequately controlled, or which requires
   treatment with immunoglobulins, systemic monoclonal antibodies (or derivatives of
   monoclonal antibodies), systemic immunosuppressants, or plasmapheresis during the
   study

   - Participants with a bleeding disorder that is not under adequate control (including a
   platelet count <50,000 or international normalized ratio [INR] >1.5 for participants
   who are not on anticoagulant treatment, example, warfarin). Participants who are on
   anticoagulant therapy should have their anticoagulant status optimized and be on a
   stable dose for 4 weeks before Screening. Participants who are on anticoagulant
   therapy are not permitted to participate in cerebrospinal fluid (CSF) assessments

   - Any other medical conditions (example, cardiac, respiratory, gastrointestinal, renal
   disease) which are not stably and adequately controlled, or which in the opinion of
   the investigator(s) could affect the participant's safety or interfere with the study
   assessments

   - Participation in a clinical study involving any therapeutic monoclonal antibody,
   protein derived from a monoclonal antibody, immunoglobulin therapy, or vaccine within
   6 months before screening unless it can be documented that the participant was
   randomized to placebo

   - Participation in a clinical study involving any anti-amyloid therapies (including any
   monoclonal antibody therapies and any β-site amyloid precursor protein cleaving enzyme
   [BACE] inhibitor therapies) unless it can be documented that the participant only
   received placebo

   - Participants who have any known prior exposure to lecanemab

   - Participants who were dosed in a clinical study involving any new chemical entities
   for AD within 6 months prior to screening unless it can be documented that the
   participant was in a placebo treatment arm

Extension Phase: Exclusion Criteria

   - Participants who discontinued early from the Core Study

   - Participants who develop the following conditions from the time of Screening for the
   Core Study to the start of the Extension Phase

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

      - Any psychiatric diagnosis or symptoms, (example, hallucinations, major
      depression, or delusions) that could interfere with study procedures in the
      participant

      - 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 MRI scanners)

      - Other significant pathological findings on brain MRI during the Core Study
      including but not limited to: cerebral contusion, encephalomalacia, aneurysms,
      vascular malformations, or infective lesions; evidence of multiple lacunar
      infarcts or stroke involving a major vascular territory, severe small vessel, or
      white matter disease; space occupying lesions; or brain tumors will be
      exclusionary if based on the opinion of the investigator, with consultation of
      medical monitor, these findings may interfere with the study procedures or safety

      - Hypersensitivity to BAN2401 or any of the excipients, or to any monoclonal
      antibody treatment

      - Any immunological disease which is not adequately controlled, or which requires
      chronic treatment with immunoglobulins, systemic monoclonal antibodies (or
      derivatives of monoclonal antibodies), systemic immunosuppressants, or
      plasmapheresis during the study

      - Any other clinically significant abnormalities in physical examination, vital
      signs, laboratory tests, or ECG, which in the opinion of the investigator require
      further investigation or treatment or which may interfere with study procedures
      or safety.

      - Malignant neoplasms (except for basal or squamous cell carcinoma in situ of the
      skin, or localized prostate cancer in male participants) that are not stably and
      adequately controlled or which, based on the opinion of the investigator, may
      interfere with the participant's safety or participation in the study

      - Any other medical conditions (example, cardiac, respiratory, gastrointestinal,
      renal disease) which are not stably and adequately controlled, or which in the
      opinion of the investigator(s) could affect the participant's safety or interfere
      with the study assessments

      - Severe visual or hearing impairment that would prevent the participant from
      performing psychometric tests accurately