A Study to Evaluate Safety, Tolerability, and Efficacy of Lecanemab in Subjects With Early Alzheimer's Disease

Key Inclusion Criteria (Core Study) for Mild Cognitive Impairment due to Alzheimer's Disease \- Intermediate likelihood: 1. Subjects who meet the National Institute of Aging - Alzheimer's Association (NIA-AA) core clinical criteria for mild cognitive impairment due to Alzheimer's disease - intermediate likelihood 2. Subjects who have a CDR score of 0.5 and a Memory Box score of 0.5 or greater at Screening and Baseline 3. Subjects who report a history of subjective memory decline with gradual onset and slow progression over the last one year before Screening; MUST be corroborated by an informant Key Inclusion Criteria (Core Study) for Mild Alzheimer's Disease Dementia: 1. Subjects who meet the NIA-AA core clinical criteria for probable Alzheimer's disease dementia 2. Subjects who have a CDR score of 0.5-1.0 and a Memory Box score of 0.5 or greater at Screening and Baseline Inclusion Criteria (Core Study) that must be met by all subjects: 1. Subjects with 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 II (WMS-IV LMII): 1. Less than or equal to 15 for age 50 to 64 years 2. Less than or equal to 12 for age 65 to 69 years 3. Less than or equal to 11 for age 70 to 74 years 4. Less than or equal to 9 for age 75 to 79 years 5. Less than or equal to 7 for age 80 to 90 years 2. Positive amyloid load as indicated by PET or CSF assessment 1. PET assessment of imaging agent uptake into brain 2. CSF assessment of Aβ(1-42) 3. Age between 50 and 90 years, inclusive 4. Mini Mental State Examination (MMSE) score equal to or greater than 22, and equal to or less than 30, at Screening and Baseline 5. Body Mass Index (BMI) greater than 17 and less than 35 at Screening or Baseline 6. Females must not be lactating or pregnant at Screening or Baseline (as documented by a negative beta-human chorionic gonadotropin assay \[ß-hCG\]). A separate baseline assessment is required if a negative screening pregnancy test was obtained more than 72 hours before the first dose of study drug. 7. Subjects on acetylcholinesterase inhibitor or memantine therapy or both for AD must be on a stable dose for at least 12 weeks prior to Baseline. Treatment naive subjects can be entered into the study. Unless otherwise stated, subjects must have been on stable doses of all other permitted concomitant medications (ie, non-AD related) for at least 4 weeks prior to Baseline. 8. Subjects must have identified caregivers/informants 9. Subjects must provide written informed consent Inclusion Criteria (Extension Phase): 1. Subjects who have completed Visit 42 (Week 79) of the Core Study or who discontinued study drug during the Core Study due to any of the following reasons: 1. Alzheimer's Related Imaging Abnormality-Edema (ARIA-E) 2. Amyloid related imaging abnormality hemorrhage (ARIA-H) (superficial siderosis, macrohemorrhage, or symptomatic microhemorrhage) 3. Prohibited or restricted medications that were prohibited during Core Study conduct but are no longer prohibited in the Extension Phase 4. Subjects who were APOE4 positive and receiving treatment with lecanemab 10 mg/kg biweekly 5. Any reason for discontinuation not related to prohibited medications, including any AE that was considered not related to study drug, and that was not severe or life-threatening 2. Must continue to have an identified caregiver or informant who is willing and able to provide follow-up information on the subject throughout the course of the Extension Phase 3. Provide written informed consent. If a subject lacks capacity to consent in the investigator's opinion, the subject'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). 4. Must be able to physically attend clinic visits and be willing and able to comply with all aspects of the protocol Key Exclusion Criteria (Core study): 1. Any neurological condition that may be contributing to cognitive impairment above and beyond that caused by the subject's AD 2. History of transient ischemic attacks (TIA), stroke, or seizures within 12 months of Screening 3. Any psychiatric diagnosis or symptoms, (e.g., hallucinations, major depression, or delusions) that could interfere with study procedures in the subject 4. Geriatric Depression Scale (GDS) score ≥8 at Screening 5. Contraindications to MRI scanning, including cardiac pacemaker/ defibrillator, ferromagnetic metal implants, e,g., in skull and cardiac devices other than those approved as safe for use in MR scanners 6. Evidence of other clinically significant lesions that could indicate a dementia diagnosis other than AD on brain MRI at Screening, or other significant pathological findings on brain MRI at Screening 7. A prolonged QT/QTc interval (QTc greater than 450 ms) as demonstrated by a repeated electrocardiogram (ECG) 8. Certain other specified medical conditions 9. Severe visual or hearing impairment that would prevent the subject from performing psychometric tests accurately Exclusion Criteria (Extension Phase): 1. Subjects who discontinued from the study drug or from the Core Study for reasons other than the following: 1. ARIA-E 2. ARIA-H (superficial siderosis, macrohemorrhage, or symptomatic microhemorrhage) 3. Prohibited or restricted medications that were prohibited during Core Study conduct but are no longer prohibited in the Extension Phase 4. Subjects who were APOE4 positive and receiving treatment with lecanemab 10 mg/kg biweekly 5. AE that was considered not related to study drug, and that was not severe or life-threatening 2. Females of childbearing potential who do not agree to use a highly effective method of contraception 3. Severe visual or hearing impairment that would prevent the subject from performing psychometric tests accurately.