The purpose of this study will be to evaluate the safety and tolerability of lecanemab at sequentially ascending doses in subjects with mild to moderate Alzheimer's disease (AD).

This will be a multicenter, double-blind, randomized, placebo-controlled study in subjects with mild to moderate Alzheimer's disease. The study will comprise separate single dose ascending (SAD) and multiple dose ascending (MAD) parts designed to allow the MAD part to be initiated while the SAD part is ongoing.

Inclusion:

1. Clinical diagnosis of probable mild to moderate Alzheimer's disease (AD) by National Institute of Neurological and Communicative Disorders and Stroke and the Alzheimer's Disease and Related Disorders Association Alzheimer's (NINCDS-ADRDA) criteria.
2. A Mini Mental State Examination (MMSE) score of 16 to 28, inclusive, at Screening. Subjects recruited to the first 2 SAD cohorts should have an MMSE of > 22.
3. Where symptomatic treatment of Alzheimer's disease (AD) is clinically indicated, subjects must be on stable treatment (e.g., with an anticholinesterase inhibitor [AChEI] and/or memantine) for at least 12 weeks prior to the Screening visit.
4. On stable doses of all other prescribed medications for at least 4 weeks prior to the screening visit.

Exclusion:

1. Any neurological condition that could be contributing to cognitive impairment above and beyond that caused by the subject's Alzheimer's disease (AD).
2. Any psychiatric diagnosis or symptoms, e.g hallucinations, major depression, or delusions, that could interfere with assessment of cognition in the subject.
3. History of transient ischemic attack (TIA), stroke, or seizures within 12 months of Screening.
4. 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.
5. Other significant pathological findings on brain MRI at Screening, including but not limited to: more than 3 micro-hemorrhages, single macro-hemorrhage; evidence of vasogenic edema; evidence of cerebral contusion, encephalomalacia, aneurysms, vascular malformations or space occupying lesions.