The purpose of this study is to evaluate the safety, tolerability and pharmacokinetics of KHK6640, given as a single dose in Japanese patients with Mild to Moderate Alzheimer's Disease.

Inclusion Criteria:

• Patients with mild to moderate Alzheimer's Disease
• Body weight ≥ 40 kg and < 100 kg
• Clinical Dementia Rating (CDR) score of 1 or 2
• Mini Mental State Examination (MMSE) score of ≥ 17 amd ≤ 26

Exclusion Criteria:

• Previous active treatment with an Alzheimer's Disease immunotherapy in an investigational study
• Use of another investigational drug within 4 months prior to the enrollment
• Subjects who meet National Institute of Neurological Disorders and Stroke/Association Internationale pour la Recherche et l'Enseignement en Neurosciences (NINDS/AIREN) criteria for vascular dementia
• Subjects with a history of presence of clinically significant seizures, brain trauma, transient ischemic attack, and/or cerebrovascular disease
• Subjects with a presence of a neurological condition that could be contributing to cognitive impairment above and beyond that caused by the subject's Alzheimer's Disease
• Subjects who have evidence of infection, tumor, or other clinically significant lesions that could indicate a dementia diagnosis other than Alzheimer's Disease