The purpose of this study is to evaluate the safety, tolerability and pharmacokinetics of KHK6640, given as a single dose and as multiple doses in patients with Prodromal Alzheimer's Disease (AD) or Mild to Moderate AD.

Inclusion Criteria:

• Patients with prodromal AD or mild to moderate AD
• Clinical Dementia Rating (CDR) score of 0.5, 1.0, or 2.0
• Have a cognitive impairment
• Low Aβ and high Tau in Cerebrospinal fluid (CSF)
• Mini Mental State Examination (MMSE) score > 16 at Screening

Exclusion Criteria:

• Previous active treatment with an AD immunotherapy in an investigational study
• Use of another investigational drug within 30 days of screening
• History or presence of clinically significant seizures, brain trauma, transient ischemic attack, and/or cerebrovascular disease
• Presence of a neurological condition that could be contributing to cognitive impairment above and beyond that caused by the subject's AD
• Evidence of infection, tumor, or other clinically significant lesions that could indicate a dementia diagnosis other than AD