Inclusion Criteria:

• Diagnosis of Alzheimer's Disease.
• MMSE 12-26 inclusive.
• If on existing anti-dementia therapy, have been on a stable dose of anti-dementia therapy (cholinesterase inhibitors and/or memantine) for at least 60 days prior to dosing in study.
• If not taking existing anti-dementia therapy, have not received therapy with cholinesterase inhibitors and/or memantine within 60 days prior to dosing in this study.

Exclusion Criteria:

• Have major structural brain disease (e.g., ischemic infarcts, subdural hematoma, hemorrhage, hydrocephalus, brain tumors, multiple subcortical ischemic lesions, or a single lesion in a critical region [e.g., thalamus, hippocampus]).
• Have any major medical illness or unstable medical condition within six months of screening that may interfere with the patient's ability to comply with study procedures and abide by study restrictions.
• Have not been on a stable dose of anti-dementia therapy for at least 60 days prior to dosing or intend to start anti-dementia therapy during the double blind portion of the study.
• Reside in a nursing home or assisted care facility with need for 24-hour care and supervision.