This research is designed to obtain efficacy and safety information for 123-I IMPY as an imaging biomarker for Alzheimer's disease (AD). The distribution of this agent will be measured by obtaining single photon emission computed tomography (SPECT) images of the brain serially over time to determine the relative localization of the radiopharmaceutical in regions of the cortex relative to background regions and develop an optimal technique of radiotracer administration (bolus or bolus with constant infusion). The researchers will then evaluate the utility of 123-I IMPY and SPECT in AD patients as an early diagnostic tool and subsequently serial evaluations of AD patients will be performed to determine if this technique may be useful as a tool for evaluation of progressive brain β-amyloid deposition in AD.

All study procedures will be conducted at the Institute for Neurodegenerative Disorders (IND) and Molecular NeuroImanging (MNI) in New Haven, CT. Approximately 25 patients with mild to moderate Alzheimer's disease (AD) and 20 healthy controls will be recruited to participate in this study. AD patients will be eligible to participate if they have a diagnosis of AD of less than 3 years duration. Healthy controls will be examined to ensure that there is no evidence of neurodegenerative changes including cognitive decline. All subjects will undergo written informed consent and a screening evaluation including baseline clinical laboratory testing, a baseline physical and neurological evaluation and baseline cognitive evaluations. Subjects will be asked to undergo either a bolus injection or bolus injection followed by continuous infusion of 123-I IMPY. Following injection, subjects will undergo serial SPECT imaging scans and serial venous plasma sampling for measurement of 123-I IMPY in plasma (both protein bound and free) over a 3.5 - 8 hour period. The imaging analyses will be performed by an image-processing specialist who will remain masked to the procedures employed with each imaging acquisition. The primary imaging outcome measure will be the brain regional distribution volumes expressed as a brain tissue to plasma ratio of the radioligand, 123-I IMPY.

Inclusion Criteria:

• The participant is 50 years or older.
• Written informed consent is obtained.
• Participants have a clinical diagnosis of Alzheimer's disease based on National Institute of Neurological and Communicative Disorders and Stroke/Alzheimer's Disease and Related Disorders Association (NINCDS/ADRA) criteria.
• Mini-Mental Status Exam score > 16 and < 25.
• Patients have a diagnosis of probable AD for < 3 years prior to screening.

Exclusion Criteria:

• The subject has signs or symptoms of another neurodegenerative disease including Parkinson's disease, diffuse Lewy body dementia, or history of significant cerebrovascular disease.
• Subjects with an iodine allergy.
• The subject has a clinically significant clinical laboratory value and/or medical or psychiatric illness.
• The subject has any disorder that may interfere with drug absorption distribution, metabolism, or excretion (including gastrointestinal surgery).
• The subject has evidence of clinically significant thyroid disease, gastrointestinal, cardiovascular, hepatic, renal, hematologic, neoplastic, endocrine, neurologic, immunodeficiency, pulmonary, or other medical or psychiatric disorder.
• The subject has received an investigational drug within 60 days before the screening visit.
• Pregnancy