Single Photon Emission Computed Tomography (SPECT) Imaging Study to Develop a Biomarker for Alzheimer's Disease

Inclusion Criteria: Alzheimer's Subject Selection: subjects who have a clinical diagnosis of mild to moderate Alzheimer's disease will be recruited for this study. The following criteria will be met for inclusion of AD subjects in this study: * 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/ADRDA) criteria. * CDR score ≥0.5 and \<2. * Modified Hachinski Ischemia Scale score of ≤ 4. * For females, non-child bearing potential or negative urine and blood pregnancy test on day of 123-I MNI-388 or 123-I MNI-390 injection. Exclusion Criteria: Alzheimer's subjects will be excluded from participation for the following reasons: * The subject has signs or symptoms of another neurodegenerative disease including Parkinson's disease, diffuse Lewy body dementia, or history of significant cerebrovascular disease. * The subject has a clinically significant abnormal laboratory value and/or clinically significant unstable 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 gastrointestinal, cardiovascular, hepatic, renal, hematological, neoplastic, endocrine, neurological, immunodeficiency, pulmonary, or other disorder or disease. * Clinically significant MRI evidence of vascular disease or alternative neurologic disorder * Pregnancy Concomitant therapy: patients may remain on a stable dose of anticholinesterase medication during the study. Use of donepezil (Aricept®), rivastigmine (Exelon®), galantamine (Razadyne®), tacrine (Cognex®), and memantine (Namenda®) are permitted during the study. Doses of these compounds must be stable for 2 months prior to the study. At each visit after the screening visit, the investigator will ask the patient whether any medications including OTC medications, were taken since the previous visit. Use of antipsychotics for agitation and benzodiazepines for insomnia or anxiety is permitted. However, the use of these agents is not permitted within the 8 hours prior to administration of cognitive testing. Inclusion Criteria: Healthy Control Subject Selection: Healthy control subjects who have no neurological disease will be recruited for this study. The following criteria will be met for inclusion of healthy control subjects in this study: * The participant is 50 years or older. * Written informed consent is obtained. * Negative history of neurological or psychiatric illness based on evaluation by a research physician. * CDR score 0. * For females, non-child bearing potential or negative urine and blood pregnancy test on day of 123-I MNI-388 or 123-I MNI-390 injection. Exclusion Criteria: Healthy control subjects will be excluded from participation for the following reasons: * The subject has a clinically significant abnormal laboratory value and/or clinically significant unstable 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 gastrointestinal, cardiovascular, hepatic, renal, hematological, neoplastic, endocrine, neurological, immunodeficiency, pulmonary, or other disorder or disease. * The subject has participated in another clinical study within the previous 30 days. * Pregnancy * Clinically significant MRI evidence of vascular disease or neurologic disorder