The molecular and cellular details underlying Alzheimer's disease (AD) are being defined at an accelerating pace, leading to new therapeutic discovery efforts that target reduction of A-beta or ADDLs (A-beta-derived diffusible ligands), the neurotoxic oligomeric assemblies of A-beta 1-42. Elevated ADDL levels correlate with synaptic dysfunction, and the appearance of ADDLs in brain tissue coincides with cognitive decline in Tg2576 AD mice, a transgenic AD animal model. Recent studies have demonstrated that ADDLs are 70-fold elevated in AD brain tissue, and that ADDLs interfere directly with synaptic function, learning and memory. It is possible that even low levels of ADDLs in serum samples may predict cognitive decline. This proposal aims to develop diagnostic tests that can measure ADDL levels in human fluid and tissue samples. Such tests will help establish a connection between ADDL levels and cognitive deficits. A serum or cerebrospinal fluid (CSF) ADDL diagnostic will be a valuable tool for identification of relevant patients with AD and mild cognitive impairment (MCI) as clinical trial participants. For testing of drugs that lower Abeta or ADDL levels, a quantitative ADDL diagnostic will be pivotal to establish that symptomatic efficacy parallels a drop in ADDL levels. Acumen and its collaborators have developed sensitive ELISA-based technology that detects ADDLs at femtomolar levels in human CSF samples. Experiments with ADDL-selective antibodies have provided preliminary evidence that ADDLs are present in plasma samples from Tg2576 mice and Down's syndrome patients. These encouraging preliminary data serve as the foundation for this proposal to: 1) optimize ADDL detection capabilities, 2) optimize an ADDL diagnostic assay format suitable as a reference test for human clinical samples, and 3) validate the hypothesis that ADDL levels correlate with human cognitive deficits. Objectives one and two will be the focus of Phase I, and objective three will be the focus of Phase II.