Project Summary. At present, there are no efficacious therapies for Alzheimer’s disease (AD) to halt or slow disease progression, which is attributed, at least in part, to the lack of translational cross-species molecular imaging tools suitable for use in both preclinical disease models and in humans to facilitate drug discovery and development. Therefore, the development of translatable imaging biomarkers for non-invasive assessment of disease progression and therapeutic efficacy hold promises to fill this urgent and unmet medical need. N-Methyl-D-aspartate receptor (NMDAR) plays a pivotal role in the synaptic transmission and synaptic plasticity thought to underlie learning and memory. NMDAR activation has been implicated in AD related to synaptic dysfunction, which leads to gradual loss of synaptic function and correlates clinically with the progressive decline in cognition/memory of AD patients. Therefore, in vivo imaging of GluN2B is of paramount translational relevance as it would allow the assessment of GluN2B distribution and expression of AD patients, enabling correlational study between NMDAR-mediated excitotoxicity and neuroimmune factors during AD progression and providing target engagement tool to support novel AD pharmacotherapy. Herein, we propose the use of a novel imaging strategy for monitoring detrimental extrasynaptic NMDAR function in AD. Our strategy entails the use of a novel positron emission tomography (PET) tracer, [11C]Me-NB1, that targets GluN2B-carrying NMDAR. To date, [11C]Me-NB1 is the only GluN2B PET tracer that has been successfully validated in naïve animals and recently tested in a small number of healthy subjects. Our preliminary studies have shown that [11C]Me-NB1-PET is highly suited for non-invasive mapping of the GluN2B subunit, with excellent target affinity and selectivity across different species. However, non-invasive assessment of the distribution and expression of GluN2B has not yet been conducted in AD patients and clear evidence is still deficient in demonstrating the underlying mechanisms and a relationship between NMDAR dysfunction and AD. The availability of [11C]Me-NB1 now provides new opportunity to fill this fundamental knowledge gap. Therefore, we have developed three objectives and hypothesis in this work: (1) A kinetic modeling approach and absolute quantification, and/or simplified graphical methods of [11C]Me-NB1 can be established in healthy controls; (2) PET quantification method will be translated into AD patients and we expect increased GluN2B binding in vitro and in vivo by [11C]Me-NB1-PET is positively correlated with AD- related brain regions in AD patients (versus controls); (3) As proof-of-concept, in transgenic AD mouse models, we anticipate that [11C]Me-NB1-PET can directly monitor GluN2B changes and enable target occupancy studies during novel AD therapy. Overall impact: The role of glutamate-induced neurotoxicity in AD renders NMDAR subtype GluN2B a potential drug and imagin...