SUMMARY Alzheimer's disease (AD) is the most common form of dementia in older adults and the 6th leading cause of death in the United States. Unfortunately, there are currently no FDA approved disease-modifying therapeutics available for AD, and definitive diagnosis is only possible via postmortem analysis. The limited number of sensitive biomarkers enabling detection of early, functionally relevant, neuromolecular changes in AD is a major impediment to improving diagnosis and validating novel therapeutics. Although data from genome wide association studies and immunohistochemistry have implicated neuroinflammation and a loss of healthy innate immune function as major AD risk factors, biomarkers to investigate neuro-immune function preceding and during AD remain extremely limited. Positron Emission Tomography (PET) is an highly sensitive molecular imaging modality well suited to studying such biomarkers longitudinally, with established utility for non-invasive in vivo interrogation of biochemical processes. Existing PET imaging biomarkers of neuroinflammation (e.g., the translocator protein 18 kDa [TSPO]) suffer from significant drawbacks, including a poorly elucidated functional role and/or expression across multiple cell types in the central nervous system (CNS), which complicates image analysis. Within the CNS, adenosine diphosphate receptor (P2Y12R) expression is restricted to microglia, the innate immune effector cells of the brain. Generally considered a biomarker of homeostatic microglia, P2Y12R expression drives chemotaxis and activation-associated morphological changes. Importantly, P2Y12R expression decreases in both acute and chronic neuroinflammation, as shown in rodents after lipopolysaccharide challenge and in AD transgenic mice. Moreover, postmortem human brain tissue from advanced AD patients demonstrated global reduction in P2Y12R expression, with a near total absence on microglia surrounding amyloid-beta plaques. While P2Y12R is an extremely well characterized pharmacological target, there are currently no CNS-penetrable PET radiotracers for this biomarker. We have identified the P2Y12R antagonist AZD1283 as a promising potential PET tracer based on its high affinity and favorable lipophilicity (measured LogD 2.8) indicative of its high likelihood for CNS uptake. Upon optimization of the cyanation radiochemistry outlined herein, we propose to assess the in vivo kinetics, biodistribution, stability, and specificity of [11C]AZD1283 in healthy mice, as well as the ability of [11C]AZD1283 to detect alterations in P2Y12R expression in human AD postmortem tissue using autoradiography (Aim 1). Additionally, we will interrogate the dynamics of microglial P2Y12R expression in two murine models of acute and chronic neuroinflammation (Aim 2). Completion of these aims will result in development and characterization of the first P2Y12R PET tracer suitable for in vivo PET imaging, with high potential for clinical translation. Ultimately, such...