Project Summary: We will develop the first subtype-selective positron emission tomography (PET) probe for α-amino- 3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR) in the ionotropic glutamate system. Dysfunction of AMPAR is implicated in the physiopathology of neurological diseases such as schizophrenia, depression, epilepsy and Parkinson’s disease. Pharmacological modulation of AMPAR prevents excessive neuronal activation, representing an attractive therapeutic approach. As a non-invasive chemical probe, PET is capable of quantifying biochemical processes in vivo, and a suitable AMPAR PET probe would substantially improve our understanding of AMPAR-based ionotropic glutamate signaling under normal and disease conditions otherwise inaccessible by ex vivo (destructive) analysis. To date, no successful examples have been demonstrated to image AMPAR, representing a significant deficiency of our ability to study this target in vivo. Therefore, we propose to develop a PET probe that can fill this void, as the first translational AMPAR imaging tool. As pan AMPAR antagonists are often accompanied by debilitating adverse effects and have a very narrow therapeutic dosing window, one recent advance focuses on subtype-selective AMPAR antagonists via modulating transmembrane AMPA regulatory proteins (TARPs). The PI and his team have pioneered the development of the first subtype-selective AMPAR PET probe targeting AMPAR subunit TARP ɣ8, [11C]JNJ-486 at MGH. [11C]JNJ-486 showed high in vitro specific binding and target selectivity towards AMPAR TARP ɣ8 subunit, but was discontinued due to low brain penetration. Through our established HEK293 cell-based Ca2+ flux fluorescent assay, we identified a second generation chemical lead. This compound showed high potency and high subtype selectivity. An 11C- isotopologue was then synthesized and preliminary PET studies confirmed that we have overcome the two major obstacles for AMPAR probe development, namely: 1) reasonable brain uptake and 2) regional-specific uptake. Though this lead is a promising template for the development of new TARP ɣ8-targeted in vivo chemical tool, PET probes with improved potency & selectivity, and increased binding potentials (Bmax/Kd) are needed for optimal imaging and quantification of subtype-selective AMPAR in translational cross-species imaging studies. In this proposal, we will design and prepare a series of carefully chosen subtype-selective AMPAR inhibitors, label top candidates with 11C or 18F, and evaluate their ability to quantify AMPAR during drug challenges in rodents and nonhuman primates. The impact of this work is not only to develop the first potent and selective AMPAR PET probe for the study of disease-related biological processes, but also ultimately, to prepare this in vivo tool for potential clinical translation and monitor target response of AMPAR therapeutic agents in the brain. Relevance: This proposal has the potential to improve public health and help patients suffering from CNS disorders/neurodegenerative diseases through the discovery of neurotherapeutics using AMPAR PET probes.

Project Narrative Positron Emission Tomography (PET) is a medical technique that creates images of biochemical processes occurring in vivo, where radioactive chemical probes are injected and the distributions of those molecules are measured using a PET scanner. Data can be acquired in animals, in research subjects and eventually in patient care. As the burden of neurological disorders and mental illnesses in the United States and worldwide is high, there is a critical need to develop novel PET chemical probes to image illness-related biological processes in the brain. This work will develop key chemical tools designed to advance the scientific understanding of subtype- selective AMPA receptor in the ionotropic glutamate system and facilitate the discovery of neurotherapeutics.