Preclinical Evaluation of Azabenzimidazole-Based PET Radioligands for -8 Dependent Transmembrane AMPA Receptor Regulatory Protein Imaging

AMPA glutamate receptors (AMPARs) play a pivotal role in excitatory neurotransmission, particularly in the hippocampus where the TARP gamma-8 subunit is enriched and serves as a target for emerging anti-epileptic drugs. To enable in vivo visualization of TARP gamma-8 distribution and expression by positron emission tomography (PET), this study focuses on the development of novel (18) F-labeled TARP gamma-8 inhibitors and their corresponding precursors, stemming from the azabenzimidazole scaffold. The resulting radioligands [(18) F]TARP-2204 and [(18) F]TARP-2205 were successfully synthesized with acceptable radiochemical yield, high molar activity, and excellent radiochemical purity. In vitro autoradiography demonstrates high level of specific binding of [(18) F]TARP-2205 to TARP gamma-8 in both rat and nonhuman primate brain tissues. However, unexpected radiodefluorination in PET imaging studies of rodents emphasizes the need for further structural refinement. This work serves as an excellent starting point for the development of future (18) F-labeled TARP gamma-8 PET tracers, offering valuable insights into medicinal chemistry design, radiosynthesis and subsequent PET evaluation.