Synthesis and characterization of C-labeled benzyl amidine derivatives as PET radioligands for GluN2B subunit of the NMDA receptors.

GluN2B-containing NMDA receptors (NMDARs) play fundamental roles in learning and memory, although they are also associated with various brain disorders. In this study, we synthesized and evaluated three C-labeled N-benzyl amidine derivatives 2-[ C]methoxybenzyl) cinnamamidine ([ C]CBA), N-(2-[ C]methoxybenzyl)-2-naphthamidine ([ C]NBA), and N-(2-[ C]methoxybenzyl)quinoline-3-carboxamidine ([ C]QBA) as PET radioligands for these receptors. The C-benzyl amidines were synthesized via conventional methylation of corresponding des-methyl precursors with [ C]CH I. In vitro binding characteristics were examined in brain sagittal sections using various GluN2B modulators and off-target ligands. Further, in vivo brain distribution studies were performed in normal mice. The C-labeled benzyl amidines showed high-specific binding to the GluN2B subunit at in vitro. In particular, the quinoline derivative [ C]QBA had the best binding properties in terms of high-brain localization to GluN2B-rich regions and specificity to the GluN2B subunit. Conversely, these C-radioligands showed the brain distributions were inconsistent with GluN2B expression in biodistribution experiments. The majority of the radiolabeled compounds were identified as metabolized forms of which amido derivatives seemed to be the major species. Although these C-ligands had high-specific binding to the GluN2B subunit, significant improvement in metabolic stability is necessary for successful positron emission tomography (PET) imaging of the GluN2B subunit of NMDARs.