New N-aryl-N′-(3-(substituted)phenyl)-N′-methylguanidines as leads to potential PET radioligands for imaging the open NMDA receptor

An expansive set of N-aryl-N′-(3-(substituted)phenyl)-N′-methylguanidines was prepared in a search for new leads to prospective PET ligands for imaging of the open channel of the N-methyl-d-aspartate (NMDA) receptor in vivo. The N-aryl rings and their substituents were varied, whereas the N-methyl group was maintained as a site for potential labeling with the positron-emitter, carbon-11 (t1/2 = 20.4 min). At micromolar concentration, over half of the prepared compounds strongly inhibited the binding of [3H]TCP to its binding site in the open NMDA receptor in vitro. Four ligands displayed affinities that are similar or superior to those of the promising SPECT radioligand ([123I]CNS1261). The 3′-dimethylamino (19; Ki 36.7 nM), 3′-trifluoromethyl (20; Ki 18.3 nM) and 3′-methylthio (2; Ki 39.8 nM) derivatives of N-1-naphthyl-N′-(phenyl)-N′-methylguanidine were identified as especially attractive leads for PET radioligand development.