Binding characterization of N-(2-chloro-5-thiomethylphenyl)-N'-(3-[ H] methoxy phenyl)-N'-methylguanidine ([ H]GMOM), a non-competitive N-methyl-D-aspartate (NMDA) receptor antagonist.

Labeled with carbon-11, N-(2-chloro-5-thiomethylphenyl)-N'-(3-methoxyphenyl)-N'-methylguanidine ([ C]GMOM) is currently the only positron emission tomography (PET) tracer that has shown selectivity for the ion-channel site of N-methyl-D-aspartate (NMDA) receptors in human imaging studies. The present study reports on the selectivity profile and in vitro binding properties of GMOM. The compound was screened on a panel of 80 targets, and labeled with tritium ([ H]GMOM). The binding properties of [ H]GMOM were compared to those of the reference ion-channel ligand [ H](+)-dizocilpine maleate ([ H]MK-801), in a set of concentration-response, homologous and heterologous inhibition, and association kinetics assays, performed with repeatedly washed rat forebrain preparations. GMOM was at least 70-fold more selective for NMDA receptors compared to all other targets examined. In homologous inhibition and concentration-response assays, the binding of [ H]GMOM was regulated by NMDA receptor agonists, albeit in a less prominent manner compared to [ H]MK-801. Scatchard transformation of homologous inhibition data produced concave upward curves for [ H]GMOM and [ H]MK-801. The radioligands showed bi-exponential association kinetics in the presence of 100 μmol L l-glutamate/30 μmol L glycine. [ H]GMOM (3 nmol L and 10 nmol L ) was inhibited with dual affinity by (+)-MK-801, (R,S)-ketamine and memantine, in both presence and absence of agonists. [ H]MK-801 (2 nmol L ) was inhibited in a monophasic manner by GMOM under baseline and combined agonist conditions, with an IC value of ~19 nmol L . The non-linear Scatchard plots, biphasic inhibition by open channel blockers, and bi-exponential kinetics of [ H]GMOM indicate a complex mechanism of interaction with the NMDA receptor ionophore. The implications for quantifying the PET signal of [ C]GMOM are discussed.