Binding characterization of N-(2-chloro-5-thiomethylphenyl)- N′-(3-[3H]3 methoxy phenyl)-N′-methylguanidine ([3H]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-11]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-3]GMOM). The binding properties of [H-3]GMOM were compared to those of the reference ion-channel ligand [H-3](+)-dizocilpine maleate ([H-3]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-3] GMOM was regulated by NMDA receptor agonists, albeit in a less prominent manner compared to [H-3]MK-801. Scatchard transformation of homologous inhibition data produced concave upward curves for [H-3]GMOM and [H-3]MK-801. The radioli-gands showed bi-exponential association kinetics in the presence of 100 mu mot L-1L-glutamate/30 mu mol L-1 glycine. [H-3]GMOM (3 nmol L-1 and 10 nmol L-1) was inhibited with dual affinity by (+)-MK-801, (R,S)-ketamine and memantine, in both presence and absence of agonists. [H-3]MK-801 (2 nmol L-1) was inhibited in a monophasic manner by GMOM under baseline and combined agonist conditions, with an IC50 value of similar to 19 nmol L-1. The non-linear Scatchard plots, biphasic inhibition by open channel blockers, and bi-exponential kinetics of [H-3]GMOM indicate a complex mechanism of interaction with the NMDA receptor ionophore. The implications for quantifying the PET signal of [C-11]GMOM are discussed.