Development of N -[4-[6-(Isopropylamino)pyrimidin-4-yl]-1,3-thiazol-2-yl]- N -methyl-4-[ 11 C]methylbenzamide for Positron Emission Tomography Imaging of Metabotropic Glutamate 1 Receptor in Monkey Brain

Three novel 4-substituted benzamides have been synthesized as potential ligands for the positron emission tomography (PET) imaging of metabotropic glutamate 1 (mGlu1) receptor in the brain. Of these compounds, N-(4-(6-(isopropylamino)pyrimidin-4-yl)-1,3-thiazol-2-yl)-N,4-dimethylbenzamide (4) exhibited the highest binding affinity (K(i) = 13.6 nM) for mGlu1 and was subsequently labeled with carbon-11. In vitro autoradiography using rat brain sections showed that [(11)C]4 binding was consistent with the distribution of mGlu1, with high specific binding in the cerebellum and thalamus. PET studies with [(11)C]4 in monkey showed a high brain uptake and a kinetic profile suitable for quantitative analysis. Pretreatment with a mGlu1-selective ligand 16 largely decreased the brain uptake, indicating high in vivo specific binding of [(11)C]4 to mGlu1. In metabolite analysis, only unchanged [(11)C]4 was found in the brain. [(11)C]4 is a useful PET ligand for the imaging and quantitative analysis of mGlu1 in monkey brain and merits further evaluation in humans.