Synthesis, Evaluation, and Radiolabeling of New Potent Positive Allosteric Modulators of the Metabotropic Glutamate Receptor 2 as Potential Tracers for Positron Emission Tomography Imaging

The synthesis and in vitro and in vivo evaluation of a new series of 7-(phenylpiperidinyl)-1,2,4-triazolo[4,3-a]pyridines, which were conveniently radiolabeled with carbon-11, as potential positron emission tomography (PET) radiotracers for in vivo imaging of the allosteric binding site of the metabotropic glutamate (mGlu) receptor subtype 2 are described. The synthesized compounds proved to be potent and selective positive allosteric modulators (PAMs) of the mGlu receptor 2 (mGluR2) in a [35S]GTPγS binding assay and were able to displace an mGluR2 PAM radioligand, which we had previously developed, with IC50 values in the low nanomolar range. The most promising candidates were radiolabeled and subjected to biodistribution studies and radiometabolite analysis in rats. Preliminary small-animal PET (μPET) studies in rats indicated that [11C]20f binds specifically and reversibly to an mGluR2 allosteric site, strongly suggesting that it is a promising candidate for PET imaging of mGluR2 in the brain.