Synthesis and preliminary studies of (11)C-labeled tetrahydro-1,7-naphthyridine-2-carboxamides for PET imaging of metabotropic glutamate receptor 2

Selective modulation of metabotropic glutamate receptor 2 (mGlu(2)) represents a novel therapeutic approach for treating brain disorders, including schizophrenia, depression, Parkinson's disease (PD), Alzheimer's disease (AD), drug abuse and addiction. Imaging mGlu(2) using positron emission tomography (PET) would allow for in vivo quantification under physiological and pathological conditions and facilitate drug discovery by enabling target engagement studies. In this paper, we aimed to develop a novel specific radioligand derived from negative allosteric modulators (NAMs) for PET imaging of mGlu(2). Methods. A focused small molecule library of mGlu(2) NAMs with tetrahydro naphthyridine scaffold was synthesized for pharmacology and physicochemical evaluation. GIRK dose-response assays and CNS panel binding selectivity assays were performed to study the affinity and selectivity of mGlu(2) NAMs, among which compounds 14a and 14b were selected as PET ligand candidates. Autoradiography in SD rat brain sections was used to confirm the in vitro binding specificity and selectivity of [(11)C]14a and [(11)C]14b towards mGlu(2). In vivo binding specificity was then studied by PET imaging. Whole body biodistribution study and radiometabolite analysis were conducted to demonstrate the pharmacokinetic properties of [(11)C]14b as most promising PET mGlu(2) PET ligand. Results. mGlu(2) NAMs 14a-14g were synthesized in 14%-20% yields in five steps. NAMs 14a and 14b were selected to be the most promising ligands due to their high affinity in GIRK dose-response assays. [(11)C]14a and [(11)C]14b displayed similar heterogeneous distribution by autoradiography, consistent with mGlu(2) expression in the brain. While PET imaging study showed good brain permeability for both tracers, compound [(11)C]14b demonstrated superior binding specificity compared to [(11)C]14a. Further radiometabolite analysis of [(11)C]14b showed excellent stability in the brain. Conclusions. Compound 14b exhibited high affinity and excellent subtype selectivity, which was then evaluated by in vitro autoradiography and in vivo PET imaging study after labeling with carbon-11. Ligand [(11)C]14b, which we named [(11)C]MG2-1904, demonstrated high brain uptake and excellent in vitro/in vivo specific binding towards mGlu(2) with high metabolic stability in the brain. As proof-of-concept, our preliminary work demonstrated a successful example of visualizing mGlu(2) in vivo derived from NAMs, which represents a promising chemotype for further development and optimization aimed for clinical translation.