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

Abstract

Selective modulation of metabotropic glutamate receptor 2 (mGlu₂) 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 mGlu₂ using positron emission tomography (PET) would allow for <i>in vivo</i> 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 mGlu₂. <b>Methods.</b> A focused small molecule library of mGlu₂ 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 mGlu₂ NAMs, among which compounds <b>14a</b> and <b>14b</b> were selected as PET ligand candidates. Autoradiography in SD rat brain sections was used to confirm the <i>in vitro</i> binding specificity and selectivity of [¹¹C]<b>14a</b> and [¹¹C]<b>14b</b> towards mGlu₂. <i>In vivo</i> binding specificity was then studied by PET imaging. Whole body biodistribution study and radiometabolite analysis were conducted to demonstrate the pharmacokinetic properties of [¹¹C]<b>14b</b> as most promising PET mGlu₂ PET ligand. <b>Results.</b> mGlu₂ NAMs <b>14a-14g</b> were synthesized in 14%-20% yields in five steps. NAMs <b>14a</b> and <b>14b</b> were selected to be the most promising ligands due to their high affinity in GIRK dose-response assays. [¹¹C]<b>14a</b> and [¹¹C]<b>14b</b> displayed similar heterogeneous distribution by autoradiography, consistent with mGlu₂ expression in the brain. While PET imaging study showed good brain permeability for both tracers, compound [¹¹C]<b>14b</b> demonstrated superior binding specificity compared to [¹¹C]<b>14a</b>. Further radiometabolite analysis of [¹¹C]<b>14b</b> showed excellent stability in the brain. <b>Conclusions.</b> Compound <b>14b</b> exhibited high affinity and excellent subtype selectivity, which was then evaluated by <i>in vitro</i> autoradiography and <i>in vivo</i> PET imaging study after labeling with carbon-11. Ligand [¹¹C]<b>14b</b>, which we named [¹¹C]MG2-1904, demonstrated high brain uptake and excellent <i>in vitro</i>/<i>in vivo</i> specific binding towards mGlu₂ with high metabolic stability in the brain. As proof-of-concept, our preliminary work demonstrated a successful example of visualizing mGlu₂<i>in vivo</i> derived from NAMs, which represents a promising chemotype for further development and optimization aimed for clinical translation.