Radiosynthesis and preclinical evaluation of a carbon-11 labeled PET ligand for imaging metabotropic glutamate receptor 7.

Am J Nucl Med Mol Imaging 2024;14(5):306-315 www.ajnmmi.us /ISSN:2160-8407/ajnmmi0158664 Original Article Radiosynthesis and preclinical evaluation of a carbon-11 labeled PET ligand for imaging metabotropic glutamate receptor 7 Yinlong Li1,2*, Zhiwei Xiao1,2*, Wakana Mori3*, Jiyun Sun2, Tomoteru Yamasaki3, Jian Rong1,2, Masayuki Fujinaga3, Jiahui Chen1,2, Katsushi Kumata3, Chunyu Zhao1,2, Yiding Zhang3, Thomas L Collier1,2, Kuan Hu3, Lin Xie3, Xin Zhou1, Wei Zhang1, Zhendong Song1, Yabiao Gao1, Zhenkun Sun6, Kuo Zhang1, Jimmy S Patel1,4, Chongzhao Ran5, Ahmad Chaudhary1, Douglas J Sheffler7, Nicholas DP Cosford7, Linqi Zhang1, Chuangyan Zhai1, Ahmed Haider1,2, Hongjie Yuan6, Ming-Rong Zhang3, Steven H Liang1,2 Department of Radiology and Imaging Sciences, Emory University, Atlanta, GA 30322, USA; 2Division of Nuclear Medicine and Molecular Imaging, Massachusetts General Hospital & Department of Radiology, Harvard Medical School, Boston, MA 02114, USA; 3Department of Advanced Nuclear Medicine Sciences, Institute for Quantum Medical Sciences, National Institutes for Quantum Science and Technology, Chiba 263-8555, Japan; 4Department of Radiation Oncology, Winship Cancer Institute of Emory University, Atlanta, GA 30322, USA; 5Athinoula A. Martinos Center for Biomedical Imaging, Department of Radiology, Massachusetts General Hospital and Harvard Medical School, Boston, MA 02114, USA; 6Department of Pharmacology and Chemical Biology, Emory University School of Medicine, Atlanta, GA 30322, USA; 7Cancer Molecular Therapeutics Program and Conrad Prebys Center for Chemical Genomics, Sanford-Burnham Prebys Medical Discovery Institute, La Jolla, CA 92037, USA. *Equal contributors. 1 Received June 22, 2024; Accepted August 29, 2024; Epub October 15, 2024; Published October 30, 2024 Abstract: Metabotropic glutamate receptor 7 (mGlu7) is a G protein-coupled receptor that is preferentially found in the active zone of neurotransmitter release in the central nervous system (CNS). mGlu7 plays a vital role in memory, learning, and neuronal development, rendering it a potential target for treating epilepsy, depression, and anxiety. The development of noninvasive imaging ligands targeting mGlu7 could help elucidate the functional significance of mGlu7 and accelerate drug discovery for neurological and psychiatric disorders. In this report, a novel carbon-11 labeled positron emission tomography (PET) tracer designated [11C]18 (codenamed MG7-2109) was synthesized via 11C-methylation in 23% decay-corrected radiochemical yield (RCY). In vitro serum stability, serum protein binding, in vitro autoradiography and ex vivo biodistribution studies of [11C]18 were conducted. Preliminary PET imaging results revealed a homogeneous distribution of [11C]18 and rapid clearance in rodent brains. This study provides valuable insights into the development of mGlu7-targeted PET tracer based on an isoxazolo(5,4-c)pyridine scaffold. Keywords: mGlu7, negative allosteric modulator, positron emission tomography, radioligand Introduction Glutamate is the primary excitatory neurotransmitter in the mammalian central nervous system (CNS), orchestrating physiological processes involved in memory formation, synaptic plasticity, and neurodevelopment. Depending on their structures and physiological functions, two classes of glutamate receptors are identified as metabotropic glutamate receptors (mGlus) and ionotropic glutamate receptors (iGlus) [1]. iGlus are ligand-gated ion channels that regulate the excitatory neurotransmission rapidly, whereas mGlus are G protein-coupled receptors modulating signal transduction cascades, including the second messengers, ion channels and other independent pathways [2, 3]. To date, eight mGlus subtypes are reported and further divided into three subgroups according to sequence homology, cell signaling transduction and pharmacology [4, 5]. Group I (mGlu1 and mGlu5) are mainly expressed postsynaptic to activate phospholipase C or adenylyl cyclase (ACs). In general, both group II (mGlu2 and mGlu3) and III (mGlu4 and mGlu6-8) are located preand postsynaptic and have similar mechanism of action, which inhibit ACs and regulate ion channels [1, 6]. Among group III, mGlu7 is widely distributed in the CNS. The striatum, hippocampus, thalamus and neocortex are the most abundant regions with mGlu7 expression. As auto- or hetero-receptors, mGlu7 activation results in an attenuated release of the endogenous neurotransmitters, glutamate or gamma-aminobutyric acid (GABA) in the presynaptic regions of glutamatergic or GABAergic terminals, respectively [3, 5, 7, 8]. Several reports uncovered that mGlu7 plays an important role in the processes of learning and memory [9-11]. Of note, dysregulation of mGlu7 signaling can be observed under some neuropathological conditions, such as Alzheimer’s disease (AD) [12], Parkinson’s disease (PD) [13], Huntington’s disease (HD) [14] and Rett syndrome [15]. Thus, mGlu7 has been studied as a potential therapeutic target in several animal models of neurodegenerative diseases [16-20]. In addition to endogenous agonists like glutamate, several exogenous mGlu7 agonists and antagonists have been found and used in mGlu7 studies (Figure 1A, 1B). In recent years, several allosteric mGlu7 modulators, both positive and negative, have attracted more attention and some progress has been made owing to their high selectivity and reduced side effects (shown in Figure 1C, 1D) [6, 7, 21, https://doi.org/10.62347/PUAI9230 mGlu7 radiotracer Figure 1. Representative mGlu7 modulators. 22]. However, these molecules have not been used as clinical drugs for reasons including low subtype-selectivity, poor bioavailability and inability to cross the bloodbrain barrier (BBB). Positron emission tomography (PET) is a highly sensitive imaging technology for pre-clinical and clinical functional molecular imaging. After the appropriate radiolabeled ligand is injected in a non-pharmacological dose, threedimensional images, including the concentration and location information of the radioligand, could be obtained to reveal the physiological state of the target through non-invasive data collection and reconstruction [23-25]. Although the development of mGlu7-selective PET tracers will contribute to the mechanism study of mGlu7-associated disease and drug discovery, only one PET tracer derived from mGlu7 negative allosteric modulator (NAM), [11C]MMPIP was developed and evaluated in rodents. Due to its moderate affinity to mGlu7 (26 nM) [26] and the existing radioactive metabolites in the brain, [11C]MMPIP cannot be used for the quantitative assessment of mGlu7 307 [27]. Therefore, compound 18 (MG7-2109) (Figure 2) [26], with improved affinity to mGlu7 (reported IC50 = 12 nM) was selected as a candidate compound in this work. Radiosynthesis of [11C]18 followed by in vitro autoradiography, ex vivo biodistribution and PET studies on rodent brains were conducted to assess its performance for imaging (...truncated)