Identification of a PET radioligand for metabotropic glutamate receptor subtype 1 (mGluR1) imaging in rat brain

Abstract

Introduction: Glutamate is an important excitatory neurotransmitter in mammalian central nervous system. After it is released from the presynaptic vesicles, it binds to glutamate receptors to acts as the first messenger. Among the 8 subtypes metabotropic glutamate receptors, metabotropic glutamate receptor subtype 1 (mGluR1) is widely distributed in the brain, especially in the cerebellum. mGluR1 regulates the release of glutamate and -aminobutyric acid (GABA) in the postsynaptic region to control synaptic transmission, neuronal excitability and brain plasticity. Thus, mGluR1 is related with various neurological and psychiatric diseases, such as anxiety, depression, epilepsy, Parkinsons disease, and neuropathic pain. Due to its important roles in central nervous system, mGluR1 is an important target for drug development and imaging, of course. Hence, I synthesized fluorine-18 labeled quinoline radioligand (3-ethyl-2-fluoro-[18F]quinolin-6-yl)((1S,4S)-4-methoxycyclohexyl) methanone ([18F]7a) as a PET tracer for selective mGluR1 imaging and its trans- isomer (3-ethyl-2-fluoro-[18F]quinolin-6-yl)((1R,4R)-4-methoxycyclohexyl)methanone ([18F]7b), and evaluated their physico-chemical properties in rodents.

Methods: A chloroquinoline precursor (6) and cold product (7a and 7b) were synthesized, and the radioactive [18F]7a and [18F]7b were labeled by nucleophilic substitution reaction. The resulting radioactive compounds were purified using semi-preparative high performance liquid chromatography (HPLC). The radiolabeling efficiencies and radio-chemical purities of [18F]7a and [18F]7b were measured using radio-TLC and analytical HPLC. In vitro experiments confirmed lipophilicity and stability of [18F]7a. Biodistribution and metabolites analysis were performed on BALB/c mice, and small animal PET and autoradiography studies were carried out to investigate the specific binding of [18F]7a to mGluR1 in normal Sprague-Dawley rat brains.

Results: [18F]7a and [18F]7b were obtained with high radiochemical purities (>99%) and specific activities (63-246 GBq/μmol for [18F]7a and 30.5-93.2 GBq/μmol for [18F]7b). The distribution coefficient value (Log D7.4) of [18F]7a was 3.24, and the initial brain uptake of [18F]7a at 10 min was over 4% of injected dose per gram in BALB/c mice. According to PET and ex vivo autoradiography studies in Sprague-Dawley rats, [18F]7a showed wide distribution in the whole brain and the highest uptake in the cerebellum which is a mGluR1-rich region. Hippocampus, thalamus, and striatum also showed higher uptake of radioactivity than other brain regions. Pre-treatment of unlabeled 7a and mGluR1-specific antagonist JNJ16259685 blocked the binding of [18F]7a to mGluR1, homogeneous distribution of radioactivity throughout the whole brain showed nonspecific binding of [18F]7a. However, pre-treatment of mGluR5-specific antagonist ABP688 did not reduce the uptake of [18F]7a in the rat brain. The trans isomer [18F]7b which has low affinity to mGluR1 showed no specific distribution in rat brain in all PET images, even if it also entered into rats brain rapidly. Conclusions: [18F]7a and [18F]7b were successfully prepared using a chloro-derivative precursor and cyclotron produced 18F anion, and formulated as an injection for animal study. In vitro and in vivo experiments using rodents showed [18F]7a had specific binding to mGluR1 in the brain. However, [18F]7b having trans configuration on cyclohexyl ring of quinoline derivative was nonspecific to mGluR1. Thus, it was proved that [18F]7a is a promising PET radioligand for mGluR1 imaging.