Shimoda, Yoko  ,  Yamasaki, Tomoteru  ,  Fujinaga, Masayuki  ,  Ogawa, Masanao  ,  Yusuke, Kurihara  ,  Nengaki, Nobuki  ,  Kumata, Katsushi  ,  Yui, Joji  ,  Hatori, Akiko  ,  Xie, Lin  ,  Zhang, Yiding  ,  Kawamura, Kazunori  ,  Ming-Rong, Zhang

Objectives: Metabotropic glutamate receptor 5 (mGluR5) is characterized as a G protein coupled receptor and is known as one of regulators for excitatory neurotransmission, which is involved in central nervous system (CNS) disorders, such as fragile X syndrome, Parkinson’s disease, and depression. Therefore, mGluR5 is an important target for development of new pharmaceuticals against these CNS disorders. Recently, 3-[5-(pyridine-2-yl)2H-tetrazol-2-yl]benzonitrile (1) has been reported as a potential compound for mGluR5 (Ki = 190 nM). In this study, we attempt to develop novel a PET ligand for mGluR5 using 1 as a lead compound. Results and Discussions: 1. In vitro binding assay: Five new analogs of 1 were designed and synthesized (Figure 1). Their affinity for mGluR5 was estimated using a binding assay with the mGluR5-seletive radioligand (E)-[11C]ABP688 in rat brain homogenate. Of these analogs, compounds 2a and 2b showed relatively high affinity (2a: Ki = 10.8 nM; 2b: Ki = 9.4 nM) for mGluR5. 2. Radiochemistry: [11C]2a and [11C]2b were synthesized by reacting desmethyl or arylstannyl precursor with [11C]CH3I. Starting from 14.8–22.2 GBq [11C]CO2, 0.58–0.91 GBq [11C]2a was produced with 14 ± 4% (n = 3) radiochemical yield at 32 min after the end of bombardment (EOB). Starting from 22.2–28.1 GBq [11C]CO2, 0.86–2.99 GBq [11C]2b was acquired with 25 ± 14% (n = 11) radiochemical yield at 32 min after EOB. In the final product solution, the specific activity of [11C]2a and [11C]2b was 57–71 and 52–190 GBq/µmol, respectively. Radiochemical purity of both ligands was higher than 98% at the end of synthesis.3. In vitro Autoradiography: In vitro autoradiography with [11C]2a or [11C]2b was performed using rat brain sections. In the control sections, distribution pattern of both radioligands was heterogeneously exhibited, which corresponded with the biological localization of mGluR5. In blocking study, co-incubation with mGluR5-selective MPEP (1.0 μM) dramatically diminished the accumulations of radioactivity. The specific binding of [11C]2b was slightly higher than that of [11C]2a. Therefore, we decided to perform further evaluation of [11C]2b using PET assessment.4. Small-animal PET study: In control rats, the uptake of [11C]2b immediately reached maximum levels in the brain regions after injection and quickly decreased after that. The uptake ratio of each region against cerebellum, a reference region for mGluR5, under the equilibrium state was 3.0 ± 0.1 in the striatum, 2.4 ± 0.1 in the hippocampus, 2.3 ± 0.1 in the frontal cortex, and 2.0 ± 0.1 in the thalamus, respectively. By treatment with MPEP, these ratio reduced by 1.3–1.6 in each brain region. As shown in Figure 2, the parametric PET images could identify regional differences in specific binding to mGluR5, demonstrating that PET with [11C]2b could provide significant signal for mGluR5 in brain regions.Conclusions: We have found out compound  2b as a promising PET ligand candidate for mGluR5 by in vitro binding assay and subsequently synthesized [11C]2b. PET study with [11C]2b in rat brain was possible to visualize regional distribution of mGluR5. [11C]2b is a potential PET ligand for imaging of mGluR5 in brain.
Ninth Japan-China Joint Seminar on Radiopharmaceutical Chemistry

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