@misc{oai:repo.qst.go.jp:00082857,
 author = {A. Yu, Qingzhen and Zhang, Ming-Rong and Liang, Huan and Zhang, Ming-Rong and Liang, Huan},
 month = {May},
 note = {Objectives: AMPA (α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid) receptor is an important protein in humans that is implicated in the increased risk of neurological diseases including epilepsy and schizophrenia. Transmembrane AMPA Receptor Regulatory Proteins (TARPs) are a recently discovered family of proteins that modulate the activity of AMPA receptors. TARPs exhibit regiospecific expression in the brain, leading to physiological differentiation of the AMPA receptor activity. TARP γ-8 dependent AMPA receptors enrich primarily in the hippocampus. 6-(2-Cyclobutyl-5-methyl-3H-imidazo[4,5-b]pyridin-3-yl)benzo[d]thiazol-2(3H)-one (pIC50 value of 9.6)[1] is one of the most highly potent and selective TARP γ-8 antagonists. Herein we report the synthesis of 6-(5-methyl-3H-imidazo[4,5-b]pyridin-3-yl)benzo[d]thiazol-2(3H)-ones as TARP γ-8 radiotracer candidates and 6-(2-cyclobutyl-5-[11C]methyl-3H-imidazo[4,5-b]pyridin-3-yl)benzo[d]thiazol-2(3H)-one as a novel potent PET ligand for imaging of TARP γ-8 for AMPA receptor.

Methods: The standard 6-(2-cyclobutyl-5-methyl-3H-imidazo[4,5-b]pyridin-3-yl)benzo[d]thiazol-2(3H)-one (Compound A), its analogs and the precursor 6-(2-cyclobutyl-5-(tributylstannyl)-3H-imidazo[4,5-b]pyridin-3-yl)benzo[d]thiazol-2(3H)-one (8) were synthesized from 6-nitrobenzo[d]thiazol-2(3H)-one and 2-chloro-6-methyl-3-nitropyridine according to the literature methods with modifications. The target tracer [11C]compound A was prepared from SnBu3-compound A with  [11C]CH3I through Pd-catalyzed methylation under basic condition and isolated by HPLC combined with solid-phase extraction in automated 11C-radiosynthesis module. Generally, 6-aminobenzo[d]thiazol-2(3H)-one 2 was obtained in 93% yield by reducing commercially available reagent 1 in the presence of 10%Pd/C and H2, which generated the key intermediate 3 in 76% through aromatic nucleophilic substitution reaction between 2-chloro-6-methyl-3-nitropyridine and compound 2. The following reduction reaction under Pd/H2 system, as well as the Cu(OAc)2/HOAc mediated condensation and oxidation reactions with cyclobutanecarbaldehyde afforded the standard compound A in 63% over 2 steps. The compounds B-F were prepared in the similar manners.  For the preparation of precursor 8, similarly, the key intermediate 6 was obtained in 76% by the reduction of Fe/NH4Cl instead of Pd/C-H2 system, due to the sensitivity of bromo group of 5. Compound 7 was constructed in 82% yield from 6 and cyclobutanecarbaldehyde by the similar condensation and oxidation reactions as compound A. The subsequent Pd catalyzed stannulation generated 8 as a precursor in 36% yield. The radiosynthesis was carried out by heating a mixture of precursor (1.0 mg), [11C]CH3I, Pd(PPh3)4 (15 mol%), P(o-tol)3 (0.6 mg), CuBr (0.5 mg) and CsF (1.7 mg) in NMP at 100 oC for 10 min.

Results: The standard compound A and precursor were obtained in 45% yield over 4 steps and 17% yield over 6 steps, respectively. The analogs B-F were prepared in the varying yields from 15% to 25%. The desired PET ligand [11C]compound A was achieved in 2% isolated radiochemical yield (relative to [11C]CH3I, non-decay corrected). The radiochemical purity of the tracer was more than 99%, and the molar activity was higher than 37 GBq/µmol at end of synthesis (EOS). [11C]compound A was evaluated in the SD rat brain by PET with a dynamic 0-60 min scan, which showed limited brain uptake and specific binding in the hippocampus region in vivo (Tarp g8 rich region).

Conclusion: We have described facile synthetic routes to compound A and its analogs, as well as [11C]compound A, which may facilitate future development and evaluation of TARP γ-8 dependent AMPA tracers., eSRS},
 title = {Development of a novel PET tracer for imaging of γ-8 dependent transmembrane AMPA receptor regulatory protein},
 year = {2021}
}