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J Med Chem. 2006 May 4;49(9):2735-42.

[2-11C]isopropyl-, [1-11C]ethyl-, and [11C]methyl-labeled phenoxyphenyl acetamide derivatives as positron emission tomography ligands for the peripheral benzodiazepine receptor: radiosynthesis, uptake, and in vivo binding in brain.

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Department of Medical Imaging, National Institute of Radiological Sciences, Inage-ku, Chiba 263-8555, Japan.


The peripheral benzodiazepine receptor (PBR) is widely expressed in peripheral tissues, blood cells, and in glia cells in the brain. We have previously developed two positron emission tomography (PET) ligands, N-(2-[(11)C],5-dimethoxybenzyl)-N-(5-fluoro-2-phenoxyphenyl)acetamide ([(11)C]2) and its [(18)F]fluoroethyl analogue ([(18)F]6), for the current investigation of PBR in the human brain. The aim of this study was to label the potent PBR agonist N-(4-chloro-2-phenoxyphenyl)-N-(isopropoxybenzyl)acetamide (3) and its ethyl (7) and methyl (8) homologues with (11)C and to evaluate them as PET ligands for PBR with mice, rats, and monkeys. Ligands [(11)C]3, [(11)C]7, and [(11)C]8 were synthesized by alkylation of phenol precursor 9 with 2-[2-(11)C]iodopropane ([(11)C]10), [1-(11)C]iodoethane ([(11)C]11), and [(11)C]iodomethane ([(11)C]12), respectively. The alkylating agent [(11)C]10 or [(11)C]11 was prepared by reacting CH(3)MgBr with [(11)C]CO(2), followed by reduction with LiAlH(4) and iodination with HI. In vitro quantitative autoradiography determined that 3, 7, and 8 had potent binding affinities (K(i) = 0.07-0.19 nM) for PBR in the rat brain. These [(11)C]ligands could pass across the blood-brain barrier and enter the rat brain (0.17-0.32% of injected dose per gram wet tissue). Ex vivo autoradiography showed that the [(11)C]ligands preferably distributed in the olfactory bulb and cerebellum, two regions with richer PBR density in the rat brain. The co-injection of PBR-selective 2 reduced the [(11)C]ligand binding in the two regions, suggesting that binding in the rat brain was specific to PBR. PET study determined that the [(11)C]ligands preferably accumulate in the occipital cortex of the monkey brain, a region with a high density of PBR in the primate brain. Moreover, in vivo binding of the methyl homologue [(11)C]8 in the monkey brain could be inhibited by PBR-selective 2 or 1, indicating that some of the [(11)C]8 binding was due to PBR. Metabolite analysis demonstrated that these [(11)C]ligands were metabolized by debenzylation to polar products mainly in the plasma.

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