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Quantitative Analysis of Peripheral Benzodiazepine Receptor in the Human Brain Using PET with ¹¹C-AC-5216

¹ Molecular Neuroimaging Group, Molecular Imaging Center, National Institute of Radiological Sciences, Chiba, Japan; ² Department of Clinical Neuroscience and Therapeutics, Hiroshima University Graduate School of Biomedical Sciences, Hiroshima, Japan; and ³ Molecular Probe Group, Molecular Imaging Center, National Institute of Radiological Sciences, Chiba, Japan

Correspondence: For correspondence or reprints contact: Hiroshi Ito, Molecular Neuroimaging Group, Molecular Imaging Center, National Institute of Radiological Sciences 4-9-1, Anagawa, Inage-ku, Chiba, 263-8555, Japan. E-mail: hito{at}nirs.go.jp

Peripheral benzodiazepine receptor (PBR) is upregulated in activated glial cells and is therefore a useful biomarker for inflammation in the brain and neurodegenerative disorders. We developed a new PET radioligand, ¹¹C-AC-N-benzyl-N-ethyl-2-(7-methyl-8-oxo-2-pheyl-7,8-dihydro-9H-purin-9-yl)acetamide (¹¹C-AC-5216), that allows the imaging and quantification of PBRs in monkey and mouse brains. The aim of this study was to evaluate a quantification method of ¹¹C-AC-5216 binding in the human brain. Methods: A 90-min dynamic PET scan was obtained for each of 12 healthy men after an intravenous injection of ¹¹C-AC-5216. Regions of interest were drawn on several brain regions. Binding potential, compared with nondisplaceable uptake (BP_ND), was calculated by a nonlinear least-squares fitting (NLS) method with the 2-tissue-compartment model, and total volume of distribution (V_T) was estimated by NLS and graphical analysis methods. Results: BP_ND was highest in the thalamus (4.6 ± 1.0) and lowest in the striatum (3.5 ± 0.7). V_T obtained by NLS or graphical analysis showed regional distribution similar to BP_ND. However, there was no correlation between BP_ND and V_T because of the interindividual variation of K₁/k₂. BP_ND obtained with data from a scan time of 60 min was in good agreement with that from a scan time of 90 min (r = 0.87). Conclusion: Regional distribution of ¹¹C-AC-5216 was in good agreement with previous PET studies of PBRs in the human brain. BP_ND is more appropriate for estimating ¹¹C-AC-5216 binding than is V_T because of the interindividual variation of K₁/k₂. ¹¹C-AC-5216 is a promising PET ligand for quantifying PBR in the human brain.

Key Words: ¹¹C-AC-5216 • peripheral benzodiazepine receptor • microglia • human brain • positron emission tomography

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