Evaluation of N‐benzyl‐N‐[11C]methyl‐2‐ (7‐methyl‐8‐oxo‐2‐phenyl‐7,8‐dihydro‐9H‐purin‐9‐yl)acetamide ([11C]DAC) as a novel translocator protein (18 kDa) radioligand in kainic acid‐lesioned rat

Abstract

The aim of this study was to evaluate N-benzyl-N-[11C]methyl-2-(7-methyl-8-oxo-2-phenyl-7,8-dihydro-9H-purin-9-yl)acetamide ([11C]DAC) as a new translocator protein (18 kDa) [TSPO, formerly known as the peripheral-type benzodiazepine receptor (PBR)] positron emission tomography (PET) ligand in normal mice and unilateral kainic acid (KA)-lesioned rats. DAC is a derivative of AC-5216, which is a potent and selective PET ligand for the clinical investigation of TSPO. The binding affinity and selectivity of DAC for TSPO were similar to those of AC-5216, and DAC was less lipophilic than AC-5216. The distribution pattern of [11C]DAC was in agreement with TSPO distribution in rodents. No radioactive metabolite of [11C]DAC was found in the mouse brain, although it was metabolized rapidly in mouse plasma. Using small-animal PET, we examined the in vivo binding of [11C]DAC for TSPO in KA-lesioned rats. [11C]DAC and [11C]AC-5216 exhibited similar brain uptake in the lesioned and nonlesioned striatum, respectively. The binding of [11C]DAC to TSPO was increased significantly in the lesioned striatum, and [(11)C]DAC showed good contrast between the lesioned and nonlesioned striatum (the maximum ratio was about threefold). In displacement experiments, the uptake of [11C]DAC in the lesioned striatum was eventually blocked using an excess of either unlabeled DAC or PK11195 injected. [11C]DAC had high in vivo specific binding to TSPO in the injured rat brain. Therefore, [11C]DAC is a useful PET ligand for TSPO imaging, and its specific binding to TSPO is suitable as a new biomarker for brain injury.