Synthesis and preliminary biological evaluation of a novel PET tracer for α synuclein PET Imaging

Abstract [Background] Accumulation of α-synuclein is a major hallmark of Parkinson’s disease. The development of PET tracers to visualize aggregated α-synuclein is useful for early diagnosis and treatment of Parkinson's disease.[Purpose] A small molecule compound based on benzothiazole scaffolds, 2-((3-fluorobenzyl)thio)-6-(3-[fluorine-18] propoxy)benzo[d]thiazole ,denoted as 18F-YM,were prepared and labeled using Cu(II) mediated radiofluorination methods respectively. The imaging properties of the tracer were primarily evaluated through PET imaging at A53T mice and normal mice. Also, the imaging properties of the probe was also investigated through biodistribution experiments as well as ex vivo autoradiography and pathological analysis.[Methods] Through chemical synthesis, compounds Sn-YM and 19F-YM were obtained. Compound Sn-YM was labeled with 18F using organic tin fluoride method, and the resulting product 18F-YM was verified by high performance liquid chromatography. The in vitro stability and octanol–water partition coefficient of 18F-YM were determined. Finally, small animal Micro PET imaging was used to assess the affinity of 18F-YM for α-synuclein, and autoradiography, pathological analysis, and biodistribution were used to validate the results of small animal Micro PET imaging.[Results] 18F labeled small molecule compound was prepared in nearly 1 hour and obtained with an undecayed yield greater than 10% and the radiochemical purity was greater than 95%. In vivo PET imaging revealed that more radioactivity was significantly detected in the brain of A53T mice than those of normal mice after administration of 18F-YM. Quantitative analysis showed that the uptake values in the brain of A53T mice and normal mice were 2.35±0.06 %ID/g and 1.38±0.15 %ID/g.Furthermore, ex vivo autoradiography and histological examination confirmed the possibility of detection of aggregated α-synuclein in thalamus, substantia nigra and striatum using 18F-YM. A biodistribution study in normal mice found that 18F- YM was quickly cleaned from the brain of normal mice. It indicates that the non-specific binding of 18F- YM in the brain is low, which allowed for obtaining good contrast images. [Conclusion] The preclinical study demonstrated that the benzothiazole analog，18F- YM，owned preferable imaging prosperities. It may be a new candidate for α-synuclein PET imaging.

摘要【背景】α-突触核蛋白（α-synuclein）聚集是帕金森病（Parkinson’s disease）的核心病理特征之一。开发可可视化聚集态α-突触核蛋白的正电子发射断层显像（PET）示踪剂，对帕金森病的早期诊断与治疗具有重要价值。【目的】本研究设计并合成了一种基于苯并噻唑（benzothiazole）骨架的小分子化合物2-((3-氟苄基)硫基)-6-(3-[氟-18]丙氧基)苯并[d]噻唑，命名为18F-YM，分别采用铜(II)介导的放射性氟化法进行标记。随后，通过对A53T小鼠与正常小鼠开展PET显像，初步评价该示踪剂的成像性能；同时通过生物分布实验、离体放射自显影及病理学分析，进一步探究该探针的成像特性。【方法】通过化学合成手段制备得到Sn-YM与19F-YM两种化合物。采用有机锡氟化法对Sn-YM进行18F标记，所得产物18F-YM经高效液相色谱法进行纯度验证。同时测定了18F-YM的体外稳定性及正辛醇-水分配系数。最后，通过小动物微型PET（Micro PET）显像评估18F-YM与α-突触核蛋白的结合亲和力，并通过放射自显影、病理学分析及生物分布实验验证小动物PET显像结果。【结果】18F标记的小分子化合物制备耗时近1小时，未校正放化产率大于10%，放化纯度高于95%。活体PET显像结果显示，尾静脉注射18F-YM后，A53T小鼠脑内的放射性摄取量显著高于正常小鼠。定量分析表明，A53T小鼠与正常小鼠的脑摄取值分别为2.35±0.06 %ID/g及1.38±0.15 %ID/g。此外，离体放射自显影与组织学检查证实，18F-YM可检测丘脑、黑质及纹状体中的聚集态α-突触核蛋白。正常小鼠的生物分布实验结果显示，18F-YM可快速从正常小鼠脑组织中清除，表明其在脑内的非特异性结合较低，可获得对比度优异的显像图像。【结论】本临床前研究证实，苯并噻唑类衍生物18F-YM具备优良的成像性能，有望成为用于α-突触核蛋白PET显像的新型候选示踪剂。