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神经炎症是中枢神经系统(central nervous system,CNS)内的一种炎症和适应性反应,通常涉及小胶质细胞的激活,是多种神经系统疾病的病理标志。小胶质细胞的轻度激活具有神经保护作用,可以改善神经变性的早期症状,修复损伤;但过度激活会引起细胞因子表达失调,进而加速神经退行性改变[1]。评估CNS异常的“金标准”如尸检、手术病理等均存在一定的局限性。传统影像方法无法显示炎症相关的生理病理过程,已不能满足需要。PET是在分子影像水平上对活体进行成像,灵敏度高,可以用放射性核素对小胶质细胞进行标记,在疾病发展的临床过程中观察CNS的异常[2],对CNS疾病的诊断有着独特的优势。在神经炎症显像方面,转位蛋白(translocator protein,TSPO)放射性配体的研究非常深入[3]。
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11C-PK11195是第一代TSPO放射性配体,针对11C-PK11195的研究在初期提供了许多关于CNS疾病中小胶质细胞活化的重要观点[6]。R型11C-PK11195的保留时间优于S型,因此11C-(R)-PK11195作为经典的TSPO配体,在CNS疾病研究中的应用最为广泛[7]。然而11C-(R)-PK11195亲脂性logD7.4=3.97,高于最佳值2.2[8],其血浆蛋白结合率较高,与TSPO中等的亲和力使得图像信噪比较差,大脑通透性也较低,且用于标记的放射性核素11C的半衰期较短,其临床应用受到一定限制。
为了提高检测细微病变的灵敏度,第二代TSPO放射性配体被研发出来,如11C-PBR28、11C-DPA713、18F-DPA714、18F-PBR06、11C-vinpocetine、18F-FEPPA等。其中,11C-DPA713是最有潜力的第二代放射性配体之一,它的非特异性结合水平较低,在4种放射性配体的对比研究中,11C-DPA713在人脑中的信噪比最高[9]。但第二代放射性配体对TSPO基因外显子4(rs6971)的多态性敏感,根据其敏感程度将TSPO分为高亲和力(high-affinity binders,HABs)、混合亲和力(mixed-affinity binders,MABs)、低亲和力(low-affinity binders,LABs)3种类型[10]。TSPO密度相同但基因型不同的个体会产生不同的PET信号,因此成像前需要进行额外的TSPO基因分型, 以排除摄取量过低的LABs,并对HABs和MABs的结合水平进行适当校正[11]。
第三代TSPO放射性配体如11C-ER176、18F-GE180可以弥补这一局限性,它们对rs6971多态性的敏感性较低。11C-ER176是11C-(R)-PK11195的异构体,其表现出了许多优于母体的特性。11C-ER176具有更高的亲和力和稍低的亲脂性(logD7.4=3.55),具有高信噪比和适当的药代动力学特性。它在猴脑和人脑[12]中都具有高特异性结合的特性,在体外测试时对rs6971几乎不敏感。在一个对9名健康志愿者进行的11C-ER176显像的小型研究中,11C-ER176表现出了一定的敏感性,在LABs个体中的全脑结合潜力(binding potential,BPND)约为1.4±0.8,远低于HABs的4.2±1.3[12]。但11C-ER176在LABs中的BPND与广泛使用的11C-PBR28在HABs中的BPND大致相同,这表明其应用时无需排除LABs患者,并且可能具有更高的灵敏度[9]。因此,有研究者认为,11C-ER176是目前用于量化TSPO的最佳放射性配体[13]。新近研发出的18F-LW223、 18F-PBR316、18F-CB251在体外试验中也对rs6971基因多态性不敏感,适用于进一步的生物学和临床研究[14-16]。由表1可以得知历代代表性示踪剂的结构式及其特点。
表 1 历代转位蛋白放射性配体代表性示踪剂的结构式及其特点
Table 1. Structural formula and characteristics of representative tracers of transposable proteins in past dynasties
靶向转位蛋白正电子示踪剂在神经炎症显像方面的应用
The application of targeted translocator protein positron tracers in neuroinflammation imaging
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摘要: 神经炎症是许多中枢神经系统疾病共有的病理表现,而小胶质细胞会在中枢神经系统受到损伤后率先产生应答。相对分子质量为18 000的转位蛋白(TSPO)在神经炎症发生时高表达于小胶质细胞,这一特性使得该蛋白适用于评估脑内小胶质细胞的活化增生情况。从靶向TSPO放射性配体研发至今,已有60多种靶向TSPO的放射性配体被研发用于神经炎症疾病的研究,以期对其病理过程有更深入的了解。笔者就靶向TSPO正电子示踪剂在神经炎症显像方面的应用进行综述。
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关键词:
- 神经炎 /
- 正电子发射断层显像术 /
- 小胶质细胞 /
- 转位蛋白 /
- TSPO放射性配体
Abstract: Neuroinflammation is a common pathological manifestation of many central nervous system diseases, and microglia are the first to respond to damage to the central nervous system. The relative molecular mass of 18 000 translocator protein (TSPO) is highly expressed in microglia at the onset of neuroinflammation, a property that makes it suitable for assessing the activation of microglia proliferation in the brain. Since the development of radioligands targeting TSPO, more than 60 radioligands targeting TSPO have been developed for the study of neuroinflammatory diseases in order to gain a better understanding of their pathological processes. The author reviews the use of targeted TSPO positron tracers for neuroinflammatory imaging.-
Key words:
- Neuritis /
- Positron-emission tomography /
- Microglia /
- Translocator protein /
- TSPO radioligands
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表 1 历代转位蛋白放射性配体代表性示踪剂的结构式及其特点
Table 1. Structural formula and characteristics of representative tracers of transposable proteins in past dynasties
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