18F标记融合双肽iRGD-CPCR4-2进行多靶点分子显像研究

According to the pathogenesis of the tumor, three receptors, integrin αvβ3, neuropilin-l and CXCR4 receptor which play an important role in the process of tumor growth and metastasis，are chosen for multi-targeted molecular imaging. The peptide iRGD is conjugated to CPCR4-2 and labeled with radionuclide F-18 to obatain the molecular probe 18F-iRGD-CPCR4-2. Due to the binding between the ligands and receptors, the probes accumulate at the surface of the tumor cells and then enter into the cell along with the penetration the tissue and cells by iRGD peptide. As a result, a large number of 18F-iRGD-CPCR4-2 retain intracellularly and bind the receptors specifically， avoiding the probes and the receptors dissociation and elution from the cell membrane.The binding intracellularly and on membrane superimposes and enhances the ratio of target/non-targe.The spacing of the two ligands is adjusted by the polyethylene glycol (PEG), making them binding of the corresponding receptors respectively. Still they have the opportunity of bivalent binding to the receptors on the cell membrane or intracellular at the same time. Thus it increases the total number of binding sites, enhances binding force and improves tumor detection capability, thus enhances the sensitivity of PET imaging. This project is a joint design of molecular targeting and cellular transduction, and is also an innovation of receptor imaging.

本课题是分子靶向与细胞转导的联合设计，是受体显像的创新模式。根据肿瘤的发病机制具有多靶点的特征，本课题选择肿瘤生长和转移过程中发挥重要作用的三个受体整合素αvβ3、neuropili-1和CXCR4受体为靶点进行多靶分子显像。将它们的配基多肽iRGD和CPCR4-2融合并进行核素F-18标记得到探针18F-iRGD-CPCR4-2，利用iRGD的载体作用穿透组织和细胞，实现18F-iRGD-CPCR4-2探针和受体的特异性结合并进入细胞与细胞内受体结合，使大量的探针滞留于细胞内，避免探针与受体的解离与洗脱，实现细胞膜与细胞内分子靶向的叠加，提高靶/非靶比值。利用聚乙二醇（PEG）调节两个配基iRGD和CPCR4-2的空间间距，使得它们互不影响各自与相应受体的结合，同时有机会与细胞膜或者细胞内受体进行双价结合，这样就增加了结合位点数，增强结合力和提高肿瘤探测能力，从而提高PET显像的灵敏度。

肿瘤受体显像具有无创性,能够在活体状态下进行细胞分子水平研究,是目前分子影像的重要发展方向之一。以多肽为代表的配基具有许多独特的优势,具有广阔的临床应用前景，本文采用中间体[18F]-SFB和[18F]-NFP分别标记了小肽iRGD和融合双肽，采用[18F]AlF螯合法标记了肽CPCR4-2。通过MicroPET来研究各个肽的生理学特性。研究发现iRGD在肿瘤部位有较大的浓聚，随着时间的延长，其摄取量依然保持在高水平。超低水平的正常脑组织本底以及高选择性靶向使其具备成为脑胶质瘤靶向PET分子探针的潜能,可用以弥补18F-FDG显像在检测烦内肿瘤方面存在的明显缺陷。本研究也显示直至到静脉注射18F-iRGD后120分钟均未见骨骼显影,提示18F-iRGD在体内很稳定,无脱氟现象，研究显示由于18F-iRGD通过肝胆系统及泌尿系统排泄导致双肾及膀胱内出现大量放射性分布,这使腹部结构显示不清,因此提示18F-iRGD可能不利于腹部肿瘤的检测。而18F-CPCR4-2的本底较高，其在肿瘤部位的摄取也不是很多，可能是引入NOTA基团使得多肽与受体的结合力收到了影响。融合双肽的标记实验显示[18F]-NFP是一种应用潜力更大的中间体，采用这种方法减少了HPLC的分离步骤。本课题在研究过程中也发现18F标记多肽存在较多困难在现有的基础上,进一步完善多肽的标记方法,研究出更简便、快捷的标记方法,逐渐向临床应用转化是未来的研究方向。同时如何进一步提高肿瘤对靶向多肽的摄取量及结合强度,以更灵敏地检测肿瘤病灶,也是未来须认真思考的问题。