基于Self-peptide和Fe5C2构建的高敏感MR分子探针对肿瘤血管的MR靶向成像研究

Blood vessels is the main way for the spread of malignancy, and the accurate assessment of tumor Microvessel density (MVD) in vivo is of great significance for the classification and the transfer tendency evaluation of malignancy, which have been possible based on MR molecular imaging. However, the low relaxivity of the MR contrast agent and the low targeting delivery efficiency of the probe limit the research of MR molecular imaging noticeably. The saturation magnetization intensity of Fe5C2 is 140 emu/g, which is 2 times higher than that of Fe3O4. The particles display an r2 relaxivity of 464.02 mM - 1s- 1, which is far higher than that of Fe3O4 (100 mM - 1s- 1) and is among the highest of all MR probes reported. Nanoparticles coated with amphiphilic block copolymer PS-b-PAA can self-assemble into magnetic micelles containing multiple Fe5C2 in aqueous solution, which further improve the relaxivity of Fe5C2. Shell cross-link of PAA can further improve the stability of the nano micelles. Self-peptide is a kind of CD47 equivalent peptides composed of 21 amino acids, which can inhibit the phagocytosis of the macrophages and RES and combine with integrin high expressed in tumor blood vessels with high affinity. Based on that discussed above, this project want to synthesize a targeted MR molecular probe with high sensitivity based on Fe5C2, PS-b-PAA and Self-peptide, to achieve targeted MR imaging of tumor blood vessels in vivo.

血行转移是恶性肿瘤的主要播散途径，肿瘤微血管密度(Microvessel density, MVD)的评估对其分级和转移倾向评价具有重要意义，MR分子成像使其成为可能。但对比剂较低的弛豫效能和靶向投递效能限制了MR分子成像的研究。Fe5C2的饱和磁化强度为140 emu/g，是Fe3O4的2倍，其T2弛豫效能为464.02 mM-1s-1，远远高于Fe3O4的100 mM-1s-1，是迄今报道弛豫效能最高的纳米粒。由两嵌段共聚物PS-b-PAA修饰的纳米粒在水溶液中能自组装成含多个Fe5C2的胶束，进一步提高其弛豫效能。PAA段壳交联能进一步增强胶束的稳定性。Self-peptide是由21个氨基酸组成的CD47分子等效多肽，能抑制巨噬细胞和网状内皮系统的吞噬、清除，并能与高表达于肿瘤血管的αvβ3高亲和力结合。本研究拟基于Fe5C2、PS-b-PAA和Self-peptide构建MR靶向的高敏感、长循环分子探针，以期实现肿瘤血管MR靶向成像。

血行转移是恶性肿瘤的主要播散途径，肿瘤微血管密度的评估对恶性肿瘤的分级和转移倾向评价具有重要意义，MR分子成像使其成为可能。但目前分子探针较低的弛豫效能和靶向投递效能限制了MR分子成像在肿瘤微血管评价领域的研究。为了改善上述问题，本研究采用高温热解法合成了形貌均一、粒径均匀的晶态铁纳米粒（粒径约为10 nm），进一步通过聚己内酯-聚乙二醇（poly(caprolactone)-block-poly(ethylene glycol), PCL-PEG）表面修饰、自组装、硅烷偶联剂(3-巯基丙基)三甲氧基硅烷((3-Mercaptopropyl)trimethoxysilane, MPTMS)壳交联、CD47类似多肽self-peptide偶联，最终得到一类具有多核/壳结构的MR分子探针Fe@PCL-PEG@SiO2-Self-peptide，并进行了理化性质表征及体内、外肿瘤血管MR分子成像研究。结果表明该分子探针具有较强的弛豫效能，其r2为658.24 mM-1S-1，远远高于Fe3O4的100 mM-1S-1，同时该探针表现出较好的生物相容性。由于Self-peptide的抗巨噬细胞吞噬作用，以及壳交联导致的纳米粒较好的结构稳定性，探针的血液循环时间显著延长，使其被动靶向作用得到优化。同时由于探针对肿瘤血管内皮细胞较好的主动靶向，探针在体内、外成像实验中均表现出优异的靶向性和亲和力。以上实验结果证明通过本研究的顺利实施，成功制备了一种性能非常优异的MR分子影像探针，能显著提高肿瘤血管MR分子成像的敏感性，具有非常好的应用前景，为肿瘤血管的早期成像及评价提供一条新的途径，进而基于肿瘤微血管密度评价肿瘤的分级和转移倾向，最终改善恶性肿瘤患者的预后。