双级pH超灵敏响应的可生物正交点击反应的铁基T1型磁共振成像造影剂的构建研究

MRI technique has been widely used in clinical diagnosis of cancer since it is capable of providing a high-resolution three-dimensional image without invasion. To cover the shortage of intrinsic low-sensitivity, MRI contrast agents are generally employed to enhance the effectiveness of imaging. However, the available contrast agents on clinic are far from the requirements for precise diagnosis of cancer at early stage. With the purpose of developing MRI contrast agents with high safety and excellent capability of providing significant contrast effect, ultra-small iron oxide nanoparticles are used as a basic element to design the smart iron-based T1 MRI contrast agents according to the fundamental theories of contrast agents for imaging combined with the characteristics of tumor tissues. On consideration of endowing contrast agents with high specificity to tumor tissues to lead obviously enhanced MRI signal and enhancing the accumulation of them at tumor location, one strategy of dually ultra-sensitive pH-responsiveness is proposed to realize the high permeability and significant improvement of MRI signal at tumor location through multi-stage delivery from large size to small size together with transition mechanism from hydrophobicity to hydrophilicity. Moreover, the other strategy of bioorthogonal click reaction with favorable bio-safety and high efficiency is presented to achieve universally efficient targeting of contrast agents to tumor cells with high specificity, and simultaneously improve their rotational correlation time τR, which would further enhance the accumulation of contrast agents and gain specific improvement of MRI signal at tumor location. On basis of the combination of the two strategies, the smart MRI contrast agents are anticipated to strengthen the contrast effect of tumor tissues significantly. This study aims to explore the general rules and intrinsic mechanism for constructing T1 MRI contrast agents with high effectiveness, and accumulates theoretic foundations and experimental basis for designing MRI contrast agents with universal targeting function for various tumors.

现有MRI造影剂难以实现肿瘤早期诊断，本项目以发展安全且能提供高成像对比度的造影剂满足肿瘤早期诊断为目标，采用超小氧化铁纳米颗粒为基本单元，依据造影剂成像的基本理论并结合肿瘤组织自身特点，从赋予造影剂对肿瘤组织高特异性MRI信号增强，及有效提高造影剂在肿瘤部位的富集量出发，构建智能型铁基T1型MRI造影剂。一方面，设计双级pH超灵敏响应策略，以“大变小”分级递送和疏亲水转换的方式同时实现造影剂在肿瘤部位的高渗透富集和高特异性MRI信号增强。另一方面，设计安全高效的生物正交点击反应策略，通过对肿瘤细胞的普适性高效高特异性靶向，进一步提高造影剂在肿瘤部位的富集量和实现因旋转相关时间τR增加而完成的高特异性MRI信号增强。结合此两种策略将有效提高造影剂对肿瘤组织的造影成像对比度。本研究旨在探索构建高效T1型MRI造影剂的一般规律和内在机理，并为设计普适性靶向的MRI造影剂提供理论依据和实验基础。

肿瘤早期诊断对于实现肿瘤的有效治疗与遏制具有重要价值，但现有MRI造影剂难以实现肿瘤的早期精确诊断。为了更好地实现MRI造影剂在肿瘤早期诊断上的应用，本研究从优良生物安全性和提高造影成像对比度的角度出发，采用T1型极小氧化铁纳米颗粒（ESIONPs）为基本构建单元，依据肿瘤组织本身的特点进行合理系统的设计，以构建安全高效的铁基MRI造影剂。其一，研究pH响应型纳米组装体的构建及其用于ESIONPs的纵向和横向弛豫率（r1和r2）的协同调控。在该研究中，利用肿瘤部位的微酸环境作为一个刺激源，我们设计和构建了基于ESIONPs的三种纳米组装体，用于探究对ESIONPs的r1和r2进行协同调控的可行性。这些关于r1和r2的协同调控可用于指导刺激响应型造影剂的设计和构建。其二，研究了pH响应型可切换的ESIONPs造影剂。该研究构建了ESIONPs System，其在肿瘤弱酸微环境时，所包含的ESIONPs-PEG-PGA和ESIONPs-PEG-PDC表面分别带有正电荷和负电荷，会通过静电相互作用立即引发ESIONPs的聚集，不仅可实现T1造影信号到T2造影信号的切换，特异性地激活在肿瘤部位的T2造影增强效果，而且可增加造影剂在肿瘤部位的沉积，从而有效提高肿瘤部位的T2造影效果。其三，研究了还原响应型可切换的ESIONPs造影剂。该研究设计并合成了一种还原响应引发聚集的造影剂ESIONPs-s-s-PEG-FA，在肿瘤部位的还原环境下，可激活而实现T1造影信号到T2造影信号的切换。该造影剂对肿瘤部位有高度特异性和选择性，可实现高效的造影成像。本项目系统探索了基于ESIONPs构建高效铁基MRI造影剂的一般规律和内在机理，为构建新一代高效MRI造影剂提供了理论依据和实验基础。