Prussian Blue Nanoparticles as Cellular T1 MRI Contrast Agents

描述(申请人提供)：这项建议旨在开发普鲁士蓝纳米颗粒(PBNPs)作为新一代T1加权磁共振造影剂(CA)，具有高弛豫度、长血液循环时间和穿透细胞膜的能力。普鲁士蓝(PB)是非理想式Fe4III[FeII(CN)6]3.nH2O(n=14-16)，其中两个不同的铁中心Fe~(3+)(高自旋S=5/2)和Fe~(2+)(低自旋S=0)通过CN-基团桥联。在PB的晶体结构中，四分之一(25%)的FeII(CN)6单元从晶格中消失，在充满水分子的结构中产生了一个巨大的空腔。缺失的FeII(CN)6单元还使得Fe3+中心被一个水分子和五个CN-基团配位，从而产生了一个活跃的内球驰豫机制来增强T1驰豫。由于在扩展的三维网络中CN-基团同时配位于Fe3+和Fe2+中心的强烈配位场效应，CN-配体和Fe3+/Fe2+离子被完全锁定在各自的晶格位置，不能从结构中释放。因此，对于任何化合物，PB具有有史以来最低的溶度积常数(KSP=10-41)。我们发现，用Mn2+或/和Gd3+离子取代晶格中的部分Fe3+离子，可以形成锰或Gd掺杂的纳米粒子，即Mn@PBNPs和Gd@PBNPs，其R1驰豫系数显著增加。此外，与小分子Gd3+-螯合物相比，PBNPs在溶液中的结构刚性和更低的滚动率有助于在这个新的纳米平台上进行额外的T1加权MRI对比度增强。我们的目标是：(I)探索通过调整纳米颗粒尺寸、Mn2+和/和Gd3+掺杂水平以及表面小分子或聚合物涂层来优化R1弛豫度的方法；(Ii)系统地研究细胞摄取和细胞成像的特征以及图像引导药物传递应用的潜力；以及(Iii)同时将Mn2+和/和Gd3+离子与放射性核素Ga-67或Ga-68结合在一起，用于MRI-SPECT和MRI-PET双峰成像应用。 我们将努力验证以下四个假设：1)普鲁士蓝纳米粒子，当适当地定制和设计时，将有效地减少散装水中质子的纵向弛豫时间。将Mn2+或/和Gd3+掺入该纳米平台将显著提高R1弛豫度；2)普鲁士蓝纳米颗粒将被细胞内化，无毒性，在细胞成像和运送小分子试剂方面有效；3)普鲁士蓝纳米颗粒将在体内成为有效的T1加权磁共振造影剂；4)顺磁性锰(II)或/和Gd(III)离子与放射性核素Ga-67或Ga-68同时掺入普鲁士蓝纳米颗粒，将产生有效的双峰造影剂，用于体外和体内MRI-SPECT和MRI-PET成像。 影响我们探索PBNPs的方法是史无前例的，代表着新一代CA设计的范式转变。这项拟议研究中出现的新范式将被证明是革命性的，而不是进化的，因为它增加了一类新型颗粒T1加权MRI CA的R1弛豫度，因此将具有很高的潜力在MRI诊断成像方面产生重大突破，甚至可能完全改变这一研究领域的格局。

DESCRIPTION (provided by applicant): This proposal is aimed at developing Prussian blue nanoparticles (PBNPs) as a new generation of T1-weighted MRI contrast agents (CAs) with high relaxivity, long blood circulation times and ability to penetrate the cell membrane. Prussian blue (PB) is iron(III) hexacyanoferrate(II) with anidealized formula Fe4III[FeII(CN)6]3.nH2O (n=14-16) in which two different iron centers, Fe3+ (high-spin S=5/2) and Fe2+ (low-spin S=0) are bridged by the CN- groups. In the crystal structure of PB, a quarter (25%) of the FeII(CN)6 unit is absent from the crystal lattice, creating a large cavity inside the structure that is filled with water molecules. The missing FeII(CN)6 unit also causes the Fe3+ center to be coordinated by one water molecule and five CN- groups, thus giving rise to an active inner-sphere relaxation mechanism for enhancing the T1 relaxation. Due to the strong ligand-field effect and simultaneous coordination of the CN- group to both the Fe3+ and Fe2+ centers in the extended 3D network, the CN- ligand and the Fe3+/Fe2+ ions are completely locked in their lattice positions and cannot be released from the structure. As a result, PB has the lowest solubility product constant ever measured for any compound (Ksp=10-41). We have found that replacement of some of the Fe3+ ions with Mn2+ or/and Gd3+ ions in the crystal lattice can form the manganese- or gadolinium-incorporated nanoparticles, Mn@PBNPs and Gd@PBNPs with significantly increased r1 relaxivity. Besides, the structural rigidity and reduced tumbling rates of PBNPs in solution, as compared to the small molecular Gd3+-chelates, can contribute to additional T1-weighted MRI contrast enhancement in this new nanoplatform. Our goals are: (i) to explore methods for optimizing the r1 relaxivity by adjusting the nanoparticle size, level of Mn2+- or/and Gd3+-doping, and surface coating with small molecules or polymers; (ii) to systematically investigate the characteristics of cellular uptake and cellular imaging as well as potential for image-guided drug delivery applications; and (iii) to simultaneously incorporate Mn2+ or/and Gd3+ ions along with the radionuclide Ga-67 or Ga-68 for MRI-SPECT and MRI-PET bimodal imaging applications. We will endeavor to test the following four hypotheses: 1) Prussian blue nanoparticles, when properly tailored and engineered, will be effective in reducing the longitudinal relaxation time of protons from bulk water. Incorporation of Mn2+ or/and Gd3+ into this nanoplatform will significantly increase the r1 relaxivity; 2) Prussian blue nanoparticles will be internalized by cells, exhibit no toxicity and be effective in cellular imaging and in delivering small-molecular agents; 3) Prussian blue nanoparticles will be effective T1-weighted MRI contrast agents in vivo; and 4) Simultaneous incorporation of paramagnetic ions of manganese(II) or/and gadolinium(III) along with the radionuclide Ga-67 or Ga-68 into Prussian blue nanoparticles will produce effective bimodal contrast agents for in vitro and in vivo MRI-SPECT and MRI-PET imaging. Impact Our approach to exploring PBNPs as novel T1-weighted MRI is unprecedented and represents a paradigm shift in the design of new-generation CAs. The new paradigm that will emerge from this proposed research will prove to be revolutionary rather than evolutionary for increasing r1 relaxivity in a novel class of particulate T1-weighted MRI CAs, and thus will have high potential to produce a major breakthrough in MRI diagnostic imaging and may even completely change the landscape in this area of research.