Synthesis of Metal-Free Magnetic Resonance Imaging Contrast Agents

描述(申请人提供)：这个项目的总体目标是开发生物稳定的有机自由基作为磁共振成像(MRI)的造影剂(CA)。这种不含金属的有机CA将在对比剂增强MRI方面取得突破，同时避免与有毒金属离子相关的风险。虽然这些药物将特别有利于肾功能受损的患者，这些患者在使用顺磁性Gd螯合物(GBCA)进行磁共振检查后患肾源性系统性纤维化(NSF)的风险增加，但新的药物也可能使普通民众受益。这项拟议工作的目标是合成能够提供高质量活体磁共振图像的有机自由基造影剂(Orcas)。磁共振成像实用ORCA开发的长期障碍是设计和合成分子大小适中的顺磁性有机化合物，这些化合物具有足够长的活体寿命、高1H水弛豫(R1)和高含水率 溶解度。我们提出了一种设计策略，从高度抗还原的氮氧自由基制备虎鲸，并利用关于树枝状大分子和聚乙二醇化的先验知识，特别是它们在药物设计中的应用，以获得具有优化MRI性能的试剂。拟议的项目有三个具体目标。具体目标1：优化虎鲸的1H水弛豫度r1(在3特斯拉下)和亲水性，使分子r1=10 mms，至少是临床cas(分子r1≈5 mm‾？S‾？)的两倍。虎鲸是由氮氧化物与第四代聚丙基亚胺(PPI-G4)树枝状大分子通过聚乙二醇化与长的线形和/或支化的聚乙二醇链连接而成。拟议的工作将为具体目标2提供基础：使用R1≥10 mM‾？S‾？分子优化逆戟鲸的分子大小，以获得半径为1 nm的≤rse≤2 nm的试剂，该试剂将经历有效的肾脏过滤/排泄。将测定虎鲸的体外毒性和水弛豫度，以及影响分子R1的性质，如旋转相关时间和电子自旋弛豫时间，以确定四种最好的虎鲸。具体目标3：评价虎鲸在活体MRI中的应用效果。在3特斯拉的临床范围内，将通过正常小鼠的MRI和荷瘤小鼠的动态增强磁共振成像(DCE)来确定所选择的四种虎鲸的最低有效剂量、时间进程和肿瘤血管通透性(转移常数Ktras)。ORCA的初步毒性将通过组织学检查进行评估。选择最有效的ORCA，用荧光探针标记，然后通过电子顺磁共振和荧光光谱进行体外生物分布研究。这些研究将确定ORCA是否适合进一步发展为人类使用。基于生物稳定的有机自由基的无金属磁共振造影剂的开发将拓展生物医学成像的前沿。有机合成的模块化使得能够针对特定的性质(氧化还原、溶解性和毒性)进行有效的结构修饰。这种有机试剂将改善疾病的早期检测和诊断。

DESCRIPTION (provided by applicant): The overall goal of this project is the development of biostable organic radicals as contrast agent (CA) for magnetic resonance imaging (MRI). Such metal-free, organic-based CAs will provide a breakthrough in contrast-enhanced MRI while avoiding the risk associated with toxic metal ions. While the agents would especially benefit patients with impaired kidney function, who are at increased risk of developing nephrogenic systemic fibrosis (NSF) following administration of paramagnetic gadolinium chelates (GBCAs) for MRI procedures, the new agents may also benefit the general population. The objective of the proposed work is to synthesize organic radical contrast agents (ORCAs) that provide high quality in vivo MR images. The long-standing obstacle in the development of a practical ORCA for MRI is the design and synthesis of paramagnetic organic compounds of moderate molecular size that possess sufficiently long in vivo lifetime, high 1H water relaxivity (r1), and high water solubility. We propose a design strategy to prepare ORCAs from nitroxide radicals that are highly resistant to reduction and exploit prior knowledge about dendrimers and PEGylation, particularly their applications to drug design, to achieve agents with optimized MRI properties. The proposed project has three specific aims. Specific aim 1: optimize 1H water relaxivity r1 (at 3 Tesla) and hydrophilicity of ORCAs to achieve molecular r1 = 10 mMs, at least twice that of the clinical CAs (molecular r1 ≈ 5 mM‾¹s‾¹). The ORCAs are derived from nitroxides conjuated to a generation 4 polypropylenimine (PPI-G4) dendrimer through PEGylation with long linear and/or branched PEG chains. The proposed work will provide a foundation for specific aim 2: optimize molecular size of the ORCAs with molecular r1 ≥ 10 mM‾¹s‾¹ to achieve agents with th radii 1 nm ≤ rSE ≤ 2 nm that would undergo efficient renal filtration/excretion. In vitro toxicty and water relaxivity of the ORCAs, as well as the properties that affect molecular r1, such as rotational correlation time and electron spin relaxation time, will be determined, to identify the four best ORCAs. Specific aim 3: evaluate effectiveness of ORCAs for in vivo MRI. Lowest effective dose, time course, and tumor vascular permeability (transfer constant Ktrans) for the four selected ORCAs will be determined at the clinical field of 3 Tesla by MRI of normal mice and by dynamic contrast enhanced (DCE) MRI of mice bearing tumor xenografts. Preliminary toxicity of the ORCA will be evaluated by histological examination. The most effective ORCA will be selected, labeled with a fluorescence probe, and then subjected to ex vivo biodistribution studies by electron paramagnetic resonance and fluorescence spectroscopy. These studies will determine suitability of ORCA for further development towards human use. Development of metal-free MRI contrast agents based on biostable organic radicals will expand the frontier of biomedical imaging. Modularity of organic synthesis enables efficient structural modifications for specific property tuning (redox, solubility, and toxicity). Such organic-based agents will improve early detection and diagnosis of diseases.