A novel design platform for diaCEST agents

摘要 磁共振成像（MRI）是最重要的临床成像方式之一，因为它可以 以非侵入性方式提供软组织的高分辨率图像。钆对比剂增强 通过缩短组织水质子的T1和T2弛豫时间来提高图像对比度。或者，图像对比度 可以使用化学交换饱和转移（CEST）机制产生。CEST要求存在 具有不同NMR化学位移的至少两个缓慢交换的质子池，并通过 将饱和1H自旋从小池（试剂）转移到大池（组织）水池。关键参数 在CEST中，交换速率常数（kex）和交换的化学位移（频率）分离 池（Δω）。CEST要求慢交换条件，（kex ≤ Δω）。CEST试剂可以是外源性的或内源性的 抗磁性分子（diaCEST）或具有不稳定质子的顺磁性金属络合物（paraCEST）。ParaCEST 智能体表现出较大的频差，很容易满足CEST的要求，但他们有潜力 体内金属释放导致的毒性。DiaCEST试剂因其化学性质而不含金属，但其化学性质 DIaCEST试剂与本体水的转移分离通常小于5 ppm。这个小频率 分离对目前的diaCEST试剂造成了严重的限制，例如由于光谱特性而导致的选择性差。 与其他交换质子共振重叠，来自组织背景磁化的强干扰 转移和直接水饱和度。在这里，我们建议开发一种新的diaCEST平台技术， 通过依赖于缓慢交换、高度低场移位来克服当前DIaCEST代理的缺点， (15 ppm）存在于结构受限的单质子化的围萘衍生物中的质子。 拟定药物的合成和体外表征将在具体目标1中完成。目标 具体目标2的目的是证明这些探针在小鼠成像实验中的体内适用性。的 所提出的代理将提供优于现有CEST代理的几个优点包括改进的CEST效率， 高选择性，不受蛋白质和脂肪的干扰，由于组织较弱，更容易进行数据分析 磁化传递效应和不对称性，显著降低或消除了直接水饱和度，pH- 独立的CEST效应和由于不含金属的组合物而改善的安全性。该平台将允许 设计和合成非特异性试剂，而不受pH值的干扰， 通过对萘骨架的修饰来实现靶向和响应性MR剂。

Abstract Magnetic resonance imaging (MRI) is one of the most important clinical imaging modalities because it can provide high resolution images of soft tissues in a non-invasive manner. Gadolinium contrast media enhance image contrast by shortening the T1 and T2 relaxation times of tissue water protons. Alternatively, image contrast can be generated using chemical exchange saturation transfer (CEST) mechanism. CEST requires the existence of at least two slowly exchanging pools of protons with different NMR chemical shifts, and generates contrast by transferring saturated 1H spins from the small pool (agent) to the bulk (tissue) water pool. The key parameters in CEST are the exchange rate constant (kex) and the chemical shift (frequency) separation of the exchanging pools (Δω). CEST requires slow exchange condition, (kex ≤ Δω). CEST agents can be exogenous or endogenous diamagnetic molecules (diaCEST) or paramagnetic metal complexes (paraCEST) with labile protons. ParaCEST agents exhibit large frequency difference that easily satisfies the requirement for CEST but they have potential toxicity due to in vivo metal release. DiaCEST agents are metal free by their chemical nature but the chemical shift separation of diaCEST agents is usually less than 5 ppm from the bulk water. This small frequency separation poses severe limitations on current diaCEST agents such as poor selectivity due to the spectral overlap with other exchanging proton resonances, strong interference from tissue background magnetization transfer and direct water saturation. Here we propose to develop a new diaCEST platform technology that would overcome the shortcomings of current diaCEST agents by relying on slowly exchanging, highly downfield shifted (15 ppm from water) protons present in structurally constrained monoprotonated peri-naphthalene derivatives. Synthesis and in vitro characterization of the proposed agents will be accomplished in Specific Aim 1. The goal of Specific Aim 2 is to demonstrate the in vivo applicability of these probes in imaging experiments in mice. The proposed agents would offer several advantages over existing CEST agents including improved CEST efficiency, high selectivity without interference from proteins and fats, easier data analysis due to weaker tissue magnetization transfer effect and asymmetry, significantly reduced or eliminated direct water saturation, pH- independent CEST effect and improved safety profile due to metal free composition. This platform would allow the design and synthesis of non-specific agents without the confounding effect of pH as well as the construction of targeted and responsive MR agents through the modification of the naphthalene backbone.