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高分辨率软组织图像。Gd造影剂增强 通过缩短组织水质子的T1和T2弛豫时间来实现图像对比度。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需要存在 至少两个具有不同核磁共振化学位移的缓慢交换的质子池，并通过 将饱和的1H自旋从小水池(药剂)转移到大水池(组织水池)。关键参数 在CEST中是交换速率常数(Kex)和交换的化学位移(频率)分离 池(Δω)。CEST需要缓慢的交换条件(KEX≤Δω)。CEST药物可以是外源性的也可以是内源性的 具有不稳定质子的抗磁性分子(DiaCEST)或顺磁性金属络合物(ParCEST)。ParaCEST 药剂表现出很大的频差，很容易满足CEST的要求，但它们有潜力 体内金属释放所致的毒性。DiaCEST试剂的化学性质是不含金属的，但化学物质 DiaCEST药剂与散装水的移位分离度通常小于5ppm。这个小频率 分离对目前的diaCEST试剂构成了严重的限制，例如由于光谱的原因导致选择性较差 与其他交换质子共振重叠，受到组织背景磁化的强烈干扰 转移和直接含水饱和度。在这里，我们建议开发一种新的diaCEST平台技术，该技术将 通过依靠缓慢的交换、高度的下场转移来克服当前diaCEST代理的缺点 (15ppm来自水)存在于结构受限的单质子化的周萘衍生物中的质子。 建议的试剂的合成和体外表征将在特定的目标1完成。 具体目的2是证明这些探针在小鼠体内的成像实验中的适用性。这个 与现有的CEST代理相比，建议的代理将提供几个优势，包括提高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- 由于不含金属的成分，独立的CEST效应和改进的安全配置。这个平台将允许 无pH混杂影响的非特异性试剂的设计合成及结构 通过对萘主链的修饰，合成具有靶向性和响应性的核磁共振试剂。

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.