Fluorinated Paramagnetic Probes for Magnetic Resonance Imaging and Spectroscopy

含氟化合物在化学和生物学方面的研究很有吸引力，因为19-F核非常容易用核磁共振技术进行研究。其高的核磁共振灵敏度和大的化学位移范围，加上在纤维素和活体中几乎为零的背景信号，使19-F核磁共振研究本质上非常吸引人。一些报告描述了使用19-F磁共振波谱来跟踪19-F标记药物的命运，例如抗癌药物5-氟尿嘧啶的代谢和药代动力学。此外，已经发表了几项研究，定义了可能与大生物分子相联系的全氟化化合物，从而能够跟踪结合物，例如在肌动蛋白聚合过程中监测标记的肌动蛋白或肌球蛋白。与放射性标记化合物相比，核磁共振报告的主要优势是货架期长，缺乏放射性，能够区分起始化合物和从它们衍生出来的产品。事实上，使用现代成像方法(化学位移成像)，频率选择性方法的使用只允许监测具有特定化学位移的化合物，这种方法称为化学位移成像(CSI)。这些最近的CSI研究的一个限制方面是，观察氟化底物需要高浓度。这一限制在19-F光谱学研究中不是什么问题。这项工作的另一个关键限制特征是19-F核的缓慢弛豫时间，特别是对于三氟基团，其中弛豫时间约为几秒；这限制了在给定时间段内可以获得的扫描次数，因为需要等待较长的重复时间。由于信号/噪声随采集的扫描次数的平方根而变化，因此松弛速率需要增加数量级。这个问题的一个解决方案是将19-F标记放在一个稳定的大环络合物中，靠近顺磁性的稀土离子，从而使观察到的完整的19-F原子核的驰豫时间大大缩短。一个明显的类比是Gd质子磁共振造影剂，自1988年以来，相关的大环络合物已用于临床，用于数千万次磁共振扫描；它们在体内的浓度高达0.2mMkg-1时具有良好的耐受性。在新的氟标记的络合物中，稀土(Ln)离子将是不同的，允许引入强弛豫离子(例如，Dy或Tb)或引起大的偶极移动的Ln离子，但不会太大地扩大观察到的共振，例如Yb和Eu。各种新的含有一个或多个19-F标记的顺磁性稀土配合物将被制备出来，并评估它们在体外、活细胞内以及最终在体内进行的光谱和成像研究的有效性。还将评估使用这种方法跟踪基因表达的可能性。

Studies of compounds containing fluorine in chemistry and biology are attractive because the 19-F nucleus is very amenable to study by nuclear magnetic resonance (NMR) techniques. Its high NMR sensitivity and large chemical shift range , accompanied by a near-zero background signal in cellulo and in vivo, make 19-F NMR studies intrinsically very attractive. Several reports have described the use of 19-F magnetic resonance spectroscopy to track the fate of 19-F labelled drugs, e.g. the metabolism and pharmacokinetics of the anti-cancer drug 5-fluorouracil. Moreover, several studies have been published defining perfluorinated compounds that may be linked to large biomolecules, allowing the tracking of the conjugate, e.g. monitoring labelled actin or myosin during the polymerisation of actin . Key advantages of NMR reporters over radiolabelled compounds are the long shelf-life, lack of radioactivity and the ability to distinguish between starting compounds and products derived from them. Indeed, using a modern imaging method (chemical shift imaging) the use of frequency selective methods allows only those compounds with a particular chemical shift to be monitored, in a method called chemical shift imaging (CSI).A limiting aspect of these recent CSI studies is that high concentrations are required to observe the fluorinated substrates. This restriction is less of an issue in 19-F spectrosocpy studies. Another critical limiting feature of this work is the slow relaxation time of the 19-F nucleus, especially for trifluoro groups, where relaxation times are of the order of seconds; this limits the number of scans that can be acquired in a given period, owing to the need to wait for a long repetition time. As signal/noise varies with the square root of the number of scans acquired, an order of magnitude increase in the rate of relaxation is required. A solution to this problem is to place the 19-F label close to a paramagnetic lanthanide ion, in a stable macrocyclic complex, leading to much shorter relaxation times for the integral 19-F nucleus to be observed. An obvious analogy is with Gd proton MRI contrast agents, for which related macrocyclic complexes have been used clinically since 1988, in tens of millions of MR scans; they are well tolerated in the body at concentrations of up 0.2 mMkg-1. The lanthanide (Ln) ion will be varied in the new F-labelled complexes, allowing the introduction either of strongly relaxing ions (e.g. dysprosium or terbium) or of Ln ions that cause large dipolar shifts, but do not broaden the observed resonance too much, e.g. ytterbium and europium. Various new paramagnetic lanthanide complexes will be prepared containing one or more 19-F labels, and their utility assessed for spectroscopic and imaging studies evaluated in vitro, inside living cells and ultimately, in vivo. The possibility of using this methodology to track gene expression will also be assessed.