Extension of PET radiochemistry toolkit. Novel 18F-fluorinated building blocks and procedures for indirect labeling

近年来，新出现的晚期放射性氟化方法显著提高了许多PET探针的可及性。然而，反应条件往往与敏感化合物不相容，如次生代谢物和蛋白质。此外，由于放射性标记底物的合成往往具有挑战性，后期氟化方法并不总是很适合于PET示踪剂体内参数的优化(稳定性、靶标的选择性、药物动力学的调节，如消除速率和路线、血白蛋白结合等)。在这种情况下，使用相同的构建块来制备不同的放射性标记化合物的模块化方法是可取的。此外，尽管人们已经知道了大量的18F-氟化积木，但只有一小部分很容易获得。其中最关键的问题之一是需要对大多数假体基团进行高效液相分离，从而延长生产时间，降低RCys，并使合成转移到远程控制的合成单元中，阻碍了cGMP的生产和放射性示踪剂的临床应用。该项目的目的是进一步开发一种模块化(间接)的放射性氟化方法，目的是简化和加快新型PET探针的开发。因此，这一应用的一个重点是已经建立的放射性标记的积木的应用，如[18F]氟苯腈氧化物和[18F]氟苯甲醛。计划工作的另一个重要目标是开发具有多种反应性和耦合特性的新型易获得的假体基团。理想情况下，这些假体基团将通过固相萃取纯化来分离，避免了繁琐和耗时的高效液相色谱纯化。为此，简化的18F-氟化方法，如“极简法”和“盒上法”，将被用于高活性的吡啶取代季铵盐前体的放射性标记。高极性和带正电荷的标签将使制备的假体基团能够通过固相萃取进行简单的纯化。该项目的另一个重点是开发用于未修饰的多肽和蛋白质的特定部位放射性标记的放射性标记协议。在这里，对于新的方法，18F标记的引入是通过将人工基团与N-末端、二硫键或肽骨架以及与半胱氨酸和酪氨酸残基偶联来计划的。新的假体基团和新的放射性标记方案的实用性将通过临床相关探针的制备以及模型多肽和蛋白质的标记来证实。重要的是，应将开发的方案转化为自动合成模块，从而最终实现临床相关示踪剂制备的新程序的应用。

Emerging late-stage radiofluorination methods significantly improved the accessibility of numerous PET probes in recent years. However, reaction conditions are often incompatible with sensitive compounds, like secondary metabolites and proteins. Furthermore, owing to the often challenging synthesis of radiolabeling substrates, late-stage fluorination methods are not always well suited for the optimization of in vivo parameters of PET-tracers (stability, selectivity for the target, modulation of pharmacokinetics like elimination rate and route, blood albumin binding, etc.). In this case a modular approach using the same building blocks for the preparation of different radiolabeled compounds is preferable. Furthermore, whereas numerous 18F-fluorinated building blocks are already known only a few of them are easily accessible. One of the most critical points is the necessity of the HPLC isolation of the majority of prosthetic groups increasing the production time, lowering RCYs and complicating synthesis transfer into a remote-controlled synthesis unit impeding the cGMP production and clinical application of radiotracers. The purpose of this project is the further development of a modular (indirect) approach to radiofluorination with the aim to simplify and accelerate the development of the novel PET probes. Consequently, one focus of this application is the application of already established radiolabeled building blocks like [18F]fluorobenzonitrile oxides and [18F]fluorobenzaldehydes. Another important aim of planned work is to develop novel easily accessible prosthetic groups with versatile reactivities and coupling properties. Ideally these prosthetic groups will be isolated by SPE purification avoiding the application of cumbersome and time consuming HPLC purification. To this end simplified 18F-fluorination methods like the “minimalist” and “on-cartridge” protocols will be used for radiolabeling of the highly activated pyridine-substituted quaternary ammonium salt precursors. The highly polar and positively charged onium tag will enable the simple purification of the prepared prosthetic groups by solid phase extraction. A further emphasis of this project is the development of radiolabeling protocols for a site-specific radiolabeling of unmodified peptides and proteins. Here for the novel approaches the introduction of the 18F label is planned by coupling of the prosthetic group with the N-terminus, disulfide bonds or peptide backbone, as well as with Cys and Tyr residues. The practicality of the novel prosthetic groups and novel radiolabeling protocols will be confirmed by the preparation of clinically relevant probes as well as by labeling of model peptides and proteins. Importantly, the developed protocols should be translated into automated synthesis modules enabling finally the application of the novel procedures for the preparation of clinically relevant tracers.