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

近年来，新兴的晚期放射性氟化方法显着提高了许多PET探针的可及性。然而，反应条件往往与敏感化合物不相容，如次级代谢物和蛋白质。此外，由于放射性标记底物的合成通常具有挑战性，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，药物代动力学的调节，如消除率和途径，血液白蛋白结合等）。在这种情况下，优选使用相同构建块制备不同放射性标记化合物的模块化方法。此外，虽然已经知道了许多含18f氟的积木，但其中只有少数是容易获得的。其中最关键的一点是大多数假体基团需要HPLC分离，增加了生产时间，降低了rcy，并且复杂的合成转移到遥控合成单元，阻碍了放射性示踪剂的cGMP生产和临床应用。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，旨在简化和加速新型PET探针的开发。因此，该应用程序的一个重点是应用已经建立的放射性标记构建块，如[18F]氟苯腈氧化物和[18F]氟苯醛。计划工作的另一个重要目标是开发具有多种反应性和偶联性的新型易于获取的假体基团。理想情况下，这些假体基团将通过SPE纯化分离，避免使用繁琐和耗时的HPLC纯化。为此目的，将使用简化的18f氟化方法，如“极简”和“盒上”方案，对高度活化的吡啶取代季铵盐前体进行放射性标记。高极性和带正电的onium标签使所制备的假体基通过固相萃取纯化变得简单。该项目的另一个重点是为未修饰肽和蛋白质的位点特异性放射性标记制定放射性标记方案。这里的新方法是通过将假基与n端、二硫键或肽主链以及Cys和Tyr残基偶联来引入18F标签。新型假体基团和新型放射性标记方案的实用性将通过制备临床相关探针以及标记模型肽和蛋白质来证实。重要的是，开发的协议应该转化为自动合成模块，最终能够应用新程序制备临床相关的示踪剂。

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.