Abstract
The increasing application of positron emission tomography (PET) in nuclear medicine has stimulated the extensive development of a multitude of novel and versatile bioorthogonal conjugation techniques especially for the radiolabeling of biologically active high molecular weight compounds like peptides, proteins or antibodies. Taking into consideration that the introduction of fluorine-18 (t1/2 = 109.8 min) proceeds under harsh conditions, radiolabeling of these biologically active molecules represents an outstanding challenge and is of enormous interest. Special attention has to be paid to the method of 18F-introduction. It should proceed in a regioselective manner under mild physiological conditions, in an acceptable time span, with high yields and high specific activities. For these reasons and due to the high number of functional groups found in these compounds, a specific labeling procedure has to be developed for every bioactive macromolecule. Bioorthogonal strategies including the Cu-assisted Huisgen cycloaddition and its copper-free click variant, both Staudinger Ligations or the tetrazine-click reaction have been successfully applied and represent valuable alternatives for the selective introduction of fluorine-18 to overcome the afore mentioned obstacles. This comprehensive review deals with the progress and illustrates the latest developments in the field of bioorthogonal labeling with the focus on the preparation of radiofluorinated building blocks and tracers for molecular imaging.
Highlights
Positron emission tomography (PET) is an outstanding instrument for the quantification and localization of physiological as well as pathophysiological activities and processes in vivo which were analyzed by tracing the appropriate biochemical fundamentals [1]
The basics of PET rely on the coincidental detection of annihilation photons in nearly 180° originating from positrons of the parent radionuclide inside the tracer [2]
Tracers for PET imaging purposes are restricted by the kind of fluorinated molecules that researchers can prepare
Summary
Positron emission tomography (PET) is an outstanding instrument for the quantification and localization of physiological as well as pathophysiological activities and processes in vivo which were analyzed by tracing the appropriate biochemical fundamentals [1]. Bioorthogonal reactions in general, and click chemistry in particular, are generic terms for a set of labeling reactions, which make use of several selective and modular building blocks and enable chemoselective ligations to radiolabel biologically relevant compounds. In this context, the Cu(I)-mediated triazole formation from azides and terminal alkynes according to the 1,3-dipolar Huisgen cycloaddition is a powerful ligation reaction, due to its high degree of specificity and the biocompatibility of both starting materials. Click chemistry is a very attractive approach for the design and synthesis of novel potent radiotracers for molecular imaging purposes
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