A Polar18F-Labeled Amino Acid Derivative for Click Labeling of Biomolecules

Comparison Study of Two Differently Clicked 18F-Folates-Lipophilicity Plays a Key Role

Within the last decade, several folate-based radiopharmaceuticals for Single Photon Emission Computed Tomography (SPECT) and Positron Emission Tomography (PET) have been evaluated; however, there is still a lack of suitable ¹⁸F-folates for clinical PET imaging. Herein, we report the synthesis and evaluation of two novel ¹⁸F-folates employing strain-promoted and copper-catalyzed click chemistry. Furthermore, the influence of both click-methods on lipophilicity and pharmacokinetics of the ¹⁸F-folates was investigated. ¹⁸F-Ala-folate and ¹⁸F-DBCO-folate were both stable in human serum albumin. In vitro studies proved their high affinity to the folate receptor (FR). The lipophilic character of the strain-promoted clicked ¹⁸F-DBCO-folate (logD = 0.6) contributed to a higher non-specific binding in cell internalization studies. In the following in vivo PET imaging studies, FR-positive tumors could not be visualized in a maximum intensity projection images. Compared with ¹⁸F-DBCO-folate, ¹⁸F-Ala-folate (logD = −1.4), synthesized by the copper-catalyzed click reaction, exhibited reduced lipophilicity, and as a result an improved in vivo performance and a clear-cut visualization of FR-positive tumors. In view of high radiochemical yield, radiochemical purity and favorable pharmacokinetics, ¹⁸F-Ala-folate is expected to be a promising candidate for FR-PET imaging.

Fluorine-18 labelled building blocks for PET tracer synthesis

This review presents a comprehensive overview of the synthesis and application of fluorine-18 labelled building blocks since 2010.

Small Molecule Radiopharmaceuticals - A Review of Current Approaches

Radiopharmaceuticals are an integral component of nuclear medicine and are widely applied in diagnostics and therapy. Though widely applied, the development of an “ideal” radiopharmaceutical can be challenging. Issues such as specificity, selectivity, sensitivity, and feasible chemistry challenge the design and synthesis of radiopharmaceuticals. Over time, strategies to address the issues have evolved by making use of new technological advances in the fields of biology and chemistry. This review presents the application of few advances in design and synthesis of radiopharmaceuticals. The topics covered are bivalent ligand approach and lipidization as part of design modifications for enhanced selectivity and sensitivity and novel synthetic strategies for optimized chemistry and radiolabeling of radiopharmaceuticals.

A 18F-labeled dibenzocyclooctyne (DBCO) derivative for copper-free click labeling of biomolecules

The new prosthetic group ¹⁸F-TEG-DBCO (dibenzocyclooctyne) can be prepared within a total reaction time of 60 min including purification with an overall yield (n.d.c.) of 34 ± 5%. Copper-free click cycloadditions with various biomolecule-azides resulted in very high RCYs under mild conditions.

18F-labeling using click cycloadditions

Due to expanding applications of positron emission tomography (PET) there is a demand for developing new techniques to introduce fluorine-18 (t_1/2 = 109.8 min). Considering that most novel PET tracers are sensitive biomolecules and that direct introduction of fluorine-18 often needs harsh conditions, the insertion of ¹⁸F in those molecules poses an exceeding challenge. Two major challenges during ¹⁸F-labeling are a regioselective introduction and a fast and high yielding way under mild conditions. Furthermore, attention has to be paid to functionalities, which are usually present in complex structures of the target molecule. The Cu-catalyzed azide-alkyne cycloaddition (CuAAC) and several copper-free click reactions represent such methods for radiolabeling of sensitive molecules under the above-mentioned criteria. This minireview will provide a quick overview about the development of novel ¹⁸F-labeled prosthetic groups for click cycloadditions and will summarize recent trends in copper-catalyzed and copper-free click ¹⁸F-cycloadditions.