Click chemistry: a transformative technology in nuclear medicine

Abstract

The 2022 Nobel Prize in Chemistry was awarded to Professors K. Barry Sharpless, Morten Meldal and Carolyn Bertozzi for their pioneering roles in the advent of click chemistry. Sharpless and Meldal worked to develop the canonical click reaction-the copper-catalyzed azide-alkyne cycloaddition-while Bertozzi opened new frontiers with the creation of the bioorthogonal strain-promoted azide-alkyne cycloaddition. These two reactions have revolutionized chemical and biological science by facilitating selective, high yielding, rapid and clean ligations and by providing unprecedented ways to manipulate living systems. Click chemistry has affected every aspect of chemistry and chemical biology, but few disciplines have been impacted as much as radiopharmaceutical chemistry. The importance of speed and selectivity in radiochemistry make it an almost tailor-made application of click chemistry. In this Perspective, we discuss the ways in which the copper-catalyzed azide-alkyne cycloaddition, the strain-promoted azide-alkyne cycloaddition and a handful of 'next-generation' click reactions have transformed radiopharmaceutical chemistry, both as tools for more efficient radiosyntheses and as linchpins of technologies that have the potential to improve nuclear medicine.

Figure 1.

Schematic of (A) the copper-catalyzed azide-alkyne cycloaddition (CuAAC) and (B) the strain-promoted azide-alkyne cycloaddition (SPAAC). Magenta spheres represent cargoes.

Figure 2.

The use of the CuAAC ligation to (A) synthesize an integrin-targeted multimodal PET/NIRF imaging agent, (B) increase the specific activity of ¹⁸F-labeled peptides, (C) radiolabel a vector with a [^99mTc]Tc(CO)₃ core via the ‘click-to-chelate’ strategy, and (D) create an ²¹¹At-labeled probe in which the radiohalogen is appended to the triazole moiety. Red and purple spheres represent cargoes.

Figure 3.

(A) The structure of [⁶⁸Ga]Ga-Trivehexin; (B) PET images acquired 120 minutes after the administration of 105 MBq of [⁶⁸Ga]Ga-Trivhexin to a patient with α_vβ₆-expressing pancreatic ductal adenocarcinoma showing uptake in a primary tumor as well as several metastatic lesions. Figure 3B was reprinted under the Creative Commons Attribution 4.0 International License (CC BY 4.0) terms from reference 22.

Figure 4.

The use of the SPAAC ligation to create (A) ¹⁸F-labeled GRPR-targeting peptides, (B) ⁶⁴Cu-labeled nanoparticles, and (C) ⁶⁴Cu-labeled somatostatin receptor-targeting peptides.

Figure 5.

Four approaches that harness the SPAAC ligation to facilitate the site-specific bioconjugation of antibodies. In each case, the site-specificity is predicated on (A) the incorporation of the unnatural amino acid N^ε−2-azideoethyloxycarbonyl-L-lysine, (B) the incorporation of the unnatural amino acid p-azidomethyl phenylalanine, (C) the chemoenzymatic manipulation of the heavy chain glycans to append azide-modified sugars, and (D) the selective ligation of a branched, azide-containing perfluorophenyl ester with the K188 residues of the light chain.

Figure 6.

(A) The traceless Staudinger ligation; (B) ¹⁸F-labeling via the sydnone-alkyne cycloaddition; (C) ¹⁸F-labeling via sulfur-fluoride exchange (SuFEx) chemistry; (D) the bioconjugation of a [⁶⁸Ga]Ga-DOTA-moiety via the RIKEN reaction. Colored spheres represent cargoes.

Figure 7.

(A) Schematic of the inverse electron-demand Diels-Alder (IEDDA) ligation; (B) schematic of the method devised by Ploegh and coworkers that harnesses [¹⁸F]FDG and the IEDDA ligation for the site-specific radiolabeling of antibody fragments. Orange and blue spheres represent cargoes.

Figure 8.

(A) Schematic of in vivo pretargeting based on the IEDDA ligation; (B) structure of [²²⁵Ac]Ac-DOTA-PEG₇-Tz; (C) timeline of an approach to theranostic pretargeting that leverages a ⁶⁴Cu-labeled tetrazine for PET imaging and a ⁶⁷Cu-labeled tetrazine for targeted radionuclide therapy (top) as well as the relationship between the image-derived cumulative activity of the ⁶⁴Cu-labeled tetrazine in the tumor and the tumor’s response to therapy with the ⁶⁷Cu-labeled tetrazine (bottom). Figure 8A is an original figure created with BioRender.com. Figure 8C was reprinted under the Creative Commons Attribution-NonCommercial-NoDerivatives License 4.0 (CC BY-NC-ND 4.0) terms of reference 46.