Novel chemoselective (18)F-radiolabeling of thiol-containing biomolecules under mild aqueous conditions

Radiolabeled Affibody Molecules for PET Imaging

Owing to their ease of engineering and production, chemical stability, size, and high target affinity and specificity, radiolabeled affibody molecules have been recognized as very promising molecular imaging probes in both preclinical and clinical settings. Herein we describe the methods for the preparation of affibody-chelator conjugates and their subsequent radiolabeling with 18F-AlF, 68Ga, 89Zr.

Fluorine-18 and Radiometal Labeling of Biomolecules via Disulfide Rebridging

Biomolecules labeled with positron-emitting radionuclides like fluorine-18 or radiometals like copper-64 and zirconium-89 are increasingly employed in nuclear medicine for diagnosis purposes. Given the fragility and complexity of these compounds, their labeling requires mild conditions. Besides, it is essential to develop methods inducing minimal modification of the tertiary structure, as it is fundamental for the biological activity of such complex entities. Given these requirements, disulfide rebridging represents a promising possibility since it allows protein modification as well as conservation of the tertiary structure. In this context, we have developed an original radiofluorinated dibromopyridazine dione prosthetic group for labeling of disulfide-containing biomolecules via rebridging. We employed it to radiolabel octreotide, a somatostatin analogue, and to radiolabel fragment antigen binding (Fab) targeting programmed death-ligand 1 (PD-L1), whose properties were then evaluated in vitro and in vivo by positron emission tomography (PET) imaging. We next extended our strategy to the radiolabeling of cetuximab, a monoclonal antibody, with various radiometals commonly used in PET imaging (zirconium-89, copper-64) by developing various rebridging molecules bearing the appropriate chelators. The stabilities of the radiolabeled antibody conjugates were assessed in biological conditions.

Synthesis of 4-Deoxy-4-Fluoro-d-Sedoheptulose: A Promising New Sugar to Apply the Principle of Metabolic Trapping

Fluorinated carbohydrates are important tools for understanding the deregulation of metabolic fluxes and pathways. Fluorinating specific positions within the sugar scaffold can lead to enhanced metabolic stability and subsequent metabolic trapping in cells. This principle has, however, never been applied to study the metabolism of the rare sugars of the pentose phosphate pathway (PPP). In this study, two fluorinated derivatives of d-sedoheptulose were designed and synthesized: 4-deoxy-4-fluoro-d-sedoheptulose (4DFS) and 3-deoxy-3-fluoro-d-sedoheptulose (3DFS). Both sugars are taken up by human fibroblasts but only 4DFS is phosphorylated. Fluorination of d-sedoheptulose at C-4 effectively halts the enzymatic degradation by transaldolase and transketolase. 4DFS thus has a high potential as a new PPP imaging probe based on the principle of metabolic trapping. Therefore, the synthesis of potential radiolabeling precursors for 4DFS for future radiofluorinations with fluorine-18 is presented.

Rapid 18F-labeling via Pd-catalyzed S-arylation in aqueous medium

We report radiolabeling of thiol-containing substrates via Pd-catalyzed S-arylation with 2-[¹⁸F]fluoro-5-iodopyridine, which is readily accessible using the "minimalist" radiofluorination method. The practicality of the procedure was confirmed by preparation of a novel PSMA-specific PET-tracer as well as labeling of glutathione, Aβ oligomer-binding RD2 peptide, bovine serum albumin and PSMA I&S.

Synthesis and application of a thiol-reactive HBED-type chelator for development of easy-to-produce Ga-radiopharmaceutical kits and imaging probes

In radiopharmaceutical syntheses, maleimide is commonly used for linking thiol-bearing bioactive molecules to metal-complexing ligands (chelators). However, due to instability of the resulting linkage, phenyloxadiazolyl methylsulfone (PODS) was developed as an alternative to maleimide. This coupling strategy has never been attempted with HBED which is a powerful chelator for gallium-radiolabeling especially at ambient temperature. Here we present HBED-CC-PODS as a bifunctional chelator scaffold for the site-selective conjugation of thiol-bearing vectors and [68Ga]Ga-radiolabeling.

The emerging role of radionuclide molecular imaging of HER2 expression in breast cancer

Targeting of human epidermal growth factor type 2 (HER2) using monoclonal antibodies, antibody-drug conjugates and tyrosine kinase inhibitors extends survival of patients with HER2-expressing metastatic breast cancer. High expression of HER2 is a predictive biomarker for such specific treatment. Accurate determination of HER2 expression level is necessary for stratification of patients to targeted therapy. Non-invasive in vivo radionuclide molecular imaging of HER2 has a potential of repetitive measurements, addressing issues of heterogeneous expression and conversion of HER2 status during disease progression or in response to therapy. Imaging probes based of several classes of targeting proteins are currently in preclinical and early clinical development. Both preclinical and clinical data suggest that the most promising are imaging agents based on small proteins, such as single domain antibodies or engineered scaffold proteins. These agents permit a very specific high-contrast imaging at the day of injection.

One-Step Synthesis of [18F]Fluoro-4-(vinylsulfonyl)benzene: A Thiol Reactive Synthon for Selective Radiofluorination of Peptides

Radiolabeled peptide-based molecular imaging probes exploit the advantages of large biologics and small molecules, providing both exquisite selectivity and favorable pharmacokinetic properties. Here, we report an operationally simple and broadly applicable approach for the ¹⁸F-fluorination of unprotected peptides via a new radiosynthon, [¹⁸F]fluoro-4-(vinylsulfonyl)benzene. This reagent demonstrates excellent chemoselectivity at the cysteine residue and rapid ¹⁸F-labeling of a diverse scope of peptides to generate stable thioether constructs.

Thiol-Reactive PODS-Bearing Bifunctional Chelators for the Development of EGFR-Targeting [18F]AlF-Affibody Conjugates

Site-selective bioconjugation of cysteine-containing peptides and proteins is currently achieved via a maleimide–thiol reaction (Michael addition). When maleimide-functionalized chelators are used and the resulting bioconjugates are subsequently radiolabeled, instability has been observed both during radiosynthesis and post-injection in vivo, reducing radiochemical yield and negatively impacting performance. Recently, a phenyloxadiazolyl methylsulfone derivative (PODS) was proposed as an alternative to maleimide for the site-selective conjugation and radiolabeling of proteins, demonstrating improved in vitro stability and in vivo performance. Therefore, we have synthesized two novel PODS-bearing bifunctional chelators (NOTA-PODS and NODAGA-PODS) and attached them to the EGFR-targeting affibody molecule Z_EGFR:03115. After radiolabeling with the aluminum fluoride complex ([¹⁸F]AlF), both conjugates showed good stability in murine serum. When injected in high EGFR-expressing tumor-bearing mice, [¹⁸F]AlF-NOTA-PODS-Z_EGFR:03115 and [¹⁸F]AlF-NODAGA-PODS-Z_EGFR:03115 showed similar pharmacokinetics and a specific tumor uptake of 14.1 ± 5.3% and 16.7 ± 4.5% ID/g at 1 h post-injection, respectively. The current results are encouraging for using PODS as an alternative to maleimide-based thiol-selective bioconjugation reactions.

Affibody Molecules as Targeting Vectors for PET Imaging

Affibody molecules are small (58 amino acids) engineered scaffold proteins that can be selected to bind to a large variety of proteins with a high affinity. Their small size and high affinity make them attractive as targeting vectors for molecular imaging. High-affinity affibody binders have been selected for several cancer-associated molecular targets. Preclinical studies have shown that radiolabeled affibody molecules can provide highly specific and sensitive imaging on the day of injection; however, for a few targets, imaging on the next day further increased the imaging sensitivity. A phase I/II clinical trial showed that ⁶⁸Ga-labeled affibody molecules permit an accurate and specific measurement of HER2 expression in breast cancer metastases. This paper provides an overview of the factors influencing the biodistribution and targeting properties of affibody molecules and the chemistry of their labeling using positron emitters.

One-step 18F-fluorination of smart positron emission tomography tracer for sensing furin activity in tumors

INTRODUCTION

Peptide analogues have attracted considerable attention in the field of developing novel positron emission tomography (PET) imaging agents due to their unique properties. Nevertheless, the complicated radiolabeling process and fast metabolism usually pose challenges to the clinical applications of peptide-based molecular probes. Herein a novel PET tracer containing a specific peptide sequence Arg-Val-Arg-Arg (RVRR), Acetyl-Arg-Val-Arg-Arg-Cys(StBu)-Gly(AMB[¹⁸F]F₃)-CBT ([¹⁸F]1), was designed and radiosynthesized using a simple and convenient one-step ¹⁸F-fluorination procedure. The smart tracer can be activated by the protease furin and then undergoes an intermolecular cyclization reaction in tumor cells, leading to improved PET imaging efficiency of tumor.

METHODS

The radiosynthesis of the target tracer [¹⁸F]1 and the control tracer [¹⁸F]1-ctrl was performed under facile conditions in pyridazine-HCl buffer (pH~2.5) at 80 °C within 30 min. The enzyme-controlled condensation was studied for non-radioactive compound 1 in the human breast cancer cell lysates (MDA-MB-468). The cellular uptake of [¹⁸F]1 and [¹⁸F]1-ctrl was studied and compared by measuring the activity in MDA-MB-468 cells using a γ-counter after incubation with 37 kBq of [¹⁸F]1 or [¹⁸F]1-ctrl, respectively. In vivo behavior of [¹⁸F]1 was examined through PET imaging of MDA-MB-468 tumor-bearing mice and compared with that of [¹⁸F]1-ctrl as well as that of [¹⁸F]1 co-injected with non-radioactive compound 1.

RESULTS

The tracer [¹⁸F]1 was obtained with a high radiochemical yield (RCY) of 42.5 ± 1.47% and an excellent radiochemical purity (RCP > 99%). Under the activation of furin and GSH, the tracer suffered a condensation reaction to form dimers and then self-assembled into nanoparticles to produce enduring signal. The cellular uptake of [¹⁸F]1 and [¹⁸F]1-ctrl was determined to be 10.2 ± 0.37 and 1.19 ± 0.25%ID at 120 min, respectively. For in vivo PET imaging, [¹⁸F]1 exhibited the optimum tumor uptake of 2.39 ± 0.31%ID/g and the tumor-to-muscle uptake ratio of 2.93 ± 0.92 at 10 min post injection. Co-injection of [¹⁸F]1 and non-radioactive compound 1 produced a high tumor uptake ranging from 2.83 ± 0.23%ID/g to 3.40 ± 0.18%ID/g at 10 min and 60 min post injection, respectively.

CONCLUSIONS

The one-step labeling method of tracer [¹⁸F]1 showed advantage in simplifying the radiolabeling process with high RCY, which could enable a real kit process for the synthesis of ¹⁸F-radiopharmaceuticals and was significant for the large-scale production of tracers for clinical applications. PET imaging results suggested that the tracer [¹⁸F]1 had good tumor uptake and the co-injection strategy of [¹⁸F]1 with 1 could enhance the imaging signal in tumor.

Influence of Residualizing Properties of the Radiolabel on Radionuclide Molecular Imaging of HER3 Using Affibody Molecules

Human epidermal growth factor receptor type 3 (HER3) is an emerging therapeutic target in several malignancies. To select potential responders to HER3-targeted therapy, radionuclide molecular imaging of HER3 expression using affibody molecules could be performed. Due to physiological expression of HER3 in normal organs, high imaging contrast remains challenging. Due to slow internalization of affibody molecules by cancer cells, we hypothesized that labeling (HE)₃-Z_HER3:08698-DOTAGA affibody molecule with non-residualizing [¹²⁵I]-N-succinimidyl-4-iodobenzoate (PIB) label would improve the tumor-to-normal organs ratios compared to previously reported residualizing radiometal labels. The [¹²⁵I]I-PIB-(HE)₃-Z_HER3:08698-DOTAGA was compared side-by-side with [¹¹¹In]In-(HE)₃-Z_HER3:08698-DOTAGA. Both conjugates demonstrated specific high-affinity binding to HER3-expressing BxPC-3 and DU145 cancer cells. Biodistribution in mice bearing BxPC-3 xenografts at 4 and 24 h pi showed faster clearance of the [¹²⁵I]I-PIB label compared to the indium-111 label from most tissues, except blood. This resulted in higher tumor-to-organ ratios in HER3-expressing organs for [¹²⁵I]I-PIB-(HE)₃-Z_HER3:08698-DOTAGA at 4 h, providing the tumor-to-liver ratio of 2.4 ± 0.3. The tumor uptake of both conjugates was specific, however, it was lower for the [¹²⁵I]I-PIB label. In conclusion, the use of non-residualizing [¹²⁵I]I-PIB label for HER3-targeting affibody molecule provided higher tumor-to-liver ratio than the indium-111 label, however, further improvement in tumor uptake and retention is needed.

18F-Trifluoromethanesulfinate Enables Direct C-H 18F-Trifluoromethylation of Native Aromatic Residues in Peptides

18F labeling strategies for unmodified peptides with [18F]fluoride require 18F-labeled prosthetics for bioconjugation more often with cysteine thiols or lysine amines. Here we explore selective radical chemistry to target aromatic residues applying C-H 18F-trifluoromethylation. We report a one-step route to [18F]CF3SO2NH4 from [18F]fluoride and its application to direct [18F]CF3 incorporation at tryptophan or tyrosine residues using unmodified peptides as complex as recombinant human insulin. The fully automated radiosynthesis of octreotide[Trp(2-CF218F)] enables in vivo positron emission tomography imaging.

Efficient 125I-radiolabeling of biomolecules using a strain-promoted oxidation-controlled cyclooctyne-1,2-quinone cycloaddition reaction

We report a novel 1,2-catechol based radioiodinated precursor for radioiodination of bicyclo[6.1.0]nonyne (BCN) installed biologically active molecules using a strain-promoted oxidation-controlled cyclooctyne-1,2-quinone cycloaddition reaction (SPOCQ) under ambient conditions. Compared to the reported methodologies, the new strategy demonstrates some clear advantages, including high in vitro and in vivo stability, high radiochemical yield, and exceptionally fast reaction kinetics at micro-molar concentration.

Chemoselective 18F-incorporation into pyridyl acyltrifluoroborates for rapid radiolabelling of peptides and proteins at room temperature

A new prosthetic group is reported for quantitative ¹⁸F-labelling of peptides and proteins based on the chemoselective ligation of potassium acyltrifluoroborates (KATs) and hydroxylamines without any detectable ¹⁸F/¹⁹F isotope exchange at the KAT moiety.

Recent Advances in Bioorthogonal Click Chemistry for Efficient Synthesis of Radiotracers and Radiopharmaceuticals

In recent years, several catalyst-free site-specific reactions have been investigated for the efficient conjugation of biomolecules, nanomaterials, and living cells. Representative functional group pairs for these reactions include the following: (1) azide and cyclooctyne for strain-promoted cycloaddition reaction, (2) tetrazine and trans-alkene for inverse-electron-demand-Diels–Alder reaction, and (3) electrophilic heterocycles and cysteine for rapid condensation/addition reaction. Due to their excellent specificities and high reaction rates, these conjugation methods have been utilized for the labeling of radioisotopes (e.g., radiohalogens, radiometals) to various target molecules. The radiolabeled products prepared by these methods have been applied to preclinical research, such as in vivo molecular imaging, pharmacokinetic studies, and radiation therapy of cancer cells. In this review, we explain the basics of these chemical reactions and introduce their recent applications in the field of radiopharmacy and chemical biology. In addition, we discuss the significance, current challenges, and prospects of using bioorthogonal conjugation reactions.

Arylation Chemistry for Bioconjugation

Bioconjugation chemistry has been used to prepare modified biomolecules with functions beyond what nature intended. Central to these techniques is the development of highly efficient and selective bioconjugation reactions that operate under mild, biomolecule compatible conditions. Methods that form a nucleophile-sp2 carbon bond show promise for creating bioconjugates with new modifications, sometimes resulting in molecules with unparalleled functions. Here we outline and review sulfur, nitrogen, selenium, oxygen, and carbon arylative bioconjugation strategies and their applications to modify peptides, proteins, sugars, and nucleic acids.

Synthesis and Bioconjugation of Thiol-Reactive Reagents for the Creation of Site-Selectively Modified Immunoconjugates

Maleimide-bearing bifunctional probes have been employed for decades for the site-selective modification of thiols in biomolecules, especially antibodies. Yet maleimide-based conjugates display limited stability in vivo because the succinimidyl thioether linkage can undergo a retro-Michael reaction. This, of course, can lead to the release of the radioactive payload or its exchange with thiol-bearing biomolecules in circulation. Both of these processes can produce elevated activity concentrations in healthy organs as well as decreased activity concentrations in target tissues, resulting in reduced imaging contrast and lower therapeutic ratios. In 2018, we reported the creation of a modular, stable, and easily accessible phenyloxadiazolyl methyl sulfone reagent - dubbed 'PODS' - as a platform for thiol-based bioconjugations. We have clearly demonstrated that PODS-based site-selective bioconjugations reproducibly and robustly create homogenous, well-defined, highly immunoreactive, and highly stable radioimmunoconjugates. Furthermore, preclinical experiments in murine models of colorectal cancer have shown that these site-selectively labeled radioimmunoconjugates exhibit far superior in vivo performance compared to radiolabeled antibodies synthesized via maleimide-based conjugations. In this protocol, we will describe the four-step synthesis of PODS, the creation of a bifunctional PODS-bearing variant of the ubiquitous chelator DOTA (PODS-DOTA), and the conjugation of PODS-DOTA to the HER2-targeting antibody trastuzumab.

Rapid nickel(ii)-promoted cysteine S-arylation with arylboronic acids

Nickel salts catalyze fast cysteine arylation with 2-nitroarylboronic acids. The process uses cheap, readily-available reagents and allows introduction of diverse chemical handles.

Development of a new thiol-reactive prosthetic group for site-specific labeling of biomolecules with radioactive iodine

In this report, we describe the radiosynthesis of a new thiol-targeting prosthetic group for efficient radioactive iodine labeling of biomolecules. Radioiodination using the precursor 3 was performed to obtain ¹²⁵I-labeled tetrazole 4b with high radiochemical yield (73%) and radiochemical purity. Using the radiolabeled 4b, a single free cysteine containing peptide and human serum albumin were labeled with ¹²⁵I in modest-to-good radiochemical yields (65-99%) under mildly reactive conditions. A biodistribution study of [¹²⁵I]7 in normal ICR mice exhibited lower thyroid uptake values than those of ¹²⁵I-labeled human serum albumin prepared via a traditional radiolabeling method. Thus, [¹²⁵I]7 could be employed as an effective radiotracer for molecular imaging and biodistribution studies. The results clearly demonstrate that 4b has the potential to be effectively implemented as a prosthetic group in the preparation of radiolabeled biomolecules.

Thiol-Reactive Bifunctional Chelators for the Creation of Site-Selectively Modified Radioimmunoconjugates with Improved Stability

Maleimide-bearing bifunctional chelators have been used extensively for the site-selective bioconjugation and radiolabeling of peptides and proteins. However, bioconjugates obtained using these constructs inevitably suffer from limited stability in vivo, a trait that translates into suboptimal activity concentrations in target tissues and higher uptake levels in healthy, nontarget tissues. To circumvent this issue, phenyloxadiazolyl methylsulfones have previously been reported as alternatives to maleimides for thiol-based ligations, but these constructs have scarcely been used in the field of radiochemistry. In this report, we describe the synthesis of two thiol-reactive bifunctional chelators for ⁸⁹Zr and ¹⁷⁷Lu based on a new, easy-to-make phenyloxadiazolyl methylsulfone reagent, PODS. Radioimmunoconjugates created using these novel bifunctional chelators displayed in vitro stability that was higher than that of their maleimide-derived cousins. More importantly, positron emission tomography imaging in murine models of cancer revealed that a ⁸⁹Zr-labeled radioimmunoconjugate created using a PODS-bearing bifunctional chelator produced a rate of uptake in nontarget tissues that is significantly lower than that of its analogous maleimide-based counterpart.

18F-Trifluoromethylation of Unmodified Peptides with 5-18F-(Trifluoromethyl)dibenzothiophenium Trifluoromethanesulfonate

The 18F-labeling of 5-(trifluoromethyl)-dibenzothiophenium trifluoromethanesulfonate, commonly referred to as the Umemoto reagent, has been accomplished applying a halogen exchange 18F-fluorination with 18F-fluoride, followed by oxidative cyclization with Oxone and trifluoromethanesulfonic anhydride. This new 18F-reagent allows for the direct chemoselective 18F-labeling of unmodified peptides at the thiol cysteine residue.

18 F-Labeling of Sensitive Biomolecules for Positron Emission Tomography

Positron emission tomography (PET) imaging study of fluorine-18 labeled biomolecules is an emerging and rapidly growing area for preclinical and clinical research. The present review focuses on recent advances in radiochemical methods for incorporating fluorine-18 into biomolecules via "direct" or "indirect" bioconjugation. Recently developed prosthetic groups and pre-targeting strategies, as well as representative examples in 18 F-labeling of biomolecules in PET imaging research studies are highlighted.

Synthesis of [18 F]Fluoroarenes by Nucleophilic Radiofluorination of N-Arylsydnones

A practical method for radiofluorination of anilines with [18 F]fluoride via N-arylsydnone intermediates is described. These precursors are stable, easy to handle and facilitate direct and regioselective 18 F-labeling to prepare [18 F]fluoroarenes. The value of this methodology is further highlighted by successful application to prepare an 18 F-labeled neuropeptide.

Small Prosthetic Groups in 18F-Radiochemistry: Useful Auxiliaries for the Design of 18F-PET Tracers

Prosthetic group (PG) applications in ¹⁸F-radiochemistry play a pivotal role among current ¹⁸F^-labeling techniques for the development and availability of ¹⁸F-labeled imaging probes for PET (Wahl, 2002) (¹). The introduction and popularization of PGs in the mid-80s by pioneers in ¹⁸F-radiochemistry has profoundly changed the landscape of available tracers for PET and has led to a multitude of new imaging agents based on simple and efficiently synthesized PGs. Because of the chemical nature of anionic ¹⁸F^- (apart from electrophilic low specific activity ¹⁸F-fluorine), radiochemistry before the introduction of PGs was limited to simple nucleophilic substitutions of leaving group containing precursor molecules. These precursors were not always available, and some target compounds were either hard to synthesize or not obtainable at all. Even with the advent of recently introduced "late-stage fluorination" techniques for the ¹⁸F-fluorination of deactivated aromatic systems, PGs will continue to play a central role in ¹⁸F-radiochemistry because of their robust and almost universal usability. The importance of PGs in radiochemistry is shown by its current significance in tracer development and exemplified by an overview of selected methodologies for PG attachment to PET tracer molecules. Especially, click-chemistry approaches to PG conjugation, while furthering the historical evolution of PGs in PET tracer design, play a most influential role in modern PG utilization. All earlier and recent multifaceted approaches in PG development have significantly enriched the contingent of modern ¹⁸F-radiochemistry procedures and will continue to do so.

Direct 11CN-Labeling of Unprotected Peptides via Palladium-Mediated Sequential Cross-Coupling Reactions

A practical procedure for ¹¹CN-labeling of unprotected peptides has been developed. The method was shown to be highly chemoselective for cysteine over other potentially nucleophilic residues, and the radiolabeled products were synthesized and purified in less than 15 min. Appropriate for biomedical applications, the method could be used on an extremely small scale (20 nmol) with a high radiochemical yield. The success of the protocol stems from the use of a Pd-reagent based on a dihaloarene, which enables direct "nucleophile-nucleophile" coupling of the peptide and [¹¹C]cyanide by temporal separation of nucleophile addition.

Irreversible Cysteine-Selective Protein Labeling Employing Modular Electrophilic Tetrafluoroethylation Reagents

Fluoroalkylation reagents based on hypervalent iodine are widely used to transfer fluoroalkyl moieties to various nucleophiles. However, the transferred groups have so far been limited to simple structural motifs. We herein report a reagent featuring a secondary amine that can be converted to amide, sulfonamide, and tertiary amine derivatives in one step. The resulting reagents bear manifold functional groups, many of which would not be compatible with the original synthetic pathway. Exploiting this structural versatility and the known high reactivity toward thiols, the new-generation reagents were used in bioconjugation with an artificial retro-aldolase, containing an exposed cysteine and a reactive catalytic lysine. Whereas commercial reagents based on maleimide and iodoacetamide labeled both sites, the iodanes exclusively modified the cysteine residue. The study thus demonstrates that modular fluoroalkylation reagents can be used as tools for cysteine-selective bioconjugation.

Click Chemistry and Radiochemistry: The First 10 Years

The advent of click chemistry has had a profound influence on almost all branches of chemical science. This is particularly true of radiochemistry and the synthesis of agents for positron emission tomography (PET), single photon emission computed tomography (SPECT), and targeted radiotherapy. The selectivity, ease, rapidity, and modularity of click ligations make them nearly ideally suited for the construction of radiotracers, a process that often involves working with biomolecules in aqueous conditions with inexorably decaying radioisotopes. In the following pages, our goal is to provide a broad overview of the first 10 years of research at the intersection of click chemistry and radiochemistry. The discussion will focus on four areas that we believe underscore the critical advantages provided by click chemistry: (i) the use of prosthetic groups for radiolabeling reactions, (ii) the creation of coordination scaffolds for radiometals, (iii) the site-specific radiolabeling of proteins and peptides, and (iv) the development of strategies for in vivo pretargeting. Particular emphasis will be placed on the four most prevalent click reactions-the Cu-catalyzed azide-alkyne cycloaddition (CuAAC), the strain-promoted azide-alkyne cycloaddition (SPAAC), the inverse electron demand Diels-Alder reaction (IEDDA), and the Staudinger ligation-although less well-known click ligations will be discussed as well. Ultimately, it is our hope that this review will not only serve to educate readers but will also act as a springboard, inspiring synthetic chemists and radiochemists alike to harness click chemistry in even more innovative and ambitious ways as we embark upon the second decade of this fruitful collaboration.