Recent Advances in Bioorthogonal Click Chemistry for Enhanced PET and SPECT Radiochemistry

Click Chemistry for Biofunctional Polymers: From Observing to Steering Cell Behavior

Click chemistry has become one of the most powerful construction tools in the field of organic chemistry, materials science, and polymer science, as it offers hassle-free platforms for the high-yielding synthesis of novel materials and easy functionalization strategies. The absence of harsh reaction conditions or complicated workup procedures allowed the rapid development of novel biofunctional polymeric materials, such as biopolymers, tailor-made polymer surfaces, stimulus-responsive polymers, etc. In this review, we discuss various types of click reactions─including azide-alkyne cycloadditions, nucleophilic and radical thiol click reactions, a range of cycloadditions (Diels-Alder, tetrazole, nitrile oxide, etc.), sulfur fluoride exchange (SuFEx) click reaction, and oxime-hydrazone click reactions─and their use for the formation and study of biofunctional polymers. Following that, we discuss state-of-the-art biological applications of "click"-biofunctionalized polymers, including both passive applications (e.g., biosensing and bioimaging) and "active" ones that aim to direct changes in biosystems, e.g., for drug delivery, antiviral action, and tissue engineering. In conclusion, we have outlined future directions and existing challenges of click-based polymers for medicinal chemistry and clinical applications.

Aptamer-Gadolinium Conjugates for Targeted Magnetic Resonance Imaging of Early-Stage Bladder Cancer

Bladder cancer (BCa) poses a significant threat to human health, with early-stage diagnosis being particularly valuable yet challenging due to the limited availability of highly efficient targeted contrast agents. In this study, we have developed a novel aptamer-gadolinium conjugate (Apt-Gd) designed as a targeted contrast agent for the magnetic resonance imaging (MRI) of early-stage BCa. The synthesis of Apt-Gd involved the direct conjugation of aptamers with chelating agents through a bioorthogonal reaction, followed by gadolinium chelation. Notably, Apt-Gd exhibited high longitudinal relaxivity, exceptional BCa specificity, and good biocompatibility. Furthermore, our research revealed that intravesical instillation of Apt-Gd effectively enhanced the stability of aptamers and boosted the local contrast enhancement in BCa-bearing mice. The utilization of Apt-Gd yielded a satisfactory signal-to-noise ratio during BCa imaging and significantly prolonged the imaging time window. On the whole, the accurate detection of early-stage BCa using Apt-Gd was achieved for the first time, making a breakthrough in BCa diagnosis with promising potential for practical clinical applications.

Recent advances in triazole synthesis via click chemistry and their pharmacological applications: A review

Click chemistry is a flexible method featuring only the most feasible and efficient chemical reactions. The synthesis of 1,2,3-triazole from azides and terminal acetylenes using copper(I) as a catalyst is an extremely powerful reaction due to the extreme dependability, good selectivity, and biocompatibility of the starting materials. Triazole molecules are more than simple passive linkers; through hydrogen bonding and dipole interactions, they rapidly bind with biological targets. Its applications in drug development are expanding, ranging from target-oriented in situ chemistry and combinatorial mechanisms for lead generation to bioconjugation methods to study proteins and DNA. The click chemistry has frequently been used to speed up drug discovery and optimization processes in the past few years. The click chemistry reaction based on copper-catalyzed azide-alkyne cycloaddition (CuAAC) is a biochemical process with applications in medicinal chemistry and chemical biology. Thus, click reactions are an essential component of the toolkit for medicinal chemistry and help medicinal chemists overcome the barriers in chemical reactions, increase throughput, and improve the standards of compound libraries. The review highlights the recent advancements in the copper-catalyzed azide-alkyne cycloaddition (CuAAC) click chemistry approach for synthesizing biologically important triazole moieties with a greater emphasis on synthesis methodologies and pharmacological applications. Additionally, the triazole-based FDA-approved drugs are also discussed with their mode of action to highlight the importance of the click chemistry approach in synthesizing the bioactive triazole compounds.

Synthesis of C3-Substituted N1-tert-Butyl 1,2,4-Triazinium Salts via the Liebeskind-Srogl Reaction for Fluorogenic Labeling of Live Cells

We recently described the development and application of a new bioorthogonal conjugation, the triazinium ligation. To explore the wider application of this reaction, in this work, we introduce a general method for synthesizing C3-substituted triazinium salts based on the Liebeskind-Srogl cross-coupling reaction and catalytic thioether reduction. These methods enabled the synthesis of triazinium derivatives for investigating the effect of different substituents on the ligation kinetics and stability of the compounds under biologically relevant conditions. Finally, we demonstrate that the combination of a coumarin fluorophore attached to position C3 with a C5-(4-methoxyphenyl) substituent yields a fluorogenic triazinium probe suitable for no-wash, live-cell labeling. The developed methodology represents a promising synthetic approach to the late-stage modification of triazinium salts, potentially widening their applications in bioorthogonal reactions.

Sulfonated Hydroxyaryl-Tetrazines with Increased pKa for Accelerated Bioorthogonal Click-to-Release Reactions in Cells

Bioorthogonal reactions that enable switching molecular functions by breaking chemical bonds have gained prominence, with the tetrazine-mediated cleavage of trans-cyclooctene caged compounds (click-to-release) being particularly noteworthy for its high versatility, biocompatibility, and fast reaction rates. Despite several recent advances, the development of highly reactive tetrazines enabling quantitative elimination from trans-cyclooctene linkers remains challenging. In this study, we present the synthesis and application of sulfo-tetrazines, a class of derivatives featuring phenolic hydroxyl groups with increased acidity constants (pKa). This unique property leads to accelerated elimination and complete release of the caged molecules within minutes. Moreover, the inclusion of sulfonate groups provides a valuable synthetic handle, enabling further derivatization into sulfonamides, modified with diverse substituents. Significantly, we demonstrate the utility of sulfo-tetrazines in efficiently activating fluorogenic compounds and prodrugs in living cells, offering exciting prospects for their application in bioorthogonal chemistry.

Scission-Enhanced Molecular Imaging (SEMI)

Positron emission tomography (PET) imaging methods have advanced our understanding of human biology, while targeted radiotherapeutic drug treatments are now routinely used clinically. The field is expected to grow considerably based on an expanding repertoire of available affinity ligands, radionuclides, conjugation chemistries, and their FDA approvals. With this increasing use, strategies for dose reduction have become of high interest to protect patients from unnecessary and off-target toxicity. Here, we describe a simple and powerful method, scission-enhanced molecular imaging (SEMI). The technique allows for rapid corporeal elimination of radionuclides once imaging or theranostic treatment is completed and relies on "click-to-release" bioorthogonal linkers.

Enabling Universal Access to Rapid and Stable Tetrazine Bioorthogonal Probes through Triazolyl-Tetrazine Formation

Despite the immense potential of tetrazine bioorthogonal chemistry in biomedical research, the in vivo performance of tetrazine probes is challenged by the inverse correlation between the physiological stability and reactivity of tetrazines. Additionally, the synthesis of functionalized tetrazines is often complex and requires specialized reagents. To overcome these issues, we present a novel tetrazine scaffold-triazolyl-tetrazine-that can be readily synthesized from shelf-stable ethynyl-tetrazines and azides. Triazolyl-tetrazines exhibit improved physiological stability along with high reactivity. We showcase the effectiveness of this approach by creating cell-permeable probes for protein labeling and live cell imaging, as well as efficiently producing 18F-labeled molecular probes for positron emission tomography imaging. By utilizing the readily available pool of functionalized azides, we envisage that this modular approach will provide universal accessibility to tetrazine bioorthogonal tools, facilitating applications in biomedicine and materials science.

Recent Progress in Synthesis of 99mTc-labeled Complexes with Nitroimidazoles as SPECT Probes for Targeting Tumor Hypoxia

The majority of solid tumors have hypoxia, or low oxygen levels, which is one of the hallmarks of cancer. Hypoxia was found to relate to cancer metastases and resistance to therapies, therefore, detection of hypoxia plays an important role in the process of cancer prognosis and treatment. Single-photon emission computed tomography (SPECT) is a non-invasive imaging technique using gamma-emitting radiopharmaceuticals to visualize biological activities within the body. SPECT is also applied for the detection of tumor hypoxia with the development of hypoxia-targeting radiopharmaceuticals. Radiopharmaceuticals containing nitroimidazole moieties have received increasing attention due to their bio-reducible characteristics which make the radiopharmaceuticals accumulate in the hypoxia regions. This review summarizes the recent development of 99mTc-labeled radiopharmaceuticals bearing nitroimidazoles for SPECT imaging of tumor hypoxia including the synthetic methods and results of animal studies.

The development of 177Lu-DOTA-CC-PSMA following a unified "Click Chemistry" protocol of synthesizing metal nuclide-conjugated radiopharmaceuticals

BACKGROUND

Currently, the synthesis pathway of metal nuclide-labeled radiopharmaceuticals is mainly divided into two steps: first, connecting the chelator with the target molecule, and second, labeling the metal nuclide to the chelator. However, the second step of the reaction to label the metal nuclide requires high temperature (90-100 °C), which tends to denature and inactivate the target molecule, leading to loss of biological activities, especially the targeting ability. A feasible solution may be the click chemistry labeling method, which consists of reacting a metal nuclide with a chelating agent to generate an intermediate and then synthesizing a radiopharmaceutical agent via the click chemistry intermediate and the target molecule-alkyne compound. In this study, through the click chemistry of 177Lu-DOTA-N3 with prostate-specific membrane antigen (PSMA)-alkyne compound, 177Lu-labeled PSMA-targeted molecular probe was synthesized and evaluated for its potential to be cleared from the bloodstream and rapidly distributed to tissues and organs, achieving a high target/non-target ratio. 177Lu-PSMA-617 was utilized as an analogue for comparison in terms of synthesizing efficiency and PSMA-targeting ability.

RESULTS

A novel 177Lu-labeled PSMA radioligand was successfully synthesized through the click chemistry of 177Lu-DOTA-N3 with PSMA-alkyne compound, and abbreviated as 177Lu-DOTA-CC-PSMA, achieving a radiochemical yield of 77.07% ± 0.03% (n = 6) and a radiochemical purity of 97.62% ± 1.49% (n = 6) when purified by SepPak C18 column. Notably, 177Lu-DOTA-CC-PSMA was characterized as a hydrophilic compound that exhibited stability at room temperature and commendable pharmacokinetic properties, such as the superior uptake (19.75 ± 3.02%ID/g at 0.5 h) and retention (9.14 ± 3.16%ID/g at 24 h) within xenografts of 22Rv1 tumor-bearing mice. SPECT/CT imaging indicated that radioactivity in both kidneys and bladder was essentially eliminated after 24 h, while 177Lu-DOTA-CC-PSMA was further enriched and retained in PSMA-expressing tumors, resulting in the high target/non-target ratio.

CONCLUSION

This study demonstrated the potential of click chemistry to unify the synthesis of metal radiopharmaceuticals, and 177Lu-DOTA-CC-PSMA was found for rapid clearance and appropriate chemical stability as a PSMA-targeted radioligand.

Synthesis and evaluation of novel trifunctional chelating agents for pretargeting approach using albumin binder to improve tumor accumulation

INTRODUCTION

The pretargeting approach consists of in vivo ligation between pre-injected antibodies and low-molecular-weight radiolabeled effectors. The advantage of the pretargeting approach is to improve a tumor-to-background ratio, but the disadvantage is to compromise tumor accumulation. In this study, we applied albumin binder (ALB) to the pretargeting approach to overcome low tumor accumulation.

METHODS

We synthesized two novel trifunctional effectors containing an ALB moiety, a chelator, and a different tetrazine and two corresponding effectors without an ALB moiety. Albumin-binding assays and stability assays were performed using 111In-labeled effectors. Measurements of reaction rate constant were conducted using 111In-labeled effectors and anti-HER2 antibody trastuzumab modified by trans-cyclooctene, which drives the click reaction with tetrazine. Biodistribution studies using HER2-expressing tumor-bearing mice were performed with or without the pretargeting approach.

RESULTS

In albumin-binding assays, ALB-containing effectors exhibited a marked binding to albumin. Two ALB-containing effectors showed the difference in the reactivity and the slight difference in the stability. In biodistribution studies without the pretargeting approach, two ALB-containing effectors showed different pharmacokinetics in blood retention. With the pretargeting approach, the tumor accumulation was improved by the introduction of ALB and the highest tumor accumulation was observed in using the ALB-containing effector with higher blood retention.

CONCLUSION

These results suggest that the application of ALB to the pretargeting approach is effective to improve tumor accumulation, and the structure of tetrazine influences the utility of ALB-containing effectors.

Protein click chemistry and its potential for medical applications

A revolution in chemical biology occurred with the introduction of click chemistry. Click chemistry plays an important role in protein chemistry modifications, providing specific, sensitive, rapid, and easy-to-handle methods. Under physiological conditions, click chemistry often overlaps with bioorthogonal chemistry, defined as reactions that occur rapidly and selectively without interfering with biological processes. Click chemistry is used for the posttranslational modification of proteins based on covalent bond formations. With the contribution of click reactions, selective modification of proteins would be developed, representing an alternative to other technologies in preparing new proteins or enzymes for studying specific protein functions in different biological processes. Click-modified proteins have potential in diverse applications such as imaging, labeling, sensing, drug design, and enzyme technology. Due to the promising role of proteins in disease diagnosis and therapy, this review aims to highlight the growing applications of click strategies in protein chemistry over the last two decades, with a special emphasis on medicinal applications.

What is the future of click chemistry in drug discovery and development?

INTRODUCTION

The concept of click chemistry was introduced in 2001 as an effective, efficient, and sustainable approach to making functional groups harnessing the thermodynamic properties of a set of known chemical reactions that are based on nature. Some of the most common examples include reactions that produce 1,2,3-triazoles, which have been used with great success in drug discovery and development, and in chemical biology. The reactions unite two molecules quickly and irreversibly, and the reactions can be performed inside living cells, without harming the cell.

AREAS COVERED

The main focus of this perspective is the future of click chemistry in drug discovery and development, exemplified by novel click chemistry approaches and other aspects of the drug development enterprise, like SPAAC and analogous techniques, PROTACs, as well as diversity-oriented click chemistry.

EXPERT OPINION

Drug discovery and development has benefited enormously from the amazing advances that have been made in the field of click chemistry since 2001. The methods most likely to have the most future applications include metal-catalyzed azide-alkyne cycloadditions giving 1,2,3-triazoles, SPAAC for medical diagnostics and vaccine development, other congeners, Sulfur-Fluoride Exchange (SuFEx) and Diversity-Oriented Clicking (DOC), a concept with diverse molecular methodology with the potential for obtaining extensive molecular diversity.

Exploring the Potential of Nanogels: From Drug Carriers to Radiopharmaceutical Agents

Nanogels open up access to a wide range of applications and offer among others hopeful approaches for use in the field of biomedicine. This review provides a brief overview of current developments of nanogels in general, particularly in the fields of drug delivery, therapeutic applications, tissue engineering, and sensor systems. Specifically, cyclodextrin (CD)-based nanogels are important because they have exceptional complexation properties and are highly biocompatible. Nanogels as a whole and CD-based nanogels in particular can be customized in a wide range of sizes and equipped with a desired surface charge as well as containing additional molecules inside and outside, such as dyes, solubility-mediating groups or even biological vector molecules for pharmaceutical targeting. Currently, biological investigations are mainly carried out in vitro, but more and more in vivo applications are gaining importance. Modern molecular imaging methods are increasingly being used for the latter. Due to an extremely high sensitivity and the possibility of obtaining quantitative data on pharmacokinetic and pharmacodynamic properties, nuclear methods such as single photon emission computed tomography (SPECT) and positron emission tomography (PET) using radiolabeled compounds are particularly suitable here. The use of radiolabeled nanogels for imaging, but also for therapy, is being discussed.

Click Chemistry and Radiochemistry: An Update

The term "click chemistry" describes a class of organic transformations that were developed to make chemical synthesis simpler and easier, in essence allowing chemists to combine molecular subunits as if they were puzzle pieces. Over the last 25 years, the click chemistry toolbox has swelled from the canonical copper-catalyzed azide-alkyne cycloaddition to encompass an array of ligations, including bioorthogonal variants, such as the strain-promoted azide-alkyne cycloaddition and the inverse electron-demand Diels-Alder reaction. Without question, the rise of click chemistry has impacted all areas of chemical and biological science. Yet the unique traits of radiopharmaceutical chemistry have made it particularly fertile ground for this technology. In this update, we seek to provide a comprehensive guide to recent developments at the intersection of click chemistry and radiopharmaceutical chemistry and to illuminate several exciting trends in the field, including the use of emergent click transformations in radiosynthesis, the clinical translation of novel probes synthesized using click chemistry, and the advent of click-based in vivo pretargeting.

Novel Imaging Probes: From Design to Applications

Molecular imaging has emerged as a powerful tool for clinical diagnosis [...].

A Brief Guide to Preparing a Peptide-Drug Conjugate

Peptide-drug conjugates (PDCs) have recently emerged as interesting hybrid constructs not only for targeted therapy, but also for the early diagnosis of different pathologies. In most cases, the crucial step in the PDC synthesis is the final conjugation step, where a specific drug is bound to a particular peptide-/peptidomimetic-targeting unit. Thus, this concept paper aims to give a short guide to determining the finest conjugation reaction, by considering in particular the reaction conditions, the stability of the linker and the major pros and cons of each reaction. Based on the recent PDCs reported in literature, the most common and efficient conjugation methods will be systematically presented and compared, generating a short guide to consult while planning the synthesis of a novel peptide-drug conjugate.

Click-derived multifunctional metal complexes for diverse applications

The Click reaction that involves Cu(I)-catalyzed azide-alkyne cycloaddition (CuAAC) serves as the most potent and highly dependable tool for the development of many complex architectures. It has paved the way for the synthesis of numerous drug molecules with enhanced synthetic flexibility, reliability, specificity and modularity. It is all about bringing two different molecular entities together to achieve the required molecular properties. The utilization of Click chemistry has been well demonstrated in organic synthesis, particularly in reactions that involve biocompatible precursors. In pharmaceutical research, Click chemistry is extensively utilized for drug delivery applications. The exhibited bio-compatibility and dormancy towards other biological components under cellular environments makes Click chemistry an identified boon in bio-medical research. In this review, various click-derived transition metal complexes are discussed in terms of their applications and uniqueness. The scope of this chemistry towards other streams of applied sciences is also discussed.