Copper-Triggered Bioorthogonal Cleavage Reactions for Reversible Protein and Cell Surface Modifications

Imaging and Downstaging Bladder Cancer with the 177Lu-Labeled Bioorthogonal Nanoprobe

Efforts on bladder cancer treatment have been shifting from extensive surgery to organ preservation in the past decade. To this end, we herein develop a multifunctional nanoagent for bladder cancer downstaging and bladder-preserving therapy by integrating mucosa penetration, reduced off-target effects, and internal irradiation therapy into a nanodrug. Specifically, an iron oxide nanoparticle was used as a carrier that was coated with hyaluronic acid (HA) for facilitating mucosa penetration. Dibenzocyclooctyne (DBCO) was introduced into the HA coating layer to react through bioorthogonal reaction with azide as an artificial receptor of bladder cancer cells, to improve the cellular internalization of the nanoprobe labeled with 177Lu. Through magnetic resonance imaging, the targeted imaging of both nonmuscle-invasive bladder cancer (NMIBC) and muscle-invasive bladder cancer (MIBC) was realized after intravesical instillation of the multifunctional probe, both NMIBC and MIBC were found downstaged, and the metastasis was inhibited, which demonstrates the potential of the multifunctional nanoprobe for bladder preservation in bladder cancer treatment.

Spotlight on ideal target antigens and resistance in antibody-drug conjugates: Strategies for competitive advancement

Antibody-drug conjugates (ADCs) represent a novel and promising approach in targeted therapy, uniting the specificity of antibodies that recognize specific antigens with payloads, all connected by the stable linker. These conjugates combine the best targeted and cytotoxic therapies, offering the killing effect of precisely targeting specific antigens and the potent cell-killing power of small molecule drugs. The targeted approach minimizes the off-target toxicities associated with the payloads and broadens the therapeutic window, enhancing the efficacy and safety profile of cancer treatments. Within precision oncology, ADCs have garnered significant attention as a cutting-edge research area and have been approved to treat a range of malignant tumors. Correspondingly, the issue of resistance to ADCs has gradually come to the fore. Any dysfunction in the steps leading to the ADCs' action within tumor cells can lead to the development of resistance. A deeper understanding of resistance mechanisms may be crucial for developing novel ADCs and exploring combination therapy strategies, which could further enhance the clinical efficacy of ADCs in cancer treatment. This review outlines the brief historical development and mechanism of ADCs and discusses the impact of their key components on the activity of ADCs. Furthermore, it provides a detailed account of the application of ADCs with various target antigens in cancer therapy, the categorization of potential resistance mechanisms, and the current state of combination therapies. Looking forward, breakthroughs in overcoming technical barriers, selecting differentiated target antigens, and enhancing resistance management and combination therapy strategies will broaden the therapeutic indications for ADCs. These progresses are anticipated to advance cancer treatment and yield benefits for patients.

Development of Biocompatible Cu(I)-Microdevices for Bioorthogonal Uncaging and Click Reactions

Transition-metal-catalyzed bioorthogonal reactions emerged a decade ago as a novel strategy to implement spatiotemporal control over enzymatic functions and pharmacological interventions. The use of this methodology in experimental therapy is driven by the ambition of improving the tolerability and PK properties of clinically-used therapeutic agents. The preclinical potential of bioorthogonal catalysis has been validated in vitro and in vivo with the in situ generation of a broad range of drugs, including cytotoxic agents, anti-inflammatory drugs and anxiolytics. In this article, we report our investigations towards the preparation of solid-supported Cu(I)-microdevices and their application in bioorthogonal uncaging and click reactions. A range of ligand-functionalized polymeric devices and off-on Cu(I)-sensitive sensors were developed and tested under conditions compatible with life. Last, we present a preliminary exploration of their use for the synthesis of PROTACs through CuAAC assembly of two heterofunctional mating units.

Genetically encoded bioorthogonal tryptophan decaging in living cells

Tryptophan (Trp) plays a critical role in the regulation of protein structure, interactions and functions through its π system and indole N-H group. A generalizable method for blocking and rescuing Trp interactions would enable the gain-of-function manipulation of various Trp-containing proteins in vivo, but generating such a platform remains challenging. Here we develop a genetically encoded N1-vinyl-caged Trp capable of rapid and bioorthogonal decaging through an optimized inverse electron-demand Diels-Alder reaction, allowing site-specific activation of Trp on a protein of interest in living cells. This chemical activation of a genetically encoded caged-tryptophan (Trp-CAGE) strategy enables precise activation of the Trp of interest underlying diverse important molecular interactions. We demonstrate the utility of Trp-CAGE across various protein families, such as catalase-peroxidases and kinases, as translation initiators and posttranslational modification readers, allowing the modulation of epigenetic signalling in a temporally controlled manner. Coupled with computer-aided prediction, our strategy paves the way for bioorthogonal Trp activation on more than 28,000 candidate proteins within their native cellular settings.

Interplay of chloride levels and palladium(ii)-catalyzed O-deallenylation bioorthogonal uncaging reactions

The palladium-mediated uncaging reaction of allene substrates remains a promising yet often overlooked strategy in the realm of bioorthogonal chemistry. This method exhibits high kinetic rates, rivaling those of the widely employed allylic and propargylic protecting groups. In this study, we investigate into the mechanistic aspects of the C-O bond-cleavage deallenylation reaction, examining how chloride levels influence the kinetics when triggered by Pd(ii) complexes. Focusing on the deallenylation of 1,2-allenyl protected 4-methylumbelliferone promoted by Allyl2Pd2Cl2, our findings reveal that reaction rates are higher in environments with lower chloride concentrations, mirroring intracellular conditions, compared to elevated chloride concentrations typical of extracellular conditions. Through kinetic and spectroscopic experiments, combined with DFT calculations, we uncover a detailed mechanism that identifies AllylPd(H2O)2 as the predominant active species. These insights provide the basis for the design of π-allylpalladium catalysts suited for selective uncaging within specific cellular environments, potentially enhancing targeted therapeutic applications.

Drug conjugates for the treatment of lung cancer: from drug discovery to clinical practice

A drug conjugate consists of a cytotoxic drug bound via a linker to a targeted ligand, allowing the targeted delivery of the drug to one or more tumor sites. This approach simultaneously reduces drug toxicity and increases efficacy, with a powerful combination of efficient killing and precise targeting. Antibody‒drug conjugates (ADCs) are the best-known type of drug conjugate, combining the specificity of antibodies with the cytotoxicity of chemotherapeutic drugs to reduce adverse reactions by preferentially targeting the payload to the tumor. The structure of ADCs has also provided inspiration for the development of additional drug conjugates. In recent years, drug conjugates such as ADCs, peptide‒drug conjugates (PDCs) and radionuclide drug conjugates (RDCs) have been approved by the Food and Drug Administration (FDA). The scope and application of drug conjugates have been expanding, including combination therapy and precise drug delivery, and a variety of new conjugation technology concepts have emerged. Additionally, new conjugation technology-based drugs have been developed in industry. In addition to chemotherapy, targeted therapy and immunotherapy, drug conjugate therapy has undergone continuous development and made significant progress in treating lung cancer in recent years, offering a promising strategy for the treatment of this disease. In this review, we discuss recent advances in the use of drug conjugates for lung cancer treatment, including structure-based drug design, mechanisms of action, clinical trials, and side effects. Furthermore, challenges, potential approaches and future prospects are presented.

Development of antibody-drug conjugates in cancer: Overview and prospects

In recent years, remarkable breakthroughs have been reported on antibody-drug conjugates (ADCs), with 15 ADCs successfully entering the market over the past decade. This substantial development has positioned ADCs as one of the fastest-growing domains in the realm of anticancer drugs, demonstrating their efficacy in treating a wide array of malignancies. Nonetheless, there is still an unmet clinical need for wider application, better efficacy, and fewer side effects of ADCs. An ADC generally comprises an antibody, a linker and a payload, and the combination has profound effects on drug structure, pharmacokinetic profile and efficacy. Hence, optimization of the key components provides an opportunity to develop ADCs with higher potency and fewer side effects. In this review, we comprehensively reviewed the current development and the prospects of ADC, provided an analysis of marketed ADCs and the ongoing pipelines globally as well as in China, highlighted several ADC platforms and technologies specific to different pharmaceutical enterprises and biotech companies, and also discussed the new related technologies, possibility of next-generation ADCs and the directions of clinical research.

Controllable and Reversible Assembly of Nanofiber from Natural Macromolecules via Protonation and Deprotonation

Nanofiber is the critical building block for many biological systems to perform various functions. Artificial assembly of molecules into nanofibers in a controllable and reversible manner will create "smart" functions to mimic those of their natural analogues and fabricate new functional materials, but remains an open challenge especially for nature macromolecules. Herein, the controllable and reversible assembly of nanofiber (CSNF) from natural macromolecules with oppositely charged groups are successfully realized by protonation and deprotonation of charged groups. By controlling the electrostatic interaction via protonation and deprotonation, the size and morphology of the assembled nanostructures can be precisely controlled. A strong electrostatic interaction contributes to large nanofiber with high strength, while poor electrostatic interaction produces finer nanofiber or nanoparticle. And especially, the assembly, disassembly, and reassembly of the nanofiber occurs reversibly through protonation and deprotonation, thereby paving a new way for precisely controlling the assembly process and structure of nanofiber. The reversible assembly allows the nanostructure to dynamically reorganize in response to subtle perturbation of environment. The as-prepared CSNF is mechanical strong and can be used as a nano building block to fabricate high-strength film, wire, and straw. This study offers many opportunities for the biomimetic synthesis of new functional materials.

Designing Bioorthogonal Reactions for Biomedical Applications

Bioorthogonal reactions are a class of chemical reactions that can be carried out in living organisms without interfering with other reactions, possessing high yield, high selectivity, and high efficiency. Since the first proposal of the conception by Professor Carolyn Bertozzi in 2003, bioorthogonal chemistry has attracted great attention and has been quickly developed. As an important chemical biology tool, bioorthogonal reactions have been applied broadly in biomedicine, including bio-labeling, nucleic acid functionalization, drug discovery, drug activation, synthesis of antibody-drug conjugates, and proteolysis-targeting chimeras. Given this, we summarized the basic knowledge, development history, research status, and prospects of bioorthogonal reactions and their biomedical applications. The main purpose of this paper is to furnish an overview of the intriguing bioorthogonal reactions in a variety of biomedical applications and to provide guidance for the design of novel reactions to enrich bioorthogonal chemistry toolkits.

Bioorthogonal chemistry for prodrug activation in vivo

Prodrugs have emerged as a major strategy for addressing clinical challenges by improving drug pharmacokinetics, reducing toxicity, and enhancing treatment efficacy. The emergence of new bioorthogonal chemistry has greatly facilitated the development of prodrug strategies, enabling their activation through chemical and physical stimuli. This "on-demand" activation using bioorthogonal chemistry has revolutionized the research and development of prodrugs. Consequently, prodrug activation has garnered significant attention and emerged as an exciting field of translational research. This review summarizes the latest advancements in prodrug activation by utilizing bioorthogonal chemistry and mainly focuses on the activation of small-molecule prodrugs and antibody-drug conjugates. In addition, this review also discusses the opportunities and challenges of translating these advancements into clinical practice.

Peripheral neuropathy associated with monomethyl auristatin E-based antibody-drug conjugates

Since the successful approval of gemtuzumab ozogamicin, antibody-drug conjugates (ADCs) have emerged as a pivotal category of targeted therapies for cancer. Among these ADCs, the use of monomethyl auristatin E (MMAE) as a payload is prevalent in the development of ADC drugs, which has significantly improved overall therapeutic efficacy against various malignancies. However, increasing clinical observations have raised concerns regarding the potential nervous system toxicity associated with MMAE-based ADCs. Specifically, a higher incidence of peripheral neuropathy has been reported in ADCs incorporating MMAE as payloads. Considering the increasing global use of MMAE-based ADCs, it is imperative to provide an inclusive overview of diagnostic and management strategies for this adverse event. In this review, we examine current information and what future research directions are required to better understand and manage this type of clinical challenge.

Transition Metal Catalysis in Living Cells: Progress, Challenges, and Novel Supramolecular Solutions

The importance of transition metal catalysis is exemplified by its wide range of applications, for example in the synthesis of chemicals, natural products, and pharmaceuticals. However, one relatively new application is for carrying out new-to-nature reactions inside living cells. The complex environment of a living cell is not welcoming to transition metal catalysts, as a diverse range of biological components have the potential to inhibit or deactivate the catalyst. Here we review the current progress in the field of transition metal catalysis, and evaluation of catalysis efficiency in living cells and under biological (relevant) conditions. Catalyst poisoning is a ubiquitous problem in this field, and we propose that future research into the development of physical and kinetic protection strategies may provide a route to improve the reactivity of catalysts in cells.

Biodegradable nanoemulsion-based bioorthogonal nanocatalysts for intracellular generation of anticancer therapeutics

Bioorthogonal catalysis mediated by transition metal catalysts (TMCs) provides controlled in situ activation of prodrugs through chemical reactions that do not interfere with cellular bioprocesses. The direct use of 'naked' TMCs in biological environments can have issues of solubility, deactivation, and toxicity. Here, we demonstrate the design and application of a biodegradable nanoemulsion-based scaffold stabilized by a cationic polymer that encapsulates a palladium-based TMC, generating bioorthogonal nanocatalyst "polyzymes". These nanocatalysts enhance the stability and catalytic activity of the TMCs while maintaining excellent mammalian cell biocompatibility. The therapeutic potential of these nanocatalysts was demonstrated through efficient activation of a non-toxic prodrug into an active chemotherapeutic drug, leading to efficient killing of cancer cells.

Metal-Mediated, Autolytic Amide Bond Cleavage: A Strategy for the Selective, Metal Complexation-Catalyzed, Controlled Release of Metallodrugs

Activation of metalloprodrugs or prodrug activation using transition metal catalysts represents emerging strategies for drug development; however, they are frequently hampered by poor spatiotemporal control and limited catalytic turnover. Here, we demonstrate that metal complex-mediated, autolytic release of active metallodrugs can be successfully employed to prepare clinical grade (radio-)pharmaceuticals. Optimization of the Lewis-acidic metal ion, chelate, amino acid linker, and biological targeting vector provides means to release peptide-based (radio-)metallopharmaceuticals in solution and from the solid phase using metal-mediated, autolytic amide bond cleavage (MMAAC). Our findings indicate that coordinative polarization of an amide bond by strong, trivalent Lewis acids such as Ga3+ and Sc3+ adjacent to serine results in the N, O acyl shift and hydrolysis of the corresponding ester without dissociation of the corresponding metal complex. Compound [68Ga]Ga-10, incorporating a cleavable and noncleavable functionalization, was used to demonstrate that only the amide bond-adjacent serine effectively triggered hydrolysis in solution and from the solid phase. The corresponding solid-phase released compound [68Ga]Ga-8 demonstrated superior in vivo performance in a mouse tumor model compared to [68Ga]Ga-8 produced using conventional, solution-phase radiolabeling. A second proof-of-concept system, [67Ga]Ga-17A (serine-linked) and [67Ga]Ga-17B (glycine-linked) binding to serum albumin via the incorporated ibuprofen moiety, was also synthesized. These constructs demonstrated that complete hydrolysis of the corresponding [68Ga]Ga-NOTA complex from [67Ga]Ga-17A can be achieved in naïve mice within 12 h, as traceable in urine and blood metabolites. The glycine-linked control [68Ga]Ga-17B remained intact. Conclusively, MMAAC provides an attractive tool for selective, thermal, and metal ion-mediated control of metallodrug activation compatible with biological conditions.

Catalyst-free thiazolidine formation chemistry enables the facile construction of peptide/protein-cell conjugates (PCCs) at physiological pH

Although numerous genetic, chemical, and physical strategies have been developed to remodel the cell surface landscape for basic research and the development of live cell-based therapeutics, new chemical modification strategies capable of decorating cells with various genetically/non-genetically encodable molecules are still urgently needed. Herein, we describe a remarkably simple and robust chemical strategy for cell surface modifications by revisiting the classical thiazolidine formation chemistry. Cell surfaces harbouring aldehydes can be chemoselectively conjugated with molecules containing a 1,2-aminothiol moiety at physiological pH without the need to use any toxic catalysts and complicated chemical synthesis. Through the combined use of thiazolidine formation and the SpyCatcher-SpyTag system, we have further developed a SpyCatcher-SpyTag Chemistry Assisted Cell Surface Engineering (SpyCASE) platform, providing a modular approach for the construction of large protein-cell conjugates (PCCs) in their native state. Thiazolidine-bridged molecules can also be detached from the surface again through a biocompatible Pd-catalyzed bond scission reaction, enabling reversible modification of living cell surfaces. In addition, this approach allows us to modulate specific cell-cell interactions and generate NK cell-based PCCs to selectively target/kill several EGFR-positive cancer cells in vitro. Overall, this study provides an underappreciated but useful chemical tool to decorate cells with tailor-made functionalities.

The Dawn of a New Era: Targeting the "Undruggables" with Antibody-Based Therapeutics

The high selectivity and affinity of antibodies toward their antigens have made them a highly valuable tool in disease therapy, diagnosis, and basic research. A plethora of chemical and genetic approaches have been devised to make antibodies accessible to more "undruggable" targets and equipped with new functions of illustrating or regulating biological processes more precisely. In this Review, in addition to introducing how naked antibodies and various antibody conjugates (such as antibody-drug conjugates, antibody-oligonucleotide conjugates, antibody-enzyme conjugates, etc.) work in therapeutic applications, special attention has been paid to how chemistry tools have helped to optimize the therapeutic outcome (i.e., with enhanced efficacy and reduced side effects) or facilitate the multifunctionalization of antibodies, with a focus on emerging fields such as targeted protein degradation, real-time live-cell imaging, catalytic labeling or decaging with spatiotemporal control as well as the engagement of antibodies inside cells. With advances in modern chemistry and biotechnology, well-designed antibodies and their derivatives via size miniaturization or multifunctionalization together with efficient delivery systems have emerged, which have gradually improved our understanding of important biological processes and paved the way to pursue novel targets for potential treatments of various diseases.

Expanding Transition Metal-Mediated Bioorthogonal Decaging to Include C-C Bond Cleavage Reactions

The ability to control the activation of prodrugs by transition metals has been shown to have great potential for controlled drug release in cancer cells. However, the strategies developed so far promote the cleavage of C-O or C-N bonds, which limits the scope of drugs to only those that present amino or hydroxyl groups. Here, we report the decaging of an ortho-quinone prodrug, a propargylated β-lapachone derivative, through a palladium-mediated C-C bond cleavage. The reaction's kinetic and mechanistic behavior was studied under biological conditions along with computer modeling. The results indicate that palladium (II) is the active species for the depropargylation reaction, activating the triple bond for nucleophilic attack by a water molecule before the C-C bond cleavage takes place. Palladium iodide nanoparticles were found to efficiently trigger the C-C bond cleavage reaction under biocompatible conditions. In drug activation assays in cells, the protected analogue of β-lapachone was activated by nontoxic amounts of nanoparticles, which restored drug toxicity. The palladium-mediated ortho-quinone prodrug activation was further demonstrated in zebrafish tumor xenografts, which resulted in a significant anti-tumoral effect. This work expands the transition-metal-mediated bioorthogonal decaging toolbox to include cleavage of C-C bonds and payloads that were previously not accessible by conventional strategies.

Boosting Endogenous Copper(I) for Biologically Safe and Efficient Bioorthogonal Catalysis via Self-Adaptive Metal-Organic Frameworks

As one of the most typical bioorthogonal reactions, the Cu(I)-catalyzed azide-alkyne 1,3-cycloaddition (CuAAC) reaction has received worldwide attention in intracellular transformation of prodrugs due to its high efficiency and selectivity. However, the exogenous Cu catalysts may disturb Cu homeostasis and cause side effects to normal tissues. What is more, the intratumoral Cu(I) is insufficient to efficiently catalyze the intracellular CuAAC reaction due to oncogene-induced labile Cu(I) deficiency. Herein, in order to boost the endogenous Cu(I) level for intracellular drug synthesis through the bioorthogonal reaction, a self-adaptive bioorthogonal catalysis system was constructed by encapsulating prodrugs and sodium ascorbate within adenosine triphosphate aptamer-functionalized metal-organic framework nanoparticles. The system presents specificity to tumor cells and does not require exogenous Cu catalysts, thereby leading to high anti-tumor efficacy and minimal side effects both in vitro and in vivo. This work will open up a new opportunity for developing biosafe and high-performance bioorthogonal catalysis systems.

NIR-II Light Leveraged Dual Drug Synthesis for Orthotopic Combination Therapy

Pd-catalyzed bioorthogonal bond cleavage reactions are widely used and frequently reported. It is circumscribed by low reaction efficiency, which may encumber the therapeutic outcome when applied to physiological environments. Herein, an NIR-II light promoted integrated catalyst (CuS@PDA/Pd) (PDA - polydopamine) is designed to accelerate the reaction efficiency and achieve a dual bioorthogonal reaction for combination therapy. As NIR-II light can penetrate deeply into tissue, the Pd-mediated cleavage reaction can be promoted both in vitro and in vivo by the photothermal properties of CuS, beneficial to orthotopic 4T1 tumor treatment. In addition, CuS also catalyzes the synthesis of active resveratrol analogs by the CuAAC reaction. These simultaneously produced anticancer agents result in enhanced antitumor cytotoxicity in comparison to the single treatments. This is a fascinating study to devise an integrated catalyst boosted by NIR-II light for dual bioorthogonal catalysis, which may provide the impetus for efficient bioorthogonal combination therapy in vivo.

Recent advances in developing active targeting and multi-functional drug delivery systems via bioorthogonal chemistry

Bioorthogonal chemistry reactions occur in physiological conditions without interfering with normal physiological processes. Through metabolic engineering, bioorthogonal groups can be tagged onto cell membranes, which selectively attach to cargos with paired groups via bioorthogonal reactions. Due to its simplicity, high efficiency, and specificity, bioorthogonal chemistry has demonstrated great application potential in drug delivery. On the one hand, bioorthogonal reactions improve therapeutic agent delivery to target sites, overcoming off-target distribution. On the other hand, nanoparticles and biomolecules can be linked to cell membranes by bioorthogonal reactions, providing approaches to developing multi-functional drug delivery systems (DDSs). In this review, we first describe the principle of labeling cells or pathogenic microorganisms with bioorthogonal groups. We then highlight recent breakthroughs in developing active targeting DDSs to tumors, immune systems, or bacteria by bioorthogonal chemistry, as well as applications of bioorthogonal chemistry in developing functional bio-inspired DDSs (biomimetic DDSs, cell-based DDSs, bacteria-based and phage-based DDSs) and hydrogels. Finally, we discuss the difficulties and prospective direction of bioorthogonal chemistry in drug delivery. We expect this review will help us understand the latest advances in the development of active targeting and multi-functional DDSs using bioorthogonal chemistry and inspire innovative applications of bioorthogonal chemistry in developing smart DDSs for disease treatment.

Exporting Homogeneous Transition Metal Catalysts to Biological Habitats

The possibility of performing designed transition-metal catalyzed reactions in biological and living contexts can open unprecedented opportunities to interrogate and interfere with biology. However, the task is far from obvious, in part because of the presumed incompatibly between organometallic chemistry and complex aqueous environments. Nonetheless, in the past decade there has been a steady progress in this research area, and several transition-metal (TM)-catalyzed bioorthogonal and biocompatible reactions have been developed. These reactions encompass a wide range of mechanistic profiles, which are very different from those used by natural metalloenzymes. Herein we present a summary of the latest progress in the field of TM-catalyzed bioorthogonal reactions, with a special focus on those triggered by activation of multiple carbon-carbon bonds.

Engineered Polymer-Supported Biorthogonal Nanocatalysts Using Flash Nanoprecipitation

Transition-metal catalysts (TMCs) effect bioorthogonal transformations that enable the generation of therapeutic agents in situ, minimizing off-target effects. The encapsulation of insoluble TMCs into polymeric nanoparticles to generate "polyzymes" has vastly expanded their applicability in biological environments by enhancing catalyst solubility and stability. However, commonly used precipitation approaches provide limited encapsulation efficiency in polyzyme fabrication and result in a low catalytic activity. Herein, we report the creation of polyzymes with increased catalyst loading and optimized turnover efficiency using flash nanoprecipitation (FNP). Polyzymes with controlled size and catalyst loading were fabricated by tuning the process conditions of FNP. The biological applicability of polyzymes was demonstrated by efficiently transforming a non-toxic prodrug into the active drug within cancer cells.

Synthesis of Polydiynes via an Unexpected Dimerization/Polymerization Sequence of C3 Propargylic Electrophiles

Here, we describe the unexpected discovery of a Cu-catalyzed condensation polymerization reaction of propargylic electrophiles (CPPE) that transforms simple C3 building blocks into polydiynes of C6 repeating units. This reaction was achieved by a simple system composed of a copper acetylide initiator and an electron-rich phosphine ligand. Alkyne polymers (up to 33.8 kg/mol) were produced in good yields and exclusive regioselectivity with high functional group compatibility. Hydrogenation of the product afforded a new polyolefin-type backbone, while base-mediated isomerization led to a new type of dienyne-based electron-deficient conjugated polymer. Mechanistic studies revealed a new α-α selective Cu-catalyzed dimerization pathway of the C3 unit, followed by in situ organocopper-mediated chain-growth propagation. These insights not only provide an important understanding of the Cu-catalyzed CPPE of C3, C4, and C6 monomers in general but also lead to a significantly improved synthesis of polydiynes from simpler starting materials with handles for the incorporation of an α-end functional group.

Fluorogenic Cyclopropenones for Multicomponent, Real-Time Imaging

Fluorogenic bioorthogonal reactions enable biomolecule visualization in real time. These reactions comprise reporters that "light up" upon reaction with complementary partners. While the spectrum of fluorogenic chemistries is expanding, few transformations are compatible with live cells due to cross-reactivities or insufficient signal turn-on. To address the need for more suitable chemistries for cellular imaging, we developed a fluorogenic reaction featuring cyclopropenone reporters and phosphines. The transformation involves regioselective activation and cyclization of cyclopropenones to form coumarin products. With optimal probes, the reaction provides >1600-fold signal turn-on, one of the highest fluorescence enhancements reported to date. The bioorthogonal motifs were evaluated in vitro and in cells. The reaction was also found to be compatible with other common fluorogenic transformations, enabling multicomponent, real-time imaging. Collectively, these data suggest that the cyclopropenone-phosphine reaction will bolster efforts to track biomolecule targets in their native settings.

Controlled Intracellular Polymerization for Cancer Treatment

Numerous prodrugs have been developed and used for cancer treatments to reduce side effects and promote efficacy. In this work, we have developed a new photoactivatable prodrug system based on intracellular photoinduced electron transfer-reversible addition-fragmentation chain-transfer (PET-RAFT) polymerization. This unique polymerization process provided a platform for the synthesis of structure-predictable polymers with well-defined structures in living cells. The intracellularly generated poly(N,N-dimethylacrylamide)s were found to induce cell cycle arrest, apoptosis, and necroptosis, inhibit cell proliferation, and reduce cancer cell motilities. This polymerization-based "prodrug" system efficiently inhibits tumor growth and metastasis both in vitro and in vivo and will promote the development of targeted and directed cancer chemotherapy.

Deactivation of a dimeric DNA-binding peptide through a palladium-mediated self-immolative cleavage

Herein, we describe an approach for the on-demand disassembly of dimeric peptides using a palladium-mediated cleavage of a designed self-immolative linker. The utility of the strategy is demonstrated for the case of dimeric basic regions of bZIP transcription factors. While the dimer binds designed DNA sequences with good affinities, the peptide–DNA complex can be readily dismounted by addition of palladium reagents that trigger the cleavage of the spacer, and the release of unfunctional monomeric peptides.

A metallic trigger for disassembling and cancelling function.

Phosphine-mediated three-component bioconjugation of amino- and azidosaccharides in ionic liquids

Bioconjugation of carbohydrates has been a challenging task because of their chemical, functional, and structural diversities, and no single chemical modification tool can be universally applicable to all the target substrates in different environments.

Antibody-drug conjugates: Recent advances in linker chemistry

Antibody–drug conjugates (ADCs) are gradually revolutionizing clinical cancer therapy. The antibody–drug conjugate linker molecule determines both the efficacy and the adverse effects, and so has a major influence on the fate of ADCs. An ideal linker should be stable in the circulatory system and release the cytotoxic payload specifically in the tumor. However, existing linkers often release payloads nonspecifically and inevitably lead to off-target toxicity. This defect is becoming an increasingly important factor that restricts the development of ADCs. The pursuit of ADCs with optimal therapeutic windows has resulted in remarkable progress in the discovery and development of novel linkers. The present review summarizes the advance of the chemical trigger, linker‒antibody attachment and linker‒payload attachment over the last 5 years, and describes the ADMET properties of ADCs. This work also helps clarify future developmental directions for the linkers.

Controlled In-Cell Generation of Active Palladium(0) Species for Bioorthogonal Decaging

Owing to their bioorthogonality, transition metals have become very popular in the development of biocompatible bond-cleavage reactions. However, many approaches require design and synthesis of complex ligands or formulation of nanoparticles which often perform poorly in living cells. This work reports on a method for the generation of an active palladium species that triggers bond-cleaving reactions inside living cells. We utilized the water-soluble Na2PdCl4 as a simple source of Pd(II) which can be intracellularly reduced by sodium ascorbate to the active Pd(0) species. Once generated, Pd(0) triggers the cleavage of allyl ether and carbamate caging groups leading to the release of biologically active molecules. These findings do not only expand the toolbox of available bioorthogonal dissociative reactions but also provide an additional strategy for controlling the reactivity of Pd species involved in Pd-mediated bioorthogonal reactions.

In situ activation of therapeutics through bioorthogonal catalysis

Bioorthogonal chemistry refers to any chemical reactions that can occur inside of living systems without interfering with native biochemical processes, which has become a promising strategy for modulating biological processes. The development of synthetic metal-based catalysts to perform bioorthogonal reactions has significantly expanded the toolkit of bioorthogonal chemistry for medicinal chemistry and synthetic biology. A wide range of homogeneous and heterogeneous transition metal catalysts (TMCs) have been reported, mediating different transformations such as cycloaddition reactions, as well as bond forming and cleaving reactions. However, the direct application of 'naked' TMCs in complex biological media poses numerous challenges, including poor water solubility, toxicity and catalyst deactivation. Incorporating TMCs into nanomaterials to create bioorthogonal nanocatalysts can solubilize and stabilize catalyst molecules, with the decoration of the nanocatalysts used to provide spatiotemporal control of catalysis. This review presents an overview of the advances in the creation of bioorthogonal nanocatalysts, highlighting different choice of nano-scaffolds, and the therapeutic and diagnostic applications.

Catalytically Cleavable Detergent for Membrane Protein Studies

Throughout the in vitro studies of membrane proteins (MPs), proper detergents are essential for the preparation of stable aqueous samples. To date, universally applicable detergents have not yet been reported to accommodate the distinct requirements for the highly diversified MPs and at the different stages of MP manipulation. Detergent exchange often has to be performed. We report herein the catalytically cleavable detergents (CatCDs) that can be efficiently removed to facilitate a complete exchange. To this end, functional groups, like propargyl and allyl, are introduced as branched chains or built in the hydrophobic chain close to the hydrophilic head. The representative CatCDs can be used as usual detergents in the extraction and purification of MPs and later be removed upon the addition of catalytic palladium. Mediated by CatCD-1, reconstitution of a transporter protein MsbA into a series of detergents was achieved. The extension of these designs could facilitate the future optimization of other biophysics studies.

Bioorthogonal catalytic patch

Bioorthogonal catalysis mediated by transition metals has inspired a new subfield of artificial chemistry complementary to enzymatic reactions, enabling the selective labelling of biomolecules or in situ synthesis of bioactive agents via non-natural processes. However, the effective deployment of bioorthogonal catalysis in vivo remains challenging, mired by the safety concerns of metal toxicity or complicated procedures to administer catalysts. Here, we describe a bioorthogonal catalytic device comprising a microneedle array patch integrated with Pd nanoparticles deposited on TiO₂ nanosheets. This device is robust and removable, and can mediate the local conversion of caged substrates into their active states in high-level living systems. In particular, we show that such a patch can promote the activation of a prodrug at subcutaneous tumour sites, restoring its parent drug's therapeutic anticancer properties. This in situ applied device potentiates local treatment efficacy and eliminates off-target prodrug activation and dose-dependent side effects in healthy organs or distant tissues.

Bioorthogonal strategies for the in vivo synthesis or release of drugs

The site-specific delivery of antitumor agents is a rapidly developing field that relies on prodrug activation and uncaging strategies. For this purpose, a wide range of homogeneous and heterogeneous biocompatible activators/catalysts have been developed to convert caged drugs with low toxicity and high stability in physiological settings into active substances in a bioorthogonal manner. The current methods allow for the site-specific delivery of activators and prodrugs to organelles, target cells, or tumors in living organisms. Here, we present an overview of the latest advances in catalytic drugs, highlighting the expanding toolbox of bioorthogonal activation strategies made possible by transition metals acting as activators or catalysts.

Bioorthogonal Azide-Thioalkyne Cycloaddition Catalyzed by Photoactivatable Ruthenium(II) Complexes

Tailored ruthenium sandwich complexes bearing photoresponsive arene ligands can efficiently promote azide–thioalkyne cycloaddition (RuAtAC) when irradiated with UV light. The reactions can be performed in a bioorthogonal manner in aqueous mixtures containing biological components. The strategy can also be applied for the selective modification of biopolymers, such as DNA or peptides. Importantly, this ruthenium‐based technology and the standard copper‐catalyzed azide–alkyne cycloaddition (CuAAC) proved to be compatible and mutually orthogonal.

Genetic Code Expansion: A Brief History and Perspective

Since the establishment of site-specific mutagenesis of single amino acids to interrogate protein function in the 1970s, biochemists have sought to tailor protein structure in the native cell environment. Fine-tuning the chemical properties of proteins is an indispensable way to address fundamental mechanistic questions. Unnatural amino acids (UAAs) offer the possibility to expand beyond the 20 naturally occurring amino acids in most species and install new and useful chemical functions. Here, we review the literature about advances in UAA incorporation technology from chemoenzymatic aminoacylation of modified tRNAs to in vitro translation systems to genetic encoding of UAAs in the native cell environment and whole organisms. We discuss innovative applications of the UAA technology to challenges in bioengineering and medicine.

The promising role of antibody drug conjugate in cancer therapy: Combining targeting ability with cytotoxicity effectively

Introduction

Traditional cancer therapy has many disadvantages such as low selectivity and high toxicity of chemotherapy, as well as insufficient efficacy of targeted therapy. To enhance the cytotoxic effect and targeting ability, while reducing the toxicity of antitumor drugs, an antibody drug conjugate (ADC) was developed to deliver small molecular cytotoxic payloads directly to tumor cells by binding to specific antibodies via linkers.

Method

By reviewing published literature and the current progress of ADCs, we aimed to summarize the basic characteristics, clinical progress, and challenges of ADCs to provide a reference for clinical practice and further research.

Results

ADC is a conjugate composed of three fundamental components, including monoclonal antibodies, cytotoxic payloads, and stable linkers. The mechanisms of ADC including the classical internalization pathway, antitumor activity of antibodies, bystander effect, and non‐internalizing mechanism. With the development of new drugs and advances in technology, various ADCs have achieved clinical efficacy. To date, nine ADCs have received US Food and Drug Administration (FDA) approval in the field of hematologic tumors and solid tumors, which have become routine clinical treatments.

Conclusion

ADC has changed traditional treatment patterns for cancer patients, which enable the same treatment for pancreatic cancer patients and promote individualized precision treatment. Further exploration of indications could focus on early‐stage cancer patients and combined therapy settings. Besides, the mechanisms of drug resistance, manufacturing techniques, optimized treatment regimens, and appropriate patient selection remain the major topics.

Unleashing the Power of Bond Cleavage Chemistry in Living Systems

Bioorthogonal cleavage chemistry has been rapidly emerging as a powerful tool for manipulation and gain-of-function studies of biomolecules in living systems. While the initial bond formation-centered bioorthogonal reactions have been widely adopted for labeling, tracing, and capturing biomolecules, the newly developed bond cleavage-enabled bioorthogonal reactions have opened new possibilities for rescuing small molecules as well as biomacromolecules in living systems, allowing multidimensional controls over biological processes in vitro and in vivo. In this Outlook, we first summarized the development and applications of bioorthogonal cleavage reactions (BCRs) that restore the functions of chemical structures as well as more complex networks, including the liberation of prodrugs, release of bioconjugates, and in situ reactivation of intracellular proteins. As we embarked on this fruitful progress, we outlined the unmet scientific needs and future directions along this exciting avenue. We believe that the potential of BCRs will be further unleashed when combined with other frontier technologies, such as genetic code expansion and proximity-enabled chemical labeling.

Bioorthogonal Reactions of Triarylphosphines and Related Analogues

Bioorthogonal phosphines were introduced in the context of the Staudinger ligation over 20 years ago. Since that time, phosphine probes have been used in myriad applications to tag azide-functionalized biomolecules. The Staudinger ligation also paved the way for the development of other phosphorus-based chemistries, many of which are widely employed in biological experiments. Several reviews have highlighted early achievements in the design and application of bioorthogonal phosphines. This review summarizes more recent advances in the field. We discuss innovations in classic Staudinger-like transformations that have enabled new biological pursuits. We also highlight relative newcomers to the bioorthogonal stage, including the cyclopropenone-phosphine ligation and the phospha-Michael reaction. The review concludes with chemoselective reactions involving phosphite and phosphonite ligations. For each transformation, we describe the overall mechanism and scope. We also showcase efforts to fine-tune the reagents for specific functions. We further describe recent applications of the chemistries in biological settings. Collectively, these examples underscore the versatility and breadth of bioorthogonal phosphine reagents.

Clip Chemistry: Diverse (Bio)(macro)molecular and Material Function through Breaking Covalent Bonds

In the two decades since the introduction of the "click chemistry" concept, the toolbox of "click reactions" has continually expanded, enabling chemists, materials scientists, and biologists to rapidly and selectively build complexity for their applications of interest. Similarly, selective and efficient covalent bond breaking reactions have provided and will continue to provide transformative advances. Here, we review key examples and applications of efficient, selective covalent bond cleavage reactions, which we refer to herein as "clip reactions." The strategic application of clip reactions offers opportunities to tailor the compositions and structures of complex (bio)(macro)molecular systems with exquisite control. Working in concert, click chemistry and clip chemistry offer scientists and engineers powerful methods to address next-generation challenges across the chemical sciences.

"One-stitch" bioorthogonal prodrug activation based on cross-linked lipoic acid nanocapsules

Bioorthogonal prodrug activation is fascinating but suffers from staggered administration of prodrug and trigger, which would not only reduce the therapeutic effect but bring great inconvenience for clinical application. Herein, we report a new cross-linked lipoic acid nanocapsules (cLANCs) based two-component bioorthogonal nanosystem for "one-stitch" prodrug activation. Due to the reversible stability of cLANCs, the loaded prodrug and trigger cannot release in advance while can react upon arrival in the tumor tissue. Moreover, the cLANCs would be degraded into dihydrolipoic acid in tumor cells to potentiate the anticancer effect of the drug synthesized in situ. The data showed that the new bioorthogonal system held a killing effect 1.63 times higher than that of parent drug 3 against human colorectal tumor cells (HT29) and a tumor inhibitory rate 34.2% higher than that of 3 against HT29 tumor xenograft model with negligible side effects. The biodistribution study showed that the "one-stitch" prodrug activation exhibited a selective accumulation of 3 in the tumor tissue compared with free 3 group (34.2 μg vs 3.56 μg of 3/g of tissue). This two-component bioorthogonal nanosystem based on cross-linked lipoic acid nanocapsules constitutes the first example of "one-stitch" bioorthogonal prodrug activation.

Bioorthogonal chemistry

Bioorthogonal chemistry represents a class of high-yielding chemical reactions that proceed rapidly and selectively in biological environments without side reactions towards endogenous functional groups. Rooted in the principles of physical organic chemistry, bioorthogonal reactions are intrinsically selective transformations not commonly found in biology. Key reactions include native chemical ligation and the Staudinger ligation, copper-catalysed azide-alkyne cycloaddition, strain-promoted [3 + 2] reactions, tetrazine ligation, metal-catalysed coupling reactions, oxime and hydrazone ligations as well as photoinducible bioorthogonal reactions. Bioorthogonal chemistry has significant overlap with the broader field of 'click chemistry' - high-yielding reactions that are wide in scope and simple to perform, as recently exemplified by sulfuryl fluoride exchange chemistry. The underlying mechanisms of these transformations and their optimal conditions are described in this Primer, followed by discussion of how bioorthogonal chemistry has become essential to the fields of biomedical imaging, medicinal chemistry, protein synthesis, polymer science, materials science and surface science. The applications of bioorthogonal chemistry are diverse and include genetic code expansion and metabolic engineering, drug target identification, antibody-drug conjugation and drug delivery. This Primer describes standards for reproducibility and data deposition, outlines how current limitations are driving new research directions and discusses new opportunities for applying bioorthogonal chemistry to emerging problems in biology and biomedicine.