Bioorthogonal catalysis for biomedical applications

Signature of click chemistry in advanced techniques for cancer therapeutics

Click chemistry has made a revolution in the field of chemical biology owing to its high efficiency, specificity, and mild reaction conditions. The copper(i)-catalyzed azide-alkyne cycloaddition (CuAAC) and strain-promoted [3 + 2] azide-alkyne cycloaddition (SPAAC) stand out as the most popular click reactions that construct a stable triazole ring by reacting an azide with an alkyne. These two reactions represent an ideal choice for biological applications due to its specificity, reliability, and biocompatibility. As a powerful modular synthetic approach for creating new molecular entities, it has seen increasing use in anticancer drug discovery. The present "state of the art" focuses mainly on the signature of click chemistry (CuAAC and SPAAC) in advanced techniques for cancer therapeutics, which includes cancer immunotherapy, antibody-drug conjugates, development of proteolysis-targeting chimeras, targeted dual-agent combination therapy for cancer, exosome modification for cancer therapy, and photodynamic therapy (PDT).

Exploring New Bioorthogonal Catalysts: Scaffold Diversity in Catalysis for Chemical Biology

Bioorthogonal catalysis has revolutionized the field of chemical biology by enabling selective and controlled chemical transformations within living systems. Research has converged on the development of innovative catalyst scaffolds, seeking to broaden the scope of bioorthogonal reactions, boost their efficiency, and surpass the limitations of conventional catalysts. This review provides a comprehensive overview of the latest advancements in bioorthogonal catalyst research based on different scaffold materials. Through an in-depth analysis of fabrication strategies and applications of bioorthogonal catalysts, this review discusses the design principles, mechanisms of action, and applications of these novel catalysts in chemical biology. Current challenges and future directions in exploring the scaffold diversity are also highlighted. The integration of diverse catalyst scaffolds offers exciting prospects for precise manipulation of biomolecules and the development of innovative therapeutic strategies in chemical biology. In addition, the review fills in the gaps in previous reviews, such as in fully summarizing the presented scaffold materials applied in bioorthogonal catalysts, emphasizing the potential impact on advancing bioorthogonal chemistry, and offering prospects for future development in this field.

Bioorthogonal Synthetic Chemistry Enabled by Visible-Light Photocatalysis

The field of bioorthogonal chemistry has revolutionized our ability to interrogate and manipulate biological systems at the molecular level. However, the range of chemical reactions that can operate efficiently in biological environments without interfering with the native cellular machinery, remains limited. In this context, the rapidly growing area of photocatalysis offers a promising avenue for developing new type of bioorthogonal tools. The inherent mildness, tunability, chemoselectivity, and external controllability of photocatalytic transformations make them particularly well-suited for applications in biological and living systems. This minireview summarizes recent advances in bioorthogonal photocatalytic technologies, with a particular focus on their potential to enable the selective generation of designed products within biologically relevant or living settings.

Light-Controlled Bioorthogonal Chemistry Altered Natural Killer Cell Activity for Boosted Adoptive Immunotherapy

Natural killer (NK) cell-based immunotherapy has received much attention in recent years. However, its practical application is still suffering from the decreased function and inadequate infiltration of NK cells in the immunosuppressive microenvironment of solid tumors. Herein, we construct light-responsive porphyrin Fe array-armed NK cells (denoted as NK@p-Fe) for cell behavior modulation via bioorthogonal catalysis. By installing cholesterol-modified porphyrin Fe molecules on the NK cell surface, a catalytic array with light-harvesting capabilities is formed. This functionality transforms NK cells into cellular factories capable of catalyzing the production of active agents in a light-controlled manner. NK@p-Fe can generate the active antineoplastic drug doxorubicin through bioorthogonal reactions to enhance the cytotoxic function of NK cells. Beyond drug synthesis, NK@p-Fe can also bioorthogonally catalyze the production of the FDA-approved immune agonist imiquimod (IMQ). The activated immune agonist plays a dual role, inducing dendritic cell maturation for NK cell activation and reshaping the tumor immunosuppressive microenvironment for NK cell infiltration. This work represents a paradigm for the modulation of adoptive cell behaviors to boost cancer immunotherapy by bioorthogonal catalysis.

Palladium-Mediated Bioorthogonal System for Prodrug Activation of N-Benzylbenzamide-Containing Tubulin Polymerization Inhibitors for the Treatment of Solid Tumors

Bioorthogonal cleavage reactions have been developed as an intriguing strategy to enhance the safety of chemotherapeutics. Aiming to reduce the toxicity and improve the targeted release properties of the colchicine binding site inhibitors (CBSIs) based on previous work, a series of biologically inert prodrugs were further designed and synthesized through a bioorthogonal prodrug strategy. The therapeutic effects of prodrugs could be "turned-on" once combined with palladium resins. Particularly, prodrug 2b was 68.3-fold less cytotoxic compared to the parent compound, while its cytotoxicity was recovered in situ in the presence of palladium resins. Mechanism studies confirmed that 2b inhibited cell growth in the same manner as CBSIs. More importantly, in vivo efficacy studies demonstrated the efficient activation of 2b by palladium resins, resulting in significant inhibition of tumor growth (63.2%). These results suggest that prodrug 2b with improved safety and targeted release property catalyzed by a Pd-mediated bioorthogonal cleavage reaction deserves further investigation.

Smart Bioorthogonal Nanozymes: From Rational Design to Appropriate Bioapplications

Bioorthogonal chemistry has provided an elaborate arsenal to manipulate native biological processes in living systems. As the great advancement of nanotechnology in recent years, bioorthogonal nanozymes are innovated to tackle the challenges that emerged in practical biomedical applications. Bioorthogonal nanozymes are uniquely positioned owing to their advantages of high customizability and tunability, as well as good adaptability to biological systems, which bring exciting opportunities for biomedical applications. More intriguingly, the great advancement in nanotechnology offers an exciting opportunity for innovating bioorthogonal catalytic materials. In this comprehensive review, the significant progresses of bioorthogonal nanozymes are discussed with both spatiotemporal controllability and high performance in living systems, and highlight their design principles and recent rapid applications. The remaining challenges and future perspectives are then outlined along this thriving field. It is expected that this review will inspire and promote the design of novel bioorthogonal nanozymes, and facilitate their clinical translation.

Designing intelligent bioorthogonal nanozymes: Recent advances of stimuli-responsive catalytic systems for biomedical applications

Bioorthogonal nanozymes have emerged as a potent tool in biomedicine due to their unique ability to perform enzymatic reactions that do not interfere with native biochemical processes. The integration of stimuli-responsive mechanisms into these nanozymes has further expanded their potential, allowing for controlled activation and targeted delivery. As such, intelligent bioorthogonal nanozymes have received more and more attention in developing therapeutic approaches. This review provides a comprehensive overview of the recent advances in the development and application of stimuli-responsive bioorthogonal nanozymes. By summarizing the design outlines for anchoring bioorthogonal nanozymes with stimuli-responsive capability, this review seeks to offer valuable insights and guidance for the rational design of these remarkable materials. This review highlights the significant progress made in this exciting field with different types of stimuli and the various applications. Additionally, it also examines the current challenges and limitations in the design, synthesis, and application of these systems, and proposes potential solutions and research directions. This review aims to stimulate further research toward the development of more efficient and versatile stimuli-responsive bioorthogonal nanozymes for biomedical applications.

Engineering of bioorthogonal polyzymes through polymer sidechain design

Synthetic polymer scaffolds can encapsulate transition metal catalysts (TMCs) to provide bioorthogonal nanocatalysts. These 'polyzymes' catalyze the in situ generation of therapeutic agents without disrupting native biological processes. The design and modification of polymer scaffolds in these polyzymes can enhance the catalytic performance of TMCs in biological environments. In this study, we explore the hydrophobic design space of an oxanorborneneimide-based polymer by varying the length of its carbon side chain to engineer bioorthogonal polyzymes. Activity studies indicate that modulating the hydrophobicity of the polymer scaffold can be used to enhance the catalyst loading efficacy, catalytic activity, and serum stability of polyzymes. These findings provide insight into the structural elements contributing to improving polymeric nanocatalysts for a variety of applications.

Engineered Leukocyte Biomimetic Colorimetric Sensor Enables High-Efficient Detection of Tumor Cells Based on Bioorthogonal Chemistry

Accurate detection of heterogeneous circulating tumor cells (CTCs) is critical as they can make tumor cells more aggressive, drug-resistant, and metastasizing. Although the leukocyte membrane coating strategy is promising in meeting the challenge of detecting heterogeneous CTCs due to its inherent antiadhesive properties, it is still limited by the reduction or loss of expression of known markers. Bioorthogonal glycol-metabolic engineering is expected to break down this barrier by feeding the cells with sugar derivatives with a unique functional group to establish artificial targets on the surface of tumor cells. Herein, an engineered leukocyte biomimetic colorimetric sensor was accordingly fabricated for high-efficient detection of heterogeneous CTCs. Compared with conventional leukocyte membrane coating, the sensor could covalently bound to the heterogeneous CTCs models fed with Ac4ManNAz in vitro through the synergy of bioorthogonal chemistry and metabolic glycoengineering, ignoring the phenotypic changes of heterogeneous CTCs. Meanwhile, a sandwich structure composed of leukocyte biomimetic layer/CTCs/MoS2 nanosheet was formed for visual detection of HeLa cells as low as 10 cells mL-1. Overall, this approach can overcome the dependence of conventional cell membrane biomimetic technology on specific cell phenotypes and provide a new viewpoint to highly efficiently detect heterogeneous CTCs.

Albumin-ruthenium catalyst conjugate for bio-orthogonal uncaging of alloc group

The employment of antibodies as a targeted drug delivery vehicle has proven successful which is exemplified by the emergence of antibody-drug conjugates (ADCs). However, ADCs are not without their shortcomings. Improvements may be made to the ADC platform by decoupling the cytotoxic drug from the delivery vehicle and conjugating an organometallic catalyst in its place. The resulting protein-metal catalyst conjugate was designed to uncage the masked cytotoxin administered as a separate entity. Macropinocytosis of albumin by cancerous cells suggests the potential of albumin acting as the tumor-targeting delivery vehicle. Herein reported are the first preparation and demonstration of ruthenium catalysts with cyclopentadienyl and quinoline-based ligands conjugated to albumin. The effective uncaging abilities were demonstrated on allyloxy carbamate (alloc)-protected rhodamine 110 and doxorubicin, providing a promising catalytic scaffold for the advancement of selective drug delivery methods in the future.

Nanoparticle-Catalyzed Transamination under Tumor Microenvironment Conditions: A Novel Tool to Disrupt the Pool of Amino Acids and GSSG in Cancer Cells

Catalytic cancer therapy targets cancer cells by exploiting the specific characteristics of the tumor microenvironment (TME). TME-based catalytic strategies rely on the use of molecules already present in the TME. Amino groups seem to be a suitable target, given the abundance of proteins and peptides in biological environments. Here we show that catalytic CuFe2O4 nanoparticles are able to foster transaminations with different amino acids and pyruvate, another key molecule present in the TME. We observed a significant in cellulo decrease in glutamine and alanine levels up to 48 h after treatment. In addition, we found that di- and tripeptides also undergo catalytic transamination, thereby extending the range of the effects to other molecules such as glutathione disulfide (GSSG). Mechanistic calculations for GSSG transamination revealed the formation of an imine between the oxo group of pyruvate and the free -NH2 group of GSSG. Our results highlight transamination as alternative to the existing toolbox of catalytic therapies.

Modular Fabrication of Bioorthogonal Nanozymes for Biomedical Applications

The incorporation of transition metal catalysts (TMCs) into nanoscaffolds generates nanocatalysts that replicate key aspects of enzymatic behavior. The TMCs can access bioorthogonal chemistry unavailable to living systems. These bioorthogonal nanozymes can be employed as in situ "factories" for generating bioactive molecules where needed. The generation of effective bioorthogonal nanozymes requires co-engineering of the TMC and the nanometric scaffold. This review presents an overview of recent advances in the field of bioorthogonal nanozymes, focusing on modular design aspects of both nanomaterial and catalyst and how they synergistically work together for in situ uncaging of imaging and therapeutic agents.

Polarization of macrophages to an anti-cancer phenotype through in situ uncaging of a TLR 7/8 agonist using bioorthogonal nanozymes

Macrophages are plastic cells of the immune system that can be broadly classified as having pro-inflammatory (M1-like) or anti-inflammatory (M2-like) phenotypes. M2-like macrophages are often associated with cancers and can promote cancer growth and create an immune-suppressive tumor microenvironment. Repolarizing macrophages from M2-like to M1-like phenotype provides a crucial strategy for anticancer immunotherapy. Imiquimod is an FDA-approved small molecule that can polarize macrophages by activating toll-like receptor 7/8 (TLR 7/8) located inside lysosomes. However, the non-specific inflammation that results from the drug has limited its systemic application. To overcome this issue, we report the use of gold nanoparticle-based bioorthogonal nanozymes for the conversion of an inactive, imiquimod-based prodrug to an active compound for macrophage re-education from anti- to pro-inflammatory phenotypes. The nanozymes were delivered to macrophages through endocytosis, where they uncaged pro-imiquimod in situ. The generation of imiquimod resulted in the expression of pro-inflammatory cytokines. The re-educated M1-like macrophages feature enhanced phagocytosis of cancer cells, leading to efficient macrophage-based tumor cell killing.

Intracellular Synthesis of Indoles Enabled by Visible-Light Photocatalysis

Performing abiotic synthetic transformations in live cell environments represents a new, promising approach to interrogate and manipulate biology and to uncover new types of biomedical tools. We now found that photocatalytic bond-forming reactions can be added to the toolbox of bioorthogonal synthetic chemistry. Specifically, we demonstrate that exogenous styryl aryl azides can be converted into indoles inside living mammalian cells under photocatalytic conditions.

Designer Nanoreactors for Bioorthogonal Catalysis

The evolutionary complexity of compartmentalized biostructures (such as cells and organelles) endows life-sustaining multistep chemical cascades and intricate living functionalities. Relatively, within a very short time span, a synthetic paradigm has resulted in tremendous growth in controlling the materials at different length scales (molecular, nano, micro, and macro), improving mechanistic understanding and setting the design principals toward different compositions, configurations, and structures, and in turn fine-tuning their optoelectronic and catalytic properties for targeted applications. Bioorthogonal catalysis offers a highly versatile toolkit for biochemical modulation and the capability to perform new-to-nature reactions inside living systems, endowing augmented functions. However, conventional catalysts have limitations to control the reactions under physiological conditions due to the hostile bioenvironment. The present account details the development of bioapplicable multicomponent designer nanoreactors (NRs), where the compositions, morphologies, interfacial active sites, and microenvironments around different metal nanocatalysts can be precisely controlled by novel nanospace-confined chemistries. Different architectures of porous, hollow, and open-mouth silica-based nano-housings facilitate the accommodation, protection, and selective access of different nanoscale metal-based catalytic sites. The modular porosity/composition, optical transparency, thermal insulation, and nontoxicity of silica are highly useful. Moreover, large macropores or cavities can also be occupied by enzymes (for chemoenzymatic cascades) and selectivity enhancers (for stimuli-responsive gating) along with the metal nanocatalysts. Further, it is crucial to selectively activate and control catalytic reactions by a remotely operable biocompatible energy source. Integration of highly coupled plasmonic (Au) components having few-nanometer structural features (gaps, cavities, and junctions as electromagnetic hot-spots) endows an opportunity to efficiently harness low-power NIR light and selectively supply energy to the interfacial catalytic sites through localized photothermal and electronic effects. Different plasmonically integrated NRs with customizable plasmonic-catalytic components, cavities inside bilayer nanospaces, and metal-laminated nanocrystals inside hollow silica can perform NIR-/light-induced catalytic reactions in complex media including living cells. In addition, magnetothermia-induced NRs by selective growth of catalytic metals on a pre-installed superparamagnetic iron-oxide core inside a hollow-porous silica shell endowed the opportunity to apply AMF as a bioorthogonal stimulus to promote catalytic reactions. By combining "plasmonic-catalytic" and "magnetic-catalytic" components within a single NR, two distinct reaction steps can be desirably controlled by two energy sources (NIR light and AMF) of distinct energy regimes. The capability to perform multistep organic molecular transformations in harmony with the natural living system will reveal novel reaction schemes for in cellulo synthesis of active drug and bioimaging probes. Well-designed nanoscale discrete architectures of NRs can facilitate spatiotemporal control over abiotic chemical synthesis without adversely affecting the cell viability. However, in-depth understanding of heterogeneous surface catalytic reactions, rate induction mechanisms, selectivity control pathways, and targeted nanobio interactions is necessary. The broad field of biomedical engineering can hugely benefit from the aid of novel nanomaterials with chemistry-based designs and the synthesis of engineered NRs performing unique bioorthogonal chemistry functions.

Nanoconstruction on Living Cell Surfaces with Cucurbit[7]uril-Based Supramolecular Polymer Chemistry: Toward Cell-Based Delivery of Bio-Orthogonal Catalytic Systems

Employing living cells as carriers to transport transition metal-based catalysts for target-specific bio-orthogonal catalysis represents a cutting-edge approach in advancing precision biomedical applications. One of the initial hurdles in this endeavor involves effectively attaching the catalysts to the carrier cells while preserving the cells' innate ability to interact with biological systems and maintaining the unaltered catalytic activity. In this study, we have developed an innovative layer-by-layer method that leverages a noncovalent interaction between cucurbit[7]uril and adamantane as the primary driving force for crafting polymeric nanostructures on the surfaces of these carrier cells. The strong binding affinity between the host-guest pair ensures the creation of a durable polymer coating on the cell surfaces. Meanwhile, the layer-by-layer process offers high adaptability, facilitating the efficient loading of bio-orthogonal catalysts onto cell surfaces. Importantly, the polymeric coating shows no discernible impact on the cells' physiological characteristics, including their tropism, migration, and differentiation, while preserving the effectiveness of the bio-orthogonal catalysts.

An in situ dual-anchoring strategy for enhanced immobilization of PD-L1 to treat autoimmune diseases

Immune checkpoints play key roles in maintaining self-tolerance. Targeted potentiation of the checkpoint molecule PD-L1 through in situ manipulation offers clinical promise for patients with autoimmune diseases. However, the therapeutic effects of these approaches are often compromised by limited specificity and inadequate expression. Here, we report a two-step dual-anchor coupling strategy for enhanced immobilization of PD-L1 on target endogenous cells by integrating bioorthogonal chemistry and physical insertion of the cell membrane. In both type 1 diabetes and rheumatoid arthritis mouse models, we demonstrate that this approach leads to elevated and sustained conjugation of PD-L1 on target cells, resulting in significant suppression of autoreactive immune cell activation, recruitment of regulatory T cells, and systematic reshaping of the immune environment. Furthermore, it restores glucose homeostasis in type 1 diabetic mice for over 100 days. This specific in situ bioengineering approach potentiates the functions of PD-L1 and represents its translational potential.

Progress in controllable bioorthogonal catalysis for prodrug activation

Bioorthogonal catalysis, a class of catalytic reactions that are mediated by abiotic metals and proceed in biological environments without interfering with native biochemical reactions, has gained ever-increasing momentum in prodrug delivery over the past few decades. Albeit great progress has been attained in developing new bioorthogonal catalytic reactions and optimizing the catalytic performance of transition metal catalysts (TMCs), the use of TMCs to activate chemotherapeutics at the site of interest in vivo remains a challenging endeavor. To translate the bioorthogonal catalysis-mediated prodrug activation paradigm from flasks to animals, TMCs with targeting capability and stimulus-responsive behavior have been well-designed to perform chemical transformations in a controlled manner within highly complex biochemical systems, rendering on-demand drug activation to mitigate off-target toxicity. Here, we review the recent advances in the development of controllable bioorthogonal catalysis systems, with an emphasis on different strategies for engineering TMCs to achieve precise control over prodrug activation. Furthermore, we outline the envisaged challenges and discuss future directions of controllable bioorthogonal catalysis for disease therapy.

Bioorthogonal Disruption of Pyroptosis Checkpoint for High-Efficiency Pyroptosis Cancer Therapy

Pyroptosis is an inflammatory form of programmed cell death that holds great promise in cancer therapy. However, autophagy as the crucial pyroptosis checkpoint and the self-protective mechanism of cancer cells significantly weakens the therapeutic efficiency. Here, a bioorthogonal pyroptosis nanoregulator is constructed to induce pyroptosis and disrupt the checkpoint, enabling high-efficiency pyroptosis cancer therapy. The nanoregulator allows the in situ synthesis and accumulation of the photosensitizer PpIX in the mitochondria of cancer cells to directly produce mitochondrial ROS, thus triggering pyroptosis. Meanwhile, the in situ generated autophagy inhibitor via palladium-catalyzed bioorthogonal chemistry can disrupt the pyroptosis checkpoint to boost the pyroptosis efficacy. With the biomimetic cancer cell membrane coating, this platform for modulating pyroptosis presents specificity to cancer cells and poses no harm to normal tissue, resulting in a highly efficient and safe antitumor treatment. To our knowledge, this is the first report on a disrupting intrinsic protective mechanism of cancer cells for tumor pyroptosis therapy. This work highlights that autophagy as a checkpoint plays a key regulative role in pyroptosis therapy, which would motivate the future design of therapeutic regimens.

Nanomaterials for Fighting Multidrug-Resistant Biofilm Infections

Multidrug-resistant bacterial infections represent a dire threat to global health. The development of antibiotic resistance in bacteria coupled with the lack of development of new antibiotics is creating infections requiring antibiotics of last resort, and even some infections for which we have no available treatment. Biofilm-based infections present some of the most challenging targets for treatment. The biofilm matrix provides a physical barrier that can impede access of antibiotics and antimicrobials to resident bacteria. The phenotypic diversity found in biofilms further exacerbates the difficulty of eliminating infections, with quiescent "persister" cells evading therapeutics and re-initiating infections after treatment. Nanomaterials provide a tool for combatting these refractory biofilm infections. The distinctive size regime and physical properties of nanomaterials provide them with the capability to penetrate and disrupt biofilms. Nanomaterials can also access antimicrobial pathways inaccessible to conventional antimicrobials, providing a synergistic strategy for treating biofilm infections. This review will summarize key challenges presented by antibiotic resistance and biofilms when treating infection and provide selected examples of how nanomaterials are being used to address these challenges.

Oxidative Desymmetrization Enables the Concise Synthesis of a trans-Cyclooctene Linker for Bioorthogonal Bond Cleavage

Modified trans-cyclooctenes (TCO) are capable of highly efficient molecular manipulations in biological environments, driven by the bioorthogonal reaction with tetrazines (Tz). The development of click-cleavable TCO has fueled the field of in vivo chemistry and enabled the design of therapeutic strategies that have already started to enter the clinic. A key element for most of these approaches is the implementation of a cleavable TCO linker. So far, only one member of this class has been developed, a compound that requires a high synthetic effort, mainly to fulfill the multilayered demands on its chemical structure. To tackle this limitation, we developed a dioxolane-fused cleavable TCO linker (dcTCO) that can be prepared in only five steps by applying an oxidative desymmetrization to achieve diastereoselective introduction of the required functionalities. Based on investigation of the structure, reaction kinetics, stability, and hydrophilicity of dcTCO, we demonstrate its bioorthogonal application in the design of a caged prodrug that can be activated by in-situ Tz-triggered cleavage to achieve a remarkable >1000-fold increase in cytotoxicity.

A Cell Selective Fluoride-Activated MOF Biomimetic Platform for Prodrug Synthesis and Enhanced Synergistic Cancer Therapy

As a burgeoning bioorthogonal reaction, the fluoride-mediated desilylation is capable of prodrug activation. However, due to the reactions lack of cell selectivity and unitary therapy modality, this strongly impedes their biomedical applications. Herein, we construct a cancer cell-selective biomimetic metal-organic framework (MOF)-F platform for prodrug activation and enhanced synergistic chemodynamic therapy (CDT). With cancer cell membranes camouflage, the designed biomimetic nanocatalyst displays preferential accumulation to homotypic cancer cells. Then, pH-responsive nanocatalyst releases fluoride ions and ferric ions. For activation of our designed prodrug tert-butyldimethyl silyl (TBS)-hydroxycamptothecin (TBSO-CPT), fluoride ions can desilylate TBS and cleave the designed silyl ether linker to synthesize the OH-CPT (10-hydroxycamptothecin) drug molecule, which effectively kills cancer cells. Intriguingly, the bioorthogonal-synthesized OH-CPT drug upregulates intracellular H₂O₂ by activating nicotinamide adenine dinucleotide phosphate oxidase (NOX), amplifying the released iron induced Fenton reaction for synergistic CDT. Both in vitro and in vivo studies demonstrate our strategy presents a versatile fluoride-activated bioorthogonal catalyst for cancer cell-selective drug synthesis. Our work may accelerate the biomedical applications of fluoride-activated bioorthogonal chemistry.

Bioorthogonal nanozymes: an emerging strategy for disease therapy

Transition metal catalysts (TMCs), capable of performing bioorthogonal reactions, have been engineered to trigger the formation of bioactive molecules in a controlled manner for biomedical applications. However, the widespread use of TMCs based biorthogonal reactions in vivo is still largely limited owing to their toxicity, poor stability, and insufficient targeting properties. The emergence of nanozymes (nanomaterials with enzyme-like activity), especially bioorthogonal nanozymes that combine the bioorthogonal catalytic activity of TMCs, the physicochemical properties of nanomaterials, and the enzymatic properties of classical nanozymes potentially provide opportunities to address these challenges. Thus, they can be used as multifunctional catalytic platforms for disease treatment and will be far-reaching. In this review, we first briefly recall the classical TMC-based bioorthogonal reactions. Furthermore, this review highlights the diverse strategies for manufacturing bioorthogonal nanozymes and their potential for therapeutic applications, with the goal of facilitating bioorthogonal catalysis in the clinic. Finally, we present challenges and the prospects of bioorthogonal nanozymes in bioorthogonal chemistry.

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.

Tetrazine-Triggered Bioorthogonal Cleavage of trans-Cyclooctene-Caged Phenols Using a Minimal Self-Immolative Linker Strategy

Bond-cleavage reactions triggered by bioorthogonal tetrazine ligation have emerged as strategies to chemically control the function of (bio)molecules and achieve activation of prodrugs in living systems. While most of these approaches make use of caged amines, current methods for the release of phenols are limited by unfavorable reaction kinetics or insufficient stability of the Tz-responsive reactants. To address this issue, we have implemented a self-immolative linker that enables the connection of cleavable trans-cyclooctenes (TCO) and phenols via carbamate linkages. Based on detailed investigation of the reaction mechanism with several Tz, revealing up to 96 % elimination after 2 hours, we have developed a TCO-caged prodrug with 750-fold reduced cytotoxicity compared to the parent drug and achieved in situ activation upon Tz/TCO click-to-release.

A Stimuli-Responsive Small-Molecule Metal-Carrying Prochelator: A Novel Prodrug Design Strategy for Metal Complexes

Selective activation of prodrugs is an important approach to reduce the side effects of disease treatment. We report a prodrug design concept for metal complexes, termed "metal-carrying prochelator", which can co-carry a metal ion and chelator within a single small-molecule compound and remain inert until it undergoes a specifically triggered intramolecular chelation to synthesize a bioactive metal complex in situ for targeted therapy. As a proof-of-concept, we designed a H₂ O₂ -responsive small-molecule prochelator, DPBD, based on the strong chelator diethyldithiocarbamate (DTC) and copper. DPBD can carry Cu²⁺ (DPBD-Cu) and respond to elevated H₂ O₂ levels in tumor cells by releasing DTC, which rapidly chelates Cu²⁺ from DPBD-Cu affording a DTC-copper complex with high cytotoxicity, realizing potent antitumor efficacy with low systemic toxicity. Thus, with its unique intramolecularly triggered activation mechanism, this concept based on a small-molecule metal-carrying prochelator can help in the prodrug design of metal complexes.

Bioorthogonal micellar nanoreactors for prodrug cancer therapy using an inverse-electron-demand Diels-Alder reaction

Block copolymer micelles functionalized with tetrazine groups can act as nanoreactors to activate a trans-cyclooctene-functionalized prodrug for releasing anticancer drugs via a bioorthogonal inverse-electron-demand Diels-Alder (IEDDA) reaction. In addition, the IEDDA reaction can be accelerated in the micellar nanoreactor system compared to the free tetrazine system. Moreover, In vivo prodrug activation in a mouse tumor model led to the inhibition of tumor growth without significant systemic toxicity. These results demonstrated their potential for applications as bioorthogonal micellar nanoreactors for cancer chemotherapy.