Bioorthogonal Removal of 3-Isocyanopropyl Groups Enables the Controlled Release of Fluorophores and Drugs in Vivo

Mutually Orthogonal Bioorthogonal Reactions: Selective Chemistries for Labeling Multiple Biomolecules Simultaneously

Bioorthogonal click chemistry has played a transformative role in many research fields, including chemistry, biology, and medicine. Click reactions are crucial to produce increasingly complex bioconjugates, to visualize and manipulate biomolecules in living systems and for various applications in bioengineering and drug delivery. As biological (model) systems grow more complex, researchers have an increasing need for using multiple orthogonal click reactions simultaneously. In this review, we will introduce the most common bioorthogonal reactions and discuss their orthogonal use on the basis of their mechanism and electronic or steric tuning. We provide an overview of strategies to create reaction orthogonality and show recent examples of mutual orthogonal chemistry used for simultaneous biomolecule labeling. We end by discussing some considerations for the type of chemistry needed for labeling biomolecules in a system of choice.

Click Chemistry: Reaction Rates and Their Suitability for Biomedical Applications

Click chemistry has become a commonly used synthetic method due to the simplicity, efficiency, and high selectivity of this class of chemical reactions. Since their initial discovery, further click chemistry methods have been identified and added to the toolbox of click chemistry reactions for biomedical applications. However, selecting the most suitable reaction for a specific application is often challenging, as multiple factors must be considered, including selectivity, reactivity, biocompatibility, and stability. Thus, this review provides an overview of the benefits and limitations of well-established click chemistry reactions with a particular focus on the importance of considering reaction rates, an often overlooked criterion with little available guidance. The importance of understanding each click chemistry reaction beyond simply the reaction speed is discussed comprehensively with reference to recent biomedical research which utilized click chemistry. This review aims to provide a practical resource for researchers to guide the selection of click chemistry classes for different biomedical applications.

Computational Organic Chemistry: The Frontier for Understanding and Designing Bioorthogonal Cycloadditions

Computational organic chemistry has become a valuable tool in the field of bioorthogonal chemistry, offering insights and aiding in the progression of this branch of chemistry. In this review, I present an overview of computational work in this field, including an exploration of both the primary computational analysis methods used and their application in the main areas of bioorthogonal chemistry: (3 + 2) and [4 + 2] cycloadditions. In the context of (3 + 2) cycloadditions, detailed studies of electronic effects have informed the evolution of cycloalkyne/1,3-dipole cycloadditions. Through computational techniques, researchers have found ways to adjust the electronic structure via hyperconjugation to enhance reactions without compromising stability. For [4 + 2] cycloadditions, methods such as distortion/interaction analysis and energy decomposition analysis have been beneficial, leading to the development of bioorthogonal reactants with improved reactivity and the creation of orthogonal reaction pairs. To conclude, I touch upon the emerging fields of cheminformatics and machine learning, which promise to play a role in future reaction discovery and optimization.

Unlocking Amides: A General Method for the Self-Immolative Release of Amide-Containing Molecules

The controlled liberation of molecules from a constructed framework is a subject of profound interest across various chemical fields. It allows for the masking of a molecule's properties and precise deployment upon a single controllable release event. While numerous methodologies have been developed for amines, alcohols, and thiols, approaches for utilising amides as payload-release handles are still in their early stages of development, despite the prevalence of amides in therapeutic compounds and materials. Herein, is presented a comprehensive strategy for the controlled and selective release of a diverse range of amides with stable linkers. The versatility of this approach is demonstrated by its successful application in the targeted release of various amide-containing drugs in their natural form via the use of commonly used trigger motifs, such as dipeptides or glycosides. As a proof of concept, the FDA-approved antibiotic linezolid has been successfully converted into a prodrug form and released selectively only in the presence of the trigger event. Significantly, in its prodrug state, no activity against Mycobacterium tuberculosis was exhibited. Linezolid's full potential was achieved only upon controlled release, where an equipotent efficacy to the free linezolid control was demonstrated, thus emphasising the immense potential of this method.

Click chemistry in the development of PROTACs

Proteolysis-targeting chimeras or PROTACs are hetero-bifunctional molecules designed to mediate the disposal of a target protein via recruitment of the ubiquitination-proteasome degradation machinery. Because of the chimeric nature of such molecules, their synthesis requires a key step of "assembling" whether in the lab or in situ. Furthermore, targeted PROTACs often are hetero-trifunctional and require a second "assembling" step. Click chemistry has the unique advantages of tethering two or more molecular entities of choice under near physiological conditions and therefore has been applied to the development of PROTACs in various ways. This review provides a succinct summary of this field with a critical analysis of various factors that need to be considered for optimal results. Specifically, we examine issues including applications of click chemistry in in situ assembly for improved delivery, conjugation with a targeting group for selectivity, rapid synthesis for linker optimization, and lysosomal degradation of extracellular and membrane-associated proteins. We also examine reaction kinetics issues whenever possible or warranted.

Tetrazine-Isonitrile Bioorthogonal Fluorogenic Reactions Enable Multiplex Labeling and Wash-Free Bioimaging of Live Cells

Developing fluorogenic probes for simultaneous live cell labeling of multiple targets is crucial for understanding complex cellular events. The emerging [4+1] cycloaddition between tetrazine and isonitriles holds promise as a bioorthogonal tool, yet existing tetrazine probes lack reactivity and fluorogenicity. Here, we present the development of a series of tetrazine-functionalized bioorthogonal probes. By incorporating pyrazole adducts into the fluorophore scaffolds, the post-reacted probes displayed remarkable fluorescence turn-on ratios, up to 3184-fold. Moreover, these modifications are generalizable to various fluorophores, enabling a broad emission range from 473 to 659 nm. Quantum chemical calculations further elucidate the turn-on mechanisms. These probes enable the simultaneous labeling of multiple targets in live cells, without the need for a washing step. Consequently, our findings pave the way for advanced multiplex imaging and detection techniques for cellular studies.

Pyranthiones/Pyrones: "Click and Release" Donors for Subcellular Hydrogen Sulfide Delivery and Labeling

Hydrogen sulfide (H2 S), one of the most important gasotransmitters, plays a critical role in endogenous signaling pathways of many diseases. However, developing H2 S donors with both tunable release kinetics and high release efficiency for subcellular delivery has been challenging. Here, we describe a click and release reaction between pyrone/pyranthiones and bicyclononyne (BCN). This reaction features a release of CO2 /COS with second-order rate constants comparable to Strain-Promoted Azide-Alkyne Cycloaddition reactions (SPAACs). Interestingly, pyranthiones showed enhanced reaction rates compared to their pyrone counterparts. We investigated pyrone biorthogonality and demonstrated their utility in protein labeling applications. Moreover, we synthesized substituted pyranthiones with H2 S release kinetics that can address the range of physiologically relevant H2 S dynamics in cells and achieved quantitative H2 S release efficiency in vitro. Finally, we explored the potential of pyranthiones as H2 S/COS donors for mitochondrial-targeted H2 S delivery in living cells.

Enamine N-oxides: Design, Synthesis, and Function in Bioorthogonal Reactions

Enamine N-oxides act as a chemical linchpin bridging two bioorthogonal associative and dissociative reactions. This article describes the design of enamine N-oxides; their synthesis through the retro-Cope elimination reaction; the use of solvent, hyperconjugation, strain, and rehybridization effects to achieve bioorthogonal reactivity; and their rapid reductive cleavage with diboron reagents. The coordinated assembly and disassembly of the enamine N-oxide motif constitutes a powerful chemical operation that enables the attachment and detachment of small molecules from biomacromolecules in a biological setting.

Critical Design Strategies Supporting Optimized Drug Release from Polymer-Drug Conjugates

The importance of an adequate linking moiety design that allows controlled drug(s) release at the desired site of action is extensively studied for polymer-drug conjugates (PDCs). Redox-responsive self-immolative linkers bearing disulfide moieties (SS-SIL) represent a powerful strategy for intracellular drug delivery; however, the influence of drug structural features and linker-associated spacers on release kinetics remains relatively unexplored. The influence of drug/spacer chemical structure and the chemical group available for conjugation on drug release and the biological effect of resultant PDCs is evaluated. A "design of experiments" tool is implemented to develop a liquid chromatography-mass spectrometry method to perform the comprehensive characterization required for this systematic study. The obtained fit-for-purpose analytical protocol enables the quantification of low drug concentrations in drug release studies and the elucidation of metabolite presence. and provides the first data that clarifies how drug structural features influence the drug release from SS-SIL and demonstrates the non-universal nature of the SS-SIL. The importance of rigorous linker characterization in understanding structure-function correlations between linkers, drug chemical functionalities, and in vitro release kinetics from a rationally-designed polymer-drug nanoconjugate, a critical strategic crafting methodology that should remain under consideration when using a reductive environment as an endogenous drug release trigger.

Bioorthogonal Bond Cleavage Chemistry for On-demand Prodrug Activation: Opportunities and Challenges

Time- and space-resolved drug delivery is highly demanded for cancer treatment, which, however, can barely be achieved with a traditional prodrug strategy. In recent years, the prodrug strategy based on a bioorthogonal bond cleavage chemistry has emerged with the advantages of high temporospatial resolution over drug activation and homogeneous activation irrespective of individual heterogeneity. In the past five years, tremendous progress has been witnessed in this field with one such bioorthogonal prodrug entering Phase II clinical trials. This Perspective aims to highlight these new advances (2019-2023) and critically discuss their pros and cons. In addition, the remaining challenges and potential strategic directions for future progress will also be included.

Bioorthogonal dissociative rhenium(i) photosensitisers for controlled immunogenic cell death induction

Photosensitisers for photoimmunotherapy with high spatiotemporal controllability are rare. In this work, we designed rhenium(i) polypyridine complexes modified with a tetrazine unit via a bioorthogonally activatable carbamate linker as bioorthogonally dissociative photosensitisers for the controlled induction of immunogenic cell death (ICD). The complexes displayed increased emission intensities and singlet oxygen (1O2) generation efficiencies upon reaction with trans-cyclooct-4-enol (TCO-OH) due to the separation of the quenching tetrazine unit from the rhenium(i) polypyridine core. One of the complexes containing a poly(ethylene glycol) (PEG) group exhibited negligible dark cytotoxicity but showed greatly enhanced (photo)cytotoxic activity towards TCO-OH-pretreated cells upon light irradiation. The reason is that TCO-OH allowed the synergistic release of the more cytotoxic rhenium(i) aminomethylpyridine complex and increased 1O2 generation. Importantly, the treatment induced a cascade of events, including lysosomal dysfunction, autophagy suppression and ICD. To the best of our knowledge, this is the very first example of using bioorthogonal dissociation reactions as a trigger to realise photoinduced ICD, opening up new avenues for the development of innovative photoimmunotherapeutic agents.

A Bioorthogonal Decaging Chemistry of N-Oxide and Silylborane for Prodrug Activation both In Vitro and In Vivo

Bioorthogonal decaging chemistry with both fast kinetics and high efficiency is highly demanded for in vivo applications but remains very sporadic. Herein, we describe a new bioorthogonal decaging chemistry between N-oxide and silylborane. A simple replacement of "C" in boronic acid with "Si" was able to substantially accelerate the N-oxide decaging kinetics by 106 fold (k2: up to 103 M-1 s-1). Moreover, a new N-oxide-masked self-immolative spacer was developed for the traceless release of various payloads upon clicking with silylborane with fast kinetics and high efficiency (>90%). Impressively, one such N-oxide-based self-assembled bioorthogonal nano-prodrug in combination with silylborane led to significantly enhanced tumor suppression effects as compared to the parent drug in a 4T1 mouse breast tumor model. In aggregate, this new bioorthogonal click-and-release chemistry is featured with fast kinetics and high efficiency and is perceived to find widespread applications in chemical biology and drug delivery.

Isonitrile-Proline - A Versatile Handle for the Chemoselective Derivatization of Collagen Peptides

Functional groups that allow for chemoselective and bioorthogonal derivatization are valuable tools for the labelling of peptides and proteins. The isonitrile is such a group but synthetic methods for its incorporation into peptides by solid-phase peptide synthesis are not known. Here, we introduce (4S)- and (4R)-isonitrileproline (Inp) as building blocks for solid-phase peptide synthesis. Conformational studies of (4S)- and (4R)-Inp and thermal stability analysis of Inp-containing collagen triple helices revealed that the isonitrile group exerts a stereoelectronic gauche effect. We showcase the value of Inp for bioorthogonal labelling by derivatization of Inp-containing collagen model peptides (CMPs). Dual labelling with a pair of bioorthogonal reactions of a CMP containing Inp and azidoproline residues further highlights the versatility of the new isonitrile-containing amino acids.

Merging the Isonitrile-Tetrazine (4+1) Cycloaddition and the Ugi Four-Component Reaction into a Single Multicomponent Process

Multicomponent reactions are of utmost importance at generating a unique, wide, and complex chemical space. Herein we describe a novel multicomponent approach based on the combination of the isonitrile-tetrazine (4+1) cycloaddition and the Ugi four-component reaction to generate pyrazole amide derivatives. The scope of the reaction as well as mechanistic insights governing the 4H-pyrazol-4-imine tautomerization are provided. This multicomponent process provides access to a new chemical space of pyrazole amide derivatives and offers a tool for peptide modification and stapling.

Orthogonal Inverse-Electron-Demand Cycloaddition Reactions Controlled by Frontier Molecular Orbital Interactions

Chemoselective pairs of bioorthogonal reactants enable the simultaneous labeling of several biomolecules. Here, we access orthogonal click reactions by exploiting differences in frontier molecular orbital interaction energies in transition states. We establish that five-membered cyclic dienes are inert to isonitriles but readily react with strained alkynes, while tetrazines with bulky substituents readily react with isonitriles. Strained alkynes show an opposite reactivity pattern. The approach was demonstrated by orthogonally labeling two proteins with different fluorophores.

Instant Macrocyclizations via Multicomponent Reactions

Macrocycles fascinate chemists due to both their structure and their applications. However, we still lack efficient and sustainable synthetic methods, giving us straightforward access to them. Herein, a rapid macrocyclization utilizing a two-step, one-pot approach based on orthogonal multicomponent reaction (MCR) tactics is introduced. This synthetic protocol, which is based on Ugi and Groebke-Blackburn-Bienaymé reactions with isocyanides tethered to alkyl tosylates, yields medium sized macrocycles that are otherwise difficult to achieve. Single crystal structures reveal conformational reorganization via intramolecular hydrogen bonding, and modeling studies profile the synthesized libraries.

Biocompatible Lysine Protecting Groups for the Chemoenzymatic Synthesis of K48/K63 Heterotypic and Branched Ubiquitin Chains

The elucidation of emerging biological functions of heterotypic and branched ubiquitin (Ub) chains requires new strategies for their preparation with defined lengths and connectivity. While in vitro enzymatic assembly using expressed E1-activating and E2-conjugating enzymes can deliver homotypic chains, the synthesis of branched chains typically requires extensive mutations of lysines or other sequence modifications. The combination of K48- and K63-biased E2-conjugating enzymes and two new carbamate protecting groups-pyridoxal 5'-phosphate (PLP)-cleavable aminobutanamide carbamate (Abac group) and periodate-cleavable aminobutanol carbamate (Aboc group)-provides a strategy for the synthesis of heterotypic and branched Ub trimers, tetramers, and pentamers. The Abac- and Aboc-protected lysines are readily prepared and incorporated into synthetic ubiquitin monomers. As these masking groups contain a basic amine, they preserve the overall charge and properties of the Ub structure, facilitating folding and enzymatic conjugations. These protecting groups can be chemoselectively removed from folded Ub chains and monomers by buffered solutions of PLP or NaIO4. Through the incorporation of a cleavable C-terminal His-tag on the Ub acceptor, the entire process of chain building, iterative Abac deprotections, and global Aboc cleavage can be conducted on a resin support, obviating the need for handling and purification of the intermediate oligomers. Simple modulation of the Ub monomers affords various K48/K63 branched chains, including tetramers and pentamers not previously accessible by synthetic or biochemical methods.

Isonitrile-Tetrazine Click-and-Release Chemistry for Controlling RNA-Guided Nucleic Acid Cleavage

With the increasing demand for the regulation of CRISPR systems, a considerable number of studies have been conducted to control their excessive activity levels. In this context, we propose a method that involves a bioorthogonal cleavage reaction between isonitrile and tetrazine to modulate the cleavage activity of the CRISPR system. Importantly, isonitrile demonstrates significant potential for modifying sgRNAs, making it a promising candidate for bioorthogonal reactions, a phenomenon that has not been previously reported. Our approach utilizes the 3-isocyanopropyl-carbonate group as a caging group to deactivate the CRISPR systems, while tetrazine acts as an activator to restore their activities. Through the implementation of post-synthetic modifications and click-and-release chemistry, we have successfully achieved the regulation of RNA-guided nucleic acid cleavage, which holds great promise for controlling gene editing in human cells.

Structural Modification Endows Small-Molecular SN38 Derivatives with Multifaceted Functions

As a camptothecin derivative, 7-ethyl-10-hydroxycamptothecin (SN38) combats cancer by inhibiting topoisomerase I. SN38 is one of the most active compounds among camptothecin derivatives. In addition, SN38 is also a theranostic reagent due to its intrinsic fluorescence. However, the poor water solubility, high systemic toxicity and limited action against drug resistance and metastasis of tumor cells of SN38 indicates that there is great space for the structural modification of SN38. From the perspective of chemical modification, this paper summarizes the progress of SN38 in improving solubility, increasing activity, reducing toxicity and possessing multifunction and analyzes the strategies of structure modification to provide a reference for drug development based on SN38.

Triazine Chalcogenones from Thiocyanate or Selenocyanate Addition to Tetrazine Ligands in Ruthenium Arene Complexes

The chemistry of 1,2,4,5-tetrazines has attracted considerable interest both from a synthetic and applicative standpoint. Recently, regioselective reactions with alkynes and alkenes have been reported to be favored once the tetrazine ring is coordinated to Re(I), Ru(II), and Ir(III) centers. Aiming to further explore the effects of metal coordination, herein, we unveil the unexplored reactivity of tetrazines with chalcogenocyanate anions. Thus, ruthenium(II) tetrazine complexes, [RuCl{κ²N-3-(2-pyridyl)-6-R-1,2,4,5-tetrazine}(η⁶-arene)]⁺ (arene = p-cymene, R = H, [1a]⁺, R = Me, [1b]⁺, R = 2-pyridyl, [1c]⁺; arene = C₆Me₆, R = H, [1d]⁺, R = Me, [1e]⁺; PF₆^- salts), reacted quantitatively and in mild conditions with M(ECN) salts (M = Na, K, Bu₄N; E = O, S, Se). The addition of thiocyanate or selenocyanate to the tetrazine ligand is regioselective and afforded, via N₂ release, 1,2,4-triazine-5-chalcogenone heterocycles, the one with selenium being unprecedented. The novel ruthenium complexes [RuCl{κ²N-(2-pyridyl)}{triazine chalcogenone}(η⁶-arene)] 2a-e (sulfur), 3b, 3d, and 3e (selenium) were characterized by analytical (CHNS analyses, conductivity), spectroscopic (IR, multinuclear and two-dimensional (2D) NMR), and spectrometric (electrospray ionization mass spectrometry (ESI-MS)) techniques. According to density functional theory (DFT) calculations, the nucleophilic attack of SCN^- on the tetrazine ring is kinetically driven. Compound 2b is selectively and reversibly mono-protonated on the triazine ring by HCl or other strong acids, affording a single tautomer. When reactions of chalcogenocyanates were performed on the 2,2'-bipyridine (bpy) complex [RuCl(bpy)(η⁶-p-cymene)]⁺, the chloride substitution products [Ru(ECN)(bpy)(η⁶-p-cymene)]⁺ (E = O, [4]⁺; E = S, [5]⁺; E = Se, [6]⁺) were obtained in 82-90% yields (PF₆^- salts). Combined spectroscopic data (IR, ¹H/¹³C/⁷⁷Se NMR) was revealed to be a useful tool to study the linkage isomerism of the chalcogenocyanate ligand in [4-6]⁺.

Mutual leveraging of proximity effects and click chemistry in chemical biology

Nature has the remarkable ability to realize reactions under physiological conditions that normally would require high temperature and other forcing conditions. In doing so, often proximity effects such as simultaneous binding of two reactants in the same pocket and/or strategic positioning of catalytic functional groups are used as ways to achieve otherwise kinetically challenging reactions. Though true biomimicry is challenging, there have been many beautiful examples of how to leverage proximity effects in realizing reactions that otherwise would not readily happen under near-physiological conditions. Along this line, click chemistry is often used to endow proximity effects, and proximity effects are also used to further leverage the facile and bioorthogonal nature of click chemistry. This review brings otherwise seemingly unrelated topics in chemical biology and drug discovery under one unifying theme of mutual leveraging of proximity effects and click chemistry and aims to critically analyze the biomimicry use of such leveraging effects as powerful approaches in chemical biology and drug discovery. We hope that this review demonstrates the power of employing mutual leveraging proximity effects and click chemistry and inspires the development of new strategies that will address unmet needs in chemistry and biology.

Bioorthogonal Functionalization of Material Surfaces with Bioactive Molecules

The functionalization of material surfaces with biologically active molecules is crucial for enabling technologies in life sciences, biotechnology, and medicine. However, achieving biocompatibility and bioorthogonality with current synthetic methods remains a challenge. We report herein a novel surface functionalization method that proceeds chemoselectively and without a free transition metal catalyst. In this method, a coating is first formed via the tyrosinase-catalyzed putative polymerization of a tetrazine-containing catecholamine (DOPA-Tet). One or more types of molecule of interest containing trans-cyclooctene are then grafted onto the coating via tetrazine ligation. The entire process proceeds under physiological conditions and is suitable for grafting bioactive molecules with diverse functions and structural complexities. Utilizing this method, we functionalized material surfaces with enzymes (alkaline phosphatase, glucose oxidase, and horseradish peroxidase), a cyclic peptide (cyclo[Arg-Gly-Asp-D-Phe-Lys], or c(RGDfK)), and an antibiotic (vancomycin). Colorimetric assays confirmed the maintenance of the biocatalytic activities of the grafted enzymes on the surface. We established the mammalian cytocompatibility of the functionalized materials with fibroblasts. Surface functionalization with c(RGDfK) showed improved fibroblast cell morphology and cytoskeletal organization. Microbiological studies with Staphylococcus aureus indicated that surfaces coated using DOPA-Tet inhibit the formation of biofilms. Vancomycin-grafted surfaces additionally display significant inhibition of planktonic S. aureus growth.

Photoclick and Release: Co-activation of Carbon Monoxide and a Fluorescent Self-reporter, COS or Sulfonamide with Fast Kinetics

Bioorthogonal prodrugs with both fast reaction kinetics and multiple outputs are highly desirable but are only found sporadically. Herein, we report a novel photoclick-and-release strategy for the co-activation of carbon monoxide and a self-reporter, carbonyl sulfide, or sulfonamide with fast reaction kinetics (k: 1.4-22.6 M^-1  s^-1 ). Such a photoclick-and-release strategy was successfully applied in live cells to deliver carbon monoxide and a fluorescent self-reporter, both of which exhibited pronounced antiproliferative activity against 4T1 cancer cells. It is conceivable that this photoclick-and-release strategy could find applications in other fields, in which a controlled bond cleavage is preferred.

Metabolic glycoengineering - exploring glycosylation with bioorthogonal chemistry

Glycans are involved in numerous biological recognition events. Being secondary gene products, their labeling by genetic methods - comparable to GFP labeling of proteins - is not possible. To overcome this limitation, metabolic glycoengineering (MGE, also known as metabolic oligosaccharide engineering, MOE) has been developed. In this approach, cells or organisms are treated with synthetic carbohydrate derivatives that are modified with a chemical reporter group. In the cytosol, the compounds are metabolized and incorporated into newly synthesized glycoconjugates. Subsequently, the reporter groups can be further derivatized in a bioorthogonal ligation reaction. In this way, glycans can be visualized or isolated. Furthermore, diverse targeting strategies have been developed to direct drugs, nanoparticles, or whole cells to a desired location. This review summarizes research in the field of MGE carried out in recent years. After an introduction to the bioorthogonal ligation reactions that have been used in in connection with MGE, an overview on carbohydrate derivatives for MGE is given. The last part of the review focuses on the many applications of MGE starting from mammalian cells to experiments with animals and other organisms.

In Vivo Applications of Bioorthogonal Reactions: Chemistry and Targeting Mechanisms

Bioorthogonal chemistry involves selective biocompatible reactions between functional groups that are not normally present in biology. It has been used to probe biomolecules in living systems, and has advanced biomedical strategies such as diagnostics and therapeutics. In this review, the challenges and opportunities encountered when translating in vitro bioorthogonal approaches to in vivo settings are presented, with a focus on methods to deliver the bioorthogonal reaction components. These methods include metabolic bioengineering, active targeting, passive targeting, and simultaneously used strategies. The suitability of bioorthogonal ligation reactions and bond cleavage reactions for in vivo applications is critically appraised, and practical considerations such as the optimum scheduling regimen in pretargeting approaches are discussed. Finally, we present our own perspectives for this area and identify what, in our view, are the key challenges that must be overcome to maximise the impact of these approaches.

A tetrazine-responsive isonitrile-caged photosensitiser for site-specific photodynamic therapy

We report herein a versatile and efficient bioorthogonal strategy to actualise targeted delivery and site-specific activation of photosensitisers for precise antitumoural photodynamic therapy. The strategy involved the use of an isonitrile-caged distyryl boron dipyrromethene-based photosensitiser, labelled as NC-DSBDP, of which the photoactivities could be specifically activated upon conversion of the meso ester substituent to carboxylate initiated by the [4 + 1] cycloaddition with a tetrazine derivative. By using two tetrazines conjugated with a galactose moiety or the GE11 peptide, labelled as gal-Tz and GE11-Tz, we could selectively label the cancer cells overexpressed with the asialoglycoprotein receptor and the epidermal growth factor receptor respectively. Upon encountering the internalised NC-DSBDP, these tetrazines triggered the "ester-to-carboxylate" transformation of this compound, activating its fluorescence and reactive oxygen species generation inside the target cells. The bioorthogonal activation was also demonstrated in vivo, leading to effective photo-eradication of the tumour in nude mice.

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.

External stimuli-responsive gasotransmitter prodrugs: Chemistry and spatiotemporal release

Gasotransmitters like nitric oxide, carbon monoxide, and hydrogen sulfide with unique pleiotropic pharmacological effects in mammals are an emerging therapeutic modality for different human diseases including cancer, infection, ischemia-reperfusion injuries, and inflammation; however, their clinical translation is hampered by the lack of a reliable delivery form, which delivers such gasotransmitters to the action site with precisely controlled dosage. The external stimuli-responsive prodrug strategy has shown tremendous potential in developing gasotransmitter prodrugs, which affords precise temporospatial control and better dose control compared with endogenous stimuli-sensitive prodrugs. The promising external stimuli employed for gasotransmitter activation range from photo, ultrasound, and bioorthogonal click chemistry to exogenous enzymes. Herein, we highlight the recent development of external stimuli-mediated decaging chemistry for the temporospatial delivery of gasotransmitters including nitric oxide, carbon monoxide, hydrogen sulfide and sulfur dioxide, and discuss the pros and cons of different designs.

Bioorthogonal Click and Release: A General, Rapid, Chemically Revertible Bioconjugation Strategy Employing Enamine N-oxides

A chemically revertible bioconjugation strategy featuring a new bioorthogonal dissociative reaction employing enamine N-oxides is described. The reaction is rapid, complete, directional, traceless, and displays a broad substrate scope. Reaction rates for cleavage of fluorophores from proteins are on the order of 82 M-1s-1, and the reaction is relatively insensitive to common aqueous buffers and pHs between 4 and 10. Diboron reagents with bidentate and tridentate ligands also effectively reduce the enamine N-oxide to induce dissociation and compound release. This reaction can be paired with the corresponding bioorthogonal hydroamination reaction to afford an integrated system of bioorthogonal click and release via an enamine N-oxide linchpin with a minimal footprint. The tandem associative and dissociative reactions are useful for the transient attachment of proteins and small molecules with access to a discrete, isolable intermediate. We demonstrate the effectiveness of this revertible transformation on cells using chemically cleavable antibody-drug conjugates.

Chitosan Hydrogels with Embedded Thermo- and pH-Responsive Microgels as a Potential Carrier for Controlled Release of Drugs

We report a promising strategy based on chitosan (CS) hydrogels and dual temperature- and pH-responsive poly(N-isopropylacrylamide-co-methacrylic acid) (PNIPAM-co-MAA) microgels to facilitate release of a model drug, moxifloxacin (MFX). In this protocol, first, the microgels were prepared using a free radical copolymerization method, and subsequently, these carboxyl-group-rich soft particles were incorporated inside the hydrogel matrix using an EDC-NHS amidation method. Interestingly, the resulting microgel-embedded hydrogel composites (MG-HG) acting as a double barrier system largely reduced the drug release rate and prolonged the delivery time for up to 68 h, which was significantly longer than that obtained using microgels or hydrogels alone (20 h). On account of the dual-responsive features of the embedded microgels and the variation of water-solubility of drug molecules as a function of pH, MFX could be released in a controllable manner by regulating the temperature and pH of the delivery medium. The release kinetics followed a Korsmeyer-Peppas model, and the drug delivery mechanism was described by Fickian diffusion. Both the gel precursors and the hydrogel composites exhibited low cytotoxicity against mammalian cell lines (HeLa and HEK-293) and no deleterious hemolytic activity up to a certain higher concentration, indicating excellent biocompatibility of the materials. Thus, the unprecedented combination of modularity of physical properties caused by soft particle entrapment, unique macromolecular architecture, biocompatibility, and the general utility of the stimuli-responsive polymers offers a great promise to use these composite materials in drug delivery applications.

Biosynthesis of Isonitrile- and Alkyne-Containing Natural Products

Natural products are a diverse class of biologically produced compounds that participate in fundamental biological processes such as cell signaling, nutrient acquisition, and interference competition. Unique triple-bond functionalities like isonitriles and alkynes often drive bioactivity and may serve as indicators of novel chemical logic and enzymatic machinery. Yet, the biosynthetic underpinnings of these groups remain only partially understood, constraining the opportunity to rationally engineer biomolecules with these functionalities for applications in pharmaceuticals, bioorthogonal chemistry, and other value-added chemical processes. Here, we focus our review on characterized biosynthetic pathways for isonitrile and alkyne functionalities, their bioorthogonal transformations, and prospects for engineering their biosynthetic machinery for biotechnological applications.

Derivatization based on tetrazine scaffolds: synthesis of tetrazine derivatives and their biomedical applications

The recent advances in tetrazine scaffold-based derivatizations have been summarized. The advantages and limitations of derivatization methods and applications of the developed tetrazine derivatives in bioorthogonal chemistry have been highlighted.

Recent advances in self-immolative linkers and their applications in polymeric reporting systems

Interest in self-immolative chemistry has grown over the past decade with more research groups harnessing the versatility to control the release of a compound from a larger chemical entity, given...

On-Demand Activation of a Bioorthogonal Prodrug of SN-38 with Fast Reaction Kinetics and High Releasing Efficiency In Vivo

Although a myriad of bioorthogonal prodrugs have been developed, very few of them present both fast reaction kinetics and complete cleavage. Herein, we report a new bioorthogonal prodrug strategy with both fast reaction kinetics (k₂: ∼10³ M^-1 s^-1) and complete cleavage (>90% within minutes) using the bioorthogonal reaction pair of N-oxide and boron reagent. Distinctively, an innovative 1,6-elimination-based self-immolative linker is masked by N-oxide, which can be bioorthogonally demasked by a boron reagent for the release of both amino and hydroxy-containing payload in live cells. Such a strategy was applied to prepare a bioorthogonal prodrug for a camptothecin derivative, SN-38, resulting in 10-fold weakened cytotoxicity against A549 cells, 300-fold enhanced water solubility, and "on-demand" activation upon a click reaction both in vitro and in vivo. This novel bioorthogonal prodrug strategy presents significant advances over the existing ones and may find wide applications in drug delivery in the future.

Isonitrile induced bioorthogonal activation of fluorophores and mutually orthogonal cleavage in live cells

Fluorophores with different emission wavelengths were efficiently quenched by a tert-butyl terminated tetrazylmethyl group and activated by an isonitrile-tetrazine click-to-release reaction. Nucleic acid templated chemistry significantly accelerated this bioorthogonal cleavage. Moreover, two mutually orthogonal fluorogenic cleavage reactions were simultaneously conducted in live cells for the first time.

The diversity of the coordination bond generated a POSS-based fluorescent probe for the reversible detection of Cu(II), Fe(III) and amino acids

In recent years, it has been found that Cu²⁺, Fe³⁺, and amino acids play an irreplaceable and subtle role in organisms and have attracted the considerable attention of many researchers. Therefore, it is vital to design visual indicators to reveal the relationships between metal ions and amino acids. However, there have been few reports on this vigorous subject. Fortunately, based on the different coordination effects between metal ions and boron groups, we have designed an accessible fluorescent probe (PSI-A). Borane was introduced as an ion-sensitive group to form a novel POSS-based fluorescent probe, which achieves fascinating performance, in situ dynamic multiple detection, excellent photostability, and enervative biological toxicity. PSI-A exhibited predominant selectivity and sensitivity to Cu²⁺/amino acids and Fe³⁺/amino acids sequence reactions in HepG2 cells and zebrafish. The fluorescence of PSI-A was quenched by Cu²⁺, which can be recovered by adding Asp, Ser, Arg, Ace or Trp. Additionally, the fluorescence of PSI-A quenched by Fe³⁺ can be restored after adding Asp. PSI-A is available to monitor Cu²⁺/amino acids and Fe³⁺/amino acids sequence reactions and can be repeated for at least three consecutive cycles without a fatigued performance. Therefore, this multifunctional fluorescent probe may have prospective application potentials in the biological field.

EGFR-targeted prodrug activation using bioorthogonal alkene-azide click-and-release chemistry

Epidermal growth factor receptor (EGFR) is overexpressed in many cancers and therefore serves as an excellent target for prodrug activation. Functionalised trans-cyclooctenes (TCO) were conjugated to an EGFR antibody (cetuximab), providing a reagent for pre-targeting and localisation of the bioorthogonal reagent. The TCOs react with a 4-azidobenzyl carbamate doxorubicin prodrug via a [3 + 2]-cycloaddition and subsequent self-immolation leads to release of doxorubicin (click-and-release). In vitro cell-based assays demonstrated proof-of-concept, that cetuximab conjugated to highly strained TCO (AB-d-TCO) could bind to the EGFR in a melanoma cell line, and selectively activate the doxorubicin prodrug. In a non-EGFR expressing melanoma cell line, no significant prodrug activation was observed. In vivo experiments using this combination of AB-d-TCO and the azido-doxorubicin prodrug in a murine melanoma model revealed no significant anti-tumour activity or increased survival, suggesting there was insufficient prodrug activation and drug release at the tumour site.

Dynamic Nucleophilic Aromatic Substitution of Tetrazines

A dynamic nucleophilic aromatic substitution of tetrazines (SN Tz) is presented herein. It combines all the advantages of dynamic covalent chemistry with the versatility of the tetrazine moiety. Indeed, libraries of compounds or sophisticated molecular structures can be easily obtained, which are susceptible to post-functionalization by inverse electron demand Diels-Alder (IEDDA) reaction, which also locks the exchange. Additionally, the structures obtained can be disassembled upon the application of the right stimulus, either UV irradiation or a suitable chemical reagent. Moreover, SN Tz is compatible with the imine chemistry of anilines. The high potential of this methodology has been proved by building two responsive supramolecular systems: A macrocycle that displays a light-induced release of acetylcholine; and a truncated [4+6] tetrahedral shape-persistent fluorescent cage, which is disassembled by thiols unless it is post-stabilized by IEDDA.

Enabling In Vivo Photocatalytic Activation of Rapid Bioorthogonal Chemistry by Repurposing Silicon-Rhodamine Fluorophores as Cytocompatible Far-Red Photocatalysts

Chromophores that absorb in the tissue-penetrant far-red/near-infrared window have long served as photocatalysts to generate singlet oxygen for photodynamic therapy. However, the cytotoxicity and side reactions associated with singlet oxygen sensitization have posed a problem for using long-wavelength photocatalysis to initiate other types of chemical reactions in biological environments. Herein, silicon-Rhodamine compounds (SiRs) are described as photocatalysts for inducing rapid bioorthogonal chemistry using 660 nm light through the oxidation of a dihydrotetrazine to a tetrazine in the presence of trans-cyclooctene dienophiles. SiRs have been commonly used as fluorophores for bioimaging but have not been applied to catalyze chemical reactions. A series of SiR derivatives were evaluated, and the Janelia Fluor-SiR dyes were found to be especially effective in catalyzing photooxidation (typically 3%). A dihydrotetrazine/tetrazine pair is described that displays high stability in both oxidation states. A protein that was site-selectively modified by trans-cyclooctene was quantitatively conjugated upon exposure to 660 nm light and a dihydrotetrazine. By contrast, a previously described methylene blue catalyst was found to rapidly degrade the protein. SiR-red light photocatalysis was used to cross-link hyaluronic acid derivatives functionalized by dihydrotetrazine and trans-cyclooctenes, enabling 3D culture of human prostate cancer cells. Photoinducible hydrogel formation could also be carried out in live mice through subcutaneous injection of a Cy7-labeled hydrogel precursor solution, followed by brief irradiation to produce a stable hydrogel. This cytocompatible method for using red light photocatalysis to activate bioorthogonal chemistry is anticipated to find broad applications where spatiotemporal control is needed in biological environments.

Recent Developments and Strategies for Mutually Orthogonal Bioorthogonal Reactions

Over the past decade, several different metal-free bioorthogonal reactions have been developed to enable simultaneous double-click labeling with minimal-to-no competing cross-reactivities; such transformations are termed 'mutually orthogonal'. More recently, several examples of successful triple ligation strategies have also been described. In this minireview, we discuss selected aspects of the development of orthogonal bioorthogonal reactions over the past decade, including general strategies to drive future innovations to achieve simultaneous, mutually orthogonal click reactions in one pot.

A Cationic Micelle as In Vivo Catalyst for Tumor-Localized Cleavage Chemistry

The emerging strategies of accelerating the cleavage reaction in tumors through locally enriching the reactants is promising. Yet, the applications are limited due to the lack of the tumor-selectivity for most of the reactants. Here we explored an alternative approach to leverage the rate constant by locally inducing an in vivo catalyst. We found that the desilylation-induced cleavage chemistry could be catalyzed in vivo by cationic micelles, and accelerated over 1400-fold under physiological condition. This micelle-catalyzed controlled release platform is demonstrated by the release of a 6-hydroxyl-quinoline-2-benzothiazole derivative (HQB) in two cancer cell lines and a NIR dye in mouse tumor xenografts. Through intravenous injection of a pH-sensitive polymer micelles, we successfully applied this strategy to a prodrug activation of hydroxyl camptothecin (OH-CPT) in tumors. Its "decaging" efficiency is 42-fold to that without cationic micelles-mediated catalysis. This micelle-catalyzed desilylation strategy unveils the potential that micelle may act beyond a carrier but a catalyst for local perturbing or activation.

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.

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.

Transition-Metal-Mediated versus Tetrazine-Triggered Bioorthogonal Release Reactions: Direct Comparison and Combinations Thereof

Bioorthogonal cleavage reactions are gaining popularity in chemically inducible prodrug activation and in the control of biomolecular functions. Despite similar applications, these reactions were developed and optimized on different substrates and under different experimental conditions. Reported herein is a side-by-side comparison of palladium-, ruthenium- and tetrazine-triggered release reactions, which aims at comparing the reaction kinetics, efficiency and overall advantages and limitations of the methods. In addition, we disclose the possibility of mutual combination of the cleavage reactions. Finally, we compare the efficiency of the bioorthogonal deprotections in cellular experiments, which revealed that among the three methods investigated, the palladium- and the tetrazine-promoted reaction can be used for efficient prodrug activation, but only the tetrazine-triggered reactions proceed efficiently inside cells.

Isocyanides: Promising Functionalities in Bioorthogonal Labeling of Biomolecules

Isocyanides have drawn increasing attention in biological applications due to their attractive properties and unique reactivities, which can undergo various reactions, such as multicomponent reactions, α-addition reactions, [4 + 1] cycloaddition reactions, and the reaction scope keeps expanding. In addition to acting as reactants for the preparation of structurally interesting and diverse N-heterocycles or peptidomimetics, this type of functionality may be a good choice in the labeling and modulation of biomolecules due to the high biocompatibility and small size to minimize modifications on the parent molecule. It has been demonstrated that isocyanides can participate in biomolecule labeling through three strategies, including the two-component bioorthogonal reaction, multicomponent reaction, and metal chelation. Among them, the isocyanide-tetrazine reaction has been better studied recently, augmenting the potency of isocyanide as a bioorthogonal handle. This review will focus on the recent progress in isocyanide chemistry for labeling of biomolecules. Meanwhile, methods to introduce isocyano groups into biomacromolecules are also described to facilitate wider applications of this unique functionality.