Triggered Drug Release from an Antibody-Drug Conjugate Using Fast "Click-to-Release" Chemistry in Mice

Reactive Oxygen Species-Instructed Supramolecular Assemblies Enable Bioorthogonally Activatable Protein Degradation for Pancreatic Cancer

Proteolysis-Targeting Chimeras (PROTACs) represent a transformative therapeutic platform for targeted protein degradation across diverse disease indications. However, their potent catalytic activity in normal tissues raises significant concerns regarding off-target toxicity. Here, we present a novel supramolecular self-assembly platform for the bioorthogonal control of PROTAC prodrug activation, enabling tumor-specific protein degradation with minimized systemic toxicity. By exploiting the overproduction of reactive oxygen species (ROS) in pancreatic cancer cells, the supramolecular self-assembly approach selectively accumulates bioorthogonal reaction triggers within the targeted malignant cells, which subsequently facilitates the spatiotemporally controlled activation of the bioorthogonally caged PROTAC. This tumor-selective activation mechanism demonstrates enhanced degradation efficiency in pancreatic cancer cells compared to normal cells. In vivo studies reveal potent tumor growth inhibition with complete preservation of major organ histology, confirming the therapeutic index enhancement achieved through a controllable activation strategy. This biomimetic activation platform establishes a generalizable framework for safer PROTAC-based therapies by integrating tumor-specific microenvironmental cues with bioorthogonal reaction engineering.

Advances in the Delivery, Activation and Therapeutics Applications of Bioorthogonal Prodrugs

Traditional prodrug strategies have been leveraged to overcome many inherent drawbacks of active native drugs in the drug research and development. However, endogenous stimuli such as specific microenvironment or enzymes are relied on to achieve the prodrug activation, resulting in unintended drug release and systemic toxicity. Alternatively, bioorthogonal cleavage reaction-enabled bioorthogonal prodrugs activation via exogenous triggers has emerged as a valuable approach, featuring spatiotemporally controlled drug release. Such bioorthogonal prodrug strategies would ensure targeted drug delivery and/or in situ generation, further circumventing systemic toxicity or premature elimination of active drugs. In recent years, metal-free bioorthogonal cleavage reactions with fast kinetics have boomed in the bioorthogonal prodrug design. Meanwhile, transition-metal-catalyzed and photocatalytic deprotection reactions have also been developed to trigger prodrug activation in biological systems. Besides traditional small molecule prodrugs, gasotransmitters have been successfully delivered to specific organelles or cells via bioorthogonal reactions, and nanosystems have been devised into bioorthogonal triggers as well. Herein, we present an overview of the latest advances in these bioorthogonally-uncaged prodrugs, focused on the delivery, activation and therapeutics applications.

Harnessing nanoparticles and bioorthogonal chemistries for improving precision of nuclear medicine

The convergence of nanotechnology, radiopharmaceutical development and molecular imaging has unveiled exciting opportunities for the progress of innovative diagnostic and therapeutic strategies, paving the way for significant advancements in biomedical research, especially in relation to cancer. For example, the use of highly sensitive and quantitative nuclear imaging techniques including PET and SPECT, together with nanoparticles for tumour imaging and therapy has recently expanded rapidly. While the long circulating properties of many nanomaterials are beneficial for prodrug chemotherapy formulations, due to the constant decay processes involved in nuclear medicines, directly labelled materials result in prolonged systemic radiation exposure and reduced therapeutic indices due to the unfavourable target-to-background ratios. This is due to the tendency for long circulating nanomaterials to distribute within the blood to other organs, such as the liver and spleen. The recent integration of bioorthogonal chemistry with nanotechnology and molecular imaging/radiotherapy has revolutionized the field by allowing the decoupling of the targeting molecule (i.e. nanomaterial with a bioorthogonal tag) and the imaging/therapeutic radioisotope. In this way, the detection/therapeutic element can be administered as a secondary "chase" molecule that contains the bioorthogonal partner, thereby creating an avenue to improve therapeutic index and provide imaging and treatments with reduced risk. This review will provide an overview of the progress made thus far in the field of nuclear imaging and radiotherapy for cancer using the combination of nanomaterials and bioorthogonal chemistry. We also provide a critical evaluation of the challenges and opportunities for using these approaches to better understand disease and treatment mechanisms, with the potential for downstream clinical translation.

Sydnonimines: synthesis, properties and applications in chemical biology

Sydnonimines are intriguing compounds belonging to the mesoionic family. To date, only a limited number of research groups have studied their chemistry and use in organic synthesis, medicinal chemistry and chemical biology. This review aims at providing an overview of the synthesis and the properties of sydnonimines and the most recent developments in their use as tools for chemical biology. The recent discovery that sydnonimines can act as a dipole to undergo bioorthogonal click-and-release reactions with cycloalkynes has stimulated a renewed interest from the scientific community. Given the high potential of these mesoionics, we believe that major developments are to be expected in the field of bioorthogonal chemistry.

A biorthogonal chemistry approach for high-contrast antibody imaging of lymphoma at early time points

BACKGROUND

Monoclonal antibodies are highly specific for their targets making them effective for cancer therapy. However, their large molecular weight causes slow blood clearance, often requiring weeks to be removed from circulation. This limitation affects companion nuclear imaging and antibody-based diagnostics, necessitating delayed imaging. We report the expansion of a methodology improving positron emission tomography (PET) contrast of the lymphoma biomarker CD20 at early time points after radiolabeled antibody administration. Intact radioimmunoconjugates are allowed to stay in circulation long enough to accumulate in tumors, and then, using a chemical trigger, we induced rapid clearance of the radioactivity from non-target tissues by cleaving the linker between the antibody and the radioactivity. For brevity, we refer to the this as the Tetrazine KnockOut (TKO) method which uses the transcyclooctene-tetrazine (TCO-Tz) reaction, wherein an antibody is conjugated with linker containing TCO and a radioisotope.

RESULTS

We optimized the TCO linker with several different radioisotopes and evaluated the ability of tetrazines to knockout radioactivity from circulating antibodies. We explored several cell types and antibodies with varying internalization rates, to characterize the parameters of TKO and tested [89Zr]Zr-DFO-TCO-rituximab in a lymphoma model with PET imaging after Tz or vehicle administration. Treatment with Tz induced > 70% cleavage of the TCO linker in vitro within 30 min. Internalizing radioimmunoconjugates exhibited similar cellular uptake with Tz compared to vehicle, whereas decreased uptake was seen with slowly internalizing antibodies. In rodents, Tz rapidly liberated the radioactivity from the antibody, cleared from the blood, and accumulated in the bladder. TKO resulted in > 50% decreased radioactivity in non-target organs following Tz injection. No decrease in tumor uptake was observed when rate of antibody internalization is higher in a lymphoma model, and the target-to-background ratio increased by > twofold in comparison with Tz nontreated groups at 24 h.

CONCLUSION

The TKO approach potentiates early imaging of rituximab radioimmunoconjugates and has translational potential for lymphoma imaging.

The Legionella pneumophila peptidoglycan recycling kinase, AmgK, is essential for survival and replication inside host alveolar macrophages

Bacterial cells are surrounded by a dynamic cell wall which in part is made up of a mesh-like peptidoglycan (PG) layer that provides the cell with structural integrity and resilience. In Gram-positive bacteria, this layer is thick and robust, whereas in Gram-negative bacteria, it is thinner and flexible as the cell is supported by an additional outer membrane. PG undergoes continuous turnover, with degradation products being recycled to maintain cell wall homeostasis. Some Gram-negative species can bypass de novo PG biosynthesis, relying instead on PG recycling to sustain growth and division. Legionella pneumophila (hereafter Legionella), the causative agent of Legionnaires' disease, encodes such recycling machinery within its genome. This study investigates the biochemical, genetic, and pathogenic roles of PG recycling in Legionella. Previously, we have shown that PG can be visualized in both model and native systems using a combination of N-acetylmuramic acid (NAM) probes and PG recycling programs. Here, two PG recycling gene homologs in the Legionella genome lpg0296 (amgK) and lpg0295 (murU) were identified and characterized; chemical biology strategies were used to rigorously track the incorporation of "click"-PG-probes. Deletion of amgK abolished PG labeling, while genetic complementation restored labeling. Additionally, copper-free click chemistry with ultra-fast tetrazine-NAM probes enabled live-cell PG labeling. The data suggest that amgK contributes to the pathogenicity of the organism, as amgK deletion increased Legionella's susceptibility to antibiotics and significantly reduced Legionella's ability to replicate in host alveolar macrophages. An intracellular replication assay demonstrated that while PG recycling is not essential for internalization, successful replication of Legionella within MH-S murine alveolar macrophages requires functional amgK. These findings underscore the essential role of AmgK in Legionella's intracellular survival, emphasizing the importance of PG recycling in pathogenicity, and establish a foundation for developing novel Legionella-specific antibiotic strategies.

Conditional Control of Benzylguanine Reaction with the Self-Labeling SNAP-tag Protein

SNAP-tag, a mutant of the O6-alkylguanine-DNA-alkyltransferase, self-labels by reacting with benzylguanine (BG) substrates, thereby forming a thioether bond. SNAP-tag has been genetically fused to a wide range of proteins of interest in order to covalently modify them. In the context of both diagnostic and therapeutic applications, as well as use as a biological recording device, precise control in a spatial and temporal fashion over the covalent bond-forming reaction is desired to direct inputs, readouts, or therapeutic actions to specific locations, at specific time points, in cells and organisms. Here, we introduce a comprehensive suite of six caged BG molecules: one light-triggered and five others that can be activated through various chemical and biochemical stimuli, such as small molecules, transition metal catalysts, reactive oxygen species, and enzymes. These molecules are unable to react with SNAP-tag until the trigger is present, which leads to near complete SNAP-tag conjugation, as illustrated both in biochemical assays and on human cell surfaces. This approach holds promise for targeted therapeutic assembly at disease sites, offering the potential to reduce off-target effects and toxicity through precise trigger titration.

Recent Advances in Smart Linkage Strategies for Developing Drug Conjugates for Targeted Delivery

Targeted drug delivery systems effectively solve the problem of off-target toxicity of chemotherapeutic drugs by combining chemotherapeutic drugs with antibodies or peptides, thereby promoting drug targeting to the tumor site and bringing further hope for cancer treatment. The development of stimulus-responsive smart linkage technologies has led to the emergence of drug conjugates. Linkage technologies play a crucial role in the design, synthesis, and in vivo circulation of drug conjugates, as they determine the release of cytotoxic drugs from the conjugates and their subsequent therapeutic efficacy. This article reviews some of the smart linkage strategies used in designing drug conjugates, with a focus on the tumor microenvironment and exogenous stimuli as conditions influencing controlled drug release. This review introduces linker classifications and cleavage mechanisms, discusses modular linkers that promote the efficient synthesis of conjugates, and discusses the differences between linkage strategies. Furthermore, this article focuses on the implementation of self-assembly in drug conjugates, which is currently of great interest. Related concepts are introduced and relevant examples of their applications are provided. Furthermore, a comprehensive discourse is presented on the challenges that may arise in the research and clinical implementation of diverse linkage strategies, along with the associated enhancement measures. Finally, the factors that should be considered when designing linkage strategies for drug conjugates are summarized, offering strategies and ideas for scientists involved in drug conjugate research. It is particularly noteworthy that appropriate linkage strategies allow for the intracellular release of drugs after internalization of the conjugates, thereby maximizing their tumor cell-killing effect.

Toward Realization of Bioorthogonal Chemistry in the Clinic

In the last decade, the use of bioorthogonal chemistry toward medical applications has increased tremendously. Besides being useful for the production of pharmaceuticals, the efficient, nontoxic reactions open possibilities for the development of therapies that rely on in vivo chemistry between two bioorthogonal components. Here we discuss the latest developments in bioorthogonal chemistry, with a focus on their use in living organisms, the translation from model systems to humans, and the challenges encountered during preclinical development. We aim to provide the reader a broad presentation of the current state of the art and demonstrate the numerous possibilities that bioorthogonal reactions have for clinical use, now and in the near future.

A Tetrazine Amplification System for Visual Detection of Trace Analytes via Click-Release Reactions

Achieving visual detection of analytes at ultra-low concentrations in complex mixtures remains a persistent challenge. While sophisticated techniques offer single-molecule sensitivity, practical hurdles remain, necessitating tailored signal amplification systems for direct visual detection. In this study, we develop a strategy for the visualized detection of tetrazine through a "click-release-oxidation-cycle" (CROC) cascade amplification process. We systematically describe the construction and synthesis of this system, the kinetic process of click release, the kinetics of oxidation to tetrazine and its cascade amplification effect in trace amounts of tetrazine. This system is capable of amplifying the signal of tetrazine at a concentration as low as 2 nM by 105-fold, thereby providing a clearly visible purple signal. Finally, as proof of concept, we successfully apply this method to visually detect trace β-galactosidase (β-gal) and Pd2+.

Synthesis and crystal structure of 2,2,2-tri-chloroethyl N-{4-[6-(1-hy-droxy-eth-yl)-1,2,4,5-tetra-zin-3-yl]benz-yl}carbamate

An orthogonally addressable 3,6-disubstituted 1,2,4,5-tetra-zine, namely 2,2,2-tri-chloro-ethyl N-{4-[6-(1-hy-droxy-eth-yl)-1,2,4,5-tetra-zin-3-yl]benz-yl}carbamate (C14H14Cl3N5O3), was synthesized and characterized by single-crystal X-ray diffraction. The tetra-zine comprises a free hydroxyl and a 2,2,2-tri-chloro-eth-oxy-carbonyl protected amino group, which gives rise to hydrogen-bonding inter-actions each making the tetra-zine highly linked in the solid state. The carbamate moieties form inter-molecular hydrogen bonds, stacking the tetra-zine mol-ecules above each other, while lateral hydrogen bonds are formed between a tetra-zine N atom and a hydroxyl group, the latter inter-action being a scarcely explored structural feature of 1,2,4,5-tetra-zines.

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

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

Transforming Aryl-Tetrazines into Bioorthogonal Scissors for Systematic Cleavage of trans-Cyclooctenes

Bioorthogonal bond-cleavage reactions have emerged as a powerful tool for precise spatiotemporal control of (bio)molecular function in the biological context. Among these chemistries, the tetrazine-triggered elimination of cleavable trans-cyclooctenes (click-to-release) stands out due to high reaction rates, versatility, and selectivity. Despite an increasing understanding of the underlying mechanisms, application of this reaction remains limited by the cumulative performance trade-offs (i.e., click kinetics, release kinetics, release yield) of existing tools. Efficient release has been restricted to tetrazine scaffolds with comparatively low click reactivity, while highly reactive aryl-tetrazines give only minimal release. By introducing hydroxyl groups onto phenyl- and pyridyl-tetrazine scaffolds, we have developed a new class of 'bioorthogonal scissors' with unique chemical performance. We demonstrate that hydroxyaryl-tetrazines achieve near-quantitative release upon accelerated click reaction with cleavable trans-cyclooctenes, as exemplified by click-triggered activation of a caged prodrug, intramitochondrial cleavage of a fluorogenic probe (turn-on) in live cells, and rapid intracellular bioorthogonal disassembly (turn-off) of a ligand-dye conjugate.

Key considerations based on pharmacokinetic/pharmacodynamic in the design of antibody-drug conjugates

Background

Antibody-drug conjugate (ADC) is an anticancer drug that links toxins to specifically targeted antibodies via linkers, offering the advantages of high target specificity and high cytotoxicity. However, complexity of its structural composition poses a greater difficulty for drug design studies.

Objectives

Pharmacokinetic/pharmacodynamic (PK/PD) based consideration of ADCs has increasingly become a hot research topic for optimal drug design in recent years, providing possible ideas for obtaining ADCs with desirable properties.

Methods

From the assessment of the ADC action process based on PK/PD, we introduce the main research strategies of ADCs. In addition, we investigated the strategies to solve the prominent problems of ADC in the clinic in recent years, and summarized and evaluated the specific ways to optimize various problems of ADC based on the PK/PD model from two perspectives of optimizing the structure and properties of the drugs themselves. Through the selection of target antigen, the optimization of the linker, the optimization of novel small molecule toxins as payload, the optimization of ADC, overcoming the multi-drug resistance of ADC, improving the ADC tumor penetration of ADC, surface modification of ADC and surface bystander effect of ADC provide a more comprehensive and accurate framework for designing new ADCs.

Results

We've expounded comprehensively on applying pharmacokinetics or pharmacodynamics while designing ADC to obtain higher efficacy and fewer side effects. From the ADC's PK/PD property while coming into play in vivo and the PK/PD study strategy, to specific ADC optimization methods and recommendations based on PK/PD, it has been study-approved that the PK/PD properties exert a subtle role in the development of ADC, whether in preclinical trials or clinical promotion.

Conclusion

The study of PK/PD unfolds the detailed mechanism of ADC action, making it easier to control related parameters in the process of designing ADC, limited efficacy and inevitable off-target toxicity remain a challenging bottleneck.

Ortho-functionalized pyridinyl-tetrazines break the inverse correlation between click reactivity and cleavage yields in click-to-release chemistry

The bioorthogonal tetrazine-triggered cleavage of trans-cyclooctene(TCO)-linked payloads has strong potential for widespread use in drug delivery and in particular in click-cleavable antibody-drug conjugates (ADCs). However, clinical translation is hampered by an inverse correlation between click reactivity and payload release yield, requiring high doses of less reactive tetrazines to drive in vivo TCO reactions and payload release to completion. Herein we report that the cause for the low release when using the highly reactive bis-(2-pyridinyl)-tetrazine is the stability of the initially formed 4,5-dihydropyridazine product, precluding tautomerization to the releasing 1,4-dihydropyridazine tautomer. We demonstrate that efficient tautomerization and payload elimination can be achieved by ortho-substituting bis-pyridinyl-tetrazines with hydrogen-bonding hydroxyl or amido groups, achieving a.o. release yields of 96% with 18-fold more reactive tetrazines. Applied to on-tumor activation of a click-cleavable ADC in mice, these tetrazines afforded near-quantitative ADC conversion at a ca. 10- to 20-fold lower dose than what was previously needed, resulting in a strong therapeutic response.

A Targeted Click-to-Release Activation of the Last-Resort Antibiotic Colistin Reduces its Renal Cell Toxicity

The use of highly potent but very toxic antibiotics such as colistin has become inevitable due to the rise of antimicrobial resistance. We aimed for a chemically-triggered, controlled release of colistin at the infection site to lower its systemic toxicity by harnessing the power of click-to-release reactions. Kinetic experiments with nine tetrazines and three dienophiles demonstrated a fast release via an inverse-electron-demand Diels-Alder reaction between trans-cyclooctene (TCO) and the amine-functionalised tetrazine Tz7. The antibiotic activity of colistin against Escherichia coli was masked by TCO units, but restored upon reaction with d-Ubi-Tz, a tetrazine functionalised with the bacterial binding peptide d-Ubi29-41. While standard TCO did not improve toxicity against human proximal tubular kidney HK-2 cells, the installation of an aspartic acid-modified TCO masking group reduced the overall charge of the peptide and entry to the kidney cells, thereby dramatically lowering its toxicity. The analog Col-(TCO-Asp)1 had favourable pharmacokinetic properties in mice and was successfully activated locally in the lung by d-Ubi-Tz in an in vivo infection model, whereas it remained inactive and non-harmful without the chemical trigger. This study constitutes the first example of a systemically acting two-component antibiotic with improved drug tolerability.

Conditional PROTAC: Recent Strategies for Modulating Targeted Protein Degradation

Proteolysis-targeting chimeras (PROTACs) have emerged as a promising technology for inducing targeted protein degradation by leveraging the intrinsic ubiquitin-proteasome system (UPS). While the potential druggability of PROTACs toward undruggable proteins has accelerated their rapid development and the wide-range of applications across diverse disease contexts, off-tissue effects and side-effects of PROTACs have recently received attentions to improve their efficacy. To address these issues, spatial or temporal target protein degradation by PROTACs has been spotlighted. In this review, we explore chemical strategies for modulating protein degradation in a cell type-specific (spatio-) and time-specific (temporal-) manner, thereby offering insights for expanding PROTAC applications to overcome the current limitations of target protein degradation strategy.

Chemistry-driven translocation of glycosylated proteins in mice

Cell surface glycans form various "glycan patterns" consisting of different types of glycan molecules, thus enabling strong and selective cell-to-cell recognition. We previously conjugated different N-glycans to human serum albumin to construct glycoalbumins mimicking natural glycan patterns that could selectively recognize target cells or control excretion pathways in mice. Here, we develop an innovative glycoalbumin capable of undergoing transformation and remodeling of its glycan pattern in vivo, which induces its translocation from the initial target to a second one. Replacing α(2,3)-sialylated N-glycans on glycoalbumin with galactosylated glycans induces the translocation of the glycoalbumin from blood or tumors to the intestine in mice. Such "in vivo glycan pattern remodeling" strategy can be used as a drug delivery system to promote excretion of a drug or medical radionuclide from the tumor after treatment, thereby preventing prolonged exposure leading to adverse effects. Alternatively, this study provides a potential strategy for using a single glycoalbumin for the simultaneous treatment of multiple diseases in a patient.

Scission-Enhanced Molecular Imaging (SEMI)

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

Stimuli-Responsive Polymers at the Interface with Biology

There has been growing interest in polymeric systems that break down or undergo property changes in response to stimuli. Such polymers can play important roles in biological systems, where they can be used to control the release of therapeutics, modulate imaging signals, actuate movement, or direct the growth of cells. In this Perspective, after discussing the most important stimuli relevant to biological applications, we will present a selection of recent exciting developments. The growing importance of stimuli-responsive polysaccharides will be discussed, followed by a variety of stimuli-responsive polymeric systems for the delivery of small molecule drugs and nucleic acids. Switchable polymers for the emerging area of therapeutic response measurement in theranostics will be described. Then, the diverse functions that can be achieved using hydrogels cross-linked covalently, as well as by various dynamic approaches will be presented. Finally, we will discuss some of the challenges and future perspectives for the field.

On Demand Bioorthogonal Switching of an Antibody-Conjugated SPECT Probe to a Cytotoxic Payload: from Imaging to Therapy

Antibody-drug conjugates (ADCs) for the treatment of cancer aim to achieve selective delivery of a cytotoxic payload to tumor cells while sparing normal tissue. In vivo, multiple tumor-dependent and -independent processes act on ADCs and their released payloads to impact tumor-versus-normal delivery, often resulting in a poor therapeutic window. An ADC with a labeled payload would make synchronous correlations between distribution and tissue-specific pharmacological effects possible, empowering preclinical and clinical efforts to improve tumor-selective delivery; however, few methods to label small molecules without destroying their pharmacological activity exist. Herein, we present a bioorthogonal switch approach that allows a radiolabel attached to an ADC payload to be removed tracelessly at will. We exemplify this approach with a potent DNA-damaging agent, the pyrrolobenzodiazepine (PBD) dimer, delivered as an antibody conjugate targeted to lung tumor cells. The radiometal chelating group, DOTA, was attached via a novel trans-cyclooctene (TCO)-caged self-immolative para-aminobenzyl (PAB) linker to the PBD, stably attenuating payload activity and allowing tracking of biodistribution in tumor-bearing mice via SPECT-CT imaging (live) or gamma counting (post-mortem). Following TCO-PAB-DOTA reaction with tetrazines optimized for extra- and intracellular reactivity, the label was removed to reveal the unmodified PBD dimer capable of inducing potent tumor cell killing in vitro and in mouse xenografts. The switchable antibody radio-drug conjugate (ArDC) we describe integrates, but decouples, the two functions of a theranostic given that it can serve as a diagnostic for payload delivery in the labeled state, but can be switched on demand to a therapeutic agent (an ADC).

Pharmacokinetic-Pharmacodynamic Analysis of pH-Responsive Doxorubicin-Releasing Micelles with Anticancer Activity

This study aimed to investigate the effect of in vivo pH-responsive doxorubicin (DOX) release and the targetability of pilot molecules in folic acid (FA)-modified micelles using a pharmacokinetic-pharmacodynamic (PK-PD) model. The time profiles of intratumoral DOX concentrations in Walker256 tumor-bearing rats were monitored using a microdialysis probe, followed by compartmental analysis, to evaluate intratumoral tissue pharmacokinetics. Maximal DOX was released from micelles 350 min after the administration of pH-responsive DOX-releasing micelles. However, FA modification of the micelles shortened the time to peak drug concentration to 150 min. Additionally, FA modification resulted in a 27-fold increase in the tumor inflow rate constant. Walker256 tumor-bearing rats were subsequently treated with DOX, pH-responsive DOX-releasing micelles, and pH-responsive DOX-releasing FA-modified micelles to monitor the tumor growth-time profiles. An intratumoral threshold concentration of DOX (55-64 ng/g tumor) was introduced into the drug efficacy compartment to construct a PD model, followed by PK-PD analysis of the tumor growth-time profiles. Similar results of threshold concentration and drug potency of DOX were obtained across all three formulations. Cell proliferation was delayed as the drug delivery ability of DOX was improved. The PK model, which was developed using the microdialysis method, revealed the intratumoral pH-responsive DOX distribution profiles. This facilitated the estimation of intratumoral PK parameters. The PD model with threshold concentrations contributed to the estimation of PD parameters in the three formulations, with consistent mechanisms observed. We believe that our PK-PD model can objectively assess the contributions of pH-responsive release ability and pilot molecule targetability to pharmacological effects.

Trans-cyclooctene-a Swiss army knife for bioorthogonal chemistry: exploring the synthesis, reactivity, and applications in biomedical breakthroughs

BACKGROUND

Trans-cyclooctenes (TCOs) are highly strained alkenes with remarkable reactivity towards tetrazines (Tzs) in inverse electron-demand Diels-Alder reactions. Since their discovery as bioorthogonal reaction partners, novel TCO derivatives have been developed to improve their reactivity, stability, and hydrophilicity, thus expanding their utility in diverse applications.

MAIN BODY

TCOs have garnered significant interest for their applications in biomedical settings. In chemical biology, TCOs serve as tools for bioconjugation, enabling the precise labeling and manipulation of biomolecules. Moreover, their role in nuclear medicine is substantial, with TCOs employed in the radiolabeling of peptides and other biomolecules. This has led to their utilization in pretargeted nuclear imaging and therapy, where they function as both bioorthogonal tags and radiotracers, facilitating targeted disease diagnosis and treatment. Beyond these applications, TCOs have been used in targeted cancer therapy through a "click-to-release" approach, in which they act as key components to selectively deliver therapeutic agents to cancer cells, thereby enhancing treatment efficacy while minimizing off-target effects. However, the search for a suitable TCO scaffold with an appropriate balance between stability and reactivity remains a challenge.

CONCLUSIONS

This review paper provides a comprehensive overview of the current state of knowledge regarding the synthesis of TCOs, and its challenges, and their development throughout the years. We describe their wide ranging applications as radiolabeled prosthetic groups for radiolabeling, as bioorthogonal tags for pretargeted imaging and therapy, and targeted drug delivery, with the aim of showcasing the versatility and potential of TCOs as valuable tools in advancing biomedical research and applications.

Advances in the Synthesis of Bioorthogonal Reagents: s-Tetrazines, 1,2,4-Triazines, Cyclooctynes, Heterocycloheptynes, and trans-Cyclooctenes

Aligned with the increasing importance of bioorthogonal chemistry has been an increasing demand for more potent, affordable, multifunctional, and programmable bioorthogonal reagents. More advanced synthetic chemistry techniques, including transition-metal-catalyzed cross-coupling reactions, C-H activation, photoinduced chemistry, and continuous flow chemistry, have been employed in synthesizing novel bioorthogonal reagents for universal purposes. We discuss herein recent developments regarding the synthesis of popular bioorthogonal reagents, with a focus on s-tetrazines, 1,2,4-triazines, trans-cyclooctenes, cyclooctynes, hetero-cycloheptynes, and -trans-cycloheptenes. This review aims to summarize and discuss the most representative synthetic approaches of these reagents and their derivatives that are useful in bioorthogonal chemistry. The preparation of these molecules and their derivatives utilizes both classical approaches as well as the latest organic chemistry methodologies.

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.

Bioorthogonal "Click and Release" Reaction-Triggered Aggregation of Gold Nanoparticles Combined with Released Lonidamine for Enhanced Cancer Photothermal Therapy

Cancer has been the most deadly disease, and 13 million cancer casualties are estimated to occur each year by 2030. Gold nanoparticles (AuNPs)-based photothermal therapy (PTT) has attracted great interest due to its high spatiotemporal controllability and noninvasiveness. Due to the trade-off between particle size and photothermal efficiency of AuNPs, rational design is needed to realize aggregation of AuNPs into larger particles with desirable NIR adsorption in tumor site. Exploiting the bioorthogonal "Click and Release" (BCR) reaction between iminosydnone and cycloalkyne, aggregation of AuNPs can be achieved and attractively accompanied by the release of chemotherapeutic drug purposed to photothermal synergizing. We synthesize iminosydnone-lonidamine (ImLND) as a prodrug and choose dibenzocyclooctyne (DBCO) as the trigger of BCR reaction. A PEGylated AuNPs-based two-component nanoplatform consisting of prodrug-loaded AuNPs-ImLND and tumor-targeting peptide RGD-conjugated AuNPs-DBCO-RGD is designed. In the therapeutic regimen, AuNPs-DBCO-RGD are intravenously injected first for tumor-specific enrichment and retention. Once the arrival of AuNPs-ImLND injected later at tumor site, highly photothermally active nanoaggregates of AuNPs are formed via the BCR reaction between ImLND and DBCO. The simultaneous release of lonidamine further enhanced the therapeutic performance by sensitizing cancer cells to PTT.

Minimalist Tetrazine N-Acetyl Muramic Acid Probes for Rapid and Efficient Labeling of Commensal and Pathogenic Peptidoglycans in Living Bacterial Culture and During Macrophage Invasion

N-Acetyl muramic acid (NAM) probes containing alkyne or azide groups are commonly used to investigate aspects of cell wall synthesis because of their small size and ability to incorporate into bacterial peptidoglycan (PG). However, copper-catalyzed alkyne-azide cycloaddition (CuAAC) reactions are not compatible with live cells, and strain-promoted alkyne-azide cycloaddition (SPAAC) reaction rates are modest and, therefore, not as desirable for tracking the temporal alterations of bacterial cell growth, remodeling, and division. Alternatively, the tetrazine-trans-cyclooctene ligation (Tz-TCO), which is the fastest known bioorthogonal reaction and not cytotoxic, allows for rapid live-cell labeling of PG at biologically relevant time scales and concentrations. Previous work to increase reaction kinetics on the PG surface by using tetrazine probes was limited because of low incorporation of the probe. Described here are new approaches to construct a minimalist tetrazine (Tz)-NAM probe utilizing recent advancements in asymmetric tetrazine synthesis. This minimalist Tz-NAM probe was successfully incorporated into pathogenic and commensal bacterial PG where fixed and rapid live-cell, no-wash labeling was successful in both free bacterial cultures and in coculture with human macrophages. Overall, this probe allows for expeditious labeling of bacterial PG, thereby making it an exceptional tool for monitoring PG biosynthesis for the development of new antibiotic screens. The versatility and selectivity of this probe will allow for real-time interrogation of the interactions of bacterial pathogens in a human host and will serve a broader utility for studying glycans in multiple complex biological systems.

A High-Efficiency Bioorthogonal Tumor-Membrane Reactor for In Situ Selective and Sustained Prodrug Activation

The site-specific activation of bioorthogonal prodrugs has provided great opportunities for reducing the severe side effects of chemotherapy. However, the precise control of activation location, sustained drug production at the target site, and high bioorthogonal reaction efficiency in vivo remain great challenges. Here, we propose the construction of tumor cell membrane reactors in vivo to solve the above problems. Specifically, tumor-targeted liposomes with efficient membrane fusion capabilities are generated to install the bioorthogonal trigger, the amphiphilic tetrazine derivative, on the surface of tumor cells. These predecorated tumor cells act as many living reactors, transforming the tumor into a "drug factory" that in situ activates an externally delivered bioorthogonal prodrug, for example intratumorally injected transcyclooctene-caged doxorubicin. In contrast to the rapid elimination of cargo that is encapsulated and delivered by liposomes, these reactors permit stable retention of bioorthogonal triggers in tumor for 96 h after a single dose of liposomes via intravenous injection, allowing sustained generation of doxorubicin. Interestingly, an additional supplement of liposomes will compensate for the trigger consumed by the reaction and significantly improve the efficiency of the local reaction. This strategy provides a solution to the efficacy versus safety dilemma of tumor chemotherapy.

Clicking in harmony: exploring the bio-orthogonal overlap in click chemistry

In the quest to scrutinize and modify biological systems, the global research community has continued to explore bio-orthogonal click reactions, a set of reactions exclusively targeting non-native molecules within biological systems. These methodologies have brought about a paradigm shift, demonstrating the feasibility of artificial chemical reactions occurring on cellular surfaces, in the cell cytosol, or within the body - an accomplishment challenging to achieve with the majority of conventional chemical reactions. This review delves into the principles of bio-orthogonal click chemistry, contrasting metal-catalyzed and metal-free reactions of bio-orthogonal nature. It comprehensively explores mechanistic details and applications, highlighting the versatility and potential of this methodology in diverse scientific contexts, from cell labelling to biosensing and polymer synthesis. Researchers globally continue to advance this powerful tool for precise and selective manipulation of biomolecules in complex biological systems.

Click chemistry in the synthesis of antibody-drug conjugates

Antibody-Drug Conjugates (ADC) are a new class of anticancer therapeutics with immense potential. They have been rapidly advancing in the last two decades. This fast speed of development has become possible due to several new technologies and methods. One of them is Click Chemistry, an approach that was created only two decades ago, but already is actively utilized for bioconjugation, material science and drug discovery. In this review, we researched the impact of Click Chemistry reactions on the synthesis and development of ADCs. The information about the most frequently utilized reactions, such as Michael's addition, Copper-catalyzed azide-alkyne [3+2] cycloaddition (CuAAC), Strain-promoted azide-alkyne [3+2] cycloaddition (SPAAC), oxime bond formation, hydrazine-iso-Pictet-Spengler Ligation (HIPS), Diels-Alder reactions have been summarized. The implementation of thiol-maleimide Click Chemistry reaction in the synthesis of numerous FDA-approved Antibody-Drug Conjugates has been reported. The data amassed in the present review provides better understanding of the importance of Click Chemistry in the synthesis, development and improvement of the Antibody-Drug Conjugates and it will be helpful for further researches related to ADCs.

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.

A Dual-Mechanism Targeted Bioorthogonal Prodrug Therapy

Bioorthogonal prodrug therapies offer an intriguing two-component system that features enhanced circulating stability and controlled activation on demand. Current strategies often deliver either the prodrug or its complementary activator to the tumor with a monomechanism targeted mechanism, which cannot achieve the desired antitumor efficacy and safety profile. The orchestration of two distinct and orthogonal mechanisms should overcome the hierarchical heterogeneity of solid tumors to improve the delivery efficiency of both components simultaneously for bio-orthogonal prodrug therapies. We herein developed a dual-mechanism targeted bioorthogonal prodrug therapy by integrating two orthogonal, receptor-independent tumor-targeting strategies. We first employed the endogenous albumin transport system to generate the in situ albumin-bound, bioorthogonal-caged doxorubicin prodrug with extended plasma circulation and selective accumulation at the tumor site. We then employed enzyme-instructed self-assembly (EISA) to specifically enrich the bioorthogonal activators within tumor cells. As each targeted delivery mode induced an intrinsic pharmacokinetic profile, further optimization of the administration sequence according to their pharmacokinetics allowed the spatiotemporally controlled prodrug activation on-target and on-demand. Taken together, by orchestrating two discrete and receptor-independent targeting strategies, we developed an all-small-molecule based bioorthogonal prodrug system for dual-mechanism targeted anticancer therapies to maximize therapeutic efficacy and minimize adverse drug reactions for chemotherapeutic agents.

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.

Bioorthogonally Assisted Phototherapy: Recent Advances and Prospects

Photoresponsive materials offer excellent spatiotemporal control over biological processes and the emerging phototherapeutic methods are expected to have significant effects on targeted cancer therapies. Recent examples show that combination of photoactivatable approaches with bioorthogonal chemistry enhances the precision of targeted phototherapies and profound implications are foreseen particularly in the treatment of disperse/diffuse tumors. The extra level of on-target selectivity and improved spatial/temporal control considerably intensified related bioorthogonally assisted phototherapy research. The anticipated growth of further developments in the field justifies the timeliness of a brief summary of the state of the art.

A Concise Synthetic Approach to Highly Reactive Click-to-Release Trans-Cyclooctene Linkers

An increase in the click-to-release reaction rate between cleavable trans-cyclooctenes (TCO) and tetrazines would be beneficial for drug delivery applications. In this work, we have developed a short and stereoselective synthesis route towards highly reactive sTCOs that serve as cleavable linkers, affording quantitative tetrazine-triggered payload release. In addition, the fivefold more reactive sTCO exhibited the same in vivo stability as current TCO linkers when used as antibody linkers in circulation in mice.

A Lysosome-Targeted Tetrazine for Organelle-Specific Click-to-Release Chemistry in Antigen Presenting Cells

Bioorthogonal deprotections are readily used to control biological function in a cell-specific manner. To further improve the spatial resolution of these reactions, we here present a lysosome-targeted tetrazine for an organelle-specific deprotection reaction. We show that trans-cyclooctene deprotection with this reagent can be used to control the biological activity of ligands for invariant natural killer T cells in the lysosome to shed light on the processing pathway in antigen presenting cells. We then use the lysosome-targeted tetrazine to show that long peptide antigens used for CD8⁺ T cell activation do not pass through this organelle, suggesting a role for the earlier endosomal compartments for their processing.

Ligand-Directed Photocatalysts and Far-Red Light Enable Catalytic Bioorthogonal Uncaging inside Live Cells

Described are ligand-directed catalysts for live-cell, photocatalytic activation of bioorthogonal chemistry. Catalytic groups are localized via a tethered ligand either to DNA or to tubulin, and red light (660 nm) photocatalysis is used to initiate a cascade of DHTz oxidation, intramolecular Diels-Alder reaction, and elimination to release phenolic compounds. Silarhodamine (SiR) dyes, more conventionally used as biological fluorophores, serve as photocatalysts that have high cytocompatibility and produce minimal singlet oxygen. Commercially available conjugates of Hoechst dye (SiR-H) and docetaxel (SiR-T) are used to localize SiR to the nucleus and microtubules, respectively. Computation was used to assist the design of a new class of redox-activated photocage to release either phenol or n-CA4, a microtubule-destabilizing agent. In model studies, uncaging is complete within 5 min using only 2 μM SiR and 40 μM photocage. In situ spectroscopic studies support a mechanism involving rapid intramolecular Diels-Alder reaction and a rate-determining elimination step. In cellular studies, this uncaging process is successful at low concentrations of both the photocage (25 nM) and the SiR-H dye (500 nM). Uncaging n-CA4 causes microtubule depolymerization and an accompanying reduction in cell area. Control studies demonstrate that SiR-H catalyzes uncaging inside the cell, and not in the extracellular environment. With SiR-T, the same dye serves as a photocatalyst and the fluorescent reporter for microtubule depolymerization, and with confocal microscopy, it was possible to visualize microtubule depolymerization in real time as the result of photocatalytic uncaging in live cells.

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.

Readily Accessible Strained Difunctionalized trans-Cyclooctenes with Fast Click and Release Capabilities

The click reaction between a functionalized trans-cyclooctene (TCO) and a tetrazine (Tz) is a compelling method for bioorthogonal conjugation in combination with payload releasing capabilities. However, the synthesis of difunctionalized TCOs remains challenging. As a result, these compounds are poorly accessible, which impedes the development of novel applications. In this work, the scalable and accessible synthesis of a new bioorthogonal difunctionalized TCO is reported in only four single selective high yielding steps starting from commercially available compounds. The TCO-Tz click reaction was assessed and revealed excellent kinetic rates and subsequently payload release was shown with various functionalized derivatives. Tetrazine triggered release of carbonate and carbamate payloads was demonstrated up to 100 % release efficiency and local drug release was shown in a cellular toxicity study which revealed a >20-fold increase in cytotoxicity.

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.

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.

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.

Initial Steps towards Spatiotemporal Signaling through Biomaterials Using Click-to-Release Chemistry

The process of wound healing is a tightly controlled cascade of events, where severe skin wounds are resolved via scar tissue. This fibrotic response may be diminished by applying anti-fibrotic factors to the wound, thereby stimulating regeneration over scarring. The development of tunable biomaterials that enable spatiotemporal control over the release of anti-fibrotics would greatly benefit wound healing. Herein, harnessing the power of click-to-release chemistry for regenerative medicine, we demonstrate the feasibility of such an approach. For this purpose, one side of a bis-N-hydroxysuccinimide-trans-cyclooctene (TCO) linker was functionalized with human epidermal growth factor (hEGF), an important regulator during wound healing, whereas on the other side a carrier protein was conjugated—either type I collagen scaffolds or bovine serum albumin (BSA). Mass spectrometry demonstrated the coupling of hEGF–TCO and indicated a release following exposure to dimethyl-tetrazine. Type I collagen scaffolds could be functionalized with the hEGF–TCO complex as demonstrated by immunofluorescence staining and Western blotting. The hEGF–TCO complex was also successfully ligated to BSA and the partial release of hEGF upon dimethyl-tetrazine exposure was observed through Western blotting. This work establishes the potential of click-to-release chemistry for the development of pro-regenerative biomaterials.

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.

Targeted Molecular Construct for Bioorthogonal Theranostics of PD-L1-Expressing Cancer Cells

Molecular targeting of tumor-overexpressed oncoproteins can improve the selectivity and tolerability of anticancer therapies. The immunoinhibitory membrane protein programmed death ligand 1 (PD-L1) is highly expressed on certain tumor types, which masks malignant cells from T cell recognition and creates an optimal environment for the cancer to thrive and spread. We report here a ligand-tetrazine conjugate (LTzC) armed with a PD-L1 small molecule inhibitor to selectively target PD-L1-expressing cancer cells and inhibit PD-L1 function and conjugated to a tetrazine module and a lipoyl group to incorporate bioorthogonal reactivities and an oxidative stress enhancer into the construct. By pairing LTzC with an imaging probe, we have established a "track-&-tag" system for selective labeling of PD-L1 both on and in living cells using click chemistry. We have further shown the specificity and versatility of LTzC by click-to-release activation of prodrugs and selective killing of PD-L1-expressing breast cancer cells, offering a new multimodal approach to "track-&-treat" malignant cells that are capable of evading the immune system.

Development of Photoremovable Linkers as a Novel Strategy to Improve the Pharmacokinetics of Drug Conjugates and Their Potential Application in Antibody-Drug Conjugates for Cancer Therapy

Although there have been extensive research and progress on the discovery of anticancer drug over the years, the application of these drugs as stand-alone therapy has been limited by their off-target toxicities, poor pharmacokinetic properties, and low therapeutic index. Targeted drug delivery, especially drug conjugate, has been recognized as a technology that can bring forth a new generation of therapeutics with improved efficacy and reduced side effects for cancer treatment. The linker in a drug conjugate is of essential importance because it impacts the circulation time of the conjugate and the release of the drug for full activity at the target site. Recently, the light-triggered linker has attracted a lot of attention due to its spatiotemporal controllability and attractive prospects of improving the overall pharmacokinetics of the conjugate. In this paper, the latest developments of UV- and IR-triggered linkers and their application and potential in drug conjugate development are reviewed. Some of the most-well-researched photoresponsive structural moieties, such as UV-triggered coumarin, ortho-nitrobenzyl group (ONB), thioacetal ortho-nitrobenzaldehyde (TNB), photocaged C40-oxidized abasic site (PC4AP), and IR-triggered cyanine and BODIPY, are included for discussion. These photoremovable linkers show better physical and chemical stabilities and can undergo rapid cleavage upon irradiation. Very importantly, the drug conjugates containing these linkers exhibit reduced off-target toxicity and overall better pharmacokinetic properties. The progress on photoactive antibody-drug conjugates, such as antibody-drug conjugates (ADC) and antibody-photoabsorber conjugate (APC), as precision medicine in clinical cancer treatment is highlighted.

Photoaffinity labeling and bioorthogonal ligation: Two critical tools for designing "Fish Hooks" to scout for target proteins

Small molecules remain an important category of therapeutic agents. Their binding to different proteins can lead to both desired and undesired biological effects. Identification of the proteins that a drug binds to has become an important step in drug development because it can lead to safer and more effective drugs. Parent bioactive molecules can be converted to appropriate probes that allow for visualization and identification of their target proteins. Typically, these probes are designed and synthesized utilizing some or all of five major tools; a photoactivatable group, a reporter tag, a linker, an affinity tag, and a bioorthogonal handle. This review covers two of the most challenging tools, photoactivation and bioorthogonal ligation. We provide a historical and theoretical background along with synthetic routes to prepare them. In addition, the review provides comparative analyses of the available tools that can assist decision making when designing such probes. A survey of most recent literature reports is included as well to identify recent trends in the field.