Expanding room for tetrazine ligations in the in vivo chemistry toolbox

Unveiling the photophysical mechanistic mysteries of tetrazine-functionalized fluorogenic labels

Tetrazine-based fluorogenic labels are widely utilized in medical and biological studies, exhibiting substantial fluorescence enhancement (FE) following tetrazine degradation through bio-orthogonal reactions. However, the underlying mechanisms driving this fluorogenic response remain only partially resolved, particularly regarding the diminished FE efficiency in the deep-red and near-infrared (NIR) regions. This knowledge gap has impeded efforts to optimize these labels for extended emission wavelengths and improved FE ratios. This review offers a photophysical perspective, discussing the fluorescence quenching pathways (i.e., energy flows and charge separation) that regulate the fluorogenic properties exhibited in various types of tetrazine labels. Moreover, this work examines the emerging role of intramolecular rotations in certain tetrazine-based structures and the integration of additional quencher units. The proposed alternative quenching channel offers the potential to surpass traditional wavelength constraints while achieving improved FE. By examining these photophysical mechanisms, this review aims to advance the understanding of tetrazine-functionalized fluorogenic labels and provide guiding principles for their future design and practical applications.

Rapid Construction of 18F-Triazolyl-tetrazines through the Click Reaction

Due to the fast reaction rate, 18F-labeled tetrazines have been widely applied in positron emission tomography (PET) imaging in cancer research and drug discovery. In this work, several functional 18F-triazolyl-tetrazines were rapidly obtained through an optimized copper-catalyzed alkyene-azide cycloaddition reaction system in >99% radiochemical conversions. Notably, the commonly used 18F-labeled azides were isolated through cartridges and directly used for cycloadditions, which greatly simplified the labeling procedure. The assembled triazolyl-tetrazines demonstrated high in vitro stability and reaction kinetics, exhibiting considerable potential for the development of PET agents.

Visible and near-infrared light-induced photoclick reactions

Photoclick reactions combine the advantages offered by light-driven processes, that is, non-invasive and high spatiotemporal control, with classical click chemistry and have found applications ranging from surface functionalization, polymer conjugation, photocrosslinking, protein labelling and bioimaging. Despite these advances, most photoclick reactions typically require near-ultraviolet (UV) and mid-UV light to proceed. UV light can trigger undesirable responses, including cellular apoptosis, and therefore, visible and near-infrared light-induced photoclick reaction systems are highly desirable. Shifting to a longer wavelength can also reduce degradation of the photoclick reagents and products. Several strategies have been used to induce a bathochromic shift in the wavelength of irradiation-initiating photoclick reactions. For instance, the extension of the conjugated π-system, triplet-triplet energy transfer, multi-photon excitation, upconversion technology, photocatalytic and photoinitiation approaches, and designs involving photocages have all been used to achieve this goal. Current design strategies, recent advances and the outlook for long wavelength-driven photoclick reactions are presented.

Bioorthogonal Cycloadditions of C3-Trifluoromethylated 1,2,4-Triazines with trans-Cyclooctenes

1,2,4-triazines are a valuable class of heterodienes that can be employed in inverse electron-demand Diels-Alder reactions. However, their broader application in bioorthogonal chemistry is limited due to their low reactivity. This article focuses on 3-(trifluoromethyl)-1,2,4-triazines, which can be efficiently prepared in a one-pot reaction from NH-1,2,3-triazoles. These triazines are highly reactive in reactions with strained cyclooctenes, giving second-order rate constants as high as 230 M-1 s-1. Despite their high reactivity, the compounds remain sufficiently stable under biologically relevant conditions. We show that some of the compounds are fluorogenic, a property of potential use in bioimaging. In addition, we demonstrate the successful application of the triazines in labeling model biomolecules. Our work shows that the reactivity of 1,2,4-triazines can be enhanced by the 3-CF3-substitution, which we consider an important step toward the wider use of this promising class of reagents.

Fluorescein-Based SynNotch Adaptors for Regulating Gene Expression Responses to Diverse Extracellular Cues

We introduce an adaptor-based strategy for regulating fluorescein-binding synthetic Notch (SynNotch) receptors using ligands based on conjugates of fluorescein isomers and analogs. To develop a versatile system, we evaluated the surface expression and activities of multiple constructs containing distinct extracellular fluorescein-binding domains. Using an optimized receptor, we devised ways to regulate signaling via fluorescein-based chemical transformations, including an approach based on a bio-orthogonal chemical ligation and a spatially controllable strategy via the photo-patterned uncaging of an o -nitrobenzyl-caged fluorescein conjugate. We further demonstrate that fluorescein-conjugated extracellular matrix (ECM)-binding peptides can regulate SynNotch activity depending on the folding state of collagen-based ECM networks. Treatment with these conjugates enabled cells to distinguish between folded versus denatured collagen proteins and enact dose-dependent gene expression responses depending on the nature of the signaling adaptors presented. To demonstrate the utility of these tools, we applied them to control the myogenic conversion of fibroblasts into myocytes with spatial and temporal precision and in response to denatured collagen-I, a biomarker of multiple pathological states. Overall, we introduce an optimized fluorescein-binding SynNotch as a versatile tool for regulating transcriptional responses to extracellular ligands based on the widely used and clinically-approved fluorescein dye.

Selective activation of prodrugs in breast cancer using metabolic glycoengineering and the tetrazine ligation bioorthogonal reaction

Increasing the selectivity of chemotherapies by converting them into prodrugs that can be activated at the tumour site decreases their side effects and allows discrimination between cancerous and non-cancerous cells. Herein, the use of metabolic glycoengineering (MGE) to selectively label MCF-7 breast cancer cells with tetrazine (Tz) activators for subsequent activation of prodrugs containing the trans-cyclooctene (TCO) moiety by a bioorthogonal reaction is demonstrated. Three novel Tz-modified monosaccharides, Ac4ManNTz 7, Ac4GalNTz 8, and Ac4SiaTz 16, were used for expression of the Tz activator within sialic-acid rich breast cancer cells' surface glycans through MGE. Tz expression on breast cancer cells (MCF-7) was evaluated versus the non-cancerous L929 fibroblasts showing a concentration-dependant effect and excellent selectivity with ≥35-fold Tz expression on the MCF-7 cells versus the non-cancerous L929 fibroblasts. Next, a novel TCO-N-mustard prodrug and a TCO-doxorubicin prodrug were analyzed in vitro on the Tz-bioengineered cells to probe our hypothesis that these could be activated via a bioorthogonal reaction. Selective prodrug activation and restoration of cytotoxicity were demonstrated for the MCF-7 breast cancer cells versus the non-cancerous L929 cells. Restoration of the parent drug's cytotoxicity was shown to be dependent on the level of Tz expression where the Ac4ManNTz 7 and Ac4GalNTz 8 derivatives (20 µM) lead to the highest Tz expression and full restoration of the parent drug's cytotoxicity. This work suggests the feasibility of combining MGE and tetrazine ligation for selective prodrug activation in breast cancer.

Glutathione Mediates Control of Dual Differential Bio-orthogonal Labelling of Biomolecules

Traditional approaches to bio-orthogonal reaction discovery have focused on developing reagent pairs that react with each other faster than they are metabolically degraded. Glutathione (GSH) is typically responsible for the deactivation of most bio-orthogonal reagents. Here we demonstrate that GSH promotes a Cu-catalysed (3+2) cycloaddition reaction between an ynamine and an azide. We show that GSH acts as a redox modulator to control the Cu oxidation state in these cycloadditions. Rate enhancement of this reaction is specific for ynamine substrates and is tuneable by the Cu:GSH ratio. This unique GSH-mediated reactivity gradient is then utilised in the dual sequential bio-orthogonal labelling of peptides and oligonucleotides via two distinct chemoselective (3+2) cycloadditions.

Glutathione Mediates Control of Dual Differential Bio-orthogonal Labelling of Biomolecules

Traditional approaches to bio-orthogonal reaction discovery have focused on developing reagent pairs that react with each other faster than they are metabolically degraded. Glutathione (GSH) is typically responsible for the deactivation of most bio-orthogonal reagents. Here we demonstrate that GSH promotes a Cu-catalysed (3+2) cycloaddition reaction between an ynamine and an azide. We show that GSH acts as a redox modulator to control the Cu oxidation state in these cycloadditions. Rate enhancement of this reaction is specific for ynamine substrates and is tuneable by the Cu:GSH ratio. This unique GSH-mediated reactivity gradient is then utilised in the dual sequential bio-orthogonal labelling of peptides and oligonucleotides via two distinct chemoselective (3+2) cycloadditions.

Chemical and Biological Engineering Strategies to Make and Modify Next-Generation Hydrogel Biomaterials

There is a growing interest in the development of technologies to probe and direct in vitro cellular function for fundamental organoid and stem cell biology, functional tissue and metabolic engineering, and biotherapeutic formulation. Recapitulating many critical aspects of the native cellular niche, hydrogel biomaterials have proven to be a defining platform technology in this space, catapulting biological investigation from traditional two-dimensional (2D) culture into the 3D world. Seeking to better emulate the dynamic heterogeneity characteristic of all living tissues, global efforts over the last several years have centered around upgrading hydrogel design from relatively simple and static architectures into stimuli-responsive and spatiotemporally evolvable niches. Towards this end, advances from traditionally disparate fields including bioorthogonal click chemistry, chemoenzymatic synthesis, and DNA nanotechnology have been co-opted and integrated to construct 4D-tunable systems that undergo preprogrammed functional changes in response to user-defined inputs. In this Review, we highlight how advances in synthetic, semisynthetic, and bio-based chemistries have played a critical role in the triggered creation and customization of next-generation hydrogel biomaterials. We also chart how these advances stand to energize the translational pipeline of hydrogels from bench to market and close with an outlook on outstanding opportunities and challenges that lay ahead.

Ru- and Ir-complex decorated periodic mesoporous organosilicas as sensitizers for artificial photosynthesis

A versatile and facile strategy based on an inverse electron demand Diels-Alder reaction between 5-norbornen-2-yltriethoxysilane and a tetrazine derivative has been established for the synthesis of a new triethoxysilane precursor containing dipyridylpyridazine units. Such a precursor has been incorporated into the mesostructure of an ethylene-bridged periodic mesoporous organosilica (PMO) material through a one-pot synthesis via a co-condensation method. Upon attachment of Ru- and Ir-complexes to the pendant N-chelating heterocyclic ligands, the resulting decorated PMOs have acted as photosensitizers in artificial photosynthetic systems. The deposition of Pt on these PMOs has allowed us to obtain efficient photocatalytic materials for the hydrogen evolution reaction as a result of electron transfer from the light harvesting Ru- and Ir-complexes to the supported Pt nanoparticles through methyl viologen as an electron relay. They have exhibited total turnover number values of 573 and 846, respectively, under visible light irradiation. The role played by each component and the stability of the photocatalytic systems have been discussed. The present approach paves the way to the synthesis of different materials with coordination sites capable of forming surface complexes to be applied as sensitizers and catalysts.

Optimization of Direct Aromatic 18F-Labeling of Tetrazines

Radiolabeling of tetrazines has gained increasing attention due to their important role in pretargeted imaging or therapy. The most commonly used radionuclide in PET imaging is fluorine-18. For this reason, we have recently developed a method which enables the direct aromatic ¹⁸F-fluorination of tetrazines using stannane precursors through copper-mediated fluorinations. Herein, we further optimized this labeling procedure. 3-(3-fluorophenyl)-1,2,4,5-tetrazine was chosen for this purpose because of its high reactivity and respective limited stability during the labeling process. By optimizing parameters such as elution conditions, precursor amount, catalyst, time or temperature, the radiochemical yield (RCY) could be increased by approximately 30%. These conditions were then applied to optimize the RCY of a recently successfully developed and promising pretargeting imaging agent. This agent could be isolated in a decay corrected RCY of 14 ± 3% and Am of 201 ± 30 GBq/µmol in a synthesis time of 70 min. Consequently, the RCY increased by 27%.

Synergistic Experimental and Computational Investigation of the Bioorthogonal Reactivity of Substituted Aryltetrazines

Tetrazines (Tz) have been applied as bioorthogonal agents for various biomedical applications, including pretargeted imaging approaches. In radioimmunoimaging, pretargeting increases the target-to-background ratio while simultaneously reducing the radiation burden. We have recently reported a strategy to directly ¹⁸F-label highly reactive tetrazines based on a 3-(3-fluorophenyl)-Tz core structure. Herein, we report a kinetic study on this versatile scaffold. A library of 40 different tetrazines was prepared, fully characterized, and investigated with an emphasis on second-order rate constants for the reaction with trans-cyclooctene (TCO). Our results reveal the effects of various substitution patterns and moreover demonstrate the importance of measuring reactivities in the solvent of interest, as click rates in different solvents do not necessarily correlate well. In particular, we report that tetrazines modified in the 2-position of the phenyl substituent show high intrinsic reactivity toward TCO, which is diminished in aqueous systems by unfavorable solvent effects. The obtained results enable the prediction of the bioorthogonal reactivity and thereby facilitate the development of the next generation of substituted aryltetrazines for in vivo applications.

Development of 18F-Labeled Bispyridyl Tetrazines for In Vivo Pretargeted PET Imaging

Pretargeted PET imaging is an emerging and fast-developing method to monitor immuno-oncology strategies. Currently, tetrazine ligation is considered the most promising bioorthogonal reaction for pretargeting in vivo. Recently, we have developed a method to ¹⁸F-label ultrareactive tetrazines by copper-mediated fluorinations. However, bispyridyl tetrazines—one of the most promising structures for in vivo pretargeted applications—were inaccessible using this strategy. We believed that our successful efforts to ¹⁸F-label H-tetrazines using low basic labeling conditions could also be used to label bispyridyl tetrazines via aliphatic nucleophilic substitution. Here, we report the first direct ¹⁸F-labeling of bispyridyl tetrazines, their optimization for in vivo use, as well as their successful application in pretargeted PET imaging. This strategy resulted in the design of [¹⁸F]45, which could be labeled in a satisfactorily radiochemical yield (RCY = 16%), molar activity (A_m = 57 GBq/µmol), and high radiochemical purity (RCP > 98%). The [¹⁸F]45 displayed a target-to-background ratio comparable to previously successfully applied tracers for pretargeted imaging. This study showed that bispyridyl tetrazines can be developed into pretargeted imaging agents. These structures allow an easy chemical modification of ¹⁸F-labeled tetrazines, paving the road toward highly functionalized pretargeting tools. Moreover, bispyridyl tetrazines led to near-instant drug release of iTCO-tetrazine-based ‘click-to-release’ reactions. Consequently, ¹⁸F-labeled bispyridyl tetrazines bear the possibility to quantify such release in vivo in the future.

Chemical approaches for investigating site-specific protein S-fatty acylation

Protein S-fatty acylation or S-palmitoylation is a reversible and regulated lipid post-translational modification (PTM) in eukaryotes. Loss-of-function mutagenesis studies have suggested important roles for protein S-fatty acylation in many fundamental biological pathways in development, neurobiology, and immunity that are also associated with human diseases. However, the hydrophobicity and reversibility of this PTM have made site-specific gain-of-function studies more challenging to investigate. In this review, we summarize recent chemical biology approaches and methods that have enabled site-specific gain-of-function studies of protein S-fatty acylation and the investigation of the mechanisms and significance of this PTM in eukaryotic biology.

Dual-Modality PET-SPECT Image-Guided Pretargeting Delivery in HER2(+) Breast Cancer Models

Pretargeted drug delivery has been explored for decades as a promising approach in cancer therapy. An image-guided pretargeting strategy significantly enhances the intrinsic advantages of this approach since imaging the pretargeting step can be used for diagnostic purposes, while imaging of the drug delivery step can be utilized to evaluate drug distribution and assess therapeutic response. A trastuzumab (Tz)-based HER2 pretargeting component (Tz-TCO-[89Zr-DFO]) was developed by conjugating with trans-cyclooctene (TCO) bioorthogonal click chemistry functional groups and deferoxamine (DFO) to enable radiolabeling with a 89Zr PET tracer. The drug delivery component (HSA-DM1-Tt-[99mTc-HyNic]) was developed by conjugating human serum albumin (HSA) with mertansine (DM1), tetrazine (Tt) functional groups, and a HyNic chelator and radiolabeling with 99mTc. For ex vivo biodistribution studies, pretargeting and delivery components (without drug) were administered subsequently to mice bearing human HER2(+) breast cancer xenografts, and a high tumor uptake of Tz-TCO-[89Zr-DFO] (26.4% ID/g) and HSA-Tt-[99mTc-HyNic] (4.6% ID/g) was detected at 24 h postinjection. In vivo treatment studies were performed in the same HER2(+) breast cancer model using PET-SPECT image guidance. The increased tumor uptake of the pretargeting and drug delivery components was detected by PET-CT and SPECT-CT, respectively. The study showed a significant 92% reduction of the relative tumor volume in treated mice (RTV = 0.08 in 26 days), compared to the untreated control mice (RTV = 1.78 in 11 days) and to mice treated with only HSA-DM1-Tt-[99mTc-HyNic] (RTV = 1.88 in 16 days). Multimodality PET-SPECT image-guided and pretargeted drug delivery can be utilized to maximize efficacy, predict therapeutic response, and minimize systemic toxicity.

Development of the First Aliphatic 18F-Labeled Tetrazine Suitable for Pretargeted PET Imaging-Expanding the Bioorthogonal Tool Box

Pretargeted imaging of nanomedicines have attracted considerable interest because it has the potential to increase imaging contrast while reducing radiation burden to healthy tissue. Currently, the tetrazine ligation is the fastest bioorthogonal reaction for this strategy and, consequently, the state-of-art choice for in vivo chemistry. We have recently identified key properties for tetrazines in pretargeting. We have also developed a method to ¹⁸F-label reactive tetrazines using an aliphatic nucleophilic substitution strategy. Here, we combined this knowledge and developed an ¹⁸F-labeled tetrazine for pretargeted imaging. In order to develop this ligand, a small SAR study was performed. The most promising compound was selected for labeling and subsequent positron-emission-tomography in vivo imaging. Radiolabeling was achieved in satisfactory yields, molar activities, and high radiochemical purities. [¹⁸F]15 displayed favorable pharmacokinetics and remarkable target-to-background ratios-as early as 1 h post injection. We believe that this agent could be a promising candidate for translation into clinical studies.

Reaction Scope of Methyl 1,2,3-Triazine-5-carboxylate with Amidines and the Impact of C4/C6 Substitution

A comprehensive study of the reaction scope of methyl 1,2,3-triazine-5-carboxylate (3a) with alkyl and aryl amidines is disclosed, reacting at room temperature at remarkable rates (<5 min, 0.1 M in CH₃CN) nearly 10000-fold faster than that of unsubstituted 1,2,3-triazine and providing the product pyrimidines in high yields. C4 Methyl substitution of the 1,2,3-triazine (3b) had little effect on the rate of the reaction, whereas C4/C6 dimethyl substitution (3c) slowed the room-temperature reaction (<24 h, 0.25 M) but displayed an unaltered scope, providing the product pyrimidines in similarly high yields. Measured second-order rate constants of the reaction of 3a-c, the corresponding nitriles 3e and 3f, and 1,2,3-triazine itself (3d) with benzamidine and substituted derivatives quantitated the remarkable reactivity of 3a and 3e, verified the inverse electron demand nature of the reaction (Hammett ρ = -1.50 for substituted amidines, ρ = +7.9 for 5-substituted 1,2,3-triazine), and provided a quantitative measure of the impact of 4-methyl and 4,6-dimethyl substitution on the reactivity of the methyl 1,2,3-triazine-5-carboxylate and 5-cyano-1,2,3-triazine core heterocycles.

Bioorthogonal Reactions Utilizing Nitrones as Versatile Dipoles in Cycloaddition Reactions

Bioorthogonal chemical reactions have emerged as convenient and rapid methods for incorporating unnatural functionality into living systems. Different prototype reactions have been optimized for use in biological settings. Optimization of 3 + 2 dipolar cycloadditions involving nitrones has resulted in highly efficient reaction conditions for bioorthogonal chemistry. Through substitution at the nitrone carbon or nitrogen atom, stereoelectronic tuning of the reactivity of the dipole has assisted in optimizing reactivity. Nitrones have been shown to react rapidly with cyclooctynes with bimolecular rate constants approaching k₂ = 10² M^-1 s^-1, which are among the fastest bioorthogonal reactions reported (McKay et al. Org. Biomol. Chem. 2012, 10, 3066-3070). Nitrones have also been shown to react with trans-cyclooctenes (TCO) in strain-promoted TCO-nitrone cycloadditions reactions. Copper catalyzed reactions involving alkynes and nitrones have also been optimized for applications in biology. This review provides a comprehensive accounting of the different bioorthogonal reactions that have been developed using nitrones as versatile reactants, and provides some recent examples of applications for probing biological systems.

Site-Specific Lipidation Enhances IFITM3 Membrane Interactions and Antiviral Activity

Interferon-induced transmembrane proteins (IFITMs) are S-palmitoylated proteins in vertebrates that restrict a diverse range of viruses. S-palmitoylated IFITM3 in particular engages incoming virus particles, prevents their cytoplasmic entry, and accelerates their lysosomal clearance by host cells. However, how S-palmitoylation modulates the structure and biophysical characteristics of IFITM3 to promote its antiviral activity remains unclear. To investigate how site-specific S-palmitoylation controls IFITM3 antiviral activity, we employed computational, chemical, and biophysical approaches to demonstrate that site-specific lipidation of cysteine 72 enhances the antiviral activity of IFITM3 by modulating its conformation and interaction with lipid membranes. Collectively, our results demonstrate that site-specific S-palmitoylation of IFITM3 directly alters its biophysical properties and activity in cells to prevent virus infection.

Structurally Redesigned Bioorthogonal Reagents for Mitochondria-Specific Prodrug Activation

The development of abiotic chemical reactions that can be performed in an organelle-specific manner can provide new opportunities in drug delivery and cell and chemical biology. However, due to the complexity of the cellular environment, this remains a significant challenge. Here, we introduce structurally redesigned bioorthogonal tetrazine reagents that spontaneously accumulate in mitochondria of live mammalian cells. The attributes leading to their efficient accumulation in the organelle were optimized to include the right combination of lipophilicity and positive delocalized charge. The best performing mitochondriotropic tetrazines enable subcellular chemical release of TCO-caged compounds as we show using fluorogenic substrates and mitochondrial uncoupler niclosamide. Our work demonstrates that a shrewd redesign of common bioorthogonal reagents can lead to their transformation into organelle-specific probes, opening the possibility to activate prodrugs and manipulate biological processes at the subcellular level by using purely chemical tools.

Fluorescence Quenching Effects of Tetrazines and Their Diels-Alder Products: Mechanistic Insight Toward Fluorogenic Efficiency

Inverse electron demand Diels-Alder reactions between s-tetrazines and strained dienophiles have numerous applications in fluorescent labeling of biomolecules. Herein, we investigate the effect of the dienophile on the fluorescence enhancement obtained upon reaction with a tetrazine-quenched fluorophore and study the possible mechanisms of fluorescence quenching by both the tetrazine and its reaction products. The dihydropyridazine obtained from reaction with a strained cyclooctene shows a residual fluorescence quenching effect, greater than that exerted by the pyridazine arising from reaction with the analogous alkyne. Linear and ultrabroadband two-dimensional electronic spectroscopy experiments reveal that resonance energy transfer is the mechanism responsible for the fluorescence quenching effect of tetrazines, whereas a mechanism involving more intimate electronic coupling, likely photoinduced electron transfer, is responsible for the quenching effect of the dihydropyridazine. These studies uncover parameters that can be tuned to maximize fluorogenic efficiency in bioconjugation reactions and reveal that strained alkynes are better reaction partners for achieving maximum contrast ratio.

Developing bioorthogonal probes to span a spectrum of reactivities

Bioorthogonal chemistries enable researchers to interrogate biomolecules in living systems. These reactions are highly selective and biocompatible and can be performed in many complex environments. However, like any organic transformation, there is no perfect bioorthogonal reaction. Choosing the "best fit" for a desired application is critical. Correspondingly, there must be a variety of chemistries-spanning a spectrum of rates and other features-to choose from. Over the past few years, significant strides have been made towards not only expanding the number of bioorthogonal chemistries, but also fine-tuning existing reactions for particular applications. In this Review, we highlight recent advances in bioorthogonal reaction development, focusing on how physical organic chemistry principles have guided probe design. The continued expansion of this toolset will provide more precisely tuned reagents for manipulating bonds in distinct environments.

Synthetic Sphingolipids with 1,2-Pyridazine Appendages Improve Antiproliferative Activity in Human Cancer Cell Lines

A synthetic sphingolipid related to a ring-constrained hydroxymethyl pyrrolidine analog of FTY720 that was known to starve cancer cells to death was chemically modified to include a series of alkoxy-tethered 3,6-substituted 1,2-pyridazines. These derivatives exhibited excellent antiproliferative activity against eight human cancer cell lines from four different cancer types. A 2.5- to 9-fold reduction in IC₅₀ in these cell lines was observed relative to the lead compound, which lacked the appended heterocycle.

Recent Advances in Bioorthogonal Click Chemistry for Efficient Synthesis of Radiotracers and Radiopharmaceuticals

In recent years, several catalyst-free site-specific reactions have been investigated for the efficient conjugation of biomolecules, nanomaterials, and living cells. Representative functional group pairs for these reactions include the following: (1) azide and cyclooctyne for strain-promoted cycloaddition reaction, (2) tetrazine and trans-alkene for inverse-electron-demand-Diels–Alder reaction, and (3) electrophilic heterocycles and cysteine for rapid condensation/addition reaction. Due to their excellent specificities and high reaction rates, these conjugation methods have been utilized for the labeling of radioisotopes (e.g., radiohalogens, radiometals) to various target molecules. The radiolabeled products prepared by these methods have been applied to preclinical research, such as in vivo molecular imaging, pharmacokinetic studies, and radiation therapy of cancer cells. In this review, we explain the basics of these chemical reactions and introduce their recent applications in the field of radiopharmacy and chemical biology. In addition, we discuss the significance, current challenges, and prospects of using bioorthogonal conjugation reactions.

Inverse electron demand Diels-Alder (IEDDA) reactions in peptide chemistry

Click chemistry is applied to selectively modify, lable and ligate peptides for their use as therapeutics, in biomaterials or analytical investigations. The inverse electron demand Diels-Alder (IEDDA) reaction is a catalyst-free click reaction with pronounced chemoselectivity and fast reaction rates. Applications and achievements of the IEDDA reaction in peptide chemistry since 2008 are described in this review.

Click to enter: activation of oligo-arginine cell-penetrating peptides by bioorthogonal tetrazine ligations

Cell-penetrating peptides are able to transport a wide variety of cargo across cell membranes. Although promising, they are not often considered for therapeutic purposes as they lack controllable activity and cell selectivity. We have developed an activation strategy based on a split octa-arginine cell-penetrating peptide (CPP) that can be activated by means of bioorthogonal ligation. To this end we prepared two non-penetrating tetra-arginine halves, functionalized either with a tetrazine or with a complementary bicyclo[6.1.0]nonyne (BCN) group. We demonstrate that an active octa-arginine can be reconstituted in situ upon mixing the complementary split peptides. The resulting activated peptide is taken up as efficiently as the well-established cell-penetrating peptide octa-arginine. The activation of the oligo-arginines can also be achieved using trans-cyclooctene (TCO) as a ligation partner, while norbornene appears too kinetically slow for use in situ. We further show that this strategy can be applied successfully to transport a large protein into living cells. Our results validate a promising first step in achieving control over cell penetration and to use CPPs for therapeutic approaches.

Crystal structure of 3,6-bis-(pyridin-2-yl)-1,4-di-hydro-1,2,4,5-tetra-zine

The structure of the title compound, C12H10N6, at 100 K has monoclinic (P21/n) symmetry. Crystals were obtained as a yellow solid by reduction of 3,6-bis-(pyridin-2-yl)-1,2,4,5-tetra-zine. The structure displays inter-molecular hydrogen bonding of the N-H⋯N type, ordering mol-ecules into infinite ribbons extending along the [100] direction.

Accelerated Blood Clearance of Antibodies by Nanosized Click Antidotes

Long blood half-life is one of the advantages of antibodies over small molecule drugs. At the same time, prolonged half-life is a problem for imaging applications or in the case of antibody-induced toxicities. There is a substantial need for antidotes that can quickly clear antibodies from systemic circulation and peripheral tissues. Engineered nanoparticles exhibit intrinsic affinity for clearance organs (mainly liver and spleen). trans-Cyclooctene (TCO) and methyltetrazine (MTZ) are versatile copper-free click chemistry components that are extensively being used for in vivo bioorthogonal couplings. To test the ability of nanoparticles to eliminate antibodies, we prepared a set of click-modified, clinically relevant antidotes based on several classes of drug carriers: phospholipid-PEG micelles, bovine serum albumin (BSA), and cross-linked dextran iron oxide (CLIO) nanoparticles. Mice were injected with IRDye 800CW-labeled, click-modified IgG followed by a click-modified antidote or PBS (control), and the levels of the IgG were monitored up to 72 h postinjection. Long-circulating lipid micelles produced a spike in IgG levels at 1 h, decreased IgG levels at 24 h, and did not decrease the area under the curve (AUC) and IgG accumulation in main organs. Long-circulating BSA decreased IgG levels at 1 and 24 h, decreased the AUC, but did not significantly decrease organ accumulation. Long-circulating CLIO nanoworms increased IgG levels at 1 h, decreased IgG levels at 24 h, did not decrease the AUC, and did not decrease the organ accumulation. On the other hand, short-circulating CLIO nanoparticles decreased IgG levels at 1 and 24 h, significantly decreasing the AUC and accumulation in the main organs. Multiple doses of CLIO and BSA were not able to completely eliminate the antibody from blood, despite the click reactivity of the residual IgG, likely due to exchange of IgG between blood and tissue compartments. Pharmacokinetic modeling suggests that short antidote half-life and fast click reaction rate should result in higher IgG depletion efficiency. Short-circulating click-modified nanocarriers are the most effective antidotes for elimination of antibodies from blood. This study sets a stage for future development of antidotes based on nanomedicine.

Highly Stable and Selective Tetrazines for the Coordination-Assisted Bioorthogonal Ligation with Vinylboronic Acids

Bioorthogonal reactions are selective transformations that are not affected by any biological functional group and are widely used for chemical modification of biomolecules. Recently, we reported that vinylboronic acids (VBAs) gave exceptionally high reaction rates in the bioorthogonal inverse electron-demand Diels–Alder (iEDDA) reaction with tetrazines bearing a boron-coordinating pyridyl moiety compared to tetrazines lacking such a substituent. In this integrated experimental and theoretical study, we show how the reaction rate of the VBA-tetrazine ligation can be accelerated by shifting the equilibrium from boronic acid to the boronate anion in the reaction mixture. Quantum chemical activation strain analyses reveal that this rate enhancement is a direct consequence of the excellent electron-donating capability of the boronate anion in which the π HOMO is pushed to a higher energy due to the net negative potential of this species. We have explored the second-order rate constants of several tetrazines containing potential VBA-coordinating hydroxyl substituents. We observed an increase in rate constants of several orders of magnitude compared to the tetrazines lacking a hydroxyl substituent. Furthermore, we find the hydroxyl-substituted tetrazines to be more selective toward VBAs than toward the commonly used bioorthogonal reactant norbornene, and more stable in aqueous environment than the previously studied tetrazines containing a pyridyl substituent.

A Simple Mannose-Coated Poly (p-Phenylene Ethynylene) for Qualitative Bacterial Capturing

A mannose-functionalized poly (p-phenylene ethynylene) was rationally designed to achieve selective detection of bacteria. The polymer was constructed as a signaling unit and was modified by attaching aminoethyl mannose using the carboxylic acid group at the end of the linker. Incubation of Escherichia coli with the polymer yielded fluorescent bacteria aggregates through polyvalent interactions. The utility of the mannose functionalized polymer to detect E. coli expressing functional FimH mannose-specific lectin on their surface was also demonstrated. The sugar units displayed on the surface of the polymer retained their functional ability to interact with mannose-binding lectin. To determine the optimum binding time, we measured the fluorescence intensity of the polymer-bacteria suspension at intervals. Our results showed that binding in this system will reach an optimum level within 30 min of incubation. The polymer’s affinity for bacteria has been demonstrated and bacteria with a concentration of 10³ CFU mL⁻¹ can be detected by this system.

Enzymatic Fluorination of Biotin and Tetrazine Conjugates for Pretargeting Approaches to Positron Emission Tomography Imaging

The use of radiolabelled antibodies and antibody-derived recombinant constructs has shown promise for both imaging and therapeutic use. In this context, the biotin-avidin/streptavidin pairing, along with the inverse-electron-demand Diels-Alder (iEDDA) reaction, have found application in pretargeting approaches for positron emission tomography (PET). This study reports the fluorinase-mediated transhalogenation [5'-chloro-5'-deoxyadenosine (ClDA) substrates to 5'-fluoro-5'-deoxyadenosine (FDA) products] of two antibody pretargeting tools, a FDA-PEG-tetrazine and a [¹⁸ F]FDA-PEG-biotin, and each is assessed either for its compatibility towards iEDDA ligation to trans-cyclooctene or for its affinity to avidin. A protocol to avoid radiolytically promoted oxidation of biotin during the synthesis of [¹⁸ F]FDA-PEG-biotin was developed. The study adds to the repertoire of conjugates for use in fluorinase-catalysed radiosynthesis for PET and shows that the fluorinase will accept a wide range of ClDA substrates tethered at C-2 of the adenine ring with a PEGylated cargo. The method is exceptional because the nucleophilic reaction with [¹⁸ F]fluoride takes place in water at neutral pH and at ambient temperature.

Nature-Inspired Bioorthogonal Reaction: Development of β-Caryophyllene as a Chemical Reporter in Tetrazine Ligation

A nature-inspired bioorthogonal reaction has been developed, hinging on an inverse-electron-demand Diels-Alder reaction of tetrazine with β-caryophyllene. Readily accessible from the cheap starting material through a scalable synthesis, the newly developed β-caryophyllene chemical reporter displays appealing reaction kinetics and excellent biocompatibility, which renders it applicable to both in vitro protein labeling and live cell imaging. Moreover, it can be used orthogonally to the strain-promoted alkyne-azide cycloaddition for dual protein labeling. This work not only provides an alternative to the existing bioorthogonal reaction toolbox, but also opens a new avenue to utilize naturally occurring scaffolds as bioorthogonal chemical reporters.

Bioorthogonal Strategies for Engineering Extracellular Matrices

Hydrogels are commonly used as engineered extracellular matrix (ECM) mimics in applications ranging from tissue engineering to in vitro disease models. Ideal mechanisms used to crosslink ECM-mimicking hydrogels do not interfere with the biology of the system. However, most common hydrogel crosslinking chemistries exhibit some form of cross-reactivity. The field of bio-orthogonal chemistry has arisen to address the need for highly specific and robust reactions in biological contexts. Accordingly, bio-orthogonal crosslinking strategies have been incorporated into hydrogel design, allowing for gentle and efficient encapsulation of cells in various hydrogel materials. Furthermore, the selective nature of bio-orthogonal chemistries can permit dynamic modification of hydrogel materials in the presence of live cells and other biomolecules to alter matrix mechanical properties and biochemistry on demand. In this review, we provide an overview of bio-orthogonal strategies used to prepare cell-encapsulating hydrogels and highlight the potential applications of bio-orthogonal chemistries in the design of dynamic engineered ECMs.

15N4-1,2,4,5-tetrazines as potential molecular tags: Integrating bioorthogonal chemistry with hyperpolarization and unearthing para-N2

Hyperpolarized magnetic resonance (HP-MR) is a powerful, sensitive, and noninvasive approach to visualize molecular structure, function, and dynamics in vitro and in vivo. Current applications of HP-MR mostly rely on hyperpolarization of target compounds in dedicated hyperpolarizers because biomolecules can typically not be hyperpolarized directly in vivo. The injected hyperpolarized probes often undergo multiple metabolic pathways in living systems, and it remains challenging to localize and identify specific targets with high chemical selectivity. To address these current limitations in HP-MR, we report a novel hyperpolarization tagging strategy that integrates bioorthogonal chemistry and hyperpolarization to achieve the specific hyperpolarization of targets. This strategy is demonstrated by studies of hyperpolarized 15N4-1,2,4,5-tetrazines, which undergo rapid and selective cycloaddition with cyclooctyne to provide hyperpolarized 15N2-containing cycloaddition products and hyperpolarized 15N2 gas. This work not only suggests great potential of 15N4-1,2,4,5-tetrazines as molecular tags in HP-MR imaging (HP-MRI) but also supports the production of hyperpolarized para-15N2 gas, a biologically and medically innocuous gas with great potential for HP-MRI. This bioorthogonal reaction-based hyperpolarization tagging strategy enables a new class of in vitro and in vivo applications.

Vinylboronic Acids as Efficient Bioorthogonal Reactants for Tetrazine Labeling in Living Cells

Bioorthogonal chemistry can be used for the selective modification of biomolecules without interfering with any other functionality present in the cell. The tetrazine ligation is very suitable as a bioorthogonal reaction because of its selectivity and high reaction rates with several alkenes and alkynes. Recently, we described vinylboronic acids (VBAs) as novel hydrophilic bioorthogonal moieties that react efficiently with dipyridyl-s-tetrazines and used them for protein modification in cell lysate. It is not clear, however, whether VBAs are suitable for labeling experiments in living cells because of the possible coordination with, for example, vicinal carbohydrate diols. Here, we evaluated VBAs as bioorthogonal reactants for labeling of proteins in living cells using an irreversible inhibitor of the proteasome and compared the reactivity to that of an inhibitor containing norbornene, a widely used reactant for the tetrazine ligation. No large differences were observed between the VBA and norbornene probes in a two-step labeling approach with a cell-penetrable fluorescent tetrazine, indicating that the VBA gives little or no side reactions with diols and can be used efficiently for protein labeling in living cells.

Benzimidazole covalent probes and the gastric H(+)/K(+)-ATPase as a model system for protein labeling in a copper-free setting

Affinity probes are useful tools for determining molecular targets and elucidating mechanism of action for novel, bioactive compounds. In the case of covalent inhibitors, activity based probes are particularly valuable for ensuring acceptable selectivity margins. However, there is a variety of bioorthogonal chemistry reactions available for modifying compounds of interest with clickable tags. Here, we describe a direct comparison of tetrazine ligation and strain promoted azide-alkyne cycloaddition using benzimidazole based probes to bind their known target, the gastric proton pump, ATP4A. This study validates the use of chemical probes for target identification and illustrates the superior efficiency of tetrazine ligation for copper-free click systems. In addition, we have identified several novel binding partners of benzimidazole probes: Isoform 2 of deleted in malignant brain tumors 1 protein (DMBT1) and three uncharacterized proteins.

Direct Synthesis of β-Aminoenals through Reaction of 1,2,3-Triazine with Secondary Amines

Simple and direct nucleophilic addition of secondary amines, including imidazole, to 1,2,3-triazine under mild reaction conditions (THF, 25-65 °C, 12-48 h), requiring no additives, cleanly provides β-aminoenals 4 in good yields (21 examples, 31-79%). The reaction proceeds by amine nucleophilic addition to C4 of the 1,2,3-triazine, in situ loss of N₂, and subsequent imine hydrolysis to provide 4.

1,2,4-Triazine-Modified 2'-Deoxyuridine Triphosphate for Efficient Bioorthogonal Fluorescent Labeling of DNA

In order to establish the Diels-Alder reaction with inverse electron demand for postsynthetic DNA modification, a 1,2,4-triazine-modified 2'-deoxyuridine triphosphate was synthesized. The bioorthogonally reactive 1,2,4-triazine group was attached at the 5-position of 2'-deoxyuridine by a flexible alkyl linker to facilitate its acceptance by DNA polymerases. The screening of four DNA polymerases showed successful primer extensions, using a mixture of dATP, dGTP, dCTP, and the modified 2'-deoxyuridine triphosphate, by using KOD XL or Vent polymerase. The triazine moiety was stable under the conditions of primer extension, which was evidenced by labeling with a BCN-modified rhodamine at room temperature in yields of up to 82 %. Two or three modified bases could be incorporated in quantitative yields when the modification sites were separated by three base pairs. These results establish the 1,2,4-triazene group as a bioorthogonally reactive moiety in DNA, thereby replacing the problematic 1,2,4,5-tetrazine for postsynthetic labeling by the Diels-Alder reaction with inverse electron demand.

The discovery of pyridinium 1,2,4-triazines with enhanced performance in bioconjugation reactions

1,2,4-Triazines have recently been identified as versatile dienes participating in the inverse electron-demand Diels-Alder reaction with strained dienophiles. However, their widespread utility in bioconjugation reactions is still limited. Herein, we report a systematic study on the reactivity of various 1,2,4-triazines with trans-cyclooctenes showing that the structure of both the triazine and the dienophile significantly affect the reaction rate. Our kinetic study led to the discovery of novel cationic 1,2,4-triazines with superior properties for bioconjugation reactions. We have developed an efficient method that enables their late-stage functionalization and allows for easy access to various useful heterobifunctional scaffolds. In addition, these charged dienes form unprecedented fluorescent products upon reaction with trans-cyclooctenes and can be used for fluorogenic labeling of subcellular compartments in live cells.

Iluminated by foreign letters - Strategies for site-specific cyclopropene modification of large functional RNAs via in vitro transcription

The synthesis of sequence-specifically modified long RNA molecules, which cannot entirely be prepared via solid phase synthesis methods is experimentally challenging. We are using a new approach based on an expanded genetic alphabet preparing site-specifically modified RNA molecules via standard in vitro transcription. In this report, the site-specific labeling of functional RNAs, in particular ribozymes and a long non-coding RNA with cyclopropene moieties, is presented. We provide detailed instructions for RNA labeling via in vitro transcription and include required analytical methods to verify production and identity of the transcript. We further present post-transcriptional inverse electron demand Diels-Alder cycloaddition reactions on the cyclopropene-modified sequences and discuss applications of the genetic alphabet expansion transcription for in vitro preparation of labeled functional RNAs with complex foldings. In detail, the glmS and CPEB3 ribozymes were site-specifically decorated with methyl cyclopropene moieties using the unnatural TPT3^CP triphosphate and were proven to be still functional. In addition, the structurally complex A region of the Xist lncRNA (401nt) was site-specifically modified with methyl cyclopropene and detected by fluorescence after cycloaddition reaction with a tetrazine-BODIPY conjugate.