Kinetic Control of Complexity in Multiple Dynamic Libraries

Multiple dynamic libraries of compounds are generated when more than one reversible reaction comes into play. Commonly, two or more orthogonal reversible reactions are used, leading to non-communicating dynamic libraries which share no building blocks. Only a few examples of communicating libraries have been reported, and in all those cases, building blocks are reversibly exchanged from one library to the other, constituting an antiparallel dynamic covalent system. Herein we report that communication between two different dynamic libraries through an irreversible process is also possible. Indeed, alkyl amines cancel the dynamic regime on the nucleophilic substitution of tetrazines, generating kinetically inert compounds. Interestingly, such amine can be part of another dynamic library, an imine-amine exchange. Thus, both libraries are interconnected with each other by an irreversible process which leads to kinetically inert structures that contain parts from both libraries, causing a collapse of the complexity. Additionally, a latent irreversible intercommunication could be developed. In such a way, a stable molecular system with specific host-guest and fluorescence properties, could be irreversibly transformed when the right stimulus was applied, triggering the cancellation of the original supramolecular and luminescent properties and the emergence of new ones.

Tetrazine-Linked Covalent Organic Frameworks With Acid Sensing and Photocatalytic Activity

The first synthesis and comprehensive characterization of two vinyl tetrazine-linked covalent organic frameworks (COF), TA-COF-1 and TA-COF-2, are reported. These materials exhibit high crystallinity and high specific surface areas of 1323 and 1114 m2 g-1. The COFs demonstrate favorable band positions and narrow band gaps suitable for light-driven applications. These advantages enable TA-COFs to act as reusable metal-free photocatalysts in the arylboronic acids oxidation and light-induced coupling of benzylamines. In addition, these TA-COFs show acid sensing capabilities, exhibiting visible and reversible color changes upon exposure to HCl solution, HCl vapor, and NH3 vapor. Further, the TA-COFs outperform a wide range of previously reported COF photocathodes. The tetrazine linker in the COF skeleton represents a significant advancement in the field of COF synthesis, enhancing the separation efficiency of charge carriers during the photoreaction and contributing to their photocathodic properties. TA-COFs can also degrade 5-nitro-1,2,4-triazol-3-one (NTO), an insensitive explosive present in industrial wastewater, in 20 min in a sunlight-driven photocatalytic process; thus, revealing dual functionality of the protonated TA-COFs as both photodegradation and Brønsted acid catalysts. This pioneering work opens new avenues for harnessing the potential of the tetrazine linker in COF-based materials, facilitating advances in catalysis, sensing, and other related fields.

Aqueous Zn-Tetrazine Batteries with Cooperative Zn2+/H+ Insertion

We present the investigation of 1,2,4,5-tetrazine derivatives as low-cost and synthetically modular organic electrode materials in rechargeable aqueous Zn-ion batteries (AZIBs). The substituents at the 3,6-positions of tetrazine were found to be critical for cycling stability. While heteroatom substituents (chloro, methoxy, and pyrazole) lead to the rapid decomposition of electrode materials in the electrolyte, the installation of phenyl groups enhances the cycling stability via π-π stacking. Spectroscopic characterization suggests a cooperative Zn2+ and H+ insertion mechanism. This unique cooperativity of Zn2+ and H+ leads to a steady discharge plateau in contrast to the undesirable sloping voltage profile typically observed in Zn-organic batteries.

A Tetrazine-Caged Carbon-Dipyrromethene as a Bioorthogonally Activatable Fluorescent Probe

A water-soluble 1,2,4,5-tetrazine-substituted carbon-dipyrromethene (C-DIPY) was synthesized from the previously reported carbonyl pyrrole dimer through a two-step procedure. Owing to the presence of a tetrazine moiety, the fluorescence emission of this compound was largely quenched in phosphate-buffered saline at pH 7.4. Upon addition of a bicyclo[6.1.0]non-4-yne (BCN) derivative, the tetrazine-based quenching component of the compound was disrupted through the inverse electron-demand Diels-Alder reaction to restore the fluorescence in up to 6.6-fold. This bioorthogonal activation was also demonstrated using U-87 MG human glioblastoma cells, in which the fluorescence intensity of this C-DIPY could be enhanced by 8.7-fold upon post-incubation with the BCN derivative. The results showed that this tetrazine-caged C-DIPY can serve as a bioorthogonally activatable fluorescent probe for bioimaging. The compound, however, was found to reside preferentially in the lysosomes instead of the mitochondria of the cells as predicted based on its cationic character, which could be attributed to its energy-dependent endocytic cellular uptake pathway, for which lysosomes are the end station.

Synthesis, Fluorine-18 Radiolabeling, and In Vivo PET Imaging of a Hydrophilic Fluorosulfotetrazine

The development of ¹⁸F-fluorotetrazines, suitable for the radiolabeling of biologics such as proteins and antibodies by IEDDA ligation, represents a major challenge, especially for pre-targeting applications. The hydrophilicity of the tetrazine has clearly become a crucial parameter for the performance of in vivo chemistry. In this study, we present the design, the synthesis, the radiosynthesis, the physicochemical characterization, the in vitro and in vivo stability, as well as the pharmacokinetics and the biodistribution determined by PET imaging in healthy animals of an original hydrophilic ¹⁸F-fluorosulfotetrazine. This tetrazine was prepared and radiolabelled with fluorine-18 according to a three-step procedure, starting from propargylic butanesultone as the precursor. The propargylic sultone was converted into the corresponding propargylic fluorosulfonate by a ring-opening reaction with ^18/19F-fluoride. Propargylic ^18/19F-fluorosulfonate was then subject to a CuACC reaction with an azidotetrazine, followed by oxidation. The overall automated radiosynthesis afforded the ¹⁸F-fluorosulfotetrazine in 29-35% DCY, within 90-95 min. The experimental LogP and LogD_7.4 values of -1.27 ± 0.02 and -1.70 ± 0.02, respectively, confirmed the hydrophilicity of the ¹⁸F-fluorosulfotetrazine. In vitro and in vivo studies displayed a total stability of the ¹⁸F-fluorosulfotetrazine without any traces of metabolization, the absence of non-specific retention in all organs, and the appropriate pharmacokinetics for pre-targeting applications.

Head-to-Tail Dimerization of N-Heterocyclic Diazoolefins

The head-to-tail dimerization of N-heterocyclic diazoolefins is described. The products of these formal (3+3) cycloaddition reactions are strongly reducing quinoidal tetrazines. Oxidation of the tetrazines occurs in a stepwise fashion, and we were able to isolate a stable radical cation and diamagnetic dications. The latter are also accessible by oxidative dimerization of diazoolefins.

Facile de Novo Sequencing of Tetrazine-Cyclized Peptides through UV-Induced Ring-Opening and Cleavage from the Solid Phase

While most FDA-approved peptide drugs are cyclic, the robust cyclization chemistry of peptides and the deconvolution of cyclic peptide sequences by using tandem mass spectrometry render cyclic peptide drug discovery difficult. Here we present the successful design of cyclic peptides on solid phase that addresses both of these problems. We demonstrate that this peptide cyclization method using dichloro-s-tetrazine on solid phase allows successful cyclization of a panel of random peptide sequences with various charges and hydrophobicities. The cyclic peptides can be linearized and cleaved from the solid phase by simple UV light irradiation, and we demonstrate that accurate sequence information can be obtained for the UV-cleaved linearized peptides by using tandem mass spectrometry. The tetrazine linker used in the cyclic peptides can further be explored for inverse electron-demand Diels-Alder (IEDDA) reactions for screening or bioconjugation applications in the future.

Two-Factor Fluorogenicity of Tetrazine-Modified Cyanine-Styryl Dyes for Bioorthogonal Labelling of DNA

Two green fluorescent tetrazine-modified cyanine-styryl dyes were synthesized for bioorthogonal labelling of DNA by means of the Diels-Alder reaction with inverse electron demand. With DNA as target biopolymer the fluorescence of these dyes is released by two factors: (i) sterically by their interaction with DNA, and (ii) structurally via the conjugated tetrazine as quencher moiety. As a result, the reaction with bicyclononyne-modified DNA is significantly accelerated up to ≥284,000 M^-1  s^-1 , and the fluorescence turn-on is enhanced up to 560 by the two-factor fluorogenicity. These dyes are cell permeable even in low concentrations and undergo fluorogenic reactions with BCN-modified DNA in living HeLa cells. The two-factor fluorescence release improves the signal-to-noise ratio such that washing procedures prior to cell imaging are not needed, which is a great advantage for live cell imaging of DNA and RNA in the future.

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.

An approach to develop personalized radiopharmaceuticals by modifying 2-[18F]fluoro-2-deoxy-D-glucose (2-[18F]FDG)

BACKGROUND

A challenge for modern medicine is the development of clinical protocols for precise diagnosis and therapy. This study aimed to propose a simple method for modification of 2-[18F]FDG used routinely in hospitals in a way, appropriate for patients' personalized radiopharmaceuticals approach.

MATERIAL AND METHODS

For the purposes of the presented study chemo selective method for indirect radiofluorination was applauded to custom synthesized aminooxy- and hydrazine-functionalized tetrazines for 18F-glycolation via oxime or hydrazone formation. 2-[18F]FDG produced with medical baby cyclotron in Nuclear Medicine Clinic at the University Hospital St. Marina-Varna, was used. Thin layer chromatography (TLC) and radio TLC were used to follow the progress of synthesis and to determine radio chemical yield (RCY).

RESULTS

The 2-[18F]FDG was modified with two bifunctional tetrazines aminooxy-acetic acid-6-(2-aminooxy-acetoxy)-[1,2,4,5] tetrazin-3-yl ester (Tz1) and {3-[4-(6-phenyl-[1,2,4,5]tetrazin-3-yl)-phenoxy]-propyl}-hydrazine (Tz2) via oxime and hydrazone formation. The radiolabeling was carried out as one-pot reaction with following parameters: temperature 70-75°C; catalyst p- diaminobenzene (Cat.); pH = 4.2; time 30 minutes; RCY = 70-99%. The radiolabeled tetrazines are appropriate for further bioorthogonal (pretargeting) strategy by click reactions with trans-cyclooctene conjugated bioactive molecules. The methodology is applicable to standard clinical conditions.

Synthesis and radiolabeling of a polar [125 I]I-1,2,4,5-tetrazine

Pretargeting imaging has gained a lot of prominence, due to its excellent bioorthogonality and improved imaging contrast compared to conventional imaging. A new iodo tetrazine (Tz) derivative has been synthesized and further developed into the corresponding iodine-125 (¹²⁵ I) analog (12), via the trimethylstannane precursor. Radiolabeling with either N-chlorosuccinimide or chloramine-T, in either MeCN or MeOH proceeded with a radiochemical conversion (RCC) of >80%. Subsequent deprotection only proved successful, among the tested conditions, when the radiolabeled Tz was stirred in 6-M HCl_(aq.) at 60°C for 2.5 h. To the best of our knowledge, this is the first H-tetrazine labeled with iodine. In vivo investigations on the pretargeting ability of 12 are currently under way.

Generating Efficient Methanomethylophilus alvus Pyrrolysyl-tRNA Synthetases for Structurally Diverse Non-Canonical Amino Acids

Genetic code expansion (GCE) technologies commonly use the pyrrolysyl-tRNA synthetase (PylRS)/tRNAPyl pairs from Methanosarcina mazei (Mm) and Methanosarcina barkeri (Mb) for site-specific incorporation of non-canonical amino acids (ncAAs) into proteins. Recently a homologous PylRS/tRNAPyl pair from Candidatus Methanomethylophilus alvus Mx1201 (Ma) was developed that, lacking the N-terminal tRNA-recognition domain of most PylRSs, overcomes insolubility, instability, and proteolysis issues seen with Mb/Mm PylRSs. An open question is how to alter Ma PylRS specificity to encode specific ncAAs with high efficiency. Prior work focused on "transplanting" ncAA substrate specificity by reconstructing the same active site mutations found in functional Mm/Mb PylRSs in Ma PylRS. Here, we found that this strategy produced low-efficiency Ma PylRSs for encoding three structurally diverse ncAAs: acridonyl-alanine (Acd), 3-nitro-tyrosine, and m-methyl-tetrazinyl-phenylalanine (Tet3.0-Me). On the other hand, efficient Ma PylRS variants were generated by a conventional life/death selection process from a large library of active site mutants: for Acd encoding, one variant was highly functional in HEK293T cells at just 10 μM Acd; for nitroY encoding, two variants also encoded 3-chloro, 3-bromo-, and 3-iodo-tyrosine at high efficiency; and for Tet-3.0-Me, all variants were more functional at lower ncAA concentrations. All Ma PylRS variants identified through selection had at least two different active site residues when compared with their Mb PylRS counterparts. We conclude that Ma and Mm/Mb PylRSs are sufficiently different that "active site transplantation" yields suboptimal Ma GCE systems. This work establishes a paradigm for expanding the utility of the promising Ma PylRS/tRNAPyl GCE platform.

Bioorthogonal Chemistry Approach for the Theranostics of GRPR-Expressing Cancers

Several gastrin-releasing peptide receptor (GRPR) antagonists with improved in vivo behavior have been recently developed and tested in the clinic. However, despite the generally mild side effects of peptide receptor radionuclide therapy (PRRT), toxicity has been observed due to high doses delivered to nontarget tissues, especially in the kidneys and pancreas. Previous experiences with radiolabeled peptides opened a unique opportunity to explore GRPR pretargeting using clickable bombesin antagonists. Toward this goal, we used clickable DOTA-like radiocomplexes which have been previously evaluated by our group. We functionalized a potent GRPR antagonist with a clickable TCO moiety using two different linkers. These precursors were then studied to select the compound with the highest GRPR binding affinity and the best pharmacokinetics to finally explore the advantages of the devised pretargeting approach. Our results provided an important proof of concept toward the development of bioorthogonal approaches to GRPR-expressing cancers, which are worth investigating further to improve the in vivo results. Moreover, the use of clickable GRPR antagonists and DOTA/DOTAGA derivatives allows for fine-tuning of their pharmacokinetics and metabolic stability, leading to a versatile synthesis of new libraries of (radio)conjugates useful for the development of theranostic tools toward GRPR-expressing tumors.

Antibody-Based In Vivo Imaging of Central Nervous System Targets-Evaluation of a Pretargeting Approach Utilizing a TCO-Conjugated Brain Shuttle Antibody and Radiolabeled Tetrazines

Bioorthogonal pretargeted imaging using the inverse-electron-demand Diels-Alder (IEDDA) reaction between a tetrazine (Tz) and a trans-cyclooctene (TCO) represents an attractive strategy for molecular imaging via antibodies. The advantages of using a pretargeted imaging approach are on the one hand the possibility to achieve a high signal-to-noise ratio and imaging contrast; on the other hand, the method allows the uncoupling of the biological half-life of antibodies from the physical half-life of short-lived radionuclides. A brain-penetrating antibody (mAb) specific for β-amyloid (Aβ) plaques was functionalized with TCO moieties for pretargeted labeling of Aβ plaques in vitro, ex vivo, and in vivo by a tritium-labeled Tz. The overall aim was to explore the applicability of mAbs for brain imaging, using a preclinical model system. In vitro clicked mAb-TCO-Tz was able to pass the blood-brain barrier of transgenic PS2APP mice and specifically visualize Aβ plaques ex vivo. Further experiments showed that click reactivity of the mAb-TCO construct in vivo persisted up to 3 days after injection by labeling Aβ plaques ex vivo after incubation of brain sections with the Tz in vitro. An attempted in vivo click reaction between injected mAb-TCO and Tz did not lead to significant labeling of Aβ plaques, most probably due to unfavorable in vivo properties of the used Tz and a long half-life of the mAb-TCO in the blood stream. This study clearly demonstrates that pretargeted imaging of CNS targets via antibody-based click chemistry is a viable approach. Further experiments are warranted to optimize the balance between stability and reactivity of all reactants, particularly the Tz.

Functionalization of Radiolabeled Antibodies to Enhance Peripheral Clearance for High Contrast Brain Imaging

Small molecule imaging agents such as [¹¹C]PiB, which bind to the core of insoluble amyloid-β (Aβ) fibrils, are useful tools in Alzheimer’s disease (AD) research, diagnostics, and drug development. However, the [¹¹C]PiB PET signal saturates early in the disease progression and does not detect soluble or diffuse Aβ pathology which are believed to play important roles in the disease progression. Antibodies, modified into a bispecific format to enter the brain via receptor-mediated transcytosis, could be a suitable alternative because of their diversity and high specificity for their target. However, the circulation time of these antibodies is long, resulting in an extended exposure to radiation and low imaging contrast. Here, we explore two alternative strategies to enhance imaging contrast by increasing clearance of the antibody ligand from blood. The bispecific Aβ targeting antibody RmAb158-scFv8D3 and the monospecific RmAb158 were radiolabeled and functionalized with either α-d-mannopyranosylphenyl isothiocyanate (mannose) or with trans-cyclooctene (TCO). While mannose can directly mediate antibody clearance via the liver, TCO-modified antibody clearance was induced by injection of a tetrazine-functionalized, liver-targeting clearing agent (CA). In vivo experiments in wild type and AD transgenic mice demonstrated the ability of both strategies to drastically shorten the circulation time of RmAb158, while they had limited effect on the bispecific variant RmAb158-8D3. Furthermore, single photon emission computed tomography imaging with TCO-[¹²⁵I]I-RmAb158 in AD mice showed higher contrast 1 day after injection of the tetrazine-functionalized CA. In conclusion, strategies to enhance the clearance of antibody-based imaging ligands could allow imaging at earlier time points and thereby open the possibility to combine antibodies with short-lived radionuclides such as fluorine-18.

An Alliance of Polynitrogen Heterocycles: Novel Energetic Tetrazinedioxide-Hydroxytetrazole-Based Materials

Energetic materials constitute one of the most important subtypes of functional materials used for various applications. A promising approach for the construction of novel thermally stable high-energy materials is based on an assembly of polynitrogen biheterocyclic scaffolds. Herein, we report on the design and synthesis of a new series of high-nitrogen energetic salts comprising the C-C linked 6-aminotetrazinedioxide and hydroxytetrazole frameworks. Synthesized materials were thoroughly characterized by IR and multinuclear NMR spectroscopy, elemental analysis, single-crystal X-ray diffraction and differential scanning calorimetry. As a result of a vast amount of the formed intra- and intermolecular hydrogen bonds, prepared ammonium and amino-1,2,4-triazolium salts are thermally stable and have good densities of 1.75–1.78 g·cm⁻³. All synthesized compounds show high detonation performance, reaching that of benchmark RDX. At the same time, as compared to RDX, investigated salts are less friction sensitive due to the formed net of hydrogen bonds. Overall, reported functional materials represent a novel perspective subclass of secondary explosives and unveil further opportunities for an assembly of biheterocyclic next-generation energetic materials.

Stoichiometric Post-Modification of Hydrogel Microparticles Dictates Neural Stem Cell Fate in Microporous Annealed Particle Scaffolds

Microporous annealed particle (MAP) scaffolds are generated from assembled hydrogel microparticles (microgels). It has been previously demonstrated that MAP scaffold are porous, biocompatible, and recruit neural progenitor cells (NPCs) to the stroke cavity after injection into the stroke core. Here, the goal is to study NPC fate inside MAP scaffolds in vitro. To create plain microgels that can later be converted to contain different types of bioactivities, the inverse electron-demand Diels-Alder reaction between tetrazine and norbornene is utilized, which allows the post-modification of plain microgels stoichiometrically. As a result of adhesive peptide attachment, NPC spreading leads to contractile force generation which can be recorded by tracking microgel displacement. Alternatively, non-adhesive peptide integration results in neurosphere formation that grows within the void space of MAP scaffolds. Although the formed neurospheres do not impose a contractile force on the scaffolds, they are seen to continuously transverse the scaffolds. It is concluded that MAP scaffolds  can be engineered to either promote neurogenesis or enhance stemness depending on the chosen post-modifications of the microgels, which can be key in modulating their phenotypes in various applications in vivo.

Radical-Bridged Ln4 Metallocene Complexes with Strong Magnetic Coupling and a Large Coercive Field

Inducing magnetic coupling between 4f elements is an ongoing challenge. To overcome this formidable difficulty, we incorporate highly delocalized tetrazinyl radicals, which strongly couple with f-block metallocenes to form discrete tetranuclear complexes. Synthesis, structure, and magnetic properties of two tetranuclear [(Cp*₂ Ln)₄ (tz^. )₄ ]⋅3(C₆ H₆ ) (Cp*=pentamethylcyclopentadienyl; tz=1,2,4,5-tetrazine; Ln=Dy, Gd) complexes are reported. An in-depth examination of their magnetic properties through magnetic susceptibility measurements as well as computational studies support a highly sought-after radical-induced "giant-spin" model. Strong exchange interactions between the Ln^III ions and tz^. radicals lead to a strong magnet-like behaviour in this molecular magnet with a large coercive field of 30 kOe.

Non-Chromatographic Purification of Synthetic RNA Using Bio-Orthogonal Chemistry

Solid-phase synthesis of RNA oligonucleotides over 100 nt in length remains challenging due to the complexity of purification of the target strands from the failure sequences. This article describes a non-chromatographic procedure that will enable routine solid-phase synthesis and purification of long RNA strands. The optimized five-step process is based on bio-orthogonal inverse electron demand Diels-Alder chemistry between trans-cyclooctene (TCO) and tetrazine (Tz), and entails solid-phase synthesis of RNA on a photo-labile support. The target oligonucleotide strands are selectively tagged with Tz while on-support. After photocleavage from the solid support, the target oligonucleotide strands can be captured and purified from the failure sequences using immobilized TCO. The approach can be applied for purification of 76-nt long tRNA and 101-nt long sgRNA for CRISPR experiments. Purity of the isolated oligonucleotides should be evaluated using gel electrophoresis, while functional fidelity of the sgRNA should be confirmed using CRISPR-Cas9 experiments. © 2021 Wiley Periodicals LLC. Basic Protocol: Five-step non-chromatographic purification of synthetic RNA oligonucleotides Support Protocol 1: Synthesis of the components that are required for the non-chromatographic purification of long RNA oligonucleotides. Support Protocol 2: Solid-phase RNA synthesis.

Dynamic Nucleophilic Aromatic Substitution of Tetrazines

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

IEDDA: An Attractive Bioorthogonal Reaction for Biomedical Applications

The pretargeting strategy has recently emerged in order to overcome the limitations of direct targeting, mainly in the field of radioimmunotherapy (RIT). This strategy is directly dependent on chemical reactions, namely bioorthogonal reactions, which have been developed for their ability to occur under physiological conditions. The Staudinger ligation, the copper catalyzed azide-alkyne cycloaddition (CuAAC) and the strain-promoted [3 + 2] azide–alkyne cycloaddition (SPAAC) were the first bioorthogonal reactions introduced in the literature. However, due to their incomplete biocompatibility and slow kinetics, the inverse-electron demand Diels-Alder (IEDDA) reaction was advanced in 2008 by Blackman et al. as an optimal bioorthogonal reaction. The IEDDA is the fastest bioorthogonal reaction known so far. Its biocompatibility and ideal kinetics are very appealing for pretargeting applications. The use of a trans-cyclooctene (TCO) and a tetrazine (Tz) in the reaction encouraged researchers to study them deeply. It was found that both reagents are sensitive to acidic or basic conditions. Furthermore, TCO is photosensitive and can be isomerized to its cis-conformation via a radical catalyzed reaction. Unfortunately, the cis-conformer is significantly less reactive toward tetrazine than the trans-conformation. Therefore, extensive research has been carried out to optimize both click reagents and to employ the IEDDA bioorthogonal reaction in biomedical applications.

Overview of Syntheses and Molecular-Design Strategies for Tetrazine-Based Fluorogenic Probes

Various bioorthogonal chemistries have been used for fluorescent imaging owing to the advantageous reactions they employ. Recent advances in bioorthogonal chemistry have revolutionized labeling strategies for fluorescence imaging, with inverse electron demand Diels-Alder (iEDDA) reactions in particular attracting recent attention owing to their fast kinetics and excellent specificity. One of the most interesting features of the iEDDA labeling strategy is that tetrazine-functionalized dyes are known to act as fluorogenic probes. In this review, we will focus on the synthesis, molecular-design strategies, and bioimaging applications of tetrazine-functionalized fluorogenic probes. Traditional Pinner reaction and "Pinner-like" reactions for tetrazine synthesis are discussed here, as well as metal-catalyzed C-C bond formations with convenient tetrazine intermediates and the fabrication of tetrazine-conjugated fluorophores. In addition, four different quenching mechanisms for tetrazine-modified fluorophores are presented.

Click-Functionalization of a Poly(Tetrazine-co-Fluorene)-Conjugated Polymer with a Series of trans-Cyclooctene Derivatives

A soluble poly(tetrazine) polymer was prepared via Suzuki polycondensation of 3,6-bis(5-bromofuran-2-yl)-1,2,4,5-tetrazine and a fluorene diboronate derivative. It can undergo efficient and quantitative post-polymerization inverse-electron-demand Diels-Alder click reactions with a variety of trans-cyclooctene (TCO) derivatives. The resulting polymers were oxidized to convert dihydropyridazine rings into pyridazines. The absorption spectra of the product polymers, both before and after oxidation, showed hypsochromic shifts that correlated with steric hindrance of the appended side chains. They also exhibited a significantly enhanced fluorescence intensity relative to the original poly(tetrazine). While gel-permeation chromatography indicated that the product polymers exhibited longer retention times, NMR end-group analysis showed that the polymers retained relatively constant degrees of polymerization. Graft copolymers were easily prepared via reaction with TCO-functionalized poly(ethylene glycol) chains and a cross-linked foam was produced by reacting the poly(tetrazine) with a bis-TCO crosslinker.

Asymmetric Ring Opening in a Tetrazine-Based Ligand Affords a Tetranuclear Opto-Magnetic Ytterbium Complex

We report the formation of a tetranuclear lanthanide cluster, [Yb₄ (bpzch)₂ (fod)₁₀ ] (1), which occurs from a serendipitous ring opening of the functionalised tetrazine bridging ligand, bpztz (3,6-dipyrazin-2-yl-1,2,4,5-tetrazine) upon reacting with Yb(fod)₃ (fod^- =6,6,7,7,8,8,8-heptafluoro-2,2-dimethyl-3,5-octandionate). Compound 1 was structurally elucidated via single-crystal X-ray crystallography and subsequently magnetically and spectroscopically characterised to analyse its magnetisation dynamics and its luminescence behaviour. Computational studies validate the observed M_J energy levels attained by spectroscopy and provides a clearer picture of the slow relaxation of the magnetisation dynamics and relaxation pathways. These studies demonstrate that 1 acts as a single-molecule magnet (SMM) under an applied magnetic field in which the relaxation occurs via a combination of Raman, direct, and quantum tunnelling processes, a behaviour further rationalised analysing the luminescent properties. This marks the first lanthanide-containing molecule that forms by means of an asymmetric tetrazine decomposition.

Evaluation of [64Cu]Cu-NOTA-PEG7-H-Tz for Pretargeted Imaging in LS174T Xenografts-Comparison to [111In]In-DOTA-PEG11-BisPy-Tz

Pretargeted nuclear imaging for the diagnosis of various cancers is an emerging and fast developing field. The tetrazine ligation is currently considered the most promising reaction in this respect. Monoclonal antibodies are often the preferred choice as pretargeting vector due to their outstanding targeting properties. In this work, we evaluated the performance of [64Cu]Cu-NOTA-PEG7-H-Tz using a setup we previously used for [111In]In-DOTA-PEG11-BisPy-Tz, thereby allowing for comparison of the performance of these two promising pretargeting imaging agents. The evaluation included a comparison of the physicochemical properties of the compounds and their performance in an ex vivo blocking assay. Finally, [64Cu]Cu-NOTA-PEG7-H-Tz was evaluated in a pretargeted imaging study and compared to [111In]In-DOTA-PEG11-BisPy-Tz. Despite minor differences, this study indicated that both evaluated tetrazines are equally suited for pretargeted imaging.

Fluorine-18-Labeled Antibody Ligands for PET Imaging of Amyloid-β in Brain

Antibodies are attractive as radioligands due to their outstanding specificity and high affinity, but their inability to cross the blood-brain barrier (BBB) limits their use for CNS targets. To enhance brain distribution, amyloid-β (Aβ) antibodies were fused to a transferrin receptor (TfR) antibody fragment, enabling receptor mediated transport across the BBB. The aim of this study was to label these bispecific antibodies with fluorine-18 and use them for Aβ PET imaging. Bispecific antibody ligands RmAb158-scFv8D3 and Tribody A2, both targeting Aβ and TfR, were functionalized with trans-cyclooctene (TCO) groups and conjugated with ¹⁸F-labeled tetrazines through an inverse electron demand Diels-Alder reaction performed at ambient temperature. ¹⁸F-labeling did not affect antibody binding in vitro, and initial brain uptake was high. Conjugates with the first tetrazine variant ([¹⁸F]T1) displayed high uptake in bone, indicating extensive defluorination, a problem that was resolved with the second and third tetrazine variants ([¹⁸F]T2 and [¹⁸F]T3). Although the antibody ligands' half-life in blood was too long to optimally match the physical half-life of fluorine-18 (t_1/2 = 110 min), [¹⁸F]T3-Tribody A2 PET seemed to discriminate transgenic mice (tg-ArcSwe) with Aβ deposits from wild-type mice 12 h after injection. This study demonstrates that ¹⁸F-labeling of bispecific, brain penetrating antibodies is feasible and, with further optimization, could be used for CNS PET imaging.

A General Strategy to Design Highly Fluorogenic Far-Red and Near-Infrared Tetrazine Bioorthogonal Probes

Highly fluorogenic tetrazine bioorthogonal probes emitting at near-infrared wavelengths are in strong demand for biomedical imaging applications. Herein, we have developed a strategy for forming a palette of novel Huaxi-Fluor probes in situ, whose fluorescence increases hundreds of times upon forming the bioorthogonal reaction product, pyridazine. The resulting probes show large Stokes shifts and high quantum yields. Manipulating the conjugate length and pull-push strength in the fluorophore skeleton allows the emission wavelength to be fine-tuned from 556 to 728 nm. The highly photo-stable and biocompatible probes are suitable for visualizing organelles in live cells without a washing step and for imaging of tumors in live small animals to depths of 500 μm by two-photon excitation.

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.

Activation and Delivery of Tetrazine-Responsive Bioorthogonal Prodrugs

Prodrugs, which remain inert until they are activated under appropriate conditions at the target site, have emerged as an attractive alternative to drugs that lack selectivity and show off-target effects. Prodrugs have traditionally been activated by enzymes, pH or other trigger factors associated with the disease. In recent years, bioorthogonal chemistry has allowed the creation of prodrugs that can be chemically activated with spatio-temporal precision. In particular, tetrazine-responsive bioorthogonal reactions can rapidly activate prodrugs with excellent biocompatibility. This review summarized the recent development of tetrazine bioorthogonal cleavage reaction and great promise for prodrug systems.

Bioorthogonal Turn-On BODIPY-Peptide Photosensitizers for Tailored Photodynamic Therapy

Photodynamic therapy (PDT) leads to cancer remission via the production of cytotoxic species under photosensitizer (PS) irradiation. However, concomitant damage and dark toxicity can both hinder its use. With this in mind, we have implemented a versatile peptide-based platform of bioorthogonally activatable BODIPY-tetrazine PSs. Confocal microscopy and phototoxicity studies demonstrated that the incorporation of the PS, as a bifunctional module, into a peptide enabled spatial and conditional control of singlet oxygen (1 O2 ) generation. Comparing subcellular distribution, PS confined in the cytoplasmic membrane achieved the highest toxicities (IC50 =0.096±0.003 μm) after activation and without apparent dark toxicity. Our tunable approach will inspire novel probes towards smart PDT.

Derivatives of 3,6-Bis(3-aminofurazan-4-ylamino)-1,2,4,5-tetrazine: Excellent Energetic Properties with Lower Sensitivities

To find a balance between energy and safety, a series of compounds based on azo-, azoxy-, 1,4,2,5-dioxadiazene-, and 3,6-diamino-1,2,4,5-tetrazine-bridged bis(aminofurazan) were designed and synthesized. These compounds were analyzed by nitro group charges (Q_nitro) and bond dissociation energy (BDE) calculations, which are related to sensitivity and stability. Based on the calculated results, derivatives of 3,6-bis(3-aminofurazan-4-ylamino)-1,2,4,5-tetrazine have the largest values for -Q_nitro and BDE of all of the bis(aminofurazan) compounds. This shows that compounds based on 3,6-bis(3-aminofurazan-4-ylamino)-1,2,4,5-tetrazine have the lowest sensitivities and best stabilities, which has been substantiated by experiments. Additionally, their explosive properties remain essentially competitive with compounds based on azo-, azoxy-, and 1,4,2,5-dioxadiazene-bridged bis(aminofurazan). Hirshfeld surface calculations were also performed to better understand the relationship between the molecular structure and stability/sensitivity. This work highlights the value of 3,6-diamino-1,2,4,5-tetrazine as a linker to achieve good balance between safety and energy.

Metabolism of a Bioorthogonal PET Tracer Candidate [19F/18F]SiFA-Tetrazine in Mouse Liver Microsomes: Biotransformation Pathways and Defluorination Investigated by UHPLC-HRMS

Organofluorosilicon based ¹⁸F-radiolabeling is an efficient method for incorporating fluorine-18 into ¹⁸F-radiopharmaceuticals for positron emission tomography (PET) by ¹⁹F/¹⁸F isotopic exchange (IE). The first PET radiopharmaceutical, ¹⁸F-SiFAlin-TATE, radiolabeled with a silicon-based [¹⁸F]fluoride acceptor (SiFA), namely, a para-substituted di-tert-butyl[¹⁸F]fluorosilylbenzene, has entered clinical trials, and is paving the way for other potential [¹⁸F]SiFA-labeled radiopharmaceuticals for diagnostic use. In this study, we report the in vitro metabolism of an oxime-linked SiFA tetrazine (SiFA–Tz), a new PET-radiotracer candidate, recently evaluated for pretargeted PET imaging and macromolecule labeling. Metabolism of SiFA–Tz was studied in mouse liver microsomes (MLM) for elucidating its major biotransformation pathways. Nontargeted screening by ultrahigh performance liquid chromatography high-resolution mass spectrometry (UHPLC-HRMS) was utilized for detection of unknown metabolites. The oxime bond between the SiFA and Tz groups forms two geometric (E/Z) isomers, which underwent the same biotransformations, but unexpectedly with different kinetics. In total, nine proposed metabolites of SiFA–Tz from phase I and II reactions were detected, five of which were defluorinated in MLMs, elucidating the metabolic pathway leading to previously reported defluorination of [¹⁸F]SiFA–Tz in vivo. Based on the HRMS studies a biotransformation pathway is proposed: hydroxylation (+O) to tert-butyl group adjacent to the silicon, followed by oxidative defluorination (+OH/-F) cleaving the fluorine off the silicon. Interestingly, eight proposed metabolites of a reduced dihydrotetrazine analogue, SiFA–H₂Tz, from phase I and II reactions were additionally detected. To the best of our knowledge, this is the first reported comprehensive investigation of enzyme mediated metabolic pathway of tetrazines and para-substituted di-tert-butylfluorosilylbenzene fluoride acceptors, providing novel structural information on the biotransformation and fragmentation patterns of radiotracers bearing these structural motifs. By investigating the metabolism preceding defluorination, structurally optimized new SiFA compounds can be designed for expanding the portfolio of efficient ¹⁹F/¹⁸F isotopic exchange labeling probes for PET imaging.

Intertwined Detection and Recognition Roles of Tetrazine in Synergistic Anion-π and H-bond Based Anion Receptor

The intrinsic properties of tetrazine as a π-anion receptor and as an on/off recognition probe merged with H-bond ability of an urea motif into a single architecture constitutes a new generation of well-defined anion receptors. Complexation properties directly benefit from the dual and synergistic contribution of tetrazine and urea. In this study, we report on the synthesis and assessment of binding properties to anions of diverse geometries. Association constants have been predicted by theoretical calculations and evaluated by multiple and complementary experimental techniques including electrospray-mass tandem spectroscopy, NMR, UV-visible, steady state fluorescence spectroscopies and time resolved fluorescence. These results provide the basis for a better understanding of both the complexation and the anion-dependent quenching mechanism.

Click by Click Microporous Annealed Particle (MAP) Scaffolds

Macroporous scaffolds are being increasingly used in regenerative medicine and tissue repair. While the recently developed microporous annealed particle (MAP) scaffolds have overcome issues with injectability and in situ hydrogel formation, limitations with respect to tunability to be able to manipulate hydrogel strength and rigidity for broad applications still exist. To address these key issues, here hydrogel microparticles (HMPs) of hyaluronic acid (HA) are synthesized using the thiol-norbornene click reaction and then HMPs are subsequently annealed into a porous scaffold using the tetrazine-norbornene click reaction. This assembly method allows for straightforward tuning of bulk scaffold rigidity by varying the tetrazine to norbornene ratio, with increasing tetrazine resulting in increasing scaffold storage modulus, Young's modulus, and maximum stress. These changes are independent of void fraction. Further incorporation of human dermal fibroblasts throughout the porous scaffold reveals the biocompatibility of this annealing strategy as well as differences in proliferation and cell-occupied volume. Finally, injection of porous HA-Tet MAP scaffolds into an ischemic stroke model shows this chemistry is biocompatible in vivo with reduced levels of inflammation and astrogliosis as previously demonstrated for other crosslinking chemistries.

Ultra-fast cycling for multiplexed cellular fluorescence imaging

Rapid analysis of single and scant cell populations is essential in modern diagnostics, yet existing methods are often limited and slow. Here we describe an ultra-fast, highly efficient cycling method for the analysis of single cells based on unique linkers for tetrazine (Tz) / trans-cyclooctene (TCO) mediated quenching. Surprisingly, the quenching reaction rates were more than 3 orders of magnitude faster (t1/2 < 1 sec) than predicted. This allowed multi-cycle staining and immune cell profiling within an hour, leveraging the accelerated kinetics to open new diagnostic possibilities for rapid cellular analyses.

Improving Tumor-to-Background Contrast through Hydrophilic Tetrazines: The Construction of 18 F-Labeled PET Agents Targeting Nonsmall Cell Lung Carcinoma

Bioorthogonal reactions have been widely used in the biomedical field. ¹⁸ F-TCO/Tetrazine ligation is the most reactive radiolabelled inverse electron demand Diels-Alder reaction, but its application had been limited due to modest contrast ratios of the resulting conjugates. Herein, we describe the use of hydrophilic tetrazines to improve tumor-to-background contrast of neurotensin receptor targeted PET agents. PET agents were constructed using a rapid Diels-Alder reaction of the radiolabeled trans-cyclooctene (¹⁸ F-sTCO) with neurotensin (NT) conjugates of a 3,6-diaryltetrazine, 3-methyl-6-aryltetrazine, and a derivative of 3,6-di(2-hydroxyethyl)tetrazine. Although cell binding assays demonstrated all agents have comparable binding affinity, the conjugate derived from 3,6-di(2-hydroxyethyl)tetrazine demonstrated the highest tumor to muscle contrast, followed by conjugates of the 3-methyl-6-aryltetrazine and 3,6-diaryltetrazine.

Evaluation of Organo [18F]Fluorosilicon Tetrazine as a Prosthetic Group for the Synthesis of PET Radiotracers

Fluorine-18 is the most widely used positron emission tomography (PET) radionuclide currently in clinical application, due to its optimal nuclear properties. The synthesis of ¹⁸F-labeled radiotracers often requires harsh reaction conditions, limiting the use of sensitive bio- and macromolecules as precursors for direct radiolabeling with fluorine-18. We aimed to develop a milder and efficient in vitro and in vivo labeling method for trans-cyclooctene (TCO) functionalized proteins, through the bioorthogonal inverse-electron demand Diels-Alder (IEDDA) reaction with fluorine-18 radiolabeled tetrazine ([¹⁸F]SiFA-Tz). Here, we used TCO-modified bovine serum albumin (BSA) as the model protein, and isotopic exchange (IE) (¹⁹F/¹⁸F) chemistry as the labeling strategy. The radiolabeling of albumin-TCO with [¹⁸F]SiFA-Tz ([¹⁸F]6), providing [¹⁸F]fluoroalbumin ([¹⁸F]10) in high radiochemical yield (99.1 ± 0.2%, n = 3) and a molar activity (MA) of 1.1 GBq/µmol, confirmed the applicability of [¹⁸F]6 as a quick in vitro fluorination reagent for the TCO functionalized proteins. While the biological evaluation of [¹⁸F]6 demonstrated defluorination in vivo, limiting the utility for pretargeted applications, the in vivo stability of the radiotracer was dramatically improved when [¹⁸F]6 was used for the radiolabeling of albumin-TCO ([¹⁸F]10) in vitro, prior to administration. Due to the detected defluorination in vivo, structural optimization of the prosthetic group for improved stability is needed before further biological studies and application of pretargeted PET imaging.

Thermodynamically Stable Mesoporous C3 N7 and C3 N6 with Ordered Structure and Their Excellent Performance for Oxygen Reduction Reaction

Carbon nitrides with a high N/C atomic ratio (>2) are expected to offer superior basicity and unique electronic properties. However, the synthesis of these nanostructures is highly challenging since many parts of the CN frameworks in the carbon nitride should be replaced with thermodynamically less stable NN frameworks as the nitrogen content increases. Thermodynamically stable C₃ N₇ and C₃ N₆ with an ordered mesoporous structure are synthesized at 250 and 300 °C respectively via a pyrolysis process of 5-amino-1H-tetrazole (5-ATTZ). Polymerization of the precursor to the ordered mesoporous C₃ N₇ and C₃ N₆ is clearly proved by X-ray and electron diffraction analyses. A combined analysis including diverse spectroscopy and FDMNES and density functional theory (DFT) calculations demonstrates that the NN bonds are stabilized in the form of tetrazine and/or triazole moieties in the C₃ N₇ and C₃ N₆ . The ordered mesoporous C₃ N₇ represents the better oxygen reduction reaction (ORR) performances (onset potential: 0.81 V vs reversible hydrogen electrode (RHE), electron transfer number: 3.9 at 0.5 V vs RHE) than graphitic carbon nitride (g-C₃ N₄ ) and the ordered mesoporous C₃ N₆ . The study on the mechanism of ORR suggests that nitrogen atoms in the tetrazine moiety of the ordered mesoporous C₃ N₇ act as active sites for its improved ORR activity.

Tetrazine-Mediated Bioorthogonal System for Prodrug Activation, Photothermal Therapy, and Optoacoustic Imaging

Bioorthogonal "bond cleavage" reactions hold great promise in a variety of biological applications such as controlled activation of the drug and probe, while the application of these biocompatible reactions in living animals is still in its infancy and has yet to be further explored. Herein we demonstrate a nanosized and two-component bioorthogonal system for tumor inhibition through the combined action of chemo- and photothermal therapy. The trigger of the system was fabricated by immobilizing PEGylated tetrazine on the gold nanorods, and the bioorthogonal prodrug was synthesized by caging the drug camptothecin with vinyl ether, followed by encapsulating it with phospholipid liposomes. The tetrazine-based trigger effectively mediates the bioorthogonal reaction and triggers the release of camptothecin for chemotherapy, and the gold nanorods exhibit high photothermal capability for photothermal therapy and for three-dimensional optoacoustic imaging. Upon injection into tumor-bearing mice, the two components accumulate in the tumor region and carry out a bioorthogonal reaction therein, hence releasing the parent drug. The combined actions of chemo- and photothermal therapy greatly inhibited tumor growth in mice. This strategy may afford a promising approach for achieving controlled release of an active drug in vivo through an alternative external stimulus-a bioorthogonal reaction.

Immobilization of Proteins with Controlled Load and Orientation

Biomaterials based on immobilized proteins are key elements of many biomedical and industrial technologies. However, applications are limited by an inability to precisely construct materials of high homogeneity and defined content. We present here a general "protein-limited immobilization" strategy by combining the rapid, bioorthogonal, and biocompatible properties of a tetrazine-strained trans-cyclooctene reaction with genetic code expansion to site-specifically place the tetrazine into a protein. For the first time, we use this strategy to immobilize defined amounts of oriented proteins onto beads and flat surfaces in under 5 min at submicromolar concentrations without compromising activity. This approach opens the door to generating and studying diverse protein-based biomaterials that are much more precisely defined and characterized, providing a greater ability to engineer properties across a wide range of applications.

Photo-induced and Rapid Labeling of Tetrazine-Bearing Proteins via Cyclopropenone-Caged Bicyclononynes

Inverse electron‐demand Diels–Alder cycloadditions (iEDDAC) between tetrazines and strained alkenes/alkynes have emerged as essential tools for studying and manipulating biomolecules. A light‐triggered version of iEDDAC (photo‐iEDDAC) is presented that confers spatio‐temporal control to bioorthogonal labeling in vitro and in cellulo. A cyclopropenone‐caged dibenzoannulated bicyclo[6.1.0]nonyne probe (photo‐DMBO) was designed that is unreactive towards tetrazines before light‐activation, but engages in iEDDAC after irradiation at 365 nm. Aminoacyl tRNA synthetase/tRNA pairs were discovered for efficient site‐specific incorporation of tetrazine‐containing amino acids into proteins in living cells. In situ light activation of photo‐DMBO conjugates allows labeling of tetrazine‐modified proteins in living E. coli. This allows proteins in living cells to be modified in a spatio‐temporally controlled manner and may be extended to photo‐induced and site‐specific protein labeling in animals.

Dual Radionuclide Theranostic Pretargeting

Recent years have played witness to the advent of nuclear theranostics: the synergistic use of "matched pair" radiopharmaceuticals for diagnostic imaging and targeted radiotherapy. In this investigation, we report the extension of this concept to in vivo pretargeting based on the rapid and bioorthogonal inverse electron demand Diels-Alder reaction between tetrazine (Tz) and trans-cyclooctene (TCO). We demonstrate that a single injection of a TCO-modified immunoconjugate can be used as a platform for pretargeted PET imaging and radiotherapy via the sequential administration of a pair of Tz-bearing radioligands labeled with the positron-emitting radiometal copper-64 (t_1/2 ≈ 12.7 h) and the beta-emitting radiometal lutetium-177 (t_1/2 ≈ 6.7 days). More specifically, a mouse model of human colorectal carcinoma received a dose of the A33 antigen-targeting immunoconjugate huA33-TCO, followed 24 and 48 h later by injections of [⁶⁴Cu]Cu-SarAr-Tz and [¹⁷⁷Lu]Lu-DOTA-PEG₇-Tz, respectively. This approach produces high activity concentrations of both radioligands in tumor tissue (16.4 ± 2.7 %ID/g for [⁶⁴Cu]Cu-SarAr-Tz at 48 h post-injection and 18.1 ± 2.1 %ID/g for [¹⁷⁷Lu]Lu-DOTA-PEG₇-Tz at 120 h post-injection) as well as promising tumor-to-healthy organ activity concentration ratios. Ultimately, we believe that this work could not only have important implications in nuclear theranostics-most excitingly with isotopologue-based radioligand pairs such as [⁶⁴Cu]Cu-SarAr-Tz and [⁶⁷Cu]Cu-SarAr-Tz-but also in the delivery of fractionated doses during pretargeted radioimmunotherapy.

Conditional Singlet Oxygen Generation through a Bioorthogonal DNA-targeted Tetrazine Reaction

We report the use of bioorthogonal reactions as an original strategy in photodynamic therapy to achieve conditional phototoxicity and specific subcellular localization simultaneously. Our novel halogenated BODIPY-tetrazine probes only become efficient photosensitizers (Φ_Δ ≈0.50) through an intracellular inverse-electron-demand Diels-Alder reaction with a suitable dienophile. Ab initio computations reveal an activation-dependent change in decay channels that controls ¹ O₂ generation. Our bioorthogonal approach also enables spatial control. As a proof-of-concept, we demonstrate the feasibility of the selective activation of our dormant photosensitizer in cellular nuclei, causing cancer cell death upon irradiation. Thus, our dual biorthogonal, activatable photosensitizers open new venues to combat current limitations of photodynamic therapy.

Tetrazine-Triggered Release of Carboxylic-Acid-Containing Molecules for Activation of an Anti-inflammatory Drug

In addition to its use for the study of biomolecules in living systems, bioorthogonal chemistry has emerged as a promising strategy to enable protein or drug activation in a spatially and temporally controlled manner. This study demonstrates the application of a bioorthogonal inverse electron-demand Diels-Alder (iEDDA) reaction to cleave trans-cyclooctene (TCO) and vinyl protecting groups from carboxylic acid-containing molecules. The tetrazine-mediated decaging reaction proceeded under biocompatible conditions with fast reaction kinetics (<2 min). The anti-inflammatory activity of ketoprofen was successfully reinstated after decaging of the nontoxic TCOprodrug in live macrophages. Overall, this work expands the scope of functional groups and the application of decaging reactions to a new class of drugs.

Tetrazine-Triggered Release of Carboxylic-Acid-Containing Molecules for Activation of an Anti-inflammatory Drug

In addition to its use for the study of biomolecules in living systems, bioorthogonal chemistry has emerged as a promising strategy to enable protein or drug activation in a spatially and temporally controlled manner. This study demonstrates the application of a bioorthogonal inverse electron-demand Diels-Alder (iEDDA) reaction to cleave trans-cyclooctene (TCO) and vinyl protecting groups from carboxylic acid-containing molecules. The tetrazine-mediated decaging reaction proceeded under biocompatible conditions with fast reaction kinetics (<2 min). The anti-inflammatory activity of ketoprofen was successfully reinstated after decaging of the nontoxic TCOprodrug in live macrophages. Overall, this work expands the scope of functional groups and the application of decaging reactions to a new class of drugs.

Toward the Optimization of Click-Mediated Pretargeted Radioimmunotherapy

Pretargeted radioimmunotherapy (PRIT) based on the inverse electron demand Diels-Alder reaction has shown promise in murine models of disease, yet the radiation dosimetry of this approach must be optimized to make it a viable clinical option. To this end, we have leveraged two recent developments in pretargeted imaging-dendritic scaffolds and masking agents-to improve the dosimetric profile of a proof-of-concept PRIT system that is based on the huA33 antibody, a ¹⁷⁷Lu-labeled tetrazine radioligand ([¹⁷⁷Lu]Lu-DOTA-PEG₇-Tz), and a mouse model of A33 antigen-expressing colorectal carcinoma. Pretargeting using an huA33 immunoconjugate bearing a trans-cyclooctene-decorated dendritic scaffold (^sshuA33-DEN-TCO) produced significantly higher tumoral activity concentrations at 120 h post-injection (23.0 ± 2.2 %ID/g) than those achieved with an analogous, dendrimer-lacking immunoconjugate (12.7 ± 2.6 %ID/g). However, pretargeting using ^sshuA33-DEN-TCO also resulted in increased activity concentrations in the blood at the same time point (1.9 ± 0.4 %ID/g) compared to the dendrimer-lacking construct (0.7 ± 0.2 %ID/g), thereby curtailing improvements to the tumor-to-blood therapeutic ratio of the system. In order to circumvent this issue, a tetrazine-labeled, dextran-based masking agent (Tz-DP) was injected prior to the radioligand to prevent the ligation between [¹⁷⁷Lu]Lu-DOTA-PEG₇-Tz and circulating immunoconjugate. This approach dramatically decreased the absorbed dose to the blood but also attenuated the absorbed dose to the tumor and increased the absorbed dose to the lungs. Ultimately, these data suggest that dendritic scaffolds and masking agents could be used to improve the dosimetry of PRIT, but the combination of these technologies will require extensive optimization.

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.

The Emerging Role of Tetrazines in Drug-Activation Chemistries

Traditionally, prodrug activation has been limited to enzymatic triggers or gross physiological aberrations, such as pH, that offer low selectivity and control over dosage. In recent years, the field of prodrug activation chemistry has been transformed by the use of bioorthogonal reactions that can be carried out under biological conditions at sub-millimolar concentrations, with the tetrazine-mediated inverse electron demand Diels-Alder reaction amongst the most recognised. Their high reaction rates, chemoselectivity and excellent biocompatibility make tetrazines ideal small molecules for activating prodrugs. Recently the tetrazine moiety has been used as a prodrug for a pyridazine thus broadening the scope of prodrug systems. This article discusses the concept of using tetrazines as small-molecule activators for prodrugs, and provides an overview of tetrazine-based prodrug systems, with a particular focus on the recently reported prodrug-prodrug activation strategy.

Site-specific conjugation allows modulation of click reaction stoichiometry for pretargeted SPECT imaging

Antibody pretargeting is a promising strategy for improving molecular imaging, wherein the separation in time of antibody targeting and radiolabeling can lead to rapid attainment of high contrast, potentially increased sensitivity, and reduced patient radiation exposure. The inverse electron demand Diels-Alder ‘click’ reaction between trans-cyclooctene (TCO) conjugated antibodies and radiolabeled tetrazines presents an ideal platform for pretargeted imaging due to rapid reaction kinetics, bioorthogonality, and potential for optimization of both slow and fast clearing components. Herein, we evaluated a series of anti-human epidermal growth factor receptor 2 (HER2) pretargeting antibodies containing distinct molar ratios of site-specifically incorporated TCO. The effect of stoichiometry on tissue distribution was assessed for pretargeting TCO-modified antibodies (monitored by ¹²⁵I) and subsequent accumulation of an ¹¹¹In-labeled tetrazine in a therapeutically relevant HER2+tumor-bearing mouse model. Single photon emission computed tomography (SPECT) imaging was also employed to assess tumor imaging at various TCO-to-monoclonal antibody (mAb) ratios. Increasing TCO-to-mAb molar ratios correlated with increased in vivo click reaction efficiency evident by increased tumor distribution and systemic exposure of ¹¹¹In-labeled tetrazines. The pharmacokinetics of TCO-modified antibodies did not vary with stoichiometry. Pretargeted SPECT imaging of HER2-expressing tumors using ¹¹¹In-labeled tetrazine demonstrated robust click reaction with circulating antibody at ~2 hours and good tumor delineation for both the 2 and 6 TCO-to-mAb ratio variants at 24 hours, consistent with a limited cell-surface pool of pretargeted antibody and benefit from further distribution and internalization. To our knowledge, this represents the first reported systematic analysis of how pretargeted imaging is affected solely by variation in click reaction stoichiometry through site-specific conjugation chemistry.

Microcontact Printing Patterning of an HOPG Surface by an Inverse Electron Demand Diels-Alder Reaction

The chemical modification of an sp² hybridized carbon surface in a controllable manner is very challenging but also crucial for many applications. An inverse electron demand Diels-Alder (IEDDA) reaction using microcontact printing technique is introduced to spatially control the modification of a highly ordered pyrolytic graphite (HOPG) surface under ambient conditions. The covalent modification was characterized by Raman spectroscopy, XPS, and SECM. Tetrazine derivatives can effectively react with an HOPG surface and with microcontact printing methods resulting in spatially patterned surfaces being produced with micrometer-scale resolution.