An Extended Approach for the Development of Fluorogenic trans-Cyclooctene-Tetrazine Cycloadditions

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

BACKGROUND

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

MAIN BODY

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

CONCLUSIONS

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

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

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

Recent Advances in Bioorthogonal Ligation and Bioconjugation

The desire to create biomolecules modified with functionalities that go beyond nature's toolbox has resulted in the development of biocompatible and selective methodologies and reagents, each with different scope and limitations. In this overview, we highlight recent advances in the field of bioconjugation from 2016 to 2023. First, (metal-mediated) protein functionalization by exploiting the specific reactivity of amino acids will be discussed, followed by novel bioorthogonal reagents for bioconjugation of modified biomolecules.

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

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

Versatile naphthalimide tetrazines for fluorogenic bioorthogonal labelling

Fluorescent probes for biological imaging have revealed much about the functions of biomolecules in health and disease. Fluorogenic probes, which are fluorescent only upon a bioorthogonal reaction with a specific partner, are particularly advantageous as they ensure that fluorescent signals observed in biological imaging arise solely from the intended target. In this work, we report the first series of naphthalimide tetrazines for bioorthogonal fluorogenic labelling. We establish that all of these compounds can be used for imaging through photophysical, analytical and biological studies. The best candidate was Np6mTz, where the tetrazine ring is appended to the naphthalimide at its 6-position via a phenyl linker in a meta configuration. Taking our synthetic scaffold, we generated two targeted variants, LysoNpTz and MitoNpTz, which successfully localized within the lysosomes and mitochondria respectively, without the requirement of genetic modification. In addition, the naphthalimide tetrazine system was used for the no-wash imaging of insulin amyloid fibrils in vitro, providing a new method that can monitor their growth kinetics and morphology. Since our synthetic approach is simple and modular, these new naphthalimide tetrazines provide a novel scaffold for a range of bioorthogonal tetrazine-based imaging agents for selective staining and sensing of biomolecules.

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.

1,2,4,5-Tetrazine derivatives as components and precursors of photo- and electroactive materials

Synthetic approaches to 3,6-disubstituted-1,2,4,5-tetrazine systems are analyzed, and their properties attractive to practical applications in photo- and electroactive materials are overviewed.

Tetrazine Carbon Nanotubes for Pretargeted In Vivo "Click-to-Release" Bioorthogonal Tumour Imaging

The bioorthogonal inverse‐electron‐demand Diels–Alder (IEDDA) cleavage reaction between tetrazine and trans‐cyclooctene (TCO) is a powerful way to control the release of bioactive agents and imaging probes. In this study, a pretargeted activation strategy using single‐walled carbon nanotubes (SWCNTs) that bear tetrazines (TZ@SWCNTs) and a TCO‐caged molecule was used to deliver active effector molecules. To optimize a turn‐on signal by using in vivo fluorescence imaging, we developed a new fluorogenic near‐infrared probe that can be activated by bioorthogonal chemistry and image tumours in mice by caging hemicyanine with TCO (tHCA). With our pretargeting strategy, we have shown selective doxorubicin prodrug activation and instantaneous fluorescence imaging in living cells. By combining a tHCA probe and a pretargeted bioorthogonal approach, real‐time, non‐invasive tumour visualization with a high target‐to‐background ratio was achieved in a xenograft mice tumour model. The combined advantages of enhanced stability, kinetics and biocompatibility, and the superior pharmacokinetics of tetrazine‐functionalised SWCNTs could allow application of targeted bioorthogonal decaging approaches with minimal off‐site activation of fluorophore/drug.

A Systematic Study of Coumarin-Tetrazine Light-Up Probes for Bioorthogonal Fluorescence Imaging

Fluorescent probes that light-up upon reaction with complementary bioorthogonal reagents are superior tools for no-wash fluorogenic bioimaging applications. In this work, a thorough study is presented on a set of seventeen structurally diverse coumarin-tetrazine probes that produce fluorescent dyes with exceptional turn-on ratios when reacted with trans-cyclooctene (TCO) and bicyclononyne (BCN) dienophiles. In general, formation of the fully aromatic pyridazine-containing dyes resulting from the reaction with BCN was found superior in terms of fluorogenicity. However, evaluation of the probes in cellular imaging experiments revealed that other factors, such as reaction kinetics and good cell permeability, prevail over the fluorescence turn-on properties. The best compound identified in this study showed excellent performance in live cell-labeling experiments and enabled no-wash fluorogenic imaging on a timescale of seconds.

Expanding the Scope of Antibody Rebridging with New Pyridazinedione-TCO Constructs

Disulfide rebridging methods have recently emerged as a route to hinge region-specific antibody modification, and there exist numerous examples of successful rebridging chemistry applied to clinically relevant human IgG1 antibodies. Here, dibromopyridazinedione disulfide rebridging is adapted to fast trans-cyclooctene/tetrazine (TCO/Tz) bioorthogonal ligations and extended beyond therapeutic human IgG1 antibodies for the first time to include mouse and rat monoclonal antibodies integral to multiplexed analytical diagnostics. In spite of a common architecture, only a subset of antibody host species and IgG isotype subclasses can be rebridged, highlighting the intricate relationship between hinge region sequence, structure, biological activity, and the conjugation chemistry of IgG antibodies.

Microscope laser assisted photooxidative activation of bioorthogonal ClickOx probes

A photoactivatable fluorogenic tetrazine-rhodaphenothiazine probe was synthesized and studied in light-assisted, bioorthogonal labeling schemes. Experimental results revealed that the bioorthogonally conjugated probe efficiently sensitizes ¹O₂ generation upon illumination with green or orange light and undergoes self-oxidation leading to an intensely fluorescent sulfoxide product. An added value of the present probe is that it is also suitable for STED super-resolution microscopy using a 660 nm depletion laser.

Flow Photochemical Syntheses of trans-Cyclooctenes and trans-Cycloheptenes Driven by Metal Complexation

trans-Cyclooctenes and trans-cycloheptenes have long been the subject of physical organic study, but the broader application had been limited by synthetic accessibility. This account describes the development of a general, flow photochemical method for the preparative synthesis of trans-cycloalkene derivatives. Here, photoisom erization takes place in a closed-loop flow reactor where the reaction mixture is continuously cycled through Ag(I) on silica gel. Selective complexation of the trans-isomer by Ag(I) during flow drives an otherwise unfavorable isomeric ratio toward the trans-isomer. Analogous photoreactions under batch-conditions are low yielding, and flow chemistry is necessary in order to obtain trans-cycloalkenes in preparatively useful yields. The applications of the method to bioorthogonal chemistry and stereospecific transannulation chemistry are described.

Bioorthogonal Fluorescence Turn-On Labeling Based on Bicyclononyne-Tetrazine Cycloaddition Reactions that Form Pyridazine Products

Fluorogenic bioorthogonal reactions enable visualization of biomolecules with excellent signal-to-noise ratio. A bicyclononyne-tetrazine ligation that produces fluorescent pyridazine products has been developed. In stark contrast to previous approaches, the formation of the dye is an inherent result of the chemical reaction and no additional fluorophores are needed in the reagents. The crucial structural elements that determine dye formation are electron-donating groups present in the starting tetrazine unit. The newly formed pyridazine fluorophores show interesting photophysical properties the fluorescence intensity increase in the reaction can reach an excellent 900-fold. Model imaging experiments demonstrate the application potential of this new fluorogenic bioorthogonal reaction.

An Extended Approach for the Development of Fluorogenic trans-Cyclooctene-Tetrazine Cycloadditions

Inverse‐electron‐demand Diels–Alder (iEDDA) cycloaddition between 1,2,4,5‐tetrazines and strained dienophiles belongs among the most popular bioconjugation reactions. In addition to its fast kinetics, this cycloaddition can be tailored to produce fluorescent products from non‐fluorescent starting materials. Here we show that even the reaction intermediates formed in iEDDA cycloaddition can lead to the formation of new types of fluorophores. The influence of various substituents on their photophysical properties and the generality of the approach with use of various trans‐cyclooctene derivatives were studied. Model bioimaging experiments demonstrate the application potential of fluorogenic iEDDA cycloaddition.