Highly reactive trans-cyclooctene tags with improved stability for Diels-Alder chemistry in living systems

Development of a novel antibody-tetrazine conjugate for bioorthogonal pretargeting

Recently, bioorthogonal chemistry based on the Inverse Electron-Demand Diels-Alder (IEDDA) cycloaddition between 1,2,4,5-tetrazines and trans-cyclooctene (TCO) analogues added an interesting dimension to molecular imaging. Until now, antibodies (Abs) were tagged with TCO and after pretargeting they were reacted with tetrazines substituted with reporters. However, TCO tags have the tendency to degrade under physiological conditions, and due to their hydrophobic nature are buried within the protein. This results in loss of reactivity and a low Ab functional loading. To circumvent these problems, we report for the first time an approach in which tetrazines are used as tags for antibody (Ab) modification, and TCO as the imaging agent. We developed a new Ab-tetrazine conjugate, which displays a high functional loading, good stability and reactivity. We utilized this immunoconjugate for live-cell imaging together with novel TCO probes, resulting in selective and rapid labeling of SKOV-3 cells. Our approach may be useful for in vivo pretargeted imaging.

Design, Synthesis, and Evaluation of a Low-Molecular-Weight (11)C-Labeled Tetrazine for Pretargeted PET Imaging Applying Bioorthogonal in Vivo Click Chemistry

A low-molecular-weight tetrazine labeled with the short-lived positron emitter carbon-11 was developed as a bioorthogonal PET probe for pretargeted imaging. A method for efficient and fast synthesis of this imaging agent is presented using radiolabeling of a readily available precursor. High reactivity with trans-cyclooctenes was observed and in vivo investigations including PET/MR scanning showed homogeneous biodistribution, good metabolic stability, and rapid excretion in naive mice. These properties are key to the success of bioorthogonal (11)C-PET imaging, which has been shown in a simple pretargeting experiment using TCO-modified mesoporous silica nanoparticles. Overall, this (11)C-labeled tetrazine represents a highly versatile and advantageous chemical tool for bioorthogonal PET imaging and enables pretargeting approaches using carbon-11 for the first time.

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

The use of a bioorthogonal reaction for the selective cleavage of tumor-bound antibody-drug conjugates (ADCs) would represent a powerful new tool for ADC therapy, as it would not rely on the currently used intracellular biological activation mechanisms, thereby expanding the scope to noninternalizing cancer targets. Here we report that the recently developed inverse-electron-demand Diels-Alder pyridazine elimination reaction can provoke rapid and self-immolative release of doxorubicin from an ADC in vitro and in tumor-bearing mice.

Rapid Bioorthogonal Chemistry Turn-on through Enzymatic or Long Wavelength Photocatalytic Activation of Tetrazine Ligation

Rapid bioorthogonal reactivity can be induced by controllable, catalytic stimuli using air as the oxidant. Methylene blue (4 μM) irradiated with red light (660 nm) catalyzes the rapid oxidation of a dihydrotetrazine to a tetrazine thereby turning on reactivity toward trans-cyclooctene dienophiles. Alternately, the aerial oxidation of dihydrotetrazines can be efficiently catalyzed by nanomolar levels of horseradish peroxidase under peroxide-free conditions. Selection of dihydrotetrazine/tetrazine pairs of sufficient kinetic stability in aerobic aqueous solutions is key to the success of these approaches. In this work, polymer fibers carrying latent dihydrotetrazines were catalytically activated and covalently modified by trans-cyclooctene conjugates of small molecules, peptides, and proteins. In addition to visualization with fluorophores, fibers conjugated to a cell adhesive peptide exhibited a dramatically increased ability to mediate contact guidance of cells.

Click Chemistry-Mediated Nanosensors for Biochemical Assays

Click chemistry combined with functional nanoparticles have drawn increasing attention in biochemical assays because they are promising in developing biosensors with effective signal transformation/amplification and straightforward signal readout for clinical diagnostic assays. In this review, we focus on the latest advances of biochemical assays based on Cu (I)-catalyzed 1, 3-dipolar cycloaddition of azides and alkynes (CuAAC)-mediated nanosensors, as well as the functionalization of nanoprobes based on click chemistry. Nanoprobes including gold nanoparticles, quantum dots, magnetic nanoparticles and carbon nanomaterials are covered. We discuss the advantages of click chemistry-mediated nanosensors for biochemical assays, and give perspectives on the development of click chemistry-mediated approaches for clinical diagnosis and other biomedical applications.

Conformationally Strained trans-Cyclooctene (sTCO) Enables the Rapid Construction of (18)F-PET Probes via Tetrazine Ligation

The bioorthogonal reaction between tetrazines and trans-cyclooctenes is a method for the rapid construction of F-18 probes for PET imaging. Described here is a second generation ¹⁸F-labeling system based on a conformationally strained trans-cyclooctene (sTCO)—a dienophile that is approximately 2 orders of magnitude more reactive than conventional TCO dienophiles. Starting from a readily prepared tosylate precursor, an ¹⁸F labeled sTCO derivative (¹⁸F-sTCO) could be synthesized in 29.3 +/- 5.1% isolated yield and with high specific activity. Tetrazine ligation was carried out with a cyclic RGD-conjugate of a diphenyl-s-tetrazine analogue (RGD-Tz) chosen from a diene class with an excellent combination of fast reactivity and stability both for the diene as well as the Diels-Alder adduct. For both the tetrazine and the sTCO, mini-PEG spacers were included to enhance solubility and improve the in vivo distribution profile of the resulting probe. Extremely fast reactivity (up to 2.86 x 10⁵ M^-1s^-1 at 25 °C in water) has been observed in kinetic studies in the reaction of sTCO with diphenyl-s-tetrazine derivatives. A kinetic study on sTCO diastereomers in 55:45 MeOH:water showed that the syn-diastereomer displayed slightly faster reactivity than the anti-diastereomer. An ¹⁸F-sTCO conjugate with RGD-Tz demonstrated prominent and persistent tumor uptake in vivo with good tumor-to-background contrast. Unlike most radiolabeled RGD peptides, the tumor uptake of this PET agent increased from 5.3 +/- 0.2% ID/g at 1 h post injection (p.i.), to 8.9 +/- 0.5% ID/g at 4 h p.i., providing evidence for prolonged blood circulation. These findings suggest that tetrazine ligations employing ¹⁸F-sTCO should serve as a powerful and general platform for the rapid construction of peptide or protein derived PET agents.

Efficient low-cost preparation of trans-cyclooctenes using a simplified flow setup for photoisomerization

Abstract

Bioorthogonal ligations have emerged as highly versatile chemical tools for biomedical research. The exceptionally fast reaction between 1,2,4,5-tetrazines and trans-cyclooctenes (TCOs), also known as tetrazine ligation, is frequently used in this regard. Growing numbers of applications for the tetrazine ligation led to an increased demand for TCO compounds, whose commercial availability is still very limited. Reported photochemical procedures for the preparation of TCOs using flow chemistry are straightforward and high yielding but require expensive equipment. Within this contribution, we present the construction and characterization of a low-cost flow photoreactor assembled from readily accessible components. Syntheses of all commonly used trans-cyclooctene derivatives were successfully carried out using the described system. We are convinced that the presented system for photoisomerization will promote access to bioorthogonally reactive TCO derivatives.

Graphical abstract

Electronic supplementary material

The online version of this article (doi:10.1007/s00706-016-1668-z) contains supplementary material, which is available to authorized users.

Photo-Triggered Click Chemistry for Biological Applications

In the last decade and a half, numerous bioorthogonal reactions have been developed with a goal to study biological processes in their native environment, i.e., in living cells and animals. Among them, the photo-triggered reactions offer several unique advantages including operational simplicity with the use of light rather than toxic metal catalysts and ligands, and exceptional spatiotemporal control through the application of an appropriate light source with pre-selected wavelength, light intensity and exposure time. While the photoinduced reactions have been studied extensively in materials research, e.g., on macromolecular surface, the adaptation of these reactions for chemical biology applications is still in its infancy. In this chapter, we review the recent efforts in the discovery and optimization the photo-triggered bioorthogonal reactions, with a focus on those that have shown broad utility in biological systems. We discuss in each cases the chemical and mechanistic background, the kinetics of the reactions and the biological applicability together with the limiting factors.

(99m)Tc-bioorthogonal click chemistry reagent for in vivo pretargeted imaging

Metal-free click chemistry has become an important tool for pretargeted approaches in the molecular imaging field. The application of bioorthogonal click chemistry between a pretargeted trans-cyclooctene (TCO) derivatized monoclonal antibody (mAb) and a (99m)Tc-modified 1,2,4,5-tetrazine for tumor imaging was examined in vitro and in vivo. The HYNIC tetrazine compound was synthesized and structurally characterized, confirming its identity. Radiolabeling studies demonstrated that the HYNIC tetrazine was labeled with (99m)Tc at an efficiency of >95% and was radiochemically stable. (99m)Tc-HYNIC tetrazine reacted with the TCO-CC49 mAb in vitro demonstrating its selective reactivity. In vivo biodistribution studies revealed non-specific liver and GI uptake due to the hydrophobic property of the compound, however pretargeted SPECT imaging studies demonstrated tumor visualization confirming the success of the cycloaddition reaction in vivo. These results demonstrated the potential of (99m)Tc-HYNIC-tetrazine for tumor imaging with pretargeted mAbs.

A New Highly Reactive and Low Lipophilicity Fluorine-18 Labeled Tetrazine Derivative for Pretargeted PET Imaging

A new (18)F-labeled tetrazine derivative was developed aiming at optimal radiochemistry, fast reaction kinetics in inverse electron-demand Diels-Alder cycloaddition (IEDDA), and favorable pharmacokinetics for in vivo bioorthogonal chemistry. The radiolabeling of the tetrazine was achieved in high yield, purity, and specific activity under mild reaction conditions via conjugation with 5-[(18)F]fluoro-5-deoxyribose, providing a glycosylated tetrazine derivative with low lipophilicity. The (18)F-tetrazine showed fast reaction kinetics toward the most commonly used dienophiles in IEDDA reactions. It exhibited excellent chemical and enzymatic stability in mouse plasma and in phosphate-buffered saline (pH 7.41). Biodistribution in mice revealed favorable pharmacokinetics with major elimination via urinary excretion. The results indicate that the glycosylated (18)F-labeled tetrazine is an excellent candidate for in vivo bioorthogonal chemistry applications in pretargeted PET imaging approaches.

In situ activation of a doxorubicin prodrug using imaging-capable nanoparticles

A general strategy for image-guided prodrug activation using fluorescently-labeled magnetic iron oxide nanoparticles is described.

A biocompatible inverse electron demand Diels–Alder reaction of aldehyde and tetrazine promoted by proline

A biocompatible inverse electron demand Diels–Alder reaction of aldehyde and tetrazine mediated by l-proline is disclosed, with apparent k₂ up to 13.8 M⁻¹ s⁻¹.

Inverse Electron-Demand Diels-Alder Bioorthogonal Reactions

Bioorthogonal reactions have been widely used over the last 10 years for imaging, detection, diagnostics, drug delivery, and biomaterials. Tetrazine reactions are a recently developed class of inverse electron-demand Diels-Alder reactions used in bioorthogonal applications. Given their rapid tunable reaction rate and highly fluorogenic properties, tetrazine bioorthogonal reactions have come to be considered highly attractive tools for elucidating biological functions and messages in vitro and in vivo. In this chapter, we present recent advances expanding the scope of precursor reactivity and we introduce new biomedical methodology based on bioorthogonal tetrazine chemistry. We specifically highlight novel applications for different kinds of biomolecules, including nucleic acid, protein, antibodies, lipids, glycans, and bioactive small molecules, in the areas of imaging, detection, and diagnostics. We also briefly present other recently developed inverse electron-demand Diels-Alder bioorthogonal reactions. Lastly, we consider future directions and potential roles that inverse electron-demand Diels-Alder reactions may play in the fields of bioorthogonal and biomedical chemistry.

Optimization of a Pretargeted Strategy for the PET Imaging of Colorectal Carcinoma via the Modulation of Radioligand Pharmacokinetics

Pretargeted PET imaging has emerged as an effective strategy for merging the exquisite selectivity of antibody-based targeting vectors with the rapid pharmacokinetics of radiolabeled small molecules. We previously reported the development of a strategy for the pretargeted PET imaging of colorectal cancer based on the bioorthogonal inverse electron demand Diels-Alder reaction between a tetrazine-bearing radioligand and a transcyclooctene-modified huA33 immunoconjugate. Although this method effectively delineated tumor tissue, its clinical potential was limited by the somewhat sluggish clearance of the radioligand through the gastrointestinal tract. Herein, we report the development and in vivo validation of a pretargeted strategy for the PET imaging of colorectal carcinoma with dramatically improved pharmacokinetics. Two novel tetrazine constructs, Tz-PEG7-NOTA and Tz-SarAr, were synthesized, characterized, and radiolabeled with (64)Cu in high yield (>90%) and radiochemical purity (>99%). PET imaging and biodistribution experiments in healthy mice revealed that although (64)Cu-Tz-PEG7-NOTA is cleared via both the gastrointestinal and urinary tracts, (64)Cu-Tz-SarAr is rapidly excreted by the renal system alone. On this basis, (64)Cu-Tz-SarAr was selected for further in vivo evaluation. To this end, mice bearing A33 antigen-expressing SW1222 human colorectal carcinoma xenografts were administered huA33-TCO, and the immunoconjugate was given 24 h to accumulate at the tumor and clear from the blood, after which (64)Cu-Tz-SarAr was administered via intravenous tail vein injection. PET imaging and biodistribution experiments revealed specific uptake of the radiotracer in the tumor at early time points (5.6 ± 0.7 %ID/g at 1 h p.i.), high tumor-to-background activity ratios, and rapid elimination of unclicked radioligand. Importantly, experiments with longer antibody accumulation intervals (48 and 120 h) yielded slight decreases in tumoral uptake but also concomitant increases in tumor-to-blood activity concentration ratios. This new strategy offers dosimetric benefits as well, yielding a total effective dose of 0.041 rem/mCi, far below the doses produced by directly labeled (64)Cu-NOTA-huA33 (0.133 rem/mCi) and (89)Zr-DFO-huA33 (1.54 rem/mCi). Ultimately, this pretargeted PET imaging strategy boasts a dramatically improved pharmacokinetic profile compared to our first generation system and is capable of clearly delineating tumor tissue with high image contrast at only a fraction of the radiation dose created by directly labeled radioimmunoconjugates.

Systemic Fluorescence Imaging of Zebrafish Glycans with Bioorthogonal Chemistry

Vertebrate glycans constitute a large, important, and dynamic set of post-translational modifications that are notoriously difficult to manipulate and image. Although the chemical reporter strategy has been used in conjunction with bioorthogonal chemistry to image the external glycosylation state of live zebrafish and detect tumor-associated glycans in mice, the ability to image glycans systemically within a live organism has remained elusive. Here, we report a method that combines the metabolic incorporation of a cyclooctyne-functionalized sialic acid derivative with a ligation reaction of a fluorogenic tetrazine, allowing for the imaging of sialylated glycoconjugates within live zebrafish embryos.

Systematic Evaluation of Bioorthogonal Reactions in Live Cells with Clickable HaloTag Ligands: Implications for Intracellular Imaging

Bioorthogonal reactions, including the strain-promoted azide–alkyne cycloaddition (SPAAC) and inverse electron demand Diels–Alder (iEDDA) reactions, have become increasingly popular for live-cell imaging applications. However, the stability and reactivity of reagents has never been systematically explored in the context of a living cell. Here we report a universal, organelle-targetable system based on HaloTag protein technology for directly comparing bioorthogonal reagent reactivity, specificity, and stability using clickable HaloTag ligands in various subcellular compartments. This system enabled a detailed comparison of the bioorthogonal reactions in live cells and informed the selection of optimal reagents and conditions for live-cell imaging studies. We found that the reaction of sTCO with monosubstituted tetrazines is the fastest reaction in cells; however, both reagents have stability issues. To address this, we introduced a new variant of sTCO, Ag-sTCO, which has much improved stability and can be used directly in cells for rapid bioorthogonal reactions with tetrazines. Utilization of Ag complexes of conformationally strained trans-cyclooctenes should greatly expand their usefulness especially when paired with less reactive, more stable tetrazines.

Diabody Pretargeting with Click Chemistry In Vivo

Radioimmunotherapy and nuclear imaging (immuno-PET/SPECT) of cancer with radiometal-labeled antibody fragments or peptides is hampered by low tumor-to-kidney ratios because of high renal radiometal retention. Therefore, we developed and evaluated a pretargeting strategy using click chemistry in vivo to reduce kidney uptake and avoid unwanted radiation toxicity. We focused on the bioorthogonal reaction between a trans-cyclooctene (TCO)-functionalized TAG72 targeting diabody, AVP04-07, and a low-molecular-weight radiolabeled tetrazine probe that was previously shown to have low kidney retention and relatively fast renal clearance.

METHODS

AVP04-07 diabodies were functionalized with TCO tags, and in vitro immunoreactivity toward bovine submaxillary mucin and tetrazine reactivity were assessed. Next, pretargeting biodistribution studies were performed in LS174T tumor-bearing mice with AVP04-07-TCO(n) (where n indicates the number of TCO groups per diabody) and radiolabeled tetrazine to optimize the TCO modification grade (0, 1.8, or 4.7 TCO groups per diabody) and the (177)Lu-tetrazine dose (0.1, 1.0, or 10 Eq with respect to the diabody). Radiolabeled tetrazine was injected at 47 h after diabody injection, and mice were euthanized 3 h later. A pretargeting SPECT/CT study with (111)In-tetrazine was performed with the optimized conditions.

RESULTS

Immunoreactivity for native AVP04-07 was similar to that for TCO-functionalized AVP04-07, and the latter reacted efficiently with radiolabeled tetrazine in vitro. The combination of the pretargeting component AVP04-07 functionalized with 4.7 TCO groups and 1 Eq of (177)Lu-tetrazine with respect to the diabody showed the most promising biodistribution. Specifically, high (177)Lu-tetrazine tumor uptake (6.9 percentage injected dose/g) was observed with low renal retention, yielding a tumor-to-kidney ratio of 5.7. SPECT/CT imaging confirmed the predominant accumulation of radiolabeled tetrazine in the tumor and low nontumor retention.

CONCLUSION

Pretargeting provides an alternative radioimmunotherapy and nuclear imaging strategy by overcoming the high renal retention of low-molecular-weight radiometal tumor-homing agents through the separate administration of a tumor-homing agent and a radioactive probe with fast clearance.

Design, Preparation, and Characterization of PNA-Based Hybridization Probes for Affibody-Molecule-Mediated Pretargeting

In radioimmunotherapy, the contrast between tumor and normal tissue can be improved by using a pretargeting strategy with a primary targeting agent, which is conjugated to a recognition tag, and a secondary radiolabeled molecule binding specifically to the recognition tag. The secondary molecule is injected after the targeting agent has accumulated in the tumor and is designed to have a favorable biodistribution profile, with fast clearance from blood and low uptake in normal tissues. In this study, we have designed and evaluated two complementary peptide nucleic acid (PNA)-based probes for specific and high-affinity association in vivo. An anti-HER2 Affibody-PNA chimera, Z(HER2:342)-SR-HP1, was produced by a semisynthetic approach using sortase A catalyzed ligation of a recombinantly produced Affibody molecule to a PNA-based HP1-probe assembled using solid-phase chemistry. A complementary HP2 probe carrying a DOTA chelator and a tyrosine for dual radiolabeling was prepared by solid-phase synthesis. Circular dichroism (CD) spectroscopy and UV thermal melts showed that the probes can hybridize to form a structured duplex with a very high melting temperature (T(m)), both in HP1:HP2 and in Z(HER2:342)-SR-HP1:HP2 (T(m) = 86-88 °C), and the high binding affinity between Z(HER2:342)-SR-HP1 and HP2 was confirmed in a surface plasmon resonance (SPR)-based binding study. Following a moderately fast association (1.7 × 10(5) M(-1) s(-1)), the dissociation of the probes was extremely slow and <5% dissociation was observed after 17 h. The equilibrium dissociation constant (K(D)) for Z(HER2:342)-SR-HP1:HP2 binding to HER2 was estimated by SPR to be 212 pM, suggesting that the conjugation to PNA does not impair Affibody binding to HER2. The biodistribution profiles of (111)In- and (125)I-labeled HP2 were measured in NMRI mice, showing very fast blood clearance rates and low accumulation of radioactivity in kidneys and other organs. The measured radioactivity in blood was 0.63 ± 0.15 and 0.41 ± 0.15%ID/g for (125)I- and (111)In-HP2, respectively, at 1 h p.i., and at 4 h p.i., the kidney accumulation of radioactivity was 0.17 ± 0.04%ID/g for (125)I-HP2 and 3.83 ± 0.39%ID/g for (111)In-HP2. Taken together, the results suggest that a PNA-based system has suitable biophysical and in vivo properties and is a promising approach for pretargeting of Affibody molecules.

Storable N-phenylcarbamate palladacycles for rapid functionalization of an alkyne-encoded protein

Here we report the synthesis of storable N-phenylcarbamate palladacycles that showed robust reactivity in the cross-coupling reaction with an alkyne-encoded protein with a second-order rate constant approaching 19 770 ± 930 M(-1) s(-1).

Two-step protein labeling by using lipoic acid ligase with norbornene substrates and subsequent inverse-electron demand Diels-Alder reaction

Inverse-electron-demand Diels-Alder cycloaddition (DAinv ) between strained alkenes and tetrazines is a highly bio-orthogonal reaction that has been applied in the specific labeling of biomolecules. In this work we present a two-step labeling protocol for the site-specific labeling of proteins based on attachment of a highly stable norbornene derivative to a specific peptide sequence by using a mutant of the enzyme lipoic acid ligase A (LplA(W37V) ), followed by the covalent attachment of tetrazine-modified fluorophores to the norbornene moiety through the bio-orthogonal DAinv  . We investigated 15 different norbornene derivatives for their selective enzymatic attachment to a 13-residue lipoic acid acceptor peptide (LAP) by using a standardized HPLC protocol. Finally, we used this two-step labeling strategy to label proteins in cell lysates in a site-specific manner and performed cell-surface labeling on living cells.

Development of an enzymatic pretargeting strategy for dual-modality imaging

A pretargeted imaging strategy based on the HaloTag dehalogenase enzyme is described. Here, a HaloTag-Trastuzumab conjugate has been used as the primary agent targeting HER2 expression, and three new radiolabelled HaloTag ligands have been used as secondary agents, two of which offer dual-modality (SPECT/optical) imaging capability.

Improved metabolic stability for 18F PET probes rapidly constructed via tetrazine trans-cyclooctene ligation

The fast kinetics and bioorthogonal nature of the tetrazine trans-cyclooctene (TCO) ligation makes it a unique tool for PET probe construction. In this study, we report the development of an (18)F-labeling system based on a CF3-substituted diphenyl-s-tetrazine derivative with the aim of maintaining high reactivity while increasing in vivo stability. c(RGDyK) was tagged by a CF3-substituted diphenyl-s-tetrazine derivative via EDC-mediated coupling. The resulting tetrazine-RGD conjugate was combined with a (19)F-labeled TCO derivative to give HPLC standards. The analogous (18)F-labeled TCO derivative was combined with the diphenyl-s-tetrazine-RGD at μM concentration. The resulting tracer was subjected to in vivo metabolic stability assessment, and microPET studies in murine U87MG xenograft models. The diphenyl-s-tetrazine-RGD combines with an (18)F-labeled TCO in high yields (>97% decay-corrected on the basis of TCO) using only 4 equiv of tetrazine-RGD relative to the (18)F-labeled TCO (concentration calculated based on product's specific activity). The radiochemical purity of the (18)F-RGD peptides was >95% and the specific activity was 111 GBq/μmol. Noninvasive microPET experiments demonstrated that (18)F-RGD had integrin-specific tumor uptake in subcutaneous U87MG glioma. In vivo metabolic stability of (18)F-RGD in blood, urine, and major organs showed two major peaks: one corresponded to the Diels-Alder conjugate and the other was identified as the aromatized analog. A CF3-substituted diphenyl-s-tetrazine displays excellent speed and efficiency in (18)F-PET probe construction, providing nearly quantitative (18)F labeling within minutes at low micromolar concentrations. The resulting conjugates display improved in vivo metabolic stability relative to our previously described system.

Harnessing the bioorthogonal inverse electron demand Diels-Alder cycloaddition for pretargeted PET imaging

Due to their exquisite affinity and specificity, antibodies have become extremely promising vectors for the delivery of radioisotopes to cancer cells for PET imaging. However, the necessity of labeling antibodies with radionuclides with long physical half-lives often results in high background radiation dose rates to non-target tissues. In order to circumvent this issue, we have employed a pretargeted PET imaging strategy based on the inverse electron demand Diels-Alder cycloaddition reaction. The methodology decouples the antibody from the radioactivity and thus exploits the positive characteristics of antibodies, while eschewing their pharmacokinetic drawbacks. The system is composed of four steps: (1) the injection of a mAb-trans-cyclooctene (TCO) conjugate; (2) a localization time period during which the antibody accumulates in the tumor and clears from the blood; (3) the injection of the radiolabeled tetrazine; and (4) the in vivo click ligation of the components followed by the clearance of excess radioligand. In the example presented in the work at hand, a (64)Cu-NOTA-labeled tetrazine radioligand and a trans-cyclooctene-conjugated humanized antibody (huA33) were successfully used to delineate SW1222 colorectal cancer tumors with high tumor-to-background contrast. Further, the pretargeting methodology produces high quality images at only a fraction of the radiation dose to non-target tissue created by radioimmunoconjugates directly labeled with (64)Cu or (89)Zr. Ultimately, the modularity of this protocol is one of its greatest assets, as the trans-cyclooctene moiety can be appended to any non-internalizing antibody, and the tetrazine can be attached to a wide variety of radioisotopes.

Enhancing reactivity for bioorthogonal pretargeting by unmasking antibody-conjugated trans-cyclooctenes

The bioorthogonal cycloaddition reaction between tetrazine and trans-cyclooctene (TCO) is rapidly growing in use for molecular imaging and cell-based diagnostics. We have surprisingly uncovered that the majority of TCOs conjugated to monoclonal antibodies using standard amine-coupling procedures are nonreactive. We show that antibody-bound TCOs are not inactivated by trans-cis isomerization and that the bulky cycloaddition reaction is not sterically hindered. Instead, TCOs are likely masked by hydrophobic interactions with the antibody. We show that introducing TCO via hydrophilic poly(ethylene glycol) (PEG) linkers can fully preserve reactivity, resulting in >5-fold enhancement in functional density without affecting antibody binding. This is accomplished using a novel dual bioorthogonal approach in which heterobifunctional dibenzylcyclooctyne (DBCO)-PEG-TCO molecules are reacted with azido-antibodies. Improved imaging capabilities are demonstrated for different cancer biomarkers using tetrazine-modified fluorophore and quantum dot probes. We believe that the PEG linkers prevent TCOs from burying within the antibody during conjugation, which could be relevant to other bioorthogonal tags and biomolecules. We expect the improved TCO reactivity obtained using the reported methods will significantly advance bioorthogonal pretargeting applications.

Implantable biomaterial based on click chemistry for targeting small molecules

Specific and targeted delivery of medical therapies continues to be a challenge for the optimal treatment of multiple medical conditions. Technological advances permit physicians to target most sites of the body. However, after the intervention, physicians rely on systemic medications that need frequent dosing and may have noxious side effects. A novel system combining the temporal flexibility of systemic drug delivery and the spatial control of injectable biomaterials would improve the spatiotemporal control of medical therapies. Here we present an implantable biomaterial that harnesses in vivo click chemistry to enhance the delivery of suitable small molecules by an order of magnitude. The results demonstrate a simple and modular method to modify a biomaterial with small molecules in vitro and present an example of a polysaccharide modified hours after in vivo implantation. This approach provides the ability to precisely control the moment when biochemical and/or physical signals may appear in an implanted biomaterial. This is the first step towards the construction of a biomaterial that enhances the spatial location of systemic small molecules via in vivo chemical delivery.

Pretargeted imaging and radioimmunotherapy of cancer using antibodies and bioorthogonal chemistry

Selective delivery of radionuclides to tumors may be accomplished using a two-step approach, in which in the first step the tumor is pretargeted with an unlabeled antibody construct and in the second step the tumor is targeted with a radiolabeled small molecule. This results in a more rapid clearance of the radioactivity from normal tissues due to the fast pharmacokinetics of the small molecule as compared to antibodies. In the last decade, several pretargeting approaches have been tested, which have shown improved tumor-to-background ratios and thus improved imaging and therapy as compared to directly labeled antibodies. In this review, we will discuss the strategies and applications in (pre-)clinical studies of pretargeting concepts based on the use of bispecific antibodies, which are capable of binding to both a target antigen and a radiolabeled peptide. So far, three generations of the bispecific antibody-based pretargeting approach have been studied. The first clinical studies have shown the feasibility and potential for these pretargeting systems to detect and treat tumor lesions. However, to fully integrate the pretargeting approach in clinic, further research should focus on the best regime and pretargeting protocol. Additionally, recent developments in the use of bioorthogonal chemistry for pretargeting of tumors suggest that this chemical pretargeting approach is an attractive alternative strategy for the detection and treatment of tumor lesions.

Bioorthogonal prodrug activation driven by a strain-promoted 1,3-dipolar cycloaddition

Bioorthogonal prodrug activation controlled by the reaction of a trans-cyclooctene with an azide-functionalized prodrug is presented.

Due to the formation of hydrolysis-susceptible adducts, the 1,3-dipolar cycloaddition between an azide and strained trans-cyclooctene (TCO) has been disregarded in the field of bioorthogonal chemistry. We report a method which uses the instability of the adducts to our advantage in a prodrug activation strategy. The reaction of trans-cyclooctenol (TCO-OH) with a model prodrug resulted in a rapid 1,3-dipolar cycloaddition with second-order rates of 0.017 M^–1 s^–1 and 0.027 M^–1 s^–1 for the equatorial and axial isomers, respectively, resulting in release of the active compound. ¹H NMR studies showed that activation proceeded via a triazoline and imine, both of which are rapidly hydrolyzed to release the model drug. Cytotoxicity of a doxorubicin prodrug was restored in vitro upon activation with TCO-OH, while with cis-cyclooctenol (CCO-OH) no activation was observed. The data also demonstrates the potential of this reaction in organic synthesis as a mild orthogonal protecting group strategy for amino and hydroxyl groups.

Fast and sequence-specific palladium-mediated cross-coupling reaction identified from phage display

Fast and specific bioorthogonal reactions are highly desirable because they provide efficient tracking of biomolecules that are present in low abundance and/or involved in fast dynamic process in living systems. Toward this end, classic strategy involves the optimization of substrate structures and reaction conditions in test tubes, testing their compatibility with biological systems, devising synthetic biology schemes to introduce the modified substrates into living cells or organisms, and finally validating the superior kinetics for enhanced capacity in tracking biomolecules in vivo--a lengthy process often mired by unexpected results. Here, we report a streamlined approach in which the "microenvironment" of a bioorthogonal chemical reporter is exploited directly in biological systems via phage-assisted interrogation of reactivity (PAIR) to optimize not only reaction kinetics but also specificity. Using the PAIR strategy, we identified a short alkyne-containing peptide sequence showing fast kinetics (k2=13,000±2000 M(-1) s(-1)) in a palladium-mediated cross-coupling reaction. Site-directed mutagenesis studies suggested that the residues surrounding the alkyne moiety facilitate the assembly of a key palladium-alkyne intermediate along the reaction pathway. When this peptide sequence was inserted into the extracellular domain of epidermal growth factor receptor (EGFR), this reactive sequence directed the specific labeling of EGFR in live mammalian cells.

Two rapid catalyst-free click reactions for in vivo protein labeling of genetically encoded strained alkene/alkyne functionalities

Detailed kinetic analyses of inverse electron-demand Diels–Alder cycloaddition and nitrilimine-alkene/alkyne 1,3-diploar cycloaddition reactions were conducted and the reactions were applied for rapid protein bioconjugation. When reacted with a tetrazine or a diaryl nitrilimine, strained alkene/alkyne entities including norbornene, trans-cyclooctene, and cyclooctyne displayed rapid kinetics. To apply these “click” reactions for site-specific protein labeling, five tyrosine derivatives that contain a norbornene, trans-cyclooctene, or cyclooctyne entity were genetically encoded into proteins in Escherichia coli using an engineered pyrrolysyl-tRNA synthetase-tRNA_CUA^Pyl pair. Proteins bearing these noncanonical amino acids were successively labeled with a fluorescein tetrazine dye and a diaryl nitrilimine both in vitro and in living cells.

Interfacial Bioorthogonal Cross-Linking

Described herein is interfacial bioorthogonal cross-linking, the use of bioorthogonal chemistry to create and pattern biomaterials through diffusion-controlled gelation at the liquid-gel interface. The basis is a rapid (k₂ 284000 M^–1 s^–1) reaction between strained trans-cyclooctene (TCO) and tetrazine (Tz) derivatives. Syringe delivery of Tz-functionalized hyaluronic acid (HA-Tz) to a bath of bis-TCO cross-linker instantly creates microspheres with a cross-linked shell through which bis-TCO diffuses freely to introduce further cross-linking at the interface. Tags can be introduced with 3D resolution without external triggers or templates. Water-filled hydrogel channels were prepared by simply reversing the order of addition. Prostate cancer cells encapsulated in the microspheres have 99% viability, proliferate readily, and form aggregated clusters. This process is projected to be useful in the fabrication of cell-instructive matrices for in vitro tissue models.

Trans-cyclooctene tag with improved properties for tumor pretargeting with the diels-alder reaction

Radioimmunotherapy (RIT) of solid tumors is hampered by low tumor-to-nontumor (T/NT) ratios of the radiolabeled monoclonal antibodies resulting in low tumor doses in patients. Pretargeting technologies can improve the effectiveness of RIT in cancer therapy by increasing this ratio. We showed that a pretargeting strategy employing in vivo chemistry in combination with clearing agents, proceeds efficiently in tumor-bearing mice resulting in high T/NT ratios. A dosimetry study indicated that the chemical pretargeting technology, which centered on the bioorthogonal Diels-Alder click reaction between a radiolabeled tetrazine probe and a trans-cyclooctene-oxymethylbenzamide-tagged CC49 antibody (CC49-TCO(1)), can match the performance of clinically validated high-affinity biological pretargeting approaches in mice ( Rossin J Nucl Med. 2013 , 54 , 1989 - 1995 ). Nevertheless, the increased protein surface hydrophobicity of CC49-TCO(1) led to a relatively rapid blood clearance and concomitant reduced tumor uptake compared to native CC49 antibody. Here, we present the in vivo evaluation of a TCO-oxymethylacetamide-tagged CC49 antibody (CC49-TCO(2)), which is highly reactive toward tetrazines and less hydrophobic than CC49-TCO(1). CC49-TCO(2) was administered to healthy mice to determine its blood clearance and the in vivo stability of the TCO. Next, pretargeting biodistribution and SPECT studies with CC49-TCO(2), tetrazine-functionalized clearing agent, and radiolabeled tetrazine were carried out in nude mice bearing colon carcinoma xenografts (LS174T). CC49-TCO(2) had an increased circulation half-life, a 1.5-fold higher tumor uptake, and a 2.6-fold improved in vivo TCO stability compared to the more hydrophobic TCO-benzamide-CC49. As a consequence, and despite the 2-fold lower reactivity of CC49-TCO(2) toward tetrazines compared with CC49-TCO(1), administration of radiolabeled tetrazine afforded a significantly increased tumor accumulation and improved T/NT ratios in mice pretargeted with CC49-TCO(2). In conclusion, the TCO-acetamide derivative represents a large improvement in in vivo Diels-Alder pretargeting, possibly enabling application in larger animals and eventually humans.

An efficient and mild oxidant for the synthesis of s-tetrazines

PhI(OAc)₂ serves as a mild and effective oxidant for the synthesis of s-tetrazine derivatives- molecules of emerging significance to the field of bioorthogonal chemistry. This reagent serves as a complementary oxidant to harsher nitrous reagents. Use of PhI(OAc)₂ improves the synthesis of 5-amino-di(pyridin-2-yl)-s-tetrazine, a molecule that has been broadly used for cellular imaging and nuclear medicine. The generality of PhI(OAc)₂ as the oxidant for tetrazine synthesis is demonstrated for nine tetrazines in 75-98% yield.

Photoclick chemistry: a fluorogenic light-triggered in vivo ligation reaction

The ability to use chemical reactivity to monitor and control biomolecular processes with a spatial and temporal precision motivated the development of light-triggered in vivo chemistries. To this end, the photoinduced tetrazole-alkene cycloaddition, also termed 'photoclick chemistry' offers a very rapid chemical ligation platform for the manipulation of biomolecules and matrices in vivo. Here we outline the recent developments in the optimization of this chemistry, ranging from the search for substrates that offer two-photon photoactivatability, superior reaction kinetics, and/or genetic encodability, to the study of the reaction mechanism. The applications of the photoclick chemistry in protein labeling in vitro and in vivo as well as in preparing 'smart' hydrogels for 3D cell culture are highlighted.

Development of a (18) F-labeled tetrazine with favorable pharmacokinetics for bioorthogonal PET imaging

A low-molecular-weight (18) F-labeled tetrazine derivative was developed as a highly versatile tool for bioorthogonal PET imaging. Prosthetic groups and undesired carrying of (18) F through additional steps were evaded by direct (18) F-fluorination of an appropriate tetrazine precursor. Reaction kinetics of the cycloaddition with trans-cyclooctenes were investigated by applying quantum chemical calculations and stopped-flow measurements in human plasma; the results indicated that the labeled tetrazine is suitable as a bioorthogonal probe for the imaging of dienophile-tagged (bio)molecules. In vitro and in vivo investigations revealed high stability and PET/MRI in mice showed fast homogeneous biodistribution of the (18) F-labeled tetrazine that also passes the blood-brain barrier. An in vivo click experiment confirmed the bioorthogonal behavior of this novel tetrazine probe. Due to favorable chemical and pharmacokinetic properties this bioorthogonal agent should find application in bioimaging and biomedical research.

In vivo evaluation of (18)F-labeled TCO for pre-targeted PET imaging in the brain

INTRODUCTION

The tetrazine-trans-cylooctene cycloaddition using radiolabeled tetrazine or radiolabeled trans-cyclooctene (TCO) has been reported to be a very fast, selective and bioorthogonal reaction that could be useful for in vivo radiolabeling of molecules. We wanted to evaluate the in vivo biodistribution profile and brain uptake of (18)F-labeled TCO ([(18)F]TCO) to assess its potential for pre-targeted imaging in the brain.

METHODS

We evaluated the in vivo behavior of [(18)F]TCO via an ex vivo biodistribution study complemented by in vivo μPET imaging at 5, 30, 60, 90, 120 and 240 min post tracer injection. An in vivo metabolite study was performed at 5 min, 30 min and 120 min post [(18)F]TCO injection by RP-HPLC analysis of plasma and brain extracts. Incubation with human liver microsomes was performed to further evaluate the metabolite profile of the tracer.

RESULTS

μPET imaging and ex-vivo biodistribution revealed an high initial brain uptake of [(18)F]TCO (3.8%ID/g at 5 min pi) followed by a washout to 3.0%ID/g at 30 min pi. Subsequently the brain uptake increased again to 3.7%ID/g at 120 min pi followed by a slow washout until 240 min pi (2.9%ID/g). Autoradiography confirmed homogenous brain uptake. On the μPET images bone uptake became gradually visible after 120 min pi and was clearly visible at 240 min pi. The metabolite study revealed a fast metabolization of [(18)F]TCO in plasma and brain into three main polar radiometabolites.

CONCLUSIONS

Although [(18)F]TCO has previously been described to be a useful tracer for radiolabeling of tetrazine modified targeting molecules, our study indicates that its utility for in vivo chemistry and pre-targeted imaging will be limited. Although [(18)F]TCO clearly enters the brain, it is quickly metabolized with a non-specific accumulation of radioactivity in the brain and bone.

Finding the right (bioorthogonal) chemistry

Bioorthogonal chemistries can be used to tag diverse classes of biomolecules in cells and other complex environments. With over 20 unique transformations now available, though, selecting an appropriate reaction for a given experiment is challenging. In this article, we compare and contrast the most common classes of bioorthogonal chemistries and provide a framework for matching the reactions with downstream applications. We also discuss ongoing efforts to identify novel biocompatible reactions and methods to control their reactivity. The continued expansion of the bioorthogonal toolkit will provide new insights into biomolecule networks and functions and thus refine our understanding of living systems.

Synthesis and reactivity comparisons of 1-methyl-3-substituted cyclopropene mini-tags for tetrazine bioorthogonal reactions

Substituted cyclopropenes have recently attracted attention as stable "mini-tags" that are highly reactive dienophiles with the bioorthogonal tetrazine functional group. Despite this interest, the synthesis of stable cyclopropenes is not trivial and their reactivity patterns are poorly understood. Here, the synthesis and comparison of the reactivity of a series of 1-methyl-3-substituted cyclopropenes with different functional handles is described. The rates at which the various substituted cyclopropenes undergo Diels-Alder cycloadditions with 1,2,4,5-tetrazines were measured. Depending on the substituents, the rates of cycloadditions vary by over two orders of magnitude. The substituents also have a dramatic effect on aqueous stability. An outcome of these studies is the discovery of a novel 3-amidomethyl substituted methylcyclopropene tag that reacts twice as fast as the fastest previously disclosed 1-methyl-3-substituted cyclopropene while retaining excellent aqueous stability. Furthermore, this new cyclopropene is better suited for bioconjugation applications and this is demonstrated through using DNA templated tetrazine ligations. The effect of tetrazine structure on cyclopropene reaction rate was also studied. Surprisingly, 3-amidomethyl substituted methylcyclopropene reacts faster than trans-cyclooctenol with a sterically hindered and extremely stable tert-butyl substituted tetrazine. Density functional theory calculations and the distortion/interaction analysis of activation energies provide insights into the origins of these reactivity differences and a guide to the development of future tetrazine coupling partners. The newly disclosed cyclopropenes have kinetic and stability advantages compared to previously reported dienophiles and will be highly useful for applications in organic synthesis, bioorthogonal reactions, and materials science.

The nitrilimine-alkene cycloaddition is an ultra rapid click reaction

The transient formation of nitrilimine in aqueous conditions is greatly influenced by pH and chloride. In basic conditions (pH 10) with no chloride, a diarylnitrilimine precursor readily ionizes to form diarylnitrilimine that reacts almost instantly with an acrylamide-containing protein and fluorescently labels it.