Expanding Transition Metal-Mediated Bioorthogonal Decaging to Include C-C Bond Cleavage Reactions.

The ability to control the activation of prodrugs by transition metals has been shown to have great potential for controlled drug release in cancer cells. However, the strategies developed so far promote the cleavage of C-O or C-N bonds, which limits the scope of drugs to only those that present amino or hydroxyl groups. Here, we report the decaging of an ortho-quinone prodrug, a propargylated ß-lapachone derivative, through a palladium-mediated C-C bond cleavage. The reaction's kinetic and mechanistic behavior was studied under biological conditions along with computer modeling. The results indicate that palladium (II) is the active species for the depropargylation reaction, activating the triple bond for nucleophilic attack by a water molecule before the C-C bond cleavage takes place. Palladium iodide nanoparticles were found to efficiently trigger the C-C bond cleavage reaction under biocompatible conditions. In drug activation assays in cells, the protected analogue of ß-lapachone was activated by nontoxic amounts of nanoparticles, which restored drug toxicity. The palladium-mediated ortho-quinone prodrug activation was further demonstrated in zebrafish tumor xenografts, which resulted in a significant anti-tumoral effect. This work expands the transition-metal-mediated bioorthogonal decaging toolbox to include cleavage of C-C bonds and payloads that were previously not accessible by conventional strategies.

Arylethynyltrifluoroborate Dienophiles for on Demand Activation of IEDDA Reactions.

Strained alkenes and alkynes are the predominant dienophiles used in inverse electron demand Diels-Alder (IEDDA) reactions. However, their instability, cross-reactivity, and accessibility are problematic. Unstrained dienophiles, although physiologically stable and synthetically accessible, react with tetrazines significantly slower relative to strained variants. Here we report the development of potassium arylethynyltrifluoroborates as unstrained dienophiles for fast, chemically triggered IEDDA reactions. By varying the substituents on the tetrazine (e.g., pyridyl- to benzyl-substituents), cycloaddition kinetics can vary from fast (k2 = 21 M-1 s-1) to no reaction with an alkyne-BF3 dienophile. The reported system was applied to protein labeling both in the test tube and fixed cells and even enabled mutually orthogonal labeling of two distinct proteins.

In Vivo Pretargeting Based on Cysteine-Selective Antibody Modification with IEDDA Bioorthogonal Handles for Click Chemistry.

Pretargeted imaging has emerged as an effective multistep strategy aiming to improve imaging contrast and reduce patient radiation exposure through decoupling of the radioactivity from the targeting vector. The inverse electron-demand Diels-Alder (IEDDA) reaction between a trans-cyclooctene (TCO)-conjugated antibody and a labeled tetrazine holds great promise for pretargeted imaging applications due to its bioorthogonality, rapid kinetics under mild conditions, and formation of stable products. Herein, we describe the use of functionalized carbonylacrylic reagents for site-specific incorporation of TCO onto a human epidermal growth factor receptor 2 (HER2) antibody (THIOMAB) containing an engineered unpaired cysteine residue, generating homogeneous conjugates. Precise labeling of THIOMAB-TCO with a fluorescent or radiolabeled tetrazine revealed the potential of the TCO-functionalized antibody for imaging the HER2 after pretargeting in a cellular context in a HER2 positive breast cancer cell line. Control studies with MDA-MD-231 cells, which do not express HER2, further confirmed the target specificity of the modified antibody. THIOMAB-TCO was also evaluated in vivo after pretargeting and subsequent administration of an 111In-labeled tetrazine. Biodistribution studies in breast cancer tumor-bearing mice showed a significant activity accumulation on HER2+ tumors, which was 2.6-fold higher than in HER2- tumors. Additionally, biodistribution studies with THIOMAB without the TCO handle also resulted in a decreased uptake of 111In-DOTA-Tz on HER2+ tumors. Altogether, these results clearly indicate the occurrence of the click reaction at the tumor site, i.e., pretargeting of SK-BR-3 HER2-expressing cells with THIOMAB-TCO and reaction through the TCO moiety present in the antibody. The combined advantages of site-selectivity and stability of TCO tagged-antibodies could allow application of biorthogonal chemistry strategies for pretargeting imaging with minimal side-reactions and background.

Mechanistic insights into transition metal-mediated bioorthogonal uncaging reactions.

Cleavage of C-O and C-N bonds mediated by transition metals is a promising bioorthogonal approach to rescue the activity of caged molecules, such as proteins and cytotoxic drugs, under biological conditions. However, the precise mechanism of such uncaging reactions remains elusive. This review provides mechanistic insights into metal-mediated bond-cleavage reactions, with the goals of understanding the main factors that influence the reaction and aiding the rational development of new caging groups/catalysts for chemical biology and drug-delivery applications.

Platinum-Triggered Bond-Cleavage of Pentynoyl Amide and N-Propargyl Handles for Drug-Activation.

The ability to create ways to control drug activation at specific tissues while sparing healthy tissues remains a major challenge. The administration of exogenous target-specific triggers offers the potential for traceless release of active drugs on tumor sites from antibody-drug conjugates (ADCs) and caged prodrugs. We have developed a metal-mediated bond-cleavage reaction that uses platinum complexes [K2PtCl4 or Cisplatin (CisPt)] for drug activation. Key to the success of the reaction is a water-promoted activation process that triggers the reactivity of the platinum complexes. Under these conditions, the decaging of pentynoyl tertiary amides and N-propargyls occurs rapidly in aqueous systems. In cells, the protected analogues of cytotoxic drugs 5-fluorouracil (5-FU) and monomethyl auristatin E (MMAE) are partially activated by nontoxic amounts of platinum salts. Additionally, a noninternalizing ADC built with a pentynoyl traceless linker that features a tertiary amide protected MMAE was also decaged in the presence of platinum salts for extracellular drug release in cancer cells. Finally, CisPt-mediated prodrug activation of a propargyl derivative of 5-FU was shown in a colorectal zebrafish xenograft model that led to significant reductions in tumor size. Overall, our results reveal a new metal-based cleavable reaction that expands the application of platinum complexes beyond those in catalysis and cancer 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.

Development of a self-immolative linker for tetrazine-triggered release of alcohols in cells.

Bioorthogonal decaging reactions are a promising strategy for prodrug activation because they involve bond cleavage to release a molecule of interest. The trans-cyclooctene (TCO)-tetrazine inverse electron-demand Diels-Alder reaction has been widely applied in vivo for decaging of amine prodrugs, however, the release of alcohol-containing bioactive compounds has been less well studied. Here, we report a TCO-carbamate benzyl ether self-immolative linker for the release of OH-molecules upon reaction with a tetrazine trigger. The benzyl ether linker proved to be highly stable and can rapidly liberate alcohols under physiological conditions upon reaction with tetrazines. The mechanism and decaging yield were systematically examined by fluorescence and HPLC analysis by using a fluorogenic TCO-benzyl ether-coumarin probe and different 3,6-substituted tetrazine derivatives. This study revealed that decaging occurs rapidly (t1/2 = 27 min) and the cycloaddition step happens within seconds (t1/2 = 7 s) with reaction rates of ≈100 M-1 s-1. Importantly, the reaction is compatible with living organisms as demonstrated by the decaging of a prodrug of the antibacterial compound triclosan in the presence of live E. Coli, that resulted in complete cell killing by action of the released "OH-active drug". Overall, this work describes a new linker for masking alcohol functionality that can be rapidly reinstated through tetrazine-triggered decaging.

A Fluorogenic Probe for Cell Surface Phosphatidylserine Using an Intramolecular Indicator Displacement Sensing Mechanism.

The detection of externalized phosphatidylserine (PS) on the cell surface is commonly used to distinguish between living, apoptotic, and necrotic cells. The tools of choice for many researchers to study apoptosis are annexin V-fluorophore conjugates. However, the use of this 35 kDa protein is associated with several drawbacks, including temperature sensitivity, Ca2+ dependence, and slow binding kinetics. Herein, a fluorogenic probe for cell surface PS, P-IID, is described, which operates by an intramolecular indicator displacement (IID) mechanism. An intramolecularly bound coumarin indicator is released in the presence of cell surface PS, leading to a fluorescence "turn-on" response. P-IID demonstrates superior performance when compared to annexin V, for both fluorescence imaging and flow cytometry. This allows P-IID to be used in time-lapse imaging of apoptosis using confocal laser scanning microscopy and demonstrates the utility of the IID mechanism in live cells.

Chemo- and Regioselective Lysine Modification on Native Proteins.

Site-selective chemical conjugation of synthetic molecules to proteins expands their functional and therapeutic capacity. Current protein modification methods, based on synthetic and biochemical technologies, can achieve site selectivity, but these techniques often require extensive sequence engineering or are restricted to the N- or C-terminus. Here we show the computer-assisted design of sulfonyl acrylate reagents for the modification of a single lysine residue on native protein sequences. This feature of the designed sulfonyl acrylates, together with the innate and subtle reactivity differences conferred by the unique local microenvironment surrounding each lysine, contribute to the observed regioselectivity of the reaction. Moreover, this site selectivity was predicted computationally, where the lysine with the lowest p Ka was the kinetically favored residue at slightly basic pH. Chemoselectivity was also observed as the reagent reacted preferentially at lysine, even in those cases when other nucleophilic residues such as cysteine were present. The reaction is fast and proceeds using a single molar equivalent of the sulfonyl acrylate reagent under biocompatible conditions (37 °C, pH 8.0). This technology was demonstrated by the quantitative and irreversible modification of five different proteins including the clinically used therapeutic antibody Trastuzumab without prior sequence engineering. Importantly, their native secondary structure and functionality is retained after the modification. This regioselective lysine modification method allows for further bioconjugation through aza-Michael addition to the acrylate electrophile that is generated by spontaneous elimination of methanesulfinic acid upon lysine labeling. We showed that a protein-antibody conjugate bearing a site-specifically installed fluorophore at lysine could be used for selective imaging of apoptotic cells and detection of Her2+ cells, respectively. This simple, robust method does not require genetic engineering and may be generally used for accessing diverse, well-defined protein conjugates for basic biology and therapeutic studies.

Radical-Mediated Thiol-Ene Strategy: Photoactivation of Thiol-Containing Drugs in Cancer Cells.

Photoactivated drugs provide an opportunity to improve efficacy alongside reducing side-effects in the treatment of severe diseases such as cancer. Described herein is a photoactivation decaging method of isobutylene-caged thiols through a UV-initiated thiol-ene reaction. The method was demonstrated with an isobutylene-caged cysteine, cyclic disulfide-peptide, and thiol-containing drug, all of which were rapidly and efficiently released under mild UV irradiation in the presence of thiol sources and a photoinitiator. Importantly, it is shown that the activity of histone deacetylase inhibitor largazole can be switched off when stapled, but selectively switched on within cancer cells when irradiated with non-phototoxic light.

Vinyl Ether/Tetrazine Pair for the Traceless Release of Alcohols in Cells.

The cleavage of a protecting group from a protein or drug under bioorthogonal conditions enables accurate spatiotemporal control over protein or drug activity. Disclosed herein is that vinyl ethers serve as protecting groups for alcohol-containing molecules and as reagents for bioorthogonal bond-cleavage reactions. A vinyl ether moiety was installed in a range of molecules, including amino acids, a monosaccharide, a fluorophore, and an analogue of the cytotoxic drug duocarmycin. Tetrazine-mediated decaging proceeded under biocompatible conditions with good yields and reasonable kinetics. Importantly, the nontoxic, vinyl ether duocarmycin double prodrug was successfully decaged in live cells to reinstate cytotoxicity. This bioorthogonal reaction presents broad applicability and may be suitable for in vivo applications.

Multisite Thrombus Imaging and Fibrin Content Estimation With a Single Whole-Body PET Scan in Rats.

OBJECTIVE: Thrombosis is a leading cause of morbidity and mortality worldwide. Current diagnostic strategies rely on imaging modalities that are specific for distinct vascular territories, but a thrombus-specific whole-body imaging approach is still missing. Moreover, imaging techniques to assess thrombus composition are underdeveloped, although therapeutic strategies may benefit from such technology. Therefore, our goal was to test whether positron emission tomography (PET) with the fibrin-binding probe (64)Cu-FBP8 allows multisite thrombus detection and fibrin content estimation. APPROACH AND RESULTS: Thrombosis was induced in Sprague-Dawley rats (n=32) by ferric chloride application on both carotid artery and femoral vein. (64)Cu-FBP8-PET/CT imaging was performed 1, 3, or 7 days after thrombosis to detect thrombus location and to evaluate age-dependent changes in target uptake. Ex vivo biodistribution, autoradiography, and histopathology were performed to validate imaging results. Arterial and venous thrombi were localized on fused PET/CT images with high accuracy (97.6%; 95% confidence interval, 92-100). A single whole-body PET/MR imaging session was sufficient to reveal the location of both arterial and venous thrombi after (64)Cu-FBP8 administration. PET imaging showed that probe uptake was greater in younger clots than in older ones for both arterial and venous thrombosis (P<0.0001). Quantitative histopathology revealed an age-dependent reduction of thrombus fibrin content (P<0.001), consistent with PET results. Biodistribution and autoradiography further confirmed the imaging findings. CONCLUSIONS: We demonstrated that (64)Cu-FBP8-PET is a feasible approach for whole-body thrombus detection and that molecular imaging of fibrin can provide, noninvasively, insight into clot composition.

A Minimal, Unstrained S-Allyl Handle for Pre-Targeting Diels-Alder Bioorthogonal Labeling in Live Cells.

The unstrained S-allyl cysteine amino acid was site-specifically installed on apoptosis protein biomarkers and was further used as a chemical handle and ligation partner for 1,2,4,5-tetrazines by means of an inverse-electron-demand Diels-Alder reaction. We demonstrate the utility of this minimal handle for the efficient labeling of apoptotic cells using a fluorogenic tetrazine dye in a pre-targeting approach. The small size, easy chemical installation, and selective reactivity of the S-allyl handle towards tetrazines should be readily extendable to other proteins and biomolecules, which could facilitate their labeling within live cells.

A 99mTc(CO)3 -labeled benzylguanidine with persistent heart uptake.

We describe the synthesis and biological evaluation of the cationic (99m)Tc-tricarbonyl complex fac-[(99m)Tc(CO)3 (κ(3) -L1)](+) (Tc1) anchored by a pyrazole-diamine-methylbenzylguanidine-based ligand (L1), as potentially useful for myocardial imaging. The rhenium complex fac-[Re(CO)3 (κ(3)-L1)](+) (Re1) was prepared and characterized as a 'cold' surrogate of the radioactive complex. Cell uptake studies in a neuroblastoma cell line suggest that Tc1 uptake mechanism is related to the norepinephrine transporter (NET). Tissue distribution studies in CD1 mice showed that Tc1 presents high initial heart uptake and a slow washout from the heart (7.8 ± 1.3% injected dose per gram (ID/g), 30-min post-injection (p.i.); 6.3 ± 1.3% ID/g, 60-min p.i.), with heart to blood ratios of 11.8 and 9.0 at 30- and 60-min p.i., respectively. The uptake mechanism of Tc1 appears to be similar to that of metaiodobenzylguanidine (MIBG), as it can be reduced by coinjection with nonradioactive MIBG. The biodistribution profile of Tc2, where the benzylguanidine pharmacophore is absent, corroborates the fact that Tc1 does not accumulate in the heart by a simple diffusion mechanism but rather by a NET-mediated mechanism. The results confirm those obtained in the cell assays. Despite the persistent heart uptake found for Tc1, the high hepatic and renal uptake remains to be improved.

Selective Inhibition of Bruton's Tyrosine Kinase by a Designed Covalent Ligand Leads to Potent Therapeutic Efficacy in Blood Cancers Relative to Clinically Used Inhibitors.

Bruton's tyrosine kinase (BTK) is a member of the TEC-family kinases and crucial for the proliferation and differentiation of B-cells. We evaluated the therapeutic potential of a covalent inhibitor (JS25) with nanomolar potency against BTK and with a more desirable selectivity and inhibitory profile compared to the FDA-approved BTK inhibitors ibrutinib and acalabrutinib. Structural prediction of the BTK/JS25 complex revealed sequestration of Tyr551 that leads to BTK's inactivation. JS25 also inhibited the proliferation of myeloid and lymphoid B-cell cancer cell lines. Its therapeutic potential was further tested against ibrutinib in preclinical models of B-cell cancers. JS25 treatment induced a more pronounced cell death in a murine xenograft model of Burkitt's lymphoma, causing a 30-40% reduction of the subcutaneous tumor and an overall reduction in the percentage of metastasis and secondary tumor formation. In a patient model of diffuse large B-cell lymphoma, the drug response of JS25 was higher than that of ibrutinib, leading to a 64% "on-target" efficacy. Finally, in zebrafish patient-derived xenografts of chronic lymphocytic leukemia, JS25 was faster and more effective in decreasing tumor burden, producing superior therapeutic effects compared to ibrutinib. We expect JS25 to become therapeutically relevant as a BTK inhibitor and to find applications in the treatment of hematological cancers and other pathologies with unmet clinical treatment.

Re and Tc tricarbonyl complexes: from the suppression of NO biosynthesis in macrophages to in vivo targeting of inducible nitric oxide synthase.

The in vivo molecular imaging of nitric oxide synthase (NOS), the enzyme responsible for the catalytic oxidation of l-arginine to citrulline and nitric oxide (NO), by noninvasive modalities could provide valuable insights into NO/NOS-related diseases. Aiming at the design of innovative (99m)Tc(I) complexes for targeting inducible NOS (iNOS) in vivo by SPECT imaging, herein we describe a set of novel (99m)Tc(CO)3 complexes (2-5) and the corresponding rhenium surrogates (2a-5a) containing the NOS inhibitor N(ω)-nitro-l-arginine. The latter is linked through its α-NH2 or α-COOH group and an alkyl spacer of variable length to the metal center. The complexes 2a (propyl spacer) and 3a (hexyl spacer), in which the α-NH2 group of the inhibitor is involved in the conjugation to the metal center, presented remarkable affinity for purified iNOS, being similar to that of the free nonconjugated inhibitor (K(i) = 3-8 µM) in the case of 3a (K(i) = 6 µM). 2a and 3a are the first examples of organometallic complexes that permeate through RAW 264.7 macrophage cell membranes, interacting specifically with the target enzyme, as confirmed by the suppression of NO biosynthesis in LPS-treated macrophages (2a, ca. 30% inhibition; 3a, ca. 50% inhibition). The (99m)Tc(I)-complexes 2 and 3, stable both in vitro and in vivo, also presented the ability to cross cell membranes, as demonstrated by internalization studies in the same cell model. The biodistribution studies in LPS-pretreated mature female C57BL6 mice have shown that 2 presented an overall higher uptake in most tissues of the LPS-treated mice compared to the control group (30 min postinjection). This increase is significant in lung (3.98 ± 0.63 vs to 0.99 ± 0.13%ID/g), which is known to be the organ with the highest iNOS expression after LPS treatment. These results suggest that the higher uptake in that organ may be related to iNOS upregulation.

Syntheses of bifunctional 2,3-diamino propionic acid-based chelators as small and strong tripod ligands for the labelling of biomolecules with (99m)Tc.

The labelling of targeting biomolecules requires small and hydrophilic complexes in order to not affect the binding properties of the vectors. 2,3-Diamino propionic acid (dap) is a small and strong, albeit scarcely used, tripod ligand for the fac-[(99m)Tc(CO)(3)](+) moiety. We have introduced at the alpha-carbon atom in the basic dap structure various second functionalities such as carboxylato, amino and alpha-amino acid groups via various spacers in order to yield bifunctional chelators. These dap derivatives can be coupled to targeting molecules for application in molecular imaging. Full characterizations of the bifunctional chelators, X-ray structures of intermediates and of one rhenium complex, as well as labelling studies with (99m)Tc, are presented.

Human monocytes but not dendritic cells are killed by blocking of autocrine cyclooxygenase activity.

Dendritic cells (DCs), in peripheral tissues, derive mostly from blood precursors that differentiate into DCs under the influence of the local microenvironment. Monocytes constitute the main known DC precursors in blood and their infiltration into tissues is up-regulated during inflammation. During this process, the local production of mediators, like prostaglandins (PGs), influence significantly DC differentiation and function. In the present paper we show that treatment of blood adherent mononuclear cells with 10microM indomethacin, a dose achieved in human therapeutic settings, causes monocytes' progressive death but does not affect DCs viability or cell surface phenotype. This resistance of DCs was observed both for cells differentiated in vitro from blood monocytes and for a population with DCs characteristics already present in blood. This phenomenon could affect the local balance of antigen-presenting cells, influence the induction and pattern of immune responses developed under the treatment with non-steroidal anti-inflammatory drugs and, therefore, deserves further investigation.

Norbornene Probes for the Detection of Cysteine Sulfenic Acid in Cells.

Norbornene derivatives were validated as probes for cysteine sulfenic acid on proteins and in live cells. Trapping sulfenic acids with norbornene probes is highly selective and revealed a different reactivity profile than the traditional dimedone reagent. The norbornene probe also revealed a superior chemoselectivity when compared to a commonly used dimedone probe. Together, these results advance the study of cysteine oxidation in biological systems.

Azabicyclic vinyl sulfones for residue-specific dual protein labelling.

We have developed [2.2.1]azabicyclic vinyl sulfone reagents that simultaneously enable cysteine-selective protein modification and introduce a handle for further bioorthogonal ligation. The reaction is fast and selective for cysteine relative to other amino acids that have nucleophilic side-chains, and the formed products are stable in human plasma and are moderately resistant to retro Diels-Alder degradation reactions. A model biotinylated [2.2.1]azabicyclic vinyl sulfone reagent was shown to efficiently label two cysteine-tagged proteins, ubiquitin and C2Am, under mild conditions (1-5 equiv. of reagent in NaPi pH 7.0, room temperature, 30 min). The resulting thioether-linked conjugates were stable and retained the native activity of the proteins. Finally, the dienophile present in the azabicyclic moiety on a functionalised C2Am protein could be fluorescently labelled through an inverse electron demand Diels-Alder reaction in cells to allow selective apoptosis imaging. The combined advantages of directness, site-specificity and easy preparation mean [2.2.1]azabicyclic vinyl sulfones can be used for residue-specific dual protein labelling/construction strategies with minimal perturbation of native function based simply on the attachment of an [2.2.1]azabicyclic moiety to cysteine.