High-Mannose Oligosaccharide Hemimimetics that Recapitulate the Conformation and Binding Mode to Concanavalin A, DC-SIGN and Langerin.

The "carbohydrate chemical mimicry" exhibited by sp2 -iminosugars has been utilized to develop practical syntheses for analogs of the branched high-mannose-type oligosaccharides (HMOs) Man3 and Man5 . In these compounds, the terminal nonreducing Man residues have been substituted with 5,6-oxomethylidenemannonojirimycin (OMJ) motifs. The resulting oligomannoside hemimimetic accurately reproduce the structure, configuration, and conformational behavior of the original mannooligosaccharides, as confirmed by NMR and computational techniques. Binding studies with mannose binding lectins, including concanavalin A, DC-SIGN, and langerin, by enzyme-linked lectin assay and surface plasmon resonance revealed significant variations in their ability to accommodate the OMJ unit in the mannose binding site. Intriguingly, OMJMan segments demonstrated "in line" heteromultivalent effects during binding to the three lectins. Similar to the mannobiose (Man2 ) branches in HMOs, the binding modes involving the external or internal monosaccharide unit at the carbohydrate binding-domain exist in equilibrium, facilitating sliding and recapture processes. This equilibrium, which influences the multivalent binding of HMOs, can be finely modulated upon incorporation of the OMJ sp2 -iminosugar caps. As a proof of concept, the affinity and selectivity towards DC-SIGN and langerin were adjustable by presenting the OMJMan epitope in platforms with diverse architectures and valencies.

Cell-free expression and characterization of multivalent rhamnose-binding lectins using bio-layer interferometry.

Lectins are important biological tools for binding glycans, but recombinant protein expression poses challenges for some lectin classes, limiting the pace of discovery and characterization. To discover and engineer lectins with new functions, workflows amenable to rapid expression and subsequent characterization are needed. Here, we present bacterial cell-free expression as a means for efficient, small-scale expression of multivalent, disulfide bond-rich, rhamnose-binding lectins. Furthermore, we demonstrate that the cell-free expressed lectins can be directly coupled with bio-layer interferometry analysis, either in solution or immobilized on the sensor, to measure interaction with carbohydrate ligands without purification. This workflow enables the determination of lectin substrate specificity and estimation of binding affinity. Overall, we believe that this method will enable high-throughput expression, screening, and characterization of new and engineered multivalent lectins for applications in synthetic glycobiology.

Advanced high-affinity glycoconjugate ligands of galectins.

Galectins are proteins of the family of human lectins. By binding terminal galactose units of cell surface glycans, they moderate biological and pathological processes such as cell signaling, cell adhesion, apoptosis, fibrosis, carcinogenesis, and metabolic disorders. The binding of monovalent glycans to galectins is usually relatively weak. Therefore, the presentation of carbohydrate ligands on multivalent scaffolds can efficiently increase and/or discriminate the affinity of the glycoconjugate to different galectins. A library of glycoclusters and glycodendrimers with various structural presentations of the common functionalized N-acetyllactosamine ligand was prepared to evaluate how the mode of presentation affects the affinity and selectivity to the two most abundant galectins, galectin-1 (Gal-1) and galectin-3 (Gal-3). In addition, the effect of a one- to two-unit carbohydrate spacer on the affinity of the glycoconjugates was determined. A new design of the biolayer interferometry (BLI) method with specific AVI-tagged constructs was used to determine the affinity to galectins, and compared with the gold-standard method of isothermal titration calorimetry (ITC). This study reveals new routes to low nanomolar glycoconjugate inhibitors of galectins of interest for biomedical research.

Multivalent glycocyclopeptides: conjugation methods and biological applications.

Click chemistry was extensively used to decorate synthetic multivalent scaffolds with glycans to mimic the cell surface glycocalyx and to develop applications in glycosciences. Conjugation methods such as oxime ligation, copper(I)-catalyzed alkyne-azide cycloaddition, thiol-ene coupling, squaramide coupling or Lansbury aspartylation proved particularly suitable to achieve this purpose. This review summarizes the synthetic strategies that can be used either in a stepwise manner or in an orthogonal one-pot approach, to conjugate multiple copies of identical or different glycans to cyclopeptide scaffolds (namely multivalent glycocyclopeptides) having different size, valency, geometry and molecular composition. The second part of this review will describe the potential of these structures to interact with various carbohydrate binding proteins or to stimulate immunity against tumor cells.

Characterization of the interaction of multivalent glycosylated ligands with bacterial lectins by biolayer interferometry.

The study of multivalent carbohydrate-protein interactions remains highly complicated and sometimes rendered impossible due to aggregation problems. Biolayer interferometry is emerging as a tool to monitor such complex interactions. In this study, various glycoclusters and dendrimers were prepared and evaluated as ligands for lectins produced by pathogenic bacteria Pseudomonas aeruginosa (LecA and Lec B) and Burkholderia ambifaria (BambL). Reliable kinetic and thermodynamic parameters could be measured, and immobilization of either lectin or ligands resulted in high quality data. The methods gave results in full agreement with previous isothermal titration calorimetry experiments, and presented strong advantages because they require less quantity and purity for the biomolecules.

Targeting Tn-Antigen-Positive Human Tumors with a Recombinant Human Macrophage Galactose C-Type Lectin.

Alterations in glycosylation cause the emergence of tumor-associated carbohydrate antigens (TACAs) during tumorigenesis. Truncation of O-glycans reveals the Thomsen nouveau (Tn) antigen, an N-acetylgalactosamine (GalNAc) frequently attached to serine or threonine amino acids, that is accessible on the surface of cancer cells but not on healthy cells. Interestingly, GalNac can be recognized by macrophage galactose lectin (MGL), a type C lectin receptor expressed in immune cells. In this study, recombinant MGL fragments were tested in vitro for their cancer cell-targeting efficiency by flow cytometry and confocal microscopy and in vivo after administration of fluorescent MGL to tumor-bearing mice. Our results demonstrate the ability of MGL to target Tn-positive human tumors without inducing toxicity. This outcome makes MGL, a fragment of a normal human protein, the first vector candidate for in vivo diagnosis and imaging of human tumors and, possibly, for therapeutic applications.

Homo- and Heterovalent Neoglycoproteins as Ligands for Bacterial Lectins.

Click chemistry gives access to unlimited set of multivalent glycoconjugates to explore carbohydrate-protein interactions and discover high affinity ligands. In this study, we have created supramolecular systems based on a carrier protein that was grafted by Cu(I)-catalyzed azide-alkyne cycloaddition with tetravalent glycodendrons presenting αGal, ßGal and/or αFuc. Binding studies of the homo- (4 a-c) and heterovalent (5) neoglycoproteins (neoGPs) with the LecA and LecB lectins from P. aeruginosa has first confirmed the interest of the multivalent presentation of glycodendrons by the carrier protein (IC50 up to 2.8 nM). Moreover, these studies have shown that the heterovalent display of glycans (5) allows the interaction with both lectins (IC50 of 10 nM) despite the presence of unspecific moieties, and even with similar efficiency for LecB. These results demonstrate the potential of multivalent and multispecific neoGPs as a promising strategy to fight against resistant pathogens.

Correction: Antibody recruiting molecules (ARMs): synthetic immunotherapeutics to fight cancer.

[This corrects the article DOI: 10.1039/D1CB00007A.].

Antibody recruiting molecules (ARMs): synthetic immunotherapeutics to fight cancer.

Antibody-recruiting molecules (ARMs) are one of the most promising tools to redirect the immune response towards cancer cells. In this review, we aim to highlight the recent advances in the field. We will illustrate the advantages of different ARM approaches and emphasize the importance of a multivalent presentation of the binding units.

Multivalent Presentations of Glycomimetic Inhibitor of the Adhesion of Fungal Pathogen Candida albicans to Human Buccal Epithelial Cells.

Candida albicans causes some of the most prevalent hospital-acquired fungal infections, particularly threatening for immunocompromised patients. C. albicans strongly adheres to the surface of epithelial cells so that subsequent colonization and biofilm formation can take place. Divalent galactoside glycomimetic 1 was found to be a potent inhibitor of the adhesion of C. albicans to buccal epithelial cells. In this work, we explore the effect of multivalent presentations of glycomimetic 1 on its ability to inhibit yeast adhesion and biofilm formation. Tetra-, hexa-, and hexadecavalent displays of compound 1 were built on RAFT cyclopeptide- and polylysine-based scaffolds with a highly efficient and modular synthesis. Biological evaluation revealed that the scaffold choice significantly influences the activity of the lower valency conjugates, with compound 16, constructed on a tetravalent polylysine scaffold, found to inhibit the adhesion of C. albicans to human buccal epithelial cells more effectively than the glycomimetic 1; however, the latter performed better in the biofilm reduction assays. Interestingly, the higher valency glycoconjugates did not outperform the anti-adhesion activity of the original compound 1, and no significant effect of the core scaffold could be appreciated. SEM images of C. albicans cells treated with compounds 1, 14, and 16 revealed significant differences in the aggregation patterns of the yeast cells.

Structural influence of antibody recruiting glycodendrimers (ARGs) on antitumoral cytotoxicity.

The recruitment of endogenous antibodies against cancer cells has become a reliable antitumoral immunotherapeutic alternative over the last decade. The covalent attachment of antibody and tumor binding modules (ABM and TBM) within a single, well-defined synthetic molecule was indeed demonstrated to promote the formation of an interacting ternary complex between both the antibodies and the targeted cell, which usually results in the simultaneous immune-mediated cellular destruction. In a preliminary study, we have described the first Antibody Recruiting Glycodendrimers (ARGs), combining cRGD as ligands for the αVß3-expressing melanoma cell line M21 and Rha as ligand for natural IgM, and demonstrated that multivalency is an essential requirement to form this complex. In the present study, we synthesized a new series of ARGs composed of ABMs, i.e. self-condensed rhamnosylated cyclopeptide and polylysine dendrimer, which have been conjugated to the TBM with or without spacer. Flow cytometry and confocal microscopy experiments with human serum and different cell lines revealed that the ABM geometry significantly influences the ternary complex formation in M21, whereas no significant binding occurs in BT 549 having low integrin expression. In addition, we demonstrate with a cellular viability assay that ARGs induce high level of cytotoxicity against M21 which is also in close correlation with the ABM structure. In particular, we have shown that ARG combining cyclopeptide core and branches, with or without spacer, induce 40-57% of selective cytotoxicity against M21 cells in the presence of human serum as the unique source of immunity effectors. Finally, we also highlight that the spacer between ABM and TBM enables an increase of the immune-mediate cytotoxicity even with ABM of lower valency.

Clicked Bifunctional Dendrimeric and Cyclopeptidic Addressable Redox Scaffolds for the Functionalization of Carbon Nanotubes with Redox Molecules and Enzymes.

Carbon nanotube electrodes were modified with ferrocene and laccase using two different click reactions strategies and taking advantage of bifunctional dendrimers and cyclopeptides. Using diazonium functionalization and the efficiency of oxime ligation, the combination of both multiwalled carbon nanotube surfaces and modified dendrimers or cyclopeptides allows the access to a high surface coverage of ferrocene in the order of 50 nmol cm-2, a 50-fold increase compared to a classic click reaction without oxime ligation of these highly branched macromolecules. Furthermore, this original immobilization strategy allows the immobilization of mono- and bi-functionalized active multicopper enzymes, laccases, via copper(I)-catalyzed azide-alkyne cycloaddition. Electrochemical studies underline the high efficiency of the oxime-ligated dendrimers or cyclopeptides for the immobilization of redox entities on surfaces while being detrimental to electron tunneling with enzyme active sites despite controlled orientation.

Development of C-type lectin-oriented surfaces for high avidity glycoconjugates: towards mimicking multivalent interactions on the cell surface.

Multivalent interactions between complex carbohydrates and oligomeric C-type lectins govern a wide range of immune responses. Up to date, standard SPR (surface plasmon resonance) competitive assays have largely been to evaluate binding properties from monosaccharide units (low affinity, mM) to multivalent elemental antagonists (moderate affinity, µM). Herein, we report typical case-studies of SPR competitive assays showing that they underestimate the potency of glycoclusters to inhibit the interaction between DC-SIGN and immobilized glycoconjugates. This paper describes the design and implementation of a SPR direct interaction over DC-SIGN oriented surfaces, extendable to other C-type lectin surfaces as such Langerin. This setup provides an overview of intrinsic avidity generation emanating simultaneously from multivalent glycoclusters and from DC-SIGN tetramers organized in nanoclusters at the cell membrane. To do so, covalent biospecific capture of DC-SIGN via StreptagII/StrepTactin interaction preserves tetrameric DC-SIGN, accessibility and topology of its active sites, that would have been dissociated using standard EDC-NHS procedure under acidic conditions. From the tested glycoclusters libraries, we demonstrated that the scaffold architecture, the valency and the glycomimetic-based ligand are crucial to reach nanomolar affinities for DC-SIGN. The glycocluster 3·D illustrates the tightest binding partner in this set for a DC-SIGN surface (KD = 18 nM). Moreover, the selectivity at monovalent scale of glycomimetic D can be easily analyzed at multivalent scale comparing its binding over different C-type lectin immobilized surfaces. This approach may give rise to novel insights into the multivalent binding mechanisms responsible for avidity and make a major contribution to the full characterization of the binding potency of promising specific and multivalent immodulators.

Chromones bearing amino acid residues: Easily accessible and potent inhibitors of the breast cancer resistance protein ABCG2.

The Breast Cancer Resistance Protein (BCRP/ABCG2) belongs to the G class of ABC (ATP-Binding Cassette) proteins, which is known as one of the main transporters involved in the multidrug resistance (MDR) phenotype that confer resistance to anticancer drugs. The aim of this study was to design, synthesize and develop new potent and selective inhibitors of BCRP that can be used to abolish MDR and potentialize clinically used anticancer agents. In previous reports, we showed the importance of chromone scaffold and hydrophobicity for the inhibition of ABC transporters. In the present study we report the design and development of chromones linked to one or two amino acids residues that are either hydrophobic or found in the structure of FTC, one of most potent (but highly toxic) inhibitors of BCRP. Herewith, we report the synthesis and evaluation of 13 compounds. The studied molecules were found to be not toxic and showed strong inhibition activity as well as high selectivity toward BCRP. The highest activity was obtained with the chromone bearing a valine residue (9c) which showed an inhibition activity against BCRP of 50 nM. The rationalization of the inhibition potential of the most active derivatives was performed through docking studies. Taken together, the ease of synthesis and the biological profile of these compounds render them as promising candidates for further development in the field of anticancer therapy.

Chemical synthesis and immunological evaluation of new generation multivalent anticancer vaccines based on a Tn antigen analogue.

Tumor associated carbohydrate antigens (TACAs), such as the Tn antigen, have emerged as key targets for the development of synthetic anticancer vaccines. However, the induction of potent and functional immune responses has been challenging and, in most cases, unsuccessful. Herein, we report the design, synthesis and immunological evaluation in mice of Tn-based vaccine candidates with multivalent presentation of the Tn antigen (up to 16 copies), both in its native serine-linked display (Tn-Ser) and as an oxime-linked Tn analogue (Tn-oxime). The high valent vaccine prototypes were synthesized through a late-stage convergent assembly (Tn-Ser construct) and a versatile divergent strategy (Tn-oxime analogue), using chemoselective click-type chemistry. The hexadecavalent Tn-oxime construct induced robust, Tn-specific humoral and CD4+/CD8+ cellular responses, with antibodies able to bind the Tn antigen on the MCF7 cancer cell surface. The superior synthetic accessibility and immunological properties of this fully-synthetic vaccine prototype makes it a compelling candidate for further advancement towards safe and effective synthetic anticancer vaccines.

Multifunctional Glycoconjugates for Recruiting Natural Antibodies against Cancer Cells.

Invited for the cover of this issue is Olivier Renaudet and co-workers at the Université Grenoble Alpes and funded by the European Research Council (CoG "LEGO'" no. 647938). The image illustrates a synthetic chemist playing with supramolecular structures to kill cancer cells by using natural antibodies present in the blood stream. Read the full text of the article at 10.1002/chem.201903327.

Multifunctional Glycoconjugates for Recruiting Natural Antibodies against Cancer Cells.

We have developed a fully synthetic and multifunctional antibody-recruiting molecule (ARM) to guide natural antibodies already present in the blood stream against cancer cells without pre-immunization. Our ARM is composed of antibody and tumor binding modules (i.e., ABM and TBM) displaying clustered rhamnose and cyclo-RGD, respectively. By using a stepwise approach, we have first demonstrated the importance of multivalency for efficient recognition with naturel IgM and αv ß3 integrin expressing M21 tumor cell line. Once covalently conjugated by click chemistry, we confirmed by flow cytometry and confocal microscopy that the recognition properties of both the ABM and TBM are conserved, and more importantly, that the resulting ARM promotes the formation of a ternary complex between natural IgM and cancer cells, which is required for the stimulation of the cytotoxic immune response in vivo. Due to the efficiency of the synthetic process, a larger diversity of heterovalent ligands could be easily explored by using the same multivalent approach and could open new perspectives in this field.

Optimization of the chromone scaffold through QSAR and docking studies: Identification of potent inhibitors of ABCG2.

The membrane transporter BCRP/ABCG2 has emerged as a privileged biological target for the development of small compounds capable of abolishing multidrug resistance. In this context, the chromone skeleton was found as an excellent scaffold for the design of ABCG2 inhibitors. With the aims of optimizing and developing more potent modulators of the transporter, we herewith propose a multidisciplinary medicinal chemistry approach performed on this promising scaffold. A quantitative structure-activity relationship (QSAR) study on a series of chromone derivatives was first carried out, giving a robust model that was next applied to the design of 13 novel compounds derived from this nucleus. Two of the most active according to the model's prediction, namely compounds 22 (5-((3,5-dibromobenzyl)oxy)-N-(2-(5-methoxy-1H-indol-3-yl)ethyl)-4-oxo-4H-chromene-2-carboxamide) and 31 (5-((2,4-dibromobenzyl)oxy)-N-(2-(5-methoxy-1H-indol-3-yl)ethyl)-4-oxo-4H-chromene-2-carboxamide), were synthesized and had their biological potency evaluated by experimental assays, confirming their high inhibitory activity against ABCG2 (experimental EC50 below 0.10 µM). A supplementary docking study was then conducted on the newly designed derivatives, proposing possible binding modes of these novel molecules in the putative ligand-binding site of the transporter and explaining why the two aforementioned compounds exerted the best activity according to biological data. Results from this study are recommended as references for further research in hopes of discovering new potent inhibitors of ABCG2.

Multivalent glycoligands with lectin/enzyme dual specificity: self-deliverable glycosidase regulators.

Multivalent mannosides with inherent macrophage recognition abilities, built on ß-cyclodextrin, RAFT cyclopeptide or peptide dendrimer cores, trigger selective inhibition of lysosomal ß-glucocerebrosidase or α-mannosidase depending on valency and topology, offering new opportunities in multitargeted drug design.