Growing impact of sialic acid-containing glycans in future drug discovery

In nature, almost all cells are covered with a complex array of glycan chain namely sialic acids or nuraminic acids, a negatively charged nine carbon sugars which is considered for their great therapeutic importance since long back. Owing to its presence at the terminal end of lipid bilayer (commonly known as terminal sugars), the well-defined sialosides or sialoconjugates have served pivotal role on the cell surfaces and thus, the sialic acid-containing glycans can modulate and mediate a number of imperative cellular interactions. Understanding of the sialo-protein interaction and their roles in vertebrates in regard of normal physiology, pathological variance, and evolution has indeed a noteworthy journey in medicine. In this tutorial review, we present a concise overview about the structure, linkages in chemical diversity, biological significance followed by chemical and enzymatic modification/synthesis of sialic acid containing glycans. A more focus is attempted about the recent advances, opportunity, and more over growing impact of sialosides and sialoconjugates in future drug discovery and development.

Oseltamivir modified bovine serum albumin inhibits neuraminidase activity and accumulates virion particles to disturb influenza virus replication

The preparation of oseltamivir-bovine serum albumin conjugate (OS-BSA) for use as a multivalent influenza neuraminidase (NA) inhibitor is reported. Briefly, the oseltamivir azidohexyl ester was synthesized and covalently bound via an orthogonal attachment to bicyclononyne-modified BSA using copper-free click chemistry. Primary antiviral assays on NA protein and cellular levels showed that the synthetic multivalent OS-BSA conjugate was a more effective inhibitor than monomeric OS azidohexyl ester. Further investigation of the antiviral mechanism found that the prepared OS-BSA could not only be used as a multivalent NA inhibitor but also acted as an adsorbent for the aggregation of virion particles, contributing to the inhibition of the influenza viral replication cycle. Our findings provide insight into the antiviral mechanism of multivalent NA inhibitors and form a basis for the development of novel antiviral agents.

Novel β-Cyclodextrin-Based Heptavalent Glycyrrhetinic Acid Conjugates: Synthesis, Characterization, and Anti-Influenza Activity

In our continuing efforts toward the design of novel pentacyclic triterpene derivatives as potential anti-influenza virus entry inhibitors, a series of homogeneous heptavalent glycyrrhetinic acid derivatives based on β-cyclodextrin scaffold were designed and synthesized by click chemistry. The structure was unambiguously characterized by NMR, IR, and MALDI-TOF-MS measurements. Seven conjugates showed sufficient inhibitory activity against influenza virus infection based on the cytopathic effect reduction assay with IC50 values in the micromolar range. The interactions of conjugate 37, the most potent compound (IC50 = 2.86 μM, CC50 > 100 μM), with the influenza virus were investigated using the hemagglutination inhibition assay. Moreover, the surface plasmon resonance assay further confirmed that compound 37 bound to the influenza HA protein specifically with a dissociation constant of 5.15 × 10−7 M. Our results suggest the promising role of β-cyclodextrin as a scaffold for preparing a variety of multivalent compounds as influenza entry inhibitors.

Multivalent peptide dendrimers inhibit the fusion of viral-cellular membranes and the cellular NF-κB signaling pathway

The binding of the influenza A virus (IAV) to host cells is multivalent interactions between the hemagglutinin (HA) trimer and sialic acid residues on the cell surface, which present a challenge for the development of efficient antiviral drugs interfering with the entry of IAV into host cells. In this study, a number of multivalent peptide dendrimers targeting the HA2 subunit of HA to block the fusion between viral-cellular membranes have been created, of which FMOC-4-KKWK showed the lowest cytotoxicity, while in the nanomolar concentration range of antiviral effects. In addition to being active against a panel of various subtypes of influenza viruses, these dendrimers reduced the levels of NF-κB in RAW 264.7 cells and inhibited the overexpression of proinflammatory cytokines of TNF-α, IL-1β, and IL-6 that are associated with the influenza infection. Further tests in mice infected with a lethal dose of PR8 virus showed that these dendrimers increased the survival rate of mice, and reduced the viral load in the lungs. Significantly, this is the first report describing peptide dendrimers that target the HA2 subunit of IAV, differing from those using carbohydrates as ligands to block the adsorption of viruses to host cells.

The disulfide bond as a key motif for the construction of multivalent glycoclusters

S-Glycosylated dendrons having a thioacetate group in their focal points led to multivalent glycoclusters by spontaneous O2-oxidation of sulfides.

Cu(I)-Catalyzed Click Chemistry in Glycoscience and Their Diverse Applications

Copper(I)-catalyzed 1,3-dipolar cycloaddition between organic azides and terminal alkynes, commonly known as CuAAC or click chemistry, has been identified as one of the most successful, versatile, reliable, and modular strategies for the rapid and regioselective construction of 1,4-disubstituted 1,2,3-triazoles as diversely functionalized molecules. Carbohydrates, an integral part of living cells, have several fascinating features, including their structural diversity, biocompatibility, bioavailability, hydrophilicity, and superior ADME properties with minimal toxicity, which support increased demand to explore them as versatile scaffolds for easy access to diverse glycohybrids and well-defined glycoconjugates for complete chemical, biochemical, and pharmacological investigations. This review highlights the successful development of CuAAC or click chemistry in emerging areas of glycoscience, including the synthesis of triazole appended carbohydrate-containing molecular architectures (mainly glycohybrids, glycoconjugates, glycopolymers, glycopeptides, glycoproteins, glycolipids, glycoclusters, and glycodendrimers through regioselective triazole forming modular and bio-orthogonal coupling protocols). It discusses the widespread applications of these glycoproducts as enzyme inhibitors in drug discovery and development, sensing, gelation, chelation, glycosylation, and catalysis. This review also covers the impact of click chemistry and provides future perspectives on its role in various emerging disciplines of science and technology.

Concise and Reliable Syntheses of Glycodendrimers via Self-Activating Click Chemistry: A Robust Strategy for Mimicking Multivalent Glycan-Pathogen Interactions

Individual interactions between glycans and their receptors are usually weak, although these weak interactions can combine to realize a strong interaction (multivalency). Such multivalency plays a crucial role in the recognition of host cells by pathogens. Glycodendrimers are useful materials for the reconstruction of this multivalent interaction. However, the introduction of a large number of glycans to a dendrimer core is fraught with difficulties. We herein synthesized antipathogenic glycodendrimers using the self-activating click chemistry (SACC) method developed by our group. The excellent reactivity of SACC enabled the efficient preparation of sialyl glycan and Gb3 glycan dendrimers, which exhibited strong avidity toward hemagglutinin on influenza virus and Shiga toxin B subunit produced by Escherichia coli, respectively. We demonstrated the usefulness of SACC-based glycodendrimers as antipathogenic compounds.

Selected Research Topics of the Dondoni Group over the Last Two Decades (2000–2020)

From a selection of research topics carried out in our laboratory during the last twenty years it becomes apparent that our main target was the discovery of new or improved synthetic methods together with new properties. Our efforts were made with the aim of being of some utility to other fields of research, with particular emphasis to glycobiology and heterocyle-based bioorganic chemistry. We performed new chemistry mainly in the field of carbohydrate manipulations taking as a primary rule the simplicity and efficiency manners. Toward this end, modern synthetic tools and approaches were employed such as heterocyle-based transformations, multicomponent reactions, organocatalysis, click azide–alkyne cycloadditions, reactions in ionic liquids, click photoinduced thiol-ene coupling, and click sulfur–fluoride exchange chemistry. With these potent methodologies in hand, the syntheses of carbohydrate containing amino acids up to proteins glycosylation were performed.

1 Heterocyclic Glycoconjugates and Amino Acids

2 Triazole-Linked Oligonucleotides: Application of Click CuAAC

3 Non-Natural Glycosyl Amino Acids

4 Non-Natural Oligosaccharides

5 Calixarene-Based Glycoclusters

6 Carbohydrate-Based Building Blocks

7 Homoazasugars and Aza-C-disaccharides

8 Synthesis of Glycodendrimers

9 Peptide and Protein Glycoconjugates

10 Conclusions

Calix[4]API-s: fully functionalized calix[4]arene-based facial active pharmaceutical ingredients

This mini-review covers 25 fully functionalized facial calix[4]arene-based symmetrical and conical cyclic tetramers with significant (comparable to established therapeutic agents) anticancer and anti-infective activities. The main role of the calixarene scaffold in these calix[4]arene-based active pharmaceutical ingredients (calix[4]API-s) is to replicate embedded phenolic units in the cyclic tetramers. So, probably owing to the multivalency, facial, conical structures of calix[4]API-s and synergistic effect of their four replicated units, they can be considered as effective bioactive agents.

Enhanced Inhibition of Influenza A Virus Adhesion by Di- and Trivalent Hemagglutinin Inhibitors

Multivalent carbohydrate-based ligands were synthesized and evaluated as inhibitors of the adhesion protein HA of the influenza A virus (IAV). HA relies on multivalency for strong viral adhesion. While viral adhesion inhibition by large polymeric molecules has proven viable, limited success was reached for smaller multivalent compounds. By linking of sialylated LAcNAc units to di- and trivalent scaffolds, inhibitors were obtained with an up to 428-fold enhanced inhibition in various assays.

Triazolated calix[4]arenes from 2-azidoethylated precursors: is there a difference in the way the triazoles are attached to narrow rims?

A large series of narrow-rim 4-R-1-triazolated calix[4]arenes was prepared, and these compounds were compared in terms of their cation-binding ability with the ‘inverted’ 1-R-4-triazolated calix[4]arenes.

A new and informative [a,b,c,d] nomenclature for one-pot multistep transformations: a simple tool to measure synthetic efficiency

Domino, cascade and tandem reactions constitute the most efficient and creative chemical transformations with a huge domain of synthetic utility and applications. A number of reactions may be achieved in a single pot, accompanied by the formation of new rings and new bonds, leading towards higher molecular complexity. A lack of one unified, yet informative descriptor often understates the synthetic ingenuity of certain highly creative transformations. In this review, we propose a new tetra-coordinated [a,b,c,d] nomenclature which takes into account and displays the basic parameters which generally indicate the level of efficiency of a chemical transformation. An almost exhaustive set of one-pot multistep reactions may be described by this system and this review is an attempt to display the one-pot multistep transformations reported from our group and to classify them based on our proposed descriptor.

Nanoscale materials for probing the biological functions of the glycocalyx

Glycans are among the most intriguing carriers of biological information in living systems. The structures of glycans not only convey the cells' physiological state, but also regulate cellular communication and responses by engaging receptors on neighboring cells and in the extracellular matrix. The assembly of simple monosaccharide building blocks into linear or branched oligo- and polysaccharides gives rise to a large repertoire of diverse glycan structures. Despite their structural complexity, individual glycans rarely engage their protein partners with high affinity. Yet, glycans modulate biological processes with exquisite selectivity and specificity. To correctly evaluate glycan interactions and their biological consequences, one needs to look beyond individual glycan structures and consider the entirety of the cell-surface landscape. There, glycans are presented on protein scaffolds, or are linked directly to membrane lipids, forming a complex, hierarchically organized network with specialized functions, called the glycocalyx. Nanoscale glycomaterials, which can mimic the various components of the glycocalyx, have been instrumental in revealing how the presentation of glycans can influence their biological functions. In this review, we wish to highlight some recent developments in this area, while placing emphasis on the applications of glycomaterials providing new insights into the mechanisms through which glycans mediate cellular functions.

Synthesis of a New Series of Sialylated Homo- and Heterovalent Glycoclusters by using Orthogonal Ligations

The synthesis of heteroglycoclusters (hGCs) is being subjected to rising interest, owing to their potential applications in glycobiology. In this paper, we report an efficient and straightforward convergent protocol based on orthogonal chemoselective ligations to prepare structurally well-defined cyclopeptide-based homo- and heterovalent glycoconjugates displaying 5-N-acetyl-neuraminic acid (Neu5Ac), galactose (Gal), and/or N-acetyl glucosamine (GlcNAc). We first used copper-catalyzed azide-alkyne cycloaddition and/or thiol-ene coupling to conjugate propargylated α-sialic acid 3, β-GlcNAc thiol 5, and β-Gal thiol 6 onto cyclopeptide scaffolds 7-9 to prepare tetravalent homoglycoclusters (10-12) and hGCs (13-14) with 2:2 combinations of sugars. In addition, we have demonstrated that 1,2-diethoxycyclobutene-3,4-dione can be used as a bivalent linker to prepare various octavalent hGCs (16, 19, and 20) in a controlled manner from these tetravalent structures.

Cu-Catalyzed Click Reaction in Carbohydrate Chemistry

Cu(I)-catalyzed azide-alkyne 1,3-dipolar cycloaddition (CuAAC), popularly known as the "click reaction", serves as the most potent and highly dependable tool for facile construction of simple to complex architectures at the molecular level. Click-knitted threads of two exclusively different molecular entities have created some really interesting structures for more than 15 years with a broad spectrum of applicability, including in the fascinating fields of synthetic chemistry, medicinal science, biochemistry, pharmacology, material science, and catalysis. The unique properties of the carbohydrate moiety and the advantages of highly chemo- and regioselective click chemistry, such as mild reaction conditions, efficient performance with a wide range of solvents, and compatibility with different functionalities, together produce miraculous neoglycoconjugates and neoglycopolymers with various synthetic, biological, and pharmaceutical applications. In this review we highlight the successful advancement of Cu(I)-catalyzed click chemistry in glycoscience and its applications as well as future scope in different streams of applied sciences.

Pentacyclic triterpenes grafted on CD cores to interfere with influenza virus entry: A dramatic multivalent effect

Multivalent effect plays an important role in biological processes, particularly in the specific recognition of virus with its host cell during the first step of infection. Here we report the synthesis of multivalent pentacyclic triterpene grafted on cyclodextrin core and potency of against influenza entry activity. Nine star-shaped compounds containing six, seven and eight pentacyclic triterpene pharmacophore on cyclodextrin scaffold were prepared by way of copper-catalyzed azide-alkyl cycloaddition reaction under microwave activation. Some of the multimers exhibited much potent antiviral activity against H1N1 virus (A/WSN/33), even equivalent or superior to oseltamivir. The most active compound 31, a heptavalent oleanolic acid-β-cyclodextrin conjugate, shows an up to 125-fold potency enhancement by its IC50 value over the corresponding monovalent conjugate and oleanolic acid, disclosing a clear multivalent effect. Further studies show that three compounds 31-33 exhibited broad spectrum inhibitory activity against other two human influenza A/JX/312 (H3N2) and A/HN/1222 (H3N2) viruses with the IC50 values at 2.47-14.90 μM. Most importantly, we found that compound 31, one of the best representative conjugate, binds tightly to the viral envelope hemagglutinin with a dissociation constant of KD = 2.08 μM, disrupting the interaction of hemagglutinin with the sialic acid receptor and thus the attachment of viruses to host cells. Our study might establish a strategy for the design of new pharmaceutical agents based on multivalency so as to block influenza virus entry into host cells.

Synthesis and biological properties of multivalent iminosugars

Clustering 1-deoxynojirimycin (DNJ), first isolated from white mulberry, and other iminosugars around various scaffolds gave strong glycosidase inhibitors.

Calixarene-based chemosensors by means of click chemistry

Click chemistry, a new strategy for organic chemistry, has been widely used in the chemical modification of calixarenes because of its reliability, specificity, biocompatibility, and efficiency. Click-derived triazoles also play a critical role in sensing ions and molecules. This in-depth review provides an overview of calixarene-based chemosensors that incorporate click-derived triazoles, and their three characteristics (chromogenic, fluorescence, and wettability) are reviewed.

Protein surface recognition by calixarenes

The present review summarizes recently developed calixarene derivatives for protein surface recognition which are able to identify, inhibit, and separate specific proteins.

Design and synthesis of glycoprotein-based multivalent glyco-ligands for influenza hemagglutinin and human galectin-3

We report a facile synthesis of glycoprotein-based glyco-ligands and their binding with influenza hemagglutinin and human galectin-3. Human serum albumin (HSA) was used as the scaffold and an Asn-linked complex type N-glycan prepared from chicken eggs was used as the glycan building block. It was found that Cu(I)-catalyzed alkyne-azide cycloaddition reaction (click chemistry) between the alkyne-labeled glycan and the azide-tagged HSA led to an efficient formation of the glycoconjugates. The density of glycan ligands on the protein scaffold was readily varied by changing the molar ratios of the two reactants. Binding studies indicated that the sialylated and desialylated multivalent glycoligands could selectively bind to influenza hemagglutinin and human galectin-3, respectively, with high affinity. In the two glycan-lectin interactions, a clear multivalent effect was observed. Moreover, a cell-based assay showed that the synthetic multivalent glyco-ligands could efficiently inhibit the attachment of galectin-3 to human prostate cancer and lung cancer cell lines. This study suggests that the synthetic glycoprotein-based glyco-ligands can be useful for different applications, including blocking the function of galectin-3 in cancer metastasis.

Analysis of carbohydrates and glycoconjugates by matrix-assisted laser desorption/ionization mass spectrometry: an update for 2007-2008

This review is the fifth update of the original review, published in 1999, on the application of MALDI mass spectrometry to the analysis of carbohydrates and glycoconjugates and brings coverage of the literature to the end of 2008. The first section of the review covers fundamental studies, fragmentation of carbohydrate ions, use of derivatives and new software developments for analysis of carbohydrate spectra. Among newer areas of method development are glycan arrays, MALDI imaging and the use of ion mobility spectrometry. The second section of the review discusses applications of MALDI MS to the analysis of different types of carbohydrate. Specific compound classes that are covered include carbohydrate polymers from plants, N- and O-linked glycans from glycoproteins, biopharmaceuticals, glycated proteins, glycolipids, glycosides and various other natural products. There is a short section on the use of MALDI mass spectrometry for the study of enzymes involved in glycan processing and a section on the use of MALDI MS to monitor products of the chemical synthesis of carbohydrates with emphasis on carbohydrate-protein complexes and glycodendrimers. Corresponding analyses by electrospray ionization now appear to outnumber those performed by MALDI and the amount of literature makes a comprehensive review on this technique impractical. However, most of the work relating to sample preparation and glycan synthesis is equally relevant to electrospray and, consequently, those proposing analyses by electrospray should also find material in this review of interest.

Design and synthesis of high-avidity tetravalent glycoclusters as probes for Sambucus sieboldiana agglutinin and characterization of their binding properties

We designed and synthesized three tetravalent sialo-glycoclusters that had different separations between the terminal sialic acids and the linking carboxy groups of the ethylene glycol bis(β-aminoethyl ether)-N,N,N',N'-tetraacetate scaffold to serve as ligands for the sialic acid-binding lectin Sambucus sieboldiana agglutinin (SSA). The interaction between each glycocluster and SSA was characterized by hemagglutination inhibition, quantitative precipitation, and double-diffusion assays. For the precipitation assays, the precipitin curves indicated that the ligands and SSA bound in either a 1:1 or a 1:2 ratio, i.e., stoichiometrically. The strong interactions of these sialo-glycoclusters with SSA could be ascribed to a combination of multivalency and spacer effects. We also assessed the nature of the ligand-SSA complexes by isothermal titration calorimetry and dynamic light scattering. The results of those experiments indicated that formation of intermolecular complexes occurred at less than stoichiometric ratios of ligand to SSA concentrations and that, as the concentrations of the ligands increased, larger cross-linked aggregates formed. Large aggregates that were present concurrently with visible precipitation and with a particle size centered at ~600 to 800 nm were identified by dynamic light scattering.

Exhaustive glycosylation, PEGylation, and glutathionylation of a [G4]-ene(48) dendrimer via photoinduced thiol-ene coupling

We report in this paper the use of free-radical thiol-ene coupling (TEC) for the introduction of carbohydrate, poly(ethylene glycol), and peptide fragments at the periphery of an alkene functional dendrimer. Four different sugar thiols including glucose, mannose, lactose and sialic acid, two PEGylated thiols and the natural tripeptide glutathione were reacted with a fourth generation alkene functional dendrimer [G4]-ene(48) upon irradiation at λ(max) 365 nm. In all cases, the (1)H NMR spectra of the crude reaction mixture revealed the complete disappearance of alkene proton signals indicating the quantitative conversion of all 48 alkene groups of the dendrimer. With one exception only, all dendrimer conjugates were isolated in high yields (70-94%), validating the high efficiency of multiple TEC reactions on a single substrate. All isolated and purified compounds were analyzed by MALDI-TOF spectrometry and gave spectra consistent with the assigned structure.

Synthesis of a glycomimetic oligonucleotide conjugate by 1,3-dipolar cycloaddition

A glycomimetic oligonucleotide conjugate bearing four galactose residues on a mannose core is -synthesized using oligonucleotide solid-phase synthesis and Cu(I)-catalyzed azide-alkyne 1,3-dipolar cycloaddition (CuAAC, or "click" chemistry). To achieve this purpose, new building blocks (including the solid support and phosphoramidites) are synthesized and used on a DNA synthesizer to generate a tetraalkyne oligonucleotide, which is then conjugated with a galactose azide derivative by click chemistry to afford the desired 3'-tetragalactosyl-mannose oligonucleotide conjugate. The procedures described in this chapter provide a general approach for the synthesis of novel glycoconjugates that can be immobilized to a DNA chip via DNA-directed immobilization to study, for example, their multivalent interactions with lectins in cellular targeting/uptake, etc.

Heterocycles in organic synthesis: thiazoles and triazoles as exemplar cases of synthetic auxiliaries

This Perspective article illustrates the key role of thiazole and triazole in the work carried out in the author's laboratory over three decades and deals with the synthesis of carbohydrate-based bioactive molecules. The first part reports on the development of synthetic strategies exploiting the use of various thiazole-based reagents and the ready conversion of thiazole into the formyl group. After describing the chain elongation of monosaccharides into higher-carbon homologues, the synthesis of target natural and non-natural carbohydrates, or their ultimate precursors, is presented. These include some sphingoids, neuraminic and destomic acids, lincosamine, various 3-deoxy-2-ulosonic acids (KDO, KDN, iso-Neu4Ac), iminosugars (nojirimycin, mannojirimycin, galactostatin) and homoazasugars. Also prepared were the disaccharide subunit of bleomycin A(2) and the side-chain of taxol and taxotere.((R)) The use of 1,2,3-triazole is discussed in the second part of the paper. The service of this heterocycle that is easily formed by the Cu(i)-catalyzed azide-alkyne cycloaddition (CuAAC) is considered in light of its use as a robust linker (a sort of keystone) of complex and diverse molecular architectures. Thus, the assembly of triazole-linked glycosyl amino acids, non-natural nucleotides, 1,6-oligomannosides, sialoside clusters on calixarene platfom via CuAAC is described and the biological relevance of these compounds is discussed in brief.