Synthesis of a phenyl thio-β-d-galactopyranoside library from 1,5-difluoro-2,4-dinitrobenzene: discovery of efficient and selective monosaccharide inhibitors of galectin-7

New syntheses of thiosaccharides utilizing substitution reactions

Novel synthetic methods published since 2005 affording carbohydrates containing sulfur atom(s) are reviewed. The review is divided to subchapters based on the position of sulfur atom(s) in the sugar molecule. Only those methods that take advantage of substitution are discussed.

Targeting galectin-driven regulatory circuits in cancer and fibrosis

Galectins are a family of endogenous glycan-binding proteins that have crucial roles in a broad range of physiological and pathological processes. As a group, these proteins use both extracellular and intracellular mechanisms as well as glycan-dependent and independent pathways to reprogramme the fate and function of numerous cell types. Given their multifunctional roles in both tissue fibrosis and cancer, galectins have been identified as potential therapeutic targets for these disorders. Here, we focus on the therapeutic relevance of galectins, particularly galectin 1 (GAL1), GAL3 and GAL9 to tumour progression and fibrotic diseases. We consider an array of galectin-targeted strategies, including small-molecule carbohydrate inhibitors, natural polysaccharides and their derivatives, peptides, peptidomimetics and biological agents (notably, neutralizing monoclonal antibodies and truncated galectins) and discuss their mechanisms of action, selectivity and therapeutic potential in preclinical models of fibrosis and cancer. We also review the results of clinical trials that aim to evaluate the efficacy of galectin inhibitors in patients with idiopathic pulmonary fibrosis, nonalcoholic steatohepatitis and cancer. The rapid pace of glycobiology research, combined with the acute need for drugs to alleviate fibrotic inflammation and overcome resistance to anticancer therapies, will accelerate the translation of anti-galectin therapeutics into clinical practice.

Glycobiology of cellular expiry: Decrypting the role of glycan-lectin regulatory complex and therapeutic strategies focusing on cancer

Often the outer leaflets of living cells bear a coat of glycosylated proteins, which primarily regulates cellular processes. Glycosylation of such proteins occurs as part of their post-translational modification. Within the endoplasmic reticulum, glycosylation enables the attachment of specific oligosaccharide moieties such as, 'glycan' to the transmembrane receptor proteins which confers precise biological information for governing the cell fate. The nature and degree of glycosylation of cell surface receptors are regulated by a bunch of glycosyl transferases and glycosidases which fine-tune attachment or detachment of glycan moieties. In classical death receptors, upregulation of glycosylation by glycosyl transferases is capable of inducing cell death in T cells, tumor cells, etc. Thus, any deregulated alternation at surface glycosylation of these death receptors can result in life-threatening disorder like cancer. In addition, transmembrane glycoproteins and lectin receptors can transduce intracellular signals for cell death execution. Exogenous interaction of lectins with glycan containing death receptors signals for cell death initiation by modulating downstream signalings. Subsequently, endogenous glycan-lectin interplay aids in the customization and implementation of the cell death program. Lastly, the glycan-lectin recognition system dictates the removal of apoptotic cells by sending accurate signals to the extracellular milieu. Since glycosylation has proven to be a biomarker of cellular death and disease progression; glycans serve as specific therapeutic targets of cancers. In this context, we are reviewing the molecular mechanisms of the glycan-lectin regulatory network as an integral part of cell death machinery in cancer to target them for successful therapeutic and clinical approaches.

Investigations on the binding specificity of β-galactoside analogues with human galectin-1 using molecular dynamics simulations

Galectin-1 (Gal-1) is the first member of galectin family, which has a carbohydrate recognition domain, specifically binds towards β-galactoside containing oligosaccharides. Owing its association with carbohydrates, Gal-1 is involved in many biological processes such as cell signaling, adhesion and pathological pathways such as metastasis, apoptosis and increased tumour cell survival. The development of β-galactoside based inhibitors would help to control the Gal-1 expression. In the current study, we carried out molecular dynamics (MD) simulations to examine the structural and dynamic behaviour Gal-1-thiodigalactoside (TDG), Gal-1-lactobionic acid (LBA) and Gal-1-beta-(1→6)-galactobiose (G16G) complexes. The analysis of glycosidic torsional angles revealed that β-galactoside analogues TDG and LBA have a single binding mode (BM1) whereas G16G has two binding modes (BM1 and BM2) for interacting with Gal-1 protein. We have computed the binding free energies for the complexes Gal-1-TDG, Gal-1-LBA and Gal-1-G16G using MM/PBSA and are -6.45, -6.22 and -3.08 kcal/mol, respectively. This trend agrees well with experiments that the binding of Gal-1 with TDG is stronger than LBA. Further analysis revealed that the interactions due to direct and water-mediated hydrogen bonds play a significant role to the structural stability of the complexes. The result obtained from this study is useful to formulate a set of rules and derive pharmacophore-based features for designing inhibitors against galectin-1.Communicated by Ramaswamy H. Sarma.

Discovery and Optimization of the First Highly Effective and Orally Available Galectin-3 Inhibitors for Treatment of Fibrotic Disease

Galectin-3 is a carbohydrate-binding protein central to regulating mechanisms of diseases such as fibrosis, cancer, metabolic, inflammatory, and heart disease. We recently found a high affinity (nM) thiodigalactoside GB0139 which currently is in clinical development (PhIIb) as an inhaled treatment of idiopathic pulmonary fibrosis. To enable treatment of systemically galectin-3 driven disease, we here present the first series of selective galectin-3 inhibitors combining high affinity (nM) with oral bioavailability. This was achieved by optimizing galectin-3 specificity and physical chemical parameters for a series of disubstituted monogalactosides. Further characterization showed that this class of compounds reduced profibrotic gene expression in liver myofibroblasts and displayed antifibrotic activity in CCl₄-induced liver fibrosis and bleomycin-induced lung fibrosis mouse models. On the basis of the overall pharmacokinetic, pharmacodynamic, and safety profile, GB1211 was selected as the clinical candidate and is currently in phase IIa clinical trials as a potential therapy for liver cirrhosis and cancer.

Design, Synthesis, and Anticancer Activity of a Selenium-Containing Galectin-3 and Galectin-9N Inhibitor

Galectins are soluble β-D-galactoside-binding proteins whose implication in cancer progression and disease outcome makes them prominent targets for therapeutic intervention. In this frame, the development of small inhibitors that block selectively the activity of galectins represents an important strategy for cancer therapy which is, however, still relatively underdeveloped. To this end, we designed here a rationally and efficiently novel diglycosylated compound, characterized by a selenoglycoside bond and the presence of a lipophilic benzyl group at both saccharide residues. The relatively high binding affinity of the new compound to the carbohydrate recognition domain of two galectins, galectin 3 and galectin 9, its good antiproliferative and anti-migration activity towards melanoma cells, as well as its anti-angiogenesis properties, pave the way for its further development as an anticancer agent.

Functions and Inhibition of Galectin-7, an Emerging Target in Cellular Pathophysiology

Galectin-7 is a soluble unglycosylated lectin that is able to bind specifically to β-galactosides. It has been described to be involved in apoptosis, proliferation and differentiation, but also in cell adhesion and migration. Several disorders and diseases are discussed by covering the aforementioned biological processes. Structural features of galectin-7 are discussed as well as targeting the protein intracellularly or extracellularly. The exact molecular mechanisms that lie behind many biological processes involving galectin-7 are not known. It is therefore useful to come up with chemical probes or tools in order to obtain knowledge of the physiological processes. The objective of this review is to summarize the roles and functions of galectin-7 in the human body, providing reasons why it is necessary to design inhibitors for galectin-7, to give the reader structural insights and describe its current inhibitors.

Structure-Guided Design of d-Galactal Derivatives with High Affinity and Selectivity for the Galectin-8 N-Terminal Domain

Galectin-8 is a carbohydrate-binding protein that plays a crucial role in tumor progression and metastasis, antibacterial autophagy, modulation of the immune system, and bone remodeling. The design, synthesis, and protein affinity evaluation of a set of C-3 substituted benzimidazole and quinoline d-galactal derivatives identified a d-galactal-benzimidazole hybrid as a selective ligand for the galectin-8 N-terminal domain (galectin-8N), with a K d of 48 μM and 15-fold selectivity over galectin-3 and even better selectivity over the other mammalian galectins. X-ray structural analysis of galectin-8N in complex with one benzimidazole- and one quinoline-galactal derivative at 1.52 and 2.1 Å together with molecular dynamics simulations and quantum mechanical calculations of galectin-8N in complex with the benzimidazole derivative revealed orbital overlap between a NH LUMO of Arg45 with electron rich HOMOs of the olefin and O4 of the d-galactal. Such overlap is hypothesized to contribute to the high affinity of the d-galactal-derived ligands for galectin-8N. A (3-(4,5-dimethylthiazol-2-yl)-5-(3- carboxymethoxyphenyl)-2-(4-sulfophenyl)-2H-tetrazolium) (MTS) assay evaluation of the d-galactal-benzimidazole hybrid and an analogous galactoside derivative on a panel of cell lines with MTS assay showed no effect on cell viability up to 100 μM concentration. A subsequent functional assay using the MDA-MB-231 cell line demonstrated that the d-galactal-benzimidazole hybrid and the analogous galactoside derivative reduced the secretion of the proinflammatory cytokines interleukin-6 (IL-6) and IL-8 in a dose-dependent manner. Therefore, these compounds represent potential probes for galectin-8N pharmacology investigations and possibly promising leads for the design and synthesis of potent and selective galectin-8 inhibitors as potential antitumor and anti-inflammatory agents.

Selective Monovalent Galectin-8 Ligands Based on 3-Lactoylgalactoside

Galectin-8 has gained attention as a potential new pharmacological target for the treatment of various diseases, including cancer, inflammation, and disorders associated with bone mass reduction. To that end, new molecular probes are needed in order to better understand its role and its functions. Herein we aimed to improve the affinity and target selectivity of a recently published galectin-8 ligand, 3-O-[1-carboxyethyl]-β-d-galactopyranoside, by introducing modifications at positions 1 and 3 of the galactose. Affinity data measured by fluorescence polarization show that the most potent compound reached a K_D of 12 μM. Furthermore, reasonable selectivity versus other galectins was achieved, making the highlighted compound a promising lead for the development of new selective and potent ligands for galectin-8 as molecular probes to examine the protein's role in cell-based and in vivo studies.

Benzimidazole-galactosides bind selectively to the Galectin-8 N-Terminal domain: Structure-based design and optimisation

We have obtained the X-ray crystal structure of the galectin-8 N-terminal domain (galectin-8N) with a previously reported quinoline-galactoside ligand at a resolution of 1.6 Å. Based on this X-ray structure, a collection of galactosides derivatised at O3 with triazole, benzimidazole, benzothiazole, and benzoxazole moieties were designed and synthesised. This led to the discovery of a 3-O-(N-methylbenzimidazolylmethyl)-galactoside with a K_d of 1.8 μM for galectin-8N, the most potent selective synthetic galectin-8N ligand to date. Molecular dynamics simulations showed that benzimidazole-galactoside derivatives bind the non-conserved amino acid Gln47, accounting for the higher selectivity for galectin-8N. Galectin-8 is a carbohydrate-binding protein that plays a key role in pathological lymphangiogenesis, modulation of the immune system, and autophagy. Thus, the benzimidazole-derivatised galactosides represent promising compounds for studies of the pathological implications of galectin-8, as well as a starting point for the development of anti-tumour and anti-inflammatory therapeutics targeting galectin-8.

Coupling of N-tosylhydrazones with tetrazoles: synthesis of 2-β-D-glycopyranosylmethyl-5-substituted-2H-tetrazole type glycomimetics

Coupling reactions of O-peracylated 2,6-anhydro-aldose tosylhydrazones (C-(β-d-glycopyranosyl)formaldehyde tosylhydrazones) with tetrazoles were studied under metal-free conditions using thermic or microwave activation in the presence of different bases. The reactions proved highly regioselective and gave the corresponding, up-to-now unknown 2-β-d-glycopyranosylmethyl-2H-tetrazoles in 7-67% yields. The method can be applied to get new types of disaccharide mimetics, 5-glycosyl-2-glycopyranosylmethyl-2H-tetrazoles, as well. Galectin binding studies with C-(β-d-galactopyranosyl)formaldehyde tosylhydrazone and 2-(β-d-galactopyranosylmethyl)-5-phenyl-2H-tetrazole revealed no significant inhibition of any of these lectins.

Targeting the CRD F-face of Human Galectin-3 and Allosterically Modulating Glycan Binding by Angiostatic PTX008 and a Structurally Optimized Derivative

Calix[4]arene PTX008 is an angiostatic agent that inhibits tumor growth in mice by binding to galectin-1, a β-galactoside-binding lectin. To assess the affinity profile of PTX008 for galectins, we used ¹⁵ N,¹ H HSQC NMR spectroscopy to show that PTX008 also binds to galectin-3 (Gal-3), albeit more weakly. We identified the contact site for PTX008 on the F-face of the Gal-3 carbohydrate recognition domain. STD NMR revealed that the hydrophobic phenyl ring crown of the calixarene is the binding epitope. With this information, we performed molecular modeling of the complex to assist in improving the rather low affinity of PTX008 for Gal-3. By removing the N-dimethyl alkyl chain amide groups, we produced PTX013 whose reduced alkyl chain length and polar character led to an approximately eightfold stronger binding than PTX008. PTX013 also binds Gal-1 more strongly than PTX008, whereas neither interacts strongly, if at all, with Gal-7. In addition, PTX013, like PTX008, is an allosteric inhibitor of galectin binding to the canonical ligand lactose. This study broadens the scope for galectin targeting by calixarene-based compounds and opens the perspective for selective galectin blocking.

Epimers Switch Galectin-9 Domain Selectivity: 3N-Aryl Galactosides Bind the C-Terminal and Gulosides Bind the N-Terminal

A series of 3-deoxy-3-N-arylated-β-d-galactoside and -guloside derivatives have been synthesized by cesium fluoride/trimetylsilylaryl triflate-mediated benzyne generation and N-arylation of 3-deoxy-3-amino-β-d-galactosides and -gulosides, respectively. Evaluation as ligands to galectin-1, 2, 3, 4N (N-terminal domain), 4C (C-terminal domain), 7, 8N, 8C, 9C, and 9N revealed that the galactosides selectively bound galectin-9C, whereas the gulosides selectively bound galectin-9N. Hence, the N-aryl group induces galectin-9 selectivity and the ligand 3C-configuration acts as an epimeric selectivity switch between the two domains of galectin-9. Furthermore, MD simulations revealed that galacto derivatives in galectin-9C and gulo derivatives in galectin-9N find stable poses with specific interactions, which proposes a possible explanation to the gal/gulo 9C/9N selectivity.

3-Substituted 1-Naphthamidomethyl-C-galactosyls Interact with Two Unique Sub-sites for High-Affinity and High-Selectivity Inhibition of Galectin-3

The galectins are a family of galactose-binding proteins playing key roles in inflammatory processes and cancer. However, they are structurally very closely related, and discovery of highly selective inhibitors is challenging. In this work, we report the design of novel inhibitors binding to a subsite unique to galectin-3, which confers both high selectivity and affinity towards galectin-3. Olefin cross metathesis between allyl β-C-galactopyranosyl and 1-vinylnaphthalenes or acylation of aminomethyl β-C-galactopyranosyl with 1-naphthoic acid derivatives gave C-galactopyranosyls carrying 1-naphthamide structural elements that interacted favorably with a galectin-3 unique subsite according to molecular modeling and X-ray structural analysis of two inhibitor-galectin-3 complexes. Affinities were down to sub-µM and selectivities over galectin-1, 2, 4 N-terminal domain, 4 C-terminal domain, 7, 8 N-terminal domain, 9 N-terminal domain, and 9 C-terminal domain were high. These results show that high affinity and selectivity for a single galectin can be achieved by targeting unique subsites, which holds promise for further development of small and selective galectin inhibitors.

Human Galectin-1 and Its Inhibitors: Privileged Target for Cancer and HIV

Galectin 1(Gal-1), a β-galactoside binding mammalian lectin of 14KDa, is implicated in many signalling pathways, immune responses associated with cancer progression and immune disorders. Inhibition of human Gal-1 has been regarded as one of the potential therapeutic approaches for the treatment of cancer, as it plays a major role in tumour development and metastasis by modulating various biological functions viz. apoptosis, angiogenesis, migration, cell immune escape. Gal-1 is considered as a biomarker in diagnosis, prognosis and treatment condition. The overexpression of Gal-1 is well established and seen in many types of cancer progression like osteosarcoma, breast, lung, prostate, melanoma, etc. Gal-1 greatly accelerates the binding kinetics of HIV-1 to susceptible cells, leading to faster viral entry and a more robust viral replication by specific binding of CD4 cells. Hence, the Gal-1 is considered a promising molecular target for the development of new therapeutic drugs for cancer and HIV. The present review laid emphasis on structural insights and functional role of Gal-1 in the disease, current Gal-1 inhibitors and future prospects in the design of specific Gal-1 inhibitors.

Combining Click Reactions for the One-Pot Synthesis of Modular Biomolecule Mimetics

Here, we report on the first combined one-pot use of the two so-called "click reactions": the thiol-ene coupling and the copper-catalyzed alkyne-azide cycloaddition. These reactions were employed in an alternating and one-pot fashion to combine appropriately functionalized monomeric carbohydrate building blocks to create mimics of trisaccharides and tetrasaccharides as single anomers, with only minimal purification necessary. The deprotected oligosaccharide mimics were found to bind both plant lectins and human galectin-3.

A Galactoside-Binding Protein Tricked into Binding Unnatural Pyranose Derivatives: 3-Deoxy-3-Methyl-Gulosides Selectively Inhibit Galectin-1

Galectins are a family of galactoside-recognizing proteins involved in different galectin-subtype-specific inflammatory and tumor-promoting processes, which motivates the development of inhibitors that are more selective galectin inhibitors than natural ligand fragments. Here, we describe the synthesis and evaluation of 3-C-methyl-gulopyranoside derivatives and their evaluation as galectin inhibitors. Methyl 3-deoxy-3-C-(hydroxymethyl)-β-d-gulopyranoside showed 7-fold better affinity for galectin-1 than the natural monosaccharide fragment analog methyl β-d-galactopyranoside, as well as a high selectivity over galectin-2, 3, 4, 7, 8, and 9. Derivatization of the 3-C-hydroxymethyl into amides gave gulosides with improved selectivities and affinities; methyl 3-deoxy-3-C-(methyl-2,3,4,5,6-pentafluorobenzamide)-β-d-gulopyranoside had K_d 700 µM for galectin-1, while not binding any other galectin.

Aminopyrimidine-galactose hybrids are highly selective galectin-3 inhibitors

Galactopyranosides with aryl-aminopyrimidine moieties at O3 inhibit the tumor and immunity-related galectin-3 with high selectivity over other galectins.

Galectins are a family of carbohydrate recognition proteins involved in, among other things, modulating cell signalling and cell–environment interactions, giving them roles in several pathologies like cancer and idiopathic lung fibrosis. Hence, developing new galectin inhibitors with high affinity and high selectivity is important to be able to target such diseases. Most existing galectin inhibitors have a disaccharide scaffold, but there has been success as of late in developing monogalactoside inhibitors such as α-arylthioglycosides. Here, we report aminopyrimidine-derivatised galactosides as good galectin-3 inhibitors with affinities down to 1.7 μM and a more than 300-fold selectivity over galectin-1. Mutant studies replacing Arg144 in galectin-3 with lysine and serine support the hypothesis that the binding of the derivatives involves interactions with Arg144. Molecular dynamics simulations converged to stable poses of the inhibitor aminopyrimidine moiety with polar interactions with Asp148 and Ser237, while the aryl-aminopyrimidine ring stacked onto the side chain of Arg144. Hence, combining an aminopyrimidine motif with a phenyl α-thiogalactoside motif offers an attractive route towards highly selective galectin-3 inhibitors.

Stereo- and regioselective hydroboration of 1-exo-methylene pyranoses: discovery of aryltriazolylmethyl C-galactopyranosides as selective galectin-1 inhibitors

Galectins are carbohydrate recognition proteins that bind carbohydrates containing galactose and are involved in cell signaling and cellular interactions, involving them in several diseases. We present the synthesis of (aryltriazolyl)methyl galactopyranoside galectin inhibitors using a highly diastereoselective hydroboration of C1-exo-methylene pyranosides giving inhibitors with fourfold or better selectivity for galectin-1 over galectin-3, -4C (C-terminal CRD), -4N (N-terminal CRD), -7, -8C, -8N, -9C, and -9N and dissociation constants down to 170 µM.

Decoding the sweet regulation of apoptosis: the role of glycosylation and galectins in apoptotic signaling pathways

Glycosylation and glycan-binding proteins such as galectins play an important role in the control of cell death signaling. Strikingly, very little attention has been given so far to the understanding of the molecular details behind this key regulatory network. Glycans attached to the death receptors such as CD95 and TRAIL-Rs, either alone or in a complex with galectins, might promote or inhibit apoptotic signals. However, we have just started to decode the functions of galectins in the modulation of extrinsic and intrinsic apoptosis. In this work, we have discussed the current understanding of the glycosylation-galectin regulatory network in CD95- as well as TRAIL-R-induced apoptosis and therapeutic strategies based on targeting galectins in cancer.

Quinoline-galactose hybrids bind selectively with high affinity to a galectin-8 N-terminal domain

Quinolines, indolizines, and coumarins are well known structural elements in many biologically active molecules. In this report, we have developed straightforward methods to incorporate quinoline, indolizine, and coumarin structures into galactoside derivatives under robust reaction conditions for the discovery of glycomimetic inhibitors of the galectin family of proteins that are involved in immunological and tumor-promoting biological processes. Evaluation of the quinoline, indolizine and coumarin-derivatised galactosides as inhibitors of the human galectin-1, 2, 3, 4N (N-terminal domain), 4C (C-terminal domain), 7, 8N, 8C, 9N, and 9C revealed quinoline derivatives that selectively bound galectin-8N, a galectin with key roles in lymphangiogenesis, tumor progression, and autophagy, with up to nearly 60-fold affinity improvements relative to methyl β-d-galactopyranoside. Molecular dynamics simulations proposed an interaction mode in which Arg59 had moved 2.5 Å and in which an inhibitor carboxylate and quinoline nitrogen formed structure-stabilizing water-mediated hydrogen bonds. The compounds were demonstrated to be non-toxic in an MTT assay with several breast cancer cell lines and one normal cell line. The improved affinity, selectivity, and low cytotoxicity suggest that the quinoline-galactoside derivatives provide an attractive starting point for the development of galectin-8N inhibitors potentially interfering with pathological lymphangiogenesis, autophagy, and tumor progression.

Substituted polyfluoroaryl interactions with an arginine side chain in galectin-3 are governed by steric-, desolvation and electronic conjugation effects

In the β-d-galactopyranoside-binding protein galectin-3, synthetic inhibitors substituted at the 3-position of a thiodigalactoside core cause the formation of an aglycone binding pocket through the displacement of an arginine residue (Arg144) from its position in the apoprotein. To examine in detail the role of different molecular interactions in this pocket, we have synthesized a series of nine 3-(4-(2,3,5,6-tetrafluorophenyl)-1,2,3-triazol-1-yl)-thiogalactosides with different para substituents and measured their affinities to galectin-3 using a fluorescence polarization assay. High-resolution crystal structures (<1.3 Å) have been determined for five of the ligands in complex with the C-terminal domain of galectin-3. The binding affinities are rationalised with the help of the three-dimensional structures and quantum-mechanical calculations. Three effects seem to be involved: Firstly, the binding pocket is too small for the largest ligands with ethyl and methyl. Secondly, for the other ligands, the affinity seems to be determined mainly by desolvation effects, disfavouring the polar substituents, but this is partly counteracted by the cation-π interaction with Arg144, which stacks on top of the substituted tetrafluorophenyl group in all complexes. The results provide detailed insight into interactions of fluorinated phenyl moieties with arginine-containing protein binding sites and the complex interplay of different energetic components in defining the binding affinity.

Galectin-Carbohydrate Interactions in Biomedicine and Biotechnology

Cellular communication events are mediated by interactions between cell-surface sugars and lectins, which are carbohydrate-binding proteins. Galectins are β-galactosyl-binding lectins that bridge molecules by their sugar moieties, forming a signaling and adhesion network. Severe changes in glycosylation and galectin expression accompany major processes in oncogenesis, cardiovascular disorders, and other pathologies, making galectins attractive therapeutic targets. Here we discuss advanced strategies of chemo-enzymatic carbohydrate synthesis for creating lead glycomimetics and (neo-)glycoconjugates for galectin-1 and -3 targeting in biomedicine and biotechnology. We will describe the challenges and bottlenecks on the route into biomedical and biotechnological practice and present the first clinical candidates. The coming era will see an exciting translation of selective well-defined high-affinity galectin ligands from bench to bedside.

Aromatic heterocycle galectin-1 interactions for selective single-digit nM affinity ligands

A series of 3-triazole-thiogalactosides and 3,3′-triazole-thiodigalactosides substituted with different five-membered heterocycles at the C-4 triazole position were found to have high selectivity for galectin-1. Initial studies on the 3-triazole-thiogalactosides indicated that five membered heterocycles in general gave increased affinity for galectin-1 and improved selectivity over galectin-3. The selectivity profile was similar for thiodigalactosides exemplified by 3,3′ substituted thien-3-yltriazole and thiazol-2-yltriazole, both having single-digit nM galectin-1 affinity and almost 10-fold galectin-1 selectivity. The binding interactions of a thiodigalactoside based galectin-1 inhibitor with two thien-3-yltriazole moieties were studied with X-ray crystallography. One of the thiophene moieties was positioned deeper into the pocket than previously reported phenyltriazoles and formed close contacts with Val31, Ser29, Gly124, and Asp123. The affinity and structural analysis thus revealed that steric and electronic optimization of five-membered aromatic heterocycle binding in a narrow galectin-1 subsite confers high affinity and selectivity.

A series of 3-triazole-thiogalactosides and 3,3′-triazole-thiodigalactosides substituted with different five-membered heterocycles at the C-4 triazole position were found to have high selectivity for galectin-1.

Galectins as Molecular Targets for Therapeutic Intervention

Galectins are a family of small, highly conserved, molecular effectors that mediate various biological processes, including chemotaxis and angiogenesis, and that function by interacting with various cell surface glycoconjugates, usually targeting β-galactoside epitopes. Because of their significant involvement in various biological functions and pathologies, galectins have become a focus of therapeutic discovery for clinical intervention against cancer, among other pathological disorders. In this review, we focus on understanding galectin structure-function relationships, their mechanisms of action on the molecular level, and targeting them for therapeutic intervention against cancer.

Assessing the stability of free-energy perturbation calculations by performing variations in the method

We have calculated relative binding affinities for eight tetrafluorophenyl-triazole-thiogalactoside inhibitors of galectin-3 with the alchemical free-energy perturbation approach. We obtain a mean absolute deviation from experimental estimates of only 2-3 kJ/mol and a correlation coefficient (R²) of 0.5-0.8 for seven relative affinities spanning a range of up to 11 kJ/mol. We also studied the effect of using different methods to calculate the charges of the inhibitor and different sizes of the perturbed group (the atoms that are described by soft-core potentials and are allowed to have differing coordinates). However, the various approaches gave rather similar results and it is not possible to point out one approach as consistently and significantly better than the others. Instead, we suggest that such small and reasonable variations in the computational method can be used to check how stable the calculated results are and to obtain a more accurate estimate of the uncertainty than if performing only one calculation with a single computational setup.

Thiodigalactoside-Bovine Serum Albumin Conjugates as High-Potency Inhibitors of Galectin-3: An Outstanding Example of Multivalent Presentation of Small Molecule Inhibitors

Galectin inhibitors are urgently needed to understand the mode of action and druggability of different galectins, but potent and selective agents still evade researchers. Small-sized inhibitors based on thiodigalactoside (TDG) have shown their potential while modifications at their C3 position indicated a strategy to improve selectivity and potency. Considering the role of galectins as glycoprotein traffic police, involved in multivalent bridging interactions, we aimed to create multivalent versions of the potent TDG inhibitors. We herein present for the first time the multivalent attachment of a TDG derivative using bovine serum albumin (BSA) as the scaffold. An efficient synthetic method is presented to obtain a novel type of neoglycosylated proteins loaded with different numbers of TDG moieties. A polyethylene glycol (PEG)-spacer is introduced between the TDG and the protein scaffold maintaining appropriate accessibility for an adequate galectin interaction. The novel conjugates were evaluated in galectin binding and inhibition studies in vitro. The conjugate with a moderate density of 19 conjugated TDGs was identified as one of the most potent multivalent Gal-3 inhibitors so far, with a clear demonstration of the benefit of a multivalent ligand presentation. The described method may facilitate the development of specific galectin inhibitors and their application in biomedical research.

Monosaccharide Derivatives with Low-Nanomolar Lectin Affinity and High Selectivity Based on Combined Fluorine-Amide, Phenyl-Arginine, Sulfur-π, and Halogen Bond Interactions

The design of small and high-affinity lectin inhibitors remains a major challenge because the natural ligand binding sites of lectin are often shallow and have polar character. Herein we report that derivatizing galactose with un-natural structural elements that form multiple non-natural lectin-ligand interactions (orthogonal multipolar fluorine-amide, phenyl-arginine, sulfur-π, and halogen bond) can provide inhibitors with extraordinary affinity (low nanomolar) for the model lectin, galectin-3, which is more than five orders of magnitude higher than the parent galactose; moreover, is selective over other galectins.

Systematic Tuning of Fluoro-galectin-3 Interactions Provides Thiodigalactoside Derivatives with Single-Digit nM Affinity and High Selectivity

Symmetrical and asymmetrical fluorinated phenyltriazolyl-thiodigalactoside derivatives have been synthesized and evaluated as inhibitors of galectin-1 and galectin-3. Systematic tuning of the phenyltriazolyl-thiodigalactosides' fluoro-interactions with galectin-3 led to the discovery of inhibitors with exceptional affinities (K_d down to 1-2 nM) in symmetrically substituted thiodigalactosides as well as unsurpassed combination of high affinity (K_d 7.5 nM) and selectivity (46-fold) over galectin-1 for asymmetrical thiodigalactosides by carrying one trifluorphenyltriazole and one coumaryl moiety. Studies of the inhibitor-galectin complexes with isothermal titration calorimetry and X-ray crystallography revealed the importance of fluoro-amide interaction for affinity and for selectivity. Finally, the high affinity of the discovered inhibitors required two competitive titration assay tools to be developed: a new high affinity fluorescent probe for competitive fluorescent polarization and a competitive ligand optimal for analyzing high affinity galectin-3 inhibitors with competitive isothermal titration calorimetry.

Intracellular galectin-7 expression in cancer cells results from an autocrine transcriptional mechanism and endocytosis of extracellular galectin-7

The β-galactoside binding protein galectin-7 (gal-7) is constitutively expressed at abnormally high levels in the outside milieu and intracellular compartments of many types of epithelial cancer cells, most notably in aggressive forms of ovarian and breast cancer. It is thus of utmost importance to understand how gal-7 traffics between both intracellular and extracellular compartments to develop novel drugs that target the protumorigenic functions of galectin-7. In the present work, we report that extracellular gal-7 plays a central role in controlling intracellular gal-7 in cells. It does so via two distinct yet complementary mechanisms: firstly by increasing the transcriptional activation of lgals7 gene transcription, and secondly via re-entry into the cells. Increased mRNA levels were dose- and time-dependent and occur in all cell lines tested, including ovarian and breast cancer cell lines. Addition of recombinant gal-7 to MDA-MB-231 transfected with a luciferase reporter vector containing response elements of the lgals7 promoter indicated that increased mRNA level of lgals7 occurs via de novo gene transcription. Re-entry of extracellular gal-7 inside cells was rapid, and reached cytosolic and mitochondrial compartments. Taken together, these findings reveal the existence of a positive self-amplification pathway that regulates intracellular gal-7 expression in breast and ovarian cancer cells.

Lactose Binding Induces Opposing Dynamics Changes in Human Galectins Revealed by NMR-Based Hydrogen-Deuterium Exchange

Galectins are β-galactoside-binding proteins implicated in a myriad of biological functions. Despite their highly conserved carbohydrate binding motifs with essentially identical structures, their affinities for lactose, a common galectin inhibitor, vary significantly. Here, we aimed to examine the molecular basis of differential lactose affinities amongst galectins using solution-based techniques. Consistent dissociation constants of lactose binding were derived from nuclear magnetic resonance (NMR) spectroscopy, intrinsic tryptophan fluorescence, isothermal titration calorimetry and bio-layer interferometry for human galectin-1 (hGal1), galectin-7 (hGal7), and the N-terminal and C-terminal domains of galectin-8 (hGal8^NTD and hGal8^CTD, respectively). Furthermore, the dissociation rates of lactose binding were extracted from NMR lineshape analyses. Structural mapping of chemical shift perturbations revealed long-range perturbations upon lactose binding for hGal1 and hGal8^NTD. We further demonstrated using the NMR-based hydrogen–deuterium exchange (HDX) that lactose binding increases the exchange rates of residues located on the opposite side of the ligand-binding pocket for hGal1 and hGal8^NTD, indicative of allostery. Additionally, lactose binding induces significant stabilisation of hGal8^CTD across the entire domain. Our results suggested that lactose binding reduced the internal dynamics of hGal8^CTD on a very slow timescale (minutes and slower) at the expense of reduced binding affinity due to the unfavourable loss of conformational entropy.

Galactose-amidine derivatives as selective antagonists of galectin-9

The family of galectin proteins involved in adhesion, growth regulation, immunity, and inflammatory events are important targets for development of small molecule antagonists. Here, N-sulfonyl amidine galactopyranoside derivatives obtained via a multicomponent reaction between galactose alkyne derivatives, sulfonyl azides, and amines were evaluated as antagonists of galectin-1, -2, -3, -4N (N-terminal domain), -4C (C-terminal domain), -8N, -9N, and -9C in a competitive fluorescence polarization assay. Highly selective compounds against galectin-9N with up to 30-fold improved affinity compared to the reference methyl β-d-galactopyranoside were identified. Molecular dynamics simulation suggested that the selectivity and affinity for galectin-9N originate from the N-sulfonyl amidine moieties forming tridentate hydrogen bonds to two asparagine side chains and one phenyl stacking edge-to-face to an arginine side chain. These selective galectin-9N antagonists are of significant value as chemical tools for studying galectin-9 biology and chemistry as well as possible starting structures for the discovery of galectin-9-targeting drugs influencing, e.g., immune regulation.

Galectin-3-Binding Glycomimetics that Strongly Reduce Bleomycin-Induced Lung Fibrosis and Modulate Intracellular Glycan Recognition

Discovery of glycan-competitive galectin-3-binding compounds that attenuate lung fibrosis in a murine model and that block intracellular galectin-3 accumulation at damaged vesicles, hence revealing galectin-3-glycan interactions involved in fibrosis progression and in intracellular galectin-3 activities, is reported. 3,3'-Bis-(4-aryltriazol-1-yl)thiodigalactosides were synthesized and evaluated as antagonists of galectin-1, -2, -3, and -4 N-terminal, -4 C-terminal, -7 and -8 N-terminal, -9 N-terminal, and -9 C-terminal domains. Compounds displaying low-nanomolar affinities for galectins-1 and -3 were identified in a competitive fluorescence anisotropy assay. X-ray structural analysis of selected compounds in complex with galectin-3, together with galectin-3 mutant binding experiments, revealed that both the aryltriazolyl moieties and fluoro substituents on the compounds are involved in key interactions responsible for exceptional affinities towards galectin-3. The most potent galectin-3 antagonist was demonstrated to act in an assay monitoring galectin-3 accumulation upon amitriptyline-induced vesicle damage, visualizing a biochemically/medically relevant intracellular lectin-carbohydrate binding event and that it can be blocked by a small molecule. The same antagonist administered intratracheally attenuated bleomycin-induced pulmonary fibrosis in a mouse model with a dose/response profile comparing favorably with that of oral administration of the marketed antifibrotic compound pirfenidone.

Low or No Inhibitory Potency of the Canonical Galectin Carbohydrate-binding Site by Pectins and Galactomannans

Some complex plant-derived polysaccharides, such as modified citrus pectins and galactomannans, have been shown to have promising anti-inflammatory and anti-cancer effects. Most reports propose or claim that these effects are due to interaction of the polysaccharides with galectins because the polysaccharides contain galactose-containing side chains that might bind this class of lectin. However, their direct binding to and/or inhibition of the evolutionarily conserved galactoside-binding site of galectins has not been demonstrated. Using a well established fluorescence anisotropy assay, we tested the direct interaction of several such polysaccharides with physiological concentrations of a panel of galectins. The bioactive pectic samples tested were very poor inhibitors of the canonical galactoside-binding site for the tested galectins, with IC₅₀ values >10 mg/ml for a few or in most cases no inhibitory activity at all. The galactomannan Davanat® was more active, albeit not a strong inhibitor (IC₅₀ values ranging from 3 to 20 mg/ml depending on the galectin). Pure synthetic oligosaccharide fragments found in the side chains and backbone of pectins and galactomannans were additionally tested. The most commonly found galactan configuration in pectins had no inhibition of the galectins tested. Galactosylated tri- and pentamannosides, representing the structure of Davanat®, had an inhibitory effect of galectins comparable with that of free galactose. Further evaluation using cell-based assays, indirectly linked to galectin-3 inhibition, showed no inhibition of galectin-3 by the polysaccharides. These data suggest that the physiological effects of these plant polysaccharides are not due to inhibition of the canonical galectin carbohydrate-binding site.

Galectin-1 inhibitors and their potential therapeutic applications: a patent review

INTRODUCTION

Galectins have affinity for β-galactosides. Human galectin-1 is ubiquitously expressed in the body and its expression level can be a marker in disease. Targeted inhibition of galectin-1 gives potential for treatment of inflammatory disorders and anti-cancer therapeutics.

AREAS COVERED

This review discusses progress in galectin-1 inhibitor discovery and development. Patent applications pertaining to galectin-1 inhibitors are categorised as monovalent- and multivalent-carbohydrate-based inhibitors, peptides- and peptidomimetics. Furthermore, the potential of galectin-1 protein as a therapeutic is discussed along with consideration of the unique challenges that galectin-1 presents, including its monomer-dimer equilibrium and oxidized and reduced forms, with regard to delivering an intact protein to a pathologically relevant site.

EXPERT OPINION

Significant evidence implicates galectin-1's involvement in cancer progression, inflammation, and host-pathogen interactions. Conserved sequence similarity of the carbohydrate-binding sites of different galectins makes design of specific antagonists (blocking agents/inhibitors of function) difficult. Key challenges pertaining to the therapeutic use of galectin-1 are its monomer-dimer equilibrium, its redox state, and delivery of intact galectin-1 to the desired site. Developing modified forms of galectin-1 has resulted in increased stability and functional potency. Gene and protein therapy approaches that deliver the protein toward the target are under exploration as is exploitation of different inhibitor scaffolds.

Diversity-oriented synthesis of fused thioglycosyl benzo[e][1,4]oxathiepin-5-ones and benzo[f][1,4]thiazepin-5(2H)-ones by a sequence of palladium-catalyzed glycosyl thiol arylation and deprotection–lactonization reactions

An efficient synthesis of thioglycosylated benzo[e][1,4]-oxathiepin-5-one and benzothiazepinone derivatives has been reported.

Biophysical characterization of lectin–glycan interactions for therapeutics, vaccines and targeted drug-delivery

Lectin–glycan interactions play a role in biological processes, host–pathogen interactions and in disease. A more detailed understanding of these interactions is not only useful for the elucidation of their biological function but can also be applied in immunology, drug development and delivery and diagnostics. We review some commonly used biophysical techniques for studying lectin–glycan interactions; namely: frontal affinity chromatography, glycan/lectin microarray, surface plasmon resonance, electrochemical impedance spectroscopy, isothermal titration calorimetry, fluorescent assays, enzyme linked lectin sorbent assay and saturation transfer difference nuclear magnetic resonance spectroscopy. Each method is evaluated on efficiency, cost and throughput. We also consider the advantages and limitations of each technique and provide examples of their application in biology, drug discovery and delivery, immunology, glycoprofiling and biosensing.

Design and synthesis of hydrolytically stable multivalent ligands bearing thiodigalactoside analogues for peanut lectin and human galectin-3 binding

Herein, we describe the design and synthesis of a novel family of hydrolytically stable glycoclusters bearing thiodigalactoside (TDG) analogues as recognition elements of β-galactoside binding lectins. The TDG analogue was synthesized by thioglycosylation of a 6-S-acetyl-α-D-glucosyl bromide with the isothiouronium salt of 2,3,4,6-tetra-O-acetyl-β-D-galactose. Further propargylation of the TDG analogue allowed the coupling to azido-functionalized oligosaccharide scaffolds through copper(I)-catalyzed azide-alkyne cycloaddition (CuAAC) under microwave activation. The final mono-, di-, and tetravalent ligands were resistant to enzymatic hydrolisis by Escherichia coli β-galactosidase. Binding affinities to peanut agglutinin and human galectin-3 were measured by isothermal titration calorimetry which showed K(a) constants in the micromolar range as well as a multivalent effect. Monovalent ligand exhibited a binding affinity higher than that of thiodigalactoside. Docking studies performed with a model ligand on both β-galactoside binding lectins showed additional interactions between the triazole ring and lectin amino acid residues, suggesting a positive effect of this aromatic residue on the biological activity.

Recent advances toward the development of inhibitors to attenuate tumor metastasis via the interruption of lectin-ligand interactions

Aberrant glycosylation is a well-recognized phenomenon that occurs on the surface of tumor cells, and the overexpression of a number of ligands (such as TF, sialyl Tn, and sialyl Lewis X) has been correlated to a worse prognosis for the patient. These unique carbohydrate structures play an integral role in cell-cell communication and have also been associated with more metastatic cancer phenotypes, which can result from binding to lectins present on cell surfaces. The most well studied metastasis-associated lectins are the galectins and selectins, which have been correlated to adhesion, neoangiogenesis, and immune-cell evasion processes. In order to slow the rate of metastatic lesion formation, a number of approaches have been successfully developed which involve interfering with the tumor lectin-substrate binding event. Through the generation of inhibitors, or by attenuating lectin and/or carbohydrate expression, promising results have been observed both in vitro and in vivo. This article briefly summarizes the involvement of lectins in the metastatic process and also describes different approaches used to prevent these undesirable carbohydrate-lectin binding events, which should ultimately lead to improvement in current cancer therapies.

Galectins in epithelial functions

Galectins are a family of animal lectins comprising 15 members in vertebrates. These proteins are involved in many biological processes including epithelial homeostasis and tumor progression by displaying intracellular and extracellular activities. Hence Galectins can be found either in the cytoplasm or the nucleus, associated with membranes or in the extracellular matrix. Current studies aim at understanding the roles of Galectins in cell-cell and cell-matrix adhesion, cellular polarity and motility. This review discusses recent progress in defining the specificities and mechanisms of action of Galectins as cell regulators in epithelial cells. Physiological, cellular and molecular aspects of Galectin specificities will be treated successively.

Inhibitors of Galectins and Implications for Structure-Based Design of Galectin-Specific Therapeutics

Galectins are a family of galactoside-specific lectins that are involved in a myriad of metabolic and disease processes. Due to roles in cancer and inflammatory and heart diseases, galectins are attractive targets for drug development. Over the last two decades, various strategies have been used to inhibit galectins, including polysaccharide-based therapeutics, multivalent display of saccharides, peptides, peptidomimetics, and saccharide-modifications. Primarily due to galectin carbohydrate binding sites having high sequence identities, the design and development of selective inhibitors targeting particular galectins, thereby addressing specific disease states, is challenging. Furthermore, the use of different inhibition assays by research groups has hindered systematic assessment of the relative selectivity and affinity of inhibitors. This review summarises the status of current inhibitors, strategies, and novel scaffolds that exploit subtle differences in galectin structures that, in conjunction with increasing available data on multiple galectins, is enabling the feasible design of effective and specific inhibitors of galectins.

Structure-based optimization of angiostatic agent 6DBF7, an allosteric antagonist of galectin-1

Galectin-1 (gal-1), which binds β-galactoside groups on various cell surface receptors, is crucial to cell adhesion and migration, and is found to be elevated in several cancers. Previously, we reported on 6DBF7, a dibenzofuran (DBF)-based peptidomimetic of the gal-1 antagonist anginex. In the present study, we used a structure-based approach to optimize 6DBF7. Initial NMR studies showed that 6DBF7 binds to gal-1 on one side of the β-sandwich away from the lectin's carbohydrate binding site. Although an alanine scan of 6DBF7 showed that the two cationic groups (lysines) in the partial peptide are crucial to its angiostatic activity, it is the hydrophobic face of the amphipath that appears to interact directly with the surface of gal-1. Based on this structural information, we designed and tested additional DBF analogs. In particular, substitution of the C-terminal Asp for alanine and branched alkyl side chains (Val, Leu, Ile) for linear ones (Nle, Nva) rendered the greatest improvements in activity. Flow cytometry with gal-1(-/-) splenocytes showed that 6DBF7 and two of its more potent analogs (DB16 and DB21) can fully inhibit fluorescein isothiocyanate-gal-1 binding. Moreover, heteronuclear single-quantum coherence NMR titrations showed that the presence of DB16 decreases gal-1 affinity for lactose, indicating that the peptidomimetic targets gal-1 as a noncompetitive, allosteric inhibitor of glycan binding. Using tumor mouse models (B16F10 melanoma, LS174 lung, and MA148 ovarian), we found that DB21 inhibits tumor angiogenesis and tumor growth significantly better than 6DBF7, DB16, or anginex. DB21 is currently being developed further and holds promise for the management of human cancer in the clinic.

Antitumor agent calixarene 0118 targets human galectin-1 as an allosteric inhibitor of carbohydrate binding

Calix[4]arene compound 0118 is an angiostatic agent that inhibits tumor growth in mice. Although 0118 is a topomimetic of galectin-1-targeting angiostatic amphipathic peptide Anginex, we had yet to prove that 0118 targets galectin-1. Galectin-1 is involved in pathological disorders like tumor endothelial cell adhesion and migration and therefore presents a relevant target for therapeutic intervention against cancer. Here, (15)N-(1)H HSQC NMR spectroscopy demonstrates that 0118 indeed targets galectin-1 at a site away from the lectin's carbohydrate binding site and thereby attenuates lactose binding to the lectin. Flow cytometry and agglutination assays show that 0118 attenuates binding of galectin-1 to cell surface glycans, and the inhibition of cell proliferation by 0118 is found to be correlated with the cellular expression of the lectin. In general, our data indicate that 0118 targets galectin-1 as an allosteric inhibitor of glycan/carbohydrate binding. This work contributes to the clinical development of antitumor calixarene compound 0118.