Origins of Temperature-Dependent Anomeric Selectivity in Glycosylations with an L-Idose Thioglycoside.

L-Idose thioglycosides are useful glycosyl donors for the construction of glycosaminoglycan oligosaccharides. When activated with NIS and catalytic TMSOTf in the presence of methanol, the stereoselectivity of O-glycosylation displays an intriguing dependence on the reaction temperature, with an increased preference for formation of the α-glycoside at higher temperatures. Using a combination of vt-NMR spectroscopy and DFT calculations, we show how a simple mechanistic model, based on competing reactions of the iodinated thioglycoside, can explain the main features of the temperature dependence. In this model, the increased selectivity at high temperature is attributed to differences among the entropy and energy terms of the competing reaction pathways. Neighbouring-group participation (giving an intermediate acyloxonium ion) plays an increasingly dominant role as temperature is raised. The general features of this kinetic regime may also apply more broadly to other glycosylations that likewise favour α-glycoside formation at high temperature.

Synthesis of Uronic Acid 1-Azasugars as Putative Inhibitors of α-Iduronidase, ß-Glucuronidase and Heparanase.

1-Azasugar analogues of l-iduronic acid (l-IdoA) and d-glucuronic acid (d-GlcA) and their corresponding enantiomers have been synthesized as potential pharmacological chaperones for mucopolysaccharidosis I (MPS I), a lysosomal storage disease caused by mutations in the gene encoding α-iduronidase (IDUA). The compounds were efficiently synthesized in nine or ten steps from d- or l-arabinose, and the structures were confirmed by X-ray crystallographic analysis of key intermediates. All compounds were inactive against IDUA, although l-IdoA-configured 8 moderately inhibited ß-glucuronidase (ß-GLU). The d-GlcA-configured 9 was a potent inhibitor of ß-GLU and a moderate inhibitor of the endo-ß-glucuronidase heparanase. Co-crystallization of 9 with heparanase revealed that the endocyclic nitrogen of 9 forms close interactions with both the catalytic acid and catalytic nucleophile.

Evidence for Multiple Binding Modes in the Initial Contact Between SARS-CoV-2 Spike S1 Protein and Cell Surface Glycans.

Infection of host cells by SARS-CoV-2 begins with recognition by the virus S (spike) protein of cell surface heparan sulfate (HS), tethering the virus to the extracellular matrix environment, and causing the subunit S1-RBD to undergo a conformational change into the 'open' conformation. These two events promote the binding of S1-RBD to the angiotensin converting enzyme 2 (ACE2) receptor, a preliminary step toward viral-cell membrane fusion. Combining ligand-based NMR spectroscopy with molecular dynamics, oligosaccharide analogues were used to explore the interactions between S1-RBD of SARS CoV-2 and HS, revealing several low-specificity binding modes and previously unidentified potential sites for the binding of extended HS polysaccharide chains. The evidence for multiple binding modes also suggest that highly specific inhibitors will not be optimal against protein S but, rather, diverse HS-based structures, characterized by high affinity and including multi-valent compounds, may be required.

Identification of a Pentasaccharide Lead Compound with High Affinity to the SARS-CoV-2 Spike Protein via In Silico Screening.

The spike (S) protein on the surface of the SARS-CoV-2 virus is critical to mediate fusion with the host cell membrane through interaction with angiotensin-converting enzyme 2 (ACE2). Additionally, heparan sulfate (HS) on the host cell surface acts as an attachment factor to facilitate the binding of the S receptor binding domain (RBD) to the ACE2 receptor. Aiming at interfering with the HS-RBD interaction to protect against SARS-CoV-2 infection, we have established a pentasaccharide library composed of 14,112 compounds covering the possible sulfate substitutions on the three sugar units (GlcA, IdoA, and GlcN) of HS. The library was used for virtual screening against RBD domains of SARS-CoV-2. Molecular modeling was carried out to evaluate the potential antiviral properties of the top-hit pentasaccharide focusing on the interactive regions around the interface of RBD-HS-ACE2. The lead pentasaccharide with the highest affinity for RBD was analyzed via drug-likeness calculations, showing better predicted druggable profiles than those currently reported for RBD-binding HS mimetics. The results provide significant information for the development of HS-mimetics as anti-SARS-CoV-2 agents.

Tumour cell-activated platelets modulate the immunological activity of CD4+, CD8+, and NK cells, which is efficiently antagonized by heparin.

Platelets, key players in haemostasis, are progressively investigated with respect to their role in immunity and inflammation. Although the platelet support to haematogenous cancer cell metastasis has been the subject of multiple studies, their impact on anti-cancer immunity remains unaddressed. Here, we investigated the immunomodulatory potential of platelets upon their activation by MDA-MB-231 breast cancer cells in various in vitro approaches. We provide evidence that platelets as well as their tumour cell-induced releasates increased the ratio of regulatory T cells, shaping an immunosuppressive phenotype in isolated CD4+ cultures. The influence on CD8+ T cells was assessed by detecting the expression of activation markers CD25/CD69 and release of cytolytic and pro-inflammatory proteins. Notably, the platelet preparations differentially influenced CD8+ T cell activation, while platelets were found to inhibit the activation of CD8+ T cells, platelet releasates, in contrast, supported their activation. Furthermore, the NK cell cytolytic activity was attenuated by platelet releasates. Low molecular weight heparin (LMWH), the guideline-based anticoagulant for cancer-associated thrombotic events, is known to interfere with tumour cell-induced platelet activation. Thus, we aimed to investigate whether, unfractionated heparin, LMWH or novel synthetic heparin mimetics can also reverse the immunosuppressive platelet effects. The releasate-mediated alteration in immune cell activity was efficiently abrogated by heparin, while the synthetic heparin mimetics partly outperformed the commercial heparin derivatives. This is the first report on the effects of heparin on rebalancing immunosuppression in an oncological context emerging as a novel aspect in heparin anti-tumour activities.

Structural Insights into Pixatimod (PG545) Inhibition of Heparanase, a Key Enzyme in Cancer and Viral Infections.

Pixatimod (PG545), a heparan sulfate (HS) mimetic and anticancer agent currently in clinical trials, is a potent inhibitor of heparanase. Heparanase is an endo-ß-glucuronidase that degrades HS in the extracellular matrix and basement membranes and is implicated in numerous pathological processes such as cancer and viral infections, including SARS-CoV-2. To understand how PG545 interacts with heparanase, we firstly carried out a conformational analysis through a combination of NMR experiments and molecular modelling which showed that the reducing end ß-D-glucose residue of PG545 adopts a distorted conformation. This was followed by docking and molecular dynamics simulations to study the interactions of PG545 with heparanase, revealing that PG545 is able to block the active site by binding in different conformations, with the cholestanol side-chain making important hydrophobic interactions. While PG545 blocks its natural substrate HS from binding to the active site, small synthetic heparanase substrates are only partially excluded, and thus pentasaccharide or larger substrates are preferred for assaying this class of inhibitor. This study provides new insights for the design of next-generation heparanase inhibitors and substrates.

An Improved Protocol for the Stereoselective Synthesis of ß-d-Glycosyl Fluorides from 2-O-Acyl Thioglycosides.

A safe and operationally simple protocol for the preparation of ß-d-glycosyl fluorides is presented. We demonstrate that a precise combination of XtalFluor-M, N-bromosuccinimide, and Et3N·3HF can mediate facile, high-yielding, and diastereoselective conversions of 2-O-acyl thioglycosides to ß-d- and other 1,2-trans glycosyl fluorides. The key roles of these reagents are dissected in this work, as is the impact of their interplay on the fluorination stereoselectivity.

Development of Improved Synthetic Routes to Pixatimod (PG545), a Sulfated Oligosaccharide-Steroid Conjugate.

The heparan sulfate (HS) mimetic pixatimod (PG545) is a highly potent inhibitor of angiogenesis, tumor growth, and metastasis currently in clinical trials for cancer. PG545 has also demonstrated potent antiviral activity against numerous HS-dependent viruses, including SARS-CoV-2, and shows promise as an antiviral drug for the treatment of COVID-19. Structurally, PG545 consists of a fully sulfated tetrasaccharide conjugated to the steroid 5α-cholestan-3ß-ol. The reported synthesis of PG545 suffers from a low yield and poor selectivity in the critical glycosylation step. Given its clinical importance, new efficient routes for the synthesis of PG545 and analogues were developed. Particular attention was given to improving the key glycosylation step by using more stable protecting groups and optimized glycosyl donors.

From Cancer to COVID-19: A Perspective on Targeting Heparan Sulfate-Protein Interactions.

Heparan sulfate (HS) is a complex, polyanionic polysaccharide ubiquitously expressed on cell surfaces and in the extracellular matrix. HS interacts with numerous proteins to mediate a vast array of biological and pathological processes. Inhibition of HS-protein interactions is thus an attractive approach for new therapeutic development for cancer and infectious diseases, including COVID-19; however, synthesis of well-defined native HS oligosaccharides remains challenging. This has aroused significant interest in the development of HS mimetics which are more synthetically tractable and have fewer side effects, such as undesired anticoagulant activity. This account provides a perspective on the design and synthesis of different classes of HS mimetics with useful properties, and the development of various assays and molecular modelling tools to progress our understanding of their interactions with HS-binding proteins.

Anticoagulant Heparin Mimetics via RAFT Polymerization.

Heparin, a sulfated polysaccharide derived from animal sources, is the most commonly used parenteral anticoagulant drug, but it suffers from significant safety and supply issues. Herein, we describe the preparation of heparin mimetic homo- and copolymers via the reversible addition-fragmentation chain transfer (RAFT) polymerization in water of commercially available (non-carbohydrate) sulfonated and carboxylated monomers. The anticoagulant activities of the polymers were assessed by activated partial thromboplastin time (APTT), thrombin clotting time (TCT), and for the more promising polymers, thrombin generation, antifactor Xa, and antifactor IIa assays. Sulfonated homopolymers studied herein displayed low cytotoxicity and significant anticoagulant activity in APTT, TCT, and thrombin generation assays. In addition, copolymers of sodium styrenesulfonate and acrylic acid [poly(SSS-co-AA)] displayed unprecedented antifactor IIa activity. This study demonstrates the potential of RAFT polymers as alternative anticoagulants for biomedical applications.

GlycoTorch Vina: Docking Designed and Tested for Glycosaminoglycans.

Glycosaminoglycans (GAGs) are a family of anionic carbohydrates that play an essential role in the physiology and pathology of all eukaryotic life forms. Experimental determination of GAG-protein complexes is challenging due to their difficult isolation from biological sources, natural heterogeneity, and conformational flexibility-including possible ring puckering of sulfated iduronic acid from 1C4 to 2SO conformation. To overcome these challenges, we present GlycoTorch Vina (GTV), a molecular docking tool based on the carbohydrate docking program VinaCarb (VC). Our program is unique in that it contains parameters to model 2SO sugars while also supporting glycosidic linkages specific to GAGs. We discuss how crystallographic models of carbohydrates can be biased by the choice of refinement software and structural dictionaries. To overcome these variations, we carefully curated 12 of the best available GAG and GAG-like crystal structures (ranging from tetra- to octasaccharides or longer) obtained from the PDB-REDO server and refined using the same protocol. Both GTV and VC produced pose predictions with a mean root-mean-square deviation (RMSD) of 3.1 Å from the native crystal structure-a statistically significant improvement when compared to AutoDock Vina (4.5 Å) and the commercial software Glide (5.9 Å). Examples of how real-space correlation coefficients can be used to better assess the accuracy of docking pose predictions are given. Comparisons between statistical distributions of empirical "salt bridge" interactions, relevant to GAGs, were compared to density functional theory (DFT) studies of model salt bridges, and water-mediated salt bridges; however, there was generally a poor agreement between these data. Water bridges appear to play an important, yet poorly understood, role in the structures of GAG-protein complexes. To aid in the rapid prototyping of future pose scoring functions, we include a module that allows users to include their own torsional and nonbonded parameters.

PI-88 and Related Heparan Sulfate Mimetics.

The heparan sulfate mimetic PI-88 (muparfostat) is a complex mixture of sulfated oligosaccharides that was identified in the late 1990s as a potent inhibitor of heparanase. In preclinical animal models it was shown to block angiogenesis, metastasis and tumor growth, and subsequently became the first heparanase inhibitor to enter clinical trials for cancer. It progressed to Phase III trials but ultimately was not approved for use. Herein we summarize the preparation, physicochemical and biological properties of PI-88, and discuss preclinical/clinical and structure-activity relationship studies. In addition, we discuss the PI-88-inspired development of related HS mimetic heparanase inhibitors with improved properties, ultimately leading to the discovery of PG545 (pixatimod) which is currently in clinical trials.

Standardizing the use of fast-field cycling NMR relaxometry for measuring hydrological connectivity inside the soil.

Hydrological connectivity inside the soil (HCS) is applied to study the effects of heterogeneities in complex environmental systems. It refers to both the spatial patterns inside the soil (i.e., structural connectivity [SC]) and the physical-chemical processes at a molecular level (i.e., functional connectivity [FC]). NMR relaxometry has been already applied to assess both SC and FC components of the HCS by defining SC and FC indexes. Here, fast-field cycling NMR relaxometry has been applied on a water suspended soil and a sediment to optimize the conditions to standardize the technique. Proton Larmor frequencies (ωL ) from 0.01 to 25 MHz were used on samples suspended in three different rates of Milli-Q grade water. The application of different magnetic fields revealed that the T1 values of the sediment sample are always shorter than those measured for the soil sample. This difference was attributed to the soil erosion processes limiting FC by reducing the size of macropores. For the soil sample, analyses showed that both structural and functional connectivity indexes can be assumed ωL independent. For the sediment sample, the connectivity indexes resulted ωL independent only for ωL  ≥ 0.1 MHz. This could be due either to instrumental problems for ωL  < 0.1 MHz or to a real magnetic field effect when a finer textured sample is examined. Further research is required in this area. Finally, the experiments revealed that the optimal water amount to obtain reliable results was corresponding to the water holding capacity.

Cross-Species Analysis of Glycosaminoglycan Binding Proteins Reveals Some Animal Models Are "More Equal" than Others.

Glycosaminoglycan (GAG) mimetics are synthetic or semi-synthetic analogues of heparin or heparan sulfate, which are designed to interact with GAG binding sites on proteins. The preclinical stages of drug development rely on efficacy and toxicity assessment in animals and aim to apply these findings to clinical studies. However, such data may not always reflect the human situation possibly because the GAG binding site on the protein ligand in animals and humans could differ. Possible inter-species differences in the GAG-binding sites on antithrombin III, heparanase, and chemokines of the CCL and CXCL families were examined by sequence alignments, molecular modelling and assessment of surface electrostatic potentials to determine if one species of laboratory animal is likely to result in more clinically relevant data than another. For each protein, current understanding of GAG binding is reviewed from a protein structure and function perspective. This combinatorial analysis shows chemokine dimers and oligomers can present different GAG binding surfaces for the same target protein, whereas a cleft-like GAG binding site will differently influence the types of GAG structures that bind and the species preferable for preclinical work. Such analyses will allow an informed choice of animal(s) for preclinical studies of GAG mimetic drugs.

In Vitro Enzymatic Digestibility of Glutaraldehyde-Crosslinked Chitosan Nanoparticles in Lysozyme Solution and Their Applicability in Pulmonary Drug Delivery.

Herein, the degradation of low molecular weight chitosan (CS), with 92% degree of deacetylation (DD), and its nanoparticles (NP) has been investigated in 0.2 mg/mL lysozyme solution at 37 °C. The CS nanoparticles were prepared using glutaraldehyde crosslinking of chitosan in a water-in-oil emulsion system. The morphological characterization of CS particles was carried out using scanning electron microscopy (SEM) and Transmission Electron Microscopy (TEM) techniques. Using attenuated total reflectance Fourier transform infrared (ATR-FTIR) and UV-VIS spectroscopy, the structural integrity of CS and its NPs in lysozyme solution were monitored. The CS powder showed characteristic FTIR bands around 1150 cm-1 associated with the glycosidic bridges (C-O-C bonds) before and after lysozyme treatment for 10 weeks, which indicated no CS degradation. The glutaraldehyde crosslinked CS NPs showed very weak bands associated with the glycosidic bonds in lysozyme solution. Interestingly, the UV-VIS spectroscopic data showed some degradation of CS NPs in lysozyme solution. The results of this study indicate that CS with a high DD and its NPs crosslinked with glutaraldehyde were not degradable in lysozyme solution and thus unsuitable for pulmonary drug delivery. Further studies are warranted to understand the complete degradation of CS and its NPs to ensure their application in pulmonary drug delivery.

Heparin mimetics with anticoagulant activity.

Heparin, a sulfated polysaccharide belonging to the glycosaminoglycan family, has been widely used as an anticoagulant drug for decades and remains the most commonly used parenteral anticoagulant in adults and children. However, heparin has important clinical limitations and is derived from animal sources which pose significant safety and supply problems. The ever growing shortage of the raw material for heparin manufacturing may become a very significant issue in the future. These global limitations have prompted much research, especially following the recent well-publicized contamination scandal, into the development of alternative anticoagulants derived from non-animal and/or totally synthetic sources that mimic the structural features and properties of heparin. Such compounds, termed heparin mimetics, are also needed as anticoagulant materials for use in biomedical applications (e.g., stents, grafts, implants etc.). This review encompasses the development of heparin mimetics of various structural classes, including synthetic polymers and non-carbohydrate small molecules as well as sulfated oligo- and polysaccharides, and fondaparinux derivatives and conjugates, with a focus on developments in the past 10 years.

Structural and conformational studies of the heparan sulfate mimetic PI-88.

The heparan sulfate mimetic PI-88 is a complex mixture of sulfated oligosaccharides with anti-metastatic and anti-angiogenic activity due to its potent inhibition of heparanase and heparan sulfate-dependent angiogenic growth factors. It was recently in Phase III clinical trials for postresection hepatocellular carcinoma. The major oligosaccharide constituents of PI-88 were prepared for the first time by sulfonation of individually purified phosphorylated oligosaccharides isolated from the PI-88 precursor. PI-88 and its components were subjected to detailed 1D and 2D NMR spectroscopic analysis. The spectra of the individual components greatly assisted the assignment of minor resonances in the 1H NMR spectrum of PI-88. The data also showed that the majority of the oligosaccharides in PI-88 are fully sulfated and that undersulfated species present are largely due to anomeric desulfation. The solution conformation of the phosphomannopentaose sulfate (major component) of PI-88 was then determined by a combination of molecular dynamics simulations and NOE measurements which may provide insights into its binding interactions with target proteins.

Prophylactic Antiheparanase Activity by PG545 Is Antiviral In Vitro and Protects against Ross River Virus Disease in Mice.

Recently we reported on the efficacy of pentosan polysulfate (PPS), a heparan sulfate mimetic, to reduce the recruitment of inflammatory infiltrates and protect the cartilage matrix from degradation in Ross River virus (RRV)-infected PPS-treated mice. Here, we describe both prophylactic and therapeutic treatment with PG545, a low-molecular-weight heparan sulfate mimetic, for arthritogenic alphaviral infection. We first assessed antiviral activity in vitro through a 50% plaque reduction assay. Increasing concentrations of PG545 inhibited plaque formation prior to viral adsorption in viral strains RRV T48, Barmah Forest virus 2193, East/Central/South African chikungunya virus (CHIKV), and Asian CHIKV, suggesting a strong antiviral mode of action. The viral particle-compound dissociation constant was then evaluated through isothermal titration calorimetry. Furthermore, prophylactic RRV-infected PG545-treated mice had reduced viral titers in target organs corresponding to lower clinical scores of limb weakness and immune infiltrate recruitment. At peak disease, PG545-treated RRV-infected mice had lower concentrations of the matrix-degrading enzyme heparanase in conjunction with a protective effect on tissue morphology, as seen in the histopathology of skeletal muscle. Enzyme-linked immunosorbent assay quantification of cartilage oligomeric matrix protein and cross-linked C-telopeptides of type II collagen as well as knee histopathology showed increased matrix protein degradation and cartilage erosion in RRV-infected phosphate-buffered saline-treated mice compared to their PG545-treated RRV-infected counterparts. Taken together, these findings suggest that PG545 has a direct antiviral effect on arthritogenic alphaviral infection and curtails RRV-induced inflammatory disease when administered as a prophylaxis.

Synthesis and mass spectrometric analysis of disaccharides from methanolysis of heparan sulfate.

The quantification of heparan sulfate (HS) in biological matrices, e.g., urine, cerebrospinal fluid, tissue samples etc., is of great importance for the diagnosis and prognosis of several of the mucopolysaccharidosis (MPS) disorders, which are lysosomal storage diseases of impaired glycosaminoglycan metabolism. The development of suitable assays for this purpose is challenging due to the high molecular weight and complexity of HS. Recent efforts towards this goal include the acid catalysed methanolysis of HS, which desulfates the polymer and results in the formation of disaccharide cleavage products which can be detected and quantified by LC-MS/MS. We have synthesized a library of 12 HS-derived disaccharides as methanolysis standards via the stereoselective 1,2-cis glycosylation of suitably protected GlcA and IdoA acceptors with a 2-deoxy-2-azido thioglucoside donor. This facilitated identification of the major peaks in the LC-MS/MS chromatograms, and potentially will allow the monitoring of specific metabolites as surrogate markers for genotype. This work also paves the way towards a fully quantitative LC-MS/MS assay for HS via the preparation of a suitably labelled derivative.

The Development of Assays for Heparanase Enzymatic Activity: Towards a Gold Standard.

The enzyme heparanase, an endo-ß-glucuronidase, degrades heparan sulfate (HS) chains on the cell surface and in the extracellular matrix. Heparanase regulates numerous biological processes that drive tumour growth, metastasis and angiogenesis. In addition to its key role in cancer progression, it has also been implicated in an ever-growing number of other diseases, particularly those associated with inflammation. The importance of heparanase in biology has led to numerous efforts over the years to develop assays to monitor its activity and to screen for new inhibitors as potential drug candidates. Despite these efforts and the commercialization of a few kits, most heparanase assays are still complex, labour intensive, costly or have limited application. Herein we review the various methods for assaying heparanase enzymatic activity, focusing on recent developments towards new assays that hold the promise of accelerating research into this important enzyme.