Effects on molecular conformation and anticoagulant activities of 1,6-anhydrosugars at the reducing terminal of antithrombin-binding octasaccharides isolated from low-molecular-weight heparin enoxaparin

Promising Drug Fondaparinux for the Treatment of COVID-19: an In Silico Analysis of Low Molecular Weight Heparin, Direct Oral Anticoagulant, and Antiplatelet Drug Interactions with Host Protease Furin

PURPOSE

As of July 2022, the COVID-19 pandemic has affected over 555 million worldwide confirmed cases and caused more than 6.3 million deaths. The studies showed that the D-dimer levels were increased in non-survivors compared to survivors and heparin treatment has begun to be administered to the patients in severe clinics. As we knew that the entrance of SARS-CoV-2 to the host cell needs to be facilitated by host proteases; we published our hypothesis that heparin as a serine protease inhibitor may block the interaction between spike protein receptor-binding domain and host proteases. In our study, we aimed to investigate the interactions between not only heparins but also other antiplatelet and anticoagulant drugs including fondaparinux.

METHODS

In this study, docking studies were carried out to evaluate the interactions between low molecular weight heparins (LMWHs) (enoxaparin, dalteparin, tinzaparin), direct oral anticoagulant, and antiplatelet drugs with host proteases. Molecular docking studies were performed by using Schrödinger molecular modeling software. 3D structures of the ligands were obtained from the 2D structures by assigning the OPLS-2005 force field using the Maestro 12.7. The 3D crystal structure of the furin complexed with an inhibitor, 2,5-dideoksistreptamin derivative, was extracted from the Protein Data Bank (PDB ID: 5MIM). Docking studies were carried out using the Grid-based Ligand Docking with Energetics module of the Schrödinger Software.

RESULTS

The docking studies revealed that fondaparinux was the most relevant molecule to interact with furin with a docking score of - 12.74. It showed better interaction than the natural ligand of furin with an increased score compared to the docking score of - 8.155 of the natural ligand. AnaGA*IsA structure representing LMWH structure has shown a docking score of - 11.562 which was also better than the score of the natural ligand of furin.

CONCLUSION

Our findings have shown that LMWHs and fondaparinux can be used for their possible antiviral effects in COVID-19 patients. Our results have shown that in accordance with heparin and LMWH, fondaparinux can also be a candidate for "drug repurposing" in COVID-19 therapy, not only because of their anticoagulant but also possible antiviral effects.

Scientific considerations in the regulatory approval of generic (or biosimilar) version of enoxaparin sodium - A lifesaving carbohydrate polymer

Enoxaparin sodium (Clexane®/Klexane®/Lovenox®) is one amongst the few drugs that have assumed a central role as drug of treatment and/or prevention against thromboembolic complications during COVID-19. The increase in demand resulting in many generic (or biosimilar) versions entering the market has increased the risks of quality and safety (including immunogenicity) related issues. Under the circumstances, development of stringent regulatory approaches has received much attention as investigation of new drug delivery systems for improved therapeutic activity. As one of the measures to increase quality testing and ensure uninterrupted supply of this life-saving drug globally, determination of enoxaparin molecular weight (MW) has been added in the United States Pharmacopoeia (USP) monograph for enoxaparin sodium. In addition, the presence of a unique 1,6-anhydro-ring structure at the reducing end of about 15-25% of the poly (oligo) saccharide chains of the generic (or biosimilar) product has been set as a mandatory requirement. This article presents an overview of the scientific considerations in the quality manufacturing and testing of the generic (or biosimilar) enoxaparin for regulatory review and approval. In certain cases of strong analytical similarity (structural and functional), abandonment of in vivo testing in animals and humans represents a major advancement in the approval of generic (or biosimilar) version of innovator enoxaparin sodium (lovenox®, injections).

Thiol-Ene Reaction of Heparin Allyl Ester, Heparin 4-Vinylbenzyl Ester and Enoxaparin

Heparin allyl ester and heparin 4-vinylbenzyl ester were prepared and examined for their potential for thiol-ene reaction using both free radical initiators and photochemistry. While both undergo reaction with mercaptoacetic acid, the allyl ester adduct proved to be somewhat more labile. Several more examples of adducts from heparin 4-vinylbenzyl ester are reported. Similar reactions on enoxaparin, where the reaction site is solely at the non-reducing end of the molecule, are also reported. These reactions may show promise as a strategy in the development of drug conjugates.

Commercial Low Molecular Weight Heparins - Patent Ecosystem and Technology Paradigm for Quality Characterization

Heparin is a subject of ever-growing interest for laboratory researchers and pharmaceutical industry. One of the driving factors is its critical life-saving drug status, which during the COVID-19 pandemic has assumed a central role in disease treatment and/or prevention. Apart, heparin is one amongst few drugs enjoying a "demand constant" status. In 2020, heparin market size was valued to US$6.5 bn., and given the ongoing stability in the COVID-19 health crisis, it is expected to reach US$11.43 bn. by 2027 with yearly growth rate momentum (CAGR) of 3.9% during the forecast period (Pepi et al., Mol Cell Proteomics 20:100,025, 2021). As patent is a limited monopoly, every year, many patents on low molecular weight heparin (LMWH; a chemically or enzymatically degraded product of unfractionated heparin) are losing market exclusivity worldwide, inviting the generic/biosimilar drug manufacturers to capture market share with cheaper drug products. By tracking patent expiration, drugs in patent litigation, regulatory setbacks for innovator companies (such as those seeking data exclusivity or patent term extension), or other unexpected events affecting market demand and competition, generics can make investment decisions in manufacturing off-patent LMWH drug products of commercial significance. However, given the US Food and Drug Administration (FDA), European Medicine Agency (EMA), Drug Regulatory Authority of Pakistan (DRAP), and other regulatory authorities scientifically rigorous standards for generic/biosimilar LMWH drug products marketing approval, the market is secured and momentous for drug makers that could demonstrate through scientific and clinical dataset that the generic/biosimilar LMWH drug product is of the same quality and purity as the innovator drug product. This study presents an overview of the patent landscape of commercially available LMWHs and advanced analytical techniques for their structural and biochemical characterization for quality control and quality assurance during manufacturing and post-marketing. The study also covers FDA, EMA, Health Canada, and DRAP's current approaches to evaluating the generic/biosimilar LMWH drug products for quality, safety including immunogenicity, and efficacy.

Conformational Analysis of Heparin-Analogue Pentasaccharides by Nuclear Magnetic Resonance Spectroscopy and Molecular Dynamics Simulations

Elucidation and improvement of the blood coagulant properties of heparin are the focus of intense research. In this study, we performed conformational analysis using nuclear magnetic resonance (NMR) spectroscopy and molecular dynamics (MD) simulations on the heparin pentasaccharide analogue idraparinux, its disulfonatomethyl analogue, which features a slightly improved blood coagulation property, and a trisulfonatomethyl analogue, in which the activity has been totally abolished. As the ring conformation of the G subunit has been suggested as a major determinant of the biological properties, we analyzed the sugar ring conformations and dynamics of the interglycosidic linkages. We found that the conformation of the G ring is dominated by the ²S_O skewed boat next to the ¹C₄ chair in all three derivatives. Both the thermodynamics and the kinetics of the conformational states were found to be highly similar in the three derivatives. Molecular kinetic analysis showed that the ²S_O skewed boat state of the G ring is equally favorable in the three analogues, resulting in similar ²S_O populations. Also, the transition kinetics from the ¹C₄ chair to the ²S_O skewed boat was found to be comparable in the derivatives, which indicates a similar energy barrier between the two states of the G subunit. We also identified a slower conformational transition between the dominant ⁴C₁ chair and the boat conformations on the E subunit. Both G and E ring flips are also accompanied by changes along the interglycosidic linkages, which take place highly synchronously with the ring flips. These findings indicate that conformational plasticity of the G ring and the dominance of the ²S_O skewed boat populations do not necessarily warrant the biological activity of the derivatives and hence the impact of other factors also needs to be considered.

NMR Characterization of the Interactions Between Glycosaminoglycans and Proteins

Glycosaminoglycans (GAGs) constitute a considerable fraction of the glycoconjugates found on cellular membranes and in the extracellular matrix of virtually all mammalian tissues. The essential role of GAG-protein interactions in the regulation of physiological processes has been recognized for decades. However, the underlying molecular basis of these interactions has only emerged since 1990s. The binding specificity of GAGs is encoded in their primary structures, but ultimately depends on how their functional groups are presented to a protein in the three-dimensional space. This review focuses on the application of NMR spectroscopy on the characterization of the GAG-protein interactions. Examples of interpretation of the complex mechanism and characterization of structural motifs involved in the GAG-protein interactions are given. Selected families of GAG-binding proteins investigated using NMR are also described.

Degeneracy of the Antithrombin Binding Sequence in Heparin: 2-O-Sulfated Iduronic Acid Can Replace the Critical Glucuronic Acid

Heparin binds to and activates antithrombin (AT) through a specific pentasaccharide sequence, in which a trisaccharide subsite, containing glucuronic acid (GlcA), has been considered as the initiator in the recognition of the polysaccharide by the protein. Recently it was suggested that sulfated iduronic acid (IdoA2S) could replace this "canonical" GlcA. Indeed, a heparin octasaccharidic sequence obtained by chemoenzymatic synthesis, in which GlcA is replaced with IdoA2S, has been found to similarly bind to and activate antithrombin. By using saturation-transfer-difference (STD) NMR, NOEs, transferred NOEs (tr-NOEs) NMR and molecular dynamics, we show that, upon binding to AT, this IdoA2S unit develops comparable interactions with AT as GlcA. Interestingly, two IdoA2S units, both present in a ¹ C₄ -² S₀ equilibrium in the unbound saccharide, shift to full ² S₀ and full ¹ C₄ upon binding to antithrombin, providing the best illustration of the critical role of iduronic acid conformational flexibility in biological systems.

Recent Developments in the Separation of Low Molecular Weight Heparin Anticoagulants

The general function of anticoagulants is to prevent blood clotting and growing of the existing clots in blood vessels. In recent years, there has been a significant improvement in developing methods of prevention as well as pharmacologic and surgical treatment of thrombosis. For over the last two decades, low molecular weight heparins (LMWHs) have found their application in the antithrombotic diseases treatment. These types of drugs are widely used in clinical therapy. Despite the biological and medical importance of LMWHs, they have not been completely characterized in terms of their chemical structure. Due to both, the structural complexity of these anticoagulants and the presence of impurities, their structural characterization requires the employment of advanced analytical techniques. Since separation techniques play the key role in these endeavors, this review will focus on the presentation of recent developments in the separation of LMWH anticoagulants.

Analysis of sulfates on low molecular weight heparin using mass spectrometry: structural characterization of enoxaparin

INTRODUCTION

Structural characterization of low molecular weight heparin (LMWH) is critical to meet biosimilarity standards. In this context, the review focuses on structural analysis of labile sulfates attached to the side-groups of LMWH using mass spectrometry. A comprehensive review of this topic will help readers to identify key strategies for tackling the problem related to sulfate loss. At the same time, various mass spectrometry techniques are presented to facilitate compositional analysis of LMWH, mainly enoxaparin. Areas covered: This review summarizes findings on mass spectrometry application for LMWH, including modulation of sulfates, using enzymology and sample preparation approaches. Furthermore, popular open-source software packages for automated spectral data interpretation are also discussed. Successful use of LC/MS can decipher structural composition for LMWH and help evaluate their sameness or biosimilarity with the innovator molecule. Overall, the literature has been searched using PubMed by typing various search queries such as 'enoxaparin', 'mass spectrometry', 'low molecular weight heparin', 'structural characterization', etc. Expert commentary: This section highlights clinically relevant areas that need improvement to achieve satisfactory commercialization of LMWHs. It also primarily emphasizes the advancements in instrumentation related to mass spectrometry, and discusses building automated software for data interpretation and analysis.

Recognition and Conformational Properties of an Alternative Antithrombin Binding Sequence Obtained by Chemoenzymatic Synthesis

Heparin is a highly sulfated glycosaminoglycan (GAG) of natural origin used as an anticoagulant and antithrombotic drug. These properties are principally based on the binding and activation of antithrombin (AT) through the pentasaccharide sequence GlcNAc/NS,6S-GlcA-GlcNS,3,6S-IdoA2S-GlcNS,6S (AGA*IA). Literature data show that the population of the 2 S0 ring conformation of the 2-O-sulfo-α-l-iduronic acid (IdoA2S) motif correlates with the affinity and activation of AT. It was recently demonstrated that two synthetic AGA*IA-containing hexasaccharides (one G unit added at the reducing end), differing in the degree of sulfation of the IdoA unit, show comparable affinity and ability to activate AT, despite a different conformation of the IdoA residue. In this paper, the binding of these two glycans to AT was studied by isothermal titration microcalorimetry (ITC), transferred (tr-) NOESY, saturation transfer difference (STD) NMR spectroscopy and molecular dynamics (MD) simulations. Results indicated that both the IdoA2S and the IdoA units assume a 2 S0 conformation when bound with AT, and so present a common binding epitope for the two glycans, centred on the AGA*IA sequence.

A review of the two major regulatory pathways for non-proprietary low-molecular-weight heparins

With the expiry or pending expiry of originator low-molecular-weight heparin (LMWH) patents, pharmaceutical companies have invested in developing non-proprietary versions of LMWHs. LMWHs are manufactured by depolymerising highly purified unfractionated heparin. In contrast to traditional synthetic drugs with well-defined chemical structures, LMWHs contain complex oligosaccharide mixtures and the different manufacturing processes for LMWHs add to the heterogeneity in their physicochemical properties such that the European Medicines Agency (EMA) and the US Food and Drug Administration (FDA) consider existing originator LMWHs to be distinct medicinal entities that are not clinically interchangeable. The FDA views LMWHs as drugs and has approved two non-proprietary (generic) LMWHs, using the Abbreviated New Drug Application pathway. In contrast, the World Health Organization and the EMA view LMWHs as biological medicines. Therefore, the EMA and also the Scientific and Standardization Subcommittee on Anticoagulation of the International Society on Thrombosis and Haemostasis and the South Asian Society of Atherosclerosis and Thrombosis have all published specific guidelines for assessing non-proprietary (biosimilar) LMWHs. This manuscript reviews why there are two distinct pathways for approving non-proprietary LMWHs. Available literature on non-proprietary LMWHs approved in some jurisdictions is also reviewed in order to assess whether they satisfy the requirements for LMWHs in the three guidance documents. The review also highlights some of the significant difficulties the two pathways pose for manufacturers and an urgent need to develop a consensus governing the manufacture and regulation of non-proprietary LMWHs to make them more widely available.

Quantitative analysis of antithrombin III binding site in low molecular weight heparins by exhausetive heparinases digestion and capillary electrophoresis

The antithrombin III (ATIII)-binding site, which contains a special 3-O-sulfated, N-sulfated glucosamine residue with or without 6-O-sulfation, is mainly responsible for the anticoagulant activity of heparin. Undergoing the chemical depolymerization process, the preservation of the ATIII-binding site in low molecular weight heparins (LMWHs) are varied leading to the fluctuation of the anticoagulant activity. Herein we report a capillary electrophoresis (CE) method in combination with heparinase digestion and affinity chromatography for the measurement of molar percentage of ATIII-binding site of LMWHs. After exhaustively digesting LMWHs with the mixture of heparinase I, II and III, almost all the resulting oligosaccharide building blocks, including the three 3-O-sulfated tetrasaccharides derived from the ATIII-binding site, were resolved by CE separation. The peak area of each building block permits quantification of the molar percentage of the ATIII-binding site. The peaks corresponding to the 3-O-sulfated tetrasaccharides were assigned based on the linear relationship between the electrophoretic mobilities of the oligosaccharides and their charge to mass ratios. The peak assignment was further confirmed by analysis of the high ATIII affinity fractions, which contains much high 3-O-sulfated tetrasaccharides. With the method, the molar percentage of the ATIII-binding site of enoxaparin from different batches and different manufactures were measured and compared. It was demonstrated that the CE method provides more precise data for assessing the anti-FXa activity than that of the biochemical assay method.

The US regulatory and pharmacopeia response to the global heparin contamination crisis

The contamination of the widely used lifesaving anticoagulant drug heparin in 2007 has drawn renewed attention to the challenges that are associated with the characterization, quality control and standardization of complex biological medicines from natural sources. Heparin is a linear, highly sulfated polysaccharide consisting of alternating glucosamine and uronic acid monosaccharide residues. Heparin has been used successfully as an injectable antithrombotic medicine since the 1930s, and its isolation from animal sources (primarily porcine intestine) as well as its manufacturing processes have not changed substantially since its introduction. The 2007 heparin contamination crisis resulted in several deaths in the United States and hundreds of adverse reactions worldwide, revealing the vulnerability of a complex global supply chain to sophisticated adulteration. This Perspective discusses how the US Food and Drug Administration (FDA), the United States Pharmacopeial Convention (USP) and international stakeholders collaborated to redefine quality expectations for heparin, thus making an important natural product better controlled and less susceptible to economically motivated adulteration.

Recent innovations in the structural analysis of heparin

Heparin, the widely used anticoagulant drug, is unusual among major pharmaceutical agents being neither single chemical entity nor a defined mixture of compounds. Its composition, while conforming to approximate average disaccharide composition or sulfation levels, exhibits heterogeneity and variability depending on the source, as well as its geographical origin. Furthermore, individual polysaccharide chains, whose physico-chemical properties are extremely similar, cannot be separated with current state-of-the-art techniques, presenting a challenge to those interested in the quality control of heparin, in ensuring its provenance and safety, and those with an interest in investigating the relationships between its structure and biological activity. The review consists of two main sections: The first is the Introduction, comprising (i) The History, Occurrence and Use of Heparin and (ii) Approaches to Structure-Activity Relationships. The second section is Improved Techniques for Structural Analysis, comprising; (i) Separation and Identification, (ii) Spectroscopic Methods, (iii) Enzymatic Approaches and (iv) Other Physico-Chemical Approaches. The ~60 references cover recent technological advances in the study of heparin structural analysis, largely since 2010.

Investigating Glycol-Split-Heparin-Derived Inhibitors of Heparanase: A Study of Synthetic Trisaccharides

Heparanase is the only known endoglycosidase able to cleave heparan sulfate. Roneparstat and necuparanib, heparanase inhibitors obtained from heparin and currently being tested in man as a potential drugs against cancer, contain in their structure glycol-split uronic acid moieties probably responsible for their strong inhibitory activity. We describe here the total chemical synthesis of the trisaccharide GlcNS6S-GlcA-1,6anGlcNS (1) and its glycol-split (gs) counterpart GlcNS6S-gsGlcA-1,6anGlcNS (2) from glucose. As expected, in a heparanase inhibition assay, compound 2 is one order of magnitude more potent than 1. Using molecular modeling techniques we have created a 3D model of 1 and 2 that has been validated by NOESY NMR experiments. The pure synthetic oligosaccharides have allowed the first in depth study of the conformation of a glycol-split glucuronic acid. Introducing a glycol-split unit in the structure of 1 increases the conformational flexibility and shortens the distance between the two glucosamine motives, thus promoting interaction with heparanase. However, comparing the relative activities of 2 and roneparstat, we can conclude that the glycol-split motive is not the only determinant of the strong inhibitory effect of roneparstat.

Pharmacology of Heparin and Related Drugs

Heparin has been recognized as a valuable anticoagulant and antithrombotic for several decades and is still widely used in clinical practice for a variety of indications. The anticoagulant activity of heparin is mainly attributable to the action of a specific pentasaccharide sequence that acts in concert with antithrombin, a plasma coagulation factor inhibitor. This observation has led to the development of synthetic heparin mimetics for clinical use. However, it is increasingly recognized that heparin has many other pharmacological properties, including but not limited to antiviral, anti-inflammatory, and antimetastatic actions. Many of these activities are independent of its anticoagulant activity, although the mechanisms of these other activities are currently less well defined. Nonetheless, heparin is being exploited for clinical uses beyond anticoagulation and developed for a wide range of clinical disorders. This article provides a "state of the art" review of our current understanding of the pharmacology of heparin and related drugs and an overview of the status of development of such drugs.

Uncovering the Relationship between Sulphation Patterns and Conformation of Iduronic Acid in Heparan Sulphate

The L-iduronic acid (IdoA) residue is a critically important structural component in heparan sulphate polysaccharide for the biological functions. The pyranose ring of IdoA is present in (1)C4-chair, (2)SO-skew boat, and less frequently, in (4)C1-chair conformations. Here, we analyzed the conformation of IdoA residue in eight hexasaccharides by NMR. The data demonstrate a correlation between the conformation of IdoA and sulphations in the surrounding saccharide residues. For the 2-O-sulpho IdoA residue, a high degree of sulphation on neighboring residues drives ring dynamics towards the (2)SO-skew boat conformer. In contrast, the nonsulphated IdoA residue is pushed towards the (1)C4-chair conformer when the neighboring residues are highly sulphated. Our data suggest that the conformation of IdoA is regulated by the sulphation pattern of nearby saccharides that is genetically controlled by the heparan sulphate biosynthetic pathway.

Anticoagulant sodium alginate sulfates and their mussel-inspired heparin-mimetic coatings

We synthesized novel sodium alginate sulfates (SASs) with different sulfation degrees. All the SASs, DA-g-SASs, and coated substrates had good anticoagulant properties and biocompatibilit.

Analytical and statistical comparability of generic enoxaparins from the US market with the originator product

Low-molecular-weight heparins (LMWHs) are complex anticoagulant drugs, made from heparin porcine mucosa starting material. Enoxaparin sodium manufactured by Sanofi is one of the most widely prescribed LMWHs and has been used since 1993 in the USA. In 2010, US Food and Drug Administration approval for supplying generic enoxaparin was granted to Sandoz and subsequently to Amphastar. Little is known, however, of the differences in composition of these preparations. In this study, samples from several batches of generic enoxaparins were purchased on the US market and analyzed with state of the art methodologies, including disaccharide building blocks quantification, nuclear magnetic resonance (NMR), and a combination of orthogonal separation techniques. Direct high-performance liquid chromatography analysis of the different enoxaparin batches revealed distinct process fingerprints associated with each manufacturer. Disaccharide building block analysis showed differences in the degree of sulfation, the presence of glycoserine derivatives, as well as in proportions of disaccharides. Results were compared by statistical approaches using multivariate analysis with a partial least squares discriminant analysis methodology. The variations were statistically significant and allowed a clear distinction to be made between the enoxaparin batches according to their manufacturer. These results were further confirmed by orthogonal analytical techniques, including NMR, which revealed compositional differences of oligosaccharides both in low- and high-affinity antithrombin fractions of enoxaparin.

Antithrombin-binding oligosaccharides: structural diversities in a unique function?

Heparin-antithrombin interaction is one of the most documented examples of heparin/protein complexes. The specific heparin sequence responsible for the binding corresponds to a pentasaccharide sequence with an internal 3-O-sulfated glucosamine residue. Moreover, the position of the pentasaccharide along the chain as well as the structure of the neighbor units affects the affinity to antithrombin. The development of separation and purification techniques, in conjunction with physico-chemical approaches (mostly NMR), allowed to characterize several structural variants of antithrombin-binding oligosaccharides, both in the free state and in complex with antithrombin. The article provides an overview of the studies that lead to the elucidation of the mechanism of interaction as well as acquiring new knowledge in heparin biosynthesis.

In-vitro suppression of IL-6 and IL-8 release from human pulmonary epithelial cells by non-anticoagulant fraction of enoxaparin

BACKGROUND

Enoxaparin, a mixture of anticoagulant and non-anticoagulant fractions, is widely used as an anticoagulant agent. However, it is also reported to possess anti-inflammatory properties. Our study indicated that enoxaparin inhibits the release of IL-6 and IL-8 from A549 pulmonary epithelial cells. Their release causes extensive lung tissue damage. The use of enoxaparin as an anti-inflammatory agent is hampered due to the risk of bleeding associated with its anticoagulant fractions. Therefore, we aimed to identify the fraction responsible for the observed anti-inflammatory effect of enoxaparin and to determine the relationship between its structure and biological activities.

METHODS

A549 pulmonary epithelial cells were pre-treated in the presence of enoxaparin and its fractions. The levels of IL-6 and IL-8 released from the trypsin-stimulated cells were measured by ELISA. The anticoagulant activity of the fraction responsible for the effect of enoxaparin was determined using an anti-factor-Xa assay. The fraction was structurally characterised using nuclear magnetic resonance. The fraction was 2-O, 6-O or N-desulfated to determine the position of sulfate groups required for the inhibition of interleukins. High-performance size-exclusion chromatography was performed to rule out that the observed effect was due to the interaction between the fraction and trypsin or interleukins.

RESULTS

Enoxaparin (60 μg/mL) inhibited the release of IL-6 and IL-8 by >30%. The fraction responsible for this effect of enoxaparin was found to be a disaccharide composed of α-L-iduronic-acid and α-D-glucosamine-6-sulfate. It (15 μg/mL) inhibited the release of interleukins by >70%. The 6-O sulphate groups were responsible for its anti-inflammatory effect. The fraction did not bind to trypsin or interleukins, suggesting the effect was not due to an artefact of the experimental model.

CONCLUSION

The identified disaccharide has no anticoagulant activity and therefore eliminates the risk of bleeding associated with enoxaparin. Future in-vivo studies should be designed to validate findings of the current study.

Susceptibility of enoxaparin reducing end amino sugars to periodate oxidation

There is a growing interest on glycol-split low-molecular weight heparins (gs-LMWHs), obtained by periodate oxidation of LMWHs, optionally followed by borohydride reduction, as potential anticancer and anti-inflammatory drugs. However, their structural characterization is still a challenging task, mainly because of the high microheterogeneity of the starting material. In addition, susceptibility to oxidation of some end-groups of LMWHs induces additional heterogeneity, making analysis of gs-LMWHs more complex. In our previous study we showed that 1,6-anhydro-d-mannosamine N-sulfate was affected by periodate, while its epimer 1,6-anhydro-d-glucosamine N-sulfate was resistant. In order to understand the apparently anomalous behavior of terminal 1,6-anhydro-d-mannosamine N-sulfate residues, in the present work we have studied by NMR spectroscopy and LC/MS the behavior of the reducing end amino sugar residues of the tetrasaccharides, isolated from the LMWH enoxaparin, in the presence of periodate. Their molecular mechanics conformational characterization has been also performed. We have shown that the C(2)-C(3) bond of the 1,6-anhydro-d-mannosamine residue can be split by periodate despite the N-substitution. Moreover, we have found that both terminal d-mannosamine N-sulfate and d-glucosamine N-sulfate, lacking the 1,6-anhydro-bridge, can be also oxidized by periodate but with a significantly lower rate. The present results suggest that the cis-e-/a-position of OH and NHSO3(-) groups of N-sulfated 1,6-anhydro-d-mannosamine is not the only factor that makes these end residues susceptible to the oxidation. The 1,6-anhydro-bridge that 'blocks' the ring conformation appears another crucial factor for oxidation to occur. Moreover, we have shown that controlling the reaction time could permit to selectively split non-sulfated iduronic acids of enoxaparin chains without oxidizing terminal amino sugar residues, a finding that may be useful to obtain more structurally homogeneous gs-LMHWs.

Re-visiting the structure of heparin

The sulfated polysaccharide heparin has been used as a life-saving anticoagulant in clinics well before its detailed structure was known. This mini-review is a survey of the evolution in the discovery of the primary and secondary structure of heparin. Highlights in this history include elucidation and synthesis of the specific sequence that binds to antithrombin, the development of low-molecular-weight heparins currently used as antithrombotic drugs, and the most promising start of chemo-enzymatic synthesis. Special emphasis is given to peculiar conformational properties contributing to interaction with proteins that modulate different biological properties.

Bottom-up low molecular weight heparin analysis using liquid chromatography-Fourier transform mass spectrometry for extensive characterization

Low molecular weight heparins (LMWHs) are heterogeneous, polydisperse, and highly negatively charged mixtures of glycosaminoglycan chains prescribed as anticoagulants. The detailed characterization of LMWH is important for the drug quality assurance and for new drug research and development. In this study, online hydrophilic interaction chromatography (HILIC) Fourier transform mass spectrometry (FTMS) was applied to analyze the oligosaccharide fragments of LMWHs generated by heparin lyase II digestion. More than 40 oligosaccharide fragments of LMWH were quantified and used to compare LMWHs prepared by three different manufacturers. The quantified fragment structures included unsaturated disaccharides/oligosaccharides arising from the prominent repeating units of these LMWHs, 3-O-sulfo containing tetrasaccharides arising from their antithrombin III binding sites, 1,6-anhydro ring-containing oligosaccharides formed during their manufacture, saturated uronic acid oligosaccharides coming from some chain nonreducing ends, and oxidized linkage region oligosaccharides coming from some chain reducing ends. This bottom-up approach provides rich detailed structural analysis and quantitative information with high accuracy and reproducibility. When combined with the top-down approach, HILIC LC-FTMS based analysis should be suitable for the advanced quality control and quality assurance in LMWH production.

Chemoenzymatic synthesis and structural characterization of 2-O-sulfated glucuronic acid-containing heparan sulfate hexasaccharides

Heparan sulfate and heparin are highly sulfated polysaccharides that consist of a repeating disaccharide unit of glucosamine and glucuronic or iduronic acid. The 2-O-sulfated iduronic acid (IdoA2S) residue is commonly found in heparan sulfate and heparin; however, 2-O-sulfated glucuronic acid (GlcA2S) is a less abundant monosaccharide (∼<5% of total saccharides). Here, we report the synthesis of three GlcA2S-containing hexasaccharides using a chemoenzymatic approach. For comparison purposes, additional IdoA2S-containing hexasaccharides were synthesized. Nuclear magnetic resonance analyses were performed to obtain full chemical shift assignments for the GlcA2S- and IdoA2S-hexasaccharides. These data show that GlcA2S is a more structurally rigid saccharide residue than IdoA2S. The antithrombin (AT) binding affinities of a GlcA2S- and an IdoA2S-hexasaccharide were determined by affinity co-electrophoresis. In contrast to IdoA2S-hexasaccharides, the GlcA2S-hexasaccharide does not bind to AT, confirming that the presence of IdoA2S is critically important for the anticoagulant activity. The availability of pure synthetic GlcA2S-containing oligosaccharides will allow the investigation of the structure and activity relationships of individual sites in heparin or heparan sulfate.

Hydroxyl-proton hydrogen bonding in the heparin oligosaccharide Arixtra in aqueous solution

Heparin is best known for its anticoagulant activity, which is mediated by the binding of a specific pentasaccharide sequence to the protease inhibitor antithrombin-III (AT-III). Although heparin oligosaccharides are thought to be flexible in aqueous solution, the recent discovery of a hydrogen bond between the sulfamate (NHSO3(-)) proton and the adjacent 3-O-sulfo group of the 3,6-O-sulfated N-sulfoglucosamine residue of the Arixtra (fondaparinux sodium) pentasaccharide demonstrates that definable elements of local structure are accessed. Molecular dynamics simulations of Arixtra suggest the presence of additional hydrogen bonds involving the C3-OH groups of the glucuronic acid and 2-O-sulfo-iduronic acid residues. NMR measurements of temperature coefficients, chemical shift differences, and solvent exchange rate constants provide experimental confirmation of these hydrogen bonds. We note that the extraction of rate constants from cross-peak buildup curves in 2D exchange spectroscopy is complicated by the presence of radiation damping in aqueous solution. A straightforward model is presented that explicitly takes into account the effects of radiation damping on the water proton relaxation and is sufficiently robust to provide an accurate measure of the proton exchange rate between the analyte hydroxyl protons and water.

New targets for glycosaminoglycans and glycosaminoglycans as novel targets

Biological functions of a variety of proteins are mediated via their interaction with glycosaminoglycans (GAGs). The structural diversity within the wide GAG landscape provides individual interaction sites for a multitude of proteins involved in several pathophysiological processes. This 'GAG angle' of such proteins as well as their specific GAG ligands give rise to novel therapeutic concepts for drug development. Current glycomic technologies to elucidate the glycan structure-function relationships, methods to investigate the selectivity and specificity of glycan-protein interactions and existing therapeutic approaches to interfere with GAG-protein interactions are discussed.

Heparin dodecasaccharide containing two antithrombin-binding pentasaccharides: structural features and biological properties

The antithrombin (AT) binding properties of heparin and low molecular weight heparins are strongly associated to the presence of the pentasaccharide sequence AGA*IA (A(NAc,6S)-GlcUA-A(NS,3,6S)-I(2S)-A(NS,6S)). By using the highly chemoselective depolymerization to prepare new ultra low molecular weight heparin and coupling it with the original separation techniques, it was possible to isolate a polysaccharide with a biosynthetically unexpected structure and excellent antithrombotic properties. It consisted of a dodecasaccharide containing an unsaturated uronate unit at the nonreducing end and two contiguous AT-binding sequences separated by a nonsulfated iduronate residue. This novel oligosaccharide was characterized by NMR spectroscopy, and its binding with AT was determined by fluorescence titration, NMR, and LC-MS. The dodecasaccharide displayed a significantly increased anti-FXa activity compared with those of the pentasaccharide, fondaparinux, and low molecular weight heparin enoxaparin.

An unusual antithrombin-binding heparin octasaccharide with an additional 3-O-sulfated glucosamine in the active pentasaccharide sequence

The 3-O-sulfation of N-sulfated glucosamine is the last event in the biosynthesis of heparin/heparan sulfate, giving rise to the antithrombin-binding pentasaccharide sequence AGA*IA, which is largely associated with the antithrombotic activity of these molecules. The aim of the present study was the structural and biochemical characterization of a previously unreported AGA*IA*-containing octasaccharide isolated from the very-low-molecular-mass heparin semuloparin, in which both glucosamine residues of the pentasaccharide moiety located at the non-reducing end bear 3-O-sulfate groups. Two-dimensional and STD (saturation transfer difference) NMR experiments clearly confirmed its structure and identified its ligand epitope binding to antithrombin. The molecular conformation of the octasaccharide-antithrombin complex has been determined by NMR experiments and docking/energy minimization. The presence of the second 3-O-sulfated glucosamine in the octasaccharide induced more than one order of magnitude increase in affinity to antithrombin compared to the pentasaccharide AGA*IA.

Characterizing the microstructure of heparin and heparan sulfate using N-sulfoglucosamine 1H and 15N NMR chemical shift analysis

Heparin and heparan sulfate (HS) are members of a biologically important group of highly anionic linear polysaccharides called glycosaminoglycans (GAGs). Because of their structural complexity, the molecular-level characterization of heparin and HS continues to be a challenge. The work presented herein describes an emerging approach for the analysis of unfractionated and low molecular weight heparins, as well as porcine and human-derived HS. This approach utilizes the untapped potential of (15)N NMR to characterize these preparations through detection of the NH resonances of N-sulfo-glucosamine residues. The sulfamate group (1)H and (15)N chemical shifts of six GAG microenvironments were assigned based on the critical comparison of selectively modified heparin derivatives, NMR measurements for a library of heparin-derived oligosaccharide standards, and an in-depth NMR analysis of the low molecular weight heparin enoxaparin through systematic investigation of the chemical exchange properties of NH resonances and residue-specific assignments using the [(1)H,(15)N] HSQC-TOCSY experiment. The sulfamate microenvironments characterized in this study include GlcNS(6S)-UA(2S), ΔUA(2S)-GlcNS(6S), GlcNS(3S)(6S)-UA(2S), GlcNS-UA, GlcNS(6S)-red(α), and 1,6-anhydro GlcNS demonstrating the utility of [(1)H,(15)N] HSQC NMR spectra to provide a spectroscopic fingerprint reflecting the composition of intact GAGs and low molecular weight heparin preparations.

Top-down approach for the direct characterization of low molecular weight heparins using LC-FT-MS

Low molecular heparins (LMWHs) are structurally complex, heterogeneous, polydisperse, and highly negatively charged mixtures of polysaccharides. The direct characterization of LMWH is a major challenge for currently available analytical technologies. Electrospray ionization (ESI) liquid chromatography-mass spectrometry (LC-MS) is a powerful tool for the characterization complex biological samples in the fields of proteomics, metabolomics, and glycomics. LC-MS has been applied to the analysis of heparin oligosaccharides, separated by size exclusion, reversed phase ion-pairing chromatography, and chip-based amide hydrophilic interaction chromatography (HILIC). However, there have been limited applications of ESI-LC-MS for the direct characterization of intact LMWHs (top-down analysis) due to their structural complexity, low ionization efficiency, and sulfate loss. Here we present a simple and reliable HILIC-Fourier transform (FT)-ESI-MS platform to characterize and compare two currently marketed LMWH products using the top-down approach requiring no special sample preparation steps. This HILIC system relies on cross-linked diol rather than amide chemistry, affording highly resolved chromatographic separations using a relatively high percentage of acetonitrile in the mobile phase, resulting in stable and high efficiency ionization. Bioinformatics software (GlycReSoft 1.0) was used to automatically assign structures within 5-ppm mass accuracy.

Sulfamate proton solvent exchange in heparin oligosaccharides: evidence for a persistent hydrogen bond in the antithrombin-binding pentasaccharide Arixtra

Sulfamate groups (NHSO(3)(-)) are important structural elements in the glycosaminoglycans (GAGs) heparin and heparan sulfate (HS). In this work, proton nuclear magnetic resonance (NMR) line-shape analysis is used to explore the solvent exchange properties of the sulfamate NH groups within heparin-related mono-, di-, tetra- and pentasaccharides as a function of pH and temperature. The results of these experiments identified a persistent hydrogen bond within the Arixtra (fondaparinux sodium) pentasaccharide between the internal glucosamine sulfamate NH and the adjacent 3-O-sulfo group. This discovery provides new insights into the solution structure of the Arixtra pentasaccharide and suggests that 3-O-sulfation of the heparin N-sulfoglucosamine (GlcNS) residues pre-organize the secondary structure in a way that facilitates binding to antithrombin-III. NMR studies of the GlcNS NH groups can provide important information about heparin structure complementary to that available from NMR spectral analysis of the carbon-bound protons.

Binding of heparin-dependent antibodies to PF4 modified by enoxaparin oligosaccharides: evaluation by surface plasmon resonance and serotonin release assay

BACKGROUND

The minimal structural requirements of low-molecular-weight heparins that determine the risk of developing heparin-induced thrombocytopenia (HIT) are not fully defined.

OBJECTIVES

The ability of enoxaparin-derived oligosaccharides (OS) to induce platelet activation and exposure of platelet-factor 4 (PF4) epitopes recognized by antibodies developed in HIT was studied by surface plasmon resonance (SPR) and serotonin release assay.

RESULTS

Decasaccharides with ≥ 11 sulfate groups induced platelet activation in the presence of plasma from patients with confirmed HIT. Serotonin release of > 80% without full inhibition at 100 μg mL(-1) was achieved with decasaccharides containing 14 or 15 sulfate groups, 2 dodecasaccharides and 2 tetradecasaccharides. An SPR method was developed using purified PF4 immobilized on carboxymethylated dextran. Antibodies from all HIT samples bound to PF4/heparin in SPR assays with resonance units (RU) ratio of 109-173 with HIT plasma vs. 88-93 with control plasma. RU ratios > 100 were measured when PF4 was pre-incubated with OS with ≥ 10 saccharide units and one octasaccharide containing 10 sulfate groups. RU ratios > 140, similar to those measured when PF4 was pre-incubated with unfractionated heparin or enoxaparin, were obtained with purified dodeca- and tetradecasaccharides. RU values strongly correlated with the number of sulfate groups in the decasaccharides tested (r = 0.93, P = 0.02).

CONCLUSIONS

LMWHs with fragments > 10 saccharides and a large number of sulfate groups are more likely to be associated with a higher risk of HIT. These structure-activity relationships were independent of the ability of the OS to bind antithrombin.

A highly efficient tree structure for the biosynthesis of heparan sulfate accounts for the commonly observed disaccharides and suggests a mechanism for domain synthesis

The form of the biosynthetic pathway of the biologically and medically important polysaccharides heparan sulfate (HS) and the closely related heparin remain obscure despite significant progress characterising the biosynthetic machinery. Considering possible biosynthetic schemes using a graph approach and applying known constraints of enzyme order and specificity, a previously unreported system with a highly efficient tree structure emerged with two features: (1) All commonly occurring HS disaccharides could be synthesised through a common route, the major branch. (2) The least common disaccharides also occurred on a separate common branch, termed here the minor branch. This suggested that the relative abundance of these two sets of structures were the result of the specificity of a single enzyme (HS epimerase) acting at an early point in the scheme, to convert GlcA-GlcNS to IdoA-GlcNS in preference to converting GlcA-GlcNAc to IdoA-GlcNAc. A third key finding was that the common substrates for 3-O-sulfation all lie on the same (major) branch. The proposed scheme is consistent with a wide body of experiments comprising both biochemical data and results from HS biosynthetic enzyme knockout experiments in the literature. The major branch also contains a bifurcation, providing a choice of two distinct backbone geometries with the same charge. Further development of this novel biosynthetic scheme, in which frame shifts in the site of action of the enzymes were permitted to occur, while maintaining their order of action, suggested a mechanism by which domains could be generated, or further modification blocked. The relationship between the proposed pathway and the geometric and charge possibilities it allows were also explored.

Molecular architecture of heparin and heparan sulfate: Recent developments in solution structural studies

The study of the relationship between the complex structures and numerous physiological functions of the glycosaminoglycans (GAGs) heparin and heparan sulfate (HS) has continued to thrive in the past decade. Though it is clear that the monosaccharide sequences of these polysaccharides must determine their ability to modulate the action of growth factors, morphogens, chemokines, cytokines, and many other extracellular proteins, the exact details of this dependence still prove elusive. Sequence determines the 3D structure of GAGs at more than one level; detailed sequences of highly sulfated regions may influence affinity for specific proteins in some cases, but in addition attention has been called to the importance of the length and spacing of these highly sulfated sequences, which are separated by unsulfated domains. Within the sulfated “S-domains”, the internal dynamics of the conformationally flexible iduronate pyranose ring have continued to interest NMR spectroscopists and molecular modelers. New studies of the relative degrees of flexibility of sulfated and unsulfated domains lead to an overall model of heparin/HS in which protein-binding, highly sulfated S-domains with well-defined conformations are separated by more flexible NA-domains.