Synthetic heparin derivatives as new anticoagulant drugs

Protecting group principles suited to late stage functionalization and global deprotection in oligosaccharide synthesis

Chemical synthesis is a powerful tool to access homogeneous complex glycans, which relies on protecting group (PG) chemistry. However, the overall efficiency of chemical glycan assembly is still low when compared to oligonucleotide or oligopeptide synthesis. There have been many contributions giving rise to collective improvement in carbohydrate synthesis that includes PG manipulation and stereoselective glycoside formation and some of this chemistry has been transferred to the solid phase or adapted for programmable one pot synthesis approaches. However, after all glycoside bond formation reactions are completed, the global deprotection (GD) required to give the desired target OS can be challenging. Difficulties observed in the removal of permanent PGs to release the desired glycans can be due to the number and diversity of PGs present in the protected OSs, nature and structural complexity of glycans, etc. Here, we have reviewed the difficulties associated with the removal of PGs from densely protected OSs to obtain their free glycans. In particularly, this review focuses on the challenges associated with hydrogenolysis of benzyl groups, saponification of esters and functional group interconversion such as oxidation/reduction that are commonly performed in GD stage. More generally, problems observed in the removal of permanent PGs is reviewed herein, including benzyl, acyl (levulinoyl, acetyl), N-trichloroacetyl, N-2,2,2-trichloroethoxycarbonyl, N-phthaloyl etc. from a number of fully protected OSs to release the free sugar, that have been previously reported in the literature.

Multiple-therapy strategies via polysaccharides-based nano-systems in fighting cancer

Polysaccharide-based biomaterials (e.g., chitosan, dextran, hyaluronic acid, chondroitin sulfate and heparin) have received great attention in healthcare, particularly in drug delivery for tumor therapy. They are naturally abundant and available, outstandingly biodegradable and biocompatible, and they generally have negligible toxicity and low immunogenicity. In addition, they are easily chemically or physically modified. Therefore, PSs-based nanoparticles (NPs) have been extensively investigated for the enhancement of tumor treatment. In this review, we introduce the synthetic pathways of amphiphilic PS derivatives, which allow the constructs to self-assemble into NPs with various structures. We especially offer an overview of the emerging applications of self-assembled PSs-based NPs in tumor chemotherapy, photothermal therapy (PTT), photodynamic therapy (PDT), gene therapy and immunotherapy. We believe that this review can provide criteria for a rational and molecular level-based design of PS-based NPs, and comprehensive insight into the potential of PS-based NPs used in multiple cancer therapies.

In vitro and in vivo anticoagulant activity of heparin-like biomacromolecules and the mechanism analysis for heparin-mimicking activity

Heparin-like biomacromolecules (HepLBm), exhibiting similar chemical structure and biological properties to heparin, can be obtained by modifying either synthetic biopolymers or natural biomacromolecules with physical or chemical methods. In this work, a low-cost and biocompatible sodium alginate was chosen as a model biomacromolecule to design anticoagulant HepLBm with a similar sulfation degree to heparin. FTIR, ¹H NMR, and element analysis data were used to confirm the chemical structure of HepLBm. Hemolysis tests, clotting time, complement activation, and contact activation tests were carried out to determine the in vitro anticoagulant activity of HepLBm. In addition, systematic studies of blood cell count, coagulation function, and histopathology were performed to demonstrate the in vivo anticoagulant activity and toxicity of HepLBm with SD rat experiments. Furthermore, a series of linear molecules containing carboxyl groups, sulfonic groups, and hydroxyl groups were selected and their clotting time was tested to provide a mechanism analysis for the excellent anticoagulant activity of HepLBm. With the excellent in vitro/in vivo anticoagulant activity, good biocompatibility, and low cost, the HepLBm synthesized in this work would have great potential for substitution of heparin in many application fields, such as the surface modification of biomedical devices, extracorporeal anticoagulants, and other clinical fields.

Chemical synthesis of glycosaminoglycan-mimetic polymers

This review describes several general chemical approaches for the preparation of glycosaminoglycan (GAG)-mimetic polymers based on different backbones and sidechains, and highlights the importance of these synthetic GAG-mimetic polymers in controlling key biofunctions.

Synthetic Oligosaccharide Libraries and Microarray Technology: A Powerful Combination for the Success of Current Glycosaminoglycan Interactomics

Glycosaminoglycans (GAGs) are extracellular matrix and/or cell-surface sulfated glycans crucial to the regulation of various signaling proteins, the functions of which are essential in many pathophysiological systems. Because structural heterogeneity is high in GAG chains and purification is difficult, the use of structurally defined GAG oligosaccharides from natural sources as molecular models in both biophysical and pharmacological assays is limited. To overcome this obstacle, GAG-like oligosaccharides of well-defined structures are currently being synthesized by chemical and/or enzymatic means in many research groups around the world. These synthetic GAG oligosaccharides serve as useful molecular tools in studies of GAG-protein interactions. In this review, besides discussing the commonest routes used for the synthesis of GAG oligosaccharides, we also survey some libraries of these synthetic models currently available for research and discuss their activities in interaction studies with functional proteins, especially through the microarray approach.

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.

Development of low molecular weight heparin based nanoparticles for metastatic breast cancer therapy

Tumor metastasis is the primary obstacle in cancer treatment and is always the leading cause of human death. And heparin and its derivatives are potential anti-metastatic agents with good biocompatibility. In this work, low molecular weight heparin (LMWH) based LMWH-Cholesterol (LHC) conjugates were prepared for intravenous delivery of doxorubicin (DOX). The DOX/LHC nanoparticles (DOX/LHC NPs) exhibited a spherical shape with a mean diameter of 135.5±2.2nm and had a longer circulation time than that of DOX. The in vitro results confirmed that the DOX/LHC NPs was more effectively taken up by 4T1 cells and showed a stronger anti-metastatic effect by cell invasion and cell migration compared with DOX. Meanwhile, DOX/LHC NPs also exhibited superior anti-metastatic effects in the pulmonary metastasis model compared with other groups. The reason may be account for the synergistic effect between the cytotoxic drug of DOX and its drug carrier of LMWH based nanoparticles, which is capable of anti-metastatic and anti-angiogenic efficiency. Thus DOX/LHC nanoparticles could be a promising anti-metastatic drug delivery system for postoperative chemotherapy.

Synthesis and Thrombin, Factor Xa and U46619 Inhibitory Effects of Non-Amidino and Amidino N²-Thiophenecarbonyl- and N²-Tosylanthranilamides

Thrombin (factor IIa) and factor Xa (FXa) are key enzymes at the junction of the intrinsic and extrinsic coagulation pathways and are the most attractive pharmacological targets for the development of novel anticoagulants. Twenty non-amidino N²-thiophencarbonyl- and N²-tosyl anthranilamides 1–20 and six amidino N²-thiophencarbonyl- and N²-tosylanthranilamides 21–26 were synthesized to evaluate their activated partial thromboplastin time (aPTT) and prothrombin time (PT) using human plasma at a concentration of 30 µg/mL in vitro. As a result, compounds 5, 9, and 21–23 were selected to study the further antithrombotic activity. The anticoagulant properties of 5, 9, and 21–23 significantly exhibited a concentration-dependent prolongation of in vitro PT and aPTT, in vivo bleeding time, and ex vivo clotting time. These compounds concentration-dependently inhibited the activities of thrombin and FXa and inhibited the generation of thrombin and FXa in human endothelial cells. In addition, data showed that 5, 9, and 21–23 significantly inhibited thrombin catalyzed fibrin polymerization and mouse platelet aggregation and inhibited platelet aggregation induced by U46619 in vitro and ex vivo. Among the derivatives evaluated, N-(3′-amidinophenyl)-2-((thiophen-2′′-yl)carbonylamino)benzamide (21) was the most active compound.

Synthetic Glycopolymers for Highly Efficient Differentiation of Embryonic Stem Cells into Neurons: Lipo- or Not?

To realize the potential application of embryonic stem cells (ESCs) for the treatment of neurodegenerative diseases, it is a prerequisite to develop an effective strategy for the neural differentiation of ESCs so as to obtain adequate amount of neurons. Considering the efficacy of glycosaminoglycans (GAG) and their disadvantages (e.g., structure heterogeneity and impurity), GAG-mimicking glycopolymers (designed polymers containing functional units similar to natural GAG) with or without phospholipid groups were synthesized in the present work and their ability to promote neural differentiation of mouse ESCs (mESCs) was investigated. It was found that the lipid-anchored GAG-mimicking glycopolymers (lipo-pSGF) retained on the membrane of mESCs rather than being internalized by cells after 1 h of incubation. Besides, lipo-pSGF showed better activity in promoting neural differentiation. The expression of the neural-specific maker β3-tubulin in lipo-pSGF-treated cells was ∼3.8- and ∼1.9-fold higher compared to natural heparin- and pSGF-treated cells at day 14. The likely mechanism involved in lipo-pSGF-mediated neural differentiation was further investigated by analyzing its effect on fibroblast growth factor 2 (FGF2)-mediated extracellular signal-regulated kinases 1 and 2 (ERK1/2) signaling pathway which is important for neural differentiation of ESCs. Lipo-pSGF was found to efficiently bind FGF2 and enhance the phosphorylation of ERK1/2, thus promoting neural differentiation. These findings demonstrated that engineering of cell surface glycan using our synthetic lipo-glycopolymer is a highly efficient approach for neural differentiation of ESCs and this strategy can be applied for the regulation of other cellular activities mediated by cell membrane receptors.

Redox-sensitive self-assembled nanoparticles based on alpha-tocopherol succinate-modified heparin for intracellular delivery of paclitaxel

To remedy the problems riddled in cancer chemotherapy, such as poor solubility, low selectivity, and insufficient intra-cellular release of drugs, novel heparin-based redox-sensitive polymeric nanoparticles were developed. The amphiphilic polymer, heparin-alpha-tocopherol succinate (Hep-cys-TOS) was synthesized by grafting hydrophobic TOS to heparin using cystamine as the redox-sensitive linker, which could self-assemble into nanoparticles in phosphate buffer saline (PBS) with low critical aggregation concentration (CAC) values ranging from 0.026 to 0.093mg/mL. Paclitaxel (PTX)-loaded Hep-cys-TOS nanoparticles were prepared via a dialysis method, exhibiting a high drug-loading efficiency of 18.99%. Physicochemical properties of the optimized formulation were characterized by dynamic light scattering (DLS), transmission electron microscope (TEM) and differential scanning calorimetry (DSC). Subsequently, the redox-sensitivity of Hep-cys-TOS nanoparticles was confirmed by the changes in size distribution, morphology and appearance after dithiothreitol (DTT) treatment. Besides, the in vitro release of PTX from Hep-cys-TOS nanoparticles also exhibited a redox-triggered profile. Also, the uptake behavior and pathways of coumarin 6-loaded Hep-cys-TOS nanoparticles were investigated, suggesting the nanoparticles could be taken into MCF-7 cells in energy-dependent, caveolae-mediated and cholesterol-dependent endocytosis manners. Later, MTT assays of different PTX-free and PTX-loaded formulations revealed the desirable safety of PTX-free nanoparticles and the enhanced anti-cancer activity of PTX-loaded Hep-cys-TOS nanoparticles (IC₅₀=0.79μg/mL). Apoptosis study indicated the redox-sensitive formulation could induce more apoptosis of MCF-7 cells than insensitive one (55.2% vs. 41.7%), showing the importance of intracellular burst release of PTX. Subsequently, the hemolytic toxicity confirmed the safety of the nanoparticles for intravenous administration. The results indicated the developed redox-sensitive nanoparticles were promising as intracellular drug delivery vehicles for cancer treatment.

Endoglycosidases for the Synthesis of Polysaccharides and Glycoconjugates

Recent advances in glycobiology have implicated essential roles of oligosaccharides and glycoconjugates in many important biological recognition processes, including intracellular signaling, cell adhesion, cell differentiation, cancer progression, host-pathogen interactions, and immune responses. A detailed understanding of the biological functions, as well as the development of carbohydrate-based therapeutics, often requires structurally well-defined oligosaccharides and glycoconjugates, which are usually difficult to isolate in pure form from natural sources. To meet with this urgent need, chemical and chemoenzymatic synthesis has become increasingly important as the major means to provide homogeneous compounds for functional glycocomics studies and for drug/vaccine development. Chemoenzymatic synthesis, an approach that combines chemical synthesis and enzymatic manipulations, is often the method of choice for constructing complex oligosaccharides and glycoconjugates that are otherwise difficult to achieve by purely chemical synthesis. Among these, endoglycosidases, a class of glycosidases that hydrolyze internal glycosidic bonds in glycoconjugates and polysaccharides, are emerging as a very attractive class of enzymes for synthetic purposes, due to their transglycosylation activity and their capability of transferring oligosaccharide units en bloc in a single step, in contrast to the limitation of monosaccharide transfers by common glycosyltransferases. In this chapter, we provide an overview on the application of endoglycosidases for the synthesis of complex carbohydrates, including oligosaccharides, polysaccharides, glycoproteins, glycolipids, proteoglycans, and other biologically relevant polysaccharides. The scope, limitation, and future directions of endoglycosidase-catalyzed synthesis are discussed.

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.

Synthesis and anticoagulant activity of polyureas containing sulfated carbohydrates

Polyurea-based synthetic glycopolymers containing sulfated glucose, mannose, glucosamine, or lactose as pendant groups have been synthesized by step-growth polymerization of hexamethylene diisocyanate and corresponding secondary diamines. The obtained polymers were characterized by gel permeation chromatography, nuclear magnetic resonance spectroscopy, and Fourier transform infrared spectroscopy. The nonsulfated polymers showed similar results to the commercially available biomaterial polyurethane TECOFLEX in a platelet adhesion assay. The average degree of sulfation after reaction with SO₃ was calculated from elemental analysis and found to be between three and four −OSO₃ groups per saccharide. The blood-compatibility of the synthetic polymers was measured using activated partial thromboplastin time, prothrombin time, thrombin time, anti-IIa, and anti-Xa assays. Activated partial thromboplastin time, prothrombin time, and thrombin time results indicated that the mannose and lactose based polymers had the highest anticoagulant activities among all the sulfated polymers. The mechanism of action of the polymers appears to be mediated via an anti-IIa pathway rather than an anti-Xa pathway.

Affinity-based design of a synthetic universal reversal agent for heparin anticoagulants

Heparin-based anticoagulant drugs have been widely used for the prevention of blood clotting during surgical procedures and for the treatment of thromboembolic events. However, bleeding risks associated with these anticoagulants demand continuous monitoring and neutralization with suitable antidotes. Protamine, the only clinically approved antidote to heparin, has shown adverse effects and ineffectiveness against low-molecular weight heparins and fondaparinux, a heparin-related medication. Alternative approaches based on cationic molecules and recombinant proteins have several drawbacks including limited efficacy, toxicity, immunogenicity, and high cost. Thus, there is an unmet clinical need for safer, rapid, predictable, and cost-effective anticoagulant-reversal agents for all clinically used heparins. We report a design strategy for a fully synthetic dendritic polymer-based universal heparin reversal agent (UHRA) that makes use of multivalent presentation of branched cationic heparin binding groups (HBGs). Optimization of the UHRA design was aided by isothermal titration calorimetry studies, biocompatibility evaluation, and heparin neutralization analysis. By controlling the scaffold's molecular weight, the nature of the protective shell, and the presentation of HBGs on the polymer scaffold, we arrived at lead UHRA molecules that completely neutralized the activity of all clinically used heparins. The optimized UHRA molecules demonstrated superior efficacy and safety profiles and mitigated heparin-induced bleeding in animal models. This new polymer therapeutic may benefit patients undergoing high-risk surgical procedures and has potential for the treatment of anticoagulant-related bleeding problems.

Efficient synthesis of a library of heparin tri- and tetrasaccharides relevant to the substrate of heparanase

A robust glycosylation protocol was fixed to construct the GlcN–(1α→4)-GlcA/IdoA linkagesen routeto heparin oligosaccharides.

5-Chlorothiophene-2-carboxylic acid [(S)-2-[2-methyl-3-(2-oxopyrrolidin-1-yl)benzenesulfonylamino]-3-(4-methylpiperazin-1-yl)-3-oxopropyl]amide (SAR107375), a selective and potent orally active dual thrombin and factor Xa inhibitor

Compound 15 (SAR107375), a novel potent dual thrombin and factor Xa inhibitor resulted from a rational optimization process. Starting from compound 14, with low factor Xa and modest anti-thrombin inhibitory activities (IC50's of 3.5 and 0.39 μM, respectively), both activities were considerably improved, notably through the incorporation of a neutral chlorothiophene P1 fragment and tuning of P2 and P3-P4 fragments. Final optimization of metabolic stability with microsomes led to the identification of 15, which displays strong activity in vitro vs factor Xa and thrombin (with Ki's of 1 and 8 nM, respectively). In addition 15 presents good selectivity versus related serine proteases (roughly 300-fold), including trypsin (1000-fold), and is very active (0.39 μM) in the thrombin generation time (TGT) coagulation assay in human platelet rich plasma (PRP). Potent in vivo activity in a rat model of venous thrombosis following iv and, more importantly, po administration was also observed (ED50 of 0.07 and 2.8 mg/kg, respectively). Bleeding liability was reduced in the rat wire coil model, more relevant to arterial thrombosis, with 15 (blood loss increase of 2-fold relative to the ED80 value) compared to rivaroxaban 2 and dabigatran etexilate 1a.

Investigational anticoagulants for hematological conditions: a new generation of therapies

INTRODUCTION

The introduction of novel anticoagulants has had contrasting effects on the agents in the pipeline, fueling the development of some and sinking the others. The complexity of the coagulation cascade offers interesting inhibition choices that might become valid treatment options.

AREAS COVERED

This review will highlight some of the anticoagulants in the pipeline. Following the success of the direct thrombin and FXa inhibitors already in the market, new agents are being tested. These include AZD0837, betrixaban, letaxaban, darexaban, and LY517717. Targeting other components of the hemostatic pathway might lead to better safety profiles without influencing efficacy. Inhibitors to FVIIa-tissue factor (FVIIa/TF) complex, FIX, FXI, and FXII are being assessed. New inspiring inhibitors are antisense oligonucleotides (ASOs) and aptamers. These are highly specific agents with readily reversible effect and might be engineered to inhibit any coagulation factor. Currently tested ASOs and aptamers are inhibitors of FXI, FXII, thrombin, FIXa, and platelet GPIV.

EXPERT OPINION

Some of the agents in the pipeline offer valid treatment option for long-term therapy, overcoming some of the drawbacks of the novel anticoagulants. Research is being driven by an expanding market in the anticoagulation field that has been unexploited for a long time.

Application of near-infrared spectroscopy combined with multivariate analysis in monitoring of crude heparin purification process

Ion-exchange chromatography is a widely used purification technology in the heparin manufacturing process. To improve the efficiency and understand the process directly, a rapid and equally precise method needs to be developed to measure heparin concentration in chromatography process. Here, two robust partial least squares regression (PLS-R) models were established for quantification of heparin based on the near-infrared (NIR) spectroscopy with 80 samples of adsorption process and 76 samples of elution process. Several variables selection algorithms, including correlation coefficient method, successive projection algorithm (SPA) and interval partial least squares (iPLSs), were performed to remove non-informative variables. The results showed that the correlation coefficient of validation (Rp) and the residual predictive deviation (RPD) corresponded to 0.957 and 3.4472 for adsorption process, 0.968 and 3.9849 for elution process, respectively. The approach was found considerable potential for real-time monitoring the heparin concentration of chromatography process.

Heparosan-derived heparan sulfate/heparin-like compounds: one kind of potential therapeutic agents

Heparan sulfate (HS) is a highly sulfated glycosaminoglycan and exists in all animal tissues. HS and heparin are very similar, except that heparin has higher level of sulfation and higher content of iduronic acid. Despite the fact that it is a century-old drug, heparin remains as a top choice for treating thrombotic disorders. Pharmaceutical heparin is derived from porcine intestine or bovine lung via a long supply chain. This supply chain is vulnerable to the contamination of animal pathogens. Therefore, new methods for manufacturing heparin or heparin-like substances devoid of animal tissues have been explored by many researchers, among which, modifications of heparosan, the capsular polysaccharide of Escherichia coli K5 strain, is one of the promising approaches. Heparosan has a structure similar to unmodified backbone of natural HS and heparin. It is feasible to obtain HS or heparin derivatives by modifying heparosan with chemical or enzymatic methods. These derivatives display different biological activities, such as anticoagulant, anti-inflammatory, anticancer, and antiviral activities. This review focuses on the recent studies of synthesis, activity, and structure-activity relationship of HS/heparin-like derivatives prepared from heparosan.

New anticoagulants - promising and failed developments

New direct and indirect acting factor Xa (FXa) and thrombin inhibitors are being developed to overcome the downsides of the conventional anticoagulants - unfractionated and low molecular weight heparins and vitamin K antagonists. Ximelagatran and idraparinux failed to demonstrate an acceptable safety profile. Rivaroxaban and dabigatran are approved for the post-operative prevention of thromboembolic complications after elective hip or knee replacement surgery; dabigatran is approved for the prevention of embolism in patients with atrial fibrillation in an increasing number of countries. Several novel indirect antithrombin-dependent anticoagulants as well as antithrombin-independent oral direct FXa and thrombin inhibitors are investigated in multiple indications for the prophylaxis and treatment of venous thromboembolism and the prophylaxis of arterial thrombotic disorders. Quality-adjusted life years costs and incremental cost-effectiveness ratios are relatively high at present, but may decrease after approval of more new anticoagulants for additional indications.

A closer look at the nitrogen next door: 1H-15N NMR methods for glycosaminoglycan structural characterization

Recently, experimental conditions were presented for the detection of the N-sulfoglucosamine (GlcNS) NHSO(3)(-) or sulfamate (1)H and (15)N NMR resonances of the pharmaceutically and biologically important glycosaminoglycan (GAG) heparin in aqueous solution. In the present work, we explore further the applicability of nitrogen-bound proton detection to provide structural information for GAGs. Compared to the detection of (15)N chemical shifts of aminosugars through long-range couplings using the IMPACT-HNMBC pulse sequence, the more sensitive two-dimensional (1)H-(15)N HSQC-TOCSY experiments provided additional structural data. The IMPACT-HNMBC experiment remains a powerful tool as demonstrated by the spectrum measured for the unsubstituted amine of 3-O-sulfoglucosamine (GlcN(3S)), which cannot be observed with the (1)H-(15)N HSQC-TOCSY experiment due to the fast exchange of the amino group protons with solvent. The (1)H-(15)N HSQC-TOCSY NMR spectrum reported for the mixture of model compounds GlcNS and N-acetylglucosamine (GlcNAc) demonstrate the broad utility of this approach. Measurements for the synthetic pentasaccharide drug Arixtra® (Fondaparinux sodium) in aqueous solution illustrate the power of this NMR pulse sequence for structural characterization of highly similar N-sulfoglucosamine residues in GAG-derived oligosaccharides.

Synthesis of heparin oligosaccharides and their interaction with eosinophil-derived neurotoxin

A convenient route for the synthesis of heparin oligosaccharides involving regioselective protection of D-glucosamine and a concise preparation of rare L-ido sugars from diacetone α-D-glucose is described. Stereoselective coupling of a D-glucosamine-derived trichloroacetimidate with a 1,6-anhydro-β-L-idopyranosyl 4-alcohol gave the desired α-linked disaccharide, which was used as repeating unit for dual chain elongation and termination. Stepwise assembly from the reducing to the non-reducing end with a D-glucosamine-derived monosaccharide as starting unit furnished the oligosaccharide skeletons having different chain lengths. A series of functional group transformations afforded the expected heparin oligosaccharides with 3, 5 and 7 sugar units. Interaction of these oligosaccharides with eosinophil-derived neurotoxin (EDN), a cationic ribonuclease and a mediator produced by human eosinophils, was further investigated. The results revealed that at 5 μg mL(-1), the heptasaccharide has sufficiently strong interference to block EDN binding to Beas-2B cells. The tri- and pentasaccharides have moderate inhibitory properties at 50 μg mL(-1) concentration, but no inhibition has been observed at 10 μg mL(-1). The IC(50) values of the tri-, penta- and heptasaccharides are 69.4, 47.2 and 0.225 μg mL(-1), respectively.

Specificity and selectivity profile of EP217609: a new neutralizable dual-action anticoagulant that targets thrombin and factor Xa

EP217609 is a new dual-action parenteral anticoagulant that combines an indirect factor Xa inhibitor (fondaparinux analog) and a direct thrombin inhibitor (α-NAPAP analog) in a single molecule together with a biotin tag to allow avidin neutralization. EP217609 exhibits an unprecedented pharmacologic profile in showing high bioavailability, long plasma half-life, and potent antithrombotic activity in animals without the complications of thrombin rebound. Here we report the exceptional specificity and selectivity profile of EP217609. EP217609 inhibited thrombin with rapid kinetics (k(on) > 10(7)M(-1)s(-1)), a high affinity (K(I) = 30-40pM), and more than 1000-fold selectivity over other coagulation and fibrinolytic protease targets, comparing favorably with the best direct thrombin inhibitors known. EP217609 bound antithrombin with high affinity (K(D) = 30nM) and activated the serpin to rapidly (k(ass) ∼ 10(6)M(-1)s(-1)) and selectively (> 20-fold) inhibit factor Xa. The dual inhibitor moieties of EP217609 acted largely independently with only modest linkage effects of ligand occupancy of one inhibitor moiety on the potency of the other (∼ 5-fold). In contrast, avidin binding effectively neutralized the potency of both inhibitor moieties (20- to 100-fold). These findings demonstrate the superior anticoagulant efficacy and rapid avidin neutralizability of EP217609 compared with anticoagulants that target thrombin or factor Xa alone.

Designing nonsaccharide, allosteric activators of antithrombin for accelerated inhibition of factor Xa

Antithrombin is a key regulator of coagulation and prime target of heparins, clinically used anticoagulants. Heparins induce a two-step conformational activation of antithrombin, a process that has remained challenging to target with molecules devoid of the antithrombin-binding pentasaccharide DEFGH. Computational screening of a focused library led to the design of two tetra-sulfated N-arylacyl tetrahydroisoquinoline variants as potential nonsaccharide activators of antithrombin. A high yielding synthetic scheme based on Horner-Wadsworth-Emmons or Pictet-Spengler reactions was developed to facilitate the functionalization of the tetrahydoisoquinoline ring, which upon further amidation, deprotection, and sulfation gave the targeted nonsaccharide activators. Spectrofluorometric measurement of affinity displayed antithrombin binding affinities in the low to high micromolar range at pH 6.0, I 0.05, 25 °C. Measurement of second-order rate constants of antithrombin inhibition of factor Xa in the presence and absence of the designed activators showed antithrombin activation in the range of 8-80-fold in the pH 6.0 buffer. This work puts forward 20c, a novel tetra-sulfated N-arylacyl tetrahydroisoquinoline-based molecule, that activates AT only 3.8-fold less than that achieved with DEFGH, suggesting a strong possibility of rationally designing sulfated organic molecules as clinically relevant AT activators.

Catalytic Effects of Different Heparin Analogs on the Hydrolysis of Auramine O

Glycosaminoglycans, such as heparin, are a class of biomaterials with a variety of important biological functions. They were found to catalyze the hydrolysis of 4,4'-(imidocarbonyl)-bis(N,N-dimethylaniline) monohydrochloride to 4,4'-bis(dimethy-lamino) benzophenone, but the catalytic mechanism of this interesting reaction is poorly understood, mainly due to the structural complexity of polysaccharides. In order to study such an unusual reaction, a pentasaccharide, fondaparinux, with defined chemical structure was chosen as a probe to investigate the hydrolytic characteristics, along with other heparin analogs. The hydrolytic reaction was catalyzed by fondaparinux, low-molecular-weight heparins, heparin and different desulfated heparins at various rates. The present results demonstrated that the length of polysaccharide chain, the N- or O-sulfo groups are critical to the hydrolysis of 4,4'-(imidocarbonyl)-bis(N,N-dimethy-laniline) monohydrochloride in addition to pH and ionic strength, which provides new insights into the catalytic phenomenon of polysaccharides.

Modular synthesis of heparan sulfate oligosaccharides for structure-activity relationship studies

Although hundreds of heparan sulfate binding proteins have been identified and implicated in a myriad of physiological and pathological processes, very little information is known about the ligand requirements for binding and mediating biological activities by these proteins. This difficulty results from a lack of technology for establishing structure-activity relationships, which in turn is due to the structural complexity of natural heparan sulfate (HS) and difficulties of preparing well-defined HS oligosaccharides. To address this deficiency, we developed a modular approach for the parallel combinatorial synthesis of HS oligosaccharides that utilizes a relatively small number of selectively protected disaccharide building blocks, which can easily be converted into glycosyl donors and acceptors. The utility of the modular building blocks has been demonstrated by the preparation of a library of 12 oligosaccharides, which has been employed to probe the structural features of HS for inhibiting the protease, BACE-1. The complex variations in activity with structural changes support the view that important functional information is embedded in HS sequences. Furthermore, the most active derivative provides an attractive lead compound for the preparation of more potent compounds, which may find use as a therapeutic agent for Alzheimer's disease.

Opportunities and challenges in synthetic oligosaccharide and glycoconjugate research

Synthetic oligosaccharides and glycoconjugates are increasingly used as probes for biological research and as lead compounds for drug and vaccine discovery. These endeavors are, however, complicated by a lack of general methods for the routine preparation of this important class of compounds. Recent development such as one-pot multi-step protecting group manipulations, the use of unified monosaccharide building blocks, the introduction of stereoselective glycosylation protocols, and convergent strategies for oligosaccharide assembly, are beginning to address these problems. Furthermore, oligosaccharide synthesis can be facilitated by chemo-enzymatic methods, which employ a range of glycosyl transferases to modify a synthetic oligosaccharide precursor. Glycosynthases, which are mutant glycosidases, that can readily form glycosidic linkages are addressing a lack of a wide range glycosyltransferases. The power of carbohydrate chemistry is highlighted by an ability to synthesize glycoproteins.

Differentiation of 3-O-sulfated heparin disaccharide isomers: identification of structural aspects of the heparin CCL2 binding motif

The presence of 3-O-sulfated glucosamine residues in heparin or heparan sulfate plays a role in binding to antithrombin III and HSV infection. In this study, tandem mass spectrometry was used to differentiate between two heparin disaccharide isomers containing variable sulfate at C6 in a common disaccharide and C3 in a more rare one. The dissociation patterns shown by MS(2) and MS(3) were clearly distinguishable between the isomers, allowing their differentiation and quantitation. Using this technique, we show that an octasaccharide with 11 sulfate groups with high affinity for inflammatory chemokine CCL2 does not contain 3-O-sulfated disaccharides.

Structural and thermodynamic analysis of thrombin:suramin interaction in solution and crystal phases

Suramin is a hexasulfonated naphthylurea which has been recently characterized as a non-competitive inhibitor of human alpha-thrombin activity over fibrinogen, although its binding site and mode of interaction with the enzyme remain elusive. Here, we determined two X-ray structure of the thrombin:suramin complex, refined at 2.4 A resolution. While a single thrombin:suramin complex was found in the asymmetric unit cell of the crystal, some of the crystallographic contacts with symmetrically related molecules are mediated by both the enzyme and the ligand. Molecular dynamics simulations with the 1:1 complex demonstrate a large rearrangement of suramin in the complex, but with the protein scaffold and the more extensive protein-ligand regions keep unchanged. Small-angle X-ray scattering measurements at high micromolar concentration demonstrate a suramin-induced dimerization of the enzyme. These data indicating a dissimilar binding mode in the monomeric and oligomeric states, with a monomeric, 1:1 complex to be more likely to exist at the thrombin physiological, nanomolar concentration range. Collectively, close understanding on the structural basis for interaction is given which might establish a basis for design of suramin analogues targeting thrombin.

Reversible biotinylated oligosaccharides: a new approach for a better management of anticoagulant therapy

OBJECTIVE

In order to obtain a neutralizable antithrombotic, a chimeric molecule (SSR126517E) containing the sequence of a long-lasting antithrombin (AT)-dependent anti-factor Xa pentasaccharide, idraparinux, linked to a biotin molecule was synthesized and tested for anticoagulant and antithrombotic activity.

METHODS

SSR126517E was tested in several models in vitro and in vivo for its pharmacological properties as well as its ability to be neutralized by avidin.

RESULTS

SSR126517E displayed exactly the same properties as idraparinux. In vitro, SSR126517E had a very high affinity for AT (K(d) < 1 nm) and showed a potent anti-FXa effect and inhibition of thrombin generation with IC(50) values similar to those of idraparinux. Ex vivo, after intravenous administration to rats, SSR126517E produced a potent and long-lasting anti-FXa effect comparable to that obtained with idraparinux; as with idraparinux, the subcutaneous bioavailability was 100%. In vivo, SSR126517E was a potent antithrombotic in rat and mouse venous and arterial thrombosis models. Direct comparison in rats showed that SSR126517E was as active as idraparinux, when administered at the same molar dose. Furthermore, injection of avidin triggered the immediate elimination of SSR126517E from the bloodstream, resulting in complete neutralization of the antithrombotic activity of SSR126517E.

CONCLUSIONS

These results show for the first time that coupling an oligosaccharide with biotin has no effect on the former's pharmacokinetic and pharmacologic properties and renders neutralization easy by injection of avidin.

Antithrombin-binding octasaccharides and role of extensions of the active pentasaccharide sequence in the specificity and strength of interaction. Evidence for very high affinity induced by an unusual glucuronic acid residue

The antithrombotic activity of low molecular weight heparins (LMWHs) is largely associated with the antithrombin (AT)-binding pentasaccharide sequence AGA(*)IA (GlcN(NAc/NS,6S)-GlcA-GlcN(NS,3,6S)-IdoUA(2S)-GlcN(NS,6S)). The location of the AGA(*)IA sequences along the LMWH chains is also expected to influence binding to AT. This study was aimed at investigating the role of the structure and molecular conformation of different disaccharide extensions on both sides of the AGA(*)IA sequence in modulating the affinity for AT. Four high purity octasaccharides isolated by size exclusion chromatography, high pressure liquid chromatography, and AT-affinity chromatography from the LMWH enoxaparin were selected for the study. All the four octasaccharides terminate at their nonreducing end with 4,5-unsaturated uronic acid residues (DeltaU). In two octasaccharides, AGA(*)IA was elongated at the reducing end by units IdoUA(2S)-GlcN(NS,6S) (OCTA-1) or IdoUA-GlcN(NAc,6S) (OCTA-2). In the other two octasaccharides (OCTA-3 and OCTA-4), AGA(*)IA was elongated at the nonreducing side by units GlcN(NS,6S)-IdoUA and GlcN(NS,6S)-GlcA, respectively. Extensions increased the affinity for AT of octasaccharides with respect to pentasaccharide AGA(*)IA, as also confirmed by fluorescence titration. Two-dimensional NMR and docking studies clearly indicated that, although elongation of the AGA(*)IA sequence does not substantially modify the bound conformation of the AGA(*)IA segment, extensions promote additional contacts with the protein. It should be noted that, as not previously reported, the unusual GlcA residue that precedes the AGA(*)IA sequence in OCTA-4 induced an unexpected 1 order of magnitude increase in the affinity to AT with respect to its IdoUA-containing homolog OCTA-3. Such a residue was found to orientate its two hydroxyl groups at close distance to residues of the protein. Besides the well established ionic interactions, nonionic interactions may thus contribute to strengthen oligosaccharide-AT complexes.

Electrochemical recognition of synthetic heparin mimetic at liquid/liquid microinterfaces

Electrochemically controlled molecular recognition of a synthetic heparin mimetic, Arixtra, at nitrobenzene/water microinterfaces was investigated to obtain a greater understanding of interfacial recognition and sensing of heparin and its analogues with biomedical importance. In contrast to unfractionated heparin, this synthetic pentasaccharide that mimics the unique Antithrombin III binding domain of heparin possesses well-defined structure and ionic charge to enable quantitative interpretation of cyclic voltammetric/chronoamperometric responses based on the interfacial recognition at micropipet electrodes. Arixtra is electrochemically extracted from the water phase into the bulk nitrobenzene phase containing highly lipophilic ionophores, methyltridodecylammonium or dimethyldioctadecylammonium. Numerical analysis of the kinetically controlled cyclic voltammograms demonstrates for the first time that formal potentials and standard rate constants of polyion transfer at liquid/liquid interfaces are ionophore dependent. Moreover, octadecylammonium and octadecylguanidinium are introduced as new, simple ionophores to model recognition sites of heparin-binding proteins at liquid/liquid interfaces. In comparison to octadecyltrimethylammonium, the best ionophore for heparin recognition at liquid/liquid interfaces reported so far, these new ionophores dramatically facilitate Arixtra adsorption at the interfaces. With a saline solution at physiological pH, an Arixtra molecule is selectively and cooperatively bound to 5 molecules of the guanidinium ionophore, suggesting hydrogen-bond-directed interactions of each guanidinium with a few of 10 negatively charged sulfo or carboxyl groups of Arixtra at the interfaces.