Pharmacological Properties of a Low Molecular Weight Butyryl Heparin Derivative (C4-CY 216) with Long Lasting Effects

We have investigated the pharmacological properties of an O-acylated butyryl derivative of the low molecular weight heparin CY 216 (C4-CY 216). In a purified system the ability of C4-CY 216 to catalyze thrombin and factor Xa inhibition was comparable to that of CY 216. The antithrombin and antifactor Xa catalytic efficiencies of C4-CY 216 were reduced 217 and 12 times respectively when albumin (10 mg ml-1) was added to the reagents, while those of CY 216 were essentially unchanged. In plasma, the antifactor Xa specific activity of C4-CY 216 was close to that of CY 216 but the antithrombin specific activity was 2 times lower. After bolus and continuous intravenous injection to rabbits, the clearances of the two activities of C4-CY 216 were on average half the corresponding values of CY 216. After subcutaneous injection, the bioavailability of C4-CY 216 was comparable to that of CY 216. C4-CY 216 was as potent as CY 216 in preventing venous thrombosis in the thromboplastin-Wessler model and the duration of the antithrombotic effect was longer than that of the parent compound. The chemical alteration of CY 216 did not enhance the prohaemorrhagic effect in the rat tail transection model. Therefore, the new concept of heparin derivative having a low clearance and long lasting effects that we have recently reported for unfractionated heparin may also be applied to a low molecular weight heparin.

Pharmacodynamic Properties of Long Lasting Butyryl Heparin Derivatives in the Rabbit

This paper reports on the pharmacodynamic properties of butyryl derivatives of unfractionated heparin (C4-UH) and of low molecular weight heparin (C4-CY 216) after bolus intravenous injection, constant infusion and subcutaneous administration to rabbits. The pharmacodynamic properties of the two butyryl derivatives were compared to those of the parent compounds, unfractionated heparin (UH) and low molecular weight heparin (CY 216). After bolus intravenous injection of increasing doses, the disposition of the butyryl derivatives were comparable to that of their parent compounds up to 3 mg kg-1. Over this dose, their clearances became 2 to 3 times lower. These long lasting properties were confirmed by constant intravenous infusion experiments. After subcutaneous administration, the bioavailability of C4-UH remained low (10%) at any dose while that of C4-CY 216 ranged from 42 to 120%. If these findings are confirmed in man, these new derivatives open the possibility of treating established deep vein thrombosis with only one daily injection of a butyryl derivative of low molecular weight heparin.

Further Studies on the Mechanisms for the Antithrombotic Effects of Sulfated Polysaccharides in Rabbits

A recent study (Fernandez et al., Thromb. Haemostas. 1987; 57: 286-93) demonstrated that when rabbits were injected with the minimum weight of a variety of glycosaminoglycans required to inhibit tissue factor-induced thrombus formation by —80%, exogenous thrombin was inactivated —twice as fast in the post-treatment plasmas as the pre-treatment plasmas. In this study, we investigated the relationship between inhibition of thrombus formation and the extent of thrombin inhibition ex vivo. We also investigated the relationship between inhibition of thrombus formation and inhibition of prothrombin activation ex vivo. Four sulfated polysaccharides (SPS) which influence coagulation in a variety of ways were used in this study. Unfractionated heparin and the fraction of heparin with high affinity to antithrombin III potentiate the antiproteinase activity of antithrombin III. Pentosan polysulfate potentiates the activity of heparin cofactor II. At less than 10 pg/ml of plasma, all three SPS also inhibit intrinsic prothrombin activation. The fourth agent, dermatan sulfate, potentiates the activity of heparin cofactor II but fails to inhibit intrinsic prothrombin activation even at concentrations which exceed 60 pg/ml of plasma. Inhibition of thrombus formation by each sulfated polysaccharides was linearly related to the extent of thrombin inhibition achieved ex vivo. These observations confirm the utility of catalysis of thrombin inhibition as an index for assessing antithrombotic potential of glycosaminoglycans and other sulfated polysaccharides in rabbits. With the exception of pentosan polysulfate, there was no clear relationship between inhibition of thrombus formation and inhibition of prothrombin activation ex vivo.

Standard Heparin Enhances the Antithrombotic Activity of Dermatan Sulfate in the Rabbit but CY 216 Does Not

Standard heparin (SH) and dermatan sulfate (DS) two gly- cosaminoglycans with different pharmacological targets are effective antithrombotic agents in the rabbit. We have investigated the antithrombotic activity of the association DS plus SH. It was found that doses as low as 25 µg/kg for DS and 10 µg/kg for SH were ineffective when injected separately but generated a high and significant antithrombotic activity when injected together. These results were confirmed when higher doses of each compound were delivered in association. Further experiments were performed to determine if the enhancement of the antithrombotic activity of DS by HS resulted from its anti-factor Ha or antifactor Xa activity or from its moiety without affinity to AT III. A low molecular weight heparin (CY 216) with an anti-factor Xa/ anti-factor Ha ratio of 5, the synthetic pentasaccharide bearing the minimum binding sequence to antithrombin III, and a low affinity fraction of SH to AT III did not increase the antithrombotic activity of DS; in contrast a high affinity fraction of SH to AT III had the same effect than SH. We conclude that the enhancement of the antithrombotic activity of DS by SH mainly results from its anti-factor IIa activity.

The Structure of Synthetic Oligosaccharides in Relation to Factor IXa Inhibition

We investigated the effect of various oligosaccharides (OS) on the inhibition of factor IXa by antithrombin (AT) in a purified system. The OS comprised the AT-binding pentasaccharide sequence prolonged by saccharide chains with various lengths and charges. We show that factor IXa inhibition depended on the molecular weight of the OS. Factor IXa was not inhibited by the AT-binding pentasaccharide alone, but was inhibited if it was prolonged by a sulphated dodecasaccharide at the non-reducing end. The overall charge was also important since factor IXa inhibition was negligible if the pentasaccharide was prolonged by a non-sulphated dodecasaccharide. Using compounds containing a non-sulphated spacer, we showed that the central part of the OS was not critical. This study therefore demonstrates that the minimal OS structure necessary for catalysing factor IXa inhibition by AT is close to that required for catalysing thrombin inhibition.

SR123781A, a Synthetic Heparin Mimetic

SR123781A, a synthetic hexadecasaccharide comprising an anti-thrombin (AT) binding domain, a thrombin binding domain, and a neutral methylated hexasaccharide sequence, was obtained from glucose through a convergent synthesis. SR123781A showed high affinity for human AT (Kd = 58 ± 22 nM) and was a potent catalyst of its inhibitory effect with regard to factor Xa (IC50 = 77 ± 5 ng/ml – 297 ± 13 U/mg) and thrombin (IC50 = 4.0 ± 0.5 ng/ml – 150 ± 30 U/mg). SR123781A which acted exclusively via AT (no effect via heparin cofactor II at a concentration of 6 g/ml) inhibited thrombin generation occurring via both the extrinsic and intrinsic pathways in vitro in human plasma. SR123781A did not compete for 3H-heparin binding to PF4 and did not activate platelets in the presence of plasma from patients with heparin-induced thrombocytopenia. After intravenous or subcutaneous administration to rats, rabbits or baboons, SR123781A displayed prolonged anti-factor Xa and anti-factor IIa activity ex vivo. After intravenous injection to baboons, decreases of the anti-factor Xa and anti-thrombin activities were parallel and disappeared with the same pharmacodynamics. Intravenous administrations of SR123781A strongly inhibited thrombus formation in an experimental model of thromboplastin-induced venous thrombosis in rats with an ED50 value of 18 ± 0.1 g/kg (vs 77 ± 3 g/kg for heparin). SR123781A inhibited arterial thrombus formation induced on a silk thread in an arterio-venous shunt and in the vena cava (ED50 values of 225 ± 10 and 27 ± 8 g/kg, respectively). Compared to standard and low molecular weight heparin and to presently used drugs, SR123781A exhibited a highly favourable anti-thrombotic/bleeding ratio therefore showing that it might be considered as a promising compound in the treatment and prevention of various thrombotic diseases.

Pharmacokinetics of New Synthetic Heparin Mimetics

Pharmacokinetics of oligosaccharides displaying various affinities for antithrombin (AT) allowed us to show that there was a close relationship between the plasma half-life of these antithrombotic oligosaccharides and their affinity for AT. Recently, we have described new heparin mimetics comprising an AT binding domain and a thrombin binding domain separated by a neutral methylated saccharide sequence. These compounds displayed strong anti-Xa and anti-IIA activities and, in contrast to heparin, escaped neutralisation by platelet factor 4. The aim of this work was to compare the pharmacokinetics of several of these heparin mimetics in rats. These compounds differed by their length, charge and affinity for AT (AT-binding domain). The results obtained indicate that the prolongation of the AT binding domain did not modify significantly their affinities for AT. However, an increase in the number of charges leads to a decrease in the half-life. When a methylated spacer was added in order to minimise the non-specific interactions to other proteins, half-lives of the heparin mimetic were in the same range than that of the pentasaccharide used as an AT binding domain.

In conclusion, this study shows the influence of the charge of the oligosaccharides on their pharmacokinetics and underlines the importance of minimising their non-specific binding to plasma proteins in order to obtain compounds with predictive pharmacokinetics.

Synthesis, radiolabeling with fluorine-18 and preliminary in vivo evaluation of a heparan sulphate mimetic as potent angiogenesis and heparanase inhibitor for cancer applications

A rationally designed, fully synthetic, octasaccharide-based, HS mimetic has been synthesized, in vitro characterized, labeled with fluorine-18, and in vivo imaged with PET in rats.

EP42675, a synthetic parenteral dual-action anticoagulant: pharmacokinetics, pharmacodynamics, and absence of interactions with antiplatelet drugs

BACKGROUND

EP42675 is a first-in-class, synthetic, parenteral, anticoagulant combining in a single molecule a direct thrombin inhibitor and an indirect factor Xa(FXa) inhibitor.

OBJECTIVES

To investigate the safety, pharmacokinetics, and pharmacodynamics of EP42675 and its interaction with aspirin, clopidogrel, and unfractionated heparin (UFH).

SUBJECTS AND METHODS

In study 1, healthy male subjects were administered intravenously single-ascending doses (1-10 mg) of EP42675 or placebo. In study 2, healthy male subjects were administered intravenously a single dose of 5 mg EP42675 on day 1 followed by oral administration of aspirin (100 mg) and clopidogrel (75 mg) once daily from day 8 to 21. On day 15, a second dose of 5 mg EP42675 was administered, and subjects were then randomized to receive a single dose of UFH (30 or 60 IU kg(-1) ) or placebo.

RESULTS AND CONCLUSIONS

Mild bleedings were the only drug-related adverse events. EP42675 pharmacokinetics were dose-proportional and characterized by a low clearance, a small volume of distribution, a long terminal half-life. EP42675 pharmacodynamics were characterized by a long-lasting, dose-dependent increase in activated clotting time, ecarin clotting time, thrombin time, anti-FXa activity, activated partial thromboplastin time, prothrombin time, and a decrease in endogenous thrombin potential, measured by a thrombin generation test. Dose-dependent additive effects were seen with UFH on coagulation tests. EP42675 had no additive effect on the inhibition of platelet aggregation induced by aspirin and clopidogrel. These results warrant further clinical development of this new class of anticoagulant.

Synthesis of N-acetyl-muramyl-L-alanyl-D-glutamic-alpha-amide(MDP) or -alpha-methyl ester derivatives, bearing a lipophilic group at the C-terminal peptide end

We report the synthesis of nine lipophilic derivatives of N-acetyl-muramyl-L-alanyl-D-glutamic-alpha-amide (MDP) or -alpha-methyl ester in which the gamma-carboxyl function of the D-glutamyl residue is either esterified by a medium chain alcohol or substituted by an L-alanyl residue esterified by a medium or long chain alcohol. A new method is described which easily allows one to obtain derivatives of MDP, bearing a free or substituted amino-acyl or peptidyl residue on the gamma-carboxyl function.

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.

Comparative effects of two synthetic oligosaccharides on platelet activation induced by plasma from HIT patients

Heparin-induced thrombocytopenia (HIT) is a serious secondary event encountered in the clinical use of heparin. HIT results from the consumption of platelets that are immunologically activated by antibodies directed against complexes formed by platelet factor 4 (PF4) and sulfated polysaccharides that activate platelet aggregation, leading to paradoxical, life-threatening thrombosis. There is strong evidence that the ability of heparin and related compounds to induce HIT is closely linked to the structure of the polysaccharide, and particularly to its negative charge and to the length of the molecule. To test this hypothesis, we synthesized two sulfated oligosaccharides: SanOrg123781, a 16-mer, presenting two terminal charged domains separated by a 7-mer neutral linker, and SR121903, a highly sulfated 17-mer. Both of them displayed strong anti-factor (F) Xa and anti-FIIa activities but their affinities for PF4 were markedly different. SR121903 displaced PF4-bound heparin, whereas SanOrg123781 did not, underlining the importance of the charge of the molecule for the interaction with PF4. Platelet studies, in the presence of HIT serum, showed that SR121903 induced the secretion of platelet-dense granules (measured by the release of serotonin) whereas SanOrg123781 did not, a result in accordance with an absence of affinity of this molecule for PF4. These results were confirmed by measurements of platelet activation by flow cytometry (measured by annexin V binding, CD62 detection and activation of the GpIIb-IIIa complexes). In conclusion, we have demonstrated the importance of the charge of the polysaccharides in the HIT-induced platelet reactions measured by diverse methods, of which some are described for this purpose for the first time.

The structure of synthetic oligosaccharides in relation to factor IXa inhibition

We investigated the effect of various oligosaccharides (OS) on the inhibition of factor IXa by antithrombin (AT) in a purified system. The OS comprised the AT-binding pentasaccharide sequence prolonged by saccharide chains with various lengths and charges. We show that factor IXa inhibition depended on the molecular weight of the OS. Factor IXa was not inhibited by the AT-binding pentasaccharide alone, but was inhibited if it was prolonged by a sulphated dodecasaccharide at the non-reducing end. The overall charge was also important since factor IXa inhibition was negligible if the pentasaccharide was prolonged by a non-sulphated dodecasaccharide. Using compounds containing a non-sulphated spacer, we showed that the central part of the OS was not critical. This study therefore demonstrates that the minimal OS structure necessary for catalysing factor IXa inhibition by AT is close to that required for catalysing thrombin inhibition.

Pharmacokinetics of new synthetic heparin mimetics

Pharmacokinetics of oligosaccharides displaying various affinities for antithrombin (AT) allowed us to show that there was a close relationship between the plasma half-life of these antithrombotic oligosaccharides and their affinity for AT. Recently, we have described new heparin mimetics comprising an AT binding domain and a thrombin binding domain separated by a neutral methylated saccharide sequence. These compounds displayed strong anti-Xa and antii-IIA activities and, in contrast to heparin, escaped neutralisation by platelet factor 4. The aim of this work was to compare the pharmacokinetics of several of these heparin mimetics in rats. These compounds differed by their length, charge and affinity for AT (AT-binding domain). The results obtained indicate that the prolongation of the AT binding domain did not modify significantly their affinities for AT. However, an increase in the number of charges leads to a decrease in the half-life. When a methylated spacer was added in order to minimise the non-specific interactions to other proteins, half-lives of the heparin mimetic were in the same range than that of the pentasaccharide used as an AT binding domain. In conclusion, this study shows the influence of the charge of the oligosaccharides on their pharmacokinetics and underlinesthe importance of minimising their non-specific binding to plasma proteins in order to obtain compounds with predictive pharmacokinetics.

Synthesis of conformationally locked L-iduronic acid derivatives: direct evidence for a critical role of the skew-boat 2S0 conformer in the activation of antithrombin by heparin

We have used organic synthesis to understand the role of L-iduronic acid conformational flexibility in the activation of antithrombin by heparin. Among known synthetic analogues of the genuine pentasaccharidic sequence representing the antithrombin binding site of heparin, we have selected as a reference compound the methylated anti-factor Xa pentasaccharide 1. As in the genuine original fragment, the single L-iduronic acid moiety of this molecule exists in water solution as an equilibrium between three conformers 1C4, 4C1 and 2S0. We have thus synthesized three analogues of 1, in which the L-iduronic acid unit is locked in one of these three fixed conformations. A covalent two atom bridge between carbon atoms two and five of L-iduronic acid was first introduced to lock the pseudorotational itinerary of the pyranoid ring around the 2S0 form. A key compound to achieve this connection was the D-glucose derivative 5 in which the H-5 hydrogen atom has been replaced by a vinyl group, which is a progenitor of the carboxylic acid. Selective manipulations of this molecule resulted in the 2S0-type pentasaccharide 23. Starting from the D-glucose derivative 28, a covalent two atom bridge was now built up between carbon atoms three and five to lock the L-iduronic acid moiety around the 1C4 chair form conformation, and the 1C4-type pentasaccharide 43 was synthesized. Finally the L-iduronic acid containing disaccharide 58 which, due to the presence of the methoxymethyl substituent at position five adopts a 4C1 conformation, was directly used to synthesize the 4C1-type pentasaccharide 61. The locked pentasaccharide 23 showed about the same activity as the reference compound 1 in an antithrombin-mediated anti-Xa assay, whereas the two pentasaccharides 43 and 61 displayed very low activity. These results clearly establish the critical importance of the 2S0 conformation of L-iduronic acid in the activation of antithrombin by heparin.

Conformation of heparin pentasaccharide bound to antithrombin III

The interaction, in aqueous solution, of the synthetic pentasaccharide AGA*IA(M) (GlcN,6-SO(3)alpha 1-4GlcA beta 1-4GlcN,3,6-SO(3)alpha 1-4IdoA,2-SO(3)alpha 1-4GlcN,6-SO(3)alpha OMe; where GlcN,6-SO(3) is 2-deoxy-2-sulphamino-alpha-D-glucopyranosyl 6-sulphate, IdoA is l-iduronic acid and IdoA2-SO(3) is L-iduronic acid 2-sulphate), which exactly reproduces the structure of the specific binding sequence of heparin and heparan sulphate for antithrombin III, has been studied by NMR. In the presence of antithrombin there were marked changes in the chemical shifts and nuclear Overhauser effects (NOEs), compared with the free state. On the basis of the optimized geometry of the pentasaccharide the transferred NOEs were interpreted with full relaxation and conformational exchange matrix analysis. An analysis of the three-dimensional structures of the pentasaccharide in the free state, and in the complex, revealed the binding to be accompanied by dihedral angle variation at the A-G and I-A(M) (where G, I, A and A(M) are beta-d-glucuronic acid, 2-O-sulphated alpha-L-iduronic acid, N,6-O-sulphated alpha-D-glucosamine and the alpha-methyl-glycoside of A respectively) glycosidic linkages. Evidence is also provided that the protein drives the conformation of the 2-O-sulphated iduronic acid residue towards the skewed (2)S(0) form.

Experimental proof for the structure of a thrombin-inhibiting heparin molecule

Kinetic studies of thrombin inhibition by antithrombin in the presence of heparin have shown that thrombin binds to heparin in a preformed heparin-antithrombin complex. To study the relative position of the thrombin binding domain and the antithrombin binding domain on a heparin molecule we have designed and synthesized heparin mimetics, which structurally are very similar to the genuine polysaccharide. Their inhibitory properties with respect to factor Xa and thrombin provide experimental evidence that in heparin the thrombin binding domain must be located at the nonreducing end of the antithrombin binding domain to observe thrombin inhibition. As expected, factor Xa inhibition is not affected by elongation of the antithrombin binding pentasaccharide sequence, regardless of the position in which this elongation takes place.

SR123781A, a synthetic heparin mimetic

SR123781A, a synthetic hexadecasaccharide comprising an antithrombin (AT) binding domain, a thrombin binding domain, and a neutral methylated hexasaccharide sequence, was obtained from glucose through a convergent synthesis. SR123781A showed high affinity for human AT (Kd = 58 +/- 22 nM) and was a potent catalyst of its inhibitory effect with regard to factor Xa (IC50) = 77 +/- 5 ng/ml - 297 +/- 13 U/mg) and thrombin (IC50 = 4.0 +/- 0.5 ng/ml - 150 +/- 30 U/mg). SR123781A which acted exclusively via AT (no effect via heparin cofactor II at a concentration of 6 microg/ml) inhibited thrombin generation occurring via both the extrinsic and intrinsic pathways in vitro in human plasma. SR123781A did not compete for 3H-heparin binding to PF4 and did not activate platelets in the presence of plasma from patients with heparin-induced thrombocytopenia. After intravenous or subcutaneous administration to rats, rabbits or baboons, SR123781A displayed prolonged anti-factor Xa and anti-factor IIa activity ex vivo. After intravenous injection to baboons, decreases of the anti-factor Xa and anti-thrombin activities were parallel and disappeared with the same pharmacodynamics. Intravenous administrations of SR123781A strongly inhibited thrombus formation in an experimental model of thromboplastin-induced venous thrombosis in rats with an ED50 value of 18 +/- 0.1 microg/kg (vs 77 +/- 3 microg/kg for heparin). SR123781A inhibited arterial thrombus formation induced on a silk thread in an arterio-venous shunt and in the vena cava (ED50 values of 225 +/- 10 and 27 +/- 8 microg/kg, respectively). Compared to standard and low molecular weight heparin and to presently used drugs, SR123781A exhibited a highly favourable antithrombotic/bleeding ratio therefore showing that it might be considered as a promising compound in the treatment and prevention of various thrombotic diseases.

The effect of a reducing-end extension on pentasaccharide binding by antithrombin

Antithrombin requires heparin for efficient inhibition of the final two proteinases of the blood coagulation cascade, factor Xa and thrombin. Antithrombin binds heparin via a specific pentasaccharide domain in a two-step mechanism whereby initial weak binding is followed by a conformational change and subsequent tight binding. The goal of this study is to investigate the role of a reducing-end extension in the binding of the longer oligosaccharides that contain the cognate pentasaccharide sequence. We determined the antithrombin binding properties of a synthetic heptasaccharide containing the natural pentasaccharide sequence (DEFGH) and an additional reducing-end disaccharide (DEFGHG'H'). Binding at low ionic strength is unaffected by the disaccharide addition, but at ionic strengths >/=0.2 the mode of heptasaccharide binding changes resulting in a 2-fold increase in affinity due to a decrease in the off-rate caused by a greater nonionic contribution to binding. Molecular modeling of possible binding modes for the heptasaccharide at high ionic strength indicates a possible shift in position of the pentasaccharide domain to occupy the extended heparin-binding site. This conclusion supports the likely presence of a range of sequences that can bind to and activate antithrombin in the natural heparan sulfates that line the vascular endothelium.

Oligosaccharides corresponding to the regular sequence of heparin: chemical synthesis and interaction with FGF-2

It has been proposed that oligosaccharides corresponding to the so-called regular region of heparin/heparan sulfate (HS) bind to fibroblast growth factor-2 (FGF-2). In order to explore the molecular basis of FGF/HS interaction, we describe here the chemical synthesis of a tetra and a hexasaccharide, prepared as methyl glycosides, corresponding to the regular sequence of heparin. The strategy relies on the efficient preparation of three building blocks: a seeding block, an elongating block and a capping block. The hexasaccharide inhibited the binding of FGF-2 on its receptor on human aorta vascular smooth muscle cells with an IC50 value (16+/-1.2 microg/mL) close to that of standard heparin (14.8+/-0.5 microg/mL) whereas the tetrasaccharide was much less potent (IC50 = 127+/-10.5 microg/mL). The hexasaccharide and heparin, inhibited in a dose-dependent manner FGF-2 (30 nM) induced proliferation (IC50 = 23.7+/-1.6 and 30.1+/-1.3 microg/mL, respectively). Under the same experimental conditions, the tetrasaccharide only slightly inhibited the mitogenic effect of FGF-2 (IC50 > 100 microg/mL).

A synthetic heparan sulfate pentasaccharide, exclusively containing L-iduronic acid, displays higher affinity for FGF-2 than its D-glucuronic acid-containing isomers

It has been suggested that the FGF-2 binding site on heparan sulfate chains is a trisulfated pentasaccharide containing three hexuronic acid units. The configuration at C-5 of two of them being undetermined, we have synthesized the four possible pentasaccharides, and have evaluated their FGF-2 binding affinity through in vitro biological assays. The pentasaccharide containing L-iduronic acid as the sole hexuronic acid showed higher affinity for FGF-2 than the other pentasaccharides, where one hexuronic acid unit at least is D-glucuronic acid.

[New antithrombotic oligosaccharides]

In the early eighties, following breakthroughs in oligosaccharide chemistry, the total chemical synthesis of pentasaccharides has been achieved, representing the antithrombin binding domain of heparin (the active site). The selective inhibitors of coagulation factor Xa thus obtained represent a new type of antithrombotic drugs. In a further step, based on the knowledge of the mechanism of antithrombin activation by heparin, oligosaccharides (pentadeca- to eicosasaccharides), comprising an antithrombin binding domain prolonged by a thrombin binding domain, were designed and synthesised in the Sanofi group. These compounds inhibit both factor Xa and thrombin, in the presence of antithrombin. Endowed with the full anticoagulant activity of heparin but devoid of undesired non-specific interactions, particularly with platelet factor 4 (PF4), they might represent "the ideal heparin-like antithrombotic".

Assessment through chemical synthesis of the size of the heparin sequence involved in thrombin inhibition

Deca- to eicosasaccharides having the generic structure methyl(sodium 2,3-di-O-methyl-4-O-sodium sulfonato-alpha-L-idopyranosyluronate)-(1-->4)-[(2,3,6-tri-O-sodiu m sulfonato-alpha-D-glucopyranosyl)-(1-->4)-(sodium 2,3-di-O-methyl-alpha-L-idopyranosyluronate)-(1-->4)]n-2,3,6-tri-O-sodiu m sulfonato-alpha-D-glucopyranoside have been synthesized from a single disaccharide precursor. All of them bind to and activate antithrombin. When n < or = 6 only Factor Xa inhibition is observed, whereas when n > 6 Factor Xa and thrombin are both inhibited in the presence of antithrombin. These results indicate that, in heparin, the sequence involved in antithrombin-catalyzed thrombin inhibition is a pentadeca- or a hexadecasaccharide.

Identification of a hexasaccharide sequence able to inhibit thrombin and suitable for 'polymerisation'

Three hexasaccharides, having from low to very high affinity for antithrombin, were synthesised from disaccharide building block precursors. One of them, methyl(sodium 2,3-di-O-methyl-4-O- sodium sulfonato-alpha-L-idopyranosyluronate)-(1-->4)-[(2,3,6-tri-O-sodiu m sulfonato-alpha-D-glucopyranosyl)-(1-->4)-(sodium 2,3-di-O-methyl-alpha-L-idopyranosyluronate)-(1-->4)]2-2,3,6-tri-O-sodiu m sulfonato-alpha-D-glucopyranoside, obtainable from a single disaccharide building block precursor, constitutes a good starting point for obtaining simple oligosaccharidic heparin mimetics able to inhibit the two coagulation factors thrombin and Factor Xa.

New synthetic heparin mimetics able to inhibit thrombin and factor Xa

Synthetic pentadeca-, heptadeca- and nonadecasaccharides, comprising an antithrombin III (AT III) binding pentasaccharide prolonged at the non-reducing end by a thrombin binding domain have been obtained. The pentadecasaccharide is the shortest oligosaccharide able to catalyse thrombin inhibition by AT III. The nonadecasaccharide is a more potent thrombin inhibitor than standard heparin.

Synthetic oligosaccharides having various functional domains: potent and potentially safe heparin mimetics

A synthetic heptadecasaccharide, comprising an antithrombin III binding domain, a thrombin binding domain, and a neutral methylated hexasaccharide sequence, was obtained through a convergent synthesis. This compound displayed in vitro anticoagulant properties similar to that of standard heparin but, in contrast with heparin, escaped neutralization by platelet factor 4, a protein released by activated platelets.

Synthesis of thrombin-inhibiting heparin mimetics without side effects

Unwanted side effects of pharmacologically active compounds can usually be eliminated by structural modifications. But the complex heterogeneous structure of the polysaccharide heparin has limited this approach to fragmentation, leading to slightly better-tolerated heparin preparations of low molecular mass. Despite this improvement, heparin-induced thrombocytopaenia (HIT), related to an interaction with platelet factor 4 (PF4) and, to a lesser extent, haemorrhages, remain significant side effects of heparinotherapy. Breakthroughs in oligosaccharide chemistry made possible the total synthesis of the pentasaccharide antithrombin-binding site of heparin. This pentasaccharide represents a new family of potential antithrombotic drugs, devoid of thrombin inhibitory properties, and free of undesired interactions with blood and vessel components. To obtain more potent and well-tolerated antithrombotic drugs, we wished to synthesize heparin mimetics able to inhibit thrombin, that is, longer oligosaccharides. Like thrombin inhibition, undesired interactions are directly correlated to the charge and the size of the molecules, so we had to design structures that were able to discriminate between thrombin and other proteins, particularly PF4. Here we describe the use of multistep converging synthesis to obtain sulphated oligosaccharides that meet these requirements.

Mechanism of heparin activation of antithrombin: evidence for an induced-fit model of allosteric activation involving two interaction subsites

The anticoagulant activation of the serpin antithrombin by heparin pentasaccharide DEFGH was previously shown to involve trisaccharide DEF first binding and inducing activation of the serpin, followed by disaccharide GH binding and stabilizing the activated state [Petitou et al. (1997) Glycobiology 7, 323-327; Desai et al. (1998) J. Biol. Chem. 273, 7478-7487]. In the present study, the role of conformational changes and charged residues of the GH disaccharide in the allosteric activation mechanism was investigated with variant pentasaccharides modified in the GH disaccharide. Perturbation of the conformational equilibrium of iduronate residue G through replacement of the nonessential 3-OH of this residue with -H resulted in parallel decreases in the fraction of residue G in the skew boat conformer (from 64 to 24%) and in the association constant for pentasaccharide binding to antithrombin [(2.6 +/- 0.3)-fold], consistent with selective binding of the skew boat conformer to the serpin. Introduction of an additional sulfate group to the 3-OH of residue H flanking a putative charge cluster in the GH disaccharide greatly enhanced the affinity for the serpin by approximately 35-fold with only a small increase in the fraction of residue G in the skew boat conformation (from 64 to 85%). The salt dependence of binding, together with a recent X-ray structure of the antithrombin-pentasaccharide complex, suggested that the majority of the enhanced affinity of the latter pentasaccharide was due to direct electrostatic and hydrogen-bonding interactions of the H residue 3-O-sulfate with antithrombin. All variant pentasaccharides produced a normal enhancement of antithrombin fluoresence and normal acceleration of factor Xa inhibition by the serpin at saturating levels, indicating that conformational activation of antithrombin was not affected by the pentasaccharide modifications. Rapid kinetic studies were consistent with the altered affinities of the variant pentasaccharides resulting mostly from perturbed interactions of the reducing-end GH disaccharide with the activated antithrombin conformation and minimally to an altered binding of the nonreducing-end DEF trisaccharide to the native serpin conformation. Together, these results support a model in which the conformational flexibility of the G residue facilitates conversion to the skew boat conformer and thereby allows charged groups of the GH disaccharide to bind and stabilize the activated antithrombin conformation that is induced by the DEF trisaccharide.

Introducing a C-interglycosidic bond in a biologically active pentasaccharide hardly affects its biological properties

We describe here the synthesis and the biological activity of a 'C-pentasaccharide', a new analogue of the antithrombin III (AT III) binding region of heparin containing a methylene bridge in place of an interglycosidic oxygen atom. The affinity for AT III and the anti-factor Xa activity of this compound have been compared with that of the corresponding selected 'O-pentasaccharide'. Such a structural modification slightly decreased the affinity of this compound for AT III as well as its anti-factor Xa activity (880 +/- 40 anti-Xa units versus 1180 +/- 30 anti-Xa units for the C-pentasaccharide and the O-pentasaccharide, respectively). This compound therefore represents the first example of a new class of anti-factor Xa pentasaccharides containing a C-interglycosidic bond.

Biological properties of synthetic glycoconjugate mimics of heparin comprising different molecular spacers

The in vitro antithrombotic activity of synthetic glycoconjugates I and II, comprising a flexible polyethylene glycol type and a rigid polyglucose type spacer, respectively, are compared to heparin.

Synthesis of a 3-deoxy-L-iduronic acid containing heparin pentasaccharide to probe the conformation of the antithrombin III binding sequence

We report in this work the total synthesis of a close analogue of the pentasaccharide active site of heparin, in which the L-iduronic acid residue has been deoxygenated at position three. 1H NMR studies demonstrated that, as anticipated, such a modification induces a shift of the conformational equilibrium toward 1C4 (contribution to the conformational equilibrium rises from 37% to 65%) and a substantial decrease of the affinity for antithrombin III (Kd 0.154 microM versus 0.050 microM).

Biochemical and pharmacological properties of SANORG 34006, a potent and long-acting synthetic pentasaccharide

SANORG 34006 is a new sulfated pentasaccharide obtained by chemical synthesis. It is an analog of the "synthetic pentasaccharide" (SR 90107/ ORG 31540) which represents the antithrombin (AT) binding site of heparin. SANORG 34006 showed a higher affinity to human AT than SR 90107/ORG 31540 (kd = 1.4 +/- 0.3 v 48 +/- 11 nmol/L), and it is a potent and selective catalyst of the inhibitory effect of AT on factor Xa (1,240 +/- 15 anti-factor Xa U/mg v 850 +/- 27 anti-factor Xa U/mg for SR 90107/ORG 31540). In vitro, SANORG 34006 inhibited thrombin generation occurring via both the extrinsic and intrinsic pathway. After intravenous (IV) or subcutaneous (SC) administration to rabbits, SANORG 34006 displayed a long-lasting anti-factor Xa activity and inhibition of thrombin generation (TG) ex vivo. SANORG 34006 was slowly eliminated after IV or SC administration to rats, rabbits, and baboons, showed exceptionally long half-lives (between 9.2 hours in rats and 61.9 hours in baboons), and revealed an SC bioavailability near 100%. SANORG 34006 displayed antithrombotic activity by virtue of its potentiation of the anti-factor Xa activity of AT. It strongly inhibited thrombus formation in experimental models of thromboplastin/stasis-induced venous thrombosis in rats (IV) and rabbits (SC) (ED50 values = 40.0 +/- 3.4 and 105.0 +/- 9.4 nmol/kg, respectively). The duration of its antithrombotic effects closely paralleled the ex vivo anti-factor Xa activity. SANORG 34006 enhanced rt-PA-induced thrombolysis and inhibited accretion of 125I-fibrinogen onto a preformed thrombus in the rabbit jugular vein suggesting that concomitant use of SANORG 34006 during rt-PA therapy might be helpful in facilitating thrombolysis and preventing fibrin accretion onto the thrombus under lysis. Contrary to standard heparin, SANORG 34006 did not enhance bleeding in a rabbit ear incision model at a dose that equals 10 times the antithrombotic ED50 in this species and, therefore, exhibited a favorable therapeutic index. We suggest that SANORG 34006 is a promising compound in the treatment and prevention of various thrombotic diseases.

Mechanism of heparin activation of antithrombin. Role of individual residues of the pentasaccharide activating sequence in the recognition of native and activated states of antithrombin

To determine the role of individual saccharide residues of a specific heparin pentasaccharide, denoted DEFGH, in the allosteric activation of the serpin, antithrombin, we studied the effect of deleting pentasaccharide residues on this activation. Binding, spectroscopic, and kinetic analyses demonstrated that deletion of reducing-end residues G and H or nonreducing-end residue D produced variable losses in pentasaccharide binding energy of approximately 15-75% but did not affect the oligosaccharide's ability to conformationally activate the serpin or to enhance the rate at which the serpin inhibited factor Xa. Rapid kinetic studies revealed that elimination of the reducing-end disaccharide marginally affected binding to the native low-heparin-affinity conformational state of antithrombin but greatly affected the conversion of the serpin to the activated high-heparin- affinity state, although the activated conformation was still favored. In contrast, removal of the nonreducing- end residue D drastically affected the initial low-heparin-affinity interaction so as to favor an alternative activation pathway wherein the oligosaccharide shifted a preexisiting equilibrium between native and activated serpin conformations in favor of the activated state. These results demonstrate that the nonreducing-end residues of the pentasaccharide function both to recognize the native low-heparin-affinity conformation of antithrombin and to induce and stabilize the activated high-heparin-affinity conformation. Residues at the reducing-end, however, poorly recognize the native conformation and instead function primarily to bind and stabilize the activated antithrombin conformation. Together, these findings establish an important role of the heparin pentasaccharide sequence in preferential binding and stabilization of the activated conformational state of the serpin.

Heparin accelerates the inhibition of cathepsin G by mucus proteinase inhibitor: potent effect of O-butyrylated heparin

Heparin tightly binds cathepsin G and so protects the enzyme from inhibition by alpha1-antichymotrypsin, alpha1-proteinase inhibitor and eglin c, three proteins which do not bind heparin [Ermolieff J., Boudier C., Laine A., Meyer B. and Bieth J.G. (1994) J. Biol. Chem. 269, 29502-29508]. Here we show that heparin no longer protects cathepsin G from inhibition when the enzyme is reacted with mucus proteinase inhibitor (MPI), a heparin-binding protein. Heparin fragments of Mr=4500 and 8100 and O-butyrylated heparin of Mr=8000 form tight complexes with cathepsin G (Kd=0.5-2.2 nM) and MPI (Kd=0. 4-0.8 muM) and accelerate the MPI-promoted inhibition of cathepsin G by a factor of 17-26. They also accelerate the inhibition of neutrophil elastase and pancreatic chymotrypsin. The rate acceleration is due to the binding of heparin to MPI. Butyrylation of heparin slightly decreases its affinity for cathepsin G and MPI but sharply decreases the ionic interactions between the positively charged proteins and the negatively charged polyanion. The butyrylated heparin derivative is the best rate accelerator: it increases the rate constant for the MPI-induced inhibition of cathepsin G and elastase by factors of 26 and 23, respectively. This, together with the fact that it has a good bioavailability and a very low anticoagulant activity, suggests that it might be an adjuvant of MPI-based therapy of cystic fibrosis.

The anticoagulant activation of antithrombin by heparin

Antithrombin, a plasma serpin, is relatively inactive as an inhibitor of the coagulation proteases until it binds to the heparan side chains that line the microvasculature. The binding specifically occurs to a core pentasaccharide present both in the heparans and in their therapeutic derivative heparin. The accompanying conformational change of antithrombin is revealed in a 2.9-A structure of a dimer of latent and active antithrombins, each in complex with the high-affinity pentasaccharide. Inhibitory activation results from a shift in the main sheet of the molecule from a partially six-stranded to a five-stranded form, with extrusion of the reactive center loop to give a more exposed orientation. There is a tilting and elongation of helix D with the formation of a 2-turn helix P between the C and D helices. Concomitant conformational changes at the heparin binding site explain both the initial tight binding of antithrombin to the heparans and the subsequent release of the antithrombin-protease complex into the circulation. The pentasaccharide binds by hydrogen bonding of its sulfates and carboxylates to Arg-129 and Lys-125 in the D-helix, to Arg-46 and Arg-47 in the A-helix, to Lys-114 and Glu-113 in the P-helix, and to Lys-11 and Arg-13 in a cleft formed by the amino terminus. This clear definition of the binding site will provide a structural basis for developing heparin analogues that are more specific toward their intended target antithrombin and therefore less likely to exhibit side effects.

New orally active non-peptide fibrinogen receptor (GpIIb-IIIa) antagonists: identification of ethyl 3-[N-[4-[4-[amino[(ethoxycarbonyl) imino]methyl]phenyl]-1,3-thiazol-2-yl]-N-[1-[(ethoxycarbonyl)methyl]pip erid -4-yl]amino]propionate (SR 121787) as a potent and long-acting antithrombotic agent

The platelet fibrinogen receptor GpIIb-IIIa is currently considered a target of choice for drugs used in the prevention and treatment of thrombosis. Ethyl 3-[N-[4-[4-[amino[(ethoxycarbonyl)-imino] methyl]phenyl]-1,3-thiazol-2-yl]-N-[1-[(ethoxycarbonyl)methyl] piperid-4-yl] amino]propionate (6, SR 121787) is a new antiaggregating agent which generates in vivo the corresponding diacid 19d (SR 121566), non-peptide GpIIb-IIIa antagonist. In vitro, 19d inhibited ADP-induced aggregation of human and baboon platelets (IC50 = 46 +/- 11 and 54 +/- 6 nM, respectively), and on human platelets, 19d antagonized the binding of 125I-labeled fibrinogen (IC50 = 19.2 +/- 6.2 nM). Ex vivo, 8 h after an i.v. administration of 19d (100 micrograms/kg, i.v.) to baboons, ADP-induced aggregation was strongly inhibited (more than 90%). At 8 h, the ED50 value was 24 +/- 3.3 micrograms/kg), and even 24 h after the administration of a single dose of 100 micrograms/kg of 19d, platelet aggregation was still significantly inhibited (50 +/- 6% inhibition, P < 0.05). In the same species, the oral administration of 500 micrograms/kg of 6 produced a nearly complete inhibition of aggregation for up to 8 h (ED50 at 8 h was 193 +/- 20 micrograms/kg). After an oral dose of 2 mg/kg of 6, an antiaggregating effect was still observed at 24 h (44 +/- 12% inhibition, P < 0.05). 6 was well tolerated in animals, showing that, on the basis of these studies, it is a suitable candidate for development as an orally active antithrombotic agent.

Combined NMR and molecular modeling study of an iduronic acid-containing trisaccharide related to antithrombotic heparin fragments

An iduronic acid-containing trisaccharide, methyl-O-(4-O-methyl-2,3,6-tri-O-sulfo-alpha-D-glucopyranosyl-(1-->4)-O- (2-O-sulfo-alpha-L-idopyranosyluronic acid)-(1-->4)-O-2,6-di-O-sulfo-alpha-D-glucopyranoside, related to antithrombotic heparin fragments has been subjected to a combined NMR and molecular modeling investigation. The conformational behavior of the two constituting disaccharide segments was investigated using a systematic grid search approach with the MM3 force field along with the proper parameters for the sulfate ester group. The exploration of the potential energy surfaces of the trisaccharide was performed through the use of the CICADA methods interfaced with the MM3 force field. In all cases, the 2-O-sulfo-alpha-L-iduronate moiety was given the three favored ring conformations (1)C4, (4)C1, and (2)S0. Conformations were clustered into families, four of which are likely to exhibit significant occupancy in solution. The different low-energy conformational families display different orientations at the glycosidic linkages and/or different ring shapes for the iduronate ring. The (2)S0 conformation is the major one for the 2-O-sulfo-alpha-L-iduronate but is still in equilibrium with the (1)C4 ring shape. The occurrence of such a conformational equilibrium in solution was probed via high-resolution NMR spectroscopy through measurements of coupling constants and NOE build-up. These results are in keeping with the observation that 2-O-sulfated pentasaccharides display a similar affinity for antithrombin III as their 2-N-sulfated counterparts.

Synthesis and pharmacological properties of a close analogue of an antithrombotic pentasaccharide (SR 90107A/ORG 31540)

The synthetic pentasaccharide (1) corresponding to the heparin sequence which binds to, and activates, antithrombin III (AT III) is a potent antithrombotic compound in several animal models of venous thrombosis. We describe here the preparation and the pharmacological properties of 34, an analogue of oligosaccharide 1 with the latter's N-sulfates being replaced by sulfate esters and hydroxyl groups being methylated. These structural modifications allow a simpler and more efficient synthesis of such anionic oligosaccharides. Affinity for human AT III, anti-factor Xa activity, ability to inhibit thrombin generation, antithrombotic activity in a rat model of venous thrombosis, and elimination half-life in the rat have been determined for 1 and 34. Surprisingly, introduction of O-sulfates in place of N-sulfates, and methylation of hydroxyl groups, contributes to reinforce the binding to AT III, resulting in an improved pharmacological profile.

The synthetic pentasaccharide SR 90107A/Org 31540 does not release lipase activity into the plasma

The present study was designed to find out whether the synthetic pentasaccharide SR 90107A/Org 31540, which is presently being evaluated in clinical trials as an antithrombotic agent, influences lipoprotein metabolism in rats as determined by plasma triglyceride (TG) lipase activity. A comparison with three clinically used sulphated polysaccharides-unfractionated heparin (UFH), low molecular weight heparin (LMWH) and pentosan polysulphate (PPS)- was performed. UFH evoked a dose-dependent increase in plasma TG lipase activity which plateaued at doses > or = 1 mg/kg i.v.. PPS and LMWH demonstrated a lower efficacy than heparin at 0.3 and 1 mg/kg i.v., but the maximum lipase releasing effect at 3 mg/kg i.v. was identical for UFH, PPS and LMWH. SR 90107A/Org 31540 did not release TG lipase activity at single i.v. doses up to 3 mg/kg. Repeated-dose experiments with SR 90107A/Org 31540 (1 mg/kg s.c. for 9 days) revealed no influence on the lipase releasing effect of UFH (1 mg/kg i.v. on day 10). These results demonstrate that SR 90107A/Org 31540 does not influence lipid metabolism in rats through lipase release, suggesting that SR 90107A/Org 31540 may offer an advantage over UFH and LMWH in clinical situations where an anticoagulant/antithrombotic effect is desired, but both an increase in plasma free fatty acids and atherogenic alterations of lipoprotein metabolism are considered harmful.

A unique trisaccharide sequence in heparin mediates the early step of antithrombin III activation

Spectrofluorimetry experiments using synthetic trisaccharides indicate that in compounds that display affinity for antithrombin III (AT-III), a unique trisaccharide sequence plays the key role in the early recognition, and the first step of AT-III activation. Added to previous observations, these new results suggest that the two-step binding mechanism previously proposed (Olson et al., J. Biol. Chem., 1981, 256, 11073-11079) might involve, in the first place, a conformational change of the protein, induced by the trisasaccharide -->4)-O-(6-O-sulfo-2-sulfoamino-2-deoxy-alpha-D-glucopyranosyl)-(1 -->4)- O-(beta-D-glucopyranosyluronic acid)-(1-->4)-O-(3,6-di-O-sulfo-2-sulfoamino-2- deoxy-alpha-D-glucopyranosyl)-(1-->, then would follow the fitting which ends in the locked complex. These observations support the new paradigm invoking specific oligosaccharide sequences in selective interactions of glycosaminoglycans and proteins.

Biochemical and pharmacological properties of SANORG 32701. Comparison with the "synthetic pentasaccharide' (SR 90107/ORG 31540) and standard heparin

SANORG 32701 is a new sulfated pentasaccharide obtained by total chemical synthesis. It is analogue of the "synthetic pentasaccharide" (SR 90107/ORG 31540), which represents the antithrombin III (AT-III) binding site of heparin. Like SR 90107, it shows high affinity for human AT-III (Kd = 3.7 +/- 0.7 nmol/L) and is a potent catalyst of its inhibitory effect with regard to factor Xa (1100 +/- 31 versus 850 +/- 27 anti-Xa U/mg for SR 90107). SANORG 32701 inhibited thrombin generation occurring via both the extrinsic and intrinsic pathways in vitro. After intravenous or subcutaneous administration to rabbits or rats, SANORG 32701 displayed prolonged anti-factor Xa activity and inhibition of thrombin generation ex vivo. SANORG 32701 was slowly eliminated, showing elimination half-lives between 2.8 and 4.9 hours with different doses. SANORG 32701 displayed antithrombotic activity by virtue of its potentiation of the anti-factor Xa activity of AT-III. It strongly inhibited thrombus formation in an experimental model of thromboplastin-induced venous thrombosis in rats (intravenously) and rabbits (subcutaneously) (ED50 values were 25.5 +/- 4.1 and 91 +/- 12.7 nmol/kg, respectively). SANORG 32701 inhibited the accretion of fibrinogen I 125 to a preformed thrombus in the rabbit jugular vein and significantly reduced thrombus growth occurring after electrical stimulation of the rabbit carotid artery. In the rabbit, intravenous injection of SANORG 32701 enhanced tissue plasminogen activator (TPA)-induced thrombolysis, suggesting that concomitant use of SANORG 32701 during TPA therapy may be helpful in preventing thrombus accretion, thus facilitating clot lysis. In the rat, SANORG 32701 potently inhibited thrombus formation induced on a silk thread in an arteriovenous shunt and in the vena cava. Compared with standard heparin, SANORG 32701 (1000 nmol/kg IV) caused only minimal bleeding enhancement and exhibited a favorable antithrombotic activity/ bleeding risk ratio, therefore showing that it might be considered as a promising compound in the treatment and prevention of various thrombotic diseases.

Glycosaminoglycans enhance megakaryocytopoiesis by modifying the activities of hematopoietic growth regulators

We have previously reported that heparin is capable of stimulating in vitro and in vivo megakaryocytopoiesis in mice and has a thrombopoietic effect when given in chronic immune thrombocytopenic purpura and that heparin and several other glycosaminoglycans (GAGs) promote the growth of human megakaryoblastic cell lines in the presence of serum. We show here that GAGs, including heparan sulfate (HS), chondroitin sulfate (CS), dermatan sulfate (DS), and hyaluronic acid (HA), also stimulate in vitro growth of murine megakaryocyte progenitors and augment the diameter of individual megakaryocytes in the presence of serum. However, in a serum-free agar system, the GAGs alone had no effect on megakaryocyte colony formation, suggesting that GAGs cooperate with some serum factor(s) to exert their activity. We also show that heparin significantly potentiates the megakaryocytopoietic activity of C-Mpl ligand and interleukin (IL)-6 but not IL3, GM-CSF, SCF, and Epo. In addition, the GAGs significantly neutralize the inhibitory action of platelet factor 4 (PF4) and transforming growth factor beta 1 (TGF beta 1) on megakaryocyte colony growth. These results demonstrate a stimulating activity of GAGs on megakaryocytopoiesis by modifying the activity of several growth-regulating factors.

L-iduronic acid derivatives as glycosyl donors

O-[Methyl (2-O-acetyl-3-O-benzyl-4-O-levulinyl-alpha, and beta-L-idopyranosid)uronate] trichloroacetimidate and the corresponding n-pentenyl glycosides are efficient L-iduronic acid glycosyl donors. Both have been used for the high-yielding synthesis of basic disaccharide blocks which are useful for the subsequent synthesis of complex oligosaccharides related to heparin/heparan sulfate, and dermatan sulfate. In contrast, the corresponding thioethyl glycosides, thiophenyl glycosides, and fluoride, did not yield the expected disaccharides.

Rational design of synthetic heparin analogues with tailor-made coagulation factor inhibitory activity

Computer modelling of the antithrombin III-heparin-thrombin complex inspired the synthesis of novel glycoconjugates, whose factor Xa and thrombin inhibitory activities can be adjusted in a rational way, leading to anticoagulants with unprecedented characteristics.

Mechanism of acceleration of antithrombin-proteinase reactions by low affinity heparin. Role of the antithrombin binding pentasaccharide in heparin rate enhancement

The role of the sequence-specific pentasaccharide region of high affinity heparin (HAH) in heparin acceleration of antithrombin-proteinase reactions was elucidated by determining the accelerating mechanism of low affinity heparin (LAH) lacking this sequence. LAH was shown to be free of HAH (< 0.001%) from the lack of exchange of added fluorescein-labeled HAH into LAH after separating the polysaccharides by antithrombin-agarose chromatography. Fluorescence titrations showed that LAH bound to antithrombin with a 1000-fold weaker affinity (KD 19 +/- 6 microM) and 5-6-fold smaller fluorescence enhancement (8 +/- 3%) than HAH. LAH accelerated the antithrombin-thrombin reaction with a bell-shaped dependence on heparin concentration resembling that of HAH, but with the bell-shaped curve shifted to approximately 100-fold higher polysaccharide concentrations and with a approximately 100-fold reduced maximal accelerating effect. Rapid kinetic studies indicated these differences arose from a reverse order of assembly of an intermediate heparin-thrombin-antithrombin ternary complex and diminished ability of LAH to bridge antithrombin and thrombin in this complex, as compared to HAH. By contrast, LAH and HAH both accelerated the antithrombin-factor Xa reaction with a simple saturable dependence on heparin or inhibitor concentrations which paralleled the formation of an antithrombin-heparin binary complex. The maximal accelerations of the two heparins in this case correlated with the inhibitor fluorescence enhancements induced by the polysaccharides, consistent with the accelerations arising from conformational activation of antithrombin. 1H NMR difference spectroscopy of antithrombin complexes with LAH and HAH and competitive binding studies were consistent with LAH accelerating activity being mediated by binding to the same site on the inhibitor as HAH. These results demonstrate that LAH accelerates antithrombin-proteinase reactions by bridging and conformational activation mechanisms similar to those of HAH, with the reduced magnitude of LAH accelerations resulting both from a decreased antithrombin affinity and the inability to induce a full activating conformational change in the inhibitor.