Synthetic heparin pentasaccharide depolymerization by heparinase I: molecular and biological implications

Discovery of exolytic heparinases and their catalytic mechanism and potential application

Heparinases (Hepases) are critical tools for the studies of highly heterogeneous heparin (HP)/heparan sulfate (HS). However, exolytic heparinases urgently needed for the sequencing of HP/HS chains remain undiscovered. Herein, a type of exolytic heparinases (exoHepases) is identified from the genomes of different bacteria. These exoHepases share almost no homology with known Hepases and prefer to digest HP rather than HS chains by sequentially releasing unsaturated disaccharides from their reducing ends. The structural study of an exoHepase (BIexoHep) shows that an N-terminal conserved DUF4962 superfamily domain is essential to the enzyme activities of these exoHepases, which is involved in the formation of a unique L-shaped catalytic cavity controlling the sequential digestion of substrates through electrostatic interactions. Further, several HP octasaccharides have been preliminarily sequenced by using BIexoHep. Overall, this study fills the research gap of exoHepases and provides urgently needed tools for the structural and functional studies of HP/HS chains.

Anticoagulants and Their Reversal

In an exciting era with many alternatives to the old anticoagulants heparin and warfarin emerging on the scene, awareness of the possibility to reverse their effect is mandatory. In this review, the traditional antidotes for warfarin (vitamin K, plasma, and prothrombin complex concentrate) and for heparin (protamine) are described together with the newer alternatives (recombinant activated factor VII, concatameric peptides, and recombinant platelet factor 4). For some of the newer anticoagulants, possible antidotes have been identified, whereas other alternatives have been discarded. There is a very limited experience of deamino-d-arginine vasopressin or a von Willebrand factor VIII concentrate to counteract hirudin. The small direct thrombin inhibitors may be reversed with activated prothrombin complex concentrate but not with recombinant activated factor VII, whereas the latter agent appears to be effective against the pentasaccharides and the recombinant nematode anticoagulant protein C2. Additional options that may become available in the future are also discussed briefly.

New Anti-thrombotic Agents: Emphasis on Hemorrhagic Complications and Their Management

Our advanced knowledge of coagulation has led to the synthesis of novel procoagulant substances, such as recombinant activated factor VII (rFVIIa; NovoSeven, Novo Nordisk, Bagsvaerd, Denmark). Similarly, in-depth understanding of the interaction between anticoagulant proteins and their natural inhibitors has led to the synthesis of various novel anticoagulants. Novel anticoagulants are characterized by highly specific coagulation-inhibiting activities and, frequently, a complete lack of effective antidotes. This lack of antidotes is particularly important in the case of novel inhibitors with extended half-lives; for example, idraparinux may produce effective anticoagulation for as long as one week after subcutaneous administration. As novel anticoagulants complete licensing evaluations and are used in clinical practice, the likelihood of anticoagulant-associated hemorrhage will increase. This will require physicians to have an understanding of the mechanism of action of these anticoagulants, and to have an advanced degree of knowledge of the potential specific and nonspecific inhibitors of these anticoagulant agents. This paper will briefly review the biochemistry of coagulation, focusing on the complexes inhibited by currently available and novel anticoagulants. Specific and nonspecific prohemostatic agents will be reviewed and discussed. The ability of nonspecific procoagulant agents (particularly rFVIIa) to reverse the effects of novel anticoagulants will also be reviewed.

Activation of platelets by heparin-induced thrombocytopenia antibodies in the serotonin release assay is not dependent on the presence of heparin

The serotonin release assay (SRA) tests for antibodies responsible for heparin-induced thrombocytopenia (HIT). By definition, SRA-positive antibodies cause platelet serotonin release in vitro, in the presence of low concentrations of heparin, but not with excess heparin. Many SRA-positive sera activate platelets in the presence of saline without drug, either as a result of residual heparin in the specimen, or because of intrinsic features of the HIT antibodies. The present experiments show that neither exhaustive heparinase treatment, nor chromatographic removal of heparin abrogates the spontaneous platelet activation caused by these HIT antibodies. This is the first study to systematically demonstrate that in vitro activity of HIT antibodies can be independent of heparin. In addition, T-gel chromatography demonstrated differences among fractions of enzyme-linked-immunosorbent assay (ELISA)-positive HIT antibodies within individual specimens. Certain ELISA-positive fractions had SRA activity while others did not, and the SRA activity was not proportional to HIT antibody ELISA titer. These data suggest that antibodies formed as a result of heparin treatment are heterogeneous, and that some can contribute to the pathogenesis of HIT even when heparin is no longer present.

Short- and long-acting synthetic pentasaccharides as antithrombotic agents

Fondaparinux sodium (Arixtra; GlaxoSmithKline) is the first of a new class of antithrombotic agents. It is a chemically synthesised pentasaccharide mimicking the site of heparin that binds to antithrombin. It is purely a factor Xa inhibitor and an inhibitor of thrombin generation that requires binding to antithrombin. Fondaparinux sodium differs from heparin, low-molecular-weight heparin and heparinoids, and cannot be used interchangeably. It has been approved in the US and Europe for the prophylaxis of venous thrombosis after orthopaedic surgery by a fixed dose of 2.5 mg/day without monitoring. Using this pentasaccharide as a backbone, other structures have been synthesised. Idraparinux sodium (Sanofi-Aventis) differs structurally from fondaparinux sodium as it has additional methyl groups, a long half-life, and once-weekly administration. Both drugs are being developed as antithrombotics for venous and arterial thrombosis, acute coronary syndrome, stroke and as adjuncts to thrombolytic therapy.

New anticoagulants: beyond heparin, low-molecular-weight heparin and warfarin

The limitations of traditional anticoagulants, heparin and warfarin, have prompted the development of new anticoagulant drugs for prevention and treatment of both venous and arterial thromboembolism. After a brief review of thrombogenesis and its regulation, this paper focuses on new anticoagulant agents in more advanced stages of clinical testing.

New anticoagulant drugs: the Seventh ACCP Conference on Antithrombotic and Thrombolytic Therapy

This article about new anticoagulant drugs is part of the seventh American College of Chest Physicians Conference on Antithrombotic and Thrombolytic Therapy: Evidence-Based Guidelines. The limitations of existing oral and parenteral anticoagulant agents have prompted a search for novel agents. Focusing on new anticoagulant drugs for the prevention and treatment of arterial and venous thrombosis, this article (1) reviews arterial and venous thrombogenesis, (2) discusses the regulation of coagulation, (3) describes the pathways for testing new anticoagulant agents, (4) describes new anticoagulant strategies focusing primarily on agents in phase II or III clinical testing, and (5) provides clinical perspective as to which of these new strategies is most likely to succeed.

Anticoagulant and antiprotease profiles of a novel natural heparinomimetic mannopentaose phosphate sulfate (PI-88)

Heparinomimetic mannopentaose phosphate sulfate (PI-88) (Progen Industries Ltd. Brisbane, Australia), currently developed as an anticoagulant and antiproliferative agent, mainly is composed of a pentomannan. However, tetrasaccharide and disaccharide components are also present. The molecular profile and the anticoagulant potency of PI-88 are investigated in this study. Gel permeation chromatography and polyacrylamide gel electrophoresis analyses were carried out to determine the molecular profile and separation of components of PI-88, respectively. Potentiation of antithrombin III (ATIII) and heparin cofactor-II (HC-II) activity were measured using chromogenic substrate assay. In order to determine anticoagulant and antiprotease effects of PI-88, various global anticoagulant tests, such as the prothrombin time (PT), activated partial thromboplastin time (APTT), thrombin time (TT), Hep-test (Haemachem Inc., St. Louis), ecarin clotting time (ECT), activated clotting time (ACT), and thromboelastography (TEG) were used. Anti-Xa and anti-IIa activities also were measured. The effect of PI-88 on the release of tissue factor pathway inhibitor (TFPI) was performed in nonhuman primates who received PI-88 and in endothelial cell culture systems. The relative susceptibility of PI-88 to heparinase I, protamine sulfate (PS), and platelet factor 4 (PF4) also was evaluated. The high-performance liquid chromatography profiles of PI-88 showed that its average molecular weight is approximately 2300 Da. Separation and gradient electrophoretic patterns of PI-88 showed that it is composed of five different fractions. This agent activates HC-II through inhibiting the thrombin generation but not inhibiting ATIII. Although PI-88 produced a concentration-dependent prolongation of all of the clotting tests, ECT gave the best correlation in the dose-response curve (ECT, r2 = 0.94; TT, r2 = 0.84; APTT, r2 = 0.69). Heparinomimetic mannopentaose phosphate sulfate (PI-88) exhibited marked inhibition of FIIa, but not of FXa. Heparinase I failed to produce significant neutralization of PI-88 in all the assays used, whereas PS and PF4 partially neutralized the effects of this compound. Heparinomimetic mannopentaose phosphate sulfate (PI-88) produced fivefold increase in the TFPI levels at 15 minutes after intravenous (IV) injection to primates. The incubation of PI-88 in endothelial cell culture system also showed a strong effect on TFPI release. These results suggest that PI-88 exhibited strong antithrombotic and anticoagulant activity in addition to its known antiproliferative properties. Because of the molecular characteristics and the dual nature of the pharmacologic action of PI-88, this agent represents an attractive pharmacologic agent for the control of thrombotic and proliferative disorders.