Thrombin inhibitors as anticoagulant agents

Combined administration of barbourin--albumin and hirudin--albumin fusion proteins limits fibrin(ogen) deposition on the rabbit balloon-injured aorta

INTRODUCTION

There are continuing needs for new antithrombotic agents and procedures. We hypothesized that the slowly cleared recombinant fusion proteins barbourin--albumin (BLAH6) and hirudin--albumin (HLAH6) would be effective in limiting fibrin(ogen) and/or platelet deposition in a rabbit model of arterial injury.

MATERIALS AND METHODS

Recombinant fusion proteins were expressed in Pichia pastoris fermenter cultures and purified by nickel-chelate affinity chromatography. They were injected intravenously into rabbits prior to blood sampling and platelet aggregometry, assessment of deposition of 125I-fibrin(ogen) and 51Cr-platelet onto the balloon-injured thoracic aorta, electron microscopy (EM) and immunohistochemistry of aortic sections, and determination of bleeding time following a standardized ear incision.

RESULTS AND CONCLUSIONS

BLAH6 administration elicited a dose- and time-dependent inhibition of platelet aggregation in post-injection whole blood samples, and reduced both fibrin(ogen) and platelet deposition on the injured aorta, although the former effect was both more durable and more significant than the latter. In contrast, HLAH6 injection reduced fibrin(ogen) but not platelet deposition. Doses of the two proteins ineffective in preventing fibrin(ogen) deposition when given alone were effective when combined, suggesting at least additive effects. Immunohistochemistry and EM supported the radioactive deposition studies, while bleeding times were decreased with combined BLAH6 and HLAH6 administration compared to HLAH6 alone in a rabbit ear bleeding model. The data show that these fusion proteins exert an antithrombotic effect in vivo and may indicate that combined low-dose administration of antiplatelet and antithrombin agents could offer safety advantages in the treatment of thrombosis.

Novel anticoagulant polyethylenimine: inhibition of thrombin-catalyzed fibrin formation

Hypercoagulability is often associated with a variety of disease states, leading to cardiovascular complications. Polyethylenimine (PEI) prolonged prothrombin time, demonstrating its anticoagulant potential. In vitro, PEI at low concentration (nM) significantly blocked thrombin-catalyzed fibrin formation, accounting for its mode of anticoagulation. The uncompetitive inhibition by PEI of fibrin formation was independent of the concentration of fibrinogen (FBG), thrombin, or NaCl. PEI showed no effect on thrombin amidolytic activity, suggesting that the blockade of thrombin interaction with FBG could account for the inhibition on fibrin formation. PEI drastically depressed rabbit brain thromboplastin procoagulation monitored by a single-stage clotting assay using human plasma. In a THP-1 monocytic hypercoagulation model, a 4-h exposure to bacterial endotoxin or Ca(2+) ionophore A23187, respectively, resulted in a 5- or 10-fold enhancement in monocytic tissue factor (mTF) procoagulation. mTF hypercoagulation was offset by PEI included in the assay mixture. PEI showed the potential to arrest mTF hypercoagulation with IC(50) around 1.2 nM. Using a chromogenic assay to dissect the extrinsic pathway, we further assessed whether PEI has any effect on other clotting factors. PEI was not an inhibitor for either FVIIa or FXa, having no effect on not only the amidolytic but also their corresponding functionally catalytic activities. Although PEI upregulated TF-dependent FVII activation under the low-salt condition, the effective downstream inhibition of fibrin formation readily abolished and overrode the upstream enhancement, demonstrating the overall anticoagulation. PEI could present a new class of anticoagulant.

Capillary electrophoresis and enzyme solid phase assay for examining the purity of a synthetic heparin proteoglycan-like conjugate and identifying binding to basic fibroblast growth factor

Interaction of basic fibroblast growth factor (bFGF) with heparin/heparan sulfate proteoglycans protects the growth factor against proteolytic degradation and is essential for its cellular activity. Although the structural requirements of heparin and heparan sulfate for the high-affinity binding to bFGF have been extensively examined, studies on intact heparin proteoglycans are limited. In this report, the purity and the binding ability of a heparin proteoglycan-like molecule-the heparin-bovine serum albumin (heparin-BSA) conjugate-was examined using capillary zone electrophoresis (CZE). Furthermore, the affinity of bFGF binding to the heparin-BSA conjugate was studied using an enzyme solid-phase assay. Chondroitin sulfate, dermatan sulfate, hyaluronan, heparan sulfate and variously sulfated disaccharides derived from heparin and heparan sulfate were also studied for their ability to compete with the binding of bFGF to heparin. Heparin-BSA conjugate was synthesized by reductive amination and, following precipitation with 1.5 vols of ethanol-sodium acetate, it was obtained free of contaminating heparin. Heparin-BSA-bFGF conjugate was obtained following incubation of heparin-BSA with bFGF for 2 h at 37 degrees C. Intact heparin, heparin-BSA and heparin-BSA-bFGF conjugates were completely resolved by CZE using 50 mM phosphate, pH 3.5, as operating buffer, reversed polarity (30 kV) and detection at 232 nm. Competitive solid phase assay showed that, among the glycosaminoglycans tested, heparin exhibits the highest affinity binding to bFGF (IC(50) = 6.4 nM). Heparan sulfate showed a lower affinity as compared with that of heparin, whereas all other glycosaminoglycans and heparin/heparan sulfate-derived disaccharides tested showed minute effects. The developed CZE method is rapid and accurate and can be easily used to identify bFGF-interacting heparin preparations of biopharmaceutical importance.

Antithrombotic effect of Glycyrrhizin, a plant-derived thrombin inhibitor

Glycyrrhizin (GL), an anti-inflammatory compound isolated from licorice (Glycyrrhiza glabra), has been previously identified as a thrombin inhibitor (Francischetti et al., Biochem Biophys Res Commun 1997;235:259-63). Here we report the in vivo effects of GL upon two experimental models of induced thrombosis in rats. Intravenous administration of GL caused a dose-dependent reduction in thrombus size on a venous thrombosis model that combines stasis and hypercoagulability. It was observed that GL doses of 180 mg/kg body weight produced 93% decrease on thrombus weight. This effect showed a time-dependent pattern being significantly reduced when the thrombogenic stimulus was applied 60 min after drug administration. GL was also able to prevent thrombosis using an arteriovenous shunt model. GL doses of 180 and 360 mg/kg decreased the thrombus weight by 35 and 90%, respectively. Accordingly, the APTT ex vivo was enhanced by 1.5- and 4.3-fold at GL doses of 180 and 360 mg/kg, respectively. In addition, GL doses above 90 mg/kg caused significant hemorrhagic effect. In contrast with heparin, GL did not potentiate the inhibitory activity of antithrombin III or heparin cofactor II towards thrombin. Altogether, data indicate that GL is an effective thrombin inhibitor in vivo, which may account for its other known pharmacological properties.

Pharmacologic preservation of the hemostatic system during cardiac surgery

Bleeding after cardiac surgery remains a major potential problem. Numerous pharmacologic approaches to attenuating hemostatic system activation in cardiac surgery patients have been studied to further improve patient management. Therapeutic approaches studied include inhibiting thrombin generation or activation, preserving platelet function, and decreasing the need for transfusion of allogeneic blood products. Pharmacologic approaches to reduce bleeding and transfusion requirements in cardiac surgery patients are based on either preventing or reversing the defects associated with the CPB-induced coagulopathy. The increasing use of platelet inhibitors (clopidogrel and IIb/IIIa receptor antagonists) and new anticoagulants (low-molecular weight heparins, pentasaccharide, recombinant hirudin, bivalirudin, and argatroban) also pose interesting problems in managing cardiac surgery patients. Aprotinin and lysine analogues (epsilon-aminocaproic acid and tranexamic acid) have become mainstay therapeutic agents to prevent bleeding and the potential need for allogeneic transfusion. Newer therapies that are important to consider include the potential of recombinant activated factor VIIa as a therapy for refractory bleeding after cardiac surgery.

Ecarin clotting time is sensitive to heparinoids: comparison of two different techniques

Ecarin clotting time (ECT) is currently developed for the specific monitoring of antithrombin drugs, such as hirudin, argatroban, and hirulog. Aqueous reagent and dry chemistry technology have become available for ECT monitoring of antithrombin agents. Currently, many heparinoids and heparinomimetic drugs are being developed. These agents activate heparin cofactor II (HCII) and primarily mediate their effects by inhibiting thrombin. Although the test is specific for antithrombin agents, heparin cofactor II-mediated thrombin inhibitors are capable of prolonging the ECT. In order to study the relative effects of some of these agents, ECT was measured in human plasma supplemented with Pl-88 (a sulfated pentomanose; Progen Industries Limited, Sydney, Australia), aprosulate, pentosan polysulfate, dermatan sulfate, unfractionated heparin (UFH), and recombinant hirudin (r-hirudin). All agents were supplemented to the citrated-pooled plasma prepared from 10 healthy volunteers at a graded dosage of 0 to 100 microg/ml. These techniques gave comparable results for all of the agents used (PI-88, r2 = 0.99; r-hirudin, r2 = 0.98; UFH, r2 = 0.98; dermatan sulfate, r2 = 0.95; aprosulate, r2 = 0.95; pentosan polysulfate, r2 = 0.94). The relative anticoagulant effects of various agents used on ECT varied widely, exhibiting their potency in the following order: r-hirudin = pentosan polysulfate > dermatan sulfate > PI-88 > aprosulate > UFH. The sensitivity of ECT was adjusted by varying the concentration of the ecarin reagent. The results suggest that HCII-mediated inhibition of thrombin can be detected by using ECT reagents.

Inhibition of prothrombin activation by bothrojaracin, a C-type lectin from Bothrops jararaca venom

Bothrojaracin is a potent and specific alpha-thrombin inhibitor (Kd approximately 0.6 nM) isolated from Bothrops jararaca venom. It binds to both of thrombin's anion-binding exosites (1 and 2), thus inhibiting the ability of the enzyme to act upon several natural macromolecular substrates, such as fibrinogen, platelet receptor, protein C, and factor V. Additionally, bothrojaracin interacts with prothrombin (Kd approximately 30 nM), as previously determined by a solid-phase assay. However, there is no information concerning the effect of this interaction on prothrombin activation and whether the binding of bothrojaracin can occur in plasma. Here, we show that bothrojaracin specifically interacts with prothrombin in human plasma. It is an effective anticoagulant after activation of the intrinsic pathway of blood coagulation, and analysis of prothrombin conversion in plasma shows that bothrojaracin strongly reduces alpha-thrombin formation. To determine whether this effect is due exclusively to inhibition of feedback reactions involving the thrombin-induced activation of factors V and VIII, we analyzed the effect of bothrojaracin on the activation of purified prothrombin by Oxyuranus scutellatus venom. As with plasma, bothrojaracin greatly inhibited thrombin formation, suggesting a direct interference in the prothrombin activation by the enzyme found in this venom (scuterin, a prothrombin activator described as a factor Xa/factor Va-like complex). Altogether, we suggest that bothrojaracin exerts its anticoagulant effect in plasma by two distinct mechanisms: (1) it binds generated thrombin and inhibits exosite 1 dependent activities such as fibrinogen clotting and factor V activation, and (2) it interacts with prothrombin and decreases its proteolytic activation. Thus, bothrojaracin may be useful in the search for thrombin inhibitors that bind both the zymogen and the active enzyme.

Use of direct thrombin inhibitors in acute coronary syndrome

OBJECTIVE

This paper examines the rationale for using direct thrombin inhibitors in the management of acute coronary syndrome (ACS).

BACKGROUND

With traditional management of ACS using aspirin and unfractionated heparin (UH), refractory angina and new myocardial infarction (MI) continue to develop. Growing understanding of the pathophysiology of ACS has led to the search for more effective therapies directed toward preventing formation of fibrin- and platelet-rich thrombi in the coronary arteries. Current pharmacologic approaches include use of direct thrombin inhibitors (lepirudin, desirudin, and bivalirudin).

METHODS

We reviewed all published clinical trials abstracted in MEDLINE from 1966 to April 2000, excluding pilot studies enrolling <500 patients.

RESULTS

Use of lepirudin at medium doses (0.4-mg/kg bolus + 0.15 mg/kg/h) resulted in lower rates of death, new MI, and refractory angina at 7 days compared with UH (3.0% vs 6.5%; P = 0.047), although the incidence of minor bleeding was increased (7.6% vs 4.5%; P < 0.05). Bivalirudin was as effective as UH in preventing complications after percutaneous coronary intervention (11.4% vs 12.2%; NS) and carried a lower bleeding risk (7.8% vs 19.2%; NS); however, its use in the management of ACS has not been studied. Desirudin used at low doses (0.1-mg/kg bolus + 0.1 mg/kg/h) in large-scale clinical trials in patients with acute MI treated with alteplase or streptokinase appeared to be at least as effective as UH (8.9% vs 9.8% at 30 days; NS). However, its therapeutic index was narrow, since it was associated with significantly more moderate bleeding (8.8% vs 7.7%; P < 0.05).

CONCLUSIONS

All clinical trials to date have studied relatively short-term use (3-5 days) of direct thrombin inhibitors, and long-term benefits on morbidity and mortality have not been demonstrated. Until further data are available, direct thrombin inhibitors should be restricted to use as a possible alternative in patients who require anticoagulant therapy but experience UH-induced thrombocytopenia.