Animal Models Of Thrombosis Research Articles

Investigation into the mechanism(s) of antithrombotic effects of carbon monoxide releasing molecule-3 (CORM-3)

Carbon monoxide (CO) like nitric oxide (NO) has been recognized as activator of soluble guanylate cyclase (sGC) in many physiological functions. Studies, which demonstrate the mechanisms by which CO inhibits platelet aggregation in in vivo models, are few. Here we investigated the possible involvement of sGC, NO, plasminogen activator inhibitor (PAI-1) and p38 MAP Kinase in antithrombotic effects of CO released by a novel, water-soluble, CO releasing molecule-3 (CORM-3) using rat. The effects of CORM-3 on in vitro and ex vivo platelet aggregation induced by thrombin as well as in in vivo thrombosis models were studied. When added to rat washed platelets in in vitro study, CORM-3 (100 and 200 μM) inhibited thrombin-induced platelet aggregation. Similarly, antiplatelet effect was also observed when 3 mg/kg i.v. infusion of CORM-3 administered for 10 minutes in ex vivo study using rat. Interestingly, in presence of inhibitor of sGC (ODQ, 10 mg/kg, i.p.) and inhibitor of nitric oxide synthase (L-NAME, 30 mg/kg, i.p.), inhibition of thrombin-induced aggregation by CORM-3 was significantly blocked. Notably, in presence of inhibitor of K ATP channel (glibenclamide, 10 mg/kg, i.p.) and p38 MAP Kinase (SCIO-469, 1 mg/kg, i.p.), inhibition of aggregation by CORM-3 was not blocked. In in vivo studies using animal models of thrombosis, we found that CORM-3-mediated antithrombotic effect was dependent on activation of sGC, NO and suppression of PAI-1 in arterial thrombosis and Arterio-Venous (A-V) shunt models. Therefore, we concluded that antithrombotic activity of CORM-3 may be mediated by activation of sGC, NO and inhibition of PAI-1.

Apolipoprotein E receptor 2 is involved in the thrombotic complications in a murine model of the antiphospholipid syndrome

AbstractAntiphospholipid (aPL)/anti-β2 glycoprotein I (anti-β2GPI) antibodies stimulates tissue factor (TF) expression within vasculature and in blood cells, thereby leading to increased thrombosis. Several cellular receptors have been proposed to mediate these effects, but no convincing evidence for the involvement of a specific one has been provided. We investigated the role of Apolipoprotein E receptor 2 (ApoER2′) on the pathogenic effects of a patient-derived polyclonal aPL IgG preparation (IgG-APS), a murine anti-β2GPI monoclonal antibody (E7) and of a constructed dimeric β2GPI I (dimer), which in vitro mimics β2GPI-antibody immune complexes, using an animal model of thrombosis, and ApoER2-deficient (−/−) mice. In wild type mice, IgG-APS, E7 and the dimer increased thrombus formation, carotid artery TF activity as well as peritoneal macrophage TF activity/expression. Those pathogenic effects were significantly reduced in ApoER2 (−/−) mice. In addition, those effects induced by the IgG-APS, by E7 and by the dimer were inhibited by treatment of wild-type mice with soluble binding domain 1 of ApoER2 (sBD1). Altogether these data show that ApoER2 is involved in pathogenesis of antiphospholipids antibodies.

Patients with severe factor XI deficiency have a reduced incidence of deep-vein thrombosis

Factor XI (FXI) plays a dual role in haemostasis and thrombosis. It contributes to thrombin generation and promotes inhibition of fibrinolysis. Severe FXI deficiency was shown to confer protection against arterial and venous thrombosis in animal models without compromising haemostasis. We have previously shown that patients with severe FXI deficiency have a low incidence of ischaemic stroke, but display the usual incidence of myocardial infarction. In the present study, we compared the incidence of deep-vein thrombosis (DVT) in 219 unrelated patients with severe FXI deficiency aged 20-94 to the incidence in a large population-based study. No cases of DVT were observed in the FXI-deficient cohort, a result that is significantly lower than the expected number (4.68) computed from the population-based study. The low incidence remains statistically significant when compared to three other population-based studies. These data suggest that severe FXI deficiency provides protection against DVT.

Atherosclerosis and Thrombosis: Insights from Large Animal Models

Atherosclerosis and its thrombotic complications are responsible for remarkably high numbers of deaths. The combination of in vitro, ex vivo, and in vivo experimental approaches has largely contributed to a better understanding of the mechanisms underlying the atherothrombotic process. Indeed, different animal models have been implemented in atherosclerosis and thrombosis research in order to provide new insights into the mechanisms that have already been outlined in isolated cells and protein studies. Yet, although no model completely mimics the human pathology, large animal models have demonstrated better suitability for translation to humans. Indeed, direct translation from mice to humans should be taken with caution because of the well-reported species-related differences. This paper provides an overview of the available atherothrombotic-like animal models, with a particular focus on large animal models of thrombosis and atherosclerosis, and examines their applicability for translational research purposes as well as highlights species-related differences with humans.

A Rabbit Model of Thrombosis on Atherosclerotic Lesions

Thrombus formation on a disrupted atherosclerotic plaque is a key event that leads to atherothrombosis. Because thrombus is induced by chemical or physical injury of normal arteries in most animal models of thrombosis, the mechanisms of thrombogenesis and thrombus growth in atherosclerotic vessels should be investigated in diseased arteries of appropriate models. Pathological findings of human atherothrombosis suggest that tissue factor, an initiator of the coagulation cascade, significantly affects enhanced platelet aggregation and fibrin formation after plaque disruption. We established a rabbit model of atherothrombosis based on human pathology in which differences in thrombus formation between normal and atherosclerotic arteries, factors contributing to thrombus growth, and mechanisms of plaque erosion can be investigated. Emerging transgenic and stem cell technologies should also provide an invaluable rabbit experimental model in the near future.

Laser-induced endothelial cell activation supports fibrin formation

Laser-induced vessel wall injury leads to rapid thrombus formation in an animal thrombosis model. The target of laser injury is the endothelium. We monitored calcium mobilization to assess activation of the laser-targeted cells. Infusion of Fluo-4 AM, a calcium-sensitive fluorochrome, into the mouse circulation resulted in dye uptake in the endothelium and circulating hematopoietic cells. Laser injury in mice treated with eptifibatide to inhibit platelet accumulation resulted in rapid calcium mobilization within the endothelium. Calcium mobilization correlated with the secretion of lysosomal-associated membrane protein 1, a marker of endothelium activation. In the absence of eptifibatide, endothelium activation preceded platelet accumulation. Laser activation of human umbilical vein endothelial cells loaded with Fluo-4 resulted in a rapid increase in calcium mobilization associated cell fluorescence similar to that induced by adenosine diphosphate (10 μM) or thrombin (1 U/mL). Laser activation of human umbilical vein endothelial cells in the presence of corn trypsin inhibitor treated human plasma devoid of platelets and cell microparticles led to fibrin formation that was inhibited by an inhibitory monoclonal anti-tissue factor antibody. Thus laser injury leads to rapid endothelial cell activation. The laser activated endothelial cells can support formation of tenase and prothrombinase and may be a source of activated tissue factor as well.

New Insights Into Factor XI Function: From Biochemistry to Animal Models

AbstractAbstract SCI-20Factor XI (fXI) is the zymogen of an enzyme (fXIa) that contributes to blood coagulation through activation of factor IX (fIX). FXI has structural and mechanistic features that distinguish it from the vitamin K-dependent proteases of coagulation. The protein is a dimer of identical 80 kDa subunits, each containing four apple domains (A1-A4) that form a platform at the base of the trypsin-like protease domain. The apple domains contain binding sites for fIX, platelet receptors, and high molecular weight kininogen. FXI is converted to fXIa by cleavage of a single bond on each subunit, unmasking exosites required for fIX binding. Conversion of fXI to fXIa proceeds through an intermediate with only one activated subunit (1/2-fXIa). 1/2-fXIa, and monomeric forms of fXIa, activate fIX in a manner similar to fully activated fXIa, indicating each subunit functions as a complete enzyme. The importance of the dimeric structure of fXI is not clear at this point. It may facilitate activation, or allow fXIa to bind simultaneously to fIX and a surface (a platelet for example) at a wound site.Congenital fXI deficiency is associated with a variable propensity to bleed excessively after trauma to certain tissues. Symptoms are usually milder than in fIX deficiency (hemophilia B), and many affected individuals are asymptomatic. In the cascade-waterfall model of coagulation, fXI is activated by factor XIIa (fXIIa) during a process called contact activation. However, current models often omit contact activation, because fXII deficiency is not associated with abnormal hemostasis. Thrombin activates fXI, providing an explanation for normal hemostasis in fXII deficiency. In contrast to its modest role in hemostasis, fXI may serve an important role in thromboembolic diseases. High fXI levels are a risk factor for arterial and venous thrombosis in humans; and deficiency or inhibition of fXI confers resistance to thrombosis in animal models. FXI deficient mice are as resistant to arterial thrombosis as fIX deficient mice, or wild type mice treated with a supra-therapeutic dose of heparin. In arterial thrombosis models in mice, rabbits and baboons, lack of fXI activity results in instability of platelet rich thrombi, preventing vessel occlusion. FXI deficiency also prolongs survival and lessens the severity of disseminated intravascular coagulation in a mouse polymicrobial sepsis model. Interestingly, mice with combined deficiencies of fXI and fIX are more resistant to arterial thrombus formation than mice deficient in only one of these proteins, indicating fXIa has proteolytic targets other than fIX.The observation that fXII deficient mice are resistant to arterial thrombosis suggests that activation of fXI by contact activation, while unnecessary for hemostasis, contributes to thrombin generation in some pathologic processes. If the observations in mice apply to thromboembolism in humans, then fXIa and/or fXIIa may be excellent targets for novel antithrombotic strategies. In contrast to drugs such as heparin and warfarin, agents targeting fXIa or fXIIa would likely be associated with relatively few bleeding complications, and could be employed in clinical situations where anticoagulation therapy is currently contraindicated. Disclosures:No relevant conflicts of interest to declare.

Dabigatran: An Oral Novel Potent Reversible Nonpeptide Inhibitor of Thrombin

Dabigatran is a highly selective, reversible, and potent thrombin inhibitor and is orally available as the prodrug, dabigatran etexilate. It has shown antithrombotic efficacy in animal models of thrombosis, with a rapid onset of action and predictable pharmacodynamic response. Peak plasma concentrations of dabigatran occur 1 to 2 hours after ingestion of the prodrug. The terminal half-life of dabigatran is 12 to 14 hours in elderly volunteers. Dabigatran is not metabolized by cytochrome P450 isoenzymes and does not interact with food. Dabigatran has a low potential for drug-drug interactions and is predominantly renally excreted. Dabigatran etexilate as chronic therapy effectively prevents the recurrence of venous thromboembolism and cardioembolic stroke. For the first time, it has been demonstrated clinically that there may be an effective and safe alternative to warfarin.

Advances in understanding the pathogenesis of chronic thromboembolic pulmonary hypertension

Chronic thromboembolic pulmonary hypertension (CTEPH) comprises organizing thrombotic obstructions in the pulmonary arteries. While roughly 40% of CTEPH cases are not preceded by a venous thromboembolic event, 0.1-5.1% of acute pulmonary thromboemboli evolve into organized obstructions of the pulmonary artery. In patients with predominantly proximal disease, surgical pulmonary endarterectomy provides a potential cure of the disease. For years, the scientific debate of CTEPH was mainly focused around its thromboembolic nature because of striking dissimilarities to classical venous thromboembolism, for example, the lack of risk factors for venous thrombosis, the lack of clinically apparent pulmonary embolism in many patients, the difficulty to reproduce the disease in animal models of thrombosis, and the nature of the pulmonary vascular obstructions that are dissociated from the degree of hemodynamic compromise. Recent studies have confirmed an association between venous thromboembolism and the evolution of CTEPH, and have shed light on disease-modifying conditions.

Variable region heavy chain glycosylation determines the anticoagulant activity of a factor VIII antibody

The mechanism of action of antibodies inhibiting partially factor VIII (FVIII) activity (type II inhibitor) is still poorly understood. We produced an unusual type II monoclonal antibody, called LE2E9, derived from a patient with mild haemophilia A. The antibody displayed several unexpected structural and functional properties such as glycosylation in the variable region, binding to the FVIII C1 domain, inhibition of maximum 80-90% FVIII activity when in excess over FVIII, and prevention of FVIII binding to von Willebrand factor (VWF). Those unusual characteristics of the antibody prompted multidisciplinary studies to determine its mechanism of action and the role of the FVIII C1 domain. Enzymatic deglycosylation and site-directed mutagenesis indicated that the oligosaccharides do not determine the affinity of the antibody but enhanced its FVIII neutralizing activity. Modification of glycosylation in the variable region of antibodies therefore contributes to the diversity of FVIII type II inhibition and provides a novel strategy with which to modulate the functional activity of antibodies. Investigation of the FVIII C1 domain function led to identification of mutations located in that domain and impairing FVIII binding to VWF as a common cause of mild/moderate haemophilia A. Finally, the cloning of human monoclonal antibodies inhibiting partially FVIII activity opened the way to evaluate such antibodies as a novel type of anticoagulant drug. This concept was validated in animal models of thrombosis. Those studies are expected to have a significant impact on the optimization of treatments for patients with inhibitor as well as for patients with thrombophilia.

Delivery of Glycoprotein IIb/IIIa Inhibitor Therapy for Percutaneous Coronary Intervention

A major goal of any antithrombotic regimen administered during percutaneous coronary intervention (PCI) is the preservation of coronary microvascular perfusion, which is critical for myocardial survival. In addition to macrovascular thrombosis, microembolization into the downstream coronary artery bed that occurs during spontaneous plaque rupture and PCI plays a prominent role in the development of microvascular dysfunction that leads to myocardial infarction. In addition to physical obstruction of the lumen by the embolus, vasoconstriction and edema due to inflammation also impair microvascular flow.1 These events likely occur more frequently during PCI performed in the setting of unstable coronary syndromes.2 With Doppler guidewire technology, it was estimated that an average of 25 embolic events occurred during primary PCI for ST-segment elevation myocardial infarction.3 Article see p 784 Platelet aggregates are important components of coronary microemboli, along with leukocytes, erythrocytes, platelet-leukocyte aggregates, cholesterol crystals, and hyalin. Platelets and leukocytes within microemboli play a particularly important role in the pathophysiological changes of blood flow by promoting in situ microvascular thrombosis, vasoconstriction, and inflammation. Interestingly, the composition of the embolized material is similar in the settings of spontaneous and catheter-induced plaque rupture.1 In animal models, distal microembolization of inert microspheres is associated with an immediate reduction in flow due to vessel occlusion, followed by enhanced blood flow due to the effects of adenosine released from the embolized myocardium into the adjacent noninvolved myocardium.4 Microembolization results in overall reduced flow reserve and the progressive loss of contractile function, which often reverses within weeks. The degree of myocardial contractile dysfunction is disproportionate to the degree of infarction and is the result of inflammation.5 However, in the clinical condition, a more complex pathophysiology exists due to the influence of multiple components of the emboli. Platelets aggregate when fibrinogen binds to the active …

Tissue factor and thrombosis: The clot starts here.

Thrombosis, or complications from thrombosis, currently occupies the top three positions in the cardiovascular causes of morbidity and mortality in the developed world. There are a limited number of safe and effective drugs to prevent and treat thrombosis. Animal models of thrombosis are necessary to better understand the complex components and interactions involved in the formation of a clot. Tissue factor (TF) is required for the initiation of blood coagulation and likely plays a key role in both arterial and venous thrombosis. Understanding the role of TF in thrombosis may permit the development of new antithrombotic drugs. This review will focus on the role of TF in in vivo models of thrombosis.

Long-term administration of traditional kampo medicine shimotsuto, juzentaihoto and unseiin inhibits experimental thrombosis in mice

Traditional Chinese herbal medicines (Kampo) are used to improve flow characteristics of blood (Oketsu). We assumed that by preventing stagnation of blood, these medicines may be beneficial not only in venous but in arterial thrombotic conditions. The present study aimed to assess the antithrombotic effect of three Kampo, using well-established in vitro and animal models of thrombosis. Western-style highfat diet containing 1% Kampo (Shimotsuto, Juzentaihoto or Unseiin) was administered to C57BL/6 mice for 12 weeks. The effect on thrombus formation by laser irradiation of the carotid artery of mice was assessed. In addition the ex-vivo technique of shear-induced platelet reactivity measurement (haemostatometry) and the in vivo test of endothelial function (flowmediated vasodilation) were also used to assess the mechanism of antithrombotic effect. All three medicines have significantly inhibited arterial thrombus formation in mice. According to our studies, the mechanism of antithrombotic effect is based on the inhibition of shear-induced platelet reac- tivity and stimulation of endothelial function (Unseiin). It is assumed that the common ingredients Japanese Angelica Root, Cnidium Rhizome, Peony Root and Rehmannia Root could be responsible for the observed antithrombotic effect.

A Rice Variety (HYJA-Ri4) Enhances Arterial Thrombus Formation in an Animal Model of Thrombosis

Epidemiological evidence exists for a causative role of inappropriate diets in the development and clinical outcome of thrombotic diseases. Our previous studies have shown that some fruits and vegetables have antithrombotic activity. As rice is a principal ingredient of everyday food in Japan, the aim of the present study was to determine rice varieties with antithrombotic effect. Western-style high-fat diet containing flour prepared from whole-grain rice of different varieties was fed to atherosclerosis-prone (apolipoprotein E- and low density lipoprotein receptor-deficient) mice for 3 months. A laser-induced carotid-artery thrombosis model was used. None of the five tested rice varieties showed an antithrombotic effect. The effect of HYJA-Ri1, HYJA-Ri2, HYJA-Ri3 and HYJA-Ri5 rice on experimental thrombus formation did not differ significantly from the controls. However, HYJA-Ri4 rice significantly enhanced the thrombotic reaction (p = 0.0078), thus having a prothrombotic effect. Our findings suggest that HYJA-Ri4 rice should not be included in an antithrombotic diet.

Combination of FXI Antisense Oligonucleotide and Plavix® Treatment Results in Enhanced Antithrombotic Activity without Increased Risk of Bleeding in Mouse Models of Thrombosis.

Abstract 4173Factor XI (FXI) is a serine protease produced in the liver that contributes to thrombin generation via the intrinsic coagulation pathway. Also, it is a key component of an amplification pathway that is thought to sustain thrombin production at a wound site to maintain fibrin clot integrity. Loss of function mutations in the human FXI gene results in FXI deficiency, a disorder which is associated with only mild bleeding. In addition, high levels of FXI are a risk factor for thrombosis. We have previously presented the antithrombotic efficacy and safety of antisense oligonucleotide (ASO) mediated FXI depletion in various animal models of thrombosis. We have demonstrated that in combination with Lovenox®, FXI ASO treatment increases antithrombotic activity but does not increase bleeding risk. The purpose of the current study was to evaluate the relative risk/benefit of ASO mediated Factor XI depletion in combination with the platelet antagonist Plavix®. The “risk” component was bleeding tendency as measured by blood volume loss following tail nick, and the “benefit” component was antithrombotic effect in arterial and venous mouse models of thrombosis. Our study investigated a 20 mg/kg dose of FXI ASO in combination with a dose response of Plavix® and indicates that antisense mediated FXI depletion provides enhanced antithrombotic protection when given in combination with Plavix®. The combination of FXI ASO and Plavix® did not result in increased bleeding tendency. In contrast, treatment with a small molecule inhibitor of factor Xa in combination with Plavix® significantly increased bleeding. The approach used in this study is being used to define the ability of FXI ASO to combine with standard of care agents for the treatment of thrombosis. Disclosures:Gao:Isis Pharmaceuticals, Inc.: Employment. Crosby:Isis Pharmaceuticals, Inc.: Employment. MacLeod:Isis Pharmaceuticals: Employment. Bhattacharjee:Isis Pharmaceuticals, Inc.: Employment. Lowenberg:Isis Pharmaceuticals, Inc.: Consultancy. Levi:Isis Pharmaceuticals, Inc.: Consultancy. Monia:Isis Pharmaceuticals, Inc.: Employment.

Animal models of thrombosis

The thrombotic response to vascular injury is an important clinical problem that mediates most vascular disease complications. Thrombus formation involves an integrated response that is influenced by blood flow, multiple cell types, and numerous circulating factors. As a result, modeling of this complex response using in-vitro or in-silico strategies is insufficient. The use of animal models of thrombosis provides a critical tool for the discovery and initial testing of novel therapies for vascular thrombosis. The literature from 2008 to the present provides significant advances in regard to novel models of arterial thrombosis, novel mechanisms underlying thrombus formation, new models and mechanisms related to thrombotic stroke, and preclinical advances in therapeutics for vascular thrombosis. The formation of occlusive thrombi is complex, involving the integration of many molecular interactions and cell types at the site of vascular injury. The identification of strategies to suppress occlusive thrombus formation without undermining normal hemostatic function is the primary goal of this area of study.

Carboxypeptidase U (TAFIa): a new drug target for fibrinolytic therapy?

Procarboxypeptidase U (TAFI) is a recently discovered plasma procarboxypeptidase that upon activation by thrombin or thrombin-thrombomodulin turns into a potent antifibrinolytic enzyme. Its prominent bridging function between coagulation and fibrinolysis raised the interest of many research groups and of the pharmaceutical industry. The development of carboxypeptidase U (CPU) inhibitors as profibrinolytic agents is an attractive concept and possibilities for rational drug design will become more readily available in the near future as a result of the recently published crystal structure. Numerous studies have been performed and many of them show beneficial effects of CPU inhibitors for the improvement of endogenous fibrinolysis in different animal sepsis and thrombosis models. CPU inhibitors combined with tissue-type plasminogen activator (t-PA) seem to increase the efficiency of pharmacological thrombolysis allowing lower dosing of t-PA and subsequently fewer bleeding complications. This review will focus on recently obtained in vivo data and the benefits/risks of targeting CPU for the treatment of thrombotic disorders.

The Tissue-Type Plasminogen Activator Story

Milestones in the development of tissue-type plasminogen activator (t-PA) as a fibrin-specific thrombolytic agent include: purification of human t-PA from the culture fluid of the Bowes melanoma cell line, elucidation of the molecular basis of fibrin-specific plasminogen activation, first experimental animal models of thrombosis, first patient (renal allograft) treated with melanoma t-PA, pilot studies in patients with acute myocardial infarction, cloning and expression of recombinant t-PA providing sufficient amounts for large scale clinical use, and demonstration of its therapeutic benefit in large multicenter clinical trials.

High spatial resolution magnetic resonance imaging of experimental cerebral venous thrombosis with a blood pool contrast agent

Purpose The purpose of this study was to investigate the feasibility of clot visualization in small sinus and cortical veins with contrast enhanced MRA in a cerebral venous thrombosis animal model using a blood pool contrast agent, Gadofosveset, and high spatial resolution imaging. Material and methods For induction of cerebral venous thrombosis a recently developed combined interventional and microsurgical model was used. Cerebral sinus and cortical vein thrombosis was induced in six pigs. Two further pigs died during the procedure. Standard structural, time-of-flight- and phase contrast-angiograms were followed by fast time resolved high resolution 3D MRA (4D MRA) and subsequent high spatial resolution 3D MRA in the equilibrium phase with and without addition of parallel imaging. Visualization of the clots using the different sequences was subjectively compared and contrast-to-noise ratio (CNR) was assessed. Results In the remaining six animals the procedure and MR-imaging protocol including administration of Gadofosveset was successfully completed. The 3D high resolution MRA in the equilibrium phase without the addition of parallel imaging was superior to all the other applied MR measurement techniques in terms of visualization of the clots. Only applying this sequence bridging vein thromboses were also seen as a small filling defect with a high CNR of >18. Conclusion Only the non-accelerated high spatial resolution 3D MRA in the equilibrium in conjunction with the blood pool agent Gadofosveset allows for high-contrast visualization of very small clots in the cerebral sinus and cortical veins. Statement clinical impact Detection of cortical vein thrombosis is of high clinical impact. Conventional MRI sequences often fail to visualize the clot. We could demonstrate that, in contrast to conventional sequences, with high spatial resolution 3D MRA in the equilibrium in conjunction with the blood pool agent Gadofosveset very small clots in the cerebral sinus and cortical veins could be successfully visualized. We think that with the presented approach cortical vein thrombosis might also be sufficiently visualized in patients.

Defibrotide Augments the Anticoagulant Actions of Heparin and Low Molecular Weight Heparins

Defibrotide represents a polydeoxyribonucleotide derived antithrombotic and antiischemic drug, which has been used in the management of vascular disorders and is currently being developed in other clinical indications. Defibrotide is a polyelectrolyte-based agent with target effects on endothelium, platelets, and blood cells. In addition, the aptameric consensus sequences in the nucleotides exhibit inhibitory effects towards thrombin and related proteases. In the anticoagulant assays defibrotide exhibits relatively weak effects (<5 USP U/mg). These studies were undertaken to study whether there is an interaction between defibrotide and unfractionated heparin (UFH) in various systems of anticoagulation. The interaction of defibrotide with commercially available low molecular weight heparins (LMWHs), enoxaparin and dalteparin, was also studied. For the first investigation, to evaluate the effect of defibrotide on the anticoagulant effects of UFH, native whole blood freshly drawn from human volunteers (n = 20) was supplemented with UFH at a fixed concentration of 5 μg/mL (0.8 U/mL), and graded amounts of defibrotide were added in a concentration range of 12.5 – 100 μg/mL. The whole blood celite Activated Clotting Time test (ACT) and the thrombin generation markers fibrinopeptide A (FPA), thrombin-antithrombin complex (TAT), and prothrombin fragment 1.2 (F1.2) were measured. Parallel controls with saline were included. While defibrotide did not produce a significant prolongation of the ACT compared to saline (128 ± 9 s vs 132 ± 7 s), it produced a concentration-dependent increase in the heparinized whole blood leading to an almost doubling of the anticoagulant action of UFH (248 ± 19 s vs 418 ± 21 s). Additional studies carried out by varying the concentrations of the two agents also revealed supraadditive to synergistic effects. Defibrotide also augmented the inhibitory effects of UFH on thrombin generation markers in a concentration-dependent fashion. Similar studies carried out with the two LMWHs did not reveal a similar interaction in the anticoagulant assays such as the ACT; however, significant interactions between defibrotide and the LMWHs were observed in the thrombin generation studies. For the second investigation, studies were carried out using plasma samples collected from heparinized patients (aPTT of 50 – 100 s). These studies also revealed that supplementation of defibrotide augmented the anticoagulant effects of UFH in a concentration-dependent fashion. While defibrotide at 12.5 μg/mL did not significantly increase the aPTT of normal plasma, when supplemented to heparinized plasmas (n = 50 with aPTT of 64.6 ± 14.0 s) it produced a strong prolongation of the clotting time (96.1 ± 20.6 s). In the third investigation, animal models of thrombosis including the rat jugular vein clamping model, demonstrated an augmentation of the antithrombotic effects of intravenously administered UFH by defibrotide. However, no augmentation of the hemorrhagic effect was observed in the rat tail bleeding model. These studies demonstrate that defibrotide exhibits a strong anticoagulant interaction with UFH and to a lesser degree LMWH. While the combination of defibrotide and UFH exhibits enhanced anticoagulant/antithrombotic activities, it does not exhibit any alteration of the hemorrhagic profile. These studies clearly suggest that defibrotide can be combined with UFH to achieve a superior anticoagulant approach with better safety/efficacy profile.