The status of new anticoagulants

Platelet-Depletion of Whole Blood Reveals That Platelets Potentiate the Release of IL-8 From Leukocytes Into Plasma in a Thrombin-Dependent Manner

Objective

In a recent study, we found an elevated level of interleukin 8 (IL-8) in response to bacterial incubation in thrombin-sufficient human whole blood anticoagulated by the fibrin polymerization blocking peptide GPRP. Whether thrombin directly activated leukocytes or mediated the release via thrombin-dependent activation of platelets remains unresolved. Herein, we addressed the role of thrombin and platelets in IL-8 release.

Methods

We separated platelets from whole blood using a combination of 0.7% (w/v) citrate and GPRP for attenuating the hemostatic response during the separation of platelets. Cytokine responses were compared in whole blood and platelet-depleted blood upon Escherichia coli incubation. Cytokine responses were also profiled with and without reconstitution of either platelets or the supernatant from activated platelets.

Results

Platelets were not activated during the separation process but responded to stimuli upon re-calcification. Plasma levels of IL-1β, IL-1Ra, IL-6, IL-8, IP-10, MIP-1α, and MIP-1β were significantly reduced in platelet-depleted blood compared to whole blood, but recovered in the presence of platelets, or with the supernatant of activated platelets. The leukocyte fraction and platelets were each found to contribute to the elevation of IL-8 at around 5 ng/ml; however, if combined, the release of IL-8 increased to 26 ng/ml. This process was dependent on thrombin since the levels of IL-8 remained at 5 ng/ml in whole blood if thrombin was blocked. Intracellular staining revealed that monocytes were the main source for IL-8 expression.

Conclusion

Our findings suggest that the release of IL-8 is mediated by the leukocytes, mainly monocytes, but potentiated via thrombin-dependent activation of platelets.

Heparin: new life for an old drug

Heparin is one of the oldest drugs, which nevertheless remains in widespread clinical use as an inhibitor of blood coagulation. The history of its identification a century ago unfolded amid one of the most fascinating scientific controversies turning around the distribution of credit for its discovery. The composition, purification and structure-function relationship of this naturally occurring glycosaminoglycan regarding its classical role as anticoagulant will be dealt with before proceeding to discuss its therapeutic potential in, among other, inflammatory and infectious disease, cancer treatment, cystic fibrosis and Alzheimer's disease. The first bibliographic reference hit using the words 'nanomedicine' and 'heparin' is as recent as 2008. Since then, nanomedical applications of heparin have experienced an exponential growth that will be discussed in detail, with particular emphasis on its antimalarial activity. Some of the most intriguing potential applications of heparin nanomedicines will be exposed, such as those contemplating the delivery of drugs to the mosquito stages of malaria parasites.

Rivaroxaban for thromboprophylaxis after total hip or knee arthroplasty: a meta-analysis with trial sequential analysis of randomized controlled trials

Venous thromboembolism (VTE) is the most widespread severe complication after total hip arthroplasty (THA) and total knee arthroplasty (TKA). We conducted this meta-analysis to further validate the benefits and harms of rivaroxaban use for thromboprophylaxis after THA or TKA. We thoroughly searched PubMed, EMBASE, and the Cochrane Central Register of Controlled Trials. Trial sequential analysis (TSA) was applied to test the robustness of our findings and to obtain a more conservative estimation. Of 316 articles screened, nine studies were included. Compared with enoxaparin, rivaroxaban significantly reduced symptomatic VTE (P = 0.0001) and symptomatic deep vein thrombosis (DVT; P = 0.0001) but not symptomatic pulmonary embolism (P = 0.57). Furthermore, rivaroxaban was not associated with an increase in all-cause mortality, clinically relevant non-major bleeding and postoperative wound infection. However, the findings were accompanied by an increase in major bleeding (P = 0.02). The TSA demonstrated that the cumulative z-curve crossed the traditional boundary but not the trial sequential monitoring boundary and did not reach the required information size for major bleeding. Rivaroxaban was more beneficial than enoxaparin for preventing symptomatic DVT but increased the risk of major bleeding. According to the TSA results, more evidence is needed to verify the risk of major bleeding with rivaroxaban.

Oral direct factor Xa inhibitor versus enoxaparin for thromboprophylaxis after hip or knee arthroplasty: Systemic review, traditional meta-analysis, dose-response meta-analysis and network meta-analysis

OBJECTIVES

To analyze the efficacy and safety of direct factor Xa inhibitors for thromboprophylaxis after total hip or knee replacement. To delineate the dose response effect of direct factor Xa inhibitors. To compare the efficacy between any two direct factor Xa inhibitors.

DESIGN

Systemic review, traditional meta-analysis, dose-response meta-analysis and network meta-analysis.

DATA SOURCE

PubMed, EMBASE and Cochrane Library.

STUDY SELECTION

Randomized controlled trials of rivaroxaban, apixaban, betrixaban, darexaban and edoxaban were compared with enoxaparin for thromboprophylaxis after total hip or knee replacement. Two reviewers independently checked the quality of RCTs. Another two investigators independently extracted data. The primary efficacy outcomes (composite of deep venous thrombosis, non-fatal pulmonary embolism and death of all causes) and the primary bleeding outcomes (major bleeding and non-major but clinically relevant bleeding) were summarized for meta-analysis. Stata software was used for traditional meta-analysis and dose-response meta-analysis, and Winbugs software was used for network meta-analysis.

RESULTS

Twenty trials with 38,507 subjects in the intention-to-treat population were included. Compared with enoxaparin, the risk of total venous thromboembolism was lower with rivaroxaban (relative risk 0.70, 95% confidence interval 0.60 to 0.81), apixaban (0.62, 0.47 to 0.81), and edoxaban (0.62, 0.39 to 0.97) and similar to darexaban (0.96, 0.84 to 1.11) and betrixaban (1.28, 0.97 to 1.68). Compared with enoxaparin, the risk of major or clinically relevant non-major bleeding was higher with rivaroxaban (1.52, 1.14 to 2.02), lower with betrixaban (0.34, 0.14 to 0.84) and similar to apixaban (0.88, 0.73 to 1.05), darexaban (0.85, 0.66 to 1.09) or edoxaban (1.30, 0.72 to 2.33). The risk of major and clinically relevant non-major bleeding of rivaroxaban had a linear relationship with its treatment doses; the risk of total venous thromboembolism of betrixaban and darexaban had linear relationships with their respective treatment doses. There was no linear nor non-liner relationships between the effect of apixaban and its treatment dose. The ranking of total venous thromboembolism risk from low to high was: rivaroxaban, apixaban, edoxaban, enoxaparin, darexaban, and betrixaban. The ranking of major and clinically relevant non-major bleeding from low to high was: betrixaban, enoxaparin, darexaban, edoxaban, apixaban, and rivaroxaban.

CONCLUSIONS

Direct oral factor Xa inhibitors are more effective to prevent venous thromboembolism after total hip or knee replacement. Their anticoagulant effect was not necessarily compromised with a higher bleeding risk. Rivaroxaban, apixaban and edoxaban showed a better anticoagulant effect, as compared with enoxaparin. Rivaroxaban had a higher bleeding rate, while apixaban and edoxaban did not show significantly higher bleeding risks.

Use of hGluc/tdTomato pair for sensitive BRET sensing of protease with high solution media tolerance

Due to the complicated media, monitoring proteases in real physiological environments is still a big challenge. Bioluminescence resonance energy transfer (BRET) is one of the promising techniques but its application is limited by the susceptibility to buffer composition, which might cause serious errors for the assay. Herein we report a novel combination of BRET pair with humanized Gaussia luciferase (hGluc) and highly bright red fluorescence protein tdTomato for sensitive and robust protease activity determination. As a result, the hGluc/tdTomato BRET pair showed much better tolerance to buffer composition, pH and sample matrices, and wide spectral separation (Δλ:~110 nm). With the protease sensor built with this pair, the detection limit for enterokinase reached 2.1 pM in pure buffer and 3.3 pM in 3% serum. The proposed pair would find broad use in both in vitro and in vivo assays, especially for samples with complicated matrix.

Sulfated pentagalloylglucoside is a potent, allosteric, and selective inhibitor of factor XIa

Inhibition of factor XIa (FXIa) is a novel paradigm for developing anticoagulants without major bleeding consequences. We present the discovery of sulfated pentagalloylglucoside (6) as a highly selective inhibitor of human FXIa. Biochemical screening of a focused library led to the identification of 6, a sulfated aromatic mimetic of heparin. Inhibitor 6 displayed a potency of 551 nM against FXIa, which was at least 200-fold more selective than other relevant enzymes. It also prevented activation of factor IX and prolonged human plasma and whole blood clotting. Inhibitor 6 reduced V(MAX) of FXIa hydrolysis of chromogenic substrate without affecting the K(M), suggesting an allosteric mechanism. Competitive studies showed that 6 bound in the heparin-binding site of FXIa. No allosteric small molecule has been discovered to date that exhibits equivalent potency against FXIa. Inhibitor 6 is expected to open up a major route to allosteric FXIa anticoagulants with clinical relevance.

Novel direct-acting anticoagulants for risk reduction in ACS

Acute coronary syndrome (ACS) is a devastating adverse cardiovascular event with a massive burden on patient morbility and mortality, as well as the economy. Approximately 1.2 million people are hospitalized annually for ACS in the United States, with direct medical costs estimated at $150 billion in 2009. Rehospitalization is common, often as the result of recurrence of the initial event or complications of ACS or its therapy. Thrombosis is central to the pathogenesis of ACS. The current standard of care includes dual antiplatelet therapy, which reduces platelet activation and aggregation, integral steps for forming a thrombus. However, antiplatelet therapy does not prevent continued thrombin generation or the deposition of fibrin in the clot and residual risk of a recurrent event remains high. New oral anticoagulants offer a mechanism of action that is different from and complementary to that of antiplatelet agents. The ATLAS ACS-TIMI 51 (Anti-Xa Therapy to Lower Cardiovascular Events in Addition to Standard Therapy in Subjects with Acute Coronary Syndrome-Thrombolysis in Myocardial Infarction 51) trial, using rivaroxaban, is the first trial of the new oral anticoagulants to show a benefit when added to antiplatelet therapy in reducing ACS events and mortality. While there was more major bleeding with the addition of rivaroxaban, fatal bleeding was not increased. These agents, if added to the current standard of care, might substantially reduce the high clinical and economic consequences of ACS.

Antithrombotic effect of a novel protein from Fusarium sp. CPCC 480097 in a rat model of artery-vein bypass thrombosis

CONTEXT

The incidence and mortality of thrombotic disorders are rapidly increasing throughout the world. Therefore, attempts have been made to develop new anticoagulant and antithrombotic drugs. Our previous studies showed that a novel protein, named Fu-P, had fibrinogenolytic activity and much higher fibrinolytic activity on the fibrin plate than urokinase in vitro.

OBJECTIVE

The antithrombotic activities of Fu-P in vivo are investigated here for the first time.

MATERIALS AND METHODS

Antithrombotic activity of Fu-P was studied in a rat model of artery-vein bypass thrombosis. The anticoagulant activity of Fu-P was measured by clotting assay of activated partial thrombinplastin time and prothrombin time (PT). The effects of Fu-P on the factor Xa and thrombin were assayed using the chromogenic substrate S-2765 and S-2238.

RESULTS

Intravenous injection of Fu-P produced a 58.4% inhibition ratio of thrombus formation at 0.1 mg/kg body weight, while heparin produced 42.5% inhibition ratio of thrombus formation at 0.6 mg/kg body weight. Fu-P significantly prolonged fibrinogen clotting time, activated partial thrombinplastin time and thrombin time, which also prolonged PT. The inhibition assay of the coagulant factors using chromogenic substrates S-2238 and S-2765 showed that Fu-P was not the inhibitor of the thrombin and Xa.

DISCUSSION AND CONCLUSION

These findings demonstrated that the novel fibrinolytic enzyme (Fu-P) might also be used as a natural agent for thrombolytic therapy or thrombosis prevention.

Antithrombotic therapy in neonates and children: Antithrombotic Therapy and Prevention of Thrombosis, 9th ed: American College of Chest Physicians Evidence-Based Clinical Practice Guidelines

BACKGROUND

Neonates and children differ from adults in physiology, pharmacologic responses to drugs, epidemiology, and long-term consequences of thrombosis. This guideline addresses optimal strategies for the management of thrombosis in neonates and children.

METHODS

The methods of this guideline follow those described in the Methodology for the Development of Antithrombotic Therapy and Prevention of Thrombosis Guidelines: Antithrombotic Therapy and Prevention of Thrombosis, 9th ed: American College of Chest Physicians Evidence-Based Clinical Practice Guidelines.

RESULTS

We suggest that where possible, pediatric hematologists with experience in thromboembolism manage pediatric patients with thromboembolism (Grade 2C). When this is not possible, we suggest a combination of a neonatologist/pediatrician and adult hematologist supported by consultation with an experienced pediatric hematologist (Grade 2C). We suggest that therapeutic unfractionated heparin in children is titrated to achieve a target anti-Xa range of 0.35 to 0.7 units/mL or an activated partial thromboplastin time range that correlates to this anti-Xa range or to a protamine titration range of 0.2 to 0.4 units/mL (Grade 2C). For neonates and children receiving either daily or bid therapeutic low-molecular-weight heparin, we suggest that the drug be monitored to a target range of 0.5 to 1.0 units/mL in a sample taken 4 to 6 h after subcutaneous injection or, alternatively, 0.5 to 0.8 units/mL in a sample taken 2 to 6 h after subcutaneous injection (Grade 2C).

CONCLUSIONS

The evidence supporting most recommendations for antithrombotic therapy in neonates and children remains weak. Studies addressing appropriate drug target ranges and monitoring requirements are urgently required in addition to site- and clinical situation-specific thrombosis management strategies.

Peptide-derivatized albumins that inhibit fibrin polymerization

Synthetic peptides patterned on sequences that appear during thrombin proteolysis of fibrinogen are known to influence fibrin formation in very different ways. A-Knob sequences (GPR-) inhibit polymerization, but B-knob sequences (GHR-) can actually enhance the process. We now report that when such peptides are attached to albumin carriers, both knob conjugates inhibit fibrin formation. In contrast, the 2-aminoethylthiol-albumin conjugate control enhances the polymerization to the same degree as albumin. The peptide AHRPam, which is known to bind exclusively to the βC holes of fibrinogen/fibrin, nullifies the inhibitory effects of the GHRPYGGGCam-albumin conjugate on fibrin polymerization, indicating that the inhibition was exclusively due to interactions with βC holes. AHRPam was much less effective in countering inhibition by the GPRPGGGGCam-albumin conjugate, suggesting that the observed effects with this conjugate involve mainly the γC holes of fibrin/fibrinogen. This study demonstrates that peptides modeled on fibrin polymerization knobs tethered to albumin retain their capacity to interact with fibrinogen/fibrin and may prove useful as inhibitors of clotting in vivo.

Update on anti-coagulation in atrial fibrillation

Atrial fibrillation (AF), the most common clinically relevant arrhythmia, affects 2.2 million individuals in the USA and 4.5 million in Europe, resulting in significant morbidity and mortality. Pharmacotherapy aimed at controlling both heart rate and rhythm is employed to relieve AF symptoms, though debate continues about which approach is preferable. AF prevalence rises with age from 0.4% to 1% in the general population to 11% in those aged >70 years. AF is associated with a pro-thrombotic state and other comorbidities; age, hypertension, heart failure and diabetes mellitus all play a key role in AF pathogenesis. Anti-coagulation is essential for stroke prevention in patients with AF and is recommended for patients with one or more risk factors for stroke. Used within the recommended therapeutic range, warfarin and other vitamin K antagonists decrease the incidence of stroke and mortality in AF patients. Warfarin remains under-used, however, because of the perceived high risk of haemorrhage, narrow therapeutic window and need for regular monitoring. Several novel anti-coagulants show promise in AF-related stroke prevention. In particular, the novel, oral, direct thrombin inhibitor, dabigatran etexilate, recently licensed by the US Food and Drug Administration (FDA) and Health Canada has shown improved efficacy and safety compared with warfarin for stroke prevention in AF, and has the potential to replace warfarin in this indication. The increasing number of new therapeutic options, including improved anti-arrhythmic agents, novel anti-coagulants and more accessible ablation techniques, are likely to deliver better care for AF patients in the near future.

Rigidification of the autolysis loop enhances Na(+) binding to thrombin

Binding of Na(+) to thrombin ensures high activity toward physiological substrates and optimizes the procoagulant and prothrombotic roles of the enzyme in vivo. Under physiological conditions of pH and temperature, the binding affinity of Na(+) is weak due to large heat capacity and enthalpy changes associated with binding, and the K(d)=80 mM ensures only 64% saturation of the site at the concentration of Na(+) in the blood (140 mM). Residues controlling Na(+) binding and activation have been identified. Yet, attempts to improve the interaction of Na(+) with thrombin and possibly increase catalytic activity under physiological conditions have so far been unsuccessful. Here we report how replacement of the flexible autolysis loop of human thrombin with the homologous rigid domain of the murine enzyme results in a drastic (up to 10-fold) increase in Na(+) affinity and a significant improvement in the catalytic activity of the enzyme. Rigidification of the autolysis loop abolishes the heat capacity change associated with Na(+) binding observed in the wild-type and also increases the stability of thrombin. These findings have general relevance to protein engineering studies of clotting proteases and trypsin-like enzymes.

Alternatives to warfarin--the next generation of anticoagulants

Anticoagulation therapy plays an important role in the management of cardiovascular disease. Currently, oral anticoagulation therapy is reliant on vitamin K antagonists (VKA). In clinical practice, VKA present several limitations including a narrow therapeutic window and frequent drug and food interactions. Despite the clear clinical need for alternative anticoagulants it is only within the last decade that significant progress has been made. These new anticoagulants target specific factors in the hemostatic network and appear to overcome some of the difficulties seen with VKA. Many have now progressed to phase III clinical trials including patients with cardiovascular disease. This review aims to highlight the exciting progress that has been made in the development of these new anticoagulants. It will focus on the key agents that have demonstrated the most promise in clinical trials to date. This will include data on cardiovascular indications for anticoagulant therapy. Finally, the review aims to analyze the future prospects for these new agents. Several issues remain to be addressed for these agents to finally replace vitamin K antagonists as the mainstay of anticoagulant therapy.

Mechanism of the anticoagulant activity of thrombin mutant W215A/E217A

The thrombin mutant W215A/E217A (WE) is a potent anticoagulant both in vitro and in vivo. Previous x-ray structural studies have shown that WE assumes a partially collapsed conformation that is similar to the inactive E* form, which explains its drastically reduced activity toward substrate. Whether this collapsed conformation is genuine, rather than the result of crystal packing or the mutation introduced in the critical 215-217 beta-strand, and whether binding of thrombomodulin to exosite I can allosterically shift the E* form to the active E form to restore activity toward protein C are issues of considerable mechanistic importance to improve the design of an anticoagulant thrombin mutant for therapeutic applications. Here we present four crystal structures of WE in the human and murine forms that confirm the collapsed conformation reported previously under different experimental conditions and crystal packing. We also present structures of human and murine WE bound to exosite I with a fragment of the platelet receptor PAR1, which is unable to shift WE to the E form. These structural findings, along with kinetic and calorimetry data, indicate that WE is strongly stabilized in the E* form and explain why binding of ligands to exosite I has only a modest effect on the E*-E equilibrium for this mutant. The E* --> E transition requires the combined binding of thrombomodulin and protein C and restores activity of the mutant WE in the anticoagulant pathway.

Rational design of novel red-shifted BRET pairs: Platforms for real-time single-chain protease biosensors

Bioluminescence resonance energy transfer (BRET) systems to date have been dominated by use of blue-green Renilla luciferase (Rluc) as the energy donor. Although effective in many cases, the expense and unfavorable biochemical attributes of the substrate (phenylcoelenterazine) limit utility of Rluc-based BRET systems. Herein we report a series of novel BRET pairs based on luciferases that utilize D-luciferin, resulting in red-shifted photonic outputs, favorable biochemical attributes, and increased efficacy. We developed a modified Förster equation to predict optimal BRET luciferase donor-fluorophore pairs and identified tdTomato as the optimal red fluorophore acceptor for click beetle green luciferase (CBG). A prototypical single-chain protease biosensor, capable of reporting on executioner caspase activity in live cells and in real-time, was generated by inserting a DEVD linker between CBG and tdTomato and validated in vitro with recombinant caspases and in cellulo with apoptosis-sensitive and -resistant cell lines. High signal-to-noise ratios ( approximately 33) and Z' factors (0.85) were observed in live cell longitudinal studies, sufficient for high-throughput screening. Thus, we illustrate a general methodology for the rational design of new BRET systems and provide a novel single-chain BRET protease biosensor that is long lived, red-shifted, and utilizes D-luciferin.

Chemotherapeutic agents doxorubicin and epirubicin induce a procoagulant phenotype on endothelial cells and blood monocytes

BACKGROUND

Although chemotherapy is associated with an increased risk of thrombosis, the pathogenic mechanisms by which chemotherapeutic agents exert prothrombotic effects are unclear.

OBJECTIVES

In this study we explored the possibility that chemotherapeutic agents doxorubicin, epirubicin, 5-fluorouracil and methotrexate induce a procoagulant phenotype on vascular endothelial cells and/or on blood monocytes.

METHODS

Thrombin generation was measured in defibrinated plasma exposed to chemotherapy-treated human umbilical vein endothelial cells (HUVECs). Tissue factor activity assays were performed on chemotherapy-treated HUVECs and blood monocytes. The effects of chemotherapy drugs on phosphatidylserine exposure and the protein C pathway were also measured.

RESULTS

Exposure of defibrinated plasma to either doxorubicin- or epirubicin-treated HUVECs resulted in an increase in plasma thrombin generation. The procoagulant activity of doxorubicin- and epirubicin-treated HUVECs reflects an increase in tissue factor activity and phosphatidylserine exposure. Doxorubicin-mediated increase in tissue factor activity is related to increased levels of phosphatidylserine rather than to protein disulfide isomerase activity, and is likely to involve reactive oxygen species generation. Unlike doxorubicin, epirubicin does not have an impact on the protein C anticoagulant pathway. Interestingly, neither methotrextate nor 5-fluorouracil altered endothelial or monocyte hemostatic properties.

CONCLUSIONS

These studies suggest that doxorubicin and epirubicin have the greatest 'prothrombotic potential' by virtue of their ability to alter endothelial and monocyte hemostatic properties.

Heparin-induced thrombocytopenia--therapeutic concentrations of danaparoid, unlike fondaparinux and direct thrombin inhibitors, inhibit formation of platelet factor 4-heparin complexes

BACKGROUND

Treatment of heparin-induced thrombocytopenia (HIT), a disorder in which anti-platelet factor 4 (PF4)-heparin antibodies cause platelet activation and hypercoagulability, requires alternative (non-heparin) anticoagulation. Treatment options include direct thrombin inhibitors [lepirudin and argatroban (approved), and bivalirudin], danaparoid (approved) (mixture of anticoagulant glycosaminoglycans), or fondaparinux (synthetic heparin-mimicking pentasaccharide). PF4-heparin complexes form at optimal stoichiometric ratios.

OBJECTIVES

To compare the effects of these various non-heparin anticoagulants in disrupting the formation of PF4-heparin complexes, and PF4-containing immune complexes.

PATIENTS/METHODS

Sera were obtained from patients with serologically confirmed HIT. The effects of the alternative anticoagulants on PF4 and PF4-heparin complex interactions with platelets, as well as HIT antibody binding and platelet activation, were investigated.

RESULTS

Danaparoid at very low concentrations increased PF4 binding to platelets. In therapeutic concentrations, however, it decreased PF4 binding to platelets (P = 0.0004), displaced PF4-heparin complexes from platelets (P = 0.0033) and PF4 from the surface of a PF4-transfected HEK-293 EBNA cell line expressing the PF4 receptor CXCR3-B (P = 0.0408), reduced PF4-heparin complex size (P = 0.025), inhibited HIT antibody binding to PF4-heparin complexes (P = 0.001), and prevented platelet activation by HIT antibodies (P = 0.046). Although fondaparinux also interfered with PF4 binding to platelets, HIT antibody binding to PF4-heparin complexes, and activation of platelets by HIT antibodies, these effects occurred only at supratherapeutic concentrations. The direct thrombin inhibitors had no effect at any concentrations.

CONCLUSIONS

Danaparoid uniquely interferes with the pathogenesis of HIT by disrupting PF4-containing immune complexes at therapeutic dose concentrations. It is possible that these effects contribute to its therapeutic efficacy.

Bemiparin: second-generation, low-molecular-weight heparin for treatment and prophylaxis of venous thromboembolism

Low-molecular-weight heparins (LMWHs) form a heterogeneous group of compounds that exhibit an extended range of pharmacodynamic profiles and, potentially, different anti-thrombotic properties. Bemiparin has the lowest MW (3600 Da), the longest half-life (5.3 h) and the highest anti-FXa/anti-FIIa activity ratio (8:1) of any second-generation LMWH. The safety and efficacy of bemiparin has been demonstrated in several studies and it is currently licensed for treatment and prophylaxis of venous thromboembolism (VTE), as well as for the prevention of clotting in the extracorporeal circuit during hemodialysis. In particular, bemiparin is the only LMWH licensed in Europe for starting thromboprophylaxis after either general or orthopedic surgery. Results from multicenter pharmacoeconomic studies in the Spanish Health Care System indicate that bemiparin is more cost effective than enoxaparin for the prevention of VTE in total knee replacement and may be a safe, cost-saving alternative to unfractionated heparin in the short-term treatment of VTE, and a safe cost-neutral alternative to oral anticoagulant therapy in long-term treatment. In the near future, information from ongoing clinical trials could be key to establishing the potential of bemiparin in different clinical settings.

Update on atrial fibrillation: part I

Atrial fibrillation (AF) is an epidemic, affecting 1% to 1.5% of the population in the developed world. Projected data from the population-based studies suggest that the prevalence of AF will grow at least 3-fold by 2050. The health and economic burden imposed by AF and AF-related morbidity is enormous. Atrial fibrillation has a multiplicity of causes ranging from genetic to degenerative, but hypertension and heart failure are the commonest and epidemiologically most prevalent conditions associated with AF as both have been shown to create an arrhythmogenic substrate. Several theories emerged regarding the mechanism of AF, which can be combined into two groups: the single focus hypothesis and the multiple sources hypothesis. Several lines of evidence point to the relevance of both hypotheses to the mechanism of AF, probably with a different degree of involvement depending on the variety of AF (paroxysmal or persistent). Sustained AF alters electrophysiological and structural properties of the atrial myocardium such that the atria become more susceptible to the initiation and maintenance of the arrhythmia, a process known as atrial remodeling. Angiotensin II has been recognized as a key element in atrial remodeling in association with AF opening the possibility of exploitation of "upstream" therapies to prevent or delay atrial remodeling. The clinical significance of AF lies predominantly in a 5-fold increased risk of stroke. The limitations of warfarin prompted the development of new antithrombotic drugs, which include anticoagulants, such as direct oral thrombin inhibitors (dabigatran) and factor Xa inhibitors (rivaroxaban, apixaban). Novel mechanical approaches for the prevention of cardioembolic stroke have recently been evaluated: percutaneous left atrial appendage occluders, minimally invasive surgical isolation of the left atrial appendage, and implantation of carotid filtering devices.

Thrombin

Thrombin is a Na+-activated, allosteric serine protease that plays opposing functional roles in blood coagulation. Binding of Na+ is the major driving force behind the procoagulant, prothrombotic and signaling functions of the enzyme, but is dispensable for cleavage of the anticoagulant protein C. The anticoagulant function of thrombin is under the allosteric control of the cofactor thrombomodulin. Much has been learned on the mechanism of Na+ binding and recognition of natural substrates by thrombin. Recent structural advances have shed light on the remarkable molecular plasticity of this enzyme and the molecular underpinnings of thrombin allostery mediated by binding to exosite I and the Na+ site. This review summarizes our current understanding of the molecular basis of thrombin function and allosteric regulation. The basic information emerging from recent structural, mutagenesis and kinetic investigation of this important enzyme is that thrombin exists in three forms, E*, E and E:Na+, that interconvert under the influence of ligand binding to distinct domains. The transition between the Na+ -free slow from E and the Na+ -bound fast form E:Na+ involves the structure of the enzyme as a whole, and so does the interconversion between the two Na+ -free forms E* and E. E* is most likely an inactive form of thrombin, unable to interact with Na + and substrate. The complexity of thrombin function and regulation has gained this enzyme pre-eminence as the prototypic allosteric serine protease. Thrombin is now looked upon as a model system for the quantitative analysis of biologically important enzymes.

Monitoring fondaparinux with the Sonoclot

Fondaparinux is a new anticoagulant that interacts with antithrombin III and activated coagulation factor X resulting in an inhibition of the coagulation system. It has been successful in doses of 2.5 mg for thromboprophylaxis as well as in higher therapeutic doses of 5-7.5 mg. No optimal method for monitoring the effects of fondaparinux has been proposed. The aim of the present study was to investigate whether a viscoelastic coagulation analyzer, the Sonoclot (Sienco, Denver, Colorado, USA), could be used for in-vitro monitoring of fondaparinux. Different concentrations of fondaparinux were added in vitro to whole blood taken from eight volunteers. The blood samples mixed with the various amounts of fondaparinux were analyzed using the Sonoclot. The whole-blood activated partial thromboplastin time with the Hemochron Jr (ITC, Edison, New Jersey, USA) was used as the reference coagulation analysis. All analyses were started expeditiously, within 30 s from sampling, and were performed at 37 degrees C. The values of the Sonoclot parameter clot rate, which measures the rate of fibrin formation, fibrin polymerization and platelet-fibrin interactions, were significantly correlated to increasing concentrations of fondaparinux (R = -0.90). The Sonoclot parameters of activated coagulation time, time to peak and clot retraction had weaker, but still significant, correlations to fondaparinux concentrations. At prophylactic doses (0.38 microg/ml blood) the clot rate decreased 15% compared with the initial unanticoagulated value, whereas at therapeutic doses (1.53 microg/ml blood) there was a 27% decrease. In conclusion, the Sonoclot parameter clot rate could be of clinical value to individualize the fondaparinux dosage, especially the higher, therapeutic, dosages.

Tick anti-hemostatics: targets for future vaccines and therapeutics

For ticks, a significant obstacle in obtaining a blood meal is counteracting the hemostatic system of the host. To this end, ticks have developed a broad array of anti-hemostatics, which is reflected in the presence of structurally related tick proteins with different functions. Disruption of blood flow which blocks successful tick feeding makes anti-hemostatics attractive targets for anti-tick vaccines. Moreover, the limited number of drugs currently available for a range of important cardio-vascular diseases makes ticks a potential source of novel therapeutics. This review aims to summarize the key features of tick anti-hemostatics, their structures, mode of action and possible future application as vaccines and novel therapeutic agents.

An economic model of adverse events and costs for oral anticoagulants used for atrial fibrillation

OBJECTIVE

To construct a semi-Markov model to compare health outcomes and medical costs associated with warfarin and a second anticoagulant over 1- and 5-year periods.

DESIGN

We posited a hypothetical cohort of 10,000 identical 70-year-old patients with atrial fibrillation. We posited 20 scenarios for events that included four possibilities for ischemic strokes (mild, moderate, severe, death) and 16 possibilities for hemorrhages. The model allowed for four levels of International Normalized Ratio. Event rates were based on outcomes in clinical trials and observational studies. Costs were estimated from the perspective of the third-party payer.

RESULTS

The greatest cost-generating events were virtually the same for the two drugs and included severe stroke ($1,758,548 for 1 year for both drugs), moderate stroke ($380,355 for 1 year for both drugs), and severe lower gastrointestinal (GI) hemorrhage ($193,804 for 1 year for warfarin and $193,474 for second drug). The least costly events for both drugs were mild intracranial or intracerebral hemorrhage ($7584 for warfarin and $4314 for second drug) and fatal upper GI hemorrhage ($16,781 and $16,752). Total costs for adverse events over 5 years were similar: $18,330,662 for warfarin and $17,102,847 for the second drug. Fatalities for 5 years were 123 for warfarin and 101 for the non-warfarin drug. Varying assumptions for nursing home care and numbers of ischemic strokes and hemorrhages generated the widest variation in costs.

LIMITATIONS

We did not account for out-of-pocket expenses, 'pain and suffering' costs, or variation across practice settings.

CONCLUSIONS

There was substantial variation in numbers and costs of adverse events across 20 scenarios, and for fatalities between the two drugs, but variation in costs between the two drugs was modest.

Variegin, a novel fast and tight binding thrombin inhibitor from the tropical bont tick

Tick saliva contains potent antihemostatic molecules that help ticks obtain their enormous blood meal during prolonged feeding. We isolated thrombin inhibitors present in the salivary gland extract from partially fed female Amblyomma variegatum, the tropical bont tick, and characterized the most potent, variegin, one of the smallest (32 residues) thrombin inhibitors found in nature. Full-length variegin and two truncated variants were chemically synthesized. Despite its small size and flexible structure, variegin binds thrombin with strong affinity (K(i) approximately 10.4 pM) and high specificity. Results using the truncated variants indicated that the seven residues at the N terminus affected the binding kinetics; when removed, the binding characteristics changed from fast to slow. Further, the thrombin active site binding moiety of variegin is in the region of residues 8-14, and the exosite-I binding moiety is within residues 15-32. Our results show that variegin is structurally and functionally similar to the rationally designed thrombin inhibitor, hirulog. However, compared with hirulog, variegin is a more potent inhibitor, and its inhibitory activity is largely retained after cleavage by thrombin.

Factor Xa or thrombin: is thrombin a better target?

The limitations of the vitamin K antagonists have prompted the development of new oral anticoagulants that target specific clotting enzymes. Most of the novel agents currently under development target either thrombin or factor Xa. As the final effector of blood coagulation and the most potent platelet agonist, thrombin is a logical target for new oral anticoagulants. Clinical trials with parenteral direct thrombin inhibitors revealed that the therapeutic window is wider with reversible inhibitors than with irreversible inhibitors. The results of clinical trials with ximelagatran, an orally active prodrug of melagatran, a reversible direct thrombin inhibitor, validate thrombin as a target. Although ximelagatran was withdrawn from the market because of hepatotoxicity, newer oral thrombin inhibitors, such as dabigatran etexilate, are filling the void. Several oral factor Xa inhibitors also are being tested. Is thrombin a better target for new oral anticoagulants than factor Xa? Only time will tell!

Thrombin as procoagulant and anticoagulant

Thrombin is a Na(+)-activated, allosteric serine protease that plays opposing functional roles in blood coagulation. Binding of Na(+) is the major driving force behind the procoagulant, prothrombotic and signaling functions of the enzyme, but is dispensable for cleavage of the anticoagulant protein C. This basic regulatory feature of thrombin has fostered the rational engineering of mutants with selectively compromised fibrinogen and PAR1 cleavage. The discovery of the Na(+) effect on thrombin interaction with substrates and the mapping of functional epitopes by Ala scanning mutagenesis have provided a rational and effective strategy for dissociating the procoagulant and anticoagulant activities of the enzyme. Thrombin mutants with selectively compromised activity toward fibrinogen and PAR1 are effective in vivo as anticoagulant and antithrombotic agents.

Serpins in thrombosis, hemostasis and fibrinolysis

Hemostasis and fibrinolysis, the biological processes that maintain proper blood flow, are the consequence of a complex series of cascading enzymatic reactions. Serine proteases involved in these processes are regulated by feedback loops, local cofactor molecules, and serine protease inhibitors (serpins). The delicate balance between proteolytic and inhibitory reactions in hemostasis and fibrinolysis, described by the coagulation, protein C and fibrinolytic pathways, can be disrupted, resulting in the pathological conditions of thrombosis or abnormal bleeding. Medicine capitalizes on the importance of serpins, using therapeutics to manipulate the serpin-protease reactions for the treatment and prevention of thrombosis and hemorrhage. Therefore, investigation of serpins, their cofactors, and their structure-function relationships is imperative for the development of state-of-the-art pharmaceuticals for the selective fine-tuning of hemostasis and fibrinolysis. This review describes key serpins important in the regulation of these pathways: antithrombin, heparin cofactor II, protein Z-dependent protease inhibitor, alpha(1)-protease inhibitor, protein C inhibitor, alpha(2)-antiplasmin and plasminogen activator inhibitor-1. We focus on the biological function, the important structural elements, their known non-hemostatic roles, the pathologies related to deficiencies or dysfunction, and the therapeutic roles of specific serpins.

Latest medical treatment strategies for venous thromboembolism

Anticoagulant therapy with unfractionated heparin (UFH) followed by warfarin prevents thrombus extension, reduces the risk of recurrent thrombosis and prevents death in patients with venous thromboembolism (VTE). Low-molecular-weight heparin (LMWH) has replaced UFH as the preferred initial anticoagulant therapy for VTE because it is as effective and safe as UFH, but does not require laboratory monitoring and is less likely to cause immune thrombocytopenia and osteoporosis. More recently, fondaparinux has been shown to be an effective and safe alternative to LMWH and several new parenteral anticoagulants are being evaluated. The most important unmet need in the anticoagulant management of VTE is a replacement for warfarin. New oral anticoagulants that selectively target individual steps in the coagulation cascade have been shown to be effective for the long-term treatment of VTE in Phase II and III trials and are likely to become available in the near future.

Antithrombotic and anticoagulant effects of the direct thrombin inhibitor dabigatran, and its oral prodrug, dabigatran etexilate, in a rabbit model of venous thrombosis

BACKGROUND

Oral anticoagulant therapies targeted at thrombin are being developed to overcome limitations associated with current standard therapies.

OBJECTIVES

This study was undertaken to assess and compare the antithrombotic and anticoagulant effects of the novel, selective and reversible, direct thrombin inhibitor (DTI), dabigatran, and its oral prodrug dabigatran etexilate, to that of unfractionated heparin (UFH), hirudin and melagatran using a rabbit model of venous thrombosis.

METHODS

A rabbit model of venous thrombosis consisting of endothelial damage with blood flow reduction was used with minor modifications.

RESULTS

All compounds demonstrated a dose-dependent reduction in thrombus formation following i.v. administration with complete or almost complete inhibition at the highest doses. Dabigatran (in the dose range 0.03-0.5 mg kg(-1)) had a 50% effective dose of 0.066 mg kg(-1). By comparison, UFH (5-50 U kg(-1)), hirudin (0.01-0.05 mg kg(-1)) and melagatran (0.01-0.3 mg kg(-1)) had a 50% effective dose of 9.8 U kg(-1), 0.016 mg kg(-1) and 0.058 mg kg(-1), respectively. Similarly, oral dabigatran etexilate (1-20 mg kg(-1)) inhibited thrombus formation in a dose-dependent manner. Maximum inhibition was achieved within 1 h of administration, suggesting a rapid onset of action. For both routes of administration, inhibition of thrombus formation directly correlated with prolongation of the activated partial thromboplastin time.

CONCLUSIONS

These findings demonstrate the potent anticoagulant and antithrombotic activity of dabigatran as a selective thrombin inhibitor in a rabbit model of venous thrombosis. Notably, dose-dependent and long-lasting antithrombotic efficacy was observed after application of its oral form dabigatran etexilate, which is currently undergoing phase III clinical development.

Effect of the anti-factor Xa and anti-factor IIa activities of low-molecular-weight heparins upon the phases of thrombin generation

BACKGROUND

Low-molecular-weight heparins (LMWHs), derived from unfractionated heparin (UFH) by different depolymerization procedures, vary in both their relative abilities to enhance the inhibition of FXa (anti-FXa) and thrombin (anti-FIIa), and in their physicochemical properties.

OBJECTIVE

We aimed to profile the inhibition of thrombin generation induced by bemiparin, enoxaparin, nadroparin, dalteparin and tinzaparin in platelet-rich plasma (PRP), and to compare them with UFH and fondaparinux (a synthetic pentasaccharide that specifically enhances FXa inhibition).

METHODS

Different LMWHs, UFH or fondaparinux were added to normal PRP. Thereafter, tissue factor-triggered thrombin generation was assessed using the Thrombogram-Thrombinoscope assay.

RESULTS

At equivalent anti-FIIa activity concentrations, LMWHs and UFH exhibited similar inhibitory effects upon thrombin generation. However, when used at equivalent anti-FXa activity concentrations, tinzaparin was significantly more active than the other LMWHs at inhibiting thrombin generation, and had similar activity to that of UFH. Enoxaparin, nadroparin and dalteparin all showed similar inhibitory activities. In these experiments, bemiparin exhibited the lowest inhibitory effect on thrombin generation of all the LMWHs. At 0.1 microg mL(-1) (0.093 anti-FXa IU mL(-1)), fondaparinux inhibited the rate of thrombin generation by 50%. A 7-fold higher concentration of fondaparinux was required to inhibit the endogenous thrombin potential by 50%.

CONCLUSIONS

LMWHs have a variable inhibitory effect on thrombin generation in vitro when compared by anti-FXa activity, but are similar when compared by their anti-FIIa activities. The rate of thrombin generation during the propagation phase, rather than the endogenous thrombin potential, is more sensitive to the anticoagulant activity of fondaparinux and the polysaccharide chains of LMWHs possessing only anti-FXa activity.

Oral, direct factor Xa inhibitors in development for the prevention and treatment of thromboembolic diseases

Anticoagulants are recommended for the prevention and treatment of a wide variety of thromboembolic events. Although existing anticoagulants are effective, their use is limited by parenteral administration or the requirement for frequent monitoring and subsequent dose adjustment. Therefore, there is an urgent need for novel, oral agents with a predictable anticoagulant action. Because of its key position in the coagulation cascade and its limited roles outside of coagulation, Factor Xa has emerged as an attractive target for novel anticoagulants. As a result, the past decade has witnessed an explosion of research into small-molecule, oral, direct Factor Xa inhibitors, and several are now in clinical development. Rivaroxaban, LY517717, YM150, apixaban, PRT054021, and DU-176b, among others, have shown considerable promise; rivaroxaban is currently furthest ahead in its developmental program, having entered phase III in 3 indications. It is hoped that, before long, these anticoagulants will allow us to enter an era of convenient, oral anticoagulation, without the need for regular monitoring or dose adjustment.

Stroke in atrial fibrillation: update on pathophysiology, new antithrombotic therapies, and evolution of procedures and devices

Atrial fibrillation (AF) is said to be an epidemic, affecting 1%-1.5% of the population in the developed world. The clinical significance of AF lies predominantly in a 5-fold increased risk of stroke. Strokes associated with AF are usually more severe and confer increased risk of morbidity, mortality, and poor functional outcome. Despite the advent of promising experimental therapies for selected patients with acute stroke, pharmacological primary prevention remains the best approach to reducing the burden of stroke. New antithrombotic drugs include both parenteral agents (e.g. a long-acting factor Xa inhibitor idraparinux) and oral anticoagulants, such as oral factor Xa inhibitors and direct oral thrombin inhibitors (ximelagatran, dabigatran). Ximelagatran had shown significant potential as a possible replacement to warfarin therapy, but has been withdrawn because of potential liver toxicity. Its congener dabigatran appears to have a better safety profile and has recently entered a phase III randomized clinical trial in AF. Oral factor Xa inhibitors (rivaroxaban, apixaban, YM150) inhibit factor Xa directly, without antithrombin III mediation, and may prove to be more potent and safe. Selective inhibitors of specific coagulation factors involved in the initiation and propagation of the coagulation cascade (factor IXa, factor VIIa, circulating tissue factor) are at an early stage of development. Additional new agents with hypothetical, although not yet proven, anticoagulation benefits include nematode anticoagulant peptide (NAPc2), protein C derivatives, and soluble thrombomodulin. A battery of novel mechanical approaches for the prevention of cardioembolic stroke has recently been evaluated, including various models of percutaneous left atrial appendage occluders which block the connection between the left atrium and the left atrial appendage, minimally invasive surgical isolation of the left atrial appendage, and implantation of the carotid filtering devices which divert large emboli from the internal to the external carotid artery, preventing the embolic material from reaching intracranial circulation. Despite recent advances and promising new approaches, prevention of recurrent AF may be one of the best protections against AF-related stroke and may reduce the prevalence of stroke by almost 25%. Improved pharmacological and nonpharmacological rhythm control strategies for AF as well as primary prevention of AF with 'upstream' therapy and risk factor modification are likely to produce a larger effect on the reduction of stroke rates in the general population than will specific interventions.