Pentasaccharide Fondaparinux Research Articles

Atomic Level Description of the Immune Complex That Causes Heparin-Induced Thrombocytopenia (HIT)

Heparin-induced thrombocytopenia (HIT) is thrombotic disorder caused by immune complexes containing antibodies to an antigen composed of platelet factor 4 (PF4) and heparin or cellular glycosaminoglycans (GAGs). The structure of these immune complexes and how their composition might contribute to the difference between pathogenic and non-pathogenic anti-PF4 antibodies are unknown. To address these questions, we solved the crystal structures of human recombinant PF4 in complex with Fabs derived from KKO (a murine monoclonal HIT-like antibody that competes with pathogenic human HIT antibodies) and RTO (an isotype-matched non-HIT anti-PF4 antibody) combined with the crystal structure of PF4 complexed with the heparin-mimic pentasaccharide fondaparinux as a model sugar. The PF4 tetramer is asymmetric and is capable of accommodating only two fondaparinux molecules. Fondaparinux binds between monomers A, B and C or between monomers A, C, and D, which stabilizes the AB/CD and AC/BD associations and the resultant tetramer. KKO-Fab binds to the PF4 tetramer by making contacts with now identified residues within each of three PF4 monomers, indicating that tetramerization of PF4 is a critical initiating step in antigen formation. Mutations in the putative KKO epitopes in PF4 abolished antibody binding.Unexpectedly, RTO-Fab binds to the PF4 monomer between the AB dimer interface. Importantly, the amino acid sequence recognized by RTO and KKO show considerable overlap. However, the epitope for RTO is obscured upon tetramer formation, in direct contrast to binding of KKO, which requires tetramer formation to bind. Binding of RTO to the PF4 monomer prevents formation of AB dimers and subsequent tetramerization. In support of these findings, preincubation of PF4 with RTO inhibits KKO induced platelet activation and platelet aggregation in vitro. Based on the analyses of crystal lattices, we propose a new model of the heparin/PF4 complex, in which PF4 tetramers cluster around a semi-rigid linear heparin subunit. Clustering of PF4 on heparin might be required for apposition of sufficient HIT antibodies to induce persistent activation of cellular FcgIIA receptors. Heparin and pathogenic HIT antibodies collaborate to stabilize the ternary immune complex, which leads to the disappearance of binding sites for at least some non-pathogenic HIT antibodies. The balance between anti-monomer and anti-tetramer PF4 antibodies may help determine the probability of clinical disease. This model also helps to explain why RTO-like anti-PF4 antibodies are found so commonly in asymptomatic patients exposed to heparin and why fondaparinux may be antigenic but rarely causes HIT, whereas longer heparin fragments and GAGs extend and render the holo-complex more stable and thereby foster the formation of pathogenic immune complexes. In summary, these crystallographic studies lead to a new model to explain the formation of pathogenic immune complexes that lead to HIT. The inhibitory effect of the anti-PF4 antibody RTO provides a structural basis for the development of new diagnostics and non-anticoagulant therapeutics. DisclosuresNo relevant conflicts of interest to declare.

Relieving Inhibition of Prothrombinase By TFPIα: a Procoagulant Activity of Unfractionated, Low Molecular Weight, and Nonanticoagulant Heparins

Introduction: Prothrombinase, the complex of factor Xa (FXa) and factor Va (FVa), is inhibited by tissue factor pathway inhibitor (TFPI)α during the initiation of coagulation (Wood JP et al, PNAS 2013). Efficient inhibition of thrombin generation by prothrombinase requires an interaction between the TFPIα basic C-terminus and an acidic region of the FVa B-domain. This acidic region is present in FXa-activated FVa and FVa released from activated platelets, but is rapidly removed by thrombin. Thus, prothrombinase inhibition only occurs during the initiation phase of thrombin generation. As the exosite interaction is charge-dependent, large negatively charged molecules, including unfractionated heparin (UFH), block it, prevent prothrombinase inhibition, and promote thrombin generation. Studies using the negatively charged molecule polyphosphate have suggested a size requirement for blocking this TFPIα activity (Smith SA et al, Blood2010). A similar size-dependence may exist with heparins and could have clinical implications, as currently-used heparins range from long (unfractionated heparin; UFH) to medium (low molecular weight heparins; LMWHs) to short (the antithrombin-binding pentasaccharide fondaparinux). Studies were performed to assess the ability of the LMWHs enoxaparin and dalteparin, fondaparinux, and the nonanticoagulant heparin 2-O, 3-O desulfated heparin (ODSH) to block TFPIα and promote thrombin generation through this mechanism.Methods: TFPIα inhibition of thrombin generation by prothrombinase, assembled with a form of FVa containing the acidic region of the B domain, was measured in the absence or presence of UFH, enoxaparin, dalteparin, fondaparinux, and ODSH. The effect of these compounds on the direct inhibition of FXa by TFPIα was measured using a FXa chromogenic substrate. The effect of these compounds on thrombin generation in plasma was measured by calibrated automated thrombography using human plasma immunodepleted of antithrombin III and heparin cofactor II (AT3/HCII-depleted plasma).Results: TFPIα inhibited prothrombinase activity (IC50 = 6.8 nM), and UFH blocked this inhibition (IC50 = 12.5 nM or 14.9 nM at 0.5 or 1 U/mL, respectively). Enoxaparin (0.8 U/mL; IC50 = 30.3 nM) and dalteparin (1 U/mL; IC50 = 29.7 nM) appeared to be more effective at reversing TFPIα inhibition. The reason for this apparent enhanced effect of LMWHs compared to UFH is not clear, as UFH and the LMWHs similarly enhanced the direct inhibition of FXa by TFPIα, and the differential activity was also observed when heparins were normalized to saccharide concentration. The same pattern was observed when measuring thrombin generation in AT3/HCII-depleted plasma, with LMWHs being more procoagulant than UFH. Consistent with TFPIα inhibition being charge-dependent, ODSH promoted thrombin generation similarly to LMWHs in both purified systems and AT3/HCII-depleted plasma. In contrast, clinical doses of fondaparinux had no effect in any assay. In a purified system, ~1000 times the clinical dose of fondaparinux was required to promote thrombin generation.Conclusion: There is a size-dependence for blocking TFPIα inhibition of prothrombinase using heparins, as the pentasaccharide has no effect. However, both LMWHs and UFH are sufficiently long to express this procoagulant activity at therapeutic doses. In addition, the nonanticoagulant heparin ODSH blocks prothrombinase inhibition by TFPIα. This procoagulant activity is likely most clinically relevant under conditions of antithrombin deficiency, which may result from sepsis, liver failure, or administration of L-asparaginase. Under any of these conditions, UFH, LMWHs, and ODSH may have unanticipated procoagulant activity mediated by blocking TFPIα. DisclosuresCamire:Pfizer: Consultancy, Patents & Royalties, Research Funding. Mast:Novo Nordisk: Research Funding.

Binding of anti–platelet factor 4/heparin antibodies depends on the thermodynamics of conformational changes in platelet factor 4

AbstractThe chemokine platelet factor 4 (PF4) undergoes conformational changes when complexing with polyanions. This can induce the antibody-mediated adverse drug effect of heparin-induced thrombocytopenia (HIT). Understanding why the endogenous protein PF4 becomes immunogenic when complexing with heparin is important for the development of other negatively charged drugs and may also hint toward more general mechanisms underlying the induction of autoantibodies to other proteins. By circular dichroism spectroscopy, atomic force microscopy, and isothermal titration calorimetry we characterized the interaction of PF4 with unfractionated heparin (UFH), its 16-, 8-, and 6-mer subfractions, low-molecular-weight heparin (LMWH), and the pentasaccharide fondaparinux. To bind anti-PF4/heparin antibodies, PF4/heparin complexes require (1) an increase in PF4 antiparallel β-sheets exceeding ∼30% (achieved by UFH, LMWH, 16-, 8-, 6-mer), (2) formation of multimolecular complexes (UFH, 16-, 8-mer), and (3) energy (needed for a conformational change), which is released by binding of ≥11-mer heparins to PF4, but not by smaller heparins. These findings may help to synthesize safer heparins. Beyond PF4 and HIT, the methods applied in the current study may be relevant to unravel mechanisms making other endogenous proteins more vulnerable to undergo conformational changes with little energy requirement (eg, point mutations and post-translational modifications) and thereby predisposing them to become immunogenic.

Antiplatelet therapy and anticoagulation in acute coronary syndrome: current evidence

In recent years, the prognosis of patients with an acute coronary syndrome (ACS) has significantly improved. This can mainly be attributed to the implementation of primary percutaneous coronary intervention (PCI). Apart from mechanical reperfusion, an optimal medical strategy is of great importance. Antiplatelet and antithrombotic therapies in particular play a crucial role in the management of patients with ACS. New options in antiplatelet therapy are more potent P2Y12 inhibitors in addition to acetylsalicylic acid and clopidogrel. Furthermore, anticoagulant therapy before, during and after PCI can be performed by the use of unfractionated heparin, low molecular weight heparins, such as enoxaparin, the synthetic pentasaccharide fondaparinux and the direct thrombin inhibitor bivalirudin with or without additional administration of glycoprotein IIb/IIIa inhibitors. In this article, data on antiplatelet and anticoagulant therapy are presented and the current evidence is discussed.

Total synthesis of anticoagulant pentasaccharide fondaparinux.

The anticoagulant pentasaccharide fondaparinux was synthesized using an improved and optimized synthetic strategy including a convergent [3+2] coupling approach, orthogonal protecting groups and various glycosyl donors. The new methods of glycosylation were also used for controlling the stereochemical configuration and improving the yield of the glycosylation. In addition, HPLC and NMR methods to monitor the process of total synthesis of fondaparinux were employed. This work provides a comprehensive elaboration for the synthesis and analysis of fondaparinux based on related literature, as well as abundant information for the synthesis of heparin-like oligosaccharides.

Testing of potential glycan-based heparanase inhibitors in a fluorescence activity assay using either bacterial heparinase II or human heparanase

Heparanase, an endo-β-glucuronidase cleaving heparan sulfate (HS) chains at cell surfaces and in the extracellular matrix (ECM), is involved in angiogenesis, tumor progression and metastasis as well as in inflammation and kidney dysfunction. Therefore, heparanase is considered a promising therapeutic target and diagnostic marker. Recently, we have developed a simple, rapid, fully automatable fluorimetric activity assay using the synthetic sulfated pentasaccharide fondaparinux as substrate and bacterial heparinase II (HEP-II) instead of human heparanase (hHEP). The aim of this study was to evaluate this assay for inhibitor testing as well as to check whether the assay principle is applicable to measure the activity and inhibition, respectively, of the actual target enzyme hHEP.Besides the known hHEP inhibitor suramin and the antiinflammatory and antimetastatic PS3, two series of β-1,3-glucan sulfates differing in their chain length and degree of sulfation, further semisynthetic sulfated glycans, and two sulfated polysaccharides isolated from algae were included to examine structure–activity relationships.The inhibitory activity of sulfated glycans showed to be greatly dependent on both their degree of sulfation and their basic glycan structure, but independent of their molecular size. The β-1,3-glucan sulfates were superior to suramin as well as to the other glycans with similar degree of sulfations. The most active inhibitor was found to be the β-1,3-glucan sulfate PS3 (IC50=0.017μM). By using hHEP instead of HEP-II comparable results were obtained. With an IC50 being about 160 times lower than that of suramin, PS3 exhibited again the strongest inhibitory effects. Inhibition of hHEP may therefore contribute to the potent antiinflammatory and antimetastatic activities of PS3 in vivo. In conclusion, the fluorimetric hHEP activity assay proved to be a simple, fully automatable tool for testing potential inhibitors. In case of HS mimetic inhibitors, the assay variant with HEP-II may provide a fast and inexpensive option for initial screening purposes.

Anticoagulation during and after acute coronary syndrome

Current antithrombotic therapy in patients with acute coronary syndrome (ACS) comprises antiplatelet and anticoagulant therapy. Dual antiplatelet therapy composed of aspirin plus a third generation P2Y12 inhibitor (prasugrel or ticagrelor) represents the gold standard, while aspirin plus second generation P2Y12 inhibitor (clopidogrel) may be used as an alternative in the presence of contraindications for third generation P2Y12 inhibitors and/or a high risk of bleeding. Unfractionated heparin (UFH) has been the unchallenged mainstay in anticoagulation for ACS for many decades and is still widely used in patients with ACS treated interventionally. Novel alternative parenteral anticoagulant strategies include the low molecular weight heparin enoxaparin and the synthetic pentasaccharide fondaparinux. Both of these agents share advantages over UFH particularly in medically treated patients with ACS not scheduled for PCI. The direct parenteral factor IIa (thrombin) inhibitor bivalirudin, when used as sole anticoagulant in patients with ACS undergoing PCI, is as effective as the regimen of UFH plus GPIIb/IIIa inhibitor in NSTEMI and superior to the latter regimen in patients with STEMI. The novel approach of a long-term low dose factor Xa inhibition with rivaroxaban in the post ACS phase even further reduced cardiovascular mortality in a clinical trial but has yet to be established in daily clinical practice in the setting of third generation P2Y12 inhibitors. This review discusses currently clinically established anticoagulants for the treatment of ACS alongside with novel approaches such as rivaroxaban.

Identification of the binding site for fondaparinux on Beta2-glycoprotein I

Antiphospholipid syndrome (APS) is an autoimmune disease with clinical manifestations of thrombosis and pregnancy complications. Beta2-glycoprotein I (β2GPI) is the major antigen for the APS-related antibodies. Heparin, low-molecular weight heparin and the synthetic pentasaccharide fondaparinux are commonly used for prophylaxis and treatment of thrombosis in patients with antiphospholipid syndrome. These antithrombotic drugs bind and activate antithrombin III to inactivate blood clotting proteases. Heparin and heparin derivatives might have a direct beneficial effect in APS via binding to β2GPI and interfering with prothrombotic properties of β2GPI/antibody complexes. We compared fondaparinux to heparin regarding its ability to bind β2GPI and inhibit the binding of β2GPI/antibody complexes to negatively charged phospholipids and endothelial cells. Although heparin and fondaparinux bind β2GPI at therapeutically relevant doses, neither fondaparinux nor heparin was efficient in inhibition of the binding of β2GPI/antibody complexes to negatively charged phospholipids and endothelial cells. Our studies suggest that these drugs do not act on pathological properties of β2GPI/antibody complexes, emphasizing the need for a new treatment specific for β2GPI-related thrombosis in APS. We observed that the binding interface of fondaparinux on β2GPI does not include the lysine residues known to be critical for binding of heparin. The docking model of the β2GPI complex with fondaparinux is in agreement with multiple experimental observations.

Cutaneous reactions to heparin therapy: when are they caused by heparin allergy?

Introduction: Little is known about the incidence and causes of heparin-induced skin lesions. The most commonly reported causes are delayed-type hypersensitivity reactions. We describe 3 patients who were referred to our staff between March and October 2009 for suspected heparin allergies. All were scheduled to undergo major surgery (cardiovascular or orthopedic). Materials and methods: All 3 patients reported the development of itchy, erythematous rashes a few days after the subcutaneous administration of heparin (nadroparin calcium in cases 1 and 2, unspecified in case 3). Each of them underwent a diagnostic work-up for heparin allergy, which included prick and intradermal tests with commonly used heparins and patch testing with undiluted heparins and disinfectants. Results: Patch tests with disinfectants were negative in all 3 cases. In case 2, all allergological tests were negative. In cases 1 and 3, delayed positivity emerged for nadroparin calcium and at least one other heparin tested. Intravenous and/or subcutaneous provocation testing was done with an alternative heparin which produced negative results in skin tests (heparin sodium in case 1, pentasaccharide fondaparinux in case 3). In both cases the alternative drug was tolerated. After our evaluation, all 3 patients underwent surgery with no heparin-related complications. Discussion: The presenting clinical features in these 3 cases provided no information on which reactions were likely to be allergic: all 3 patients presented with similar local delayed reaction. The allergic reactions were identified only after cutaneous testing.

Synthesis of Fondaparinux: modular synthesis investigation for heparin synthesis

The anti-thromboembolic pentasaccharide Fondaparinux was synthesized in 36 steps for the longest linear route, with 0.017% overall yield from d-glucose. Only three kinds of protecting groups were used for hydroxyl protection, Bn, Ac, and Bz, to accomplish this complex synthesis without decreasing the synthetic efficiency. Three l-idopyranosyl donors were investigated. Thioethyl glycoside is an efficient donor for l-idopyranosyl glycosylation with full α-selectivity, while l-idopyranosyl trichloroacetimidate resulted in poor α/β selectivity. A practical synthesis of key intermediate 1,6-anhydro-l-idopyranose 17 by H+/β-CD catalyst was developed.

Cyr61 is a target for heparin in reducing MV3 melanoma cell adhesion and migration via the integrin VLA-4

The integrin VLA-4 is important for the metastatic dissemination of melanoma cells. We could recently show that heparin can block VLA-4 binding, which contributes, next to blocking P- and L-selectin, to the understanding of antimetastatic activities of heparin. The matricellular ligand Cyr61, secreted by numerous tumours, is responsible for increased tumourigenicity and metastasis. This has been attributed to Cyr61 binding to, and thus activating integrins. However, a VLA-4/Cyr61 axis has not yet been reported. Since Cyr61 possesses heparin binding capabilities, Cyr61 can be supposed as potential target for heparin to indirectly interfere with integrin functions. The present in vitro studies address (i) the existence of a Cyr61/VLA-4 axis and (ii) the functional relevance of heparin interference via Cyr61. The C-terminal module III of Cyr61 could be exposed as nanomolar affine binding site for VLA-4. A shRNA-based knockdown of Cyr61 in MV3 human melanoma cells reduced VLA-4-mediated cell binding to VCAM-1, migration on fibronectin, and integrin signalling functions significantly. Using a biosensor approach we provide insight into heparin interference with this process. The low-molecular-weight heparin tinzaparin, but not the pentasaccharide fondaparinux, binds module IV of Cyr61 with micromolar affinity. But tinzaparin cannot interfere with Cyr61 accumulation onto syndecan-4, indicating different Cyr61 binding sites for heparin and other GAGs. Nonetheless, tinzaparin affects the VLA-4 binding and signalling functions selectively via Cyr61 already at very low concentration most likely by blocking the cellular secreted free Cyr61. This study emphasises Cyr61 as promising, and hitherto not considered target for heparin to selectively influence integrin functions.

Parenteral anticoagulants in heart disease: Current status and perspectives (Section II)

Anticoagulants are a mainstay of cardiovascular therapy, and parenteral anticoagulants have widespread use in cardiology, especially in acute situations. Parenteral anticoagulants include unfractionated heparin, low-molecular-weight heparins, the synthetic pentasaccharides fondaparinux, idraparinux and idrabiotaparinux, and parenteral direct thrombin inhibitors. The several shortcomings of unfractionated heparin and of low-molecular-weight heparins have prompted the development of the other newer agents. Here we review the mechanisms of action, pharmacological properties and side effects of parenteral anticoagulants used in the management of coronary heart disease treated with or without percutaneous coronary interventions, cardioversion for atrial fibrillation, and prosthetic heart valves and valve repair. Using an evidence-based approach, we describe the results of completed clinical trials, highlight ongoing research with currently available agents, and recommend therapeutic options for specific heart diseases.

Oberflächliche Venenthrombose

This review summarizes the epidemiology, clinical aspects, diagnosis and new therapeutic options of the superficial venous thrombosis. An important new finding is, that the thrombophlebitis is actually a thrombosis, which occurs in an intra- or epifascial vein. It can be associated with deep venous thrombosis or pulmonary embolism, thus carrying considerable risk. Other underlying diseases such as underlying tumors or infections as well as acquired or inherited disorders of coagulation must be considered. New placebo-controlled studies show a clear benefit for low molecular weight heparin or the pentasaccharide fondaparinux. In addition to the medical treatment, compression is recommended for superficial venous thrombosis in varicose veins, but not in normal veins.

Antithrombin-Independent Effects of Heparins on Fibrin Clot Nanostructure

Because of the widespread clinical use of heparins, their effects on the enzymatic cascade are very well known. In contrast, little is known about the direct effect of heparins on the nanostructure of fibrin fibers, even though this nanostructure plays a major role in the mechanical strength and lysis of clots. This lack of reliable data can be correlated with the lack of a nonintrusive, quantitative method to determine this structure. We recently developed such a method that allows the simultaneous determination of the average fiber radius and the protein content using spectrometric data. In this study, we assessed the nanostructure of fibrin in a system composed of human thrombin and fibrinogen. We provide quantitative evidence showing that both unfractionated heparin and low molecular weight heparin directly alter the nanostructure of fibrin fibers independent of their other actions on the coagulation cascade; as expected, the pentasaccharide fondaparinux has no effect. Our results show that in addition to the effect of heparin on the coagulation cascade, modifications of the fibrin nanostructure may also contribute to improved fibrinolysis.

Fondaparinux-Associated Heparin-Induced Thrombocytopenia

Heparin-induced thrombocytopenia (HIT) is an immune-mediated clinical syndrome of thrombocytopenia and thrombosis occurring after exposure to heparin. HIT most commonly occurs after exposure to unfractionated heparin but has also been reported with low-molecular-weight heparin. To date, there have been very few reports of HIT with the pentasaccharide fondaparinux, and some have proposed fondaparinux as a treatment for HIT. This article presents two cases of HIT associated with fondaparinux use.

Application, tolerance and safety of fondaparinux therapy in a German hospital: A prospective single-centre experience

Introduction The pentasaccharide fondaparinux is widely approved for prophylaxis and treatment of thromboembolic diseases and therapy of acute coronary syndrome. It is also used off-label in patients with acute, suspected or antecedent heparin-induced thrombocytopenia (HIT). The aim of this prospective observational cohort study was to document fondaparinux’ prescription practice, tolerance and therapy safety in a representative mixed German single-centre patient cohort. Patients and Methods Between 09/2008 – 04/2009, 231 consecutive patients treated with fondaparinux were enrolled. Medical data were obtained from patient's records. The patients were clinically screened for thrombosis (Wells score), sequelae of HIT (4T's score), and bleeding complications (ISTH-criteria) and subjected to further assessment (i.e. sonography, HIT-diagnostics), if necessary. The mortality rate was assessed 30 days after therapy start. Results Overall, 153/231 patients had a prophylactic, 74/231 patients a therapeutic, and 4/231 patients a successive prophylactic/therapeutic indication. In 11/231 patients fondaparinux was used due to suspected/antecedent HIT, in 5/231 patients due to a previous cutaneous delayed-type hypersensitivity to heparins. Other indications were rare. Three new/progressive thromboses were detected. No cases of HIT, major bleedings, or fatalities occurred. Conclusions Fondaparinux was well tolerated and was safe in prophylaxis and therapy; prescriptions mostly followed the current approval guidelines and were rarely related to HIT-associated indications ( < 5% of prescriptions), which is in contrast to previous study results in the U.S. ( > 94% of prescriptions were HIT-associated). A trend towards an individualised fondaparinux use based on the compound's inherent properties and the patients’ risk profiles, i.e., antecedent HIT, bone fractures, heparin allergy, was observed.

The Future of Anticoagulant Therapy

Major advances in the development of oral anticoagulants haveresulted in considerable progress towards the goal of safe and ef-fective oral anticoagulants that do not require frequent monitor-ing or dose adjustment, and have minimal food/drug interactionsas highlighted recently in several reports [1–7] and summarized inrecent reviews in Cardiovascular Therapeutics and other journals[1–4]. Indirect inhibitors of factor Xa and factor IIa such as low-molecular weight heparin (LMWH) and the pentasaccharide fon-daparinux represent improvements over traditional drugs, such asunfractionated heparin, for acute treatment of VTE, constituting amore targeted anticoagulant approach, with predictable pharma-cokinetic profiles, and no requirement for monitoring [6,7]. Vita-min K antagonist, with its inherent limitations in terms of multi-ple food and drug interactions and frequent need for monitoring,remains the only oral anticoagulant approved for long-term sec-ondary Thromboprophylaxis in VTE. The oral direct thrombin in-hibitor ximelagatran was withdrawn from the world market dueto safety concerns. Newer anticoagulant drugs such as oral directthrombin inhibitors (dabigatran), oral direct factor Xa inhibitors(rivaroxaban, apixaban, and several others) are being introducedinto the European and Canadian markets and have the potentialto greatly expand oral antithrombotics for both acute and long-term treatment of VTE, and for the prevention of stroke in atrialfibrillation patients [1–9]. See Figure 1 for historical progressionin anticoagulation and Figure 2 for the structure of the first oraldirect anti-Xa and the first direct anti-IIa.The current anti-thrombotic (anti-coagulant) strategies are ex-pected to change the landscape of thrombosis management dra-matically. A number of these new oral anticoagulant therapies areset to enter the market, raising a number of questions: How willit evolve? What are the future scenarios? Will it be cost effective?With so many drugs in development, will it turn into another ‘me-too’ market? Or, will one drug dominate? Can heparin, LMWHswarfarin be replaced?In a comprehensive RECORD clinical trial program involvingover 12,500 patients, rivaroxaban, a direct factor Xa inhibitor,taken as a once-daily tablet, demonstrated improved efficacy of upto 50% over the existing standard of care: the injectable, LMWHenoxaparin [8,9]. Rivaroxaban (Figure 2) is the only oral antico-agulant to have demonstrated superior efficacy over enoxaparin,the mainstay of current Thromboprophylaxis [8,9].Results of the RECORD 1 and 3 trials, which compared ri- The futureof all neworal factorXa or IIainhibitorswill dependon therelativeefficacy,safety, andcost.varoxaban with enoxaparin, demonstrated that prophylactic treat-ment with rivaroxaban was associated with a 70% relative riskreduction (RRR) in the development of venous blood clots inhip replacement patients (2.6% absolute risk reduction) and 49%RRR in knee replacement patients (9.2% absolute risk reduction),whilst demonstrating comparable bleeding and side effect profiles[2,3]. With its efficacy advantages and convenient oral administra-tion, which requires no coagulation monitoring, rivaroxaban hasthe potential to have a significant impact on clinical practice [3–7].In March 2009, the FDA Advisory panel recommended theapproval of rivaroxaban (Xarelto, Johnson & Johnson, NewBrunswick, NJ, USA), which would be the first new oral antico-agulant since the approval of warfarin in 1954 for the prophylaxisof deep vein thrombosis (DVT) and pulmonary embolism (PE) inpatients undergoing hip and knee replacement surgery. The rec-ommendation is based on data from the four phase 3 RECORD tri-als, which together compared the factor Xa inhibitor rivaroxabanat 10 mg per day, with the subcutaneously administered LMWHenoxaparin for prevention of venous thromboembolism after hipor knee surgery in more than 12,500 patients [6–9].In October 2010, the FDA approved the first new oral di-rect anti-IIa anticoagulant in 55 years, dabigatran (marketedby Boehringer Ingelheim Pharmaceuticals, Ingelheim, Germany,under the trade name Pradaxa) for stroke prevention in pa-tients with nonvalvular atrial fibrillation. As demonstrated inthe 18,113 patients RE-LY trial, Pradaxa 150 mg taken twicedaily significantly reduced stroke and systemic embolism by35% beyond the reduction achieved with warfarin, the cur-rent standard of care for patients with nonvalvular atrial fib-rillation. Pradaxa 150 mg twice daily significantly reducedboth ischemic and hemorrhagic strokes compared with war-farin. Treatment with Pradaxa does not require blood monitor-ing or related dose adjustments and has no recommended di-etary restrictions. The FDA also approved Pradaxa 75 mg twicedaily for the small sub-set of patients who have severe renalimpairment.

Preventing Postsurgical Venous Thromboembolism: Pharmacological Approaches

The use of antithrombotic drugs for the prevention of venous thromboembolism (VTE) in patients undergoing surgery is presently based on solid principles and high-level scientific evidence. This article reviews current strategies of pharmacological thromboprophylaxis. The level of VTE risk following surgery depends on a variety of factors that the surgeon should take into account, including the type of surgery and the presence of additional risk factors, such as elderly age and cancer. In patients undergoing minor general surgery, early mobilization is sufficient as prophylaxis, whereas in those undergoing major general surgery, thromboprophylaxis with low molecular weight heparin (LMWH), low-dose unfractionated heparin, or the pentasaccharide fondaparinux is recommended. Patients undergoing major orthopedic surgery have a particularly high risk of VTE, and routine thromboprophylaxis with LMWH, fondaparinux, or a vitamin K antagonist (international normalized ratio target: 2.0 to 3.0) is the standard of care in this group of patients. Recently, two new oral anticoagulants, rivaroxaban (a factor Xa inhibitor) and dabigatran etexilate (a direct thrombin inhibitor) have been licensed to be used for thromboprophylaxis after orthopedic surgery in Europe. Mechanical methods of thromboprophylaxis (compression stockings, intermittent pneumatic compression, vena cava filters), not discussed in detail in this review, should always be considered in patients at high thrombotic risk, in association with the pharmacological strategies, or in cases of contraindications to anticoagulants, as in patients or procedures at high risk of bleeding.

Evidence based venous thromboprophylaxis in patients undergoing total hip replacement (THR), total knee replacement (TKR) and hip fracture surgery (HFS)

Patients undergoing total hip, knee replacement and hip fracture surgery are at high risk of developing venous thromboembolism. Incidence of deep vein thrombosis ranges from 40% to 70% and 1% to 7% for fatal pulmonary embolism. Venous thromboprophylaxis comprises mechanical and pharmacological intervention but for added protection in major orthopaedic surgery, most national and international guidelines advise a combined regimen of both modalities. Reportedly, 40% of such patients do not receive pharmacological prophylaxis because of the increased risk of bleeding. When considering pharmacological prophylaxis, a trade off between the benefits of reducing venous thromboembolism and the potential harms of bleeding and haematoma formation must be balanced. Anti-embolism stockings, intermittent pneumatic pump and foot impulse devices are the main mechanical methods but are often collectively addressed as mechanical prophylaxis as no difference in efficacy exists between these devices. The National Institute for Health and Clinical Excellence advises that one of those devices be commenced preoperatively and continued until the patients have no significant reduced immobility. Anti-embolism stockings (knee or thigh) are to be applied with caution. Mechanical prophylaxis is particularly important in hip/knee and hip fracture surgery, when patients are not protected by pharmacological prophylaxis. Unfractionated heparin, low molecular weight heparin, pentasaccharide fondaparinux and old and new oral anticoagulants are the mainstay of pharmacological prophylaxis. In the absence of contraindication, it is recommended that pharmacological prophylaxis is timely initiated postoperatively. Extended pharmacological prophylaxis is recommended for 28–35 days for hip replacement and hip fracture surgery and 10–14 days for knee replacement surgery.

Rational Design of Anticoagulant Drugs Using Oligosaccharide Chemistry

For a long time, heparin and low molecular weight heparins have been the drugs of choice for the management of thrombosis. Discovery of the antithrombin binding domain in heparin, a critical element in the anticoagulant activity of this polysaccharide, allowed a rational approach based on medicinal carbohydrate chemistry in the design of new anticoagulants. The fully synthetic pentasaccharide fondaparinux that selectively targets blood coagulation factor Xa was first to be developed as a drug. Fondaparinux was followed by various heparin mimicking oligosaccharides prepared with a view to replace polydisperse heparin and low molecular weight heparins by structurally-defined anticoagulants with no unwanted side-effects.