Selective Factor Xa Inhibitor Research Articles

Role of current and emerging antithrombotics in thrombosis and cancer

In the nearly 130 years since Trousseau first described migratory thrombophlebitis in cancer patients, thromboembolism has become a well-established presenting sign and complication of cancer. The coagulation system is activated in cancer and is further amplified by treatment with chemotherapy, radiation or surgery. Hypercoagulation is documented in virtually all cancer types, albeit at different rates, and is the second leading cause of death in cancer patients. The relationship between clotting activation and carcinogenesis supports the view of cancer as a hypercoagulable state and holds implications for the development of thrombosis, enhancement of tumor growth and risk of poor clinical outcomes. Although it is well recognized that cancer can activate the coagulation cascade, it is less well known that activation of the coagulation system may also support tumor progression. Additionally, platelet activation in cancer patients and its impact on tumor progression and metastasis further expand the role of the hemostatic system in malignancy. The problem of thrombosis in patients with metastatic diseases is a serious concern for clinicians. This review explores the mechanisms and clinical implications of coagulation and platelet activation in cancer. The prevention and treatment of venous thromboembolism in cancer will also be discussed by reviewing data from key clinical investigations. Finally, the emerging role of low-molecular-weight heparin as an antineoplastic agent will be explored. Warfarin and unfractionated heparin have been in clinical use for more than 50 years. Both are effective anticoagulants, but their use is associated with a number of impediments, including the need for intensive coagulation monitoring, wide variation in dose-response relationships, multiple drug interactions (in the case of warfarin), and serious immune-mediated thrombocytopenia (in the case of heparin). The introduction of low-molecular weight heparin advanced anticoagulation therapy by enhancing efficacy and eliminating the need for intensive coagulation monitoring. Fondaparinux, the first selective factor Xa inhibitor, represents yet another improvement in anticoagulation therapy. By binding rapidly and strongly to antithrombin, its sole physiologic target in plasma, fondaparinux catalyzes specifically the inhibition of factor Xa, which results in effective and linear dose-dependent inhibition of thrombin generation. Additionally, efficient inhibition of factor Xa activity impairs the activation of tissue factor/factor VIIa complex leading to downregulation of procoagulant state, pro-angiogenesis, and proinflammatory factors induced by tissue factor/factor VIIa. Furthermore, a number of orally active direct antithrombin and anti-factor Xa are in advanced clinical development for various thromboembolic disorders.

Initial treatment of venous thromboembolism

Immediate anticoagulant treatment is essential to reduce morbidity and mortality in patients with acute venous thromboembolism (VTE). Currently, rapid anticoagulation can only be achieved with parenteral anticoagulants, such as heparin or low-molecular-weight heparin (LMWH). Weight-adjusted LMWH is the treatment of choice, because it produces predictable anticoagulation and does not require coagulation monitoring. If heparin is used, the activated partial thromboplastin time must be monitored and the heparin dose adjusted to ensure a therapeutic level of anticoagulation. Heparin is recommended for patients with renal impairment and for those at high risk of bleeding. The selective factor Xa inhibitor fondaparinux is a recently introduced alternative to heparin or LMWH for initial VTE treatment. Heparin, LMWH, or fondaparinux should be given for at least five to seven days. Vitamin K antagonists should be initiated on the first day, or as soon as possible, in patients who are candidates for an oral anticoagulant. An oral anticoagulant agent to be used without laboratory monitoring for both acute and long-term treatment of VTE remains an unsolved clinical need in the treatment of VTE.

Inhibition of Factors Xa and Iia Results in Synergistic Inhibition of Thrombin Generation with Minimal Clot Time Elevation.

Inhibition of either coagulation factor Xa (FXa) or thrombin (FIIa) alters ex vivo biomarkers of coagulation and decreases thrombus size in animal models of experimental thrombosis. The objective of this study was to determine if combining FXa and FIIa inhibitors would synergistically reduce the magnitude of IIa generated without elevating markers of bleeding. A synergistic combination may translate into lower doses, more effective anticoagulation and better safety. To predict the FXa and FIIa inhibitor combinations with the maximum potential for synergy, PD 0313052, a potent, selective FXa inhibitor, and argatroban, a potent FIIa inhibitor, were each tested independently and in combination using an in vitro thrombin generation assay. Individually, PD 0313052 and argatroban reduced total thrombin generation (TG) in a concentration dependent manner with IC50's of 497±148 and 882±193 nM, respectively. Subsequently, PD 0313052 and argatroban were combined in 96 well plates at concentrations ranging from 0.125x to 8x their respective IC50 concentrations. Analysis using the Bliss Independence Model identified statistically significant synergistic activity, with the greatest increase (33%) over simple additivity at 249 and 441 nM FXa to FIIa, respectively, both below their respective IC50 concentrations. Furthermore, combinations of PD 0313052 and argatroban were evaluated in an assay measuring activated clotting time (ACT). Although both PD 0313052 and argatroban dose-dependently elongated the ACT, the combination of 0.5x the TG IC50 concentrations, which demonstrated the greatest synergy in the TG assay, showed the smallest increase in the ACT, prolonging clotting time by only 15%. These data demonstrate that the combination of a specific factor Xa inhibitor and a specific IIa inhibitor can synergistically reduce thrombin generation without appreciable elevation of ACT, suggesting that dual inhibition of FXa and FIIa, using relatively low doses of each, may provide efficacious and safe treatment for thromboembolic diseases.

Fondaparinux Combined with Intermittent Pneumatic Compression (IPC) Versus IPC Alone in the Prevention of Venous Thromboembolism after Major Abdominal Surgery: The Randomized APOLLO Study.

Background: The selective factor Xa inhibitor fondaparinux has been shown to be at least as effective and as safe as the low-molecular-weight heparin dalteparin for venous thromboembolism (VTE) prevention after major abdominal surgery (Agnelli GA, et al. Br J Surg, 2005, In press). The benefit of fondaparinux in addition to intermittent pneumatic compression (IPC) in VTE prevention after abdominal surgery has not been evaluated.Objective: We performed a randomized, double-blind, placebo-controlled, superiority trial to compare the efficacy and safety of fondaparinux in conjunction with IPC versus IPC alone in patients undergoing major abdominal surgery.Methods: Patients aged at least 40 years undergoing abdominal surgery of at least 45 minutes were included. Patients at highest risk of VTE, requiring pharmacologic prophylaxis in addition to IPC were excluded at the investigator's discretion. Patients were randomized to receive either fondaparinux 2.5 mg or placebo once daily subcutaneously for 5 to 9 days, starting 6 to 8 hours postoperatively. All patients received IPC. The primary efficacy outcome was the composite of deep-vein thrombosis detected by mandatory bilateral venography, or documented symptomatic deep-vein thrombosis or pulmonary embolism up to day 10. The main safety outcome was major bleeding during the treatment period. A blinded independent committee adjudicated all these outcomes. Follow-up lasted 32 days.Results: Of the 1309 patients randomized between November 2001 and October 2004 (fondaparinux+IPC, n=650; placebo+IPC, n=659), 842 (64.3%) were evaluable for efficacy. The treatment groups were comparable with regard to VTE risk factors, demographic and surgical characteristics; 82% had at least one VTE risk factor (over and above being at least 40 years old and undergoing abdominal surgery). Fondaparinux significantly reduced the VTE rate from 5.3% (22/418) with placebo to 1.7% (7/424), an odds ratio reduction of 69.8% (95% CI: 27.9 to 87.3; p=0.004). Similarly, fondaparinux significantly reduced the proximal deep-vein thrombosis rate from 1.7% (7/417) to 0.2% (1/423; p=0.037). Major bleeding occurred in 1.6% (10/635) and 0.2% (1/650) of fondaparinux- and placebo-treated patients, respectively (p=0.006). None of the bleeding events were fatal or involved a critical organ.Conclusions: Fondaparinux combined with IPC was significantly more effective than IPC alone for VTE prevention after major abdominal surgery. Although the bleeding risk was increased with fondaparinux compared with placebo, this risk was low and consistent with that observed in previous fondaparinux studies in surgical patients.

Clinical Efficacy and Safety of Fondaparinux in the Prevention and Treatment of Venous and Arterial Thrombosis

Fondaparinux (Arixtra) is a synthetic, selective Factor Xa inhibitor. Its pharmacokinetic profile allows once-daily subcutaneous administration of the drug without any laboratory monitoring. The benefit-to-risk ratio of fondaparinux in the prevention of venous thromboembolism has been extensively studied in both surgical and acutely ill medical patients at risk of thrombosis. Its efficacy and safety have also been investigated in the initial treatment of symptomatic deep-vein thrombosis and pulmonary embolism. Finally, a number of Phase II trials investigated the safety and efficacy of fondaparinux in patients with acute coronary syndromes, and a large Phase III program is ongoing in this setting. This review focuses on the use of fondaparinux in the prevention and treatment of thromboembolic disorders.

Monitoring anticoagulation by fondaparinux: determination of anti factor Xa-level

In contrast to low molecular weight heparin and unfractionated heparin fondaparinux is a selective synthetic factor Xa inhibitor. It was approved for thromboprophylaxis after major orthopaedic surgery three years ago. Because of its excellent bioavailability after subcutaneous application once daily independent of body weight (within the range of 50-100 kg) monitoring of the substance is usually not necessary. Since its recent approval for therapeutic use in acute venous thrombosis monitoring might be useful in special clinical situations (e. g. suspected over- or underdosing, renal insufficiency or severe bleeding complications). Based on standard laboratory methods for the chromogenic determination of the anti-factor Xa activity of low molecular weight heparin several chromogenic substrate assays were published for monitoring. Only fondaparinux should be used for calibration and measured fondaparinux levels should be expressed in mug/ml of the fondaparinux calibrator. Because of its single synthetic chemical entity, a linear dose/response relationship over a wide concentration range is observed and measurements are of high precision.

The safety of fondaparinux for the prevention and treatment of venous thromboembolism

Fondaparinux is the first synthetic selective Factor Xa inhibitor. Along with its antithrombotic efficacy, the safety of fondaparinux has been documented in several Phase II and III clinical trials, including the prevention of venous thromboembolism in patients undergoing major orthopaedic surgery or high-risk abdominal surgery, or in acutely ill medical patients with restricted mobility, and the treatment of patients with deep-vein thrombosis and pulmonary embolism. In all these indications, the safety of fondaparinux used according to its registered regimen was similar to that of reference comparators. In conclusion, due to its superior efficacy and satisfactory safety, fondaparinux may substantially improve the prevention and treatment of venous thrombosis.

Effect of the novel direct factor Xa inhibitor DX-9065a on thrombin generation and inhibition among patients with stable atherosclerotic coronary artery disease

Introduction Thrombin, a pluripotential effector enzyme with prothrombotic, proinflammatory, and mitogenic properties, plays a pivotal role in the pathobiology and clinical expression of atherothrombotic coronary artery disease. Existing anticoagulant drugs have not been shown to attenuate thrombin generation or activity consistently. We sought to investigate the effect of DX-9065a on thrombin generation and inhibition in patients with stable CAD. DX-9065a is a small-molecule, synthetic, direct inhibitor of factor Xa. Materials and methods Peripheral venous blood samples were collected serially during and after administration of either placebo or 1 of 4 weight-adjusted regimens of DX-9065a, in 73 patients with stable CAD participating in the XaNADU-1B study. Results and conclusions At baseline, the median (25th, 75th) prothrombin activation fragment 1.2 (F1.2) level was 2.56 (2.05, 3.20) nmol/L, and the median d-dimer level was 0.26 (0.19, 0.38) μg FEU/L. There were significant relationships between measured plasma DX-9065a concentrations and both F1.2 (4.9% decrease for each doubling of DX-9065a) ( P < 0.0001) and d-dimer (5.5% decrease for each doubling of DX-9065a) ( P = 0.001). F1.2 was suppressed (below baseline) at 96 h after administration of DX-9065a. Coronary thrombotic events did not occur during or after study drug administration. DX-9065a, the first in a class of small-molecule, direct, selective and reversible factor Xa inhibitors, reduces thrombin generation and fibrin formation among patients with stable CAD. The effect is concentration-dependent and persists for at least 96 h following drug cessation, without biochemical or clinical evidence of rebound.

Synthesis and Structure−Activity Relationships of Novel Selective Factor Xa Inhibitors with a Tetrahydroisoquinoline Ring

A series of novel 2,7-disubstituted tetrahydroisoquinoline derivatives were designed and synthesized. Among these derivatives, compounds 1 and 2 exhibited potent inhibitory activity against factor Xa (FXa) and good selectivity with respect to other serine proteases (thrombin, plasmin, and trypsin). In addition, compound 2 exhibited potent anti-FXa activity after intravenous and oral administration to cynomolgus monkeys, showed a dose-dependent antithrombotic effect at 0.1, 0.3, and 1 mg kg(-1) h(-1) in a rat model of venous thrombosis, and significantly reduced the size of brain infarction in a middle cerebral artery occlusion model at a dose of 0.1 mg kg(-1) h(-1). These results suggest that compound 2 (JTV-803) is likely to be useful as both a venous and arterial antithrombotic agent.

Discovery of Novel Tetrahydroisoquinoline Derivatives as Potent and Selective Factor Xa Inhibitors.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Prophylaxis, Diagnosis and Therapy of Surgery-Related Complications in Orthopedic and Trauma Surgery

Selective factor Xa inhibition by the synthetic pentasaccharide fondaparinux is an efficient way of preventing venous thrombosis. Fondaparinux has been shown to reduce the incidence of venous thromboembolism (VTE) by 50% compared with the low-molecular-weight heparin enoxaparin. The aim of the CHANGE study was to evaluate the management of thrombosis prevention with fondaparinux, and the diagnosis and treatment of complications following major orthopedic surgery. Nationwide observational study (“CHANGE”) in 111 centers in Germany. Patients underwent elective knee or hip replacement surgery or surgery on account of fractures of the hip or lower limb. In total, 8,331 patients were documented, of which 7,787 were suitable for analysis. The mean age was 68.5 years; for 50.8% of the patients additional risk factors for VTE were reported. The most frequent main diagnoses by ICD10 were coxarthrosis (41.8%), gonarthrosis (32.8%), and fracture of femur (16.7%). 52.6% of the patients underwent hip surgery, 35.3% knee surgery, and 11.5% surgery for fractures. The mean hospital stay lasted 18 days, with only small differences between the different types of surgery. The overall complication rate was 1.68% (n = 131, 95% confidence interval [CI]: 1.42/1.99) with 0.47% VTE (n = 37, 95% CI: 0.34/0.65, 31 deep vein thromboses, five pulmonary embolisms, one not specified) and 1.2% bleedings (n = 94, 95% CI: 0.99/1.47). Most of the bleedings occurred in the surgical field. The bleeding rate was highest in the knee surgery group, and also VTE occurred more frequently following knee surgery. During the whole study, no heparin-induced thrombocytopenia was observed. The observational study CHANGE confirms the results of the published preauthorization studies with fondaparinux, with a complication rate nearly half that previously reported. Using fondaparinux for the prevention of VTE leads to improved quality of care in orthopedic patients.

First experience with direct, selective factor Xa inhibition in patients with non-ST-elevation acute coronary syndromes: results of the XaNADU-ACS Trial

Unfractionated heparin is widely used in patients with non-ST-elevation acute coronary syndromes but has important limitations. Anticoagulants with predictable kinetics and anticoagulant effects, better efficacy, and greater safety are needed. To investigate the efficacy and safety of a direct, selective factor Xa inhibitor, DX-9065a (Daiichi Pharmaceuticals LTD, Inc.) compared with heparin, in patients with non-ST-elevation acute coronary syndromes. Patients (n = 402) from the USA, Canada, and Japan were randomized to blinded, weight-adjusted heparin, low-dose DX-9065a, or high-dose DX-9065a. The primary efficacy endpoint of death, myocardial infarction, urgent revascularization, or ischemia on continuous ST-segment monitoring occurred in 33.6%, 34.3%, and 31.3% of patients assigned to heparin, low-dose DX-9065a, and high-dose DX-9065a (P = 0.91 for heparin vs. combined DX-9065a). The composite of death, myocardial infarction, or urgent revascularization occurred in 19.5%, 19.3%, and 11.9% (P = 0.125 for heparin vs. high-dose DX-9065a) of patients; major or minor bleeding occurred in 7.7%, 4.2%, and 7.0% of patients; and major bleeding in 3.3%, 0.8%, and 0.9% of patients. Higher concentrations of DX-9065a were associated with a lower likelihood of ischemic events (P = 0.03) and a non-significant tendency toward a higher likelihood of major bleeding (P = 0.32). In this small phase II trial, there was a non-significant tendency toward a reduction in ischemic events and bleeding with DX-9065a compared with heparin in patients with acute coronary syndromes. The absence of an effect on ST-monitor ischemia warrants further investigation. These data provide the rationale for adequately powered studies of DX-9065a in acute coronary syndromes or percutaneous intervention.

Rationale for the use of antifactor Xa in the treatment and prevention of venous and arterial thromboembolic events

The design and synthesis of new antithrombotic agents have led to numerous recent clinical trials to investigate their efficacy and safety. Analysis of the data from these trials, especially when the results were not totally expected, has provided interesting information, allowing a better understanding of the pathophysiology of thrombosis in various clinical situations. The aim of the present manuscript is to present the hypotheses on thrombogenesis generated by some of these recent clinical trials, notably those investigating the antithrombotic efficacy of a new synthetic and selective factor Xa inhibitor, fondaparinux. The antithrombotic efficacy of fondaparinux was recently investigated in a number of trials in the prevention and treatment of venous and arterial thrombotic disorders. In each case, the concept of the clinical efficacy of a selective factor Xa inhibitor has been proven. These trials have also clarified the implication and mode of action of factor Xa in these various types of thrombotic events. In the light of these trials, we discuss the clinical efficacy of such a selective factor Xa inhibitor, and other specific points such as the apparent lack of dose-dependency of its antithrombotic effect.

From heparins to factor Xa inhibitors and beyond

Despite some disadvantages, unfractionated heparin (UFH) and oral anticoagulants have been the only anticoagulants for prophylaxis and therapy of thromboembolic disorders for several decades. Based on the increasing knowledge of the structure and pharmacology of heparin, low molecular weight heparins (LMWH) have been developed in the 1980s. Compared to UFH, their advantages are mainly based on their reduced nonspecific binding to proteins and cells resulting in improved pharmacokinetics. In 1991, LMWH were declared as the most efficient prophylaxis in high-risk patients. Although the use of LMWH is increasing and they are today also applied for therapy and in other indications like acute coronary syndrome, they are considered not optimal concerning efficacy and safety. With the approval of fondaparinux for the prevention of venous thromboembolic disease in high-risk orthopedic patients, there might be a paradigm shift in the field of anticoagulants. Fondaparinux, a synthetic, chemically defined pentasaccharide, is the first selective inhibitor of factor Xa. By its highly specific binding to antithrombin, it selectively inhibits factor Xa and consequently prevents thrombin generation. In contrast to UFH and LMWH, it does not bind to any other cells and other proteins than antithrombin. This leads to a favourable linear pharmacokinetic profile, allowing once-daily subcutaneous application of a fixed dose without monitoring in thromboembolism prophylaxis. In addition to the evaluation of fondaparinux for further indications, chemical modifications of this pentasaccharide such as the long-acting idraparinux are currently under investigation.

Fondaparinux: an update on new study results

Fondaparinux (Arixtra) is the first selective factor Xa inhibitor approved for use in thromboprophylaxis after orthopaedic surgery. New recently completed trials have also demonstrated the potential of fondaparinux in the prevention of venous thromboembolism (VTE) in other surgical and medical settings and in the treatment of established VTE. In the randomized double-blind PEGASUS study in high-risk abdominal surgery patients, fondaparinux reduced the incidence of VTE from 6.1% with dalteparin to 4.6% (odds ratio reduction = 25.8%, P = 0.14), without increasing the bleeding risk. In the randomized double-blind ARTEMIS trial in acutely ill medical patients, fondaparinux reduced the incidence of VTE from 10.5% with placebo to 5.6% (odds ratio reduction = 49.5%, P = 0.029), without increasing the bleeding risk; there was no pulmonary embolism in the fondaparinux group compared with five, all fatal, in the placebo group (P = 0.029). In the two MATISSE trials, both the efficacy and safety of once daily fondaparinux were at least as good as enoxaparin in the treatment of deep-vein thrombosis (MATISSE-DVT) and unfractionated heparin in the treatment of pulmonary embolism (MATISSE-PE). In patients with coronary artery disease, promising results were obtained in phase II trials and large phase III trials are ongoing. In conclusion, fondaparinux may further improve and simplify the prevention and treatment of thrombosis in a large range of medical and surgical settings.

Venous thromboembolism prophylaxis guidelines: Use by primary care physicians

Venous thromboembolism (VTE), a prevalent, costly medical condition, is one of the most common causes of death in the United States. Although risk factors for VTE are well known, thromboembolic events cannot be predicted because patients are asymptomatic and screening methods have limitations. Anticoagulant therapy (eg, low-molecular-weight heparin, unfractionated heparin, selective factor Xa inhibitors) has proved effective for preventing thromboembolism, including deep vein thrombosis and pulmonary embolism. While quality care for VTE entails prophylaxis for all relevant patients, many high-risk patients are undertreated or treated incorrectly. Both primary and secondary prevention of VTE remain inadequate for several reasons, including lack of awareness of the American College of Chest Physicians guidelines, of the seriousness of VTE, of the benefits of prophylaxis, and of the relatively low risk of bleeding complications. To provide appropriate treatment, physicians must assess the numbers and types of risk factors for each patient, the underlying illness or surgical procedure, and the benefits and risks of possible therapies. The problem of VTE will grow as the US population ages, as surgery is performed on increasingly sick patients, and as the length of hospital stays continues to decrease.

Überwachung der Antikoagulanzientherapie mit Fondaparinux

ZusammenfassungIm Gegensatz zu niedermolekularem und unfraktioniertem Heparin besitzt Fondaparinux, ein synthetisch hergestelltes Pentasaccharid, einen selektiv hemmenden Effekt auf den Gerinnungsfaktor X. Die Substanz ist seit drei Jahren zur Hochrisikothromboseprophylaxe nach großen orthopädischen Operationen zugelassen. Auf Grund der ausgezeichneten Bioverfügbarkeit nach subkutaner Gabe, die einmal pro Tag in körpergewichtsunabhängiger Dosierung (bei einem Körpergewicht von 50-100 kg) erfolgen kann, ist normalerweise ein Monitoring nicht erforderlich. Insbesondere seit der europaweiten Zulassung zur Therapie venöser Thrombosen ist jedoch im Ausnahmefall (z. B. Verdacht auf Fehldosierung, bei Niereninsuffizienz oder Blutungen) ein Monitoring im Routinelabor sinnvoll.Inzwischen wurden chromogene Substratmethoden, basierend auf zur Verfügung stehenden Monitoring-Methoden zur Messung des Faktor Xa inhibierenden Effektes der niedermolekularen Heparine, publiziert. Unter Berücksichtigung dieser Publikationen sollte Fondaparinux als Kalibrator verwendet und die Einheiten in μg/ml angegeben werden. Wegen der synthetischen Herstellung der Substanz findet sich eine lineare Dosis/Wirkungsbeziehung in einem weiten Konzentrationsbereich, so dass eine sehr gute Präzision der Methodik an Vollautomaten erreicht werden kann.

Inhibition of Factor Xa

Anticoagulant therapy plays an important role in the prevention and treatment of pathologic arterial and venous thrombosis. There is increasing enthusiasm in the inhibition of Factor Xa as a target to achieve therapeutic anticoagulation because of its central and 'upstream' position in the coagulation process. The indirect, selective, parenteral Factor Xa inhibitor fondaparinux sodium (synthetic pentasaccharide) has been studied extensively in the prevention and treatment of venous thromboembolism. In an overview of four studies in patients undergoing major orthopedic surgery, fondaparinux sodium was associated with a 55% reduction in recurrent thromboembolism, albeit with a modest increase in bleeding. Preliminary results from phase II studies of fondaparinux sodium in patients with ST-elevation and non-ST-elevation acute coronary syndromes have been promising and have led to the initiation of two large phase III trials, which are currently underway. Idraparinux sodium, a long-acting synthetic pentasaccharide, is currently being investigated as a once-weekly alternative to other long-term anticoagulants. DX-9065a and razaxaban are two of many direct selective Factor Xa inhibitors currently in development. DX-9065a has been studied in phase II trials in patients undergoing percutaneous coronary intervention and in those with non-ST-elevation acute coronary syndromes. Razaxaban has been studied in a phase II trial in patients who have undergone orthopedic surgery. Data from these trials, although preliminary and based on small numbers of patients, suggest that direct selective Factor Xa inhibition may provide effective anticoagulation, perhaps without excessive bleeding. Inhibition of Factor Xa is a promising target for the prevention and treatment of thrombosis in both the venous and arterial circulation. Ongoing investigation with numerous oral and parenteral inhibitors of Factor Xa will establish the potential of Factor Xa as a target for therapeutic anticoagulation.

Effect of fondaparinux on platelet activation in the presence of heparin-dependent antibodies: a blinded comparative multicenter study with unfractionated heparin

AbstractHeparin-induced thrombocytopenia (HIT) is a complication of heparin therapy caused by antibodies against a complex of platelet factor 4 and heparin. Fondaparinux (Arixtra) is a new synthetic selective factor Xa inhibitor. We performed a serologic study to determine the cross-reactivity of HIT sera with fondaparinux. Using a prospective, blinded study design, 39 clinically and serologically confirmed sera from patients with HIT and 15 control sera were sent to 3 different laboratories, each of which specialized in a particular HIT assay. These include the serotonin release assay, heparin-induced platelet agglutination assay, and platelet aggregation assay. Two of 82 assays (2.4%) performed in the presence of control sera were positive, both with unfractionated heparin. In the presence of HIT sera, 75 of 94 (79.8%) evaluable assays were positive with unfractionated heparin; fondaparinux was significantly (P < .001) less reactive than unfractionated heparin, only 3 of 91 evaluable assays (3.3%) being positive. Using flow cytometry, unlike unfractionated heparin, fondaparinux did not induce the binding of PAC1 and anti-CD62 monoclonal antibodies or of annexin V to platelets with HIT sera. Together, these results suggest that fondaparinux is nonreactive to HIT sera and raise the possibility that the drug may be used for prophylaxis and treatment of thrombosis in patients with a history of HIT.

Advances in Anticoagulation Therapy: The Role of Selective Inhibitors of Factor Xa and Thrombin in Thromboprophylaxis after Major Orthopedic Surgery

A new generation of antithrombotic agents that target a single enzyme within the procoagulant cascade is making its way into mainstream clinical practice. Borrowing selectively from the properties of their parent anticoagulant, unfractionated heparin, as well as from those of peptide anticoagulants from reptile or insect venoms, designers of the new drugs have created targeted inhibitors of thrombin, factor Xa, or other specific factors in the procoagulant pathways. These new agents promise efficacy and safety profiles far more favorable than those of conventional anticoagulants for thromboprophylaxis after major orthopedic surgery and require no laboratory monitoring of drug efficacy in this setting. Ximelagatran, the oral "prodrug" of the direct thrombin inhibitor melagatran, is in phase III of clinical development. Clinical trials using various dosages of ximelagatran, sometimes preceded by subcutaneous melagatran, as thromboprophylaxis after total hip or knee replacement surgery have suggested efficacy equal to or better than that of warfarin and enoxaparin. The best dosing regimen and optimal timing of first dose for melagatran and ximelagatran remain to be determined, as do the mechanism and impact of drug disturbance of hepatic function. Fondaparinux, a selective, synthetic inhibitor of factor Xa, has been shown in large clinical trials to be superior to low-molecular-weight heparins and is approved as a fixed once-daily subcutaneous 2.5-mg dose for thromboprophylaxis for hip or knee replacement surgery and after hip fracture repair. Fixed-dose fondaparinux 2.5 mg initiated 6 to 8 hours after surgery achieved superior efficacy and comparable safety in head-to-head comparisons with enoxaparin for the prevention of venous thromboembolism after major orthopedic surgery. Fondaparinux is the only agent approved for use in hip fracture patients in the United States at this time and has recently gained approval for extended prophylaxis in this patient population.