Short- and long-acting synthetic pentasaccharides

Pentasaccharides for the prevention of venous thromboembolism

BACKGROUND

Venous thromboembolism (VTE) is a common condition with potentially serious and life-threatening consequences. The standard method of thromboprophylaxis uses an anticoagulant such as low molecular weight heparin (LMWH) or warfarin. In recent years, another type of anticoagulant, pentasaccharide, an indirect factor Xa inhibitor, has shown good anticoagulative effect in clinical trials. Three types of pentasaccharides are available: short-acting fondaparinux, long-acting idraparinux and idrabiotaparinux. Pentasaccharides cause little heparin-induced thrombocytopenia and are better tolerated than unfractionated heparin, LMWH and warfarin. However, no consensus has been reached on whether pentasaccharides are superior or inferior to other anticoagulative methods.

OBJECTIVES

To assess effects of pentasaccharides versus other methods of thromboembolic prevention (thromboprophylaxis) in people who require anticoagulant treatment to prevent venous thromboembolism.

SEARCH METHODS

The Cochrane Vascular Information Specialist (CIS) searched the Specialised Register (last searched March 2016) and the Cochrane Central Register of Controlled Trials (CENTRAL; 2016, Issue 2). The CIS searched trial databases for details of ongoing and unpublished studies. Review authors searched LILACS (Latin American and Caribbean Health Sciences) and the reference lists of relevant studies and reviews identified by electronic searches.

SELECTION CRITERIA

We included randomised controlled trials on any type of pentasaccharide versus other anticoagulation methods (pharmaceutical or mechanical) for VTE prevention.

DATA COLLECTION AND ANALYSIS

Two review authors independently selected trials, assessed methodological quality and extracted data in predesigned tables.

MAIN RESULTS

We included in this review 25 studies with a total of 21,004 participants. All investigated fondaparinux for VTE prevention; none investigated idraparinux or idrabiotaparinux. Studies included participants undergoing abdominal surgery, thoracic surgery, bariatric surgery or coronary bypass surgery; acutely ill hospitalised medical patients; people requiring rigid or semirigid immobilisation; and those with superficial venous thrombosis. Most studies focused on orthopaedic patients. We lowered the quality of the evidence because of heterogeneity between studies and a small number of events causing imprecision.When comparing fondaparinux with placebo, we found less total VTE (risk ratio (RR) 0.24, 95% confidence interval (CI) 0.15 to 0.38; 5717 participants; 8 studies; I2 = 64%; P < 0.00001), less symptomatic VTE (RR 0.15, 95% CI 0.06 to 0.36; 6503 participants; 8 studies; I2 = 0%; P < 0.0001), less total DVT (RR 0.25, 95% CI 0.15 to 0.40; 5715 participants; 8 studies; I2 = 67%; P < 0.00001), less proximal DVT (RR 0.12, 95% CI 0.04 to 0.39; 2746 participants; 7 studies; I2 = 64%; P = 0.0004) and less total pulmonary embolism (PE) (RR 0.16, 95% CI 0.04 to 0.62; 6412 participants; 8 studies; I2 = 0%; P = 0.008) in the fondaparinux group. The quality of the evidence was moderate for total VTE, total DVT and proximal DVT, and high for symptomatic VTE and total PE.When fondaparinux was compared with LMWH, analyses indicated that fondaparinux reduced total VTE and DVT (RR 0.55, 95% CI 0.42 to 0.73; 9339 participants; 11 studies; I2 = 64%; P < 0.0001; and RR 0.54, 95% CI 0.40 to 0.71; 9356 participants; 10 studies; I2 = 67%; P < 0.0001, respectively), and showed a trend toward reduced proximal DVT (RR 0.58, 95% CI 0.33 to 1.02; 8361 participants; 9 studies; I2 = 53%; P = 0.06). Symptomatic VTE (RR 1.03, 95% CI 0.65 to 1.63; 12240 participants; 9 studies; I2 = 35%; P = 0.90) and total PE (RR 1.24, 95% CI 0.65 to 2.34; 12350 participants; 10 studies; I2 = 0%; P = 0.51) indicated no difference between fondaparinux and LMWH. The quality of the evidence was moderate for total VTE, symptomatic VTE, total DVT and total PE, and low for proximal DVT.We showed that fondaparinux increased major bleeding compared with both placebo and LWMH (RR 2.56, 95% CI 1.48 to 4.44; 6659 participants; 8 studies; I2 = 0%; P = 0.0008; moderate-quality evidence; and RR 1.38, 95% CI 1.09 to 1.75; 12,501 participants; 11 studies; I2 = 24%; P = 0.008; high-quality evidence, respectively). All-cause mortality was not different between fondaparinux and placebo or LMWH (RR 0.76, 95% CI 0.48 to 1.22; 6674 participants; 8 studies; I2 = 14%; P = 0.26; moderate-quality evidence; and RR 0.88, 95% CI 0.63 to 1.22; 12,400 participants; 11 studies; I2 = 0%; P = 0.44; moderate-quality evidence, respectively).One study compared fondaparinux with variable and fixed (1 mg per day) doses of warfarin after elective hip or knee replacement surgery and showed no difference in primary and secondary outcomes between fondaparinux and both variable and fixed doses of warfarin. The quality of the evidence was very low. One small study compared fondaparinux with edoxaban in patients with severe renal impairment undergoing lower-limb orthopaedic surgery and reported no thromboembolic events, major bleeding events or deaths in either group. The quality of the evidence was very low. One small study compared fondaparinux with mechanical thromboprophylaxis. Results showed no difference in total VTE and total DVT between fondaparinux and mechanical thromboprophylaxis. This study reported no cases pertaining to the other outcomes of this review. The quality of the evidence was low.There were insufficient studies to permit meaningful conclusions for subgroups of clinical conditions other than orthopaedic surgery.

AUTHORS' CONCLUSIONS

Moderate to high quality evidence shows that fondaparinux is effective for short-term prevention of VTE when compared with placebo. It can reduce total VTE, DVT, total PE and symptomatic VTE, and does not demonstrate a reduction in deaths compared with placebo. Low to moderate quality evidence shows that fondaparinux is more effective for short-term VTE prevention when compared with LMWH. It can reduce total VTE and total DVT and does not demonstrate a reduction in deaths when compared with LMWH. However, at the same time, moderate to high quality evidence shows that fondaparinux increases major bleeding when compared with placebo and LMWH. Therefore, when fondaparinux is chosen for the prevention of VTE, attention should be paid to the person's bleeding and thrombosis risks. Most data were derived from patients undergoing orthopaedic surgery. Therefore, the conclusion predominantly pertains to these patients. Data on fondaparinux for other clinical conditions are sparse.

Current and future antithrombotic agents in children

Thromboembolic complications are increasing in children, and the use of anticoagulation has seen a dramatic increase despite the lack of randomized clinical trials. The most widely used agents in children are heparin and warfarin, however these agents have limitations that are exaggerated in children. This has led to the use of newer agents with improved pharmacologic properties such as low-molecular-weight heparin, however, the use of novel agents such as direct thrombin inhibitors has been limited to case reports. These agents, however, have potential advantages over heparin, low-molecular-weight heparin and warfarin. Current clinical trials are in progress to define the proper dose of two such agents--argatroban (Argatroban, GlaxoSmithKline) and bivalirudin (Angiomax, The Medicines Company). The selective Factor Xa inhibitor fondaparinux (Arixtra, Sanofi-Synthelabo) has not been used in children; however, there are situations in which this agent may be advantageous. This review will discuss the currently available agents, with an emphasis on those that are novel and their potential uses in children.

Egyptian experience of reliability of 4T's score in diagnosis of heparin induced thrombocytopenia syndrome

To evaluate the utility of the 4Ts clinical scoring system as a pretest probability method for detection of heparin-induced thrombocytopenia (HIT). Medical and surgical inpatients and outpatients at Kasr El Eini hospital. This single-centre series of 50 HIT testing referrals assessed combination of clinical score (thrombocytopenia, timing, thrombosis, other causes of thrombocytopenia not evident; 4T's), Heparin platelet factor 4 (H-PF4) rapid particle gel immunoassay (PaGIA) and 14C serotonin release assay (SRA) to develop a practical and well tolerated diagnostic strategy for HIT. Sixteen patients (32%) had a low 4T's score, 26 (52%) had an intermediate score and only eight (16%) had a high score. A positive H-PF4 by PaGIA was seen in seven patients (14%). As might be anticipated, the likelihood of obtaining a positive H-PF4 by PaGIA increased with an increasing clinical score, with positive H-PF4 by PaGIA results in low, intermediate and high scoring patients of 6.25, 7.7 and 50%, respectively. The positive predictive value of a positive PaGIA was 92%. The negative predictive value was 100%. Five patients (10%) in our cohort had a positive SRA. All patients with a positive SRA were included in the intermediate (two of 26 patients, 7.7%) or high (three of eight patients, 37.5%) score groups. The negative predictive value of a low 4T's score was 100%, effectively ruling out HIT. A low 4Ts score supports low probability of HIT based on the results of the PaGIA and SRA. Overall, the interrater reliability of the scoring system was fair.

Principles of Anticoagulation and New Therapeutic Agents in Atrial Fibrillation

Anticoagulation is required in cardiac arrhythmias, specifically atrial fibrillation (AF) and atrial flutter, to reduce the risk of thromboembolism. Principles of anticoagulation of both AF and atrial flutter are similar because the location and nature of the arrhythmias are similar. Approximately 2 million people in the United States are affected by AF, and the prevalence is expected to exceed 10 million by the year 2050. Warfarin is known to reduce stroke risk by 68% in patients with AF and is the most effective agent for this indication, although it is not without risk. Antithrombotic therapy with antiplatelets or anticoagulants is recommended for most patients with AF. This review discusses the principles of anticoagulation and the mechanism of action, pharmacologic profile, and phase of development of the therapeutic agents used as anticoagulants.

New pharmacotherapy for stroke prevention in atrial fibrillation: update 2010

Atrial fibrillation (AF) is the most commonly encountered arrhythmia in clinical practice and is associated with substantial morbidity and mortality. Its prevalence increases with age, affecting about 1% of patients aged <60 years and almost 10% of patients >80 years. AF is associated with a fivefold increasing risk of embolism or stroke with absolute risk ranging from less than 1% to 20% per year, depending on patient age and the presence of clinical risk factors including congestive heart failure, systemic hypertension, diabetes mellitus and prior history of cardioembolic events. Vitamin K antagonists (VKAs) and acetyl salicylic acid are currently the only licensed antithrombotic therapies for stroke prevention in patients with AF. Anticoagulants are very effective for stroke prevention in patients with AF, overall a 64% relative risk reduction. Nonetheless, approximately 50% of patients with AF who have an indication for VKA receive anticoagulant therapy, of which only 50% maintain adequate therapeutic ranges. Furthermore, 50% will discontinue VKAs within 3 to 5 years regardless of appropriate international normalized ratio control. Underutilization of VKAs is related, in part, to their numerous limitations and difficulty in maintaining adequate therapeutic control, prompting the development of new antithrombotic strategies that are equally effective and safer, and easier to manage than VKAs. This review focuses on new antithrombotic therapies for stroke prevention in patients with AF.

Reversible biotinylated oligosaccharides: a new approach for a better management of anticoagulant therapy

OBJECTIVE

In order to obtain a neutralizable antithrombotic, a chimeric molecule (SSR126517E) containing the sequence of a long-lasting antithrombin (AT)-dependent anti-factor Xa pentasaccharide, idraparinux, linked to a biotin molecule was synthesized and tested for anticoagulant and antithrombotic activity.

METHODS

SSR126517E was tested in several models in vitro and in vivo for its pharmacological properties as well as its ability to be neutralized by avidin.

RESULTS

SSR126517E displayed exactly the same properties as idraparinux. In vitro, SSR126517E had a very high affinity for AT (K(d) < 1 nm) and showed a potent anti-FXa effect and inhibition of thrombin generation with IC(50) values similar to those of idraparinux. Ex vivo, after intravenous administration to rats, SSR126517E produced a potent and long-lasting anti-FXa effect comparable to that obtained with idraparinux; as with idraparinux, the subcutaneous bioavailability was 100%. In vivo, SSR126517E was a potent antithrombotic in rat and mouse venous and arterial thrombosis models. Direct comparison in rats showed that SSR126517E was as active as idraparinux, when administered at the same molar dose. Furthermore, injection of avidin triggered the immediate elimination of SSR126517E from the bloodstream, resulting in complete neutralization of the antithrombotic activity of SSR126517E.

CONCLUSIONS

These results show for the first time that coupling an oligosaccharide with biotin has no effect on the former's pharmacokinetic and pharmacologic properties and renders neutralization easy by injection of avidin.

Advancement in antithrombotics for stroke prevention in atrial fibrillation

The focus of this review is the evolving field of antithrombotic drug therapy for stroke prevention in patients with atrial fibrillation (AF). The current standard of therapy includes warfarin, acenocoumarol and phenprocoumon which have proven efficacy by reducing stroke by 68% against placebo. However, a narrow therapeutic index, wide variation in metabolism, and numerous food and drug interactions have limited their clinical application to only 50% of the indicated population. Newer agents such as direct thrombin inhibitors, factor Xa inhibitors, factor IX inhibitors, tissue factor inhibitors and a novel vitamin K antagonist are being developed to overcome the limitations of current agents. The direct thrombin inhibitor dabigatran is farthest along in development. Further clinical trial testing, and eventual incorporation into clinical practice will depend on safety, efficacy and cost. Development of a novel vitamin K antagonist with better INR control will challenge the newer mechanistic agents in their quest to replace the existing vitamin K antagonists. Till then, the large unfilled gap to replace conventional agents remains open. This review will assess all these agents, and compare their mechanism of action, stage of development and pharmacologic profile.

Cibles des médicaments antithrombotiques

New antithrombotic agents are being developed not only to improve efficacy, but also to increase safety in comparison with widely used conventional agents such as the oral anticoagulants. New anticoagulant, antiplatelet, and profibrinolytic compounds are currently under study in drug development programs, and most of those in phase II or III of development are derived from the observation of natural phenomena and merely mimic processes developed by mammalians, including humans, to avoid thrombosis, or by blood-sucking insects or animals to prevent coagulation of the blood their are feeding on. By contrast, drug candidates identified by means of rigorous research and designed to target new pathways and achieve direct and specific inhibition of factors that are presumed to play an important role in thrombogenesis have generally failed to show any benefit and sometimes even induce deleterious effects. The clinical development of new drugs, even those mimicking natural phenomena, improves our knowledge of the pathogenesis of thrombosis and sheds light, retrospectively, on previous conceptual errors. The improvement in our basic knowledge and the development of new types of drugs suggest that, in contrast to the current antithrombotic compounds that are used in a broad range of clinical settings, use of new drugs should be restricted to specific situations in which their mechanisms of action are predicted to deliver the highest medical benefit. A major obstacle resides in the fact that current drug development programs are still required to comply with long obsolete guidelines based on the characteristics of first-generation antithrombotic agents, and that do not take into account the specific mechanisms of action of new drugs. This situation should change, however, and new antithrombotic drugs should soon be able to benefit from adapted development programs that will make it possible to determine their optimal risk-benefit ratio.

Argatroban Dosing of Patients with Heparin-Induced Thrombocytopenia and an Elevated aPTT Due to Antiphospholipid Antibody Syndrome

Objective:

To describe the clinical characteristics, management, and outcomes of patients with heparin-induced thrombocytopenia with thrombosis (HITTS) or without thrombosis (HIT) who also had an elevated baseline activated partial thromboplastin time (aPTT) due to antiphospholipid antibody syndrome (APS).

Case Summary:

Four patients with HIT/HITTS and an elevated baseline aPTT due to APS were identified. Two patients had venous thrombosis, 1 had limb ischemia, and 1 had isolated HIT. All 4 were managed with a weight-based fixed dose of argatroban without laboratory monitoring. None of the patients had thrombotic or bleeding complications once therapy was initiated.

Discussion:

Management of patients with HIT/HITTS and an abnormal baseline aPTT due to APS is problematic. We review alternative management strategies, such as monitoring direct thrombin inhibitors with the ecarin clotting time or thrombin inhibition time or using an alternative anticoagulant, such as fondaparinux. As of March 13, 2006, none of these management strategies has been evaluated in a clinical trial for this patient population. We report the successful use of weight-based, fixed-dose argatroban without laboratory monitoring in patients with APS.

Conclusions:

Use of a fixed-dose argatroban regimen without laboratory monitoring is a potential management strategy for patients with HIT/HITTS and an elevated baseline aPTT due to APS.

Fondaparinux is a safe alternative in case of heparin intolerance during pregnancy

Heparin is the drug of choice for the treatment or the prevention of thromboembolic disease during pregnancy. However, treatment options are limited when heparin cannot be used because of hypersensitivity skin reactions. Despite the recent availability of new anticoagulant agents, data relating to their use during pregnancy are lacking. This report describes the successful management with fondaparinux, during 150 days, of a pregnant patient with protein S deficiency and prior deep vein thrombosis (DVT) who developed heparin and danaparoid hypersensitivity.

Anticoagulation strategies in continuous renal replacement therapy: can the choice be evidence based?

OBJECTIVES

Critical illness increases the tendency to both coagulation and bleeding, complicating anticoagulation for continuous renal replacement therapy (CRRT). We analyzed strategies for anticoagulation in CRRT concerning implementation, efficacy and safety to provide evidence-based recommendations for clinical practice.

METHODS

We carried out a systematic review of the literature published before June 2005. Studies were rated at five levels to create recommendation grades from A to E, A being the highest. Grades are labeled with minus if the study design was limited by size or comparability of groups. Data extracted were those on implementation, efficacy (circuit survival), safety (bleeding) and monitoring of anticoagulation.

RESULTS

Due to the quality of the studies recommendation grades are low. If bleeding risk is not increased, unfractionated heparin (activated partial thromboplastin time, APTT, 1-1.4 times normal) or low molecular weight heparin (anti-Xa 0.25-0.35 IU/l) are recommended (grade E). If facilities are adequate, regional anticoagulation with citrate may be preferred (grade C). If bleeding risk is increased, anticoagulation with citrate is recommended (grade D(-)). CRRT without anticoagulation can be considered when coagulopathy is present (grade D(-)). If clotting tendency is increased predilution or the addition of prostaglandins to heparin may be helpful (grade C(-)).

CONCLUSION

Anticoagulation for CRRT must be tailored to patient characteristics and local facilities. The implementation of regional anticoagulation with citrate is worthwhile to reduce bleeding risk. Future trials should be randomized and should have sufficient power and well defined endpoints to compensate for the complexity of critical illness-related pro- and anticoagulant forces. An international consensus to define clinical endpoints is advocated.

New trends in anticoagulant treatments

Classic anticoagulant drugs are very effective, save lives and have been used for more than 50 years. Nevertheless, some drawbacks are encountered in their routine clinical use. Recently, pharmaceutical research has developed new drugs, some of which are already on the market. This is the case of fondaparinux, a pentasaccharide which can interact with antithrombin, thus inhibiting factor Xa. Modification of its structure (idraparinux) has led to more stable binding with antithrombin and to an increase in its half-life allowing for once-a-week administration. Another important oral compound is ximelagatran which directly binds thrombin and blocks its catalytic site. There is no need for laboratory control, and phase II and phase III studies are encouraging. Thus, in the next few years, we may witness great changes in the treatment of patients with thromboembolic disorders.

New antithrombotics in the treatment of thromboembolic disease

New insights into the in vivo function of the coagulation system have resulted in the development of new antithrombotic agents with a more specific point of impact and more reliable pharmacological properties. Inhibition of coagulation can be achieved by specific thrombin inhibitors (e.g. melagatran), agents specifically targeted at factor Xa (e.g. pentasaccharides), and by tissue factor/factor VIIa blockers (e.g. recombinant NAPc2). In some situations, these agents are more effective and safer than the antithrombotics currently used. However, the major difference may be that the newer generation of antithrombotics does not require repeated laboratory control and dose adjustment to obtain a stable level of anticoagulation; therefore, they are more suitable for application in clinical practice. In this overview, we will briefly discuss the various new compounds that have recently become available or that will be introduced into clinical medicine in the near future and their relative efficacy and safety in comparison to currently used antithrombotic agents.

Management of the anticoagulated patient

Excessive bleeding or thrombosis is a preeminent concern for all surgeons. Patients may be at risk because of medical therapy, underlying disease, or complications related to both. An understanding of the coagulation cascade--mechanisms and tests of function--permits a rational, if not always complete, basis for a plan of therapy. Newer anticoagulation medications are changing how thrombotic complications, such a VTE or graft occlusion, are treated or prevented. This entire area is undergoing rapid evolution, and the approaches that have been standard for decades soon will be supplanted. Ultimately, however, the most important assessment is made at the bedside by the clinician.

Management of thrombosis in cancer: primary prevention and secondary prophylaxis

Although traditional anticoagulant regimens are highly effective and safe in most patients with venous thromboembolism (VTE), the aggressive natural history of VTE and the high risk of serious bleeding in cancer patients can complicate the management of VTE. In addition, because few clinical trials have focused on the prevention and treatment of VTE in this unique patient population, many clinical questions regarding the care of cancer patients with VTE remain unanswered. Currently, low-molecular-weight heparins and oral vitamin K antagonists are the most commonly used agents for the primary and secondary prophylaxis of VTE in patients with or without cancer, but their use is associated with pharmacological and practical limitations. This review will provide an up-to-date summary of the clinical trials that have addressed the management of VTE in patients with cancer. A brief discussion of the potential application of novel anticoagulants in these clinical settings is also included.

Blood coagulation and its regulation by anticoagulant pathways: genetic pathogenesis of bleeding and thrombotic diseases

Blood coagulation and its regulation by anticoagulant pathways: genetic pathogenesis Platelet-mediated primary haemostasis and blood coagulation have evolved as important defence mechanisms against bleeding. The formation of the platelet plug provides the initial occlusion of the vascular lesion. This is temporally co-ordinated with the activation of the coagulation system, which occurs in response to the rupture of endothelium and the exposure of blood to the extravascular tissue. The reactions of blood coagulation are carefully controlled by several anticoagulant mechanisms and under normal conditions they prevail over the procoagulant forces. Genetic or acquired disturbances of the natural balance between the pro- and anticoagulant systems may result in bleeding or thrombotic diseases.

Novel therapies for the prevention of stroke

The health and economic burden of stroke to society is enormous. Pharmacological therapies remain the primary stroke prevention strategy for the vast majority. Several existing and newer pharmacological agents aimed at the treatment of hypertension and lowering cholesterol are proving to be effective. For example, the antiplatelet agent clopidogrel has reduced end points in the secondary prevention of stroke, as have combinations of aspirin with traditional therapies, including dipyramidole. The direct oral thrombin inhibitor ximelagatran is a novel oral anticoagulant that has shown significant potential as a possible replacement to warfarin therapy, for the prevention of stroke for patients with non-valvular atrial fibrillation. Additional novel agents with hypothetical, although not yet proven, benefits in stroke prevention include fish oils, homocysteine-lowering therapy and anti-inflammatory agents. Finally, a controversial novel polypill, which would include fixed combinations of several pharmacological agents, may yet become a realistic and promising stroke prevention option.

The Management of Patients With Heparin-Induced Thrombocytopenia Who Require Anticoagulant Therapy

For patients with heparin-induced thrombocytopenia (HIT), reexposure to heparin is generally not recommended. However, these patients are likely to require anticoagulation therapy at some point in the future. During acute HIT, when thrombocytopenia and anti-heparin-platelet factor 4 antibodies (or HIT antibodies) are present, therapy with heparin must be avoided. In patients with subacute HIT, when platelets have recovered but HIT antibodies are still present, therapy with heparin should be avoided. In patients with a remote history of HIT, when HIT antibodies have cleared, heparin reexposure may be safe, although recurrent HIT has been described in some patients. For all of these patients, the use of alternate anticoagulant agents, including direct thrombin inhibitors and anti-Xa agents, is preferable. There is an increasing amount of data supporting the use of these alternative agents in a wide variety of clinical circumstances, including thromboprophylaxis and treatment of acute thrombosis. Except for a few clinical situations, it is generally possible to avoid heparin reexposure in patients with a history of HIT.

A selective, slow binding inhibitor of factor VIIa binds to a nonstandard active site conformation and attenuates thrombus formation in vivo

The serine protease factor VIIa (FVIIa) in complex with its cellular cofactor tissue factor (TF) initiates the blood coagulation reactions. TF.FVIIa is also implicated in thrombosis-related disorders and constitutes an appealing therapeutic target for treatment of cardiovascular diseases. To this end, we generated the FVIIa active site inhibitor G17905, which displayed great potency toward TF.FVIIa (Ki = 0.35 +/- 0.11 nM). G17905 did not appreciably inhibit 12 of the 14 examined trypsin-like serine proteases, consistent with its TF.FVIIa-specific activity in clotting assays. The crystal structure of the FVIIa.G17905 complex provides insight into the molecular basis of the high selectivity. It shows that, compared with other serine proteases, FVIIa is uniquely equipped to accommodate conformational disturbances in the Gln217-Gly219 region caused by the ortho-hydroxy group of the inhibitor's aminobenzamidine moiety located in the S1 recognition pocket. Moreover, the structure revealed a novel, nonstandard conformation of FVIIa active site in the region of the oxyanion hole, a "flipped" Lys192-Gly193 peptide bond. Macromolecular substrate activation assays demonstrated that G17905 is a noncompetitive, slow-binding inhibitor. Nevertheless, G17905 effectively inhibited thrombus formation in a baboon arterio-venous shunt model, reducing platelet and fibrin deposition by approximately 70% at 0.4 mg/kg + 0.1 mg/kg/min infusion. Therefore, the in vitro potency of G17905, characterized by slow binding kinetics, correlated with efficacious antithrombotic activity in vivo.

Current Issues in Anticoagulation

Anticoagulation therapy with unfractionated heparin, low-molecular-weight heparins and oral vitamin K antagonists is currently the mainstay of treatment and prevention of thromboembolic disorders (such as deep vein thrombosis, pulmonary embolism and stroke prevention in patients with atrial fibrillation). Although these therapies have proven benefits, they also have important limitations that result in their underuse in routine clinical practice. Consequently, many patients identified by guidelines as requiring anticoagulant therapy receive no or inadequate treatment. Heparins require parenteral administration and pose the risk of heparin-induced thrombocytopenia. Vitamin K antagonists have a narrow separation of antithrombotic and haemorrhagic effects and numerous food and drug-drug interactions, and require frequent coagulation monitoring and dose adjustment to ensure effective antithrombotic protection while minimizing the risk of bleeding complications. In response to these limitations, several new anticoagulants have recently been developed, including selective factor Xa inhibitors such as fondaparinux and ximelagatran, the first oral agent in the new class of direct thrombin inhibitors and the first new oral anticoagulant for almost 60 years. Ximelagatran possesses many of the properties of an ideal agent for anticoagulation therapy. With its oral formulation, consistent and predictable pharmacological profile and no coagulation monitoring, ximelagatran has the potential to increase the use and duration of anticoagulation treatment in thromboembolic disorders and to reduce the burden associated with long-term management.

Traitement de la maladie thromboembolique veineuse chez les patients souffrant de cancers

PURPOSE

Venous tromboembolic treatment in patients with cancer can be a clinical dilemma. Comorbid conditions, significant risk of recurrence of bleeding associated with warfarin, difficult venous access, are some of the factors that often complicate anticoagulants therapy in patients with cancer.

EXEGESIS

Low molecular weight heparin has replaced unfractionated heparin as the first line treatment in the majority of patients with venous thromboembolism and cancer, in hospital or safely at home. Recent trial demonstrated that long-term low molecular weight heparin administrated over a 6-month period reduced the rate of recurrence venous thromboembolism with non increase in bleeding compared with oral anticoagulant therapy. Placement of an inferior vena caval filter should be reserved for patients with active or very high risk of bleeding, but oncologist should consider these sobering results in such patients. Whether anticoagulants might also improve cancer survival rates independent of their effect on thromboembolism deserves further investigation.

PERSPECTIVE

In future, new antithrombotic agents such as oral direct thrombin or long-acting synthetic factor Xa inhibitor may be useful in these patients.

Initial Treatment of Venous Thromboembolism

Adequate initial anticoagulant therapy of deep venous thrombosis (DVT) is required to prevent thrombus growth and pulmonary embolism (PE). Intravenous unfractionated heparin (UFH) is being replaced by low-molecular-weight heparin (LMWH) as the anticoagulant of choice for initial treatment of venous thromboembolism (VTE). Both agents are relatively safe and effective when used to treat VTE, with LMWH suitable for outpatient therapy because of improved bioavailability and more predictable anticoagulant response. Serious potential complications of heparin therapy, such as heparin-induced thrombocytopenia (HIT) and osteoporosis, seem less common with LMWH. The potential for fetal harm and changes in maternal physiology complicate the treatment of VTE during pregnancy. Although systemic thrombolysis is used in patients with massive PE and in some patients with proximal DVT, controversy persists with respect to appropriate patient selection for this intervention.

[Therapeutic approaches to acute pulmonary embolism]

The diagnosis of pulmonary embolism (PE) remains a considerable challenge to any physician. Irrespective of the diagnostic progress, the prevalence of fatal PE in autopsy studies is still about one third. Introducing sufficient anticoagulant therapy, mortality due to PE can be decreased from about 30% to 2-8%. Therefore, immediate anticoagulant therapy should be given, if PE is clinically suspected. Initial anticoagulation by low-molecular-weight heparins is as effective as unfractionated heparin in non-massive PE. In patients suffering from massive PE, thrombolytic treatment is indicated. Whether patients with submassive PE and/or elevated cardial troponins should also receive thrombolytic treatment, is still under debate. After PE has been established, vitamin-k-antagonists are the current standard of secondary prophylaxis.

New Therapeutic Approaches in Pulmonary Embolism

Pulmonary embolism as a part of venous thromboembolic disease has a broad spectrum of clinical presentations from minimal disease to life-threatening right heart failure. Therapy has to be guided by the risk associated with the individual clinical state of the patient. As long as hemodynamics are entirely stable, anticoagulation is given in order to prevent early or late recurrence, thereby allowing for endogeneous thrombolysis and recovery. In hemodynamically instable patients, i.e. patients under cardiopulmonary resuscitation or in shock, there is the need for a rapid reduction of thrombus mass in order to restore right ventricular function. Systemic thrombolysis is the most feasible modality to reduce the thrombus burden of the pulmonary circulation in the short term. For hemodynamically stable patients with right ventricular dysfunction as assessed by echocardiography, there is still some controversy as to whether thrombolysis improves the long-term outcome. At the least, thrombolysis may positively modify the short-term course of acute disease in patients with an extremely low risk of bleeding. When the acute phase has been overcome, secondary prophylaxis with vitamin K antagonists has to be given. The duration of secondary prophylaxis requires an individual assessment of both the risk of recurrence and the risk of bleeding. In the near future, new anticoagulant drugs such as direct thrombin and factor Xa inhibitors will offer new treatment modalities for the acute phase as well as for secondary prophylaxis.