Anti-factor IIa Research Articles

Evaluation of the changes on hemostatic parameters induced by valdecoxib in male Wistar rats

The effects of the cyclooxygenase (COX)-2 selective inhibitor, valdecoxib, on blood coagulation parameters were evaluated, along with aspirin in male Wistar rats. Groups of animals were administered a daily oral dose of 10 mg/kg rat of valdecoxib, 100 mg/kg rat of aspirin and the vehicle alone during 4 weeks. Blood samples were collected at the end of 1, 2, 3 and 4 weeks of administration period and the plasma concentrations of valdecoxib were determined by RP-HPLC giving mean values of 101.1, 113.5, 164.0 and 184.6 ng/mL, respectively. The same plasma samples were used for the analysis of hematological parameters and the results compared to the controls. Valdecoxib induced significant activated partial thromboplastin time reduction (18%) after 2 weeks and prothrombin time reduction (12.2%) after 3 weeks (P<0.05). There were no significant changes in the platelet count and fibrinogen levels. The anti-factor Xa and anti-factor IIa activities showed slight reductions, but only significant for the anti-factor Xa on the 3rd week (6.7%). The results showed that valdecoxib at the dose tested with the plasmatic concentrations induced some changes in the hemostatic function of rats, which can be helpful to understand the side effects and the safe use of the drug.

Validation of the Antifactor Xa and Antifactor IIa Assays for the Potency Assessment of Nadroparin in Pharmaceutical Formulations

The low-molecular weight heparin nadroparin calcium is used clinically for the prevention and treatment of venous and arterial thrombosis. The antifactor Xa and antifactor IIa assays were validated by investigating the parameters of range, linearity (r2 = 0.9905 and r2 = 0.9914, respectively) precision, accuracy, and robustness. The 2 methods incorporated a chromogenic endpoint and detection at 405 nm, yielding good results with detection limits of 0.004 and 0.01 IU/mL and quantitation limits of 0.01 and 0.03 IU/mL, respectively, for the antifactor Xa and antifactor IIa assays. Nadroparin calcium pharmaceutical products were evaluated by the antifactor Xa assay and the antifactor IIa assay, giving potencies between 93.86 and 109.88%, with an antifactor Xa/antifactor IIa ratio between 3.2 and 3.7. The results demonstrated the validity of the assays that are useful methodologies for the routine quality control of nadroparin in pharmaceutical formulations.

Age-related differences in heparin response

Introduction Major physiological differences in the coagulation system throughout childhood, compared to adults, are well documented. However, the impact of this on anticoagulant drugs is unknown. This study aimed to determine whether heparin therapeutic range determination is affected by the age of plasma donors and whether age-specific therapeutic ranges for heparin therapy may need to be considered in the clinical setting. Materials and methods Plasma samples were obtained from healthy children and adults, and pooled into age-specific pools. These were spiked with different concentrations of heparin and APTT; Anti-Factor Xa and Anti-Factor IIa were measured using standard techniques. The experiments were repeated using three separate plasma pools, and results expressed as means with standard deviations. Results and conclusions The results show clear age-related differences in APTT for the same Anti-Factor Xa heparin concentration. The differences were more pronounced in younger children, with higher APTT for same Anti-Factor Xa. The Anti-Factor IIa activity of heparin for a given Anti-Factor Xa activity also differed between age-specific plasma pools. This study suggests that when using heparin in children, basic assumptions about the drug mechanism of action and implications for therapeutic ranges need to be considered.

Heparin Octasaccharides Inhibit Angiogenesis In vivo

In previous experiments, we showed that heparin oligosaccharides inhibit the angiogenic cytokine fibroblast growth factor-2. Here, we present the first in vivo study of size-fractionated heparin oligosaccharides in four models of angiogenesis that are progressively less dependent on fibroblast growth factor-2. Heparin oligosaccharides were prepared using size-exclusion gel filtration chromatography and characterized through depolymerization and strong anion exchange high-performance liquid chromatography. Size-defined oligosaccharides (20 mg/kg/d) were given to mice bearing s.c. sponges that were injected with fibroblast growth factor-2 (100 ng/d). After 14 days, octasaccharides and decasaccharides reduced the microvessel density to levels below control. In a second experiment, HEC-FGF2 human endometrial cancer cells that overexpress fibroblast growth factor-2 were implanted in a hollow fiber placed s.c. in vivo. Oligosaccharides were given at 20 mg/kg/d for 2 weeks and the data again showed that octasaccharides significantly reduced microvessel density around the fiber (P = 0.03). In a more complex model, where angiogenesis was induced by a broad spectrum of growth factors, including vascular endothelial growth factor, we implanted H460 lung carcinoma cells in hollow fibers and treated the animals with oligosaccharides at 20 mg/kg/d over 3 weeks. Octasaccharides reduced the microvessel density to that of control. Preliminary investigation of 6-O-desulfated heparins showed that these also had antiangiogenic activity. Finally, we examined the inhibitory potential of hexasaccharides and octasaccharides given at 20 mg/kg/d and these inhibited the growth of H460 lung carcinoma in vivo. At clinically attainable concentrations, significant anticoagulation (activated partial thromboplastin time, anti-factor Xa, and anti-factor IIa) was not observed in vitro unless species containing > or =16 saccharide residues were investigated. Thus, our preclinical data show that heparin octasaccharides represent novel antiangiogenic compounds that can be given without the anticoagulant effects of low molecular weight heparin.

Fondaparinux: A potential new therapy for HIT

Heparin has been the mainstay of treatment and prevention of venous and arterial thromboembolism for many years. Its use, however, is associated with a serious and potentially fatal immunological drug reaction termed heparin-induced thrombocytopenia (HIT). Current treatment consists of discontinuing heparin therapy and the administration of an alternate anticoagulant (e.g. danaparoid, lepirudin, bivalirudin or argatroban). Fondaparinux is a novel synthetic heparin pentasaccharide capable of inhibiting factor Xa via the action of antithrombin (AT) but devoid of anti-factor IIa (thrombin) activity. Although the drug is identical in structure to the pentasaccharide domain found on unfractionated heparin (UH), it is too small to be recognized by the majority of heparin-reactive antibodies. It is theoretically an excellent candidate agent for the treatment of HIT. Currently, fondaparinux is licensed for orthopaedic venous thromboprophylaxis but not for the treatment of HIT. Successes in the use of fondaparinux in the treatment of HIT, as demonstrated in recent published case reports, warrant further study in larger controlled trials for this indication.

Clinical use of enoxaparin in the management of non-ST segment elevation acute coronary syndromes

Enoxaparin (Lovenox®; Roule-Poulenc Rorer, Inc.), a low molecular weight heparin (LMWH), is commonly used in the management of non-ST-segment elevation acute coronary syndromes (NSTE ACS) based on clinical trial outcomes. It is one of a group of glycosaminoglycan compounds that accelerate the inactivation of factor Xa by inducing a conformational change in antithrombin. In contrast to unfractionated heparin (UFH), LMWH have greater bioavailability, a more predictable anticoagulant response, longer half-life and a higher proportion of anti-factor Xa to anti-factor IIa activity. As a consequence, laboratory monitoring of the anticoagulant effect is typically unnecessary. Antithrombin therapy with LMWH or UFH has the highest-level recommendation (IA) in the 2002 professional guidelines for the management of unstable angina and non-ST-elevation myocardial infarction, where enoxaparin has a IIA recommendation over UFH unless early coronary artery bypass surgery is planned. In a recent systematic overview of > 20,000 patients with NSTE ACS from six clinical trials, including conservative and invasively managed patients, enoxaparin provided a statistically significant reduction in 30-day death or nonfatal myocardial infarction (MI) compared with UFH with no significant excess in transfusions, or major bleeding. These data support the role of enoxaparin as an anti-coagulant in patients with NSTE ACS.

When Heparins Promote Thrombosis

Heparin-induced thrombocytopenia (HIT; sometimes known as HIT type II) is a serious, immune system–mediated complication of heparin therapy often resulting in devastating thromboembolic outcomes. Although nomenclature distinctions have been made historically between this condition and the non–immune system–mediated, asymptomatic transient drop in platelet count in some patients receiving heparin (sometimes known as HIT type I), the term “HIT” is now preferably reserved for the immune system–mediated condition.1 An estimated 1 in 100 patients who receive unfractionated heparin for at least 5 days will develop HIT-associated thrombosis.2 The pervasive use of heparins makes HIT one of the most important adverse drug reactions confronting physicians. Heparin is routinely used for thromboprophylaxis or treatment in many clinical settings, including cardiovascular surgery and interventional procedures, acute coronary syndromes, venous thromboembolism, atrial fibrillation, peripheral occlusive disease, dialysis, and extracorporeal circulation. It is among the most frequently prescribed medications in the United States, with >1 trillion units used3 and 12 million patients treated4 annually. Because thrombocytopenia is common in hospitalized patients, occurring in up to 58% of critically ill patients, and can be caused by a variety of factors,5 HIT unfortunately often remains unrecognized. However, consistent with standard clinical practice for life-threatening conditions, HIT should be suspected in a heparin-treated patient who has thrombocytopenia with or without thrombosis. Increased awareness and a high degree of suspicion for HIT are critical to ensure its prompt recognition, diagnosis, and treatment. Advances in understanding the pathophysiology of HIT and its natural history have led to current treatment recommendations—specifically, that heparin must be discontinued immediately and alternative anticoagulation must be initiated.6 Herein we review the pathogenesis, frequency, natural history, diagnosis, and treatment of HIT. Unfractionated heparin is a heterogeneous group of negatively charged, sulfated glycosaminoglycans (molecular weight 3000 to 30 000 Da) from animal …

Validation of the anti-factor IIa assay and potency assessment of enoxaparin in pharmaceutical formulations

The low molecular weight heparin enoxaparin sodium is used clinically for the prevention and treatment of venous and arterial thrombosis. An anti-factor IIa assay was applied and validated for the potency evaluation of the pharmaceutical formulations. Investigating the parameters of range, linearity ( r 2 = 0.9912), precision, accuracy and robustness, the biological assay incorporated a chromogenic end-point and detection at 405 nm. The method yielded good results with a detection limit of 0.01 IU/ml and a quantitation limit of 0.03 IU/ml. Sodium enoxaparin pharmaceutical products were evaluated by the anti-factor Xa assay and the anti-factor IIa assay giving potencies between 97.18% and 106.50%, with an anti-factor Xa/anti-factor IIa ratio between 3.9 and 4.2. The results demonstrated the validity of the anti-factor IIa assay that together with the anti-factor Xa are useful methodologies for the routine quality control of enoxaparin in pharmaceutical products.

Comparison of Two Low-Molecular-Weight Heparins for DVT and PE

Each of the low-molecular-weight heparins (LMWHs) available today differs in anti-factor Xa and anti-factor IIa activities. To learn whether equivalence can be assumed between two available LMWH preparations, Canadian researchers enrolled 505 adults (mean age, 58) with deep-vein thrombosis or pulmonary embolism in a multicenter randomized …

Thrombin generation for the control of heparin treatment, comparison with the activated partial thromboplastin time

Heparin can be quantified with antifactor Xa and IIa tests (aXa, aIIa) but the anticoagulant power of heparin depends upon plasma properties as well as upon heparin concentrations and thus differs between subjects. Measuring the effect, as with the activated partial thromboplastin time (APTT) therefore is clinically more relevant. Here we investigate the use of the endogenous thrombin potential (ETP) for this purpose. In 12 volunteers 9000 IU of four heparins of different mol. wt distributions were injected. Samples were taken at 11 time points between 0 and 24 h. With the exception of the 0 and 24-h time points, heparin could be demonstrated by its aIIa and aXa activity in virtually all samples. The APTT showed the effect of this heparin in 34% of the samples; the ETP in 80%. This is partly due to the wide margins of the normal values, caused by large interindividual variation [coefficient of variation (CV) approximately 12% for the APTT, approximately 17% for the ETP]. The intraindividual variation is much smaller (CV approximately 4% for the APTT, approximately 5% for the ETP). Relative to the baseline value of the individual, the heparin effect was recognized by the APTT in 55% of the cases and by the ETP in 98%. There were no large differences between the different types of heparin.

Activity of a synthetic hexadecasaccharide (SanOrg123781A) in a pig model of arterial thrombosis

The activity of SanOrg123781A, a new synthetic antithrombotic drug inhibiting both factor Xa and thrombin through antithrombin (AT), was compared to that of unfractionated heparin (UFH) and of the synthetic pentasaccharide (fondaparinux, SP) in an ex vivo arterial thrombosis model in the pig. Six groups of four pigs were administered intravenously with SanOrg123781A (1, 3, 10 and 30 nmol kg(-1)), UFH (30 nmol kg(-1)) or SP (30 nmol kg(-1)). In this arterial model in which platelet thrombus was formed on a thrombogenic surface under a constant high shear rate, UFH and SP had moderate antithrombotic effects while SanOrg123781A exhibited a strong, dose-dependent inhibitory activity on platelet adhesion and platelet thrombus formation. In contrast to UFH, SanOrg123781A did not modify the activated partial thromboplastin time (aPTT) even at 30 nmol kg(-1), but strongly inhibited thrombin generation. At the same dose, despite a lower antithrombotic activity than SanOrg123781A, UFH significantly affected all the coagulation parameters. Taken together, these results show that SanOrg123781A, due to its potent and selective antifactor Xa and antifactor IIa activities is a promising new antithrombotic agent even in arterial setting.

Extending the role of antithrombotic agents: an example based on the low-molecular-weight heparin, tinzaparin.

We illustrate a strategy for developing an antithrombotic agent based on the low-molecular-weight heparin (LMWH) tinzaparin experience. After anti-factor Xa and IIa activity pharmacokinetic characterization in healthy volunteers, clinical studies first explored the doses and then confirmed thrombosis prevention effects in postoperative (general and hip or knee replacement surgery) settings as compared with placebo and active treatments. This experience and additional dose-finding assessments led to large clinical studies verifying the effectiveness of tinzaparin in the treatment of deep vein thrombosis and acute pulmonary embolism. Subgroup analyses from these studies and preclinical experiments suggested a role for tinzaparin in patients with malignancy who are at high risk for thromboembolic complications. Challenging patient populations and other thrombotic disorders spawned interest in tinzaparin studies in the obese, the hemodialysis, the stroke, peripheral angioplasty, and the pregnant patient as well as in children with thromboembolic disorders. New therapeutic challenges were addressed with a bridging study for patients requiring interruption of oral anticoagulant therapy, a study of patients undergoing cardioversion for atrial fibrillation, and outpatient venous thromboembolism treatment studies. Efficient antithrombotic development programs not only build on traditional indications but elaborate on new therapeutic hypotheses generated from clinical studies, new therapeutic areas, and on-going basic science research programs.

Review of bemiparin sodium – a new second-generation low molecular weight heparinand its applications in venous thromboembolism

Bemiparin sodium (Hibor®, Ivor®, Zivor®, Badyket®, Laboratorios Farmaceuticos Rovi SA) is a new second-generation low molecular weight heparin (LMWH). Bemiparin has the lowest mean molecular weight (3600 Da), the longest half-life (5.3 h) and the largest antifactor Xa:antifactor IIa ratio (8:1) of all LMWHs. Bemiparin promotes a greater release of tissue factor pathway inhibitor than unfractionated heparin (UFH) or dalteparin. These properties could result in a more favourable efficacy:safety ratio than the currently marketed LMWHs. Bemiparin 2500 IU/day was as effective as UFH for preventing venous thromboembolism (VTE) in moderate risk abdominal surgery. Bemiparin 3500 IU/day significantly reduced VTE compared to UFH in high-risk hip replacement surgery. Bemiparin 3500 IU/day started postoperatively was as effective as enoxaparin 4000 IU/day started preoperatively in total knee arthroplasty, with a trend towards a lower rate of proximal deep vein thrombosis (DVT), pulmonary embolism and symptomatic VTE. In patients with acute DVT, bemiparin was more effective than UFH in thrombus mass reduction and at least as effective as UFH for the prevention of clinical recurrence. Bemiparin was as effective as UFH for clot prevention during haemodialysis. The use of bemiparin was associated with a lower incidence of major and minor bleeding as compared to UFH in abdominal surgery. When compared with enoxaparin in orthopaedic surgery, a lower rate of complications at injection site was observed.

Correlation of Point-of-Care ecarin clotting time versus activated clotting time with bivalirudin concentrations

To rapidly and specifically monitor the anticoagulant effect of direct thrombin inhibitors, a thrombin inhibitor management (TIM) point-of-care test was developed (PharmaNetics, Inc., Morrisville, North Carolina), based upon the ecarin clotting time (ECT). This method utilizes the enzyme ecarin (from the venom of Echis carinatus) to convert prothrombin to meizothrombin. Meizothrombin catalyzes the conversion of fibrinogen to fibrin and is rapidly inhibited by direct thrombin inhibitors, such as bivalirudin. This assay has the particular advantage of being relatively specific for the effect of direct thrombin inhibitors, because heparins are poor inhibitors of meizothrombin. 1,2 The TIM-ECT test card may provide a more accurate measurement of direct thrombin inhibitor activity than current point-of-care assays. The purpose of this study was to compare the TIM-ECT test and 2 available activated clotting time (ACT) methods with a central laboratory anti-Factor IIa assay for monitoring bivalirudin-mediated anticoagulation. ••• Consecutive patients who underwent non-emergency percutaneous coronary intervention (PCI) with bivalirudin at The Cleveland Clinic Foundation were enrolled into the study. The protocol was approved by the institutional review board, and all patients gave written informed consent. Patients were not allowed to receive lepirudin, argatroban, abciximab, epti fibatide, or tirofiban 72 hours before PCI. All unfractionated heparin was discontinued 12 hours before PCI and all fractionated heparins 24 hours before PCI. All patients received preprocedural aspirin. Information regarding patient demographics, laboratory data, procedural details, and serial postprocedural creatine kinase (CK, CK-MB) was prospectively collected. Clinical outcomes were assessed to 30-day follow-up. Arterial blood samples were collected at baseline (sample 1), after bivalirudin bolus and infusion initiation (sample 2), after glycoprotein IIb/IIIa inhibitor bolus and infusion initiation (sample 3), during the procedure (sample 4), and before sheath removal (sample 5). Samples were procured from a nonheparinized arterial sheath and were placed into a noncitrated tube (3 ml) and a 3.2% sodium citrate tube (2.7 ml). Immediately at collection, the noncitrated whole blood samples were tested with the Hemochron ACT (International Technidyne, Edison, New Jersey), CoaguChek Pro/DM ACT (Roche Diagnostics, Indianapolis, Indiana), and the TIM-ECT. After this, the citrated whole blood was tested with the TIM-ECT. The remaining citrated blood was promptly centrifuged, and the plasma was aliquoted into cryovials and frozen at 70°C for the bivalirudin concentration assay. The ACT measures the time for whole blood to clot in the presence of an activating substance. 3,4 The Hemochron tube-based system uses a magnet in a rotating glass specimen tube that contains diatomaceous earth.5 As the blood clots, the magnet is displaced from the tube’ s bottom, thereby activating a proximity switch. Hemochron ACT values were truncated at a maximum of 999 seconds. The Pro/DM ACT is a cartridge-based system that uses tissue factor and sulfatides as activating substances. 6 The blood sample is drawn by capillary action into the reagent chamber, where it mixes with the chemical activators. The time from application of blood to cessation of flow within the chamber is detected by an optical system and converted to an ACT equivalent. Pro/DM clotting times were truncated at a maximum of 500 seconds.

Maladie thrombo-embolique veineuse : médicaments antithrombotiques d’aujourd’hui et de demain

Two new classes of anticoagulants are actually developed which would change in the near future our strategies for the prevention and the treatment of venous thromboembolic events. These two classes are the anti-factor Xa and anti-factor IIa (direct antithrombin) agents. Among the anti factor Xa, the pentasaccharides are initiating their clinical use. Fondaparinux is a synthetic form of the natural pentasaccharide, its pharmacokinetics allows one s.c. administration/24 hours. It is active in prevention and treatment of venous thromboembolic and coronary thrombotic events. A modified form (idraparinux) whose pharmacokinetics allows one administration only once a week should have the same type of efficacy. Among direct antithrombin agents, hirudin and derivatives have been developed in the past decade with a limited use due to several drawback. More recently synthetic direct antithrombins modified to allow oral route have been developed, the most advanced in development, melagatran, is active in the prevention and treatment of venous thromboembolic and coronary thrombotic events. It could allow (if confirmed by clinical trials) a complete oral treatment of deep vein thrombosis without any biological monitoring. Melagatran is also active in the prevention of arterial thromboembolic events on atrial fibrillation. But other molecular forms of synthetic orally active direct antithrombin are also in development. Besides these important changes in our therapeutics which should appear in a near future, molecules aimed at other target are also tested: the most advanced are those antagonizing the initial phase of tissue factor activation of factor VII but other strategies are being tested such as stimulation of fibrinolysis. These new drugs at our disposal to treat venous thromboembolism should modify completely our handling of the patients. But additionally the numerous clinical trials necessary to prove the efficacy of the drugs, modify our understanding in the implication of the coagulation and in the physiopathogeny of thrombotic events.

Influence of Lepirudin, Argatroban, and Melagatran on Prothrombin Time and Additional Effect of Oral Anticoagulation

Oral anticoagulation (OAC) with coumarin derivatives decreases the activities of coagulation factors II, VII, IX, and X by inhibiting their vitamin K-dependent carboxylation (1). The effectiveness and safety of therapy with oral anticoagulants for primary or secondary prophylaxis of thromboembolism are usually monitored by the prothrombin time (PT), expressed as international normalized ratio (INR), prothrombin ratio, or percentage of normal (2)(3). INR was standardized through large international reference studies (4)(5)(6)(7). The direct thrombin inhibitors (DTIs) lepirudin and argatroban are used to achieve effective anticoagulation in patients with heparin-induced thrombocytopenia with or without thrombosis (type II) (8)(9)(10). Melagatran is currently under investigation in clinical trials (11)(12)(13)(14). DTIs prolong clotting times in PT assays and therefore interfere with oral anticoagulants (15)(16)(17)(18). During treatment of deep venous thrombosis, heparins or DTIs are switched to oral anticoagulants. During treatment for invasive diagnostics or surgery, patients on oral anticoagulant therapy may temporarily be switched to a DTI. Decreased thrombin activity in the plasma of these patients leads to prolongations of the PT (15)(16)(18). These additive effects make it difficult to adjust dosage of either of the drugs during concomitant use. In the case of heparins, additive effects are antagonized by addition of protamine or heparinase to PT reagents (19). For DTIs, such antagonists are missing. Antibodies against hirudin have been unsuitable for neutralizing the drug’s anti-factor IIa effects because of polyclonality, producing neutralizing or enhancing antibodies, depending on the individual (20)(21). Argatroban and melagatran are small molecules and have not been reported to be antigenic. Without the ability to eliminate the additive effects of DTIs and oral anticoagulants on PT, it is important to …

Further evidence that periodate cleavage of heparin occurs primarily through the antithrombin binding site

Porcine mucosal heparin was fragmented into low-molecular-weight (LMW) heparin by treatment of periodate-oxidized heparin with sodium hydroxide, followed by reduction with sodium borohydride and acid hydrolysis. Gradient polyacrylamide gel electrophoresis analysis showed a mixture of heparin fragments with an average size of eight disaccharide units. 1D 1H NMR showed two-thirds of the N-acetyl groups were lost on periodate cleavage, suggesting cleavage had occurred at the glucopyranosyluronic acid (Glc pA) and idopyranosyluronic acid (Ido pA) residues located within and adjacent to the antithrombin III (ATIII) binding site. The N-acetyl glucopyranose (Glc pNAc) residue was lost on workup. The Glc pA residue, within the ATIII binding site, is on the non-reducing side of the N-sulfo, 3, 6- O-sulfo glycopyranosylamine (Glc pNS3S6S) residue. Thus, periodate cleaved heparin should be enriched in Glc pNS3S6S residues. Two-dimensional correlation spectroscopy (2D COSY) confirmed that LMW heparin prepared through periodate cleavage contained Glc pNS3S6S at its non-reducing end. As expected, this LMW heparin also showed reduced ATIII mediated anti-factor IIa and anti-factor Xa activities.

Heparin pentasaccharide.

Fondaparinux (a synthetic heparin analogue) (Sanofi-Synthelabo; Paris, France and Organon Research; Oss, The Netherlands) is the subject of intense recent clinical evaluation for the prevention and treatment of venous and arterial thromboembolism. The drug replicates the sulphated antithrombin-binding pentasaccharide sequence in heparin and induces potent and specific antithrombin-mediated anti-Xa activity with excellent bioavailability and a long circulating half-life of 18 hours that makes it ideal for once-daily subcutaneous dosing. Its very short chain length ensures this heparin pentasaccharide (PS) is devoid of anti-factor IIa activity. No need for laboratory monitoring is anticipated. Fondaparinux does not cross-react ex vivo with the anti-platelet antibodies responsible for heparin-induced thrombocytopenia. Fondaparinux was evaluated in four large, randomized, placebo-controlled, double-blind phase III trials of deep vein thrombosis prevention after major joint surgery where the PS given after surgery was compared with a low molecular weight heparin (LMWH). LMWH was started before surgery in two comparisons and soon after surgery in the others. The trials shared the same blindly adjudicated efficacy and safety endpoints: efficacy was measured by recording subclinical deep vein thrombosis detected by screening with bilateral venography, plus clinically suspected and confirmed symptomatic thrombosis and embolism; safety was indicated by the rate of major bleeding. Bleeding was considered major if it caused death or reoperation, affected an internal organ, or was overt and associated with a bleeding index of 2 or more. By comparison with LMWH, 2.5 mg/d of the PS beginning 4 to 8 hours after wound closure reduced venous thromboembolism rates by 56% and 26% after elective hip replacement, 63% after knee replacement, and 62% after hip fracture surgery. In three studies and overall, the effect was statistically very significant and included similarly reduced rates of proximal deep vein thrombosis. In absolute terms, the DVT rates with PS are the lowest yet seen after major joint surgery. Trends toward more major bleeding with PS in three studies were statistically significant in one trial. PS did not increase risks from reoperation, internal bleeding, or death because of bleeding, because between-group differences were caused entirely by an excess of patients with a raised bleeding index. Post hoc analysis suggests this excess can be explained by too-early postoperative drug administration and may be avoided without loss of efficacy by giving the first PS injection 6 to 8 hours after surgery. Results of phase III treatment trials for DVT/PE will soon be available, but studies in coronary artery disease are less advanced.

Pharmacokinetics and pharmacodynamics of the prophylactic dose of enoxaparin once daily over 4 days in patients with renal impairment

The pharmacokinetics of the low-molecular-weight heparin enoxaparin were evaluated in 12 healthy volunteers and 36 patients with mild, moderate or severe renal impairment. Enoxaparin was administered once daily by subcutaneous injections at a dose of 40 mg for 4 days and venous blood samples were taken over a 5-day period to determine antifactor Xa and antifactor IIa activity and the activated partial thromboplastin time. Adverse events were also recorded. The results for anti-Xa activity showed that the rate of absorption of enoxaparin was similar across the four groups of study participants. The elimination half-life increased with the degree of renal impairment, and this relationship was more evident after repeated dosing. Anti-Xa exposure was not significantly different between healthy volunteers and patients with mild or moderate renal impairment, but was significantly increased in patients with severe renal impairment (creatinine clearance ≤30 ml/min). Anti-Xa clearance decreased with the degree of renal impairment after repeated dosing, but only the difference between patients with severe renal impairment and healthy volunteers was statistically significant, the clearance on Day 4 being 39% lower in patients with severe renal impairment than in healthy volunteers ( P=.0001). Anti-IIa activity was low in all study participants, and the activated partial thromboplastin time was not significantly different between the study groups. In conclusion, the clearance of enoxaparin is reduced in patients with severe renal impairment. Dose adjustment of enoxaparin may need to be recommended in these patients, but no recommendation can be made in patients with mild or moderate renal impairment.

SR123781A, a Synthetic Heparin Mimetic

SR123781A, a synthetic hexadecasaccharide comprising an antithrombin (AT) binding domain, a thrombin binding domain, and a neutral methylated hexasaccharide sequence, was obtained from glucose through a convergent synthesis. SR123781A showed high affinity for human AT (Kd = 58 +/- 22 nM) and was a potent catalyst of its inhibitory effect with regard to factor Xa (IC50) = 77 +/- 5 ng/ml - 297 +/- 13 U/mg) and thrombin (IC50 = 4.0 +/- 0.5 ng/ml - 150 +/- 30 U/mg). SR123781A which acted exclusively via AT (no effect via heparin cofactor II at a concentration of 6 microg/ml) inhibited thrombin generation occurring via both the extrinsic and intrinsic pathways in vitro in human plasma. SR123781A did not compete for 3H-heparin binding to PF4 and did not activate platelets in the presence of plasma from patients with heparin-induced thrombocytopenia. After intravenous or subcutaneous administration to rats, rabbits or baboons, SR123781A displayed prolonged anti-factor Xa and anti-factor IIa activity ex vivo. After intravenous injection to baboons, decreases of the anti-factor Xa and anti-thrombin activities were parallel and disappeared with the same pharmacodynamics. Intravenous administrations of SR123781A strongly inhibited thrombus formation in an experimental model of thromboplastin-induced venous thrombosis in rats with an ED50 value of 18 +/- 0.1 microg/kg (vs 77 +/- 3 microg/kg for heparin). SR123781A inhibited arterial thrombus formation induced on a silk thread in an arterio-venous shunt and in the vena cava (ED50 values of 225 +/- 10 and 27 +/- 8 microg/kg, respectively). Compared to standard and low molecular weight heparin and to presently used drugs, SR123781A exhibited a highly favourable antithrombotic/bleeding ratio therefore showing that it might be considered as a promising compound in the treatment and prevention of various thrombotic diseases.