Monitoring of anticoagulant effects of direct thrombin inhibitors

Monitoring of direct thrombin inhibitors with the activated partial thromboplastin time (aPTT) is limited by poor linearity and reproducibility. Recently, direct prothrombin activation methods have been developed for coagulation analysis: ecarin clotting time (ECT) and prothrombinase-induced clotting time (PiCT). Laboratory monitoring of the direct thrombin inhibitors lepirudin, argatroban, and melagatran was analyzed and compared with monitoring unfractionated heparin (UFH). Plasma samples of six healthy donors were spiked with lepirudin and argatroban extending to 3000 ng/mL, melagatran extending to 1000 ng/mL, and UFH up to 0.48 IU/mL. Clotting times of aPTT (two reagents), ECT, PiCT, and prothrombin time were determined in a KC 10, a micro instrument. At 3000 ng/mL ECT values were 339.1 +/- 25.0 seconds with lepirudin and 457.5 +/- 29.5 seconds with argatroban. ECT was 586.0 +/- 38.2 seconds with 1000 ng/mL melagatran. The PiCT method provided clotting times of 137.0 +/- 8.4 seconds with UFH, 128.0 +/- 23.4 seconds with lepirudin, and 151 +/- 23.9 seconds with argatroban, and 153.5 +/- 9.9 seconds with melagatran, with the concentrations mentioned. ECT is more sensitive to therapeutic drug concentration ranges than aPTT (prolongations of 3-7 versus 2-3). PiCT yields comparable results with direct thrombin inhibitors and UFH. This method could therefore be suitable for monitoring both drug groups.     

Direct thrombin inhibitors

Direct thrombin inhibitors interact with thrombin and block its catalytic activity on a wide range of substrates. Their action is in contrast to heparin and its derivatives, which inhibit thrombin and other coagulation serine proteases via antithrombin, and to the warfarin-type drugs that interfere with synthesis of the precursors of the coagulation serine proteases. There are three direct thrombin inhibitors approved for clinical use at present (lepirudin, bivalirudin, argatroban) and another in advanced clinical testing (melagatran/ximelagatran). The chemical structure, kinetics of thrombin inhibition, pharmacokinetics, and clinical use of each of these agents are discussed.     

Direct thrombin inhibitors in cardiac disease

Most acute coronary syndromes (ACS) are triggered by platelet-rich thrombus superimposed on disrupted atherosclerotic plaque. Thrombin and platelets both play a role in this process. Whereas unfractionated heparin and aspirin have served as cornerstones in the treatment of ACS, several limitations of heparin provide the impetus to seek out better anticoagulants. Direct thrombin inhibitors such as bivalirudin, hirudin, and argatroban offer several pharmacologic advantages over heparin. Additionally, bivalirudin also appears to provide clinical advantages over unfractionated heparin therapy in ACS patients and those undergoing percutaneous coronary intervention.     

Inhibition of thrombin generation in recalcified plasma.

Eight inhibitors of thrombin generation were compared in recalcified unfrozen plasma. Individual or pooled normal citrated plasma was supplemented on polystyrol flat-bottom wells (23 degrees C) with increasing concentrations of low-molecular-weight heparin, heparin, danaparoid, fondaparinux, hirudin, argatroban, corn trypsin inhibitor, or aprotinin. Thrombin was generated by addition of 5 microl fresh 250 mmol/l CaCl2 to 50 microl plasma in polystyrol flat-bottom wells and incubation for 20 min at 37 degrees C (recalcified coagulation activity assay). Arginine stopped hemostasis activation and then the generated thrombin activity was specifically quantified. The approximate 50% inhibitory concentrations of plasmatic anticoagulants for individual or pooled normal plasma are, respectively, 0.6 or 3.7 mIU/ml low-molecular-weight heparin, 0.3 or 1.6 mIU/ml heparin, 0.7 or 6.1 mU/ml danaparoid, 0.023 or 0.18 microg/ml fondaparinux, 75 or 230 pg/ml hirudin, 0.026 or 0.24 microg/ml argatroban, 1 or 2 U/ml corn trypsin inhibitor, and 2 or 4 KIU/ml aprotinin. The 50% inhibitory concentration values for corn trypsin inhibitor or aprotinin at plasmatic concentrations above 4-100 U/ml might increase pathologically the thrombin generation. The recalcified coagulation activity assay is a sensitive method to measure prothrombotic tendencies of blood or subtle concentrations of any plasmatic anticoagulant. It is suggested to analyze the individual patient's sensibility to certain plasmatic anticoagulants.     

Differential effects of direct thrombin inhibitors and antithrombin-dependent anticoagulants on the dynamics of clot formation.

New anticoagulants, including the direct thrombin inhibitors (DTIs) and fondaparinux, are increasingly replacing unfractionated heparin and enoxaparin. We examined the effects of argatroban (n = 60), bivalirudin (n = 44), heparin (n = 14), enoxaparin (n = 22), and fondaparinux (n = 24) on clot formation utilizing thromboelastography. Blood samples containing anticoagulants at clinically relevant concentrations were prepared ex vivo and analyzed using kaolin or tissue factor activation. Thromboelastography parameters of clot initiation (R), clot propagation (K and angle), clot rigidity (maximum amplitude) and clot elasticity (G) were compared between anticoagulants. Thromboelastography was also performed on blood from eight patients receiving anticoagulants. Each anticoagulant exerted significant concentration-dependent effects on R, K and angle. Only heparin, enoxaparin, and fondaparinux significantly affected maximum amplitude and G. Significant differences existed for all parameters between heparin and each anticoagulant and between fondaparinux and each DTI (P < 0.001), and for angle, maximum amplitude, and G between enoxaparin and each DTI (P < 0.008). Thromboelastography responses in ex-vivo samples and patient samples were comparable. In conclusion, whereas argatroban, bivalirudin, heparin, enoxaparin and fondaparinux each delay clot formation, the DTIs do not alter clot rigidity or elasticity. The reduced bleeding reported with DTIs versus heparin may relate to the fact that clots form with normal rigidity and elasticity.     

Ecarin based coagulation testing

Ecarin is derived from venom of Echis carinatus, and will activate prothrombin into meizothrombin which will then cleave fibrinogen to result in clot formation. Ecarin based testing has been described for decades, but these assays were typically restricted to reference or speciality coagulation laboratories. This test was initially described for the assessment of direct thrombin inhibitors (eg, bivalirudin lepirudin, or argatroban) and was not affected by heparins or heparinoids. Ecarin based assays were rarely used for anticoagulation monitoring until the emergence of the direct oral thrombin inhibitor dabigatran etexilate in 2010. As this test was mentioned in the prescribing information for dabigatran etexilate, there was increased interest for use by clinical laboratories as the preferred method for assessing the anticoagulant effect of this drug. The purpose of this document is to review the current status of ecarin based assays for assessing dabigatran. This is with the understanding that these methods can also be exploited for determining the anticoagulation effect of parenteral direct thrombin inhibitors, such as argatroban and bivalirudin.     

Effects of vitamin K antagonist phenprocoumon on activated partial thromboplastin time measurement of direct thrombin inhibitors.

The activated partial thromboplastin time (aPTT) is currently the most common test used to measure the anticoagulation intensity of heparins and direct thrombin inhibitors (DTIs). Vitamin K antagonists variably affect aPTT reagents. Interactions between heparin and DTIs occur during concurrent therapy. Three DTIs (lepirudin, argatroban, melagatran) and one unfractionated heparin (liquemin) were added to normal plasma (NP) samples (n = 23) and to vitamin K antagonist plasma (VKAP) samples (n = 23) of patients treated with phenprocoumon. Lepirudin and argatroban were added at concentrations from 300 to 3000 ng/ml, melagatran from 30 to 1000 ng/ml, and unfractionated heparin from 0.016 to 0.48 IU/ml. Wave parameters of clotting time and aPTT ratio curves were evaluated by multivariate analysis for inhibitors, aPTT reagents and NP and VKAP samples. Normal ranges resulting from NP samples were 34.5 +/- 1.0 s with Pathromtin SL and 33.9 +/- 0.8 s with Platelin LS. Normal ranges using VKAP were 52.8 +/- 2.6 s (Pathromtin SL) and 44.2 +/- 1.1 s (Platelin LS) (P < 0.0001). Variance analysis showed that inhibitors, plasmas (NP versus VKAP) and reagents influenced the wave characteristics of aPTT (s) (P < 0.0001) and aPTT ratios (P < 0.0001). Distinct statistical differences between aPTT reagents on one hand and normal versus vitamin K antagonist plasma on the other hand make a comparison of reported aPTT results difficult, especially during overlapping therapy with vitamin K antagonists.     

Recombinant activated factor VII effectively reverses the anticoagulant effects of heparin, enoxaparin, fondaparinux, argatroban, and bivalirudin ex vivo as measured using thromboelastography.

Bleeding is the major adverse reaction to anticoagulants, leading to significant morbidity and even mortality. Protamine is a specific antidote for heparin yet is only partially effective for enoxaparin, and the activated factor X inhibitor fondaparinux and the direct thrombin inhibitors argatroban and bivalirudin lack specific antidotes. We evaluated the ability of recombinant activated factor VII (rFVIIa), a general hemostatic agent, to reverse the anticoagulant effects of heparin, enoxaparin, fondaparinux, argatroban, and bivalirudin, as measured by thromboelastography. Whole-blood samples containing each test anticoagulant, with or without rFVIIa 1.5-4.5 microg/ml, were prepared ex vivo (n >or= 48, each anticoagulant) and analyzed by thromboelastography. The thromboelastography parameters of clot initiation, propagation, rigidity and elasticity were compared for the ex-vivo samples for each anticoagulant. The reversal ability of rFVIIa was also assessed using the standard clinical assay used to monitor each anticoagulant. Thromboelastography was performed on blood from eight stably anticoagulated patients, with and without exogenous rFVIIa. For each anticoagulant, rFVIIa significantly improved and, in some cases, completely normalized all thromboelastography parameters (P < 0.001). rFVIIa significantly (P < 0.01) decreased the activated partial thromboplastin time for argatroban-containing, bivalirudin-containing, or heparin-containing blood yet did not affect the anti-activated factor X levels for enoxaparin-containing or fondaparinux-containing blood. By thromboelastography, rFVIIa exerted generally similar reversal effects on the anticoagulated patient samples as on the ex-vivo samples. In conclusion, rFVIIa effectively reverses the anticoagulant effects of heparin, enoxaparin, fondaparinux, argatroban, and bivalirudin, and should be considered for patients with excessive bleeding associated with these anticoagulants.     

Greater inhibitory effects of bivalirudin compared with unfractionated heparin plus eptifibitide on thrombin-induced platelet activation

OBJECTIVES: The effects of antithrombotic agents on the activation of platelets by thrombin were determined in blood from patients (n=12) with symptomatic coronary artery disease. METHODS: Blood obtained immediately before cardiac catheterization was incubated in vitro with therapeutic concentrations of unfractionated heparin (1.2 and 2.0 U/ml) alone and in combination with eptifibatide (unfractionated heparin 1.2 U/ml+eptifibatide, 1.7 microg/ml) or bivalirudin (8 and 14 microg/ml). An activated clotting time was determined. Platelet activation was induced with thrombin (0, 5, 15, and 40 U/ml) and assessed with the use of flow cytometry. Fibrin polymerization was prevented by the peptide Gly-Pro-Arg-Pro. The activation of glycoprotein IIb-IIIa and surface expression of P-selectin were identified with antibodies (PAC-1 and anti-CD62). RESULTS: The activated clotting time was 258+/-13 s with 1.2 U/ml unfractionated heparin alone, 396+/-8 s with unfractionated heparin+eptifibatide, and 348+/-9 s with 8 microg/ml bivalirudin. The binding of PAC-1 (reflecting the potential to aggregate) was decreased most effectively by bivalirudin (percentage of platelets binding PAC-1 in response to 15 U/ml thrombin-unfractionated heparin=54+/-7%, unfractionated heparin+eptifibatide=12+/-4%, bivalirudin=1+/-0.3% of platelets, P<0.05 for bivalirudin compared with unfractionated heparin or unfractionated heparin+eptifibatide and unfractionated heparin+eptifibatide compared with unfractionated heparin). Bivalirudin prevented thrombin-induced surface expression of P-selectin more effectively than unfractionated heparin alone or unfractionated heparin+eptifibatide (percentage of platelets expressing P-selectin in response to 15 U/ml thrombin-unfractionated heparin alone=74+/-5%, unfractionated heparin+eptifibatide=53+/-7%, bivalirudin=1+/-0.3%, P<0.05 for bivalirudin compared with unfractionated heparin or unfractionated heparin+eptifibatide). CONCLUSION: Comparison between pharmacologic concentrations shown to be therapeutic demonstrated that bivalirudin inhibited thrombin-induced activation of platelets to a greater extent than unfractionated heparin alone or in combination with eptifibatide.     

Percutaneous interventions in patients with immune-mediated heparin-induced thrombocytopenia

The use of unfractionated heparin, the traditional antithrombotic agent during percutaneous coronary interventions (PCI), is associated with the risk of heparin-induced thrombocytopenia, a rare but often fatal clinical condition. This article focuses on several issues related to heparin-induced immune-mediated thrombocytopenia (HIT, type II) and alternative modes of periprocedural anticoagulation in patients with suspected or known HIT. The hypercoagulable state characterizing HIT, along with mechanical plaque disruption resulting from PCI place patients with HIT at particular risk of thrombosis during PCI. Given that a diagnosis of HIT precludes any further use of heparin, other treatment modalities are essential. Direct thrombin inhibitors are the drugs of choice in this challenging situation. These agents offer several advantages as anticoagulants for patients with HIT: (1) the ability to inhibit both thrombin that is bound to fibrin (clot-bound thrombin) and fluid-phase free thrombin; (2) rapid achievement of steady state; and (3) no cross-reactivity with HIT antibodies. Recent data on the use of bivalirudin, lepirudin, and argatroban in the setting of PCI in patients with HIT are encouraging. Optimal dosing regimens for argatroban, lepirudin, and bivalirudin should be further established in PCI patients.     

Hydroxyethyl starch enhances argatroban-mediated decreases in clot propagation and strength by diminishing thrombin-fibrinogen interactions.

Direct thrombin inhibitors (DTIs) have been administered for anticoagulation during cardiopulmonary bypass for patients with heparin-induced thrombocytopenia. While DTIs prolonged clot initiation and decreased clot propagation, clot strength did not change. Hydroxyethyl starches (HES), however, significantly decreased clot propagation and strength. We hypothesized that DTI with HES could significantly decrease hemostasis more than DTI alone. Plasma was exposed to 0 or 5 microg/ml argatroban with 0 or 30% dilution with 0.9% NaCl, 10% pentastarch or 6% Voluven. Additional argatroban-exposed samples diluted with HES had addition of alpha-thrombin (0.25 U/ml) and fibrinogen (150 mg/ml). Clot kinetics were determined via thrombelastography. While dilution with 0.9% NaCl significantly (P < 0.05) decreased the clot strength by 17% compared with samples only exposed to argatroban, dilution with pentastarch and Voluven significantly (P < 0.05) markedly decreased clot strength (53 and 78%, respectively). Voluven dilution significantly increased the time to clot initiation and decreased the velocity of clot propagation compared with samples only exposed to argatroban. Addition of alpha-thrombin/fibrinogen restored clot strength. DTI/HES administration diminished hemostasis to a greater extent than DTI exposure alone. Further investigation is warranted to determine whether this therapeutic approach can improve the safety of anticoagulation during cardiopulmonary bypass in patients with heparin-induced thrombocytopenia.     

Heparininduzierte Thrombozytopenie

Heparin-induced thrombocytopenia (HIT) is an adverse drug reaction that carries an increased risk of thromboembolic complications. HIT is caused by platelet-activating antibodies directed against a complex of platelet factor 4 (PF4) and heparin. HIT typically manifests in the second week after initiation of heparin therapy with a platelet count reduction of more than 50% of the highest level after the start of heparin administration as well as thromboembolic events. The clinical probability can be calculated by the 4 T’s score. The laboratory diagnosis of HIT is based on confirmation of PF4/heparin antibodies or on functional tests that provide evidence of heparin-dependent platelet-activating antibodies. A low 4 T’s score and negative HIT test virtually rule out the presence of HIT. Patients with acute HIT require anticoagulation with a compatible anticoagulant in a therapeutic dose. The drugs currently available for this include the direct thrombin inhibitors argatroban, lepirudin, bivalirudin, and desirudin and the indirect factor Xa inhibitors danaparoid and fondaparinux.     

Evaluation of intravenous direct thrombin inhibitor monitoring tests: Correlation with plasma concentrations and clinical outcomes in hospitalized patients

Journal of Thrombosis and Thrombolysis - The parenterally administered direct thrombin inhibitors (DTIs) argatroban and bivalirudin are effective anticoagulants for acute heparin-induced...     

Direct thrombin inhibitors in the treatment of immune-mediated heparin-induced thrombocytopenia

Heparin was first discovered in 1916 and at present is used in more than 12 million patients a year. In the 1950s, several physicians noticed an uncommon paradoxical phenomenon in which heparin appeared to function as a procoagulant instead of an anticoagulant. This phenomenon is now known as the immune-mediated heparin-induced thrombocytopenia (HIT) and thrombosis syndrome (HITTS). Our understanding of this syndrome has evolved over the last 2 to 3 decades, and therapeutic options are arising. This article will focus on the most extensively studied therapy for HIT, which is the class of drugs known as the direct thrombin inhibitors. Specifically, we will focus on the mechanisms by which direct thrombin inhibitors may be useful in this syndrome, the evidence for their use, and the unique characteristics of the two FDA-approved agents in this class, lepirudin and argatroban.     

The effects of glycosaminoglycans on coagulation: a thromboelastographic study.

Endogenous heparinoids impair coagulation, evidenced by thrombelastography in cirrhotic patients with bacterial infection, but it is not clear which glycosaminoglycans can be detected by native and heparinase-modified thrombelastography. To assess the effects of different glycosaminoglycans on thrombelastography parameters and the reversibility of these effects by heparinase-I-modified thrombelastography. Twenty volunteers were enrolled. Solutions of heparan sulphate, dermatan sulphate, and chondroitin-4-sulphate were prepared at 'equivalent' concentrations, based on the composition and anticoagulant activity of danaparoid. Serial dilutions of each glycosaminoglycan were prepared to achieve 1.0, 0.5, 0.1, and 0.05 U/ml. Native and heparinase-modified thrombelastography, anti-activated factor X activity and heparin cofactor II activity were evaluated at each concentration. A statistically significant heparin-like effect was seen with 1 and 0.5 U/ml heparan sulphate, and 1 and 0.5 U/ml dermatan sulphate, which was completely reversed by heparinase-modified thrombelastography. Anti-activated factor X activity was significantly increased in samples containing heparan and dermatan sulphates. The heparin cofactor II activity decreased with 1.0 and 0.5 U/ml dermatan sulphate and chondroitin-4-sulphate, but not with heparan sulphate. Heparan and dermatan sulphates affect haemostasis when added to whole blood in vitro, detectable by native thrombelastography and completely reversed by heparinase-I-modified thrombelastography. They may therefore be responsible for the heparin-like effect seen by thrombelastography in patients with cirrhosis and bacterial infection.     

Treatment of Patients with a History of Heparin-Induced Thrombocytopenia and Anti-Lepirudin Antibodies with Argatroban

Patients with heparin-induced thrombocytopenia (HIT) type II require anticoagulation with non-heparin immediate acting anticoagulants. Danaparoid may cross react with HIT-antibodies and lepirudin may generate anti-lepirudin antibodies influencing anticoagulation. We hypothesised, that the synthetic small molecular thrombin inhibitor argatroban does not induce immunoglobulins reacting towards lepirudin in patients with anti-lepirudin antibodies in the history and that titration of the anticoagulation may be easier with argatroban. We report on the treatment of four patients of a study, which was terminated prematurely due to official warnings for a repeated use of lepirudin. Two patients each received argatroban and lepirudin intravenously. A blinded assessor adjusted the doses of the anticoagulants to 1.5–3.0 fold prolongation of the aPTT. Ecarin clotting time (ECT), concentrations of lepirudin (ELISA) and of argatroban (gaschromatoraphy with mass spectrometry,) and the generation of lepirudin antibodies (ELISA) were measured. APTT-adjusted dosages for argatroban was 2.0–2.6 μg/kg⋅ min and for lepirudin 48–149 μ g/kg⋅ h. ECT was prolonged 2.1 to 4.5-fold with lepirudin and 4 to 7-fold with argatroban. The concentration of lepirudin ranged between 750 and 1500 ng/ml and of argatroban between 400 and 1100 ng/ml. Patients on argatroban did not generate immunoglobulin IgG reacting towards lepirudin in contrast to both patients on lepirudin who developed anti-lepirudin antibodies. Both treatments were well tolerated. Despite the low number of patients argatroban seems to lead to a more stable anticoagulant response than lepirudin resulting in a lower variability of the dosage for prophylaxis or treatment of thromboembolism of patients with a history of HIT and lepirudin antibodies. An erratum to this article can be found at .     

Hirudin in heparin-induced thrombocytopenia

Heparin-induced thrombocytopenia (HIT), a serious side effect of heparin treatment, requires alternative anticoagulation in most affected patients. The recombinant hirudin (r-hirudin) lepirudin has been approved for this purpose after two prospective trials in laboratory-confirmed HIT patients. Other drugs available for this purpose are danaparoid sodium (a heparinoid) and argatroban, a synthetic direct thrombin inhibitor. In this article, recommendations for optimal use of r-hirudin in HIT are given, covering therapy in uncomplicated patients as well as in special situations such as heparin reexposure of HIT patients. Because lepirudin's half-life depends on renal function, it may vary between 1 and 200 hours, which requires individual dose adjustments. Lepirudin compares favorably with danaparoid, based on retrospective data. No direct comparisons of lepirudin with argatroban are available, but argatroban might offer advantages in patients with renal failure, because it is mainly eliminated hepatically. Major hemorrhage, the main risk of lepirudin treatment, occurring in about 15% of patients, makes close monitoring important. New monitoring tools, such as the ecarin clotting time (ECT), might further reduce bleeding risks. Antihirudin antibodies, which can alter the pharmacokinetics as well as the pharmacodynamics of hirudin, can also be countered by close monitoring and appropriate dose adjustments. Whereas hirudins have not yet managed to gain importance in non-HIT indications such as unstable coronary syndromes, they have a major role to play in the treatment of HIT. The choice between the available drugs for HIT, namely lepirudin, danaparoid, and argatroban, has to be made according to the clinical presentation of the patient.     

Effects of recombinant activated factor VII on thrombin-mediated feedback activation of coagulation

Thrombin is a key hemostatic enzyme, which propagates its own generation by activating factors V, VIII, and XI. Sustained thrombin generation also activates thrombin-activatable fibrinolysis inhibitor (TAFI), which stabilizes fibrin clot against fibrinolysis. Recombinant activated factor VII (rFVIIa) is considered a novel hemostatic intervention for refractory bleeding, but rebleeding episodes related to fibrinolysis still occur. The present study aimed to investigate the antifibrinolytic effects of rFVIIa in relation to thrombin generation. Using thrombelastography, the effects of rFVIIa on thrombin-activated fibrin formation and on fibrinolysis induced by tissue plasminogen activator were evaluated in various factor-deficient plasma samples. A Thrombinoscope was used to quantitate thrombin generation. Thrombin increased antifibrinolytic activity in a concentration-dependent manner as demonstrated by a longer clot lysis time. In plasma deficient in factors V, VIII, IX, X, or XI, clot lysis occurred early (< 20 min), and rFVIIa addition had minimal effect, except for improved antifibrinolytic effect in factor-XI-deficient plasma. A normal clot lysis time was observed in factor-XIII-deficient or dual antithrombin/factor-VIII-deficient plasma. Inhibition of TAFI increased the rate of fibrinolysis. Thrombin generation was delayed or decreased in single factor-deficient plasma except for factor XIII deficiency. After rFVIIa addition, the peak thrombin generation reached over 100 nmol/l in factor-XI-deficient plasma, but not in plasma deficient in factors V, VIII, IX, or X. Thrombin generation and subsequent activation of TAFI were important for clot stability. We conclude that rFVIIa therapy does not compensate for increased susceptibility to fibrinolysis due to lack of factor(s) necessary for the formation of tenase and prothrombinase.     

Direct thrombin antagonists

BACKGROUND: Inhibiting thrombin as a key enzyme of the coagulation cascade is therapeutically useful in thromboembolic diseases. In coronary thrombosis, direct thrombin inhibitors promise to be useful for an efficacious therapy. Hirudin and recombinant or synthetic mimetics like hirulog, argatroban and melagatran have proven their efficacy in clinical studies. CONCLUSION: Therapy with direct thrombin inhibitors such as hirudin and analogous substances reduces coronary events. Moreover, the agents are useful for therapy of thromboembolic diseases, especially in the case of heparin induced thrombocytopenia type II.     

Large inter-individual variation of the pharmacodynamic effect of anticoagulant drugs on thrombin generation

Anticoagulation by a standard dosage of an inhibitor of thrombin generation presupposes predictable pharmacokinetics and pharmacodynamics of the anticoagulant. We determined the inter-individual variation of the effect on thrombin generation of a fixed concentration of direct and antithrombin-mediated inhibitors of thrombin and factor Xa. Thrombin generation was determined by calibrated automated thrombinography in platelet-poor plasma from 44 apparently healthy subjects which was spiked with fixed concentrations of otamixaban, melagatran, unfractionated heparin, dermatan sulfate and pentasaccharide. The variability of the inhibitory effect of the different anticoagulants within the population was determined using the coefficient of variation, i.e. the standard deviation expressed as a percentage of the mean. The inter-individual coefficients of variation of the endogenous thrombin potential and peak height before inhibition were 18% and 16%, respectively and became 20%-24% and 24%-43% after inhibition. The average inhibition of endogenous thrombin potential and peak height (ETP, peak) brought about by the anticoagulants was respectively: otamixaban (27%, 83%), melagatran (56%, 63%), unfractionated heparin (43%, 58%), dermatan sulfate (68%, 57%) and pentasaccharide (25%, 67%). This study demonstrates that the addition of a fixed concentration of any type of anticoagulant tested causes an inhibition that is highly variable from one individual to another. In this respect there is no difference between direct inhibitors of thrombin and factor Xa and heparin(-like) inhibitors acting on the same factors.