Article in press - uncorrected proof

J Lab Med 2009;33(2):121–127 2009 by Walter de Gruyter • Berlin • New York. DOI 10.1515/JLM.2009.023 2009/23

Ha¨ mostaseologie Redaktion: C. Schambeck

Factor II- and factor Xa-inhibitors for prevention and treatment of thromboses Faktor II- und Faktor Xa-Inhibitoren in der Pra¨ vention und Therapie von Thrombosen

Matthias Orth* tiven Thromboseprophylaxe sowie die Mo¨ glichkeiten eines Dosismonitorings vorgestellt. Vinzenz von Paul Kliniken gGmbH, Institut fu¨r Laboratoriumsmedizin, Stuttgart, Germany Schlu¨ sselwo¨ rter: Anti Xa-Inhibitoren; aPTT; direkte Thrombininhibitoren; Ecarin chromogener Assay; Thera- Abstract piekontrolle; -induced Clotting Time (PiCT). The desired properties of an ideal should be reliable action, a defined, selective mode of action, a wide therapeutic window, and predictable pharmaco- Introduction kinetics. Numerous new substances were developed as a substitute for oral and . Owing Major surgery, such as total hip replacement and total to their superiority in clinical studies, some of these drugs knee replacement, is associated with a very high risk of have already been approved. The hemostaseological developing deep vein thrombosis (DVT) and pulmonary laboratory is now challenged to detect a sufficient anti- embolism. Certain medical conditions, such as chronic coagulation while avoiding bleeding by overdosing. The (AF), are associated with embolic stroke. purpose of this review is to describe novel anticoagulants Therefore, prophylactic anticoagulation is the standard of in current use and the possible tests for drug monitoring care in these patients and is supported by Grade 1A in -induced thrombocytopenia and in the peri- evidence recommendations w1x. Thromboprophylaxis has operative prevention of thromboses. been obtained either by the injection of heparin or other pentasaccharides or by the oral administration of vitamin Keywords: anti-Xa inhibitors; aPTT; direct K antagonists, in particular . Heparin, a inhibitors; ecarin chromogenic assay; monitoring; polydisperse mixture of glycosaminoglycan chains, is prothrombinase-induced clotting time. used without fractionation (‘‘unfractionated heparin’’, UFH) or after chemical degradation and fractionation Zusammenfassung (‘‘low molecular weight heparin’’, LMWH). In most cases, Ziel eines idealen Antikoagulanz ist eine zuverla¨ ssige sufficient anticoagulation can be achieved by once a day Wirkung, ein definierter selektiver Wirkungsmechanis- administration of oral anticoagulants or of LMWH. UFH, mus, eine große therapeutische Breite und eine vorher- an alternative to LMWH, is used less frequently owing to sagbare Pharmakokinetik. Als Ersatz fu¨ r die oralen the much higher risk of developing heparin-induced Antikoagulantien und Heparine wurden verschiedene thrombocytopenia, a potentially life threatening situation w x neue Substanzen entwickelt, und aufgrund ihrer U¨ berle- 2 , and because of multiple injections needed daily. The genheit in klinischen Studien teilweise bereits zugelas- advantage of UFH is the rapid onset of action. The readily sen. Das ha¨ mostaseologische Labor ist nun gefordert, available drug monitoring, and the availability of an eine ausreichende Antikoagulation festzustellen bzw. antidote, which makes it the treatment of choice in U¨ berdosierungen zu erkennen. In diesem Review werden situations, such as patients with renal insufficiency or in die neuen Antikoagulantien zur Therapie der Heparin- cardiac surgery. induzierten Thrombozytopenie (HIT2) und zur periopera- Coagulation monitoring has formed the cornerstone to the effective management of patients receiving oral anti- *Correspondence: PD Dr. Matthias Orth, Vinzenz von Paul coagulants (‘‘vitamin K antagonists’’, VKAs), but the mon- Kliniken gGmbH, Institut fu¨ r Laboratoriumsmedizin, Adlerstr. 9, itoring of VKAs can be challenging: VKAs suffer from a 70199 Stuttgart, Germany very narrow therapeutic range. In addition, the pharmaco- Tel.: q49 (0711) 64892760 Fax: q49 (0711) 64892688 kinetics is effected by two common polymorphisms of E-Mail: [email protected] the VKORC1 and the CYP2C9 genes w3x. However, only Article in press - uncorrected proof

122 Orth: Factor II- and factor Xa-inhibitors

;40% of the individual dose needed for stable antico- techniques (in particular for replacing molecules formerly agulation can be explained by these polymorphisms. obtained from animal sources) and combinatory chem- Other factors are very important, such as vitamin K istry. The latter embrace small molecules designed derived from the diet, the absorption of vitamin K from specifically either to block the catalytic activity of coagu- the chyme (which is affected by, e.g., pancreas insuffi- lation enzymes or to interact directly with anticoagulant ciency) and the production of vitamin K by intestinal proteins. bacteria (which can be easily diminished by antibiotic Different experimental models of blood coagulation, treatment). Owing to the slow onset of action, its unpre- such as the ‘‘waterfall-cascade’’, the ‘‘autoprothrombin dictable pharmacology and numerous food and drug model’’, the cell-based model of coagulation and the interactions, monitoring and dose adjusting of VKAs is cumulative 4-component model wtissue factor (TF)-medi- mandatory and is achieved by international normalized ated thrombin generation, TF activation of the blood ratio (INR) testing from citrated plasma in the hemo- coagulation proteome, TF-activated and contact path- staseological laboratory, or less frequent, from whole way-inhibited whole blood in vitro, and bloodshed form (capillary) blood in a point-of-care testing (POCT) setting. microvascular wounds in vivoxw6x, were the basis for the While the therapy with UFH is routinely adjusted either development of substances able to modulate the coagu- by the aPTT or the thrombin time, monitoring of LMWH lation pathway at almost every step. is only performed in a subset of patients. The test per- In short, replacing heparin or VKA in the perioperative formed, anti-FXa, measures the inhibition of the activated prophylaxis of venous thrombo-embolism (VTE) or the coagulation factor Xa. Monitoring of LMWH therapy is prevention of stroke in chronic atrial fibrillation (CAF) has advisable during pregnancy, obesity and renal insuffi- not been the primary focus in the development of sub- ciency (however, kidney failure is a contraindication for stances inhibiting tissue factor/FVII, FIX, or FVIII. The some LMWH). Given highly variable inter- and intra-indi- potential advantages of these ‘‘upstream targets’’ are vidual pharmacokinetics due to subcutaneous injection, additional features, such as the inhibition of metastatic due to absorption of the highly charged heparin to sev- disease (e.g., in rNAPc2), or the effects on sepsis (e.g., eral surfaces, and due to the heterogeneity of heparins, in rTFPI). The number of these drugs under current devel- the results of a single anti-FXa analysis in the monitoring opment is very small. of LMWH therapy can only give an estimate for adjusting Current clinical data support that factor IIa and factor the correct dosing. From an analytical standpoint, how- Xa are the ideal targets for anticoagulation in terms of ever, the testing per se is highly reproducible with a varia- efficacy and safety, and numerous drugs were developed tion coefficient between 1% and 7%. A disadvantage of or are under current development to modulate thrombin heparins is the dependence of raw material from and factor Xa. animal sources: most of the unprocessed heparin is First evaluation of novel anticoagulants was carried out obtained from porcine mucosa and the recent adultera- in short-term indications, such as the prevention VTE after tion of these raw materials led to a worldwide recall of total hip or knee replacement surgery. This approach many heparins w4x. is ideal for anticoagulant drug development, because The significant limitations of VKAs and of heparins and studies with well-defined efficacy (proof of principle) and the increasing need for safe anticoagulation owing to the safety endpoints can be carried out in a population in higher incidence both of hip and knee surgery and of AF which there is a relatively high rate of VTE events, and in older patients due to the current demographic devel- bleeding can be monitored and controlled in a hospital opment (the current number of patients with AF is 5.8 environment w7x. As a consequence of the short-term Mio and this will increase to ;11 Mio in 2040 w5x), foster observation periods so far, the new oral anticoagulants the development of new anticoagulants for both short- are only approved for the postoperative anticoagulation. term and long-term use. The ideal anticoagulant should However, long-term studies (phase III studies) are cur- be for oral administration, should have a qualified repro- rently performed and approval for the outpatient use ducible production process, should be free from non- (such as AF) is expected for some of these drugs in hemorrhagic side effects, should have predictable the near future w8x. This will be a new challenge for pharmacokinetics and pharmacodynamics, should not the hemostaseological laboratory: these drugs have suffer from interaction with drugs or food intake, should the potential to interfere with many coagulation tests in have a wide therapeutic window, should have an appro- current use and anticoagulation monitoring will be nec- priate dose selection for the indication of interest and no essary under certain conditions, such as compliance requirement for monitoring while giving a reliable anti- control, in cases of overdose and for tapering therapy coagulation effect. Advantageous is a specific (and pre- from one agent to another. dictive) mode of action and, in particular in substances In the following, the novel anticoagulants used for with long half-life, the availability of a safe antidote w6x. perioperative anticoagulation and for the treatment of Around 60 years ago, heparin and were heparin-induced thrombocytopenia (HIT2) as well as their detected by chance and have been used for decades potential interferences with hemostaseological tests will before their mode of action was discovered. Today, be reviewed. For a review of the effects of novel anti- structure-based drug design enables the development of coagulants in patients with coronary symptoms, see Di the novel anticoagulant drugs employing recombinant Nisio et al. w9x. Article in press - uncorrected proof

Orth: Factor II- and factor Xa-inhibitors 123

Direct thrombin inhibitors regard to fewer VTE and major bleeding, and because hepatotoxicity and major adverse cardiovascular events Direct thrombin inhibitors (DTIs) act independently of were observed in several studies and mela- and can inhibit thrombin bound to fibrin or gatran were withdrawn from all markets and from further fibrin degradation products. Bivalent DTIs (including development in February 2006 w14x. and ) block thrombin at the active site Since 2008, is approved for VTE prevention and at exosite 1. Univalent DTIs, such as , after total knee replacement or total hip replacement melagatran (and its precursor ximelagatran), and dabi- w15x. No significant differences in the incidence of liver gatraban bind at the active site only. Native hirudin and enzyme elevation and acute coronary events were recombinant hirudins ( and desirudin) form an observed across this phase III program. Current dosing irreversible 1:1 stoichiometric complex with thrombin. is approximately 220 mg daily. An ongoing phase IIII The synthetic hirudin bivalirudin, however, binds at the study (RE-LY) for the prevention of stroke in patients with active site and at exosite 1. After binding, it is cleaved AF compares the effects of dabigatran vs. . A by thrombin and the active site functions of thrombin are total of 18,000 patients are enrolled in the study and restored making bivalirudin a reversible DTI. Similar to results are expected in 2010 w16x. bivalirudin, argatroban and melagatran dissociate from Perioperative application of thrombin inhibitors thrombin leaving a small amount of free thrombin avail- requires more caution than the application of LMWH w9x: able for hemostatic interaction w9x. timing of the first dose has to be early enough for suffi- DTIs also have an antiplatelet effect by reducing cient prevention of VTE but dosing has to consider the thrombin-mediated activation of platelets. DTIs do not increased bleeding risk, which is of particular concern in bind to other plasma proteins, which gives them a more patients with intracerebral surgery or from peridural predictive response than UFH and, because of their inhi- catheters. bition of fibrin-bound thrombin, make them more effec- tive than LMWH. Factor Xa inhibitors The plasma half-life of recombinant hirudins w9, 10x, bivalirudin and argatroban w11x is short (from 25 to Thrombin has other important roles besides its effects on 60 min). The substances are cleared by the kidneys with plasmatic coagulation, such as the promotion of platelet the exception of argatroban, which is cleared by the liver. activation via activation of protease-activated receptors The primary use of these substances is the anticoagu- on the platelet. Thrombin is a potent vasoconstrictor and lation of patients with HIT2. In these patients, the dis- a mitogen. Therefore, selective inhibition of FXa may be continuation of heparin is mandatory and the use of a better target than thrombin because FXa has fewer alternative anticoagulants is recommended. The rationale functions outside the plasmatic coagulation system, and for the use of DTIs is the intense thrombin activity by not inhibiting thrombin activity directly such agents observed in these patients w12x. might allow traces of thrombin to escape neutralization Lepirudin and argatroban are approved for HIT2. Both and thus inhibition of FXa may have effi- substances are effective in reducing the rate of throm- cacy at doses that are not associated with excessive botic events from 15% (in historic controls) to approxi- bleeding and may cause fewer side effects than the inhi- mately 4%. However, the rate of serious bleeding was bition of thrombin w17x. This hypothesis is supported by much higher in patients receiving lepirudin, both in com- large-scale, randomized studies with heparin-based parison to historic controls and in comparison to patients anticoagulants: the efficacy of different FII/FXa inhibitors treated with argatroban. Another drawback of lepirudin is over LMWH is higher in substances with a higher selec- its immunogenicity: approximately 40–50% of patients tivity for FXa compared to FII w7x. Factor Xa is generated develop antibodies to lepirudin within 5 days. These anti- via both the intrinsic and the extrinsic pathway and is bodies can prolong the half-life of lepirudin and lead to the rate-limiting step for the propagation of thrombin severe bleeding and, after re-exposure with lepirudin, generation. fatal anaphylaxis has been described. Neutralizing anti- Unlike thrombin inhibitors, direct FXa inhibitors have bodies are not observed. Argatroban does not appear to not been associated with rebound thrombin generation be immunogenic w13x. Bivalirudin is approved for anti- after termination of treatment. Rebound thrombin gen- coagulation during percutaneous coronary intervention eration is a phenomenon observed after treatment with only and is sometimes used for ‘‘off-label’’ treatment of different thrombin inhibitors, such as ximelagatran and HIT2. argatroban, with dalteparin (a LMWH), and with UFHs For stroke prevention in patients with AF or the pre- w18, 19x. vention of DVT after orthopedic surgery, few oral direct thrombin inhibitors have undergone clinical studies. Indirect factor Xa inhibitors in current use These include ximelagatran and dabigatran w14x. Xime- lagatran is a prodrug and its active metabolite melagatran , a fully synthetic molecule, is a sulfated has also been compared with regard to efficacy and pentasaccharide that is approved for preventing throm- safety versus enoxaparin and dalteparin. However, xime- bus formation in patients with acute coronary syn- lagatran and melagatran were not superior to LMWH with dromes, in the treatment of VTE, and in the prevention Article in press - uncorrected proof

124 Orth: Factor II- and factor Xa-inhibitors of VTE in patients undergoing major surgery. It is given Because idraparinux has a very long duration of action, subcutaneously in a single dose of 2.5 mg/day. Owing to a biotinylated version of this drug (SSR 126517) is also its renal clearance, renal insufficiency warrants special in clinical development for the prevention of stroke and caution. The risk of developing HIT2 is much lower than systemic embolism in cases of AF. The biotinylation does under UFH. However, as expected by its chemical struc- not change the half-life of the drug significantly and ture, there have been few reports of patients developing enables the application of a very efficient procedure in HIT2 w20x. case of bleeding episodes. The addition of the The , which is obtained from moiety would allow the rapid removal of the drug from similar animal sources as heparin, is approved for pro- the circulation by affinity chromatography employing avi- phylaxis and treatment of thrombosis in patients with HIT din. SSR 126517 is currently tested in phase III trials in in the European Union, Australia, and Canada. Danapa- comparison to VKAs. Noteworthy is the potential in vitro roid inhibits thrombin generation primarily by inhibition of interference of the biotinylated drug with a wide array of FXa with a half-life of ;24 h. A dose-response relation- tests currently in use in the clinical laboratory employing ship is predictable after intravenous as well as subcuta- biotin- technology. neous administration. Monitoring of treatment in therapeutic dose is required in patients with impaired Oral, direct factor Xa inhibitors in development renal function, very low or high body weight, life-threat- ening thrombosis, unexpected bleeding complications, Numerous oral, direct FXa inhibitors are in various stages or severe illness w21x. Similar to heparin, danaparoid of clinical development, such as LY517717 (Lilly), YM150 requires antithrombin for its major anticoagulant effect. (Astellas), DU-176b (Daiichi Sankyo), (Bristol- w x There are no antidotes for fondaparinux and danaparoid. Myers Squibb) 26 , and (Millenium Pharmaceuticals).

Direct factor Xa inhibitors in current use

All selective direct FXa inhibitors in current use are fully Monitoring of novel anticoagulants synthetic molecules. The only oral FXa inhibitor currently approved is . It is approved for the prevention Monitoring of FXa inhibitors of VTE after major orthopedic surgery. Rivaroxaban has The response of different FXa activity and clotting tests a half-life of up to 12 h in elderly subjects, and up to 9 h are very different for the FXa inhibitors in current use. For at steady state in healthy young subjects w22x. One-third example, under stable anticoagulation with fondaparinux, of the unchanged active drug is eliminated renally and the prothrombin time is increased only by approximately two-thirds of the drug is metabolized hepatically. For the 10% and the aPTT by approximately 20%, while anti- prevention of VTE after major orthopedic surgery, several coagulation with rivaroxaban prolongs the prothrombin studies showed higher efficacy for the prevention of both time by approximately 160% but the aPTT by 80% only asymptomatic and clinically relevant, symptomatic VTE w27x. and similar safety of oral rivaroxaban 10 mg compared In patients with fondaparinux and danaparoid, adjust- to subcutaneous enoxaparin 40 mg w23x. Large trials are ing the dose is necessary in patients with renal impair- currently under way to compare the effects of long-term ment and is recommended in older patients and in anticoagulation with rivaroxaban with warfarin in patients patients with very low or very high body weight. Special with acute, symptomatic proximal DVT (EINSTEIN-DVT caution must be given to thrombocytopenic patients and w x study) 8 and in patients with AF (ROCKET-AF study). to intensive care situations – both situations found fre- quently in HIT2. Similar to LMWH, fondaparinux and Parenteral, indirect factor Xa inhibitors danaparoid do not affect the prothrombin time and have in development only weak, if any, effects on aPTT. The activity and the concentration of these substances can be assessed by Idraparinux is a hypermethylated, long-acting pentasac- the anti-FXa assay (chromogenic or clot-based). How- charide, allowing once-weekly dosing. Idraparinux ever, the international standard that was developed for (2.5–10 mg once weekly) had a similar efficacy (com- measuring anti-FXa activity of LMWH should not be posite of VTE and all-cause mortality, and change in used: it is recommended that special calibrators and thrombotic burden) as warfarin and demonstrated dose- controls (which are commercially available) are used for dependent increases in major bleeding w24x. In a large the determination of anti-FXa in patients receiving fon- trial studying the lowest dose of idraparinux (2.5 mg once daparinux and danaparoid. Laboratory information on the weekly) (VAN GOGH study) in patients (ns2904) with type of anticoagulant used is mandatory when requesting DVT or pulmonary embolism (PE) who received treatment anti-FXa testing. for 3 or 6 months, more bleeding episodes were In the treatment of heparin-induced thrombocyto- observed and the target of non-inferiority for the treat- penia with danaparoid, anti-FXa should be F1.0 anti-Xa ment of PE was not reached w25x. units/mL 5–10 min after bolus injection and 0.5–0.8 anti- Article in press - uncorrected proof

Orth: Factor II- and factor Xa-inhibitors 125

Xa units/mL during steady-state infusion. Fondaparinux plasma levels cause only a minor change in the aPTT. To is not approved for this indication; therefore, no dosage avoid severe bleeding, the ECT or even preferable, the recommendations are given. ecarin chromogenic assay (ECA) is more reliable for DTI Rivaroxaban can be monitored by prothrombin time. monitoring under situations with high-dose therapy (e.g., Plasma concentration of rivaroxaban in blood drawn 4 h during cardiopulmonary bypass surgery) w30x. Recent after the intake of rivaroxaban shows a very close linear reports indicate that overdosing is a major problem with correlation with prothrombin time (in seconds). However, DTIs in HIT2 patients when using the recommended these data are only available for neoplastin (Roche Diag- dosing schemes and prevention of thromboses with nostica/Stago). Obviously, the Quick test (in%) as well as significantly lower concentrations of DTIs might be more the INR is not suited for this indication. aPTT and Heptest appropriate w21, 31x. are also prolonged after rivaroxaban but are not One pitfall of DTI dosing is a low prothrombin level. recommended. Anti-FXa testing is not suitable as no Severe acquired prothrombin deficiency, which can occur calibrators are available and no close correlation was with disseminated intravascular coagulation, with hepatic observed in therapeutic rivaroxaban concentrations w22x. dysfunction, or with VKA therapy, might result in elevated The Ecarin clotting time (ECT), a test for factor IIa, is not aPTTs, leading to inappropriate dose reductions of DTIs affected by therapeutic doses of rivaroxaban. and to thromboses w32x. If low prothrombin levels are The novel FXa inhibitors can interfere with antithrombin expected, lepirudin may be better monitored either by determination: antithrombin is either determined by direct measurement of lepirudin levels by ELISA w33x thrombin-based tests or by FXa-based tests. In FXa- (therapeutic range 0.5–1.0 mg/mL) or by the ECA, which based tests, excess FXa and heparin are added to the provides a linear dose-response curve for all DTIs inde- sample and the quantity of unbound FXa is determined, pendent of prothrombin concentration. The commercially e.g., by cleavage of a chromogenic substrate. The chro- available, automated ECA seems to overcome the inher- mogen released is inversely proportional to the anti- ent problems associated with monitoring of DTIs by aPTT thrombin activity of the plasma sample. The presence of and ECT w34x. FXa inhibitors in the plasma sample will decrease FXa Transition from DTIs to VKA can be challenging: where- and falsely indicate a higher antithrombin activity com- as lepirudin and bivalirudin have only minor effects, arga- pared to samples without FXa inhibitors (e.g., see w22x). troban causes substantial prothrombin time prolongation The difference, however, is not sufficient to use the anti- w29x. This can cause interpretative confusion and poten- thrombin assay for dose monitoring of rivaroxaban. tially triggering inappropriate plasma transfusions in the Thrombin-based antithrombin tests are preferred in transition to VKAs. The explanation for the disproportion- patients treated with selective FXa inhibitors. ate prolongation of prothrombin time for argatroban relates to its much higher molar concentrations required Monitoring of direct thrombin inhibitors for aPTT prolongation which is ;15- to 20-fold com- pared to lepirudin. Prothrombin time is sensitive to very DTIs with predominant renal clearance, such as recom- high molar concentrations of DTIs w21x. It is recommend- binant hirudins, melagatran and dabigatran, are likely to ed to stop argatroban and continue with VKAs only when, accumulate in patients with impaired renal function. under the combination of argatroban and VKA, the INR Patients with severe renal impairment have been exclud- has increased above 4 using Quick-type reagents. ed from clinical trials, and clinical data on dosing of DTIs Owrens-type reagents, which are used frequently in in renal impairment are scarce. Bivalirudin is only partially Scandinavia, Austria, and Japan, are only affected from excreted by the kidneys, but its half-life is prolonged with high dose therapy with argatroban. severe renal impairment. A comparable interference with antithrombin determi- Argatroban requires dose adjustment in patients with nation is also observed in DTIs: in thrombin-based tests, hepatic dysfunction owing to its hepatic clearance. Since (bovine) thrombin and heparin are added and the quantity there is no specific antidote for rapidly reversing the of unbound thrombin is determined, e.g., by cleavage of effect of DTIs, monitoring of patients at risk is important. a chromogenic substrate. The chromogen released is Preliminary data suggest that recombinant factor VIIa has inversely proportional to the antithrombin activity of the a limited capacity to reverse DTIs w28x. plasma sample. The presence of DTIs in the plasma sam- aPTT testing is appropriate for monitoring of low ther- ple will decrease thrombin and falsely indicate a higher apeutic doses of recombinant hirudins and argatroban. antithrombin activity compared to samples without DTIs. However, DTI effects on the aPTT are dependent on the FXa-based tests should be preferred in patients treated reagent and the clotting device used w29x. It is recom- with DTIs. mended that each laboratory generates its own standard Only limited experience is available with the prothrom- DTI dose-response curve for its aPTT reagent using binase-induced clotting time for the dosing of these ‘‘spiked’’ plasma to define the range over which the aPTT novel anticoagulants. This clotting assay is sensitive to reliably reflects changes in the DTI plasma concentration factor Xa and factor IIa inhibitors. An almost linear dose- w21x. At higher concentrations, the DTI dose-aPTT- response and high sensitivity of the assay in the one-step response curve flattens, and even major changes in modification was found for UFH, LMWHs, recombinant Article in press - uncorrected proof

126 Orth: Factor II- and factor Xa-inhibitors hirudins, and argatroban. Fondaparinux, however, administration of the factor Xa inhibitor rivaroxaban in the showed a non-linear dose-response w35, 36x and no data treatment of patients with acute symptomatic deep vein are available for rivaroxaban. thrombosis: the Einstein-DVT Dose-Ranging Study. Blood 2008;112:2242–7. 9. Di Nisio M, Middeldorp S, Buller HR. Direct thrombin inhib- itors. N Engl J Med 2005;353:1028–40. Conclusions 10. Taketomi T, Szlam F, Vinten-Johansen J, Levy JH, Tanaka KA. Thrombin-activated thrombelastography for evaluation Different novel specific inhibitors of FXa or of thrombin of thrombin interaction with thrombin inhibitors. Blood Coa- have been introduced previously for short-term indica- gul 2007;18:761–7. 11. Raaz U, Maegdefessel L, Buerke M, Janusch M, Werdan K, tions, such as the prevention of VTE after total hip or Schlitt A. 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