J. I. Weitz

Comparison of 1 month with 3 months of anticoagulation for a first episode of venous thromboembolism associated with a transient risk factor

Summary.  Background: The risk of recurrence is lower after treatment of an episode of venous thromboembolism associated with a transient risk factor, such as recent surgery, than after an episode associated with a permanent, or no, risk factor. Retrospective analyses suggest that 1 month of anticoagulation is adequate for patients whose venous thromboembolic event was provoked by a transient risk factor. Methods: In this double‐blind study, patients who had completed 1 month of anticoagulant therapy for a first episode of venous thromboembolism provoked by a transient risk factor were randomly assigned to continue warfarin or to placebo for an additional 2 months. Our goal was to determine if the duration of treatment could be reduced without increasing the rate of recurrent venous thromboembolism during 11 months of follow‐up. Results:  Of 84 patients assigned to placebo, five (6.0%) had recurrent venous thromboembolism, compared with three of 81 (3.7%) assigned to warfarin, resulting in an absolute risk difference of 2.3%[95% confidence interval (CI) − 5.2, 10.0]. The incidence of recurrent venous thromboembolism after discontinuation of warfarin was 6.8% per patient‐year in those who received warfarin for 1 month and 3.2% per patient‐year in those who received warfarin for 3 months (rate difference of 3.6% per patient‐year; 95% CI − 3.8, 11.0). There were no major bleeds in either group. Conclusion: Duration of anticoagulant therapy for venous thromboembolism provoked by a transient risk factor should not be reduced from 3 months to 1 month as this is likely to increase recurrent venous thromboembolism without achieving a clinically important decrease in bleeding.

Comparison of three-factor and four-factor prothrombin complex concentrates regarding reversal of the anticoagulant effects of rivaroxaban in healthy volunteers

Four‐factor prothrombin complex concentrates (PCCs), which contain factor II, FVII, FIX, and FX, have shown the potential to reverse the anticoagulant effect of rivaroxaban in healthy volunteers. The purpose of this study was to determine whether a three‐factor PCC, which contains little FVII, has a similar effect.

When and how to use antidotes for the reversal of direct oral anticoagulants: guidance from the SSC of the ISTH.

J . H . LEVY ,* W. AGENO,† N. C . CHAN,‡ M. CROWTHER ,§ P . VERHAMME ¶ and J . I . WE ITZ ,§ FOR THE SUBCOMMITTEE ON CONTROL OF ANT ICOAGULAT ION *Duke University School of Medicine, Durham, NC, USA; †University of Insubria, Varese, Italy; ‡Monash University, Clayton, Vic., Australia; §McMaster University and the Thrombosis and Atherosclerosis Research Institute, Hamilton, ON, Canada; and ¶University of Leuven, Leuven, Belgium

Laboratory assessment of the anticoagulant effects of the next generation of oral anticoagulants

Summary.  In contrast to vitamin K antagonists, which reduce the functional levels of several coagulation factors, the new oral anticoagulants specifically target either thrombin or factor Xa. These new agents have such predictable pharmacokinetics and pharmacodynamics that routine coagulation monitoring is unnecessary. However, there are still some situations in which measurement of anticoagulant effect may be required. The coagulation assays that are used to monitor heparin derivatives or vitamin K antagonists may not always accurately reflect the anticoagulant activity of the new oral anticoagulants, and specialized assays may be needed. In this article, we: (i) identify situations in which assessment of anticoagulant effect may aid treatment decisions; (ii) describe the effects of the new oral anticoagulants on the various coagulation tests; (iii) review the specialized coagulation assays that have been developed to measure the anticoagulant effects of the new oral anticoagulants; and (iv) provide a clinical perspective on the role of coagulation testing in the clinical management of patients treated with the new oral anticoagulants.

Medical device-induced thrombosis: what causes it and how can we prevent it?

Blood‐contacting medical devices, such as vascular grafts, stents, heart valves, and catheters, are often used to treat cardiovascular diseases. Thrombus formation is a common cause of failure of these devices. This study (i) examines the interface between devices and blood, (ii) reviews the pathogenesis of clotting on blood‐contacting medical devices, (iii) describes contemporary methods to prevent thrombosis on blood‐contacting medical devices, (iv) explains why some anticoagulants are better than others for prevention of thrombosis on medical devices, and (v) identifies future directions in biomaterial research for prevention of thrombosis on blood‐contacting medical devices.

Factor Xa or thrombin: Is thrombin a better target?

Summary.  The limitations of the vitamin K antagonists have prompted the development of new oral anticoagulants that target specific clotting enzymes. Most of the novel agents currently under development target either thrombin or factor Xa. As the final effector of blood coagulation and the most potent platelet agonist, thrombin is a logical target for new oral anticoagulants. Clinical trials with parenteral direct thrombin inhibitors revealed that the therapeutic window is wider with reversible inhibitors than with irreversible inhibitors. The results of clinical trials with ximelagatran, an orally active prodrug of melagatran, a reversible direct thrombin inhibitor, validate thrombin as a target. Although ximelagatran was withdrawn from the market because of hepatotoxicity, newer oral thrombin inhibitors, such as dabigatran etexilate, are filling the void. Several oral factor Xa inhibitors also are being tested. Is thrombin a better target for new oral anticoagulants than factor Xa? Only time will tell!

Oral apixaban for the treatment of venous thromboembolism in cancer patients: results from the AMPLIFY trial.

The AMPLIFY trial compared apixaban with enoxaparin followed by warfarin for the treatment of acute venous thromboembolism (VTE).

Elevated tissue factor procoagulant activity in CD133‐positive cancer cells

Clinical and experimental evidence suggests a parallel between increasing cancer aggressiveness and procoagulant tendencies, which are often attributable to high levels of tissue factor (TF). TF is upregulated on the surface of cancer cells and their derived microvesicles due to a combined impact of oncogenic events and tumor microenvironment [1–4] and thereby becomes involved in cancer progression, both as the principal initiator of the blood coagulation cascade and as a signaling receptor [1,2,5,6]. These TF activities are implicated in tumor growth, angiogenesis and metastasis [1,2,5,6], a finding congruent with the anticancer effects of anticoagulation revealed in recent clinical trials [1,7]. It remains unclear which cancer cells harbor biologically relevant TF. It has recently come to light that the capacity of cancer cells to initiate tumor growth is not universal, but rather can only be executed by a small minority of specialized cancer cells (fewer than 1%), often referred to as cancer stem cells (CSCs) [8]. Identification of CSCs in several solid tumors has been made possible by the recent discovery of their molecular markers, of which expression of CD133 (prominin-1) represents one of the best-known paradigms [9–12]. CD133 belongs to a family of five-transmembrane cell-surface glycoproteins commonly localized to membrane protrusions of various progenitors [9,13]. While CD133-positive cells have been identified as CSCs in several solid tumors, e.g. of the brain [10], colon [12] and in melanoma [11], it is unclear whether CD133 expression plays a causative, contributive or correlative role in the formation of the CSC population. Still, CD133-expressing CSCs have been implicated in a number of key processes, including tumor repopulation, resistance to therapy [14], increased aggressiveness [10] and angiogenesis [15]. It is intriguing to note that various recent reports point either to cancer cells with increased expression of TF or to cancer cells harboring the CSC marker CD133 as particularly central to cancer progression [4,16]. We reasoned that this may signify a deeper interrelationship between these two properties. In order to examine this in more detail, we tested the expression of CD133 and TF in the highly tumorigenic squamous cell carcinoma cell line A431, which is known to express considerable procoagulant activity [17]. We used Caco-2 cells as a positive control due to their curiously high CD133 expression [coupled with a paradoxically poor tumorforming capacity and undetectable TF expression (our unpubl. obs.)]. Interestingly, whole cell lysates of the pooled A431 cell population were found to contain appreciable amounts of TF [3], but surprisingly little detectable CD133 (Fig. 1A). However, flow cytometry analysis revealed that A431 are heterogeneous in that a small subset of these cells (0.5%) stained strongly with the CD133 antibody (Fig. 1B). In order to better understand the significance of this heterogeneity, A431 cells were separated immunomagnetically into CD133-positive and CD133-negative fractions (using the CD133 MACS system, Miltenyi Biotech, Auburn, CA, USA), each of which was then tested for TF content. Interestingly, CD133-positive A431 cells expressed a 5to 6-fold greater amount of TF antigen on their surfaces than their CD133-negative counterparts did (Fig. 1C) [3]. This was paralleled by a corresponding increase in the TF-dependent procoagulant activity (TF-PCA) [3,18] of CD133-positive A431 cells relative to cells lacking this stem cell marker. This latter assay measures the ability of the TF/ factor (F) VIIa complex to generate FXa-dependent, prothrombin-activating proteolytic activity [18] on the surface of A431 cancer cells. Thus we found that in a subset of A431 cancer cells CD133 is coexpressed with high levels of TF. As the CD133-positive (CSC) cancer cell subset is thought to drive tumor initiation/ formation events, we asked whether TF was required for the manifestation of these properties in vivo. Immunodeficient (SCID) mice were injected with A431 cells and treated with CNTO 859, a neutralizing TF-directed antibody that blocks Correspondence: Janusz Rak, McGill University, Montreal Children s Hospital Research Institute, Place Toulon, 4060 Ste Catherine West, PT-232, Montreal, Quebec, H3Z 2Z3, Canada. Tel.: +1 514 412 4400 ext. 22342; fax: +1 514 412-4331; e-mail: janusz.rak@mcgill.ca

The benefit-to-risk profile of melagatran is superior to that of hirudin in a rabbit arterial thrombosis prevention and bleeding model

Summary.  Although hirudin is better than heparin at preventing recurrent ischemia in patients with unstable angina, hirudin produces more bleeding. The purpose of this study was to use a rabbit arterial thrombosis prevention and ear bleeding model to determine whether for equivalent efficacy, melagatran, a synthetic direct thrombin inhibitor, is safer than hirudin. A combination of balloon injury and stasis was used to induce thrombosis in the distal aorta, and patency and blood flow were continuously monitored with ultrasonic flow probes. Rabbits were randomized to melagatran (in total doses of 78–313 nmol kg−1), hirudin (in total doses of 18–107 nmol kg−1), or saline over 90 min. To assess safety, blood loss from standardized ear incisions was measured. Both melagatran and hirudin produced dose‐dependent increases in patency and blood flow. At doses that maintained the highest levels of patency, however, melagatran produced 2–3‐fold less bleeding than hirudin. Thus, at maximally effective doses, melagatran causes less bleeding than hirudin in this model. These findings raise the possibility that some direct thrombin inhibitors are safer than others.

Real-world variability in dabigatran levels in patients with atrial fibrillation.

(Randomized Evaluation of Long-Term Anticoagulation Therapy). J Am Coll Cardiol 2014; 63: 321–8. 9 Douxfils J, Dogne JM, Mullier F, Chatelain B, Ronquist-Nii Y, Malmstrom RE, Hjemdahl P. Comparison of calibrated dilute thrombin time and aPTT tests with LC-MS/MS for the therapeutic monitoring of patients treated with dabigatran etexilate. Thromb Haemost 2013; 110: 543–9. 10 Douxfils J, Mullier F, Robert S, Chatelain C, Chatelain B, Dogne JM. Impact of dabigatran on a large panel of routine or specific coagulation assays. Laboratory recommendations for monitoring of dabigatran etexilate. Thromb Haemost 2012; 107: 985–97. 11 Antovic JP, Skeppholm M, Eintrei J, Boija EE, Soderblom L, Norberg EM, Onelov L, Ronquist-Nii Y, Pohanka A, Beck O, Hjemdahl P, Malmstrom RE. Evaluation of coagulation assays versus LC-MS/MS for determinations of dabigatran concentrations in plasma. Eur J Clin Pharmacol 2013; 69: 1875–81. 12 Skeppholm M, Hjemdahl P, Antovic JP, Muhrbeck J, Eintrei J, Ronquist-Nii Y, Pohanka A, Beck O, Malmstrom RE. On the monitoring of dabigatran treatment in “real life” patients with atrial fibrillation. Thromb Res 2014; 134: 783–9. 13 Hawes EM, Deal AM, Funk-Adcock D, Gosselin R, Jeanneret C, Cook AM, Taylor JM, Whinna HC, Winkler AM, Moll S. Performance of coagulation tests in patients on therapeutic doses of dabigatran: a cross-sectional pharmacodynamic study based on peak and trough plasma levels. J Thromb Haemost 2013; 11: 1493–502. 14 Rao RB. Regarding the effect of dabigatran plasma concentrations. J Am Coll Cardiol 2014; 63: 2885.