Background and objective Rivaroxaban – an oral, direct Factor Xa inhibitor – inhibits thrombus formation and growth in animal models. We have investigated the effects of rivaroxaban on thrombolysis because impaired fibrinolysis is a risk factor for venous thrombosis and it occurs more often in patients who had a myocardial infarction. As the propensity of a clot to be degraded depends on its structure, we tested the effects of rivaroxaban on clot structure and degradability by tissue plasminogen activator (t-PA). This was done in the absence and presence of thrombomodulin because the thrombin - thrombomodulin complex is the activator of TAFI (thrombin-activatable fibrinolysis inhibitor), a potent inhibitor of fibrinolysis. Methods Clots were formed in a microchamber in the presence or absence of rivaroxaban at pharmacological concentrations (0.15 and 0.25 μg/ml). Clot structure was analyzed by confocal microscopy, and permeability calculated by measuring flow rates. Degradation was evaluated by the amount of D-dimers in the eluate of clot perfused with t-PA, in the presence or absence of thrombomodulin. Results Microscopy showed that clots formed in the presence of rivaroxaban had thicker fibers and a looser fibrin structure with larger pores than controls, leading to increased permeation rate (Darcy constant 2.16-fold and 2.45-fold higher than controls with rivaroxaban at 0.15 μg/ml and 0.25 μg/ml, respectively). This clot structure modification renders the clots more susceptible to fibrinolytic enzymes. The degradation of clots containing 0.15 μg/ml of rivaroxaban was 3.6-fold higher than that of control clots, after 90 minutes perfusion with t-PA. In addition, when clots are formed in the presence of thrombomodulin, the degradability is decreased in control, while In the presence of rivaroxaban, fibrin degradation remains enhanced. Conclusion Rivaroxaban increased thrombolysis by t-PA. This was due to a decrease in thrombin generation. Two mechanisms are involved: modification of clot structure, which makes it more accessible to thrombolytic enzymes; and decrease in TAFI activation by the thrombomodulin–thrombin complex. This property of rivaroxaban may contribute to its antithrombotic effect.

1064 Poster Board I-86 Introduction: Decreased fibrinolysis has been reported in venous thrombosis. Thrombus degradation depends on its structure: thicker fibrin fibers are permeable to blood flow and highly susceptible to fibrinolytic enzymes, while thinner fibers are poorly permeable to flow and are resistant to fibrinolysis. Thrombin concentration present at the time of gelation profoundly influences fibrin clot structure: decrease in thrombin generation leads to the formation of thick fibrin fibers and to a decrease in activation of thrombin-activated fibrinolysis inhibitor (TAFI). Rivaroxaban, an oral direct factor Xa inhibitor, is in late stage clinical development for the prevention and treatment of venous and arterial thrombosis. The objective of this study was to evaluate the effect of Rivaroxaban on whole blood (WB) clot structure and degradability by t-PA. Compared to plasma clots, WB clots might better represent the in vivo formed thrombi. Methods: 1- Clots were formed by adding to WB or to corresponding plasma, low concentration of tissue factor and calcium in the presence or absence of Rivaroxaban at therapeutic concentrations (0.15 and 0.25 μg/ml). 2- Clot permeability was calculated by measuring the flow rate of liquid through the clot. It was expressed as Darcy constant. 3- Clot degradability was evaluated by D dimers generation during clot perfusion with plasminogen and tissue-type plasminogen activator (t-PA). Results: 1- In the absence of Rivaroxaban, WB clots had a lower porosity than that of corresponding plasma clots: Darcy constant of WB clots was 3.1 –fold lower than that of plasma clots. This decreased porosity of WB clots leads to thrombolysis resistance by preventing access of fibrinolytic enzymes to fibrin network: D dimers generation in t-PA-perfused clots for 60 min was 38 -fold lower in WB clots compared to plasma clots. 2- Rivaroxaban increased the permeation rate of WB clots and thrombolysis by t-PA: the addition of Rivaroxaban at 0.15 μg/ml in WB (corresponding in fact to plasma concentration of 0.25 μg/ml), increased the Darcy constant by 5.5 –fold and the clot degradability in 60 min by 108 -fold. These effects of Rivaroxaban were higher in WB clots than in corresponding plasma clots, as Rivaroxaban at 0.25 μg/ml in plasma clots increased the Darcy constant by 2.5-fold and clot degradation by 9.6-fold. In the presence of Rivaroxaban, the Darcy constant and the degradability of WB clots and of plasma clots were nearly identical. 3- To explain the greater efficacy of Rivaroxaban on WB permeation constants and thrombolysis in comparison to plasma clots a) we tested the possibility for Rivaroxaban to reduce the entrapment of red blood cells (RBC) into the network of fibrin as RBC can be responsible for fibrin pore occlusion. This possibility was excluded since Rivaroxaban had no effect on clot permeation rate in clots formed by clotting purified fibrinogen with thrombin in the presence or in the absence of RBC (condition in which there is no generation of thrombin): RBC induced a 2.5 times decrease in permeation rate due to entrapment of RBC into fibrin network, regardless of presence or absence of Rivaroxaban. b) we analyzed the effect of RBC on thrombin generation and its modification by Rivaroxaban: the addition of 0.1 ml RBC diluted ½ to 0,2 ml plasma increased the thrombin generation (540 % of control without RBC). This is probably due to exposure of phosphatidyl serine at surface of RBC during thrombin generation. The increase in thrombin generation by RBC was reduced to 140 % in presence of Rivaroxaban at 0.15 μg/ml. This is explained by Rivaroxaban's inhibition of factor Xa bound to cells. Conclusion: Thrombin generation was greater in WB than in plasma, leading to a lower porosity and degradability of WB clots as compared to plasma clots. Rivaroxaban, by decreasing thrombin generation, increased clot permeability and degradability to the same level in WB clots and plasma clots. This property of Rivaroxaban may contribute to its antithrombotic effect. This study received a support from Bayer-Schering-Pharma France. Disclosures: No relevant conflicts of interest to declare.