Effect Of Dabigatran and Rivaroxiban On Thrombomodulin Mediated Activation Of Protein C and Thrombin Activated Fibrinolysis Inhibitor (TAFI). Potential Clinical Implications

Introduction The new oral anticoagulant agents Dabigatran and Rivaroxiban mainly target thrombin and factor Xa to mediate their anticoagulant effects respectively. The inhibition of thrombin also results in the modulation of thrombins regulatory functions including the thrombin thrombomodulin mediated processes whic play a central role in maintaining hemostasis. The inhibition of FXa also results in compromising Xa mediated regulatory functions which are mediated by protease activated receptors. This study was designed to investigate the effects of active form Dabigatran and Rivaroxiban on the activation process of protein C and TAFI by thrombin-thrombomodulin complex using various experimental approaches. Materials and Methods The active form of Dabigatran was synthesized. While Rivaroxiban was extracted from commercially available tablets. Both agents were dissolved in appropriate solution matrices at a stock concentration of 100ug/ml. Thrombin-thrombomodulin mediated activation of protein C and TAFI were measured using specific chromogenic substrate based methods at a concentration of 0-10ug/ml in different matrices. The activation of Protein C and TAFI was also measured using mass spectrometric (SELDI TOF) and immunoblotting methods. Additionally plasma samples from patients treated with either Rivaroxaban and Dabigatran were evaluated for various laboratory parameters including Protein C functionality and functional TAFI levels. Results Dabigatran produced a strong inhibition of the thrombin-thrombomodulin mediated generation of both the activated protein C and TAFI (IC50<1.0ug/ml) whereas Rivaroxaban did not produce any inhibition of the activation of either of these proteases. Dabigatran also inhibited the amidolytic actions of thrombin-thrombomodulin complex whereas Rivaroxiban did not produce any effect. Neither Dabigatran nor Rivaroxiban produced a direct inhibition of activated protein C or TAFI at concentrations of up to 10ug/ml. Mass spectrometric and immunoblotting methods showed that Dabigatran blocked the activation of both TAFI and activation of Protein C by thrombin-thrombomodulin complex. The plasma samples obtained from Rivaraoxaban treated patients did not show any decreased functioanlity of either the Protein C or TAFI as measured by chromogenic substrate methods. The inhibition of protein C and TAFI was found to be concentration dependent. Conclusions The persistant inhibition of thrombin and its regulatory effects by Dabigatran may differentiate its pharmacodynamic profile from Rivaroxiban. Since thrombin plays several regulatory functions, its persistent inhibition may compromise hemostatic regulation. The observed adverse events such as the reported higher incidence of myocardial infarction signal in Dabigatran treated patients and may be related to the indiscriminate inhibition of thrombin. Clinical Implications Beside the mediation of thrombus formation, thrombin and its complex with thrombomodulin play a very important role in the regulation of hemostatic processes. A persistent inhibition of thrombin may therefore lead to physiologic dysregulation which may result in adverse events such as bleeding and occlusive disorders. Disclosures: No relevant conflicts of interest to declare.