The paradoxical antifibrinolytic effect of dabigatran and argatroban in the presence of soluble thrombomodulin is unrelated to protein C-dependent increase of thrombin generation.

Background: Anticoagulants stimulate fibrinolysis in vitro, mainly by inhibiting thrombin-mediated TAFI activation. Surprisingly, however, direct thrombin inhibitors (DTIs) inhibit fibrinolysis and enhance thrombin generation in vitro when tested in the presence of high thrombomodulin (TM) concentrations. Because the paradoxical effect on thrombin generation was shown to be protein C (PC)-dependent, we investigated the role of PC in the antifibrinolytic effect of two DTIs, dabigatran and argatroban.

Methods And Results: In the presence of 10 nM TM, both dabigatran (0.5 μM) and argatroban (1 μM) prolonged clot lysis time and enhanced thrombin generation. This notwithstanding, the DTIs inhibited thrombin-mediated TAFI activation, peak TAFIa activity being reduced by >60%. A specific feature of TAFI activation curve in the presence of DTIs was a much slower disappearance of TAFIa activity, which was likely the cause of fibrinolysis inhibition. The addition of an anti-PC antibody (αPC) nullified the paradoxical effect of DTIs on thrombin generation but influenced neither TAFI activation nor the fibrinolysis time.

Conclusions: Our results suggest that the inhibition of PC activation by DTIs in the presence of TM, while enhancing thrombin generation, has no effect on thrombin-mediated TAFI activation. The inhibition of fibrinolysis by DTIs can be explained by the prolonged activation of TAFI resulting from the sustained release of thrombin from thrombin-DTI complex. While the clinical relevance of these findings needs to be investigated by in vivo studies, our data might help understanding the role of the different players in the regulation of thrombin generation, TAFI activation and fibrinolysis resistance.