The Direct Thrombin Inhibitors Dabigatran and Lepirudin Inhibit GPIbα-mediated Platelet Aggregation

Objectives: The direct thrombin inhibitor (DTI) dabigatran is a non-vitamin K antagonist oral anticoagulant for the prevention of stroke and systemic embolism in patients with non-valvular atrial fibrillation. In addition to its antithrombotic efficacy it has been suggested that dabigatran treatment of patients with atrial fibrillation also exerts some prothrombotic effect due to fostering the ligation of thrombin to its high affinity receptor GPIbα on the platelet surface, thereby enhancing ristocetin-mediated platelet aggregation in whole blood. On the other hand, we provided evidence that a member of another class of DTIs, lepirudin, stimulates the inhibitory cGMP/soluble guanylate cyclase pathway in human platelets. Therefore, we investigated the effect of lepirudin and dabigatran spiked to platelets from healthy volunteers on GPIbα-mediated platelet aggregation and agglutination.

Methods: Lepirudin, active dabigatran or PPACK was spiked to isolated human and murine platelets (platelet-rich plasma, washed platelets). Platelet aggregation, platelet-based thrombin generation (calibrated automated thrombography according to Hemker HC et al. Thromb. Haemost. 2010; Jurk K et al. JTH 2011), cAMP/cGMP formation and VASP S239 phosphorylation were assessed in response to ristocetin, ristocetin/VWF or in response to the GPIbα-selective agonist echicetin-beads (Navdaev A et al. PLoSOne 2014). In addition, the effect of lepirudin and dabigatran on ADP- or on TXA₂ mimetic U46619-induced aggregation of human platelets in platelet-rich plasma was investigated.

Results: Ristocetin-induced aggregation of platelets in the presence or absence of plasma was significantly inhibited by lepirudin, dabigatran and PPACK. In contrast, ristocetin-mediated agglutination of washed platelets was not affected by the tested DTIs. The inhibitory effect of the tested DTIs on aggregation was confirmed by using the GPIbα-specific agonist echicetin-beads for human and murine platelets. Thrombin was not generated in response to GPIbα-mediated platelet activation and therefore did not explain the inhibitory effect of the DTIs. ADP- or TXA₂-induced platelet aggregation was not affected by the tested DTIs. Therapeutic concentration of lepirudin and dabigatran did not show any significant effect on platelet VASP S239 phosphorylation as well as on cGMP and cAMP levels.

Conclusions: Our data suggest that the DTIs lepirudin and dabigatran directly impaired GPIbα-dependent platelet activation. Thus, these DTIs may act not only as anticoagulants but also as distinct anti-platelet agents, facilitating their beneficial anti-thrombotic effects.