Differential profiles of thrombin inhibitors (heparin, hirudin, bivalirudin, and dabigatran) in the thrombin generation assay and thromboelastography in vitro

Thrombin is a central enzyme in hemostasis and thrombosis, and a proven target for anticoagulant therapies. We compared four marketed and representative thrombin inhibitors, heparin, hirudin, bivalirudin, and dabigatran, in in-vitro spike-in assays that covered their therapeutic ranges. The assays employed were low tissue factor (1 pmol/l)-triggered thrombin generation assay (TGA) with plasma and 1:8000 Recombiplastin-triggered thromboelastography (TEG) with whole blood, with or without tissue plasminogen activator (tPA)-induced fibrinolysis. The three direct thrombin inhibitors (DTIs) prolonged TGA lag time and TEG clotting time (R) with a potency stack-ranking of hirudin>dabigatran approximately equal to bivalirudin. Heparin had the most steep concentration-response curve for both parameters. In TGA, 1-2 μmol/l dabigatran or hirudin resulted in complete inhibition on peak, slope, and endogenous thrombin potential, whereas bivalirudin had no effect on these parameters up to 10 μmol/l. All three DTIs, but not heparin, displayed a paradoxical increase in peak and slope in the low concentration range. In TEG, whereas all four agents reduced clot strength (maximal amplitude) in synergy with tPA, hirudin was the only DTI that reduced maximal amplitude appreciably without tPA. Dabigatran had the strongest potentiating effect on tPA-induced fibrinolytic activity (Ly30). With regard to the effects on coagulation and clot strength (lag time, R, and maximal amplitude) in the respective therapeutic range, dabigatran elicited the most modest changes. In summary, our observations highlight the distinct features of each agent in thrombin generation, coagulation, and fibrinolysis. The contrasts between the agents are consistent with their known properties and are informative on efforts to define the optimal profiles of new anticoagulants.