Abstract 264: Differential Profiles of Thrombin Inhibitors (Dabigatran, Hirudin, Bivalirudin and Heparin) in the Thrombin Generation Assay (TGA) and Thromboelastography (TEG) in Vitro
Thrombin is a central enzyme in haemostasis and thrombosis, and a proven target for anticoagulant therapies. Different classes of thrombin inhibitors, while exerting therapeutic benefits in most clinical trials, have different indications, dosing regimens, and bleeding complications. To gain more insight into the underlying mechanisms for their differential clinical profiles, we compared four marketed and representative agents, including dabigatran, hirudin, bivalirudin (direct thrombin inhibitors, DTIs), and heparin (an indirect thrombin inhibitor), in two in vitro spike-in assays with concentration titrations covering their therapeutic ranges. The two assays were the Thrombinoscope TGA with plasma, triggered by low tissue factor (1 nM TF), and TEG with whole blood, triggered by 1:8000 Recombiplastin (equivalent to low TF), with or without a threshold level of tPA to induce fibrinolysis. In TGA, the largest effect was prolongation of lag time, with the potency of the three DTIs rank-ordered as hirudin>dabigatran>bivalirudin; regarding peak, slope, and ETP, while complete inhibition was achieved with 1-2 μM dabigatran or hirudin, bivalirudin had no effect even at 4 μM, possibly due to its short half life in plasma. In TEG, the three DTIs prolonged clotting time (R) in the same rank order as TGA; for clot strength (MA), while all four agents reduced MA in synergy with tPA, only hirudin reduced MA without tPA, likely due to its highest potency. With tPA-induced fibrinolytic activity (Ly30), dabigatran and bivalirudin enhanced Ly30 (dabigatran>bivalirudin), but hirudin and heparin did not. This contrast might involve differential access to clot-bound thrombin. Heparin had a steep dose-response curve for both lag time in TGA and R in TEG, which is in line with its very narrow therapeutic index. All three DTIs, but not heparin, displayed the previously reported paradoxical increase in peak and slope in TGA in the low concentration range, suggesting this is indeed a class effect of DTI. In summary, our observations highlight the distinct features of each agent in thrombin generation, coagulation, and fibrinolysis. These results in combination with known clinical properties are informative on efforts to define the optimal profiles of new anticoagulants.
- © 2012 by American Heart Association, Inc.