Inhibitory effect of various thrombin inhibitors on shear-induced platelet function and dynamic coagulation

Abstract

We assessed the effects of active site-directed, fibrinogen recognition exosite (FRE)-directed and bifunctional thrombin inhibitors, on shear-induced platelet reactivity (adhesion/aggregation) and dynamic coagulation (coagulation of flowing blood). An in vitro test for shear-induced haemostatic plug formation and dynamic coagulation (haemostatometry) was employed using non-anticoagulated rat blood. The active site-directed inhibitors (argatroban, P891, P899) caused inhibition of platelet reactivity and coagulation at 1-, 100- and 100-μM concentrations, respectively. Bifunctional inhibitors (P553, P1053) exerted inhibitory effects at 0.1 μM. A dimeric bifunctional inhibitor P824 caused significant inhibition at 1 μM. The FRE-directed inhibitor (P960) inhibited shear-induced platelet reactivity at 10 μM but the dynamic coagulation at 1 μM. Combination of active site-directed argatroban and FRE-directed P960 did not show any synergistic effect. The most potent inhibition was observed in monomeric bifunctional inhibitors. The inhibitory effects were compared with the K i values against human thrombin and with the IC 50 values against fibrin clot formation. The minimum effective concentrations on shear-induced platelet reactivity and dynamic coagulation were comparable with the IC 50 values, but not with the K i values.