fibrinopeptide-a and napsagatran

Clot-bound thrombin remains active and is less accessible to heparin-antithrombin III than fluid-phase thrombin. To determine whether clot-bound human thrombin is more susceptible to inactivation by direct thrombin inhibitors, the activity of a novel synthetic competitive thrombin inhibitor Ro 46-6240, recombinant hirudin and unfractionated heparin were compared with fluid-phase thrombin and clot-bound thrombin. Fibrinopeptide A generated in human plasma was used as an index of thrombin activity. Hirudin was the most potent inhibitor of fluid-phase and clot-bound thrombin. However, Ro 46-6240 inhibited clot-bound thrombin three times more potently than fluid-phase thrombin (IC50 19 vs 56 ng/ml) while hirudin was two times (IC50 8 vs 3 ng/ml) and heparin six times (IC50 1,205 vs 200 ng/ml) less active against clot-bound thrombin compared with fluid-phase thrombin. The relative selectivity for clot-bound thrombin is not a unique property of Ro 46-6240 since two other synthetic thrombin inhibitors tested inhibited clot-bound thrombin more effectively than fluid-phase thrombin and a third was equally active against both forms of thrombin. In contrast, the affinities of two chromogenic substrates were similar for both forms of thrombin. This study shows that direct thrombin inhibitors inhibit clot-bound thrombin more potently than heparin and suggests that an apparent selectivity for clot-bound thrombin can be achieved with some synthetic thrombin inhibitors. Further studies have to show whether the high potency of these direct thrombin inhibitor translates into antithrombotic efficacy in clinical situations with pre-existing clots.

Topics: Binding, Competitive; Blood Coagulation; Chromogenic Compounds; Fibrinopeptide A; Heparin; Hirudins; Humans; Naphthalenes; Piperidines; Recombinant Proteins; Thrombin

TNF-alpha induces changes in endothelial cell functions, such as upregulation of tissue factor, resulting in endothelial procoagulant activity which may play a role in disseminated intravascular coagulation. The procoagulant activity of TNF-alpha-stimulated endothelial cell monolayers was studied in a human ex vivo native (nonanticoagulated) blood flow system using the three thrombin inhibitors recombinant hirudin, Ro 46-6240, and heparin. Under venous blood flow conditions (shear rate 65 s-1) recombinant hirudin, Ro 46-6240, and heparin inhibited fibrin deposition on the endothelial cells by 50% at concentrations of 14, 28, and 412 ng/ml, respectively. The highest tested concentrations of the thrombin inhibitors reduced the postchamber fibrinopeptide A levels from 713 +/- 69 to < 70 ng/ml. Surprisingly, even at relatively high inhibitor concentrations, some local fibrin deposits were found on TNF-alpha-stimulated cells, suggesting that some endothelial cells possess higher procoagulant activity than others. Therefore, the surface expression pattern of tissue factor, the primary initiator of coagulation in this system, was examined by immunogold-silver staining. The results showed that the tissue factor density on the cell surface varied strongly among TNF-alpha-stimulated endothelial cells. Using TNF receptor-selective agonistic mutants of TNF-alpha, it was demonstrated further that the heterogenous surface expression of tissue factor was mediated entirely by the 55-kD TNF receptor and did not involve the 75-kD TNF receptor. We conclude that in this system TNF-alpha induces heterogenous tissue factor expression which may lead to a high local thrombin concentration, such that even in the presence of thrombin inhibitors focal fibrin deposition occurs.

Topics: Blood Circulation; Blood Coagulation; Blood Platelets; Cell Membrane; Dose-Response Relationship, Drug; Endothelium, Vascular; Fibrin; Fibrinopeptide A; Heparin; Hirudins; Humans; Immunohistochemistry; Leukocytes; Naphthalenes; Piperidines; Thrombin; Thromboplastin; Tumor Necrosis Factor-alpha; Umbilical Veins