The role of GPIIb/IIIa in tissue factor induced coagulation

Background: Severe sepsis still carries a high mortality rate despite advantages in intensive care medicine and antimicrobial therapy. The inflammatory and procoagulant host response to infection are intricately linked and interactions between platelets, leukocytes and endothelium play a central role in the pathogenesis of septic shock and disseminated intravascular coagulation (DIC), which is tissue factor (TF) mediated. Recombinant human activated protein C has recently been shown to blunt DIC and improve mortality in sepsis (Bernard et al. NEJM 2001). This provides a strong rationale to further investigate alternative therapeutic strategies to dampen inflammation and coagulation in endotoxemia. It is well known that platelets enhance TF expression by monocytes. In addition, in vitro studies suggest that the fibrinogen receptor on platelets, the glycoprotein (GPIIb/IIIa) could play a role in TF-induced coagulation.. Most importantly, blockade of GPIIb/IIIa decreased monocyte TF expression and consumption of coagulation factors in rabbits challenged with endotoxin (LPS) resulting in improved mortality. Hypothesis: Blockade of GPIIbIIIa could inhibit TF mediated thrombin generation, as shown in vitro and in rabbits. The Trial objective is to explore the role of GPIIb/IIIa in TF-induced coagulation in humans. In particular to investigate whether blockade of GPIIbIIIa decreases LPS mediated thrombin formation and its consequences in a standardized model of human endotoxemia. Secondary objectives are To explore whether GPIIb/IIIa inhibitors decrease platelet -leukocyte co-aggregation, tissue factor expression and systemic inflammation Aim of the trial: To compare the effect of an i.v. bolus of 2ng/kg endotoxin with and without blockade of the GPIIbIIIa molecule, in order to characterize the role of the GPIIb/IIIa receptor in tissue factor induced coagulation in humans. A double blind, randomized, placebo-controlled trial will be conducted in three parallel groups. Outcome variables will be measured by enzyme immuno assays, flowcytometry, and quantitative PCR analysis.

Objectives Activated platelets facilitate thrombin generation by providing a catalytic surface on which coagulation activation occurs. The glycoprotein (GP) IIb/IIIa receptor might play a major role in this process as shown by in vitro and animal experiments. However, it is controversial whether the GPIIb/IIIa receptor facilitates TF-induced thrombin generation in humans as well. We therefore investigated whether two clinically used GPIIb/IIIa antagonists (tirofiban and eptifibatide) may blunt TF-induced coagulation in humans. Methods and Results: Thirty male volunteers received 2ng/kg endotoxin and standard doses of eptifibatide, tirofiban or placebo over 5 hours in a randomized, double-blind, placebo-controlled, double-dummy parallel-group trial. Markers of thrombin generation (prothrombin fragment 1+2, thrombin-antithrombin complexes), fibrinolysis (D-dimer, plasmin-antiplasmin complexes) as well as inflammatory markers (interleukin-6, tumor necrosis factor-a) were measured by enzyme linked immunoasssays, TF-mRNA expression was quantified by RT-PCR. Neither eptifibatide nor tirofiban influenced LPS-induced coagulation activation or fibrinolytic activity. Additionally, increase of TNF-a and IL-6 was similarly in all groups. Conclusion GPIIb/IIIa blockade with eptifibatide or tirofiban did not influence TF-induced coagulation activation in human low grade endotoxemia. Thus, in clinical practice, both drugs seem to primarily act as anti-platelet agents rather than by interference with tissue factor driven thrombin generation.