Effects of the glycoprotein IIb/IIIa receptor antagonist c7E3 Fab and anticoagulants on platelet aggregation and thrombin potential under high coagulant challenge in vitro

Collagen/endogenous thrombin-induced platelet aggregation in whole blood samples

The aim of the study was to investigate the individual and combined effects of collagen (3.5 microg/ml) and endogenously generated thrombin (due to addition of 0.35 pmol/l tissue factor) on platelet aggregation in the physiological environment of whole blood by means of the impedance method. Lag times were significantly shorter when a combination of collagen and endogenous thrombin was used to provoke platelet aggregation (41.9 +/- 16.3 s) compared with collagen (173.8 +/- 52.1 s, P < 0.0001) or endogenous thrombin (94.3 +/- 43.6 s, P < 0.001). Amplitudes and slopes were the lowest in collagen-induced experiments (2.83 +/- 1.59 Omega and 1.79 +/- 0.45 Omega/min, respectively), whereas they were approximately the same in endogenous thrombin-induced experiments whether collagen was present or not (13.7 +/- 3.1 versus 11.2 +/- 4.0 Omega and 6.3 +/- 2.8 versus 5.6 +/- 2.3 Omega/min, respectively). No synergistic effect of collagen and endogenous thrombin on the clot formation process was observed by means of thrombelastometry. Moreover, thrombin potentials in tissue factor-activated plasma samples were approximately the same whether collagen was present or not (834 +/- 67 versus 809 +/- 63 nmol/l.min). In conclusion, endogenously generated thrombin is a potent platelet agonist in whole blood, and a combination of collagen and endogenous thrombin synergistically shortens the lag time until the onset of platelet aggregation.

Combined effects of melagatran and eptifibatide on platelet aggregation inhibition but not thrombin generation inhibition

The aim of our study was to investigate the combined in vitro effects of melagatran and eptifibatide on platelet aggregation and thrombin generation under low and high coagulant challenge in tissue-factor-activated, platelet-rich plasma. Increasing amounts of melagatran dose-dependently decreased prothrombin fragment 1.2 and activated factor X values, and dose-dependently prolonged the lag phase until the onset of platelet aggregation. Eptifibatide exerted a dose-dependent anti-aggregating effect under both high and low coagulant challenge. The combination of melagatran and eptifibatide resulted in significant additive prolongation of the lag phase until the onset of platelet aggregation, which was more pronounced under low coagulant challenge. Under low, but not under high, coagulant challenge, the combination of melagatran and eptifibatide had a significant additive inhibitory effect on platelet aggregation. No additive effects on decreasing prothrombin fragment 1.2 and activated factor X values were observed with combined administration of the drugs. The present study demonstrates the additive effect of melagatran and eptifibatide on platelet aggregation inhibition and on prolongation of the lag phase until the onset of platelet aggregation.

Activation of the inflammation, coagulation, and fibrinolysis systems, without influence of abciximab infusion in patients with non-ST-elevation acute coronary syndromes treated with dalteparin: a GUSTO IV substudy

BACKGROUND

In acute coronary syndromes, the inflammation and the coagulation systems are activated, implying an impaired outcome. In addition to platelet inhibition, recent evidence suggests that the glycoprotein IIb/IIIa receptor inhibitor abciximab attenuates inflammation and coagulation activity.

METHODS

The Swedish Global Utilization of Strategies To open Occluded arteries-IV (GUSTO-IV) substudy included 404 patients with non-ST-elevation acute coronary syndromes. In addition to aspirin and dalteparin, all patients were randomized to receive abciximab infusion for 24 hours or 48 hours or corresponding placebo without early coronary revascularization. Plasma samples were obtained at baseline and 24, 48, and 72 hours.

RESULTS

The median levels of the coagulation markers thrombin/antithrombin complex and soluble fibrin increased significantly from 3.1 to 3.7 ug/L (baseline to peak; P <.001) and from 20 to 23 nmol/L (P <.001), respectively. The fibrinolysis marker, tissue plasminogen-activator, also increased its median levels, from 11.7 to 17.5 ug/L (P <.001), whereas the median level of plasminogen-activator-inhibitor was unchanged. The inflammatory markers interleukin-6, C-reactive protein, and fibrinogen also increased their median levels (5.4-7.8 ng/L, P <.001; 4.4-8.7 mg/L, P <.001; 3.3-3.9 g/L, P <.001). However, there were no differences in median levels or in changes of median levels of any marker at any point between the placebo group and any of the abciximab groups.

CONCLUSIONS

In non-ST-elevation acute coronary syndrome, there was a simultaneous activation of the inflammation, coagulation, and fibrinolysis systems, despite aspirin and dalteparin treatment. Prolonged treatment with abciximab had no influence of the activation of these systems.

Effect of anticoagulants and antiplatelet agents on platelet-mediated thrombin generation

The present study was designed to determine the mechanism by which and extent to which antagonists of glycoprotein IIbIIIa (GPIIbIIIa or alpha beta ) or activated factor X (FXa) activity block tissue factor-initiated thrombin generation by prothrombinase complexes assembled on the surface of activated platelets. In the presence of high concentrations of GPIIbIIIa antagonists, which eliminate platelet aggregation but not activation, there is still a substantial amount of thrombin produced. In contrast, specific antagonists of the coagulation cascade lead to abolition of both thrombin generation and platelet aggregation. In addition, inhibitors with similar inhibitory activity (Ki) against purified human FXa require a much broader range of concentrations (a variation of 10 000-fold or more) to reduce the amount of thrombin produced in a platelet-rich plasma assay. At the doses tested, inhibitors with greater potency in prevention of thrombin production in the platelet-rich plasma assay were effective in vivo antithrombotics in an animal model system, whereas a lower potency compound did not reduce thrombus mass. Therefore, inhibition of FXa within platelet bound prothrombinase rather than inhibition of purified FXa in solution may be a better predictor of antithrombotic efficacy. In addition, all the studied anticoagulants fared better than the antiplatelet agents in reducing thrombin generation.

Respective roles of factors II, VII, IX, and X in the procoagulant activity of FEIBA

Activated prothrombin complex concentrates (APCCs) are effective in the therapy of bleeding episodes in hemophilic patients with inhibitors. We investigated the respective roles of factor II, factor VII, factor IX, and factor X in the procoagulant activity of the APCC FEIBA. Factor II, factor VII, factor IX, and factor X were reduced in platelet-poor plasma, and the thrombin potential (TP) was determined using a chromogenic substrate in the absence or presence of FEIBA. Reduction of factor II resulted in a significant decrease of the TP without influencing the lag phase until the onset of thrombin generation. The reduction of factor VII showed no effect on the TP, but resulted in a prolongation of the lag phase. Changes of factor IX or factor X concentrations showed neither an effect on the TP nor on lag phases. Our study demonstrates that thrombin generation in the presence of FEIBA mainly depends on prothrombin.