Comparison of the antithrombotic effects and bleeding risk of fractionated aurin tricarboxylic acid and the GPIIb/IIIa antagonist GR144053 in a hamster model of stenosis

Inhibitors of fibrinolytic components play different roles in the formation and removal of arterial thrombus in mice

Plasminogen activator inhibitor-1 (PAI-1) and alpha2-anti-plasmin (alpha2-AP) may contribute to arterial thrombolysis resistance. The role of these components on thrombus evolution in vivo was investigated in mice deficient for PAI-1 (PAI-1(-/-)) or alpha2-AP (alpha2-AP(-/-)) or their wild-type counterparts (PAI-1(+/+), alpha2-AP(+/+)). Moreover, the influence of either PAI-1 or alpha2-AP deficiency on the results of pharmacologic inhibition of glycoprotein IIb/IIIa of platelets or thrombin was also investigated. A thrombus was induced in the murine carotid artery by endothelial injury. The alpha2-AP(-/-) mice were indistinguishable from wild-type, whereas the time to occlusion in PAI-1(-/-) was significantly prolonged to 24.9 +/- 3.7 min. Vascular patency was markedly increased in both PAI-1- and alpha2-AP-deficient mice. In separate animals, either a glycoprotein IIb/IIIa antagonist or a thrombin inhibitor was applied. The time required to occlusion was prolonged in a dose-dependent manner in all types of mice. When each compound was administered to PAI-1(-/-) mice, significant changes were observed. In conclusion, lack of PAI-1 prolongs the time to occlusion and accelerates clot lysis, whereas alpha2-AP only has an effect on spontaneous reperfusion. Consequently, the inhibition of PAI-1, but not of alpha2-AP, could enhance the effects of anti-thrombotic therapy.

Effects of fibrinogen receptor antagonist GR144053F and aurintricarboxylic acid on platelet activation and degranulation

Activated blood platelets play crucial role in restenosis due to their fundamental significance in thrombus formation. Therefore, platelets are attractive targets for the inhibition with a variety of antagonists. In this study, we present direct evidence that GR144053F [non-peptide antagonist of glycoprotein IIb-IIIa complex (GPIIb-IIIa)] inhibits activation and degranulation of human platelets, and opposes the action of aurintricarboxylic acid (ATA), the antagonist of von Willebrand factor, which augments platelet secretion. The effects of both drugs on platelet function were monitored by using various instrumental methods. Platelet-rich plasma and whole-blood aggregation was measured by using ADP and collagen as agonists. Platelet degranulation was assessed based on the expression of surface membrane activation markers: P-selectin, glycoprotein Ib, and activated GPIIb-IIIa complex. Measurements of closure time with platelet function analyzer PFA-100 enabled us to reason on primary hemostatic capacity and reflected both aggregability and adhesiveness. GR144053F markedly reduced primary hemostatic platelet response (IC(50) = 114.0 +/- 9.6 nM) under conditions that closely mimicked natural blood flow in circulation, and inhibited aggregation in platelet-rich plasma (IC(50) = 17.7 +/- 7.0 nM). It was equally potent inhibitor of platelet activation, degranulation, fibrinogen binding, platelet consumption, and aggregate formation. Also, ATA was efficient in inhibition of platelet aggregation and adhesion (by up to 50% at 100 microM), but the combined action of both drugs on primary haemostatic capacity was not additive. GR144053F suppressed the activating effects of ATA on platelet degranulation and secretion. Overall, our data indicate that GR144053F is not only the efficient blocker of fibrinogen binding to GPIIb-IIIa, but also hampers platelet degranulation and may attenuate the activating effects of ATA.

tPA, but not uPA, significantly affects antithrombotic therapy by a glycoprotein IIb/IIIa antagonist, but not by a factor Xa inhibitor

To define the interaction of fibrinolytic components with platelets or coagulation factors on thrombus formation, we investigated mouse deficient in tissue plasminogen activator (tPA -/-) or urokinase plasminogen activator (uPA -/-) and in their wild-type control (tPA +/+, uPA +/+). A thrombus was induced in the murine carotid artery using photochemical reaction. Blood flow was monitored and the time needed before the vessel became completely obstructed was within 12 min in all types of mice. When DX-9065a, a selective factor Xa inhibitor, or GR144053, a platelet glycoprotein (GP) complex IIb/IIIa antagonist was applied, the time required to occlusion was prolonged in a dose-dependent manner in all types of mice. When a factor Xa inhibitor was injected in tPA -/- mice, the estimated ED50 was not changed. However, when GR144053 was injected in tPA -/- mice, the most significant changes were observed: the estimated ED51 was 19.6 times higher than the one in tPA +/+ mice. Platelet aggregation, hemostasis tests, and bleeding times were not significantly different among the different types of mice. In conclusion, the antithrombotic effect of platelet inhibition by a GPIIb/IIIa antagonist, is severely affected by the absence or presence of tPA production. On the contrary, the inhibition of factor Xa shows a stable antithrombotic effect with or without tPA. Thus the lack of tPA, but not of uPA, significantly affects antithrombotic efficacy.

Inhibition of platelet aggregation in baboons: therapeutic implications for xenotransplantation

Activation of endothelial cells and platelet sequestration play major roles in rejection of xenografts. The histopathology of both hyperacute and acute vascular or delayed rejection of vascularized discordant xenografts is characterized by interstitial hemorrhage and intravascular thrombosis. Agents that prevent platelet activation and consequent microthrombus formation have proven beneficial in xenograft rejection but do not fully preclude vascular thrombosis. Recently, several new anti-platelet therapies have undergone extensive clinical testing for atherosclerotic thrombotic vascular disorders; other putative therapies are undergoing pre-clinical evaluation. We have investigated the effect of several of these novel agents on platelet aggregation in baboons in order to screen for future potential in xenograft rejection models.

METHODS

Drugs tested in these experiments were aurintricarboxylic acid (ATA, von Willebrand Factor-GPIb inhibitor), fucoidin (a selectin-inhibitor), 1-benzylimidazole (1-BI, thromboxane synthase antagonist), prostacyclin (PGI2, endothelial stabilizer), heparin (thrombin antagonist), nitroprusside sodium or nicotinamide (NPN or NA, both NO-donors), and eptifibatide (EFT, GPIIb/IIIa receptor antagonist). These were infused intravenously to nine baboons. Coagulation parameters and platelet counts were monitored and baboons were observed for adverse side-effects. The efficacy of these agents in inhibiting platelet aggregation was assayed in a platelet aggregometer.

RESULTS

Treatment with ATA and fucoidin resulted in complete inhibition of platelet aggregation but also in major perturbation of coagulation parameters. 1-BI and PGI2 had no effect when administered alone, but in combination resulted in moderate inhibition of aggregation without disturbance in PT or PTT. NPN and NA had no substantive effects on platelet aggregation. Heparin resulted in specific inhibition of thrombin-induced platelet aggregation and, as anticipated, was associated with moderate prolongation of PTT. Importantly, EFT caused complete inhibition of platelet aggregation without changes in coagulation. Platelet counts, fibrinogen levels, and fibrinogen degradation products remained within the normal ranges in all experiments.

CONCLUSIONS

Although excellent inhibition of platelet activation was obtained with ATA and fucoidin, clinical use may be precluded by concomitant disturbances of coagulation. Combinations of heparin and EFT may prove beneficial in preventing the thrombotic disorders associated with xenograft rejection while maintaining adequate hemostatic responses. These agents are to be evaluated in our pig-to-primate xenotransplantation models.

The interaction between components of the fibrinolytic system and GPIb/V/IX of platelets thrombus formation in mice

The interaction of fibrinolytic components with GPIb/V/IX of platelets on thrombus formation, was investigated in mice deficient in tissue type (tPA-/-), urokinase type plasminogen activator (uPA-/-) or plasminogen activator inhibitor-1 (PAI-1-/-) and in their wild type control (tPA+/+, uPA+/+, PAI-1+/+). A thrombus was induced in the murine carotid artery using a photochemical reaction. The times to occlusion after the initiation of endothelial injury in all wild type mice was within 12 min, and no significant changes in occlusion delay were observed in uPA-/- and tPA-/- mice compared to wild type mice, whereas that of PAI-1 mice were significantly prolonged (16.9+/-2.9 min, P<0.05). When high molecular weight aurintricarboxylic acid (ATA), an inhibitor of platelet glycoprotein Ib/V/IX, was administered, the time to occlusion was prolonged in a dose-dependent manner in all types of mice. However, when this compound was injected in tPA-/- mice, the most significant changes were observed: i.e. the estimated ED(50) was 20.2 times higher than that in tPA+/+ mice, but the estimated ED(50) in uPA-/- mice was not changed as compared with that of wild type mice. On the other hand, when ATA was injected in PAI-1-/- mice, the estimated ED(50) was significantly decreased (P<0.05). Platelet aggregation induced by botrocetin was not significantly different among all types of mice. The bleeding time was prolonged in a dose dependent-manner when ATA was injected in all types of mice. In conclusion, the antithrombotic effect of inhibition of platelet GPIb/V/IX is severely affected by the absence or presence of tPA-production on thrombus formation and the inhibition of PAI-1 could enhance this antithrombotic effect.