Contribution of platelets and the vessel wall to the antithrombotic effects of a single bolus injection of Fab fragments of the antiplatelet GPIIb/IIIa antibody 7E3 in a canine arterial eversion graft preparation

Short-term intravenous eptifibatide infusion combined with reduced dose recombinant tissue plasminogen activator inhibits platelet recruitment at sites of coronary artery injury

OBJECTIVES

This study was designed to determine in a dog model of coronary thrombosis whether short-term eptifibatide (Ep) combined with low-dose plasminogen activator (rt-PA) inhibits platelet recruitment at sites of endothelial damage after normalization of platelet function.

BACKGROUND

Ep plus reduced-dose rt-PA has not previously been shown to render a recanalized coronary artery resistant to platelet recruitment after normalization of platelet function.

METHOD

Inhibition of platelet recruitment was studied by scanning electron microscopy (SEM) in a canine model of left anterior descending (LAD) thrombosis. In phase I treatment groups were: 1) Ep (n = 6); 2) Ep + rt-PA (n = 6); 3) rt-PA (n = 6); and 4) placebo (n = 4). Coronary blood flow was monitored and LAD segments excised for SEM after 90-min infusion of study drug. In phase II, dogs were randomized to Ep alone (n = 5) or to Ep + rt-PA (n = 5). Coronary blood flow was monitored during and 120 min after cessation of drug when platelet function had returned to normal and LAD segments were excised.

RESULTS

All animals except placebo showed reflow. In phase I, SEM showed an absence of platelet aggregates with Ep alone and with Ep + rt-PA, but not with rt-PA alone. In phase II, SEM showed an intimal surface devoid of mural thrombus and platelet aggregates only in Ep + rt-PA treated arteries. Ep-alone treated arteries showed new platelet aggregates at sites of residual mural thrombus.

CONCLUSIONS

Short-term infusion Ep plus low-dose rt-PA acutely neutralizes the ability of damaged endothelial surfaces to recruit new platelets by inhibiting platelet aggregation and eliminating residual mural thrombus.

Anti-human vWF monoclonal antibody, AJvW-2 Fab, inhibits repetitive coronary artery thrombosis without bleeding time prolongation in dogs

The antithrombotic and antihaemostatic effects of the monoclonal antibody against human vWF (AJvW-2 Fab) were investigated in comparison with those of the monoclonal antibody against platelet GPIIb/IIIa (abciximab) in dogs. The ex vivo platelet aggregation and template bleeding time were measured before, 5, 90, 210 min and 24 h after injection of either AJvW-2 Fab or abciximab in anesthetized beagle dogs. Plasma concentration, vWF occupancy and plasma vWF antigen level were also measured by ELISA. In addition, the antithrombotic effect was evaluated in a canine model of repetitive coronary thrombosis (Folts model). AJvW-2 Fab significantly inhibited the ex vivo botrocetin-induced platelet aggregation at 0.18 mg/kg (53% plasma vWF occupancy) and also inhibited cyclic flow reductions (CFRs) at 0.06 mg/kg (31% occupancy). A significant prolongation of the bleeding time was observed at 1.8 mg/kg (95% occupancy), which was 30 times as high as the antithrombotic effective dose. Whereas, abciximab significantly inhibited both the ex vivo ADP-induced platelet aggregation and CFRs at 0.8 mg/kg, which was the minimally effective dose, also resulting in a significant prolongation of the bleeding time. These results suggest that blockade of the GPIb-vWF axis with AJvW-2 Fab leads to the inhibition of thrombus formation in the stenosed coronary arteries without less bleeding time prolongation than the GPIIb/IIIa blockade with abciximab.

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.

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

The present study compared the antithrombotic properties of fractionated aurin tricarboxylic acid (ATA), an inhibitor of platelet glycoprotein (GP) Ib, and GR144053, a GPIIb/IIIa antagonist, in a hamster model of stenosis. Endothelial cell injury in the hamster carotid artery was achieved by a 2F modified catheter. Arterial blood flow in the control groups was interrupted 5.4+/-0.9 minutes after the injury. When ATA (0.01, 0.03, 0.1, 0.3, and 1.0 mg/kg per hour) or GR144053 (0.1, 0.3, and 1.0 mg/kg per hour) were continuously infused intravenously, the time elapse before the vessel completely occluded was prolonged in a dose-dependent manner. However, all arteries in the ATA-treated groups ultimately occluded during the observation period even if the aggregation of platelets ex vivo and induced by botrocetin was completely inhibited. When either ATA (0.1 mg/kg per hour) or GR144053 (0.3 mg/kg per hour) were infused via an implanted osmotic pump together with tissue-type plasminogen activator (tPA), late patency of the reperfused artery was improved compared to that of arteries treated with TPA alone. However, the cyclic reflow pattern after reperfusion on days 0 and 1 was not reduced by the ATA treatment. The bleeding time was significantly prolonged when either ATA or GT144053 was coadministered with tPA. The treatment with ATA showed an especially marked prolongation of the bleeding time. In conclusion, both inhibition of platelet activation by ATA or GR144053 prevent arterial thrombosis and enhance the thrombolytic effect of tPA, but GR144053 was more protective in its antithrombotic effect and more effective during thrombolytic therapy than ATA.

Death and nonfatal reinfarction within the first 24 hours after presentation with an acute coronary syndrome: experience from GUSTO-IIb. Global Utilization of Strategies for Total Occlusion

BACKGROUND

A large proportion of deaths among patients with myocardial infarction occurs within the first 24 hours after presentation. It is not clear whether this phenomenon is also true of patients without ST-segment elevation who may or may not have infarction at the time of presentation. Thrombin activity may also be greatest during the first 24 hours after plaque rupture. Accordingly, this study was designed to examine the pattern of early ischemic events among patients with acute coronary syndromes and to determine whether the direct thrombin inhibitor desirudin (r-hirudin) would be most effective during this period.

METHODS AND RESULTS

Among the 11,142 patients enrolled in GUSTO-II, death or (re)infarction occurred within 24 hours in 210 patients (1.7%), representing 19% of the 1135 deaths that had occurred by 30 days. Death or (re)infarction occurred within 24 hours in 113 patients (2. 7%) with ST-segment elevation and in 97 patients without ST-segment elevation (1.2%, P <.001), representing 26% and 14% of the 30-day event rates, respectively, for the 2 enrollment strata. Among patients with ST-segment elevation, most of these events were deaths, whereas among patients without ST-segment elevation, most events were (re)infarctions. Death or (re)infarction by 24 hours occurred in 80 (1.3%) patients treated with desirudin and 130 (2.1%) patients treated with heparin (P =.01). This finding predominantly consisted of prevention of death among patients with ST-segment elevation and of (re)infarction among patients without ST-segment elevation.

CONCLUSIONS

These findings have important implications for early triage of patients with acute coronary syndromes and for the development of new therapies directed at stabilizing the unstable atherosclerotic plaque.

Effect of GR144053, a fibrinogen-receptor antagonist, on thrombus formation and vascular patency after thrombolysis by tPA in the injured carotid artery of the hamster

The antithrombotic effect of GR144053, which inhibits platelet aggregation by binding to the fibrinogen receptor (glycoprotein IIb/IIIa), was investigated in vitro and in vivo by using hamsters. This compound inhibited the platelet aggregation induced by adenosine diphosphate (ADP; 2.5 microM) with a mean inhibitory concentration (IC50) value of 2.2 +/- 0.4 x 10(-5) M. Vascular injury was inflicted in one carotid artery by using a modified catheter to produce endothelial denudation. In the control group, arterial blood flow was interrupted 4.4 +/- 2.3 min (n = 12) after the injury. When GR144053 continuously infused intravenously at doses of 0 (saline) 0.1, 0.3, and 1.0 mg/kg/h (n = 8, each), the time that elapsed before the vessel became completely obstructed was prolonged in a dose-dependent manner. In separate experiments, reperfusion could be obtained by continuous infusion of tissue-type plasminogen activator (tPA; 0.52 mg/kg) starting 30 min after the initiation of thrombus formation. When GR144053 (0.3 and 1.0 mg/kg/h) was infused in addition to tPA, the incidence of reperfusion and the later patency of the reperfused artery were much improved as compared with tPA alone. The bleeding time at the end of tPA infusion was significantly prolonged in the presence of the highest dose of GR144053. Next, neointima formation was evaluated 2 weeks after the vascular injury. When GR144053 (0.3 mg/kg/h) was continuously infused i.v. by an implanted osmotic pump for 14 days, the neointimal area was significantly reduced. In separate hamsters, the proliferating index of smooth muscle cells (SMCs) by using bromodeoxyuridine (BrdU) was investigated, and treatment with both tPA and GR144053 significantly decreased the SMC proliferation index in vivo. However, in the in vitro experiments using a hamster SMC line, GR144053 did not have an inhibitory effect on SMC proliferation. These findings suggest that GR144053 inhibits platelet activation on the injured artery and improves vascular patency after thrombolysis with tPA, which furthermore results in suppression of neointima formation.

GR144053, a fibrinogen receptor antagonist, enhances the suppression of neointima formation by losartan, an angiotensin II receptor antagonist, in the injured carotid artery of hamster

1. The present study investigated the inhibitory effect of losartan, a type 1 angiotensin II (AT1) antagonist, and of combined treatment with losartan and GR144053, a fibrinogen receptor (GPIIb/IIIa) antagonist, on neointima formation subsequent to vascular injury in the hamster carotid artery. Vascular injury was achieved by a roughened-tip 2F catheter and the neointimal area was measured up to 2 weeks inducing the injury. 2. Compared to non-treated hamsters (intimal area (IA/internal elastic laminal area (IELA) ratio = 60.3 +/- 5.9%, n = 12), losartan dissolved in drinking water (1, 3 and 10 mg kg-1 per day, n = 8 each) reduced neointimal area dose-dependently, a significant decrease (IA/IELA = 39.7 +/- 5.6%) being attained with the highest dose when it was administered from 1 day before injury. However, neointima formation was not prevented even with the highest dose of losartan when the administration was started after injury. 3. When the administration of GR144053 (1.0 mg kg-1 per hour) via an implanted osmotic pump was started 30 min before the injury and continued for the next 2 weeks, no suppression of neointima formation was observed, although platelet aggregation evoked ex vivo by adenosine diphosphate (ADP) at the end of treatment period was efficiently inhibited. 4. In separate experiments in which 5-bromo-2-deoxy-Uridine (BrdU) was used to test smooth muscle cell (SMC) proliferation 1 and 7 days after injury, the ratio of SMC proliferation in the injured area was only slightly decreased by losartan when its administration was started after the injury, despite the marked reduction of SMC proliferation when treatment was started before the injury. Treatment with GR144053 as indicated above also significantly decreased the SMC proliferating index 1 day after the injury. 5. To examine the potential benefit of the coadministration of the GPIIb/IIIa antagonist with the AT1 receptor antagonist, GR144053 (1.0 mg kg-1 per hour) was combined with post-injury treatment with losartan (10 mg kg-1 per day). This markedly reduced the proliferation of SMCs and significantly decreased the neointimal area (IA/IELA = 31.2 +/- 4.6%) measured 2 weeks following the catheterization. 6. According to the results of a time-dependent study in which GR144053 was given in combination with post injury treatment with losartan for 1, 3, 7 or 14 days, neointima formation could be reduced by treatment with GR144053 for just 7 days. 7. In conclusion, GR144053, a fibrinogen receptor antagonist, enhanced the inhibitory effect of losartan, an AT1 receptor antagonist, on neointima formation in the damaged carotid artery of hamsters.