Understanding the mechanism of platelet thrombus formation under blood flow conditions and the effect of new antiplatelet agents

Platelet aggregation induced by stirring of a platelet suspension after activation by the exogenous addition of stimulants, such as ADP, epinephrine and thrombin, has long been used as a platelet function test, and for the screening of antiplatelet agents. Since platelet aggregation has been demonstrated to be mediated exclusively by the binding of plasma fibrinogen to the activated GP IIb/IIIa, anti-GP IIb/IIIa agents, which can completely inhibit platelet aggregation, were expected to become among the most potent of antithrombotic agents. The strong preventive effects of anti-GP IIb/IIIa agents against thrombotic complications after coronary intervention, and the weaker preventive effects of the same agents against the onset of coronary thrombosis in unstable angina pectoris patients point to both the efficacy and the limitations of anti-GP IIb/IIIa antithrombotic agents. Recently, many investigators have reported that platelets play a major role in thrombus formation at sites exposed to blood flow. Several platelet-function assay systems have been developed to elucidate the mechanism of thrombus formation under blood flow conditions. Numerous studies have demonstrated that von Willebrand factor (VWF) and its interaction with its receptors on platelets, including GP Ibalpha and GP IIb/IIIa, play essential roles not only in platelet activation, but also in platelet adhesion and possibly platelet cohesion. These findings prompted us to explore whether the VWF-mediated process of thrombus formation could be exploited as a target for potential antiplatelet agents. Moreover, recent studies have demonstrated that multiple synergistic signals, including those mediated by catecholamine receptors, purine nucleotide receptors, as well as some types of collagen receptors are involved in the process of VWF-mediated platelet thrombus formation. We now have new antiplatelet agents on the horizon, targeted against thrombus formation under blood flow conditions.     

Optimal use of platelet glycoprotein IIb/IIIa receptor antagonists in patients undergoing percutaneous coronary interventions

Discovery of the central role of platelets in the pathogenesis of acute coronary syndromes (ACS) and ischaemic complications of percutaneous coronary interventions (PCI) has led to the widespread use of oral and parenteral platelet inhibitors to treat these conditions. Glycoprotein (GP) IIb/IIIa (also known as α(IIb)β(3)) receptors on the surface of platelets play an essential role in platelet aggregation and serve as a key mediator in the formation of arterial thrombus. When activated, GP IIb/IIIa receptors bind to fibrinogen, which serves as the 'final common pathway' in platelet aggregation. Of the numerous agents developed for modulating platelet activity, intravenous platelet GP IIb/IIIa receptor antagonists are the most potent. There are four agents in clinical use, including abciximab, eptifibatide, tirofiban and lamifiban, although lamifiban is not approved for use in the US. While all agents block fibrinogen binding to GP IIb/IIIa, they do so by different mechanisms. Abciximab is a humanized form of a murine monoclonal antibody directed against GP IIb/IIIa, eptifibatide is a synthetic, cyclic heptapeptide that contains a lysine-glycine-aspartic acid (KGD) sequence that mimics the arginine-glycine-aspartic acid (RGD) sequence found on GP IIb/IIIa, tirofiban is a non-peptide antagonist derived by optimization of the tyrosine analogue that structurally mimicks the RGD-containing loop of the disintegrin echistatin, and lamifiban is a synthetic, non-cyclic, non-peptide, low-molecular-weight compound. In clinical trials, use of these agents reduces ischaemic adverse cardiovascular events in patients with ACS undergoing PCI, but at a cost of increased bleeding.     

Polyamines inhibit both platelet aggregation and glycoprotein IIb/IIIa activation

Platelet aggregation is inhibited by the polyamines putrescine, spermidine, and spermine. To date, the mechanism of action has not been clearly identified. Evidence suggests that polyamines may interact with the fibrinogen receptor (GP IIb/IIIa), interfering with platelet-platelet attachment. The effect of polyamines on human platelet aggregation and GP IIb/IIIa activation was evaluated. For the aggregation experiments, platelets were obtained from heparin- or citrate-collected blood. Our results indicate that the polyamines putrescine, spermidine, and spermine cause a dose-dependent inhibition of ADP- or collagen-mediated platelet aggregation with an order of potency spermine>spermidine>putrescine. In addition, spermine arrests or inhibits thrombin-, epinephrine-, arachidonate-, or ristocetin-induced platelet aggregation. Expression of platelet membrane glycoproteins IIb, IIIa, and IX is not reduced by polyamines. However, spermine inhibits the ADP- or thrombin-induced activation of GP IIb/IIIa. It is concluded that the final step in aggregation, common to all agonists, ie, fibrinogen binding to GP IIb/IIIa, is inhibited by spermine through inhibition of the agonist-induced activation of GP IIb/IIIa that precedes fibrinogen-ligand binding.     

Targeted delivery of thrombolytic agents: role of integrin receptors

Blood clotting (formation of thrombus) plays a critical role in the evolution of a number of cardiovascular diseases. Targeted delivery of thrombolytic agents reduces the risks of hemorrhage and toxicity associated with systemic drug administration, thus offering a promising, minimally invasive approach to controlling and treating thrombosis. Platelets play a major role in the progression of thrombosis on vascular injury. Platelet integrin αIIbβ3 (GP IIb/IIIa) serves as a receptor for various proteins such as fibrinogen, vWF, fibronectin and vitronectin, as well as contributing to the adhesion and aggregation of platelets in a variety of conditions. These receptor-based targeted therapies are currently under clinical studies. Integrins and RGD-based ligands for integrins are currently being investigated in imaging and drug delivery related areas of research. RGD-targeted drugs and imaging agents have been developed either by direct conjugation of the homing peptide to the drug or by conjugation of the RGD-peptide to a carrier device containing drug molecules. This review describes the role of integrin receptors in the pathophysiology of thrombosis and its use in the targeted delivery of thrombolytic agents.     

Inhibition of platelet glycoprotein Ib and its antithrombotic potential

The platelet receptor glycoprotein (GP)Ib-IX-V complex plays a dominant role in the first steps of platelet adhesion and arterial thrombus formation. Through its interaction with the multimeric plasma protein von Willebrand factor (VWF), which is bound to the damaged subendothelial structures, GPIb-IX-V tethers the platelets from the flowing blood thereby slowing them down. This step is a prerequisite for the collagen receptors to participate in firm adhesion resulting in the formation of a first platelet layer which is the basis for further thrombus formation. Recently, other ligands for GPIb-IX-V besides the extensively studied VWF have been identified, such as: alpha-thrombin, coagulation factor XII (FXII), high molecular weight kininogen (HMWK), factor XI (FXI), integrin Mac-1 and P-selectin. In this review, the interaction of GPIb-IX-V with its different ligands is described and the anticipated or demonstrated in vivo effects are discussed.     

Inhibitors of platelets glycoprotein IIb/IIIa (GP IIb/IIIa) receptor: rationale for their use in clinical cardiology

The glycoprotein IIb/IIIa (GP IIb/IIIa) receptor is the most important receptor involved in platelet aggregation. A stable GP IIb/IIIa inhibition is required when a massive platelet activation triggers thrombosis. Three GP IIb/IIIa inhibitors are currently approved for clinical use: abciximab, tirofiban and integrilin. Their different pharmacodynamic and pharmacokinetic properties reflect a different efficacy in platelet inhibition.     

Pharmacological characteristics of solid-phase von Willebrand factor in human platelets.

1. The pharmacological characteristics of solid-phase von Willebrand factor (svWF), a novel platelet agonist, were studied. 2. Washed platelet suspensions were obtained from human blood and the effects of svWF on platelets were measured using aggregometry, phase-contrast microscopy, flow cytometry and zymography. 3. Incubation of platelets with svWF (0.2 - 1.2 microg ml(-1)) resulted in their adhesion to the ligand, while co-incubations of svWF with subthreshold concentrations of ADP, collagen and thrombin resulted in aggregation. 4. 6B4 inhibitory anti-glycoprotein (GP)Ib antibodies abolished platelet adhesion stimulated by svWF, while aggregation was reduced in the presence of 6B4 and N-Acetyl-Pen-Arg-Gly-Asp-Cys, an antagonist of GPIIb/IIIa. 5. Platelet adhesion stimulated with svWF was associated with a concentration-dependent increase in expression of GPIb, but not of GPIIb/IIIa. 6. In contrast, collagen (0.5 - 10.0 microg ml(-1)) caused down-regulation of GPIb and up-regulation of GPIIb/IIIa in platelets. 7. Solid-phase vWF (1.2 microg ml(-1)) resulted in the release of MMP-2 from platelets. 8. Inhibition of MMP-2 with phenanthroline (10 microM), but not with aspirin or apyrase, inhibited platelet adhesion stimulated with svWF. 9. In contrast, human recombinant MMP-2 potentiated both the effects of svWF on adhesion and up-regulation of GPIb. 10. Platelet adhesion and aggregation stimulated with svWF were reduced by S-nitroso-n-acetyl-penicillamine, an NO donor, and prostacyclin. 11. Thus, stimulation of human platelets with svWF leads to adhesion and aggregation that are mediated via activation of GPIb and GPIIb/IIIa, respectively. 12. Mechanisms of activation of GPIb by svWF involve the release of MMP-2, and are regulated by NO and prostacyclin.     

The role of eptifibatide in patients undergoing percutaneous coronary intervention

Glycoprotein (GP) IIb/IIIa receptor antagonists inhibit the binding of ligands to activated platelet GP IIb/IIIa receptors and, therefore, prevent the formation of platelet thrombi. They have been extensively studied in patients undergoing percutaneous coronary intervention (PCI). Eptifibatide, one of the approved GP IIb/IIIa inhibitors, is a small heptapeptide that is highly selective and rapidly dissociates from its receptor after cessation of therapy. In clinical studies, concomitant administration of eptifibatide in patients undergoing elective PCI reduced thrombotic complications in the IMPACT-II (Integrilin to Minimize Platelet Aggregation and Prevent Coronary Thrombosis II) and ESPRIT (Enhanced Suppression of the Platelet IIb/IIIa Receptor with Integrilin Therapy) trials. In the PURSUIT (Platelet Glycoprotein IIb/IIIa in Unstable Angina: Receptor Suppression Using Integrilin Therapy) trial, which included 10,948 patients with non-ST-elevation acute coronary syndromes, eptifibatide significantly reduced the primary end point of death and non-fatal myocardial infarction at 30 days compared with placebo. In patients with ST-segment elevation myocardial infarction (STEMI), eptifibatide has been studied as adjunct to primary PCI and improved epicardial flow and tissue reperfusion. Studies are now evaluating eptifibatide in high-risk patients with non-ST elevation acute coronary syndromes (NSTE-ACS) and a planned early invasive strategy in the EARLY-ACS (Eptifibatide Administration prior to Diagnostic Catherization and Revascularization to Limit Myocardial Necrosis in Acute Coronary Syndrome) trial and in patients with primary PCI for STEMI in comparison to abciximab in the EVA-AMI (Eptifibatide versus Abciximab in Primary PCI for Acute Myocardial Infarction) trial. After the completion of these trials, the value of eptifibatide in patients undergoing PCI in different indications can be determined.     

The role of eptifibatide in patients undergoing percutaneous coronary intervention

Glycoprotein (GP) IIb/IIIa receptor antagonists inhibit the binding of ligands to activated platelet GP IIb/IIIa receptors and, therefore, prevent the formation of platelet thrombi. They have been extensively studied in patients undergoing percutaneous coronary intervention (PCI). Eptifibatide, one of the approved GP IIb/IIIa inhibitors, is a small heptapeptide that is highly selective and rapidly dissociates from its receptor after cessation of therapy. In clinical studies, concomitant administration of eptifibatide in patients undergoing elective PCI reduced thrombotic complications in the IMPACT-II (Integrilin to Minimize Platelet Aggregation and Prevent Coronary Thrombosis II) and ESPRIT (Enhanced Suppression of the Platelet IIb/IIIa Receptor with Integrilin Therapy) trials. In the PURSUIT (Platelet Glycoprotein IIb/IIIa in Unstable Angina: Receptor Suppression Using Integrilin Therapy) trial, which included 10,948 patients with non-ST-elevation acute coronary syndromes, eptifibatide significantly reduced the primary end point of death and non-fatal myocardial infarction at 30 days compared with placebo. In patients with ST-segment elevation myocardial infarction (STEMI), eptifibatide has been studied as adjunct to primary PCI and improved epicardial flow and tissue reperfusion. Studies are now evaluating eptifibatide in high-risk patients with non-ST elevation acute coronary syndromes (NSTE-ACS) and a planned early invasive strategy in the EARLY-ACS (Eptifibatide Administration prior to Diagnostic Catherization and Revascularization to Limit Myocardial Necrosis in Acute Coronary Syndrome) trial and in patients with primary PCI for STEMI in comparison to abciximab in the EVA-AMI (Eptifibatide versus Abciximab in Primary PCI for Acute Myocardial Infarction) trial. After the completion of these trials, the value of etifibatide in patients undergoing PCI in different indications can be determined.     

Development of monoclonal antibodies that inhibit platelet adhesion or aggregation as potential anti-thrombotic drugs

Cardiovascular disease is the major cause of mortality in Western countries. Platelets play a crucial role in the development of arterial thrombosis and other pathophysiologies leading to clinical ischemic events. In the damaged vessel wall, platelets adhere to the subendothelium through an interaction with von Willebrand factor (VWF), which forms a bridge between subendothelial collagen and the platelet receptor glycoprotein (GP) Ib/IX/V. This reversible adhesion allows platelets to roll over the damaged area, decreasing their velocity and resulting in strong platelet activation. This leads to the conformational activation of the platelet GPIIb/IIIa receptor, fibrinogen binding and finally to platelet aggregation. As each interaction (collagen-VWF, VWF-GPIb and GPIIb/IIIa-fibrinogen) plays an essential role in primary haemostasis, loss of either of these interactions results in a bleeding diathesis, implying that interfering with these interactions might result in an anti-thrombotic effect. Whereas GPIIb/IIIa antagonists indeed are effective anti-thrombotics, it has been suggested that drugs which block the initial steps of thrombus formation (collagen-VWF or VWF-GPIb interaction) might have advantages over the ones that merely inhibit platelet aggregation. In this review we will discuss and compare the development of monoclonal antibodies (moAbs) that inhibit platelet adhesion or platelet aggregation. The effect of the moAbs in in vitro experiments, in in vivo models and in clinical trials will be described. Benefits, limitations, current applications and the future perspectives in the development of antibodies for each target will be discussed.     

New perspectives in antiplatelet therapy

Platelet activation is a complex mechanism of response to vascular injury and atherothrombotic disease, leading to thrombus formation. A wide variety of surface receptors -integrins, leucine-rich family receptors, G protein coupled receptors, tyrosine kinase receptors- and intraplatelet molecules support and regulate platelet activation. They are potential targets of antiplatelet therapy for the prevention and treatment of arterial thrombosis. Despite the overall clinical benefit of established antiplatelet drugs targeting cyclooxigenase-1 (COX-1), glycoprotein integrin αIIbβ3, and the purinergic P2Y(12) receptor of adenosine diphosphate, a significant proportion of treated patients continue to experience recurrent ischaemic events. This may be in partly attributed to insufficient inhibition of platelet activation. In addition, it should not be underestimated that these drugs are not immune from bleeding complications. The substantial progress in understating the regulation of platelet activation has played a key role in the development of novel antiplatelet agents. Current examples of drug under development and evaluation include: novel P2Y(12) receptor inhibitors (prasugrel, ticagrelor, cangrelor, and elinogrel), thrombin receptor PAR-1 antagonists (vorapaxar, atopaxar), new integrin glycoprotein IIb/IIIa inhibitors, and inhibitors targeting the thromboxane receptor (TP), phosphodiesterases, the collagen receptor glycoprotein VI, and intraplatelet signalling molecules. This review summarizes the mechanisms of action and current clinical evaluation of these novel antiplatelet agents.     

血小板糖蛋白Ⅱb/Ⅲa受体拮抗剂研究现状

血小板糖蛋白Ⅱb/Ⅲa受体是各种血小板聚集剂导致血栓形成的最后通道.综述上市和部分开发中的血小板糖蛋白Ⅱb/Ⅲa受体拮抗剂类药物.     

Intracoronary use of GP IIb/IIIa inhibitors in percutaneous coronary interventions

The glycoprotein (GP) IIb/IIIa receptor is critical to the process of platelet aggregation and thrombus formation as it serves as the final common pathway for platelet aggregation. For this reason, the development of GP IIb/IIIa inhibitors that block fibrinogen binding to the receptor has become an attractive strategy for antiplatelet therapy with an expected strong and specific effect. Presently, there are three commercially available GP IIb/IIIa inhibitors: abciximab, eptifibatide and tirofiban. All three drugs are commonly administered intravenously, and large-scale clinical trials have demonstrated a clear clinical benefit and good safety profile in patients at high risk, especially those undergoing percutaneous coronary interventions (PCI). Recently, several studies tested the intracoronary (IC) route for GP IIb/IIIa inhibitors in order to verify its safety and its possible superiority as compared to the intravenous (IV) route. The majority of the studies testing the IC route were conducted using abciximab and in patients with STEMI with better results in terms of myocardial reperfusion and infarct size and also promising results in terms of clinical outcome. On the IC administration of eptifibatide and tirofiban only some, even if promising, data are available. Larger and randomized studies are warranted to confirm the superiority of the IC route of administration of the GP IIb/IIIa inhibitors to the IV one in patients with coronary artery disease undergoing PCI.     

Inhibition of platelet adhesion to collagen as a new target for antithrombotic drugs

Platelet adhesion to a damaged blood vessel is the initial trigger for arterial hemostasis and thrombosis. Platelets adhere to the subendothelium through an interaction with von Willebrand factor (VWF), which forms a bridge between collagen within the damaged vessel wall and the platelet receptor glycoprotein Ib/V/IX (GPIb), an interaction especially important under high shear conditions[1]. This reversible adhesion allows platelets to roll over the damaged area, which is then followed by a firm adhesion mediated by the collagen receptors (alpha(2)beta(1), GPVI, ) in addition[2] resulting in platelet activation. This leads to the conformational activation of the platelet alpha(IIb)beta3 receptor, fibrinogen binding and finally to platelet aggregation. Over the past decades, modulation of platelet function has been a strategy for the control of cardiovascular disease. Lately, drugs have been developed that target the fibrinogen receptor alphaIIbbeta3 or the ADP receptor and many of these promising compounds have been tested in clinical trials. However the development of products that interfere with the first step of hemostasis, i.e. the platelet adhesion, has lagged behind. In this review we want to discuss (i) the in vivo studies that were performed with compounds that target proteins involved in different adhesion steps i.e. the VWF-GPIb-axis, the collagen-VWF axis and the collagen-collagen receptor axis and (ii) the possible advantages these putative new drugs could have over the current antiplatelet agents.     

Role of platelet glycoprotein polymorphisms in cardiovascular diseases

Atherothrombosis is the leading cause of death in western countries. Major complications of atherothrombotic disease, which are responsible for a large burden of morbidity and mortality, are acute coronary syndromes, ischemic stroke, and peripheral occlusive disease. Plaque rupture, platelet adhesion, aggregation, and thrombosis may lead to unstable angina and may progress to myocardial infarction as well as to ischemic stroke. Platelet membrane glycoprotein receptors mediate crucial reactions in acute thrombosis and chronic processes of atherogenesis. The platelet glycoprotein GP IIb/IIIa, which is the most abundant platelet receptor, also represents the drug target of a novel class of anti-platelet drugs, which includes abciximab, tirofiban, and eptifibatide. The genes encoding the three major platelet glycoprotein receptors (GP Ib/IX/V, GP Ia/IIa, and GP IIb/IIIa) are subject to considerable genetic variability. This paper reviews how polymorphisms in the platelet glycoprotein receptors affect platelet function, susceptibility to atherothrombosis and its major complications including myocardial infarction, stroke, and complications following percutaneous coronary interventions, and individual variability of drug response. Recent data on platelet glycoprotein receptor polymorphisms as modifiers of drug action and as predictors of drug response offer the perspective of individualized drug treatment. Prospective studies will show whether this approach is useful or not. As the data reviewed here show clearly, future clinical trials should routinely take into account genetic susceptibility factors and modifiers, both for study design and for predefined patient stratification.Abbreviations ACS Acute coronary syndrome - CI Confidence interval - DCA Directional coronary atherectomy - GP Glycoprotein - MACE Major adverse cardiac event - MI Myocardial infarction - PCI Percutaneous coronary intervention - PTCA Percutaneous transluminal coronary angioplasty - RR Relative risk - SCD Sudden cardiac death - VWF Von Willebrand factor     

Sphingosylphosphorylcholine,a naturally occurring lipid mediator,inhibits human platelet function

1 The lysophospholipids, lysophosphatidic acid and sphingosine 1-phosphate, have been reported to activate platelets. Here we examined effects of the naturally occurring related sphingosylphosphorylcholine (SPC) on human platelet function. 2 Platelet activation was determined as aggregation, elevation of intracellular Ca(2+) concentrations, surface expression of P-selectin, GP 53, and GP IIb/IIIa neoepitope PAC-1, and of fibrinogen binding to the platelet surface. 3 Platelets were activated by ADP (5 and 20 micro M), the thrombin receptor-activating peptide TRAP-6 (5 and 20 micro M), the thromboxane A(2) mimetic U-46619 (1 micro M) and collagen (20 and 50 micro g ml(-1)) but not by SPC (up to 20 micro M). 4 SPC concentration-dependently (IC(50) approximately 1-10 micro M) inhibited activation of washed human platelets in response to all of the above agonists, with almost complete inhibition occurring at 20 micro M SPC. 5 The SPC stereoisomers, D-erythro SPC and L-threo SPC, exhibited similar concentration-response curves in inhibiting 20 micro M ADP-induced platelet aggregation, suggesting that SPC did not act via specific lysophospholipid receptors. 6 Although SPC slightly activated platelet protein kinase A (as assessed by VASP phosphorylation), this effect could not explain the marked platelet inhibition. Possible protein kinase C inhibition also did not explain the inhibition of platelet activation by SPC. On the other hand, SPC suppressed agonist-induced Ca(2+) mobilization and phospholipase C stimulation. 7 These results indicate that the lysophospholipid SPC is an effective inhibitor of human platelet activation, apparently primarily by uncoupling agonist-activated receptors from their effectors.     

Comparative efficacy between the glycoprotein IIb/IIIa antagonists roxifiban and orbofiban in inhibiting platelet responses in flow models of thrombosis

This study was undertaken to compare the platelet binding characteristics and anti-platelet efficacy of a nonpeptide glycoprotein IIb/IIIa antagonist roxifiban with orbofiban in static and dynamic adhesion and aggregation assays. The results indicate that roxifiban binds with higher affinity to glycoprotein IIb/IIIa receptors and exhibits slower dissociation rates than orbofiban. Furthermore, the platelet inhibitory effects of roxifiban, but not orbofiban, were unaffected by changes in plasma calcium concentrations. Both agents reduced, in a concentration-dependent manner, the size of platelet thrombi deposited onto collagen I upon perfusion of heparinized blood at a shear rate of 1,500/s. At a clinically achievable concentration of 60 nM, roxifiban abrogated the formation of thrombi containing > 20 platelets per thrombus, thereby displaying comparable in vitro efficacy to that achieved by the theoretical maximal abciximab blood concentration (3.5 microg/ml) produced after standard treatment. In contrast, orbofiban, even at 500 nM, was only effective in inhibiting the formation of larger platelet thrombi (> or =150 platelets per thrombus). Pretreatment of surface-anchored platelets with roxifiban (100 nM), but not orbofiban (500 nM), inhibited monocytic THP-1 cell attachment under flow. However, this heterotypic adhesion process was also suppressed when orbofiban (500 nM) was maintained in the perfusion buffer during the entire course of flow experiment. These findings demonstrate roxifiban (unlike orbofiban) is a potent glycoprotein IIb/IIIa antagonist with a long receptor-bound lifetime and prolonged anti-platelet efficacy and may thus be beneficial for the treatment and prevention of acute ischemic syndromes.     

Eptifibatide: a potent inhibitor of the platelet receptor integrin glycoprotein IIb/IIIa

The platelet glycoprotein (GP) IIb/IIIa integrin plays a key role in mediating platelet aggregation. Blockade of the platelet GP IIb/IIIa receptor prevents arterial thrombosis in animal models much better than does aspirin. Among the most specific inhibitors in this class of drugs is eptifibatide (Integrilin^TMMillennium Pharmaceuticals, Inc.), a cyclic heptapeptide based on a peptide recognition sequence found in snake venom. Peptide inhibitors, such as eptifibatide, bind competitively to GP IIb/IIIa and have a short half-life, allowing the effect to be rapidly reversible and providing a favourable overall safety profile. Eptifibatide has been studied in a broad range of ischaemic coronary conditions including percutaneous coronary intervention (PCI), ST-segment and non-ST-segment acute myocardial infarction (MI) and unstable angina. In PCI and non-ST-segment MI, therapy with eptifibatide has been shown to reduce acute ischaemic complications without any increased risk of life-threatening adverse events. In the recently reported Enhanced Suppression of the Platelet IIb/IIIa Receptor with Integrilin Therapy (ESPRIT) trial, two 180 μg/kg boluses of eptifibatide, 10 min apart, followed by an 18 – 24 h infusion at 2 μg/kg/min given as adjunctive therapy in non-urgent PCI reduced the 30-day composite of death, MI and need for urgent target vessel revascularisation from 10.4 to 6.8% compared with placebo. These results were achieved under conditions of typical contemporary PCI, namely the implantation of second- and third-generation stents deployed at high balloon pressures along with modern adjunctive pharmacological treatment, particularly the universal use of thienopyridines and lower-dose heparin. Few significant pharmacological effects other than inhibition of platelet aggregation and the effect on bleeding time have been reported. Future research will focus on alternative clinical applications and combinations with other therapies to further improve cardiovascular outcomes.     

Phosphotyrosine signaling in platelets: lessons for vascular thrombosis

Platelet activation is crucial for normal hemostasis to arrest bleeding following vascular injury. However, excessive platelet activation in narrowed atherosclerotic blood vessels that are subject to high shear forces may initiate the onset of arterial thrombosis. When platelets come into contact with, and adhere to collagen exposed by damaged endothelium, they undergo morphological and functional changes necessary to generate a platelet-rich thrombus. This process is complex and involves precise co-ordination of various signaling pathways which lead to firm platelet adhesion to sites of tissue damage, release of granule contents from activated platelets, platelet shape change, platelet aggregation and subsequent thrombus formation and consolidation. Induction of tyrosine phosphorylation of key signaling molecules has emerged as a critical event central to stimulatory signaling pathways that generate platelet activation, but is an essential component associated with regulatory pathways that limit the extent of platelet activation. Understanding mechanisms that regulate platelet activation may contribute to the development of novel therapeutics that control common vascular diseases such as myocardial infarction and ischaemic stroke.     

Eptifibatide: a potent inhibitor of the platelet receptor integrin glycoprotein IIb/IIIa

The platelet glycoprotein (GP) IIb/IIIa integrin plays a key role in mediating platelet aggregation. Blockade of the platelet GP IIb/IIIa receptor prevents arterial thrombosis in animal models much better than does aspirin. Among the most specific inhibitors in this class of drugs is eptifibatide (Integrilin(TM), Millennium Pharmaceuticals, Inc.), a cyclic heptapeptide based on a peptide recognition sequence found in snake venom. Peptide inhibitors, such as eptifibatide, bind competitively to GP IIb/IIIa and have a short half-life, allowing the effect to be rapidly reversible and providing a favourable overall safety profile. Eptifibatide has been studied in a broad range of ischaemic coronary conditions including percutaneous coronary intervention (PCI), ST-segment and non-ST-segment acute myocardial infarction (MI) and unstable angina. In PCI and non-ST-segment MI, therapy with eptifibatide has been shown to reduce acute ischaemic complications without any increased risk of life-threatening adverse events. In the recently reported Enhanced Suppression of the Platelet IIb/IIIa Receptor with Integrilin Therapy (ESPRIT) trial, two 180 microg/kg boluses of eptifibatide, 10 min apart, followed by an 18 - 24 h infusion at 2 microg/kg/min given as adjunctive therapy in non-urgent PCI reduced the 30-day composite of death, MI and need for urgent target vessel revascularisation from 10.4 to 6.8% compared with placebo. These results were achieved under conditions of typical contemporary PCI, namely the implantation of second- and third-generation stents deployed at high balloon pressures along with modern adjunctive pharmacological treatment, particularly the universal use of thienopyridines and lower-dose heparin. Few significant pharmacological effects other than inhibition of platelet aggregation and the effect on bleeding time have been reported. Future research will focus on alternative clinical applications and combinations with other therapies to further improve cardiovascular outcomes.