Dissociation of glycoprotein IIb/IIIa antagonists from platelets does not result in fibrinogen binding or platelet aggregation

RUC-4: a novel αIIbβ3 antagonist for prehospital therapy of myocardial infarction

OBJECTIVE

Treatment of myocardial infarction within the first 1 to 2 hours with a thrombolytic agent, percutaneous coronary intervention, or an αIIbβ3 antagonist decreases mortality and the later development of heart failure. We previously reported on a novel small molecule αIIbβ3 antagonist, RUC-2, that has a unique mechanism of action. We have now developed a more potent and more soluble congener of RUC-2, RUC-4, designed to be easily administered intramuscularly by autoinjector to facilitate its use in the prehospital setting. Here, we report the properties of RUC-4 and the antiplatelet and antithrombotic effects of RUC-2 and RUC-4 in animal models.

APPROACH AND RESULTS

RUC-4 was ≈ 20% more potent than RUC-2 in inhibiting human ADP-induced platelet aggregation and much more soluble in aqueous solutions (60-80 mg/mL). It shared RUC-2's specificity for αIIbβ3 versus αVβ3, did not prime the receptor to bind fibrinogen, or induce changes in β3 identified by a conformation-specific monoclonal antibody. Both RUC-2 and RUC-4 prevented FeCl3-induced thrombotic occlusion of the carotid artery in mice and decreased microvascular thrombi in response to laser injury produced by human platelets infused into transgenic mice containing a mutated von Willebrand factor that reacts with human but not mouse platelets. Intramuscular injection of RUC-4 in nonhuman primates at 1.9 and 3.85 mg/kg led to complete inhibition of platelet aggregation within 15 minutes, with dose-dependent return of platelet aggregation after 4.5 to 24 hours.

CONCLUSIONS

RUC-4 has favorable biochemical, pharmacokinetic, pharmacodynamic, antithrombotic, and solubility properties as a prehospital therapy of myocardial infarction, but the possibility of increased bleeding with therapeutic doses remains to be evaluated.

Managing the Complications of Mild Therapeutic Hypothermia in the Cardiac Arrest Patient

Mild therapeutic hypothermia (MTH) is used to lower the core body temperature of cardiac arrest (CA) patients to 32°C from 34°C to provide improved survival and neurologic outcomes after resuscitation from in-hospital or out-of-hospital CA. Despite the improved benefits of MTH, there are potentially unforeseen complications associated during management. Although the adverse effects are transient, the clinician should be aware of the associated complications when managing the patient receiving MTH. We aim to provide the medical community comprehensive information related to the potential complications of survivors of CA receiving MTH, as it is imperative for the clinician to understand the physiologic changes that take place in the patient receiving MTH and how to prepare for them and manage them if they do occur. We hope to provide information of how to manage these potential complications through both a review of the current literature and a reflection of our own experience.

A review of the mechanisms and effectiveness of dietary polyphenols in reducing oxidative stress and thrombotic risk

Dietary sources of polyphenols, which are derivatives and/or isomers of flavones, isoflavones, flavonols, catechins and phenolic acids, possess antioxidant properties and therefore might be important in preventing oxidative-stress-induced platelet activation and attenuating adverse haemostatic function. Free radicals, including reactive oxygen and nitrogen species, promote oxidative stress, leading to platelet hyperactivation and the risk of thrombosis. The consumption of antioxidant/polyphenol rich foods might therefore impart anti-thrombotic and cardiovascular protective effects via their inhibition of platelet hyperactivation or aggregation. Most commonly-used anti-platelet drugs such as aspirin block the cyclooxygenase (COX)-1 pathway of platelet activation, similar to the action of antioxidants with respect to neutralising hydrogen peroxide (H2 O2 ), with a similar effect on thromboxane production via the COX-1 pathway. Polyphenols also target various additional platelet activation pathways (e.g. by blocking platelet-ADP, collagen receptors); thus alleviating fibrinogen binding to platelet surface (GPIIb-IIIa) receptors, reducing further platelet recruitment for aggregation and inhibiting platelet degranulation. As a result of the ability of polyphenols to target additional pathways of platelet activation, they may have the potential to substitute or complement currently used anti-platelet drugs in sedentary, obese, pre-diabetic or diabetic populations who can be resistant or sensitive to pharmacological anti-platelet therapy.

Integrin αIIbβ3: from discovery to efficacious therapeutic target

From the initial description of platelets in 1882, their propensity to aggregate and to contribute to thrombosis was apparent. Indeed, excessive platelet aggregation is associated with myocardial infarction and other thrombotic diseases whereas Glanzmann thrombasthenia, in which platelet aggregation is reduced, is a bleeding syndrome. Over the last half of the 20th century, many investigators have provided insights into the cellular and molecular basis for platelet aggregation. The major membrane protein on platelets, integrin αIIbβ3, mediates this response by rapidly transiting from its resting to an activated state in which it serves as a receptor for ligands that can bridge platelets together. Monoclonal antibodies, natural products, and small peptides were all shown to inhibit αIIbβ3 dependent platelet aggregation, and these inhibitors became the forerunners of antagonists that proceeded through preclinical testing and into large patient trials to treat acute coronary syndromes, particularly in the context of percutaneous coronary interventions. Three such αIIbβ3 antagonists, abciximab, eptifibatide, and tirofiban, received Food and Drug Administration approval. Over the past 15 years, millions of patients have been treated with these αIIbβ3 antagonists and many lives have been saved by their administration. With the side effect of increased bleeding and the development of new antithrombotic drugs, the use of αIIbβ3 antagonists is waning. Nevertheless, they are still widely used for the prevention of periprocedural thrombosis during percutaneous coronary interventions. This review focuses on the biology of αIIbβ3, the development of its antagonists, and some of the triumphs and shortcomings of αIIbβ3 antagonism.

Structure-guided design of a high-affinity platelet integrin αIIbβ3 receptor antagonist that disrupts Mg²⁺ binding to the MIDAS

An integrin found on platelets, α(IIb)β(3) mediates platelet aggregation, and α(IIb)β(3) antagonists are effective antithrombotic agents in the clinic. Ligands bind to integrins in part by coordinating a magnesium ion (Mg(2+)) located in the β subunit metal ion-dependent adhesion site (MIDAS). Drugs patterned on the integrin ligand sequence Arg-Gly-Asp have a basic moiety that binds the α(IIb) subunit and a carboxyl group that coordinates the MIDAS Mg(2+) in the β(3) subunits. They induce conformational changes in the β(3) subunit that may have negative consequences such as exposing previously hidden epitopes and inducing the active conformation of the receptor. We recently reported an inhibitor of α(IIb)β(3) (RUC-1) that binds exclusively to the α(IIb) subunit; here, we report the structure-based design and synthesis of RUC-2, a RUC-1 derivative with a ~100-fold higher affinity. RUC-2 does not induce major conformational changes in β(3) as judged by monoclonal antibody binding, light scattering, gel chromatography, electron microscopy, and a receptor priming assay. X-ray crystallography of the RUC-2-α(IIb)β(3) headpiece complex in 1 mM calcium ion (Ca(2+))/5 mM Mg(2+) at 2.6 Å revealed that RUC-2 binds to α(IIb) the way RUC-1 does, but in addition, it binds to the β(3) MIDAS residue glutamic acid 220, thus displacing Mg(2+) from the MIDAS. When the Mg(2+) concentration was increased to 20 mM, however, Mg(2+) was identified in the MIDAS and RUC-2 was absent. RUC-2's ability to inhibit ligand binding and platelet aggregation was diminished by increasing the Mg(2+) concentration. Thus, RUC-2 inhibits ligand binding by a mechanism different from that of all other α(IIb)β(3) antagonists and may offer advantages as a therapeutic agent.

Use of antiplatelet agents for prevention of ischemic stroke

Overall management to lower risk for ischemic stroke is multifaceted. Management includes measures to treat risk factors for accelerated atherosclerosis and stroke, antithrombotic therapies to lower the risk for thromboembolism, and surgery to treat a defined arterial or cardiac lesion. Treatment decisions are made on a case-by-case basis, with most patients receiving some combination of medication and recommendations for lifestyle modification. Some patients will also undergo surgical or endovascular interventions. This article discusses antithrombotic treatment for ischemic stroke prevention, placing major emphasis on the indications for and administration of antiplatelet therapy.

Comparison of the effects of pretreatment with tirofiban, clopidogrel or both on the inhibition of platelet aggregation and activation in patients with acute coronary syndromes

OBJECTIVE

We sought to compare platelet inhibition produced by three antiplatelet regimens.

METHODS AND RESULTS

Sixty NSTE-ACS patients undergoing coronary angiography treated with aspirin and enoxaparin were randomised to receive tirofiban 0.4 microg/kg/min over 30 min plus 0.15 microg/kg/min over 24 h (A), clopidogrel 600 mg (B), clopidogrel 300 mg plus tirofiban (C); blood samples were taken at baseline and 2, 6 and 24 h after the drug administration, and were analyzed by light transmission aggregometry and flow cytometry. Treatment with clopidogrel 600 mg significantly reduced P-selectin expression in comparison with tirofiban alone at all time points (group B vs. A: P < 0.0001). However tirofiban inhibited platelet aggregation significantly more than clopidogrel 600 mg during the first 6 h (group A vs. B: P < 0.0001), and the addition of clopidogrel 300 mg did not inhibit platelet aggregation any more than tirofiban alone throughout the 24 h (group C vs. A: P = NS). All of the changes over time within each group were highly significant (P < 0.0001).

CONCLUSIONS

Tirofiban leads to greater early inhibition of platelet aggregation but less suppression of P-selectin expression than clopidogrel 600 mg. The addition of clopidogrel to tirofiban does not add any anti-aggregatory effect, but reduces P-selectin expression, thus likely adding a significant biological and clinical protective effect and providing a rationale for the combined use of the two drugs.

Rapid stimulation of tyrosine phosphorylation signals downstream of G-protein-coupled receptors for thromboxane A2 in human platelets

Signals ensuing from trimeric G-protein-coupled receptors synergize to induce platelet activation. At low doses, the thromboxane A2 analogue U46619 does not activate integrin alphaIIbbeta3 or trigger platelet aggregation, but it induces shape changes. In the present study, we addressed whether low doses of U46619 trigger tyrosine phosphorylation independently of integrin alphaIIbbeta3 activation and ADP secretion, and synergize with adrenaline (epinephrine) to induce aggregation in acetylsalicylic acid (aspirin)-treated platelets. Low doses of U46619 triggered tyrosine phosphorylation of different proteins, including FAK (focal adhesion kinase), Src and Syk, independently of signals ensuing from integrin alphaIIbbeta3 or ADP receptors engaged by secreted ADP. The G(12/13)-mediated Rho/Rho-kinase pathway was also increased by low doses of U46619; however, this pathway was not upstream of tyrosine phosphorylation, because this occurred in the presence of the Rho-kinase inhibitor Y-27632. Although low doses of U46619 or adrenaline alone were unable to trigger platelet aggregation and integrin alphaIIbbeta3 activation, the combination of the two stimuli effectively induced these responses. PP2, a tyrosine kinase inhibitor, and Y-27632 inhibited platelet activation induced by low doses of U46619 plus adrenaline and, when used in combination, totally suppressed this platelet response. In addition, the two inhibitors selectively blocked tyrosine kinases and the Rho/Rho-kinase pathway respectively. These findings suggest that both tyrosine phosphorylation and the Rho/Rho-kinase pathway are required to activate platelet aggregation via G(12/13) plus G(z) signalling.

Immune thrombocytopenia caused by glycoprotein IIb/IIIa inhibitors

Agents that react with the platelet glycoprotein (GP) IIb/IIIa complex (alphaIIb/beta3 integrin) to block fibrinogen binding and platelet-platelet aggregation have been proved to be effective in reducing the incidence of complications following coronary angioplasty and are now widely used for this purpose. Acute thrombocytopenia, which is sometimes severe and life-threatening, is a recognized side effect of this class of drugs. In contrast to other types of drug-induced thrombocytopenia, this complication can occur within a few hours of a patient's first exposure to the medication. Accumulating evidence has indicated that drug-dependent antibodies, which can be naturally occurring, are the cause of platelet destruction in such individuals. In this review, we will consider the clinical aspects of thrombocytopenia resulting from sensitivity to GPIIb/IIIa inhibitors and will review evidence that the platelet destruction is antibody-mediated.

GPIIb-IIIa antagonists reduce thromboinflammatory processes in patients with acute coronary syndromes undergoing percutaneous coronary intervention

OBJECTIVE

To investigate the effects of abciximab, eptifibatide and no GPIIb-IIIa antagonist (control) on soluble CD40 ligand (sCD40L) and the formation of leukocyte-platelet aggregates (LPA) in 98 ACS patients undergoing percutaneous coronary intervention (PCI).

BACKGROUND

sCD40L and LPA are increased in patients with ACS.

METHODS

sCD40L was measured by enzyme-linked immunosorbent assay (ELISA) and LPA by whole blood flow cytometry.

RESULTS

There were no baseline differences between the three groups in sCD40L and LPA. At the end of PCI, sCD40L was unchanged in the controls, decreased by 30% (P < 0.001) in the abciximab group and by 11% (P < 0.02) in the eptifibatide group. Eighteen to 24 h after PCI, sCD40L was unchanged in the controls, reduced 30% (P < 0.001) in the abciximab-treated group and 9% (P < 0.01) in the eptifibatide-treated group. At the end of PCI, circulating monocyte-platelet aggregates (MPA) were reduced by 12% (P = NS) in the abciximab-treated group, 13% in the eptifibatide-treated group (P = NS), but slightly increased in the controls (P = NS). Eighteen to 24 h after PCI, MPA were reduced by 41% (P < 0.001) compared to baseline in the abciximab-treated group, by 23% (P = NS) in the eptifibatide-treated group, and 15% (P = NS) in the controls. In contrast to control patients presenting while on clopidogrel, control patients presenting not on clopidogrel demonstrated a reduction in sCD40L and LPA 18-24 h post-PCI (P = NS). At low receptor occupancy, GPIIb-IIIa antagonists did not augment the release of sCD40L or the number of circulating LPA.

CONCLUSIONS

GPIIb-IIIa antagonists reduce circulating sCD40L and LPA formation in patients with ACS undergoing PCI. At low receptor occupancy, GPIIb-IIIa antagonists do not activate platelets.

In vivo platelet redistribution and acute transient thrombocytopenia after eptifibatide injection in baboons

BACKGROUND

The occurrence of thrombocytopenia has been reported during clinical eptifibatide (Integrilin) therapy, but the exact mechanism is not yet established to explain the varied duration and severity of thrombocytopenia associated with glycoprotein (GP) IIb/IIIa inhibitors. We assessed the redistribution of platelets in juvenile baboons during acute transient thrombocytopenia that was observed after eptifibatide injection.

METHODS

Eptifibatide was administered intravenously to eight baboons by infusion at 20 microg/kg/min or a bolus injection of 10 mg. Platelet distribution was measured with a gamma scintillation camera using 111In-labeled autologous platelets. Platelet function and GP IIb/IIIa receptor inhibition were evaluated using the Plateletworks system. The effects of pretreatment with abciximab (0.4 mg/kg) or human immunoglobulin concentrate (0.75 g/kg) were also investigated.

RESULTS

Eptifibatide, administered as an infusion or a bolus, caused transient thrombocytopenia with uptake of platelets predominantly by the liver. The recovery of platelet aggregation was associated with the re-entry of platelets from the liver into the systemic circulation. Pretreatment with either abciximab (0.4 mg/kg) or human intravenous immunoglobulin (IVIG, 0.75 g/kg) attenuated eptifibatide-induced thrombocytopenia and the hepatic uptake of radiolabeled platelets.

CONCLUSION

Acute thrombocytopenia after eptifibatide injection was caused by the transient redistribution of platelets to the liver. Attenuation of the decrease in platelet count and hepatic sequestration by abciximab and IVIG suggests that thrombocytopenia may have been caused by ligand-induced binding site antigen induction and recognition by the reticuloendothelial system.

A perspective on the toxicological mechanisms possibly contributing to the failure of oral glycoprotein IIb/IIIa antagonists in the clinic

In clinical trials in patients with acute or unstable coronary syndromes and/or undergoing percutaneous coronary intervention, oral glycoprotein (GP) IIb/IIIa antagonists did not show therapeutic benefit over aspirin during long-term administration. Moreover, high-dose oral administration of these agents was associated with greater fatality risk compared with that of lower doses. This article postulates that continuous exposure of the GP IIb/IIIa receptor (integrin alpha(IIb)beta(3)) to these agents may result in some form of resistance or activation of other biological systems. These toxicological mechanisms may help explain some factors that could potentially contribute to the failure of these agents in clinical trials. Several hypotheses are presented: (i) modulation of platelet response because of long-term exposure to GP IIb/IIIa antagonists; (ii) role of related integrins and associated proteins to compensate for the loss of platelet activity because of dysfunctional GP IIb/IIIa receptors occupied by inhibitors; (iii) effects of the GP IIb/IIIa antagonists on other cellular systems such as the caspase and procaspase enzymes in apoptosis and possibly the ryanodine receptor involved in sarcoplasmic reticulum calcium release. These toxicological mechanisms could potentially limit the utility of these oral agents in long-term administration.

Fibrinogen receptor antagonists induce conformational changes of the human platelet glycoprotein IIb

BACKGROUND

Controversial results have been reported concerning the ability of fibrinogen receptor antagonists (fibans) to induce conformational changes in the fibrinogen receptor after binding to it as the initial step of fibrinogen binding and platelet activation.

METHODS

Platelets in citrated whole blood were stained with several pairs of anti-glycoprotein (anti-GP) IIb-directed monoclonal antibodies conjugated to phycoerythrin (PE) or indirectly labeled with Cy5. Pairs of monoclonal antibodies that induced a high-fluorescence resonance energy transfer (FRET) efficiency served as tools to detect activation-dependent changes of GP IIb after addition of adenosine diphosphate and several fibans.

RESULTS

Using the combination of the clones 5B12-PE and P2-biotin/SA-Cy5, a concentration-dependent alteration of the GP IIb conformation was observed after addition of tirofiban, eptifibatide, and lotrafiban. Magnitude and kinetics differed among the investigated substances.

CONCLUSION

The newly developed FRET assay allows the direct investigation of conformational changes of GP IIb after addition of platelet agonists or receptor ligands, as shown for three fibans.

Eptifibatide provides additional platelet inhibition in Non–ST-Elevation myocardial infarction patients already treated with aspirin and clopidogrel

OBJECTIVES

The present study hypothesis was that eptifibatide offered further antiplatelet efficacy above clopidogrel in non-ST-elevation myocardial infarction (NSTEMI) patients before an expeditive coronary intervention.

BACKGROUND

Although thienopyridines and glycoprotein (GP) IIb/IIIa antagonists are often co-prescribed in the context of NSTEMI, the antiplatelet interaction of these agents is poorly described and the superiority of GP IIb/IIIa antagonists above thienopyridine treatment alone is not clear.

METHOD

Thirty-two NSTEMI patients treated with aspirin and enoxaparin were studied using flow cytometry to define parameters of platelet activation with a panel of agonists before clopidogrel, after clopidogrel, and during an eptifibatide infusion following the clopidogrel load.

RESULTS

After platelet activation with adenosine diphosphate, thrombin receptor-activating peptide, or U46-619, relative reductions in conformationally activated GP IIb/IIIa receptor expression (evaluated with PAC-1) of 48%, 43%, and 33%, respectively (all p < 0.0001), were seen with clopidogrel, but further 80%, 78%, and 72% (all p < 0.0001) reductions were seen with eptifibatide. With the same agonists, fibrinogen binding was significantly reduced after clopidogrel by 70%, 64%, and 81% (all p < 0.0001) and again further reduced with eptifibatide by 90%, 95%, and 69% (all p < 0.0001). The total number of GP IIb/IIIa receptors (measured as P2 expression) and P-selectin expression fell after clopidogrel, after ex vivo stimulation with the same agonists; however, both parameters increased slightly during the eptifibatide infusion.

CONCLUSIONS

The activated GP IIb/IIIa expression and fibrinogen binding findings indicate that eptifibatide provides significant potent antiplatelet activity above aspirin and clopidogrel, suggesting additive immediate protection in the treatment of NSTEMI. The P2 and P-selectin findings suggest the possibility of a partial agonist and/or pro-inflammatory effect.

Reversibility versus Persistence of GPIIb/IIIa Blocker-Induced Conformational Change of GPIIb/IIIa (αIIbβ3, CD41/CD61)

Clinically used GPIIb/IIIa blockers are ligand mimetics, and thereby their binding can induce conformational changes of the platelet integrin GPIIb/IIIa. Since the reversibility of these conformational changes may be an important determinant of potential adverse effects of GPIIb/IIIa blockers, we produced a new monoclonal antibody (anti-LIBS-mAb), and by using its binding properties, we investigated the conformational changes of GPIIb/IIIa during the binding and especially the dissociation of GPIIb/IIIa blockers. Production of monoclonal antibody (mAb) clones was performed using purified GPIIb/IIIa in a high affinity conformation and using activated platelets. Clone anti-LIBS-145-mAb was chosen, since it allowed the sensitive probing of eptifibatide-induced conformational changes of GPIIb/IIIa. On resting and activated platelets and on GPIIb/IIIa-expressing Chinese hamster ovary cells, anti-LIBS-145-mAb binding returned to background binding after dissociation of eptifibatide, indicating a complete reversibility of the eptifibatide-induced conformational change. Furthermore, with the mixing of eptifibatide-preincubated and nonincubated cells, a fast reversibility could be demonstrated. However, when fibrinogen was present in a physiological concentration, the GPIIb/IIIa blocker-induced conformation was partially retained after the dissociation of eptifibatide and to the same extent binding of fibrinogen and the activation-specific mAb Pac-1 was induced. In conclusion, a fast reversibility of the conformational change of GPIIb/IIIa after dissociation of GPIIb/IIIa blockers could be demonstrated as an intrinsic property of the GPIIb/IIIa receptor. This mechanism prevents general platelet aggregation after dissociation of ligand mimetic GPIIb/IIIa blockers. Nevertheless, in the presence of fibrinogen this reversibility is not complete, which may explain some of the side effects of GPIIb/IIIa blockers, especially those of the oral GPIIb/IIIa blockers.

Abciximab: an updated review of its therapeutic use in patients with ischaemic heart disease undergoing percutaneous coronary revascularisation

Abciximab (Reopro) is an antibody fragment that dose-dependently inhibits platelet aggregation and leucocyte adhesion by binding to the glycoprotein (GP) IIb/IIIa, vitronectin and Mac-1 receptors. Abciximab (0.25 mg/kg bolus plus infusion of 0.125 micro g/kg/min for 12 hours) showed greater efficacy than tirofiban in reducing the 30-day composite endpoint of death, nonfatal myocardial infarction (MI) or urgent target-vessel revascularisation in the randomised, double-blind TARGET study in patients scheduled for stent placement. In addition, the beneficial effects of treatment with abciximab previously observed in the randomised, multicentre, placebo-controlled EPILOG and EPISTENT studies have been maintained to 1 year, with a significantly reduced incidence of ischaemic complications relative to placebo consistently observed across a range of subgroups including age, sex, bodyweight and indication for revascularisation. The incidence of the composite endpoint was reduced in patients presenting with acute MI of <48 hours' duration in comparison with either fibrinolytic therapy or stenting alone in the randomised STOPAMI and ADMIRAL trials, primarily because of a reduced requirement for urgent repeat revascularisation and reduced incidence of mortality. In the randomised, nonblind, multicentre CADILLAC trial in patients with acute myocardial infarction (MI), stenting alone was superior to percutaneous transluminal coronary angioplasty (PTCA) and stenting alone was not inferior to PTCA plus abciximab. Recent large randomised, multicentre studies (ASSENT-3 and GUSTO-V) have shown higher efficacy (on various ischaemic endpoints) of abciximab in combination with either a reduced dose of tenecteplase or reteplase compared with the fibrinolytic drug alone. TIMI grade 3 flow rates at 60 and 90 minutes in the TIMI-14 and SPEED trials were higher in patients who received abciximab in combination with either alteplase or reteplase than abciximab alone and were similar to that seen with the full-dose fibrinolytic alone. In the randomised, multicentre GUSTO IV-ACS study, no significant differences in any of the ischaemic endpoints at either 7 or 30 days in patients with acute coronary syndromes who were not scheduled to undergo early revascularisation (within 12 hours of end of infusion) were apparent between those who received abciximab (bolus and either 24- or 48-hour infusion) and those who received placebo in addition to aspirin and heparin. The most common adverse events associated with the use of abciximab are bleeding complications and thrombocytopenia, although the risk of major bleeding can be limited through adhering to current administration protocols. Treatment costs are generally higher in both stent plus abciximab and angioplasty plus abciximab groups than stent plus placebo, primarily because of the acquisition cost of abciximab. Abciximab appeared most cost beneficial in high-risk patients undergoing elective percutaneous coronary revascularisation; among lower risk patients, abciximab therapy has been associated with higher total in-hospital and 6-month medical costs than eptifibatide.

CONCLUSION

The GP IIb/IIIa receptor antagonist abciximab, when used with aspirin and heparin, has demonstrated efficacy in reducing the short- and long-term risk of ischaemic complications in patients with ischaemic heart disease undergoing percutaneous coronary intervention, when used with aspirin and heparin. High-risk patients (including those with diabetes mellitus) derive particular benefits from abciximab treatment. Abciximab remains an important therapeutic option for the prevention of complications in patients with ischaemic heart disease.

Therapeutic antagonists and conformational regulation of integrin function

Integrins are a structurally elaborate family of adhesion molecules that transmit signals bi-directionally across the plasma membrane by undergoing large-scale structural rearrangements. By regulating cell-cell and cell-matrix contacts, integrins participate in a wide range of biological processes, including development, tissue repair, angiogenesis, inflammation and haemostasis. From a therapeutic standpoint, integrins are probably the most important class of cell-adhesion receptors. Recent progress in the development of integrin antagonists has resulted in their clinical application and has shed new light on integrin biology. On the basis of their mechanism of action, small-molecule integrin antagonists fall into three different classes. Each of these classes affect the equilibria that relate integrin conformational states, but in different ways.

Effects of the glycoprotein IIb/IIIa antagonist Roxifiban on P-selectin expression, fibrinogen binding, and microaggregate formation in a phase I dose-finding study: no evidence for platelet activation during treatment with a glycoprotein IIb/IIIa antagonist

The hypothesis that glycoprotein (GP) IIb/IIIa antagonists stimulate platelets is controversial. Here, we report the results of flow cytometric measurements of platelet activation markers in a phase I dose optimization study of Roxifiban, an orally active GP IIb/IIIa antagonist. Whole blood was collected at pre-dose and during the dosing interval directly into citrate fixative so that circulating levels of platelet activation could be assessed. P-selectin expression and fibrinogen binding of single platelets were unchanged at any of the dosing intervals compared to the pre-dose values, whereas microaggregate formation was reduced. Blood was also collected in hirudin to maintain physiological calcium concentrations and stimulated with platelet agonists to test whether GP IIb/IIIa antagonists lower the threshold for platelet activation. After stimulation with a concentration range of ADP and TRAP, P-selectin expression was not altered by Roxifiban administration compared to pre-dose levels. Fibrinogen binding and microaggregate formation were reduced by Roxifiban dosing in a dose-dependent manner. Inhibition of both parameters was retained at trough and no increase above pre-dose values was observed at any time. This study provides evidence for a dose-dependent inhibition of platelet functions by an orally active GP IIb/IIIa antagonist and does not detect paradoxical activation of platelets by a GP IIb/IIIa antagonist in humans.

Effects of glycoprotein IIb/IIIa antagonists on platelet activation: development of a transfer method to mimic peak to trough receptor occupancy

Several oral glycoprotein (GP) IIb/IIIa antagonists, Sibrafiban, Orbofiban and Lotrafiban, have been studied in large phase III trials; each has failed to provide efficacy and has been associated with increased mortality. Roxifiban has pharmacokinetic and pharmacodynamic properties believed to be more favorable than the earlier oral agents. Here, we revisit the controversial hypothesis of platelet activation liabilities of GP IIb/IIIa antagonists. The effects of site occupancy by four fibans (Roxifiban, Sibrafiban, Orbofiban and Lotrafiban) on platelet activation was assessed using P-selectin expression, fibrinogen binding and microaggregate formation. All four fibans inhibited ADP and TRAP-stimulated fibrinogen binding and microaggregate formation in a concentration-dependent manner, whereas P-selectin expression was relatively unaltered. To more vigorously test for activation liabilities, the effects of transition from peak to trough receptor occupancy upon platelet stimulation was analyzed. The high affinity of Roxifiban for resting platelets precluded reduction of site occupancy by dialysis or gel filtration. A method was developed that takes advantage of the rapid equilibrium of Roxifiban between platelets and soluble GPIIb/IIIa. The platelet occupancy is controlled by the ratio of platelet GPIIb/IIIa to soluble GPIIb/IIIa. This method allows in vitro investigation of peak/trough transitions on platelet activation. A decrease in site occupancy from peak to trough of Roxifiban or Sibrafiban did not result in increased activation of platelets. The loss of platelet-bound antagonist upon incubation with purified soluble GPIIb/IIIa returned fibrinogen binding/microaggregate formation to no drug levels. In conclusion, these studies do not provide evidence for an activation liability of GPIIb/IIIa antagonists in vitro.

Anti-platelet effects of GPIIb/IIIa and P-selectin antagonism, platelet activation, and binding to neutrophils

Platelet activation with GPIIb/IIIa binding to fibrinogen, aggregation and interaction with leukocytes constitute the principal mediator of thrombosis. Although the clinical benefits of GPIIb/IIIa antagonists have been documented, the relationship between their anti-platelet properties, platelet activation and binding to leukocytes is still debated. We investigated the effects of abciximab, tirofiban, roxifiban, and an anti-P-selectin blocking monoclonal antibody (Mab) on isolated human platelet aggregation using optical aggregometer, and on platelet P-selectin and GPIIb/IIIa expression, and platelet-neutrophil binding using flow cytometry. Thrombin at 0.025 U/ml induced maximal platelet aggregation (76.3 +/- 2.6%), P-selectin expression (88.5 +/- 4%), GPIIb/IIIa activation (PAC-1 binding, 86.2 +/- 8.9%) and platelet-neutrophil binding (58.0 +/- 6.4%). The GPIIb/IIIa antagonists inhibited in a concentration-dependent manner platelet aggregation (IC50 of 100 nM for abciximab and tirofiban and 50 nM for roxifiban) and PAC-1 binding, without any effect on P-selectin. None of these agents affected significantly platelet-neutrophil binding, whereas an anti-P-selectin Mab abolished this binding and amplified the effect of abciximab on platelet aggregation. These results indicate that the effects of these GPIIb/IIIa antagonists on platelet aggregation are not related to inhibition of platelet activation, as P-selectin levels and platelet-neutrophil binding remained unaffected, and highlight the participation of P-selectin with GPIIb/IIIa in platelet aggregation.

The failure of orally administered glycoprotein IIb/IIIa inhibitors to prevent recurrent cardiac events

PURPOSE

Despite the success of intravenous glycoprotein IIb/IIIa antagonists, oral formulations have failed to show benefit and have been associated with increased mortality. To understand these findings, we performed a meta-analysis of results from four phase 3 trials.

SUBJECTS AND METHODS

Trials were identified by MEDLINE search; review of abstracts from American College of Cardiology, European Society of Cardiology, and American Heart Association scientific sessions; or querying investigators in the field. Published, phase 3, randomized, placebo-controlled trials involving more than 1000 patients with coronary artery disease that compared an oral glycoprotein IIb/IIIa antagonist with or without background aspirin versus aspirin, and that had a planned follow-up of > or =30 days, were included. Four trials met these criteria. Odds ratios (ORs) and 95% confidence intervals (CIs) were generated from results, and combined using an empirical Bayes random-effects model.

RESULTS

Among 33,326 patients, oral glycoprotein IIb/IIIa agents were associated with 31% increased mortality (OR = 1.31; 95% CI: 1.12 to 1.53; P= 0.0001). Results were similar whether the agent was added to (OR = 1.38; 95% CI: 1.15 to 1.67) or substituted for (OR = 1.37; 95% CI: 1.00 to 1.86) aspirin. Ischemic events or sudden death (OR = 1.22; 95% CI: 0.91 to 1.63) were also more common. Among patients with acute coronary syndromes, the incidence of myocardial infarction was increased (OR = 1.16; 95% CI: 1.03 to 1.29).

CONCLUSION

Oral glycoprotein IIb/IIIa inhibitor therapy is associated with increased mortality and myocardial infarction. No single explanation for these findings is satisfactory; the problem is likely to be multifactorial.

Low incidence of paradoxical platelet activation by glycoprotein IIb/IIIa inhibitors

The human platelet antigen-1 (HPA-1, Pl(A)) polymorphism has been proposed to influence the inhibitory actions of abciximab. Thus, we hypothesized that this polymorphism might also be the cause for paradoxical activation of platelets by GPIIb/IIIa inhibitors. The effects of abciximab (1-10 microg/ml), tirofiban (3-30 nM), or eptifibatide (0.3-3 microg/ml) on basal and ADP (3 microM)-induced CD62P externalization were measured in n=62 healthy blood donors and n=177 patients with stable coronary artery disease. All subjects were genotyped for the human platelet antigen-1 (HPA-1, Pl(A)) polymorphism by GALIOS(R) and fluorescence correlation spectroscopy. Although a significant platelet hyperreactivity was observed in the patients, the HPA-1 genotype did not influence basal or ADP-induced CD62P expression. A moderate (twofold) stimulation of CD62P expression by abciximab but not by tirofiban or eptifibatide was observed in one patient. Interestingly, this patient carried the HPA-1 b/b genotype. In no other subject any activation of platelets by GP IIb/IIIa inhibitors was observed and there were no statistically significant differences between HPA-1 genotypes with respect to the effects of GP IIb/IIIa inhibitors on basal or ADP-stimulated CD62P expression. It is concluded that paradoxical platelet activation by abciximab is a rare (<2%) phenomenon. HPA-1 b/b genotype might be a contributing factor but clearly does not predict platelet activation by GP IIb/IIIa inhibitors.

Leukocyte-platelet aggregation, platelet surface P-selectin, and platelet surface glycoprotein IIIa after percutaneous coronary intervention: Effects of dalteparin or unfractionated heparin in combination with abciximab

BACKGROUND

Plaque disruption with resultant platelet activation and leukocyte-platelet aggregation is a pathophysiologic process common to both acute coronary syndromes and percutaneous coronary interventions. Unfractionated heparin is a standard antithrombotic therapy in patients with both acute coronary syndromes and in those undergoing percutaneous coronary interventions. Low-molecular-weight heparins have been reported to cause less platelet activation than unfractionated heparin.

METHODS

Monocyte-platelet aggregates, neutrophil-platelet aggregates, platelet surface P-selectin, and platelet surface glycoprotein (GP) IIIa were measured serially by whole blood flow cytometry in 40 patients with unstable angina (randomly assigned to either unfractionated heparin 70 U/kg or the low-molecular-weight heparin dalteparin 60 IU/kg) undergoing coronary intervention with planned abciximab administration (in 2, one-half-dose boluses). Assays were performed at baseline, 5 minutes after administration of either type of heparin, 10 minutes after the first bolus of abciximab, 10 minutes after second bolus of abciximab, and 8 to 10 and 16 to 24 hours after administration of either heparin.

RESULTS

No significant differences in clinical outcomes were observed between patients receiving either unfractionated heparin or dalteparin. The number of circulating P-selectin-positive platelets was increased by unfractionated heparin but not dalteparin, and abciximab reversed this increase. The number of circulating P-selectin-positive platelets was reduced below baseline levels in both treatment groups 8 to 10 and 16 to 24 hours after study drug administration. At 8 to 10 and 16 to 24 hours after administration of study drug, platelet degranulation in response to iso-thrombin receptor agonist peptide 1.5 mmol/L was significantly reduced by almost 50% (compared with immediately after study drug administration). Both unfractionated heparin and dalteparin significantly increased the numbers of circulating monocyte-platelet and neutrophil-platelet aggregates, which were subsequently reduced to baseline levels after administration of the second abciximab bolus and to below baseline at both 8 to 10 and 16 to 24 hours in all patients. After both unfractionated heparin and dalteparin administration, platelet surface GP IIIa expression was significantly increased compared with baseline at both 8 to 10 and 16 to 24 hours.

CONCLUSIONS

Dalteparin in combination with abciximab in patients with unstable angina undergoing coronary intervention appears to be safe. Unfractionated heparin, but not dalteparin, degranulates platelets in patients with unstable angina. Both heparins increase the number of circulating monocyte-platelet and neutrophil-platelet aggregates. Abciximab therapy during coronary interventions rapidly reduces the number of degranulated platelets and leukocyte-platelet aggregates.