Platelet glycoprotein IIb/IIIa receptor blockers: An appropriate-use model for expediting care in acute coronary syndromes

Downstream administration of a high-dose tirofiban bolus in high-risk patients with unstable angina undergoing early percutaneous coronary intervention

BACKGROUND

The best treatment option for high-risk patients with unstable coronary syndrome is an early invasive strategy accompanied by intensive anti-platelet therapy. We tested the effect on clinical outcome of early coronary angioplasty using a high-dose bolus of tirofiban in patients with non-ST segment elevation acute coronary syndrome.

METHODS

One hundred and forty consecutive patients with unstable coronary syndrome who underwent an immediate percutaneous coronary intervention with the administration of a high (25 microg/kg) dose bolus of tirofiban followed by an 18-h infusion of 0.15 microg kg(-1) min(-1) were compared with a matched control group of 162 patients treated with abciximab. The primary endpoint of the study was the 30-day incidence of major adverse cardiac events; the secondary endpoints were the incidence of major and minor bleeding.

RESULTS

The time from admission to PCI was slightly shorter in the tirofiban group (3.9+/-4.8 vs. 4.5+/-4.4 h; P=0.26). The 30-day rate of major adverse cardiac events was similar in the two groups (6% with tirofiban and 8.6% with abciximab: OR=1.37, 95% CI=0.58-3.29, P=0.52). No major bleeding episodes were observed; the incidence of minor bleeding was 3.6% in the tirofiban group and 2.5% in the abciximab group (OR=0.68, 95% CI=0.18-2.59, P=0.74).

CONCLUSIONS

In this preliminary study, the beneficial effect of the administration of a high-dose tirofiban bolus on 30-day clinical outcomes was similar to that of abciximab in high-risk patients with unstable angina undergoing immediate percutaneous coronary intervention. The results of this therapeutic strategy should be tested in a larger randomised study.

Adjunctive Pharmacotherapy before Percutaneous Coronary Intervention in Non-ST-Elevation Acute Coronary Syndromes: The Role of Modulating Inflammation

Vascular inflammation is central to the pathogenesis of acute coronary syndromes (ACS) and the response to vascular injury after percutaneous coronary intervention (PCI). For both ACS and PCI, the magnitude of vascular inflammation is linked to adverse late clinical outcomes (e.g., death, recurrent myocardial infarction [MI] or ischemia, and restenosis). Many pharmacologic therapies with demonstrated efficacy for the treatment of ACS have anti-inflammatory properties, which are distinct from their perceived primary mechanism of action. The anti-inflammatory effects of aspirin, clopidogrel, low-molecular-weight heparin (LMWH), platelet glycoprotein (GP) IIb/IIIa receptor inhibitors, statins, and angiotensin converting enzyme (ACE) inhibitors are reviewed, and the hypothesis is generated that modulation of vascular inflammation at least in part contributes a common basis for the long-term clinical benefit ascribed to these medications. A therapeutic algorithm based on clinical risk stratification and coronary revascularization strategy is proposed for incorporating the current American College of Cardiology (ACC)/American Heart Association (AHA) guideline recommendations for treatment of patients who present with non-ST-elevation ACS.

Facilitated thrombolysis: dethrombosis?

Intracoronary thrombus formation results from the combined effects of platelet aggregation and fibrin formation. Fibrinogen plays a significant role in each of these components. Dethrombosis is a therapeutic approach that allows dissolution of a recent thrombus while avoiding the potent fibrinolytic therapies. The less aggressive strategies suggested to achieve dethrombosis are antifibrin, antiplatelet, and defibrinogenation. Glycoprotein receptor antagonists have been beneficial in patients undergoing percutaneous transluminal coronary angioplasty; however, little is known about their potency of inducing reperfusion in acute myocardial infarction. Early use of the strategies suggested to induce dethrombosis (antifibrin, antiplatelet, and defibrinogenation) may facilitate fibrinolysis and primary angioplasty and further induce reperfusion. Nevertheless, the theoretical arguments of facilitated thrombolysis (dethrombosis) have not yet been fruitful as a major clinical benefit except in patients undergoing primary percutaneous coronary intervention.

Risk stratification in non-ST segment elevation acute coronary syndromes with special focus on recent guidelines

Patients with unstable angina or non-ST segment elevation (non-Q-wave) myocardial infarction are a heterogeneous group with respect to their risk of developing clinically significant adverse events such as subsequent myocardial infarction and death. Recent guidelines promote risk stratification of these patients, targeting high-risk patients for maximal antithrombotic and antiischemic therapy and low-risk patients for early discharge. We reviewed current and future modalities for risk stratification of patients and the predictive value of these methods in context with available pharmacologic agents. Unfortunately, most of the data identifying a particular pharmacologic regimen as beneficial in high-risk patients are retrospectively derived from large trials. Until prospective studies that use markers to guide therapy are available, clinicians should be familiar with the use of these risk markers and their application to the role of a given management strategy, including pharmacologic therapy.

Percutaneous coronary intervention after subcutaneous enoxaparin pretreatment in patients with unstable angina pectoris

BACKGROUND

Subcutaneous low-molecular-weight (LMW) heparins can effectively replace unfractionated heparin in patients with unstable angina or non-Q-wave myocardial infarction. However, the optimal anticoagulation strategy for these patients when they require cardiac catheterization is still unclear. Therefore, we evaluated a new and simple strategy of anticoagulation in these patients.

METHODS AND RESULTS

A total of 451 consecutive patients with unstable angina/non-Q-wave myocardial infarction were treated for at least 48 hours with subcutaneous injections of enoxaparin (1 mg [100 IU]/kg every 12 hours, cycled at 6 AM and 6 PM). Of this unselected population, 293 patients (65%) underwent a coronary angiography within 8 hours of the morning LMW heparin injection, followed by immediate percutaneous coronary intervention (PCI) in 132 patients (28%). PCI was performed without any additional bolus of unfractionated/LMW heparin and without coagulation monitoring. Anti-Xa activity at the time of catheterization was 0.98+/-0.03 IU/mL, was >0.5 IU/mL in 97.6% of patients, and did not relate to the LMW heparin injection-to-catheterization time. There were no in-hospital abrupt closures or urgent revascularizations after PCI. The death/myocardial infarction rate at 30 days was 3.0% in the PCI group (n=132) but 6.2% in the whole population (n=451) and 10.8% in the patients not undergoing catheterization (n=158). The 30-day major bleeding rate was 0.8% in the PCI group, which was comparable to that of patients without catheterization (1.3%).

CONCLUSIONS

PCI within 8 hours of the last enoxaparin subcutaneous injection seems to be safe and effective. The safety of subcutaneous LMW heparin in combination with platelet glycoprotein IIb/IIIa blockade awaits further study.

Therapeutic adjuncts for immediate transfer to the catheterization laboratory in patients with acute coronary syndromes

Early coronary intervention in patients with non-ST-segment elevation myocardial infarction (MI) and unstable angina may be made safer and more efficacious with concomitant therapies, including glycoprotein IIb/IIIa inhibitors and low-molecular-weight heparins. Stent placement has been shown to improve procedural success and reduce major in-hospital complications when compared with balloon angioplasty alone in patients with unstable angina. However, unstable angina remains a major hazard for adverse coronary events in long-term follow-up after elective stent placement. The currently available glycoprotein IIb/IIIa inhibitors-eptifibatide, tirofiban, and abciximab--have each been shown to reduce ischemic events before percutaneous coronary intervention when administered to patients presenting with non-ST-segment elevation acute coronary syndromes in large clinical trials. The adjunctive role of low-molecular-weight heparins in this scenario has been largely unexplored. Enoxaparin, when given before angiography or percutaneous coronary intervention, has been shown to be superior to unfractionated heparin in preventing major coronary events. In this review, an algorithm for treatment of non-ST-segment elevation acute coronary syndromes is presented and the current role of these newer adjunctive pharmacotherapies is explored. In the future, combinations of these agents may prove to be most beneficial in patients undergoing early percutaneous coronary intervention.

Medical therapy of unstable angina and non-Q-wave myocardial infarction

Management of acute coronary syndromes has been the focus of increased interest in recent years. This has come about with the recognition that the majority of patients who present to the hospital with chest pain have unstable angina or non-Q-wave myocardial infarction (MI). Further, sensitive biochemical markers of myocardial necrosis, such as troponin and creatine kinase, have improved early diagnosis. Markers of inflammation such as C-reactive protein (CRP), although not in wide clinical practice, may provide an early and important marker of prognosis. The current approach to management of acute coronary syndromes is careful risk stratification so as to select appropriate medical therapies and to guide the clinician to appropriate interventions such as angiography or percutaneous coronary intervention (PCI). Established therapies such as aspirin, heparin, intravenous nitrates, and, in selected patients, beta blockers or calcium antagonists, are being used concomitantly with, or are being supplanted by, newer therapies such as low-molecular-weight heparins and glycoprotein IIb/IIIa inhibitors. The role of hydroxymethyl glutaryl coenzyme A reductase inhibitors (statins) in patients with acute coronary syndromes is being investigated and shows promise.

IIb's are not IIb's

Platelet glycoprotein (GP) IIb/IIIa receptor blockade improves clinical outcomes after percutaneous coronary intervention (PCI) and for patients who present with non-ST-segment elevation acute coronary syndromes. Although this class of therapeutic agents has been defined by a common affinity for the platelet GP IIb/IIIa receptor, the 3 currently available agents differ markedly in pharmacodynamic and pharmacokinetic profile as well as receptor affinity. Differential (separate) binding sites on the GP IIb/IIIa receptor explain the observation that abciximab binding to platelets is not influenced by either tirofiban or eptifibatide. Abciximab (ReoPro, chimeric 7E3 Fab) is a low K(d) (high affinity) agent with a very short plasma t(1/2) and a prolonged duration of action at the platelet target receptor. Eptifibatide and tirofiban are high K(d) (low affinity) agents with a relatively long plasma t(1/2) and short duration of action at the platelet target receptor. These pharmacodynamic differences underlie the phenomena of gradual redistribution in abciximab binding and smooth tapering of abciximab antiplatelet effect after discontinuation of therapy. Furthermore, abciximab demonstrates affinity for both the CD11b/18 (alpha(m)beta(2) or MAC 1) and alpha(V)beta(3) (vitronectin) receptors. Although a survival advantage in favor of abciximab has been observed after PCI in both randomized controlled trials and high-volume clinical practice, no survival benefit has been observed to date after eptifibatide or tirofiban therapy for PCI. The mechanism of survival advantage after abciximab therapy has not been defined but may be distinct from the degree of platelet GP IIb/IIIa receptor inhibition during the duration of intravenous treatment. Although this important new "class" of therapeutic agent was simplistically defined by a common affinity for the GP IIb/IIIa receptor, this solitary unifying attribute may not define agent-specific benefit.

Debate: Unstable angina - When should we intervene?

The prognosis of patients who present with non-ST segment elevation acute coronary syndromes (ACS) is guarded. These patients can be risk-stratified on the basis of symptom complex, electrocardiographic ST segment depression, obvious hemodynamic compromise and particularly on the basis of serum troponin level. An elevated troponin level determines risk and also predicts the degree of benefit from treatment with either low molecular weight heparin or platelet glycoprotein (GP) IIb/IIIa blockade. Higher risk patients should undergo early coronary angiography and myocardial revascularization as indicated and feasible. Although studies performed before the advent of coronary stenting and adjunctive platelet GP IIb/IIIa blockade suggested increased hazard for patients undergoing early intervention, recent experience cited herein supports an in-hospital and long-term clinical benefit for the aggressive approach. Here, I propose an algorithm for risk stratification and triage of appropriate patients for adjunctive pharmacotherapy and early revascularization.