Clinical Trials with Glycoprotein IIb/IIIa Receptor Antagonists in Acute Coronary Syndromes

Pharmacosimulation of delays and interruptions during administration of tirofiban: a systematic comparison between EU and US dosage regimens

Tirofiban is a glycoproteine (GP) IIb/IIIa receptor antagonist, which inhibits platelet-platelet aggregation and is a potential adjunctive antithrombotic treatment in patients with acute coronary syndromes (ACS) or high-risk percutaneous coronary interventions (PCI). It is administered intravenously as a bolus followed by continuous infusion. However, the dosage recommendations in the United States (US) and European Union (EU) differ considerably. Furthermore, in routine clinical practice, deviations from the recommendations may occur. The objective of the present study was to investigate the impact of different alterations on tirofiban plasma concentrations in US and EU administration regimens and to give suggestions for delay management in clinical practice. We therefore mathematically simulated the effects of different bolus-infusion delays and infusion interruptions in different scenarios according to the renal function. Here, we provide a systematic assessment of concentration patterns of tirofiban in the US versus EU dosage regimens. We show that differences between the two regimens have important effects on plasma drug levels. Furthermore, we demonstrate that deviations from the proper administration mode affect the concentration of tirofiban. Additionally, we calculated the optimal dosage of a second bolus to rapidly restore the initial concentration without causing overdosage. In conclusion, differences in tirofiban dosing regimens between the U.S and EU and potential infusion interruptions have important effects on drug levels that may impact on degrees of platelet inhibition and thus antithrombotic effects. Thus, the findings of our modelling studies may help to explain differences in clinical outcomes observed in previous clinical trials on tirofiban.

Ethanol enhances the inhibitory effect of an oral GPIIb/IIIa antagonist on human platelet function

Ethanol is a commonly used substance that can significantly influence platelet responses when combined with therapeutic drugs. In in vitro studies, we combined ethanol with LY309562, a novel 2,6-disubstituted isoquinolone RGD mimic that competes for fibrinogen binding to GPIIb/IIIa. Ethanol inhibits aggregation and secretion, partly by inhibiting thromboxane A(2) formation. We measured aggregation and secretion of dense granule contents by platelets labeled with [(14)C] serotonin in plasma from blood anticoagulated with FPRCH(2)Cl (PPACK). Alone, LY309562 dose-dependently inhibited aggregation induced by 10 micromol/L adenosine diphosphate, 1 microg/mL collagen, 2 micromol/L U46619 (a thromboxane A(2) mimetic), or 15 micromol/L SFLLRN (protease-activated receptor-1-activating peptide); inhibition was complete at 1 micromol/L LY309562 and partial at 0.1 micromol/L (50% inhibitory concentration [IC(50)] 0.19-0.33 micromol/L). Secretion induced by collagen, U46619, and SFLLRN was also inhibited by LY309562 (IC(50) 0.08-0.31 micromol/L). At inhibitory concentrations of LY309562, ethanol (2 or 4 mg/mL) further inhibited responses to collagen, U46619, and SFLLRN (IC(50) for aggregation 0.12-0.16 micromol/L; for secretion 0.04-0.12 micromol/L). Responses of aspirin-treated platelets to U46619 were also inhibited, indicating that ethanol was not acting solely by inhibiting thromboxane A(2) formation. Because it is likely that our results with LY309562 are representative of results with other GPIIb/IIIa antagonists, our in vitro data suggest that the concomitant use of GPIIb/IIIa antagonists and consumption of alcoholic beverages may result in further impairment of platelet participation in hemostasis and thrombosis.

Acute coronary syndromes after renal transplantation in patients with end-stage renal disease resulting from diabetes

Coronary heart disease is the leading cause of death in both diabetes mellitus and end-stage renal disease. Although renal transplantation is known to reduce mortality in end-stage renal disease, its effect on the incidence of acute coronary syndromes is unknown. Using data from the United States Renal Data System, we studied 11,369 patients with end-stage renal disease due to diabetes enrolled on the renal and renal-pancreas transplant waiting list from 1 July 1994 to 30 June 1997. Cox nonproportional hazards regression models were used to calculate the adjusted, time-dependent relative risk for the most recent hospitalization for acute coronary syndromes (including acute myocardial infarction, unstable angina, or other acute coronary syndromes, ICD9 Code 410.x or 411.x) for a given patient in the study period. Demographics and comorbidities were controlled by using data from the medical evidence form (HCFA 2728). After renal transplantation, patients had an incidence of acute coronary syndromes of 0.79% per patient year, compared to 1.67% per patient year prior to transplantation. In comparison to maintenance dialysis, renal transplantation was independently associated with a lower risk for acute coronary syndromes (hazard ratio 0.38, 95% confidence interval, 0.30-0.49). Patients with end-stage renal disease due to diabetes on the renal transplant waiting list were much less likely to be hospitalized for acute coronary syndromes after renal transplantation. The reasons for this decreased risk should be the subject of further study.

Prognostic role of hemostatic markers in acute coronary syndromes patients

Coronary thrombosis is an important determinant of prognosis in patients with acute coronary syndromes (ACS). However, the identification of patients at high-risk for progression of coronary thrombosis is difficult in part because we currently lack clinically meaningful laboratory methods for its detection. The most promising approaches involve the measurement in plasma of markers of fibrin formation and degradation. Thrombin activity, as reflected by plasma or urine concentrations of fibrinopeptide A, is increased in patients with ACS and is associated with adverse outcome. However, the use of fibrinopeptide A as a marker of fibrin formation is limited by the very short half-life of the compound, by artifact due to sample acquisition, and by extremely long turnaround times. To overcome these limitations, measurement of soluble fibrin has been proposed. We have recently explored the prognostic value of a new fibrin-specific ELISA assay for soluble fibrin in patients with ACS and found that patients with highest levels had a twofold increased risk of early and late cardiac events. Increases in plasma concentrations of cross-linked fibrin degradation products (XL-FDPs), which reflect increased fibrin turn-over, are a marker of risk for complications of myocardial infarction. However, until recently, assays for XL-FDPs lacked specificity, because they did not distinguish between fibrin and fibrinogen degradation products. Recently, fibrin-specific ELISAs have been described and a rapid whole blood assay for D-dimer has been developed. We recently validated the prognostic value of this whole blood agglutination assay in patients with ACS. The results suggest that: (1) the detection of significant activation of the coagulation and/or fibrinolytic system may be important for rapid risk stratification of patients with ACS; (2) patients with biochemical evidence of ongoing coronary thrombosis may particularly benefit from aggressive antithrombotic strategies; (3) sequential measurements of these markers may be useful to guide antithrombotic treatment during the unstable phase of coronary artery disease.

[The efficacy of platelet IIb/IIIa receptor blockers in acute coronary syndromes]

The knowledge of the central role of platelets in the pathogenesis of acute coronary syndromes, on the one hand, and the fact that aspirin is a weak antiplatelet agent on the other, have led to an intensive investigational activity in antiplatelet drugs in the last years. Actually, the literature in the last two years is inundated with studies on the use of platelet IIb/IIIa receptor blockers in different clinical settings. Agents that block the IIb/IIIa platelet receptor have shown to be useful in improving prognosis of patients with acute coronary syndromes, especially in those undergoing percutaneous coronary revascularization procedures. However, their potential risk of bleeding and their high cost have prevented them from being applied universally and routinely. Furthermore there are still some unclear issues regarding the use of these drugs such as their correct dosage, the optimal duration of treatment and the direct comparison of the efficacy of different types of IIb/IIIa blockers available. On the other hand, oral IIb/IIIa antagonists have not improved the efficacy of aspirin to date and, moreover, they have been proven to be unsafe. Finally, it is necessary to identify those patients who will obtain the greatest benefit from the treatment in order to avoid the unnecessary risks and costs that would be derived from their universal use.

Fibrinogen Receptor Antagonist-Induced Thrombocytopenia in Chimpanzee and Rhesus Monkey Associated With Preexisting Drug-Dependent Antibodies to Platelet Glycoprotein IIb/IIIa

Most clinical trials with fibrinogen receptor antagonists (FRAs) have been associated with thrombocytopenia. This report describes the occurrence of thrombocytopenia in one chimpanzee and one rhesus monkey upon administration of potent FRAs. Chimpanzee A-264 experienced profound thrombocytopenia on two occasions immediately upon intravenous administration of two different potent FRAs, L-738,167 and L-739,758. However, an equally efficacious antiaggregatory dose of another potent antagonist, L-734,217, caused no change in platelet count. These compounds did not affect platelet count in five other chimpanzees or numerous other nonhuman primates. Flow cytometric analysis showed drug-dependent antibodies (DDAbs) in the plasma of chimpanzee A-264 that bound to platelets of chimpanzees, humans, and all other primates tested only in the presence of the compounds that induced thrombocytopenia. Rhesus monkey 94-R021 experienced thrombocytopenia upon administration of a different antagonist, L-767,679, and several prodrugs that are converted into the active form, L-767,679, in the blood. More than 20 other FRAs, including those that induced thrombocytopenia in chimpanzee A-264, had no effect on platelet count in this monkey. Flow cytometric measurements again identified DDAbs that reacted with platelets of all primates tested and required the presence of L-767,679. Screening for DDAbs in the plasma of 1,032 human subjects with L-738,167 and L-739,758 demonstrated that the incidence of these preexisting antibodies in this population was 0.8% ± 0.6% and 1.1% ± 0.6%, respectively.

Fibrinogen Receptor Antagonist-Induced Thrombocytopenia in Chimpanzee and Rhesus Monkey Associated With Preexisting Drug-Dependent Antibodies to Platelet Glycoprotein IIb/IIIa

Most clinical trials with fibrinogen receptor antagonists (FRAs) have been associated with thrombocytopenia. This report describes the occurrence of thrombocytopenia in one chimpanzee and one rhesus monkey upon administration of potent FRAs. Chimpanzee A-264 experienced profound thrombocytopenia on two occasions immediately upon intravenous administration of two different potent FRAs, L-738,167 and L-739,758. However, an equally efficacious antiaggregatory dose of another potent antagonist, L-734,217, caused no change in platelet count. These compounds did not affect platelet count in five other chimpanzees or numerous other nonhuman primates. Flow cytometric analysis showed drug-dependent antibodies (DDAbs) in the plasma of chimpanzee A-264 that bound to platelets of chimpanzees, humans, and all other primates tested only in the presence of the compounds that induced thrombocytopenia. Rhesus monkey 94-R021 experienced thrombocytopenia upon administration of a different antagonist, L-767,679, and several prodrugs that are converted into the active form, L-767,679, in the blood. More than 20 other FRAs, including those that induced thrombocytopenia in chimpanzee A-264, had no effect on platelet count in this monkey. Flow cytometric measurements again identified DDAbs that reacted with platelets of all primates tested and required the presence of L-767,679. Screening for DDAbs in the plasma of 1,032 human subjects with L-738,167 and L-739,758 demonstrated that the incidence of these preexisting antibodies in this population was 0.8% ± 0.6% and 1.1% ± 0.6%, respectively.

Early risk stratification of unstable angina/non-Q myocardial infarction: biochemical markers of coronary thrombosis

Coronary thrombosis is an important determinant of prognosis in patients with acute coronary syndromes (ACS). However, the identification of patients at high-risk for progression of coronary thrombosis is difficult partly because we currently lack clinically meaningful laboratory methods for its detection. The most promising approaches involve the measurement in plasma of markers of fibrin formation and degradation. Thrombin activity, as reflected by plasma or urine concentrations of fibrinopeptide A, is increased in patients with ACS and is associated with adverse outcome. However, the use of fibrinopeptide A as a marker of fibrin formation is limited by the very short half-life of the compound, by artifact due to sample acquisition, and by extremely long turnaround times. To overcome these limitations, measurement of soluble fibrin has been proposed. We have recently explored the prognostic value of a new fibrin-specific ELISA assay for soluble fibrin in patients with ACS and found that patients with the highest levels had a 2-fold increased risk of early and late cardiac events. Increases in plasma concentrations of cross-linked fibrin degradation products (XL-FDPs), which reflect increased fibrin turn-over, are a marker of risk for complications of myocardial infarction. However, until recently, assays for XL-FDPs lacked specificity, because they did not distinguish between fibrin and fibrinogen degradation products. Recently, fibrin-specific ELISAs have been described and a rapid whole blood assay for D-dimer has been developed. We recently validated the prognostic value of this whole blood agglutination assay in patients with ACS. These results suggest that: (1) The detection of significant activation of the coagulation and/or fibrinolytic system may be important for rapid risk stratification of patients with ACS; (2) patients with biochemical evidence of ongoing coronary thrombosis may particularly benefit from aggressive antithrombotic strategies; (3) sequential measurement of these markers may be useful to guide antithrombotic treatment during the unstable phase of coronary artery disease.

Cone and plate(let) analyzer: monitoring glycoprotein IIb/IIIa antagonists and von Willebrand disease replacement therapy by testing platelet deposition under flow conditions

Exposure of citrated whole blood samples to polystyrene plates under flow conditions with the cone and plate(let) analyzer (CPA) results in surface immobilization of plasma von Willebrand factor (vWF) followed by platelet deposition. Staining the plates allows measurement of the percentage of surface covered (SC) by the adhered particles and their average size (AS). Both SC and AS parameters depend on platelet count and hematocrit level and reach maximal values after 2 minutes; only AS is shear rate dependent. Under optimal assay conditions (2 minutes at 1800 sec(-1)) normal blood samples yielded an SC of 14.9% +/- 2.5% and an AS of 39.4 +/- 5.2 microm2. Severe von Willebrand disease (eight patients) yielded low SC (5.2% +/- 2.1%), which was restored to normal by testing with vWF precoated surfaces. Glanzmann's thrombasthenia (six patients) samples demonstrated no adhesion of platelet at all. Blocking of the glycoprotein (GP) IIb/IIIa receptor by the chimeric antibody abciximab, and of the GPIb by a recombinant vWF fragment, have yielded a dose-response inhibition demonstrating the crucial role of these receptors in platelet deposition on polystyrene plates in this system. Evaluation of a patient with severe von Willebrand disease receiving replacement vWF factor VIII therapy revealed a comparable response as tested by both the CPA and the Ricof methods. In vitro testing of the GPIIb/IIIa blocking by a nonpeptidic analogue tirofiban and abciximab revealed a good correlation between the CPA test and the routine aggregometry. Eleven patients treated by abciximab after coronary angioplasty were studied by the CPA during the first 24 hours, demonstrating a marked decrease in SC and AS, with some diversity in the responses. We conclude that the CPA test is suitable for evaluation of primary hemostasis and for monitoring of anti-platelet drugs.

An overview of the results of clinical trials with glycoprotein IIb/IIIa inhibitors

The era of platelet glycoprotein (GP) IIb/IIIa receptor inhibition in cardiology was inaugurated in 1994 with the publication of the Evaluation of 7E3 for the Prevention of Ischemic Complications (EPIC) trial results. EPIC demonstrated that the GP IIb/IIIa blocker abciximab, administered as a bolus and 12-hour infusion, afforded protection against ischemic complications in high-risk patients undergoing angioplasty and atherectomy, including those with unstable angina or evolving myocardial infarction (MI). A significant reduction in the incidence of death, acute MI, or revascularization was apparent at 30 days and also sustained at 6-month and 3-year follow-up. The subsequent Evaluation in PTCA to Improve Long-Term Outcome with Abciximab GP IIb/IIIa Blockade (EPILOG) study extended these findings to the full spectrum of coronary intervention patients, confirming that abciximab provided similar benefits in low-risk patients as well. The EPILOG trial also proved that any excess bleeding risk associated with potent antiplatelet therapy could be brought down to placebo levels through the use of a low-dose, weight-adjusted heparin regimen, early vascular sheath removal, and elimination of routine postprocedural heparinization. The potential for an advantage of GP IIb/IIIa blockade in patients with refractory unstable angina/non-Q-wave MI was demonstrated in the c7E3 Fab Antiplatelet Therapy in Unstable Refractory Angina (CAPTURE) trial, which showed that a 24-hour preprocedural abciximab infusion effectively stabilized these patients, thereby enhancing the safety of intervention and reducing the 30-day incidence of ischemic events. A similar pattern of benefit has emerged from clinical trials of such other GP IIb/IIIa inhibitors as eptifibatide, lamifiban, and tirofiban. Trials are currently underway to clarify the benefits of GP IIb/IIIa blockers in patients undergoing stenting and as an adjunct to thrombolytic therapy or primary angioplasty in patients with acute MI (ST-segment elevation).

Rationale and clinical evidence for the use of GP IIb/IIIa inhibitors in acute coronary syndromes

Platelet glycoprotein (GP) IIb/IIIa blockade has the potential to advance treatment of acute coronary syndromes, both as a primary pharmacologic approach and an adjunct to interventional treatment strategies. The benefits of GP IIb/IIIa inhibition with the chimeric monoclonal antibody abciximab in preventing ischemic complications of interventional treatment have been well defined in patients with unstable angina. In the future, major therapeutic applications for this class of agents may include the stabilization of patients with unstable angina and potentially as single medical therapy, as several recently completed trials have suggested. Evidence also is accumulating on the use of GP IIb/IIIa blockade as adjunctive therapy in fibrinolytic approaches to treatment of acute myocardial infarction. Several ongoing trials are evaluating the safety and efficacy of this novel strategy. This article reflects a distillation of the views and consensus regarding the prospective use of GP IIb/IIIa inhibitors in patients with acute coronary syndromes expressed by a group of international experts convened in Davos, Switzerland, February 16, 1997. This report attempts to review clinical progress to date, formulate recommendations, and map out potentially fruitful lines of inquiry for future investigation.