c7E3 Fab Research Articles

Occurrence and clinical significance of pseudothrombocytopenia during abciximab therapy

OBJECTIVESThis study determined the incidence of pseudothrombocytopenia during abciximab therapy administered for percutaneous coronary interventions and compared the clinical course of patients with pseudothrombocytopenia with the clinical courses of patients with thrombocytopenia and patients with normal platelet counts.BACKGROUNDAlthough pseudothrombocytopenia has been previously reported during therapy with abciximab, the incidence and significance of this occurrence are unknown. The failure to differentiate pseudothrombocytopenia from thrombocytopenia could lead to unnecessary interruption of abciximab infusions or to platelet transfusions.METHODSThe incidences of pseudothrombocytopenia and thrombocytopenia were determined in four large placebo-controlled abciximab trials: c7E3 Fab Antiplatelet Therapy in Unstable Refractory Angina (CAPTURE), Evaluation of 7E3 for the Prevention of Ischemic Complications (EPIC), Evaluation of Percutaneous Transluminal Coronary Angioplasty to Improve Long-term Outcome of c7E3 GpIIb/IIIa Receptor Blockade (EPILOG) and Evaluation of Platelet IIb/IIIa Inhibitor for Stenting (EPISTENT). The clinical features, bleeding complications and major clinical outcomes of patients with pseudothrombocytopenia and those with thrombocytopenia were compared with each other and with those of patients with normal platelet count.RESULTSPseudothrombocytopenia occurred in 2.1% (95% confidence intervals [CI]: 1.7%, 2.5%) of abciximab-treated patients and in 0.6% of placebo-treated patients (p < 0.001). Thrombocytopenia occurred in 3.7% (95% CI: 3.2%, 4.2%) of abciximab-treated patients and in 1.8% (95% CI: 1.3%, 2.3%) of placebo-treated patients (p < 0.001). Patients with thrombocytopenia had significantly higher rates of major bleeding, major decreases in hemoglobin and increased transfusion requirements of both blood and platelets compared with those without thrombocytopenia. By contrast, pseudothrombocytopenic patients did not differ from patients with normal platelet counts in any of the measures of blood loss or transfusion requirements. Thrombocytopenic patients, but not those with pseudothrombocytopenia, had increased rates of revascularization at 30 days and six months. As previously reported, there was also a higher rate of death and myocardial infarction in the thrombocytopenic patients.CONCLUSIONSPseudothrombocytopenia is the cause of more than one third (36.3%) of low platelet counts in patients undergoing coronary interventions who are treated with abciximab. This study demonstrates that pseudothrombocytopenia is a benign laboratory condition that does not increase bleeding, stroke, transfusion requirements or the need for repeat revascularization. It is important to recognize pseudothrombocytopenia so that the beneficial effects of abciximab are not lost by premature termination of therapy.

C7E3 Fab Inhibits Low Shear Flow Modulated Platelet Adhesion to Endothelium and Surface-adsorbed Fibrinogen by Blocking Platelet GP IIb/IIIa as well as Endothelial Vitronectin Receptor

The c7E3 Fab reduces ischemic complications in patients undergoing high-risk coronary angioplasty or atherectomy. The present study investigated how c7E3 Fab inhibition of the platelet receptor glycoprotein IIb/IIIa and the endothelial vitronectin receptor affected platelet adhesion to endothelium and surface adsorbed fibrinogen under flow conditions. Platelet adhesion was examined using a stagnation point flow device with shear stress and shear rates up to 2.2 dynes/cm2 and 170 s(-1), respectively. Ex vivo adhesion was compared between two groups of patients with acute myocardial infarction (AMI) treated with angioplasty and stent implantation and a group of healthy controls. Only one AMI group received c7E3 Fab therapy. Patients in both groups were administered acetyl salicylic acid (ASA) and heparin. In AMI patients c7E3 Fab reduced platelet adhesion to adsorbed fibrinogen by 79% compared to AMI patients without c7E3 Fab treatment and by 74% compared to healthy controls. Thirty hours after termination of c7E3 Fab infusion adhesion had slightly recovered with an inhibition of 61% and 52% still present, respectively. Additionally, in vitro platelet adhesion to intact endothelium and to adsorbed fibrinogen was measured during superfusion with ADP stimulated platelet rich plasma of healthy controls to which c7E3 Fab was added at a final concentration fc of 20 microg/ml. In spite of ADP stimulation c7E3 Fab completely blocked platelet adhesion to adsorbed fibrinogen and, moreover, to intact endothelium. Preincubation of endothelial cells with c7E3 (fc = 20 [microg/ml) blocked adhesion of ADP-stimulated platelets by approximately 50%. Apart from the inhibition of platelet aggregation, c7E3 Fab added in vitro and given therapeutically in patients effectively blocks platelet adhesion to components of the injured as well as intact vessel wall under stagnation point flow conditions.

Acute severe thrombocytopenia after c7E3 Fab (abciximab) therapy in a patient with unstable angina and stenting of the right coronary artery. Occurrence of subacute stent thrombosis and safe readministration of the GPIIb/IIIa inhibitor tirofiban.

This paper reports a case of acute severe thrombocytopenia (platelet count: 1 x 10(9)/liter) occurring within minutes of an initial abciximab bolus during coronary angioplasty and stenting in a patient with unstable angina. After six days with platelets again in the normal range the patient developed stent thrombosis. The stent was reopened and the glycoprotein receptor inhibitor tirofiban (Aggrastat) was administered without any adverse effects on platelet count. Antibodies against heparin-platelet factor 4 complexes could be excluded. Allo- and autoantibodies (IgG, IgA, IgM) directed against platelets with and without binding of abciximab could not be detected by indirect and direct platelet fluorescence antiglobulintest. A possible activation or lysis of the platelets by abciximab could also be excluded by an in vitro bleeding test investigating the effect of abciximab on heparin and citrate blood of the patient and two healthy donors. The mechanisms of abciximab-induced thrombocytopenia in this case remain unclear. The possible mechanisms are discussed.

Use of the PFA-100™ Apparatus to Assess Platelet Function in Patients Undergoing PTCA during and after Infusion of cE3 Fab in the Presence of other Antiplatelet Agents

The PFA-100 (Dade) is a new functional whole blood analyzer, the accuracy and reliability of which have been evaluated in von Willebrand disease and during acetyl salicylate acid therapy. This new test has the advantages of rapidity and simplicity. It may be useful to monitor new antiplatelet agents, such as GPIIb/IIIa receptor antagonists. The objective of this study was to assess the PFA-100 in comparison with aggregometry and with the percentage of blockaded receptors GPIIb/IIIa during and after c7E3 Fab infusion in fifteen patients undergoing PTCA. Our results showed a change of closure time values from normal to abnormal within a small margin of flow cytometric values (60-75% of blockaded receptors), and moreover a variable platelet response to long-term low dose aspirin treatment in agreement with aggregometry. No influence with heparin was observed. In conclusion, this study shows that PFA-100 may be helpful in the decision making for additional antiaggregant therapy before PTCA or in monitoring long-term GPIIb/IIIa receptor antagonist treatment.

Assessing platelet and fibrinogen contribution to clot strength using modified thromboelastography in pregnant women.

The monoclonal antibody fragment c7E3 Fab (ReoPro), by binding to platelet surface fibrinogen receptors (glycoprotein, GPIIb/IIIa), inhibits platelet aggregation and its interaction with fibrinogen. In this study, we used thromboelastography with ReoPro to evaluate the independent contribution of fibrinogen and platelets to clot strength. Thromboelastography was performed in 21 healthy, term parturients scheduled for elective cesarean delivery with 360 microL of celite-activated whole blood and with 5 microL of (2 mg/mL) ReoPro added to 355 microL of celite-activated whole blood. The contribution of platelets to clot strength (MAplt) was derived by subtracting MAfib (maximal amplitude with ReoPro) from MAwb (maximal amplitude with whole blood). Thus, MAwb - MAfib = MAplt. The value for MAwb (mean +/- SD) was 73+/-4 mm, for MAfib it was 33+/-5 mm, and for MAplt it was 40+/-3 mm. The contribution of fibrinogen and platelets to the MAplt was 45% and 55%, respectively. Modified thromboelastography using ReoPro in healthy parturients can be used to determine the contribution of fibrinogen and platelets to blood clot strength. Determining the independent contribution of platelets and fibrinogen to the maximal amplitude of thromboelastography using c7E3 Fab may further improve the use of thromboelastography in detecting and treating coagulation defects.

Platelet Glycoprotein IIb/IIIa Inhibitors

Glycoprotein IIb/IIIa (GPIIb-IIIa) complexes (integrin αIIbβ3) mediate platelet aggregation by binding fibrinogen or von Willebrand factor (vWF), protein cofactors that form bridges between adjacent platelets. The cross-linked adhesive proteins assemble platelets into the aggregate. Agents that block the function of the GPIIb-IIIa complex of platelets constitute a powerful new generation of antithrombotic drugs.1 Among the short- and long-term aims of such drugs are (1) to provide immediate relief in the case of ongoing arterial thrombosis and (2) to eliminate excessive platelet reactivity in diseased vessels so that occlusive thrombi and restenosis do not occur, while allowing sufficient hemostasis to prevent spontaneous bleeding. It should be emphasized that stenosis and partial occlusion are both prothrombotic, with increased shear stress promoting platelet activation. Under these conditions, vWF plays a major role in the mediation of thrombus formation, interacting with GPIIb-IIIa and the adhesion receptor GPIb.2 Otherwise, fibrinogen is the major cofactor of platelet aggregation, essentially binding through a dodecapeptide sequence (aa400 to aa411) present at the carboxy terminus of each γ chain. Binding of vWF and other adhesive proteins, such as fibronectin, to GPIIb-IIIa is mediated by the Arg-Gly-Asp (RGD) sequence, a universal mediator of cellular interactions with the extracellular matrix.1 2 3 Anti–GPIIb-IIIa drugs block this final step of the platelet aggregation process. They also block the “outside-in” signaling that follows the binding of adhesive proteins to activated GPIIb-IIIa and the onset of platelet aggregation.3 This signaling may promote events such as secretion, clot retraction, and the expression of procoagulant activity; therefore, its inhibition extends the influence of anti–GPIIb-IIIa drugs beyond the blocking of platelet-to-platelet cohesion. The present review will mostly be illustrated by results obtained with abciximab (c7E3 Fab, ReoPro), a chimeric antibody fragment that is the most widely used of the new …

Attainment and maintenance of platelet inhibition through standard dosing of abciximab in diabetic and nondiabetic patients undergoing percutaneous coronary intervention.

Although the effectiveness of abciximab (c7E3 Fab; ReoPro) in large populations of patients undergoing a percutaneous coronary intervention has been consistently proved in clinical trials, it is unknown whether all patients achieve and maintain target inhibition during treatment. Diabetic patients in particular are a subgroup of patients with known underlying platelet abnormalities whose long-term response to abciximab has been shown to vary from that of nondiabetic patients. Forty-nine diabetic and 51 nondiabetic patients who received adjunctive abciximab therapy during percutaneous coronary interventions were evaluated prospectively. The degree of platelet function inhibition was determined immediately after the abciximab bolus, 8 hours after the bolus (during the 12-hour abciximab infusion), and the next morning (13 to 26 hours after the bolus) with the use of a rapid platelet function assay (Accumetrics). After the abciximab bolus, platelet function was inhibited by 95+/-4% (mean+/-SD). By 8 hours, the average percent inhibition had decreased to 88+/-9%, with 13% of patients with <80% inhibition. The next morning (mean 19 hours after the bolus), mean inhibition was 71+/-14%. A difference was not found between diabetics and nondiabetics, nor was any physiological parameter found to be predictive of the response to abciximab. Although the majority of patients achieve and maintain >/= 80% platelet inhibition during the 12-hour infusion with standard-dose abciximab, there is substantial variability among patients. Diabetic status does not appear to influence this variability.

Safety of glycoprotein IIb-IIIa inhibitors: A heart surgeon’s perspective

Platelet-mediated coronary thrombosis is the primary pathophysiologic mechanism of acute coronary syndromes (ACS) and acute ischemic complications of percutaneous coronary intervention (PCI). The final common pathway of platelet aggregation that leads to thrombotic occlusion of coronary arteries involves cross-linking of receptor glycoprotein (GP) IIb-IIIa on adjacent platelets by adhesive plasma proteins, primarily fibrinogen. Clinical trials of several GP IIb-IIIa inhibitors have demonstrated an unequivocal clinical benefit of this potent antithrombotic therapy in patients with ACS as well as in those undergoing PCI. Nevertheless, a significant number of patients with ischemic heart disease may still be expected to require elective or emergency coronary artery bypass graft (CABG) after treatment with GP IIb-IIIa inhibitors. In the emergency CABG setting, complications and platelet blockade with GP IIb-IIIa inhibitors may further enhance the already heightened risk of bleeding as compared with elective procedures. This issue became apparent in the first large clinical trial of the GP IIb-IIIa inhibitor abciximab (c7E3 Fab, ReoPro ®; Centocor, Malvern, Pa, and Eli Lilly and Co, Indianapolis, Ind) in patients undergoing high-risk PCI. In this study, mortality rates and bleeding complications were increased among patients undergoing emergency CABG after treatment with a bolus plus infusion of abciximab. Subsequent clinical experience also suggests that the potential for bleeding complications related to emergency CABG may be increased in patients treated with abciximab, particularly if the drug is discontinued within 6 hours of the operation. Higher bleeding risk with abciximab is a result of its prolonged antiplatelet effect, which is in contrast to the readily reversible platelet blockade provided by more recently developed small-molecule GP IIb-IIIa inhibitors such as the peptide eptifibatide (Integrilin ®; COR Therapeutics, South San Francisco, Calif, and Key Pharmaceuticals, Kenilworth, NJ) and the nonpeptide tirofiban HCl (MK-383, Aggrastat ®; Merck & Co, Whitehouse Station, NJ). Therefore, among patients requiring CABG after treatment with GP IIb-IIIa inhibitors, eptifibatide and tirofiban may be associated with fewer bleeding episodes than is abciximab. With recent approval of eptifibatide for patients with ACS and those scheduled for PCI and of tirofiban for patients with ACS, the number of patients receiving GP IIb-IIIa inhibitor therapy who subsequently undergo CABG is expected to increase significantly. Strategies for improved management of bleeding complications in these patients, including the choice of a GP IIb-IIIa inhibitor, are clearly needed and are discussed in detail. (Am Heart J 1999;138:S307-S316.)

Bleeding complications of glycoprotein IIb-IIIa receptor inhibitors

Large clinical studies have demonstrated an unequivocal clinical benefit of antithrombotic therapy with inhibitors of the platelet surface-membrane glycoprotein (GP) IIb-IIIa receptor in a broad range of patients with ischemic heart disease. Potent antiplatelet effects of these agents, however, may increase the risk of bleeding complications, as occurred in the first large evaluation of this therapy, the Evaluation of c7E3 for Prevention of Ischemic Complications (EPIC) trial with abciximab (c7E3 Fab; ReoPro ®; Centocor, Malvern, Pa). Although the incidence of bleeding events in subsequent studies has been reduced through the use of a low-dose, weight-adjusted heparin regimen and early removal of vascular sheaths in patients who have undergone percutaneous coronary interventions, hemorrhage continues to be the most common complication of GP IIb-IIIa inhibitor therapy. This review summarizes current experience related to bleeding complications with various GP IIb-IIIa inhibitors and suggests strategies for improved management of bleeding in patients receiving these agents. (Am Heart J 1999;138:S287-S296)

Readministration of abciximab: Interim report of the ReoPro Readministration Registry

Even with continued improvements in the technology of percutaneous coronary intervention (PCI), approximately 10% to 20% of patients undergoing PCI will require repeat procedures within 1 year. Furthermore, because of the chronic nature of coronary artery disease, many patients will require additional treatment with PCI well after an initial episode of care. Abciximab (ReoPro), a chimeric (murine/human) monoclonal antibody fragment (c7E3 Fab), has been shown to significantly improve periprocedural and long-term outcomes associated with PCI and to reduce the need for repeat target vessel revascularization. However, because the structure of abciximab is derived from an antibody, concern has been raised about subsequent repeat administration. To prospectively evaluate the safety and efficacy of abciximab readministration, we established the ReoPro Readministration Registry with the intent to determine the efficacy, human antichimeric antibody response and rates of thrombocytopenia, bleeding, intracranial hemorrhage, and anaphylaxis in at least 500 patients being retreated with abciximab. The study was conducted at 19 centers beginning in March 1997. This article details interim data that are based on the first 329 patients. Data to date indicate that readministration with abciximab is safe and efficacious and that the same indications for first-time use should apply to subsequent readministration. (Am Heart J 1999;138:S33-S38)

Acute profound thrombocytopenia following C7E3 Fab (abciximab) therapy: Case reports, review of the literature and implications for therapy

Platelets play a crucial role in the ischemic complications of percutaneous coronary procedures. The recent availability of C7E3 Fab (Abxiximab or ReoPro™), a chimeric monoclonal antibody Fab fragment directed against the platelet glycoprotein IIb/IIIa receptor, has reduced abrupt closure and other adverse events and lessened the need for revascularization procedures. As the use for this drug has increased, rare cases of severe thrombocytopenia have been revealed. From August 1995 to June 1997, 452 patients at Charleston Area Medical Center who underwent percutaneous coronary revascularization procedures and were treated with abciximab were evaluated for the development of severe thrombocytopenia (i.e., platelet count less than 20,000 within 48 hr of treatment). A review of published reports of severe thrombocytopenia was also reviewed. A review of published reports of abciximab-induced severe thrombocytopenia, as well as our three cases, reveals that: 1) the incidence is less than 0.7%; 2) the nadir platelet count (range 1,000–16,000) was noted within 2–31 hr after abciximab infusion; 3) the platelet count increases to greater than 100,000 within 12 days in all patients; 4) bleeding episodes were treated with platelet transfusion with an improvement in platelet count within 24 hr in all patients in whom they were given; and 5) in the one patient treated with γ globulin alone, no significant rise in platelet count was noted. Acute severe thrombocytopenia can occur after ReoPro™ administration. Its development is not predictable and may occur within 2 hr of administration. Thrombocytopenia, therefore, requires consideration in every patient treated with this drug. It appears prudent to obtain a platelet count 2 hr after initiating ReoPro™. If thrombocytopenia develops, then the drug can be stopped in a timely manner and platelet transfusion can be given. Am. J. Hematol. 61:205–208, 1999. © 1999 Wiley-Liss, Inc.

Effects of platelet glycoprotein IIb/IIIa inhibition with abciximab on thrombin generation and activity during percutaneous coronary intervention

Background Antagonists of the platelet glycoprotein IIb/IIIa decrease acute ischemic complications after percutaneous coronary interventions (PCI). Abciximab (c7E3 Fab, ReoPro) has been reported to decrease thrombin generation in vitro. We investigated in vivo the effect of abciximab therapy on thrombin generation, thrombin activity, and the activated clotting time (ACT) during PCI. Methods We studied 32 consecutive patients who underwent PCI for unstable coronary syndromes. Group I (n = 11) was treated with heparin plus aspirin, and group II (n = 21) was treated with heparin plus aspirin plus standard-dose abciximab, administered 5 minutes after the initial heparin bolus. Patients received a standardized heparin bolus at time 0, and arterial blood specimens for prothrombin fragment F1.2, fibrinopeptide A (FPA), and ACT were obtained from the guiding catheter at 5 minutes, 10 minutes (ACT only), 20 minutes, and at the end of the PCI. Standard-dose abciximab was administered in group II only. Each patient served as his or her own control, and the changes against the baseline were compared between the 2 groups. Results There were no significant differences between the 2 groups regarding baseline characteristics, hematocrit, and platelet count. Group I patients had higher ACT and lower F1.2 and FPA compared with group II at baseline. Subsequent measurements demonstrated a gradual decrease in FPA and F1.2 in group II; the end of procedure versus baseline changes that occurred in F1.2 were significantly different compared with group I (decrease of 0.59 ± 0.22 nmol/L in group II vs increase of 0.22 ± 0.3 nmol/L in group I, P = .04), and a trend in the same direction was evident for FPA changes (decrease of 1.46 ± 1.16 ng/mL in group II vs increase of 2.25 ± 1.58 ng/mL in group I, P = .07). The ACT response to abciximab was variable, but a 6.3% increase (+20 sec) in ACT was documented 5 minutes after abciximab bolus in group II compared with the 3.4% decrease (–10 sec) observed in group I at the same time point ( P = .1). Conclusion Addition of abciximab to heparin plus aspirin during PCI was associated with a significant decrease in thrombin generation and a borderline decrease in thrombin activity. (Am Heart J 1999;138:49-54.)

Platelet aggregatory response to platelet activating factor (PAF), ex vivo, and PAF-acetylhydrolase activity in patients with unstable angina: effect of c7E3 Fab (abciximab) therapy.

Platelet activation and aggregation is a dominant feature in the pathophysiology of unstable angina. The final step of platelet aggregation is mediated through the platelet integrin glycoprotein IIb/IIIa (GP IIb/IIIa), while abciximab (ReoPro) is one of the most potent inhibitors of this receptor. Platelet-activating factor (PAF) is a potent platelet agonist which is degraded and inactivated by PAF-acetylhydrolase (PAF-AH). The plasma form of PAF-AH is associated with lipoproteins. We studied the platelet response to the aggregatory effect of PAF, ex vivo, in relation to the plasma PAF-AH activity in 32 patients with unstable angina, as well as the effect of abciximab therapy on the above parameters. Thirty two patients with unstable angina and 25 sex- and age-matched healthy controls participated in the study. On the day of admission (day 1) 17 patients received a bolus of abciximab (0.25 mg/kg) followed by a 12-h infusion (10 micrograms/min). Platelet aggregation to both PAF and ADP, in platelet rich plasma, was successively studied in both patients receiving abciximab or remaining untreated. The plasma and HDL-associated PAF-AH activity was also determined at the same times. In the untreated patients, the PAF EC50 values were significantly lower on the day of admission, whereas the maximal percentage of aggregation was significantly higher compared to controls (p < 0.01 for both comparisons). Similar behaviour of the platelets was observed in the aggregatory effect of ADP. This aggregatory response was not significantly altered 4 days, 7 days or 1 month afterwards. In the 17 patients who received abciximab, platelet aggregation to both PAF and ADP was inhibited by 90 +/- 5 and 96 +/- 3%, respectively, 1 h after bolus. At 2 and 3 days after treatment, platelet aggregation to both agonists was significantly recovered being similar to controls. However, it was fully restored 6 days after bolus, still being significantly higher compared to controls (p < 0.01 for PAF and p < 0.003 for ADP). The total plasma PAF-AH activity in both patient groups was not different from that of controls, whereas the HDL-associated PAF-AH activity was significantly lower. The total plasma or HDL-associated enzyme activity was not altered at any time interval studied, and it was not influenced by abciximab. The increased aggregatory response of platelets to PAF and the low plasma levels of HDL-cholesterol and HDL-associated PAF-AH activity in patients with unstable angina may contribute to the severe atherosclerosis and to acute thrombosis found in these patients. Abciximab therapy may protect platelets from PAF action in vivo the first days after drug administration, but it fails to permanently restore the enhanced aggregatory response observed.

Abciximab Binding to Glycoprotein IIb-IIIa and Protein Tyrosine Phosphorylation in Human Platelets

To the Editor: We read with interest the report of Peter et al[1][1] in which the authors showed that the binding of abciximab (c7E3 Fab fragments), a new generation anti-thrombotic drug[2][2] [3][3] induced conformation changes in glycoprotein (GP) IIb-IIIa. According to these authors, these

Labeling of the Internal Pool of GP IIb-IIIa in Platelets by c7E3 Fab Fragments (abciximab): Flow and Endocytic Mechanisms Contribute to the Transport

AbstractAbciximab is a new antiplatelet therapeutic in ischemic cardiovascular disease. The drug, chimeric Fab fragments of a murine monoclonal antibody (MoAb) (c7E3), blocks GP IIb-IIIa function. However, its capacity to reach all receptor pools in platelets is unknown. Electron microscopy and immunogold labeling were used to localize abciximab in platelets of patients receiving the drug for up to 24 hours. Studies on frozen-thin sections showed that c7E3 Fab, in addition to the surface pool, also labeled the surface-connected canalicular system (SCCS) and α-granules. Analysis of gold particle distribution showed that intraplatelet labeling was not accumulative and in equilibrium with the surface pool. After short-term incubations of platelets with c7E3 Fab in vitro, gold particles were often seen in lines within thin elements of the SCCS, some of which appeared in contact with α-granules. Little labeling was associated with Glanzmann's thrombasthenia platelets, confirming that the channels contained bound and not free c7E3 Fab. Endocytosis of abciximab in clathrin-containing vesicles was visualized by double staining and constitutes an alternative mechanism of transport. The remaining free pool of GP IIb-IIIa was evaluated with the MoAb AP-2; flow cytometry showed it to be about 9% on the surface of nonstimulated platelets but 33% on thrombin-activated platelets. The ability of drugs to block all pools of GP IIb-IIIa and then to be associated with secretion-dependent residual aggregation must be considered when evaluating their efficiency in a clinical context.

Labeling of the Internal Pool of GP IIb-IIIa in Platelets by c7E3 Fab Fragments (abciximab): Flow and Endocytic Mechanisms Contribute to the Transport

Abciximab is a new antiplatelet therapeutic in ischemic cardiovascular disease. The drug, chimeric Fab fragments of a murine monoclonal antibody (MoAb) (c7E3), blocks GP IIb-IIIa function. However, its capacity to reach all receptor pools in platelets is unknown. Electron microscopy and immunogold labeling were used to localize abciximab in platelets of patients receiving the drug for up to 24 hours. Studies on frozen-thin sections showed that c7E3 Fab, in addition to the surface pool, also labeled the surface-connected canalicular system (SCCS) and alpha-granules. Analysis of gold particle distribution showed that intraplatelet labeling was not accumulative and in equilibrium with the surface pool. After short-term incubations of platelets with c7E3 Fab in vitro, gold particles were often seen in lines within thin elements of the SCCS, some of which appeared in contact with alpha-granules. Little labeling was associated with Glanzmann's thrombasthenia platelets, confirming that the channels contained bound and not free c7E3 Fab. Endocytosis of abciximab in clathrin-containing vesicles was visualized by double staining and constitutes an alternative mechanism of transport. The remaining free pool of GP IIb-IIIa was evaluated with the MoAb AP-2; flow cytometry showed it to be about 9% on the surface of nonstimulated platelets but 33% on thrombin-activated platelets. The ability of drugs to block all pools of GP IIb-IIIa and then to be associated with secretion-dependent residual aggregation must be considered when evaluating their efficiency in a clinical context.

Rapid platelet-function assay: an automated and quantitative cartridge-based method.

The platelet glycoprotein (GP) IIb/IIIa receptor is important in mediating platelet thrombus formation, and the GP IIb/IIIa antagonist abciximab (c7E3 Fab; ReoPro) is effective in preventing thrombotic ischemic cardiovascular complications of unstable angina and percutaneous coronary interventions. Small-molecule antagonists of GP IIb/IIIa based on the Arg-Gly-Asp (RGD) sequence show similar benefit, and some of these agents are orally active. However, there may be significant interindividual variation in response to such antagonists, especially with chronic oral therapy. It will be essential to balance the beneficial antithrombotic effect of these drugs with their potential for causing bleeding. In response to this need, we have developed a rapid platelet-function assay (RPFA), a point-of-care system that provides a quantitative measure of the competence of the GP IIb/IIIa receptor as reflected in the ability of platelets to agglutinate fibrinogen-coated beads. Polystyrene beads were coated with fibrinogen and placed in a cartridge along with a lyophilized peptide that activates the thrombin receptor. Anticoagulated whole blood was added to the cartridge, and then a microprocessor-controlled operation mixed the reagents and detected agglutination between platelets and coated beads. Quantitative digital results were displayed within 3 minutes. Because there is no dilution of the blood, the assay can be used to measure platelet activity in samples that have been treated with GP IIb/IIIa antagonists with high dissociation rates. RPFA results of whole-blood samples treated with different GP IIb/IIIa antagonists correlated well with both conventional turbidimetric platelet aggregation (r2=0.95) and the percentage of free GP IIb/IIIa molecules in the sample (r2=0.96). The mean difference in measurements between RPFA and aggregometry was -4% (+/-4% SD), and the mean difference in measurements between RPFA and free GP IIb/IIIa receptors was -2% (+/-6% SD). The RPFA provides rapid information on platelet function that mirrors turbidimetric platelet aggregation and reflects GP IIb/IIIa receptor blockade.

Free and Total Platelet Glycoprotein IIb/IIIa Measurement in Whole Blood by Quantitative Flow Cytometry during and after Infusion of c7E3 Fab in Patients Undergoing PTCA

A quantitative flow cytometry assay was used to evaluate the ex vivo kinetics of c7E3 Fab platelet effect in 16 patients undergoing PTCA treated with abciximab and compared with aggregometry assay. Immunolabeling of platelets was directly assessed on whole blood, using in parallel two monoclonal antibodies (Mabs) raised against GPIIIa, Mab1, the binding of which is inhibited by c7E3 Fab, and Mab2, the binding of which is not affected by c7E3 Fab. We found a severe and sustained inhibition of both GPIIb/IIIa receptors and platelet functions. The inter-individual variation in response to abciximab was low. A significant transient increase at H24 and H48 in the binding of Mab2 was found as an unexpected result, and confirmed in vitro. Results demonstrate that flow cytometry is a reliable method in agreement with aggregation. In addition, our results show that it is a standardized tool and a time-saving technique.

Binding of Abciximab to αVβ3 and Activated αMβ2 Receptors: With a Review of Platelet-Leukocyte Interactions

IntroductionMurine monoclonal antibody 7E3, as well as the derivatives of 7E3 used in vivo [7E3-F(ab’)2, 7E3 Fab’, mouse/human chimeric 7E3 Fab (c7E3 Fab; abciximab; ReoPro™)], inhibit platelet aggregation induced by physiologic and pathologic agonists by binding to the platelet glycoprotein (GP) IIb/IIIa receptor.1,2 This biological activity formed the basis of its development as an antithrombotic agent to prevent and treat plateletmediated ischemic cardiovascular disease. During its development, 7E3 was reported to also react with two other integrin receptors, the αVβ3 “vitronectin” receptor (CD51/CD61)3,4 and at least one activation-dependent conformation of the αMβ2 or “Mac-1” receptor (CD11b/CD18).5 Whereas both αVβ3 and αMβ2 have been implicated in a number of different physiologic and pathologic processes, it is possible that some effects of abciximab are due to its reactivity with one or the other of these receptors. Moreover, the reactivity of abciximab with these receptors opens up the possibility that abciximab, or other agents that inhibit these receptors, may be useful in preventing or treating disorders in which these receptors play a role. This review will address these issues.

Inhibition of angiogenesis and tumor growth by murine 7E3, the parent antibody of c7E3 Fab (abciximab; ReoPro).

Angiogenesis plays an essential role in the growth and dissemination of solid tumor cancers. The expression of endothelial cell integrin alpha(v)beta3 has been shown to increase during vascular proliferation associated with human tumors. Selective antagonists of alpha(v)beta3 can block angiogenesis and tumor growth by inducing programmed cell death in proliferating endothelial cells. Monoclonal antibody 7E3, an antagonist of the human, but not murine, integrins alpha(v)beta3 and alphaIIbbeta3 (GPIIb/IIIa), inhibits platelet aggregation. It is the parent antibody of a mouse/human chimeric antibody fragment approved for adjunctive therapy of patients undergoing percutaneous coronary interventions to prevent ischemic complications (c7E3Fab; abciximab; ReoPro). To evaluate the potential of 7E3 to inhibit human angiogenesis and tumor growth independent of its antiplatelet effects, we established integrin alpha(v)beta3-negative human melanoma tumors in full-thickness human skin grafted onto SCID mice. The resulting tumors induce a human angiogenic response as assessed by the immunoreactivity of vascular cells with monoclonal antibodies specific for human CD31. Administration of 7E3 prevented or significantly inhibited the growth of tumors, and this effect correlated with a significant reduction in the number of blood vessels supplying the tumors. These results support the previous findings that blockade of integrin alpha(v)beta3 inhibits angiogenesis and tumor growth and indicates that dual inhibitors of alpha(v)beta3 and alphaIIbbeta3 are effective in blocking tumor growth and angiogenesis.