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J Am Coll Cardiol, 1998; 32:311-319 © 1998 by the American College of Cardiology Foundation |

a Divisions of Hematology/Coagulation and Cardiology, Department of Medicine, Duke University Medical Center, Durham, North Carolina, USA
* Section of Cardiology, Wake Forest University, Winston-Salem, North Carolina, USA
Department of Cardiology, Cleveland Clinic Foundation, Cleveland, Ohio, USA
Manuscript received February 19, 1998; revised manuscript received April 7, 1998, accepted April 23, 1998.
Address for correspondence: Dr. Scott D. Berkowitz, Divisions of Hematology/Coagulation and Cardiology, Department of Medicine, Box 3471, Duke University Medical Center, Durham, North Carolina 27710
berko005{at}mc.duke.edu
| Abstract |
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Background. The development of thrombocytopenia on exposure to GPIIb/IIIa antagonists threatens the utility and economic viability of this drug class for patients with vascular disease.
Methods. We analyzed data from the Evaluation of c7E3 for the Prevention of Ischemic Complications trial (EPIC), a 2,099-patient, randomized trial of placebo, abciximab bolus or abciximab bolus plus a 12-h infusion during high-risk coronary revascularization.
Results. Thrombocytopenia (nadir platelet count <100 x 109/liter) developed in 81 patients (3.9%) during their hospital stay, with 19 (0.9%) developing severe (<50 x 109/liter) thrombocytopenia. Both thrombocytopenia and severe thrombocytopenia were more frequent in the bolus-plus-infusion arm (5.2% and 1.6%, respectively) than in the bolus-only and placebo arms combined (p = 0.020 and p = 0.025, respectively). Acute profound thrombocytopenia developed in two patients in the bolus-plus-infusion arm. Patients with thrombocytopenia experienced more unfavorable clinical outcomes than those who did not develop thrombocytopenia, regardless of treatment assignment, but those with thrombocytopenia who received abciximab had fewer worse outcomes at 30 days. Multivariable logistic modeling revealed a lower baseline platelet count, older age and lighter weight to be important predictors of thrombocytopenia. In a logistic regression model, bolus-plus-infusion treatment was a significant predictor of thrombocytopenia (p = 0.016) and remained so after adjustment for procedures and baseline risk factors (p = 0.0077).
Conclusions. Thrombocytopenia was associated with adverse clinical outcomes and excessive bleeding, but patients receiving abciximab fared better than those receiving placebo.
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In the Evaluation of c7E3 for Prevention of Ischemic Complications (EPIC) trial, the use of a monoclonal chimeric human-murine antibody Fab fragment to the platelet glycoprotein (GP) IIb/IIIa receptor (c7E3 Fab, abciximab, ReoPro), in combination with aspirin and intravenous heparin, produced a 35% reduction in the 30-day composite-event rate (death, myocardial infarction or urgent coronary revascularization) in high risk patients undergoing coronary interventions, as compared with aspirin and heparin therapy alone (5,7). Our purposes were to define in this study group the frequency of thrombocytopenia and to analyze its relation to the 30-day clinical outcomes and hemorrhagic complications.
| Methods |
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Patients at risk for bleeding complications (history of hemorrhagic diathesis, major operation within 6 weeks of enrollment, gastrointestinal or genitourinary bleeding of clinical significance within the previous 6 weeks, stroke within 2 years of enrollment or any stroke with significant residual neurologic deficit or presumed or documented history of vasculitis) were excluded from participation in the study.
All patients received aspirin and intravenous heparin and were randomly assigned to one of three treatment groups: placebo bolus plus placebo infusion; abciximab bolus (0.25 mg/kg body weight) with placebo infusion; or abciximab bolus (0.25 mg/kg) plus abciximab infusion (10 µg/min). The bolus was administered within 10 to 60 min before the procedure. The abciximab infusions were given intravenously and continuously for 12 h unless clinically contraindicated.
Defining the group with thrombocytopenia.
Platelet counts were obtained from complete blood count samples drawn into EDTA and were determined before the infusion, at 30 min and at 2, 12 and 24 h after initiation of the study agent, and then daily until discharge or day 7, whichever occurred first. The term thrombocytopenia was assigned if there was a nadir platelet count <100 x 109/liter and an independent reviewer excluded pseudothrombocytopenia. The term severe thrombocytopenia was assigned to those nadir platelet counts <50 x 109/liter. Acute thrombocytopenia was defined as a platelet count <100 x 109/liter within 24 h of the initiation of therapy. Acute severe thrombocytopenia was defined as a platelet count <50 x 109/liter within 24 h of the initiation of therapy. Acute profound thrombocytopenia was defined as platelet count <20 x 109/liter within 24 h of the initiation therapy. Pseudothrombocytopenia was defined as artifactual, in vitro thrombocytopenia detected by automated laboratory cell counters recognized by preservation of the 30-min platelet count in citrate anticoagulant, while a simultaneous platelet count in EDTA anticoagulant was low (<100 x 109/liter). Platelet clumps were identified on a peripheral blood smear in some cases, and this was used as an independent criterion for pseudothrombocytopenia. The protocol recommended that heparin and aspirin be discontinued if the platelet count decreased to
60 x 109/liter and that platelet transfusion be initiated if the platelet count decreased to <50 x 109/liter.
Bleeding events. Bleeding events were classified as major or minor according to the Thrombolysis in Myocardial Infarction trial criteria (8), and net blood loss was computed using the criteria of Landefeld et al. (9). Details have been described previously (10).
Statistical analysis. Continuous measures are expressed as the median values with interquartile ranges (25th and 75th percentiles), whereas categoric factors are expressed as percentages. Multivariable logistic modeling was applied to examine the relation between baseline patient characteristics and thrombocytopenia. The effect of abciximab bolus-plus-infusion treatment on the development of thrombocytopenia was explored using multiple logistic regression analysis to account for procedures (coronary artery bypass graft surgery, repeat catheterizations, repeat percutaneous transluminal coronary intervention, balloon pump use, stent implantation) occurring before the development of thrombocytopenia. A second logistic regression model was developed to examine the effect of thrombocytopenia on a "major bleeding" outcome after adjustment for previously identified predictors of bleeding (10). Multiple linear regression techniques were used in parallel to explore the relation between thrombocytopenia and the bleeding index in the presence of known predictors. All clinical outcomes reported are at 30 days.
| Results |
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Baseline characteristics associated with thrombocytopenia. The distributions of the key baseline characteristics for patients who developed thrombocytopenia are displayed in Table 2. Univariably, thrombocytopenia occurred more often in patients who were older, of lighter weight, of African ancestry and had peripheral vascular disease or three-vessel coronary artery disease.
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Role of thrombocytopenia in major bleeding. Predictors of bleeding in the EPIC population were previously identified (10). In the current analysis, a logistic regression model was developed to examine the effect of thrombocytopenia on the "major bleeding" outcome after adjustment for these predictors of bleeding (chi-square 300.51, p < 0.0001), and it validated thrombocytopenia as a significant additional predictor of major bleeding (chi-square 13.04, p = 0.0003). Multiple linear regression techniques used in parallel to explore the relation between thrombocytopenia and the bleeding index in the presence of known predictors proved thrombocytopenia to be a very potent predictor of an increased bleeding index (chi-square 114.98, p < 0.0001).
| Discussion |
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Thrombocytopenia is associated with excessive bleeding. Thrombocytopenia in the EPIC trial, regardless of whether the patient was treated with abciximab or not, was also a significant additional predictor of major bleeding and a very potent predictor of an increased bleeding index. Among the 2,018 patients who did not develop thrombocytopenia, more bleeding events were recorded in those who received abciximab than in those who received placebo, as would be expected with antagonism of the platelet GPIIb/IIIa receptor. Among the 81 patients who developed thrombocytopenia, treatment with abciximab was associated with less bleeding as compared with thrombocytopenia without abciximab; these patients endured fewer major bleeding events and fewer transfusions of packed red blood cells than those who received placebo. This appears counter-intuitive in light of the published report of the bleeding complications in the entire EPIC population (10). However, that study did not separately analyze patients to evaluate the effect of thrombocytopenia and focused on major bleeding unrelated to bypass surgery. Owing to the small number of patients with thrombocytopenia (81 of 2,099), the effect of thrombocytopenia on bleeding within each treatment group may have been diluted in that analysis.
Thrombocytopenia associated with abciximab infusion is distinct. A distinct subgroup of patients with thrombocytopenia who had received abciximab was observed. Patients with thrombocytopenia who received placebo were a sicker group: they had worse clinical outcomes, had a higher bleeding index (p = 0.0462) and were more frequently transfused with packed red blood cells (p = 0.018). They developed thrombocytopenia later; 24 h after initiation of treatment the frequency of thrombocytopenia among patients in the placebo group was half that of those who received abciximab (Fig. 2). In the abciximab-treated group the observed frequency of thrombocytopenia was higher, but it was quantitatively less severe.
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Potential mechanisms for abciximab-associated thrombocytopenia. The mechanism of the thrombocytopenia observed with abciximab infusion is not understood, although postulates exist. Antibodies to the GPIIb/IIIa receptor can be encountered in immune and drug-induced thrombocytopenias (14,15), but by design abciximab has no Fc portion, making untenable the hypothesis that platelets with abciximab on their GPIIb/IIIa receptors are removed from the circulation by the Fc receptors of the macrophages of the reticuloendothelial system. This does not, however, rule out the development of aggregates of platelets and c7E3 Fab and subsequent removal from the circulation. Another consideration is the potential presence of residual uncleaved c7E3 Fab2 in the infused preparation, but this appears unlikely according to data from the manufacturer (personal communication of Robert Jordan, Centocor). Because the construction of the abciximab fragment requires cleavage of the chimeric antibody across the hinge region of the molecule, a human anti-chimeric antibody (HACA) reaction to this damaged human antibody structure might occur after abciximab infusion. The development of a HACA response suggests that the potential for immune-mediated reactions exists. In the EPIC trial, the development of HACA within the first 12 weeks of exposure to abciximab was 5.8% overall, with 6.5% developing in the bolus-plus-infusion group, 5.2% in the bolus-only group and none in the placebo group (16). Also detected was a low affinity pre-immune (pre-dose) HACA reactivity in 8% of the patients in EPIC, which appeared to be specific for the Fab cleavage site of human immunoglobulins. However, no correlation was observed between pre-dose or induced HACA and thrombocytopenia (personal communication of Carrie L. Wagner, Centocor).
Binding of abciximab to the GPIIb/IIIa receptor could conceivably produce a conformational change, resulting in the expression of new epitopes, termed ligand-induced binding sites (LIBS) (17). If patients have preformed antibodies that cross react with LIBS, the platelets may become coated with immunoglobulin and be removed from the circulation. Bednar et al. (18) reported the presence in human plasma samples of preexisting immunoglobulin Gtype antibodies that selectively bound to platelets in the presence of their GPIIb/IIIa antagonist and led to thrombocytopenia.
Platelet modifications might also be created either by the presence of abciximab on the platelet surface in relation to other platelet antigens or by the ability of abciximab to bind to the vitronectin (
vß3) receptor (where it can inhibit platelet-mediated thrombin generation) or the Mac-1 (CD11b/CD18) integrin receptors of activated monocytes (19). The unusual pharmacokinetic properties of the abciximab bolus plus infusion (13,20,21) might heighten the opportunity for the evolution of these scenarios; there is an extended platelet-bound half-life, with small amounts of the drug remaining on circulating platelets for 15 days or more (16). Any of the these aforementioned developments could culminate in platelet activation and plateletplatelet interactions with consumption or removal of the platelets from the circulation by macrophages of the reticuloendothelial system.
Study limitations. This analysis has certain limitations. A definitive delineation of the cause(s) for each of the 81 cases of thrombocytopenia was not possible. The thrombocytopenia protocol was primarily designed to detect the presence of pseudothrombocytopenia and HACA-induced thrombocytopenia; poor compliance with other aspects of the workup of thrombocytopenia led to incomplete investigation of other potential causes. Clean data are not available to evaluate the presence of heparin-induced thrombocytopenia. Data on heparin use were only obtained in the catheterization laboratory; once the patient left the laboratory, only the highest and lowest rates of heparin infused during the first 3 days were collected. Plasma samples to detect antibodies to heparin were collected in the extreme minority of cases. Also, the relation between thrombocytopenia and procedure could not be defined because only the day, and not the time, of the procedures was collected. Still, our analysis represents an important description of thrombocytopenia and augments the limited knowledge developed from previous studies of thrombocytopenia in the setting of acute coronary syndromes, most of which were derived from patients treated with thrombolytic therapy (8,22,23)
Conclusions. Thrombocytopenia was a frequent covariate among patients who developed ischemia-associated morbidity and mortality; however, among patients with thrombocytopenia, treatment with abciximab was associated with fewer ischemic events and less bleeding. Independent predictors of thrombocytopenia were lower baseline platelet count, increased age, lighter weight and treatment with bolus plus infusion of abciximab. The subgroup with thrombocytopenia with abciximab treatment appeared to be distinct from other types of thrombocytopenia and could be detected by a declining platelet count in the first 24 h after therapy initiation. A platelet count performed at 2 to 4 h after the abciximab infusion is begun will identify those patients at risk of developing thrombocytopenia, especially those who will develop acute severe and acute profound thrombocytopenia.
| Acknowledgments |
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| Footnotes |
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A list of the EPIC investigators can be found in the N Engl J Med 1994;330:95661. ![]()
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