Platelet activation as a potential mechanism of GP IIb/IIIa inhibitor-induced thrombocytopenia

Critical Analysis of Thrombocytopenia Associated With Glycoprotein IIb/IIIa Inhibitors and Potential Role of Zalunfiban, a Novel Small Molecule Glycoprotein Inhibitor, in Understanding the Mechanism(s)

Thrombocytopenia is a rare but serious complication of the intravenous glycoprotein IIb/IIIa (GPIIb/IIIa; integrin αIIbβ3) receptor inhibitors (GPIs), abciximab, eptifibatide, and tirofiban. The thrombocytopenia ranges from mild (50 000-100 000 platelets/μL), to severe (20 000 to <50 000/μL), to profound (<20 000/μL). Profound thrombocytopenia appears to occur in <1% of patients receiving their first course of therapy. Thrombocytopenia can be either acute (<24 hours) or delayed (up to ~14 days). Both hemorrhagic and thrombotic complications have been reported in association with thrombocytopenia. Diagnosis requires exclusion of pseudothrombocytopenia and heparin-induced thrombocytopenia. Therapy based on the severity of thrombocytopenia and symptoms may include drug withdrawals and treatment with steroids, intravenous IgG, and platelet transfusions. Abciximab-associated thrombocytopenia is most common and due to either preformed antibodies or antibodies induced in response to abciximab (delayed). Readministration of abciximab is associated with increased risk of thrombocytopenia. Evidence also supports an immune basis for thrombocytopenia associated with the 2 small molecule GPIs. The latter bind αIIbβ3 like the natural ligands and thus induce the receptor to undergo major conformational changes that potentially create neoepitopes. Thrombocytopenia associated with these drugs is also immune-mediated, with antibodies recognizing the αIIbβ3 receptor only in the presence of the drug. It is unclear whether the antibody binding depends on the conformational change and whether the drug contributes directly to the epitope. Zalunfiban, a second-generation subcutaneous small molecule GPI, does not induce the conformational changes; therefore, data from studies of zalunfiban will provide information on the contribution of the conformational changes to the development of GPI-associated thrombocytopenia.

The Effects of Arginine-Based Supplements on Fatigue Levels following COVID-19 Infection: A Prospective Study in Romania

The purpose of this study was to examine the effects of two arginine-based supplements on the fatigue level of patients following the COVID-19 infection. This is a prospective study of the SARS-CoV-2-infected patients divided into two groups (according to family physicians' prescriptions, Group 1 of patients were treated with Astenor Energy^® containing arginine aspartate, B6 vitamin, biotin and magnesium, and Group 2 of patients were treated with Astenor Forte^® containing L-arginine and malic acid). The patients visited their family physicians from October 2021 to January 2022, complaining of physical and/or mental fatigue following the COVID-19 infection. We recorded 505 patients (146 patients in Group 1 and 359 patients in Group 2) and analyzed the fatigue level using the Fatigue Assessment Scale (FAS) through its total (FAS-T), mental (FAS-M) and physical (FAS-P) scores, at baseline and after three months of treatment. There was no difference between the two groups in terms of age (p = 0.265), but more men were included in Group 1 than in Group 2 (p = 0.001). The patients from Group 2 were significantly more likely to be treated at home than those included in Group 1 (89.7% vs. 65.1%, p < 0.0001) because of the lower severity of the COVID-19 infection (asymptomatic or mild: 82.5% vs. 48.7%, p < 0.0001). After 3 months of treatment, patients indicated no fatigue in the higher percentage compared to than at the baseline (68.7% vs. 27.7%), and the fatigue level significantly decreased both in Group 1 (median baseline 33.0 vs. follow-up 17.00, p < 0.0001) and Group 2 (median baseline 25.0 vs. follow-up 17.00, p < 0.0001). These findings suggest that supplements with L-arginine may be proposed as a remedy to restore physical and mental performance affected by the fatigue burden in people with COVID-19 or following the COVID-19 infection.

Novel Antiplatelet Agents in Cardiovascular Disease

Antiplatelet therapy reduces atherothrombotic risk and has therefore become a cornerstone in the treatment of cardiovascular disease. Aspirin, adenosine diphosphate P2Y12 receptor antagonists, glycoprotein IIb/IIIa inhibitors, and the thrombin receptor blocker vorapaxar are effective antiplatelet agents but significantly increase the risk of bleeding. Moreover, atherothrombotic events still impair the prognosis of many patients with cardiovascular disease despite established antiplatelet therapy. Over the last years, advances in the understanding of thrombus formation and hemostasis led to the discovery of various new receptors and signaling pathways of platelet activation. As a consequence, many new antiplatelet agents with high antithrombotic efficacy and supposedly only moderate effects on regular hemostasis have been developed and yielded promising results in preclinical and early clinical studies. Although their long journey from animal studies to randomized clinical trials and finally administration in daily clinical routine has just begun, some of the new agents may in the future become meaningful additions to the pharmacological armamentarium in cardiovascular disease.

Current and future antiplatelet therapies: emphasis on preserving haemostasis

Antiplatelet drugs, such as aspirin, P2Y₁₂ antagonists, and glycoprotein (GP) IIb/IIIa inhibitors, have proved to be successful in reducing the morbidity and mortality associated with arterial thrombosis. These agents are, therefore, the cornerstone of therapy for patients with acute coronary syndromes. However, these drugs all carry an inherent risk of bleeding, which is associated with adverse cardiovascular outcomes and mortality. Thus, the potential benefits of more potent, conventional antiplatelet drugs are likely be offset by the increased risk of bleeding. Data from experiments in vivo have highlighted potentially important differences between haemostasis and thrombosis, raising the prospect of developing new antiplatelet drugs that are not associated with bleeding. Indeed, in preclinical studies, several novel antiplatelet therapies that seem to inhibit thrombosis while maintaining haemostasis have been identified. These agents include inhibitors of phosphatidylinositol 3-kinase-β (PI3Kβ), protein disulfide-isomerase, activated GPIIb/IIIa, GPIIb/IIIa outside-in signalling, protease-activated receptors, and platelet GPVI-mediated adhesion pathways. In this Review, we discuss how a therapeutic ceiling has been reached with existing antiplatelet drugs, whereby increased potency is offset by elevated bleeding risk. The latest advances in our understanding of thrombus formation have informed the development of new antiplatelet drugs that are potentially safer than currently available therapies.

GPIIb/IIIa inhibitors: From bench to bedside and back to bench again

From the discovery of the platelet glycoprotein (GP) IIb/IIIa and identification of its central role in haemostasis, the integrin GPIIb/IIIa (αIIbβ3, CD41/CD61) was destined to be an anti-thrombotic target. The subsequent successful development of intravenous ligand-mimetic inhibitors occurred during a time of limited understanding of integrin physiology. Although efficient inhibitors of ligand binding, they also mimic ligand function. In the case of GPIIb/IIIa inhibitors, despite strongly inhibiting platelet aggregation, paradoxical fibrinogen binding and platelet activation can occur. The quick progression to development of small-molecule orally available inhibitors meant that this approach inherited many potential flaws, which together with a short half-life resulted in an increase in mortality and a halt to the numerous pharmaceutical development programs. Limited clinical benefits, together with the success of other anti-thrombotic drugs, in particular P2Y12 ADP receptor blockers, have also led to a restrictive use of intravenous GPIIb/ IIIa inhibitors. However, with a greater understanding of this key platelet-specific integrin, GPIIb/IIIa remains a potentially attractive target and future drug developments will be better informed by the lessons learnt from taking the current inhibitors back to the bench. This overview will review the physiology behind the inherent problems of a ligand-based integrin inhibitor design and discuss novel promising approaches for GPIIb/IIIa inhibition.

Abciximab-induced acute profound thrombocytopenia postpercutaneous coronary intervention

Abciximab (c7E3 Fab) is one of the three potent intravenous glycoprotein IIb/IIIa receptor inhibitors (along with eptifibatide and tirofiban) that have shown significant positive outcomes when used in patients with intracoronary thrombus. However, major side effects have been reported with its use including hypotension, major bleeding and thrombocytopenia. This case is a 53-year-old man presenting with acute chest pain diagnosed with non-ST-elevation myocardial infarction, who underwent percutaneous coronary intervention with abciximab and heparin infusion and developed acute profound thrombocytopenia (platelet count <20,000/L) within 9 hours of infusion. This case demonstrates the importance of routinely monitoring the platelet count prior to and 2-4 hours following abciximab infusion and differentiating other causes of acute profound thrombocytopenia, particularly pseudothrombocytopenia and heparin-induced thrombocytopenia.

Thrombocytopenia and Critical Care Medicine

Platelet transfusions play an important role in the treatment of critically ill patients. Like any blood component, however, there are various aspects of platelet transfusion therapy that need be considered by the intensivist. These include the proper dose and type of platelet component to infuse, as well as the route and method of administration. Methods to reduce the volume of the transfused platelets, for example, must ensure that the infused platelets will be functional and viable, posttransfusion. Treatment and diagnosis of the HLA alloimmunized recipient can pose a serious challenge to the clinician and an obstacle to adequate platelet therapy. An ICU patient for whom an adequate posttransfusion platelet increment cannot be achieved is at great risk of suffering a fatal hemorrhage. The ICU physician should be aware of the techniques used in modern transfusion practice to avoid having to deal with this complication. Adverse reactions to platelet transfusion include not only serologic ones, but those related to febrile and allergic complications, as well as infectious complications. The latter group includes diseases caused by infection with cytomegalovirus, bacteria, and a cadre of viruses including HIV and hepatitis. The clinical approach to thrombocytopenia in the ICU will be covered in some detail in an effort to review many of the conditions associated with recipient thrombocytopenia, including ITP, TTP, dilutional thrombocytopenia, DIC, surgery, HELLP syndrome, and drug-induced thrombocytopenia. Unfortunately the treatment approaches traditionally used are not always derived from evidence-based studies. This review covers many of these topics in an attempt to help physicians become better able to manage thrombocytopenia in the ICU and thus provide the best transfusion therapy for their patients.

Targeting integrin and integrin signaling in treating thrombosis

The critical roles of integrins in thrombosis have enabled the successful development and clinical use of the first generation of integrin antagonists as represented by abciximab (Reopro), eptifibatide (Integrilin), and tirofiban (Aggrastat). These integrin αIIbβ3 antagonists are not only potent antithrombotics but also have significant side effects. In particular, their induction of ligand-induced integrin conformational changes is associated with thrombocytopenia. Increased bleeding risk prevents integrin antagonists from being used at higher doses and in patients at risk for bleeding. To address the ligand-induced conformational changes caused by current integrin antagonists, compounds that minimally induce conformational changes in integrin αIIbβ3 have been developed. Recent studies on the mechanisms of integrin signaling suggest that selectively targeting integrin outside-in signaling mechanisms allows for potent inhibition of thrombosis, while maintaining hemostasis in animal models.

Evaluation of platelet activation in patients supported by the Jarvik 2000* high-rotational speed impeller ventricular assist device

OBJECTIVE

The Jarvik 2000 (Jarvik Heart, Inc, New York, NY) is a thumb-sized high-speed impeller pump that is used as a ventricular assist device in patients with terminal heart failure. Because the Jarvik 2000 is designed for long-term use, it is a central question whether the mechanical forces inside the pump affect blood components. This study evaluated the potential association of the high rotational speed of the Jarvik 2000 with platelet activation, which may result in thromboembolic events.

METHODS

The study group comprised patients with terminal heart failure who were supported with the Jarvik 2000. All were men and received 100 mg aspirin daily. In 8 patients, soluble platelet activation markers (P-selectin and sCD40L), platelet counts, and hemolysis markers (haptoglobin and lactate dehydrogenase levels) were determined. In 5 patients, P-selectin expression and platelet receptor glycoprotein IIb/IIIa activation were determined with flow cytometry and compared with a control group of 5 healthy men. Platelet activation was measured at various rotational device speeds.

RESULTS

After Jarvik 2000 implantation, increased hemolysis was observed, but platelet activation markers and platelet counts were not affected. Increased rotational speed (8000 to 12,000 rpm) of the device also did not result in increased platelet activation.

CONCLUSION

The Jarvik 2000 was not associated with detectable platelet activation, despite high rotational impeller speeds.

Acute profound thrombocytopenia with second exposure to eptifibatide associated with a strong antibody reaction

We present a case of eptifibatide-induced acute profound thrombocytopenia in a 64-year-old male receiving eptifibatide for the second time during percutaneous coronary intervention. Although rare, short and self-limited episodes of acute and profound thrombocytopenia have been associated with eptifibatide exposure. The thrombocytopenia is thought to be immune mediated, and assays are available to test for eptifibatide-induced platelet antibodies.

Antiaggregatory and proangiogenic effects of a novel recombinant human dual specificity anti-integrin antibody

BACKGROUND

beta(3)-Integrins are involved in platelet aggregation via alpha(IIb)beta(3) [glycoprotein (GP)IIb-GPIIIa], and in angiogenesis via endothelial alpha(V)beta(3). Cross-reactive ligands with antiaggregatory and proangiogenic effects, both desirable in peripheral vasculopathies, have not yet been described.

OBJECTIVES

In vitro and in vivo characterization of antiaggregatory and proangiogenic effects of two recombinant human Fab fragments, with emphasis on beta(3)-integrins.

METHODS

Recombinant Fab fragments were obtained by phage display technology. Specificity, affinity and IC(50) were determined by immunodot assays, enzyme-linked immunosorbent assay (ELISA), and Scatchard plot analysis, and by means of human umbilical vein endothelial cells (HUVECs). Functional analyses included ELISA for interaction with fibrinogen binding to GPIIb-GPIIIa, flow cytometry for measurement of activation parameters and competitive inhibition experiments, human platelet aggregometry, and proliferation, tube formation and the chorioallantoic membrane (CAM) assay for measurement of angiogenic effects.

RESULTS

We observed specific and high-affinity binding to an intact GPIIb-GPIIIa receptor complex of two human Fab autoantibody fragments, with no platelet activation. Dose-dependent fibrinogen binding to GPIIb-GPIIIa and platelet aggregation were completely inhibited. One Fab fragment was competitively inhibited by abciximab and its murine analog monoclonal antibody (mAb) 7E3, whereas the other Fab fragment bound to cultured HUVECs, suggesting cross-reactivity with alpha(V)beta(3), and also demonstrated proangiogenic effects in tube formation and CAM assays.

CONCLUSIONS

These Fab fragments are the first entirely human anti-GPIIb-GPIIIa Fab fragments with full antiaggregatory properties; furthermore, they do not activate platelets. The unique dual-specificity anti-beta(3)-integrin Fab fragment may represent a new tool for the study and management of peripheral arterial vasculopathies.

Thrombocytopenia in patients with an acute coronary syndrome (from the Global Registry of Acute Coronary Events [GRACE])

The incidence of thrombocytopenia after hospital admission, patient and treatment characteristics, and outcomes in patients enrolled in the prospective multinational GRACE were examined. Heparin (unfractionated or low molecular weight) and glycoprotein IIb/IIIa-inhibition can be associated with immune-mediated thrombocytopenia of clinical importance. The prevalence of thrombocytopenia in patients with acute coronary syndromes (ACSs) in general and specifically related to these therapies and associated outcomes have been studied little outside of clinical trials. Patients with an ACS were stratified into 4 groups of those with heparin-induced thrombocytopenia (HIT), those with glycoprotein IIb/IIIa-associated thrombocytopenia (GAT), those with other thrombocytopenia (not diagnosed as HIT or associated with glycoprotein inhibitors), and those with no thrombocytopenia. From June 2000 to September 2007, a total of 52,647 patients with an ACS and information for platelet count were enrolled in GRACE. Of these, 152 (0.3%) were reported to develop HIT, 324 (0.6%) developed GAT, and 368 (0.7%) developed other thrombocytopenia. Patients with HIT, GAT, or other thrombocytopenia were significantly more likely to die in the hospital versus those without these diseases (adjusted odds ratio [OR] 1.94, 95% confidence interval [CI] 1.07 to 3.53; adjusted OR 3.45, 95% CI 2.35 to 5.05; and adjusted OR 2.83, 95% CI 1.97 to 4.06, respectively). They were also more likely to experience major bleeding, (re)infarction, or stroke. In conclusion, in this large multinational registry, 1.6% of patients with ACS were reported to develop thrombocytopenia, with only 0.3% being HIT. Regardless of whether patients had clinically recognized HIT, GAT, or other thrombocytopenia, all 3 groups had significantly higher rates of major bleeding, recurrent infarction, stroke, and death.

Acute thrombocytopenia after liver transplant: role of platelet activation, thrombopoietin deficiency and response to high dose intravenous IgG treatment

BACKGROUND/AIMS

Thrombocytopenia is common after liver transplantation due to platelet sequestration secondary to hypersplenism. The aim of this study was to further investigate the causes of this condition, as well as the response of thrombocytopenia to high dose intravenous immunoglobulins.

METHODS

We retrospectively studied 73 patients who underwent liver transplantation. Out of these 73 patients, 27 had severe thrombocytopenia and were treated with high dose intravenous immunoglobulin. Additionally, we retrospectively studied 8 patients undergoing liver transplantation.

RESULTS

Our data suggest that splenomegaly is not the only factor responsible for thrombocytopenia after liver transplantion and two additional phenomena, namely, reduced platelet production due to reduced thrombopoietin levels and sustained platelets activation take part in the pathogenesis of this condition. The infusion of high dose immunoglobulins induced a safe, prompt, complete and persistent resolution of severe thrombocytopenia in more than 70% of patients.

CONCLUSIONS

Based on these findings, treatment with high dose intravenous immunoglobulins should be considered in the management of severe thrombocytopenia after liver transplant, although additional randomized trials are warranted.

Drug-induced thrombocytopenia and thrombosis: Evidence from patients receiving an oral glycoprotein IIb/IIIa inhibitor in the Orbofiban in Patients with Unstable coronary Syndromes- (OPUS-TIMI 16) trial

OBJECTIVE

To assess etiology and impact of thrombocytopenia in a large oral glycoprotein (GP) IIb/IIIa inhibitor trial.

BACKGROUND

Heparin is known to cause thrombocytopenia, and in some of these patients thrombosis. GP IIb/IIIa inhibitors are also associated with thrombocytopenia.

METHODS

The Orbofiban in Patients with Unstable Coronary Syndromes (OPUS-TIMI 16) Trial randomized 10,392 patients with ACS to the oral GP IIb/IIIa inhibitor orbofiban or placebo. Patients were followed for a minimum of ten months. Thrombocytopenia was defined prospectively as a platelet count < 80,000.

RESULTS

Thrombocytopenia was rare in the OPUS-TIMI 16 trial (0.68% at Day 30 and 0.80% at 1 year), but more common in patients treated with orbofiban (0.92%) compared with those treated with placebo (0.2%), p < 0.001. Patients who developed thrombocytopenia had higher rates of death (11.6% vs. 1.7%, p < 0.001), recurrent MI (12.1% vs. 2.8%, p < 0.001), intracranial hemorrhage (2.9% vs. 0.0%, p < 0.001), and major or severe bleeding (19.0% vs. 2.0%, p < 0.001) at 30 days (with similar results at one year).

CONCLUSION

Thrombocytopenia, though uncommon, was associated with orbofiban use and an increased risk of bleeding, but also death and MI. This study provides further evidence that drugs that lead to thrombocytopenia are, in a significant proportion of patients associated with thrombotic events.

New approaches to blockade of alpha4-integrins, proven therapeutic targets in chronic inflammation

The recruitment of leukocytes into tissue is a pivotal step in inflammation. alpha4-Integrins are adhesion receptors on circulating leukocytes that mediate attachment to the endothelium and facilitate their migration into the inflamed tissue. This multistep process is mediated by the interaction of alpha4-integrins with their counter receptors VCAM-1 and MadCAM-1 that are expressed on endothelial cells. alpha4-Integrins act as both adhesive and signaling receptors. Paxillin, a signaling adaptor molecule, binds directly to the alpha4 cytoplasmic tail and its binding is important for cell migration. Blocking the adhesive functions of alpha4-integrins has been shown to be an effective therapeutic approach in the treatment of autoimmune diseases, but also carries the risk of defects in development, hematopoiesis and immune surveillance. Interfering with alpha4 signaling by inhibiting the alpha4-paxillin interaction decreases alpha4-mediated cell migration and adhesion to VCAM-1 and MadCAM under shear flow. These in vitro effects are accompanied by a selective impairment of leukocyte migration into inflammatory sites when the alpha4-paxillin interaction is blocked in vivo. Thus, blockade of alpha4-integrin signaling may offer a novel strategy for interfering with the functions of these receptors in pathological events while sparing important physiological functions.

Recurrent Acute Profound Thrombocytopenia Related to Readministration of Eptifibatide

We describe the case of an 86-year-old woman who presented with unstable angina. She was given heparin and eptifibatide, and she underwent percutaneous coronary intervention (PCI). Shortly thereafter, she developed acute profound thrombocytopenia (6,000 platelets/mm3), which resolved after the discontinuation of heparin and eptifibatide. Four months later, she presented again with unstable angina and underwent PCI. Soon after the procedure, she again developed acute profound thrombocytopenia (2,000 platelets/mm3). To our knowledge, acute profound thrombocytopenia due to eptifibatide treatment in the same patient at two different times has not been reported before.

Frequency and management of thrombocytopenia with the glycoprotein IIb/IIIa receptor antagonists

Glycoprotein IIb/IIIa receptor antagonists (GPRAs) are widely used in the management of a variety of patients with acute coronary syndromes. Major adverse reactions to these agents include bleeding and thrombocytopenia. Immune mechanisms responsible for severe thrombocytopenia seen with GPRAs have been hypothesized for all 3 agents currently available in the United States, although specific laboratory tests are not available for use in routine practice. A review of published research for GPRA-induced thrombocytopenia (GIT) is provided. Although the incidence of severe GIT is relatively low, the implications for patients are potentially life threatening. Prompt recognition of severe thrombocytopenia is essential to facilitate the necessary care of patients. Treatment strategies include the modification of drug regimens and other interventions targeting the reduction of immediate bleeding risk and the provision of supportive care measures. A review of published research supporting the conservative use of corticosteroids and intravenous gamma globulin in this syndrome is provided. Clinicians identifying severe thrombocytopenia after GPRA exposure are encouraged to report these events, following national and institutional guidelines.

Cilostazol: Potential mechanism of action for antithrombotic effects accompanied by a low rate of bleeding

Treatment of thrombotic disease requires a delicate balance between prevention of new thrombotic events and management of bleeding complications. Various antiplatelet and anticoagulant agents have been used to this end, with varying degrees of success. Among the antiplatelet agents tested so far, cilostazol, which selectively targets phosphodiesterase III (PDE-III), has unique features. Cilostazol is classified as an antiplatelet agent because it inhibits the platelet aggregation induced by collagen, 5'-adenosine diphosphate (ADP), epinephrine, and arachidonic acid. Unlike other antiplatelet agents cilostazol not only inhibits platelet function but also improves endothelial cell function. Platelets circulate throughout the body with continuous tethering on the surface of endothelial cells. When endothelial cells are stimulated, the number of activated, or pre-conditioned, platelets in circulation increases. These platelets enhance thrombus formation at the sites of endothelial disruption. Cilostazol effectively prevents the onset of thrombotic disease, not only by direct inhibition of platelet function, but also by reducing the number of activated or pre-conditioned platelets in circulation. Secondary prevention of stroke with modest increase in bleeding complications was achieved by administration of cilostazol in the Japanese Cilostazol Stroke Prevention Study done in Japan. These results suggest that cilostazol may reduce the risk of stroke without increasing the risk of bleeding complications.

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.

Eptifibatide-associated acute, profound thrombocytopenia

OBJECTIVE

To describe 3 cases of eptifibatide-associated acute, profound thrombocytopenia.

CASE SUMMARIES

A 40-year-old black female received eptifibatide 180-microg/kg double bolus followed by a continuous infusion of 2 microg/kg/min for percutaneous coronary intervention (PCI). The platelet count decreased from 308 x 10(3)/mm3 to 2 x10(3)/mm3 4 hours after initiation of eptifibatide. Eptifibatide was discontinued and platelets were transfused. The patient developed a hematoma and petechiae. A 67-year-old white female received the same dosage regimen of eptifibatide for PCI with no serious adverse effects, with the treatment repeated one month later. At that time, she developed chest and back pain, dyspnea, wheezing, and hypotension after the first bolus. Her platelet count decreased from 334 x10(3)/mm3 to 6 x10(3)/mm3 24 hours after initiation. Eptifibatide was discontinued and platelets were transfused. The patient died due to shock. A 72-year-old white male received eptifibatide 180-microg/kg double bolus followed by a continuous infusion of 2 microg/kg/min for acute coronary syndrome. His platelet count decreased from 189 x10(3)/mm3 to 17 x10(3)/mm3, and eptifibatide was discontinued. Eptifibatide was readministered with bivalirudin for PCI once the platelet count increased to 94 x10(3)/mm3. Sixteen hours later, the platelet count decreased to 1 x 10(3)/mm3. Eptifibatide was discontinued and platelets were transfused. The patient developed a hematoma.

DISCUSSION

Acute, profound thrombocytopenia is a rare complication of glycoprotein IIb/IIIa inhibitor therapy characterized by a precipitous decline in platelet count to <20 x10(3)/mm3 within 24 hours of therapy. An objective causality assessment revealed that the adverse drug event was probable in 2 cases and possible in the other.

CONCLUSIONS

Increasing use of the glycoprotein IIb/IIIa inhibitors and enhanced recognition of the potential for acute, profound thrombocytopenia reinforce the need for more vigilant monitoring and alternative management strategies.

Inhibition of platelet activation and aggregation

It has recently been established that platelets are involved at all stages of atherosclerotic disease. A major platelet mediated process is the acute vessel closure at the site of atherosclerotic plaque rupture and there is emerging evidence for platelet adhesion to endothelial cells in the early stage of atherosclerotic disease. This, through engagement of other cells, leads to the development of the atherosclerotic plaque. Beside dietary, cholesterol- and lipid-lowering, and other pharmaceutical approaches antiplatelet therapy plays an important part in the treatment of atherosclerosis and its multifarious clinical manifestations. Antiplatelet therapy and the currently approved substances for oral (acetylsalicylic acid, dipyridamole, cilostazol, ticlopidin and clopidogrel) and parenteral (acetylsalicylic acid, abciximab, eptifibatide and tirofiban) administration are discussed in the following section. Attention is given to each single agent and its mechanism of action. Differences in pharmacodynamic and pharmacokinetic properties are elucidated and outlook on future antiplatelet strategies is discussed.

Delayed thrombocytopenia after treatment with abciximab: a distinct clinical entity associated with the immune response to the drug

BACKGROUND

Acute thrombocytopenia is a recognized side-effect of treatment with the fibrinogen receptor antagonist, abciximab, a chimeric (human/mouse) Fab fragment. The etiology of this complication is not fully understood. Generally, abciximab-induced thrombocytopenia occurs within a few hours of starting treatment with the drug. We have characterized a group of 13 patients who first developed thrombocytopenia 3-6 days after abciximab was discontinued.

OBJECTIVE

To characterize clinical and serological aspects of this newly recognized clinical entity.

PATIENTS AND METHODS

Clinical information was obtained from attending physicians and review of hospital records. Antibodies reactive with abciximab-coated platelets were characterized by flow cytometry.

RESULTS

In each patient, IgG and/or IgM antibodies reactive with abciximab-coated platelets were identified. These antibodies could be distinguished from similar antibodies present in many normal persons by two criteria-they were relatively resistant to inhibition by normal Fab fragments, and they reacted preferentially with platelets coated with 7E3, the murine monoclonal antibody from which peptide sequences in abciximab are derived. Antibodies with these characteristics were not found in pretreatment serum from three of the thrombocytopenic patients or in patients given abciximab who did not develop thrombocytopenia.

CONCLUSIONS

'Delayed thrombocytopenia' after treatment with abciximab is caused by antibodies produced in response to the drug. These antibodies may be specific for murine peptide sequences in abciximab but could recognize other target epitopes on abciximab-coated platelets. Physicians administering abciximab should be aware of this potential complication of treatment, which usually occurs after discharge from hospital.

Use of GPIIb/IIIa inhibitors in cardiovascular medicine

The GPIIb/IIIa inhibitors were the first clinically used anti-integrin therapeutics. They opened the gate to a rapidly developing area of anti-integrin targeting as a therapeutic approach to many diseases. The use of GPIIb/IIIa inhibitors in interventional cardiology is widespread and still increasing, as is the number of percutaneous coronary interventions. There seems to be a class effect of GPIIb/IIIa inhibitors in percutaneous coronary intervention, but there are major differences in the pharmacokinetics and pharmacodynamics of these agents. Currently clinically approved for parenteral use are the Fab fragment abciximab (ReoPro, Lilly) and the small-molecule GPIIb/IIIa inhibitors eptifibatide (Integrilin, Millennium/Schering-Plough) and tirofiban (Aggrastat, Merck). This review focuses on the different pharmacological properties of these agents and summarizes present and future therapeutic use of GPIIb/IIIa inhibitors in cardiovascular and other vascular diseases.

Discovery of Novel 2,8-Diazaspiro[4.5]decanes as Orally Active Glycoprotein IIb-IIIa Antagonists

In our efforts to develop orally active GPIIb-IIIa antagonists with improved pharmaceutical properties, we have utilized a novel 2,8-diazaspiro[4.5]decane scaffold as a template. We describe here our investigation of a variety of templates including spiropiperidinyl-gamma-lactams, spiropiperidinylimide, spiropiperidinylureas, and spiropiperidinylhydantoins. With the appropriate acidic and basic pharmacophores in place, each template yielded analogues with potent GPIIb-IIIa inhibitory activity. One of the compounds, 59 (CT50787), was also used to demonstrate for the first time the use of a pharmacological agent which is alphaIIbbeta3 specific to display biological activity in a lower species such as mouse and to extend bleeding times. Evaluation of the pharmacokinetic properties of selected compounds from each series in rat, dog, and cynomolgus monkey has led to the identification of 22 (CT51464), a double prodrug, with excellent pharmacokinetic properties. It exhibited good pharmacokinetic profile across species (F% = 33 (Cyno), 73 (dog), 22 (rat); t(1/2)(beta)() = 14.2 h (Cyno), 8.97 h (dog), 1.81 h (rat)). The biologically active form, 23 (CT50728), displayed inhibition of platelet aggregation in platelet rich plasma (PRP) with an IC(50) value of 53 nM in citrate buffer, 110 nM in PPACK anticoagulated PRP, and 4 nM in solid-phase GPIIb-IIIa competition binding assay (ELISA). Both 23 and 22 were stable in human liver microsomes, did not inhibit the P450 3A4 isozyme, and had low protein binding (18.22% for 23) and a desirable log P (0.45 +/- 0.06 for 22, and -0.91 +/- 0.32 for 23). It is predicted that the high oral bioavailability for these compounds in multiple species should translate into lower intra- and intersubject variability in man. The long plasma half-life of the lead is consistent with once or twice daily administration for chronic therapy. Analogue 22 (CT51464) thus appears to be a promising oral GPIIb-IIIa inhibitor with significantly improved pharmacokinetic properties over the previously described clinical candidates and may be found useful in the treatment of arterial occlusive disorders.

Platelet function and immune response

Altered platelet function may cause abnormal bleeding tendency or thrombosis. The goal of this article is to provide insights for understanding how platelet functions are related to immune response. Autoantibodies and drug-induced platelet antibodies have been demonstrated to downregulate or enhance platelet function. Drug-induced immune thrombocytopenia is an important adverse effect of glycoprotein IIb/IIIa antagonists. Platelets respond to binding of glycoprotein IIb/IIIa by partial platelet activation. This includes conformational changes of glycoprotein IIb/IIIa. Membrane changes may expose immunogenic neoantigens capable of abnormally altering immune responses. The presence of drug-dependent antibodies in an unexpectedly high frequency compared with the frequency of overt thrombocytopenia has opened a model for further studies. These may include monitoring of antiplatelet immune responses when new platelet antagonists are developed and comparisons of specific immune responses in other acute thrombocytopenias, such as those induced by quinidine or heparin and that associated with gold therapy or in acute profound thrombocytopenia, which may follow vaccination with live attenuated viruses.

Effects of 2 different antiplatelet regimens with abciximab or tirofiban on platelet function in patients undergoing coronary stenting

BACKGROUND

We sought to compare the antiplatelet effects of the glycoprotein IIb-IIIa receptor blockers abciximab or tirofiban, combined with an adjuvant therapy with clopidogrel and aspirin.

STUDY DESIGN AND METHODS

Twenty patients undergoing coronary stenting were randomly assigned to receive either abciximab or tirofiban combined with aspirin and clopidogrel. Serial blood samples were taken to assess platelet aggregation, P-selectin expression, thrombin generation, and platelet-induced endothelial cell expression of MCP-1, uPAR, and ICAM-1. Results and conclusions The therapy with aspirin plus clopidogrel attenuated agonist-induced platelet aggregation and P-selectin surface exposure (P <.05 vs aspirin monotherapy). Both tirofiban and abciximab further reduced agonist-induced platelet aggregation (P <.05), and decreased thrombin generation but had no effect on platelet alpha-granule release. None of the antithrombotic strategies significantly affected platelet-induced endothelial cell activation. Since platelet adhesion/degranulation initiates an inflammatory/mitogenic response in the vascular wall, future therapeutic strategies will have to be aimed at the inhibition of platelet release reactions.

Differential effect of the GPIIb/IIIa antagonist orbofiban on human platelet aggregate formation in vitro

Platelet microaggregates have been demonstrated in the systemic circulation of patients with atherosclerotic cardiovascular diseases. Glycoprotein IIb/IIIa antagonists have been reported to play roles in platelet activation, which may be associated with pro-thrombotic events. We report the effect of the orally active GPIIb/IIIa antagonist, orbofiban, on human platelet microaggregate formation in vitro. Laser light scatter (LSS) technology was used to monitor small, medium and large platelet aggregates formed in platelet-rich plasma in response to ADP or thrombin receptor activating peptide (TRAP)-6. ADP at 0.5 microM induced only small platelet aggregates that were prevented by orbofiban in a concentration-dependent manner. Likewise, orbofiban dissolved small aggregates after their formation. In marked contrast, in the presence of strong agonist stimulation (20 microM ADP or 3 microM TRAP-6) which generated primarily large platelet aggregates, orbofiban blocked the large aggregates while significantly augmenting small aggregates by three- to sixfold. The data suggest that GPIIb/IIIa antagonists do not induce platelet microaggregation directly but may augment small platelet microaggregate formation indirectly at conditions of strong stimuli. The percentage of the microaggregate population expressing P-selectin remained the same in the presence of orbofiban, indicating that these small microaggregates remain activated, although the mean intensity of expression was diminished, possibly reflecting the reduced size of the particles or density of P-selectin molecules. In summary, an increase in small platelet microaggregates might have contributed to pro-thrombotic events in orbofiban-treated patients.

Acute profound thrombocytopenia associated with eptifibatide therapy

An 80-year-old woman and a 79-year-old man underwent urgent percutaneous coronary intervention and received adjunctive eptifibatide. Platelet counts in both patients fell to below 20 x 10(3)/mm3 within 4 hours of eptifibatide administration. Reports in the medical literature reinforce the importance of recognizing that eptifibatide can cause acute profound thrombocytopenia. All three available glycoprotein IIb-IIIa inhibitors--abciximab, eptifibatide, and tirofiban--have been associated with the development of this disorder. Thus, clinicians should routinely monitor platelet counts in patients receiving glycoprotein IIb-IIIa inhibitors within 2-4 hours of the start of the infusion.

Eptifibatide-induced acute profound thrombocytopenia presenting as refractory hypotension

A 61-year-old woman presented with acute coronary syndrome and was given heparin and eptifibatide in conjunction with coronary angioplasty. Shortly thereafter she became profoundly thrombocytopenic (platelets 2.0 x 10(9)/L) and developed severe refractory hypotension. Heparin-induced antibodies were not detected, but the patient developed strong eptifibatide-dependent antibodies specific for platelets that appear to explain both the thrombocytopenia and the hypotensive episode.

Optimizing glycoprotein IIb/IIIa inhibition: lessons from recent randomized controlled trials

Recent randomized clinical trials with intravenous glycoprotein IIb/IIIa inhibition have provided unanticipated results, thereby questioning their role as empirical medical management for acute coronary syndromes. The lack of benefit with abciximab observed in Global Utilization of Strategies to Open Occluded Coronary Arteries IV is somewhat inconsistent with the benefits seen with this agent in coronary intervention, and the benefits of an early invasive approach incorporating tirofiban seen within Treat Angina with Aggrastat and determine Cost of Therapy with an Invasive or Conservative Strategy Thrombolysis in Myocardial Infraction 18. Additionally, a direct 'head-to-head' comparative study of abciximab and tirofiban within the setting of percutaneous coronary intervention demonstrates clinically relevant superiority with abciximab with respect to 30-day outcomes. in the setting of coronary instability and mechanical plaque disruption is more complex than initially perceived. Hence, although an abundance of evidence details the efficacy of these agents, their optimal clinical application remains somewhat challenging in light of these recent data. However, within the context of previous trial experience, the current evidence highlights several key aspects of glycoprotein IIb/IIIa inhibitor therapy that may be associated with improved patient outcomes. In particular, these trials indicate: (i) the importance of selecting high-risk patients in whom substantial clinical benefit is evident, (ii) the incorporation of these agents into an early invasive strategy, thereby matching the timing of vascular injury with maximal platelet inhibition and (iii) optimal dosing to achieve the high levels of platelet inhibition that appear to be required for efficacy with these agents.

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.

Thrombocytopenia after second exposure to abciximab is caused by antibodies that recognize abciximab-coated platelets

Thrombocytopenia, often severe, occurs in 1% to 2% of patients given the fibrinogen receptor antagonist abciximab, a chimeric Fab fragment containing murine specificity-determining and human framework sequences. The cause of this complication has not yet been defined. Studies of 9 patients who developed profound thrombocytopenia (platelets <10 x 10(9)/L [10 000/microL]) within a few hours of being given abciximab a second time showed that each had a strong immunoglobulin G (IgG) antibody that recognized platelets sensitized with abciximab. Five patients also had IgM antibodies. IgG antibodies reactive with abciximab-coated platelets were also found in 77 (74%) of 104 healthy subjects. However, the patient antibodies could be distinguished from "normal" ones in 2 ways: (1) only the patient antibodies reacted preferentially with platelets sensitized with the intact monoclonal antibody 7E3 from which the murine sequences in abciximab are derived; and (2) the "normal" antibodies could be inhibited by Fab fragments derived from normal human IgG, whereas the patient antibodies were relatively resistant to this treatment. The findings suggest that antibodies from the patients are specific for murine sequences in abciximab and are capable of causing life-threatening thrombocytopenia upon injection of this drug. The antibodies commonly found in healthy subjects are specific for the papain cleavage site of any Fab fragments and, although they react with abciximab-coated platelets, appear not to cause significant thrombocytopenia. It may be possible to identify patients at risk for developing thrombocytopenia if given abciximab by screening for antibodies that recognize 7E3-coated platelets.

Intrinsic activating properties of GP IIb/IIIa blockers

Potential intrinsic activating properties are probably the most controversially discussed issues with respect to GP IIb/IIIa blockers, especially since clinical trials with oral GP IIb/IIIa blockers revealed disappointing results. Based on the finding that currently clinically used GP IIb/IIIa blockers are ligand mimetics, experimental data are discussed, demonstrating an intrinsic activating effect of ligand mimetic GP IIb/IIIa blockers that potentially results in fibrinogen binding to alpha(IIb)beta(3) and in platelet aggregation. Furthermore, the inhibitory effect of aspirin on GP IIb/IIIa blocker-induced platelet aggregation is discussed as a clinically relevant finding. Finally, the potential association of GP IIb/IIIa blocker-induced thrombocytopenia with platelet activation is described.

Drug-induced and drug-dependent immune thrombocytopenias

Thrombocytopenia is a frequent comorbid condition in many in hospital patients. In some patients, drugs are the cause of low platelet counts. While cytotoxic effects of anti-tumor therapy are the most frequent cause, immune mechanisms should also be considered. This review addresses thrombocytopenias in four groups. Heparin-dependent thrombocytopenia (HIT), by far the most frequent drug-induced immune-mediated type of thrombocytopenia, has a unique pathogenesis and clinical consequences. HIT is a clinicopathological syndrome in which antibodies mostly directed against a multimolecular complex of platelet factor 4 and heparin cause paradoxical thromboembolic complications. The mechanisms through which heparin can enhance thrombin generation are discussed and treatment alternatives for affected patients are presented in detail. It is of primary importance to recognize these patients as early as possible and to substitute heparin with a compatible anticoagulatory drug, such as hirudin, danaparoid or argatroban. Patients seem to benefit from therapeutic doses of alternative treatment rather than from low-dose prophylactic doses. With the increasing use of glycoprotein (GP) IIb/IIIa inhibitors in patients with acute coronary syndromes, thrombocytopenias are increasingly recognized as an adverse effect of these drugs. Up to 4% of treated patients are affected. Most important, pseudothrombocytopenia, a laboratory artefact, is as frequent as real drug-induced thrombocytopenia and must be excluded before changes in treatment are considered. The pathogenesis of these thrombocytopenias is still debated; an immune mechanism involving preformed antibodies is likely. However, since these antibodies are also detectable in a high percentage of normal controls and of patients not developing thrombocytopenia, their impact is still unclear. Patients with real thrombocytopenia are at an increased risk of bleeding; treatment consists of cessation of the GP IIb/IIIa inhibitor and platelet transfusions in cases of severe hemorrhage. Classic immune thrombocytopenia can be induced by some drugs, e.g. gold, which trigger anti-platelet antibodies indistinguishable from platelet autoantibodies found in autoimmune thrombocytopenia. Drug-induced and drug-dependent immune thrombocytopenia is induced by antibodies recognizing an epitope on platelet GP formed after binding of a drug to a platelet glycoprotein. Still unresolved is whether antibody binding is the consequence of a conformational change of the antigen, the antibody, or both. These antibodies typically react with monomorphic epitopes on platelet GP, but only in the presence of the drug or a metabolite. Although several platelet GP have been identified as antibody target (GPIb/IX, GPV, GP IIb/IIIa), antibodies in an individual patient are highly specific for a single GP. Clinically, these patients present with very low platelet counts and acute, sometimes severe, hemorrhage. Treatment is restricted to withdrawal of the drug and symptomatic treatment of bleeding.

Severe thrombocytopenia possibly related to readministration of eptifibatide

Thrombocytopenia following eptifibatide therapy is a rare event, and acute severe thrombocytopenia following readministration has not been reported. We describe a case of acute severe thrombocytopenia following reexposure to eptifibatide during percutaneous coronary intervention. We continue to monitor platelet counts at 2-4 hr and 16-24 hr following administration of any glycoprotein IIb/IIIa inhibitor. Cathet Cardiovasc Intervent 2001;54:63-67.

Molecular mechanisms of drug-induced thrombocytopenia

A wide range of drugs can induce thrombocytopenia. Molecular mechanisms for the formation of specific epitopes for all the drug-dependent antibodies appear to be very similar. A restricted set of glycoproteins on the platelet surface interacts with the drugs to form neoepitopes, to which the drug-dependent antibodies bind. Molecular mapping of antigenic sites may help characterize genetic polymorphisms that predispose to the formation of the antibody binding sites. Identification of antibody binding sites will enhance our understanding of the pathogenesis of immune drug-induced thrombocytopenia.

Pharmacodynamics and safety of lefradafiban, an oral platelet glycoprotein IIb/IIIa receptor antagonist, in patients with stable coronary artery disease undergoing elective angioplasty

OBJECTIVE

Lefradafiban is the orally active prodrug of fradafiban, a glycoprotein IIb/IIIa receptor antagonist. The present phase II study aimed to determine the dose of lefradafiban that provides 80% blockade of the glycoprotein IIb/IIIa receptors by fradafiban, and to study the pharmacodynamics and safety of different doses in patients with stable angina undergoing angioplasty.

DESIGN

A double blind, placebo controlled, dose finding study.

SETTING

Four academic and community hospitals in the Netherlands.

PATIENTS

64 patients with stable coronary artery disease undergoing elective percutaneous transluminal coronary angioplasty.

INTERVENTIONS

30 mg, 45 mg, and 60 mg of lefradafiban three times daily or placebo was given for 48 hours.

MAIN OUTCOME MEASURES

The primary safety end point was the occurrence of bleeding, classified as major, minor, or insignificant according to the thrombolysis in myocardial infarction (TIMI) criteria. Efficacy indices included per cent fibrinogen receptor occupancy (FRO), ex vivo platelet aggregation, and plasma concentrations of fradafiban.

RESULTS

Administration of lefradafiban 30, 45, and 60 mg three times daily resulted in a dose dependent increase in median FRO levels of 71%, 85%, and 88%, respectively. Inhibition of platelet aggregation was closely related to FRO. There were no major bleeding events. The 60 mg lefradafiban group had a high (71%) incidence of minor and insignificant bleeding. The incidence of bleeding was 44% in the 30 mg and 45 mg groups, compared with 9% in placebo patients. Puncture site bleeding was the most common event. The odds of bleeding increased by 3% for every 1% increase in FRO.

CONCLUSIONS

Lefradafiban is an effective oral glycoprotein IIb/IIIa receptor blocker. The clinical effectiveness of doses up to 45 mg three times daily should be investigated.

Evidence of platelet activation during treatment with a GPIIb/IIIa antagonist in patients presenting with acute coronary syndromes

OBJECTIVES

The study was done to determine the role of partial agonist activity in the lack of effectiveness of the oral GPIIb/IIIa antagonist orbofiban.

BACKGROUND

Orbofiban, an oral GPIIb/IIIa antagonist, was found to increase the mortality of patients with acute coronary syndromes (ACS) in the OPUS-TIMI-16 trial, despite the fact that it is a very potent anti-platelet agent and that IV agents have proven very effective.

METHODS

Patients (n = 520) with ACS were randomized to orbofiban 30 mg, 40 mg or 50 mg twice daily or 50 mg once daily or placebo. Platelet activity was assessed in 175 patients by examining GPIIb/IIIa receptor conformation, expression of CD63 antigen, and platelet aggregation.

RESULTS

Plasma concentrations of orbofiban at the highest dose (74 +/- 6 ng/ml peak, 61 +/- 5 ng/ml trough) exceeded the IC50 for platelet aggregation to adenosine diphosphate (ADP) (29 +/- 6 ng/ml) and thrombin-activating peptide (61 +/- 18 ng/ml). Orbofiban induced a conformational change in GPIIb/IIIa detected as the displacement of the monoclonal antibody mAb2; such conformational changes have been linked to partial agonist activity. Consistent with this, platelet expression of CD63 ex vivo was significantly increased at five time points during the study. In vitro, orbofiban increased platelet aggregation to a submaximal concentration of epinephrine (67 +/- 19% vs. 27 +/- 9%, n = 5) and increased thromboxane formation when the platelet GPIIb/IIIa were clustered using monoclonal antibodies to the receptor.

CONCLUSIONS

Orbofiban is both an antagonist and a partial agonist of platelet GPIIb/IIIa. At low concentrations of the drug, this partial agonist activity may enhance platelet aggregation. Along with suboptimal plasma drug levels, these findings may help explain the lack of efficacy seen with orbofiban in patients with ACS.

Flow cytometric monitoring of glycoprotein IIb/IIIa blockade and platelet function in patients with acute myocardial infarction receiving reteplase, abciximab, and ticlopidine: continuous platelet inhibition by the combination of abciximab and ticlopidine

BACKGROUND

Improvement of thrombolysis may be achieved by concomitant strong platelet inhibition. To monitor platelet function in patients with myocardial infarction (n=46) who were treated with the fibrinolytic agent reteplase, the glycoprotein (GP) IIb/IIIa blocker abciximab, and the ADP receptor antagonist ticlopidine, we developed a flow cytometric assay.

METHODS AND RESULTS

Binding of abciximab to platelets was directly monitored as the percentage of platelets stained by a goat anti-mouse antibody. Blood drawn 10 minutes and 2 hours after the start of therapy with reteplase and abciximab and during the 12-hour infusion of abciximab demonstrated a maximal blockade of GP IIb/IIIa (10 minutes, 86.2+/-10.3%; 12 hours, 85.8+/-7.1%). Starting at 24 hours, abciximab binding gradually decreased (24 hours, 74.6+/-16.2%; 48 hours, 66.8+/-14.9%; 72 hours, 60.5+/-16.7%; 96 hours, 49.4+/-17.8%; 120 hours, 35.8+/-16. 4%; and 144 hours, 29.9+/-15.3%). Binding of a chicken anti-fibrinogen antibody to platelets, indicating the level of functional blockade of GP IIb/IIIa, was inversely correlated with the binding of abciximab (r=-0.72, P:<0.0001). In blood drawn at 10 minutes, platelet aggregation was maximally inhibited but recovered within 48 hours even if the majority of GP IIb/IIIa receptors were still blocked by abciximab. Reteplase did not influence abciximab binding and did not activate platelets, as measured by P-selectin expression, fibrinogen binding, and platelet aggregation. Platelet inhibition that was achieved during the first 24 hours by abciximab was directly maintained by additional treatment with ticlopidine.

CONCLUSIONS

Flow cytometric monitoring of platelet function allows differentiation of the effects of reteplase, abciximab, and ticlopidine. The combination of abciximab and ticlopidine is an attractive therapeutic strategy that provides a fast and continuous platelet inhibition.

Acute profound thrombocytopenia following abciximab therapy

OBJECTIVE

To report a case of acute, profound thrombocytopenia (APT), defined as an abrupt drop in platelet count to <20,000/mm3 that occurred within 24 hours of administration of an abciximab bolus, to summarize other abciximab-associated APT cases reported in the literature, to review the postulated mechanisms behind this reaction, and to emphasize the importance of periodically monitoring platelet counts after initiating abciximab therapy.

DATA SOURCES

MEDLINE and Index Medicus searches restricted to English-language literature from 1993 through June 1999 were conducted. MeSH headings included abciximab, ReoPro, thrombocytopenia, and glycoprotein IIb-IIIa (GP IIb-IIIa) inhibitors. References of the articles obtained were also reviewed.

DATA EXTRACTION

Search and evaluation were focused on published case reports and reviews of abciximab-induced APT, as well as the incidence of thrombocytopenia from the drug compared with that from other GP IIb-IIIa inhibitors.

DATA SYNTHESIS

Platelet aggregation has been identified as the structural basis for coronary thrombosis. This may lead to ischemic complications during acute coronary syndromes or following coronary intervention procedures. The use of GP IIb-IIIa inhibitors such as abciximab as antiplatelet agents has been effective in reducing these ischemic complications. We summarize 15 published cases of abciximab-associated APT gathered from data on 2,482 patients treated with the drug. Prior to suspecting abciximab, other potential causes of thrombocytopenia should be evaluated. It is important to monitor the platelet count at baseline, two hours after initiating abciximab, and 24 hours after initiation of therapy or prior to discharge, whichever comes first, to identify patients at risk for developing APT. If APT occurs and is left untreated, it can produce excessive hemorrhage and ischemia, potentially leading to death. Platelet transfusions have been more effective than immunoglobulin in the management of APT.

CONCLUSIONS

Abciximab-induced APT has a low incidence of occurrence. If it does develop and is not recognized or treated promptly, it can lead to serious hemorrhagic complications. Consequently, monitoring the platelet count two hours after initiation of the infusion is essential. If APT develops, abciximab should be discontinued and a platelet transfusion should be considered.

Emerging Therapies: Spectrum of Applications of Monoclonal Antibody Therapy

This article focuses on the recent dramatic advances in the applications of monoclonal antibody therapy to hematopoietic and neoplastic disease. The increase in the understanding of the role of growth factors and their receptors in the pathogenesis of malignancy and other undesirable hematological events taken in conjunction with the ability to produce humanized chimeric monoclonal antibodies to these targets is providing a new perspective for the treatment of leukemia, lymphoma and breast cancer, autoimmune disease and for prevention of ischemic complications. Dr. Waldmann describes approaches targeting the Her2/neu and the II-2/IL-15 receptor systems. The Her2/neu receptor is overexpressed in select breast, ovarian, gastric and pancreatic neoplasms. The use of trastuzumab (Herceptin) in the treatment of patients with breast cancer whose tumors overexpress this receptor are reviewed. The IL-2 receptor (Tac) is expressed on select malignant cells (adult T cell leukemia, hairy cell leukemia) and activated T cells involved in autoimmune disease and organ rejection. Humanized anti-Tac alone (daclizumab, Zenapax) or armed with toxins or radionuclides have been used successfully in the treatment of leukemia. Dr. Levy updates the experience with rituximab targeting CD20 on B cell lymphomas and reviews the antibodies to CD3, CD22, CD33, CD52, HLA-DR β chain and HLA-D currently in or proposed for clinical trials, including radiolabelled antibodies. In the last section, Dr. Coller reviews the therapeutic results achieved with abciximab (ReoPro), an antagonist of platelet receptor GPIIbIIIa for the prevention of restenosis in percutaneous coronary interventions and the treatment of unstable angina. The mechanism of action, pharmacology and safety and efficacy of abciximab are reviewed.