Immune thrombocytopenia caused by glycoprotein IIb/IIIa inhibitors

Drug-induced thrombocytopenia

Drug-induced thrombocytopenia was first described in the 19th century, yet our understanding of its pathogenesis continues to evolve. The list of drugs implicated in drug-induced thrombocytopenia is extensive and growing. Many, if not most, of these medications induce thrombocytopenia by immune mechanisms. Because the degree of thrombocytopenia can put patients at risk for serious bleeding, a prompt diagnosis is key to clinical management. The laboratory approach to diagnosing drug-induced thrombocytopenia is 2-pronged. First, nondrug causes of thrombocytopenia must be ruled out. Second, testing for drug-dependent platelet antibodies, available at specialized reference laboratories, often can identify the offending medication, although usually not in time for initial clinical management. Once a medication is suspected of causing thrombocytopenia, it must be discontinued promptly, and the patient should be monitored closely. Thrombocytopenia generally resolves quickly after offending medication withdrawal, and the prognosis of drug-induced thrombocytopenia is then excellent.

Eptifibatide-induced thrombocytopenia and thrombosis in humans require FcgammaRIIa and the integrin beta3 cytoplasmic domain

Thrombocytopenia and thrombosis following treatment with the integrin alphaIIbbeta3 antagonist eptifibatide are rare complications caused by patient antibodies specific for ligand-occupied alphaIIbbeta3. Whether such antibodies induce platelet clearance by simple opsonization, by inducing mild platelet activation, or both is poorly understood. To gain insight into the mechanism by which eptifibatide-dependent antibodies initiate platelet clearance, we incubated normal human platelets with patient serum containing an alphaIIbbeta3-specific, eptifibatide-dependent antibody. We observed that in the presence of eptifibatide, patient IgG induced platelet secretion and aggregation as well as tyrosine phosphorylation of the integrin beta3 cytoplasmic domain, the platelet FcgammaRIIa Fc receptor, the protein-tyrosine kinase Syk, and phospholipase Cgamma2. Each activation event was inhibited by preincubation of the platelets with Fab fragments of the FcgammaRIIa-specific mAb IV.3 or with the Src family kinase inhibitor PP2. Patient serum plus eptifibatide did not, however, activate platelets from a patient with a variant form of Glanzmann thrombasthenia that expressed normal levels of FcgammaRIIa and the alphaIIbbeta3 complex but lacked most of the beta3 cytoplasmic domain. Taken together, these data suggest a novel mechanism whereby eptifibatide-dependent antibodies engage the integrin beta3 subunit such that FcgammaRIIa and its downstream signaling components become activated, resulting in thrombocytopenia and a predisposition to thrombosis.

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.

FcgammaRII mediates platelet aggregation caused by disintegrins and GPIIb/IIIa monoclonal antibody, AP2

OBJECTIVE

Disintegrins, snake venom-derived Arg-Gly-Asp (RGD)-containing polypeptides, and GPIIb/IIIa antagonist (AP2) block fibrinogen binding to GPIIb/IIIa of activated platelets, however, the combination of these two agents caused platelet aggregation. We hypothesize that disintegrin initially binds to specific epitope of GPIIb/IIIa, causing conformational change, and the recruitment of FcgammaRII, which can be bound by AP2, and finally triggering platelet aggregation.

MATERIALS AND METHODS

We prepared human platelet suspensions and measured platelet aggregation, Ca2+ mobilization, thromboxane B2 formation, and signal transduction.

RESULTS

Disintegrin (e.g., accutin) and AP2 (a monoclonal antibody [mAb]-raised against GPIIb/IIIa) individually inhibited human platelet aggregation caused by collagen. However, as both accutin and AP2 were sequentially added into platelet suspension, platelet aggregation occurred. Accutin/AP2 caused shape change, cytosolic Ca2+ mobilization, P-selectin expression, and thromboxane A2 formation. Tirofiban, FcgammaRII mAb, or indomethacin completely inhibited platelet aggregation caused by accutin/AP2. Accutin/AP2 also caused tyrosine phosphorylation of signal molecules. Disintegrins enhanced AP2 binding to platelets, and AP2 also promoted disintegrin binding to platelets. FcgammaRII mAb inhibited the enhanced fluorescein isothiocyanate-disintegrin binding to platelet caused by AP2. Immunoprecipitation of the lysates of disintegrin/AP2-treated platelets using FcgammaRII Ab showed complex formation of GPIIb/IIIa and FcgammaRII.

CONCLUSION

FcgammaRII mediates platelet aggregation caused by disintegrin and AP2, triggering a phospholipase C, phospholipase A2, Src-, Syk kinases, and Ca2+-dependent activation process. AP2 triggers platelet aggregation via binding to accessible FcgammaRII and the conformation-altered GPIIb/IIIa caused by disintegrin.

Flow cytometry in the diagnosis of drug-induced thrombocytopenia: two illustrative cases

Drug-induced thrombocytopenia is a challenging diagnosis in the clinical practice because of the many drugs or alternative causes that may be implicated. Exact identification of such drug(s) is required to guide future management and avoid re-exposure. We describe two cases of isolated thrombocytopenia in which cytometric analysis, a readily available technique, allowed the identification of the causative drug in the context of complex therapies (rifampicin and abciximab causing late onset thrombocytopenia).

Abciximab: a reappraisal of its use in coronary care

Platelet reactivity plays a pivotal role in the pathogenesis of ischemic adverse events during and after acute coronary syndromes (ACS), and percutaneous coronary intervention (PCI). Glycoprotein (GP) IIb/IIIa inhibitors are the strongest antiplatelet agents currently available on the market and three different compounds, namely abciximab, tirofiban, and eptifibatide, have been approved for clinical use. Abciximab has been investigated in the clinical field far more extensively than the other GPIIb/IIIa inhibitors. Abciximab is an anti-integrin Fab fragment of a human - mouse chimeric monoclonal antibody with high affinity and a slow dissociation rate from the GP IIb/IIIa platelet receptor. Abciximab, given shortly before the coronary intervention, is superior to placebo in reducing the acute risk of ischemic complications (EPIC, EPISTENT, EPILOG trials); moreover, in the ISAR-REACT 2 study abciximab has been shown to reduce the risk of adverse events in patients with non ST-segment elevation ACS who are undergoing PCI even after optimal pre-treatment with 600 mg of clopidogrel. Finally, abciximab has been also used in abciximab-coated stent, with only bolus administration regimen and for direct intracoronary use with promising results that may extend and/or modify its current use in clinical practice in future.

Application of high-throughput screening to identify a novel alphaIIb-specific small- molecule inhibitor of alphaIIbbeta3-mediated platelet interaction with fibrinogen

Small-molecule alphaIIbbeta3 antagonists competitively block ligand binding by spanning between the D224 in alphaIIb and the MIDAS metal ion in beta3. They variably induce conformational changes in the receptor, which may have undesirable consequences. To identify alphaIIbbeta3 antagonists with novel structures, we tested 33 264 small molecules for their ability to inhibit the adhesion of washed platelets to immobilized fibrinogen at 16 muM. A total of 102 compounds demonstrated 50% or more inhibition, and one of these (compound 1, 265 g/mol) inhibited ADP-induced platelet aggregation (IC(50): 13+/- 5 muM), the binding of soluble fibrinogen to platelets induced by mAb AP5, and the binding of soluble fibrinogen and a cyclic RGD peptide to purified alphaIIbbeta3. Compound 1 did not affect the function of GPIb, alpha2beta1, or the other beta3 family receptor alphaVbeta3. Molecular docking simulations suggest that compound 1 interacts with alphaIIb but not beta3. Compound 1 induced partial exposure of an alphaIIb ligand-induced binding site (LIBS), but did not induce exposure of 2 beta3 LIBS. Transient exposure of purified alphaIIbbeta3 to eptifibatide, but not compound 1, enhanced fibrinogen binding ("priming"). Compound 1 provides a prototype for small molecule selective inhibition of alphaIIbbeta3, without receptor priming, via targeting alphaIIb.

Drug-induced thrombocytopenia

Drug-induced thrombocytopenia (DIT) is a relatively common clinical disorder. It is imperative to provide rapid identification and removal of the offending agent before clinically significant bleeding or, in the case of heparin, thrombosis occurs. DIT can be distinguished from idiopathic thrombocytopenic purpura, a bleeding disorder caused by thrombocytopenia not associated with a systemic disease, based on the history of drug ingestion or injection and laboratory findings. DIT disorders can be a consequence of decreased platelet production (bone marrow suppression) or accelerated platelet destruction (especially immune-mediated destruction).

Evaluation of the effect of partial splenic embolization on platelet values for liver cirrhosis patients with thrombocytopenia

AIM: To investigate the effect of partial splenic embolization (PSE) on platelet values in liver cirrhosis patients with thrombocytopenia and to determine the effective embolization area for platelet values improvement.

METHODS: Blood parameters and liver function indicators were measured on 10 liver cirrhosis patients (6 in Child-Pugh grade A and 4 in grade B) with thrombocytopenia (platelet values < 80 × 10³/μL) before embolization. Computed tomography scan was also needed in advance to acquire the splenic baseline. After 2 to 3 d, angiography and splenic embolization were performed. A second computed tomography scan was made to confirm the embolization area after 2 to 3 wk of embolization. The blood parameters of patients were also examined biweekly during the 1 year follow-up period.

RESULTS: According to the computed tomography images after partial splenic embolization, we divided all patients into two groups: low (< 30%), and high (≥ 30%) embolization area groups. The platelet values were increased by 3 times compared to baseline levels after 2 wk of embolization in high embolization area group. In addition, there were significant differences in platelet values between low and high embolization area groups. GPT values decreased significantly in all patients after 2 wk of embolization. The improvement in platelet and GPT values still persisted until 1 year after PSE. In addition, 3 of 4 (75%) Child-Pugh grade B patients progressed to grade A after 2 mo of PSE. The complication rate in < 30% and ≥ 30% embolization area groups was 50% and 100%, respectively.

CONCLUSION: Partial splenic embolization is an effective method to improve platelet values and GPT values in liver cirrhosis patients with thrombocytopenia and the ≥ 30% embolization area is meaningful for platelet values improvement. The relationship between the complication rate and embolization area needs further studies.

Glycoprotein IIb/IIIa inhibitor-induced thrombocytopenia

Thrombocyte glycoprotein IIb/IIIa inhibitors prevent fibrinogen binding and thereby thrombocyte aggregation. The inhibition of thrombocyte activation at the damaged coronary plaque is the target of the new therapeutic strategies in treating acute coronary syndrome. This reduces the ischemic complications associated with the non-STelevation myocardial infarction (NSTEMI) and percutaneous coronary intervention (PCI). Thrombocytopenia is a known complication of glycoprotein (GP) IIb/IIIa inhibitors. Although, in general, GP IIb/IIIa inhibitor-induced thrombocytopenia is a harmless side effect which responds readily to thrombocyte transfusion, it can occasionally be a very serious complication associated with serious bleeding. In addition patients developing thrombocytopenia have unfavorable outcome (e.g., death, myocardial infarction, bypass surgery or additional PCI) in comparison to patients without thrombocytopenia. Advanced age (> 65 years), low BMI and a low initial thrombocyte count (<180,000/microl) are independent risk factors of thrombocytopenia. The risk of bleeding is higher with this form of thrombocytopenia not only due to the low thrombocyte count but also to the impaired function of the remaining thrombocytes. It is important to closely monitor platelet count during GP IIb/IIIa antagonist treatment. Platelet count monitoring two, six, twelve and 24 hour after starting the treatment reveals most cases of acute thrombocytopenia. Side effects can be avoided by the early discontinuation of the GP IIb/IIIa antagonist treatment. This article reviews the diagnosis and treatment of glycoprotein IIb/IIIa inhibitor-induced thrombocytopenia and summarizes the differential diagnosis from heparin-induced thrombocytopenia and laboratory-related pseudothrombocytopenia.

Eptifibatide-induced thrombocytopenia and thrombosis

Glycoprotein (GP) IIb/IIIa inhibitors have been shown to reduce morbidity and mortality in patients with acute coronary syndromes undergoing percutaneous coronary interventions (PCI). With their widespread use, there is a growing body of literature describing adverse outcomes, including severe thrombocytopenia. Here we report a case of a 75-year-old man who presented with an ST-elevation myocardial infarction, underwent primary PCI and stenting, and subsequently developed profound thrombocytopenia and thrombosis after eptifibatide administration. This report adds to the literature regarding eptifibatide-induced thrombocytopenia and also raises the possibility of a new syndrome of eptifibatide-induced thrombosis. A case is made to examine available databases for thrombosis after administration of eptifibatide and other GPIIb/IIIa inhibitors.

Regulation of proplatelet formation and platelet release by integrin alpha IIb beta3

Mature megakaryocytes form structures called proplatelets that serve as conduits for platelet packaging and release at vascular sinusoids. Since the megakaryocyte expresses abundant levels of integrin alpha IIb beta3, we have examined a role for fibrinogen in proplatelet development and platelet release alongside that of other matrices. Primary mature murine megakaryocytes from bone marrow aspirates readily formed proplatelets when plated on fibrinogen at a degree that was significantly higher than that seen on other matrices. In addition, alpha IIb beta3 was essential for proplatelet formation on fibrinogen, as megakaryocytes failed to develop proplatelets in the presence of alpha IIb beta3 antagonists. Interestingly, inhibition of Src kinases or Ca2+ release did not inhibit proplatelet formation, indicating that alpha IIb beta3-mediated outside-in signals are not required for this response. Immunohistochemical studies demonstrated that fibrinogen is localized to the bone marrow sinusoids, a location that would allow it to readily influence platelet release. Further, thrombopoietin-stimulated alpha IIb-/- mice had a reduced increase in platelet number relative to controls. A similar observation was not observed for platelet recovery in alpha IIb-/- mice in response to antibody-induced thrombocytopenia, indicating the existence of additional pathways of regulation of proplatelet formation. These results demonstrate that fibrinogen is able to regulate proplatelet formation via integrin alpha IIb beta3.

Activation-independent, antibody-mediated removal of GPVI from circulating human platelets: development of a novel NOD/SCID mouse model to evaluate the in vivo effectiveness of anti-human platelet agents

GPVI is a 62-kDa membrane glycoprotein expressed in noncovalent association with the Fc receptor gamma chain on human and murine platelets and serves as the major activating receptor for collagen. GPVI-specific antibodies have the capacity to specifically deplete GPVI from mouse and human platelets in vivo, rendering them unresponsive to collagen and GPVI-specific agonists. Such antibodies do not remove GPVI from noncirculating platelets in vitro, however, making it difficult to evaluate their antithrombotic potential and mechanism of action, particularly in human platelets. We devised a model system in which human platelets are introduced into the retroorbital plexus of nonobese diabetic/severe combined immunodeficient (NOD/SCID) mice, allowed to circulate, and evaluated for the effects of GPVI-specific murine monoclonal antibodies (mAbs) on platelet survival and function. GPVI-specific mAbs triggered depletion of GPVI from human, but not murine, platelets. Soluble truncated human GPVI appeared concomitantly in mouse plasma. GPVI-depleted human platelets had markedly diminished responses to GPVI-specific agonists and unexpectedly exhibited somewhat depressed responses to G-protein-coupled agonists. The ability to evaluate in living mice the in vivo function and survival of circulating human platelets may prove valuable for determining mechanisms of antibody-mediated platelet passivation and aid in the development of novel anti-platelet therapeutics.

Abciximab-Associated Thrombocytopenia After Previous Tirofiban-Related Thrombocytopenia

A 62-year-old man with a history of coronary artery disease and coronary artery bypass graft, chronic heart failure, and peripheral vascular disease required percutaneous coronary intervention (PCI) after progression of shortness of breath and fatigue over 2 years. Four hours after the procedure, the patient developed hematemesis and was found to be thrombocytopenic. The thrombocytopenia was presumed to be due to the abciximab infusion the patient received during and shortly after the PCI. Further review of the patient's medical history revealed that a similar episode had occurred 11 years earlier. At that time, he was enrolled in a clinical trial comparing tirofiban and heparin in patients with unstable angina; he developed profound thrombocytopenia within 24 hours of randomization. After the study unblinding, investigators discovered that the patient received tirofiban, which was thought to be the cause of his thrombocytopenia. Both abciximab and tirofiban are glycoprotein IIb-IIIa inhibitors, and thrombocytopenia induced by this class of drugs is a serious and potentially life-threatening adverse reaction. The mechanism is not well understood but has been described as immune mediated with both ligand-mimetic agents (tirofiban and eptifibatide) and abciximab. Our patient's situation was unusual in that he developed thrombocytopenia from a ligand-mimetic agent and subsequently had a similar reaction to abciximab. To our knowledge, this case report is the first documentation of thrombocytopenia associated with both tirofiban and abciximab in a single patient, and suggests that care should be given in administering glycoprotein IIb-IIIa inhibitors of either type to patients with a history of thrombocytopenia due to one of these agents.