The Pathophysiology of Heparin-Induced Thrombocytopenia

Trivalent nanobody-based ligands mediate powerful activation of GPVI, CLEC-2, and PEAR1 in human platelets whereas FcγRIIA requires a tetravalent ligand

BACKGROUND

Clustering of the receptors glycoprotein receptor VI (GPVI), C-type lectin-like receptor 2 (CLEC-2), low-affinity immunoglobulin γ Fc region receptor II-a (FcγRIIA), and platelet endothelial aggregation receptor 1 (PEAR1) leads to powerful activation of platelets through phosphorylation of tyrosine in their cytosolic tails and initiation of downstream signaling cascades. GPVI, CLEC-2, and FcγRIIA signal through YxxL motifs that activate Syk. PEAR1 signals through a YxxM motif that activates phosphoinositide 3-kinase. Current ligands for these receptors have an undefined valency and show significant batch variation and, for some, uncertain specificity.

OBJECTIVES

We have raised nanobodies against each of these receptors and multimerized them to identify the minimum number of epitopes to achieve robust activation of human platelets.

METHODS

Divalent and trivalent nanobodies were generated using a flexible glycine-serine linker. Tetravalent nanobodies utilize a mouse Fc domain (IgG2a, which does not bind to FcγRIIA) to dimerize the divalent nanobody. Ligand affinity measurements were determined by surface plasmon resonance. Platelet aggregation, adenosine triphosphate secretion, and protein phosphorylation were analyzed using standardized methods.

RESULTS

Multimerization of the nanobodies led to a stepwise increase in affinity with divalent and higher-order nanobody oligomers having sub-nanomolar affinity. The trivalent nanobodies to GPVI, CLEC-2, and PEAR1 stimulated powerful and robust platelet aggregation, secretion, and protein phosphorylation at low nanomolar concentrations. A tetravalent nanobody was required to activate FcγRIIA with the concentration-response relationship showing a greater variability and reduced sensitivity compared with the other nanobody-based ligands, despite a sub-nanomolar binding affinity.

CONCLUSION

The multivalent nanobodies represent a series of standardized, potent agonists for platelet glycoprotein receptors. They have applications as research tools and in clinical assays.

Intracellular origin and ultrastructure of platelet-derived microparticles

Essentials Platelet microparticles play a major role in pathologies, including hemostasis and thrombosis. Platelet microparticles have been analyzed and classified based on their ultrastructure. The structure and intracellular origin of microparticles depend on the cell-activating stimulus. Thrombin-treated platelets fall apart and form microparticles that contain cellular organelles.

SUMMARY

Background Platelet-derived microparticles comprise the major population of circulating blood microparticles that play an important role in hemostasis and thrombosis. Despite numerous studies on the (patho)physiological roles of platelet-derived microparticles, mechanisms of their formation and structural details remain largely unknown. Objectives Here we studied the formation, ultrastructure and composition of platelet-derived microparticles from isolated human platelets, either quiescent or stimulated with one of the following activators: arachidonic acid, ADP, collagen, thrombin or calcium ionophore A23187. Methods Using flow cytometry, transmission and scanning electron microscopy, we analyzed the intracellular origin, structural diversity and size distributions of the subcellular particles released from platelets. Results The structure, dimensions and intracellular origin of microparticles depend on the cell-activating stimulus. The main structural groups include a vesicle surrounded by one thin membrane or multivesicular structures. Thrombin, unlike other stimuli, induced formation of microparticles not only from the platelet plasma membrane and cytoplasm but also from intracellular structures. A fraction of these vesicular particles having an intracellular origin contained organelles, such as mitochondria, glycogen granules and vacuoles. The size of platelet-derived microparticles depended on the nature of the cell-activating stimulus. Conclusion The results obtained provide a structural basis for the qualitative differences of various platelet activators, for specific physiological and pathological effects of microparticles, and for development of advanced assays.

Fondaparinux for Thromboembolic Treatment and Prophylaxis of Heparin-Induced Thrombocytopenia

Objective:

TO review literature evaluating the use of fondaparinux for thromboembolic treatment and prophylaxis in patients with heparin-induced thrombocytopenia (HIT).

Data Sources:

A MEDLINE search (1966–February 2006) was conducted using the search terms fondaparinux, heparin, low-molecular-weight heparin, and thrombocytopenia to identify English-language articles. Additional sources were identified from bibliographies of select articles and the manufacturer.

Data Synthesis:

Fondaparinux, a pentasaccharide that selectively inhibits factor Xa, has been reported to have negligible or no cross-reactivity in vitro with HIT antibodies. Thromboembolic treatment and prophylaxis with fondaparinux in patients with HIT has been described. Three cases reported patients who were successfully treated for thromboembolic events with fondaparinux after developing HIT during therapy with unfractionated heparin (UFH) or low-molecular-weight heparin (LMWH). Another report showed positive outcomes using fondaparinux for prophylaxis in a patient who had previously developed HIT after receiving UFH. Moreover, 2 case series, one using fondaparinux for prophylaxis in patients with a previous diagnosis of HIT and the other using fondaparinux for treatment in patients who developed HIT while receiving UFH or LMWH, reported normal platelet counts during fondaparinux treatment. Finally, results of a retrospective review demonstrated that fondaparinux prevented thromboembolic events or recurrent thrombocytopenia in patients with a prior HIT diagnosis.

Conclusions:

Limited data support the use of fondaparinux for thromboembolic treatment or prophylaxis in patients with antibody assay–confirmed HIT who do not have a contraindication for fondaparinux use. Randomized controlled trials have not been published; therefore, questions remain regarding efficacy, safety, optimal doses, treatment duration, and incidence of thromboembolic events when fondaparinux is used in this setting. Prospective trials evaluating the efficacy and safety of fondaparinux in this patient population need to be conducted to answer these questions.

The significance and management of thrombocytopenia in antiphospholipid syndrome

The association between antiphospholipid antibodies (aPL) and clinical problems goes beyond what is stated in the antiphospholipid syndrome (APS) classification criteria, namely thrombosis and pregnancy morbidity, and thrombocytopenia is the most common non-criteria hematologic manifestation of aPL with a frequency ranging from 20 to 50 %. Thrombocytopenia is rarely severe, and hemorrhage is far less common than thrombosis. However, when anticoagulation is considered, it may constitute a clinical problem with increased bleeding risk. Furthermore, thrombocytopenia represents a risk factor for thrombosis in aPL-positive patients. Therefore, it is important to understand the pathogenesis and the clinical associations of thrombocytopenia to build the right medical approach in aPL-positive patients. In this paper, we review the literature on aPL/APS-associated thrombocytopenia and briefly discuss the other conditions that can result in thrombocytopenia as they have commonalities with APS and their recognition is important to establish the most appropriate treatment strategy.

CLEC-2 expression is maintained on activated platelets and on platelet microparticles

The C-type lectin-like receptor CLEC-2 mediates platelet activation through a hem-immunoreceptor tyrosine-based activation motif (hemITAM). CLEC-2 initiates a Src- and Syk-dependent signaling cascade that is closely related to that of the 2 platelet ITAM receptors: glycoprotein (GP)VI and FcγRIIa. Activation of either of the ITAM receptors induces shedding of GPVI and proteolysis of the ITAM domain in FcγRIIa. In the present study, we generated monoclonal antibodies against human CLEC-2 and used these to measure CLEC-2 expression on resting and stimulated platelets and on other hematopoietic cells. We show that CLEC-2 is restricted to platelets with an average copy number of ∼2000 per cell and that activation of CLEC-2 induces proteolytic cleavage of GPVI and FcγRIIa but not of itself. We further show that CLEC-2 and GPVI are expressed on CD41+ microparticles in megakaryocyte cultures and in platelet-rich plasma, which are predominantly derived from megakaryocytes in healthy donors, whereas microparticles derived from activated platelets only express CLEC-2. Patients with rheumatoid arthritis, an inflammatory disease associated with increased microparticle production, had raised plasma levels of microparticles that expressed CLEC-2 but not GPVI. Thus, CLEC-2, unlike platelet ITAM receptors, is not regulated by proteolysis and can be used to monitor platelet-derived microparticles.

Heparin-Induced Thrombocytopenia: Is There a Role for Direct Thrombin Inhibitors in Therapy?

Heparin is widely used in the intensive care environment usually for thromboprophylaxis but also to facilitate extra-corporeal circuits such as renal replacement and ECMO. Heparin-induced thrombocytopenia (HIT) is a rare but extremely serious disorder. It is associated with significant morbidity and mortality. The diagnosis is often challenging particularly as thrombocytopenia can be caused by a number of other common conditions seen in intensive care. Unfortunately routine screening for HIT antibodies is not helpful as it is possible to have these but have no manifestation of the disease process. If the diagnosis of HIT is not considered and the patient does in fact have the disease process they are at risk of thrombotic episodes. This article reviews the pathophysiology of HIT and the challenges with making the diagnosis. We explore the role of newer anticoagulants that may have a role such as direct thrombin inhibitors.

Elevated heparin-induced antibodies are more common in diabetic patients with vascular disease

Background. Hypercoagulable disorders can lead to deep vein thrombosis (DVT), arterial thrombosis or embolization, and early or recurrent bypass graft failure. The purpose of this study was to identify whether diabetes increased the likelihood of heparin-induced platelet factor 4 antibodies in at risk vascular patients. Methods. We reviewed clinical data on 300 consecutive patients. A hypercoagulable workup was performed if patients presented with (1) early bypass/graft thrombosis (<30 days), (2) multiple bypass/graft thrombosis, and (3) a history of DVT, pulmonary embolus (PE), or native vessel thrombosis. Relevant clinical variables were analyzed and compared between patients with diabetes (DM) and without diabetes (nDM). Results. 85 patients (47 women; age 53 ± 16 years, range 16–82 years) had one of the defined conditions and underwent a hypercoagulable evaluation. Screening was done in 4.7% of patients with early bypass graft thrombosis, 60% of patients were screened because of multiple bypass or graft thrombosis, and 35.3% had a previous history of DVT, PE, or native vessel thrombosis. Of the 43 patients with DM and 42 nDM evaluated, 59 patients (69%) had an abnormal hypercoagulable profile. An elevated heparin antibody level was present in 30% of DM and 12% of nDM patients (chi-squared test P < 0.04). Additionally, DM was associated with a higher likelihood of arterial complications while nDM was associated with a higher rate of venous adverse events (chi-squared test P < 0.003). Conclusions. Diabetes is associated with a higher likelihood of developing heparin-induced antibodies and an increased combined incidence of arterial complications that include early or multiple bypass/graft thrombosis. This finding may influence the choice of anticoagulation in diabetic patients at risk with vascular disease.

Argatroban therapy in heparin-induced thrombocytopenia

Argatroban is a direct thrombin inhibitor approved for anticoagulation in heparin-induced thrombocytopenia (HIT; in several countries) and in patients with or at risk of HIT undergoing percutaneous coronary intervention (PCI; in the USA). HIT is a relatively common extreme prothrombotic condition. When HIT is reasonably suspected, an alternative anticoagulant should be promptly initiated. In historical controlled studies, argatroban reduced new thrombosis, mortality from thrombosis and the composite of death, amputation or thrombosis, without increasing bleeding. With intravenous infusion, advantages include short half-life, easy monitoring and elimination primarily by hepatobiliary (rather than renal) means. In patients undergoing PCI, argatroban with or without glycoprotein IIb/IIIa inhibition leads to high rates of procedural success with low bleeding risk. Herein we review argatroban therapy for HIT and for PCI.

Anticoagulation for pediatric mechanical circulatory support

Extracorporeal life support applications have evolved considerably in recent years. However, the blood-biomaterial interface remains incompletely understood, and management of the acute inflammatory response and coagulation pathways continues to be challenging. At present, the gold standard for anticoagulation is unfractionated heparin. Since the inception of extracorporeal life support, the mainstay for anticoagulation monitoring has been activated clotting time. However, alongside the technological evolution in extracorporeal life support, the methods for monitoring heparin have also become more sophisticated, adding additional layers of complexity to creating an ideal safe protocol for anticoagulation during extracorporeal life support. To address this, the Extracorporeal Life Support Organization has formed an Anticoagulation Task Force to help direct both a consensus statement and potential guidelines within which the multiple monitoring methods can be customized for extracorporeal life support. One key question that remains in the use of these monitoring methods is whether the objective during extracorporeal life support is to anticoagulate the circuit to prevent thrombus formation within the extracorporeal device or whether it is to systemically anticoagulate the patient. This review details all current monitoring methods and highlights how they can be used during pediatric mechanical circulatory support.

The clinical significance and risk factors of anti-platelet factor 4/heparin antibody on maintenance hemodialysis patients: a two-year prospective follow-up

BACKGROUND

Heparin-induced thrombocytopenia is an immune response mediated by anti-PF4/heparin antibody, which is clinically characterized by thrombocytopenia and thromboembolic events. In this study, a prospective and multi-center clinical investigation 1) determined the positive rate of anti-PF4/heparin antibody in maintenance hemodialysis patients in China, 2) identified the related risk factors, and 3) further explored the effect of the anti-PF4/heparin antibody on bleeding, thromboembolic events, and risk of death in the patients.

METHODS

The serum anti-PF4/heparin antibody was measured in 661 patients from nine hemodialysis centers, detected by IgG-specific ELISA and followed by confirmation with excess heparin. Risk factors of these patients were analyzed. Based on a two-year follow-up, the association between the anti-PF4/heparin antibody and bleeding, thromboembolic events, and risk of death in the patients was investigated.

RESULTS

1) The positivity rate of the anti-PF4/heparin antibody in maintenance hemodialysis patients was 5.6%. With diabetes as an independent risk factor, the positivity rate of the anti-PF4/heparin antibody decreased in the patients undergoing weekly dialyses ≥3 times. 2) The positivity rate of the anti-PF4/heparin antibody was not related to the occurrence of clinical thromboembolic events and was not a risk factor for death within two years in maintenance hemodialysis patients. 3) Negativity for the anti-PF4/heparin antibody combined with a reduction of the platelet count or combined with the administration of antiplatelet drugs yielded a significant increase in bleeding events. However, the composite determination of the anti-PF4/heparin antibody and thrombocytopenia, as well as the administration of antiplatelet drugs, was not predictive for the risk of thromboembolic events in the maintenance hemodialysis patients.

CONCLUSIONS

A single detection of the anti-PF4/heparin antibody did not predict the occurrence of clinical bleeding, thromboembolic events, or risk of death in the maintenance hemodialysis patients.

Argatroban dose reductions for suspected heparin-induced thrombocytopenia complicated by child-pugh class C liver disease

OBJECTIVE

To report our experience of reduced-dose argatroban in a patient with suspected heparin-induced thrombocytopenia (HIT) and Child-Pugh class C liver disease and review the relevant literature to summarize current recommendations on argatroban use in patients with severe liver disease.

CASE SUMMARY

A 58-year-old male with Child-Pugh class C liver disease (Model for End-Stage Liver Disease [MELD] score = 31, total bilirubin 4.5 mg/dL) and hemodialysis-dependent renal failure was hospitalized with acute deep vein thrombosis (DVT). Three days after heparin initiation for DVT, he developed thrombocytopenia. Given his heparin exposure (both for treatment of DVT and ongoing hemodialysis), HIT was suspected and all heparinoids were immediately discontinued. Argatroban was initiated for the treatment of HIT while laboratory testing for HIT antibodies and the serotonin release assay were completed. Because of the patient's advanced liver disease, the starting dose of argatroban was reduced to 0.2 μg/kg/min, with frequent monitoring of the activated partial thromboplastin time (aPTT) (goal 60-85 seconds). Despite this dose reduction, the aPTT was supratherapeutic. Following further dose reductions, a final argatroban maintenance dose of 0.05 μg/kg/min was necessary for the attainment of goal aPTTs.

DISCUSSION

Reducing the starting dose of argatroban to 0.5 μg/kg/min is recommended in patients with liver disease. Nevertheless, this recommended dose is largely based on data from patients with more moderate liver disease (eg, Child-Pugh class A or B), and dosing in more advanced liver disease remains largely unexplored. Patients with more advanced liver disease may require additional dose reductions to avoid supratherapeutic concentrations of anticoagulation agents and to minimize bleeding risk.

CONCLUSIONS

This report illustrates the importance of careful selection of argatroban dosing and appropriate aPTT monitoring in patients with severe liver disease. Excessive anticoagulation may precipitate major bleeding complications, placing patients with this complicated disease at undue risk.

The first comprehensive and quantitative analysis of human platelet protein composition allows the comparative analysis of structural and functional pathways

Antiplatelet treatment is of fundamental importance in combatting functions/dysfunction of platelets in the pathogenesis of cardiovascular and inflammatory diseases. Dysfunction of anucleate platelets is likely to be completely attributable to alterations in posttranslational modifications and protein expression. We therefore examined the proteome of platelets highly purified from fresh blood donations, using elaborate protocols to ensure negligible contamination by leukocytes, erythrocytes, and plasma. Using quantitative mass spectrometry, we created the first comprehensive and quantitative human platelet proteome, comprising almost 4000 unique proteins, estimated copy numbers for ∼ 3700 of those, and assessed intersubject (4 donors) as well as intrasubject (3 different blood samples from 1 donor) variations of the proteome. For the first time, our data allow for a systematic and weighted appraisal of protein networks and pathways in human platelets, and indicate the feasibility of differential and comprehensive proteome analyses from small blood donations. Because 85% of the platelet proteome shows no variation between healthy donors, this study represents the starting point for disease-oriented platelet proteomics. In the near future, comprehensive and quantitative comparisons between normal and well-defined dysfunctional platelets, or between platelets obtained from donors at various stages of chronic cardiovascular and inflammatory diseases will be feasible.

Anticoagulation Review

Anticoagulants, also known as antithrombotics, are among the most commonly prescribed medications in the United States. Understanding how these medications work, the propensity for interactions with other drugs, dietary factors, and disease states is important for clinicians assessing and providing care to patients in all environments. In this review, we seek to provide essential information for the home health care provider for evaluating patients receiving anticoagulants commonly prescribed in the home health care setting. The low-molecular-weight heparins and vitamin K antagonists are the most commonly used agents for outpatient anticoagulation. New agents, such as the direct factor Xa inhibitors and direct thrombin inhibitors have recently been approved with additional new agents in the approval process and development pipeline. We seek to review the most pertinent information for each of these classes of medications providing information on pharmacology, interactions with other drugs, diet, and diseases and important clinical information.

Frequency of heparin/platelet factor 4-dependent platelet antibodies in patients undergoing angioplasty and stenting for cardiovascular disease and their role for on-clopidogrel platelet reactivity

BACKGROUND

The frequency of heparin-induced platelet antibodies (H/PF4 antibodies) following heparin exposure during percutaneous intervention with stent implantation is unknown. These antibodies may activate platelets and therefore contribute to high on-clopidogrel residual platelet reactivity (HRPR).

METHODS

We screened 288 patients after angioplasty and stenting for H/PF4 antibodies by an IgG/A/M ELISA. The 44 (15.3%) positive samples were further evaluated for IgG only antibodies, by the particle gel immunoassay (PaGIA), the heparin induced platelet activation assay (HIPA) and MEA. Further, we determined on-treatment platelet reactivity by multiple electrode aggregometry (MEA) in these patients. In vivo platelet activation was assessed by P-selectin expression.

RESULTS

The prevalence of H/PF4 antibodies in the total patients' cohort was 15.3% (95% CI 11.3-20%) by the IgG/A/M ELISA, 9.4% (95% CI 6.3-13.4%) by the IgG ELISA, 11.5% (95% CI 8-15.7%) by PaGIA, 14.2% (95% CI 10.4-18.8%) by MEA, and 2.4% (95% CI 1-4.9%) by HIPA. On-treatment platelet reactivity was similar between patients without and with H/PF4 antibodies [39 AU (6-110 AU) vs. 41 AU (7-91 AU); P = 0.85]. HRPR was seen in 105 patients (37.5%), and occurred to a similar extent in patients without and with H/PF4 antibodies in all test systems (all P > 0.2). Further, there was no difference of the ELISA optical densities using the IgG/A/M or the IgG only ELISA between patients without or with HRPR (all P > 0.3). There was no significant difference of P-selectin expression between patients without or with H/PF4 antibodies (P = 0.97). Noteworthy, none of the patients who developed H/PF4 antibodies had heparin-induced thrombocytopenia or a thromboembolic event.

CONCLUSION

H/PF4 antibodies are not rare in patients undergoing angioplasty and stenting. However, these antibodies are not associated with the occurrence of HRPR.

Thrombosis in suspected heparin-induced thrombocytopenia occurs more often with high antibody levels

OBJECTIVE

The study objective was to determine whether higher antiplatelet factor 4 (PF4)/heparin antibody levels using an enzyme-linked immunosorbent assay are associated with more frequent thrombotic events in patients with clinically suspected heparin-induced thrombocytopenia. Heparin-induced thrombocytopenia is an immune-mediated adverse drug reaction. An enzyme-linked immunosorbent assay detects anti-PF4/heparin antibodies to support a suspected clinical diagnosis of heparin-induced thrombocytopenia. The utility of quantitative enzyme-linked immunosorbent assay results is uncertain.

METHODS

Our single-centered study evaluated quantitative anti-PF4/heparin antibody levels using an enzyme-linked immunosorbent assay in consecutive hospitalized patients with a clinical suspicion of heparin-induced thrombocytopenia and positive anti-PF4/heparin antibody levels between July 2003 and December 2006.

RESULTS

Overall, anti-PF4/heparin antibody values were available for 318 patients with clinically suspected heparin-induced thrombocytopenia. The median level was 0.85 optical density units (range 0.31-4.0). The overall rate of arterial or venous thrombosis was 23.3%. A 1-unit increase in anti-PF4/heparin antibody level was associated with an approximate doubling in the odds of thrombosis by 30 days (odds ratio, 1.9; 95% confidence interval, 1.5-2.6; P=.0001). The proportion of patients with pulmonary embolism increased with higher anti-PF4/heparin antibody levels.

CONCLUSION

Higher levels of anti-PF4/heparin antibody are associated with increased thrombosis risk among patients with clinically suspected heparin-induced thrombocytopenia and might have clinical utility for prediction of true heparin-induced thrombocytopenia and the development of thrombosis.

Results of a consensus meeting on the use of argatroban in patients with heparin-induced thrombocytopenia requiring antithrombotic therapy - a European Perspective

Argatroban has been introduced as an alternative parenteral anticoagulant for HIT-patients in several European countries in 2005. In 2009 a panel of experts discussed their clinical experience with argatroban balancing risks and benefits of argatroban treatment in managing the highly procoagulant status of HIT-patients. This article summarizes the main conclusions of this round table discussion. An ongoing issue is the appropriate dosing of argatroban in special patient groups. Therefore, dosing recommendations for different HIT-patient groups (ICU patients; non-ICU patients, paediatric patients, and for patients undergoing renal replacement therapies) are summarized in this consensus statement. Because of the strong correlation between argatroban dosing requirements and scores used to characterize the severity of illness (APACHE; SAPS, SOFA) suitable dosing nomograms are given. This consensus statement contributes to clinically relevant information on the appropriate use and monitoring of argatroban based on the current literature, and provides additional information from clinical experience. As the two other approved drugs for HIT, danaparoid and lepirudin are either currently not available due to manufacturing problems (danaparoid) or will be withdrawn from the market in 2012 (lepirudin), this report should guide physicians who have limited experience with argatroban how to use this drug safely in patients with HIT.

Simple Scoring System for Early Management of Heparin-Induced Thrombocytopenia

This study was performed to develop a simple scoring system to aid in the early clinical management of patients suspected of heparin-induced thrombocytopenia (HIT) with regard to decisions for continued heparin therapy. The system was designed to arrive at low (0) or possible (1) probability scores without knowledge of laboratory test results (except platelet counts) to avoid delays. As the safest clinical approach is to discontinue heparin, intermediate and high scores were combined. Critically ill VA hospital patients (n = 100) with a ≥30% fall in platelet count were assessed by platelet aggregation (PA), 14C-serotonin release assay (14C-SRA), and GTI ELISA. In this population, 53% were scored 1 and of these 43% were positive by laboratory test. Emphasizing the decision to discontinue heparin, the clinical signs of HIT were paramount for the immediate determination of a diagnosis of HIT without dependence on a positive laboratory test.

Novel anticoagulant therapy: principle and practice

Currently, there are several lines of evidence supporting the interplay between coagulation and inflammation in the propagation of various disease processes, including venous thromboembolism (VTE) and inflammatory diseases. Major advances in the development of oral anticoagulants have resulted in considerable progress toward the goal of safe and effective oral anticoagulants that do not require frequent monitoring or dose adjustment and have minimal food/drug interactions. Indirect inhibitors such as low-molecular-weight heparin (LMWH) and the pentasaccharide fondaparinux represent improvements over traditional drugs such as unfractionated heparin for acute treatment of VTE, constituting a more targeted anticoagulant approach with predictable pharmacokinetic profiles and no requirement for monitoring. Vitamin K antagonist, with its inherent limitations in terms of multiple food and drug interactions and frequent need for monitoring, remains the only oral anticoagulant approved for long-term secondary thromboprophylaxis in VTE. The oral-direct thrombin inhibitor ximelagatran was withdrawn from the world market due to safety concerns. Newer anticoagulant drugs such as parenteral pentasaccharides (idraparinux, SSR126517E), novel oral-direct thrombin inhibitors (dabigatran), oral-direct factor Xa inhibitors (rivaroxaban, apixaban, YM-150, DU-176b), and tissue factor/factor VIIa complex inhibitors have been "tailor-made" to target specific procoagulant complexes and have the potential to greatly expand oral antithrombotic targets for both acute and long-term treatment of VTE, acute coronary syndromes, and for the prevention of stroke in atrial fibrillation patients.

Profiling of heparin-induced thrombocytopenia antibody levels in patients with and without diabetes

Heparin/platelet factor 4 (H:PF4) antibodies are the causative agent in heparin-induced thrombocytopenia (HIT). The antibodies are frequently formed after exposure to heparin, most commonly without any signs of clinical HIT. Heparin-induced thrombocytopenia antibodies have been detected by enzyme-linked immunosorbent assay (ELISA) in individuals who have not been exposed to heparin. It is possible that the antibodies could be elicited by PF4 associated with endogenous, heparin-like glycosaminoglycans (GAGs). This risk would be higher in individuals with endothelial dysfunction and chronic platelet activation. In the setting of an outpatient endocrinology clinic, both diabetic and nondiabetic patients were studied and compared with healthy volunteers. Heparin/platelet factor 4 antibody titers were measured by ELISA and analyzed to determine the frequency of clinically seropositive responses, and median and interquartile ranges of baseline antibody titers. The study found no increase in frequency of ELISA-positive patients among diabetic patients. Moreover, the diabetic population had lower overall level of H:PF4 antibody titer, especially the subgroups treated with thiazolidinedione drugs or angiotensin receptor blockers. Further studies are needed to determine whether subthreshold titers of HIT antibody may be reflective of the physiological state of platelet/endothelial balance.

Heparin-induced thrombocytopenia. A contemporary clinical approach to diagnosis and management

Thrombocytopenia following heparin administration can be associated with an immune reaction, now referred to as heparin-induced thrombocytopenia (HIT). HIT is essentially a prothrombotic disorder mediated by an IgG antiplatelet factor 4/heparin antibody, which induces platelet, endothelial cell, monocyte, and other cellular activation, leading to thrombin generation and thrombotic complications. Indeed, HIT can also be regarded as a serious adverse drug effect. Importantly, HIT can be a life-threatening and limb-threatening condition frequently associated with characteristically severe and extensive thromboembolism (both venous and arterial) rather than with bleeding. This article provides an overview of HIT, with an emphasis on the clinical diagnosis and management.

Therapeutic approaches to secondary immune thrombocytopenic purpura

Secondary thrombocytopenia is similar to primary or idiopathic thrombocytopenia (ITP) in that it is characterized by reduced platelet production or increased platelet destruction resulting in platelet levels<60,000/microL. Thrombocytopenia can occur from secondary causes associated with chronic disorders or with disturbed immune function due to chronic infections, lymphoproliferative and myeloproliferative disorders, pregnancy, or autoimmune disorders. Diagnosis of secondary ITP in some cases is complex, and the thrombocytopenia can often be resolved by treating the underlying disorder to the extent this is possible. In most cases, treatment is focused on reducing platelet destruction, but, in some cases, treatment may also be directed at stimulating platelet production. The most problematic cases of thrombocytopenia may be seen in pregnant women. This review will address various agents and their utility in treating ITP from secondary causes; in addition, thrombocytopenia in pregnancy, ITP in immunodeficiency conditions, and drug-induced thrombocytopenia will be discussed. Unlike primary ITP, treatment often must be tailored to the specific circumstance underlying the secondary ITP, even if the condition itself is incurable.

Reducing harm associated with anticoagulation: practical considerations of argatroban therapy in heparin-induced thrombocytopenia

Argatroban is a hepatically metabolized, direct thrombin inhibitor used for prophylaxis or treatment of thrombosis in heparin-induced thrombocytopenia (HIT) and for patients with or at risk of HIT undergoing percutaneous coronary intervention (PCI). The objective of this review is to summarize practical considerations of argatroban therapy in HIT. The US FDA-recommended argatroban dose in HIT is 2 microg/kg/min (reduced in patients with hepatic impairment and in paediatric patients), adjusted to achieve activated partial thromboplastin times (aPTTs) 1.5-3 times baseline (not >100 seconds). Contemporary experiences indicate that reduced doses are also needed in patients with conditions associated with hepatic hypoperfusion, e.g. heart failure, yet are unnecessary for renal dysfunction, adult age, sex, race/ethnicity or obesity. Argatroban 0.5-1.2 microg/kg/min typically supports therapeutic aPTTs. The FDA-recommended dose during PCI is 25 microg/kg/min (350 microg/kg initial bolus), adjusted to achieve activated clotting times (ACTs) of 300-450 sec. For PCI, argatroban has not been investigated in hepatically impaired patients; dose adjustment is unnecessary for adult age, sex, race/ethnicity or obesity, and lesser doses may be adequate with concurrent glycoprotein IIb/IIIa inhibition. Argatroban prolongs the International Normalized Ratio, and published approaches for monitoring the argatroban-to-warfarin transition should be followed. Major bleeding with argatroban is 0-10% in the non-interventional setting and 0-5.8% periprocedurally. Argatroban has no specific antidote, and if excessive anticoagulation occurs, argatroban infusion should be stopped or reduced. Improved familiarity of healthcare professionals with argatroban therapy in HIT, including in special populations and during PCI, may facilitate reduction of harm associated with HIT (e.g. fewer thromboses) or its treatment (e.g. fewer argatroban medication errors).

Heparin-induced thrombocytopenia successfully treated with fondaparinux

We report the successful treatment of a patient with HIT-associated venous thrombosis by fondaparinux, a synthetic pentasaccharide. Although not yet approved for this indication, this new anticoagulant may be a useful alternative in the setting of HIT.

Heparin induced thrombocytopenia with thrombosis: a two step process?

Heparin induced thrombocytopaenia is an increasingly recognised prothrombotic disorder, where platelet activation by immunoglobulin G, heparin and platelet factor 4 complexes has been proposed to result in the activation of coagulation, and thrombus formation. However, this pathophysiological mechanism will not explain the predilection of patients, undergoing surgical procedures, especially orthopaedic surgeries, for the development of thrombosis secondary to heparin induced thrombocytopaenia. The article describes a two-step process, where a quantitative role for platelet factor 4 in activating monocytes at the tissue injury sites may play a role in the pathogenesis of heparin induced thrombocytopenia with thrombosis.

Platelet transfusions in heparin-induced thrombocytopenia: a report of four cases and review of the literature

BACKGROUND

Heparin-induced thrombocytopenia (HIT) is a complication of heparin therapy associated with thrombocytopenia and thrombosis. The diagnosis of HIT is based on clinical criteria and laboratory tests, including the serotonin release assay (SRA). Because HIT patients are thrombocytopenic, platelet (PLT) transfusions may be contemplated; however, many published reviews have concluded that PLT transfusions are contraindicated in HIT because they may precipitate thrombotic events. This study reports four patients with clinically suspected HIT who received PLT transfusions without complications, and the literature regarding this subject has been reviewed.

STUDY DESIGN AND METHODS

Patients with a SRA ordered for suspected HIT were retrospectively identified. Charts of patients with positive SRAs who received a PLT transfusion when HIT was clinically suspected were reviewed for evidence of PLT transfusion safety and efficacy. A comprehensive search of the published literature regarding PLT transfusions in patients with HIT was conducted.

RESULTS

A SRA was performed on 189 patients with suspected HIT. Thirteen patients tested positive and 4 of these received a PLT transfusion. No patient developed a thrombotic complication. All 4 patients had adequate posttransfusion PLT increments. Two of the 3 patients with active bleeding had cessation of bleeding after transfusion. Review of the literature revealed no case of a complication clearly attributable to PLT transfusion.

CONCLUSION

Four patients with clinically suspected HIT and a positive SRA were transfused PLTs both efficaciously and safely. These outcomes, combined with the results of the literature review, suggest that PLT transfusions should not be withheld when clinically indicated in patients with HIT.

The use of fondaparinux for the treatment of venous thromboembolism in a patient with heparin-induced thombocytopenia and thrombosis caused by heparin flushes

Heparin-induced thrombocytopenia (HIT) is an immunologic drug reaction characterized by paradoxical association with venous and arterial thrombosis. The syndrome is caused by IgG antibodies that are reactive against complexes of platelet factor 4 and heparin. Fondparinux does not bind to platelet factor 4, is structurally too short to induce an antibody response, and could in theory be a useful agent to treat HIT. A 69-year-old white female presented with a lower extremity extensive iliofemoral deep vein thrombosis after a right total knee arthroplasty and was subsequently found to have a pulmonary embolism. The patient was noted to have heparin flushes during her operation. Her platelet drop decreased >50% from baseline during initiation of antithrombotic therapy. She was started on subcutaneous fondaparinux 7.5 mg once daily injection. Her serotonin release assay and enzyme-linked immunosorbent assay for heparin antibodies were positive for HIT. Her platelet count nadir was 60 x 0(3)/mm(3) on day 5 and the platelet count rebounded after 8 days of fondaparinux therapy. No recurrent thrombotic or bleeding events were noted throughout her therapy. Anecdotal reports have shown that fondaparinux can be a useful agent to treat HIT with or without thrombosis.

The relationship between the antiphospholipid syndrome and heparin-induced thrombocytopenia

Antiphospholipid syndrome (APS) and heparin-induced thrombocytopenia (HIT) are immune-mediated thrombotic conditions caused by antibodies targeted to a protein-antigen complex. Although each disorder is attributed to two distinct antibodies, these autoimmune disorders are characterized by a similar pathogenesis that includes a hypercoagulable state, platelet activation, damage to the vascular endothelium, and inflammation. APS and HIT share similarities in the clinical presentation because each is associated with thrombocytopenia, a high risk of thrombosis in all venous and arterial sites, and catastrophic thrombotic outcomes occur if untreated. Understanding the disease process for one disorder could potentially aid in understanding the other disorder.

Contemporary standards for the diagnosis and treatment of heparin-induced thrombocytopenia (HIT)

BACKGROUND

Heparin binding to platelet factor 4 (PF4) generates a new antigenic epitope. In an unpredictable fashion, as many as approximately 17% of patients treated with unfractionated heparin (UFH) and approximately 8% treated with low-molecular-weight heparin (LMWH) subsequently develop the anti-heparin-PF4 antibodies that mediate heparin-induced thrombocytopenia and thrombosis (HIT). Very few of those patients with circulating anti-heparin-PF4 antibodies, however, progress to develop clinical HIT (referred to previously as Type II HIT). Only 20% of those who harbor antibodies ( approximately 3% of those exposed to heparin) will manifest the thrombocytopenia subsequently. Even fewer patients (0.03% to 0.09% of those exposed to heparin) experience the marked platelet activation and morbid thromboses characteristic of the HIT syndrome. The pathogenesis of heparin-induced thrombocytopenia (HIT) remains elusive. The pathophysiologic understanding to date has revolved around pathogenic anti-heparin-PF4 antibodies that trigger platelet activation, release of platelet procoagulant microparticles, and resultant thrombosis. The clinical diagnosis of HIT is confusing because current assays to detect anti-heparin-PF4 antibodies do not correlate well with the disease. Currently available assays lack either adequate sensitivity and interlaboratory reproducibility (ie, functional serotonin release assays) or specificity (ie, enzyme-linked immunosorbent assays or ELISAs).

CONCLUSIONS

Fortunately, the treatment for HIT is not confusing. The purposes of this review are as follows: (1) to examine the relevant clinical definition of HIT, (2) to explore our current understanding as to the pathogenesis of HIT, and (3) to present an algorithm for the identification and treatment of the HIT syndrome.

[Heparin-induced thrombocytopenia]

Hemorrhage is the most common and best-recognized complication of heparin treatment. However, a potentially more dangerous complication is the development of heparin-induced thrombocytopenia (HIT). All patients exposed to heparin, irrespective of the dose and route of administration, are at risk of developing HIT. It is due to the formation of antibodies against the heparin-platelet factor 4 complex, which cause secondary activation of platelets, coagulation and, finally, increased thrombin production. The main symptom is the sudden onset of thrombocytopenia involving a drop in the platelet count to less than 50% of the basal level, with or without the appearance of thrombotic complications some 5 to 14 days after the start of heparin therapy. Heparin-induced thrombocytopenia can be detected early in patients receiving heparin by monitoring the platelet count. Demonstration of heparin-dependent platelet activation using an antigen or functional assay confirms the clinical diagnosis. Once the diagnosis of HIT has been confirmed serologically or there is a high level of suspicion of HIT, heparin must be suspended and treatment with an alternative anticoagulant should be considered. This review contains a discussion of the diagnosis and treatment of this syndrome.

Reducing thrombotic complications in the perioperative setting: an update on heparin-induced thrombocytopenia

Heparins are widely used in the perioperative setting. Immune heparin-induced thrombocytopenia (HIT) is a serious, antibody-mediated complication of heparin therapy that occurs in approximately 0.5%-5% of patients treated with heparin for at least 5 days. An extremely prothrombotic disorder, HIT confers significant risks of thrombosis and devastating consequences on affected patients: approximately 38%-76% develop thrombosis, approximately 10% with thrombosis require limb amputation, and approximately 20%-30% die within a month. HIT antibodies are transient and typically disappear within 3 mo. In patients with lingering antibodies, however, re-exposure to heparin can be catastrophic. In the perioperative setting, heightened awareness is important for the prompt recognition, diagnosis, and treatment of HIT. HIT should be considered if the platelet count decreases 50% and/or thrombosis occurs 5-14 days after starting heparin, with other diagnoses excluded. On strong clinical suspicion of HIT, heparin should be discontinued and a parenteral alternative anticoagulant initiated, even before laboratory confirmation of HIT is obtained. Subsequent laboratory test results may help with the decision to continue with nonheparin therapy or switch back to heparin. Heparin avoidance in patients with current or previous HIT is feasible in most clinical situations, except perhaps in cardiovascular surgery. If the surgery cannot be delayed until HIT antibodies have disappeared, intraoperative alternative anticoagulation is recommended.

Argatroban Therapy in Women with Heparin-Induced Thrombocytopenia

OBJECTIVES

Women have increased risk of developing heparin-induced thrombocytopenia (HIT), a serious, immune-mediated prothrombotic condition, and have a worse prognosis when affected. We compared gender differences for treatment and outcomes in HIT patients administered argatroban therapy.

METHODS

From a multicenter retrospective registry of argatroban-treated patients, we identified females (n = 42) and males (n = 50) with clinically diagnosed HIT who were administered argatroban <or=10 microg/kg/min. Upon diagnosis of HIT, heparin was discontinued. Continuous intravenous argatroban was instituted, adjusted to achieve activated partial thromboplastin times (aPTTs) 1.5-3 times baseline. Between-gender comparisons were made of argatroban dosing, aPTT responses, and clinical outcomes (death, amputation, new thrombosis, major bleeding).

RESULTS

At baseline, females and males were generally well matched, excepting platelet count (medians, 101 x 10(9)/L vs. 170 x 10(9)/L, p = 0.01), with 9 (21%) females and 19 (38%) males having HIT-related thrombosis. Typically, argatroban was initiated and maintained at 1.0-1.1 mug/kg/min for approximately 6 days, irrespective of gender. No differences were detected between females and males in aPTTs during therapy (respective medians, 57.3 vs. 59.5 seconds) or time to therapeutic aPTTs (7.3 vs. 10.3 hours), or platelet count recovery (2.5 vs. 7.4 days). Of patients with available data, 20 of 35 (57%) females and 19 of 35 (54%) males were converted to warfarin. The composite end point of death, amputation, or new thrombosis within 37 days of argatroban initiation occurred in 10 (24%) females and 8 (16%) males (p = 0.43). Within 37 days, 7 (17%) females and 7 (14%) males died (2 while on argatroban), 1 (2%) female required amputation (off argatroban), and 3 (7%) females and 2 (4%) males developed new thrombosis (3 while on argatroban). Major bleeding occurred in 1 (2%) female and 1 (2%) male.

CONCLUSIONS

Argatroban can be used effectively and safely to manage HIT in females, with dosing requirements, aPTT responses, and clinical outcomes comparable to those in men. Future, larger studies are warranted in establishing the unique characteristics of HIT in females.

Fondaparinux as a Treatment Option for Heparin-Induced Thrombocytopenia

Heparin-induced thrombocytopenia (HIT) is an immune-mediated complication that can occur after exposure to heparin products. Because patients with HIT are at increased risk for thrombosis, anticoagulation is warranted. The direct thrombin inhibitors lepirudin and argatroban are approved by the United States Food and Drug Administration (FDA) for this indication. Bivalirudin, another direct thrombin inhibitor, is approved for use in patients with HIT who must undergo percutaneous coronary intervention. The synthetic pentasaccharide fondaparinux lacks FDA approval for treating patients with HIT; however, a few published reports describe its use. Furthermore, various small-scale, in vitro studies have demonstrated a lack of cross-reactivity between fondaparinux and HIT antibodies. Large, in vivo comparison trials must be performed before fondaparinux can become a standard treatment option in the setting of HIT.

Argatroban for anticoagulation in pediatric patients with heparin-induced thrombocytopenia requiring extracorporeal life support

Heparin-induced thrombocytopenia (HIT) is a well recognized and described immune phenomenon in adults. However, pediatric data on the incidence, treatment, and complications of HIT are limited. Despite the widespread use of heparin in pediatric patients and the repeated and sustained exposures to heparin in neonates and children with congenital heart disease, HIT has been infrequently recognized and under-reported in this population. HIT is an intensely procoagulant disorder and carries significant morbidity and mortality. In this case series, we describe HIT management with argatroban as alternative anticoagulation in 3 children requiring extracorporeal life support.

Investigational treatments of venous thromboembolism

The antithrombotic management of venous thromboembolism (VTE) has gone through major developments. Indirect inhibitors such as low molecular weight heparin and the pentasaccharide fondaparinux represent improvements over traditional drugs such as unfractionated heparin for acute treatment of VTE with more targeted approaches, predictable pharmacokinetic profiles and lack of need for monitoring. Vitamin K antagonists, with inherent limitations of multiple food and drug interactions and frequent need for monitoring, remain the only oral anticoagulants approved for long-term secondary thromboprophylaxis in VTE with the removal of the oral direct thrombin inhibitor ximelagatran from the world market due to safety concerns. Newer anticoagulant drugs such as parenteral pentasaccharides (idraparinux and SSR-126517-E), oral direct thrombin inhibitors (dabigatran), oral direct Factor Xa inhibitors (rivaroxaban, apixaban, YM-150 and DU-176b) and tissue factor-Factor VIIa complex inhibitors (NAPc2) are tailor-made to target specific procoagulant complexes and have the potential to greatly expand our antithrombotic armamentarium for both acute and long-term treatment of VTE, especially as non-monitored parenteral and oral anticoagulants with a wide therapeutic window and a predictable anticoagulant response.

Heparin–platelet factor 4 antibodies in patients presenting to the ED with thrombosis

BACKGROUND

Patients with heparin-platelet factor 4 (PF4) antibodies, particularly platelet-activating ones, are at risk for heparin-induced thrombocytopenia if administered heparin. We determined the heparin-PF4 antibody prevalence in emergency department (ED) patients presenting with chest pain or symptoms of thrombosis.

METHODS

Admission samples from 324 ED patients with chest pain or symptoms of thrombosis were tested for heparin-PF4 antibodies and, if positive, platelet-activating antibodies.

RESULTS

Twenty-four (7.4%; 95% confidence interval, 4.8%-10.8%) patients had heparin-PF4 antibodies. Seropositivity occurred in 18 (9.2%) of 196 patients recently (< or =6 months) hospitalized vs 6 (4.7%) of 128 not recently hospitalized (P = .19), and in 16/231 (6.9%) patients with chest pain vs 8/93 (8.6%) with other thrombosis (P = .64). Of 22 seropositive patients retested, 8 (7 recently hospitalized) had platelet-activating antibodies.

CONCLUSION

Heparin-PF4 antibody prevalence is 7.4% in ED patients with chest pain or thrombosis, with approximately 1 in 3 seropositive patients having platelet-activating antibodies. Alternative, nonheparin anticoagulation would be prudent in these at-risk patients.