Heparin-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.

Current Options in the Treatment of COVID-19: A Review

Novel Coronavirus, also known as severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), emerged in December 2019 in China and spread rapidly all around the world infecting many people. To date, no specific vaccines and drugs have been developed for this disease. Also, due to the COVID-19 pandemic and high prevalence of the infected patients, the drugs and the therapies of other past viral epidemics have been used for this disease. Many studies have been performed on the specific treatments to find whether or not they are effective on COVID-19 patients. In this review, we collected information about the most widely used drugs to treat COVID-19 (coronavirus disease 2019) belonging to groups of antivirals, antibiotics, immune modulators, and anticoagulants. Some of these compounds and drugs were used directly by inpatients, so researchers have examined others in laboratory conditions. This study considered the pros and cons of using these treatments separately and together and compared their results. By studying this review, we hope to provide useful information for researchers.

Myocardial Infarction with Limb Arterial and Venous Thrombosis in a Patient with Enoxaparin-Induced Thrombocytopenia

Patient: Female, 67-year-old

Final Diagnosis: Enoxaparin induced thrombocytopenia with life threatening thrombosis

Symptoms: Chest discomfort

Medication:—

Clinical Procedure: —

Specialty: Hematology

Pathogenic heparin-induced thrombocytopenia and thrombosis (HIT) antibodies determined by rapid functional flow cytometry

OBJECTIVES

Reliable diagnosis of heparin-induced thrombocytopenia and thrombosis (HIT) is mandatory for patient management, yet prompt determination of pathogenic antibodies remains an unmet clinical challenge. Common immunoassays carry inherent limitations and functional assays which detect antibody-mediated platelet activation are not usually readily available to routine laboratories, especially the serotonin release assay (SRA), being technically demanding, time consuming, and requires high level expertise. To overcome some of these limitations, we have developed a practical functional flow cytometric assay (FCA) for routine clinical use.

METHODS

A simple FCA is described which avoids platelet manipulation, is highly specific and sensitive compared with SRA, and provides rapid results.

RESULTS

Of the 650 consecutive samples, from HIT-suspected patients, 99 (15.3%) were positive by the PaGIA Heparin/PF4 immunoassay and 31 (4.8%) by FCA. Average platelet activation was 11-fold higher in PaGIA+/FCA+ vs PaGIA-/FCA- samples. Of 21 SRA-positive samples, 19 were FCA-positive (relative sensitivity 90.5%), and of 42 SRA-negative samples, 40 were FCA-negative (relative specificity 95.2%). The FCA showed significantly higher correlation with the clinical presentation of HIT (4Ts score) performed on 182 patients, compared with PaGIA Heparin/PF4 (ROC-plot analysis, AUC 0.93 vs 0.63, P < 0.001). At a 92% sensitivity, the assay specificity was 96%.

CONCLUSIONS

The present FCA is practical for routine testing, providing prompt reliable results for initial diagnosis and confirmation, to effectively assist in HIT patient management.

Heparin-Induced Thrombocytopenia: A Spectrum of Hemorrhage and Thrombosis—Case Presentations

Three cases of the heparin-induced thrombocytopenia syndrome are presented. Two of the patients died of thrombotic sequelae and 1 patient survived but suffered a significant bleeding complication. Several significant features are identified: (1) clinical presentation of the syndrome was apparent just before a fall in the platelet count; (2) platelet counts below 100,000/mm3 were not necessary for complications to occur; (3) antiplatelet IgM antibody was identified with the bleeding complication, as opposed to IgG in the thrombotic complication ; (4) despite cessation of heparin and institution of coumadin in 1 patient, platelet counts remained depressed and thrombotic complications continued to occur; and (5) superior vena cava thrombosis occurred in 1 patient, a condition previously unreported. These findings offer a means of suspecting the diagnosis before complications occur, raise questions regarding mechanism and management of the disorder, and add to the spectrum of thrombohemorrhagic sequelae associated with this syndrome.

Heparin-Induced Thrombocytopenia: Prevalence in a Large Cohort of Patients and Confirmed Role of PF4/Heparin Complex as the Main Antigen for Antibodies

We performed a retrospective study on the prevalence of heparin-induced thrombocytopenia (HIT) in 233 patients receiving hog mucosa heparin therapy. Of these, 82 patients received s.c. calcium heparin, 130 patient received unfractionated (UF) i.v. heparin, and 21 patients received low molecular weight heparins (LMWH). An additional four patients, referred to our consultation and diagnosed by us as having clinically active type II HIT (HIT-II) were also studied. The mean platelet count of the 233 patients receiving heparin showed a significant decrease after 2 days of heparin treatment and a following significant increase 6 days later (basal: 257 ± 147 x 109 platelets/L; day 2: 239 ± 122, p < 0.0002; day 6: 286 ± 119, p < 0.004). Of the 212 patients receiving UF heparin, 13 (6%) fulfilled the criteria for HIT-II: seven of these had received i.v. heparin (mean daily dose 26,600 ± 4,082 IU ± SD) and six had received s.c. heparin (mean daily dose 21,428:t 6,900 IU). Their mean basal platelet count was 226 ± 100 SD × 109 platelets/L and the nadir during heparin treatment was 78 ± 39 x 10 9 platelets/L. Thrombotic complications occurred in four (30.7%) of the 13 patients with HIT-II. Since the immunological mechanism has been demonstrated for HIT-II and since platelet factor 4 (PF4) was identified as the co-factor for the binding of heparin-related antibodies, we set up our own enzyme-linked immunosorbent assay (ELISA) for testing antibodies against PF4/heparin complex bound through electrostatic bridges to the solid phase. The highest binding capacity of HIT-related IgG to the multimolecular complex was obtained at 20 μg/ml for PF4 and 3 μg/ml for heparin, corresponding to 250 ng of PF4 and 42 ng of heparin in each microtiter well. Such binding was inhibited in a dose-dependent manner by increasing amounts of heparin, protamine hydrochloride, and a monoclonal antibody anti-human PF4 clone 1OB2. We observed that HIT-related antibodies bound also to PF4/LMWH complexes but the optimal PF4/glycosaminoglycan ratio appeared more critical for LMWH (enoxaparin, fraxiparin, and pamaparin) than for UF heparin. Sera from eight patients with HIT-II were tested by PF4/heparin ELISA; six of these had IgG against the complex PF4/heparin and three also had IgM. The persistence of HIT-related antibodies was investigated in three patients: in one such antibodies were still detectable 3 years after the acute episode, while in the other two, they disappeared after 6 months and 1 year, respectively. Key Words: Heparin-related anti body—Platelet factor 4 (PF4)—Heparin—Low molecular weight heparin—Thrombocytopenia—Thrombosis.

State-of-the-Art Review: Usefulness of Laboratory Techniques for Evaluating Antithrombotic Efficacy of New Therapeutic Strategies

New drugs and new therapeutic strategies are being introduced for the prevention and the management of cardiovascular and thromboembolic disorders. Extensive clinical studies and large-scale epidemiological investigations are conducted to demonstrate the efficacy of these therapeutic approaches. Laboratory assays are invaluable tools for this exploration. We discuss how the new understanding concerning the regulation of the coagulolytic equilibrium offers novel investigation tools. Parameters reflecting the activities of new drugs targeted to their impact site and presenting few side effects are available. In addition to global clotting methods and chromogenic substrate-based assays, introduction of immunoassays has allowed measurement of most of the molecular markers of hemostatic activation. We recommend use of a panel of markers exploring the endothelial damage, the blood cell involvement, the early coagulant pathways' activation (XIIa and VIIa), the thrombin-formation pathways, the fibrin formation, and the evaluation of the global fibrinolytic capacity. When related to the clinical end points, all these laboratory assays offer useful and reliable monitoring of new drugs. They contribute to the establishment of new therapeutic strategies. Key Words: Hemostasis—Activation—Antithrombotic—Therapy— Laboratory monitoring.

Lack of In Vitro Cross-Reactivity Predicts Safety of Low-Molecular Weight Heparins in Heparin-Induced Thrombocytopenia

Alternative anticoagulation in patients with heparin-induced thrombocytopenia (HIT) is often problematic. The relatively high cross-reactivity rate reported for the low-molecular-weight heparins (LMWH) has discouraged their use in this setting. This study has investigated the safety of using the LMWH Fragmin, based on a negative heparin-dependent platelet aggregation test using the latter, in patients with proven HIT. Fifty-three evaluable patients with clinical and laboratory evidence of HIT were evaluated for cross-reactivity with Fragmin using a Fragmin-dependent platelet aggregation test. In 20 of 38 patients who showed no in vitro cross-reactivity. Fragmin was substituted for unfractionated heparin. The outcome of these 20 patients was evaluated and compared to that of the remaining 33 patients, in whom anticoagulates were ceased or warfarin or Orgaran was used. Eighteen of 20 patients treated with Fragmin increased their platelet count by ≥50 x 109/l from a mean nadir of 57.9 ± 4.7 x 109/l within 2.8 ± 0.29 days following substitution of Fragmin for unfractionated heparin. Twenty-eight of the 33 remaining patients who did not receive Fragmin increased their platelet count by ≥50 x 109/l from a mean nadir of 53.0 ± 4.8 x 109/l within 3.0 ± 0.29 days. In seven patients (two treated with Fragmin), response could not be evaluated due to death within 36 h of cessation of heparin or discharge from hospital. The results indicate that in vitro cross-reactivity testing employing a heparin-dependent platelet aggregation assay can be safely used to select patients with HIT for further anticoagulation with LMWH. Key Words: Fragmin—Crossreactivity—Heparin-induced thrombocytopenia.

Trombocitopenia induzida por heparina em paciente com oclusão arterial aguda

Resumo A trombocitopenia induzida por heparina é uma complicação grave da terapêutica anticoagulante com heparina e está associada à formação de anticorpos antifator IV plaquetário. Costuma surgir a partir do quinto dia do tratamento, com queda de pelo menos 50% da contagem plaquetária. Em decorrência da ativação plaquetária concomitante, pode ocorrer quadro de trombose, venosa ou arterial, com repercussões clínicas graves. Apresentamos um caso de paciente portador de síndrome do anticorpo antifosfolípide, com quadro de oclusão arterial aguda, que foi tratado cirurgicamente e recebeu heparina não fracionada no intra e pós-operatório. No quinto dia de tratamento anticoagulante, apresentou queda maior de 50% da contagem de plaquetas em relação à contagem pré-heparina. A suspeita de trombocitopenia induzida por heparina e seus aspectos diagnósticos e terapêuticos serão abordados neste desafio terapêutico.

Use and safety of unfractionated heparin for anticoagulation during maintenance hemodialysis

Anticoagulation is essential to hemodialysis, and unfractionated heparin (UFH) is the most commonly used anticoagulant in the United States. However, there is no universally accepted standard for its administration in long-term hemodialysis. Dosage schedules vary and include weight-based protocols and low-dose protocols for those at high risk of bleeding, as well as regional anticoagulation with heparin and heparin-coated dialyzers. Adjustments are based largely on clinical signs of under- and overanticoagulation. Risks of UFH use include bleeding, heparin-induced thrombocytopenia, hypertriglyceridemia, anaphylaxis, and possibly bone mineral disease, hyperkalemia, and catheter-associated sepsis. Alternative anticoagulants include low-molecular-weight heparin, direct thrombin inhibitors, heparinoids, and citrate. Anticoagulant-free hemodialysis and peritoneal dialysis also are potential substitutes. However, some of these alternative treatments are not as available as or are more costly than UFH, are dependent on country and health care system, and present dosing challenges. When properly monitored, UFH is a relatively safe and economical choice for anticoagulation in long-term hemodialysis for most patients.

Hypothermic cardiopulmonary bypass as a determinant of late thrombocytopenia following cardiac operations in pediatric patients

BACKGROUND

Thrombocytopenia after cardiac operations is a common event in both adult and pediatric patients. Late thrombocytopenia (LTCP) is a less common event that is still without a well-recognized cause. This study explores the role of heparin-induced thrombocytopenia (HIT) and other factors (complexity of the operation, temperature management, and drug use) in determining LTCP.

METHODS

We conducted an observational study of 63 consecutive patients aged <36 months operated with or without cardiopulmonary bypass (CPB). LTCP was defined as a platelet count <100,000 cells/microl or <50% of the pre-operative count at any point in time between post-operative days 5 and 10. A diagnostic test for heparin-platelet factor 4 (PF4) antibodies was performed in patients with LTCP. Other pre- and post-operative factors were investigated for their association with LTCP.

RESULTS

LTCP occurred in 15 (24%) patients. No patient had positive heparin-PF4 antibodies. The lowest temperature on CPB was an independent predictor of LTCP, with a cut-off value at 29 degrees C (sensitivity 80%, specificity 70%). Other factors associated with LTCP were prolonged post-operative use of unfractionated heparin and milrinone. LTCP was associated with increased post-operative morbidity.

CONCLUSION

LTCP was related to a combination of factors (operation severity, degree of hypothermia during CPB, prolonged use of unfractionated heparin, and milrinone). The individual contribution of each factor seems difficult to establish. However, the degree of hypothermia during CPB and drug-associated effects were identified. HIT could be excluded in all cases.

Bilateral lower extremity gangrene requiring amputation associated with heparin-induced thrombocytopenia: a case report

Heparin is a common cause of thrombocytopenia in hospitalized patients. Between 10% and 15% of patients receiving therapeutic doses of heparin develop thrombocytopenia. Heparin-induced thrombocytopenia (HIT) can cause severe bleeding and thrombosis owing to intravascular platelet aggregation. HIT must be distinguished from other causes of thrombocytopenia. Importantly, heparin use is often associated with an early fall in the platelet count that usually occurs within the first 4 days of initiation and recovers without cessation of heparin treatment. This nonimmune heparin-associated thrombocytopenia has not been found to be associated with thrombosis and does not necessitate discontinuation of heparin. The authors present a case report of a 70-year-old man who received heparin therapy following aortic tissue valve replacement and aortic root repair with graft and developed bilateral lower extremity arterial clots 6 days postoperatively in the setting of positive heparin antibody titers. Ultimately the patient required bilateral above-knee amputations.

Bilateral lower extremity gangrene requiring amputation associated with heparin-induced thrombocytopenia--a case report

Heparin use, both prophylactically and therapeutically, is prevalent among hospitalized patients. Patients on heparin may develop a thrombocytopenia that is self-limited. Fewer patients develop a heparin-induced thrombocytopenia that can cause severe bleeding and thrombosis owing to intravascular platelet aggregation. The authors present a case report of heparin-induced thrombocytopenia in a patient who underwent aortic arch and aortic valve replacement that resulted in bilateral above-knee amputations. The patient developed limb ischemia related to heparin-associated thrombosis, but had a delay in antibody seroconversion. Early and accurate diagnosis of heparin-induced thrombocytopenia requires a high clinical suspicion and may be present despite the absence of serum antibodies.

Heparin-induced thrombocytopenia

Heparin-induced thrombocytopenia (HIT) is not only a common but also a potentially serious drug adverse effect. Unlike other drug-induced thrombocytopenias, HIT does not usually cause bleeding, but instead causes thrombosis. Thrombosis in HIT can lead to limb gangrene (requiring leg amputation) or even death. HIT is mediated by an antibody that recognizes an epitope on the platelet factor 4 (PF4)-heparin complex. The antibody-PF4-heparin complex binds to FcgammaRII receptors on the platelet surface and cross-links the receptors. This induces intense platelet activation and platelet aggregation, and simultaneously activates blood-coagulation pathways. These changes are probably the basis of the thrombotic events in HIT. Diagnosis of HIT should be made mainly on clinical criteria but should be confirmed whenever possible by laboratory tests, particularly in patients with comorbid conditions, in whom the diagnosis of HIT cannot be made with certainty without testing. The tests for HIT antibodies are either immunoassays (e.g. ELISA), or functional tests, (e.g. 14C-serotonin release assay). Once a clinical diagnosis of HIT is made, heparin should be ceased immediately and treatment with an alternative anticoagulant (such as danaparoid, r-hirudin or argatroban) commenced. This should continue for at least 5 days unless the diagnosis of HIT is subsequently proven to be incorrect. Warfarin should also be commenced when the patient is clinically stable and thrombosis is under control. There should be an overlap of a few days between warfarin and the alternative anticoagulant therapy.

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.

Determination of heparin-induced thrombocytopenia: a rapid flow cytometric assay for direct demonstration of antibody-mediated platelet activation

Heparin-induced thrombocytopenia (HIT) and thrombosis are serious complications of heparin therapy. Recently, we have reported a practical and rapid functional flow cytometric assay (FCA) for the diagnosis of HIT with high specificity and sensitivity compared with the radioactive serotonin-release assay (SRA). In the present study, we added an immune-neutralization assay to directly demonstrate the antibody-mediated process, and tested the immune compatibility of low-molecular-weight heparin (LMWH) Lovenox and the heparinoid Orgaran (danaproid) using plasma from 18 patients with HIT confirmed by both FCA and SRA. The clinical utility of this modified method is demonstrated by a pediatric patient with a complex clinical presentation who developed thrombocytopenia with multiple thromboses while on heparin therapy. ELISA and SRA (performed in three independent laboratories) for diagnosis of HIT were both negative. In contrast, the FCA for detecting activated platelets expressing anionic phospholipids, was highly and reproducibly positive with both unfractionated and LMWH. Another FCA also demonstrated the surface expression of the alpha-granule membrane p-selectin (CD62p). Compatibility testing with the heparinoid Orgaran was also positive (and with plasma from 4 of the 18 patients with HIT). Heparin was discontinued, along with full recovery of the platelet count. The capacity of the patient's plasma to activate platelets in the presence of heparin gradually decreased over 4 weeks consistent with antibody clearance. The responsible mechanism was clarified using an immune-neutralization assay, which showed a dose response neutralization of the plasma activity by antibodies against human Immunoglobulin G (IgG) and IgM. This assay was also reproducible in the 18 patients with HIT. We conclude that the functional FCA with its modification is practical, sensitive, and specific for reliable diagnosis of HIT. It can simultaneously assess the compatibility of alternative therapies and directly confirm the antibody-mediated process. Further, it is particularly useful to clarify mechanisms of thrombocytopenia and thrombosis and to direct therapy in patients with a complex presentation and confounding laboratory results who often need prompt diagnosis and treatment.

Drug-Induced Thrombocytopenia: Is it a Serious Concern for Glycoprotein IIb/IIIa Receptor Inhibitors?

Over the past decade, several glycoprotein IIb/IIIa receptor antagonists have been developed and tested clinically as adjuncts to coronary intervention and/or treatment of acute coronary syndromes. Thrombocytopenia associated with this class of compounds has been described in most large studies to date and when it occurs in combination with bleeding represents a major safety concern. Cases of thrombocytopenia caused by GP IIb/IIIa antagonists vary in their clinical presentation according to time of onset (following the first dose or delayed), severity (profound, i.e., <20,000 cells/mm(3), or mild), and may or may not be associated with clinically important bleeding. More than one etiology appears responsible for thrombocytopenia associated with GP IIb/IIIa antagonists, including acute, idiosyncratic, as well as delayed immune-mediated mechanisms. Comparison of the incidence of thrombocytopenia across the different agents currently being studied and the one agent commercially available is complicated by varying definitions of thrombocytopenia used to date; different clinical settings in which GP IIb/IIIa antagonists have been studied; use of concomitant medications such as heparin, which itself may cause thrombocytopenia; relatively infrequent occurrence of thrombocytopenia; and the limited number of patients exposed to these agents. Review of the large studies presented and published to date suggests that thrombocytopenia due specifically to GP IIb/IIIa receptor antagonists occurs in less than 5% of treated patients and may vary depending on the type of agent, concomitant therapy, and clinical scenario. Current standard management includes immediate cessation of the GP IIb/IIIa antagonist and, in severe cases, platelet transfusions. In cases with associated hemorrhage, other anticoagulants and antiplatelet agents should be discontinued and possibly reversed. There may be a role for IV IgG and steroids, especially for cases of thrombocytopenia that are immune-mediated; however, further investigations are necessary.

Heparin-induced thrombocytopenia with thrombosis: incidence, analysis of risk factors, and clinical outcomes in 108 consecutive patients treated at a single institution

Heparin-induced thrombocytopenia with thrombosis (HITT) can lead to serious morbidity and may be potentially fatal. We reviewed our experience with this entity over a 4-year period, to determine the following: 1) incidence and type of thrombosis in patients with heparin-induced thrombocytopenia (HIT), 2) clinical consequences of thrombosis, i.e., amputation, cerebrovascular accidents and death, 3) risk factors associated with development of thrombosis, and 4) impact of therapy on clinical outcomes in patients with HITT. Between 1991-1994, 108 patients were diagnosed to have HIT by heparin-induced platelet aggregation test. Thirty-two (29%) of these developed thrombotic complications, of which 20 were venous, 8 arterial, and 4 both. Five of the 32 died, 3 underwent amputations, and 3 had cerebrovascular accidents. The patients who developed thrombotic complications, when compared to those with HIT alone, were older (68.7 +/- 11.5 vs. 63.3 +/- 16 years, P = .05), had more severe thrombocytopenia (platelet count 46,300 +/- 30,400/mm3 vs. 62,500 +/- 34,400/mm3, P = .02), and developed it earlier (6.0 +/- 2.9 vs. 7.4 +/- 3.1 days, P = .03). Multivariate analysis showed that severity of thrombocytopenia and early fall in platelet count were independent risk factors for development of thrombotic complications. We did not find an association between development of thrombosis and clinical events (myocardial infarction, cardiac procedures or surgery, noncardiac surgery, and sepsis) that occurred immediately prior to onset of thrombocytopenia. Heparin was stopped in all 32 patients with HITT. Six received no additional therapy, and one received a single dose of aspirin. Three of these 7 died. The other 25 received anticoagulant or multiagent therapy, with 2 deaths. The death rate was lower in those who were treated with anticoagulant or multiagent therapy (P = .05). We conclude that: 1) Thrombotic complications occur in about 29% of hospitalized patients who develop HIT. 2) Early, severe fall in platelet count in elderly patients receiving heparin appears to be associated with development of thrombotic complications. 3) Our data do not show an association between development of thrombotic complications and clinical events immediately preceding the diagnosis of HIT. 4) In addition to discontinuation of heparin, anticoagulant or thrombolytic therapy should be considered in patients with HITT.

Anticoagulant fucoidan fractions from Fucus vesiculosus induce platelet activation in vitro

Anticoagulant fucoidan fractions of different molecular weight and sulfate content were prepared and investigated for their effects on platelet function in vitro. The fucoidan fractions were incubated with human platelet rich plasma (PRP) at concentrations of 5, 10 and 50 micrograms/ml. Platelet activation was subsequently studied by a standard aggregation assay and flow cytometric determination of the activation dependent platelet-surface markers CD62p (P-selectin, GMP-140) and CD63 (GP53). All fucoidan fractions induced irreversible platelet aggregation in a dose-dependent manner. Comparing fractions of identical molecular weight (100 kDa) the low sulfate content fucoidan FF5 (S = 7.6%) exerted a significantly greater effect than the highly sulfated fucoidan FF7 (S = 10.2%) over the whole concentration range (n = 5, P < 0.05). Among fractions of identical sulfate content fucoidan-induced platelet aggregation was also found to depend on the molecular weight of the fucoidan. At concentrations of 10 and 50 micrograms/ml the high molecular weight fraction FF7/1 (150 kDa) showed a significantly greater effect than the 50 kDa fraction FF7/3 (24.8 +/- 6.7 vs. 7.0 +/- 3.5 and 54.6 +/- 13.5 vs. 15.0 +/- 9.0%, respectively; mean +/- SD, n = 5, P < 0.05). The molecular weight dependence of the fucoidan effect was also reflected by the flow cytometric data. Coincubation of FF7/1 and FF7/3 (10 micrograms/ml) with PRP increased the number of CD62p and CD63 positive platelets by 9.0 +/- 3.3 vs. 2 +/- 1.9 and 7.1 +/- 2.4 vs. 3.2 +/- 2.6% over control values, respectively (n = 5, P < 0.05). In conclusion, our results show that the low molecular weight fucoidan FF7/3 combines potent anticoagulant and fibrinolytic properties with only minor platelet activating effects and is therefore a suitable substance for further pharmacological studies.

Pentosan-induced thrombocytopenia: support for an immune complex mechanism

Pentosan polysulphate is a low molecular weight heparinoid that is used as an anticoagulant. Because the drug also has antineoplastic properties, it has been used experimentally at the National Institutes of Health to treat metastatic malignancies. We present the case of a patient who developed thrombocytopenia resembling Type II heparin-induced thrombocytopenia (HIT) during the course of pentosan therapy. The patient's plasma demonstrated platelet reactivity both by aggregometry and 14C-serotonin release in the presence of pentosan. Heparin and other polyanions could substitute for pentosan in aggregation studies. The aggregating activity co-purified with the patient's IgG and was inhibited by pre-incubation with monoclonal antibody (MoAb) to the platelet Fc receptor. To elucidate the relationship between the platelet, the polyanion and the antibody, we measured the binding of 3H-heparin to platelets in the presence of the patient's IgG and found that it was increased 6-fold over binding in the presence of control IgG. Heparin binding was not reduced by MoAb against the Fc receptor. Taken together, these data support a model in which polyanion-antibody complexes attach to the platelet surface by the polyanion and secondarily stimulate the platelet via their Fc termini.

Heparin-induced thrombocytopenia in children

Two children aged 3 months and 14 years developed heparin-induced thrombocytopenia whilst receiving heparin. The clinical course was complicated by renal vein thrombosis in the 3-month-old infant and subcutaneous haemorrhage requiring resuscitation in the 14-year-old child. Anticoagulant therapy was discontinued immediately in the 3-month-old child. In the 14-year-old child, because of the need for continued anticoagulation therapy, low molecular weight heparin was used until warfarin could be substituted.

Dipyridamole in the Management of Severe Heparin-associated Thrombocytopenia

Severe heparin-associated thrombocytopenia (SHAT) is a rare, life-threatening condition. The aim of this prospective pilot study was to determine the safety and efficacy of dipyridamole-heparin infusion (DHI) in the management of the condition. We studied 6 patients (4 males and 2 females) aged 28 to 80 years (mean 50.5±14.2) with deep venous thrombosis and/or pulmonary embolism who developed SHAT a few days following heparin therapy. Heparin-dependent platelet aggregating factor was demonstrated ex vivo in the plasma of 4 patients. 240-300 mg of dipyridamole/day (4 mg/kg/day) was mixed with heparin in the same bag and given as a continuous intravenous infusion. Anticoagulation was continued successfully along with significant platelet recovery over a few days. This regimen was without side-effects. We conclude that DHI may provide an effective therapy for patients with SHAT.

Heparin-induced thrombocytopenia

Thrombocytopenia is a common adverse effect of heparin therapy. Two types of heparin-induced thrombocytopenia (HIT) are observed clinically--an early onset mild thrombocytopenia (Type I) in which the patients remain asymptomatic and a delayed onset severe thrombocytopenia (Type II). Patients with Type II HIT have an increased risk of thrombotic complications which frequently cause crippling disability e.g. limb amputation or even death. Type I HIT, the commoner of the two types, is believed to be due to the platelet proaggregating effect of heparin itself but Type II HIT is generally agreed to be caused by an immune mechanism, in which heparin-antibody complexes bind to platelets resulting in platelet activation, reduced platelet survival, thrombocytopenia and, in some cases, thrombosis. The diagnosis of HIT is made mainly on a clinical basis but in patients with suspected Type II HIT, laboratory test for the heparin-dependent antibody using platelet aggregometry or the two-point 14C-serotonin release method, allows confirmation of the diagnosis. In most Type I and all Type II patients, heparin should be stopped and warfarin commenced if there is a recent or new thrombosis requiring continuing anticoagulation. An alternative antithrombotic drug such as low molecular weight heparinoid (Org 10172) or dextran should be given at the same time until warfarin becomes therapeutic. The use of low molecular weight heparins (e.g. Fragmin) should be avoided unless it can be demonstrated that the HIT antibody does not cross-react with these drugs.

Insights into heparin-induced thrombocytopenia

Heparin-induced thrombocytopenia (HIT) is characterized by the presence of an antibody which can activate platelets in the presence of heparin. The in vitro Ca2+ mobilization induced by purified IgG from patients with HIT was evaluated to elucidate the platelet activation mechanism in this syndrome. HIT-IgG induced platelet activation in a heparin-dependent manner. This activation was inhibited by high viscosity, consistent with a previously documented cell-cell-mediated mechanism. We found that F(ab')2 fragments from an anti-GPIIb/IIIa monoclonal antibody in some cases, and dextran sulphate or salmon sperm DNA in all cases, could substitute for heparin, suggesting that heparin exposes a neo-antigen on the platelet surface rather than serving as the primary antigen in the process. Furthermore, heparin, dextran sulphate and salmon sperm DNA all augmented the platelet activation induced by aggregated IgG, suggesting an additional charge shielding effect. These data suggest the following mechanism for platelet activation in HIT: heparin binds to the platelet surface, exposing a neoantigen which elicits an antibody response in some patients. Antibody binding in the presence of heparin leads to platelet activation through Fc receptor mediated platelet-platelet interaction, a process augmented by the charge shielding effect of heparin.

Influence of heparins on inositol 1,4,5-trisphosphate-induced calcium mobilization in permeabilized human platelets

Heparin has been shown to prevent inositol 1,4,5-trisphosphate (IP3) binding to its receptor and to inhibit IP3-induced calcium mobilization in a variety of cells. Heparin added to whole blood at a concentration of 1 U/ml prevented thrombin-induced secretion of granule contents and irreversible aggregation of platelets. Heparin (2-15 kDa) had no inhibitory effect on IP3-induced calcium mobilization in Fura 2-loaded, saponin (10-15 micrograms/ml)-permeabilized platelets. None of the commercially available heparin preparations can induce inhibition of agonist-induced calcium mobilization in intact platelets because they are not cell permeant. Mild saponin treatment makes the membrane permeable to IP3, but restricts the action of heparins. Recent observations suggesting heparin's affinity to IP3 binding sites will be of clinical interest if effective cell permeant analogs can be developed.

Heparin as a therapy for atherosclerosis: preliminary observations on the intrapulmonary administration of low-dose heparin in the morning versus evening gauged by its effect on blood variables

Reports on clinical trials with subcutaneous and intrapulmonary administration of low-dose heparin suggest that it may be an attractive therapeutic modality for the treatment of coronary artery disease because of unprecedented reduction in mortality of treated subjects. As a preliminary to a clinical trial with low-dose intrapulmonary heparin, a pilot study was conducted on three subjects. It compares overall circadian responses of 37 blood variables following intrapulmonary administration of heparin (10,500-18,800 U) in the morning (0800 h) and in the evening (2000 h). After each of these times, blood samples, mostly at 3 h intervals for the ensuing 27 h, were analyzed for heparin, APTT, TT, functional fibrinogen, CBC, enzymes, lipids, electrolytes, and hormones. Each time series was analyzed for circadian rhythm by the least-squares fit of a 24 h cosine and circadian mesors were compared by the Bingham test of rhythm parameters. Following heparin in the evening, but not in the morning, a statistically significant increase in circulating heparin levels, as well as directional increases in APTT and TT and decreases in fibrinogen, were observed in all three subjects. Same direction changes in several other variables were also observed. It is concluded that inhalation of heparin in low-dose levels results in variable circadian effects on blood parameters measured, ranging from no changes in their levels to minimal within normal range changes, and that these effects are dependent upon the timing of dose administration. It is suggested that the timed self-administration of low-dose heparin by inhalation be seriously considered for long-term clinical trials in the treatment and prevention of atherosclerosis.

Heparin-induced thrombocytopenia: mechanism of interaction of the heparin-dependent antibody with platelets

The interaction of the heparin-dependent antibody with heparin and platelets has been studied using the sera and purified IgG of four patients with heparin-induced thrombocytopenia. Both normal platelets and Bernard-Soulier syndrome (BSS) platelets which lack glycoprotein (GP) Ib. GPV and GPIX, aggregated in response to patient serum or IgG, but only in the presence of heparin. A monoclonal antibody (Mab) against platelet Fc II receptor (IV.3) strongly inhibited the heparin-dependent aggregation of both normal and BSS platelets induced by patient sera/IgG. Inhibition by the anti-GPIb Mab (AK2) was variable and occurred only with normal platelets. Anti-GPIX Mab (FMC 25) was not inhibitory with either normal or BSS platelets. Similar results were obtained using 14C-serotonin release instead of platelet aggregation as a measure of platelet activation. These findings suggest that (1) the reaction of the heparin-dependent antibody with platelets and heparin is mediated by a Fc-dependent mechanism, (2) GPIb, GPV and GPIX are not involved in this reaction, and (3) the inhibitory effect of anti-GPIb Mab on normal platelets is due to steric interference consistent with the platelet Fc receptor being in close proximity to GPIb.

Heparin-induced thrombocytopenia

Thrombocytopenia is a frequent and sometimes insidious complication of anticoagulant therapy with heparin. Two types of heparin-induced thrombocytopenia with a distinct aetiology have been recognized. Type I is characterized by a mild thrombocytopenia of early onset which requires careful monitoring but usually not the cessation of heparin therapy. The mild thrombocytopenia is probably due to the mild pro-aggregatory properties of heparin and can be more severe in the presence of other predisposing factors, e.g. sepsis. Type II heparin-induced thrombocytopenia is more severe and usually occurs after a period of 7-10 days. Heparin therapy should be ceased immediately and other anticoagulant therapy initiated. The thrombocytopenia is believed to be due to the development of a heparin-dependent antibody that causes platelet aggregation and release. The precise mechanism of heparin-dependent antibody-platelet interaction is still not entirely clear but probably involves the binding of an antibody-heparin immune complex to the platelet Fc receptor.