Fundamental concepts in the pathobiology of heparin-induced thrombocytopenia

Unfractionated heparin versus low molecular weight heparins for avoiding heparin-induced thrombocytopenia in postoperative patients

BACKGROUND

Heparin-induced thrombocytopenia (HIT) is an adverse drug reaction presenting as a prothrombotic disorder related to antibody-mediated platelet activation. It is a paradoxical immune reaction resulting in thrombin generation in vivo, which leads to a hypercoagulable state and the potential to initiate venous or arterial thrombosis. A number of factors are thought to influence the incidence of HIT including the type and preparation of heparin (unfractionated heparin (UFH) or low molecular weight heparin (LMWH)) and the heparin-exposed patient population, with the postoperative patient population at higher risk.Although LMWH has largely replaced UFH as a front-line therapy, there is evidence supporting a lack of superiority of LMWH compared with UFH regarding prevention of deep vein thrombosis and pulmonary embolism following surgery, and similar frequencies of bleeding have been described with LMWH and UFH. The decision as to which of these two preparations of heparin to use may thus be influenced by harmful effects such as HIT. We therefore sought to determine the relative impact of UFH and LMWH on HIT in postoperative patients receiving thromboembolism prophylaxis. This is an update of a review first published in 2012.

OBJECTIVES

The objective of this review was to compare the incidence of heparin-induced thrombocytopenia (HIT) and HIT complicated by venous thromboembolism in postoperative patients exposed to unfractionated heparin (UFH) versus low molecular weight heparin (LMWH).

SEARCH METHODS

For this update, the Cochrane Vascular Information Specialist searched the Specialised Register (May 2016), CENTRAL (2016, Issue 4) and trials registries. The authors searched Lilacs (June 2016) and additional trials were sought from reference lists of relevant publications.

SELECTION CRITERIA

We included randomised controlled trials (RCTs) in which participants were postoperative patients allocated to receive prophylaxis with UFH or LMWH, in a blinded or unblinded fashion. Studies were excluded if they did not use the accepted definition of HIT. This was defined as a relative reduction in the platelet count of 50% or greater from the postoperative peak (even if the platelet count at its lowest remained greater than 150 x 10⁹/L) occurring within five to 14 days after the surgery, with or without a thrombotic event occurring in this timeframe. Additionally, we required circulating antibodies associated with the syndrome to have been investigated through laboratory assays.

DATA COLLECTION AND ANALYSIS

Two review authors independently extracted data and assessed the risk of bias. Disagreements were resolved by consensus with participation of a third author.

MAIN RESULTS

In this update, we included three trials involving 1398 postoperative participants. Participants were submitted to general surgical procedures, minor and major, and the minimum mean age was 49 years. Pooled analysis showed a significant reduction in the risk of HIT with LMWH compared with UFH (risk ratio (RR) 0.23, 95% confidence interval (CI) 0.07 to 0.73); low-quality evidence. The number needed to treat for an additional beneficial outcome (NNTB) was 59. The risk of HIT was consistently reduced comparing participants undergoing major surgical procedures exposed to LMWH or UFH (RR 0.22, 95% CI 0.06 to 0.75); low-quality evidence. The occurrence of HIT complicated by venous thromboembolism was significantly lower in participants receiving LMWH compared with UFH (RR 0.22, 95% CI 0.06 to 0.84); low-quality evidence. The NNTB was 75. Arterial thrombosis occurred in only one participant who received UFH. There were no amputations or deaths documented. Although limited evidence is available, it appears that HIT induced by both types of heparins is common in people undergoing major surgical procedures (incidence greater than 1% and less than 10%).

AUTHORS' CONCLUSIONS

This updated review demonstrated low-quality evidence of a lower incidence of HIT, and HIT complicated by venous thromboembolism, in postoperative patients undergoing thromboprophylaxis with LMWH compared with UFH. Similarily, the risk of HIT in people undergoing major surgical procedures was lower when treated with LMWH compared to UFH (low-quality evidence). The quality of the evidence was downgraded due to concerns about the risk of bias in the included studies and imprecision of the study results. These findings may support current clinical use of LMWH over UFH as front-line heparin therapy. However, our conclusions are limited and there was an unexpected paucity of RCTs including HIT as an outcome. To address the scarcity of clinically-relevant information on HIT, HIT must be included as a core harmful outcome in future RCTs of heparin.

Elucidating the specificity of non-heparin-based conformational activators of antithrombin for factor Xa inhibition

Introduction:

Antithrombin, the principal inhibitor of coagulation proteases, requires allosteric activation by its physiological cofactor, heparin or heparin sulfate to achieve physiologically permissible rates. This forms the basis of heparin's use as a clinical anticoagulant. However, heparin therapy is beset with severe complications, giving rise to the need to search new non-heparin activators of antithrombin, devoid of these complications and with favorable safety profiles.

Materials and Methods:

We chose some representative organic compounds that have been shown to be involved in coagulation modulation by affecting antithrombin and applied a blind docking protocol to find the binding energy and interactions of the modified (sulfated) versus unmodified organic scaffolds.

Results and Conclusion:

Increased sulfation plays a key role in shifting the specificity of organic compounds like quercetin, diosmin, rutin, mangiferin, isomangostin, Trapezifolixanthone and benzofuran towards the heparin binding site (HBS). However, in hesperetin and tetrahydroisoquinoline, sulfation shifts the specificity away from HBS. We have further tried to elucidate changes in the binding affinity of quercetin on account of gradual increase in the number of hydroxyl groups being substituted by sulfate groups. The results show gradual increase in binding energy with increase in sulfation. A theoretical screening approach is an ideal mechanism to predict lead molecules as activators of antithrombin and in determining the specificity for antithrombin.

Heparin-induced thrombocytopenia: a review of concepts regarding a dangerous adverse drug reaction

Heparin is a natural agent with antithrombotic action, commercially available for therapeutic use as unfractionated heparin and low molecular weight heparin. Heparin-induced thrombocytopenia (HIT) is a serious adverse reaction to heparin that promotes antibody-mediated platelet activation. HIT is defined as a relative reduction in platelet count of 50% (even when the platelet count at its lowest level is above>150 x 10(9)/L) occurring within five to 14 days after initiation of the therapy. Thrombocytopenia is the main feature that directs the clinical suspicion of the reaction and the increased risk of thromboembolic complications is the most important and paradoxical consequence. The diagnosis is a delicate issue, and requires a combination of clinical probability and laboratory tests for the detection of platelet activation induced by HIT antibodies. The absolute risk of HIT has been estimated between 1% and 5% under treatment with unfractionated heparin, and less than 1% with low molecular weight heparin. However, high-quality evidence about the risk of HIT from randomized clinical trials is scarce. In addition, information on the frequency of HIT in developing countries is not widely available. This review aims to provide a better understanding of the key features of this reaction and updated information on its frequency to health professionals and other interested parties. Knowledge, familiarity, and access to therapeutic options for the treatment of this adverse reaction are mandatory to minimize the associated risks, improving patient safety.

Unfractionated heparin versus low molecular weight heparin for avoiding heparin-induced thrombocytopenia in postoperative patients

BACKGROUND

Heparin-induced thrombocytopenia (HIT) is an adverse drug reaction presenting as a prothrombotic disorder related to antibody-mediated platelet activation. It is a poorly understood paradoxical immune reaction resulting in thrombin generation in vivo, which leads to a hypercoagulable state and the potential to initiate venous or arterial thrombosis. A number of factors are thought to influence the incidence of HIT including the type and preparation of heparin (unfractionated heparin (UFH) or low molecular weight heparin (LMWH)) and the heparin-exposed patient population, with the postoperative patient population presenting a higher risk.Although LMWH has largely replaced UFH as a front-line therapy, there is evidence supporting a lack of superiority of LMWH compared with UFH regarding prevention of deep vein thrombosis and pulmonary embolism following surgery, and similar frequencies of bleeding have been described with LMWH and UFH. The decision as to which of these two preparations of heparin to use may thus be influenced by adverse reactions such as HIT. We therefore sought to determine the relative impact of UFH and LMWH specifically on HIT in postoperative patients receiving thromboembolism prophylaxis.

OBJECTIVES

The objective of this review was to compare the incidence of HIT and HIT complicated by thrombosis in patients exposed to UFH versus LMWH in randomised controlled trials (RCTs) of postoperative heparin therapy.

SEARCH METHODS

The Cochrane Peripheral Vascular Diseases Group searched their Specialised Register (March 2012) and CENTRAL (2012, Issue 2). In addition, the authors searched LILACS (March 2012) and additional trials were sought from reference lists of relevant publications.

SELECTION CRITERIA

We were interested in comparing the incidence of HIT occurring during exposure to UFH or LMWH after any surgical intervention. Therefore, we studied RCTs in which participants were postoperative patients allocated to receive UFH or LMWH, in a blinded or unblinded fashion. Eligible studies were required to have as an outcome clinically diagnosed HIT, defined as a relative reduction in the platelet count of 50% or greater from the postoperative peak (even if the platelet count at its lowest remained > 150 x 10(9)/L) occurring within five to 14 days after the surgery, with or without a thrombotic event occurring in this timeframe. Additionally, circulating antibodies associated with the syndrome were required to have been investigated through laboratory assays.

DATA COLLECTION AND ANALYSIS

Two review authors independently extracted data and assessed the risk of bias. Disagreements were resolved by consensus with participation of a third author.

MAIN RESULTS

In total two studies involving 923 participants met all the inclusion criteria and were included in the review. Pooled analysis showed a statistically significant reduction in the risk of HIT with LMWH compared with UFH (risk ratio (RR) 0.24, 95% confidence interval (CI) 0.07 to 0.82; P = 0.02). This result suggests that patients treated with LMWH would have a relative risk reduction (RRR) of 76% in the probability of developing HIT compared with patients treated with UFH.Venous thromboembolism (VTE) complicating HIT occurred in 12 of 17 patients who developed HIT. Pooled analysis showed a statistically significant reduction in HIT complicated by VTE with LMWH compared with UFH (RR 0.20, 95% CI 0.04 to 0.90; P = 0.04). This result indicates that patients using LMWH would have a RRR of 80% for developing HIT complicated by VTE compared with patients using UFH. Arterial thrombosis occurred in only one patient who received UFH and there were no amputations or deaths documented.

AUTHORS' CONCLUSIONS

There was a lower incidence of HIT and HIT complicated by VTE in postoperative patients undergoing thromboprophylaxis with LMWH compared with UFH. This is consistent with the current clinical use of LMWH over UFH as front-line heparin therapy. However, conclusions are limited by a scarcity of high quality evidence. We did not expect the paucity of RCTs including HIT as an outcome as heparin is one of the most commonly used drugs worldwide and HIT is a life-threatening adverse drug reaction. To address the scarcity of clinically-relevant information on the topic of HIT as a whole, HIT should be included as an outcome in future RCTs of heparin, and HIT as an adverse drug reaction should be considered in clinical recommendations regarding monitoring of the platelet count for HIT.

Heparin-induced thrombocytopenia in cardiovascular patients: pathophysiology, diagnosis, and treatment

Heparin-induced thrombocytopenia is a life-threatening immune-mediated platelet activation condition that can cause arterial and venous thromboembolism. The triggering complex, platelet factor 4/heparin antibody, has several unique immunologic characteristics that have not been well elucidated until recently. In patients undergoing cardiovascular procedures such as percutaneous coronary intervention and coronary artery bypass graft surgery, the prevalence of platelet factor 4/heparin antibody is significantly higher than that in the general population. The acuity and graveness of the thromboembolic phenomenon requires early diagnosis and empirical initiation of treatment, even before confirmatory test results are available. Also, although multiple therapeutic modalities exist, the safety and efficacy of each option depends upon the clinical setting. Therefore, this review will focus on the updated pathophysiology of heparin-induced thrombocytopenia, new diagnostic criteria, and the various treatment options for cardiovascular patients with different conditions.

Recent advances in the treatment of thromboembolic diseases: Venous thromboembolism

Venous thromboembolic diseases are the major concern of rising cost of healthcare and are commonest health problem across the globe. Both genetic and acquired risk factors are believed to be strongly linked with these diseases. Commonly encountered problems to the therapy include dose fixing and routine monitoring, yet some serious problems of bleeding also necessitate the immediate need to develop new agents. The review is primarily concerned with the new developments in the treatment of thromboembolic diseases. Therapeutic applications of anticoagulants, antiplatelets, and thrombolytics have been discussed in enough detail.

Treatment of Heparin-Induced Thrombocytopenia with Drotrecogin Alfa (activated)

A patient was administered drotrecogin alfa (activated) in addition to the standard of care for presumed severe sepsis and circulatory shock. Heparin-induced thrombocytopenia (HIT) and hepatic and splenic thromboses complicated her clinical course. Because drotrecogin alfa (activated) treatment is associated with improvement in thrombotic manifestations and thrombocytopenia, it was continued as the sole antithrombotic agent after the HIT became apparent. This approach was chosen despite the patient's severe hepatic and renal dysfunction, which made the use of direct thrombin inhibitors unfavorable. She survived with a reasonable outcome and salvage of her limbs. Although this case suggests a potential role of drotrecogin alfa (activated) in the management of HIT, systematic evaluation of its efficacy in this situation is warranted.

New anticoagulants for treatment of venous thromboembolism

Treatment of venous thromboembolism (VTE) usually starts with concomitant administration of heparin or low-molecular-weight heparin (LMWH) and a vitamin K antagonist. The parenteral anticoagulant, which is given for at least 5 days, is stopped once the vitamin K antagonist produces a therapeutic level of anticoagulation. Although the introduction of LMWH has simplified the initial treatment of VTE, problems remain. LMWH must be given by daily subcutaneous (SC) injection and vitamin K antagonists require routine coagulation monitoring, which is inconvenient for patients and physicians. Recently, 3 new anticoagulants have been introduced in an attempt to overcome these limitations. These include fondaparinux and idraparinux, synthetic analogs of the pentasaccharide sequence that mediates the interaction of heparin and LMWH with antithrombin, and ximelagatran, an orally active inhibitor of thrombin. These agents produce a predictable anticoagulant response; thus, routine coagulation monitoring is unnecessary. Because they do not bind to platelets or platelet factor 4, fondaparinux and idraparinux do not cause heparin-induced thrombocytopenia (HIT). Unlike vitamin K antagonists, ximelagatran has a rapid onset of action, thereby obviating the need for concomitant administration of a parenteral anticoagulant when starting treatment. The lack of an antidote for these new agents is a drawback, particularly for idraparinux, which has a long half-life.

Transgenic mice studies demonstrate a role for platelet factor 4 in thrombosis: dissociation between anticoagulant and antithrombotic effect of heparin

The platelet-specific chemokine platelet factor 4 (PF4) is released in large amounts at sites of vascular injury. PF4 binds to heparin with high affinity, but its in vivo biologic role has not been defined. We studied the role of PF4 in thrombosis using heterozygote and homozygote PF4 knock-out mice (mPF4(+/-) and mPF4(-/-), respectively) and transgenic mice overexpressing human PF4 (hPF4(+)). None of these lines had an overt bleeding diathesis, but in a FeCl(3) carotid artery thrombosis model, all showed impaired thrombus formation. This defect in thrombus formation in the mPF4(-/-) animals was corrected by infusing hPF4 over a narrow concentration range. The thrombotic defect in the mPF4(+/-) and mPF4(-/-) animals was particularly sensitive to infusions of the negatively charged anticoagulant heparin. However, the same amount of heparin paradoxically normalized thrombus formation in the hPF4(+) animals, although these animals were anticoagulated systemically. Upon infusion of the positively charged protein, protamine sulfate, the reverse was observed with mPF4(+/-) and mPF4(-/-) animals having improved thrombosis, with the hPF4(+) animals having worsened thrombus formation. These studies support an important role for PF4 in thrombosis, and show that neutralization of PF4 is an important component of heparin's anticoagulant effect. The mechanisms underlying these observations of PF4 biology and their clinical implications remain to be determined.

Heparin-induced thrombocytopenia: a common complication in cardiac transplant recipients

BACKGROUND

Heparin-induced thrombocytopenia (HIT) is an idiosyncratic complication of heparin therapy triggered by the development of immunoglobulin G (IgG) antibodies to platelet factor 4 heparin. It typically results in a 50% decrease in platelet count. Paradoxically, although bleeding is rare, there is a high risk of venous or arterial thrombotic events. Given that many patients awaiting transplantation are exposed to heparin for prolonged periods, we sought to determine the frequency of HIT and its consequences among patients before and after cardiac transplantation.

METHODS

We reviewed retrospectively the clinical, pathologic, and laboratory databases for all patients who underwent heart transplantation at our institution between January 1, 1998, and December 31, 2000. An enzyme-linked immunoabsorption assay (ELISA) that detected IgG, IgA, and IgM antibodies directed against platelet factor 4 heparin complex confirmed the diagnosis of HIT. We analyzed bleeding and thrombotic complications and determined the influence of HIT on post-transplant outcomes.

RESULTS

An assay for HIT antibody was performed before or after transplantation in 26 of 46 patients (46% of the entire cohort). In all cases, the clinical indication for testing was thrombocytopenia. Among patients screened, HIT antibody was detected in 11 patients (39%); HIT developed in 10 of 11 patients before transplantation. The mean platelet count at diagnosis was 88,000 +/- 22,000/mm(3). Heparin-induced thrombocytopenia with thrombosis syndrome developed in 5 of 11 patients (45%). Manifestations included splenic and renal infarctions, renal artery occlusion, coronary artery embolism with myocardial infarction, pulmonary embolism, and femoral and jugular venous occlusions. Alternative pre-operative anti-coagulation included lepirudin (n = 7), argatroban (n = 1), dalteparin (n = 1), and abciximab (n = 1). Two deaths occurred in the HIT-positive group; neither bleeding nor thrombosis caused either death. Actuarial 36-month survival did not differ between HIT-positive and HIT-negative cohorts (78% and 79%, respectively).

CONCLUSION

Heparin-induced thrombocytopenia is a frequent complication among patients hospitalized for heart failure who are awaiting heart transplantation. Timely HIT-antibody screening and the use of alternative forms of systemic anti-coagulation may permit successful transplantation with intermediate survival rates comparable to those of HIT-negative recipients.

Insights from mouse models of heparin-induced thrombocytopenia and thrombosis

Heparin-induced thrombocytopenia/thrombosis (HIT/T) is the most frequent cause of drug-induced antibody-mediated thrombocytopenia, a common cause of life-and limb-threatening platelet activation and thrombosis. Although antibodies to heparin-platelet factor 4 (PF4) complexes are found in essentially all patients with HIT/T, it is unclear how antibody formation is initiated, why only a small subset of these antibodies cause disease, or the mechanism by which they initiate thrombosis. The recent development of a transgenic mouse model of HIT/T showed, for the first time in vivo, that heparin, PF4, antibodies to the heparin-PF4 complex, and Fc gamma RIIA are necessary and sufficient to recapitulate the severe thrombocytopenia and thrombosis seen in patients. This model can be expanded to systematically study individual factors important to HIT/T development in vivo, to determine their contribution to the spectrum of human disease. Several recent papers have examined the role of other contributors, such as monocyte tissue factor, endothelial cell activation, and leukocyte-platelet aggregates in the pathogenesis of the disease. Mouse models will provide a means to test new diagnostic and therapeutic approaches.

Heparin-induced thrombocytopenia with thrombotic sequelae: a review

Heparin-induced thrombocytopenia (HIT) occurs in 1-5% of patients treated with heparin. The pathogenesis involves the formation of antibodies to heparin-platelet factor 4 complexes, and the major clinical sequelae are thrombotic. Diagnosis is based on a combination of clinical and laboratory data. Treatment consists of stopping heparin, but, insofar as the risk of thrombosis remains high, treatment by alternative antithrombotic agents is indicated. Most clinical experience has been with danaparoid sodium and hirudin. The use of low-molecular-weight heparins (LMWH) in subsequent HIT episodes has been described, but is not recommended, especially with the introduction of new agents, such as oral thrombin inhibitors and pentasaccharides, which are hoped to reduce the use of heparins and the occurrence of HIT.

Heparin-induced thrombocytopenia: pathophysiology and management

Immune-mediated heparin-induced thrombocytopenia (HIT) is a well-defined syndrome. Clinical criteria (thrombocytopenia, resistance to heparin anticoagulation, or thromboses during heparin therapy) are defined, and serologic diagnostic tests are available. Earlier recognition of HIT syndrome has allowed for significant advances in therapy, leading to marked reductions in morbidity and mortality from HIT syndrome. This review addresses the epidemiology, pathobiology, and management of HIT syndrome

The presence of heparin-platelet factor 4 antibodies as a marker of hypercoagulability during hemodialysis

We have assessed the prevalence of heparin-platelet factor 4 antibodies in a group of 100 patients on chronic intermittent hemodialysis with repeated exposure to heparin in order to establish the clinical significance of an immunological response to heparin. Heparin-induced platelet reactivity was studied in a subgroup of 86 patients. Routine laboratory parameters such as plasma C-reactive protein, albumin and immunoglobulins were included. Clinical endpoints were thrombosis in blood access, change of blood access, other thromboembolic events, bleeding complications, falling platelet count, and ischemic vascular disease or death within 1 year of blood sampling. Blood access thrombosis was a frequent complication (26%), often leading to change of dialysis blood access (21%). Antibodies were detected in 3-6% of hemodialysis patients and antibody levels correlated to blood access thrombosis and change of access (p<0.05). An unexpected finding was that of increased heparin-induced platelet reactivity in 36% of the control group of 24 uremic patients prior to dialysis or exposure to heparin. The findings suggest that antibodies to heparin-platelet factor 4 may contribute to hypercoagulability during hemodialysis, leading to thrombotic complications in affected patients.