Platelet proaggregating effect of heparin: possible mechanism for non-immune heparin-associated thrombocytopenia

Pharmacology of Heparin and Related Drugs: An Update

Heparin has been used extensively as an antithrombotic and anticoagulant for close to 100 years. This anticoagulant activity is attributed mainly to the pentasaccharide sequence, which potentiates the inhibitory action of antithrombin, a major inhibitor of the coagulation cascade. More recently it has been elucidated that heparin exhibits anti-inflammatory effect via interference of the formation of neutrophil extracellular traps and this may also contribute to heparin's antithrombotic activity. This illustrates that heparin interacts with a broad range of biomolecules, exerting both anticoagulant and nonanticoagulant actions. Since our previous review, there has been an increased interest in these nonanticoagulant effects of heparin, with the beneficial role in patients infected with SARS2-coronavirus a highly topical example. This article provides an update on our previous review with more recent developments and observations made for these novel uses of heparin and an overview of the development status of heparin-based drugs. SIGNIFICANCE STATEMENT: This state-of-the-art review covers recent developments in the use of heparin and heparin-like materials as anticoagulant, now including immunothrombosis observations, and as nonanticoagulant including a role in the treatment of SARS-coronavirus and inflammatory conditions.

Pharmacology of Heparin and Related Drugs

Heparin has been recognized as a valuable anticoagulant and antithrombotic for several decades and is still widely used in clinical practice for a variety of indications. The anticoagulant activity of heparin is mainly attributable to the action of a specific pentasaccharide sequence that acts in concert with antithrombin, a plasma coagulation factor inhibitor. This observation has led to the development of synthetic heparin mimetics for clinical use. However, it is increasingly recognized that heparin has many other pharmacological properties, including but not limited to antiviral, anti-inflammatory, and antimetastatic actions. Many of these activities are independent of its anticoagulant activity, although the mechanisms of these other activities are currently less well defined. Nonetheless, heparin is being exploited for clinical uses beyond anticoagulation and developed for a wide range of clinical disorders. This article provides a "state of the art" review of our current understanding of the pharmacology of heparin and related drugs and an overview of the status of development of such drugs.

Heparin-induced thrombocytopenia

Abstract A summary of heparin-induced thrombocytopenia (HIT) is presented. HIT is an adverse drug reaction characterized by thrombocytopenia and a high risk for venous or arterial thrombosis. The frequency of HIT ranges from 1 to 5% of patients receiving heparin with exact frequencies ranging between specific agents. Interestingly, this immune-mediated syndrome is ironically associated with thrombosis, not bleeding, with thrombin formation playing a major role. It is caused by heparin-dependent, platelet-activating antibodies that identifies a self-protein, PF4, bound to heparin that results in an antibody formation. The resulting platelet activation is associated with increased thrombin generation. Typically, the platelet count fall begins 5-10 days after starting heparin, although a rapid platelet count fall can occur in a patient who has antibodies from recent heparin use. Typical causes of HIT as well as the best diagnostic studies and treatment are discussed in this review. HIT was reviewed using a pubmed™ search; google scholar™ using key words: "Heparin-induced thrombocytopenia"; "heparin", and "drug AND thrombocytopenia."

Thrombocytopenia in ESRD patients: epidemiology, mechanisms and interventional nephrology perspective

A well-functioning vascular access is essential for provision of life-sustaining dialysis treatment in patients with end-stage renal disease. Arteriovenous accesses are preferred form of vascular access. Although significant advances have been made in the field of dialysis access, arteriovenous access dysfunction remains the single most important cause of morbidity in ESRD patients. While thrombosis and stenosis of AV access are more frequently seen, hemorrhage from AV access can be life threatening with or without risk of permanent access loss. Aside from anticoagulation for comorbidities, qualitative and/or quantitative platelet abnormalities are often the predisposing factors. We describe an ESRD patient who developed new onset but severe thrombocytopenia due to metastatic small cell neuroendocrine carcinoma of lung. Given her persistent thrombocytopenia and presence of prolonged bleeding from the cannulation sites, a right internal jugular tunneled dialysis catheter was placed for continuation of maintenance dialysis. This review discusses the definition of thrombocytopenia, mechanisms of thrombocytopenia in patients with ESRD and with a special focus on implications of thrombocytopenia on dialysis access interventions. The review underscores the need for consensus with regard to cannulating AV access as well as guidelines specific to dialysis access-related endovascular intervention in the setting of thrombocytopenia and other coagulation abnormalities.

Evaluation of a pretest scoring system (4Ts) for the diagnosis of heparin-induced thrombocytopenia in a university hospital setting

The initial diagnosis of heparin-induced thrombocytopenia (HIT) is made on clinical grounds because the assays with the highest sensitivity (eg, heparin-platelet factor 4 antibody enzyme-linked immunosorbent assay [ELISA]) and specificity (eg, serotonin release assay) may not be readily available. The clinical utility of the pretest scoring system, the 4Ts, was developed and validated by Lo et al in the Journal of Thrombosis and Haemostasis in 2006. The pretest scoring system looks at the degree and timing of thrombocytopenia, thrombosis, and the possibility of other etiologies. Based on the 4T score, patients can be categorized as having a high, intermediate, or low probability of having HIT. We conducted a retrospective study of 100 consecutive patients who were tested for HIT during their hospitalization at Hahnemann University Hospital (Philadelphia, PA) in 2009. Of the 100 patients analyzed, 72, 23, and 5 patients had 4T pretest probability scores of low, intermediate, and high, respectively. A positive HIT ELISA (optical density > 1.0 unit) was detected in 0 of 72 patients (0%) in the low probability group, in 5 of 23 patients (22%) in the intermediate probability group, and in 2 of 5 patients (40%) in the high probability group. The average turnaround time for the HIT ELISA was 4 to 5 days. Fourteen (19%) of the 72 patients with a low pretest probability of HIT were treated with a direct thrombin inhibitor. Ten (71%) of the 14 patients in the low probability group treated with a direct thrombin inhibitor had a major complication of bleeding requiring blood transfusion support. In this retrospective study, a low 4T score showed 100% correlation with a negative HIT antibody assay. We recommend incorporating the 4T scoring system into institutional core measures when assessing a patient with suspected HIT, selecting only patients with intermediate to high probability for therapeutic intervention, which may translate into reduced morbidity and lower health care costs.

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.

The incidence of heparin-induced thrombocytopenia Type II in patients with subarachnoid hemorrhage treated with heparin versus enoxaparin

Object

Heparin-induced thrombocytopenia Type II (HIT II) is a serious complication that occurs in 0.2–3% of patients treated with heparin and is associated with a high risk of thrombotic events. One center recently reported an incidence of HIT II of 15% in a population of patients with aneurysmal subarachnoid hemorrhage (aSAH). Because these patients are typically exposed to heparin during angiography, controversy exists regarding whether prophylaxis with enoxaparin rather than heparin affords any reduction in the risk of developing HIT II. In this study, the authors investigated the effect of heparin compared with enoxaparin on the incidence of HIT II in patients with aSAH.

Methods

The authors reviewed the medical records of 300 patients treated for aSAH who received thromboprophylaxis with either heparin or enoxaparin, and identified patients who developed HIT II. The incidences of HIT II in the 2 treatment groups were then compared.

Results

One hundred sixty-six patients with aSAH were treated with heparin, and 134 patients were treated with enoxaparin. Sixteen (5.3%) of 300 patients met the diagnostic criteria for HIT II. Of those treated with heparin, 8 (4.8%) of 166 developed HIT II, compared with 8 (6%) of 134 treated with enoxaparin (difference not significant).

Conclusions

The authors report a lower incidence of HIT II in patients with aSAH than has previously been reported. The data also suggest that patients with aSAH who receive heparin are at no greater risk of developing HIT II than those who receive enoxaparin. This finding challenges the merit of choosing enoxaparin rather than heparin for thromboprophylaxis in patients with a SAH.

[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.

HIV infection increases the risk of heparin-induced thrombocytopenia

The incidence of heparin-induced thrombocytopenia in human immunodeficiency virus (HIV)-infected inpatients was compared with that in a control group that was not known to be infected with HIV in a retrospective cohort study. HIV-infected patients receiving heparin therapy, especially unfractionated heparin therapy, were at increased risk of developing heparin-induced thrombocytopenia, compared with HIV-uninfected patients.

Heparin-induced thrombocytopenia: acknowledging its presence in low-molecular weight heparin therapy

Low-molecular weight heparin (LMWH) is associated with a lower incidence of heparin-induced thrombocytopenia (HIT) than is unfractionated heparin. We describe a 75-year-old woman who developed HIT with thrombotic manifestations following the use of nadroparin calcium. Subsequent anticoagulation was achieved with warfarin. This case serves to highlight an important complication that cannot be ignored despite its low incidence. The majority of HIT cases are likely to occur in LMWH-treated patients because LMWH replaces unfractionated heparin for most indications. The lack of suitable alternative anticoagulant treatments for patients with HIT in Singapore is also emphasized.

Advances in the diagnosis of idiopathic thrombocytopenic purpura

Idiopathic thrombocytopenic purpura (ITP) remains a clinical diagnosis made by the exclusion of other causes of thrombocytopenia. It is based on the patient's history, physical examination, and complete blood cell count, as well as examination of the blood film. Over the last four decades, a number of platelet antibody tests have been developed to aid the diagnosis of ITP. They can be divided chronologically into three groups. Phase I assays measure a functional change in control platelets after incubation with test serum. Because their sensitivity and specificity are low, they are no longer used to diagnose ITP. Phase II assays measure platelet-associated IgG by three different approaches. They lack the ability to differentiate between pathologic and nonpathologic platelet-associated IgGs. These assays are sensitive (80% to 90%) but their specificity is too low for them to be diagnostically useful. Phase III assays are the latest development in platelet serology testing. They measure glycoprotein-specific platelet antibodies by different approaches, namely, immunoblot, immunoprecipitation, and glycoprotein immobilization. Despite their high specificity, they suffer from low sensitivity (47% to 60%), which must be improved if they are to be clinically useful for the diagnosis of ITP.

Low molecular weight heparins and heparin-induced thrombocytopenia

Heparin-induced thrombocytopenia (HIT) and HIT thrombosis syndrome (HITTS) are immune-mediated complications of clinical use of unfractionated heparin (UFH). The antibody/antigen complex is composed of heparin and platelet factor 4. This complex not only activates platelets but also the clotting system leading to thrombin generation. This explains the thrombosing tendency of these patients, and venous and arterial thromboembolisms are encountered with a morbidity and mortality of about 25-37%. The incidence of HIT is about 3% when UFH is administered therapeutically. The diagnosis is at this time based on clinical observations, especially a sudden, unexplained drop in platelet counts without other reasons. Laboratory tests can be used to confirm the clinical diagnosis, but none of the available tests is 100% reliable. There is no test that will predict HIT and no test that will signal the development of HITTS. Treatment consists of discontinuation of UFH in any form and anticoagulation with danaparoid or r-hirudin, if needed. The use of low molecular weight heparins instead of UFH could largely (not totally) alleviate the problem.

The platelet proaggregating and potentiating effects of unfractionated heparin, low molecular weight heparin and heparinoid in intensive care patients and healthy controls

Heparin binds to platelets and can cause platelet proaggregating and potentiating effects, possibly causing thrombocytopenia, particularly in patients in intensive care with hyperaggregable platelets. In this study we compared the platelet proaggregating and potentiating effects of unfractionated heparin (UH), 2 low molecular weight (LMW) heparins, enoxaparin and dalteparin, and a heparinoid, danaparoid sodium (orgaran), to platelets of an ICU patient population and a normal control group. In both populations UH caused platelet aggregation in a dose-dependent manner. This occurred in the therapeutic range of the drug, with as little as 0.5 U/ml UH. The LMW heparins caused less and the heparinoid least platelet aggregation. Generally, the aggregation observed in ICU patients was greater than in the normal population. The potentiating effects of the 4 drugs in association with physiological agonists was examined. Similar patterns of potentiation were observed in both populations, with UH causing significant enhancement of platelet aggregation, the LMW heparins intermediate and heparinoid least enhancement. There was substantial variability in the individuals' platelets' reactions to the drugs, in particular to UH. Our findings suggest that UH has the greatest effect, the low molecular weight heparins an intermediate effect and the heparinoid the least propensity to cause platelet activation.

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.

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.

Immediate effects of heparin and LMW heparin on some platelet and endothelial derived factors

Heparin and a low molecular heparin fragment, injected intravenously in volunteers, increased the plasma concentrations of platelet factor 4, but did not induce platelet activation as judged from excretion of 2,3-dinor-TxB2 (a major thromboxane A2 metabolite) and beta-thromboglobulin (btg) in urine and from btg levels in plasma. Heparin prolonged, within the normal range, the bleeding time in all six subjects. Platelet aggregation in platelet rich plasma was potentiated by both heparins, but platelet number, mean platelet volume and platelet distribution width were not affected. No evidence for endothelial release of prostacyclin was obtained as judged from urinary excretion of 2,3-dinor-6-keto-PGF1 alpha (a major prostacyclin metabolite), and plasma concentrations of tissue plasminogen activator, its inhibitor (PAI-1) and the von Willebrand-factor were unchanged.

Whole blood platelet aggregation is not affected by cigarette smoking but is sex-related

1. In normal subjects, 18-49 years old, the effects of the smoking habit (greater than 10 cigarettes/day) and the act of smoking two cigarettes over 10 min were studied on whole blood platelet aggregation (in vitro impedance method). 2. Acute smoking (n = 10) did not affect platelet aggregation to ADP, collagen or to platelet activating factor (PAF) nor thromboxane B2 production during aggregation. There was no difference between smokers (n = 13) and non-smokers (n = 10). However, aggregation to all aggregants was greater in females (n = 11) than males (n = 12) (ADP and collagen, P less than 0.001; PAF, P less than 0.01; ANOVA). 3. Although others have obtained diverse results studying platelet-rich plasma, the absence of an effect of cigarette smoking on whole blood platelet aggregation is consistent with many of those observations. Greater in vitro aggregability in females than males is consistent with the few studies of platelet-rich plasma. It seems unlikely that the role of cigarette smoking as a risk factor for ischaemic heart disease is related to a direct effect on platelet aggregability.