Thrombin generation and heparin-induced thrombocytopenia

Platelets from patients with chronic inflammation have a phenotype of chronic IL-1β release

Background

Chronic inflammation is a cardiovascular risk factor, and interleukin-1β (IL-1β) is central to the inflammatory host response. Platelets contain the NLRP3 inflammasome and are able to translate IL-1β messenger RNA (mRNA) and secrete mature IL-1β upon activation. However, the role of a chronic inflammatory environment in platelet IL-1β mRNA and protein content remains unclear.

Objectives

The aim of the current study was to investigate intracellular platelet IL-1β and IL-1β mRNA in a chronic inflammatory state.

Methods

Sixty-five patients with stable inflammation (ie, high-sensitivity C-reactive protein within predefined margins in 2 separate measurements) were stratified according to high-sensitivity C-reactive protein levels in low (0.0-0.9 mg/L), medium (1.0-2.9 mg/L), and high (3.0-9.9 mg/L) risk groups. Platelet reactivity as well as platelet IL-1β protein synthesis were studied.

Results

The highest risk group was characterized by a distinct cardiovascular risk profile and approximately 20% higher platelet counts. While platelet reactivity was not different, a reduction in intracellular platelet IL-1β mRNA and IL-1β protein levels was observed in the highest risk group and was linked to decreased platelet size and granularity. This signature suggests a phenotype of chronic IL-1β secretion and could be experimentally phenocopied by stimulation of platelets from healthy volunteers with either TRAP-6 or collagen related peptide (CRP-XL).

Conclusion

Our data suggest a phenotype of chronic IL-1β secretion by platelets in patients with chronic sterile inflammation.

Platelet phosphatidylserine is the critical mediator of thrombosis in heparin-induced thrombocytopenia

Heparin-induced thrombocytopenia (HIT) is a severe immune-mediated prothrombotic disorder caused by antibodies (Ab) reactive to complexes of platelet factor 4 and heparin. Platelets (PLT) and their interaction with different immune cells contribute to prothrombotic conditions in HIT. However, the exact mechanisms and the role of different PLT subpopulations in this prothrombotic environment remain poorly understood. In this study, we observed that HIT patient Ab induce a new PLT population that is characterized by increased P-selectin expression and phosphatidylserine (PS) externalization. Formation of this procoagulant PLT subpopulation was dependent on engagement of PLT Fc-γ-RIIA by HIT Ab and resulted in a significant increase of thrombin generation on the PLT surface. Using an ex vivo thrombosis model and multi-parameter assessment of thrombus formation, we observed that HIT Ab-induced procoagulant PLT propagated formation of large PLT aggregates, leukocyte recruitment and most importantly, fibrin network generation. These prothrombotic conditions were prevented via the upregulation of PLT intracellular cAMP with Iloprost, a clinically approved prostacyclin analogue. Additionally, the functional relevance of P-selectin and PS was dissected. While inhibition of P-selectin did not affect thrombus formation, the specific blockade of PS prevented HIT Ab-mediated thrombin generation and most importantly procoagulant PLT-mediated thrombus formation ex vivo. Taken together, our findings indicate that procoagulant PLT are critical mediators of prothrombotic conditions in HIT. Specific PS targeting could be a promising therapeutic approach to prevent thromboembolic events in HIT patients.

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.

Measurement of procoagulant platelets provides mechanistic insight and diagnostic potential in heparin-induced thrombocytopenia

BACKGROUND

Heparin-induced thrombocytopenia (HIT) is a prothrombotic, immune-mediated adverse drug reaction associated with high rates of thrombosis-related morbidity and mortality caused by FcγRIIa-activating pathogenic antibodies to PF4-heparin. Procoagulant platelets are a platelet subset that promote thrombin generation, are clinically relevant in prothrombotic diseases, and are formed when platelet G-protein-coupled receptor (GPCR) and ITAM-linked receptors are co-stimulated.

OBJECTIVES

We examined the procoagulant platelet response of healthy donors to platelet agonists in the presence of HIT plasma and determined the contribution of FcγRIIa.

PATIENTS/METHODS

Our previously established flow cytometry-based procoagulant platelet assay was modified to incorporate plasma samples, performed using FcγRIIa-responsive donor platelets. Plasma samples were serotonin-release assay-confirmed HIT (HIT+), or negative on HIT screening.

RESULTS

In response to GPCR stimulation, only HIT+ plasma produced a heparin-dependent sensitization that required active FcγRIIa. As a potential diagnostic tool, the procoagulant platelet assay achieved 98% accuracy in identifying clinically verified HIT when performed blinded to the diagnoses of a validation cohort. Samples inducing a higher procoagulant platelet response were more likely from patients with thrombotic complications. Thrombin stimulation markedly increased the procoagulant platelet response with HIT+ plasma that was heparin independent and only partially reversed by FcγRIIa blockade, possibly reflecting ongoing thrombotic risk after heparin cessation.

CONCLUSIONS

We demonstrate that HIT plasma together with platelet agonists increased the procoagulant platelet proportions, which may contribute to thrombotic risk in HIT. Targeting procoagulant platelet activation may represent a novel treatment strategy. This assay may be a rapid, clinically relevant functional assay for accurately detecting pathological HIT antibodies.

Long-term follow-up after successful treatment of vaccine-induced prothrombotic immune thrombocytopenia

BACKGROUND

Cases of ChAdOx1 nCoV-19 (AstraZeneca) vaccinated patients with thrombocytopenia, elevated D-dimer, and elevated platelet factor 4 (PF4) antibody levels with- and without thrombosis have been reported. No recommendations regarding the duration of anticoagulation have been made, because data on the long-term course beyond the first weeks is lacking.

OBJECTIVE

To report on the treatment, medical course, and longitudinal follow-up of laboratory parameters in patients with vaccine-induced prothrombotic immune thrombocytopenia (VIPIT).

PATIENTS

We followed VIPIT patients with- (n = 3) and without (n = 3) venous thromboembolism fulfilling the aforementioned laboratory criteria.

RESULTS

Elevated D-dimer (median: 35.10 μg/ml, range: 17.80-52.70), thrombocytopenia (42 G/l, 20-101), and strong positivity in the platelet factor 4 (PF4)/heparin-enzyme-immunoassay (2.42 optical density [OD], 2.06-3.13; reference range < 0.50) were present in all patients after vaccination (10 days, 7-17). Routine laboratory parameters rapidly improved upon initiation of treatment (comprising therapeutic non-heparin anticoagulation in all patients and high dose immunoglobulins ± corticosteroids in 5 patients). PF4 antibody levels slowly decreased over several weeks. Patients were discharged in good physical health (8 days, 5-13). VIPIT did not recur during follow-up (12 weeks, 8-17). Five of 6 patients fully recovered (in 2 patients thrombosis had resolved, in 1 patient exertional dyspnea persisted).

CONCLUSIONS

Remissions without sequelae can be achieved upon rapid initiation of treatment in patients with VIPIT. Platelet factor 4 antibody levels slowly decreased over several weeks but VIPIT did not recur in any of our patients. Continuation of anticoagulation in VIPIT patients at least until PF4 antibody negativity is reached seems reasonable.

Added Value of Blood Cells in Thrombin Generation Testing

The capacity of blood to form thrombin is a critical determinant of coagulability. Plasma thrombin generation (TG), a test that probes the capacity of plasma to form thrombin, has improved our knowledge of the coagulation system and shows promising utility in coagulation management. Although plasma TG gives comprehensive insights into the function of pro- and anticoagulation drivers, it does not measure the role of blood cells in TG. In this literature review, we discuss currently available continuous TG tests that can reflect the involvement of blood cells in coagulation, in particular the fluorogenic assays that allow continuous measurement in platelet-rich plasma and whole blood. We also provide an overview about the influence of blood cells on blood coagulation, with emphasis on the direct influence of blood cells on TG. Platelets accelerate the initiation and velocity of TG by phosphatidylserine exposure, granule content release and surface receptor interaction with coagulation proteins. Erythrocytes are also major providers of phosphatidylserine, and erythrocyte membranes trigger contact activation. Furthermore, leukocytes and cancer cells may be important players in cell-mediated coagulation because, under certain conditions, they express tissue factor, release procoagulant components and can induce platelet activation. We argue that testing TG in the presence of blood cells may be useful to distinguish blood cell-related coagulation disorders. However, it should also be noted that these blood cell-dependent TG assays are not clinically validated. Further standardization and validation studies are needed to explore their clinical usefulness.

Detection of Platelet-Activating Antibodies Associated with Heparin-Induced Thrombocytopenia

Heparin-induced thrombocytopenia (HIT) is a prothrombotic immune drug reaction caused by platelet-activating antibodies that in most instances recognize platelet factor 4 (PF4)/polyanion complexes. Platelet activation assays (i.e., functional assays) are more specific than immunoassays, since they are able to discern clinically relevant heparin-induced antibodies. All functional assays used for HIT diagnosis share the same principle, as they assess the ability of serum/plasma from suspected HIT patients to activate fresh platelets from healthy donors in the presence of several concentrations of heparin. Depending on the assay, donors’ platelets are stimulated either in whole blood (WB), platelet-rich plasma (PRP), or in a buffer medium (washed platelets, WP). In addition, the activation endpoint studied varies from one assay to another: platelet aggregation, membrane expression of markers of platelet activation, release of platelet granules. Tests with WP are more sensitive and serotonin release assay (SRA) is considered to be the current gold standard, but functional assays suffer from certain limitations regarding their sensitivity, specificity, complexity, and/or accessibility. However, the strict adherence to adequate preanalytical conditions, the use of selected platelet donors and the inclusion of positive and negative controls in each run are key points that ensure their performances.

Effects of argatroban, danaparoid, and fondaparinux on trombin generation in heparin-induced thrombocytopenia

There is no in vitro data on the comparison of the effects of danaparoid, argatroban and fondaparinux on thrombin generation in patients with heparin-induced thrombocytopenia. It was the study objective to compare the in vitro anticoagulant potential of argatroban, danaparoid and fondaparinux using a thrombin generation assay TGA on a mixture of control platelet-rich plasma (PRP) and HIT patient platelet-poor plasma (PPP). The plasma of seven patients with a clear HIT diagnosed at our institution was selected. Mixtures of donor PRP and patient PPP were incubated with unfractionated heparin 0.2 U.mL-1, argatroban at 600 ng.mL-1, argatroban at 400 ng.mL-1, danaparoid at 0.65 IU.mL-1 and fondaparinux at 1 [uni03BC]g.mL-1. Thrombin generation was assessed by calibrated thrombinography. The percentage of inhibition of the endogenous thrombin potential observed with argatroban at 600 ng.mL-1 was statistically significantly higher compared with those observed with fondaparinux (median: 53.6% vs. 3.9%; p = 0.031) but not compared with argatroban at 400 ng.mL-1 and danaparoid. The percentage of inhibition of the thrombin peak observed with argatroban at 600 ng.mL-1 was statistically significantly higher compared with those observed with danaparoid (median: 71.2 vs. 56.8; p = 0.031) and fondaparinux (mean: 71.2 vs. 30; p = 0.031) but not with argatroban at 400 ng.mL-1. In conclusion, the in vitro effect of argatroban and danaparoid on thrombin generation seems to corroborate the results of clinical studies of these drugs in the treatment of HIT in term of efficiency. Fondaparinux showed a very small effect on thrombin generation evaluated by calibrated thrombinography.

Functional Assays in the Diagnosis of Heparin-Induced Thrombocytopenia: A Review

A rapid and accurate diagnosis in patients with suspected heparin-induced thrombocytopenia (HIT) is essential for patient management but remains challenging. Current HIT diagnosis ideally relies on a combination of clinical information, immunoassay and functional assay results. Platelet activation assays or functional assays detect HIT antibodies that are more clinically significant. Several functional assays have been developed and evaluated in the literature. They differ in the activation endpoint studied; the technique or technology used; the platelet donor selection; the platelet suspension (washed platelets, platelet rich plasma or whole blood); the patient sample (serum or plasma); and the heparin used (type and concentrations). Inconsistencies in controls performed and associated results interpretation are common. Thresholds and performances are determined differently among papers. Functional assays suffer from interlaboratory variability. This lack of standardization limits the evaluation and the accessibility of functional assays in laboratories. In the present article, we review all the current activation endpoints, techniques and methodologies of functional assays developed for HIT diagnosis.

New platelet functional method for identification of pathogenic antibodies in HIT patients

Heparin-induced thrombocytopenia (HIT) is a thrombotic complication of heparin therapy. The most used functional method for HIT diagnosis is serotonin release assay (SRA). A different functional method based on ATP release with luciferin/luciferase long-life and stable luminescent signal is used here, which is shown to be comparable for accuracy with SRA in both negative (patients 4Ts ≤3, and negative for both anti-PF4/heparin immunoassay and SRA) and positive (4Ts >3, and positive for both PF4/heparin antibodies and SRA) patients. Our results show that ATP release is higher in washed platelets activated by sera from positive patients than in platelets activated by sera from negative patients. In conclusion, we demonstrate that ATP release assay is a valid alternative method to SRA for the identification of pathogenic anti-PF4/heparin antibodies.

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.

Anticoagulant properties of enoxaparin 400 IU/mL-40 % ethanol catheter lock solution

Unfractionated heparin (UFH) is the most widely used interdialytic lock solution but has no anti-infectious properties. Ethanol at a content ≥40 %v/v eradicates experimental biofilm but has no anticoagulant properties. In contrast to UFH, enoxaparin (Enox) can be combined with 40 % ethanol without precipitation. Enoxaparin 400 UI/mL–40 % ethanol (Enox/Eth) has antibiofilm properties and therefore has promise as an alternative lock solution. This study assessed the anticoagulant properties of Enox/Eth. Enox and Enox/Eth were diluted in whole blood at a final Enox concentration of 0.5, 1 (N = 6 samples), 1.5 (N = 4) and 2 (N = 6) IU/mL. Anti-Xa activity was determined by chromogenic assay and the inhibition of endogenous thrombin potential (ETP) by thrombinography. Quantitative data were compared by the Mann-Withney U test. For Enox concentrations of 0.5, 1, 1.5 and 2 UI/mL in whole blood samples, the mean ± SD values of the anti-Xa activity were 0.68 ± 0.09, 1.26 ± 0.14, 1.73 ± 0.30, 2.35 ± 0.32 UI/mL for Enox/Eth and 0.94 ± 0.15, 1.80 ± 0.22, 2.74 ± 0.23, 3.54 ± 0.44 UI/mL for Enox (P = 0.03, P = 0.03, P = 0.13, P = 0.03); and of the percentage of ETP inhibition was 17.36 ± 9.65, 30.27 ± 17.06, 36.5 ± 17.06, 57.82 ± 15.42 for Enox/Eth, and 42.96 ± 15.68, 68.93 ± 10.01, 83.5 ± 8.81, 91.19 ± 4.67 for Enox (P = 0.03, P = 0.03, P = 0.13, P = 0.03), respectively. The median and IQR values of Enox concentration inhibiting 50 % of ETP (IC50 ETP) were 1.8 [1.1–2.4] IU/mL for Enox/Eth and 0.7 [0.3–0.9] IU/mL for Enox, P = 0.03. Enox/Eth has strong anticoagulant activity, albeit lower than that of Enox, but with an extremely low IC50 ETP compared to the Enox concentration of non-diluted Enox/Eth.

Anti-platelet factor 4/heparin antibodies in patients with impaired graft function after liver transplantation

BACKGROUND

Heparin, the standard perioperative anticoagulant for the prevention of graft vessel thrombosis in patients undergoing liver transplantation (LT), binds to the chemokine platelet factor 4 (PF4). Antibodies that are formed against the resulting PF4/heparin complexes can induce heparin-induced thrombocytopenia. LT is a clinical situation that allows the study of T-cell dependency of immune responses because T-cell function is largely suppressed pharmacologically in these patients to prevent graft rejection.

OBJECTIVES

To investigate the immune response against PF4/heparin complexes in patients undergoing LT.

PATIENTS AND METHODS

In this prospective cohort study, 38 consecutive patients undergoing LT were systematically screened for anti-PF4/heparin antibodies (enzyme immunoassay and heparin-induced platelet aggregation assay), platelet count, liver function, and engraftment.

RESULTS

At baseline, 5 (13%) of 38 patients tested positive for anti-PF4/heparin IgG (non-platelet-activating) antibodies. By day 20, an additional 5 (15%) of 33 patients seroconverted for immunoglobulin G (two platelet-activating) antibodies. No patient developed clinical heparin-induced thrombocytopenia. Two of six patients with graft function failure had anti-PF4/heparin IgG antibodies at the time of graft function failure. Graft liver biopsy samples from these patients showed thrombotic occlusions of the microcirculation.

CONCLUSIONS

Anti-PF4/heparin IgG antibodies are generated despite strong pharmacologic suppression of T cells, indicating that T cells likely have a limited role in the immune response to PF4/heparin complexes in humans.

Structure and function of platelet receptors initiating blood clotting

At the clinical level, recent studies reveal the link between coagulation and other pathophysiological processes, including platelet activation, inflammation, cancer, the immune response, and/or infectious diseases. These links are likely to underpin the coagulopathy associated with risk factors for venous thromboembolic (VTE) and deep vein thrombosis (DVT). At the molecular level, the interactions between platelet-specific receptors and coagulation factors could help explain coagulopathy associated with aberrant platelet function, as well as revealing new approaches targeting platelet receptors in diagnosis or treatment of VTE or DVT. Glycoprotein (GP)Ibα, the major ligand-binding subunit of the platelet GPIb-IX-V complex, that binds the adhesive ligand, von Willebrand factor (VWF), is co-associated with the platelet-specific collagen receptor, GPVI. The GPIb-IX-V/GPVI adheso-signaling complex not only initiates platelet activation and aggregation (thrombus formation) in response to vascular injury or disease but GPIbα also regulates coagulation through a specific interaction with thrombin and other coagulation factors. Here, we discuss the structure and function of key platelet receptors involved in thrombus formation and coagulation in health and disease, with a particular focus on platelet GPIbα.

Antiplatelet agents can promote two-peaked thrombin generation in platelet rich plasma: mechanism and possible applications

BACKGROUND

Thrombin generation assay is a convenient and widely used method for analysis of the blood coagulation system status. Thrombin generation curve (TGC) is usually bell-shaped with a single peak, but there are exceptions. In particular, TGC in platelet-rich plasma (PRP) can sometimes have two peaks.

OBJECTIVE

We sought to understand the mechanism underlying the occurrence of two peaks in the PRP thrombin generation curve.

METHODS

Tissue factor-induced thrombin generation in PRP and platelet-poor plasma (PPP) was monitored using continuous measurement of the hydrolysis rate of the thrombin-specific fluorogenic substrate Z-Gly-Gly-Arg-AMC. Expression of phosphatidylserine (PS) and CD62P on the surface of activated platelets was measured by flow cytometry using corresponding fluorescently labeled markers.

RESULTS

The addition of the P(2)Y(12) receptor antagonist MeS-AMP (160 µM), 83 nM prostaglandin E(1) (PGE(1)), or 1.6% DMSO to PRP caused the appearance of two peaks in the TGC. The PS exposure after thrombin activation on washed platelets in a suspension supplemented with DMSO, PGE(1) or MeS-AMP was delayed, which could indicate mechanism of the second peak formation. Supplementation of PRP with 1.6% DMSO plus 830 nM PGE(1) mediated the disappearance of the second peak and decreased the amplitude of the first peak. Increasing the platelet concentration in the PRP promoted the consolidation of the two peaks into one.

CONCLUSIONS

Procoagulant tenase and prothrombinase complexes in PRP assemble on phospholipid surfaces containing PS of two types--plasma lipoproteins and the surface of activated platelets. Thrombin generation in the PRP can be two-peaked. The second peak appears in the presence of platelet antagonists as a result of delayed PS expression on platelets, which leads to delayed assembly of the membrane-dependent procoagulant complexes and a second wave of thrombin generation.

Assessment of HIT antibody complex in hip fracture patients receiving enoxaparin or unfractionated heparin

Thromboembolic disease is a common complication of hip fracture in the elderly. Anticoagulants represent a standard of care in preventing postoperative thrombotic complications following surgical fixation. We asked whether levels of antibody to heparin-platelet factor 4 (PF4) complex were differentially present in unfractionated heparin (UFH) versus Enoxaparin, following hip fracture and whether one particular subtype of antibodies was more prevalent. Plasma samples from elderly patients sustaining a hip fracture treated with either enoxaparin or UFH were collected pre- and postoperatively and analyzed using enzyme-linked immunosorbent assay (ELISA) sandwich method for the prevalence of antiheparin-PF4 antibodies and later subtyped. The prevalence of antiheparin-PF4 antibodies was higher in the UFH group especially on postoperative day 7. Patients treated with UFH showed a greater prevalence of antiheparin-PF4 antibodies and a greater prevalence of immunoglobulin G (IgG) subtype. Heparin and enoxaparin are capable of generating heparin-induced thrombocytopenia (HIT) antibodies in elderly patients undergoing orthopedic surgery but perhaps not to the same extent. When comparing low-molecular-weight heparin (LMWH) with UFH, the incidence of new antiheparin-PF4 antibody production is higher in patients treated with UFH.

Validation of whole blood impedance aggregometry as a new diagnostic tool for HIT: results of a large Australian study

Heparin-induced thrombocytopenia (HIT) remains a challenge, with diagnosis confirmed only by functional assays. The gold standard 14C-serotonin release assay (SRA) is highly sensitive but technically challenging and unsuitable for routine use. We conducted a large study to validate whole blood impedance aggregometry (WBIA) as a suitable diagnostic tool for HIT. WBIA and SRA were used to test 181 samples positive for H-PF4 antibodies by PaGIA or ELISA. Using the same high responder donor, 77 samples were positive by WBIA (aggregation with low-dose but not high-dose heparin). Using the strict definition for SRA positivity, 72 samples were true HIT. In nine samples, serotonin release with high-dose heparin dropped by > 50% but was still >20%; these were retested after a one-half dilution and 8/9 became positive. Ten other samples were discrepant between the two assays: one strongly positive (89% release) and six weakly positive samples by SRA (average release 56%) were WBIA negative. When these samples were retested using a random donor, only two remained SRA positive. Three samples were strongly WBIA positive but SRA negative; two were retested by SRA with 0.5IU/ml heparin and one became positive. Under controlled conditions, using the same selected high-responder donor, WBIA and SRA performed similarly with slightly increased sensitivity of the WBIA when using the strict definition of SRA positivity. WBIA is easy to perform with rapid turn-around time and warrants a multi-laboratory trial to complete its validation as a confirmatory assay for platelet-activating HIT antibodies.

Adverse effects of heparin

All the adverse effects of heparins are related to their wide variety of biological activities, with bleeding being the most important safety issue, resulting directly from the potency of heparin as an anticoagulant. However, it is hard to define the bleeding risk, since it depends on numerous parameters including the indication, dosage, method, and duration of heparin application, the clinical study design and definition of bleeding as well as patient characteristics and determinants of bleeding such as type of surgery and co-medication. Nonbleeding complications of heparins are caused by binding of heparin molecules to proteins other than antithrombin and to cells, which is generally more pronounced with unfractionated heparin than with low-molecular-weight heparins. Accordingly, heparin-induced thrombocytopenia, the most severe nonbleeding adverse reaction, occurs about 10 times less with low-molecular-weight heparins than with unfractionated heparin. Frequent and therefore important adverse reactions of heparins are skin lesions resulting from delayed-type hypersensitivity reactions. All the other undesirable effects are discussed as well, but they are mostly clinically irrelevant.

The Syk-kinase inhibitor R406 impairs platelet activation and monocyte tissue factor expression triggered by heparin-PF4 complex directed antibodies

BACKGROUND

Heparin-induced thrombocytopenia (HIT) is a rare but severe complication of heparin therapy in which immunoglobulin G (IgG) antibodies against the platelet factor 4-heparin complex activate platelets through the FcγRIIA receptor. Clustering of FcγRIIA initiates signaling cascades involving tyrosine kinases including the spleen tyrosine kinase (Syk). Moreover, besides the critical role of platelets, the expression of tissue factor (TF) by human monocytes triggered by HIT antibodies has been shown to contribute to the hypercoagulability and the thrombotic complications in HIT patients.

OBJECTIVES

We investigated the effect of R406, a small molecule inhibitor of Syk developed as a potential treatment of autoimmune diseases, allergic disorders and B-cell related hematological malignancies, on FcγRIIA-mediated platelet activation. To further assess the potential activity of Syk inhibitors in HIT treatment, the effect of R406 was also evaluated on HIT antibodies-induced expression of TF and procoagulant activity of monocytic cells.

RESULTS

We show that R406 is a potent inhibitor of platelet signaling and functions initiated by FcγRIIA cross-linking by specific antibodies or by sera from HIT patients. Syk inhibition efficiently prevents FcγRIIA-induced LAT phosphorylation and activation of phosphoinositide 3-kinase, Akt, phospholipase Cγ2 and p38 MAP-kinase. As a consequence, FcγRIIA-induced platelet aggregation, granule secretion and microparticles production are strongly inhibited by R406. Moreover, the Syk inhibitor efficiently impairs the expression of TF and the procoagulant activity of human monocytes triggered by HIT antibodies.

CONCLUSION

Syk inhibitors may be of therapeutic interest in the treatment of HIT by reducing HIT antibodies-mediated platelet activation and monocyte procoagulant activity.

HIT paradigms and paradoxes

The current major problem with HIT is its overdiagnosis. This concept follows from the HIT central paradigm: HIT is caused by a subset of antibodies against platelet factor 4 (PF4)/heparin complexes that have strong platelet-activating properties. Prospective studies show that only a minority of sera containing such antibodies exhibit platelet-activating properties. Ironically, the earliest tests for HIT--platelet activation assays--remain today the most diagnostically useful, particularly the washed platelet assays. But the wider application of PF4-dependent immunoassays, and their much greater sensitivity for the larger subset of non-platelet-activating (and non-HIT-inducing) antibodies, has resulted in HIT overdiagnosis in many centres. Studies of anti-PF4/heparin immunization in diverse clinical situations have provided insights into the factors that influence the HIT immune response. Besides the conundrum of anticoagulant-induced thrombosis (including its potentiation of coumarin-induced microthrombosis), HIT evinces numerous other paradoxes: (i) it is a platelet-activating disorder with venous thrombosis as its predominant clinical manifestation; (ii) 'delayed-onset' (or 'autoimmune') HIT can lead to dramatic worsening of HIT-associated thrombosis despite cessation of heparin; (iii) partial thromboplastin time (PTT) monitoring of direct thrombin inhibitor treatment - and confounding of PTT monitoring by HIT-associated consumptive coagulopathy - infers that the worst subset of HIT patients may fail this therapeutic approach; (iv) the highly sulfated pentasaccharide anticoagulant, fondaparinux, can (rarely) cause HIT yet appears to be an effective treatment for this disorder; and (v) the transience of the HIT immune response means that many patients with previous HIT can safely receive future heparin.

Heparin-induced thrombocytopenia: current status and diagnostic challenges

Heparin-induced thrombocytopenia (HIT) is a fairly common and potentially catastrophic complication of heparin therapy. Diagnosing HIT remains a challenge, as the patients at risk often have other reasons for thrombocytopenia and/or thrombosis. HIT is considered a clinicopathologic disorder whose diagnosis is generally made on the basis of both clinical criteria and the presence of "HIT antibodies" in the patient's serum or plasma. There are two basic laboratory approaches to detect HIT antibodies. The immunoassays detect antibodies based on their binding properties, whereas the functional assays detect antibodies based on their platelet-activating properties. Prompt and accurate diagnosis of HIT is imperative, as overdiagnosis exposes patients to alternative anticoagulants and their associated bleeding risks, whereas under- or delayed diagnosis leaves patients vulnerable to the thromboembolic sequelae of HIT, which can be life threatening. A critical interpretation of laboratory results by the clinician is an essential component of diagnosing HIT. This requires a keen understanding of the current concepts in the pathophysiologic mechanisms of the disease, and the application of these concepts when interpreting the results of both the functional and immunoassays. Equally important is an awareness of the strengths and weaknesses, as well as the current lack of standardization and proficiency testing, of these assays.

Heparin and Oversulfated Heparin Byproduct Induce Thrombin Generation Through Contact System Activation in Plasma of Patients With HIT

Heparin-induced thrombocytopenia with thrombosis (HITT) is the most severe side effect of heparin administration. Patients with HITT may die or have permanent sequelae such as stroke or limb amputation. Contaminated heparin is associated with anaphylactic reactions and deaths by activating the contact system. It is also associated with high incidence of HIT via a yet unknown mechanism. This study showed that although oversulfated heparin byproduct induced thrombin activities in both normal and HIT patient plasmas through the contact system activation, authentic heparin induced thrombin activities only in HIT patient plasmas containing autoantibodies against protein/ heparin complex. These data suggest that the negatively charged immunoglobulin G (IgG)/platelet factor 4 (PF4)/heparin complex activate the contact system and produce thrombin in human plasma, and thrombin partially activates the platelets allowing subsequent platelet activation through IgG/Fc receptor II signaling. The newly discovered mechanism of heparin-induced thrombin activity could explain the increased incidence of HIT in patients exposed to contaminated heparin. Furthermore, the assays used in these studies would be valuable for HIT diagnosis, prevention, and treatment.