Pharmacodynamics and Tolerance of Two Nadroparin Formulations (10,250 and 20,500 Anti Xa IU·ml-1) Delivered for 10 Days at Therapeutic Dose

Venous thromboembolism may be efficiently treated by once-a-day (o.d.) administration of a high dose of low molecular weight heparin (LMWH) instead of administration of the same total dose in two injections a day (b.i. d.). To reduce the volume of the subcutaneous (s.c.) injection, a more concentrated form of the drug is advisable. This study was designed to compare the bioavailability of 2 formulations of nadroparin containing 10,250 and 20,500 anti-Xa IU·ml-1 respectively. This was an open, randomized, cross-over study where 12 healthy volunteers (age 18-35) were enrolled. They received either 90 anti-Xa IU·kg-1 b.i. d. of the 10,250 IU preparation (treatment A), or 180 anti-Xa IU·kg-1 o.d. of the 20,500 IU preparation (treatment B) for 10 days. On day 1, the subjects were sampled between 0 and 12 h (treatment A) or between 0 and 24 h (treatment B). On day 10, they were sampled between 0 and 12 h and between 12 and 24 h (treatment A) or between 0 and 24 h (treatment B). Anti-Xa and anti-IIa activities were determined by specific chromogenic assays. The main result of the study was that the bioavailability of the anti-Xa activity of the 2 nadroparin formulations was equivalent, as shown by the comparison of the AUC0-12 h plus AUC12-24 h (treatment A) and the AUC0-24 h (treatment B), calculated on day 10. This study also allowed a number of interesting observations to be made. 1) Between day 1 and day 10, there was an accumulation of the anti-Xa activity for treatment A but not for treatment B (accumulation factors: 1.6 and 1.1 respectively); 2) On day 10, the AUC0-12 h were slightly but significantly lower than the AUC12-24 h suggesting a circadian effect for anti-Xa and anti-IIa activities; 3) the clearance of the anti-Xa activity was comparable at the 2-dose regimens, while that of the anti-IIa activity was lower in treatment B than in treatment A, indicating a significant dose effect for the pharmacodynamics of the longer heparin chains; 4) On average, the clearance of the anti-IIa activity was twice as high as that of the anti-Xa activity; 5) For treatment B, significant APTT prolongations were noticed at Tmax (prolongation factor: 1.7 ± 0.25), in relation with the anti-IIa activity (0.3 ± 0.1 IU·ml–1).

The Expression of Tissue Factor and Tissue Factor Pathway Inhibitor in Aortic Smooth Muscle Cells Is Up-regulated in Synthetic Compared to Contractile Phenotype

Tissue factor (TF) and its specific inhibitor TF pathway inhibitor (TFPI) are produced by vascular smooth muscle cells (SMCs) in vitro and are increased in vivo in atherosclerotic compared to normal vessels. Besides local regulation of the hemostatic balance, this may be related to non-hemostatic TF/protease dependent functions such as SMC proliferation, adhesion and migration. The aim of the study was to compare the expression of both proteins between the contractile (normal adult) and synthetic (neo-intimal) SMC phenotypes.

Primary cultures of SMCs isolated from rat thoracic aorta before and 10 days after balloon injury displayed stable characteristics of the contractile and synthetic phenotype, respectively. Synthetic SMCs expressed more TF mRNA than contractile SMCs, but released excess TF in the conditioned medium, so that the cell-associated TF activity measured by a factor Xa generating assay remained similar in the two subtypes. Accordingly, cell surface thrombogenicity measured under blood flow conditions was also similar. The production and release of functional TFPI was enhanced by a factor 3 to 6 (p < 0.01) in synthetic SMCs.

A difference in the quantitative expression of TF and TFPI is a new distinctive feature of SMC phenotypes. Matrix-associated TFPI derived from synthetic SMCs may serve as an anchorage for their migration and regulate protease-activated processes during neo-intima formation.

Aging and Venous Thromboembolism Influence the Pharmacodynamics of the Anti-Factor Xa and Anti-thrombin Activities of a Low Molecular Weight Heparin (Nadroparin)

Venous thromboembolism may be efficiently treated by one single daily administration of a high dose of low molecular weight heparin (LMWH). The present study investigates if the physiological deterioration of renal function associated with normal aging or the presence of an acute venous thromboembolism influences the pharmacodynamic pattern of the anti-factor Xa and anti-thrombin activities. Three groups of 12 subjects were investigated. The first 2 groups were composed of healthy volunteers differing by age (25 ± 4 and 65 ± 3 yrs) and creati-nine clearance (114 ± 15 and 62 ± 6 ml · min –1). The third group was composed of patients hospitalized for deep vein thrombosis, having a mean age of 65 ± 11 yrs and creatinine clearance of 76 ± 8 ml · min –1. Nadroparin was administered subcutaneously once daily at the dose of 180 anti-factor Xa IU.kg–1 for 6 to 10 days. Serial sampling on day 1 and on the last day of administration (day n) allowed the pharmacodynamic parameters of the anti-factor Xa and anti-thrombin activities to be compared at the begining and at the end of the treatment. The main findings were the following: (1) After repeated administration, a significant accumulation of the anti-factor Xa activity was observed in the healthy elderly and in the patients but not in the healthy young subjects (accumulation factor: 1.3). There was no evidence of accumulation of anti-thrombin activity; (2) There were significant correlations between the clearance of creatinine and the clearance of the anti-factor Xa activity but not with that of the anti-thrombin activity; (3) In the patients, the clearance of the anti-factor Xa and of the anti-thrombin activities were 1.4 and 2 times higher respectively than those calculated in the healthy elderly; (4) The mean ratio of the of anti-factor Xa and anti-thrombin clearances was close to 2 in the healthy subjects but equal to 5.4 in the patients. These results suggest that the mechanisms involved in the clearance of polysaccharide chains which support the anti-thrombin activity are different from those of the anti-factor Xa activity and that the enhanced binding properties of plasma proteins to unfractionated heparin reported in patients presenting an acute venous thromboembolism also exists for LMWH, predominantly for the anti-thrombin activity.

Microparticles from apoptotic monocytes induce transient platelet recruitment and tissue factor expression by cultured human vascular endothelial cells via a redox-sensitive mechanism

Circulating microparticles derived from different types of blood cells have been reported to impair endothelial function and to induce pro-inflammatory and prothrombotic endothelial phenotypes. Although the number of monocyte-derived microparticles (M-MPs) is elevated in the blood of patients with various inflammatory conditions, their interaction with endothelial cells has been poorly investigated so far. In this study, we produced microparticles in vitro from apoptotic human monocytes and examined the effects of their interaction with cultured human umbilical vascular endothelial cells (HUVECs). We found that low concentrations of M-MPs induced the production of reactive oxygen species (ROS), mainly anion superoxide, by the endothelial cells. At sub-toxic concentrations, M-MPs induced a rapid expression of von Willebrand factor at the cell surface, which mediated the transient attachment of non-activated platelets to the endothelium in flow conditions. In parallel, M-MPs up-regulated the expression of functional tissue factor by the endothelial cells. ROS controlled these two major changes and the process involved the phosphorylation of p38 mitogen activated protein kinase. We conclude that M-MPs may contribute to thrombotic events by producing redox signalling in endothelial cells.

Comparison of three activated protein C resistance tests in the risk assessment of venous thrombosis in non-carriers of the factor V Leiden mutation

Activated protein C resistance (APCR), measured using the original assay described by Dahlbäck, is a risk factor for venous thrombosis independent of the factor V Leiden (FVL) mutation. This assay is based on the activated partial thromboplastin time (APTT) after plasma exposure to activated protein C (APC).As this assay was sensitive to numerous interferences, new assays have been developed for FVL screening. The objectives of the study were to investigate the association of second generation assays for APCR with venous thrombosis in FVL non-carriers. One hundred ninety-seven subjects with a history of venous thrombosis and 211 controls were explored using 3 APCR assays, the original APTT-based assay (test A), an APTT-based assay with factorV depleted plasma pre-dilution (test B) and a direct factorX activation-based assay with the same pre-dilution (test C).We found that subjects with results in the lowest quartile of the APTT-based assays are at increased risk, compared to those in the highest quartile (test A Odds Ratio = 6.39; 95%CI 3.23–12.63; test B OR=2.72; 95%CI 1.50–4.94). There was no significant risk increase associated with test C results. After adjusting for FVIII levels, the ORs of tests A and B were similar (test A OR=3.22; 95%CI 1.47–7.08; test B OR=3.10; 95%CI 1.54–6.21). In conclusion, APTT-based assays, but not direct factor X activation-based assays, effectively detect the risk for venous thrombosis independent of FVL. Pre-dilution in factor V depleted plasma is an effective way to directly assess the risk independent of FVIII levels.

Antiphosphatidylethanolamine antibodies are associated with an increased odds ratio for thrombosis

A multicenter study was set up to evaluate the prevalence, clinical and biological significance of antiphosphatidylethanolamine antibodies (aPE) in thrombotic patients with or without the main known clinical and biological risk factors for thrombosis. APE and antibodies, defined as the laboratory criteria of antiphospholipid syndrome (APS) -lupus anticoagulant, anticardiolipin and anti-beta2-GPI antibodies were measured in 270 patients with thrombosis (234 venous and 37 arterial) and 236 matched controls. APE were found in 15% of thrombotic patients compared to 3% of controls (p<0.001) with no predominant isotype, no association with the main known clinical or biological risk factors for thrombosis neither with a type of thrombosis, arterial or venous. In a multivariate logistic regression analysis of antibodies, aPE showed the highest association with thrombosis (odds ratio [OR]: 4.2, p<0.001). Moreover, using a multivariate analysis in a case-control subgroup study on 158 patients, IgGaPE were found to be significantly associated with venous thrombosis (OR:6;p=0.005). Interestingly, 25 of the 40 aPE-positive patients (63%) were negative for the APS laboratory criteria. Most of them (21/25) had venous thrombosis, recurrent in ten of them. Four patients also suffered from early or late miscarriages. Our results underline the strength of the association between the presence of aPE and thrombosis and suggest their measurement in thrombotic patients, especially when lupus anticoagulant, anticardiolipin or anti-beta2-GPI antibodies are absent.

Management of direct oral anticoagulants in patients undergoing elective surgeries and invasive procedures: Updated guidelines from the French Working Group on Perioperative Hemostasis (GIHP) - September 2015

Since 2011, data on patients exposed to direct oral anticoagulants (DOAs) while undergoing invasive procedures have accumulated. At the same time, an increased hemorrhagic risk during perioperative bridging anticoagulation without thrombotic risk reduction has been demonstrated. This has led the GIHP to update their guidelines published in 2011. For scheduled procedures at low bleeding risk, it is suggested that patients interrupt DOAs the night before irrespective of type of drug and to resume therapy six hours or more after the end of the invasive procedure. For invasive procedures at high bleeding risk, it is suggested to interrupt rivaroxaban, apixaban and edoxaban three days before. Dabigatran should be interrupted according to the renal function, four days and five days if creatinine clearance is higher than 50mL/min and between 30 and 50mL/min, respectively. For invasive procedures at very high bleeding risk such as intracranial neurosurgery or neuraxial anesthesia, longer interruption times are suggested. Finally, bridging with parenteral anticoagulation and measurement of DOA concentrations can no longer routinely be used.

Spotlight on idarucizumab and its potential for the reversal of anticoagulant effects of dabigatran

Idarucizumab is the first targeted antidote of dabigatran, a direct oral anticoagulant used for prevention and treatment of venous thromboembolism and prevention of stroke in atrial fibrillation. Idarucizumab is a humanized fragment of a monoclonal antibody, which binds dabigatran reversibly with high affinity and, when administered intravenously, immediately neutralizes its anticoagulant effect. It is rapidly cleared by the kidney with captured dabigatran. In Phase I and II trials, no significant adverse events have been reported. Specifically, idarucizumab has no anticoagulant or procoagulant effect by itself. Idarucizumab is currently being evaluated in an ongoing Phase III trial, in patients treated with dabigatran presenting with severe active bleeding or requiring emergency surgery or an invasive procedure and are at high risk of bleeding. The results of the interim analysis confirm the ability of idarucizumab to neutralize dabigatran instantaneously, without rebound effect, except in rare patients with very high baseline levels of anticoagulant. Although not definitely proving clinical efficacy, due to the noncontrolled design of the trial and the heterogeneity of patient conditions, these promising results on an intermediate criterion with strong rationale have led to the approval of idarucizumab for these indications. However, several questions are unresolved. First, activity measurement of dabigatran in blood, useless in current practice, could be useful to guide the treatment and avoid over- or underutilization of the antidote; but so far, it has not been largely available in real time. Second, the translation of anticoagulant neutralization to an effect on mortality and better outcome is highly dependent on the global management of these patients, especially rapid diagnosis, supportive care, and easy access to antidote administration. Although idarucizumab represents a remarkable achievement in drug design and development, whether it will be an important step toward improved safety of patients treated with dabigatran in the real world will have to be demonstrated in the postmarketing phase.

A diagnostic approach that may help to discriminate inherited thrombocytopenia from chronic immune thrombocytopenia in adult patients

Inherited thrombocytopenia (IT) is a heterogeneous group of rare diseases that are often confused with immune thrombocytopenia (ITP). The objective of this study was to supply clinicobiological elements that allow a distinction to be drawn between IT and chronic ITP. We then compared 23 adult patients with IT and 9 patients with chronic ITP. Our study revealed six discriminating criteria: (i) an age of discovery <34 years: positive predictive value (PPV) = 88.2% [63.6; 98.5], (ii) a family history of thrombocytopenia: PPV = 100.0% [82.4; 100.0], (iii) a personal history of bleeding: PPV = 100% [76.8; 100.0], (iv) a mean platelet volume >11 fL: PPV = 93.3% [68.1; 99.8], (v) an excess of giant platelets on blood smear: 100.0% [76.8; 100.0], and (vi) a percentage >44% of platelets with a surface area >4 µm(2) in electron microscopy: PPV = 83.3% [58.6; 96.4]. If at least three of these criteria were combined, it was possible to distinguish IT from chronic ITP with 91.3% [72.0; 98.9] sensitivity and PPV = 100.0% [66.4; 100.0] specificity. The secondary objective of this study was to assess the prevalence of potential IT diagnosis in patients with chronic thrombocytopenia of uncertain origin. Applying our diagnostic approach to a series of 20 cases allowed us to estimate that 40% of them could be suffering from IT. Finally, our diagnostic approach may help to correctly distinguish IT from chronic ITP, particularly in the context of macrothrombocytopenia.

ATP-binding cassette transporter 1 (ABCA1) deficiency decreases platelet reactivity and reduces thromboxane A2 production independently of hematopoietic ABCA1

ESSENTIALS: The role of ATP-binding cassette transporter 1 (ABCA1) in platelet functions is poorly characterized. We studied the impact of ABCA1 deficiency on platelet responses in a mouse model and two Tangier patients. ABCA1-deficient platelets exhibit reduced positive feedback loop mechanisms. This reduced reactivity is dependent on external environment and independent of hematopoietic ABCA1.

SUMMARY

BACKGROUND

The ATP-binding cassette transporter ABCA1 is required for the conversion of apolipoprotein A-1 to high-density lipoprotein (HDL), and its defect causes Tangier disease, a rare disorder characterized by an absence of HDL and accumulation of cholesterol in peripheral tissues. The role of ABCA1 in platelet functions remains poorly characterized.

OBJECTIVE

To determine the role of ABCA1 in platelet functions and to clarify controversies concerning its implication in processes as fundamental as platelet phosphatidylserine exposure and control of platelet membrane lipid composition.

METHODS AND RESULTS

We studied the impact of ABCA1 deficiency on platelet responses in a mouse model and in two Tangier patients. We show that platelets in ABCA1-deficient mice are slightly larger in size and exhibit aggregation and secretion defects in response to low concentrations of thrombin and collagen. These platelets have normal cholesterol and major phospholipid composition, granule morphology, or calcium-induced phosphatidylserine exposure. Interestingly, ABCA1-deficient platelets display a reduction in positive feedback loop mechanisms, particularly in thromboxane A2 (TXA2) production. Hematopoietic chimera mice demonstrated that defective eicosanoids production, particularly TXA2, was primarily dependent on external environment and not on the hematopoietic ABCA1. Decreased aggregation and production of TXA2 and eicosanoids were also observed in platelets from Tangier patients.

CONCLUSIONS

Absence of ABCA1 and low HDL level induce reduction of platelet reactivity by decreasing positive feedback loops, particularly TXA2 production through a hematopoietic ABCA1-independent mechanism.

The class I phosphoinositide 3-kinases α and β control antiphospholipid antibodies-induced platelet activation

Antiphospholipid syndrome (APS) is an autoimmune disease characterised by the presence of antiphospholipid antibodies (aPL) associated with increased thrombotic risk and pregnancy morbidity. Although aPL are heterogeneous auto-antibodies, the major pathogenic target is the plasma protein β2-glycoprotein 1. The molecular mechanisms of platelet activation by aPL remain poorly understood. Here, we explored the role of the class IA phosphoinositide 3-kinase (PI3K) α and β isoforms in platelet activation by aPL. Compared to control IgG from healthy individuals, the IgG fraction isolated from patients with APS potentiates platelet aggregation induced by low dose of thrombin in vitro and increases platelet adhesion and thrombus growth on a collagen matrix under arterial shear rate through a mechanism involving glycoprotein Ib (GPIb) and Toll Like Receptor 2 (TLR-2). Using isoforms-selective pharmacological PI3K inhibitors and mice with megakaryocyte/platelet lineage-specific inactivation of class IA PI3K isoforms, we demonstrate a critical role of the PI3Kβ and PI3Kα isoforms in platelet activation induced by aPL. Our data show that aPL potentiate platelet activation through GPIbα and TLR-2 via a mechanism involving the class IA PI3Kα and β isoforms, which represent new potential therapeutic targets in the prevention or treatment of thrombotic events in patients with APS.

Expanding the Mutation Spectrum Affecting αIIbβ3 Integrin in Glanzmann Thrombasthenia: Screening of the ITGA2B and ITGB3 Genes in a Large International Cohort

We report the largest international study on Glanzmann thrombasthenia (GT), an inherited bleeding disorder where defects of the ITGA2B and ITGB3 genes cause quantitative or qualitative defects of the αIIbβ3 integrin, a key mediator of platelet aggregation. Sequencing of the coding regions and splice sites of both genes in members of 76 affected families identified 78 genetic variants (55 novel) suspected to cause GT. Four large deletions or duplications were found by quantitative real-time PCR. Families with mutations in either gene were indistinguishable in terms of bleeding severity that varied even among siblings. Families were grouped into type I and the rarer type II or variant forms with residual αIIbβ3 expression. Variant forms helped identify genes encoding proteins mediating integrin activation. Splicing defects and stop codons were common for both ITGA2B and ITGB3 and essentially led to a reduced or absent αIIbβ3 expression; included was a heterozygous c.1440-13_c.1440-1del in intron 14 of ITGA2B causing exon skipping in seven unrelated families. Molecular modeling revealed how many missense mutations induced subtle changes in αIIb and β3 domain structure across both subunits, thereby interfering with integrin maturation and/or function. Our study extends knowledge of GT and the pathophysiology of an integrin.

Platelet Adhesion and Thrombus Formation in Whole Blood at Arterial Shear Rate at the End of Pregnancy

PROBLEM

Platelet reactivity has not been evaluated in integrated functional testing during normal pregnancy. Here, we analysed platelet functions under arterial shear rate in comparison with static conditions.

METHOD OF STUDY

Thirty pregnant women with uncomplicated pregnancies and 30 healthy non-pregnant women were enrolled in this study. Platelet adhesion to collagen and fibrinogen and subsequent thrombus formation were measured at arterial shear rate in whole blood using a microfluidic and imaging system. Standard light transmission aggregometry, flow cytometry of activation markers in washed platelets and impedance aggregometry in whole blood were also used to assess platelet responsiveness in static conditions.

RESULTS

Compared to non-pregnant controls, thrombus formation on collagen fibres and firm platelet adhesion on fibrinogen under arterial shear rate were significantly reduced in pregnant women. Platelet aggregometry assays in suspension showed a slight increase in platelet reactivity in pregnant women.

CONCLUSION

While platelet aggregometry and platelet activation markers in static conditions show little changes in platelet reactivity, monitoring of platelet adhesion and thrombus growth on collagen or fibrinogen under flow condition in whole blood indicates a significant decrease in pregnant women compared to controls. This decrease might contribute to counteract a hypercoagulable state and to reduce the risk of arterial thrombosis.

[Laboratory coagulation tests in patients treated by direct oral anticoagulants (DOACs)]

Routine clotting time assays (Prothrombin Time/INR, activated Partial Thromboplastin Time [aPTT]) are prolonged at variable extent by direct oral anticoagulants (DOAC), according to the assay, the reagent and the type of DOAC. These assays are not reliable for monitoring the intensity of treatment and the measurement of the plasma level of the DOAC is usually not required. At high concentrations, DOAC interfere with the routine clotting assays, making them difficult to interpret. In critical situations such as major bleeding or urgent invasive procedure, the measurement of DOAC level and its kinetics are simple and useful to manage the patient.

Platelet activation and arterial peripheral serotonin turnover in cardiac remodeling associated to aortic stenosis

Peripheral serotonin (5-HT) has been involved in adverse cardiac remodeling and valve fibrosis. The peripheral levels of 5-HT mainly depend on its release from activated platelets and degradation by monoamine oxidase A (MAO-A). The SERAOPI study investigated the relationship between arterial serotoninergic system, degree of platelet activation and cardiac remodeling, in patients with aortic valve stenosis (AS). Thirty patients with severe AS and 15 control subjects underwent transthoracic echocardiography, radial, and aortic arterial blood sampling. Measurements of 5-HT and its MAO-A-dependent degradation product, 5-HIAA, were performed by HPLC. Arterial platelet activation was assessed by flow cytometry analysis of platelet surface expression of P-selectin and activated integrin GPIIb/IIIa. Activated platelets and arterial plasma 5-HT increased in AS patients as compared to control subjects (P-selectin 1.08 ± 0.2MFI vs. 0.49 ± 0.1MFI, P = 0.04; GPIIb/IIIa 0.71 ± 0.1MFI vs. 0.35 ± 0.1MFI; P = 0.0015 and arterial plasma 5-HT 11.55 ± 1.6 nM vs. 6.18 ± 0.7 nM, P = 0.028, respectively). Moreover, 5-HT was strongly correlated to left ventricular hypertrophy assessed by echocardiography. The correlation was independent of cardiovascular risk comorbidities and others echocardiographic AS parameters. Finally, plasma 5-HIAA increased in AS patients (74.64 ± 9.7 nM vs. 37.16 ± 4.1 nM; P = 0.0002) indicating a higher 5-HT degradation rate by MAO-A. Platelet activation, arterial circulating serotonin, and serotonin degradation increased in patients with AS. These observations suggest that the serotoninergic system may contribute to the pathogenesis of AS including valve fibrosis and adverse ventricular remodeling.

Management of major bleeding complications and emergency surgery in patients on long-term treatment with direct oral anticoagulants, thrombin or factor-Xa inhibitors: proposals of the working group on perioperative haemostasis (GIHP) - March 2013

Direct new oral anticoagulants (NOACs) - inhibitors of thrombin or factor Xa - are intended to be used largely in the treatment of venous thromboembolic disease or the prevention of systematic embolism in atrial fibrillation, instead of vitamin K antagonists. Like any anticoagulant treatment, they are associated with spontaneous or provoked haemorrhagic risk. Furthermore, a significant proportion of treated patients are likely to be exposed to emergency surgery or invasive procedures. Given the absence of a specific antidote, the action to be taken in these situations must be defined. The lack of data means that it is only possible to issue proposals rather than recommendations, which will evolve according to accumulated experience. The proposals presented here apply to dabigatran (Pradaxa(®)) and rivaroxaban (Xarelto(®)); data for apixaban and edoxaban are still scarce. For urgent surgery with haemorrhagic risk, the drug plasma concentration should be less or equal to 30ng/mL for dabigatran and rivaroxaban should enable surgery associated with a high bleeding risk. Beyond that, if possible, the intervention should be postponed by monitoring the drug concentration. The course to follow is then defined according to the NOAC and its concentration. If the anticoagulant dosage is not immediately available, worse propositions, based on the usual tests (prothrombin time and activated partial thromboplastin time), are presented. However, these tests do not really assess drug concentration or the risk of bleeding that depends on it. In case of serious bleeding in a critical organ, the effect of anticoagulant therapy should be reduced using a non-specific procoagulant drug as a first-line approach: activated prothrombin complex concentrate (aPCC) (FEIBA(®) 30-50U/kg) or non-activated PCC (50U/kg). In addition, for any other type of severe haemorrhage, the administration of a procoagulant drug, which is potentially thrombogenic in these patients, is discussed according to the NOAC concentration and the possibilities of mechanical haemostasis.

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.