Effect of rivaroxaban and dabigatran on platelet functions: in vitro study

Consensus report on flow cytometry for platelet function testing in thrombocytopenic patients: communication from the SSC of the ISTH

BACKGROUND

Platelet count alone does not reliably predict bleeding risk, suggesting platelet function is important to monitor in patients with thrombocytopenia. There is still an unmet need for improved platelet function diagnostics in patients with low platelet count in many clinical situations. Flow cytometry is a promising tool allowing reliable platelet function study in this setting.

OBJECTIVES

The goal of this joint project between the International Society on Thrombosis and Haemostasis (ISTH) Scientific Standardization Committee (SSC) Subcommittees on Platelet Physiology and Platelet Immunology is to provide expert consensus guidance on the use of flow cytometry for the evaluation of platelet function, particularly activation, in patients with low platelet counts.

METHODS

A literature review was performed to identify relevant questions and areas of interest. An electronic expression of interest form was thereafter announced on the ISTH webpage, followed by a survey encompassing 37 issues regarding preanalytical, analytical, postanalytical, and performance aspects. Areas of disagreement or uncertainty were identified and formed the basis for 2 focus group discussions.

RESULTS

Consensus recommendations relative to patient sample collection, preanalytical variables, sample type, platelet-count cutoff, any potential specific modification of the standard flow cytometry protocol, and results expression and reporting are proposed based on the current practices of experts in the field as well as on literature review.

CONCLUSION

The proposed consensus recommendations would allow standardization of protocols in upcoming clinical studies. The clinical utility of platelet function testing using flow cytometry to predict bleeding risk still needs rigorous multicenter outcome studies in patients with thrombocytopenia.

Antiplatelet Aggregation Properties of Cirsilineol: A Novel Inhibitor of Blood Coagulation Factor Xa

A small natural substance called cirsilineol (CSL), which was discovered in the plant Artemisia vestita, is lethal to many cancer cells and has antioxidant, anticancer, and antibacterial properties. Here, we investigated the underlying mechanisms of the antithrombotic action of CSL. We demonstrated that CSL has antithrombotic efficacy comparable to rivaroxaban, a direct blood coagulation factor Xa (FXa) inhibitor employed as a positive control, in inhibiting the enzymatic activity of FXa and the platelet aggregation induced by adenosine diphosphate (ADP) and U46619, a thromboxane A2 analog. The expression of P-selectin, the phosphorylation of myristoylated alanine-rich C kinase substrate by U46619 or ADP, and the activation of PAC-1 in platelets were inhibited by CSL. Nitric oxide production was increased by CSL in ADP- or U46619-treated human umbilical vein endothelial cells (HUVECs), although excessive endothelin-1 secretion was suppressed. CSL demonstrated strong anticoagulant and antithrombotic effects in a mouse model of arterial and pulmonary thrombosis. Our findings suggest that CSL is a potential pharmacological candidate for a novel class of anti-FXa and antiplatelet medications.

Immunothrombosis and the Role of Platelets in Venous Thromboembolic Diseases

Venous thromboembolism (VTE) is the third leading cardiovascular cause of death and is conventionally treated with anticoagulants that directly antagonize coagulation. However, recent data have demonstrated that also platelets play a crucial role in VTE pathophysiology. In the current review, we outline how platelets are involved during all stages of experimental venous thrombosis. Platelets mediate initiation of the disease by attaching to the vessel wall upon which they mediate leukocyte recruitment. This process is referred to as immunothrombosis, and within this novel concept inflammatory cells such as leukocytes and platelets directly drive the progression of VTE. In addition to their involvement in immunothrombosis, activated platelets can directly drive venous thrombosis by supporting coagulation and secreting procoagulant factors. Furthermore, fibrinolysis and vessel resolution are (partly) mediated by platelets. Finally, we summarize how conventional antiplatelet therapy can prevent experimental venous thrombosis and impacts (recurrent) VTE in humans.

Inhibition of factor Xa activity, platelet aggregation, and experimentally induced thrombosis by Sparstolonin B

BACKGROUND

Sparstolonin B (SsnB) is an isocumarin compound extracted from medicinal plants such as Sparganium stoloniferum and Scirpus yagara with well documented anti-inflammatory activity. Here we examined if SsnB also possesses antithrombotic activity and the underlying mechanisms.

METHODS

Anti-thrombotic effects of SsnB were determined by measuring in vitro/ex vivo/in vivo clotting times, platelet aggregation assay, production and activity of factor Xa, nitric oxide, and expressions of relative proteins.

RESULTS

Treatment with SsnB prolonged the clotting time of human platelet-poor serum at concentrations comparable to the clinical anticoagulant rivaroxaban (as a positive control) and inhibited human platelet aggregation induced by adenosine diphosphate (ADP) or the thromboxane A2 analog U46619. SsnB also inhibited U46619-induced and ADP-induced phosphorylation of phospholipase C (PLC)γ2/protein kinase C (PKC) and intracellular calcium mobilization, both of which are required for platelet aggregation. In addition, SsnB inhibited expression of the cell adhesion factors P-selectin and PAC-1. SsnB increased production of the vasodilator nitric oxide and suppressed secretion of the vasoconstrictor endothelin-1 from ADP- or U46619-treated human umbilical vein endothelial cells. Further, SsnB reduced coagulation factor Xa (FXa) catalytic activity and production by endothelial cells as well as FXa-induced platelet aggregation.

CONCLUSION

Finally, SsnB injection reduced thrombus formation time, number, size, and related mortality in mouse models of thromboembolism. SsnB is a promising antithrombotic agent targeting both FXa and platelet aggregation pathways, which can overcome the side effects of existing antithrombotic agents.

Factors Associated with Platelet Activation-Recent Pharmaceutical Approaches

Platelets are at the forefront of human health and disease following the advances in their research presented in past decades. Platelet activation, their most crucial function, although beneficial in the case of vascular injury, may represent the initial step for thrombotic complications characterizing various pathologic states, primarily atherosclerotic cardiovascular diseases. In this review, we initially summarize the structural and functional characteristics of platelets. Next, we focus on the process of platelet activation and its associated factors, indicating the potential molecular mechanisms involving inflammation, endothelial dysfunction, and miRs. Finally, an overview of the available antiplatelet agents is being portrayed, together with agents possessing off-set platelet-inhibitory actions, while an extensive presentation of drugs under investigation is being given.

The Impact of Dabigatran and Rivaroxaban on Variation of Platelet Activation Biomarkers and DRT Following Percutaneous Left Atrial Appendage Closure

Background: The current post-procedure antithrombotic recommendation for left atrial appendage closure (LAAC) remains empiric. This study was designed to compare variations in platelet activation biomarkers and device-related thrombosis (DRT) under different antithrombotic regimens following LAAC. Methods: This study enrolled 105 consecutive patients with atrial fibrillation who underwent LAAC successfully and received post-procedure anticoagulation with either dabigatran (N = 33) or rivaroxaban (N = 72). After 3 months of anticoagulation treatment, thromboelastogram was used to evaluate thrombin receptor-activating peptide (TRAP)-induced platelet aggregation (PA). Measurements of platelet activation biomarkers, including thrombin-antithrombin complex (TAT), P-selectin, von Willebrand disease (vWF), and CD40L, were performed immediately before the LAAC procedure and after 3 months of post-procedure anticoagulation. Repeated transesophageal echocardiography was performed to evaluate DRT during follow-ups. Results: Three (4.2%) patients in the rivaroxaban and 4 (12.1%) patients in the dabigatran group experienced DRT events (odds ratio (OR) = 0.315, 95% confidence interval (95%CI): 0.066-1.489, p = 0.129) during follow-ups. The TRAP-induced PA was statistically significantly higher in the dabigatran group (62.9% vs 59.7%, p = 0.028*). Statistically significant increases in plasma concentration of TAT, P-selectin, and vWF were observed after 3 months of exposure to dabigatran when compared with rivaroxaban. An increased expression of platelet activation biomarkers was observed in DRT subjects compared with non-DRT subjects in terms of P-selectin and vWF (65.28 ± 13.93 ng/L vs 32.14 ± 12.11 ng/L, p = 0.037; 501.92 ± 106.48 U/L vs 280.98 ± 54.10 U/L, p = 0.045; respectively). Multivariate regression analysis indicated that the use of dabigatran might be an independent predictor of DRT (p = 0.022; OR = 4.366, 95%CI: 0.434-10.839). Furthermore, the CHA2DS2-VASc score (OR = 2.076, p = 0.016) and CD40L levels (OR = 1.015, p = 0.021) were independent predictors of increased D-dimer levels. Conclusions: Post-LAAC anticoagulation with dabigatran may increase the risk of DRT by enhancing platelet reactivity. In light of this potential increased risk in DRT, the authors recommend against using dabigatran for post-procedural anticoagulation in patients who have undergone LAAC.

2021 Update of the International Council for Standardization in Haematology Recommendations for Laboratory Measurement of Direct Oral Anticoagulants

In 2018, the International Council for Standardization in Haematology (ICSH) published a consensus document providing guidance for laboratories on measuring direct oral anticoagulants (DOACs). Since that publication, several significant changes related to DOACs have occurred, including the approval of a new DOAC by the Food and Drug Administration, betrixaban, and a specific DOAC reversal agent intended for use when the reversal of anticoagulation with apixaban or rivaroxaban is needed due to life-threatening or uncontrolled bleeding, andexanet alfa. In addition, this ICSH Working Party recognized areas where additional information was warranted, including patient population considerations and updates in point-of-care testing. The information in this manuscript supplements our previous ICSH DOAC laboratory guidance document. The recommendations provided are based on (1) information from peer-reviewed publications about laboratory measurement of DOACs, (2) contributing author's personal experience/expert opinion and (3) good laboratory practice.

A case-control study indicates that coagulation imbalance is associated with arteriosclerosis and markers of endothelial dysfunction in kidney failure

Endothelial dysfunction, one of many causes of arterial changes in end-stage kidney disease (kidney failure), is a likely link between early vascular aging and the risk of thrombosis or bleeding in this condition. To evaluate this, we compared links between arterial stiffness and endothelial/coagulation factors in 55 patients receiving hemodialysis therapy and 57 age-/sex-matched control individuals. Arterial stiffness was assessed from carotid-femoral pulse wave velocity, and coagulation status from the endogenous thrombin generating potential. Markers of endothelial dysfunction (von Willebrand factor, tissue factor pathway inhibitor), neutrophil extracellular traps and tissue factor-positive extracellular vesicles were higher in patients with kidney failure. Prothrombin fragments 1 and 2, and D-dimer markers of in vivo coagulation activation were also higher. However, in vitro in the presence of platelets, endogenous thrombin generating potential was lower and its downregulation by activated protein C impaired. Antiplatelet drugs did not affect these parameters. In multiple regression analysis, prothrombin fragments 1 and 2, D-dimer, factor VIII and monocyte-derived tissue factor-positive extracellular vesicles correlated with higher carotid-femoral pulse wave velocity. In patients with kidney failure, in vivo hypercoagulability occurred with reduced thrombin generation in platelet-rich plasma, likely explaining the opposing thrombotic and bleeding tendencies in patients with kidney failure. Importantly, arteriosclerosis is more closely related to a prothrombotic state. Thus, coagulation changes plus arterial stiffness highlight a major therapeutic challenge for anticoagulant and antiplatelet drug use.

Novel factor Xa inhibitor, maslinic acid, with antiplatelet aggregation activity

As antithrombotic effects of maslinic acid (MA) have not yet been studied, MA-mediated downregulation of coagulation factor Xa (FXa) and platelet aggregation was studied. We show that MA inhibited the enzymatic activity of FXa and platelet aggregation, induced by adenosine diphosphate (ADP) and a thromboxane A₂ (TXA₂ ) analog, U46619 with a similar antithrombotic efficacy to rivaroxaban, a direct FXa inhibitor used as a positive control. Mechanistically, MA suppressed U46619- or ADP-induced phosphorylation of myristoylated alanine-rich C kinase substrate, and the expression of P-selectin, and activated PAC-1 in platelets. MA increased generation of nitric oxide, but downregulated excessive secretion of endothelin-1 in ADP- or U46619-treated human umbilical vein endothelial cells. In arterial and pulmonary thrombosis mouse model, MA showed prominent anticoagulant and antithrombotic effects. Our data suggest MA as a candidate molecule for a new class of drugs targeting anti-FXa and antiplatelet.