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Background: Andexanet alfa (andexanet) is a modified human factor Xa (FXa) approved for anticoagulation reversal in patients with life-threatening bleeding treated with rivaroxaban or apixaban. Four-factor prothrombin complex concentrates (4F-PCCs) are approved for reversal of vitamin K antagonist-induced anticoagulation but not FXa inhibitors. The mechanism and effectiveness of 4F-PCCs for FXa inhibitor reversal are unclear., Objective: To investigate the mechanism and impact of 4F-PCCs on reversal of rivaroxaban and apixaban in vitro compared to andexanet., Methods: The effect of 4F-PCCs (or individual factors) on tissue factor-initiated thrombin generation (TF-TG) was evaluated in human plasma, with or without rivaroxaban or apixaban, and compared with andexanet under the same conditions., Results: In the TF-TG assay, 4F-PCC completely reversed warfarin anticoagulation. Andexanet normalized TF-TG over a wide range of apixaban and rivaroxaban concentrations tested (19-2000 ng/mL). However, 4F-PCC (or individual factors) was unable to normalize endogenous thrombin potential (ETP) or peak thrombin (Peak) in the presence of apixaban or rivaroxaban (75-500 ng/mL). TF-TG was only normalized by 4F-PCC at inhibitor concentrations <75 ng/mL (ETP) or <37.5 ng/mL (Peak). These data can be explained by the estimated thresholds of FXa activity required to support normal TF-TG based on the inhibitor:FXa ratios and levels of uninhibited FXa. The data are consistent with healthy volunteer studies where TF-TG is not normalized until inhibitor levels are substantially decreased., Conclusions: Both the theoretical calculations and experimental data demonstrated that 4F-PCCs are only able to normalize TG over a low and narrow range of FXa inhibitor concentrations (<75 ng/mL)., (© 2020 The Authors. Research and Practice in Thrombosis and Haemostasis published by Wiley Periodicals LLC on behalf of International Society on Thrombosis and Haemostasis.)

Background: Andexanet alfa (andexanet) reverses the anticoagulant effects of factor Xa inhibitors, but it has not been assessed in clinical studies for apixaban reversal in trauma. This study evaluated andexanet for reversing apixaban anticoagulation in a porcine polytrauma model., Methods: Oral apixaban (20 mg q.d., n=21) or placebo (n=7; sham group) was administered to male pigs for 4 days before blunt liver injury and bi-lateral femur fracture. After trauma, animals were randomised 1:1:1 to a single andexanet bolus (1000 mg), a bolus (1000 mg) plus infusion (1200 mg over 2 h), or vehicle (control). Haemodynamic and coagulation variables were monitored for 5 h or until death. The primary endpoint was blood loss., Results: Mean blood loss in sham animals was 472 (standard deviation, 58) ml 12 min after injury and 658 (98) ml at 300 min, with 100% survival. Anticoagulation with apixaban significantly increased blood loss 12 min after injury [888 (133) ml, P<0.01]. Controls exhibited total blood loss of 3403 (766) ml, with 100% mortality. Andexanet bolus or bolus plus infusion significantly reduced blood loss to 1264 (205) and 1202 (95) ml, respectively), and increased survival to 100%. Haemodynamic parameters and markers of shock recovered to pre-trauma levels in andexanet-treated animals., Conclusion: Andexanet effectively reversed apixaban anticoagulation and reduced blood loss induced by severe trauma. Andexanet bolus alone had a similar impact on survival and blood loss as bolus plus infusion. Therefore, a 2 h andexanet infusion after the bolus may not be necessary to restore normal haemostatic mechanisms., (Copyright © 2019 British Journal of Anaesthesia. Published by Elsevier Ltd. All rights reserved.)

Essentials Anticoagulants prevent venous thromboembolism but may be associated with greater bleeding risks. Bivariate analysis assumes a non-linear relationship between efficacy and safety outcomes. Extended full-dose betrixaban is favorable over standard enoxaparin in bivariate endpoint. Clinicians must weigh efficacy and safety outcomes in decision-making on thromboprophylaxis., Summary: Background Among acutely ill hospitalized medical patients, extended-duration thromboprophylaxis reduces the risk of venous thromboembolism (VTE), but some pharmacologic strategies have been associated with greater risks of major bleeding, thereby offsetting the net clinical benefit (NCB). Methods To assess the risk-benefit profile of anticoagulation regimens, a previously described bivariate method that does not assume a linear risk-benefit tradeoff and can accommodate different margins for efficacy and safety was performed to simultaneously assess efficacy (symptomatic VTE) and safety (major bleeding) on the basis of data from four randomized controlled trials of extended-duration (30-46 days) versus standard-duration (6-14 days) thromboprophylaxis among 28 227 patients (EXCLAIM, ADOPT, MAGELLAN and APEX trials). Results Extended thromboprophylaxis with full-dose betrixaban (80 mg once daily) was superior in efficacy and non-inferior in safety to standard-duration enoxaparin, and showed a significantly favorable NCB, with a risk difference of - 0.51% (- 0.89% to - 0.10%) in the bivariate outcome. Extended enoxaparin was superior in efficacy and inferior in safety (bivariate outcome: 0.03% [- 0.37% to 0.43%]), whereas apixaban and rivaroxaban were non-inferior in efficacy and inferior in safety (- 0.20% [- 0.49% to 0.17%] and 0.23% [- 0.16% to 0.69%], respectively). Reduced-dose betrixaban did not show a significant difference in either efficacy or safety (0.41% [- 0.85% to 1.94%]). Conclusions In a bivariate analysis that assumes non-linear risk-benefit tradeoffs, extended prophylaxis with full-dose betrixaban was superior to standard-duration enoxaparin, whereas other regimens failed to simultaneously achieve both superiority and non-inferiority with respect to symptomatic VTE and major bleeding in the management of acutely ill hospitalized medical patients., (© 2017 International Society on Thrombosis and Haemostasis.)

Essentials There is currently no approved reversal agent for factor Xa (FXa) inhibitors Andexanet alfa has been developed to reverse the anticoagulant effects of FXa inhibitors Andexanet reduced blood loss and anticoagulation markers in rivaroxaban-anticoagulated rabbits Andexanet was well tolerated in monkeys and rats, with no evidence of prothrombotic activity SUMMARY: Background Andexanet alfa is a recombinant modified form of factor Xa (FXa), designed to bind to and reverse the anticoagulant activity of FXa inhibitors. Objectives To evaluate the ability of andexanet to reverse the anticoagulant activity of rivaroxaban, and assess its pharmacokinetics (PK) and toxicity in animal models. Methods The effects of andexanet on blood loss, anti-FXa activity, rivaroxaban unbound plasma concentrations and other coagulation parameters were assessed in a rabbit liver laceration 'treatment' model. Andexanet was administered 10 min after blood loss was initiated. The toxicity of repeated administration of andexanet (up to 60 mg kg -1 day -1 ) was assessed in cynomolgus monkeys. PK parameters were evaluated in rats and monkeys. Results Excess blood loss due to anticoagulation with rivaroxaban was significantly decreased by a single intravenous bolus administration of andexanet at 35 and 75 mg per rabbit, by 75% and 63%, respectively. This correlated with dose-dependent decreases in the unbound fraction of rivaroxaban and anti-FXa activity. Co-administration of rivaroxaban had no significant impact on the PK parameters of andexanet. Andexanet (up to 60 mg kg -1 day -1 ) was well tolerated in monkeys, with no accumulation of andexanet or rivaroxaban. There was a single occurrence of anaphylaxis, which resolved after treatment with diphenhydramine and epinephrine. There was no histological evidence of prothrombotic activity with high-dose andexanet compared with vehicle control, as measured by clot and fibrin deposition in all major organs. Conclusions These data suggest that andexanet is a promising therapy for the reversal of FXa inhibitor-induced anticoagulation, supporting clinical studies in humans., (© 2017 International Society on Thrombosis and Haemostasis.)

Although current antiplatelet therapies provide potent antithrombotic effects, their efficacy is limited by a heightened risk of bleeding and failure to affect vascular remodeling after injury. New lines of research suggest that thrombosis and hemorrhage may be uncoupled at the interface of pathways controlling thrombosis and inflammation. Here, as one remarkable example, studies using a novel and highly selective pharmacologic inhibitor of the spleen tyrosine kinase Syk [PRT060318; 2-((1R,2S)-2-aminocyclohexylamino)-4-(m-tolylamino)pyrimidine-5-carboxamide] coupled with genetic experiments, demonstrate that Syk inhibition ameliorates both the acute and chronic responses to vascular injury without affecting hemostasis. Specifically, lack of Syk (murine radiation chimeras) attenuated shear-induced thrombus formation ex vivo, and PRT060318 strongly inhibited arterial thrombosis in vivo in multiple animal species while having minimal impact on bleeding. Furthermore, leukocyte-platelet-dependent responses to vascular injury, including inflammatory cell recruitment and neointima formation, were markedly inhibited by PRT060318. Thus, Syk controls acute and long-term responses to arterial vascular injury. The therapeutic potential of Syk may be exemplary of a new class of antiatherothrombotic agents that target the interface between thrombosis and inflammation.

Platelet aggregation is a dynamic entity, capable of directing its own growth and stability via the activation of signaling cascades that lead to the expression and secretion of various secondary agonists. Here we show that the signaling pathways triggered during platelet aggregation include an intrinsic pro-thrombotic activity mediated by 2 homophilic adhesion molecules, CD84 and CD150 (SLAM [signaling lymphocyte activation molecule]), which are tyrosine phosphorylated in a platelet aggregation-dependent fashion. The 2 CD84/SLAM adapter proteins, SAP (SLAM-associated protein) and EAT-2 (EWS-activated transcript-2), were found in platelets; only SAP, however, was found to immunoprecipitate with tyrosine-phosphorylated SLAM. The immobilized extracellular domain of CD84 promoted microaggregate formation, while SAP-deficient platelets demonstrated defective spreading on immobilized CD84, demonstrating a functional role in platelets for SLAM family interactions. Finally, analysis of SLAM-deficient mice revealed an overall defect in platelet aggregation in vitro and a delayed arterial thrombotic process in vivo. The data indicate that signaling of the adhesion molecules in the SLAM family, activated by proximity during aggregation, further stabilize platelet-platelet interactions in thrombosis.

The current standard of care for the treatment of arterial thrombosis includes anticoagulants and three classes of antiplatelet agents--aspirin, thienopyridines and glycoprotein IIb-IIIa antagonists. Although these drugs have had a significant impact on morbidity and mortality in several patient populations, up to 15% of the high risk patients with acute coronary syndrome continue to suffer from ischemic events. This problem may occur, in part, because the platelets in many patients are non-responsive to aspirin and clopidogrel. Murine models now indicate that platelets are not only responsible for arterial occlusion, they are also involved in the progression of atherosclerotic disease. New opportunities have emerged identifying potential targets and strategies for drug discovery suited to address these deficiencies by more effectively modulating platelet adhesion, thrombus growth, thrombus stability and the pro-inflammatory activity of platelets. In addition, a growing need has emerged for the development of bedside devices capable of bringing personalized medicine to patients being treated with antithrombotic drugs in order to measure the pharmacodynamic activities of new therapies, to assess the activities achieved by combined antithrombotic therapy, and to identify patients that fail to respond.

Glycoprotein (GP) VI, the primary collagen receptor on platelets, has been shown to have variable expression, possibly as a consequence of immune modulation. The present study was designed to determine the mechanism by which GP VI clearance occurs. We found that direct activation of GP VI both by a GP VI-specific antibody and by GP VI ligands (collagen and convulxin) reduced binding of biotinylated convulxin to the stimulated platelets. Analysis of immunoblots of platelets and supernatants showed that the stimulated platelets contained less GP VI, while the soluble fraction contained a 57-kDa cleavage product. Stimulation of platelets with PAR-1 agonists (TRAP peptide and thrombin) also caused GP VI cleavage, although the amount of GP VI loss was less than that observed with direct GP VI ligands. The metalloproteinase (MMP) inhibitors GM6001 and TAPI prevented both the clearance of GP VI from the platelet surface and the appearance of the soluble cleavage product. Induction of GP VI cleavage caused specific down-regulation of collagen-induced platelet aggregation, providing a mechanism for the modulation of platelet responsiveness to this important platelet agonist.