Blood coagulation factor Xa as an emerging drug target

Ligand-Based Design of Selective Peptidomimetic uPA and TMPRSS2 Inhibitors with Arg Bioisosteres

Trypsin-like serine proteases are involved in many important physiological processes like blood coagulation and remodeling of the extracellular matrix. On the other hand, they are also associated with pathological conditions. The urokinase-pwlasminogen activator (uPA), which is involved in tissue remodeling, can increase the metastatic behavior of various cancer types when overexpressed and dysregulated. Another member of this protease class that received attention during the SARS-CoV 2 pandemic is TMPRSS2. It is a transmembrane serine protease, which enables cell entry of the coronavirus by processing its spike protein. A variety of different inhibitors have been published against both proteases. However, the selectivity over other trypsin-like serine proteases remains a major challenge. In the current study, we replaced the arginine moiety at the P1 site of peptidomimetic inhibitors with different bioisosteres. Enzyme inhibition studies revealed that the phenylguanidine moiety in the P1 site led to strong affinity for TMPRSS2, whereas the cyclohexylguanidine derivate potently inhibited uPA. Both inhibitors exhibited high selectivity over other structurally similar and physiologically important proteases.

Low dose rivaroxaban for the management of atherosclerotic cardiovascular disease

Atherosclerotic cardiovascular disease is characterized by some risk of major adverse events despite the availability of effective medical therapies for secondary prevention. There is emerging evidence suggesting that thrombin partly contributes to this residual risk. In fact, thrombin (i.e., activated coagulation factor II) triggers not only the conversion of fibrinogen to fibrin but also platelet activation and various pathways responsible for pro-atherogenic and/or pro-inflammatory effects through interaction with protease activated receptors. To reduce the risk associated with thrombin activation, oral anticoagulants antagonists of vitamin K showed promise, but were associated with unacceptable bleeding rates. Direct oral anticoagulants targeting the activated factors X and II carry a lower risk of bleeding than vitamin K antagonists. Rivaroxaban, a direct inhibitor of activated factor X approved at the dose of 20 mg once daily for the prevention of thromboembolic events, has been also investigated at a reduced dose of 2.5 mg twice daily in several alternative scenarios of atherosclerotic cardiovascular disease, in combination with standard of care. Current guidelines recommend that low-dose rivaroxaban is given in an adjunct to standard therapy to patients with stable atherosclerosis and acute coronary syndromes at low bleeding risk. Several studies are underway to evaluate its putative benefits in other clinical settings.

Activated Factor X Signaling Pathway via Protease-Activated Receptor 2 Is a Novel Therapeutic Target for Preventing Atrial Fibrillation

BACKGROUND

Activated factor X (FXa), which contributes to chronic inflammation via protease-activated receptor 2 (PAR2), might play an important role in atrial fibrillation (AF) arrhythmogenesis. This study aimed to assess whether PAR2 signaling contributes to AF arrhythmogenesis and whether rivaroxaban ameliorates atrial inflammation and prevents AF.Methods and Results:In Study 1, PAR2 deficient (PAR2-/-) and wild-type mice were infused with angiotensin II (Ang II) or a vehicle via an osmotic minipump for 2 weeks. In Study 2, spontaneously hypertensive rats (SHRs) were treated with rivaroxaban, warfarin, or vehicle for 2 weeks after 8 h of right atrial rapid pacing. The AF inducibility and atrial remodeling in both studies were examined. Ang II-treated PAR2-/- mice had a lower incidence of AF and less mRNA expression of collagen1 and collagen3 in the atrium compared to wild-type mice treated with Ang II. Rivaroxaban significantly reduced AF inducibility compared with warfarin or vehicle. In SHRs treated with a vehicle, rapid atrial pacing promoted gene expression of inflammatory and fibrosis-related biomarkers in the atrium. Rivaroxaban, but not warfarin, significantly reduced expression levels of these genes.

CONCLUSIONS

The FXa-PAR2 signaling pathway might contribute to AF arrhythmogenesis associated with atrial inflammation. A direct FXa inhibitor, rivaroxaban, could prevent atrial inflammation and reduce AF inducibility, probably by inhibiting the pro-inflammatory activation.

Apixaban Downregulates Endothelial Inflammatory and Prothrombotic Phenotype in an In Vitro Model of Endothelial Dysfunction in Uremia

PURPOSE

Chronic kidney disease (CKD) associates with inflammatory and prothrombotic phenotypes, resulting in higher cardiovascular risk. Factor Xa displays functions beyond coagulation, exhibiting proinflammatory effects. The aim of the present study was to investigate whether a direct FXa inhibitor protects from the endothelial dysfunction (ED) caused by uremia.

METHODS

Macro (HUVEC) and microvascular (HMEC) endothelial cells (ECs) were exposed to serum from uremic patients or healthy donors, in absence and presence of apixaban (60 ng/ml). We evaluated changes in surface VCAM-1 and ICAM-1, intracellular eNOS, reactive oxygen species (ROS), and von Willebrand Factor (VWF) production by immunofluorescence, reactivity of the extracellular matrix (ECM) towards platelets, and intracellular signaling.

RESULTS

ECs exposed to uremic serum triggered dysregulation of all the parameters. Presence of apixaban resulted in decreased expression of VCAM-1 (178 ± 14 to 89 ± 2% on HMEC and 324 ± 71 to 142 ± 25% on HUVEC) and ICAM-1 (388 ± 60 to 111 ± 10% on HMEC and 148 ± 9% to 90 ± 7% on HUVEC); increased eNOS (72 ± 8% to 95 ± 10% on HMEC); normalization of ROS levels (173 ± 21 to 114 ± 13% on HMEC and 165 ± 14 to 127 ± 7% on HUVEC); lower production of VWF (168 ± 14 to 92 ± 4% on HMEC and 151 ± 22 to 99 ± 11% on HUVEC); and decreased platelet adhesion onto ECM (134 ± 22 to 93 ± 23% on HMEC and 161 ± 14 to 117 ± 7% on HUVEC). Apixaban inhibited p38MAPK and p42/44 activation in HUVEC (139 ± 15 to 48 ± 15% and 411 ± 66 to 177 ± 57%, respectively) (p < 0.05 vs control for all parameters).

CONCLUSION

Anti-FXa strategies, such as apixaban, prevented ED caused by the uremic milieu, exhibiting anti-inflammatory and antioxidant properties and modulating the reactivity of the ECM.

Safety of direct oral anticoagulant - and antiplatelet therapy in patients with atrial fibrillation treated by carotid artery stenting

BACKGROUND

The periprocedural administration of dual antiplatelet therapy has been recommended in patients treated by carotid artery stenting. However, some patients with concurrent disease have been prescribed anticoagulants. We compared the post-operative incidence of hemorrhagic and thromboembolic events in two patient groups treated by different regimens of multi-antithrombotic agents.

METHODS

As our 31 patients had a history of nonvalvular atrial fibrillation, they had received anticoagulants; they were also treated with aspirin and clopidogrel before carotid artery stenting. The prior anticoagulant therapy was continued in 17 patients and they received vitamin K antagonist plus dual antiplatelet therapy after the procedure (group 1). Other 14 patients underwent direct oral anticoagulant plus aspirin or clopidogrel (group 2). Post-procedural hemorrhagic and thromboembolic events were compared between two groups.

RESULTS

Carotid artery stenting was angiographically successful in all patients. Complications were encountered in two group 1 patients. Post-operative image revealed a silent subarachnoid hemorrhage in one. The other presented with superior mesenteric artery occlusion 6 months after the procedure. No hemorrhagic or thromboembolic events occurred in group 2.

CONCLUSION

We concluded that the administration of a direct oral anticoagulant plus an antiplatelet agent reduced the risk for periprocedural hemorrhagic and embolic events in patients with concurrent nonvalvular atrial fibrillation who underwent carotid artery stenting.

Effects of rivaroxaban and dabigatran on local expression of coagulation and inflammatory factors within human aortic stenotic valves

BACKGROUND

Treatment with non-vitamin K antagonist oral anticoagulants (NOACs) such as dabigatran (a direct thrombin inhibitor) or rivaroxaban (a direct inhibitor of factor [F] Xa) attenuates atherosclerotic plaque progression in hypercholesterolemic mice.

PURPOSE

To evaluate the effect of NOACs application on the expression of coagulation proteins in loco within stenotic aortic valves and in valve interstitial cells (VICs) from patients with severe aortic stenosis (AS).

METHODS

Primary cultures of VICs obtained from 90 patients undergoing aortic valve replacement were stimulated with TNF-α (50 ng/mL) and pre-treated with rivaroxaban (1 and 10 ng/mL) or dabigatran (25 and 250 ng/mL). The expression of coagulation proteins was analyzed by immunofluorescence. Cytokine levels were measured by ELISA.

RESULTS

FX, FXa, FVII, thrombin and PAR1/2 were present in loco within human aortic stenotic valves. Cultured VICs exhibited constant expression of FX, TF, PAR1/2. Exposure of VICs to TNF-α caused the upregulated expression of TF, PAR1/2 and induced expression of thrombin, FVII and FXa. FX was expressed by 80% of VICs, regardless of stimulation. Cultured VICs were able to synthesize metalloproteinases 1-3, IL-6, IL-32, IL-34, osteopontin and osteocalcin, the levels of which increased under TNF-α stimulation. NOACs added to culture inhibited coagulation factor and PAR1/2 expression. Moreover, NOACs down-regulated VIC-derived proteins responsible for valve calcification and extracellular matrix remodeling.

CONCLUSIONS

NOACs at therapeutic concentrations may inhibit the effects of FXa and thrombin at in vitro level. It might be speculated that long-term treatment with rivaroxaban or dabigatran could attenuate the progression of AS in humans.

Fondaparinux pentasaccharide reduces sepsis coagulopathy and promotes survival in the baboon model of Escherichia coli sepsis

BACKGROUND

Sepsis triggers dysfunction of coagulation and fibrinolytic systems leading to disseminated intravascular coagulation (DIC) that contributes to organ failure and death. Fondaparinux (FPX) is a synthetic pentasaccharide that binds to antithrombin (AT) and selectively inhibits factor (F) Xa and other upstream coagulation proteases but not thrombin (T).

OBJECTIVES

We used a baboon model of lethal Escherichia coli sepsis to investigate the effects of FPX treatment on DIC, organ function, and outcome.

METHODS

Two experimental groups were studied: (a) E. coli challenge (n = 4); and (b) E coli plus FPX (n = 4). Bacteremia was modeled by intravenous infusion of pathogen (1-2 × 1010  CFU/kg). Fondaparinux (0.08 mg/kg) was administered subcutaneously, 3 h prior to and 8 h after bacteria infusion.

RESULTS

Bacteremia rapidly increased plasma levels of inhibitory complexes of AT with coagulation proteases. Activation markers of both intrinsic (FXIa-AT), and extrinsic (FVIIa-AT) pathways were significantly reduced in FPX-treated animals. Factor Xa-AT and TAT complexes were maximal at 4 to 8 h post challenge and reduced >50% in FPX-treated animals. Fibrinogen consumption, fibrin generation and degradation, neutrophil and complement activation, and cytokine production were strongly induced by sepsis. All parameters were significantly reduced, while platelet count was unchanged by the treatment. Fondaparinux infusion attenuated organ dysfunction, prolonged survival, and saved two of four challenged animals (log-rank Mantel-Cox test, P = .0067).

CONCLUSION

Our data indicate that FPX-mediated inhibition of coagulation prevents sepsis coagulopathy; protects against excessive complement activation, inflammation, and organ dysfunction; and provides survival benefit in E. coli sepsis.

Straight to the heart: Pleiotropic antiarrhythmic actions of oral anticoagulants

Mechanistic understanding of atrial fibrillation (AF) pathophysiology and the complex bidirectional relationship with thromboembolic risk remains limited. Oral anticoagulation is a mainstay of AF management. An emerging concept is that anticoagulants may themselves have potential pleiotropic disease-modifying effects. We here review the available evidence for hemostasis-independent actions of the oral anticoagulants on electrical and structural remodeling, and the inflammatory component of the vulnerable substrate.

Nonhemostatic Activities of Factor Xa: Are There Pleiotropic Effects of Anti-FXa Direct Oral Anticoagulants?

Factor Xa (FXa) is the key serine protease of the coagulation cascade as it is the point of convergence of the intrinsic and extrinsic pathways, leading to the formation of thrombin. Factor Xa is an established target of anticoagulation therapy, due to its central role in coagulation. Over the past years, several direct oral anticoagulants (DOACs) targeting FXa have been developed. Rivaroxaban, apixaban, and edoxaban are used in clinical practice for prevention and treatment of thrombotic diseases. Increasing evidence suggests that FXa exerts nonhemostatic cellular effects that are mediated mainly through protease-activated receptors-1 and -2 and are involved in pathophysiological conditions, such as atherosclerosis, inflammation, and fibrosis. Direct inhibition of FXa by DOACs could be beneficial in these conditions. This is a narrative review that focuses on the cellular effects of FXa in various cell types and conditions, as well as on the possible pleiotropic effects of FXa-targeting DOACs.

Design, synthesis and biological evaluation of anthranilamide derivatives as potential factor Xa (fXa) inhibitors

Factor Xa (fXa) is a crucial player in various thromboembolic disorders. Inhibition of fXa can provide safe and effective antithrombotic effects. In this study, a series of anthranilamide compounds were designed by utilizing structure-based design strategies. Optimization at P1 and P4 groups led to the discovery of compound 16g: a highly potent, selective fXa inhibitor with pronounced in vitro anticoagulant activity. Moreover, 16g also displayed excellent in vivo antithrombotic activity in the rat venous thrombosis (VT) and arteriovenous shunt (AV-SHUNT) models. The bleeding risk evaluation showed that 16g had a safer profile than that of betrixaban at 1 mg/kg and 5 mg/kg dose. Additionally, 16g also exhibited satisfactory PK profiles. Eventually, 16g was selected to investigate its effect on hypoxia-reoxygenation- induced H9C2 cell viability. MTT results showed that H9C2 cell viability can be remarkably alleviated by 16g.

Tissue factor: newer concepts in thrombosis and its role beyond thrombosis and hemostasis

For many years, the attention on tissue factor (TF) in human pathophysiology has been limited to its role as initiator of extrinsic coagulation pathway. Moreover, it was described as a glycoprotein located in several tissue including vascular wall and atherosclerotic plaque. However, in the last two decades, the discovery that TF circulates in the blood as cell-associated protein, microparticles (MPs) bound and as soluble form, is changing this old vessel-wall TF dogma. Moreover, it has been reported that TF is expressed by different cell types, even T lymphocytes and platelets, and different pathological conditions, such as acute and chronic inflammatory status, and cancer, may enhance its expression and activity. Thus, recent advances in the biology of TF have clearly indicated that beyond its known effects on blood coagulation, it is a "true surface receptor" involved in many intracellular signaling, cell-survival, gene and protein expression, proliferation, angiogenesis and tumor metastasis. Finally, therapeutic modulation of TF expression and/or activity has been tested with controversial results. This report, starting from the old point of view about TF as initiator of extrinsic coagulation pathway, briefly illustrates the more recent concepts about TF and thrombosis and finally gives an overview about its role beyond thrombosis and haemostasis focusing on the different intracellular mechanisms triggered by its activation and potentially involved in atherosclerosis.

Targeting factor Xa and thrombin: impact on coagulation and beyond

Great advances have been made in recent years in understanding the haemostatic system and the molecular and cellular basis of thrombus formation. Although directly targeting factor Xa or thrombin (factor IIa) for effective anticoagulation is now well established, evidence has emerged suggesting that factor Xa and thrombin are involved in other physiological and pathophysiological cellular processes, including inflammation. These non-haemostatic activities of factor Xa and thrombin are predominantly mediated via the activation of proteinaseactivated receptors. Studies have indicated a potential role of coagulation proteins (including factor Xa and thrombin) in the progression of disease conditions such as atherothrombosis. Preclinical studies have provided evidence for the effects of direct factor Xa or direct thrombin inhibition beyond anticoagulation, including anti-inflammatory activities and atherosclerotic plaque stabilisation. In this article, the non-haemostatic activities of factor Xa and thrombin and the effects of direct inhibition of these coagulation factors on these activities are summarised. In addition, the potential roles of factor Xa and thrombin in atherosclerosis and atherothrombosis are explored and the cardiovascular profiles of rivaroxaban, apixaban and dabigatran etexilate observed in phase III clinical studies are discussed.

Factor Xa Signaling Contributes to the Pathogenesis of Inflammatory Diseases

The coagulation protease Factor Xa (FXa) triggers a variety of signaling pathways through activation of protease-activated receptors (PARs) and non-PAR receptors. FXa-mediated signaling is strongly implicated in the pathogenesis of several inflammatory diseases including fibrosis, cardiovascular diseases, and cancer. Thus, targeting of FXa can have great clinical significance in terms of the treatment of these disorders. This review summarizes the current knowledge about the mechanism of FXa signaling in cellular and animal systems under (patho) physiological conditions for a better understanding and hence a better management of FXa-induced disorders. J. Cell. Physiol. 232: 1966-1970, 2017. © 2016 Wiley Periodicals, Inc.

Factor Xa Inhibitor Suppresses the Release of Phosphorylated HSP27 from Collagen-Stimulated Human Platelets: Inhibition of HSP27 Phosphorylation via p44/p42 MAP Kinase

Selective inhibitors of factor Xa (FXa) are widely recognized as useful therapeutic tools for stroke prevention in non-valvular atrial fibrillation or venous thrombosis. Thrombin, which is rapidly generated from pro-thrombin through the activation of factor X to FXa, acts as a potent activator of human platelets. Thus, the reduction of thrombin generation by FXa inhibitor eventually causes a suppressive effect on platelet aggregation. However, little is known whether FXa inhibitors directly affect the function of human platelets. We have previously reported that collagen induces the phosphorylation of heat shock protein 27 (HSP27), a low-molecular weight heat shock protein via Rac-dependent activation of p44/p42 mitogen-activated protein (MAP) kinase in human platelets, eventually resulting in the release of HSP27. In the present study, we investigated the direct effect of FXa inhibitor on the collagen-induced human platelet activation. Rivaroxaban as well as edoxaban significantly reduced the collagen-induced phosphorylation of both HSP27 and p44/p42 MAP kinase without affecting the platelet aggregation. Rivaroxaban significantly inhibited the release of phosphorylated HSP27 from collagen-stimulated platelets but not the secretion of platelet derived growth factor-AB. In patients administrated with rivaroxaban, the collagen-induced levels of phosphorylated HSP27 were markedly diminished after 2 days of administration, which failed to affect the platelet aggregation. These results strongly suggest that FXa inhibitor reduces the collagen-stimulated release of phosphorylated HSP27 from human platelets due to the inhibition of HSP27 phosphorylation via p44/p42 MAP kinase.

The emerging role of coagulation proteases in kidney disease

A role of coagulation proteases in kidney disease beyond their function in normal haemostasis and thrombosis has long been suspected, and studies performed in the past 15 years have provided novel insights into the mechanisms involved. The expression of protease-activated receptors (PARs) in renal cells provides a molecular link between coagulation proteases and renal cell function and revitalizes research evaluating the role of haemostasis regulators in renal disease. Renal cell-specific expression and activity of coagulation proteases, their regulators and their receptors are dynamically altered during disease processes. Furthermore, renal inflammation and tissue remodelling are not only associated, but are causally linked with altered coagulation activation and protease-dependent signalling. Intriguingly, coagulation proteases signal through more than one receptor or induce formation of receptor complexes in a cell-specific manner, emphasizing context specificity. Understanding these cell-specific signalosomes and their regulation in kidney disease is crucial to unravelling the pathophysiological relevance of coagulation regulators in renal disease. In addition, the clinical availability of small molecule targeted anticoagulants as well as the development of PAR antagonists increases the need for in-depth knowledge of the mechanisms through which coagulation proteases might regulate renal physiology.

New Insights into the Pros and Cons of the Clinical Use of Vitamin K Antagonists (VKAs) Versus Direct Oral Anticoagulants (DOACs)

Vitamin K-antagonists (VKA) are the most widely used anticoagulant drugs to treat patients at risk of arterial and venous thrombosis for the past 50 years. Due to unfavorable pharmacokinetics VKA have a small therapeutic window, require frequent monitoring, and are susceptible to drug and nutritional interactions. Additionally, the effect of VKA is not limited to coagulation, but affects all vitamin K-dependent proteins. As a consequence, VKA have detrimental side effects by enhancing medial and intimal calcification. These limitations stimulated the development of alternative anticoagulant drugs, resulting in direct oral anticoagulant (DOAC) drugs, which specifically target coagulation factor Xa and thrombin. DOACs also display non-hemostatic vascular effects via protease-activated receptors (PARs). As atherosclerosis is characterized by a hypercoagulable state indicating the involvement of activated coagulation factors in the genesis of atherosclerosis, anticoagulation could have beneficial effects on atherosclerosis. Additionally, accumulating evidence demonstrates vascular benefit from high vitamin K intake. This review gives an update on oral anticoagulant treatment on the vasculature with a special focus on calcification and vitamin K interaction.

Rivaroxaban Induces Mucosal Healing in a Rat Model of Trinitrobenzene Sulfonic Acid-Induced Colitis

OBJECTIVE

This study was designed to identify the effect of rivaroxaban, a direct factor Xa inhibitor, on trinitrobenzene sulfonic acid (TNBS)-induced colitis in rats.

MATERIALS AND METHODS

Twenty-four female Wistar rats were divided into 4 groups of 6 each. Group 1 received TNBS + rivaroxaban, group 2 received TNBS + methylprednisolone, group 3 received TNBS and group 4 received a saline enema. Colitis was induced in the rats by the intracolonic administration of TNBS. Rivaroxaban and methylprednisolone were given by oral gavage daily for 7 days. The rats were killed 7 days after the induction of colitis.

RESULTS

Rivaroxaban and methylprednisolone significantly reduced gross damage and histopathological scores. Rivaroxaban was more effective than methylprednisolone in terms of microscopic mucosal healing. Rivaroxaban attenuated the accumulation of malonyldialdehyde (MDA) and transforming growth-factor β1 (TGF-β1) and the activities of myeloperoxidase (MPO), matrix metalloproteinase-3 and tissue inhibitor of metalloproteinases-1. Methylprednisolone reduced only the activity of MPO and the accumulation of MDA and TGF-β1. Superoxide dismutase activity showed a restoration to normal levels after rivaroxaban and methylprednisolone administration.

CONCLUSIONS

Rivaroxaban showed a therapeutic effect in the TNBS model of experimental colitis, and it seemed to be at least as effective as methylprednisolone. This effect may be brought about by the inhibition of oxidative stress and metalloproteinase activity associated with tissue injury and remodeling.

Differential cellular effects of old and new oral anticoagulants: consequences to the genesis and progression of atherosclerosis

The main purpose of anticoagulants is to diminish fibrin formation, thereby decreasing the risk of venous or arterial thrombosis. Vitamin K antagonist have been used for many decades in order to achieve reduced thrombotic risk, despite major drawbacks of this class of drugs such as cumbersome dossing and monitoring of anticoagulant status. To overcome these drawbacks of VKA, new classes of anticoagulants have been developed including oral anticoagulants for direct inhibition of either thrombin or factor Xa, which can be administrated in a fixed dose without monitoring. Coagulation factors can activate cellular protease-activated receptors, thereby inducing cellular processes as inflammation, apoptosis, migration, and fibrosis. Therefore, inhibition of coagulation proteases not only attenuates fibrin formation, but may also influence pathophysiological processes like vascular calcification and atherosclerosis. Animal models revealed that VKA therapy induced both intima and media calcification and accelerated plaque vulnerability, whereas specific and direct inhibition of thrombin or factor Xa attenuated atherosclerosis. In this review we provide an overview of old and new oral anticoagulants, as well discuss potential pleiotropic effects with regard to calcification and atherosclerosis. Although translation from animal model to clinical patients seems difficult at first sight, effort should be made to fully understand the clinical implications of long-term oral anticoagulant therapy on vascular side effects.

Ensemble-based docking using biased molecular dynamics

Proteins are dynamic molecules, and understanding their movements, especially as they relate to molecular recognition and protein-ligand interactions, poses a significant challenge to structure-based drug discovery. In most instances, protein flexibility is underrepresented in computer-aided drug design due to uncertainties on how it should be accurately modeled as well as the computational cost associated with attempting to incorporate flexibility in the calculations. One approach that aims to address these issues is ensemble-based docking. With this technique, ligands are docked to an ensemble of rigid protein conformations. Molecular dynamics (MD) simulations can be used to generate the ensemble of protein conformations for the subsequent docking. Here we present a novel approach that uses biased-MD simulations to generate the docking ensemble. The MD simulations are biased toward an initial protein-ligand X-ray complex structure. The biasing maintains some of the original crystallographic pocket-ligand information and thereby enhances sampling of the more relevant conformational space of the protein. Resulting trajectories are clustered to select a representative set of protein conformations, and ligands are docked to that reduced set of conformations. Cross-docking to this ensemble and then selecting the lowest scoring pose enables reliable identification of the correct binding mode. Various levels of biasing are investigated, and the method is validated for cyclin-dependent kinase 2 and factor Xa.

Lufaxin, a novel factor Xa inhibitor from the salivary gland of the sand fly Lutzomyia longipalpis blocks protease-activated receptor 2 activation and inhibits inflammation and thrombosis in vivo

OBJECTIVE

Blood-sucking arthropods' salivary glands contain a remarkable diversity of antihemostatics. The aim of the present study was to identify the unique salivary anticoagulant of the sand fly Lutzomyia longipalpis, which remained elusive for decades.

METHODS AND RESULTS

Several L. longipalpis salivary proteins were expressed in human embryonic kidney 293 cells and screened for inhibition of blood coagulation. A novel 32.4-kDa molecule, named Lufaxin, was identified as a slow, tight, noncompetitive, and reversible inhibitor of factor Xa (FXa). Notably, Lufaxin's primary sequence does not share similarity to any physiological or salivary inhibitors of coagulation reported to date. Lufaxin is specific for FXa and does not interact with FX, Dansyl-Glu-Gly-Arg-FXa, or 15 other enzymes. In addition, Lufaxin blocks prothrombinase and increases both prothrombin time and activated partial thromboplastin time. Surface plasmon resonance experiments revealed that FXa binds Lufaxin with an equilibrium constant ≈3 nM, and isothermal titration calorimetry determined a stoichiometry of 1:1. Lufaxin also prevents protease-activated receptor 2 activation by FXa in the MDA-MB-231 cell line and abrogates edema formation triggered by injection of FXa in the paw of mice. Moreover, Lufaxin prevents FeCl(3)-induced carotid artery thrombus formation and prolongs activated partial thromboplastin time ex vivo, implying that it works as an anticoagulant in vivo. Finally, salivary gland of sand flies was found to inhibit FXa and to interact with the enzyme.

CONCLUSIONS

Lufaxin belongs to a novel family of slow-tight FXa inhibitors, which display antithrombotic and anti-inflammatory activities. It is a useful tool to understand FXa structural features and its role in prohemostatic and proinflammatory events.