Factor XI: hemostasis, thrombosis, and antithrombosis

Inhibitors of Polyphosphate and Neutrophil Extracellular Traps

The contact pathway of blood clotting has received intense interest in recent years as studies have linked it to thrombosis, inflammation, and innate immunity. Because the contact pathway plays little to no role in normal hemostasis, it has emerged as a potential target for safer thromboprotection, relative to currently approved antithrombotic drugs which all target the final common pathway of blood clotting. Research since the mid-2000s has identified polyphosphate, DNA, and RNA as important triggers of the contact pathway with roles in thrombosis, although these molecules also modulate blood clotting and inflammation via mechanisms other than the contact pathway of the clotting cascade. The most significant source of extracellular DNA in many disease settings is in the form of neutrophil extracellular traps (NETs), which have been shown to contribute to incidence and severity of thrombosis. This review summarizes known roles of extracellular polyphosphate and nucleic acids in thrombosis, with an emphasis on novel agents under current development that target the prothrombotic activities of polyphosphate and NETs.

Dual Inhibition of Factor XIIa and Factor XIa Produces a Synergistic Anticoagulant Effect

ABSTRACT

Clinical practice shows that a critical unmet need in the field of thrombosis prevention is the availability of anticoagulant therapy without bleeding risk. Inhibitors against FXIa or FXIIa have been extensively studied because of their low bleeding risk. However, whether these compounds produce synergistic effects has not yet been explored. In this study, analyses of activated partial thromboplastin time in combination with the FXIa inhibitor PN2KPI and the FXIIa inhibitor Infestin4 at different proportions were performed using the SynergyFinder tool identifying synergistic anticoagulation effects. Both an FeCl 3 -induced carotid artery thrombosis mouse model and a transient occlusion of the middle cerebral artery mouse model showed that the combination of PN2KPI and Infestin4, which are 28.57% and 6.25% of the effective dose, respectively, significantly prevents coagulation, and furthermore, dual inhibition does not cause bleeding risk.

Determination of the Potential Clinical Benefits of Small Molecule Factor XIa Inhibitors in Arterial Thrombosis

Anticoagulants are the mainstay for the prevention and treatment of thrombosis. However, bleeding complications remain a primary concern. Recent advances in understanding the contribution of activated factor XI (FXIa) in arterial thrombosis with a limited impact on hemostasis have led to the development of several FXIa-targeting modalities. Injectable agents including monoclonal antibodies and antisense oligonucleotides against FXIa have been primarily studied in venous thrombosis. The orally active small molecules that specifically inhibit the active site of FXIa are currently being investigated for their antithrombotic activity in both arteries and veins. This review focuses on a discussion of the potential clinical benefits of small molecule FXIa inhibitors, mainly asundexian and milvexian, in arterial thrombosis based on their pharmacological profiles and the compelling results of phase 2 clinical studies. The preclinical and epidemiological basis for the impact of FXIa in hemostasis and arterial thrombosis is also addressed. In recent clinical study results, asundexian appears to reduce ischemic events in patients with myocardial infarction and minor-to-moderate stroke, whereas milvexian possibly provides benefits in patients with minor stroke or high-risk transient ischemic attack (TIA). In addition, asundexian and milvexian had a minor impact on hemostasis even in combination with dual-antiplatelet therapy. Other orally active FXIa inhibitors also produce antithrombotic activity in vivo with low bleeding risk. Therefore, FXIa inhibitors might represent a new class of direct-acting oral anticoagulants (DOACs) for the treatment of thrombosis, although the explicit clinical positions of asundexian and milvexian in patients with ischemic stroke, high-risk TIA, and coronary artery disease require confirmation from the outcomes of ongoing phase 3 trials.

Phenotypic and genetic analyses of four cases of coagulation factor XII deficiency

OBJECTIVES

To identify the clinical phenotypic and molecular pathogeneses of four cases of coagulation factor XII deficiency and to deepen the cognition of this disease.

METHODS

Coagulation tests were performed through one stage of coagulation on a STAGO coagulation analyser. Coagulation factor XII antigen was detected using enzyme-linked immunosorbent assay. The species conservatism and structural change of mutant proteins were analysed using MegAlign and PYMOL. Meanwhile, missense variants and a novel splice site variant were identified using PolyPhen2 and NetGene2.

RESULTS

The four cases had an observably prolonged activated partial thromboplastin time but without obvious bleeding tendency. Their coagulation factor XII activity (FⅫ:C) and antigen (FXII:Ag) were greatly reduced. Six mutations were detected: NM_000505.4:c.398-1G>A, NP_000496.2:p.(Pro182Leu), NP_000496.2:p.(Ser479Ter), NP_000496.2:p.(Cys559Arg), NC_000005.10:g.7217_7221delinsGTCTA and NM_000505.4:c.1681-1G>A. The first five are newly discovered mutations. The two missense mutation sites were highly conservative, and their protein secondary structure changes may occur not only on the mutation sites but also on other domains. In silico analysis revealed that NP_000496.2:p.(Pro182Leu) may be BENIGN, NP_000496.2:p.(Cys559Arg) may be damaging, and that NM_000505.4:c.398-1G>A and NM_000505.4:c.1681-1G>A are crucial for splicing.

CONCLUSION

We found six types of mutations, of which five were novel. The two missense mutation sites might be closely related to the function of coagulation factor XII. The mutations were the primary culprits of factor XII deficiency.

Essential role of a carboxyl-terminal α-helix motif in the secretion of coagulation factor XI

BACKGROUND

Coagulation factor XI (FXI) is a plasma serine protease zymogen that contributes to hemostasis. However, the mechanism of its secretion remains unclear.

OBJECTIVE

To determine the molecular mechanism of FXI secretion by characterizing a novel FXI mutant identified in a FXI-deficient Japanese patient.

PATIENT/METHODS

The FXI gene (F11) was analyzed by direct sequencing. Mutant recombinant FXI (rFXI) was overexpressed in HEK293 or COS-7 cells. Western blotting and enzyme-linked immunosorbent assay were performed to examine the FXI extracellular secretion profile. Immunofluorescence microscopy was used to investigate the subcellular localization of the rFXI mutant.

RESULTS

We identified a novel homozygous frameshift mutation in F11 [c.1788dupC (p.E597Rfs*65)], resulting in a unique and extended carboxyl-terminal (C-terminal) structure in FXI. Although rFXI-E597Rfs*65 was intracellularly synthesized, its extracellular secretion was markedly reduced. Subcellular localization analysis revealed that rFXI-E597Rfs*65 was abnormally retained in the endoplasmic reticulum (ER). We generated a series of C-terminal-truncated rFXI mutants to further investigate the role of the C-terminal region in FXI secretion. Serial rFXI experiments revealed that a threonine at position 622, the fourth residue from the C-terminus, was essential for secretion. Notably, Thr622 engages in the formation of an α-helix motif, indicating the importance of the C-terminal α-helix in FXI intracellular behavior and secretion.

CONCLUSION

FXI E597Rfs*65 results in the pathogenesis of a severe secretory defect resulting from aberrant ER-to-Golgi trafficking caused by the lack of a C-terminal α-helix motif. This study demonstrates the impact of the C-terminal structure, especially the α-helix motif, on FXI secretion.

Design and synthesis of a multivalent catch-and-release assay to measure circulating FXIa

BACKGROUND

Decreased blood coagulation factor (F)XIa levels have been shown to protect from thrombosis without bleeding side effects, but less is known on effects of increased FXIa levels. Studies are hampered by lack of a reliable and robust method for FXIa quantification in blood. We aim to develop a new assay employing a unique multivalent catch-and-release system. The system selectively isolates and protects homodimeric FXIa from plasma and releases free FXIa allowing subsequent quantification.

METHODS

A dynamic multivalent construct was synthesized by complexing four identical FXIa inhibitors from the snake Bungarus Fasxiatus to avidin through desthiobiotin-PEG-linkers, allowing dissociation of FXIa by excess biotin. PEG-linker lengths were optimised for FXIa inhibitory activity and analysed by Michaelis-Menten kinetics. Finally, the catch-and-release assay was validated in buffer and plasma model systems.

RESULTS

Monovalent and multivalent inhibitor constructs were successfully obtained by total chemical synthesis. Multimerisation of Fasxiator resulted in a 30-fold increase in affinity for FXIa from 1.6 nM to 0.05 nM. With use of this system, FXIa could be quantified down to a concentration of 7 pM in buffer and 20 pM in plasma.

CONCLUSION

In this proof-of-concept study, we have shown that the catch-and-release approach is a promising technique to quantify FXIa in plasma or buffer. By binding FXIa to the multivalent construct directly after blood drawing, FXIa is hypothesized to be inaccessible for serpin inhibition or auto inactivation. This results in a close reflection of actual circulating FXIa levels at the moment of blood drawing.

Combined Congenital Hypodysfibrinogemia and Factor XI Deficiency in a Chinese Family

Both congenital hypodysfibrinogenemia and factor XI deficiency are rare coagulopathies caused by mutations within the fibrinogen and F11 genes, respectively. To investigate the pathogenesis of combined congenital hypodysfibrinogenemia with factor XI (FXI) deficiency in a Chinese family, coagulation assays, FXI activity (the 1-stage method), fibrinogen activity (the Clauss method), and antigen (prothrombin time [PT]-derived method) were performed. The sequences of fibrinogen genes and F11 were amplified by PCR and analyzed by direct sequencing. The proband as well as his grandmother, father, aunt, and sister showed a low plasma concentration of fibrinogen measured by the Clauss method and a slightly decreased result by the PT-derived method; finally, c.1097A>G in exon 8 of FGG was detected in the pedigree, which caused His340Arg mutation. His grandfather had a slightly prolonged activated partial thromboplastin time (APTT) due to low FXI activity. FXI deficiency was a compound heterozygote inherited with missense mutations of c.434A>G in exon 5 as well as c.1253G>T in exon 11 which caused HGV p.His145Arg and Gly400Val mutations, respectively. The grandfather had no qualitative or quantitative defect in fibrinogen. The proband and his father and aunt had c.434A>G at the exon 5 mutation site and no decrease in FXI activity. His mother had no fibrinogen or F11 gene mutations. Plasma fibrin polymerization was delayed. The proband in our study showed typical changes of congenital hypodysfibrinogemia in the clotting analyses with delayed fibrin polymerization, but although he was a heterozygous carrier of the c.434A>G variant in the F11 gene, he had no decrease in FXI activity and no bleeding tendency, thus questioning the pathogenicity of the identified variant in the F11 gene. To our knowledge, this is the first report of a case of combined hypodysfibrinogenemia and FXI deficiency confirmed by molecular genetic tests.

Sulfated Non-Saccharide Glycosaminoglycan Mimetics as Novel Drug Discovery Platform for Various Pathologies

Glycosaminoglycans (GAGs) are very complex, natural anionic polysaccharides. They are polymers of repeating disaccharide units of uronic acid and hexosamine residues. Owing to their template-free, spatiotemporally-controlled, and enzyme-mediated biosyntheses, GAGs possess enormous polydispersity, heterogeneity, and structural diversity which often translate into multiple biological roles. It is well documented that GAGs contribute to physiological and pathological processes by binding to proteins including serine proteases, serpins, chemokines, growth factors, and microbial proteins. Despite advances in the GAG field, the GAG-protein interface remains largely unexploited by drug discovery programs. Thus, Non-Saccharide Glycosaminoglycan Mimetics (NSGMs) have been rationally developed as a novel class of sulfated molecules that modulate GAG-protein interface to promote various biological outcomes of substantial benefit to human health. In this review, we describe the chemical, biochemical, and pharmacological aspects of recently reported NSGMs and highlight their therapeutic potentials as structurally and mechanistically novel anti-coagulants, anti-cancer agents, anti-emphysema agents, and anti-viral agents. We also describe the challenges that complicate their advancement and describe ongoing efforts to overcome these challenges with the aim of advancing the novel platform of NSGMs to clinical use.

Allosteric Inhibition as a New Mode of Action for BAY 1213790, a Neutralizing Antibody Targeting the Activated Form of Coagulation Factor XI

Factor XI (FXI), the zymogen of activated FXI (FXIa), is an attractive target for novel anticoagulants because FXI inhibition offers the potential to reduce thrombosis risk while minimizing the risk of bleeding. BAY 1213790, a novel anti-FXIa antibody, was generated using phage display technology. Crystal structure analysis of the FXIa-BAY 1213790 complex demonstrated that the tyrosine-rich complementarity-determining region 3 loop of the heavy chain of BAY 1213790 penetrated deepest into the FXIa binding epitope, forming a network of favorable interactions including a direct hydrogen bond from Tyr102 to the Gln451 sidechain (2.9 Å). The newly discovered binding epitope caused a structural rearrangement of the FXIa active site, revealing a novel allosteric mechanism of FXIa inhibition by BAY 1213790. BAY 1213790 specifically inhibited FXIa with a binding affinity of 2.4 nM, and in human plasma, prolonged activated partial thromboplastin time and inhibited thrombin generation in a concentration-dependent manner.

Recent advances in the discovery and development of factor XI/XIa inhibitors

Factor XIa (FXIa) is a serine protease homodimer that belongs to the intrinsic coagulation pathway. FXIa primarily catalyzes factor IX activation to factor IXa, which subsequently activates factor X to factor Xa in the common coagulation pathway. Growing evidence suggests that FXIa plays an important role in thrombosis with a relatively limited contribution to hemostasis. Therefore, inhibitors targeting factor XI (FXI)/FXIa system have emerged as a paradigm-shifting strategy so as to develop a new generation of anticoagulants to effectively prevent and/or treat thromboembolic diseases without the life-threatening risk of internal bleeding. Several inhibitors of FXI/FXIa proteins have been discovered or designed over the last decade including polypeptides, active site peptidomimetic inhibitors, allosteric inhibitors, antibodies, and aptamers. Antisense oligonucleotides (ASOs), which ultimately reduce the hepatic biosynthesis of FXI, have also been introduced. A phase II study, which included patients undergoing elective primary unilateral total knee arthroplasty, revealed that a specific FXI ASO effectively protects patients against venous thrombosis with a relatively limited risk of bleeding. Initial findings have also demonstrated the potential of FXI/FXIa inhibitors in sepsis, listeriosis, and arterial hypertension. This review highlights various chemical, biochemical, and pharmacological aspects of FXI/FXIa inhibitors with the goal of advancing their development toward clinical use.

Normal aPTT in children with mild factor XI deficiency

It has been suggested that persons with factor XI deficiency can have a normal activated partial thromboplastin time (aPTT). This notion is based on limited data, especially in children. Because of the central role the aPTT plays in diagnostic algorithms for bleeding disorders, it is important to know if a normal aPTT eliminates the need for factor XI activity testing. Our institutional database contains seven children with factor XI deficiency, of whom four have a normal aPTT. This supports the hypothesis that children with factor XI deficiency can have a normal aPTT. Clinicians may wish to consider this evidence when evaluating children with abnormal bleeding.

Simultaneous targeting of CD14 and factor XIa by a fusion protein consisting of an anti-CD14 antibody and the modified second domain of bikunin improves survival in rabbit sepsis models

Severe sepsis is a complex, multifactorial, and rapidly progressing disease characterized by excessive inflammation and coagulation following bacterial infection. To simultaneously suppress pro-inflammatory and pro-coagulant responses, we genetically engineered a novel fusion protein (MR1007) consisting of an anti-CD14 antibody and the modified second domain of bikunin, and evaluated the potential of MR1007 as an anti-sepsis agent. Suppressive effects of MR1007 on lipopolysaccharide (LPS)-induced inflammatory responses were assessed using peripheral blood mononuclear cells or endothelial cells. Its inhibitory activity against the coagulation factor XIa was assessed using a purified enzyme and a chromogenic substrate. Anticoagulant activity was assessed using human or rabbit plasma. Anti-inflammatory and anti-coagulant effects and/or survival benefits were evaluated in an endotoxemia model and a cecal ligation and puncture model. MR1007 inhibited LPS-induced cytokine production in peripheral blood mononuclear cells and endothelial cells, inhibited factor XIa, and exhibited anticoagulant activity. In an endotoxemia model, 0.3-3mg/kg MR1007 suppressed pro-inflammatory and pro-coagulant responses in a dose-dependent manner; at a dose of 3mg/kg, the protein improved survival even when administered 8h after the LPS injection. In addition, 10mg/kg MR1007 administered 2h post cecal ligation and puncture improved survival. However, MR1007 administered at doses up to 30mg/kg did not increase ear bleeding time or bacterial counts in the cecal ligation and puncture model. Thus, simultaneous targeting of CD14 and factor XIa improves survival in the rabbit endotoxemia and sepsis models and represents a promising approach for the treatment of severe sepsis.

Factor XIa inhibitors: A review of the patent literature

INTRODUCTION

Anticoagulants are the mainstay for prevention and/or treatment of thrombotic disorders. Each clinically used anticoagulant is associated with significant adverse consequences, especially bleeding. Factor XIa (FXIa), a key factor involved in the amplification of procoagulation signal, has been suggested as a major target for anticoagulant drug discovery because of reduced risk of bleeding.

AREAS COVERED

Our literature search uncovered dozens of industrial and academic patents on the discovery of novel FXIa/FXI inhibitors. Small peptidomimetics, sulfated glycosaminoglycan mimetics, polypeptides, antisense oligonucleotides, and monoclonal antibodies have been developed as inhibitors of FXIa. Although many agents are in early discovery/development phases, the activity and safety of a few have been evaluated in various animal models and in humans.

EXPERT OPINION

FXIa is a promising drug target for development of effective anticoagulants with limited bleeding complications. Literature reveals a major trend in the number of patent applications over the last three years. These inhibitors exploit different approaches for target inhibition. Allosteric modulation of FXIa and biosynthetic inhibition of FXI are mechanistically unique. Despite initial results in patients undergoing knee anthroplasty as with antisense oligonucleotides, major advances should be realized, particularly with respect to pharmacokinetics, for FXI/FXIa inhibitors to enter the clinic.

F11 rs2289252T and rs2036914C Polymorphisms Increase the Activity of Factor XI in Post-trauma Patients with Fractures Despite Thromboprophylaxis

OBJECTIVE

To evaluate the association between F11 rs2289252, rs2036914 polymorphisms and the activity of clotting factor XI in post-trauma patients with fractures receiving routine anticoagulation therapy for deep venous thrombosis (DVT).

METHODS

A case-control study involving 110 consecutive post-trauma patients with fractures and DVT in our hospital was conducted from April 2014 to October 2015; these patients comprised a DVT group. Another 40 sex- and age-matched patients with fractures but without DVT served as controls. Additionally, 40 sex- and age-matched healthy people were chosen as a normal group. Venous blood samples (2 mL) were drawn from all participants and genomic DNA extracted from the leukocytes of the patients with fracture-related DVT, whose genotype and allele frequency distribution of F11 gene rs2089252 and rs2036914 single nucleotide polymorphism were then assessed by a sequencing method. The activity of factor XI was measured by a solidification method in all participants, including those in control and normal groups.

RESULTS

The activity of factor XI in patients with fracture-related DVT and F11 rs2089252 CT was 1.16 times that of those with CC genotypes (P < 0.0001), whereas in patients with fracture-related DVT and F11 rs2089252 TT genotypes it was 1.32 times that of those with CC genotypes (P < 0.0001), in patients with fracture-related DVT and F11 rs2089252 T allele it was 1.24 times that of those with C allele (P < 0.05), in patients with fracture-related DVT and F11 rs2036914 CC it was 1.35 times that of those with TT genotypes, in patients with fracture-related DVT and F11 rs2036914 CT genotypes it was 1.12 times that of those with TT genotypes (P < 0.05), and in patients with fracture-related DVT F11 and rs2036914 C allele it was 1.22 times that of those with T allele (P < 0.05). The activity of factor XI was significantly higher in the control than in the normal group (P < 0.05).

CONCLUSIONS

High activity of factor XI indicates a risk of occurrence of DVT in post-trauma patients with fractures. F11 rs2089252 and rs2036914 (single nucleotide polymorphisms) are associated with activity of factors XI in such patients despite prophylaxis.

Clinical manifestations and mutation spectrum of 57 subjects with congenital factor XI deficiency in China

Congenital factor XI (FXI) deficiency is a rare bleeding disorder with unpredictable bleeding tendency. Few studies in a large cohort have been reported regarding associations between FXI activity (FXI:C) or genotypes and bleeding symptoms currently. This study characterized clinical manifestations and mutation spectrum of 57 subjects with FXI deficiency in China. Clinical data were collected and mutations were identified by direct sequencing and determined by mRNA analysis. The result revealed bleeding symptoms were only found in 12 patients (12/57, 21.1%) with severely reduced FXI:C, and prolonged bleeding post injury/surgery as well as easy bruising were the commonest bleeding manifestations presented in respective 5 cases (5/12, 41.7%). A total number of 37 mutations were identified including 19 missense mutations, 9 nonsense mutations, 6 splice site mutations and 3 small deletions. Among them, 4 missense mutations, 5 splice mutations, 3 small deletions and a nonsense mutation were newly detected. W228*, G400V, Q263* and c.1136-4delGTTG with a total frequency of 48.3% were the most four common mutations in Chinese patients. RT-PCR analysis was carried out and confirmed that both c.596-8T>A and c.1136-4delGTTG were pathogenic due to frameshift resulting in respective truncated proteins. Our findings suggested clinical manifestations had little to do with FXI:C or genotypes, which required further study. This study, the largest investigation of FXI deficiency in China revealed that the F11 mutation spectrum of Chinese population was distinct from those of other populations earlier established.

Abnormal coagulation factor VIII transcript in a Tennessee Walking Horse colt with hemophilia A

Hemophilia A is an X-chromosome-linked disorder caused by a deficiency in factor VIII (FVIII). Although foals have been diagnosed with hemophilia A based on deficiency in FVIII activity, causative gene mutations have not been identified. The genomic DNA and cDNA encoding FVIII of a Tennesee Walking Horse colt affected with hemophilia A and the genomic DNA of his dam and a normal unrelated horse were analyzed with no splice site or coding sequence abnormalities identified in any of the horses. Polymerase chain reactions (PCR) were then performed on hepatic cDNA from the affected colt and an unrelated normal horse, and no product was obtained for the sequence between and including exon 1 and exon 2 in the affected colt. Based on these results, suspected mutations were identified in the noncoding region of FVIII (intron 1), and genomic sequencing of intron 1 in the dam and the affected colt suggested maternal inheritance.

The Contact Activation System (CAS) in cord blood: Measurement of CAS components and comparison with mother's blood. A pilot study

INTRODUCTION

Classical reference data concerning the coagulation system and fibrinolysis in fetuses and newborns date back to the 1990 s. Since that time a number of methodological or other improvements have been implemented, which may cast some doubt on timeliness of the data. The study objective was to measure the levels of Contact Activation System (CAS) components by antigen, i.e. factors XII and XI (FXII, FXI), prekallikrein (PK) and high molecular weight kininogen (HMWK) in cord blood and maternal blood.

MATERIAL AND METHODS

The study group consisted of 35 healthy parturient women with an uneventful pregnancy and birth. The samples of cord blood and maternal blood were obtained immediately after delivery, before clumping the umbilical cord. The CAS components were measured by immunoenzymatic method (ELISA).

RESULTS

The median concentrations of CAS components in cord blood plasma and mother's plasma were as follow: FXII: 1.02 (0.60- 2.58) ng/mg protein vs. 0.94 (0.66-1.86) ng/mg protein (p>0.05); FXI: 2.71(0.03-8.0) ng/mg protein vs. 0.92 (0.03-10.44) ng/mg protein (p>0.05); PK: 168.78 (104.28-261.16) pg/mg protein vs. 113.44 (79.94-146.70) pg/mg protein (p>0.05); HMWK: 2169.45 (1530.64- 2539.83) ng/mg protein vs. 2857.96 (2541.52-3161.04) ng/mg protein (p<0.001).

CONCLUSIONS

1. The antigen levels of the three contact factors, i.e. FXII, FXI and PK in the cord blood of full-term and healthy fetuses were similar to those observed in mother's blood immediately after delivery. Only high molecular weight kininogen was found to be lower (accounting for 84% of the values noted in mothers). 2. Based on our measurements, we claim that the cited reference data concerning the contact factors in full-term and healthy newborns are underestimated; hence, new reference values need to be determined for each antigen and activity contact factor level.

Polyphosphate as modulator of hemostasis, thrombosis, and inflammation

Inorganic polyphosphate (polyP), a linear polymer of phosphates, is present in many infectious microorganisms and is secreted by mast cells and platelets. PolyP has recently been shown to accelerate blood clotting and slow fibrinolysis, in a manner that is highly dependent on polymer length. Very long-chain polyP (of the type present in microorganisms) is an especially potent trigger of the contact pathway, enhances the proinflammatory activity of histones, and may participate in host responses to pathogens. PolyP also inhibits complement, providing another link between polyP and inflammation/innate immunity. Platelet-size polyP (which is considerably shorter) accelerates factor V activation, opposes the anticoagulant action of tissue factor pathway inhibitor, modulates fibrin clot structure, and promotes factor XI activation. PolyP may have utility in treating bleeding. It is also a potential target for the development of antithrombotic drugs with a novel mechanism of action and potentially fewer bleeding side effects compared with conventional anticoagulants.

Creating novel activated factor XI inhibitors through fragment based lead generation and structure aided drug design

Activated factor XI (FXIa) inhibitors are anticipated to combine anticoagulant and profibrinolytic effects with a low bleeding risk. This motivated a structure aided fragment based lead generation campaign to create novel FXIa inhibitor leads. A virtual screen, based on docking experiments, was performed to generate a FXIa targeted fragment library for an NMR screen that resulted in the identification of fragments binding in the FXIa S1 binding pocket. The neutral 6-chloro-3,4-dihydro-1H-quinolin-2-one and the weakly basic quinolin-2-amine structures are novel FXIa P1 fragments. The expansion of these fragments towards the FXIa prime side binding sites was aided by solving the X-ray structures of reported FXIa inhibitors that we found to bind in the S1-S1'-S2' FXIa binding pockets. Combining the X-ray structure information from the identified S1 binding 6-chloro-3,4-dihydro-1H-quinolin-2-one fragment and the S1-S1'-S2' binding reference compounds enabled structure guided linking and expansion work to achieve one of the most potent and selective FXIa inhibitors reported to date, compound 13, with a FXIa IC50 of 1.0 nM. The hydrophilicity and large polar surface area of the potent S1-S1'-S2' binding FXIa inhibitors compromised permeability. Initial work to expand the 6-chloro-3,4-dihydro-1H-quinolin-2-one fragment towards the prime side to yield molecules with less hydrophilicity shows promise to afford potent, selective and orally bioavailable compounds.

Regulation of coagulation factor XI expression by microRNAs in the human liver

High levels of factor XI (FXI) increase the risk of thromboembolic disease. However, the genetic and environmental factors regulating FXI expression are still largely unknown. The aim of our study was to evaluate the regulation of FXI by microRNAs (miRNAs) in the human liver. In silico prediction yielded four miRNA candidates that might regulate FXI expression. HepG2 cells were transfected with miR-181a-5p, miR-23a-3p, miR-16-5p and miR-195-5p. We used mir-494, which was not predicted to bind to F11, as a negative control. Only miR-181a-5p caused a significant decrease both in FXI protein and F11 mRNA levels. In addition, transfection with a miR-181a-5p inhibitor in PLC/PRF/5 hepatic cells increased both the levels of F11 mRNA and extracellular FXI. Luciferase assays in human colon cancer cells deficient for Dicer (HCT-DK) demonstrated a direct interaction between miR-181a-5p and 3′untranslated region of F11. Additionally, F11 mRNA levels were inversely and significantly correlated with miR-181a-5p levels in 114 healthy livers, but not with miR-494. This study demonstrates that FXI expression is directly regulated by a specific miRNA, miR-181a-5p, in the human liver. Future studies are necessary to further investigate the potential consequences of miRNA dysregulation in pathologies involving FXI.

Factor XI as a target for antithrombotic therapy

Anticoagulants currently used in clinical practice to treat thromboembolic disorders are effective but increase the risk of severe bleeding because they target proteins that are essential for normal coagulation (hemostasis). Drugs with better safety profiles are required for prevention and treatment of thromboembolic disease. Coagulation factor XIa has emerged as a novel target for safer anticoagulant therapy because of its role in thrombosis and its relatively small contribution to hemostasis.

Functional investigation of a venous thromboembolism GWAS signal in a promoter region of coagulation factor XI gene

Coagulation factor XI (FXI) is essential for normal function of the intrinsic pathway of blood coagulation. A nucleotide variant (rs3756008) in the promoter region of the FXI gene was recently reported for association with venous thromboembolism. This study aimed to examine promoter activity of the rs3756008 or other variants linked with it. Luciferase assay was analyzed with minigenes including haplotypes (AA with frequency of 0.62 and TG with frequency of 0.38) of 2 completely linked nucleotide variants (rs3756008 and rs3756009) in 5'-upstream region of the FXI gene. While their expression did not differ in hepatic cell (P > 0.05), the major haplotype (AA) made a significantly more expression (P < 0.05) than the minor haplotype (TG) in human embryonic kidney 293 cells. Further luciferase analysis with additional haplotypes (artificial; TA, AG) revealed that the large expression was caused by the major allele of rs3756008 (P < 0.05), but not by that of rs3756009 (P > 0.05). We suggested that the minor allele of rs3756008 in the promoter of FXI gene could reduce its expression in kidney.