Oral Anticoagulants Beyond Warfarin.

Direct oral anticoagulants (DOACs) have largely replaced vitamin K antagonists, mostly warfarin, for the main indications for oral anticoagulation, prevention and treatment of venous thromboembolism, and prevention of embolic stroke in atrial fibrillation. While DOACs offer practical, fixed-dose anticoagulation in many patients, specific restrictions or contraindications may apply. DOACs are not sufficiently effective in high-thrombotic risk conditions such as antiphospholipid syndrome and mechanical heart valves. Patients with cancer-associated thrombosis may benefit from DOACs, but the bleeding risk, particularly in those with gastrointestinal or urogenital tumors, must be carefully weighed. In patients with frailty, excess body weight, and/or moderate-to-severe chronic kidney disease, DOACs must be cautiously administered and may require laboratory monitoring. Reversal agents have been developed and approved for life-threatening bleeding. In addition, the clinical testing of potentially safer anticoagulants such as factor XI(a) inhibitors is important to further optimize anticoagulant therapy in an increasingly elderly and frail population worldwide.

Factor XI Inhibition for the Prevention of Catheter-Associated Thrombosis in Patients With Cancer Undergoing Central Line Placement: A Phase 2 Clinical Trial.

BACKGROUND: Despite the ubiquitous utilization of central venous catheters in clinical practice, their use commonly provokes thromboembolism. No prophylactic strategy has shown sufficient efficacy to justify routine use. Coagulation factors FXI (factor XI) and FXII (factor XII) represent novel targets for device-associated thrombosis, which may mitigate bleeding risk. Our objective was to evaluate the safety and efficacy of an anti-FXI mAb (monoclonal antibody), gruticibart (AB023), in a prospective, single-arm study of patients with cancer receiving central line placement. METHODS: We enrolled ambulatory cancer patients undergoing central line placement to receive a single dose of gruticibart (2 mg/kg) administered through the venous catheter within 24 hours of placement and a follow-up surveillance ultrasound at day 14 for evaluation of catheter thrombosis. A parallel, noninterventional study was used as a comparator. RESULTS: In total, 22 subjects (n=11 per study) were enrolled. The overall incidence of catheter-associated thrombosis was 12.5% in the interventional study and 40.0% in the control study. The anti-FXI mAb, gruticibart, significantly prolonged the activated partial thromboplastin time in all subjects on day 14 compared with baseline (P<0.001). Gruticibart was well tolerated and without infusion reactions, drug-related adverse events, or clinically relevant bleeding. Platelet flow cytometry demonstrated no difference in platelet activation following administration of gruticibart. T (thrombin)-AT (antithrombin) and activated FXI-AT complexes increased following central line placement in the control study, which was not demonstrated in our intervention study. CRP (C-reactive protein) did not significantly increase on day 14 in those who received gruticibart, but it did significantly increase in the noninterventional study. CONCLUSIONS: FXI inhibition with gruticibart was well tolerated without any significant adverse or bleeding-related events and resulted in a lower incidence of catheter-associated thrombosis on surveillance ultrasound compared with the published literature and our internal control study. These findings suggest that targeting FXI could represent a safe intervention to prevent catheter thrombosis. REGISTRATION: URL: https://www.clinicaltrials.gov; Unique identifier: NCT04465760.

Pharmacology and Clinical Development of Factor XI Inhibitors.

Therapeutic anticoagulation is indicated for a variety of circumstances and conditions in several fields of medicine to prevent or treat venous and arterial thromboembolism. According to the different mechanisms of action, the available parenteral and oral anticoagulant drugs share the common principle of hampering or blocking key steps of the coagulation cascade, which unavoidably comes at the price of an increased propensity to bleed. Hemorrhagic complications affect patient prognosis both directly and indirectly (ie, by preventing the adoption of an effective antithrombotic strategy). Inhibition of factor XI (FXI) has emerged as a strategy with the potential to uncouple the pharmacological effect and the adverse events of anticoagulant therapy. This observation is based on the differential contribution of FXI to thrombus amplification, in which it plays a major role, and hemostasis, in which it plays an ancillary role in final clot consolidation. Several agents were developed to inhibit FXI at different stages (ie, suppressing biosynthesis, preventing zymogen activation, or impeding the biological action of the active form), including antisense oligonucleotides, monoclonal antibodies, small synthetic molecules, natural peptides, and aptamers. Phase 2 studies of different classes of FXI inhibitors in orthopedic surgery suggested that dose-dependent reductions in thrombotic complications are not paralleled by dose-dependent increases in bleeding compared with low-molecular-weight heparin. Likewise, the FXI inhibitor asundexian was associated with lower rates of bleeding compared with the activated factor X inhibitor apixaban in patients with atrial fibrillation, although no evidence of a therapeutic effect on stroke prevention is available so far. FXI inhibition could also be appealing for patients with other conditions, including end-stage renal disease, noncardioembolic stroke, or acute myocardial infarction, for which other phase 2 studies have been conducted. The balance between thromboprophylaxis and bleeding achieved by FXI inhibitors needs confirmation in large-scale phase 3 clinical trials powered for clinical end points. Several of such trials are ongoing or planned to define the role of FXI inhibitors in clinical practice and to clarify which FXI inhibitor may be most suited for each clinical indication. This article reviews the rationale, pharmacology, results of medium or small phase 2 studies, and future perspectives of drugs inhibiting FXI.

Diagnosis and management of factor XI alloinhibitors in patients with congenital factor XI deficiency-A large single-centre experience.

INTRODUCTION: Factor (F) XI deficiency is an inherited bleeding disorder with increased prevalence in Ashkenazi Jews where it is mainly caused by two variants, p.Glu135* (type II, leading to a null allele) and p.Phe301Leu (type III, missense variant). Inhibitor development is rare, and only seen in severe FXI deficiency (<20 IU/dL) upon exposure to plasma-based products. We report our experience of a large cohort of patients with severe FXI deficiency, including seven patients who developed FXI alloinhibitors, their presentation, natural history and subsequent perioperative management. METHODS: A single-centre retrospective database review of patients with FXI deficiency, including those who have subsequently developed inhibitors, and extraction of clinical, laboratory and genotype data, including operative management records. RESULTS: A total of 682 patients were identified with FXI deficiency, of whom 113 had FXI < 20 IU/dL and 42 had FXI ≤ 1 IU/dL. Factor XI inhibitors were seen in seven patients, six of whom were homozygous for the type II variant (prevalence of inhibitor with this genotype of 30%, risk of inhibitor upon plasma exposure 50%). FXI inhibitors were not seen, despite similar exposures, in patients with other genotypes. No alteration in bleeding phenotype occurred after inhibitor development and subsequent surgery was managed on 13 occasions with recombinant factor VIIa (rFVIIa), including low doses (15-30 µg/kg), with good haemostasis. The inhibitor spontaneously disappeared in four of seven patients over 1-22 years. CONCLUSION: FXI inhibitors were only observed in severe FXI deficient patients homozygous for p.Glu135* (null allele) upon plasma or FXI concentrate exposure, with a 30% prevalence. The bleeding phenotype was not altered and inhibitors may disappear with time. Adequate haemostasis in the perioperative setting is achievable with low doses of rFVIIa.

Real world management of individuals with severe FXI deficiency and its impact on clinical outcomes: Experience from a haemophilia treatment centre.

INTRODUCTION: The management of Factor XI deficiency is challenged by a variable association between FXI level and bleeding phenotype. Additionally, there is scarce data describing management strategies and their outcomes, specifically bleeding, thrombosis, and other complications. AIMS: To evaluate bleeding, thrombosis, and other complications in individuals with severe FXI deficiency seen in our comprehensive haemophilia treatment centre (HTC). Peri-procedural management strategies and the resulting impact on bleeding and other clinically relevant outcomes were reported. METHODS: Retrospective review of the electronic medical record of adult patients with severe FXI deficiency (< 20% activity) seen at a New York City comprehensive HTC between 2017 and 2022. Procedures, haemostatic management, and outcomes were collected and analysed. RESULTS: We identified 38 individuals (64%) females with severe FXI deficiency. The mean age was 56 ± 21 years (SD). The median FXI activity level was 3% (IQR: 1-8%). The mean BAT score was 3.1 ± 2.4; (52%) individuals did not have a history of bleeding. A total of 256 surgeries and procedures were performed. There was reduced bleeding with preventative or reactive treatment during procedures. Arterial but not venous thrombotic complications were observed. Plasma was mostly used for procedures associated with higher risk of bleeding and antifibrinolytics for procedures at sites of high fibrinolysis. CONCLUSIONS: Current management strategies pose a burden of care for these patients and manifested as nonbleeding adverse events and changes in clinical management. These findings highlight the need for novel investigation in predicting and managing bleeding for individuals with severe FXI deficiency.

New Therapeutic Targets for the Prevention and Treatment of Venous Thromboembolism With a Focus on Factor XI Inhibitors.

FXI (factor XI) and FXII (factor XII) have emerged as targets for new anticoagulants that have the potential to be both more efficacious and safer than the currently available direct oral anticoagulants for the prevention and treatment of venous thromboembolism. In this review, we discuss the role of FXI and FXII in the pathogenesis of venous thromboembolism, explain why FXI is a better target, and explain why FXI inhibitors have potential advantages over currently available anticoagulants. Finally, we describe the FXI inhibitors under development and discuss their potential to address unmet needs in venous thromboembolism management.

Factor XI/XIa inhibitors for the prevention and treatment of venous and arterial thromboembolism: A narrative review.

During the past decade, direct oral anticoagulants (DOACs) have advanced and simplified the prevention and treatment of venous thromboembolism (VTE). However, there remains a high incidence of bleeds, which calls for agents that have a reduced risk of bleeding. Factor XI (FXI) deficiency is associated with lower rates of venous thrombosis and stroke compared to the general population with a lower risk of bleeding. In conjunction with this, phase 2 studies have demonstrated safety and the potential for reduced thrombotic events with FXI inhibitors as compared to currently available medications. The aim of this review is to summarize key data on the clinical pharmacology of FXI, the latest developments in clinical trials of FXI inhibitors, and to describe the efficacy and safety profiles of FXI inhibitors for the prevention of venous and arterial thromboembolism.

A Case Report of a Hemodialysis Patient With Coagulation Factor XI and Factor XII Deficiencies.

Coagulation Factor XI (FXI) and Factor XII (FXII) deficiencies are rare. FXI deficiency is associated with a bleeding disorder, while FXII deficiency is not, but both can cause chronic prolongation of activated partial thromboplastin time and impair thrombus formation, posing great challenges for hemodialysis anticoagulation. Traditionally, heparin or low-molecular-weight heparins (LMWHs) are not considered a safe anticoagulation option for patients with increased bleeding risk. In this context, FXI and FXII have received substantial attention as targets for new anticoagulants. We present the case of a 68-year-old woman with combined FXI and FXII deficiencies who successfully underwent hemodialysis with anticoagulation using a low dose of LMWHs. This case highlights that FXI and FXII deficiencies are associated with anticoagulant effects, which can reduce the dosage of anticoagulant during hemodialysis. With careful monitoring, an appropriate dosage of LMWHs is still an acceptable option for patients with a bleeding risk.

Emulation of randomized trials of direct oral anticoagulants with claims data and implications for new Factor XI inhibitors.

Direct oral anticoagulants (DOACs) revolutionized the management of thromboembolic disorders. Clinical care may be further improved as Factor XIs undergo large-scale outcome trials. What role can non-randomized database studies play in expediting understanding of these drugs in clinical practice? The RCT-DUPLICATIVE Initiative emulated the design of eight DOAC randomized clinical trials (RCT) using non-randomized claims database studies. RCT study design parameters and measurements were closely emulated by the database studies and produced highly concordant results. The results of the single database study that did not meet all agreement metrics with the specific RCT it was emulating were aligned with a meta-analysis of six trials studying similar questions, suggesting the trial result was an outlier. Well-designed database studies using fit-for-purpose data came to the same conclusions as DOAC trials, illustrating how database studies could complement RCTs for Factor XI inhibitors-by accelerating insights in underrepresented populations, demonstrating effectiveness and safety in clinical practice, and testing broader indications.

Factor XIa inhibition as a therapeutic strategy for atherothrombosis.

When selecting an anticoagulant, clinicians consider individual patient characteristic, the treatment indication, drug pharmacology, and safety and efficacy as demonstrated in randomized trials. An ideal anticoagulant prevents thrombosis with little or no increase in bleeding. Direct oral anticoagulants represent a major advance over traditional anticoagulants (e.g., unfractionated heparin, warfarin) but still cause bleeding, particularly from the gastrointestinal tract which can limit their use. Epidemiological studies indicate that patients with congenital factor XI (FXI) deficiency have a lower risk of venous thromboembolism (VTE) and ischemic stroke (IS) than non-deficient individuals, and do not have an increased risk of spontaneous bleeding, even with severe deficiency. These observations provide the rationale for targeting FXI as a new class of anticoagulant. Multiple FXI inhibitors have been introduced and several are being evaluated in Phase III trials. In this review, we explain why drugs that target FXI may be associated with a lower risk of bleeding than currently available anticoagulants and summarize the completed and ongoing trials.

Factor XI as a new target for prevention of thromboembolism in cardiovascular disease: a meta-analysis of randomized controlled trials.

Anticoagulant therapy is a mainstay in the management of patients with cardiovascular disease. The use of conventional anticoagulants carries potential side effects, mainly bleeding. Drugs targeting Factor XI (FXI) have been investigated in randomized controlled trials as a new option with more favorable outcomes. A comprehensive literature search was conducted to identify relevant studies comparing FXI inhibitors to placebo or standard therapy. The primary outcomes were incidence of all bleeding events, major bleeding, and thromboembolism. Secondary outcomes included incidence of all adverse events (AE), serious AE, and all-cause mortality. A total of 11 studies involving 10,536 patients were included. FXI inhibitors were associated with a trend toward reduction of bleeding events and incidence of thromboembolism compared to the control group (placebo/standard therapy). There was no statistically significant difference between both groups in terms of adverse events and all-cause mortality. When compared to enoxaparin, FXI inhibitors significantly reduced the risk of bleeding events (RR = 0.42, 95% CI: 0.23-0.76, P = 0.004) and thromboembolism (RR = 0.59, 95% CI: 0.44-0.77, P = 0.001). On the other hand, when compared to DOACs, FXI inhibitors were associated with a significant reduction in bleeding events but not thromboembolism. Whereas, compared to placebo, FXI inhibitors did not increase the risk of bleeding events, adverse events, or all-cause mortality (P > 0.05). FXI inhibitors could be a safer and more potent option for prevention of thromboembolism than conventional therapy.

The foundation for investigating factor XI as a target for inhibition in human cardiovascular disease.

Anticoagulant therapy is a mainstay in the management of patients with cardiovascular disease and related conditions characterized by a heightened risk for thrombosis. Acute coronary syndrome, chronic coronary syndrome, ischemic stroke, and atrial fibrillation are the most common. In addition to their proclivity for thrombosis, each of these four conditions is also characterized by local and systemic inflammation, endothelial/endocardial injury and dysfunction, oxidative stress, impaired tissue-level reparative capabilities, and immune dysregulation that plays a critical role in linking molecular events, environmental triggers, and phenotypic expressions. Knowing that cardiovascular disease and thrombosis are complex and dynamic, can the scientific community identify a common pathway or specific point of interface susceptible to pharmacological inhibition or alteration that is likely to be safe and effective? The contact factors of coagulation may represent the proverbial "sweet spot" and are worthy of investigation. The following review provides a summary of the fundamental biochemistry of factor XI, its biological activity in thrombosis, inflammation, and angiogenesis, new targeting drugs, and a pragmatic approach to managing hemostatic requirements in clinical trials and possibly day-to-day patient care in the future.

Elevated factor XI is associated with increased risk of recurrent cerebral venous sinus thrombosis: a cohort study.

Cerebral venous sinus thrombosis (CVST) has no identified cause in 15% of cases. Elevated factors (F) VIII and FXI have been associated with thromboembolism, but data on CVST are limited. We hypothesized that elevated plasma FVIII and FXI predispose to first and recurrent CVST. In 50 CVST survivors aged < 60 years, following anticoagulant cessation and in 50 controls, we determined plasma FVIII and FXI, along with fibrin clot properties: lysis time, permeability, maximum D-dimer (D-Dmax), and maximum rate of D-dimer increase (D-Drate). We recorded CVST recurrence during a follow-up of 58.5 (55.0-60.0) months. Plasma FVIII was 22.7% higher in CVST than in controls, with elevated FVIII > 150% in 13 (26%) vs. 4 (8%) patients, respectively (p = 0.02). Median FXI tended to be higher in CVST vs. controls (110.5 [99.0-117-0]% vs. 104.5 [97.0-116.0]%, p = 0.07), while FXI > 120% was observed more commonly in the former group (12 [24%] vs. 4 [8%], respectively, p = 0.03). Patients with FVIII > 150% were less likely to achieve complete recanalization compared with the remainder (2 [15.4%] vs. 28 [75.7%], respectively; p < 0.001). Eight patients (16%) experienced CVST recurrence. They had higher baseline FXI, but not FVIII, as compared with the remainder (125.5 [114.5-140.0]% vs. 107.5 [102.0-117.0]%, respectively, p = 0.01). Patients with FXI > 120% were four times more likely to have recurrent CVST (5 [62.5%] vs. 7 [16.7%], respectively; p = 0.01). Plasma FXI > 120% could represent a novel risk factor for first and recurrent CVST. Given advances in anti-FXI agents, CVST might be another indication for this emerging treatment.

Factor XI: structure, function and therapeutic inhibition.

Arterial and venous thromboembolism is a major medical concern that requires therapeutic anticoagulation in various medical fields to prevent its drastic consequences. Despite significant advances in anticoagulant therapy, thrombosis remains a leading cause of morbidity and mortality worldwide. Traditional anticoagulants like heparin and vitamin K antagonists (VKAs) have shown efficacy in preventing and treating thrombosis but come with an inherent risk of bleeding due to their non-specific inhibition of multiple coagulation factors. Subsequent direct oral anticoagulants (DOACs), targeting specific factors such as Xa or thrombin, demonstrated improved safety profiles compared to VKAs, yet bleeding remains a concern. Accordingly, research is focused on developing anticoagulants with improved safety profiles. A safer class of anticoagulants would have broad appeal. The intrinsic pathway of coagulation, involving factor XI (FXI), has attracted attention as a potential target for safer anticoagulants. Preclinical studies and epidemiological data indicate that FXI deficiency or inhibition protects against thrombosis with minimal bleeding. Current research involves evaluating various FXI-directed strategies, and phase 2 studies have shown promising results in orthopedic surgery, atrial fibrillation, end-stage renal disease (ESRD), myocardial infarction, and ischemic stroke. Several agents, such as antisense oligonucleotides, monoclonal antibodies, small synthetic molecules, natural peptides, and aptamers, have been developed to inhibit FXI at different stages, offering potentially safer alternatives to traditional anticoagulants. However, the optimal balance between preventing thrombosis and the risk of bleeding associated with FXI inhibitors requires validation through extensive phase 3 clinical trials using definite clinical endpoints. Several of such trials are currently underway or planned to define the role of FXI inhibitors in clinical practice and determine the most suitable FXI inhibitor for each specific indication. The current review highlights the rationale behind developing FXI inhibitors, presenting the most advanced agents in development, summarizing completed clinical trials, and discussing ongoing research efforts.

Factor XI inhibition in patients with acute coronary syndrome.

A hypercoagulable condition is typical of patients with acute coronary syndrome and is a determining factor in the genesis of recurrent ischaemic events. Modern pharmacological therapies consisting of antiplatelets and anticoagulants derive their rationale for use on the pathophysiological mechanisms most commonly associated with myocardial infarction (MI); they have contributed to reducing the ischaemic risk of these patients, but left ample room for improvement. In particular, trials that have studied the association of an anticoagulant with antiplatelet drugs have provided promising results in terms of efficacy, but highlighted a significant bleeding risk. Evidence derived from experimental animal and epidemiological studies has shown how factor XI (FXI) deficiency is associated with a reduction in thrombotic events but with modest bleeding. These data added to the role that FXI plays in the coagulation cascade constituted an incipit for the pharmacological attempt to decouple thrombosis from haemostasis by means of the inhibition of this factor. The theoretical assumption that FXI inhibitor drugs may be able to reduce the ischaemic risk without significantly increasing the haemorrhagic risk makes these compounds a potential therapeutic aid for patients in secondary prevention after acute MI. To date, on these patients, we only have data from a Phase 2 trial, PACIFIC-AMI (Study to Gather Information About the Proper Dosing and Safety of the Oral FXIa Inhibitor BAY 2 433 334 in Patients Following an Acute Heart Attack). In this study, the primary endpoint-represented by the Bleeding Academic Research Consortium (BARC) composite of Type 2, 3, or 5 bleeding-showed no significant differences between the various doses of asundexian tested (10, 20, and 50 mg quoque die), and between these and placebo (asundexian all doses vs. placebo: hazard ratio, 0.98; 90% confidence interval, 0.71-1.35). The data on efficacy, however, showed neutral results, but it should be noted that the study did not have the adequate statistical power to evaluate this outcome. Valuable information could, therefore, derive in the future from the ongoing Phase 3 trial with milvexian, LIBREXIA-ACS (A Study of Milvexian in Participants After a Recent Acute Coronary Syndrome) and from any future studies that could be started by testing different molecules.

Will Factor XI Inhibitors Replace Current Anticoagulants for Stroke Prevention in Atrial Fibrillation?

PURPOSE OF REVIEW: This review provides an overview of the factor XI (FXI) inhibitor hypothesis for the development of novel anticoagulants which may be safer to those currently used in clinical practice and describes preliminary clinical data from phase 2 dose-ranging studies of patients with atrial fibrillation. RECENT FINDINGS: Recent data from phase 2 dose ranging studies demonstrate substantial reductions in bleeding with FXI pathway inhibition compared with currently approved anticoagulants. However, larger studies are necessary to demonstrate efficacy of FXI inhibition for stroke prevention in atrial fibrillation. FXI pathway inhibition holds great promise for revolutionizing the landscape of anticoagulation for atrial fibrillation, primarily by reducing bleeding risk; however, further data are necessary to demonstrate efficacy.

Reversal agents for current and forthcoming direct oral anticoagulants.

Over the past 20 years, there has been a shift from vitamin K antagonists to direct oral anticoagulants (DOACs), which include the thrombin inhibitor dabigatran and the factor Xa inhibitors apixaban, edoxaban, and rivaroxaban. Although DOACs are associated with less serious bleeding than vitamin K antagonists, bleeding still occurs with DOACs, particularly in the elderly and in those with comorbidities. Reversal of the anticoagulant effects of the DOACs may be needed in patients with serious bleeding and in those requiring urgent surgery or intervention. Reversal can be effected with specific agents, such as idarucizumab for dabigatran and andexanet alfa for apixaban, edoxaban, and rivaroxaban, or with non-specific agents, such as prothrombin complex concentrates, activated prothrombin complex concentrate, and recombinant activated factor VII. This paper (i) provides an update on when and how to reverse the DOACs, (ii) describes new reversal agents under development, and (iii) provides a strategic framework for the reversal of the factor XI inhibitors currently under investigation in phase three clinical trials.

Factor XI inhibitors: cardiovascular perspectives.

Anticoagulants are the cornerstone for prevention and treatment of thrombosis but are not completely effective, and concerns about the risk of bleeding continue to limit their uptake. Animal studies and experience from patients with genetic coagulation factor XI deficiency suggesting that this factor is more important for thrombosis than for haemostasis raises the potential for drugs that target factor XI to provide safer anticoagulation. Multiple factor XI inhibitors are currently under evaluation in clinical trials, including parenterally administered antisense oligonucleotides, monoclonal antibodies, and orally active small-molecule inhibitors. Promising results of phase 2 trials in patients undergoing major orthopaedic surgery, and in those with end-stage kidney disease, atrial fibrillation and acute coronary syndromes have led to large phase 3 trials that are currently ongoing. We here review premises for the use of these agents, results so far accrued, ongoing studies, and perspectives for future patient care.

Skeletal muscle myosin promotes coagulation by binding factor XI via its A3 domain and enhancing thrombin-induced factor XI activation.

Skeletal muscle myosin (SkM) has been shown to possess procoagulant activity; however, the mechanisms of this coagulation-enhancing activity involving plasma coagulation pathways and factors are incompletely understood. Here, we discovered direct interactions between immobilized SkM and coagulation factor XI (FXI) using biolayer interferometry (Kd = 0.2 nM). In contrast, we show that prekallikrein, a FXI homolog, did not bind to SkM, reflecting the specificity of SkM for FXI binding. We also found that the anti-FXI monoclonal antibody, mAb 1A6, which recognizes the Apple (A) 3 domain of FXI, potently inhibited binding of FXI to immobilized SkM, implying that SkM binds FXI A3 domain. In addition, we show that SkM enhanced FXI activation by thrombin in a concentration-dependent manner. We further used recombinant FXI chimeric proteins in which each of the four A domains of the heavy chain (designated A1 through A4) was individually replaced with the corresponding A domain from prekallikrein to investigate SkM-mediated enhancement of thrombin-induced FXI activation. These results indicated that activation of two FXI chimeras with substitutions of either the A3 domains or A4 domains was not enhanced by SkM, whereas substitution of the A2 domain did not reduce the thrombin-induced activation compared with wildtype FXI. These data strongly suggest that functional interaction sites on FXI for SkM involve the A3 and A4 domains. Thus, this study is the first to reveal and support the novel intrinsic blood coagulation pathway concept that the procoagulant mechanisms of SkM include FXI binding and enhancement of FXI activation by thrombin.

Discovery of Teleost Plasma Kallikrein/Coagulation Factor XI-Like Gene from Channel Catfish (Ictalurus punctatus) and the Evidence that the Protein Encoded by it Acts as a Lectin.

Mammalian plasma kallikrein (PK) and coagulation factor XI (fXI) are serine proteases that play in the kinin-kallikrein cascade and in the blood clotting pathway. These proteases share sequence homology and have four apple domains (APDs) and a serine protease domain (SPD) from their N-terminus to C-terminus. No homologs of these proteases are believed to be present in fish species, except for lobe-finned fish. Fish, however, have a unique lectin, named kalliklectin (KL), which is composed of APDs only. In the present study, we found genomic sequences encoding a protein with both APDs and SPD in a few cartilaginous and bony fishes, including the channel catfish Ictalurus punctatus, using bioinformatic analysis. Furthermore, we purified two ~ 70 kDa proteins from the blood plasma of the catfish using mannose-affinity and gel filtration chromatography sequentially. Using de novo sequencing with quadrupole time-of-flight tandem mass spectrometry, several internal amino acid sequences in these proteins were mapped onto possible PK/fXI-like sequences that are thought to be splicing variants. Exploration of APD-containing proteins in the hagfish genome database and phylogenetic analysis suggested that the PK/fXI-like gene originated from hepatocyte growth factor, and that the gene was acquired in a common ancestor of jawed fish. Synteny analysis provided evidence for chromosomal translocation around the PK/fXI-like locus that occurred in the common ancestor of holosteans and teleosts after separation from the lobe-finned fish lineage, or gene duplication into two chromosomes, followed by independent gene losses. This is the first identification of PK/fXI-like proteins in teleosts.