Fasxiator, a novel factor XIa inhibitor from snake venom, and its site-specific mutagenesis to improve potency and selectivity

Deciphering toxico-proteomics of Asiatic medically significant venomous snake species: A systematic review and interactive data dashboard

Snakebite envenomation (SBE) is a neglected tropical disease (NTD) with an approximate 1.8 million cases annually. The tremendous figure is concerning, and the currently available treatment for snakebite envenomation is antivenom. However, the current antivenom has limited cross-neutralisation activity due to the variations in snake venom composition across species and geographical locations. The proteomics of medically important venomous species is essential as they study the venom compositions within and among different species. The advancement of sophisticated proteomic approaches allows intensive investigation of snake venoms. Nevertheless, there is a need to consolidate the venom proteomics profiles and distribution analysis to examine their variability patterns. This review systematically analysed the proteomics and toxicity profiles of medically important venomous species from Asia across different geographical locations. An interactive dashboard - Asiatic Proteomics Interactive Datasets was curated to consolidate the distribution patterns of the venom compositions, serve as a comprehensive directory for large-scale comparative meta-analyses. The population proteomics demonstrate higher diversities in the predominant venom toxins. Besides, inter-regional differences were also observed in Bungarus sp., Naja sp., Calliophis sp., and Ophiophagus hannah venoms. The elapid venoms are predominated with three-finger toxins (3FTXs) and phospholipase A2 (PLA2). Intra-regional variation is only significantly observed in Naja naja venoms. Proteomics diversity is more prominent in viper venoms, with widespread dominance observed in snake venom metalloproteinase (SVMP) and snake venom serine protease (SVSP). Correlations exist between the proteomics profiles and the toxicity (LD50) of the medically important venomous species. Additionally, the predominant toxins, alongside their pathophysiological effects, were highlighted and discussed as well. The insights of interactive toxico-proteomics datasets provide comprehensive frameworks of venom dynamics and contribute to developing antivenoms for snakebite envenomation. This could reduce misdiagnosis of SBE and accelerate the researchers' data mining process.

Characterization of Kunitz-Domain Anticoagulation Peptides Derived from Acinetobacter baumannii Exotoxin Protein F6W77

Recent studies have revealed that the coagulation system plays a role in mammalian innate defense by entrapping bacteria in clots and generating antibacterial peptides. So, it is very important for the survival of bacteria to defend against the host coagulation system, which suggests that bacterial exotoxins might be a new source of anticoagulants. In this study, we analyzed the genomic sequences of Acinetobacter baumannii and a new bacterial exotoxin protein, F6W77, with five Kunitz-domains, KABP1-5, was identified. Each Kunitz-type domain features a classical six-cysteine framework reticulated by three conserved disulfide bridges, which was obviously similar to animal Kunitz-domain peptides but different from plant Kunitz-domain peptides. Anticoagulation function evaluation showed that towards the intrinsic coagulation pathway, KABP1 and KABP5 had apparently inhibitory activity, KABP4 had weak inhibitory activity, and KBAP2 and KABP3 had no effect even at a high concentration of 20 μg/mL. All five Kunitz-domain peptides, KABP1-5, had no inhibitory activity towards the extrinsic coagulation pathway. Enzyme-inhibitor experiments showed that the high-activity anticoagulant peptide KABP1 had apparently inhibitory activity towards two key coagulation factors, Xa and XIa, which was further confirmed by pull-down experiments that showed that KABP1 can bind to coagulation factors Xa and XIa directly. Structure-function relationship analyses of five Kunitz-type domain peptides showed that the arginine of the P1 site of three new bacterial anticoagulants, KABP1, KABP4 and KABP5, might be the key residue for their anticoagulation activity. In conclusion, with bioinformatics analyses, peptide recombination, and functional evaluation, we firstly found bacterial-exotoxin-derived Kunitz-type serine protease inhibitors with selectively inhibiting activity towards intrinsic coagulation pathways, and highlighted a new interaction between pathogenic bacteria and the human coagulation system.

Novel horizons in anticoagulation: the emerging role of factor XI inhibitors across different settings

Anticoagulants have long been fundamental in preventing and treating thromboembolic disorders, with a recent shift of focus towards direct oral anticoagulants, thanks to their ease of use, efficacy, and safety. Despite these advancements, bleeding complications remain a major concern with any anticoagulant, highlighting the need for safer drugs. Factor XI (FXI) inhibitors have emerged as promising agents in this regard, offering a novel approach by targeting upstream factors in the coagulation system. Phase II trials have shown encouraging outcomes, indicating a reduced bleeding risk compared to that associated with traditional anticoagulants, particularly in the context of cardiovascular disease management when combined with antiplatelet therapy. However, the variability in findings and limited efficacy data call for a cautious interpretation pending insights from phase III trials. These trials are essential for validating the potential of FXI inhibitors to balance bleeding risk reduction and maintain anticoagulant efficacy. This review explores the pharmacology, potential indications, clinical data, and future directions of FXI inhibitors, providing a perspective on their evolving role in anticoagulant therapy. It also provides a detailed analysis of data from published clinical trials on FXI inhibitors in various indications. Preliminary data from ongoing trials are also outlined. As the field moves forward, a cautiously optimistic outlook can be expected, focusing on comprehensive data from phase III trials to define the role of FXI inhibitors in various clinical scenarios.

Therapeutic Potential of FXI Inhibitors: Hype or Hope?

Significant advancements have shaped the landscape of anticoagulant therapy in the past two decades, including the introduction of direct oral anticoagulants (DOACs), characterized by favorable safety and efficacy profiles and reduced drug-to-drug or food interaction resulting in excellent patient compliance. However, residual concerns still exist with standard-of-care anticoagulant therapy, including the inability to use DOACs in several clinical settings and the need to further reduce the risk of bleeding. Recent improvements in the understanding of the mechanisms behind thrombus formation have led to the awareness that the intrinsic pathway of the coagulation cascade may play an important role in pathological thrombosis, but not in hemostasis. This has represented the rationale for targeting this pathway with factor XI (FXI) inhibitors, with the aim of uncoupling hemostasis and thrombosis. Clinical evidence from patients with FXI deficiency further supports this concept. A number of compounds with different mechanisms of action have been developed to target FXI (i.e., asundexian, abelacimab, Ionis-FXIRx, milvexian, osocimab, and Xisomab 3G). To date, the majority of available trials have not gone beyond completion of phase 2 and results are conflictive making it difficult to appraise the clinical benefit of these compounds in the different clinical settings where they have been tested (i.e., atrial fibrillation, acute ischemic stroke, acute myocardial infarction, end-stage renal disease, total knee arthroplasty). Moreover, the largest phase 3 randomized trial designed to test the efficacy of asundexian over apixaban in patients with atrial fibrillation, the OCEANIC-AF, has been prematurely stopped as a result of the inferior efficacy of asundexian. In this review we discuss the pharmacological properties and available evidence generated thus far for factor XI inhibitors, providing a perspective on the current state of these drugs.

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.

Blood Lines: Intraspecific and Interspecific Variations in Anticoagulant Actions of Agkistrodon Viperid Venoms

This study investigated the intraspecific and interspecific variability in the venom effects of Agkistrodon viperid snake species and subspecies (eleven venoms total) on plasma clotting times, fibrinogen levels, and fibrin clot strength. Significant delays in plasma clotting time were observed for A. conanti, A. contortrix mokasen, A. contortrix phaeogaster, A. howardgloydi, A. piscivorus leucostoma, and A. piscivorus piscivorus. Notably, the phylogenetically disjunct lineages A. conanti, A. contortrix mokasen, and A. howardgloydi exhibited the most potent anticoagulant effects, indicating the independent amplification of a basal trait. Inhibition assays with the activated clotting enzymes Factors XIa, IXa, Xa, and IIa (thrombin) revealed that FXa inhibition is another basal trait amplified independently on multiple occasions within the genus, but with A. howardgloydi, notably more potent than all others. Phospholipid degradation and zymogen destruction were identified as mechanisms underlying the variability in venom effects observed experimentally and in previous clinical reports. Thromboelastography demonstrated that the venoms did not clot fibrinogen directly but affected fibrin clot strength by damaging fibrinogen and that thrombin was subsequently only able to cleave into weak, unstable clots. The ability to activate Protein C, an endogenous anticoagulant enzyme, varied across species, with some venoms exceeding that of A. contortrix contortrix, which previously yielded the protein diagnostic agent Protac®. Phylogenetic analysis suggested that both fibrinogen degradation and Protein C activation were each amplified multiple times within the genus, albeit with negative correlation between these two modes of action. This study highlights the evolutionary, clinical, and biodiscovery implications of venom variability in the Agkistrodon species, underscoring their dynamic evolution, emphasising the need for tailored clinical approaches, and highlighting the potential for novel diagnostic and therapeutic developments inspired by the unique properties of snake venoms.

The Clot Thickens: Differential Coagulotoxic and Cardiotoxic Activities of Anguimorpha Lizard Venoms

Despite their evolutionary novelty, lizard venoms are much less studied in comparison to the intense research on snake venoms. While the venoms of helodermatid lizards have long been assumed to be for defensive purposes, there is increasing evidence of toxic activities more useful for predation than defence (such as paralytic neurotoxicity). This study aimed to ascertain the effects of Heloderma, Lanthanotus, and Varanus lizard venoms on the coagulation and cardiovascular systems. Anticoagulant toxicity was demonstrated for the Varanus species studied, with the venoms prolonging clotting times in human and bird plasma due to the destructive cleavage of fibrinogen. In contrast, thromboelastographic analyses on human and bird plasmas in this study demonstrated a procoagulant bioactivity for Heloderma venoms. A previous study on Heloderma venom using factor-depleted plasmas as a proxy model suggested a procoagulant factor was present that activated either Factor XI or Factor XII, but could not ascertain the precise target. Our activation studies using purified zymogens confirmed FXII activation. Comparisons of neonate and adult H. exasperatum, revealed the neonates to be more potent in the ability to activate FXII, being more similar to the venom of the smaller species H. suspectum than the adult H. exasperatum. This suggests potent FXII activation a basal trait in the genus, present in the small bodied last common ancestor. This also indicates an ontogenetic difference in prey preferences in the larger Heloderma species paralleing the change in venom biochemistry. In addition, as birds lack Factor XII, the ability to clot avian plasma suggested an additional procoagulant site of action, which was revealed to be the activation of Factor VII, with H. horridum being the most potent. This study also examined the effects upon the cardiovascular system, including the liberation of kinins from kininogen, which contributes to hypotension induction. This form of toxicity was previously described for Heloderma venoms, and was revealed in this study was to also be a pathophysiological effect of Lanthanotus and Varanus venoms. This suggests that this toxic activity was present in the venom of the last common ancestor of the anguimorph lizards, which is consistent with kallikrein enzymes being a shared toxin trait. This study therefore uncovered novel actions of anguimorph lizard venoms, not only contributing to the evolutionary biology body of knowledge but also revealing novel activities to mine for drug design lead compounds.

Pharmacological and clinical appraisal of factor XI inhibitor drugs

The evolution of anticoagulation therapy, from vitamin K antagonists to the advent of direct oral anticoagulants (DOACs) almost two decades ago, marks significant progress. Despite improved safety demonstrated in pivotal trials and post-marketing observations, persistent concerns exist, particularly regarding bleeding risk and the absence of therapeutic indications in specific subgroups or clinical contexts. Factor XI (FXI) has recently emerged as a pivotal contributor to intraluminal thrombus formation and growth, playing a limited role in sealing vessel wall injuries. Inhibiting FXI presents an opportunity to decouple thrombosis from haemostasis, addressing concerns related to bleeding events while safeguarding against thromboembolic events. Notably, FXI inhibition holds promise for patients with end-stage renal disease or cancer, where clear indications for DOACs are currently lacking. Various compounds have undergone design, testing, and progression to phase 2 clinical trials, demonstrating a generally favourable safety and tolerability profile. However, validation through large-scale phase 3 trials with sufficient power to assess both safety and efficacy outcomes is needed. This review comprehensively examines FXI inhibitors, delving into individual classes, exploring their pharmacological properties, evaluating the latest evidence from randomized trials, and offering insights into future perspectives.

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.

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 XIa Inhibitors in Anticoagulation Therapy: Recent Advances and Perspectives

Factor XIa (FXIa) in the intrinsic pathway of the coagulation process has been proven to be an effective and safe target for anticoagulant discovery with limited or no bleeding. Numerous small-molecule FXIa inhibitors (SMFIs) with various scaffolds have been identified in the early stages of drug discovery. They have served as the foundation for the recent discovery of additional promising SMFIs with improved potency, selectivity, and pharmacokinetic profiles, some of which have entered clinical trials for the treatment of thrombosis. After reviewing the coagulation process and structure of FXIa, this perspective discusses the rational or structure-based design, discovery, structure-activity relationships, and development of SMFIs disclosed in recent years. Strategies for identifying more selective and druggable SMFIs are provided, paving the way for the design and discovery of more useful SMFIs for anticoagulation therapy.

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.

Coming soon to a pharmacy near you? FXI and FXII inhibitors to prevent or treat thromboembolism

Anticoagulants have been in use for nearly a century for the treatment and prevention of venous and arterial thromboembolic disorders. The most dreaded complication of anticoagulant treatment is the occurrence of bleeding, which may be serious and even life-threatening. All available anticoagulants, which target either multiple coagulation factors or individual components of the tissue factor (TF) factor VIIa or the common pathways, have the potential to affect hemostasis and thus to increase bleeding risk in treated patients. While direct oral anticoagulants introduced an improvement in care for eligible patients in terms of safety, efficacy, and convenience of treatment, there remain unmet clinical needs for patients requiring anticoagulant drugs. Anticoagulant therapy is sometimes avoided for fear of hemorrhagic complications, and other patients are undertreated due to comorbidities and the perception of increased bleeding risk. Evidence suggests that the contact pathway of coagulation has a limited role in initiating physiologic in vivo coagulation and that it contributes to thrombosis more than it does to hemostasis. Because inhibition of the contact pathway is less likely to promote bleeding, it is an attractive target for the development of anticoagulants with improved safety. Preclinical and early clinical data indicate that novel agents that selectively target factor XI or factor XII can reduce venous and arterial thrombosis without an increase in bleeding complications.

Factor XI, a potential target for anticoagulation therapy for venous thromboembolism

Venous thromboembolism (VTE) is a common cause of mortality and disability in hospitalized patients, and anticoagulation is an essential therapeutic option. Despite the increasing use of direct oral anticoagulants, complications and adverse drug reactions still occur in patients with VTE. Within 5 years, 20% of patients with VTE experience recurrence, and 50% of patients with deep vein thrombosis develop post-thrombotic syndrome. Furthermore, bleeding due to anticoagulants is a side effect that must be addressed. Therefore, safer and more effective anticoagulant strategies with higher patient compliance are urgently needed. Available epidemiological evidence and animal studies have shown that factor XI (FXI) inhibitors can reduce thrombus size and loosen the thrombus structure with a relatively low risk of bleeding, suggesting that FXI has an important role in thrombus stabilization and is a safer target for anticoagulation. Recent clinical trial data have also shown that FXI inhibitors are as effective as enoxaparin and apixaban in preventing VTE, but with a significantly lower incidence of bleeding. Furthermore, FXI inhibitors can be administered daily or monthly; therefore, the monitoring interval can be longer. Additionally, FXI inhibitors can prolong the activated partial thromboplastin time without affecting prothrombin time, which is an easy and common test used in clinical testing, providing a cost-effective monitoring routine for patients. Consequently, the inhibition of FXI may be an effective strategy for the prevention and treatment of VTE. Enormous progress has been made in the research strategies for FXI inhibitors, with abelacimab already in phase III clinical trials and most other inhibitors in phase I or II trials. In this review, we discuss the challenges of VTE therapy, briefly describe the structure and function of FXI, summarize the latest FXI/activated FXI (FXIa) inhibitor strategies, and summarize the latest developments in clinical trials of FXI/FXIa inhibitors.

A Factor XIa Inhibitor Engineered from Banded Krait Venom Toxin: Efficacy and Safety in Rodent Models of Arterial and Venous Thrombosis

Activated factor XI (FXIa) is an important antithrombotic drug target. Clinical and pre-clinical data have demonstrated that its inhibition attenuates thrombosis with minimal risk of excessive bleeding. We isolated Fasxiator from the venom of banded krait Bungarus fasciatus and subsequently engineered FasxiatorN17R,L19E, with improved affinity (Ki = 0.9 nM) and selectivity towards FXIa. Here, we assess the in vivo efficacy and bleeding risk of rFasxiatorN17R, L19E in pre-clinical animal models. Rats injected intravenously (i.v.) with bolus rFasxiatorN17R, L19E showed the specific in vivo attenuation of the intrinsic coagulation pathway, lasting for at least 60 min. We performed the in vivo dose-ranging experiments for rFasxiatorN17R, L19E as follows: FeCl3-induced carotid artery occlusion in rats (arterial thrombosis); inferior vena cava ligation in mice (venous thrombosis); tail bleeding time in both rats and mice (bleeding risk). Head-to-head comparisons were made using therapeutic dosages of unfractionated heparin (UFH) and low-molecular-weight heparin (LMWH) for arterial and venous thrombosis, respectively. In the arterial thrombosis model, 2 mg/kg i.v. rFasxiatorN17R,L19E achieved a similar antithrombotic efficacy to that of UFH, with >3-fold lower bleeding time. In the venous thrombosis model, the 10 mg/kg subcutaneous (s.c.) injection of rFasxiatorN17R,L19E achieved similar efficacy and bleeding levels to those of LMWH enoxaparin. Overall, rFasxiatorN17R,L19E represents a promising molecule for the development of FXIa-targeting anticoagulants.

State-of-the-art review - A review on snake venom-derived antithrombotics: Potential therapeutics for COVID-19-associated thrombosis?

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), the causative agent responsible for the Coronavirus Disease-2019 (COVID-19) pandemic, has infected over 185 million individuals across 200 countries since December 2019 resulting in 4.0 million deaths. While COVID-19 is primarily associated with respiratory illnesses, an increasing number of clinical reports indicate that severely ill patients often develop thrombotic complications that are associated with increased mortality. As a consequence, treatment strategies that target COVID-associated thrombosis are of utmost clinical importance. An array of pharmacologically active compounds from natural products exhibit effects on blood coagulation pathways, and have generated interest for their potential therapeutic applications towards thrombotic diseases. In particular, a number of snake venom compounds exhibit high specificity on different blood coagulation factors and represent excellent tools that could be utilized to treat thrombosis. The aim of this review is to provide a brief summary of the current understanding of COVID-19 associated thrombosis, and highlight several snake venom compounds that could be utilized as antithrombotic agents to target this disease.

WPK5, a Novel Kunitz-Type Peptide from the Leech Whitmania pigra Inhibiting Factor XIa, and Its Loop-Replaced Mutant to Improve Potency

Kunitz-type proteins or peptides have been found in many blood-sucking animals, but the identity of them in leeches remained elusive. In the present study, five Kunitz-type peptides named WPK1-WPK5 were identified from the leech Whitmania pigra. Recombinant WPK1-WPK5 were expressed in Pichia pastoris GS115, and their inhibitory activity against Factor XIa (FXIa) was tested. WPK5 showed inhibitory activity against FXIa with an IC₅₀ value of 978.20 nM. To improve its potency, the loop replacement strategy was used. The loop 1 (TGPCRSNLER) and loop 2 (QYGGC) in WPK5 were replaced by loop 1 (TGPCRAMISR) and loop 2 (FYGGC) in PN2KPI, respectively, and the resulting peptide named WPK5-Mut showed an IC₅₀ value of 8.34 nM to FXIa, which is about 100-fold the potency of FXIa compared to that of WPK5. WPK5-Mut was further evaluated for its extensive bioactivity in vitro and in vivo. It dose-dependently prolonged APTT on both murine plasma and human plasma, and potently inhibited FeCl₃-induced carotid artery thrombosis in mice at a dose of 1.5 mg/kg. Additionally, WPK5-Mut did not show significant bleeding risk at a dose of 6 mg/kg. Together, these results showed that WPK5-Mut is a promising candidate for the development of an antithrombotic drug.

A general approach to protein folding using thermostable exoshells

In vitro protein folding is a complex process which often results in protein aggregation, low yields and low specific activity. Here we report the use of nanoscale exoshells (tES) to provide complementary nanoenvironments for the folding and release of 12 highly diverse protein substrates ranging from small protein toxins to human albumin, a dimeric protein (alkaline phosphatase), a trimeric ion channel (Omp2a) and the tetrameric tumor suppressor, p53. These proteins represent a unique diversity in size, volume, disulfide linkages, isoelectric point and multi versus monomeric nature of their functional units. Protein encapsulation within tES increased crude soluble yield (3-fold to >100-fold), functional yield (2-fold to >100-fold) and specific activity (3-fold to >100-fold) for all the proteins tested. The average soluble yield was 6.5 mg/100 mg of tES with charge complementation between the tES internal cavity and the protein substrate being the primary determinant of functional folding. Our results confirm the importance of nanoscale electrostatic effects and provide a solution for folding proteins in vitro.

DAKS1, a Kunitz Scaffold Peptide from the Venom Gland of Deinagkistrodon acutus Prevents Carotid-Artery and Middle-Cerebral-Artery Thrombosis via Targeting Factor XIa

Scaffold-based peptides (SBPs) are fragments of large proteins that are characterized by potent bioactivity, high thermostability, and low immunogenicity. Some SBPs have been approved by the FDA for human use. In the present study, we developed SBPs from the venom gland of Deinagkistrodon acutus (D. acutus) by combining transcriptome sequencing and Pfam annotation. To that end, 10 Kunitz peptides were discovered from the venom gland of D. acutus, and most of which peptides exhibited Factor XIa (FXIa) inhibitory activity. One of those, DAKS1, exhibiting strongest inhibitory activity against FXIa, was further evaluated for its anticoagulant and antithrombotic activity. DAKS1 prolonged twofold APTT at a concentration of 15 μM in vitro. DAKS1 potently inhibited thrombosis in a ferric chloride-induced carotid-artery injury model in mice at a dose of 1.3 mg/kg. Furthermore, DAKS1 prevented stroke in a transient middle cerebral-artery occlusion (tMCAO) model in mice at a dose of 2.6 mg/kg. Additionally, DAKS1 did not show significant bleeding risk at a dose of 6.5 mg/kg. Together, our results indicated that DAKS1 is a promising candidate for drug development for the treatment of thrombosis and stroke disorders.

Development of Selective FXIa Inhibitors Based on Cyclic Peptides and Their Application for Safe Anticoagulation

Coagulation factor XI (FXI) has emerged as a promising target for the development of safer anticoagulation drugs that limit the risk of severe and life-threatening bleeding. Herein, we report the first cyclic peptide-based FXI inhibitor that selectively and potently inhibits activated FXI (FXIa) in human and animal blood. The cyclic peptide inhibitor (K_i = 2.8 ± 0.5 nM) achieved anticoagulation effects that are comparable to that of the gold standard heparin applied at a therapeutic dose (0.3-0.7 IU/mL in plasma) but with a substantially broader estimated therapeutic range. We extended the plasma half-life of the peptide via PEGylation and demonstrated effective FXIa inhibition over extended periods in vivo. We validated the anticoagulant effects of the PEGylated inhibitor in an ex vivo hemodialysis model with human blood. Our work shows that FXI can be selectively targeted with peptides and provides a promising candidate for the development of a safe anticoagulation therapy.

Factor XI as a Target for New Anticoagulants

Despite advances in anticoagulant therapy, thrombosis remains the leading cause of morbidity and mortality worldwide. Heparin and vitamin K antagonists (VKAs), the first anticoagulants to be used successfully for the prevention and treatment of thrombosis, are associated with a risk of bleeding. These agents target multiple coagulation factors. Thus, by activating antithrombin, heparin mainly inhibits factor Xa and thrombin, whereas VKAs lower the levels of the vitamin K-dependent clotting factors. Direct oral anticoagulants, which have replaced VKAs for many indications, inhibit only factor Xa or thrombin. Although the direct oral anticoagulants are associated with less bleeding than VKAs, bleeding remains their major side effect. Epidemiological and animal studies have identified factor XI as a target for potentially safer anticoagulant drugs because factor XI deficiency or inhibition protects against thrombosis and is associated with little or no bleeding. Several factor XI-directed strategies are currently under investigation. This article (1) reviews the rationale for the development of factor XI inhibitors, (2) identifies the agents in most advanced stages of development, (3) describes the results of completed clinical trials and provides a summary of those underway, and (4) highlights the opportunities and challenges for this next generation of anticoagulants.

Selection and in vitro and in vivo characterization of a Kunitz protease inhibitor domain of protease nexin 2 variant that inhibits factor XIa without inhibiting plasmin

The 57-amino acid Kunitz Protease Inhibitor (KPI) domain of Protease Nexin 2 inhibits Factor XIa (FXIa) and other proteases. We previously fused KPI to human serum albumin (KPIHSA). KPIHSA inhibits coagulation Factor XIa (FXIa) 6-fold more rapidly than plasmin. We screened a bacterial expression library of KPI variants randomized at M17, and selected M17D as having the highest anti-FXIa: antiplasmin activity ratio. Expressed as HSA fusion proteins in Pichia pastoris, KPIHSA and KPI(M17D)HSA inhibited FXIa indistinguishably (K_i 9 nM) but KPI(M17D)HSA lacked detectable antiplasmin activity. Purified variant and wild-type KPIHSA were expressed and injected into mice with ferric chloride-treated carotid arteries, with or without systemic administration of tissue plasminogen activator (Tenecteplase, TNKase). The time to arterial occlusion (TTO) or reperfusion (TTR) was assessed by Doppler ultrasound. TTR did not differ between mice treated with TNKase alone or with TNKase supplemented with 38 mg/kg KPI(M17D)HSA but was significantly prolonged to >60 min in all mice treated with TNKase and 38 mg/kg KPIHSA. TTO was significantly but equally prolonged by either 38 mg/kg KPIHSA or KPI(M17D)HSA versus vehicle controls. The antiplasmin activity of KPI is relevant in vivo but its elimination did not enhance counter-thrombosis by KPI.

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.

Cloning and identification of a new multifunctional Ascaris-type peptide from the hemolymph of Buthus martensii Karsch

Only a few work have been done for peptides from non-venom gland tissues of venomous animals. Here, with the help of the whole body transcriptomic and the hemolymph proteomic data of the Chinese scorpion Buthus martensii Karsch, we identified the first Ascaris-type peptide BmHDP from scorpion hemolymph. The precursor of BmHDP has 80 residues, including a 16 residue signal peptide and a 64 residue mature peptide. The mature peptide has 10 conserved cysteines and adopts a conserved Ascaris-type fold. Using combined inclusion body refolding and biochemical identification strategies, recombinant BmHDP was obtained successfully. Protease inhibitory assays showed that BmHDP inhibited chymotrypsin apparently at a concentration of 8 nM. Patch-clamp experiments showed that BmHDP inhibited the Kv1.3 potassium channel apparently at a concentration of 1000 nM. Coagulation experiment assays showed that BmHDP inhibited intrinsic coagulation pathway apparently at a concentration of 500 nM. To the best of our knowledge, BmHDP is the first Ascaris-type peptide from scorpion hemolymph. Our work highlighted a functional link between scorpion non-venom gland peptides and venom gland toxin peptides, and suggested that scorpion hemolymph might be a new source of bioactive peptides.

Factor XI(a) inhibitors for thrombosis: an updated patent review (2016-present)

Introduction: Anticoagulation without bleeding is an ideal goal in treating thrombosis, however, this goal has not been achieved. All current anticoagulants are associated with significant bleeding which limits their safe use. Genetic and pharmacological findings indicate that factor XIa is a key player in thrombosis, yet it is a relatively marginal one in hemostasis. Thus, factor XIa and its zymogen offer a unique opportunity to develop anticoagulants with low bleeding risk.Areas covered: A survey of patent literature has retrieved more than 50 patents on the discovery of novel therapeutics targeting factor XI(a) since 2016. Small molecules, monoclonal antibodies, oligonucleotides, and polypeptides have been developed to inhibit factor XI(a). Many inhibitors are in early development and few have been evaluated in clinical trials.Expert opinion: Factor XI(a) is being actively pursued as a drug target for the development of effective and safer anticoagulants. Although many patents claiming factor XI(a) inhibitors were filed prior to 2016, recent literature reveals a moderately declining trend. Nevertheless, more agents have entered different levels of clinical trials. These agents exploit diverse mechanistic strategies for inhibition. Although further development is warranted, reaching one or more of these agents to the clinic will transform the anticoagulation therapy.

Drug Discovery on Natural Products: From Ion Channels to nAChRs, from Nature to Libraries, from Analytics to Assays

Natural extracts are complex mixtures that may be rich in useful bioactive compounds and therefore are attractive sources for new leads in drug discovery. This review describes drug discovery from natural products and in explaining this process puts the focus on ion-channel drug discovery. In particular, the identification of bioactives from natural products targeting nicotinic acetylcholine receptors (nAChRs) and serotonin type 3 receptors (5-HT3Rs) is discussed. The review is divided into three parts: "Targets," "Sources," and "Approaches." The "Targets" part will discuss the importance of ion-channel drug targets in general, and the α7-nAChR and 5-HT3Rs in particular. The "Sources" part will discuss the relevance for drug discovery of finding bioactive compounds from various natural sources such as venoms and plant extracts. The "Approaches" part will give an overview of classical and new analytical approaches that are used for the identification of new bioactive compounds with the focus on targeting ion channels. In addition, a selected overview is given of traditional venom-based drug discovery approaches and of diverse hyphenated analytical systems used for screening complex bioactive mixtures including venoms.

Thromboinflammation: challenges of therapeutically targeting coagulation and other host defense mechanisms

Thrombosis with associated inflammation (thromboinflammation) occurs commonly in a broad range of human disorders. It is well recognized clinically in the context of superficial thrombophlebitis (thrombosis and inflammation of superficial veins); however, it is more dangerous when it develops in the microvasculature of injured tissues and organs. Microvascular thrombosis with associated inflammation is well recognized in the context of sepsis and ischemia-reperfusion injury; however, it also occurs in organ transplant rejection, major trauma, severe burns, the antiphospholipid syndrome, preeclampsia, sickle cell disease, and biomaterial-induced thromboinflammation. Central to thromboinflammation is the loss of the normal antithrombotic and anti-inflammatory functions of endothelial cells, leading to dysregulation of coagulation, complement, platelet activation, and leukocyte recruitment in the microvasculature. α-Thrombin plays a critical role in coordinating thrombotic and inflammatory responses and has long been considered an attractive therapeutic target to reduce thromboinflammatory complications. This review focuses on the role of basic aspects of coagulation and α-thrombin in promoting thromboinflammatory responses and discusses insights gained from clinical trials on the effects of various inhibitors of coagulation on thromboinflammatory disorders. Studies in sepsis patients have been particularly informative because, despite using anticoagulant approaches with different pharmacological profiles, which act at distinct points in the coagulation cascade, bleeding complications continue to undermine clinical benefit. Future advances may require the development of therapeutics with primary anti-inflammatory and cytoprotective properties, which have less impact on hemostasis. This may be possible with the growing recognition that components of blood coagulation and platelets have prothrombotic and proinflammatory functions independent of their hemostatic effects.

Expression and characterization of Flavikunin: A Kunitz-type serine protease inhibitor identified in the venom gland cDNA library of Bungarus flaviceps

Trancriptomic analysis of the venom gland cDNA library of Bungarus flaviceps revealed Kunitz-type serine protease inhibitor as one of the major venom protein families with three groups A, B, C. One of the group B isoforms named Flavikunin, which lacked an extra cysteine residue involved in disulfide bond formation in β-bungarotoxin, was synthesized, cloned, and overexpressed in Escherichia coli. To decipher the structure-function relationship, the P1 residue of Flavikunin, histidine, was mutated to alanine and arginine. Purified wild-type and mutant Flavikunins were screened against serine proteases-thrombin, factor Xa, trypsin, chymotrypsin, plasmin, and elastase. The wild-type and mutant Flavikunin (H∆R) inhibited plasmin with an IC ₅₀ of 0.48 and 0.35 µM, respectively. The in-silico study showed that P1 residue of wild-type and mutant (H∆R) Flavikunin interacted with S1' and S1 site of plasmin, respectively. Thus, histidine at the P1 position was found to be involved in plasmin inhibition with mild anticoagulant activity.

Plasma contact factors as therapeutic targets

Direct oral anticoagulants (DOACs) are small molecule inhibitors of the coagulation proteases thrombin and factor Xa that demonstrate comparable efficacy to warfarin for several common indications, while causing less serious bleeding. However, because their targets are required for the normal host-response to bleeding (hemostasis), DOACs are associated with therapy-induced bleeding that limits their use in certain patient populations and clinical situations. The plasma contact factors (factor XII, factor XI, and prekallikrein) initiate blood coagulation in the activated partial thromboplastin time assay. While serving limited roles in hemostasis, pre-clinical and epidemiologic data indicate that these proteins contribute to pathologic coagulation. It is anticipated that drugs targeting the contact factors will reduce risk of thrombosis with minimal impact on hemostasis. Here, we discuss the biochemistry of contact activation, the contributions of contact factors in thrombosis, and novel antithrombotic agents targeting contact factors that are undergoing pre-clinical and early clinical testing.

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.

Engineering varied serine protease inhibitors by converting P1 site of BF9, a weakly active Kunitz-type animal toxin

Although there were a lot of weakly active animal toxins in the venoms, their values and applications are still mysterious, such as BF9, which is a Kunitz-type toxin isolated from the venom of the elapid snake Bungarus fasciatus. Here, we used BF9 to be a molecular scaffold, and engineered eight BF9-derived peptides by changing P1 site Asn17 of BF9, such as BF9-N17Y and BF9-N17T designed from the polar subfamily, BF9-N17L and BF9-N17G designed from the Non-polar subfamily, BF9-N17D designed from acidic subfamily, and BF9-N17H, BF9-N17K and BF9-N17R designed from basic subfamily. Through enzyme inhibitor experiment assays, we found a potent and selective chymotrypsin inhibitor BF9-N17Y, a potent and selective coagulation factor XIa inhibitor BF9-N17H, and two highly potent coagulation factor XIa inhibitors BF9-N17K and BF9-N17. APTT and PT assays further showed that BF9-N17H, BF9-N17K and BF9-N17R were three novel anticoagulants with selectively intrinsic coagulation pathway inhibitory activity. Considering that natural weakly active animal toxins are also a huge peptide resource, our present work might open a new window about pharmacological applications of weakly active animal toxins, which might be good templates for potent and selective molecular probe and lead drug designs.

The Kv1.3 channel-inhibitory toxin BF9 also displays anticoagulant activity via inhibition of factor XIa

The Kv1.3 channel plays potential roles in immune, inflammation and coagulation system. Many studies showed that Kv1.3 channel inhibitors have immunosuppressive and anti-inflammatory activities, but no Kv1.3 channel inhibitors have been found to have anticoagulation activities. Here, based on our previous work about Kv1.3 channel toxin peptide inhibitors, we first attempt to test anticoagulation activities of four known venom-derived Kv1.3 channel inhibitors with different structural folds: BmKTX with CSα/β structural fold, OmTx3 with CSα/α structural fold, BF9 with Kuntz-type structural fold, and SjAPI-2 with Ascaris-type structural fold. Our results showed that BmKTX and OmTx3 have no activities towards both intrinsic and extrinsic coagulation pathway, SjAPI-2 just has weak activity towards intrinsic coagulation pathway, and BF9 has potent activity towards intrinsic coagulation pathway with no apparent effect on extrinsic coagulation pathway. Enzyme and inhibitor reaction kinetics experiments further showed that BF9 inhibited intrinsic coagulation pathway-associated coagulation factor XIa, but have no apparent effects on common coagulation pathway coagulation factor IIa. Structure-activity relationship showed that Gly14, Asn17, Ala18 and Ile20 of BF9 are main residues involved in the inhibiting effect on factor XIa. To the best of our knowledge, BF9 is the first anticoagulant with Kv1.3 channel inhibitory activity. Together, our present studies found the first dual functional peptides with Kv1.3 channel and coagulation factor XIa inhibitory activities, and provided a new molecular template for the lead drug discovery towards immune and thrombosis-associated human diseases.

Factor XIa Inhibitors as New Anticoagulants

With the introduction of thrombin and factor Xa inhibitors to the oral anticoagulant market, significant improvements in both efficacy and safety have been achieved. Early clinical and preclinical data suggest that inhibitors of factor XIa can provide a still safer alternative, with expanded efficacy for arterial indications. This Perspective provides an overview of target rationale and details of the discovery and development of inhibitors of factor XIa as next generation antithrombotic agents.

Recombinant expression of Ixolaris, a Kunitz-type inhibitor from the tick salivary gland, for NMR studies

Ixolaris is an anticoagulant protein identified in the tick saliva of Ixodes scapularis. Ixolaris contains 2 Kunitz like domains and binds to Factor Xa or Factor X as a scaffold for inhibition of the Tissue Factor (TF)/Factor VIIa (FVIIa). In contrast to tissue factor pathway inhibitor (TFPI), however, Ixolaris does not bind to the active site cleft of FXa. Instead, complex formation is mediated by the FXa heparin-binding exosite. Due to its potent and long-lasting antithrombotic activity, Ixolaris is a promising agent for anticoagulant therapy. Although numerous functional studies of Ixolaris exist, three-dimensional structure of Ixolaris has not been obtained at atomic resolution. Using the pET32 vector, we successfully expressed a TRX-His₆-Ixolaris fusion protein. By combining Ni-NTA chromatography, enterokinase protease cleavage, and reverse phase HPLC (RP-HPLC), we purified isotopically labeled Ixolaris for NMR studies. 1D ¹H and 2D ¹⁵N-¹H NMR analysis yielded high quality 2D ¹⁵N-¹H HSQC spectra revealing that the recombinant protein is folded. These studies represent the first steps in obtaining high-resolution structural information by NMR for Ixolaris enabling the investigation of the molecular basis for Ixolaris-coagulation factors interactions.

Therapeutic potential of krait venom

Kraits belong to Elapideae and are widely distributed in East and South-East Asian countries. Krait venom possesses neurotoxins, membrane toxins, cardiotoxins, three finger toxins, metalloproteinases, cholinesterases, L-amino acid oxidases and serine proteases. The therapeutic potential of krait venom in pathophysiological conditions such as microbial and parasitic infections, cancer, arthritis, inflammation and blood coagulation disorder is discussed in this review. More intensive new research ventures are required to establish the therapeutic potential of krait venom in complex and emerging diseases.

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.

Exploiting the antithrombotic effect of the (pro)thrombin inhibitor bothrojaracin

Bothrojaracin is a 27 kDa C-type lectin-like protein from Bothrops jararaca snake venom. It behaves as a potent thrombin inhibitor upon high-affinity binding to thrombin exosites. Bothrojaracin also forms a stable complex with prothrombin that can be detected in human plasma. Formation of the zymogen-inhibitor complex severely decreases prothrombin activation and contributes to the anticoagulant activity of bothrojaracin. In the present study, we employed two rodent models to evaluate the antithrombotic effect of bothrojaracin in vivo: stasis-induced thrombosis and thrombin-induced pulmonary thromboembolism. It was observed that bothrojaracin interacts with rat prothrombin in plasma. Ex-vivo assays showed stable complex formation even after 24 h of a single bothrojaracin dose. As a result, bothrojaracin showed significant antithrombotic activity in a rat venous thrombosis model elicited by thromboplastin combined with stasis. The antithrombotic activity of bothrojaracin (1 mg/kg) persisted for up to 24 h and it was associated with moderate bleeding as assessed by a tail transection method. Formation of bothrojaracin-prothrombin complex has been also observed following intravenous administration of the inhibitor into mice. As a result, bothrojaracin effectively protected mice from thrombin-induced fatal thromboembolism. We conclude that bothrojaracin is a potent antithrombotic agent in vivo and may serve as a prototype for the development of new zymogen-directed drugs that could result in prolonged half-life and possible decreased hemorrhagic risk.