Corn trypsin inhibitor coating attenuates the prothrombotic properties of catheters in vitro and in vivo

Unravelling Surface Modification Strategies for Preventing Medical Device-Induced Thrombosis

The use of biomaterials in implanted medical devices remains hampered by platelet adhesion and blood coagulation. Thrombus formation is a prevalent cause of failure of these blood-contacting devices. Although systemic anticoagulant can be used to support materials and devices with poor blood compatibility, its negative effects such as an increased chance of bleeding, make materials with superior hemocompatibility extremely attractive, especially for long-term applications. This review examines blood-surface interactions, the pathogenesis of clotting on blood-contacting medical devices, popular surface modification techniques, mechanisms of action of anticoagulant coatings, and discusses future directions in biomaterial research for preventing thrombosis. In addition, this paper comprehensively reviews several novel methods that either entirely prevent interaction between material surfaces and blood components or regulate the reaction of the coagulation cascade, thrombocytes, and leukocytes.

Surface modification of polyvinyl chloride with sodium alginate/carboxymethyl chitosan and heparin for realizing the anticoagulation

Thrombosis of extracorporeal circuits causes significant morbidity and mortality worldwide. In this study, plasma treatment technology and chemical grafting method were used to construct heparinized surfaces on the PVC substrate, which could not only reduce thrombosis but also decrease the side effects of the direct injection of anticoagulants. The PVC substrate was modified by plasma treatment technology firstly to obtain the active surface with the hydroxyl groups used for grafting. Then, heparin was grafted onto the modified PVC surface using different grafting strategies to prepare different heparinized surfaces. The experimental results indicated that the sodium alginate (SA) and carboxymethyl chitosan (CCS) used as interlayers could significantly increase the graft density of heparin to improve the anticoagulant effects and hemocompatibility of heparinized surfaces. In addition, the modification of heparin can further improve the anticoagulant effects. The CCS/low-molecular-weight heparin (LWMH) surface has the best anticoagulant properties, which can prolong the activated partial thromboplastin time (APTT) values of human plasma for about 35 s, reduce the hemolysis rates to <0.3 %, and perform well in the in-vitro blood circulation test. The heparinized surfaces prepared in this work have great application potential in anticoagulant treatment for medical devices.

Histidine-rich glycoprotein attenuates catheter thrombosis

Factor XII (FXII) knockdown attenuates catheter thrombosis in rabbits. Because histidine-rich glycoprotein (HRG) modulates FXIIa activity, we hypothesized that HRG depletion would promote catheter thrombosis. To test this, rabbits were given either antisense oligonucleotides (ASOs) against HRG or FXII, a control ASO, or saline. The activated partial thromboplastin time (aPTT), prothrombin time (PT), and catheter-induced thrombin generation were determined in blood collected before and after treatment. Compared with the controls, the HRG- and FXII-directed ASOs reduced hepatic messenger RNA and plasma levels of HRG and FXII, respectively, by >90%. Although HRG knockdown shortened the aPTT by 2.5 fold, FXII knockdown prolonged it by fourfold; neither of the ASOs affected the PT. Catheter segments shortened the lag time and increased peak thrombin in the plasma from control rabbits; effects were significantly enhanced and attenuated in the plasma from rabbits given the HRG- and FXII-directed ASOs, respectively. Catheters were then inserted into the right external jugular vein of the rabbits, and the time for catheter occlusion was determined. The catheter occlusion times with the control ASO or saline were 62 ± 8 minutes and 60 ± 11 minutes, respectively. The occlusion time was significantly reduced to 34 ± 9 minutes, with HRG knockdown and significantly prolonged to 128 ± 19 minutes with FXII knockdown. HRG levels are decreased with sepsis or cancer, and such patients are prone to catheter thrombosis. Because HRG modulates catheter thrombosis, our findings suggest that HRG supplementation may prevent this problem.

Current and future strategies to monitor and manage coagulation in ECMO patients

Extracorporeal membrane oxygenation (ECMO) can provide life-saving support for critically ill patients suffering severe respiratory and/or cardiac failure. However, thrombosis and bleeding remain common and complex problems to manage. Key causes of thrombosis in ECMO patients include blood contact to pro-thrombotic and non-physiological surfaces, as well as high shearing forces in the pump and membrane oxygenator. On the other hand, adverse effects of anticoagulant, thrombocytopenia, platelet dysfunction, acquired von Willebrand syndrome, and hyperfibrinolysis are all established as causes of bleeding. Finding safe and effective anticoagulants that balance thrombosis and bleeding risk remains challenging. This review highlights commonly used anticoagulants in ECMO, including their mechanism of action, monitoring methods, strengths and limitations. It further elaborates on existing anticoagulant monitoring strategies, indicating their target range, benefits and drawbacks. Finally, it introduces several highly novel approaches to real-time anticoagulation monitoring methods including sound, optical, fluorescent, and electrical measurement as well as their working principles and future directions for research.

Rivaroxaban and apixaban are less effective than enoxaparin for the prevention of catheter-induced clotting in vitro

BACKGROUND

Central venous catheters are prone to clotting, particularly in patients with cancer. Although low-molecular-weight heparin and direct oral anticoagulants, such as apixaban and rivaroxaban, have been evaluated for the prevention of catheter thrombosis, their efficacy remains uncertain.

OBJECTIVES

Compare apixaban and rivaroxaban with enoxaparin for the prevention of catheter-induced clotting in vitro.

METHODS

To address this uncertainty, we used a well-established microplate-based assay to compare the effects of enoxaparin, apixaban, and rivaroxaban on catheter-induced thrombosis and thrombin generation in human plasma.

RESULTS

Consistent with our previous findings, catheter segments shortened the clotting time and promoted thrombin generation. When compared at concentrations with similar anti-factor Xa activity as enoxaparin, apixaban and rivaroxaban were >20-fold less potent than enoxaparin for the prevention of catheter-induced clotting and thrombin generation.

CONCLUSION

The prevention of catheter thrombosis in patients with cancer is challenging. Clinical trials are needed to compare the efficacy of low-molecular-weight heparin with that of direct oral anticoagulants both for the prevention and treatment of catheter thrombosis.

Amide-functionalized 1,2,4-Triazol-5-amines as Covalent Inhibitors of Blood Coagulation Factor XIIa and Thrombin

To counteract thrombosis, new safe and efficient antithrombotics are required. We herein report the design, synthesis, and biological activity of a series of amide-functionalized acylated 1,2,4-triazol-5-amines as selective inhibitors of blood coagulation factor XIIa and thrombin. The introduction of an amide moiety into the main scaffold of 3-aryl aminotriazoles added certain three-dimensional properties to synthesized compounds and allowed them to reach binding sites in FXIIa and thrombin previously unaddressed by non-functionalized 1,2,4-triazol-5-amines. Among synthesized compounds, one quinoxaline-derived aminotriazole bearing N-butylamide moiety inhibited FXIIa with the IC50 value of 28 nM, whereas the N-phenylamide-derived aminotriazole inhibited thrombin with the IC50 value of 41 nM. Performed mass-shift experiments and molecular modeling studies proved the covalent mechanism of FXIIa and thrombin inhibition by synthesized compounds. In plasma coagulation tests, developed aminotriazoles showed anticoagulant properties mainly affecting the intrinsic blood coagulation pathway, activation of which is associated with thrombosis but is negligible for hemostasis.

Mechanisms for Reduced Fibrin Clot Formation on Liquid-Infused Surfaces

Biomedical devices are prone to blood clot formation (thrombosis), and liquid-infused surfaces (LIS) are effective in reducing the thrombotic response. However, the mechanisms that underpin this performance, and in particular the role of the lubricant, are not well understood. In this work, it is investigated whether the mechanism of LIS action is related to i) inhibition of factor XII (FXII) activation and the contact pathway; ii) reduced fibrin density of clots formed on surfaces; iii) increased mobility of proteins or cells on the surface due to the interfacial flow of the lubricant. The chosen LIS is covalently tethered, nanostructured layers of perfluorocarbons, infused with thin films of medical-grade perfluorodecalin (tethered-liquid perfluorocarbon), prepared with chemical vapor deposition previously optimized to retain lubricant under flow. Results show that in the absence of external flow, interfacial mobility is inherently higher at the liquid-blood interface, making it a key contributor to the low thrombogenicity of LIS, as FXII activity and fibrin density are equivalent at the interface. The findings of this study advance the understanding of the anti-thrombotic behavior of LIS-coated biomedical devices for future coating design. More broadly, enhanced interfacial mobility may be an important, underexplored mechanism for the anti-fouling behavior of surface coatings.

Uninterrupted DOACs Approach for Catheter Ablation of Atrial Fibrillation: Do DOACs Levels Matter?

Most patients present for catheter ablation of atrial fibrillation (CAAF) with residual or full effect of vitamin K antagonists (VKAs) or direct oral anticoagulants (DOACs). In daily practice, it has been observed that the activated clotting time (ACT) was actually poorly sensitive to the effect of DOACs and that patients on DOACs required more unfractionated heparin (UFH) to achieve the ACT target of 300 s during the procedure, leading some authors to worry about potential overdosing. Conversely, we hypothesize that these higher doses of UFH are necessary to achieve adequate hemostasis during CAAF regardless of the residual effect of DOACs. During CAAF, thrombosis is promoted mainly by the presence of thrombogenic sheaths and catheters in the bloodstream. Preclinical data suggest that only high doses of DOACs are able to mitigate catheter-induced thrombin generation, whereas low dose UFH already do so. In addition, the effect of UFH seems to be lower in patients on DOACs, compared to patients on VKAs, explaining part of the differences observed in heparin requirements. Clinical studies could not identify increased bleeding risk in patients on DOACs compared to those on VKAs despite similar efficacy during CAAF procedures. Moreover, targeting a lower ACT was associated with an increased periprocedural thrombotic risk for both DOAC and VKA patients. Therefore, the low sensitivity of the ACT to the residual effect of DOACs should not be a major concern in its use in the interventional cardiology laboratory.

Control of Blood Coagulation by Hemocompatible Material Surfaces-A Review

Hemocompatibility of biomaterials in contact with the blood of patients is a prerequisite for the short- and long-term applications of medical devices such as cardiovascular stents, artificial heart valves, ventricular assist devices, catheters, blood linings and extracorporeal devices such as artificial kidneys (hemodialysis), extracorporeal membrane oxygenation (ECMO) and cardiopulmonary bypass. Although lower blood compatibility of materials and devices can be handled with systemic anticoagulation, its side effects, such as an increased bleeding risk, make materials that have a better hemocompatibility highly desirable, particularly in long-term applications. This review provides a short overview on the basic mechanisms of blood coagulation including plasmatic coagulation and blood platelets, as well as the activation of the complement system. Furthermore, a survey on concepts for tailoring the blood response of biomaterials to improve the hemocompatibility of medical devices is given which covers different approaches that either inhibit interaction of material surfaces with blood components completely or control the response of the coagulation system, blood platelets and leukocytes.

Biomaterial Wettability Affects Fibrin Clot Structure and Fibrinolysis

Thrombosis on blood-contacting medical devices can cause patient fatalities through device failure and unstable thrombi causing embolism. The effect of material wettability on fibrin network formation, structure, and stability is poorly understood. Under static conditions, fibrin fiber network volume and density increase in clots formed on hydrophilic compared to hydrophobic polystyrene surfaces. This correlates with reduced plasma clotting time and increased factor XIIa (FXIIa) activity. These structural differences are consistent up to 50 µm away from the material surface and are FXIIa dependent. Fibrin forms fibers immediately at the material interface on hydrophilic surfaces but are incompletely formed in the first 5 µm above hydrophobic surfaces. Additionally, fibrin clots on hydrophobic surfaces have increased susceptibility to fibrinolysis compared to clots formed on hydrophilic surfaces. Under low-flow conditions, clots are still denser on hydrophilic surfaces, but only 5 µm above the surface, showing the combined effect of the surface wettability and coagulation factor dilution with low flow. Overall, wettability affects fibrin fiber formation at material interfaces, which leads to differences in bulk fibrin clot density and susceptibility to fibrinolysis. These findings have implications for thrombus formed in stagnant or low-flow regions of medical devices and the design of nonthrombogenic materials.

High-Throughput Docking and Molecular Dynamics Simulations towards the Identification of Potential Inhibitors against Human Coagulation Factor XIIa

Human coagulation factor XIIa (FXIIa) is a trypsin-like serine protease that is involved in pathologic thrombosis. As a potential target for designing safe anticoagulants, FXIIa has received a great deal of interest in recent years. In the present study, we employed virtual high-throughput screening of 500,064 compounds within Enamine database to acquire the most potential inhibitors of FXIIa. Subsequently, 18 compounds with significant binding energy (from -65.195 to -15.726 kcal/mol) were selected, and their ADMET properties were predicted to select representative inhibitors. Three compounds (Z1225120358, Z432246974, and Z146790068) exhibited excellent binding affinity and druggability. MD simulation for FXIIa-ligand complexes was carried out to reveal the stability and inhibition mechanism of these three compounds. Through the inhibition of activated factor XIIa assay, we tested the activity of five compounds Z1225120358, Z432246974, Z45287215, Z30974175, and Z146790068, with pIC50 values of 9.3∗10⁻⁷, 3.0∗10⁻⁵, 7.8∗10⁻⁷, 8.7∗10⁻⁷, and 1.3∗10⁻⁶ M, respectively; the AMDET properties of Z45287215 and Z30974175 show not well but have better inhibition activity. We also found that compounds Z1225120358, Z45287215, Z30974175, and Z146790068 could be more inhibition of FXIIa than Z432246974. Collectively, compounds Z1225120358, Z45287215, Z30974175, and Z146790068 were anticipated to be promising drug candidates for inhibition of FXIIa.

Robust and Antiswelling Hollow Hydrogel Tube with Antibacterial and Antithrombotic Ability for Emergency Vascular Replacement

Infection and thrombosis are the two major complications in almost any indwelling intravascular catheters, leading to adverse consequences. Here, we report a robust and antiswelling hollow hydrogel tube that is prepared by copolymerizing a hydrogen-bonding (H-bonding) monomer and a zinc methacrylate (ZMA) monomer in the absence of any chemical cross-linker. The strong H-bonding interactions from the side chain of N-acryloylsemicarbazide (NASC) endow the hydrogel with high mechanical strength and swelling stability. Introduction of ZMA affords a superhydrophilic surface, and the release of a zinc ion (Zn²⁺) from the hydrogel can kill nearly 100% both of Staphylococcus aureus and Escherichia coli, indicating its excellent antibacterial ability. Importantly, the P(NASC-co-ZMA) hydrogel exhibits better antithrombosis ability due to the resistant adhesion of fibrinogen protein and platelets, as well binding calcium ions (Ca²⁺) from the blood. The hydrogel tube is used to connect the ex vivo arteriovenous shunt circuit or implanted into the left carotid artery in the rabbit model, showing a better patency rate. All of these results suggest that this hydrogel tube may mitigate infection and thrombosis complications, thus holding potential as an artificial blood vessel for emergency vascular replacement.

The rebirth of the contact pathway: a new therapeutic target

PURPOSE OF REVIEW

Anticoagulation with vitamin-K antagonists or direct oral anticoagulants is associated with a significant risk of bleeding. There is a major effort underway to develop antithrombotic drugs that have a smaller impact on hemostasis. The plasma contact proteins factor XI (FXI) and factor XII (FXII) have drawn considerable interest because they contribute to thrombosis but have limited roles in hemostasis. Here, we discuss results of preclinical and clinical trials supporting the hypothesis that the contact system contributes to thromboembolic disease.

RECENT FINDINGS

Numerous compounds targeting FXI or FXII have shown antithrombotic properties in preclinical studies. In phase 2 studies, drugs-targeting FXI or its protease form FXIa compared favorably with standard care for venous thrombosis prophylaxis in patients undergoing knee replacement. While less work has been done with FXII inhibitors, they may be particularly useful for limiting thrombosis in situations where blood comes into contact with artificial surfaces of medical devices.

SUMMARY

Inhibitors of contact activation, and particularly of FXI, are showing promise for prevention of thromboembolic disease. Larger studies are required to establish their efficacy, and to establish that they are safer than current therapy from a bleeding standpoint.

Single and multi-functional coating strategies for enhancing the biocompatibility and tissue integration of blood-contacting medical implants

Device-associated clot formation and poor tissue integration are ongoing problems with permanent and temporary implantable medical devices. These complications lead to increased rates of mortality and morbidity and impose a burden on healthcare systems. In this review, we outline the current approaches for developing single and multi-functional surface coating techniques that aim to circumvent the limitations associated with existing blood-contacting medical devices. We focus on surface coatings that possess dual hemocompatibility and biofunctionality features and discuss their advantages and shortcomings to providing a biocompatible and biodynamic interface between the medical implant and blood. Lastly, we outline the newly developed surface modification techniques that use lubricant-infused coatings and discuss their unique potential and limitations in mitigating medical device-associated complications.

Cyclic peptide FXII inhibitor provides safe anticoagulation in a thrombosis model and in artificial lungs

Inhibiting thrombosis without generating bleeding risks is a major challenge in medicine. A promising solution may be the inhibition of coagulation factor XII (FXII), because its knock-out or inhibition in animals reduced thrombosis without causing abnormal bleeding. Herein, we have engineered a macrocyclic peptide inhibitor of activated FXII (FXIIa) with sub-nanomolar activity (K_i = 370 ± 40 pM) and a high stability (t_1/2 > 5 days in plasma), allowing for the preclinical evaluation of a first synthetic FXIIa inhibitor. This 1899 Da molecule, termed FXII900, efficiently blocks FXIIa in mice, rabbits, and pigs. We found that it reduces ferric-chloride-induced experimental thrombosis in mice and suppresses blood coagulation in an extracorporeal membrane oxygenation (ECMO) setting in rabbits, all without increasing the bleeding risk. This shows that FXIIa activity is controllable in vivo with a synthetic inhibitor, and that the inhibitor FXII900 is a promising candidate for safe thromboprotection in acute medical conditions.

Toward an artificial endothelium: Development of blood-compatible surfaces for extracorporeal life support

A new generation of extracorporeal artificial organ support technologies, collectively known as extracorporeal life support (ECLS) devices, is being developed for diverse applications to include acute support for trauma-induced organ failure, transitional support for bridge to organ transplant, and terminal support for chronic diseases. Across applications, one significant complication limits the use of these life-saving devices: thrombosis, bleeding, and inflammation caused by foreign surface-induced blood interactions. To address this challenge, transdisciplinary scientists and clinicians look to the vascular endothelium as inspiration for development of new biocompatible materials for ECLS. Here, we describe clinically approved and new investigational biomaterial solutions for thrombosis, such as immobilized heparin, nitric oxide-functionalized polymers, "slippery" nonadhesive coatings, and surface endothelialization. We describe how hemocompatible materials could abrogate the use of anticoagulant drugs during ECLS and by doing so radically change treatments in critical care. Additionally, we examine several special considerations for the design of biomaterials for ECLS, including: (1) preserving function of the artificial organ, (2) longevity of use, and (3) multifaceted approaches for the diversity of device functions and applications.

Novel antithrombotic strategies for treatment of venous thromboembolism

Venous thromboembolism (VTE), which includes deep vein thrombosis (DVT) and pulmonary embolism (PE), is the third most common cause of vascular death after heart attack and stroke. Anticoagulation therapy is the cornerstone of VTE treatment. Despite such therapy, up to 50% of patients with DVT develop postthrombotic syndrome, and up to 4% of patients with PE develop chronic thromboembolic pulmonary hypertension. Therefore, better therapies are needed. Although direct oral anticoagulants are more convenient and safer than warfarin for VTE treatment, bleeding remains the major side effect, particularly in cancer patients. Factor XII and factor XI have emerged as targets for new anticoagulants that may be safer. To reduce the complications of VTE, attenuation of thrombin activatable fibrinolysis inhibitor activity is under investigation in PE patients to enhance endogenous fibrinolysis, whereas blockade of leukocyte interaction with the vessel wall is being studied to reduce the inflammation that contributes to postthrombotic syndrome in DVT patients. Focusing on these novel antithrombotic strategies, this article explains why safer anticoagulants are needed, provides the rationale for factor XII and XI as targets for such agents, reviews the data on the factor XII- and factor XI-directed anticoagulants under development, describes novel therapies to enhance fibrinolysis and decrease inflammation in PE and DVT patients, respectively, and offers insights into the opportunities for these novel VTE therapies.

Thrombogenic and Inflammatory Reactions to Biomaterials in Medical Devices

Blood-contacting medical devices of different biomaterials are often used to treat various cardiovascular diseases. Thrombus formation is a common cause of failure of cardiovascular devices. Currently, there are no clinically available biomaterials that can totally inhibit thrombosis under the more challenging environments (e.g., low flow in the venous system). Although some biomaterials reduce protein adsorption or cell adhesion, the issue of biomaterial associated with thrombosis and inflammation still exists. To better understand how to develop more thrombosis-resistant medical devices, it is essential to understand the biology and mechano-transduction of thrombus nucleation and progression. In this review, we will compare the mechanisms of thrombus development and progression in the arterial and venous systems. We will address various aspects of thrombosis, starting with biology of thrombosis, mathematical modeling to integrate the mechanism of thrombosis, and thrombus formation on medical devices. Prevention of these problems requires a multifaceted approach that involves more effective and safer thrombolytic agents but more importantly the development of novel thrombosis-resistant biomaterials mimicking the biological characteristics of the endothelium and extracellular matrix tissues that also ameliorate the development and the progression of chronic inflammation as part of the processes associated with the detrimental generation of late thrombosis and neo-atherosclerosis. Until such developments occur, engineers and clinicians must work together to develop devices that require minimal anticoagulants and thrombolytics to mitigate thrombosis and inflammation without causing serious bleeding side effects.

Serine Protease Inhibitors-New Molecules for Modification of Polymeric Biomaterials

Three serine protease inhibitors (AEBSF, soy inhibitor, α₁-antitrypsin) were covalently immobilized on the surface of three polymer prostheses with the optimized method. The immobilization efficiency ranged from 11 to 51%, depending on the chosen inhibitor and biomaterial. The highest activity for all inhibitors was observed in the case of immobilization on the surface of the polyester Uni-Graft prosthesis, and the preparations obtained showed high stability in the environment with different pH and temperature values. Modification of the Uni-Graft prosthesis surface with the synthetic AEBSF inhibitor and human α₁-antitrypsin inhibited the adhesion and multiplication of Staphylococcus aureus subs. aureus ATCC^® 25923^TM and Candida albicans from the collection of the Department of Genetics and Microbiology, UMCS. Optical profilometry analysis indicated that, after the immobilization process on the surface of AEBSF-modified Uni-Graft prostheses, there were more structures with a high number of protrusions, while the introduction of modifications with a protein inhibitor led to the smoothing of their surface.

Rivaroxaban and Dabigatran for Suppression of Mechanical Heart Valve-Induced Thrombin Generation

BACKGROUND

Patients with mechanical heart valves (MHVs) require warfarin to prevent thromboembolism. Dabigatran was less effective than warfarin in patients with MHVs, which prompted a black box warning against the use of direct oral anticoagulants for this indication. However, rivaroxaban and apixaban, which inhibit factor Xa, have not been evaluated in patients with MHVs. To determine whether rivaroxaban and apixaban would be effective, we used MHV-induced thrombin generation assays to compare them with warfarin either alone or in combination with dabigatran.

METHODS

Thrombin generation in the absence or presence of MHV leaflets or sewing ring segments (SRSs) was quantified. Studies were done in control plasma; plasma from patients on warfarin; plasma containing varying concentrations of rivaroxaban, apixaban, or dabigatran alone; or plasma containing rivaroxaban plus dabigatran.

RESULTS

Mean endogenous thrombin potential (ETP) increased 1.2-fold, 1.5-fold, and 1.8-fold in the presence of leaflets, Teflon (Terumo Aortic (Sunrise, FL)) SRSs, or Dacron (Terumo Aortic (Sunrise, FL)) SRSs, respectively. Rivaroxaban and apixaban reduced ETP at concentrations above 50 ng/mL but were less effective than warfarin. When rivaroxaban and dabigatran were combined, they suppressed ETP in a more than additive manner.

CONCLUSIONS

Whereas warfarin suppresses MHV-induced thrombin generation, MHVs induce the generation of factor Xa in concentrations that overwhelm clinically relevant concentrations of rivaroxaban or apixaban. When used in combination, rivaroxaban and dabigatran are more effective than either agent is alone, suggesting that concomitant inhibition of factor Xa and thrombin is better than inhibition of either clotting enzyme alone.

Heparin-induced thrombocytopenia complicating extracorporeal membrane oxygenation support: Review of the literature and alternative anticoagulants

Heparin-induced thrombocytopenia (HIT) is a life-threatening prothrombotic, immune-mediated complication of unfractionated heparin and low molecular weight heparin therapy. HIT is characterized by moderate thrombocytopenia 5-10 days after initial heparin exposure, detection of platelet-activating anti-platelet factor 4/heparin antibodies and an increased risk of venous and arterial thrombosis. Extracorporeal membrane oxygenation (ECMO) is a form of mechanical circulatory support used in critically ill patients with respiratory or cardiac failure. Systemic anticoagulation is used to alleviate the thrombotic complications that may occur when blood is exposed to artificial surfaces within the ECMO circuit. Therefore, when HIT complicates patients on ECMO support, it is associated with a high thrombotic morbidity and mortality. The risk for HIT correlates with the accumulative dosage of heparin exposure. In ECMO patients receiving continuous infusion of heparin for circuit patency, the risk for HIT is not neglected and must be thought of in the differential diagnosis of the appropriate clinical and laboratory circumstances. The following article reviews the current knowledge in HIT complicating ECMO patients and the alternative anticoagulation options in the presence of HIT.

The blood compatibility challenge. Part 2: Protein adsorption phenomena governing blood reactivity

The adsorption of proteins is the initiating event in the processes occurring when blood contacts a "foreign" surface in a medical device, leading inevitably to thrombus formation. Knowledge of protein adsorption in this context has accumulated over many years but remains fragmentary and incomplete. Moreover, the significance and relevance of the information for blood compatibility are not entirely agreed upon in the biomaterials research community. In this review, protein adsorption from blood is discussed under the headings "agreed upon" and "not agreed upon or not known" with respect to: protein layer composition, effects on coagulation and complement activation, effects on platelet adhesion and activation, protein conformational change and denaturation, prevention of nonspecific protein adsorption, and controlling/tailoring the protein layer composition. STATEMENT OF SIGNIFICANCE: This paper is part 2 of a series of 4 reviews discussing the problem of biomaterial associated thrombogenicity. The objective was to highlight features of broad agreement and provide commentary on those aspects of the problem that were subject to dispute. We hope that future investigators will update these reviews as new scholarship resolves the uncertainties of today.

The blood compatibility challenge. Part 4: Surface modification for hemocompatible materials: Passive and active approaches to guide blood-material interactions

Biomedical devices in the blood flow disturb the fine-tuned balance of pro- and anti-coagulant factors in blood and vessel wall. Numerous technologies have been suggested to reduce coagulant and inflammatory responses of the body towards the device material, ranging from camouflage effects to permanent activity and further to a responsive interaction with the host systems. However, not all types of modification are suitable for all types of medical products. This review has a focus on application-oriented considerations of hemocompatible surface fittings. Thus, passive versus bioactive modifications are discussed along with the control of protein adsorption, stability of the immobilization, and the type of bioactive substance, biological or synthetic. Further considerations are related to the target system, whether enzymes or cells should be addressed in arterial or venous system, or whether the blood vessel wall is addressed. Recent developments like feedback controlled or self-renewing systems for drug release or addressing cellular regulation pathways of blood platelets and endothelial cells are paradigms for a generation of blood contacting devices, which are hemocompatible by cooperation with the host system. STATEMENT OF SIGNIFICANCE: This paper is part 4 of a series of 4 reviews discussing the problem of biomaterial associated thrombogenicity. The objective was to highlight features of broad agreement and provide commentary on those aspects of the problem that were subject to dispute. We hope that future investigators will update these reviews as new scholarship resolves the uncertainties of today.

The blood compatibility challenge. Part 1: Blood-contacting medical devices: The scope of the problem

Blood-contacting medical devices are an integral part of modern medicine. Such devices may be used for only a few hours or may be implanted for life. Despite advances in biomaterial science, clotting on medical devices remains a common problem. Systemic administration of antiplatelet drugs or anticoagulants is often needed to reduce the risk of clotting. Although effective, such therapy increases the risk of bleeding, which can be fatal. This chapter (a) describes some of the commonly used blood-contacting devices and their potential complications, (b) provides an overview of the mechanisms that drive device-associated clotting, and (c) reviews the strategies employed to attenuate clotting on blood-contacting medical devices. STATEMENT OF SIGNIFICANCE: This paper is part 1 of a series of 4 reviews discussing the problem of biomaterial associated thrombogenicity. The objective was to highlight features of broad agreement and provide commentary on those aspects of the problem that were subject to dispute. We hope that future investigators will update these reviews as new scholarship resolves the uncertainties of today.

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.

Only high levels of dabigatran attenuate catheter thrombosis in vitro and in rabbits

In patients with mechanical heart valves, thromboembolic events were more frequent with dabigatran, an oral thrombin inhibitor, than with warfarin. This observation raises the possibility that dabigatran may be less effective than conventional anticoagulants in patients with other blood-contacting devices, such as catheters. To address this, we compared the capacity of dabigatran and/or heparin to inhibit catheter-induced thrombin generation in vitro and to attenuate catheter occlusion in rabbits. Using a catheter-induced thrombin generation assay, concentrations of dabigatran over 100 ng/ml prolonged the lag time and time to peak thrombin, and reduced the peak thrombin concentration and endogenous thrombin potential in a concentration-dependent fashion. Compared with saline in a rabbit model of catheter thrombosis, dabigatran prolonged the mean time to catheter occlusion by 2.9– and 1.9-fold when plasma levels were 173 and 140 ng/ml, respectively; values comparable to median peak levels in humans given dabigatran 150 mg twice daily. In contrast, low-dose dabigatran, which produced a level of 60 ng/ml; a value comparable to the trough level of dabigatran in humans, did not prolong the time to occlusion. Whereas a 70 U/kg bolus of heparin prolonged the mean time to occlusion by 3.4-fold, a 15 U/kg bolus had no effect. When low-dose dabigatran was given in combination with 15 U/kg heparin, the mean time to occlusion was prolonged by 2.7-fold. These findings suggest that only peak levels of dabigatran are sufficient to prevent catheter-induced clotting unless supplemented heparin is given.

Factor XIIa inhibition by Infestin-4: in vitro mode of action and in vivo antithrombotic benefit

Coagulation factor XII (FXII) plays a central role in initiating the intrinsic cascade of blood coagulation. Purified recombinant Human Albumin- tagged Infestin-4 (rHA-Infestin-4) is a recently described FXIIa inhibitor that displayed strong anticoagulant activity without compromising haemostasis in several animal models. We pursued detailed in vitro characterisation of rHA-Infestin-4 and demonstrated that it is a competitive inhibitor of FXIIa with slow on and off rate constants for binding (kon =5x105 M-1s-1, koff =6x10–4 s-1), it can block FXIIa activation of its physiological substrates (plasma prekallikrein and FXI), and it can inhibit ellagic acid-triggered thrombin generation in plasma. Potency and selectivity profiling in enzyme assays suggest that rHAInfestin- 4 is indeed highly potent on FXIIa (IC50=0.3 ± 0.06, 1.5 ± 0.06, 1.2 ± 0.09 nM, for human, rat, and rabbit FXIIa, respectively) with at least >100-fold selectivity against factors IIa, Xa, IXa, XIa, VIIa, and plasma kallikrein in all three species. rHA-Infestin-4 dose-dependently and markedly reduced clot weight in the arteriovenous shunt thrombosis model in rats and rabbits, accompanied with minimal increase in cuticle bleeding times in either species. rHA-Infestin-4 treatment at 5 mg/kg in rabbit resulted in a 13% reduction in ex vivo FXa activity, demonstrating a modest off-target effect. In summary, our findings confirmed and extended previous reports that inhibition of FXIIa by rHA-Infestin-4 can produce strong antithrombotic efficacy while preserving haemostasis. Our comprehensive selectivity profiling, mode of action, and kinetic studies of rHA-Infestin-4 reveal limitations of this molecule and offer new perspectives on any potential effort of discovering novel FXIIa inhibitors.

In vivo activation and functions of the protease factor XII

Combinations of proinflammatory and procoagulant reactions are the unifying principle for a variety of disorders affecting the cardiovascular system. Factor XII (FXII, Hageman factor) is a plasma protease that initiates the contact system. The biochemistry of the contact system in vitro is well understood; however, its in vivo functions are just beginning to emerge. The current review concentrates on activators and functions of the FXII-driven contact system in vivo. Elucidating its physiologic activities offers the exciting opportunity to develop strategies for the safe interference with both thrombotic and inflammatory diseases.

An omniphobic lubricant-infused coating produced by chemical vapor deposition of hydrophobic organosilanes attenuates clotting on catheter surfaces

Catheter associated thrombosis is an ongoing problem. Omniphobic coatings based on tethering biocompatible liquid lubricants on self-assembled monolayers of hydrophobic organosilanes attenuate clotting on surfaces. Herein we report an efficient, non-invasive and robust process for coating catheters with an antithrombotic, omniphobic lubricant-infused coating produced using chemical vapor deposition (CVD) of hydrophobic fluorine-based organosilanes. Compared with uncoated catheters, CVD coated catheters significantly attenuated thrombosis via the contact pathway of coagulation. When compared with the commonly used technique of liquid phase deposition (LPD) of fluorine-based organosilanes, the CVD method was more efficient and reproducible, resulted in less disruption of the outer polymeric layer of the catheters and produced greater antithrombotic activity. Therefore, omniphobic coating of catheters using the CVD method is a simple, straightforward and non-invasive procedure. This method has the potential to not only prevent catheter thrombosis, but also to prevent thrombosis on other blood-contacting medical devices.

Inhibition of Factors XI and XII for Prevention of Thrombosis Induced by Artificial Surfaces

Exposure of blood to a variety of artificial surface induces contact activation, a process that contributes to the host innate response to foreign substances. On the foreign surface, the contact factors, factor XII (FXII), and plasma prekallikrein undergo reciprocal conversion to their fully active protease forms (FXIIa and α-kallikrein, respectively) by a process supported by the cofactor high-molecular-weight kininogen. Contact activation can trigger blood coagulation by conversion of factor XI (FXI) to the protease FXIa. There is interest in developing therapeutic inhibitors to FXIa and FXIIa because these activated factors can contribute to thrombosis in certain situations. Drugs targeting these proteases may be particularly effective in thrombosis triggered by exposure of blood to the surfaces of implantable medical devices. Here, we review clinical data supporting roles for FXII and FXI in thrombosis induced by medical devices, and preclinical data suggesting that therapeutic targeting of these proteins may limit surface-induced thrombosis.

Assessment of the protein interaction between coagulation factor XII and corn trypsin inhibitor by molecular docking and biochemical validation

Essentials Corn Trypsin Inhibitor (CTI) is a selective inhibitor of coagulation Factor XII (FXII). Molecular modelling of the CTI-FXIIa complex suggested a canonical inhibitor binding mode. Mutagenesis revealed the CTI inhibitory loop and helices α1 and α2 mediate the interaction. This confirms that CTI inhibits FXII in canonical fashion and validates the molecular model.

SUMMARY

Background Corn trypsin inhibitor (CTI) has selectivity for the serine proteases coagulation factor XII and trypsin. CTI is in widespread use as a reagent that specifically inhibits the intrinsic pathway of blood coagulation but not the extrinsic pathway. Objectives To investigate the molecular basis of FXII inhibition by CTI. Methods We performed molecular docking of CTI, using its known crystal structure, with a model of the activated FXII (FXIIa) protease domain. The interaction model was verified by use of a panel of recombinant CTI variants tested for their ability to inhibit FXIIa enzymatic activity in a substrate cleavage assay. Results The docking predicted that: (i) the CTI central inhibitory loop P1 Arg34 side chain forms a salt bridge with the FXIIa S1 pocket Asp189 side chain; (ii) Trp22 from CTI helix α1 interacts with the FXIIa S3 pocket; and (iii) Arg43 from CTI helix α2 forms a salt bridge with FXIIa H1 pocket Asp60A. CTI amino acid substitution R34A negated all inhibitory activity, whereas the G32W, L35A, W22A and R42A/R43A substitutions reduced activity by large degrees of 108-fold, 41-fold, 158-fold, and 100-fold, respectively; the R27A, W37A, W39A and R42A substitutions had no effect. Synthetic peptides spanning CTI residues 20-44 had inhibitory activity that was three-fold to 4000-fold less than that of full-length CTI. Conclusions The data confirm the validity of a canonical model of the FXIIa-CTI interaction, with helix α1 (Trp22), central inhibitory loop (Arg34) and helix α2 (Arg43) of CTI being required for effective binding by contacting the S1, S3 and H1 pockets of FXIIa, respectively.

The plasma contact system, a protease cascade at the nexus of inflammation, coagulation and immunity

The contact system is a potent procoagulant and proinflammatory plasma protease cascade that is initiated by binding ("contact")-induced, auto-activation of factor XII zymogen. Formed active serine protease FXIIa then cleaves plasma prekallikrein to kallikrein that in turn liberates the mediator bradykinin from its precursor high molecular weight kininogen. Bradykinin induces inflammation with implications for host defense and innate immunity. FXIIa also triggers the intrinsic pathway of coagulation that has been shown to critically contribute to thrombosis. Vice versa, FXII deficiency impairs thrombosis in animal models without inducing abnormal excessive bleeding. Recent work has established the FXIIa-driven contact system as promising target for anticoagulant and anti-inflammatory drugs. This review focuses on the biochemistry of the contact system, its regulation by endogenous and exogenous inhibitors, and roles in disease states. This article is part of a Special Issue entitled: Proteolysis as a Regulatory Event in Pathophysiology edited by Stefan Rose-John.

Factor XI and factor XII as targets for new anticoagulants

Although the non-vitamin antagonist oral anticoagulants produce less intracranial bleeding than warfarin, serious bleeding still occurs. Therefore, the search for safer anticoagulants continues. Factor XII and factor XI have emerged as promising targets whose inhibition has the potential to prevent thrombosis with little or no disruption of hemostasis. Thus, thrombosis is attenuated in mice deficient in factor XII or factor XI and patients with congenital factor XII deficiency do not bleed and those with factor XI deficiency rarely have spontaneous bleeding. Strategies targeting factor XII and XI include antisense oligonucleotides to decrease their synthesis, inhibitory antibodies or aptamers, and small molecule inhibitors. These strategies attenuate thrombosis in various animal models and factor XI knockdown with an antisense oligonucleotide in patients undergoing knee replacement surgery reduced postoperative venous thromboembolism to a greater extent than enoxaparin without increasing bleeding. Therefore, current efforts are focused on evaluating the efficacy and safety of factor XII and factor XI directed anticoagulant strategies.

Factors XI and XII as Targets for New Anticoagulants

Compared with vitamin K antagonists, the direct oral anticoagulants (DOACs) are simpler to administer and are associated with less intracranial bleeding. Nonetheless, even with the DOACs, bleeding still occurs and many patients with atrial fibrillation fail to receive anticoagulant thromboprophylaxis because of the fear of bleeding. Therefore, there is an urgent need for safer anticoagulants. Recent investigations into the biochemistry of hemostasis and thrombosis have identified new targets for development of novel anticoagulants. Using data from complementary sources, including epidemiological studies and investigations in various animal models, the contact pathway has emerged as a potential mediator of thrombosis that plays a minor part in hemostasis. Consequently, factor (F) XII of the contact system and FXI in the intrinsic pathway have been identified as potentially safer targets of anticoagulation than thrombin or FXa. However, further studies are needed to identify which is the better target for the appropriate indication. This review highlights the evidence for focusing on FXI and FXII and examines the novel approaches directed at these new targets. These emerging strategies should address current unmet medical needs and provide new avenues by which to improve anticoagulant therapy by reducing the risk of bleeding.

Factor XIIa as a Novel Target for Thrombosis: Target Engagement Requirement and Efficacy in a Rabbit Model of Microembolic Signals

Coagulation Factor XII (FXII) plays a critical role in thrombosis. What is unclear is the level of enzyme occupancy of FXIIa that is needed for efficacy and the impact of FXIIa inhibition on cerebral embolism. A selective activated FXII (FXIIa) inhibitor, recombinant human albumin-tagged mutant Infestin-4 (rHA-Mut-inf), was generated to address these questions. rHA-Mut-inf displayed potency comparable to the original wild-type HA-Infestin-4 (human FXIIa inhibition constant = 0.07 and 0.12 nM, respectively), with markedly improved selectivity against Factor Xa (FXa) and plasmin. rHA-Mut-inf binds FXIIa, but not FXII zymogen, and competitively inhibits FXIIa protease activity. Its mode of action is hence akin to typical small-molecule inhibitors. Plasma shift and aPTT studies with rHA-Mut-inf demonstrated that calculated enzyme occupancy for FXIIa in achieving a putative aPTT doubling target in human, nonhuman primate, and rabbit is more than 99.0%. The effects of rHA-Mut-inf in carotid arterial thrombosis and microembolic signal (MES) in middle cerebral artery were assessed simultaneously in rabbits. Dose-dependent inhibition was observed for both arterial thrombosis and MES. The ED₅₀ of thrombus formation was 0.17 mg/kg i.v. rHA-Mut-inf for the integrated blood flow and 0.16 mg/kg for thrombus weight; the ED₅₀ for MES was 0.06 mg/kg. Ex vivo aPTT tracked with efficacy. In summary, our findings demonstrated that very high enzyme occupancy will be required for FXIIa active site inhibitors, highlighting the high potency and exquisite selectivity necessary for achieving efficacy in humans. Our MES studies suggest that targeting FXIIa may offer a promising strategy for stroke prevention associated with thromboembolic events.

Emerging anticoagulant strategies

Despite the introduction of direct oral anticoagulants (DOACs), the search for more effective and safer antithrombotic strategies continues. Better understanding of the pathogenesis of thrombosis has fostered 2 new approaches to achieving this goal. First, evidence that thrombin may be as important as platelets to thrombosis at sites of arterial injury and that platelets contribute to venous thrombosis has prompted trials comparing anticoagulants with aspirin for secondary prevention in arterial thrombosis and aspirin with anticoagulants for primary and secondary prevention of venous thrombosis. These studies will help identify novel treatment strategies. Second, emerging data that naturally occurring polyphosphates activate the contact system and that this system is critical for thrombus stabilization and growth have identified factor XII (FXII) and FXI as targets for new anticoagulants that may be even safer than the DOACs. Studies are needed to determine whether FXI or FXII is the better target and to compare the efficacy and safety of these new strategies with current standards of care for the prevention or treatment of thrombosis. Focusing on these advances, this article outlines how treatment strategies for thrombosis are evolving and describes the rationale and approaches to targeting FXII and FXI. These emerging anticoagulant strategies should address unmet needs and reduce the systemic underuse of anticoagulation because of the fear of bleeding.

Neutralizing blood-borne polyphosphate in vivo provides safe thromboprotection

Polyphosphate is an inorganic procoagulant polymer. Here we develop specific inhibitors of polyphosphate and show that this strategy confers thromboprotection in a factor XII-dependent manner. Recombinant Escherichia coli exopolyphosphatase (PPX) specifically degrades polyphosphate, while a PPX variant lacking domains 1 and 2 (PPX_Δ12) binds to the polymer without degrading it. Both PPX and PPX_Δ12 interfere with polyphosphate- but not tissue factor- or nucleic acid-driven thrombin formation. Targeting polyphosphate abolishes procoagulant platelet activity in a factor XII-dependent manner, reduces fibrin accumulation and impedes thrombus formation in blood under flow. PPX and PPX_Δ12 infusions in wild-type mice interfere with arterial thrombosis and protect animals from activated platelet-induced venous thromboembolism without increasing bleeding from injury sites. In contrast, targeting polyphosphate does not provide additional protection from thrombosis in factor XII-deficient animals. Our data provide a proof-of-concept approach for combating thrombotic diseases without increased bleeding risk, indicating that polyphosphate drives thrombosis via factor XII.

The inorganic procoagulant polymer polyphosphate participates in thrombosis via factor XII. Here the authors use recombinant probes that specifically bind or degrade circulating polyphosphate to protect mice in arterial and venous thrombosis models without an increased bleeding risk, the primary complication of all currently used anticoagulants.

New Infestin-4 Mutants with Increased Selectivity against Factor XIIa

Factor XIIa (fXIIa) is a serine protease that triggers the coagulation contact pathway and plays a role in thrombosis. Because it interferes with coagulation testing, the need to inhibit fXIIa exists in many cases. Infestin-4 (Inf4) is a Kazal-type inhibitor of fXIIa. Its specificity for fXIIa can be enhanced by point mutations in the protease-binding loop. We attempted to adapt Inf4 for the selective repression of the contact pathway under various in vitro conditions, e.g., during blood collection and in ‘global’ assays of tissue factor (TF)-dependent coagulation. First, we designed a set of new Inf4 mutants that, in contrast to wt-Inf4, had stabilized canonical conformations during molecular dynamics simulation. Off-target activities against factor Xa (fXa), plasmin, and other coagulation proteases were either reduced or eliminated in these recombinant mutants, as demonstrated by chromogenic assays. Interactions with fXIIa and fXa were also analyzed using protein-protein docking. Next, Mutant B, one of the most potent mutants (its K_i for fXIIa is 0.7 nM) was tested in plasma. At concentrations 5–20 μM, this mutant delayed the contact-activated generation of thrombin, as well as clotting in thromboelastography and thrombodynamics assays. In these assays, Mutant B did not affect coagulation initiated by TF, thus demonstrating sufficient selectivity and its potential practical significance as a reagent for coagulation diagnostics.