Selective factor XIIa inhibition attenuates silent brain ischemia: application of molecular imaging targeting coagulation pathway

New Triazole-Based Potent Inhibitors of Human Factor XIIa as Anticoagulants

Factor XIIa (FXIIa) functions as a plasma serine protease within the contact activation pathway. Various animal models have indicated a substantial role for FXIIa in thromboembolic diseases. Interestingly, individuals and animals with FXII deficiency seem to maintain normal hemostasis. Consequently, inhibiting FXIIa could potentially offer a viable therapeutic approach for achieving effective and safer anticoagulation without the bleeding risks associated with the existing anticoagulants. Despite the potential, only a limited number of small molecule inhibitors targeting human FXIIa have been documented. Thus, we combined a small library of 32 triazole and triazole-like molecules to be evaluated for FXIIa inhibition by using a chromogenic substrate hydrolysis assay under physiological conditions. Initial screening at 200 μM involved 18 small molecules, revealing that 4 molecules inhibited FXIIa more than 20%. In addition to being the most potent inhibitor identified in the first round, inhibitor 8 also exhibited a substantial margin of selectivity against related serine proteases, including factors XIa, Xa, and IXa. However, the molecule also inhibited thrombin with a similar potency. It also prolonged the clotting time of human plasma, as was determined in the activated partial thromboplastin time and prothrombin time assays. Subsequent structure-activity relationship studies led to the identification of several inhibitors with submicromolar activity, among which inhibitor 22 appears to demonstrate significant selectivity not only over factors IXa, Xa, and XIa, but also over thrombin. In summary, this study introduces novel triazole-based small molecules, specifically compounds 8 and 22, identified as potent and selective inhibitors of human FXIIa. The aim is to advance these inhibitors for further development as anticoagulants to provide a more effective and safer approach to preventing and/or treating thromboembolic diseases.

Factor XII Silencing Using siRNA Prevents Thrombus Formation in a Rat Model of Extracorporeal Life Support

Heparin anticoagulation increases the bleeding risk during extracorporeal life support (ECLS). This study determined whether factor XII (FXII) silencing using short interfering RNA (siRNA) can provide ECLS circuit anticoagulation without bleeding. Adult male, Sprague-Dawley rats were randomized to four groups (n = 3 each) based on anticoagulant: (1) no anticoagulant, (2) heparin, (3) FXII siRNA, or (4) nontargeting siRNA. Heparin was administered intravenously before and during ECLS. FXII or nontargeting siRNA were administered intravenously 3 days before the initiation of ECLS via lipidoid nanoparticles. The rats were placed on pumped, arteriovenous ECLS for 8 hours or until the blood flow resistance reached three times its baseline resistance. Without anticoagulant, mock-oxygenator resistance tripled within 7 ± 2 minutes. The resistance in the FXII siRNA group did not increase for 8 hours. There were no significant differences in resistance or mock-oxygenator thrombus volume between the FXII siRNA and the heparin groups. However, the bleeding time in the FXII siRNA group (3.4 ± 0.6 minutes) was significantly shorter than that in the heparin group (5.5 ± 0.5 minutes, p < 0.05). FXII silencing using siRNA provided simpler anticoagulation of ECLS circuits with reduced bleeding time as compared to heparin. http://links.lww.com/ASAIO/A937.

Triazol-1-yl Benzamides Promote Anticoagulant Activity via Inhibition of Factor XIIa

BACKGROUND

Human factor XIIa (FXIIa) is a plasma serine protease that plays a significant role in several physiological and pathological processes. Animal models have revealed an important contribution of FXIIa to thromboembolic diseases. Remarkably, animals and patients with FXII deficiency appear to have normal hemostasis. Thus, FXIIa inhibition may serve as a promising therapeutic strategy to attain safer and more effective anticoagulation. Very few small molecule inhibitors of FXIIa have been reported. We synthesized and investigated a focused library of triazol-1-yl benzamide derivatives for FXIIa inhibition.

METHODS

We chemically synthesized, characterized, and investigated a focused library of triazol- 1-yl benzamide derivatives for FXIIa inhibition. Using a standardized chromogenic substrate hydrolysis assay, the derivatives were evaluated for inhibiting human FXIIa. Their selectivity over other clotting factors was also evaluated using the corresponding substrate hydrolysis assays. The best inhibitor affinity to FXIIa was also determined using fluorescence spectroscopy. Effects on the clotting times (prothrombin time (PT) and activated partial thromboplastin time (APTT)) of human plasma were also studied.

RESULTS

We identified a specific derivative (1) as the most potent inhibitor in this series. The inhibitor exhibited nanomolar binding affinity to FXIIa. It also exhibited significant selectivity against several serine proteases. It also selectively doubled the activated partial thromboplastin time of human plasma.

CONCLUSION

Overall, this work puts forward inhibitor 1 as a potent and selective inhibitor of FXIIa for further development as an anticoagulant.

Novel contact-kinin inhibitor sylvestin targets thromboinflammation and ameliorates ischemic stroke

Ischemic stroke is a leading cause of death and disability worldwide. Increasing evidence indicates that ischemic stroke is a thromboinflammatory disease in which the contact-kinin pathway has a central role by activating pro-coagulant and pro-inflammatory processes. The blocking of distinct members of the contact-kinin pathway is a promising strategy to control ischemic stroke. Here, a plasma kallikrein and active FXII (FXIIa) inhibitor (sylvestin, contained 43 amino acids, with a molecular weight of 4790.4 Da) was first identified from forest leeches (Haemadipsa sylvestris). Testing revealed that sylvestin prolonged activated partial thromboplastin time without affecting prothrombin time. Thromboelastography and clot retraction assays further showed that it extended clotting time in whole blood and inhibited clot retraction in platelet-rich plasma. In addition, sylvestin prevented thrombosis in vivo in FeCl3-induced arterial and carrageenan-induced tail thrombosis models. The potential role of sylvestin in ischemic stroke was evaluated by transient and permanent middle cerebral artery occlusion models. Sylvestin administration profoundly protected mice from ischemic stroke by counteracting intracerebral thrombosis and inflammation. Importantly, sylvestin showed no signs of bleeding tendency. The present study identifies sylvestin is a promising contact-kinin pathway inhibitor that can proffer profound protection from ischemic stroke without increased risk of bleeding.

An Update on Safe Anticoagulation

Blood coagulation is essential to maintain the integrity of a closed circulatory system (hemostasis), but also contributes to thromboembolic occlusion of vessels (thrombosis). Thrombosis may cause deep vein thrombosis, pulmonary embolism, myocardial infarction, peripheral artery disease, and ischemic stroke, collectively the most common causes of death and disability in the developed world. Treatment for the prevention of thromboembolic diseases using anticoagulants such as heparin, coumarins, thrombin inhibitors, or antiplatelet drugs increase the risk of bleeding and are associated with an increase in potentially life-threatening hemorrhage, partially offsetting the benefits of reduced coagulation. Thus, drug development aiming at novel targets is needed to provide efficient and safe anticoagulation. Within the last decade, experimental and preclinical data have shown that some coagulation mechanisms principally differ in thrombosis and hemostasis. The plasma contact system protein factors XII and XI, high-molecular-weight kininogen, and plasma kallikrein specifically contribute to thrombosis, however, have minor, if any, role in hemostatic coagulation mechanisms. Inherited deficiency in contact system proteins is not associated with increased bleeding in humans and animal models. Therefore, targeting contact system proteins provides the exciting opportunity to interfere specifically with thromboembolic diseases without increasing the bleeding risk. Recent studies that investigated pharmacologic inhibition of contact system proteins have shown that this approach provides efficient and safe thrombo-protection that in contrast to classical anticoagulants is not associated with increased bleeding risk. This review summarizes therapeutic and conceptual developments for selective interference with pathological thrombus formation, while sparing physiologic hemostasis, that enables safe anticoagulation treatment.

Generation of a humanized FXII knock-in mouse-A powerful model system to test novel anti-thrombotic agents

BACKGROUND

Effective inhibition of thrombosis without generating bleeding risks is a major challenge in medicine. Accumulating evidence suggests that this can be achieved by inhibition of coagulation factor XII (FXII), as either its knock-out or inhibition in animal models efficiently reduced thrombosis without affecting normal hemostasis. Based on these findings, highly specific inhibitors for human FXII(a) are under development. However, currently, in vivo studies on their efficacy and safety are impeded by the lack of an optimized animal model expressing the specific target, that is, human FXII.

OBJECTIVE

The primary objective of this study is to develop and functionally characterize a humanized FXII mouse model.

METHODS

A humanized FXII mouse model was generated by replacing the murine with the human F12 gene (genetic knock-in) and tested it in in vitro coagulation assays and in in vivo thrombosis models.

RESULTS

These hF12KI mice were indistinguishable from wild-type mice in all tested assays of coagulation and platelet function in vitro and in vivo, except for reduced expression levels of hFXII compared to human plasma. Targeting FXII by the anti-human FXIIa antibody 3F7 increased activated partial thromboplastin time dose-dependently and protected hF12KI mice in an arterial thrombosis model without affecting bleeding times.

CONCLUSION

These data establish the newly generated hF12KI mouse as a powerful and unique model system for in vivo studies on anti-FXII(a) biologics, supporting the development of efficient and safe human FXII(a) inhibitors.

A model of silent brain infarction induced by endovascular intervention with balloon in cynomolgus macaques: A pilot study

Silent brain infarction is a special type of cerebral infarction, which can be detected by MRI or CT. The most patients with silent brain infarction show no symptoms, but some have mild depression, vascular dementia and other symptoms that are easily overlooked. Silent brain infarction is one of the risk factors for symptomatic cerebral infarction, it can develop into symptomatic cerebral infarction placing a heavy burden on families and society. Therefore, it's prevention and treatment should be as important as symptomatic cerebral infarction. However, the pathogenesis of silent brain infarction has not been elucidated. Studies have shown that silent brain infarction models have been established in rats and mice. But compared with other animals, non-human primates are more similar to humans in neuroanatomical structure and clinical characteristics. Therefore, this study is the first time to explore the silent brain infarction model in cynomolgus macaques. In this study, a model of silent brain infarction was established by endovascular intervention using balloon occlusion at the end of internal carotid artery for 45 min, which can lay a foundation for the future research on the pathological mechanism of silent brain infarction.

Intrinsic Pathway of Coagulation and Thrombosis

Activation of the intrinsic pathway of coagulation contributes to the pathogenesis of arterial and venous thrombosis. Critical insights into the involvement of intrinsic pathway factors have been derived from the study of gene-specific knockout animals and targeted inhibitors. Importantly, preclinical studies have indicated that targeting components of this pathway, including FXI (factor XI), FXII, and PKK (prekallikrein), reduces thrombosis with no significant effect on protective hemostatic pathways. This review highlights the advances made from studying the intrinsic pathway using gene-specific knockout animals and inhibitors in models of arterial and venous thrombosis. Development of inhibitors of activated FXI and FXII may reduce thrombosis with minimal increases in bleeding compared with current anticoagulant drugs.

Radionuclide Imaging of Atherothrombotic Diseases

Purpose of Review

A variety of approaches and molecular targets have emerged in recent years for radionuclide-based imaging of atherosclerosis and vulnerable plaque using single photon emission computed tomography (SPECT) and positron emission tomography (PET), with numerous methods focused on characterizing the mechanisms underlying plaque progression and rupture. This review highlights the ongoing developments in both the preclinical and clinical environment for radionuclide imaging of atherosclerosis and atherothrombosis.

Recent Findings

Numerous physiological processes responsible for the evolution of high-risk atherosclerotic plaque, such as inflammation, thrombosis, angiogenesis, and microcalcification, have been shown to be feasible targets for SPECT and PET imaging. For each physiological process, specific molecular markers have been identified that allow for sensitive non-invasive detection and characterization of atherosclerotic plaque.

Summary

The capabilities of SPECT and PET imaging continue to evolve for physiological evaluation of atherosclerosis. This review summarizes the latest developments related to radionuclide imaging of atherothrombotic diseases.

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.

Molecular Imaging of Atherothrombotic Diseases: Seeing Is Believing

Molecular imaging, with major advances in the development of both innovative targeted contrast agents/particles and radiotracers, as well as various imaging technologies, is a fascinating, rapidly growing field with many preclinical and clinical applications, particularly for personalized medicine. Thrombosis in either the venous or the arterial system, the latter typically caused by rupture of unstable atherosclerotic plaques, is a major determinant of mortality and morbidity in patients. However, imaging of the various thrombotic complications and the identification of plaques that are prone to rupture are at best indirect, mostly unreliable, or not available at all. The development of molecular imaging toward diagnosis and prevention of thrombotic disease holds promise for major advance in this clinically important field. Here, we review the medical need and clinical importance of direct molecular imaging of thrombi and unstable atherosclerotic plaques that are prone to rupture, thereby causing thrombotic complications such as myocardial infarction and ischemic stroke. We systematically compare the advantages/disadvantages of the various molecular imaging modalities, including X-ray computed tomography, magnetic resonance imaging, positron emission tomography, single-photon emission computed tomography, fluorescence imaging, and ultrasound. We further systematically discuss molecular targets specific for thrombi and those characterizing unstable, potentially thrombogenic atherosclerotic plaques. Finally, we provide examples for first theranostic approaches in thrombosis, combining diagnosis, targeted therapy, and monitoring of therapeutic success or failure. Overall, molecular imaging is a rapidly advancing field that holds promise of major benefits to many patients with atherothrombotic diseases.

Factor XII as a Risk Marker for Hemorrhagic Stroke: A Prospective Cohort Study

Background

Coagulation factor XII (FXII) is involved in pathological thrombus formation and is a suggested target of anticoagulants. It is unclear whether FXII levels are correlated with cardiovascular risk factors and whether they are associated with myocardial infarction or ischemic or hemorrhagic stroke. The aim of this study was to investigate the correlation between FXII and cardiovascular risk factors in the general population. We also aimed to study the associations between FXII levels and future myocardial infarction and ischemic and hemorrhagic stroke.

Methods

This prospective cohort study measured FXII levels in 1,852 randomly selected participants in a health survey performed in northern Sweden in 1994. Participants were followed until myocardial infarction, stroke, death, or until December 31, 2011.

Results

During the median follow-up of 17.9 years, 165 individuals were diagnosed with myocardial infarction, 108 with ischemic stroke, and 30 with hemorrhagic stroke. There were weak correlations between FXII and body mass index, cholesterol, and hypertension. There was no association between FXII and myocardial infarction or ischemic stroke, neither in univariable Cox regression analysis nor after adjustment for age, sex, smoking, body mass index, cholesterol, hypertension, and diabetes. In univariable Cox regression analysis, the hazard ratio for the association between FXII levels and hemorrhagic stroke was 1.42 per SD (95% confidence interval: 0.99–2.05). In the multivariable model, higher levels of FXII were associated with increased risk of hemorrhagic stroke (hazard ratio 1.51 per SD; 95% confidence interval: 1.03–2.21).

Conclusion

We found an independent association between FXII levels and the risk of hemorrhagic stroke, but not between FXII levels and ischemic stroke or myocardial infarction.

Targeting coagulation factor XII as a novel therapeutic option in brain trauma

OBJECTIVE

Traumatic brain injury is a major global public health problem for which specific therapeutic interventions are lacking. There is, therefore, a pressing need to identify innovative pathomechanism-based effective therapies for this condition. Thrombus formation in the cerebral microcirculation has been proposed to contribute to secondary brain damage by causing pericontusional ischemia, but previous studies have failed to harness this finding for therapeutic use. The aim of this study was to obtain preclinical evidence supporting the hypothesis that targeting factor XII prevents thrombus formation and has a beneficial effect on outcome after traumatic brain injury.

METHODS

We investigated the impact of genetic deficiency of factor XII and acute inhibition of activated factor XII with a single bolus injection of recombinant human albumin-fused infestin-4 (rHA-Infestin-4) on trauma-induced microvascular thrombus formation and the subsequent outcome in 2 mouse models of traumatic brain injury.

RESULTS

Our study showed that both genetic deficiency of factor XII and an inhibition of activated factor XII in mice minimize trauma-induced microvascular thrombus formation and improve outcome, as reflected by better motor function, reduced brain lesion volume, and diminished neurodegeneration. Administration of human factor XII in factor XII-deficient mice fully restored injury-induced microvascular thrombus formation and brain damage.

INTERPRETATION

The robust protective effect of rHA-Infestin-4 points to a novel treatment option that can decrease ischemic injury after traumatic brain injury without increasing bleeding tendencies. Ann Neurol 2016;79:970-982.

The Coagulation Factor XIIa Inhibitor rHA-Infestin-4 Improves Outcome after Cerebral Ischemia/Reperfusion Injury in Rats

Background and Purpose

Ischemic stroke provokes severe brain damage and remains a predominant disease in industrialized countries. The coagulation factor XII (FXII)-driven contact activation system plays a central, but not yet fully defined pathogenic role in stroke development. Here, we investigated the efficacy of the FXIIa inhibitor rHA-Infestin-4 in a rat model of ischemic stroke using both a prophylactic and a therapeutic approach.

Methods

For prophylactic treatment, animals were treated intravenously with 100 mg/kg rHA-Infestin-4 or an equal volume of saline 15 min prior to transient middle cerebral artery occlusion (tMCAO) of 90 min. For therapeutic treatment, 100 mg/kg rHA-Infestin-4, or an equal volume of saline, was administered directly after the start of reperfusion. At 24 h after tMCAO, rats were tested for neurological deficits and blood was drawn for coagulation assays. Finally, brains were removed and analyzed for infarct area and edema formation.

Results

Within prophylactic rHA-Infestin-4 treatment, infarct areas and brain edema formation were reduced accompanied by better neurological scores and survival compared to controls. Following therapeutic treatment, neurological outcome and survival were still improved although overall effects were less pronounced compared to prophylaxis.

Conclusions

With regard to the central role of the FXII-driven contact activation system in ischemic stroke, inhibition of FXIIa may represent a new and promising treatment approach to prevent cerebral ischemia/reperfusion injury.

Factor XII: a novel target for safe prevention of thrombosis and inflammation

Plasma protein factor XII (FXII) activates the procoagulant and proinflammatory contact system that drives both the kallikrein-kinin system and the intrinsic pathway of coagulation. When zymogen FXII comes into contact with negatively charged surfaces, it auto-activates to the serine proteaseactivated FXII (FXIIa). Recently, various in vivo activators of FXII have been identified including heparin, misfolded protein aggregates, polyphosphate and nucleic acids. Murine models have established a central role of FXII in arterial and venous thrombosis. Despite its central function in thrombosis, deficiency in FXII does not impair haemostasis in animals and humans. In a preclinical cardiopulmonary bypass system in large animals, the FXIIa-blocking antibody 3F7 prevented thrombosis; however, in contrast to traditional anticoagulants, bleeding was not increased. In addition to its function in thrombosis, FXIIa initiates formation of the inflammatory mediator bradykinin. This mediator increases vascular leak, causes vasodilation, and induces chemotaxis with implications for septic, anaphylactic and allergic disease states. Therefore, targeting FXIIa appears to be a promising strategy for thromboprotection without associated bleeding risks but with anti-inflammatory properties.

Hypercoagulability Is a Stronger Risk Factor for Ischaemic Stroke than for Myocardial Infarction: A Systematic Review

Background and Purpose

Hypercoagulability increases the risk of arterial thrombosis; however, this effect may differ between various manifestations of arterial disease.

Methods

In this study, we compared the effect of coagulation factors as measures of hypercoagulability on the risk of ischaemic stroke (IS) and myocardial infarction (MI) by performing a systematic review of the literature. The effect of a risk factor on IS (relative risk for IS, RR_IS) was compared with the effect on MI (RR_MI) by calculating their ratio (RRR = RR_IS/RR_MI). A relevant differential effect was considered when RRR was >1+ its own standard error (SE) or <1−SE.

Results

We identified 70 publications, describing results from 31 study populations, accounting for 351 markers of hypercoagulability. The majority (203/351, 58%) had an RRR greater than 1. A larger effect on IS risk than MI risk (RRE>1+1SE) was found in 49/343 (14%) markers. Of these, 18/49 (37%) had an RRR greater than 1+2SE. On the opposite side, a larger effect on MI risk (RRR<1-1SE) was found in only 17/343 (5%) markers.

Conclusions

These results suggest that hypercoagulability has a more pronounced effect on the risk of IS than that of MI.

Noninvasive Imaging of Early Venous Thrombosis by 19 F Magnetic Resonance Imaging With Targeted Perfluorocarbon Nanoemulsions

Background—

Noninvasive detection of deep venous thrombi and subsequent pulmonary thromboembolism is a serious medical challenge, since both incidences are difficult to identify by conventional ultrasound techniques.

Methods and Results—

Here, we report a novel technique for the sensitive and specific identification of developing thrombi using background-free 19 F magnetic resonance imaging, together with α2-antiplasmin peptide (α2 AP )–targeted perfluorocarbon nanoemulsions (PFCs) as contrast agent, which is cross-linked to fibrin by active factor XIII. Ligand functionality was ensured by mild coupling conditions using the sterol-based postinsertion technique. Developing thrombi with a diameter <0.8 mm could be visualized unequivocally in the murine inferior vena cava as hot spots in vivo by simultaneous acquisition of anatomic matching 1 H and 19 F magnetic resonance images at 9.4 T with both excellent signal-to-noise and contrast-to-noise ratios (71±22 and 17±5, respectively). Furthermore, α2 AP -PFCs could be successfully applied for the diagnosis of experimentally induced pulmonary thromboembolism. In line with the reported half-life of factor XIIIa, application of α2 AP -PFCs >60 minutes after thrombus induction no longer resulted in detectable 19 F magnetic resonance imaging signals. Corresponding results were obtained in ex vivo generated human clots. Thus, α2 AP -PFCs can visualize freshly developed thrombi that might still be susceptible to pharmacological intervention.

Conclusions—

Our results demonstrate that 1 H/ 19 F magnetic resonance imaging, together with α2 AP -PFCs, is a sensitive, noninvasive technique for the diagnosis of acute deep venous thrombi and pulmonary thromboemboli. Furthermore, ligand coupling by the sterol-based postinsertion technique represents a unique platform for the specific targeting of PFCs for in vivo 19 F magnetic resonance imaging.

New agents for thromboprotection. A role for factor XII and XIIa inhibition

Blood coagulation is essential for hemostasis, however excessive coagulation can lead to thrombosis. Factor XII starts the intrinsic coagulation pathway and contact-induced factor XII activation provides the mechanistic basis for the diagnostic aPTT clotting assay. Despite its function for fibrin formation in test tubes, patients and animals lacking factor XII have a completely normal hemostasis. The lack of a bleeding tendency observed in factor XII deficiency states is in sharp contrast to deficiencies of other components of the coagulation cascade and factor XII has been considered to have no function for coagulation in vivo. Recently, experimental animal models showed that factor XII is activated by an inorganic polymer, polyphosphate, which is released from procoagulant platelets and that polyphosphate-driven factor XII activation has an essential role in pathologic thrombus formation. Cumulatively, the data suggest to target polyphosphate, factor XII, or its activated form factor XIIa for anticoagulation. As the factor XII pathway specifically contributes to thrombosis but not to hemostasis, interference with this pathway provides a unique opportunity for safe anticoagulation that is not associated with excess bleeding. The review summarizes current knowledge on factor XII functions, activators and inhibitors.

Recent advances on plasmin inhibitors for the treatment of fibrinolysis-related disorders

Growing evidence suggests that plasmin is involved in a number of physiological processes in addition to its key role in fibrin cleavage. Plasmin inhibition is critical in preventing adverse consequences arising from plasmin overactivity, e.g., blood loss that may follow cardiac surgery. Aprotinin was widely used as an antifibrinolytic drug before its discontinuation in 2008. Tranexamic acid and ε-aminocaproic acid, two small molecule plasmin inhibitors, are currently used in the clinic. Several molecules have been designed utilizing covalent, but reversible, chemistry relying on reactive cyclohexanones, nitrile warheads, and reactive aldehyde peptidomimetics. Other major classes of plasmin inhibitors include the cyclic peptidomimetics and polypeptides of the Kunitz and Kazal-type. Allosteric inhibitors of plasmin have also been designed including small molecule lysine analogs that bind to plasmin's kringle domain(s) and sulfated glycosaminoglycan mimetics that bind to plasmin's catalytic domain. Plasmin inhibitors have also been explored for resolving other disease states including cell metastasis, cell proliferation, angiogenesis, and embryo implantation. This review highlights functional and structural aspects of plasmin inhibitors with the goal of advancing their design.

Factor XII: a drug target for safe interference with thrombosis and inflammation

Data from experimental animal models revealed an essential role for factor XII (FXII) in thrombotic occlusive diseases. In contrast to other blood coagulation factors, deficiency in the protease is not associated with abnormal bleeding from injury sites (hemostasis) in patients or in animals. Cumulatively, these findings suggest that FXII could be targeted as a new method of anticoagulation that is devoid of bleeding risks. An FXIIa-neutralizing antibody, 3F7, has been developed that inhibited thrombosis in an extracorporeal membrane oxygenation (ECMO) system as efficiently as heparin. However, in sharp contrast to heparin, 3F7 treatment was not associated with an increase in therapy-associated hemorrhage. In this review, we summarize current knowledge of FXII physiology and pharmacology.

Factor XII inhibition reduces thrombus formation in a primate thrombosis model

The plasma zymogens factor XII (fXII) and factor XI (fXI) contribute to thrombosis in a variety of mouse models. These proteins serve a limited role in hemostasis, suggesting that antithrombotic therapies targeting them may be associated with low bleeding risks. Although there is substantial epidemiologic evidence supporting a role for fXI in human thrombosis, the situation is not as clear for fXII. We generated monoclonal antibodies (9A2 and 15H8) against the human fXII heavy chain that interfere with fXII conversion to the protease factor XIIa (fXIIa). The anti-fXII antibodies were tested in models in which anti-fXI antibodies are known to have antithrombotic effects. Both anti-fXII antibodies reduced fibrin formation in human blood perfused through collagen-coated tubes. fXII-deficient mice are resistant to ferric chloride-induced arterial thrombosis, and this resistance can be reversed by infusion of human fXII. 9A2 partially blocks, and 15H8 completely blocks, the prothrombotic effect of fXII in this model. 15H8 prolonged the activated partial thromboplastin time of baboon and human plasmas. 15H8 reduced fibrin formation in collagen-coated vascular grafts inserted into arteriovenous shunts in baboons, and reduced fibrin and platelet accumulation downstream of the graft. These findings support a role for fXII in thrombus formation in primates.

Imaging Neuroinflammation - from Bench to Bedside

Neuroinflammation plays a central role in a variety of neurological diseases, including stroke, multiple sclerosis, Alzheimer’s disease, and malignant CNS neoplasms, among many other. Different cell types and molecular mediators participate in a cascade of events in the brain that is ultimately aimed at control, regeneration and repair, but leads to damage of brain tissue under pathological conditions. Non-invasive molecular imaging of key players in the inflammation cascade holds promise for identification and quantification of the disease process before it is too late for effective therapeutic intervention. In this review, we focus on molecular imaging techniques that target inflammatory cells and molecules that are of interest in neuroinflammation, especially those with high translational potential. Over the past decade, a plethora of molecular imaging agents have been developed and tested in animal models of (neuro)inflammation, and a few have been translated from bench to bedside. The most promising imaging techniques to visualize neuroinflammation include MRI, positron emission tomography (PET), single photon emission computed tomography (SPECT), and optical imaging methods. These techniques enable us to image adhesion molecules to visualize endothelial cell activation, assess leukocyte functions such as oxidative stress, granule release, and phagocytosis, and label a variety of inflammatory cells for cell tracking experiments. In addition, several cell types and their activation can be specifically targeted in vivo, and consequences of neuroinflammation such as neuronal death and demyelination can be quantified. As we continue to make progress in utilizing molecular imaging technology to study and understand neuroinflammation, increasing efforts and investment should be made to bring more of these novel imaging agents from the “bench to bedside.”

Ischemia/reperfusion injury: effect of simultaneous inhibition of plasma cascade systems versus specific complement inhibition

Ischemia/reperfusion injury (IRI) may occur from ischemia due to thrombotic occlusion, trauma or surgical interventions, including transplantation, with subsequent reestablishment of circulation. Time-dependent molecular and structural changes result from the deprivation of blood and oxygen in the affected tissue during ischemia. Upon restoration of blood flow a multifaceted network of plasma cascades is activated, including the complement-, coagulation-, kinin-, and fibrinolytic system, which plays a major role in the reperfusion-triggered inflammatory process. The plasma cascade systems are therefore promising therapeutic targets for attenuation of IRI. Earlier studies showed beneficial effects through inhibition of the complement system using specific complement inhibitors. However, pivotal roles in IRI are also attributed to other cascades. This raises the question, whether drugs, such as C1 esterase inhibitor, which regulate more than one cascade at a time, have a higher therapeutic potential. The present review discusses different therapeutic approaches ranging from specific complement inhibition to simultaneous inhibition of plasma cascade systems for reduction of IRI, gives an overview of the plasma cascade systems in IRI as well as highlights recent findings in this field.