In vitro differential inhibition of the factor XI activity assay in the setting of a lupus anticoagulant

Acquired factor XI deficiencies due to factor-specific inhibitors are rare and may be associated with lupus anticoagulant. We report a 63-year-old male with suspected postsurgical bleeding, prior surgical site infection, an isolated prolonged activated partial thromboplastin time, and a positive lupus anticoagulant. Although the factor II assay was normal, factor VIII and IX assays initially demonstrated nonparallelism with factor activity that consistently increased to normal reference ranges with serial dilutions. A discrepancy in factor XI activity results was discovered when the in-house method demonstrated undetectable activity (<3%); send-out testing using different instrument/reagent combinations revealed the presence of factor XI activity between 70% and 76%. The patient received surgical follow-up and was subsequently discharged home. Given the differential in vitro inhibition of factor XI activity on our initial in-house testing, this case highlights the importance of recognizing factor assay interference in the presence of a known lupus anticoagulant inhibitor, with strategies to mitigate potentially erroneous results.

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

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

Pharmacologic targeting of coagulation factors XII and XI by monoclonal antibodies reduces thrombosis in nitinol stents under flow

BACKGROUND

Cardiovascular implantable devices, such as vascular stents, are critical for the treatment of cardiovascular diseases. However, their success is dependent on robust and often long-term antithrombotic therapies. Yet, the current standard-of-care therapies often pose significant bleeding risks to patients. Coagulation factor (F)XI and FXII have emerged as potentially safe and efficacious targets to safely reduce pathologic thrombin generation in medical devices.

OBJECTIVES

To study the efficacy of monoclonal antibody-targeting FXII and FXI of the contact pathway in preventing vascular device-related thrombosis.

METHODS

The effects of inhibition of FXII and FXI using function-blocking monoclonal antibodies were examined in a nonhuman primate model of nitinol stent-related thrombosis under arterial and venous flow conditions.

RESULTS

We found that function-blocking antibodies of FXII and FXI reduced markers of stent-induced thrombosis in vitro and ex vivo. However, FXI inhibition resulted in more effective mitigation of thrombosis markers under varied flow conditions.

CONCLUSION

This work provides further support for the translation of contact pathway of coagulation inhibitors for their adjunctive clinical use with cardiovascular devices.

Pharmacological targeting of coagulation factor XI attenuates experimental autoimmune encephalomyelitis in mice

Multiple sclerosis (MS) is the most common causes of non-traumatic disability in young adults worldwide. MS pathophysiologies include the formation of inflammatory lesions, axonal damage and demyelination, and blood brain barrier (BBB) disruption. Coagulation proteins, including factor (F)XII, can serve as important mediators of the adaptive immune response during neuroinflammation. Indeed, plasma FXII levels are increased during relapse in relapsing-remitting MS patients, and previous studies showed that reducing FXII levels was protective in a murine model of MS, experimental autoimmune encephalomyelitis (EAE). Our objective was to determine if pharmacological targeting of FXI, a major substrate of activated FXII (FXIIa), improves neurological function and attenuates CNS damage in the setting of EAE. EAE was induced in male mice using murine myelin oligodendrocyte glycoprotein peptides combined with heat-inactivated Mycobacterium tuberculosis and pertussis toxin. Upon onset of symptoms, mice were treated every other day intravenously with anti-FXI antibody, 14E11, or saline. Disease scores were recorded daily until euthanasia for ex vivo analyses of inflammation. Compared to the vehicle control, 14E11 treatment reduced the clinical severity of EAE and total mononuclear cells, including CD11b+CD45high macrophage/microglia and CD4+ T cell numbers in brain. Following pharmacological targeting of FXI, BBB disruption was reduced, as measured by decreased axonal damage and fibrin(ogen) accumulation in the spinal cord. These data demonstrate that pharmacological inhibition of FXI reduces disease severity, immune cell migration, axonal damage, and BBB disruption in mice with EAE. Thus, therapeutic agents targeting FXI and FXII may provide a useful approach for treating autoimmune and neurologic disorders.

Pharmacological reduction of coagulation factor XI reduces macrophage accumulation and accelerates deep vein thrombosis resolution in a mouse model of venous thrombosis

BACKGROUND

Deep vein thrombosis (DVT) and post-thrombotic syndrome (PTS) remain highly prevalent despite modern medical therapy. Contact activation is a promising target for safe antithrombotic anticoagulation. The anti-factor XI (FXI) monoclonal antibody 14E11 reduces circulating levels of FXI without compromising hemostasis. The human recombinant analog, AB023, is in clinical development. The role of FXI in mediation of inflammation during DVT resolution is unknown.

OBJECTIVES

Investigate the effects of pharmacological targeting of FXI with 14E11 in an experimental model of venous thrombosis.

METHODS

Adult wild-type CD1 mice were treated with subcutaneous anti-FXI antibody (14E11, 5 mg/kg) versus saline prior to undergoing surgical constriction of the inferior vena cava (IVC). Mice were evaluated at various time points to assess thrombus weight and volume, as well as histology analysis, ferumoxytol enhanced magnetic resonance imaging (Fe-MRI), and whole blood flow cytometry.

RESULTS

14E11-treated mice had reduced thrombus weights and volumes after IVC constriction on day 7 compared to saline-treated mice. 14E11 treatment reduced circulating monocytes by flow cytometry and macrophage content within thrombi as evaluated by histologic staining and Fe-MRI. Collagen deposition was increased at day 3 while CD31 and smooth muscle cell actin expression was increased at day 7 in the thrombi of 14E11-treated mice compared to saline-treated mice.

CONCLUSION

Pharmacologic targeting of FXI enhances the early stages of experimental venous thrombus resolution in wild-type CD1 mice, and may be of interest for future clinical evaluation of the antibody in DVT and PTS.

Factor XI Inhibition for the Prevention of Venous Thromboembolism: An Update on Current Evidence and Future perspectives

During the past decade, emergence of direct oral anticoagulants (DOACs) has drastically improved the prevention of thrombosis. However, several unmet needs prevail in the field of thrombosis prevention, even in the DOACs’ era. The use of DOACs is still constrained and the drugs cannot be administered in every clinical scenario, such as an increased anticoagulant-associated bleeding risk, particularly in some specific populations (cancer – notably those with gastrointestinal or genitourinary cancer – and frail patients), the impossibility to be used in certain patients (eg, end-stage kidney failure during hemodialysis, pregnancy and breastfeeding), and their lack of efficacy in certain clinical scenarios (eg, mechanical heart valves, triple-positive antiphospholipid syndrome). Efforts to find a factor that upon antagonization prevents thrombosis but spares haemostasis have resulted in the identification of coagulation factor XI (FXI) as a therapeutic target. After briefly recapitulating the role of factor XI in the balance of haemostasis, we propose a narrative review of the key data published to date with compounds targeting factor XI to prevent thrombosis as well as the main ongoing clinical studies, opening up prospects for improving the care of patients requiring thrombosis prevention.

Abelacimab for Prevention of Venous Thromboembolism

BACKGROUND

The role of factor XI in the pathogenesis of postoperative venous thromboembolism is uncertain. Abelacimab is a monoclonal antibody that binds to factor XI and locks it in the zymogen (inactive precursor) conformation.

METHODS

In this open-label, parallel-group trial, we randomly assigned 412 patients who were undergoing total knee arthroplasty to receive one of three regimens of abelacimab (30 mg, 75 mg, or 150 mg) administered postoperatively in a single intravenous dose or to receive 40 mg of enoxaparin administered subcutaneously once daily. The primary efficacy outcome was venous thromboembolism, detected by mandatory venography of the leg involved in the operation or objective confirmation of symptomatic events. The principal safety outcome was a composite of major or clinically relevant nonmajor bleeding up to 30 days after surgery.

RESULTS

Venous thromboembolism occurred in 13 of 102 patients (13%) in the 30-mg abelacimab group, 5 of 99 patients (5%) in the 75-mg abelacimab group, and 4 of 98 patients (4%) in the 150-mg abelacimab group, as compared with 22 of 101 patients (22%) in the enoxaparin group. The 30-mg abelacimab regimen was noninferior to enoxaparin, and the 75-mg and 150-mg abelacimab regimens were superior to enoxaparin (P<0.001). Bleeding occurred in 2%, 2%, and none of the patients in the 30-mg, 75-mg, and 150-mg abelacimab groups, respectively, and in none of the patients in the enoxaparin group.

CONCLUSIONS

This trial showed that factor XI is important for the development of postoperative venous thromboembolism. Factor XI inhibition with a single intravenous dose of abelacimab after total knee arthroplasty was effective for the prevention of venous thromboembolism and was associated with a low risk of bleeding. (Funded by Anthos Therapeutics; ANT-005 TKA EudraCT number, 2019-003756-37.).

PK/PD modeling of FXI antisense oligonucleotides to bridge the dose-FXI activity relation from healthy volunteers to end-stage renal disease patients

IONIS-FXIRX (BAY2306001) is an antisense oligonucleotide that inhibits the synthesis of coagulation factor XI (FXI) and has been investigated in healthy volunteers and patients with end-stage renal disease (ESRD). FXI-LICA (BAY2976217) shares the same RNA sequence as IONIS-FXIRX but contains a GalNAc-conjugation that facilitates asialoglycoprotein receptor (ASGPR)-mediated uptake into hepatocytes. FXI-LICA has been studied in healthy volunteers and is currently investigated in patients with ESRD on hemodialysis. We present a model-informed bridging approach that facilitates the extrapolation of the dose-exposure-FXI relationship from IONIS-FXIRX to FXI-LICA in patients with ESRD and, thus, supports the selection of FX-LICA doses being investigated in patients with ESRD. A two-compartment pharmacokinetic (PK) model, with mixed first- and zero-order subcutaneous absorption and first-order elimination, was combined with an indirect response model for the inhibitory effect on the FXI synthesis rate via an effect compartment. This PK/pharmacodynamic model adequately described the median trends, as well as the interindividual variabilities for plasma drug concentration and FXI activity in healthy volunteers of IONIS-FXIRX and FXI-LICA, and in patients with ESRD of IONIS-FXIRX . The model was then used to predict dose-dependent steady-state FXI activity following repeat once-monthly doses of FXI-LICA in a virtual ESRD patient population. Under the assumption of similar ASGPR expression in patients with ESRD and healthy volunteers, doses of 40 mg, 80 mg, and 120 mg FXI-LICA are expected to cover the target range of clinical interest for steady-state FXI activity in the phase IIb study of FXI-LICA in patients with ESRD undergoing hemodialysis.

Turning Up to Eleven: Factor XI Inhibitors as Novel Agents to Maximize Safety and Maintain Efficacy in Thromboembolic Disease

Within the past decade nonvitamin K oral anticoagulants have emerged as the standard of care for the prevention and treatment of thromboembolic disorders, however safety of anticoagulants remain a concern for many patients and providers. There exists new interest in factor XI inhibition as novel therapeutic target based on observations of lower thrombotic rates and without significant bleed risk in individuals with inherited factor XI deficiency. Several classes of factor XI inhibitors including antisense oligonucleotides, monoclonal antibodies, and small molecule inhibitors have undergone preclinical studies and clinical trials in humans. Both osocimab and IONIS-FXI have been evaluated in patients undergoing orthopedic surgery and demonstrated superiority to enoxaparin without increasing major bleeding. Future studies with both these agents are ongoing, as well as the continued development of other inhibitors of factor XI. Early data regarding factor XI inhibition is encouraging as a potent anticoagulant and may offer a safer alternative compared to therapeutic currently available in contemporary practice for thromboembolic disease.

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

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

Differential Roles for the Coagulation Factors XI and XII in Regulating the Physical Biology of Fibrin

In the contact activation pathway of the coagulation, zymogen factor XII (FXII) is converted to FXIIa, which triggers activation of FXI leading to the activation of FIX and subsequent thrombin generation and fibrin formation. Feedback activation of FXI by thrombin has been shown to promote thrombin generation in a FXII-independent manner and FXIIa can bypass FXI to directly activate FX and prothrombin in the presence of highly negatively charged molecules, such as long-chain polyphosphates (LC polyP). We sought to determine whether activation of FXII or FXI differentially regulate the physical biology of fibrin formation. Fibrin formation was initiated with tissue factor, ellagic acid (EA), or LC polyP in the presence of inhibitors of FXI and FXII. Our data demonstrated that inhibition of FXI decreased the rate of fibrin formation and fiber network density, and increased the fibrin network strength and rate of fibrinolysis when gelation was initiated via the contact activation pathway with EA. FXII inhibition decreased the fibrin formation and fibrin density, and increased the fibrinolysis rate only when fibrin formation was initiated via the contact activation pathway with LC polyP. Overall, we demonstrate that inhibition of FXI and FXII distinctly alter the biophysical properties of fibrin.

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.

Factor XI and contact activation as targets for antithrombotic therapy

The most commonly used anticoagulants produce therapeutic antithrombotic effects either by inhibiting thrombin or factor Xa (FXa) or by lowering the plasma levels of the precursors of these key enzymes, prothrombin and FX. These drugs do not distinguish between thrombin generation contributing to thrombosis from thrombin generation required for hemostasis. Thus, anticoagulants increase bleeding risk, and many patients who would benefit from therapy go untreated because of comorbidities that place them at unacceptable risk for hemorrhage. Studies in animals demonstrate that components of the plasma contact activation system contribute to experimentally induced thrombosis, despite playing little or no role in hemostasis. Attention has focused on FXII, the zymogen of a protease (FXIIa) that initiates contact activation when blood is exposed to foreign surfaces, and FXI, the zymogen of the protease FXIa, which links contact activation to the thrombin generation mechanism. In the case of FXI, epidemiologic data indicate this protein contributes to stroke and venous thromboembolism, and perhaps myocardial infarction, in humans. A phase 2 trial showing that reduction of FXI may be more effective than low molecular weight heparin at preventing venous thrombosis during knee replacement surgery provides proof of concept for the premise that an antithrombotic effect can be uncoupled from an anticoagulant effect in humans by targeting components of contact activation. Here, we review data on the role of FXI and FXII in thrombosis and results of preclinical and human trials for therapies targeting these proteins.

Factor XI antisense oligonucleotide for prevention of venous thrombosis

BACKGROUND

Experimental data indicate that reducing factor XI levels attenuates thrombosis without causing bleeding, but the role of factor XI in the prevention of postoperative venous thrombosis in humans is unknown. FXI-ASO (ISIS 416858) is a second-generation antisense oligonucleotide that specifically reduces factor XI levels. We compared the efficacy and safety of FXI-ASO with those of enoxaparin in patients undergoing total knee arthroplasty.

METHODS

In this open-label, parallel-group study, we randomly assigned 300 patients who were undergoing elective primary unilateral total knee arthroplasty to receive one of two doses of FXI-ASO (200 mg or 300 mg) or 40 mg of enoxaparin once daily. The primary efficacy outcome was the incidence of venous thromboembolism (assessed by mandatory bilateral venography or report of symptomatic events). The principal safety outcome was major or clinically relevant nonmajor bleeding.

RESULTS

Around the time of surgery, the mean (±SE) factor XI levels were 0.38±0.01 units per milliliter in the 200-mg FXI-ASO group, 0.20±0.01 units per milliliter in the 300-mg FXI-ASO group, and 0.93±0.02 units per milliliter in the enoxaparin group. The primary efficacy outcome occurred in 36 of 134 patients (27%) who received the 200-mg dose of FXI-ASO and in 3 of 71 patients (4%) who received the 300-mg dose of FXI-ASO, as compared with 21 of 69 patients (30%) who received enoxaparin. The 200-mg regimen was noninferior, and the 300-mg regimen was superior, to enoxaparin (P<0.001). Bleeding occurred in 3%, 3%, and 8% of the patients in the three study groups, respectively.

CONCLUSIONS

This study showed that factor XI contributes to postoperative venous thromboembolism; reducing factor XI levels in patients undergoing elective primary unilateral total knee arthroplasty was an effective method for its prevention and appeared to be safe with respect to the risk of bleeding. (Funded by Isis Pharmaceuticals; FXI-ASO TKA ClinicalTrials.gov number, NCT01713361.).

Factor XI as a target for antithrombotic therapy

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

Dynamics of pathologic clot formation: a mathematical model

Recent studies have provided evidence of a significant role of the Hageman factor in pathologic clot formation. Since auto-activation of the Hageman factor triggers the intrinsic coagulation pathway, we study the dynamics of pathologic clot formation considering the intrinsic pathway as the predominant mechanism of this process. Our methodological approach to studying the dynamics of clot formation is based on mathematical modelling. Activation of the blood coagulation cascade, particularly its intrinsic pathway, is known to involve platelets. Therefore, equations accounting for the effects of activated platelets on the intrinsic pathway activation are included in our model. This brings about a considerable increase in the values of kinetic constants involved in the model of the principal biochemical processes resulting in clot formation. The purpose of this study is to elucidate the mechanism of pathologic clot formation. Since the time window of thrombolysis is 3-6h, we hypothesize that in many cases the rate of pathologic clot formation is much lower than that of haemostatic clot. This assumption is used to simplify the mathematical model and to estimate kinetic constants of biochemical reactions that initiate pathologic clot formation. The insights we gained from our mathematical model may lead to new approaches to the prophylaxis of pathologic clot formation. We believe that one of the most efficient ways to prevent pathologic clot formation is simultaneous inhibition of activated factors ХII and ХI.

Factor XII promotes blood coagulation independent of factor XI in the presence of long-chain polyphosphates

BACKGROUND

Inorganic polyphosphates (polyP), which are secreted by activated platelets (short-chain polyP) and accumulate in some bacteria (long-chain polyP), support the contact activation of factor XII (FXII) and accelerate the activation of FXI.

OBJECTIVES

The aim of the present study was to evaluate the role of FXI in polyP-mediated coagulation activation and experimental thrombus formation.

METHODS AND RESULTS

Pretreatment of plasma with antibodies that selectively inhibit FXI activation by activated FXII (FXIIa) or FIX) activation by activated FXI (FXIa) were not able to inhibit the procoagulant effect of long or short-chain polyP in plasma. In contrast, the FXIIa inhibitor, corn trypsin inhibitor, blocked the procoagulant effect of long and short polyP in plasma. In a purified system, long polyP significantly enhanced the rate of FXII and prekallikrein activation and the activation of FXI by thrombin but not by FXIIa. In FXI-deficient plasma, long polyP promoted clotting of plasma in an FIX-dependent manner. In a purified system, the activation of FXII and prekallikrein by long polyP promoted FIX activation and prothombin activation. In an ex vivo model of occlusive thrombus formation, inhibition of FXIIa with corn trypsin inhibitor but not of FXI with a neutralizing antibodies abolished the prothrombotic effect of long polyP.

CONCLUSIONS

We propose that long polyP promotes FXII-mediated blood coagulation bypassing FXI. Accordingly, some polyp-containing pathogens may have evolved strategies to exploit polyP-initiated FXII activation for virulence, and selective inhibition of FXII may improve the host response to pathogens.

Inhibitors of propagation of coagulation (factors VIII, IX and XI): a review of current therapeutic practice

The management of patients with congenital haemophilia who develop alloantibodies against factors of the propagation phase of blood coagulation, commonly known as inhibitors, is the most important challenge facing haemophilia caregivers at present, as this complication not only compromises the efficacy of replacement therapy but also consumes an enormous amount of economic resources. Development of inhibitors further complicates the clinical course of severe haemophilia, with a prevalence of up to 30% in patients with haemophilia A (factor VIII deficiency) and up to 5% in those with haemophilia B (factor IX deficiency) and haemophilia C (factor XI deficiency). While the short-term goal of treatment of patients who develop alloantibodies is the control of bleeding, the eradication of the inhibitor is the main long-term goal. The management of severe bleeding episodes and the eradication of the autoantibody are also the mainstays of treatment of patients with acquired haemophilia, a rare but life-threatening haemorrhagic condition characterized by the development of inhibitory autoantibodies against coagulation factor VIII. The most recent options available for treating patients with congenital haemophilia complicated by inhibitors and acquired haemophilia because of autoantibodies against factor VIII are summarized in this review article.

Factor XI and XII as antithrombotic targets

PURPOSE OF REVIEW

Arterial and venous thrombosis are major causes of morbidity and mortality, and the incidence of thromboembolic diseases increases as a population ages. Thrombi are formed by activated platelets and fibrin. The latter is a product of the plasma coagulation system. Currently available anticoagulants such as heparins, vitamin K antagonists and inhibitors of thrombin or factor Xa target enzymes of the coagulation cascade that are critical for fibrin formation. However, fibrin is also necessary for terminating blood loss at sites of vascular injury. As a result, anticoagulants currently in clinical use increase the risk of bleeding, partially offsetting the benefits of reduced thrombosis. This review focuses on new targets for anticoagulation that are associated with minimal or no therapy-associated increased bleeding.

RECENT FINDINGS

Data from experimental models using mice and clinical studies of patients with hereditary deficiencies of coagulation factors XI or XII have shown that both of these clotting factors are important for thrombosis, while having minor or no apparent roles in processes that terminate blood loss (hemostasis).

SUMMARY

Hereditary deficiency of factor XII (Hageman factor) or factor XI, plasma proteases that initiate the intrinsic pathway of coagulation, impairs thrombus formation and provides protection from vascular occlusive events, while having a minimal impact on hemostasis. As the factor XII-factor XI pathway contributes to thrombus formation to a greater extent than to normal hemostasis, pharmacological inhibition of these coagulation factors may offer the exciting possibility of anticoagulation therapies with minimal or no bleeding risk.

Coagulation factor XI as a novel target for antithrombotic treatment

Coagulation factor (F)XI was first described as a member of the contact pathway of coagulation. However, the 'classic' theory of the extrinsic and intrinsic pathway has been revised and FXI was found to be activated by thrombin and to play a role in sustained thrombin generation and fibrinolysis inhibition. Recent studies have pointed to a disproportionate role of FXI in thrombosis and hemostasis. The observations that human congenital FXI deficiency is generally accompanied by mild and injury-related bleeding, and that experimental, provoked bleeding in animals is unaffected by FXI deficiency or FXI inhibition, suggest that the FXI amplification pathway is less important for normal hemostasis in vivo. In contrast, elevated plasma levels of FXI may contribute to human thromboembolic disease and the antithrombotic efficacy of FXI inhibition has been demonstrated in numerous animal models of arterial, venous and cerebral thrombosis. Whether severe FXI deficiency in humans protects against thromboembolic events remains unclear, although some evidence exists that the occurrence of ischemic stroke or venous thrombosis is low in severely FXI-deficient patients. Because of its distinctive function in thrombosis and hemostasis, FXI is an attractive target for the treatment and prevention of thromboembolism. A novel strategy for FXI inhibition is the use of antisense technology which has been studied in various thrombosis and bleeding animal models. The results are promising and support the concept that targeting FXI might serve as a new, effective and potentially safer alternative for the treatment of thromboembolic disease in humans.

Prevention of vascular graft occlusion and thrombus-associated thrombin generation by inhibition of factor XI

The protease thrombin is required for normal hemostasis and pathologic thrombogenesis. Since the mechanism of coagulation factor XI (FXI)-dependent thrombus growth remains unclear, we investigated the contribution of FXI to thrombus formation in a primate thrombosis model. Pretreatment of baboons with a novel anti-human FXI monoclonal antibody (aXIMab; 2 mg/kg) inhibited plasma FXI by at least 99% for 10 days, and suppressed thrombin-antithrombin (TAT) complex and beta-thromboglobulin (betaTG) formation measured immediately downstream from thrombi forming within collagen-coated vascular grafts. FXI inhibition with aXIMab limited platelet and fibrin deposition in 4-mm diameter grafts without an apparent increase in D-dimer release from thrombi, and prevented the occlusion of 2-mm diameter grafts without affecting template bleeding times. In comparison, pretreatment with aspirin (32 mg/kg) prolonged bleeding times but failed to prevent graft occlusion, supporting the concept that FXI blockade may offer therapeutic advantages over other antithrombotic agents in terms of bleeding complications. In whole blood, aXIMab prevented fibrin formation in a collagen-coated flow chamber, independent of factor XII and factor VII. These data suggest that endogenous FXI contributes to arterial thrombus propagation through a striking amplification of thrombin generation at the thrombus luminal surface.

Levels of intrinsic coagulation factors and the risk of myocardial infarction among men: opposite and synergistic effects of factors XI and XII

The role of the intrinsic coagulation system on the risk of myocardial infarction is unclear. In the Study of Myocardial Infarctions Leiden (SMILE) that included 560 men younger than age 70 with a first myocardial infarction and 646 control subjects, we investigated the risk of myocardial infarction for levels of factor XI (factor XIc) and factor XII (factor XIIc). Furthermore, the risks for factor VIII activity (factor VIIIc) and factor IX activity (factor IXc) were assessed. Factor XIc was 113.0% in patients compared with 109.8% in control subjects (difference, 3.2%; 95% CI, 1.1%-5.4%). The risk of myocardial infarction adjusted for age for men in the highest quintile compared with those in the lowest quintile was 1.8-fold increased (ORadj, 1.8; 95% CI, 1.2-2.7). In contrast, factor XIIc among patients with myocardial infarction was lower than in control subjects, respectively, 93.0% and 98.6% (difference, 5.6%; 95% CI, 3.3%-7.9%). The odds ratio of myocardial infarction for men in the highest quintile versus those in the lowest quintile was 0.4 (ORadj, 0.4; 95% CI, 0.2-0.5). The highest risk was found among men with both high factor XIc and low factor XIIc (analyses in tertiles: ORadj, 6.4; 95% CI, 2.2-18.0). Factor VIIIc increased the risk of myocardial infarction although not dose dependently. Factor IXc increased the risk; odds ratio of myocardial infarction for men in the highest quintile versus those in the lowest quintile was 3.2 (ORadj, 3.2; 95% CI, 2.0-5.1). Thus, factors XIc and XIIc have opposite and synergistic effects on the risk of myocardial infarction in men; factor VIIIc and factor IXc increase the risk.

Mutation of surface residues to promote crystallization of activated factor XI as a complex with benzamidine: an essential step for the iterative structure-based design of factor XI inhibitors

Activated factor XI (FXIa) is a key enzyme in the amplification phase of the blood-coagulation cascade. Thus, a selective FXIa inhibitor may have lesser bleeding liabilities and provide a safe alternative for antithrombosis therapy to available drugs on the market. In a previous report, the crystal structures of the catalytic domain of FXIa (rhFXI(370-607)) in complex with various ecotin mutants have been described. However, ecotin forms a matrix-like interaction with rhFXI(370-607) and is impossible to displace with small-molecule inhibitors; ecotin crystals are therefore not suitable for iterative structure-based ligand design. In addition, rhFXI(370-607) did not crystallize in the presence of small-molecule ligands. In order to obtain the crystal structure of rhFXI(370-607) with a weak small-molecule ligand, namely benzamidine, several rounds of surface-residue mutation were implemented to promote crystal formation of rhFXI(370-607). A quadruple mutant of rhFXI(370-607) (rhFXI(370-607)-S434A,T475A,C482S,K437A) readily crystallized in the presence of benzamidine. The benzamidine in the preformed crystals was easily exchanged with other FXIa small-molecule inhibitors. These crystals have facilitated the structure-based design of small-molecule FXIa inhibitors.

New observations on factor XI deficiency

Factor (F) XI is an injury-related bleeding tendency that commonly occurs when trauma involves tissues rich in fibronolytic activators. Severe FXI deficiency is defined when the activity of FXI in plasma is less than 15 U dL(-1). The disorder is inherited as an autosomal recessive trait manifesting in homozygotes or compound heterozygotes, and infrequently in heterozygotes. So far 53 mutations in the gene of FXI have been described and four of them were found to be prevalent in Ashkenazi Jews, Iraqi Jews, Basques or the English population. For each of the four mutations a founder effect was discerned. Inhibitors can develop in patients with FXI level < 1U dL(-1) who were exposed to plasma which seriously complicates their management during surgery. No correction of a prolonged aPTT by normal plasma is indicative of the presence of an inhibitor. In contrast to patients with haemophilia A, severe FXI deficiency provides no protection against myocardial infarction. In patients with severe FXI deficiency undergoing surgery, fresh frozen plasma is the treatment of choice. FXI concentrates can also be used but cause thrombosis in approximately 10% of patients, particularly those with cardiovascular disease. Recombinant FVIIa has successfully prevented bleeding during or after surgery in patients with FXI inhibitors.

Synthesis of macrocyclic, potential protease inhibitors using a generic scaffold

A generic macrocyclic peptide structure 2 was designed as a potential inhibitor of a range of proteinases, by using as a basis for the design the known structures of a series of enzyme-inhibitor complexes. The macrocyclic nature of the target 2 was chosen so as to reduce the entropic advantage in the hydrolytic enzymatic step, and thereby to inhibit the function of the enzyme. The nature of the linking group was identified as a benzoxazole by molecular modeling, so as to preserve the recognized conformation of the peptide chain. The specificity of the potential inhibitor was tuned by variation of the P(1) group (by incorporating phenylalanine, aspartic acid, or lysine), to allow recognition by different enzyme classes. The targets were prepared from the bis-amino acid derivative 5, itself prepared using the Pd-catalyzed coupling of an organozinc reagent with the iodobenzothiazole 7 and subsequent macrocyclization of the open-chain derivatives 22-24 using HATU. None of the macrocylic compounds 25, 28-30, and 32 inhibited their target enzymes. NMR and MS studies on the interaction of macrocycle 29 and chymotrypsin established that compound 29 was in fact a substrate of the enzyme. This result indicated that while the design had been partially successful in identifying a compound that bound, the reduction in entropic advantage due to its macrocyclic nature was not sufficient to allow 29 to act as an inhibitor.

Successful use of recombinant factor VIIa in a patient with inhibitor secondary to severe factor XI deficiency

Factor XI (FXI) inhibitors are a rare complication of inherited FXI deficiency. We report the successful use of recombinant factor VIIa (FVIIa) in a patient with a high-responding inhibitor undergoing cataract extraction. At the time of surgery there were limited available data on the optimal management of patients with FXI deficiency. A 62-year-old Ashkenazi Jewish woman had a lifelong history of excessive bleeding secondary to severe FXI deficiency (2 U dL-1), and received FXI concentrate (FXI:C) when she underwent a colposuspension procedure. She was subsequently diagnosed with a FXI inhibitor of 16 Bethesda units (BU) when she developed a poor response to FXI:C at the time of total hip replacement. Two months later she was admitted for cataract extraction. The FXI level was < 1 U dL-1 with an inhibitor titre of 48 BU. She received 90 microg kg-1 of FVIIa immediately preoperatively followed by continuous infusion at a rate of 20 microg kg-1 h-1 for 24 h. The cataract extraction was successful and there was no excess bleeding during surgery or in the postoperative period. Mutation analysis of the FXI gene showed that the patient was homozygous for the type II genotype [exon 5, Glu117-->Ter]. The reason for the low prevalence of inhibitor formation in patients with FXI deficiency is unclear but may reflect a number of factors including reporting bias, the rarity of absent circulating FXI:C activity, and the infrequent use of FXI replacement therapy.

The inhibition of human factor Xa by plasminogen activator inhibitor type 1 in the presence of calcium ion, and its enhancement by heparin and vitronectin

Plasminogen activator inhibitor type 1 (PAI-1), a member of serine proteinase inhibitor superfamily, is known to inhibit thrombin in the presence of either heparin or vitronectin. We analyzed possible inhibitory activity of PAI-1 on human factor Xa. PAI-1 inhibited factor Xa in the presence of calcium ion (Ca2+), whereas no inhibition was observed in the absence of Ca2+. Half maximal enhancement by Ca2+ was obtained at 0.8 mM. An equimolar complex formation between factor Xa and PAI-1 in the presence of Ca2+ was observed by SDS polyacrylamide gel electrophoresis. Both unfractionated heparin and vitronectin enhanced the inhibition only in the presence of Ca2+. Apparent second-order rate constant (ki) for the inhibition of factor Xa by PAI-1 at 5 mM Ca2+ was 1.6 x 10(4) M-1 s-1, and was enhanced 3-fold by 2 u/ml of heparin (4.6 x 10(4) M-1 s-1) and 10-fold by 100 nM vitronectin (1.6 x 10(5) M-1 s-1), respectively. The interaction between Ca(2+)-bound factor Xa and PAI-1 could be important from the view of PAI-1 neutralization and enhancement of fibrinolysis.

Successful childbirth by a patient with congenital factor XI deficiency and an acquired inhibitor

Acquired inhibitors in factor XI deficiency (FXI) are rare. The presence of an inhibitor during pregnancy poses a potential haemorrhagic risk to the fetus. We report an uncomplicated pregnancy and successful childbirth by a woman with congenital FXI deficiency and an acquired inhibitor, and discuss the persistence of residual FXI activity in the presence of an inhibitor.

Effect of heparin on the inhibition of the contact system enzymes

1. One can accurately predict the contribution of each inhibitor to the total inactivation of an enzyme in plasma once its pseudo-first-order reaction rate constant and concentration are known. 2. Because the mechanism of augmentation of the inactivation rate of an enzyme by ATIII occurs via formation of an ATIII-heparin complex, the degree of potentiation can be predicted by knowing the binding capacity (sites per mole) of the heparin preparation and the concentration of heparin in the reaction (to calculate the concentration of the ATIII-heparin complex). 3. The augmentation by heparin of the inactivation rate of a particular enzyme by ATIII is dependent upon the presence of other enzymes with higher kassoc, since these would strongly compete for the ATIII-heparin complex. 4. In a plasma environment, using therapeutic levels of heparin, there is no augmentation of the inactivation rate of any of the contact enzymes.

The heparin-catalysed inhibition of human factor XIa by antithrombin III is dependent on the heparin type

The effect of various well-characterized heparin preparations on the inactivation of human Factor XIa by human antithrombin III was studied. The heparin preparations used were unfractionated heparin and four heparin fractions obtained after anion-exchange chromatography. Inactivation of Factor XIa was monitored with S2366 as chromogenic substrate and followed pseudo-first-order reaction kinetics under all reaction conditions tested. Enhancement of the rate of inhibition of Factor XIa in the presence of unfractionated heparin correlated to the binding of antithrombin III to heparin. From the kinetic data a binding constant of 0.1 microM was inferred. The maximum rate enhancement, achieved at saturating heparin concentrations, was 30-fold. The rate enhancement achieved in the presence of each of the heparin fractions could also be correlated to the binding of antithrombin III to the heparin. The binding constant inferred from the kinetic data varied from 0.10 to 0.28 microM and the number of binding sites for antithrombin III varied from 0.06 to 0.74 site per heparin molecule. The maximum rate enhancements, achieved at saturating heparin concentrations, were strongly dependent on the type of heparin used and varied from 7-fold for fraction A to 41-fold for fraction D. Therefore, although the stimulation of Factor XIa inactivation by antithrombin III could be quantitatively correlated to the binding of antithrombin III to heparin, the heparin-catalysed inhibition of Factor XIa is dependent not only upon the degree of binding of antithrombin III to heparin but also upon the type of heparin to which antithrombin III is bound.

Inactivation of human plasma kallikrein and factor XIa by protein C inhibitor

The inhibition of kallikrein and factor XIa by protein C inhibitor (PCI) was studied. The method of Suzuki et al. [Suzuki, K., Nishioka, J., & Hashimoto, S. (1983) J. Biol. Chem. 258, 163-168] for the purification of PCI was modified in order to avoid the generation of proteolytic activity and subsequent inactivation of PCI. With the use of soybean trypsin inhibitor, an efficient inhibitor of kallikrein and factor XIa, the generation of proteolytic activity was avoided. The kinetics for the inactivation of activated protein C (APC), kallikrein, and factor XIa by PCI were determined. In the absence of heparin, no inactivation of APC was observed, in contrast to kallikrein and factor XIa, which are inhibited with second-order rate constants of (11 +/- 4) X 10(4) and (0.94 +/- 0.07) X 10(4) M-1 s-1, respectively. Addition of heparin potentiated the inhibition of APC [(1.2 +/- 0.2) X 10(4) M-1 s-1] and factor XIa [(9.1 +/- 0.7) X 10(4) M-1 s-1] by PCI, whereas the inhibition of kallikrein by PCI was unchanged [(10 +/- 1) X 10(4) M-1 s-1]. The second-order rate constants for the inhibition of kallikrein or factor XIa by PCI were similar to the second-order rate constants for the inhibition of their isolated light chains by PCI, indicating a minor role for the heavy chains of both molecules in the inactivation reactions. With sodium dodecyl sulfate-polyacrylamide slab gel electrophoresis and immunoblotting, complex formation of APC, kallikrein, and factor XIa with PCI could be demonstrated. APC and kallikrein formed 1:1 molar complexes with PCI.(ABSTRACT TRUNCATED AT 250 WORDS)

Transfusion-induced specific anti-factor XI inhibitor in a patient with previously unrecognized factor XI deficiency

This article reports the rare occurrence and investigation of a specific anti-factor XI inhibitor that arose after fresh-frozen plasma infusion into a patient with previously unrecognized Factor XI deficiency. The IgG fraction of the patient's plasma that contained anti-Factor XI antibody was isolated by chromatography on DEAE-Affigel and concentrated. It was shown to exert inhibitory effect on purified Factor XI and XIa both in the activated and nonactivated partial thromboplastin time assay.

Inhibition of human blood coagulation factor XIa by C-1 inhibitor

The inactivation of activated factor XI (factor XIa) and of its isolated light chain by C-1 inhibitor was studied. Irreversible inhibition was observed in a reaction in which no reversible enzyme-inhibitor complex was formed. The second-order rate constants for the inactivation of factor XIa or its light chain by C-1 inhibitor were 2.3 X 10(3) and 2.7 X 10(3) M-1 s-1, respectively. High molecular weight kininogen did not affect the rate of inactivation. The nature of the complexes formed between factor XIa or its light chain and C-1 inhibitor was studied by using sodium dodecyl sulfate gradient polyacrylamide slab gel electrophoresis. Under nonreducing conditions, two factor XIa-C-1 inhibitor complexes were observed with apparent molecular weights of 230,000 and 300,000. Reduction of these complexes resulted in the formation of a single band with a molecular weight of 130,000. This band is also formed in the reaction of the isolated light chain of factor XIa with C-1 inhibitor. These results demonstrate that two C-1 inhibitor molecules can become bound to the light chains of a factor XIa molecule. In addition, the mechanism of interaction of factor XIa or its isolated light chain with C-1 inhibitor appears identical, and the rate of inactivation of the enzyme by C-1 inhibitor is very similar. Neither the heavy chain of factor XIa nor high molecular weight kininogen is significantly involved in the inactivation of factor XIa by C-1 inhibitor.

Acquired factor XI inhibitors in congenitally deficient patients

Four factor XI (F XI)-deficient patients are described, all of whom formed circulating anticoagulants against F X1. In the three most severely affected patients (F XI 0%-6% activity), the anticoagulant appeared to have been stimulated by plasma infusion. However, in the milder case (25% F XI activity), no infusion had been documented. The findings in these cases emphasize the diversity of F XI inhibitors in congenitally deficient patients. Awareness of the potential development of these inhibitors will be helpful in both daily management and perioperative care of such patients.

Determination of main inhibitor of activated factor XI

To study the contribution of alpha 1 antitrypsin (alpha 1AT) and antithrombin III (AT III) to the inactivation of F.XIa in plasma, we developed the assay system of F.XIa-AT III complex according to our method of F.XIa-alpha 1AT complex, and measured the levels of both complexes in the patients with disseminated intravascular coagulation (DIC) derived from various triggers. F.XIa-alpha 1AT complex level was always higher than F.XIa-AT III complex level in each patient, independently on the trigger of DIC, the administration of heparin and the levels of F.XI, alpha 1AT and AT III. These results indicate that alpha 1AT is the main inhibitor of F.XIa in plasma.

Inhibition of factor XIa by antithrombin III

The inactivation of human factor XIa by human antithrombin III was studied under pseudo-first-order reaction conditions (excess antithrombin III) both in the absence and in the presence of heparin. The time course of inhibition was followed by using polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate. After electrophoresis, proteins were blotted onto nitrocellulose and stained either for glycoprotein or for antithrombin III using antibodies against antithrombin III. Concomitant with factor XIa inactivation, two new slower migrating bands, one of which represented the intermediate complex consisting of one antithrombin III complexed with factor XIa, appeared as a transient band. Complete inactivation resulted in a single band representing the complex of factor XIa with two antithrombin III molecules. Quantitative analysis of the time course of inactivation was accomplished by measurement of the disappearance of factor XIa amidolytic activity toward the chromogenic substrate S2366. Pseudo-first-order reaction kinetics were observed throughout. The rate constant of inactivation was found to be 10(3) M-1 s-1 in the absence of heparin and 26.7 X 10(3) M-1 s-1 in the presence of saturating amounts of heparin. From the kinetic data, a binding constant (Kd) of 0.14 microM was inferred for the binding of antithrombin III to heparin. The time course of inactivation and the distribution of the reaction products observed upon gel electrophoresis are best explained assuming a mechanism of inactivation in which the two active sites present in factor XIa are inhibited in random order (i.e., independent of each other) with the same rate constant of inhibition.

Studies on the effect of serine protease inhibitors on activated contact factors. Application in amidolytic assays for factor XIIa, plasma kallikrein and factor XIa

Amidolytic assays have been developed to determine factor XIIa, factor XIa and plasma kallikrein in mixtures containing variable amounts of each enzyme. The commercially available chromogenic p-nitroanilide substrates Pro-Phe-Arg-NH-Np (S2302 or chromozym PK), Glp-Pro-Arg-NH-Np (S2366), Ile-Glu-(piperidyl)-Gly-Arg-NH-Np (S2337), and Ile-Glu-Gly-Arg-NH-Np (S2222) were tested for their suitability as substrates in these assays. The kinetic parameters for the conversion of S2302, S2222, S2337 and S2366 by beta factor XIIa, factor XIa and plasma kallikrein indicate that each active enzyme exhibits considerable activity towards a number of these substrates. This precludes direct quantification of the individual enzymes when large amounts of other activated contact factors are present. Several serine protease inhibitors have been tested for their ability to inhibit those contact factors selectively that may interfere with the factor tested for. Soybean trypsin inhibitor very efficiently inhibited kallikrein, inhibited factor XIa at moderate concentrations, but did not affect the amidolytic activity of factor XIIa. Therefore, this inhibitor can be used to abolish a kallikrein and factor XIa contribution in a factor XIIa assay. We also report the rate constants of inhibition of contact activation factors by three different chloromethyl ketones. D-Phe-Pro-Arg-CH2Cl was moderately active against contact factors (k = 2.2 X 10(3) M-1 s-1 at pH 8.3) but showed no differences in specifity. D-Phe-Phe-Arg-CH2Cl was a very efficient inhibitor of plasma kallikrein (k = 1.2 X 10(5) M-1 s-1 at pH 8.3) whereas it slowly inhibited factor XIIa (k = 1.4 X 10(3) M-1 s-1) and factor XIa (k = 0.11 X 10(3) M-1 s-1). Also Dns-Glu-Gly-Arg-CH2Cl was more reactive towards kallikrein (k = 1.6 X 10(4) M-1 s-1) than towards factor XIIa (k = 4.6 X 10(2) M-1 s-1) and factor XIa (k = 0.6 X 10(2) M-1 s-1). Since Phe-Phe-Arg-CH2Cl is highly specific for plasma kallikrein it can be used in a factor XIa assay selectively to inhibit kallikrein. Based on the catalytic efficiencies of chromogenic substrate conversion and the inhibition characteristics of serine protease inhibitors and chloromethyl ketones we were able to develop quantitative assays for factor XIIa, factor XIa and kallikrein in mixtures of contact activation factors.

Kinetics of inhibition of human plasma kallikrein by a site-specific modified inhibitor Arg15-aprotinin: evaluation using a microplate system and comparison with other proteases

Human plasma kallikrein, a product of contact-activated plasma proteolysis, is moderately inhibited by aprotinin, a small polypeptide from bovine lung that has been used as an experimental drug in human disease states. Aprotinin has a Lys residue in the P1 (reactive center) position occupying residue 15. Since kallikrein is an arginine-directed serine protease, we hypothesized that an altered form of aprotinin, Arg15-aprotinin, might be a better inhibitor. Kinetic evaluations were performed in 96-well microplates. We found that the KL (loose or Michaelis-Menten complex) was unchanged by the modification. However, the association rate constant was increased from 1.14 X 10(4) (mol/L)-1s-1 to 1.5 X 10(5) (mol/L)-1s1, thus indicating that the inhibition rate was increased 14-fold for the modified protein. The Ki (at equilibrium) was decreased from 3.2 X 10(-7) mol/L to 1.5 X 10(-8) mol/L after substituting Arg for Lys in the P1 position. Therefore, the modified inhibitor binds to plasma kallikrein more tightly than the natural protein. We also investigated the effect of Arg15-aprotinin on tissue kallikrein, plasmin, factor XIIa, factor XIa, and thrombin and found that the Ki slightly decreased from 5.1 X 10(-7) mol/L to 1.2 X 10(-7) mol/L for tissue kallikrein and slightly decreased from 2 X 10(-8) mol/L to 1 X 10(-8) mol/L for plasmin. Arg15-aprotinin did not inhibit thrombin or factor XIIa, even though both enzymes are arginine-directed serine proteases. However, factor XIa, although it was not inhibited by aprotinin, had a Ki of 3.4 X 10(-8) mol/L for Arg15-aprotinin. Therefore, Arg15-aprotinin is a more effective inhibitor of plasma kallikrein as well as factor XIa but shows minimal preference for plasmin and tissue kallikrein. This study also indicates that it is possible and practical to perform kinetic analyses directly in microplates.

Studies on a circulating anticoagulant inhibiting factor XI in a patient with congenital deficiency and carcinoma of the prostate

An inhibitor of plasma thromboplastin antecedent (PTA, factor XI), measured in coagulant and radioimmunoassays, was detected in a 60-year-old man with carcinoma of the prostate who had no evidence of a bleeding tendency. Family studies indicated that the patient was either a homozygote or a heterozygote for hereditary factor XI deficiency. In contrast to earlier described patients with factor XI deficiency in whom inhibitors were detected, the patient was unaware of having been transfused with blood or blood products at any time before the discovery of the inhibitor. The inhibitor of factor XI in the patient's plasma appeared to be predominantly in the IgG4 fraction and to be directed at a locus on the factor XI molecule other than the active site; it did not block the amidolytic properties of activated factor XI (XIa). Rather, it appeared to block adsorption of factor XI to negatively charged surfaces. The inhibitor interfered with measurement of other components of the intrinsic pathway of thrombin formation, perhaps explaining the low titres of other coagulation factors of the intrinsic system reported in patients with strong inhibitors directed against factor XI.

Inhibitors of plasma thromboplastin antecedent (factor XI): studies on mechanism of inhibition

Acquired inhibitors of plasma thromboplastin antecedent (PTA) have been described in several recent studies, which collectively emphasize the heterogeneity of PTA functions that are impaired by this class of circulating anticoagulants. To clarify whether acquired inhibitors of PTA exhibit inhibitory properties that vary widely from one patient to the next, IgG inhibitors from four patients were isolated. The effect of these inhibitors on the binding of PTA to glass, the activation of kaolin-bound PTA, the coagulant activity of both kaolin-bound and fluid phase activated PTA (aPTA), and the enzymatic activity of aPTA against a low molecular weight substrate was quantified. The relative inhibition of these individual components of PTA activation and function by the four anticoagulants was quite similar. The results indicate that the principal binding sites for acquired inhibitors of PTA are PTA domains involved with surface binding and conversion of PTA to aPTA, with less effective steric impairment of aPTA enzymatic activity.

Acquired factor XI inhibitors in two patients with hereditary factor XI deficiency

Two patients with hereditary factor XI deficiency developed inhibitors following plasma transfusions. Neither had severe spontaneous bleeding. The patients' plasmas neutralized both factor XI in plasma, purified factor XI, and purified factor XIa. The inhibitor in both patients' plasmas adsorbed to Protein A-Sepharose. The inhibitors eluted from Protein A-Sepharose were partially neutralized by kappa and lambda light chain antisera indicating that they were polyclonal IgG antibodies. Both inhibitors markedly decreased adsorption of factor XI to glass surfaces. The cleavage of factor XI by trypsin was unaffected by the inhibitors. The lack of severe spontaneous bleeding in both of these patients strongly suggests that an alternate coagulation mechanism bypassing factor XI must compensate for this severe defect.