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

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

Inhibition of Factor XI: A New Era in the Treatment of Venous Thromboembolism in Cancer Patients?

Direct oral anticoagulants against activated factor X and thrombin were the last milestone in thrombosis treatment. Step by step, they replaced antivitamin K and heparins in most of their therapeutic indications. As effective as the previous anticoagulant, the decreased but persistent risk of bleeding while using direct oral anticoagulants has created space for new therapeutics aiming to provide the same efficacy with better safety. On this basis, drug targeting factor XI emerged as an option. In particular, cancer patients might be one of the populations that will most benefit from this technical advance. In this review, after a brief presentation of the different factor IX inhibitors, we explore the potential benefit of this new treatment for cancer patients.

Factor XI inhibitors for the prevention of cardiovascular disease: A new therapeutic approach on the horizon?

Anticoagulant drugs that are currently used to prevent and/or treat thrombosis have some limitations that hinder their ability to meet specific clinical requirements. While these drugs effectively reduce the rates of thrombotic events, they simultaneously increase the risk of bleeding. Moreover, their risk-to-benefit balance is problematic in some patients, such as those with severe chronic kidney disease or those at high bleeding risk. A novel anticoagulation method, FXI inhibition has emerged as a promising alternative. It demonstrates a strong rationale for the prevention and treatment of venous thromboembolism and the potential fulfillment of unmet clinical needs in the cardiovascular field. A number of FXI inhibitors are currently undergoing clinical investigation. The objective of this review is to provide an overview of early results of research on FXI inhibitors in the cardiovascular setting, offering valuable insights into their potential role in shaping the future of anticoagulation.

A new strategy for anticoagulation: The factor XI inhibitors.

Direct oral anticoagulants (DOACs) are currently the first-choice therapy for the prevention of cardioembolic events in patients with atrial fibrillation and for the treatment of venous thromboembolism (VTE) due to their more favorable efficacy to safety profile in comparison to vitamin K antagonists (VKA). DOACs did not show a clinical benefit when used for in stroke prevention in patients with mechanic or rheumatic valves or in those who underwent transcatheter aortic valve implantation (TAVI), in the treatment of VTE in patients with antiphospholipid antibody syndrome and in prevention of VTE in medically ill patients. There are some concerns for bleeding excess at the gastrointestinal site for some, but not all, DOACs. In recent years, in order to overcome the limitations of the available DOACs and to explore the advantages of anticoagulation in additional clinical settings, the development of factor XI and factor XII inhibitors as anticoagulant agents has been proposed. Emerging data show that factor XI has a minor role in the physiological process of hemostasis and an important role in the development of thrombosis. Bleeding has been viewed for several years as an unavoidable side effect of anticoagulant therapy. The aim of factor XI inhibitors is to challenge this dogma by favoring the uncoupling between hemostasis and thrombosis. This paper provides an update on the rationale for the use of factor XI inhibitors, their pharmacological properties and the preliminary clinical findings.

[A new anticoagulant strategy: the factor XI inhibitors]. / Una nuova strategia per l'anticoagulazione: gli inibitori del fattore XI.

In the last 10 years the introduction of the direct oral anticoagulants (DOACs) has revolutionized the anticoagulant treatment, one of the cornerstones of the therapy for cardiovascular diseases. Thanks to their efficacy at least not inferior compared to vitamin K antagonists and their better safety profile, particularly with regard to intracranial bleeding, DOACs are now the first choice for the prevention of cardioembolism in patients with non-valvular atrial fibrillation and for the treatment of venous thromboembolism (VTE). Other areas of clinical use for DOACs include the prevention of VTE in orthopedic and oncology surgery and in outpatient cancer patients treated with anticancer therapy, or the use of low-dose in association with aspirin in patients with coronary or peripheral artery disease.An increased risk of gastrointestinal bleeding has been reported for some DOACs. In addition, DOACs have also experienced some failures including stroke prevention in patients with mechanical prosthetic valves or rheumatic diseases and VTE therapy in patients with antiphospholipid antibody syndrome. Also, no data are available on DOACs in some particular areas, including severe renal impairment and thrombocytopenia.In recent years, the clinical use of factor XI and factor XII inhibitors has been proposed. Currently, factor XI inhibitors have more clinical data than factor XII inhibitors. This article will report the rationale for the clinical use and the main evidences currently available on factor XI inhibitors.

Treatment of Cancer-Associated Thrombosis: Recent Advances, Unmet Needs, and Future Direction.

Cancer-associated thrombosis, with the incidence rising over the years, is associated with significant morbidity and mortality in patients with cancer. Recent advances in the treatment of cancer-associated venous thromboembolism (VTE) include the introduction of direct oral anticoagulants (DOACs), which provide a more convenient and effective option than low-molecular-weight heparin (LMWH). Nonetheless, important unmet needs remain including an increased risk of bleeding in certain patient subgroups such as those with gastroesophageal cancer, concerns about drug-drug interactions, and management of patients with severe renal impairment. Although DOACs are more convenient than LMWH, persistence can decline over time. Factor XI inhibitors have potential safety advantages over DOACs because factor XI appears to be essential for thrombosis but not hemostasis. In phase II trials, some factor XI inhibitors were superior to enoxaparin for the prevention of VTE after knee replacement surgery without increasing the risk of bleeding. Ongoing trials are assessing the efficacy and safety of factor XI inhibitors for the treatment of cancer-associated VTE.

Leveraging genetic predictors of factor XI levels to anticipate results from clinical trials.

BACKGROUND: Factor XI (FXI) is a promising therapeutic target for the prevention of thrombotic disease without increasing bleeding risk. METHODS: We performed Mendelian randomization (MR) analyses to investigate the association of genetically predicted reductions in FXI levels with risk of venous thromboembolism, ischemic stroke, bleeding outcomes, and lifespan. RESULTS: Genetically predicted reductions in FXI levels were associated with lower risk of ischemic stroke (odds ratio per 1 standard deviation (SD) lower serum FXI 0.90, 95% confidence interval 0.87-0.93, p = 1.59 × 10-11 ), and venous thromboembolism (0.54, 0.49-0.59, p = 2.13 × 10-39 ) but did not associate with increased bleeding risk (p > 0.16). Genetically predicted reductions in serum FXI levels associated with longer lifespan (0.37 years per 1 SD lower serum FXI, 0.13-0.61, p = 0.003). CONCLUSIONS: These genetic data support FXI as a potentially efficacious and safe therapeutic target and anticipate positive results from ongoing phase 3 clinical trials.

Factor XI inhibitors: cardiovascular perspectives.

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

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

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

[Evidence-based research and prognosis of novel coagulation factor â ª inhibitors in venous thromboembolism].

Thrombosis is the main cause of the development and progression of venous thromboembolism(VTE). Anticoagulant therapy is the cornerstone for the prophylaxis and treatment of VTE, and it has evolved mainly through indirect thrombin inhibitors, direct thrombin inhibitors, vitamin K antagonists and new oral anticoagulants. Although safety is improving, the risk of bleeding remains a non-negligible side effect of current anticoagulation therapy, especially in patients with higher bleeding risk. Studies of the coagulation pathway have found that FXI is not involved in the initiation of hemostasis, but promotes thrombus growth and stabilization primarily through feedback activation of FⅪ by thrombin. Further studies have found that inhibition of FⅪ significantly inhibits thrombus formation and only affects hemostasis slightly. Recent studies have confirmed the efficacy and safety of FⅪ inhibitors in the prevention of VTE in patients after knee replacement. In addition, the safety of FⅪ inhibitors has been further confirmed by the studies of FⅪ inhibitors in people with higher bleeding risk. FⅪ inhibitors may be the most promising anticoagulant drugs in the next decade.

Antiplatelet therapy after noncardioembolic ischemic stroke or transient ischemic attack.

INTRODUCTION: Antiplatelet therapy is key to prevent recurrences in patients with an acute or prior non-cardioembolic stroke or transient ischemic attack (TIA). The narrow balance between the risks of ischemic recurrence and major bleeding is a relevant clinical dilemma in this population. AREAS COVERED: This review covers the current evidence on antiplatelet therapy for patients with non-cardioembolic stroke or TIA. Randomized controlled trials of antithrombotic strategies for patients with these conditions were searched in Pubmed/Medline from 1970 to 2022. EXPERT OPINION: Numerous randomized controlled trials have defined the current indications to the use of antiplatelet drugs for patients with non-cardioembolic ischemic stroke or TIA. For the management of these subjects, single antiplatelet therapy with aspirin or clopidogrel, or the combination of aspirin and dipyridamole, is usually recommended. After an acute stroke or TIA, a short course of dual antiplatelet therapy with aspirin in combination with clopidogrel or ticagrelor should be considered. The risk of bleeding might be higher with ticagrelor, but a direct comparison with clopidogrel is not available in this setting. The introduction of newer strategies, such as dual-pathway inhibition with aspirin and a direct oral anticoagulant (including emerging factor XI inhibitors under clinical development) may open a new research avenue in this challenging area.

Pediatric severe factor XI deficiency: A multicenter study.

BACKGROUND: Factor XI (FXI) deficiency is a rare autosomal recessive bleeding disorder. Only scarce publications address its clinical features in children. The increased prevalence of FXI deficiency in Israel enabled data collection for this large multicenter cohort study. OBJECTIVE: Some hemostatic challenges may be unique or more common in children, such as bleeding in the neonatal period or trauma-related injury. The current study was designed to explore the potential impact of these differences in children with severe FXI deficiency. METHODS: Medical files of all children with FXI level under 15% followed at five tertiary centers were evaluated. The retrieved data comprised demographic and clinical characteristics, including bleeding episodes, surgical interventions, treatment strategies, as well as laboratory features. RESULTS: Sixty children, whose median age at diagnosis was 4.2 years and their median FXI level was 4%, were included. Three children experienced triggered intracranial hemorrhage (ICH) and two children had major bleeds. No bleeding complications occurred in surgeries in which hemostatic treatment consisting mostly of tranexamic acid or fresh frozen plasma was applied (n = 45). In contrast, excessive bleeding was noted in 25% of surgical procedures performed without hemostatic preparation (p = .002). CONCLUSION: This study's findings confirm the generally favorable outcome of this rare bleeding disorder, with no spontaneous bleeds or cases of perinatal ICH. Nonetheless, proper diagnosis and adequate hemostasis in the surgical setting are imperative. Unlike previous studies in adults, our pediatric study suggests an association between the severity of FXI deficiency and bleeding tendency.

A proposal for managing bleeding in patients on therapeutic factor XI(a) inhibitors.

Several drugs that reduce functional levels of the plasma protease zymogen factor XI (FXI), or that inhibit its activated form (FXIa), are being evaluated as treatments to prevent thrombosis. Based on the observation that individuals with inherited FXI deficiency have a relatively mild bleeding disorder, it is anticipated that therapeutic FXI(a) inhibitors will have a smaller impact on hemostasis than anticoagulants targeting thrombin or factor Xa. However, even if FXI(a) inhibitors are determined to be safer than currently used anticoagulants, some patients on these drugs will experience abnormal bleeding or require emergent surgery. Strategies for dealing with such situations are required. Treatment with antifibrinolytic agents and low doses of recombinant factor VIIa effectively prevent abnormal bleeding in FXI-deficient patients with alloantibody inhibitors to FXI who undergo surgery. We propose that a similar strategy can be used for patients on therapeutic FXI(a) inhibitors who are bleeding or require invasive procedures.

Factor XI as a Target for New Anticoagulants.

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

Tissue Factor-Independent Coagulation Correlates with Clinical Phenotype in Factor XI Deficiency and Replacement Therapy.

BACKGROUND: In factor XI (FXI) deficiency, bleeding cannot be predicted by routine analyses. Since FXI is involved in tissue factor (TF)-independent propagation loop of coagulation, we hypothesized that investigating the spatiotemporal separated phases of coagulation (TF-dependent and -independent) could improve diagnostics. OBJECTIVES: This article investigates the correlation of parameters describing TF-dependent and -independent coagulation with the clinical phenotype of FXI deficiency and their ability to assess hemostasis after FXI replacement. METHODS: We analyzed: (1) plasma from healthy controls (n = 53); (2) normal plasma (n = 4) spiked with increasing concentrations of a specific FXI inhibitor (C7P); (3) plasma from FXI-deficient patients (n = 24) with different clinical phenotypes (13 bleeders, 8 non-bleeders, 3 prothrombotics); (4) FXI-deficient plasma spiked with FXI concentrate (n = 6); and (5) plasma from FXI-deficient patients after FXI replacement (n = 7). Thrombin generation was measured with the reference method calibrated automated thrombogram and with Thrombodynamics (TD), a novel global assay differentiating TF-dependent and -independent coagulation. RESULTS: C7P dose-dependently decreased FXI activity, prolonged activated partial thromboplastin time, and hampered TF-independent coagulation. In FXI-deficient bleeders, TD parameters describing TF-independent propagation of coagulation and fibrin clot formation were reduced compared with controls and FXI-deficient nonbleeders and increased in FXI-deficient patients with prothrombotic phenotype. Receiver operating characteristic analysis indicated that TF-independent parameters were useful for discriminating FXI-deficient bleeders from non-bleeders. In FXI-deficient plasma spiked with FXI concentrate and in patients receiving FXI replacement, TD parameters were shifted toward hypercoagulation already at plasma FXI levels around 20%. CONCLUSION: TF-independent coagulation parameters assessed by TD have the potential to identify the clinical phenotype in FXI-deficient patients and to monitor FXI replacement therapy.

Alfaeftrenonacogue (fator XI de coagulação recombinante Fc) para Hemofilia B / Alfaeftrenonacogue (Recombinant Coagulation Factor XI) for Hemophilia B

INTRODUÇÃO: As coagulopatias hereditárias são doenças hemorrágicas resultantes da deficiência quantitativa e/ou qualitativa de um ou mais fatores da coagulação e se caracterizam pela ocorrência de hemorragias de gravidade variável, de forma espontânea e/ou pós-traumática. Segundo dados de 2015 do Perfil das Coagulopatias Hereditárias no Brasil , o número de pacientes com Hemofilia B era de 1.948 no Brasil. Como não há cura para as hemofilias, os objetivos de tratamento são prevenir e tratar hemorragias de modo a evitar artropatias incapacitantes e dano tecidual, melhorar a qualidade de vida e a sobrevida dos pacientes. As modalidades de tratamento da hemofilia B são definidas pela periodicidade com que é realizada a reposição dos fatores de coagulação IX, podendo ser sob demanda (episódico) ou profilático. O fator IX de origem plasmática faz parte do rol de tecnologias ofertada pelo SUS para o tratamento de Hemofilia B. TECNOLOGIA: Alfaeftrenonacogue (Elprolix®). PERGUNTA: O uso do Alfaeftrenonacogue é mais eficaz, seguro e custo-efetivo no tratamento de pacientes com hemofilia B quando comparado ao(s) fatores IX de coagulação derivado de plasma? EVIDÊNCIAS CIENTÍFICAS: Os estudos apresentados pelo demandante demonstram a eficácia e segurança do medicamento alfaeftrenonacogue em diminuir a taxa anualizada de sangramentos, reduzir o consumo de fator IX devido a redução de infusões, menor ocorrência do desenvolvimento de inibidores e menor incidência de eventos adversos. Entretanto, não foi apresentado nenhum estudo que comparasse o fator IX plasmático ofertado pelo SUS e alfaeftrenonacogue, o que impossibilita fazer qualquer consideração de superioridade, inferioridade ou igualdade entre as tecnologias. AVALIAÇÃO ECONÔMICA: O demandante delineou em sua proposta um estudo de custo-efetividade do alfaeftrenonacogue para tratamento de hemofilia B. O estudo demonstrou uma RCEI de R$ 7.437,00 por sangramento evitado, quando comparado supostamente com fator IX plasmático. O modelo possui grandes limitações nos dados de eficácia devido a inexistência de estudos que fazem a comparação entre as tecnologias e no levantamentos dos custos, limitando a interpretação dos resultados. AVALIAÇÃO DE IMPACTO ORÇAMENTÁRIO: O impacto orçamentário apresentado pelo demandante estimou, considerando um market-share de 30% em 5 anos, que a inclusão do alfaeftrenonacogue no SUS proporcionará um valor incremental de R$ 8,7 milhões no primeiro ano e de R$ 95,2 milhões no acumulado de 5 anos. O modelo possui limitações quando à estimativa da população, estimativa de difusão da tecnologia e a previsão de custos, que podem subestimar os resultados. MONITORAMENTO DO HORIZONTE TECNOLÓGICO: No horizonte foram detectadas três tecnologias para controlar ou prevenir episódios de sangramento em pacientes com hemofilia B, duas consideradas como terapias gênicas (Estimulador do gene F9) e um inibidor de antitrombina III já em estudos fase 3 ou 4 mas sem aprovação nos orgãos de resgistro ANVISA, EMA e FDA. CONSIDERAÇÕES: São necessárias novas evidências para melhor compreensão dos benéficos clínicos do alfaeftrenonacogue quando comparado as alternativas disponíveis atualmente no SUS. RECOMENDAÇÃO PRELIMINAR DA CONITEC: O plenário, em reunião da CONITEC realizada no dia 07 de novembro de 2018, recomendou que o tema fosse submetido à consulta pública com recomendação preliminar não favorável à incorporação do alfaeftrenonacogue (Fator XI de coagulação recombinante Fc) para tratamento de pacientes com hemofilia B. Considerou-se que há grande incerteza a respeito da eficácia do medicamento quando comparado as opções de tratamento já disponíveis no SUS, além disso a análise econômica apresentada e a análise de impacto orçamentário apresentaram limitações importantes que atribuíram elevada incerteza quanto as estimativas reais de custo-efetividade e de impacto orçamentário. CONSULTA PÚBLICA: Foram recebidas 19 contribuições técnico-científicas e 289 contribuições de experiência e opinião durante o período de consulta pública, entre 29 de novembro a 18 de dezembro de 2018. Dentre as contribuições, a maioria foram contrárias à recomendação da CONITEC. Nenhuma evidência científica adicional foi encontrada nas contribuições. Os principais argumentos abordados foram a redução de dosageme das infusões, melhora na qualidade de vida, resução dos eventos adversos e maior adesão. O plenário da CONITEC entendeu que não houve argumentação suficiente para alterar a recomendação inicial. RECOMENDAÇÃO FINAL DA CONITEC: Os membros da CONITEC presentes na 74ª reunião ordinária, nos dias 06 e 07 de fevereiro de 2019, deliberaram, por unanimidade, por não recomendação a incorporação ao SUS do medicamento alfaeftrenonacogue (Fator XI de coagulação recombinante Fc) para tratamento de pacientes com hemofilia B. Foi assinado em 07 de fevereiro o registro de deliberação nº 420/2019. DECISÃO: Não incorporar o alfaeftrenonacogue para hemofilia B, no âmbito do Sistema Único de Saúde - SUS. Dada pela Portaria nº 9, de 21 de fevereiro de 2019, publicada no Diário Oficial da União nº 38, de 22/02/2019, seção 1, página 54.

Coagulation factor XI vaccination: an alternative strategy to prevent thrombosis.

Essentials Coagulation Factor (F) XI is a safe target for the development of antithrombotics. We designed an antigen comprising the human FXI catalytic domain and diphtheria toxin T domain. Antigen immunization reduced plasma FXI activity by 54% and prevented thrombosis in mice. FXI vaccination can serve as an effective strategy for thrombosis prevention. SUMMARY: Background Coagulation factor XI serves as a signal amplifier in the intrinsic coagulation pathway. Blockade of FXI by mAbs or small-molecule inhibitors inhibits thrombosis without causing severe bleeding, which is an inherent risk of currently available antithrombotic agents. Objectives To design an FXI vaccine and assess its efficacy in inhibiting FXI activity and preventing thrombosis. Methods An FXI antigen was generated by fusing the catalytic domain of human FXI to the C-terminus of the transmembrane domain of diphtheria toxin. The anti-FXI antibody response, plasma FXI activity and antithrombotic efficacy in mice immunized with the FXI antigen were examined. Results The antigen elicited a significant antibody response against mouse FXI, and reduced the plasma FXI activity by 54.0% in mice. FXI vaccination markedly reduced the levels of coagulation and inflammation in a mouse model of inferior vena cava stenosis. Significant protective effects were also observed in mouse models of venous thrombosis and pulmonary embolism. Conclusions Our data demonstrate that FXI vaccination can serve as an effective strategy for thrombosis prevention.

In vitro comparison of the effect of two factor XI (FXI) concentrates on thrombin generation in major FXI deficiency.

INTRODUCTION: Bleeding risk in factor XI (FXI) deficiency following surgery may be reduced by treatment with either of two FXI concentrates, but indications for their use are unclear and treatment has been associated with thrombosis. AIM: To quantify and compare the effects of two different FXI concentrates on thrombin generation (TG) in major FXI deficiency (FXI:C < 15 IU dL(-1) ). METHODS: Thrombin generation was measured in controls (n = 50), FXI-deficient individuals pre and post in vitro spiking with FXI concentrates (n = 10), and in ex vivo samples following treatment with FXI concentrate (n = 3). RESULTS: Thrombin generation was significantly impaired in FXI deficiency but improved following FXI replacement in vitro and in vivo. LFB Hemoleven(®) had greater effect on TG than BPL FXI concentrate in vitro (equivalent in vivo doses 10, 20 and 30 U kg(-1) ): higher endogenous thrombin potential (ETP) (P < 0.0001), peak height (P < 0.01) velocity (P < 0.0002) and shorter lag time and time to peak (both P < 0.003). Some measurements with LFB Hemoleven(®) exceeded the reference range. At lower dose (5 U kg(-1) ), BPL FXI concentrate normalized all TG parameters and LFB Hemoleven(®) normalized the ETP but exceeded the reference range with other parameters. CONCLUSION: Both FXI concentrates improve TG in vitro in major FXI deficiency but differ in dose response, and for both products, doses lower than previously recommended normalized TG in vitro. Comparison of in vitro spiked and ex vivo samples suggest that in vitro results could be used to estimate an expected in vivo response to FXI replacement.

Safety and efficacy of factor XI (FXI) concentrate use in patients with FXI deficiency: a single-centre experience of 19 years.

AIM: Factor XI (FXI) concentrate is a pooled human plasma-derived factor concentrate used as replacement therapy for patients with FXI deficiency, which provides a predictable response and consistent haemostatic cover in emergency or elective situations. It has previously been implicated in adverse events such as thrombosis and inhibitor formation, with rare case reports of fatal incidents. We sought to establish the incidence of such complications in a retrospective case series between 1994 and 2012 at the Haemophilia Comprehensive Care Centre at Royal Free Hospital, London, UK. METHODS: Patients who received FXI concentrate had their medical records reviewed to extract information and specific adverse events recorded such as failure of treatment with further bleeding, suspected viral transfusion transmitted infection (TTI), thrombosis or inhibitor formation. RESULTS: Eighty-six patients received 242 treatment episodes of FXI concentrate. Ninety percent of treatment episodes were covered with BPL FXI concentrate and 10% with LFB Hemoleven. Twelve (5%) adverse events were recorded, with eight (3.3%) of all treatment episodes were related to persistent bleeding postconcentrate infusion and there were 4 (1.7%) non-bleeding adverse events. No viral TTIs were identified. There were two recorded inhibitors, one thrombotic event (central retinal artery occlusion) and one transfusion reaction. No patient suffering an adverse event resulted in long-term morbidity. CONCLUSION: Our experience of FXI concentrate use demonstrates infrequent minor adverse events related to its administration and is a safe product to use.