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Gene therapy and other investigational approaches for hemophilia

Gene therapy and other investigational approaches for hemophilia
W Keith Hoots, MD
Amy D Shapiro, MD
Section Editor:
Lawrence LK Leung, MD
Deputy Editor:
Jennifer S Tirnauer, MD
Literature review current through: Dec 2022. | This topic last updated: Nov 30, 2022.

INTRODUCTION — Hemophilia A (factor VIII [factor 8] deficiency) and hemophilia B (factor IX [factor 9] deficiency) are X-linked hereditary coagulation factor deficiencies that result in lifelong bleeding disorders. However, optimal management is complex, and available therapies carry a number of costs and burdens.

Hemophilia A and B are attractive candidates for gene therapy. They are both monogenic disorders that can be adequately treated by raising factor levels. The deficient factor can be produced and delivered to the circulation by various cell types. Unlike immunodeficiency syndromes, hematopoietic stem cell transplant is not required. (See "Overview of gene therapy, gene editing, and gene silencing", section on 'Inherited single gene disorders'.)

This topic discusses gene therapy for hemophilia and other investigational approaches under development.

Separate topic reviews discuss:

Function of factors VIII and IX – (See "Biology and normal function of factor VIII and factor IX".)

Hemophilia diagnosis – (See "Clinical manifestations and diagnosis of hemophilia".)

Prophylactic therapy and minor bleeding – (See "Hemophilia A and B: Routine management including prophylaxis".)

Treatment of bleeding and surgery – (See "Treatment of bleeding and perioperative management in hemophilia A and B".)

OVERVIEW OF STRATEGIES TO REDUCE BLEEDING RISK — There are two main strategies to reduce bleeding risk, each of which has several potential approaches in development that are discussed in the following sections (figure 1).

Enhancing factor activity

Gene therapy to replace the deficient factor – (See 'Gene therapy' below.)

Cellular therapy to transplant transduced cells capable of producing the deficient factor – (See 'Cellular therapy' below.)

Modifications to factor proteins such as half-life extension or subcutaneous administration – (See 'Improvements to factor products' below.)

Substitutes for the deficient factor such as bifunctional monoclonal antibodies – (See 'Substitutes for clotting factors' below.)

Decreasing natural anticoagulants

RNA therapeutics to reduce levels of targeted anticoagulants – (See 'Fitusiran (AT antisense)' below.)

Function-blocking monoclonal antibodies or small molecules – (See 'Anti-TFPI' below.)

Engineered versions of natural inhibitors – (See 'Modified inhibitors of APC' below.)

GENE THERAPY — Gene therapy is appealing because infusion of the gene therapy construct (or treated cells) can provide the deficient factor. Efforts to establish gene therapy approaches for hemophilia are underway in various countries around the world [1].

However, as stated in a 2021 review, aspects of this approach remain to be optimized and studied for longer durations, and clinician familiarity with the principles and logistics of gene therapy remains to be systematically addressed [2]. (See 'General gene therapy principles' below.)

Gene editing, which uses enzymes to alter the individual's native DNA sequence rather than introducing a new copy of the gene, is under study for possible use in hemophilia [3-5]. (See "Overview of gene therapy, gene editing, and gene silencing", section on 'Gene editing'.)

General gene therapy principles — Gene therapy is both easy to understand and remarkably complex. A guide is available with figures and a glossary to help clinicians discuss gene therapy with their patients [6].

General information is provided by professional organizations such as:

World Federation of Hemophilia (WFH;

National Hemophilia Foundation (NHF;

International Society on Thrombosis and Haemostasis (ISTH;

Access to clinical trials — Information and resources for enrolling in hemophilia gene therapy trials may be accessed by consulting:

Websites including (

A local Hemophilia Treatment Center or Centre of Excellence

United States –

United Kingdom –

Methodology — Gene therapy introduces exogenous DNA into cells that can be used to produce a deficient protein. In monogenic disorders such as hemophilia, this may involve a wild-type factor gene or a modified gene with enhanced properties such as greater activity or longer half-life. (See "Biology and normal function of factor VIII and factor IX", section on 'Naturally-occurring gain-of-function variants'.)

Target cell – Because the factor circulates in the bloodstream, the DNA can be transduced into cells with access to the circulation. Hepatocytes are considered an appropriate target cell for transduction because the liver is the site of synthesis of most clotting factors, including endogenous factor IX. Endogenous factor VIII is produced by hepatic endothelial cells. In gene therapy approaches, constructs for both F8 and F9 are transduced into hepatocytes.

Target factor level – Hemophilia is amenable to gene therapy because factor levels do not need to be raised to the normal range. Production of even a small amount of the missing factor (to raise the factor level from a baseline of 0 percent up to a level of 5 percent) is sufficient to convert the disease from one of frequent potentially life-threatening bleeding into a mild phenotype that no longer requires routine factor prophylaxis. Raising the factor level above 30 to 40 percent may be equivalent to curing the disease.

Vector – A variety of technologies may be used to introduce the F8 or F9 gene, either using ex vivo genetic modification followed by implantation of the modified cells, or direct injection of a vector carrying a construct that includes the normal gene into the patient [7-11]. Vectors that produce other coagulation factors such as factor VIIa, a protein to which individuals with hemophilia A or B have immunologic tolerance, may also be used [12,13].

Adeno-associated viruses (AAV) have become widely used vectors for gene therapy because they generally do not integrate into the nuclear genome and typically are replication-deficient, reducing the risk of insertional mutagenesis. The risk of any insertional mutagenesis awaits further human studies. Another issue with AAV vectors is that some individuals have natural immunity to specific serotypes of adenoviruses and AAV due to natural infection; preexisting neutralizing antibodies to these vectors can reduce efficacy of gene therapy, as demonstrated in some trials. (See "Overview of gene therapy, gene editing, and gene silencing", section on 'Types of vectors'.)

Eligibility – Preliminary gene therapy trials have included males with severe factor deficiency (factor activity <1 to 2 percent). Other individuals with bleeding phenotypes may be included in the future.

Individuals who have developed an inhibitor to factor have been excluded from gene therapy trials due to concerns about reduced efficacy. Eligibility may eventually be expanded to include these individuals. (See "Inhibitors in hemophilia: Mechanisms, prevalence, diagnosis, and eradication".)

Some individuals with severe liver disease or active viral hepatitis have been excluded from clinical trials due to concerns about hepatotoxicity. (See 'Potential concerns' below.)

Children have been excluded from trials due to concerns about informed consent and the likelihood that normal liver growth with proliferation of liver cells would dilute an episomal vector, reducing efficacy over time. There is also a prevailing caution about gene modification in younger children until overwhelming evidence of safety has been demonstrated in adults, adolescents, and older children. (See 'Potential concerns' below.)

Administration – Gene therapy constructs are typically administered intravenously and home to the target cells (often hepatocytes) to which the vector has tropism. The dose is typically expressed as number of vector genomes per kg of patient weight (vg/kg) [2]. This can be done as an outpatient procedure with close follow up for development of adverse effects, including short-term infusion-related reactions and longer term transaminase elevations. Efficacy measures including factor levels and bleeding events are monitored longitudinally. (See 'Clinical experience with gene therapy' below.)

Regardless of the methods used, the goal of gene therapy may not be restoration of a normal factor level, as conversion from a severe to a mild phenotype would result in significant improvement. (See "Clinical manifestations and diagnosis of hemophilia", section on 'Definitions'.)

Potential concerns — Several concerns have been raised based on reduced efficacy and/or adverse effects observed in clinical trials or in preclinical models [2,14]:

Eligibility – Some individuals may not be eligible for gene therapy due to preexisting immunity to the vector, preexisting antibodies (inhibitors) to the factor, or underlying liver disease.

Duration of effect – The longest period of follow up for any hemophilia gene therapy construct is <10 years, with the longest available follow-up data for factor IX deficiency. Thus, the durability of response has not been definitively established for any gene therapy product. Declining factor VIII levels over time have been observed in the most advanced trial in factor VIII deficiency; whether the response stabilizes over time or continues to decline will require further observation. (See 'Hemophilia A' below.)

Children – If episomal therapies are used in children, ongoing proliferation of liver cells may dilute the number of viral genomes, possibly reducing factor levels.

Variable expression – Inter-individual expression variability remains unexplained.

Long-term complications – Several years of data on toxicity are reassuring, yet longer-term data remain to be collected.

Hepatotoxity – Several studies have demonstrated increased transaminases or decreased factor levels; glucocorticoids have been used to reduce the presumed viral vector inflammatory response that can lead to vector loss.

Integration and oncogenesis – Certain vectors (such as those based on retroviruses) carry a theoretical risk of insertional mutagenesis that may cause malignancy. The risk of late genotoxicity with AAV-based therapies is felt to be reduced because these vectors do not integrate as readily into the nuclear genome [15]. Nonetheless, because the number of vector particles used to insert the transgene into the target cell for transcription (as high as 0.6 x 106 in one therapeutic strategy), even a very low rate of insertion could result in many target cells with insertional events [7].

However, even non-integrating vectors such as AAV can integrate into the nuclear genome in small amounts; the clinical significance and risk of malignancy in the long-term is not known [16,17]. One individual who received the hemophilia B gene therapy construct etranacogene dezaparvovec developed hepatocellular carcinoma; a subsequent detailed investigation concluded that the predisposing factor was not the gene therapy vector, based on no demonstration of proximal insertion of the AAV machinery near a known enhancer or promoter of clonality [18-20]. (See 'Hemophilia B' below.)

Immunity to the vector – Certain vectors may elicit a host immune response to viral proteins made by the vector, which could potentially complicate repeat dosing. Possible approaches to facilitating re-transduction in this setting are under investigation.

Transmission to other individuals – Gene therapy vectors are shed for a period of time, potentially exposing other individuals. Preclinical studies and early experience suggest that vector shedding can occur transiently in semen [21-24]. Patients who have received a gene therapy construct are counseled to abstain from sexual intercourse until it has been demonstrated that vector shedding is no longer occurring.

Clinical experience with gene therapy — Early gene therapy trials have focused on adults (males) with factor activity <1 to 2 percent who have not developed inhibitors [2,5].

Hemophilia A — The F8 gene can be modified to remove the B-domain; this is the construct used in recombinant factor VIII replacement products (see "Hemophilia A and B: Routine management including prophylaxis", section on 'Recombinant human factor VIII'). The B-domain deleted F8 gene is smaller (F8 is a very large gene that barely fits [or cannot fit] in the AAV vectors). The following gene therapy constructs have been evaluated:

Valoctocogene roxaparvovec – The first gene therapy construct for hemophilia A to be approved in the European Union was valoctocogene roxaparvovec (approved as Roctavian) in 2022 [25]. Application to the US Food and Drug Administration (FDA) is under review [14].

This construct uses a codon-optimized adeno-associated virus serotype 5 (AAV5) vector containing a gene for B-domain deleted human factor VIII (AAV5-hFVIII-SQ) [26]. The AAV5 vector (and the AAV virus on which it is based) is a non-integrating hepatotrophic vector that remains mostly episomal, although a very small amount integrates into the nuclear genome. (See "Overview of gene therapy, gene editing, and gene silencing", section on 'Integrating versus non-integrating vectors'.)

The European Union approval is for adults with severe hemophilia A without an inhibitor and with no antibodies to AAV5. The dose is 6 × 1013 vector genomes per kilogram (vg/kg), administered as a single intravenous infusion. Other aspects of administration (monitoring of factor VIII and liver function, and indications for glucocorticoids) are described in the product label [25]. Factor VIII activity is expected to rapidly increase over the first six months and then to decline (at first rapidly and then more gradually) but to maintain effective levels, as described in the studies below.  

Efficacy and safety of this construct was reported in a 2022 study involving 134 males with severe hemophilia A (baseline factor VIII activity ≤1 percent) who were followed for ≥1 year after receiving the therapy at a dose of 6 x 1013 vg per kg intravenously [27]. Individuals who were positive for anti-AAV5 capsid antibodies were not eligible to participate in the trial.

Efficacy – There were clinically meaningful improvements in bleeding rates due to sustained increases in factor VIII activity. The mean annualized bleeding rate decreased from 4.8 with prior prophylaxis to 0.8 after the gene therapy, and the mean annualized number of factor VIII infusions decreased from 136 to 2. Factor VIII activity levels (assayed by chromogenic substrate) remained increased above baseline through the end of the year (mean, 43 IU/dL). Factor VIII activity >40 IU/dL was seen in 50 participants (38 percent) and <5 IU/dL in 16 participants (12 percent).

For the 17 participants who had received ≥2 years before the data cutoff, the mean factor VIII activity was 42 percent at the end of the first year and 24 percent at the end of the second year.

Safety – All participants had at least one adverse event, mostly mild. The most common was an increase in alanine aminotransferase (in 86 percent); these elevations were treated with prednisone or prednisolone (60 mg daily, tapered over at least eight weeks). Serious adverse events related to the study drug were seen in 4 percent (mostly hypersensitivity reactions, including one anaphylaxis). There were no deaths, thromboses, or inhibitor development.

These results were consistent with earlier, smaller studies, including the declining trend in factor VIII activity after the first year [28]. The declining trend in factor VIII activity raised concerns about durability of therapy, and the FDA requested additional years of follow up prior to review [14].

SPK-8011 – This construct uses an AAV-LK3 vector with a liver-specific promoter and a codon-optimized, B-domain deleted F8 gene. In a series of 18 individuals with severe hemophilia A treated in various dose cohorts (from 5 x 1011 vg per kg to 2 x 1012 vg per kg) observed for a median of three years, 16 had preserved factor VIII expression [22]. Two individuals, both in the highest dose cohort, lost factor VIII expression due to an immune response to the AAV capsid, despite immunosuppression.

Efficacy – Peak factor levels occurred at 6 to 12 weeks after infusion. For the entire cohort, the annualized bleeding rate decreased from a median of 8.5 events per year before infusion to 0.3 events per year after infusion, a 92 percent reduction. Factor infusions decreased from a median of 58 per year to 0.6 per year. In 15 individuals who were followed for >1 year, factor activity levels were stable at 11±7 percent. Factor activity levels remained stable in 12 individuals who were followed for >2 years.

Safety – Therapy was well-tolerated, with an infusion reaction in one participant 12 hours after receiving the construct and transaminase elevations in seven. Four participants had adverse effects related to glucocorticoids used to reduce liver toxicity.

Giroctogcogene fitelparvovec – Giroctocogene fitelparvovec (formerly SB-525) has shown good efficacy in 5 individuals, based on unpublished data [29]. Additional studies are ongoing.

Other constructs – Several other gene therapy constructs for hemophilia A are in various stages of development [5,14]. Some of these have used a retroviral vector or a non-viral, plasmid-based system [21,30]. These have been effective in raising factor VIII activity levels and have generally been well-tolerated, although vector shedding remains possible [21]. (See 'Potential concerns' above.)

A variant in the F8 gene that increases expression levels was identified in 2021 and has not yet been incorporated into gene therapy constructs. (See "Biology and normal function of factor VIII and factor IX", section on 'Factor VIII Padua'.)

Hemophilia B — The F9 Padua variant (F9 p.R338L) contains a naturally occurring missense mutation in the F9 gene that increases its activity approximately 4 to 40-fold; this construct has been used in most of the factor IX gene therapy trials. It is appealing because it provides enhanced factor IX activity. Individuals are generally excluded from clinical trials if they have factor IX inhibitors or neutralizing antibodies against the viral vector used to deliver the gene therapy construct. (See "Biology and normal function of factor VIII and factor IX", section on 'Factor IX Padua'.)

The following gene therapy constructs have been evaluated:

Verbrinacogene setparvovec – Verbrinacogene setparvovec (formerly FLT180a) is a synthetic AAV vector (AAVS3, designed for increased liver transduction) containing F9 Padua. A dose-finding study in 10 males with hemophilia B and baseline factor IX activity <2 percent reported a decrease in mean annualized bleeding rate from 2.93 per year (range, 0 to 7.33) to 0.71 per year (range, 0 to 1.7) [31]. Only one person resumed factor IX replacement therapy; the other nine had factor IX activity of 28 to 279 percent. Adverse effects were as expected including transaminase elevations and side effects of immunosuppressive therapies (prednisolone, with or without tacrolimus). One individual with high factor IX activity developed a thrombosis.

Fidanacogene elaparvovec – Fidanacogene elaparvovec (formerly SPK-9001) is an AAV vector containing F9 Padua. Results from a study in 10 males with hemophilia B and baseline factor IX activity <2 percent who were treated with a single-stranded AAV construct with liver tropism and liver-specific regulatory elements (AAV SPK-9001) expressing F9 Padua produced a mean steady state factor IX expression level of 33.7 percent (range, 14 to 81 percent), and a reduction in mean annualized bleeding rate from 11.1 to 0.4 events per year [32,33]. A low dose of vector (5 x 1011 vg/kg) was administered as a means of minimizing host antiviral immunity. There were no serious adverse events and no evidence of inhibitor development. Two individuals who had transient increases in liver function tests were treated with a course of glucocorticoids.

Etranacogene dezaparvovec – Etranacogene dezaparvovec (formerly AMT-061) is an AAV5 vector containing F9 Padua with a liver-specific promoter. It was approved by the FDA in November 2022 (as Hemgenix) for adults using factor IX prophylaxis or those with a history of life-threatening bleeding or repeated serious bleeding [34]. It is given as a single intravenous dose.

Supporting evidence:

Efficacy – Initial study at a dose of 2 x 1013 vg/kg in three males with severe hemophilia B demonstrated reduced bleeding (no bleeds during the study period) and increased factor IX activity levels (mean factor IX activity of 31 percent at six weeks and 47 percent at 26 weeks) [35]. A previous study with an earlier construct (codon-optimized factor IX in the same AAV5 vector) in 10 men with hemophilia B was similarly effective [36]. The HOPE-B study, presented in abstract form, reported results from 54 patients with at least 26 weeks of follow up [37]. Mean factor IX activity was 37 percent and expression appeared to be stable; bleeds decreased accordingly. Transduction was effective even in individuals with preexisting immunity to the AAV.

Safety – Adverse events have included mild transaminase elevations in some individuals; these resolved after treatment with glucocorticoids. One individual who received this construct developed liver cancer; a detailed analysis concluded that the predisposing factor was not the gene therapy vector, as discussed above [18-20]. (See 'Potential concerns' above.)

Other constructs – Earlier studies showed efficacy of other constructs such as an AAV8 vector expressing a codon-optimized F9 gene that produced dose-dependent increases in factor IX levels in 10 individuals (range, 1 to 6 percent, versus pre-study levels of <1 percent) [38,39]. A follow-up study demonstrated that these increases persisted over a median of 3.2 years. Annualized bleeding rates decreased from 16 to 2, and four of seven patients were able to discontinue factor IX prophylaxis. Transient, asymptomatic elevations in alanine aminotransferase levels with corresponding reductions in factor IX activity and increased numbers of AAV8-reactive T lymphocytes were seen in four individuals, suggesting a loss of transduced hepatocytes. These reactions responded to administration of a short course of prednisolone. There were no other major adverse events or factor IX inhibitors [35].

CELLULAR THERAPY — Cellular therapy involves introducing intact cells into the patient rather than manipulation of coagulation factor genes. Autologous cells may be treated with a gene therapy construct outside the body and then re-introduced. Foreign cells may be enclosed in immuno-protective devices before implantation to prevent rejection.

A plasmid-based system was used to transfect autologous dermal fibroblasts with the gene for factor VIII and these cells were injected into the omentum [30]. Administration of autologous bone marrow cells transduced outside the body with a lentiviral vector is also under investigation. This strategy will likely require some degree of myeloablation before reintroduction of the ex-vivo modified hematopoietic cells into the individual. This may, in turn, raise concerns about clonal hematopoiesis, based on observations in individuals with sickle cell disease. (See "Investigational therapies for sickle cell disease", section on 'Concern about myeloid malignancy in gene therapy studies'.)

In a mouse model of hemophilia A, transplantation of liver sinusoidal endothelial cells from non-hemophilic donor animals resulted in increased factor levels and correction of the bleeding phenotype [40]. (See "Hepatocyte transplantation".)

A polymer-encapsulated human cell system to express factor VIII (SIG-001) was under development but is no longer being pursued due to development of a factor VIII inhibitor in one recipient [41-43].


Half-life extension — Several modified factor products are available with extended half-life. The major benefit is an extended interval between doses, which reduces the risks and burdens of intravenous access. Examples include recombinant fusion to the immunoglobulin Fc region or albumin, or PEGylation. (See "Hemophilia A and B: Routine management including prophylaxis", section on 'Longer lasting recombinant factor VIII' and "Hemophilia A and B: Routine management including prophylaxis", section on 'Longer-lasting recombinant factor IX'.)

A main determinant limiting extension of factor VIII half-life is its binding to von Willebrand factor (VWF) for stabilization; hence, the half-life of VWF limits the half-life extension with currently licensed technologies. (See "Biology and normal function of factor VIII and factor IX", section on 'Binding to von Willebrand factor' and "Pathophysiology of von Willebrand disease", section on 'VWF functions'.)

VWF fusion for factor VIII — An investigational recombinant factor VIII molecule circumvents the dependence on VWF and provides additional approaches to half-life extension. This product fuses B domain-deleted factor VIII to the VWF D'D3-domain (the factor VIII binding site), along with Fc and two XTEN polypeptides, which further increase stability (figure 2) [44,45].

In a preliminary clinical study, one dose of this new product, designated BIVV001 (rFVIIIFc-VWF-XTEN), was administered to 16 adults with severe hemophilia A [46]. Pharmacokinetic measurements suggested an approximately three- to fourfold increase in half-life over a standard recombinant factor VIII product:

Standard factor VIII 25 units – 9.1 hours

Standard factor VIII 65 units – 13.2 hours

BIVV001 25 units – 37.6 hours

BIVV001 65 units – 42.5 hours

At seven days after injection of the higher dose of BIVV001, the mean factor VIII level was 17 percent, suggesting that the product could potentially be administered once per week. No inhibitors were observed, and the new product was well-tolerated. Additional trials with this and other half-life extension products are ongoing [47].

Subcutaneous administration — No subcutaneous factor products are clinically available (emicizumab is given subcutaneously but is not a factor product). Subcutaneous administration would potentially eliminate the need for intravenous access or central venous catheter placement for administration of routine prophylaxis. (See "Hemophilia A and B: Routine management including prophylaxis", section on 'Emicizumab for hemophilia A'.)


Bispecific humanized monoclonal antibodies — Emicizumab is a bispecific humanized monoclonal antibody used for prophylaxis in hemophilia A. (See "Hemophilia A and B: Routine management including prophylaxis", section on 'Emicizumab for hemophilia A'.)

Mim8 is another bispecific antibody under development that also substitutes for the role of factor VIIIa in bringing together factor XIa and factor X. Like emicizumab, it can be administered subcutaneously in individuals with hemophilia A, both with and without inhibitors [48]. In hemophilia A plasma and whole blood, Mim8 normalized thrombin generation and clot formation, with potencies 13 and 18 times higher than a sequence-identical analog of emicizumab.

REDUCING NATURAL ANTICOAGULANTS — Some individuals with hemophilia who also carry a prothrombotic gene variant such as factor V Leiden (FVL) or antithrombin (AT) deficiency have been reported to have less frequent or milder bleeding. (see "Clinical manifestations and diagnosis of hemophilia", section on 'Disease severity').

These observations have led to the development of approaches that target endogenous anticoagulant proteins, in effect "rebalancing" the equilibrium between procoagulant and anticoagulant factors.

Since they increase thrombin generation generally rather than by correcting a specific factor deficiency, these therapies have the potential to reduce bleeding regardless of the deficient procoagulant.

Anti-TFPI — Tissue factor pathway inhibitor (TFPI) is a natural inhibitor of coagulation that prevents unchecked amplification of the clotting cascade. It acts in two ways, both inhibiting factor Xa and preventing generation of factor Xa by the extrinsic pathway (tissue factor and factor VIIa). (See "Overview of hemostasis", section on 'Control mechanisms and termination of clotting' and "Overview of hemostasis", section on 'Tissue factor pathway inhibitor'.)

Factor VIII and IX deficiencies reduce thrombin generation via the intrinsic pathway. Monoclonal antibodies against TFPI have the potential to improve hemostasis by allowing generation of higher local concentrations of factor Xa and thrombin by the extrinsic and common pathways. One monoclonal antibody, befovacimab, was not pursued because three individuals in an early study developed thromboses (one venous and two arterial) in the central nervous system [49].

An anti-TFPI RNA aptamer was not pursued due to high bleeding rates at the highest dose level [49,50].

Concizumab — Concizumab (previously called mAb2021) is a monoclonal antibody against TFPI that can be administered subcutaneously (as done in clinical trials) or intravenously.

The following data are available regarding efficacy and safety:

A 2019 trial involving 53 individuals (36 with hemophilia A, 9 with hemophilia A and inhibitors, and 8 with hemophilia B with inhibitors) treated with concizumab prophylaxis versus on-demand rFVIIa found a significant reduction in annualized bleeding rate with concizumab; there was an 80 to 90 percent greater reduction in bleeding than seen with on-demand rFVIIa (annualized bleeding rate with concizumab, 4.5, versus 20.4 with rFVIIa) [51]. Concizumab was given subcutaneously once daily at a starting dose of 0.15 mg/kg and increased as needed based on a defined number of breakthrough bleeding events over a period of time. Earlier studies in people with hemophilia A and B, healthy volunteers, and preclinical models demonstrated good hemostatic efficacy [52-54].

A randomized trial evaluating concizumab was put on clinical hold due to the development of thrombotic events, and a risk mitigation strategy was instituted and the study reinstated [49].

Marstacimab — Marstacimab (previously called PF-06741086) is a monoclonal antibody against TFPI that can be administered subcutaneously or intravenously [49]. Unpublished data in a series of 26 individuals with hemophilia A or B treated in various dose cohorts of marstacimab for six months suggest that the annualized bleeding rate was reduced by 80 to 96 percent across the marstacimab dose cohorts [49]. Thrombotic complications were not observed.

Reducing AT — AT is a natural anticoagulant that inhibits thrombin (factor IIa), factor Xa, and other serine proteases in the coagulation cascade such as factor IXa. Reduced AT activity can lead to a prothrombotic state. (See "Antithrombin deficiency", section on 'Pathophysiology' and "Overview of hemostasis", section on 'Antithrombin, heparin, and heparan'.)

While reducing AT could reduce bleeding risk, there is concern about prothrombotic risk, as seen with fitusiran. (See 'Fitusiran (AT antisense)' below.)

Fitusiran (AT antisense) — Fitusiran is an antisense oligonucleotide (ASO) against AT (encoded by the SERPINC1 gene) that can be administered subcutaneously. In a study of 25 individuals with moderate to severe hemophilia A or B who were treated with fitusiran and served as their own controls before and after administration, the number of bleeding events decreased (mean annualized bleeding rate, 0 versus 3); this correlated with reduced AT levels and the effect appeared to be dose-dependent [55].

In September 2017, in the open-label extension study, a patient with hemophilia A without inhibitors developed a fatal thrombotic event; the study was temporarily halted and then restarted following implementation of a risk mitigation strategy in which dose intensity was decreased [56,57]. Additional thrombotic events have been described, some unpublished [58]. A follow-up publication in 2021 did not report thrombotic complications [59]. However, the possibility exists that a prothrombotic stimulus such as severe injury or infection (such as coronavirus disease 2019 [COVID-19]) could further increase prothrombotic risk if AT concentrate is not administered.

Prior pharmacokinetic analysis showed that the drug had a short half-life in the plasma (three to five hours), but the reduction of AT levels persists for several weeks [55]. The monthly regimen used in the initial study induced a dose-dependent reduction of AT (mean maximum reduction, 70 to 89 percent from baseline). A reduction in the AT level of >75 percent from baseline resulted in mean peak thrombin values at the lower end of the range seen in individuals without hemophilia.

Modified inhibitors of APC — Activated protein C (APC) is a natural inhibitor of coagulation that proteolyzes factors such as activated factor V. FVL is a variant of factor V that is resistant to APC. (See "Factor V Leiden and activated protein C resistance", section on 'Physiology'.)

Natural inhibitors of APC exist, including protein C inhibitor (PCI) and alpha-1 antitrypsin (AAT); these are referred to as serine protease inhibitors (SERPINs). APC also has roles in inflammatory signaling; there have been concerns that disruption of APC function could have effects beyond the coagulation cascade. (See "Clinical manifestations, diagnosis, and natural history of alpha-1 antitrypsin deficiency", section on 'AAT genetics'.)

To circumvent the other roles of APC, modified versions of natural SERPINs have been created that specifically block APC activity in the coagulation cascade without disrupting other pathways, and these are under investigation as possible therapies for hemophilia.

SOCIETY GUIDELINE LINKS — Links to society and government-sponsored guidelines from selected countries and regions around the world are provided separately. (See "Society guideline links: Hemophilia A and B".)

INFORMATION FOR PATIENTS — UpToDate offers two types of patient education materials, "The Basics" and "Beyond the Basics." The Basics patient education pieces are written in plain language, at the 5th to 6th grade reading level, and they answer the four or five key questions a patient might have about a given condition. These articles are best for patients who want a general overview and who prefer short, easy-to-read materials. Beyond the Basics patient education pieces are longer, more sophisticated, and more detailed. These articles are written at the 10th to 12th grade reading level and are best for patients who want in-depth information and are comfortable with some medical jargon.

Here are the patient education articles that are relevant to this topic. We encourage you to print or e-mail these topics to your patients. (You can also locate patient education articles on a variety of subjects by searching on "patient info" and the keyword(s) of interest.)

Basics topic (see "Patient education: Hemophilia (The Basics)")


Need for new therapies – Despite numerous factor products and other prophylactic therapies, optimal management of hemophilia is complex, and available therapies carry a number of costs and burdens. Several types of investigational therapy are under development (figure 1), including gene therapy, cellular therapy, modified factor products, novel factor alternatives, and therapies to reduce natural anticoagulants. (See 'Introduction' above and 'Overview of strategies to reduce bleeding risk' above.)

Increasing factor activity

Gene therapy – Gene therapy introduces exogenous DNA into cells that can be used to produce a missing protein. Modified genes that are capable of producing a missing factor can be used. Hepatocytes are considered an ideal target cell for clotting factor genes. Some individuals may not be eligible for gene therapy due to preexisting immunity to the vector, preexisting antibodies (inhibitors) to the factor, underlying liver disease, or age (liver still growing in young children). Initial data show promising efficacy and safety, but long-term follow up is needed. Data for specific constructs are presented above. (See 'Gene therapy' above.)

Cellular therapy – Autologous cells treated outside the body with gene therapy or foreign cells can be used to produce the deficient factor. These approaches are in early stages of development. (See 'Cellular therapy' above.)

Factor half-life extension – Various approaches to extending factor half-life are in development. Recombinant longer half-life products are already in use for hemophilia A and B. (See 'Half-life extension' above and "Hemophilia A and B: Routine management including prophylaxis", section on 'Longer lasting recombinant factor VIII' and "Hemophilia A and B: Routine management including prophylaxis", section on 'Longer-lasting recombinant factor IX'.)

Factor alternatives – Bifunctional monoclonal antibodies (mAbs) that replace the function of the deficient factor are in development. Emicizumab is one such product that is already approved and in wide use for hemophilia A. (See 'Substitutes for clotting factors' above and "Hemophilia A and B: Routine management including prophylaxis", section on 'Emicizumab for hemophilia A'.)

Reducing natural anticoagulants

Anti-TFPI – mAbs against tissue factor pathway inhibitor (TFPI) such as concizumab and marstacimab allow generation of higher local concentrations of factor Xa and thrombin by the extrinsic and common pathways.

Reducing antithrombin – Fitusiran is an antisense oligonucleotide (ASO) against antithrombin (AT, encoded by SERPINC1) that prevents proteolysis of thrombin, factor Xa, and other serine proteases. Concerns about increased thrombotic risk have been raised and a revised dosing strategy has been implemented.

Modified inhibitors of APC – Modified versions of natural serine protease inhibitors (SERPINs) that specifically block activated protein C (APC) activity in the coagulation cascade without disrupting other pathways are under investigation.

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