Problems With Traditional Therapies for Hemophilia

Standard traditional treatments for hemophilia A and B involve frequent, time-consuming sessions 2 to 3 times a week for intravenous factor infusions. The biweekly or triweekly frequency is due to the short half-lives of clotting factors.1 

Although the development of extended half-life factor replacement products decreases the frequency of infusions, patients with hemophilia still encounter the lifelong burden of treatment. In addition to the time spent during and traveling to and from replacement therapy appointments, these patients still face suboptimal protection from bleeding and joint arthropathy.1 

The development of immune tolerance via neutralizing anti-drug antibodies (termed inhibitors) to factor VIII and factor IX occurs in 30% of patients with severe hemophilia A and in 5% of patients with severe hemophilia B, rendering replacement therapy ineffective. Circumventing immune tolerance requires fewer effective treatments and negatively impacts quality of life. Immune tolerance induction (ITI) therapy may eliminate high-titer inhibitors; however, this is only effective in 70% of hemophilia A cases and 30% of hemophilia B cases.1

The inherent limitations of traditional treatment of hemophilia, all of which revolve around replacement of the missing or dysfunctional protein clotting factor, fuels science-based, experimental research for better treatment options. These experimental therapies include antibody therapy, gene therapy, gene editing, RNA therapy, treatments for joint arthropathy caused by hemophilia, prophylactic bleeding management therapies, and solutions to the development of immune tolerance.  

Experimental Antibody Therapy

Emicizumab is a subcutaneous humanized monoclonal antibody that bridges activated factor IX and factor X to replace the function of the dysfunctional or missing factor VIII.2 Emicizumab can function as factor VIII in the guise of an antibody because the factor VIII-neutralizing inhibitory antibodies in individuals who have developed immune tolerance do not recognize it. It is approved for bleeding prophylaxis in patients with hemophilia type A, both with and without immune tolerance with reductions in bleeding rates of 79% and 68%, respectively.1 

There are key differences when comparing emicizumab and factor VIII replacement proteins. Factor VIII replacement proteins require active bleeding to become activated, while emicizumab is always activated, such that the clotting-based test, activated plasma thromboplastin time (aPTT), overestimates the time to clot in patients taking emicizumab. Care must be taken when administering emicizumab to patients with immune tolerance who are also taking higher doses of activated prothrombin complex concentrate (aPCC), as they may form clots. Dosing of emicizumab attempts to keep the plasma concentration of emicizumab close to 50 μg/mL, which is equivalent to 15 IU/dL factor VIII, to reduce active bleeding episodes.1

A phase 3 multi-center clinical trial of emicizumab demonstrated that the drug achieved significant effects. Overall, children with hemophilia A who had factor VIII inhibitors tolerated emicizumab well. This trial indicated that emicizumab may provide a new standard of therapy that decreases the treatment burden on young patients with hemophilia A who have immune tolerance.2 

The observation that people with hemophilia who also inherit prothrombotic mutations have milder bleeding than other hemophilia phenotypes inspired research to downregulate natural anticoagulants to restore a balanced hemostasis. One approach to do this involves using monoclonal antibodies to block tissue factor pathway inhibitor (TFPI). TFPI works in a negative feedback loop to control the production of factor X, thus inhibiting it and slowing down the initial phase of coagulation. This in turn allows for more time to produce factor X, which plays a role in the generation of thrombin. This approach may successfully treat von Willebrand disease and some of the rarer bleeding disorders such as factor VII deficiency, but there is not enough clinical trial data to confirm this.1

Four known anti-TFPI antibodies undergoing clinical trials in individuals with hemophilia A and hemophilia B both with and without immune tolerance are concizumab, marstacimab, MG1113, and BAY 1093884. BAY 1093884 development was discontinued following serious adverse events during a phase 2 clinical trial. These trials are currently working out the frequency of infusions and assessing safe therapeutic windows. Breakthrough bleeding still would require standard replacement therapies since TFPI functions in the initial phase of coagulation, which has usually passed by the time standard therapy intervention is administered.1  

Experimental RNA Therapy

Fitusiran is a double-stranded, small interfering RNA that degrades the transcript of the SERPINC1 gene via an RNA-induced silencing complex to prevent translation. SERPINC1 encodes for antithrombin, a protease inhibitor produced in the liver that deactivates thrombin, factor X, and other clotting factors. Fitusiran contains chemical modifications that target multiple transcripts containing a specific region of the SERPINC1 messenger RNA (mRNA). This prevents the translation of antithrombin, which consequently improves thrombin generation. Fitusiran remains active for weeks and thus requires only monthly administration in these trials. Phase 3 trials indicate that if fitusiran reduced antithrombin levels more than 75% from the patient’s baseline, this increased the production of thrombin to non-hemophilic levels.1

Systemic delivery of mRNA allows for internal production of the encoded protein. One experimental study showed that 1 injection of in vitro transcribed B domain-deleted factor VIII-encoding mRNA into factor VIII-deficient mice produced circulating clotting factor VIII above 5% of normal levels for a period of 3 days with a half-life of 17.9 hours and successfully curtailed bleeding episodes. Problems with mRNA therapies still require strategic solutions due to the powerful neutralizing IgG antibody response with repeated administration of the mRNA that encodes for factor VIII.3 

Experimental Gene Therapy

Following a successful series of animal trials, recombinant adeno-associated viral (AAV) vectors by which genes are transferred to the liver demonstrate success in human trials. Researchers conducting multiple ongoing clinical trials report significant therapeutic benefit, and in some cases, curative expression. Patient immune responses against the AAV prove a difficult challenge, but immunosuppressive treatments have been developed to minimize these responses.4

Twelve individuals with hemophilia B participated in a phase I/II clinical trial to study the dosage of an in vivo gene transfer, with 10 individuals demonstrating measurable levels of factor IX up to 3 years following a single infusion of the AAV vector through a peripheral vein. The 2 other participants who were treated more recently still show stable production of factor IX. Mild liver toxicity and CD8 T cell responses to the virus after administration required timely treatment with glucocorticoids to decrease liver enzyme levels. There are several current and planned gene therapy clinical trials for hemophilia B sponsored by Freeline, Pfizer, Takeda, St. Jude Children’s Research Hospital (SJCRH), Sangamo, Shenzhen Gemo-Immune Medical Institute (SGIMI), and UniQure.1

AAV gene therapy for hemophilia A presents a unique challenge due to the ~7-kb length of the coding sequence for factor VIII. This length exceeds the current ~5-kb capsid genome packaging limit for AAV vectors used to treat other conditions. There are several current and planned gene therapy clinical trials for hemophilia A underway, sponsored by Bayer, BioMarin, Medical College of Wisconsin (MCW), Sangamo, SGIMI, Takeda, Spark, and University College London (UCL)/St. Jude.1 

Using AAV gene transfer to induce immune tolerance in patients who have developed factor VIII and/or factor IX immune tolerance has potential and is currently under investigation. The possibilities of using an integrating virus like a lentivirus or genome editing approaches for long-term clotting factor expression are being examined.4 

A phase 1 trial is currently examining the use of a lentivirus vector to deliver the factor VIII gene into autologous hematopoietic stem cells (HSCs) ex vivo. The researchers would infuse these HSCs into patients with hemophilia A. Interaction with platelets that produce von Willebrand factor, which binds to factor VIII, might help to store factor VIII inside the cells until required. This tactic may also bypass the inhibitor neutralizing immune tolerance that some individuals with severe hemophilia develop.1  

Experimental Gene Editing Therapy

In vivo gene editing for hemophilia contains 3 liver-tropic AAV vectors that transport 1 of 3 components, including a right or left zinc finger nuclease (ZFN) or a normal F9 transgene. The ZFNs place normal copies of the clotting factor gene inside albumin intron 1 to produce clotting factor IX. The clustered regularly interspaced short palindromic repeats (CRISPR)/Cas9 is another gene editing system currently under investigation to achieve therapeutic levels of factor IX production and obtain a cure.5 The ZFN and CRISPR/Cas9 gene editing systems both carry an uncertain risk of off-target DNA mutagenesis, so these therapies must undergo rigorous testing prior to use in humans.1  

To maintain healthy joint function, prophylactic treatment is initiated in childhood and continued into adulthood with the aim of minimizing the number of spontaneous joint bleeds. Current prophylactic therapies do not entirely prevent progressive joint disease in patients with hemophilia. Once joint damage occurs, even if future bleeding is successfully prevented, the existing damage progresses over the patient’s lifetime. Hemophilia-related joint arthropathy progresses over many years, affecting the ankle joints first and with severity.6 

The roles that iron, inflammation, and angiogenesis play in the pathogenesis of hemophilia-related joint arthropathy may lead to future development of therapies targeting these factors to minimize joint damage caused by bleeds. Experiments with radionuclide synovectomy using a phosphorus 32-sulfur colloid to treat existing hemophilic joint arthropathy are also underway.7

Experimental Bleeding Management/Prophylactic Therapies

Emicizumab is approved for bleed prophylaxis in patients with hemophilia A with and without immune tolerance.1 The Guardian 7 clinical trial in China demonstrated the safety and effectiveness of turoctocog alfa for prophylaxis, surgical use, and immediate treatment of acute bleeding episodes in individuals with severe hemophilia A.8 

Innovative Solutions to Immune Tolerance 

Future therapies targeting immune tolerance may include prophylactic immune tolerance induction. This strategy is meant to prevent the development of factor VIII and factor IX inhibitors, allowing traditional replacement therapies to continue to work. The main topic of research for this strategy is oral tolerance induction using transplastomic lettuce that expresses factor VIII or factor IX. This lettuce, which is fused to a transmucosal carrier, can be ingested, be absorbed through the intestinal epithelium, and induce regulatory T cells within gut-associated lymphoid tissue to suppress inhibitor development.9

Other experimental gene and cell therapies to eliminate the possibilities of inhibitor development demonstrate promise in preclinical animal studies. Some of these therapies include maternal antigen transfer, genetically engineered regulatory T cells (Treg), use of immune modulatory drugs to induce Treg in vivo, and hepatic gene transfers with AAV vectors.9


  1. Butterfield JSS, Hege KM, Herzog RW, Kaczmarek R. A molecular revolution in the treatment of hemophilia. Mol Ther. 2020;28(4):997-1015. doi:10.1016/j.ymthe.2019.11.006
  2. Young G, Liesner R, Chang T, et al. A multicenter, open-label phase 3 study of emicizumab prophylaxis in children with hemophilia A with inhibitors. Blood. 2019;134(24):2127-2138. doi:10.1182/blood.2019001869
  3. Russick J, Delignat S, Milanov P, et al. Correction of bleeding in experimental severe hemophilia A by systemic delivery of factor VIII-encoding mRNA. Haematologica. 2020;105(4):1129-1137. doi:10.3324/haematol.2018.210583
  4. Nienhuis AW, Nathwani AC, Davidoff AM. Gene therapy for hemophilia. Mol Ther. 2017;25(5):1163-1167. doi:10.1016/j.ymthe.2017.03.033
  5. Li N, Kaczmarek R. Curing hemophilia: repeated treatments versus a one-off fix. Mol Ther. 2020;28(5):1229-1230. doi:10.1016/j.ymthe.2020.04.012
  6. Oldenburg J. Optimal treatment strategies for hemophilia: achievements and limitations of current prophylactic regimens. Blood. 2015;125(13):2038-2044. doi:10.1182/blood-2015-01-528414
  7. Rodriguez NI, Hoots WK. Advances in hemophilia: experimental aspects and therapy. Pediatr Clin North Am. 2008;55(2):357-376. doi:10.1016/j.pcl.2008.01.010
  8. Wu R, Sun J, Xu W, et al. Safety and efficacy of turoctocog alfa in the prevention and treatment of bleeding episodes in previously treated patients from China with severe hemophilia A: results from the Guardian 7 trial.  Ther Clin Risk Manag. 2020;16:567-578. doi:10.2147/TCRM.S243146
  9. Sherman A, Biswas M, Herzog RW. Innovative approaches for immune tolerance to factor VIII in the treatment of hemophilia A.Front Immunol. 2017;8:1604. doi:10.3389/fimmu.2017.01604

Reviewed by Harshi Dhingra, MD, on 8/11/2021.