Gene Therapy for Hemophilia: Progress and Setbacks

As the FDA delayed the approval of the world’s first gene therapy for hemophilia A, companies forged ahead with phase III trials to gather longer-term patient data. 

On December 23, 2019, California-based BioMarin Pharmaceutical announced that it had submitted a Biologics License Application (BLA) to the FDA for its hemophilia A gene therapy, valoctocogene roxaparvovec (valrox; formerly BMN 270) – the first filing of a gene therapy for hemophilia in the U.S.1 The submission was based on 3 years’ worth of data from phase I/II trials, as well as interim phase III results.

The company, and many clinicians and patients living with hemophilia, had expected an approval announcement from the FDA by the end of August 2020. Instead, the agency sent BioMarin a Complete Response Letter (CRL) on August 18, requesting an additional 2 years of safety and efficacy data from the company’s ongoing phase III BMN 270-301 trial to provide evidence of valrox’s durability in preventing bleeds.2

Differences between the phase I/II and phase III study, the FDA concluded, limited the agency’s ability to rely on the earlier-phase data to support durability of effect. “The [FDA] first informed the Company of this recommendation in the CRL having not raised this at any time during development or review,” BioMarin noted in a press release announcing the decision.

To gather the requested data, BioMarin will need to complete the phase III study, which was fully enrolled with 134 patients in November 2019. The last patient will complete 2 years of follow-up in November 2021, pushing the earliest FDA approval date back to 2022.

“The durability data on valrox are only from the phase I/II BMN 270-701 trial, which enrolled a small number of patients. Few of the phase III data have been publicly disclosed,” Stacy Croteau, MD, MMS, Medical Director of the Boston Hemophilia Center and Assistant Professor of Pediatrics at Harvard Medical School, told ASH Clinical News. “While there was a lot of anticipation of approval for a hemophilia gene therapy, I think the FDA conducted a careful review and made a prudent decision given this new therapeutic modality, the trends observed over time, and the small patient population with available data so far.”

As BioMarin continues gathering data in its single-arm phase III trial, other biotechnology companies are moving forward with alternative hemophilia gene therapies. Pennsylvania-based Spark Therapeutics, which was acquired by Roche in December 2019 but continues to operate as an independent company, is evaluating its hemophilia A gene therapy, SPK-8011, in a phase I/II trial. Pfizer and partner Sangamo Therapeutics also have a hemophilia A gene therapy in phase I/II testing and expect to begin a phase III trial by the end of 2020. Yet another company, Georgia-based Expression Therapeutics, is planning to launch a phase I trial of its ex vivo gene therapy by year’s end.

Gene therapy development for hemophilia B has trailed behind that for hemophilia A, but uniQure and Pfizer both have hemophilia B candidates in phase III testing.

With several hemophilia gene therapies poised to enter late-stage clinical trials in the coming months, ASH Clinical News spoke with Dr. Croteau and other hemophilia experts involved in gene therapy development about the outstanding questions surrounding this therapeutic modality.

A One-Gene Target

As an X-linked recessive disorder, hemophilia predominantly occurs in males. There are approximately 30,000 people living with a form of hemophilia in the U.S., and hemophilia A affects 1 in 5,000 male births every year, making it the most common form of the disease.3 Patients with hemophilia typically lack one of two proteins necessary for the blood coagulation cascade: factor VIII (FVIII) in the case of hemophilia A and factor IX (FIX) in the case of hemophilia B.

FVIII is a co-factor within the clotting cascade, encoded by the F8 gene on the X chromosome and produced in sinusoidal endothelial cells in the liver and in endothelial cells outside the liver. FIX is a serine protease encoded by the F9 gene, which also is found on the X chromosome and is made by hepatocytes.

Standard treatment for hemophilia consists of replacing the missing coagulation factor through frequent intravenous (IV) infusions to treat current bleeding episodes or prevent future ones. Patients with severe hemophilia A (those with FVIII levels ≤1 IU/dL) may require multiple IV infusions per week, making a “one-and-done” gene therapy particularly attractive.

Hemophilia is amenable to gene therapy because both the A and B types are the result of a variant in a single gene. The gene therapy candidates under investigation are designed to provide long-term expression of the missing or abnormal coagulation factor at sufficient and steady enough levels to reduce – or even eliminate – the need for exogenous factor replacement. Several gene delivery vehicles have been tested, but the therapies in phase II or III development all use a recombinant adeno-associated virus (rAAV) vector with an inserted F8 or F9 gene. The therapy is administered as a one-time IV infusion, after which the vector is taken up by hepatocytes, where the synthesis of FVIII or FIX begins.

How Long Is “Lifelong?”

The appeal of hemophilia gene therapies is that they are a single-dose treatment that corrects the factor deficiency, rather than continuously replacing it. They are potentially curative, but whether they will really offer a “cure” depends on how long their effects last.

Paul Monahan, MD, Clinical and Translational Development Lead at Spark Therapeutics, noted that, “because the AAV vector does not integrate into the host cell genome, and because the liver likely turns over about once a decade, the hemophilia field has yet to establish whether the FVIII expression will be lifelong.”

As of October 2020, BioMarin’s valrox is the only gene therapy for hemophilia A that has reported interim phase III results. On the hemophilia B side, uniQure’s etranacogene dezaparvovec (formerly AMT-061) is in phase III testing, and a lead-in study to support a pivotal trial of Pfizer’s fidanacogene elaparvovec has completed enrollment.4,5

In the phase III BMN 270-301 trial, 16 patients who received valrox in the 6×1013-vg/kg dosing cohort had been followed for at least 26 weeks after infusion.6 Their estimated median annual bleed rate (ABR) was 0, and their estimated mean ABR was 1.5, representing an 85% reduction from baseline levels, when all patients were on standard-of-care FVIII replacement. Eight patients had FVIII levels of 40 IU/dL, on the highest end of the “mild” hemophilia A range, as assayed by the chromogenic substrate assay, which measures factor activity.

In January, researchers also reported 3-year data from the phase I/II trial of valrox, in which 15 men with severe hemophilia A received doses ranging from 6×1012 vg/kg to 6×1013 vg/kg.7 Patients in the highest dose cohorts maintained an ABR of less than 1 for 3 consecutive years and had significant decreases in the number of FVIII infusions required.

However, levels of FVIII appeared to fall 12 to 18 months after valrox infusion, which raised some concerns that a single gene therapy dose may not be enough to maintain protection from bleeding in the long term. Longer-term follow-up, including from the phase III trial, which the FDA requested in its CRL, should resolve this durability question.

Meanwhile, in 3-year follow-up data presented in July at the International Society on Thrombosis and Haemostasis (ISTH) 2020 Virtual Congress, Spark Therapeutics’ SPK-8011 appeared to lead to safe and durable FVIII expression levels in men with severe hemophilia A.8

The gene therapy uses Spark’s AAV-LK03 engineered vector, including a liver-specific promoter, to express the human F8 gene, explained the trial’s principal investigator Lindsey George, MD, a physician at the Children’s Hospital of Philadelphia. A total of 14 trial participants received SPK-8011 at one of three dose levels. Two patients lost FVIII expression following vector administration, but, among the remaining 12 patients who had sustained FVIII expression and were available for the preliminary efficacy analysis, Dr. George and colleagues observed “a remarkable 91% decrease in bleeding events.”

When investigators analyzed FVIII expression in the first 5 patients to receive vector, they found that the men in this group showed preliminary evidence of stable expression at follow-up between 2 and 3.3 years. Adverse events were generally mild and included elevations in liver enzymes.

“The FVIII expression appeared to be stable and durable with SPK-8011,” Dr. George told ASH Clinical News. “These are important data for the field because it shows that FVIII, which is not naturally made in the liver, can be expressed stably in hepatocytes with an AAV gene therapy vector.”

Spark is continuing its phase I/II to establish the optimal dose of SPK-8011 and to better understand how immunosuppressive steroids affect factor expression before starting a phase III trial, according to Dr. Monahan.

Giroctocogene fitelparvovec (formerly SB-525), the product developed by Pfizer and Sangamo, is being investigated in the phase I/II Alta trial. As of June, the longest follow-up was 61 weeks.9 Of the 5 men with severe hemophilia A who received the therapy at a dose of 3×1013 vg/kg, all had sustained FVIII activity levels, with a median of 64% activity at 9 weeks post-infusion. One patient in this cohort had a grade 3 treatment-related serious adverse event of hypotension and grade 2 fever. Four participants received corticosteroids for elevated alanine aminotransferase levels, all of which fully resolved after treatment. The dose-ranging trial has a planned enrollment of 13 patients.

Pfizer also announced the pivotal phase III AFFINE trial, which is evaluating giroctocogene fitelparvovec in 63 patients with moderate to severe hemophilia A, enrolled the first patient in October 2020.

Finding the Right Candidate

In addition to questions about the durability of gene therapies, there is also uncertainty about which patients are likely to derive the most benefit from this approach. Researchers are now looking for factors that influence gene therapy uptake and FVIII expression. “Across all the hemophilia A gene therapy programs, it is fair to say that some individuals achieve quite good factor expression while others have much lower levels of expression,” Dr. Monahan commented. “We don’t yet know what fully accounts for that variability.”

Ongoing trials are all enrolling patients without anti-FVIII or FIX antibodies or inhibitors to the factor products used to treat or prevent their bleeding episodes, making the approximately 30% of people with hemophilia A and 3% of people with hemophilia B ineligible for most gene therapy platforms. Spark is developing a gene therapy, SPK-8016, specifically for individuals with hemophilia A with inhibitors that is currently in phase I/II trials. The difference between this therapy and SPK-8011 is proprietary information, Dr. Monahan told ASH Clinical News.

The trials are forging ahead – with caution. “Systemic AAV vectors were first used in hemophilia clinical trials almost 20 years ago now, but we had little confirmation that the participants had sustained expression of the factor genes encoded by the vectors,” explained Dr. George. “Over the past decade, the study of systemic AAV vectors for hemophilia and also other disorders has expanded.”

The longest follow-up demonstrating sustained vector expression with an AAV gene therapy was in the initial successful hemophilia B trial, which reported safety and efficacy data from up to 8 years.10 However, she said, “in my mind, the long-term safety of systemic AAV is an unanswered question.”

Dr. Croteau agreed. “From decades’ worth of data across hemophilia and other diseases using AAV-based gene therapy, we have not seen any significant negative long-term impacts of the therapy – particularly in the adult population,” she said. “Still, caution and attention to longitudinal safety and efficacy monitoring are needed.”

“Gene therapy is potentially a paradigm-changing strategy for hemophilia A care,” added Dr. George. “The optimism around this area is high, and there are many clinical trial programs underway so that we will soon be able to answer some of these outstanding questions.” —By Anna Azvolinsky


  1. BioMarin press release. BioMarin Submits Biologics License Application to U.S. Food and Drug Administration for Valoctocogene Roxaparvovec to Treat Hemophilia A. Accessed October 6, 2020, from
  2. BioMarin press release. BioMarin receives Complete Response Letter (CRL) from FDA for valoctocogene roxaparvovec gene therapy for severe hemophilia A. August 19, 2020. Accessed October 6, 2020, from
  3. Centers for Disease Control and Prevention. Data & statistics on hemophilia. Accessed October 6, 2020, from
  4. uniQure press release. uniQure announces achievement of target patient dosing in HOPE-B pivotal trial of AMT-061 (etranacogene dezaparvovec) in hemophilia B. Accessed October 6, 2020, from
  5. Pfizer press release. Pfizer Investor Day features significant number of pipeline advances for COVID-19 programs and across numerous therapeutic areas. Accessed October 6, 2020, from
  6. BioMarin press release. BioMarin announces that phase 3 cohort of valoctocogene roxaparvovec, gene therapy study in severe hemophilia A met pre-specified criteria for regulatory submissions in the U.S. and Europe. Accessed October 6, 2020, from
  7. Pasi KJ, Rangarajan S, Mitchell N, et al. Multiyear follow-up of AAV5-hFVIII-SQ gene therapy for hemophilia A. N Engl J Med. 2020;382:29-40.
  8. George L, Eyster E, Ragni M, et al. Phase I/II trial of SPK-8011: Stable and durable FVIII expression for >2 years with significant ABR improvements in initial dose cohorts following AAV-mediated FVIII gene transfer for hemophilia A. Abstract OC 03.5. Presented at ISTH 2020 Virtual Congress; July 12-14, 2020.
  9. Pfizer press release. Pfizer and Sangamo announce updated phase 1/2 results showing sustained factor VIII activity levels and no bleeding events or factor usage in 3e13 vg/kg cohort following giroctocogene fitelparvovec (SB-525) gene therapy. Accessed October 6, 2020, from
  10. Nathwani AC, Reiss U, Tuddenham E, et al. Adeno-associated mediated gene transfer for hemophilia B: 8-year follow-up and impact of removing “empty viral particles” on safety and efficacy of gene transfer. Blood. 2018;132(Supplement 1):491.