Studies Confirm Long-Term Benefit of Gene Therapy for Hemophilia A

In two studies presented at the International Society on Thrombosis and Haemostasis (ISTH) 2020 Virtual Congress, gene therapies for hemophilia A demonstrated durable increases in factor VIII (FVIII) expression and decreases in bleeding events, but the findings highlight several questions about this approach that remain to be answered.

Valoctocogene Roxaparvovec

In the first presentation, lead author Sylvia Fong, PhD, of BioMarin Pharmaceuticals, shared results from an exploratory sub-study of patients who were enrolled in a phase I/II trial of valoctocogene roxaparvovec, in which durability of the gene therapy was demonstrated through liver biopsies obtained at more than 2 years post-infusion.

Valoctocogene roxaparvovec is an adeno-associated virus serotype 5 (AAV5) vector containing a B-domain-deleted FVIII gene. The FVIII expression is driven by a human liver-specific promoter, Dr. Fong explained, and is characterized by a ramping-up period, with levels peaking at week 20, followed by a gradual decline.

Investigators have hypothesized that the underlying mechanism of decline is related to normal hepatocyte turnover or the kinetics of vector gene processing, but no clear cause has been determined. With this analysis, researchers evaluated the histopathology and vector DNA distribution in human liver biopsies from 2 patients who were infused with valoctocogene roxaparvovec, hoping to identify factors associated with durable FVIII expression.

These participants were infused with valoctocogene roxaparvovec at doses of 4×1013 vg/kg and 6×1012 vg/kg, and liver biopsies were obtained 140 and 201 weeks after infusion, respectively.

Histopathologic examination revealed normal liver architecture, with mild steatosis and no evidence of steatohepatitis or significant inflammation in either participant, Dr. Fong reported.

Investigators also used in situ hybridization to detect vector genomes and DNA extraction for molecular analyses. These analyses revealed increases in full-length vector genomes that could give rise to stable transcription, as well as increases in FVIII-SQ RNA. These increases were dose dependent, the authors noted.

There also was a dose-dependent increase in the number of hepatocytes that stain positive for vector genomes, at 1.3% at the 6×1012 vg/kg dose and 32% at the 4×1013 vg/kg dose. “This was very exciting to see,” Dr. Fong said. “Not only were we able to detect vector genomes more than 2 years post-dosing, there seems to be quite a bit of signals in this patient sample.”

In addition to the persistent FVIII expression demonstrated in this sub-study, the researchers noted that both participants are clinically stable, with no long-term biochemical and histologic hepatic issues more than 2 years after infusion. “Ideally, we would like to include participants across the different dose levels, with different FVIII activity level profiles, including subjects with the lowest FVIII activity level, for cross-sectional sampling,” Dr. Fong said, noting future directions for this research.

On August 18, the FDA issued a complete response letter to the manufacturer’s biologics license application indicating that valoctocogene roxaparvovec was not ready for approval because of a lack of evidence for durable long-term responses. The agency recommended 2 years of follow-up safety and efficacy data from the ongoing phase III trial. The last patient enrolled will complete 2 years of follow-up in November 2021.

Commenting on the FDA’s decision, Glenn Pierce, MD, PhD, Vice President of the World Federation of Hemophilia, said, “Gene therapy to cure hemophilia, the dream of many, remains a technology in early development. Individual patient responses are variable, unreliable, unpredictable, and in some cases not durable for the long term. We still have a lot to learn in order to harness this technology to benefit people with hemophilia around the world. I’m confident it will occur, but will take time to answer some challenging questions.”


In the second presentation, treatment with the experimental gene therapy SPK-8011 led to safe and durable FVIII expression levels for up to 3 years in patients with hemophilia A, with a “remarkable” reduction in bleeding events, according to follow-up data from a phase I/II trial.

SPK-8011 is designed to restore the body’s ability to produce FVIII, using a modified version of an AAV that contains a codon-optimized human FVIII gene under the control of a liver-specific promoter, explained lead study author Lindsey A. George, MD, from the University of Pennsylvania and Children’s Hospital of Philadelphia. However, questions about the potential for declining FVIII expression over time have raised concerns about the durability of AAV-mediated, liver-directed gene transfer.

In this analysis, Dr. George and researchers reviewed the safety and efficacy of SPK-8011 in 14 men with hemophilia A who had received an infusion at one of three dose levels: 5×1011 vg/kg, 1×1012 vg/kg, and 2×1012 vg/kg. All but 1 patient had severe hemophilia, and approximately two-thirds were treated prophylactically with FVIII or emicizumab.

Patients were followed for 1 year after vector infusion, and then underwent study visits every 3 to 6 months for 4 years to monitor long-term safety and efficacy.

At the time of presentation, study participants had been followed for an average of 27 months (range = 14-40). No patients died, and no patients developed FVIII inhibitors.

In terms of safety, 1 patient experienced an adverse event that was considered related to SPK-8011 (an acute infusion reaction that resolved). Three patients reported liver function test elevations, including one case of grade 2 transaminitis), but all events resolved.

Study authors reported that 2 of the 14 patients lost FVIII expression following vector administration, leaving 12 patients who had sustained FVIII expression available for the preliminary efficacy analysis.

When the investigators compared the number of bleeding events in the year prior and the year after SPK-8011 infusion, they reported “a remarkable 91% decrease in bleeding events,” Dr. George said. The reduction in the number of FVIII infusions dropped by 96%, she added (TABLE). The researchers also were able to identify the minimal level of FVIII activity required to eliminate spontaneous bleeding events, she said. At FVIII expression levels of >10%, the annualized bleeding rate was <1.

Durability of FVIII expression was demonstrated in the analysis of the first 5 patients to receive vector (at doses of  5×1011 vg/kg and 1×1012 vg/kg), with the men in this group showing preliminary evidence of stable expression at follow-up between 2 and 3.3 years.

In the group of 9 patients who received SPK-8011 at 2×1012 vg/kg, 2 who did not receive steroids had stable expression at 92- to 106-week follow-up. However, of the 5 who received daily oral steroids, 2 lost expression and returned to FVIII or emicizumab prophylaxis and 3 maintained expression off steroids. The remaining 2 patients who had prophylactic steroids initiated within 4 weeks of SPK-8011 infusion experienced supratherapeutic levels of FVIII that resolved after tapering off steroids.

According to these findings, infusion of SPK-8011 led to sustained FVIII expression through more than 2 years of follow-up, with no serious safety concerns, the authors noted. However, given the observed interactions between FVIII levels and steroid use, Dr. George outlined future directions for research. “Optimal vector dose and immune suppression regimens, including alternatives to daily oral steroid use, are being studied in phase I/II trials to optimize predictable, safe, efficacious, and durable FVIII expression,” Dr. George concluded.

Study authors report relationships with BioMarin Pharmaceutical, the manufacturer of valoctocogene roxaparvovec, and Spark Therapeutics, the manufacturer of SPK-8011.


  1. Fong S, Rangarajan S, Mitchell N, et al. First In Human Liver Biopsy Study Following Gene Therapy for Hemophilia A. Abstract OC 03.4. Presented at ISTH 2020 Virtual Congress; July 12-14, 2020.
  2. 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.
  3. BioMarin press release, August 19, 2020.
  4. World Federation of Hemophilia press release, August 19, 2020.