Hemophilia A (a deficiency of clotting Factor X) is the most common form of the disease, accounting for about 85% of patients.
The other type is hemophilia B (deficiency of clotting Factor VIII), and a gene therapy for this form of the disease has recently been launched – etranacogene dezaparvovec (Hemgenix), at the enormous price tag of $3.5 million.
Both products are comprised of a one-off intravenous IV infusion that delivers a functional gene via an adeno-associated virus that instructs the body to make the missing clotting factor. The hope is that this one-off infusion will act as a ‘cure’ and that the individual will be freed from life-long prophylaxis and/or treatment.
The new clinical data on valoctocogene roxaparvovec, published online in the New England Journal of Medicine, show that the beneficial effects from the gene are largely durable at 2 years, but they are anticipated to fade with time.
Two years after the one-time infusion, there remained “a significant reduction in the annualized bleeding rates” among 132 men who, at baseline, had severe hemophilia A requiring ongoing factor VIII prophylaxis, said the investigators, led by hematologist Johnny Mahlangu, MBBCh, MMed, of the University of the Witwatersrand, Johannesburg, South Africa.
However, the team predicted that median factor VIII activity would decrease below 10% of normal by year 3 or 5 depending on measurement technique, which would still translate to mild disease with an annualized bleeding rate of less than 1 episode per year.
“Although valoctocogene roxaparvovec may not eliminate bleeding, it potentially provides more consistent protection than factor VIII prophylaxis with less treatment burden,” the team said.
New questions
Data from the study “will directly inform therapeutic decision-making” in Europe, where valoctocogene roxaparvovec is already conditionally approved, and the United States, where it is awaiting approval by the FDA, says Lindsey George, MD, a hematologist and gene therapy specialist at Children’s Hospital of Philadelphia, in an accompanying editorial.
The study speaks to an ongoing concern about the durability of gene therapy for hemophilia but also raises new questions, she said.
For instance, while some patients had normal Factor VIII production and activity at 2 years, activity had dropped substantially in others, including in six men who resumed prophylaxis. “The cause of the decrease in factor VIII expression is an unanswered question,” and despite an anticipated U.S. price tag of around $2.5 million per treatment, “it is not possible [at the moment] to predict where an individual patient may fall within this range,” she writes.
Also, some subjects had elevations in liver aminotransferase levels that lasted for several months, including 2 years after infusion in 29% of subjects. Elevations in liver aminotransferase levels were treated with immune suppression for a median of 33 weeks.
“This is a unique finding with an undefined cause and long-term safety implications,” Dr. George said.
Getting to the bottom of such issues will be necessary for hemophilia gene therapy to fulfill its promise as “a one-time, lifelong, disease-ameliorating” fix for the condition, she asserted.