In the past two years, gene therapy has become available for some people with hemophilia A and hemophilia B. Gene therapy is given as a one-time dose to eliminate the need for constant factor replacement and to improve quality of life. Could gene therapy be a good choice for you or your adult child with hemophilia?

Here’s what you need to know to have a conversation with your primary health care provider or hematologist about gene therapy as a treatment option for hemophilia.

1. It Delivers a Working Copy of the Clotting Factor Gene

Hemophilia is caused by genetic mutations (changes) that prevent the body from making enough clotting factor — factor VIII (8) for hemophilia A or factor IX (9) for hemophilia B — to control bleeding.

Gene therapy uses a viral vector made from a modified virus to deliver a functional copy of the mutant (changed) gene into liver cells. In other words, gene therapy uses a customized virus to deliver a healthy gene into liver cells. Specifically, an adeno-associated virus (AAV) is used. Viruses are used as vectors to deliver gene therapy because they’re very good at getting into human cells. However, scientists remove the illness-causing abilities of these viruses to prepare them to be used as vectors.

Once the functioning gene is present, the liver can then begin making its own clotting factor. The goal of gene therapy for hemophilia is to help the body produce enough of the missing clotting factor to control bleeds.

Gene therapy is given as a one-time dose via intravenous (IV) infusion.

2. Gene Therapy Is Available for Hemophilia A and B

So far, the U.S. Food and Drug Administration (FDA) has approved two gene therapies:

• For adults with hemophilia A: Valoctocogene roxaparvovec-rvox (sold under the brand name Roctavian), approved in 2023
• For adults with hemophilia B: Etranacogene dezaparvovec-drlb (sold under the brand name Hemgenix), approved in 2022

Gene therapies for other types of bleeding disorders are currently being studied in clinical trials.

3. It’s Not Right for Everyone

Right now, gene therapy for hemophilia has only been approved for adults, people age 18 or older. So far, gene therapy has only been studied in men.

To be eligible for Roctavian, you must be an adult with severe hemophilia A. People with severe hemophilia A — about 60 percent of those with the condition — have less than 1 percent of the normal level of factor VIII in their blood. You’ll also need to be tested for antibodies, or immune system proteins, against the viral vector that’s used to deliver the gene therapy. If you have these antibodies, it means your body has encountered the virus before and has developed a defense against it. If you test positive for antibodies to the viral vector, it means Roctavian may be less effective for you, so you may not be a good candidate for the gene therapy.

To be eligible for Hemgenix, you must be an adult and either be using prophylactic (preventive) factor IX replacement therapy or have a history of life-threatening bleeds or repeated serious spontaneous bleeds.

4. Gene Therapy Has Proved Effective in Studies

Gene therapy has been proved effective in clinical studies. Not everyone will experience the same results.

Roctavian was approved based on a study involving 112 men with severe hemophilia A. The men who received Roctavian were watched for at least three years. Before receiving Roctavian, the men experienced an average of 5.4 bleeds per year. After the infusion of Roctavian, they experienced an average of 2.6 bleeds per year.

Hemgenix was approved based on two studies involving 57 men with severe or moderately severe hemophilia B. In one study, men who received Hemgenix experienced increased factor IX levels, less need for prophylactic factor replacement infusions, and a 54 percent lower annual bleeding rate (ABR). The ABR is the number of reported bleeding events divided by the number of months in the reporting time frame, then multiplied by 12.

One MyHemophiliaTeam member who participated in a gene therapy trial shared, “I haven’t taken any factor in 30 days! I’m trying to wrap my head around the idea that I don’t have to infuse anymore.”

Doctors are still unsure whether gene therapy will be less effective over time.

5. Gene Therapy Is Safe for Most People

Every medication, even those available over the counter at a pharmacy, can cause side effects. In general, gene therapy for hemophilia has been well tolerated by people given the treatment in clinical studies.

In clinical studies, the most common side effects of Roctavian for hemophilia A were:

• Slight changes in liver function
• Headache
• Nausea
• Vomiting
• Fatigue
• Abdominal pain
• Infusion-related reactions (issues due to the medication being given by injection)

Although it didn’t happen during clinical studies, researchers think there may be an increased risk of dangerous blood clots or the development of liver cancer.

In clinical studies, the most common side effects of Hemgenix for hemophilia B were:

• Higher levels of liver enzymes in the blood
• Headache
• Infusion-related reactions
• Symptoms similar to those seen in the flu

Read about what to carry in your hemophilia emergency kit in case of sudden, severe bleeds.

Is the Viral Vector Safe?

The viral vectors used to deliver gene therapy for hemophilia are harmless and can’t make you sick. Once it’s no longer needed, the viral vector will be “shed” via bodily fluids. It doesn’t stay in the body.

6. You Can Still Pass Hemophilia Genes to Children

Gene therapy only corrects the mutated gene in your body. It doesn’t fix the mutation in your genome, and you can still pass it to your children.

7. Gene Therapy Isn’t a Cure for Hemophilia

After gene therapy, it’s likely the amount of clotting factor your body produces will rise, perhaps even to normal levels. You may need fewer clotting factor replacement infusions, or you may not need them at all for some time. Whatever effects you see may not last forever — more studies are needed before researchers will know what to expect over time.

In case the effects of gene therapy start to wear off, it is not possible to repeat the treatment. Once gene therapy has been given, your body starts reacting to the adenovirus vector that transported this gene, producing substances called antibodies. These antibodies would make any repeat dose of gene therapy ineffective.

If you receive gene therapy, you should still attend regular follow-ups at your hemophilia treatment center to monitor your clotting factor levels and check for any problems.

Read the article “Hemophilia A Prognosis: What Is the Life Expectancy?”

Talk With Others Who Understand

On MyHemophiliaTeam, the social network for people with hemophilia and their loved ones, more than 6,200 members come together to ask questions, give advice, and share their stories with others who understand life with hemophilia.

Have you considered using gene therapy to treat your hemophilia? Share your thoughts in the comments below, or start a conversation by posting on your Activities page.