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How investigational therapies are studied

The purpose of this site is to provide information to patients and caregivers about the field of gene therapy and how it is being investigated for use in haemophilia.

Safety and well-being of patients are top priorities

Following many years of initial (preclinical) research, which includes laboratory, manufacturing, and animal studies, investigational new therapies may then be reviewed and approved by the appropriate regulatory agency for research studies in humans, known as clinical trials. Clinical trials include multiple phases and extensive review of data to – first and foremost – ensure the safety of patients.



Results from clinical trials provide insights into the safety, effectiveness, and appropriate use of the therapy being studied.

Potential risks of gene therapy

As with any new treatment being researched, there is the potential for unintended effects. To help identify these risks, gene therapies are studied in clinical trials under controlled conditions. While the safety of investigational gene therapy is still being studied, research to date has helped scientists learn important lessons.

Potential risks identified in gene therapy research so far include the following:

The body’s immune system

The body’s immune system could respond in unintended ways.

The job of the immune system is to defend against outside pathogens – things, such as viruses, from outside the body that could cause harm or sickness when inside the body. While this defence mechanism is normal and expected, it could cause the immune system to resist or attempt to fight off the gene therapy. This is because the immune system may see the vector, acting as the delivery vehicle for gene therapy, as something that isn’t supposed to be there, an “invader.” This may lead to immune responses in the body, such as:

  • Swelling of the liver, which, if not controlled, could lead to a decrease or loss of the factor protein made from investigational gene therapy. Treatment, such as a short course of steroids, may be required to calm the immune system.
  • The development of antibodies in response to AAV gene therapy, which could make someone ineligible for AAV gene therapy research and potential future treatments. This is because the antibodies would recognise the previously identified AAV gene therapy and escort it out of the body.
  • The development of antibodies against FVIII or FIX (also called inhibitors), which would limit the ability of gene therapy to work as desired.
  • Allergic reactions ranging from mild to severe.

Other potential risks

Other potential risks could occur related to the gene transfer itself

  • The functional gene carried by a vector may be delivered to the wrong cells. While vectors tend to be specific in the cells they target, there is still a risk that vectors could find their way into unintended cells. This could damage those cells or cause inappropriate cell growth, leading to tumours or cancer.
  • Once the AAV vector has placed the functional gene inside the nucleus, the vector’s empty shell (the “envelope”) is removed (or shed) from the body. This “vector shedding” occurs in fluids such as urine, semen, or saliva. While rare, this could lead to the formation of antibodies to the AAV vector in people who come in contact with these fluids. This could unintentionally make someone ineligible for AAV gene therapy in the future.

Studies are ongoing and participants continue to be followed. Additional risks may be identified in the future.

Learning more

If you’re interested in learning more about gene therapy clinical trials for haemophilia, speak with your doctor and visit