December 13, 2017 / Clinical Trials,Research

Understanding Gene Therapy: The Screening Process

With advancements made in Duchenne gene therapies now coming to fruition, a number of questions are being asked about who may qualify for the upcoming trials.

While the inclusion criteria of each trial will be different based on age, mutation (in some cases), and functional abilities, one of the criteria for participating in all trials will be based on a blood test to look for pre-existing antibodies to the Adeno-associated virus (AAV), the virus being used to transport the micro-dystrophin into the body. This will mean that some would not be able to receive AAV-mediated gene therapy due to their pre-existing antibodies and thus will not be eligible to participate in these early human trials.

PPMD wanted to take this opportunity to provide a Q&A with PPMD’s Chief Scientific Officer, Dr. H. Lee Sweeney, to clarify this important issue and address some of the related questions for the Duchenne community.

Q&A with PPMD’s Chief Scientific Officer, Dr. H. Lee Sweeney

Can you explain what the Adeno-associated virus (AAV) is and why it is being used in gene therapy?

AAV is a naturally occurring virus that can infect humans as well as non-human primates (mammals in general).  AAV is used in gene therapy because it is a particularly small virus which allows it to easily get into many tissues in the body including skeletal and cardiac muscle, better than any other virus explored to date.

Because it is a naturally occurring virus, there is always a chance that an individual has been infected by the virus in the past, therefore their immune system will recognize the AAV and destroy it before it can get into the relevant (targeted) tissues.

It is for this reason, screening for antibodies against the AAV will tell you:

  1. If the person has had past infection of AAV
  2. If the person’s immune system has already produced AAV antibodies that will not allow it to infect the tissues

What are “pre-existing antibodies” and why would they prevent a patient from participation?

When the immune system fights a foreign infection, one of the things the immune cells do is to make antibodies against the foreign invader. These antibodies continue to be produced long after the original infection, allowing the immune system to rapidly recognize and remove the invader if there is a second exposure.   This is good news for keeping us alive and resistant to repeat viral infections, but problematic for gene therapies which rely on efficient infection of tissues.

What could happen if someone with these antibodies received gene therapy?

If a person who has had a previous exposure to AAV receives the virus, there could be an immune reaction, preventing it from infecting the relevant or targeted tissues. The severity of the immune reaction is currently unknown, but researchers are trying to avoid these reactions, thus the reasoning for pre-screen testing.

How do they test for these antibodies?

Testing for antibodies (called antibody titer testing) is done by taking blood, diluting the blood sequentially and adding antigens (proteins) from the virus. “Titer” is another word for amount or concentration; antibody titer testing tests for the amount, or concentration, of antibodies in the blood. If a reaction is detected between the virus antigens and the blood, you know antibodies are present. If you know how much you have diluted the blood, you can then quantify the amount of antibody that is present. This is referred to as the dilution ratio. The cut-off ratio is how diluted the blood sample must be before the antibody is no longer detected. Each trial protocol will have a dilution ratio threshold that patients will need to be under, in order to be eligible for participation in that AAV gene therapy trial.

Will patients who test positive for these antibodies never be able to participate in gene therapies (trials or treatments)?

Patients who test positive for AAV antibodies may or may not be able to participate in ongoing studies, depending on the AAV antibody levels (thresholds) established by each sponsor’s clinical trial inclusion criteria.

Researchers are also looking at future possible strategies to get around the antibody issues, such as:

  1. Remove the antibodies in a process called plasmapheresis.
  2. Develop a new AAV type (sometimes referred to as a serotype) that will not cross-react with the antibodies to AAV.
  3. Discover some entirely new method of delivery (non-viral) that we haven’t thought of yet. Remember, emerging gene therapies in other diseases are facing the same issue, so this issue is likely to get a lot of attention.

Should I try and get the test done on my own so I know beforehand? Will it need to be done at the clinical running the trial?

We do not recommend patients receive any blood testing prior to screening for a trial. All testing needs to be done in a clinical lab that is supporting the trial, near the time of enrollment for the trial.  No other data would be acceptable in this circumstance.

How many patients do you suspect have these pre-existing antibodies and thus be excluded from the early studies?

It is difficult to put an exact number to this question. Much younger patients (2-3 year olds and younger for example) are unlikely to have pre-existing antibodies.  The older you get, the more likely you are to have come in contact with this virus, developing pre-existing antibodies to the virus due to exposure.

When considering screening for a gene therapy clinical trial, what other things should I be thinking about?

Each study will have its own set of inclusion and exclusion criteria for their trials. We will learn more about each set of criteria as those studies are made public. As always, please consult with your physician for additional information.

What does the future hold for getting around this problem? Will it be possible?

There is a great deal of focus on developing delivery vehicles, different versions of AAV, plasmid delivery, and you may have recently heard about the gold nanoparticle.

There are significant attempts to discover a delivery vehicle that will be safe and capable of infecting relevant target tissues.  This is critical for progress as there is speculation that repeat dosing in gene therapy approaches may be necessary over time.

Additional Resources on Gene Therapy

Learn more about PPMD’s Gene Therapy Initiative, including links to our Gene Therapy Webinar Series.

Understanding Currently Recruiting Gene Therapy Studies

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