Back to Basics: Animal Models of Inhibitors

A Q&A with David Lillicrap (Canada) and Valder Arruda (U.S.A.)

Working with animal models could lead to better pre-clinical methods of predicting inhibitor formation, providing important research support for the 10 to 15 new bleeding disorder treatments that are likely to be introduced in the next five years.

“With all the new therapies, immunogenicity is one of the questions, if not the major question, and using animal models to help understand or predict that is a very good research objective,” Dr. David Lillicrap of the Department of Pathology and Molecular Medicine, Queen’s University (Canada), says. “That’s not going to be solved next year, but it’s something we may be edging towards.”

Q: What can we still learn from animal models?
A: “The development of antibodies to replacement therapy is probably the major complication in the treatment of hemophilia, especially hemophilia A, and that makes it a major clinical concern. But you can only get so far at the moment by doing human studies. Human studies are complicated and they take a long time. So one of the important messages is that knowledge of inhibitor biology and management requires a combination of patient studies and animal work.”

Q: How do patient studies and animal models support each other?
A: “By studying animals, one can only extrapolate so far into what you’ll see in clinical situations. The vast majority of these experiments are done in mice, and because you’re looking at inbred populations of animals, you’re not replicating the genetic diversity in human populations. So that is the major concern. But animal models have obvious advantages: the mouse immune system is remarkably similar to the human system, your study population can be as big as you want, and you can work out the extremely detailed mechanisms as to how these things are happening, in ways that you could never do in human populations. You can dissect out why antibodies are occurring and understand their natural history in ways that would be ethically and pragmatically impossible in human populations.

“We are not for one second suggesting that human studies are not necessary. They absolutely are. But to accelerate knowledge in this area, the combination or synergy of animal experiments, basic experiments, and clinical experiments is where we should be looking.”

Q: What new research results do you see on the horizon?
A: “New mouse models of hemophilia [developed by Dr. Brigit Reipert of Austria] will allow a much more relevant, detailed analysis of the immune response to factor VIII, and those observations will apply to the human situation. There have already been some observations in mice that tell us about the biology of immunologic tolerance to factor VIII, so that’s an example of an animal model that is directly applicable to the biology in humans.

“One of the things happening in the hemophilia field is the large number of new products on the horizon. For each of those products, inhibitor formation is one of the potential complications. So I think it’s great that these new developments are occurring, but the risk is in the backs of our minds. With that background, the work to understand the immunogenicity, the biology, the natural history, and the management is even more important.”

Dr. Valder Arruda of the Department of Pediatrics, University of Pennsylvania and the Children’s Hospital of Philadelphia, USA, has been working with a canine model to study gene therapy. Along the way, he and his team have made an interesting observation that may provide a new approach to the treatment of inhibitors.

Q: Can you explain your work with gene therapy?
A: “Over the last decade, we’ve pursued a genotherapy approach to deliver a normal factor VIII or factor IX gene into the liver, or the muscle for factor IX, of patients with severe hemophilia. We’ve done two clinical trials in patients with severe hemophilia B, and now we’re working on a third.

“In dogs with hemophilia we found long-term expression of the transgene for factor IX, up to eight years after a single injection. The next step is to repeat the idea with factor VIII. It’s a tricky problem because factor VIII is five times larger, and the vector has limited capacity. But the dog is stable, five years after a single injection…”

Q: How can this approach be used to treat inhibitors?
A: “We’ve learned that we can actually eradicate antibodies in the presence of inhibitors to factor VIII. Up until now, we have used this approach for dogs that are immune tolerant or have a high risk of inhibitors, but don’t have them at the time they’re injected. This is a very different way to look at things. We’ve shown that we can induce immunotolerance, even with antibodies, so that the endogenous factor mimics immunotolerance induction (ITI) and works as a new source of factor VIII, thereby changing the phenotype from moderate to mild disease.

“This kind of translational research always requires time, but this is the proof of principle, that this can be done. The next step is to significantly increase the number of dogs under study and test dogs with higher-typed inhibitors, to see if there’s a level beyond which this approach is not effective.”

David Lillicrap and Valder Arruda delivered their talks during a session on Animal Models of Inhibitors held at the XXIX International Congress of the World Federation of Hemophilia, Buenos Aires, Argentina, July 10-14 2010. Other speakers included Brigit Reipert (Austria). Their state of the art paper, published in a special supplement of the WFH’s official journal Haemophilia, is available here.

Last Updated September 2010