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April 18, 2019
In 1987, scientists first uncovered the gene that is the target of mutations that causes Duchenne muscular dystrophy (DMD), a lethal muscle-wasting disease that affects about one in every 5,000 newborn boys. While the discovery of the dystrophin gene sparked hope that scientists could someday replace the mutated gene with a healthy one, turning this concept into a safe and effective treatment has been a decades-long challenge.
In drug discovery, the process of converting early biological insights into a therapy that can be tested in the clinic is known as translational research. It’s rigorous and very often painstaking work that happens behind the scenes, but the nonetheless is a critical bridge to bringing life-transforming treatments to patients. And in the journey of developing a gene therapy for DMD, scientists have had to overcome significant hurdles due to the complexity of the disease and the various challenges of engineering vector delivery systems.
“Three separate fields of study, genetics, molecular biology, and AAV vector biology, had to sufficiently mature and come together to bring a potential treatment to be tested in patients,” says Anna Tretiakova, Vice President of Translational Gene Therapy at Pfizer’s Kendall Square, Cambridge, Massachusetts research site.
From bench to bedside
The first challenge scientists faced was the size of the dystrophin gene, as it is the largest in the human genome. Since it was too large to be carried by the viral vector delivery system, adeno-associated virus (AAV), scientists worked to develop a synthetic “mini-dystrophin” gene that could be packaged inside the vector.
Once researchers solved for the packaging challenges, they had to demonstrate that the gene therapy actually resulted in expression of the missing protein (dystrophin) and that it had a lasting effect. Early experiments in the mid-90s helped affirm this possibility because they showed AAV injected locally into muscles could result in expression of the healthy gene.
But unlike other genetic conditions isolated to a single organ such as the eye or liver, DMD affects the muscles of the entire body, including the heart and respiratory muscles, which are difficult to reach with an AAV vector. “Muscle constitutes 40 percent of body mass with an extensive vascular supply,” says Michael Binks, Vice President of Rare Disease Clinical Research also based in Cambridge. “You need to get an adequate circulation of AAV to get uptake in tissues, and that requires vectors which have selectivity for muscle and a lot of virus,” he adds.
Much research has also been devoted to designing an optimal AAV delivery system that can evade specific immune responses from patients. For example, if a patient has been exposed to a particular type of AAV already, their immune system might have developed antibodies that could neutralize the effects of that vector. Without an approach for anticipating that response and working around it, there’s the possibility that the gene therapy would have a weaker than intended effect. “We have selected a vector not only for its ability to be taken up in the target tissues as efficiently as possible, but also to have a low prevalence of neutralizing antibodies in the patient population,” says Binks. Part of this translational work involves developing tests to screen the relevant population for pre-existing antibodies against AAV.
Living on hope
Engaging patients early on has been another critical aspect of developing a successful gene therapy strategy.
With DMD, where current investigational treatments are administered in children, advocacy groups work with families to understand the goals of current experimental gene therapies. “We’re not replacing the entire defective gene,” says Katherine Beaverson, Senior Director and Rare Disease Patient Advocacy Lead based at Pfizer’s Kendall Square, Cambridge site. “The ultimate goal is to lessen the symptoms of the disease and improve quality of life.”
“These communities are living on hope,” says Beaverson. “It’s really important to share what we know and don’t know so people can make the best decisions about the treatments for themselves and their families,” she says.
