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First clinical trial of gene therapy for muscular dystrophy now under way

The first gene therapy human trial in the United States for a form of muscular dystrophy is under way.

The clinical trial for Duchenne muscular dystrophy (DMD) tests the safety and effectiveness of a therapy that was developed over two decades by scientists at the University of North Carolina at Chapel Hill’s School of Medicine and the University of Pittsburgh.

The trial was launched March 28, at Columbus Children’s Hospital in Ohio, an affiliate of Ohio State University’s School of Medicine. In the trial, six boys with DMD will receive replacement genes for an essential muscle protein.

Each of the boys will receive replacement genes via injection into a bicep of one arm and a placebo in the other arm. Neither the investigators nor the participants will know which muscle got the genes. After several weeks, an analysis of the injected muscle tissue’s microscopic appearance, as well as extensive testing of the health and strength of the trial participants, will reveal whether gene therapy for DMD is likely to be safe and whether it’s likely to result in persistent production of the essential protein in muscle cells.

Muscular dystrophies are genetic disorders characterized by progressive muscle wasting and weakness that begin with microscopic changes in the muscle. As muscles degenerate over time, the person’s muscle strength declines.

Duchenne muscular dystrophy is a genetic disease that begins in early childhood, causes progressive loss of muscle strength and bulk, and usually leads to death in the 20s from respiratory or cardiac muscle failure. DMD occurs when a gene on the X chromosome fails to make the essential muscle protein dystrophin. One of nine types of muscular dystrophy, DMD primarily affects boys.

Currently, the best medical therapy can only slow the progressive muscle weakness of DMD.

The gene for dystrophin is one of the largest genes in the human body, and miniaturizing it, while retaining the crucial elements of its set of DNA instructions, has been among the greatest challenges to the gene therapy field.

The new Biostrophin therapy uses a novel combination of advanced technologies, including a miniaturized replacement dystrophin gene and nano delivery technology called biological nanoparticles. Developed from a virus known as adeno-associated virus (AAV), the nanoparticles are engineered specifically to target and carry the "minidystrophin" gene to muscle cells.

The therapy was made possible by the pioneering research in AAV by Dr. Richard Jude Samulski, professor of pharmacology and director of the Gene Therapy Center at UNC, and Dr. Xiao Xiao, a former UNC postdoctoral researcher in Samulski’s laboratory now with the University of Pittsburgh Human Gene Therapy Center and associate professor of orthopedic surgery.

Samulski has long pioneered methodologies for making viruses deliver genes. As a graduate student at the University of Florida in the early 1980s, his thesis project was developing the AAV as a vector for therapeutic genes. This work eventually led to isolation of type-2 AAV, which has been used for gene therapy trials in cystic fibrosis and in several other settings, Samulski said.

"It’s what we would call the parent virus that everybody started with."

Samulski moved to the University of Pittsburgh in 1986, joining the biology department as an assistant professor with his own laboratory. His first graduate student was Xiao, who had just come from China. Xiao focused on the lab’s AAV vector project. "We’ve continued to have productive collaborations ever since he graduated from my lab," Samulski said.

In 1993, Samulski moved to the University of North Carolina at Chapel Hill, becoming director of UNC’s new Gene Therapy Center. Xiao moved first into industry, then to UNC as a postdoctoral researcher in Samulski’s gene therapy center. In 1996, the team published a report of the first muscle gene delivery involving an AAV vector.

Xiao then returned to Pittsburgh in 1998, where he worked on muscle biology with an eye toward gene transfer, while Samulski remained focused on the vector aspects of AAV, the delivery system.

At Pittsburgh, Xiao had developed a miniaturized version of the gene for dystrophin, the muscle protein needed by people with DMD. Eager to test it in a vector, he contacted his former mentor.

"The dystrophy gene is like a long picket fence. It’s the largest gene in the human body, occupying 1 percent of the X chromosome," Samulski said. "Xiao Xiao began removing pegs, or pickets, from the fence, making it smaller and smaller but kept testing to see if it would still perform its function," Samulski said.

Xiao then began looking for a way he might move his minidystrophin gene forward clinically.

"We had just finished demonstrating that we could make clinical-grade virus for another genetic disorder called Canavan’s disease, an enzyme deficiency disorder," Samulski said. "We were the first academic institution to ever make FDA-certified AAV vectors to go into the brains of children with Canavan’s disease. So we had cut our teeth and had a bit of a track record by 2002, and that’s when Xiao approached me."

Xiao told Samulski he wanted to put his new minidystrophin gene into an AAV vector and test it.

"He wanted to know if UNC could make the virus, and that’s when I told him that we were using this virus for Canavan’s disease and for other efforts. We had started improving the vectors, and we had developed some new ones that we thought were better for muscle."

Xiao and Samulski put their projects together and formed Asklepios BioPharmaceutical Inc. (AskBio) in 2003. Along with the rights granted by UNC to Samulski’s vector technology, AskBio acquired the intellectual property rights to Xiao’s uniquely miniaturized dystrophin gene.

In July 2004, the Muscular Dystrophy Association (MDA) awarded $1.6 million to AskBio to develop gene therapy strategies for DMD.

Rounding out the AskBio team clinically is neurologist Dr. Jerry R. Mendell, co-director of the MDA clinic at Columbus Children’s Hospital; professor of pediatrics, neurology and pathology at Ohio State University’s School of Medicine; and head of the neuromuscular research program and Center for Gene Therapy at Columbus Children’s Research Institute. In the clinical trial at the Columbus Children’s Hospital’s MDA clinic, Mendell will administer the injections.

Following extensive laboratory toxicity experiments required by the U.S. Food and Drug Administration demonstrating that minidystrophin gene transfer was unlikely to harm and could ultimately benefit muscles affected by DMD, approval was granted March 3, 2006, to proceed with the human trial.

"After years of encouraging pre-clinical results, I’m excited that AskBio will help bring this promising new therapy into the clinic, and look forward with a great deal of optimism to offering this initial step toward hope for the DMD community," Samulski said.

Stephanie Crayton | EurekAlert!
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