Will Gene Therapy Change the Treatment of Muscular Dystrophy?

Thanks to decades of research, new treatments for muscular dystrophy that address the disease at the generic level may soon change the course of the disease and help more people.

Muscular dystrophy is a rare group of genetic disorders that cause muscles to waste away over time, creating an inability to move and making everyday tasks difficult, depleting quality of life, and causing early death. The disorders are caused by a defective gene located on the X chromosome that controls production of dystrophin, a protein complex found in muscle fibers.

The most common form of muscular dystrophy is Duchenne muscular dystrophy (DMD), typically seen in boys. Other forms include Becker muscular dystrophy, which is similar to DMD but milder, and limb-girdle muscular dystrophy, which affects the shoulder and pelvic muscles.

Until recently, managing symptoms was the only approach to muscular dystrophy; for example, using corticosteroids that can help strengthen muscle and slow the disease. But there are risks to long-term use of corticosteroids, including weakening of bone tissue that increases the risk of fractures.

Now several pharmaceutical companies are developing gene therapy for muscular dystrophy. Results have been mixed. Earlier this year, Sarepta Therapeutics, a biotech company headquartered in Cambridge, Massachusetts, that focuses on genetic medicine for disorders like DMD, saw its stock price drop in half after it revealed that its investigational gene therapy for DMD, SRP-9001, fell short of its primary end point in a midstage trial. Sarepta attributed these results to differences in fitness among the patients included in the study, and the company released results in May that were more promising.

Sarepta has three products marketed for the treatment of DMD: Exondys 51 (eteplirsen), Vyondys 53 (golodirsen) and Amondys 45 (casimersen). These exon-skipping therapies are indicated for treatment if certain mutations are present and are designed to increase the production of dystrophin. Sarepta also hopes to regain its muscular dystrophy pipeline momentum with a gene therapy for limb-girdle muscular dystrophy, which has no approved treatments.

Pfizer is also looking to make headway in DMD with its gene therapy, fordadistrogene movaparvovec. The company has a phase 3 trial underway and plans to enroll patients across 15 countries. The FDA is addressing outstanding questions regarding the investigational new drug application, says Nicole Kjesbo, Pharm.D., leader of the pipeline team at Prime Therapeutics, a PBM headquartered in Eagan, Minnesota.

Another gene therapy for DMD is under development by Genethon, a French gene therapy company, says Kjesbo. In April, the first patient was dosed with Genethon’s investigational treatment, GNT 0004. Solid Biosciences, a Cambridge, Massachusetts, biotech company, also has a gene therapy candidate for DMD in early-stage trials.

Several companies have therapies for muscular dystrophy in the pipeline that are not gene therapies:

• Taiho Pharmaceutical, a Japanese company, has started a phase 3 trial of its oral treatment, pizuglanstat, which is designed to dampen the inflammatory response in the muscles of DMD patients.
• Italfarmaco, an Italian company, is testing how well givinostat, an oral treatment, promotes muscle repair in DMD.
• FibroGen, headquartered in San Francisco, is testing pamrevlumab, a monoclonal antibody that inhibits connective tissue growth factor in patients with several different illnesses, including DMD. 

Jaime Rosenberg is a freelance medical writer based in Jersey City, New Jersey.