GlaxoSmithKline plc (ADR) (GSK), Sarepta Therapeutics Inc (SRPT): Could This DMD Treatment Become a Blockbuster Drug?

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Duchenne muscular dystrophy, or DMD, is a muscle-wasting disease caused by a genetic mutation in the dystrophin gene — the largest gene on the X chromosome — which affects one in every 2,400 male births. Muscle weakness in DMD patients generally spreads from the legs to the pelvis, eventually reaching the arms, neck, and other areas, confining most patients to a wheelchair by age 12 and resulting in a maximum lifespan of around 25 years.

To date, there is no effective treatment for DMD, but there could be soon, thanks to three biotech companies: Sarepta Therapeutics Inc (NASDAQ:SRPT), Prosensa Holding NV, and GlaxoSmithKline plc (ADR) (NYSE:GSK). However, Sarepta Therapeutics Inc (NASDAQ:SRPT) recently took the lead after Prosensa, which is partnered with GlaxoSmithKline plc (ADR) (NYSE:GSK), reported disappointing clinical trial data for its DMD treatment. Could Sarepta Therapeutics Inc (NASDAQ:SRPT)’s treatment finally be the game-changing, blockbuster drug that DMD patients have been waiting for?

GlaxoSmithKline plc (ADR) (NYSE:GSK)

Sarepta gains the upper hand
Prosensa and GlaxoSmithKline plc (ADR) (NYSE:GSK)’s co-developed DMD treatment, drisapersen, is intended to increase the production of dystrophin, a protein that facilitates muscle growth that is absent in DMD patients. GSK and Prosensa recently released the eagerly anticipated results from drisapersen’s phase 2 trials that concluded a year ago, but the results were fairly disappointing.

Drisapersen was shown to produce dystrophin in 72% of patients on a continuous dose, and in 59% of those who had intermittent doses of the drug. By comparison, Sarepta Therapeutics Inc (NASDAQ:SRPT)’s competing drug, eteplirsen, showed a 100% success rate in increasing the dystrophin production rate in all dosed patients. Both drisapersen and eteplirsen are genetic treatments that work at the RNA level to modify dystrophin production.

GlaxoSmithKline plc (ADR) (NYSE:GSK) had previously administered a six-minute walk test (how far a patient can walk in six minutes) to patients dosed with drisapersen, which was found to increase the boys’ average walking distance by 117 feet. Sarepta also beat Glaxo in that test as well, reporting an average walking distance increase of 152 feet with eteplirsen. However, investors should note that GlaxoSmithKline plc (ADR) (NYSE:GSK)’s testing group is significantly larger, with 53 boys, compared to the 12 tested in Sarepta’s studies.

Prosensa shareholders didn’t take the news too well, since drisapersen is the company’s most promising drug, and shares plunged 8% on Aug. 16. Glaxo was largely unaffected, but shares of Sarepta Therapeutics Inc (NASDAQ:SRPT) climbed 3%. Sarepta intends to file an FDA approval for eteplirsen early next year, but doubts have been raised regarding its ambitious plans to receive an accelerated approval.

Either treatment is forecast to generate more than $1 billion in annual revenues if approved. Prosensa and GlaxoSmithKline plc (ADR) (NYSE:GSK) intend to release the full results of its Phase III trial later this year for drisapersen, and Sarepta is expected to start a Phase III trial for eteplirsen next year.

A newcomer carves out a new niche
Although Sarepta Therapeutics Inc (NASDAQ:SRPT), Prosensa, and Glaxo are currently the biggest names in DMD treatments now, that hasn’t discouraged smaller competitors from exploring their own treatments. One notable newcomer is PTC Therapeutics, which only went public two months ago. PTC recently initiated a phase 3 clinical trial of ataluren, a treatment for patients with nonsense mutation DMD.

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