GlaxoSmithKline’s drisapersen achieved the primary objective of a mid-stage study, heating up competition with Sarepta Therapeutics  in the race for a drug that could reverse the debilitating effects of Duchenne muscular dystrophy.

Patients taking drisapersen showed a clinically meaningful difference from those on placebo after 48 weeks of treatment, according to an abstract of the study results posted on the website of CureDuchenne, an investor in Glaxo’s (NYSE: GSK) partner on the drug, Dutch biotechnology company Prosensa Therapeutics BV. Glaxo will present the data at a genetics conference in Cold Spring Harbor, New York, on Thursday.

Duchenne muscular dystrophy is a neuromuscular disease with no known cure that affects one in 3,500 newborn boys. Drisapersen has been designated by U.S. and European regulators as an orphan drug, and may eventually reach 500 million pounds ($766 million) in sales if proven to be effective and approved for commercialization, according to Fabian Wenner, an analyst at Kepler Capital Markets in Zurich.

“We are very pleased to report that the results are clinically significant,” CureDuchenne, a non-profit organization based in Newport Beach, California, said on its website. “It’s been a long 10 years and the drug is not yet approved, but we are looking forward to the Phase 3 data later this year.”

Excessive Protein

All 35 patients treated with drisapersen completed the study, with the majority of side effects related to injection site reactions and proteinuria, where excessive protein is observed in urine, according to the abstract.

Drisapersen would compete with a similar drug, eteplirsen, being developed by Sarepta of Bothell, Washington, which has reported statistically significant benefit in a mid-stage study with no serious side effects.

Sarepta shares surged 11 percent yesterday in New York. The drisapersen data was also posted on

“Prosensa data validates Sarepta’s platform,” Deutsche Bank analyst Robyn Karnauskas said in a note to investors yesterday. Eteplirsen’s better safety profile may differentiate it from drisapersen, he said.

A “small” number of boys taking drisapersen were hospitalized because of proteinuria and thrombocytopenia, characterized by a drop in the number of platelets that help blood to clot, Rohit Batta, head of global medical affairs at Glaxo’s rare disease unit, said in a message posted on a patient group’s website last month.

Glaxo spokeswoman Melinda Stubbee yesterday confirmed that four hospitalizations occurred, though she said it wasn’t in the mid-stage study. A late-stage study began in early 2011, with results to be released later this year.

Gene Mutations

Duchenne is a genetic disease caused by gene mutations that delete one or more exons, a part of the gene sequence that instructs production of dystrophin, which protects muscles. Rapid weakening of muscles can lead to boys needing wheelchairs by the age of 12.
Drisapersen is an RNA-based therapy that works by skipping an exon next to a defective exon to allow the production of dystrophin.

Some investors are concerned about the patent situation given the similarity in Prosensa and Sarepta’s technology, Karnauskas said in a note on March 18. While Sarepta challenged Prosensa’s patent in the European Union in 2011, it lost the case, allowing Prosensa to block its rival from entering the European market.

In the unlikely event that Prosensa challenges Sarepta’s U.S. patent, the two companies may arrive at a royalty-based settlement, she said.

[GSK operates its North American headquarters in RTP, N.C.]