The significant, relative improvement in walking ability was sustained when Sarepta analyzed six patients, including two patients treated with a lower, 30 mg dose of eteplirsen. Another two patients treated at the lower dose were excluded because they experienced a sudden and rapid decline in function soon after the study started. There was no difference in walking ability between patients treated exclusively with the lower eteplirsen dose compared to the control arm, Sarepta said. Sarepta continues to follow the patients in the study and will have updated results after 48 weeks of treatment ready for presentation at a medical conference in October. On a conference call Tuesday, Sarepta said if the benefit observed in the 36-week data are confirmed at 48 weeks, the company will meet with FDA to discuss regulatory options. One of those options could be an accelerated approval filing under new FDA regulations designed to speed the development of drugs for rare diseases. Sarepta would still have to conduct a larger clinical trial to confirm eteplirsen's benefit, but the trial could be done post approval. Last April, Sarepta presented data showing eteplirsen treatment for six months resulted in a statistically significant increase in the production of dystrophin compared to placebo. Dystrophin is a protein that plays key role in muscle function and repair. The genetic inability to make dystrophin is what causes muscular dystrophy. Eteplirsen is designed to "skip over" the section of damaged gene in DMD patients and therefore restore the gene's ability to produce partially functioning dystrophin. In many ways, the science Sarepta is using to find an effective treatment for DMD is similar to the work done successfully by Vertex Pharmaceuticals ( VRTX) to find new treatments for the underlying cause of cystic fibrosis, another rare, genetic disease. What was missing from the April eteplirsen data -- until Tuesday -- was evidence linking increased production of dystrophin to an improved ability to walk. The new data suggests that longer treatment is required before meaningful levels of dystrophin are produced such that it significantly delays progression of DMD and improves walking function, Sarepta said.