Although we don't believe in timing the market or panicking over market movements, we do like to keep an eye on big changes -- just in case they're material to our investing thesis.

What: Shares of Sarepta Theraputics (NASDAQ:SRPT) have fallen by 15% today after the company released disappointing results  from its Phase 2b open-label extension study of eteplirsen for the treatment of Duchenne muscular dystrophy.

So what: Sarepta has been running eteplirsen tests for more than three years. Its press release  states that "results of the six-minute walk test at 168 weeks showed continued ambulation across all patients evaluable on the test, however all patients showed a decline in distance walked on this measure since the week 144 timepoint." The study showed that six patients in the modified intent-to-treat group (taking 30 or 50 milligrams per kilogram of body weight) managed to walk 76.7 fewer meters (19.5% less) than their baseline walking ability.

On the other hand, this group did manage to outperform the placebo and delayed-treatment group by 65.4 meters on the six-minute walk test, but this is primarily due to the fact that this second cohort continued to weaken rather than stabilizing over time. The trial patients are walking, and they do not appear to be suffering any side effects from their eteplirsen doses.

The FDA has requested that Sarepta include its 168-week results in its new drug application, which is still planned for submission in mid-2015. Sarepta highlighted a number of caveats and mitigating factors to its results in its full press release, which can be found here.

Now what: This trial has been ongoing for so long that any disappointment is bound to be amplified by the amount of effort put into the study. However, the treatment is clearly doing something more than the alternative, which means that it will likely continue its progress toward approval. Investors have increasingly given up on Sarepta over the past few months, as shares prices have been cut nearly in half since last July. Nothing else in Sarepta's pipeline  is as close to a final approval as eteplirsen, but investors could potentially try to capitalize on the company's long-term prospects (drugs for influenza, Ebola, and the Marburg virus are all in Phase 1 trials at the moment) by taking advantage of this ongoing slide.