I think another key reason Mendell and Sarepta should want the 4th biopsy beyond the FDA reasons is to better understand the drug and method of action. People have speculated about all kinds of immune reactions, etc. The real question is does dystrophin continue to increase or did it for some reason decrease? Is the dropping in 6 MWT scores in due to dystrophin issues or is it strictly a matter that the boys are becoming taller and heavier. Are they really following a becker model or something else? One of the real issues is that when you think is a statistical manner, the tails steroid treated boys for duchenne and even strictly an exon 51 skip appear to be extremely wide.
The problem is "placebo effect" especially in a trial, where there are early results strongly reporting positive effects. I am very certain the drug works, but expect early reports if any from the new trial subjects will be enhanced by the placebo effect, unless there is real disappointment.
Only slightly possibly, if the ebola drug mutated, the Sarepta drug might come into play. If you recall one of the antiviral approaches Iversen took in antivirals was to target conserved regions where mutation was unlikely. Though I think at least one other drug may also do this in the ebola arena.
Also, the response time, if there were a bio engineered virus based on ebola by some terrorist group, the time to respond, with an antiviral, might go to Sarepta, but at this time the limited production availability would be an issue.