Yes it can be rejected and be eligible for compassionate use so long as eteplirsen remains in clinical trials and continues to be safe, with some hint of efficacy. While the FDA was quick to publicly respond that its press release about compassionate use yesterday had nothing to do with eteplirsen because it's been in the works for years, I don't believe it. Notwithstanding that the guidelines may have been in the works for years, the PDUFA date for eteplirsen may have been delayed just so the FDA could publish them before the eteplirsen decision - the veil of deceit cannot get much thinner.
Lots of speculation out there for a drop. Here's my speculation for a 28% drop on 12 million shares at noon: A hedge fund with a large share holding has learned something negative from an FDA insider and is dumping its shares, using the FDA news release as cover. There are many other theories for the huge drop. Just use your imagination.