I am suprised that they didn't get much upfront and only $90m on approval. Sound slike the deal that TSPT got last year. Except when TSPT got that deal TSPT has $6 a share in cash and it had a $70m market cap. ARNA has a $400m diluted market cap - so this $$$ isn't that big a deal and the upfront is about $0.45 a share.
Might be a decent short if it gets much of a pop (or a fade) seems like a SELL THE NEWS event - the best time in my experience to fade something like this is about 1/2 - 1hr after the open - after the squeeze attempt.