Cash burn rate under $50M a year. At $650M market cap this company can become a cash cow easily. Won't take too much revenue to come in to turn profits and earnings and as market cap is not in the billions earnings will be good per share. Additionally, big pharma certainly is watching this company and some big pharma will take it and all longs will be happy to have held out for that big premium.
Big pharma may have cold feet due to the Inhibitex disaster but eventually that will subside and additionally what Achillion has is multiple drugs in process of various types for Hep C. Point being they are not related and all going to fail as happened with Idenix. Idenix has a market cap close to Achillion's yet Idenix is years and years off from any product coming to market and so far Idenix track record has been to create failures. Achillion's developments have been going nicely forward.
As to a buy out, there have been a lot of recent biotech offers and buyouts recently. Things seemed to have heated up in that area again.
It is obvious why funds would be loading up on shares. Has been a good time to buy.