AMAG Pharmaceuticals, Inc. shares (NASDAQ:AMAG) gained $1.61, or 11.2 percent, to close at $16 Wednesday following an unsolicited offer from New York-based hedge fund MSMB Capital Management to buy the Lexington, Mass.-based company's outstanding stock for $18 a share in cash, a 25 percent premium above the previous closing price, in a deal worth about $378 million.