With Zejula (niraparib) well on its way to blockbuster status, Glaxosmithkline plc (GSK) paid a premium for the opportunity to pick up developer Tesaro Inc. in a deal valued at approximately $5.1 billion. But while Tesaro shares (NASDAQ:TSRO) spiked nearly to the proposed price of $75 per share – a 62 percent premium to Friday's closing price of $46.38 and a 110 percent premium to Tesaro's 30-day volume-weighted average price of $35.67 – GSK investors gave the transaction a chilly reception, pushing the stock (NYSE:GSK) down 7.8 percent to close at $38.61 for a loss of $3.26.