Amgen Inc (AMGN.O) said it plans buy back $5 billion of its shares, or about 10 percent of outstanding stock, in a modified Dutch auction, which allows shareholders to choose the price at which they are willing to sell.

The biotechnology company said it would pay between $54 and $60 for each share, sending its stock up nearly 5 percent to $57.81 in midday Nasdaq trading.

The buyback is part of a $10 billion repurchase program announced last month, but the accelerated timing and aggressiveness came as a relative surprise to investors, according to RBC Capital Markets analyst Michael Yee.

Deutsche Bank analyst Robin Karnauskas said she had expected Amgen to buy back a total of just $3.1 billion in shares by the end of next year.

Under the auction, Amgen will determine the lowest price per share that enables it to buy the $5 billion total and will pay the same price for all purchased shares.

Amgen said it is issuing senior notes to help the repurchase program, which will commence on Tuesday and expire on December 7.

Debt rating agencies Moody's and Fitch downgraded Amgen following the announcement, but Standard & Poor's affirmed its ratings.