Amgen Inc., the world's largest biotechnology company, gained the most in 17 months after the company said it's planning a $5 billion share repurchase as part of a push to return profit to stockholders. Amgen rose 5.9 percent Monday to $58.43 at the close in New York, the biggest single-day increase since June 2010. The shares of the Thousand Oaks, California-based drugmaker have gained 6.4 percent this year. The modified Dutch auction, amounting to about 10 percent of the company, is part of a current $10 billion buyback program, Amgen said in a regulatory filing today. The drugmaker will raise debt to help fund the program, part of a commitment to return at least 60 percent of net income to investors, said Eric Schmidt, an analyst with Cowen & Co. in New York. "This is a way to give money back to shareholders at a modest premium," Schmidt said in a telephone interview today. Amgen declared its first quarterly dividend this year, of 28 cents a share.