Amgen Inc. CEO Kevin Sharer, who has been at the helm of the biotech giant since 2000, will step down in May and then retire at the end of 2012, the Thousand Oaks biotech giant said.

Sharer will stay on as chairman until Dec 31, 2012, and then will retire, Amgen said in a news release Thursday. Current President and Chief Operating Officer Robert Bradway will take over as chief executive.

Amgen’s stock was up 3.53 percent to $60.69 in early afternoon trading Friday.

Sharer, who has been with the company for 20 years, oversaw significant growth during his tenure as chief executive, growing from $3.6 billion in 2000 to nearly $16 billion today. During that time Amgen saw its presence increase from 17 countries to 55 countries, the Valley-area firm said.

In October, Amgen announced plans to lay off about 380 research and development staff, including 226 in Thousand Oaks, as it restructures it R&D division to focus on drugs in the expensive late stages of development. That number represents about 2 percent of the company’s global workforce.

In a statement, Board Chairman Vance Coffman thanked Sharer “for his service in leading Amgen over the past decade.” Sharer added the company is “well positioned for the future.”