CHICAGO--(BUSINESS WIRE)--Fitch Ratings has affirmed Amgen Inc.'s (Amgen's) ratings, including the 'BBB'/'F2' Issuer Default Rating (IDR). The Rating Outlook has been revised to Stable from Negative as a result of Amgen's strong operational performance and commitment to debt leverage of 3.0x by year end. A complete list of ratings follows at the end of this release. The ratings apply to approximately $33.9 billion of debt for the pro forma May 1, 2015 debt issuance. KEY RATING DRIVERS --Amgen's