CHICAGO--(BUSINESS WIRE)--Fitch Ratings has placed the ratings of Aetna Inc. (Aetna) on Rating Watch Negative. The rating action follows Aetna's recent announcement that it has reached an agreement to acquire Humana Inc. (Humana) in a cash and stock transaction valued at approximately $37 billion. A full list of rating actions follows at the end of this release. KEY RATING DRIVERS The rating action reflects Fitch's concern about Aetna's expected financial leverage metrics following the close of