Ketchum is believed to have already started work on the UK PR account and has replaced Red Door Communications on the business.
Enbrel treats rheumatoid arthritis, an inflammatory disease that attacks the joints, and psoriasis, a recurrent skin condition.
The brand hit the national front pages last June when it was described by the Daily Mail as part of a 'cocktail of drugs' that could represent a major breakthrough in the battle against rheumatoid arthritis.
It was reported that a mix of Enbrel and methotrexate - a cheap, pre-existing drug - could make a successful combination to treat the condition.
Late last year, European approval was given for the use of Enbrel for the treatment of adults with moderate to severe plaque psoriasis for whom phototherapy or conventional systemic treatments have been inadequate or are inappropriate.
The product was first approved in the UK for rheumatoid arthritis around five years ago.
Wyeth would only say 'no comment' when contacted by PRWeek about the appointment. Ketchum and Red Door also declined to comment on their work on the brand, referring calls to Wyeth.
The company continues to use The Medical Multimedia Company to help market Enbrel in the UK.
Ketchum already works for Wyeth, having been hired to promote its Tygacil brand globally (including the UK) during 2004.
Enbrel was one of the drugs cited by Wyeth as helping to boost its global sales last year. Other treatments cited included the firm's popular antidepressant Effexor.