Amgen

(AMGN) - Get Report

shares gained early Monday following regulatory approval to market its Enbrel drug for the treatment of psoriasis.

Approval by the Food and Drug Administration late Friday was largely expected, but is nevertheless a significant move for Amgen, pitting it against

Biogen Idec's

(BIIB) - Get Report

Amevive and

Genentech's

(DNA)

Raptiva treatments in the lucrative market. Psoriasis -- a severe and painful skin condition -- affects some 4.5 million Americans.

The company said that in a phase III study, nearly half of the patients receiving Enbrel showed positive results.

Enbrel is approved to treat moderate to severe plaque psoriasis and has been previously approved to treat four other diseases, including rheumatoid arthritis.

Amgen co-markets Enbrel in the U.S. with

Wyeth

(WYE)

.

Amgen shares rose 89 cents, or 1.6%, to $57.30