Amgen Inc., the world’s biggest biotechnology company, won U.S. approval on Friday to market its rheumatoid arthritis drug Enbrel for treating the skin disorder psoriasis.

A U.S. Food and Drug Administration spokeswoman said Enbrel had been approved for moderate to severe psoriasis, an inflammatory condition that causes red, scaly patches of skin and afflicts more than 7 million people in the United States.

The injected drug has been sold for several years, in partnership with Wyeth, as a treatment for the painful joint disorder rheumatoid arthritis and a form of arthritis accompanied by psoriasis.

First quarter sales were $397 million. Analysts have said approval for psoriasis could add $700 million to $800 million per year to U.S. Enbrel sales.

Amgen officials could not immediately be reached for comment.