Swiss biotech firm

Serono

(SRA)

is bringing in a big gun to help sell its multiple sclerosis drug, Rebif, in the United States -- the pharmaceutical giant

Pfizer

(PFE) - Get Report

.

The two companies announced a marketing partnership Thursday for Rebif, which since March has been engaged in a fierce sales battle against Avonex, the rival MS drug marketed by

Biogen

(BGEN)

.

Under terms of the agreement, Pfizer, the No. 1 pharmaceutical firm in the United States, will pay Serono an up-front fee of $200 million, and will share in the drug's development and commercialization costs. In exchange, Serono will pay Pfizer an undisclosed royalty based on Rebif sales in the United States.

"We are delighted to work with Pfizer as our marketing partner for Rebif in the U.S.," said Ernesto Bertarelli, CEO of Serono, in a statement. "The combined strengths of our companies will enable us to build on the strong momentum Rebif has achieved since its launch, and to accelerate the market penetration of Rebif in the U.S."

Biogen's Avonex title as the leading MS drug in the United States is facing a serious challenge from Rebif, based on studies that show the Serono drug to be

clinically superior. It's a claim that Biogen strongly denies, but one that will ultimately be decided by the doctors who must choose which drug to recommend to their MS patients.

But already, Rebif's entry into the U.S. market has had a detrimental effect on Biogen's fortunes. The company, in early June,

reduced Avonex sales and earnings estimates for 2002, due, in part, to Rebif competition. Biogen closed Wednesday at $35.27, but is off 2.12, or 6%, in pre-market trading.

Serono has said previously that it expects 2002 Rebif sales in the United States to reach $60 million to $70 million. The company will continue to be the sole marketer of Rebif outside the United States.

Serono closed Wednesday trading at $14.33 and Pfizer at $31.03. Those prices were unchanged in pre-market trading.