The Food and Drug Administration has approved label changes for Rebif, a multiple sclerosis drug co-marketed in the U.S. by Pfizer ( PFE) and Serono Inc., a decision that could impact Biogen's ( BGEN) rival treatment Avonex. Pfizer and Serono, the U.S. affiliate of Switzerland's Serono S.A. ( SRA), said the label changes include an update that uses data directly comparing Rebif with Avonex. Those data showed that MS patients treated with Rebif were more likely to remain relapse-free after 24 and 48 weeks than were patients treated with Avonex, Pfizer and Serono said. The expanded Rebif label contains the results from a study of 677 patients with relapsing remitting MS. According to Pfizer and Serono, the study found that 75% of patients who received Rebif didn't have a relapse after 24 weeks of treatment, compared with 63% of patients treated with Avonex. At 48 weeks, 62% of Rebif patients were relapse-free vs. 52% of those treated with Avonex. According to Biogen's Web site, Avonex is used by more than 120,000 patients. The drug was launched in the U.S. in 1996 and later in Europe for the treatment of relapsing forms of MS to slow the progression of disability and reduce relapses. Avonex is marketed internationally in more than 65 countries. For the first quarter of this year, Biogen's revenue totaled $321 million. Avonex posted worldwide sales of $274 million, including $189 million in the U.S. The label change is just the latest development in the battle between the makers of the MS drugs, and this certainly isn't the first time Rebif's marketers have proclaimed their product superior to Avonex. In March 2002, just after Rebif was approved in the U.S., Biogen asked the FDA to stop Serono from saying that its drug was better than Avonex. Serono signed up Pfizer last summer to help market Rebif in the U.S. In early trading, shares of Biogen were down 56 cents to $38.30. Serono was up 15 cents at $13.75.