Patients taking an experimental multiple sclerosis drug developed by Biogen Idec Inc. (BIIB) and Elan Corp. (ELN) experienced far fewer relapses in a clinical trial, data that analysts say could give the drug a dominant market position.

The companies hope the drug, Antegren (search), will be approved by the end of November based on one-year data from a two-year trial of 942 patients.

The data, released on Monday, showed a 66 percent reduction in the relapse rate in patients taking the drug compared to patients taking a placebo. By contrast, existing therapies reduce relapse rates by 30 to 40 percent, though Antegren was not directly compared to them in a head-to-head trial.

"Almost all the signs are positive for Antegren's success," said Henry Dummett, senior health-care analyst at World Markets Research Center. "Preliminary clinical trial data have all pointed to Antegren's potential to be safer, more conveniently administered and more effective in treating MS than existing treatments."

Elan is counting on Antegren, which analysts expect to generate sales of more than $1 billion, to help it recover from a host of problems, including a regulatory investigation and a brush with bankruptcy in 2002.

Biogen is hoping the drug will take over from its multiple sclerosis (search) drug Avonex, the U.S. market leader whose growth had been slowing in the face of competition from Serono SA's Rebif.

Shares of Ireland-based Elan rose 5 percent on the New York Stock Exchange (search), 8 percent on the Irish Stock Exchange and 6 percent in London. Shares of Biogen Idec were little changed on Nasdaq.

Multiple sclerosis is an autoimmune disorder that targets the central nervous system and can cause blurred vision, weakness, poor muscle coordination, loss of memory and mental functions. It affects more than 1 million people worldwide.

Analysts expect the market for treatments to grow from about $3.5 billion currently to about $6 billion over the next few years.

Antegren is the first in a new class of treatments called selective adhesion molecule, or SAM, inhibitors. The drug is a humanized monoclonal antibody that blocks a molecule known as alpha-4 integrin and prevents inflammatory cells from escaping into the tissue of the brain.

The drug will compete with Avonex, Rebif, which Serono markets with Pfizer Inc. (PFE), Schering AG's Betaseron, and Teva Pharmaceuticals Industries Ltd.'s (TEVA) Copaxone.

While Rebif has been gaining ground against Avonex in the United States, that advance could be held in check if Biogen Idec and Elan release positive data from a second trial in which Antegren has been tested in combination with Avonex. Patients taking the combination are compared to those taking Avonex alone.

The companies declined to say exactly when they would release data from the second trial, but it will likely be around the time the drug is approved. The full results of both trials will probably be included in the drug's package label.

Andrew Galazka, head of scientific affairs at Serono, said in an interview he is still confident Rebif will be the biggest-selling multiple sclerosis drug by 2006.

The Antegren data, he said, is "incomplete and highly selective."

Analysts at ABN Amro said Serono's share price probably now reflects a realistic level of risk from Antegren based on an assumption that the new drug achieves a 30 percent market share two years after its launch.

Serious infections occurred in 2 percent of patients compared to 1 percent in the placebo group. Serious hypersensitivity occurred in 1 percent of patients.

Elan rose $1.36, or 4.88 percent, to $29.24 on the NYSE. It advanced 1.73 euros, or 8 percent, to 23.08 euros in Dublin and 1.3 euros, or 6 percent to 22.96 on the London Stock Exchange to 22.96 euros. Serono fell 2.4 percent in Switzerland and Schering fell 3 percent in Frankfurt.