The trial is important because the drug industry is searching for new ways to change the course of the debilitating autoimmune disease, rather than merely treating the symptoms, as most products on the market do. Multiple sclerosis is a common neurological disorder in which a patient’s immune system starts to attack the central nervous system. It affects an estimated 2.5 million people worldwide.

“Despite significant advances in MS therapy, many patients continue to experience disease activity,” said Dr. Ludwig Kappos, lead investigator for the Abbott and Biogen-funded study and head of MS research at University Hospital in Basel, Switzerland. “The MS community is eager for new treatment approaches.”

Biogen already has a well-known therapy on the market called Tysabri, proven to be effective by slowing progression of the disease in most patients studied and reducing MS flare-ups. That has helped the drug become a blockbuster that will generate more than $1 billion in annual sales for the first time this year.

But Tysabri also has a rare but deadly side effect. It has been linked to causing a fatal disease called progressive multifocal leukoencephalopathy, or “PML.”

As of June 7, Biogen said, 55 people have developed PML and 11 of them have died out of about 68,000 people on Tysabri. The U.S. Food and Drug Administration is closely monitoring Tysabri, but has maintained the drug’s benefits outweigh its risks.

In clinical trials so far with daclizumab, there are no known cases of PML, according to Abbott and Biogen. The drug is a monoclonal antibody that works by attaching itself to disease cells.

Biogen said enrollment of MS patients in the final stage clinical trial will be completed next year. Some analysts say daclizumab could be submitted to the FDA in the next three to four years for possible approval.