New England Journal of Medicine. A leukemia drug was about 70% more effective than a standard therapy in treating early multiple sclerosis , according to clinical trial results in today's

In multiple sclerosis, or MS, the immune system attacks myelin, the sheath that enables nerve cells to conduct impulses between the brain and other parts of the body.

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The drug, alemtuzumab, is a monoclonal antibody that depletes the body of the white blood cells that attack myelin, which are eventually replaced by new white blood cells that don't. For reasons not yet clear, though, alemtuzumab raised patients' risk of autoimmune diseases of the thyroid or platelets, and one study participant died as a result.

Closer monitoring of patients in larger trials of the drug needed before it's approved for MS should head off such problems, co-author Susan Moran, medical director at Genzyme, said Wednesday in a news conference. Genzyme co-funded the study with Bayer Schering Pharma.

In the MS trial, researchers randomly assigned 334 patients to get either alemtuzumab or Rebif, a brand of interferon beta, for three years. The alemtuzumab group received an intravenous infusion once a year, while the Rebif group injected themselves three times a week.

Participants all had relapsing-remitting MS that had been diagnosed within three years of entering the study. Relapsing-remitting MS, the initial diagnosis for 85% of people with MS, is characterized by attacks of worsening neurologic function followed by partial or complete remissions.

The researchers didn't know who was getting what treatment, but the patients did. Lead author Alasdair Coles of the University of Cambridge in England said it's impossible to prevent patients from figuring out whether they're getting alemtuzumab, because the drug has predictable, mild symptoms when infused.

Although he calls the alemtuzumab trial's results "remarkable," Stephen Hauser, a University of California-San Francisco neurologist, writes in an editorial that alemtuzumab's "toxic effects … considerably dampen any enthusiasm for its routine use in patients with multiple sclerosis until more is known about its long-term safety and sustained efficacy."

In an interview, John Richert, executive vice president for research and clinical programs at the National MS Society, noted that alemtuzumab "is many years away from being generally available as a treatment for MS."

The autoimmunity side effects "can be monitored pretty carefully," Richert said, but a larger and longer trial is needed to see whether alemtuzumab "is as strongly efficacious" as the newly published results suggest.