The FDA has approved the first of a new generation of multiple sclerosis treatments (search) that are based on fighting the biology behind the disease rather than just the symptoms.

The drug, which will be sold under the name Tysabri (natalizumab), is a monoclonal antibody (search) bioengineered from part of a mouse antibody that closely resembles a human antibody. Antibodies are disease-fighting protein molecules made by blood cells of the immune system.

The exact cause of multiple sclerosis (MS) is unknown. But the disease is thought to be triggered by a malfunction in the immune system that prompts a person’s immune system to attack the brain and/or spinal cord. The most common form of multiple sclerosis is a relapse-remitting form in which relapses or “attacks” of worsening function and pain appear, and then disappear, for months or years at a time.

Tysabri (search) appears to reduce the frequency of these multiple sclerosis attacks (search) by binding to a protein, called alpha-4-integrin, found on white blood cells and which plays a role in the immune system. Tysabri prevents the white blood cells from traveling to the brain, where they cause damage. The drug is given by intravenous injection once a month in a doctor’s office.

"This innovative treatment for multiple sclerosis represents a new approach to treating MS -- exciting news for patients with this serious disease," says acting FDA Commissioner Lester M. Crawford, MD, in a news release. "While we eagerly await long-term results from ongoing clinical trials, we have reason to believe that Tysabri will significantly reduce relapses in MS."

New Approach in Multiple Sclerosis Treatment

The FDA issued an accelerated approval of Tysabri based on the results of one year of treatment in two clinical trials. As part of this accelerated approval, the manufacturer of Tysabri will continue its clinical trial of the treatment for another year.

The first clinical trial of the drug’s safety and efficacy showed that Tysabri reduced the frequency of multiple sclerosis attacks by 66 percent compared with a placebo.

In the second clinical trial, people with multiple sclerosis who were taking Avonex, an interferon beta drug approved for multiple sclerosis treatment, but were experiencing relapses, were randomly assigned to receive Tysabri or a placebo.

The study showed that adding Tysabri to Avonex treatment reduced the frequency of multiple sclerosis attacks by 54 percent.

The most frequently reported serious side effects associated with Tysabri were infections, including pneumonia, temporary hypersensitivity reactions (such as rash, fever, low blood pressure, and chest pain), depression, and gallstones. These serious adverse reactions were uncommon.

Common side effects were generally mild and included nonserious infections (such as those affecting the urinary tract, lower respiratory tract, gastrointestinal system, and vagina), headache, depression, joint pains, and menstrual disorders.

Tysabri is marketed by Biogen Idec, Inc., of Cambridge, Massachusetts, and Elan Pharmaceuticals, Inc., of Dublin, Ireland.

By Jennifer Warner, reviewed by Brunilda Nazario, MD

SOURCES: FDA. News release, Biogen Idec and Elan Corporation.