WASHINGTON, DC (November 29, 2004) – A new biologic approach aimed at reducing the frequency of symptom flare-ups in patients suffering from relapsing forms of multiple sclerosis (MS) was licensed last week by the Food & Drug Administration (FDA).

MS is a chronic, often disabling disease of the brain and spinal cord. Natalizumab, the new product, is a monoclonal antibody bioengineered from part of a mouse antibody to closely resemble a human antibody. It is being marketed under the tradename Tysabri. The product is given intravenously once a month in a physician’s office.

According to the Multiple Sclerosis Association of America, approximately 350,000 individuals have been diagnosed with MS in the U.S., with an estimated 10,000 new cases diagnosed each year.

The most common form of MS at the time of initial diagnosis is a relapsing-remitting form, in which acute symptoms or worsening of neurologic function (referred to as “relapses,” “attacks” or “exacerbations”) occur intermittently. The symptoms can diminish or disappear for months or years between relapses.

Although the cause of MS is unknown, it is widely considered to be an autoimmune disease in which the person’s immune system attacks the brain and/or spinal cord. Tysabri appears to work by binding to these immune system cells, thus preventing them from traveling to the brain where they can cause damage.

Antibodies are proteins produced by a person’s immune system to fight foreign substances, such as infections. Monoclonal antibodies, such as natalizumab, can be produced in large quantities in cell culture in a laboratory setting. They can be designed to bind to proteins on the body’s normal cells.

By recognizing and attaching to these proteins, monoclonal antibodies can interfere with (or alter) normal or abnormal cellular responses. In this way, monoclonal antibodies may be useful in the treatment of certain diseases such as MS, according to the FDA.

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

The approval of Tysabri is based on positive results seen in patients after one year of treatment, the FDA said. The product received accelerated approval because it appears to provide substantial benefit for patients with a serious disease. As part of that approval, the manufacturer has committed to continuing its trials of this product for another year.

Tysabri was evaluated for safety and efficacy in two ongoing randomized, double-blind, placebo-controlled trials in patients with relapsing forms of MS. In the first clinical trial of the product’s safety and efficacy, the drug reduced the frequency of relapses by 66 percent relative to placebo.

In a second trial, patients who had been treated with Avonex (interferon beta-1a), an approved treatment for MS, but who had experienced one or more relapses while on Avonex, were randomized to receive Tysabri or placebo. Avonex was continued throughout the study for both groups. In this trial, natalizumab reduced the frequency of relapses by 54 percent relative to placebo.

The most frequently reported serious adverse reactions were infections, including pneumonia, temporary hypersensitivity reactions (such as rash, fever, low blood pressure, and chest pain), depression, and gallstones, the FDA said. These “serious adverse reactions” were uncommon, according to the agency.

Common adverse reactions were generally mild and included non-serious infections (such as urinary tract, lower respiratory tract, GI system, and vaginal infections), headache, depression, joint pains and menstrual disorders, the FDA said.

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