WASHINGTON — The U.S. government approved a drug Tuesday that tries a new method of attacking multiple sclerosis, an incurable disease of the central nervous system that affects 350,000 Americans.

The drug -- a monoclonal antibody produced by Cambridge, Mass.-based Biogen Idec Inc. and Irish drug-maker Elan Corp. -- is known chemically as natalizumab. During clinical trials, it was called Antegren but in the Food and Drug Administration announcement, the name was changed to Tysabri.

Earlier this month, the drug-makers reported 942 MS patients who took the drug for one year had a 66-per-cent reduction in relapses, compared with the placebo. That compares with a 30-per-cent to 35-per-cent reduction in relapses for products currently on the market. The first-year glance at the study will be followed by second-year results in early 2005.

A second trial looked at MS patients who had one or more relapses with the drug Avonex. Some received Tysabri or placebo along with Avonex. The Tysabri-Avonex treatment reduced relapse frequency by 54 per cent, compared with placebo. The most frequent serious side-effects were pneumonia, rash, fever, low blood pressure and chest pain.

The company said the MS patients in the second trial had the disease longer and were harder to treat. Most of those in the first group had been recently diagnosed.

All told, 2,800 patients with multiple sclerosis and Crohn's disease received Tysabri in placebo-controlled clinical trials. The most frequent side-effects were headache, fatigue and inflammation of the nose and pharynx.

Antibodies are proteins that the body's immune system uses to fight infection. Monoclonal antibodies are made in the lab and act just like people's own antibodies to attack foreign substances.

The FDA has approved monoclonal antibodies to battle other ailments, including cancer.

But Tuesday's approval of the monoclonal antibody treatment Tysabri was the first time the government approved the targeted therapy to treat multiple sclerosis. The approval covers intravenous delivery of the drug once a month in a doctor's office.