WASHINGTON – The 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 that 942 MS patients who took the drug for one year had a 66 percent reduction in relapses, compared with the placebo. That compares with a 30 percent to 35 percent 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 percent, compared with placebo. The most frequent serious side effects were pneumonia, rash, fever, low blood pressure and chest pain.