The Food and Drug Administration approved an eagerly awaited new multiple sclerosis drug Tuesday.

The drug, now called Tysabri, is made by Elan ( ELN) and Biogen Idec ( BIIB). Called Antegren during its development, the treatment is given intravenously once a month in a physician's office.

The approval is based on positive results seen in patients after one year of treatment, the FDA said. As part of the approval, Elan and Biogen Idec will continue clinical trials for another year.

The new drug will certainly help elevate Elan, which is trying to emerge from a financial crisis during the early part of the decade. Meanwhile, the question for Biogen -- and several competitors -- is whether the new drug will crimp sales of existing MS treatments.

In after-hours trading, BiogenIdec gained $1.22 to $58.65 and Elan rose $1.25 to $28.55.

According to the Multiple Sclerosis Association of America, some 350,000 people have been diagnosed with the disease in the U.S. An estimated 10,000 new cases are diagnosed each year, the FDA says.

The most common form of MS at the time of diagnosis is a relapsing-remitting form, in which acute symptoms or worsening of neurologic function can occur intermittently, the FDA says. The symptoms can diminish or disappear for months or years between relapses.

"The MS community is pleased that the FDA approval of Tysabri provides an additional treatment option for people with relapsing forms of MS. There are many people living with MS who may benefit from this different treatment approach," said Stephen C. Reingold of the National MS Society.

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

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