Tysabri Said to Deplete Immune-System Cells

Tysabri, the multiple-sclerosis drug withdrawn because of a link to a fatal brain infection, can deplete immune-system cells as much as does HIV, a group of researchers has concluded.

Cambridge, Mass.-based Biogen Idec Inc., which developed the drug with Elan Corp. of Dublin, Ireland, saw its shares fall nearly 6% on the news Tuesday.

Biogen and Elan plan to tell U.S. regulators next week that Tysabri should be cleared for sale because the treatment reduces MS relapses by more than two-thirds, twice the rate of existing therapies. The risk of contracting the rare nerve disorder that prompted the drug’s withdrawal last year can be managed by monitoring patients and not giving it to those with impaired immune systems, the companies said.

According to a summary posted on the website of the American Academy of Neurology, a study led by neurologists at the University of Texas found that the amount of immune-system cells in the spinal fluid of patients given Tysabri fell as low as levels seen in people infected with the virus that causes AIDS.

“This is an immunosuppressant drug and it suppresses the immune system,” said David Blaustein, a former assistant professor of medicine at Yale University who now manages healthcare investments at Suttonbrook Capital Management in New York and owns Elan shares. “It means the drug is working. It doesn’t mean Tysabri equals AIDS.”

Biogen Idec spokeswoman Amy Brockelman declined to comment until the data are presented at a meeting April 5.

Tysabri sales were voluntarily suspended in February 2005 after the drug was linked to three cases of progressive multifocal leukoencephalopathy, or PML. Two patients died from the condition.

Tysabri’s effect on the immune system may increase patients’ risk of contracting the virus that causes the disease, the researchers said in the study.

Biogen Idec shares dropped $2.85 to $47.25. Elan shares fell $1.25, or nearly 9%, to $12.70.