BOSTON (Reuters) - Biogen Idec Inc said six patients taking its multiple sclerosis drug Tysabri developed a potentially fatal brain infection in December, bringing the total number of such cases associated with the drug to 85.
The biotechnology company said that as of January 7, the overall incidence of progressive multifocal leukoecephalopathy, or PML, was 1.06 per 1,000 patients, up from 1.0 in 1,000 last year.
Biogen, which makes the drug with Irish drugmaker Elan Corp Plc, temporarily withdrew Tysabri from the market in 2005 after it was first associated with the condition; it was brought back, with stricter safety warnings, in 2006.
The drug is widely considered the most effective on the market, but its sales have been crimped by concerns over PML. The risk of the condition increases with the length of treatment.
After two years of monthly infusions, the incidence of PML is now 2.13 per 1,000 patients.
The overall rate of PML in clinical trials was 1 in 1000, a benchmark that has now been exceeded.
Of the 85 cases of PML reported, 16 patients have died, while 69 are still alive with varying degrees of disability.
Biogen is developing a test that it hopes will allow doctors to screen patients to identify which might be more likely to develop PML.
Biogen shares were down $2.02 or 2.9 percent to $66.76 in afternoon New York trading, while Elan shares were down 10 cents or 1.5 percent to $6.37.
Reporting by Toni Clarke, editing by Gerald E. McCormick
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