Biogen Idec Inc. (BIIB) disclosed five more cases of a rare brain infection in multiple sclerosis patients on Tysabri, which it sells with Elan Corp. (ELN), bringing the total number of cases to 63 as of Aug. 4.

The Cambridge, Mass., biotech company reported that the number of deaths among patients that have developed the infection--known as progressive multifocal leukoencephalopathy, or PML--remains at 12.

Sales of Tysabri are important to the future of both Elan and Biogen. The drug is considered a highly effective therapy for multiple sclerosis, but its growth has been slower than originally hoped due to concerns about the risk of PML that led to the drug's temporary withdrawal beginning in 2005.

The infection re-emerged among Tysabri patients in mid-2008, and Biogen provides monthly updates about the number cases.

The overall global PML rate is about 0.80 per 1,000 patients, the company said, which falls within the 1-in-1,000 rate previously seen in clinical trials and implied on the drug's label.

As of June 30, 52,700 patients were using the drug around the world. In total, about 71,400 patients have use the drug since its launch.

Of the total PML cases, 25 were in the U.S., 34 were in the European Union and four were in other areas. The company doesn't disclose the distribution of the 12 patient deaths.

The number of cases is important because if the infection rate climbs too high, the drug's sales growth may drop. Regulators have said that they watch the cases, but have concluded that the benefits of the medicine to MS patients outweigh the risks.

Tysabri is usually used in MS patients who don't respond to earlier therapy or in patients with aggressive cases of the disease, and is distributed under a strict risk management plan that monitors patients for PML.

The risk of the infection increases with the number of monthly infusions that he or she receives.

The most recent data update translates to a rate of 1.35 cases per 1,000 for patients on the drug for a year or longer, but rises to 1.76 per 1,000 for those on the drug for two years or longer.

Looked at another way, the rate is about 1.39 cases per 1,000 patients on the drug for between two and three years. The incidence is about 0.40 case per 1,000 patients in those using it for one to two years, and it is essentially nonexistent in patients using it for less than a year.

Shares of Biogen closed down 39 cents at $56.52 on Wednesday, while Elan's American depositary shares dropped 2 cents to $4.90.

-By Thomas Gryta, Dow Jones Newswires; 212-416-2169; thomas.gryta@dowjones.com
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