Biogen Idec Inc. (BIIB) disclosed 10 more cases of a rare brain infection among multiple sclerosis patients taking Tysabri, a medication it sells with Elan Corp. (ELN, ELN.DB), bringing the total number of affected patients to 95 as of Feb. 2.

The Weston, Mass., biotech company reported that four more of those patients have died, bringing the total to 20.

The cases of the infection--known as progressive multifocal leukoencephalopathy, or PML--are closely watched as the MS market becomes increasingly competitive.

The drug is generally regarded as highly effective, but mostly used for patients that have stopped responding to other drugs or have aggressive cases of the disease. Sales rose 16% in 2010 to $1.2 billion and 56,600 patients were on the drug at the end of December.

Regulators have said that they watch the cases of PML, but have concluded that the benefits of the medicine to MS patients outweigh the risks. Tysabri was withdrawn from the market in 2005 and relaunched in 2006 with a strict access plan that monitors patients.

There is hope that the development of a blood test may better determine the chances of patients contracting PML. Although the overall risk of the infection remains small, it can have devastating and permanent effects, so a test may make patients and physicians more comfortable with using the drug. Biogen and Elan have filed with the Food and Drug Administration to update the label of Tysabri to include information about certain antibodies in the blood being a PML risk factor.

The overall global PML rate is now at 1.16 per 1,000 patients. Although that is above the typical one-in-1,000 rate commonly cited from clinical trials of the drug, a Biogen spokeswoman said the current PML figure falls within that seen in the trials' confidence interval, a statistical tool that helps show the precision of a measurement.

The rate is about 1.64 cases per 1,000 patients on the drug for between two and three years. The incidence is about 0.41 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. The rate drops to 1.06 per 1,000 patients for those on the drug between three and four years.

-By Thomas Gryta, Dow Jones Newswires; 212-416-2169;

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