TysabriTwo patients taking the multiple sclerosis drug Tysabri have developed the type of brain infection that caused the drug to be temporarily pulled from the market a few years back.

Biogen Idec and Elan, which co-market the drug, reported the cases of progressive multifocal leukoencephalopathy in SEC filings yesterday.

These are the first cases of PML that have emerged since the drug was taken off the market in 2005 after three patients in clinical trials developed PML, and two died from the infection. (Two of the patients had MS; a third had Crohn’s, which the drug is also approved for.)

Both new cases were diagnosed this week in the EU. One patient, who had been taking Tysabri for 17 months, is now “clinically stable and ambulatory at home,” the SEC filing said. That patient had not taken other MS drugs. The other patient, on the drug for 14 months, is hospitalized and had previously taken other MS drugs.

U.S.-traded ADRs of Elan were down more than 40% in pre-market trading this morning, and Biogen was down more than 20%. It’s the second gut-punch in a week for Elan investors. Disappointing data on an experimental Alzheimer’s treatment Wyeth and Elan are developing clipped about 30% from Elan’s value.

More than 30,000 people take Tysabri, and the companies have said all along that a small risk of PML remains associated with the drug. Analysts have been impressed with how many patients have returned to Tysabri since it went back on the market.

The drug will probably remain on the market despite the new cases, the WSJ suggests. That means patients and their doctors will face a potentially difficult decision: Weighing the benefits and risks of a drug that eases the burden of MS but slightly increases the risk of developing a second illness.

Photo: iStockphoto