Another Tysabri patient has PML

Biogen Idec drug tied to disease since 2004

Biogen Idec Inc. reported late yesterday that another patient contracted a potentially fatal brain disease after taking the multiple sclerosis drug Tysabri, the third case since the biotechnology giant reintroduced the treatment two years ago. The news sent shares tumbling 12.1 percent in after-hours trading.

The Cambridge company originally launched Tysabri in 2004, but pulled it from the market just three months later after a few patients were diagnosed with progressive multifocal leukoencephalopathy, or PML. Two died. The company reintroduced Tysabri with its Irish partner, Elan Corp., in July 2006 with stricter prescribing guidelines - designed to minimize the risk of contracting the disease - and a warning that one in 1,000 patients could contract PML.

Three months ago, Biogen Idec said two additional patients had contracted PML, sparking fears that some doctors and patients will decide Tysabri isn't worth the risk.

But despite the chance of contracting the disease, many patients have been willing to use Tysabri to treat serious forms of MS and Crohn's disease when other treatments fail. As of the end of September, more than 35,500 patients were taking Tysabri, and Biogen Idec predicts 100,000 will use the drug by 2010.

Biogen Idec provided few details about the latest PML case, saying only the patient is receiving medical care.

Todd Wallack can be reached at twallack@globe.com.