U.S. biotechnology Biogen Idec. Inc and Dublin-based partner Elan Corp. said that its multiple sclerosis (MS) drug Tysabri (natalizumab) has been linked to a new case of progressive multifocal leukiencephalopathy (PML) a deadly brain disease, this is the fourth detected case in 2008.

(PML) a deadly brain infection was detected in a European patient that had been taking Tysabri as a stand-alone treatment for 26 months. According to Biogen that presented a regulatory filing the patient is under physician's care.

This is the first case detected of PML for Tysabri since its market reintroduction in 2006 after Elan and its partner, Massachusetts-based Biogen Idec Inc, created a plan to limit the side effect. The product's label warns that 1 in 1,000 users can be expected to develop PML. According to Biogen’s spokeswoman Naomi Akin the drug was taken off the market during 2005. Although the bad news, the executive justifies its use and remarked the superior effectiveness of Tysabri in preventing relapses of multiple sclerosis symptoms, and a relatively low risk of PML.

Aoki said about 35,000 patients worldwide are taking Tysabri, and that the drug in clinical trials was able to produce a 68 percent reduction in relapses.

According to statistics, about 2.5 million people worldwide have MS. Tysabri is designed to suppress the immune system – in doing so, it likely subdues disease defenses that protect the brain from the PML virus.