Biogen Idec (NASDAQ: BIIB) and Elan Corporation, plc (NYSE: ELN) today announced that the New England Journal of Medicine published research from the companies’ global risk management program that updates the risk of TYSABRI ® (natalizumab)-associated progressive multifocal leukoencephalopathy (PML), an infrequent but serious brain infection that usually leads to death or severe disability, in people with multiple sclerosis (MS). The analysis looked at three risk factors associated with a patient’s PML risk: anti-JC virus (JCV) antibody status, use of immunosuppressant (IS) therapy prior to TYSABRI initiation, and longer duration of treatment with TYSABRI. Biogen Idec and Elan developed the quantitative risk stratification algorithm to help physicians and people with MS have more confidence in their treatment decisions when considering TYSABRI, a highly effective treatment for relapsing forms of MS. “By identifying these risk factors and incorporating them into our risk stratification algorithm, we bring the advantages of personalized medicine to MS,” said Alfred Sandrock, M.D., Ph.D., senior vice president, development science and chief medical officer, Biogen Idec. “This approach to treatment is intended to help patients better understand their individual benefit-risk profiles when considering TYSABRI as a treatment option.” About the ResearchResearchers used data from clinical studies, post-marketing sources, and an independent Swedish registry to estimate the incidence of PML among TYSABRI-treated patients. Data as of Feb 29, 2012 from 212 confirmed cases of PML among 99,571 TYSABRI-treated patients were used to develop a risk stratification algorithm based on three established risk factors for PML: anti-JCV antibody status, prior use of IS therapy, and duration of treatment with TYSABRI (one to 24 months vs. 25 to 48 months). Based on the presence or absence of these risk factors, patients were divided into distinct subgroups at lower or higher risk for the development of PML. Blood samples from 5,896 MS patients who participated in three clinical studies - AFFIRM , STRATIFY-1, and the U.S. arm of the TYSABRI Global Observational Program in Safety [TYGRIS] study - as well as blood samples from patients included in the Swedish Multiple Sclerosis registry, were tested for anti-JCV antibodies.