AVONEX(R), the #1 Prescribed Multiple Sclerosis (MS) Therapy Worldwide: New MRI Data Presented.

CAMBRIDGE, Mass. -- Initial findings presented today at the 22nd Congress of the European Committee for Treatment and Research in Multiple Sclerosis (ECTRIMS) in Madrid, Spain showed that treatment with AVONEX (Interferon beta-1a) promoted a statistically significant recovery of T1-black hole lesion volume by almost 24% (p<0.05) in patients with RRMS compared to baseline. Black holes are indicative of irreversible axonal damage and correlate strongly with development of long-term disability in patients with MS. Furthermore, AVONEX showed a statistically significant improvement in measures of brain atrophy (9.7% [p<0.0001] decrease in lateral ventricular volume in patients with RRMS compared to baseline).

The objective of this study was to evaluate the effect of AVONEX on global and focal disease as measured by multiple non-conventional and conventional MRI indices in a large sample of MS patients with either relapsing-remitting (RR) or secondary-progressive MS over 1 year. This study was an open label, single-blind, post-marketing, MRI observational study. 147 patients (131 with RRMS) were included with a mean pre-study disease duration of 11.1 years and followed for an average of 13.7 months. Patients received clinical and MRI examination at baseline and at the end of follow-up.

"This study represents one of the largest non-conventional MRI studies ever performed in patients with MS," said Dr. Robert Zivadinov, Associate Professor of Neurology and Director of the Buffalo Neuroimaging and Analysis Center, part of the Jacobs Neurological Institute, SUNY Buffalo, NY. He emphasized, "that the positive effect of AVONEX was particularly evident in patients with RRMS." This cohort represents a well-defined group of patients being on AVONEX monotherapy for a mean of 4.0 years before study entry. This study was funded by a grant from Biogen Idec.

About AVONEX

AVONEX (Interferon beta-1a) is indicated for the treatment of patients with relapsing forms of multiple sclerosis to slow the accumulation of physical disability and decrease the frequency of clinical exacerbations. Patients with multiple sclerosis in whom efficacy has been demonstrated include patients who have experienced a first clinical episode and have MRI features consistent with multiple sclerosis. Safety and efficacy in patients with chronic progressive multiple sclerosis have not been established.

AVONEX is the most prescribed treatment for relapsing forms of multiple sclerosis worldwide, with more than 130,000 patients on therapy. It was launched in the U.S. in 1996 and later in Europe for the treatment of relapsing forms of MS to slow the progression of disability and reduce relapses. AVONEX is marketed internationally in more than 90 countries. AVONEX was the first treatment approved for MS patients who have their first clinical MS attack and have a brain MRI scan that suggests MS; this use was approved in Europe in 2002 and in the U.S. in 2003.

The most common side effects associated with AVONEX treatment are flu-like symptoms including myalgia, fever, fatigue, headache, chills, nausea, vomiting, pain and asthenia.

AVONEX should be used in caution with patients with depression or other mood disorders and in patients with seizure disorders. AVONEX should not be used by pregnant women. Patients with cardiac disease should be closely monitored. Patients should also be monitored for signs of hepatic injury. Routine periodic blood chemistry and hematology tests are recommended during treatment with AVONEX. Rare cases of anaphylaxis have been reported. Please see complete prescribing information available at www.AVONEX.com.

Safe Harbor/Forward Looking Statements

This press release contains forward-looking statements regarding AVONEX that are subject to a number of risks and uncertainties. These statements are based on the companies' current beliefs and expectation. No forward-looking statement can be guaranteed. Drug development involves a high degree of risk. Factors which could cause actual results to differ materially from the companies' current expectations include: the risk that unexpected concerns may arise from additional data or analysis or that the company may encounter other unexpected hurdles. For more detailed information on the risks and uncertainties associated with Biogen Idec's drug development and other activities, see the periodic reports of Biogen Idec Inc. filed with the Securities and Exchange Commission. Biogen Idec assumes no obligation to update any forward-looking statements, whether as a result of new information, future events or otherwise.