PRESS RELEASE: Phase II Clinical Trial Results for ADENTRI Published in Journal of the American College of Cardiology

Phase II Clinical Trial Results for ADENTRI Published in Journal of the American College of Cardiology

Cambridge, MA -- August 14, 2007 -- Biogen Idec has announced that positive results of a Phase II study of oral ADENTRI®, an A1 adenosine receptor antagonist, in stable heart failure patients have been published in today's issue of the Journal of the American College of Cardiology. Results showed that administration of oral ADENTRI for 10 days, in addition to standard heart failure therapy, was well tolerated and resulted in clinically significant increases in sodium excretion while preserving renal function.

"We are excited to have these data published showing the potential of ADENTRI in patients with heart failure. Worsening kidney function occurs commonly in these patients and it is typically associated with adverse outcomes," said Barry H. Greenberg, MD, Professor of Medicine and Director of the Advanced Heart Failure Program at University of California, San Diego Medical Center. "Our results demonstrate significant increases in salt excretion without significant adverse events on renal function, suggesting that ADENTRI may have the potential to improve the standard of treatment that is currently offered."

According to the American Heart Association (AHA), there are currently five million people that suffer from heart failure in the United States. In this patient population studies have demonstrated that renal function is one of the most important independent determinants of survival. Heart failure patients with renal insufficiency are also considered to be among the most challenging to treat. It is believed that worsened renal function is caused by a combination of pre-existing medical conditions and the chronic use of high-dose diuretics.

"Biogen Idec is committed to developing novel compounds for patients with cardiopulmonary diseases with high unmet need," said Barry Ticho, M.D., Ph.D, Senior Director of Medical Research at Biogen Idec. "We are moving forward with oral and intravenous formulations of ADENTRI for acute and chronic heart failure, as well as two additional compounds; LIXIVAPTAN for the treatment of hyponatremia in congestive heart failure and AVIPTADIL for the treatment of pulmonary arterial hypertension."

The trial was a randomized, double blind, placebo-controlled study that sought to assess the pharmacokinetics and pharmacologic effects of ADENTRI in heart failure patients.

Fifty patients were maintained on their usual medications, including ACE inhibitors and diuretics, and were dosed with placebo or one of four doses of ADENTRI, administered once daily for 10 days.

The study showed increases in sodium excretion above baseline and placebo beginning on Day 1, continuing over the 10-day dosing period. Notably, these effects were not accompanied by reductions in renal function nor substantial increases in potassium excretion. Trends toward beneficial effects in clinical measures of heart failure, including body weight, edema, and physician global assessment, were also observed. The safety evaluation did not reveal any significant concerns during dosing nor during 30 days of additional follow-up. The incidence of adverse events was similar to placebo, and higher doses of ADENTRI were not associated with an increase in adverse events. The pharmacokinetic profile was consistent with once daily dosing.

Preliminary results of this study were presented in 2003 at the annual meeting of the American Heart Association.

About Biogen Idec

Biogen Idec creates new standards of care in therapeutic areas with high unmet medical needs. Founded in 1978, Biogen Idec is a global leader in the discovery, development, manufacturing, and commercialization of innovative therapies. Patients in more than 90 countries benefit from Biogen Idec's significant products that address diseases such as lymphoma, multiple sclerosis, and rheumatoid arthritis.

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Safe Harbor/Forward Looking Statements

This press release contains forward-looking statements regarding ADENTRI®. These statements are based on Biogen Idec's current beliefs and expectations. The commercial potential of ADENTRI is subject to a number of risks and uncertainties, including the risk of unexpected delays or hurdles and the uncertainty of obtaining regulatory approval. Drug development and commercialization involves a high degree of risk.

For more detailed information on the risks and uncertainties associated with Biogen Idec's drug development and other activities, see Item 1A "Risk Factors" in Biogen Idec's most recent Form 10-Q filing with the Securities and Exchange Commission. These forward looking statements speak only as of the date of this press release, and we assume no obligation to update any forward-looking statements, whether as a result of new information, future events or otherwise.