This means that in the end, amongst all those exposed to Tysabri and who underwent PML screening, there were a final total of 3 cases of PML, 2 of which were fatal. In other words, after the initial reports, no new cases have been discovered across multiple sclerosis, Crohn's and rheumatoid arthritis patients dosed with Tysabri during clinical trials. This additional data will be added to the supplementary Biologics License Application submitted to the FDA for multiple sclerosis.

With no additional cases found, certainly these findings must bolster the case for Tysabri's return as a monotherapy, though to an unknown degree. As a next milestone, the FDA will rule by October 26th whether they will grant Tysabri an accelerated review (6 months maximum) or a standard review (10 months).

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Press Release

Elan and Biogen Idec Announce TYSABRI(R) Safety Evaluation Findings in Crohn's Disease and Rheumatoid Arthritis Patients

Monday October 17, 2:30 am ET

TYSABRI Safety Evaluation Complete: No New Confirmed Cases of PML

DUBLIN, Ireland & CAMBRIDGE, Mass.--(BUSINESS WIRE)--Oct. 17, 2005--Elan Corporation, plc (NYSE: ELN - News) and Biogen Idec (NASDAQ: BIIB - News) announced today that findings from their safety evaluation of TYSABRIŽ (natalizumab) in patients with Crohn's disease (CD) and rheumatoid arthritis (RA) resulted in no new confirmed cases of progressive multifocal leukoencephalopathy (PML). The companies have previously reported that findings from their safety evaluation of TYSABRI in patients with multiple sclerosis (MS) resulted in no new confirmed cases of PML. Three confirmed cases of PML were previously reported, two of which were fatal. The TYSABRI safety evaluation is now complete.

On September 26, 2005 the companies announced that they submitted a supplemental Biologics License Application for TYSABRI to the U.S. Food and Drug Administration (FDA) for the treatment of MS. The companies also recently submitted a similar data package to the European Medicines Agency.

More than 1,500 CD and RA patients from clinical trials were eligible for the safety evaluation. A total of 88% of these patients participated in the safety evaluation. In total, 98% of the patients participating in the evaluation had a neurological exam by a consultant neurologist and an MRI exam.

On February 28, 2005, Biogen Idec and Elan announced that they voluntarily suspended TYSABRI from the U.S. market and all ongoing clinical trials based on reports of PML, a rare and potentially fatal, demyelinating disease of the central nervous system.

About Elan

Elan Corporation, plc is a neuroscience-based biotechnology company committed to making a difference in the lives of patients and their families by dedicating itself to bringing innovations in science to fill significant unmet medical needs that continue to exist around the world. Elan shares trade on the New York, London and Dublin Stock Exchanges. For additional information about the company, please visit http://www.elan.com.

About Biogen Idec

Biogen Idec creates new standards of care in oncology, neurology and immunology. As a global leader in the development, manufacturing, and commercialization of novel therapies, Biogen Idec transforms scientific discoveries into advances in human healthcare. For product labeling, press releases and additional information about the company, please visit http://www.biogenidec.com.

Contact:

MEDIA CONTACTS

Elan

Davia B. Temin, 212-407-5740

or

Elizabeth Headon, 353-1-498-0300

or

Biogen Idec

Amy Brockelman, 617-914-6524

or

INVESTOR CONTACTS

Elan

Emer Reynolds, 353 1 709 4000

or

Chris Burns, 800-252-3526

or

Biogen Idec

Oscar Velastegui, 617-679-2812

Source: Elan Corporation, plc and Biogen Idec