The companies earlier reported three confirmed cases of PML, two of which were fatal.

The ongoing safety evaluation in Crohn's disease and rheumatoid arthritis is on track to be completed by the end of the summer, and the companies anticipate making submissions to regulatory authorities in early fall of 2005.

They are taking preliminary steps to restart clinical trials in MS.

Unmet Need in MS

More than 2,000 MS patients from clinical trials were eligible for the safety evaluation. To date, 91% of these MS patients participated in the safety evaluation. The remaining 9% of patients did not participate in the safety review, the companies said.

A total of 99% of patients participating in the evaluation visited their treating physician and had a neurological exam. In addition, 98% of participants had an MRI exam. The safety evaluation also included the review of any reports of potential PML in patients receiving Tysabri in the commercial setting.

"Our ongoing Tysabri safety evaluation is a rigorous medical and scientific undertaking that has been led by some of the world's leading experts in neurology and neuroradiology," said Whaijen Soo, MD, PhD, senior vice president, Medical Research, Biogen Idec.

"Given the high unmet medical need in MS and the therapeutic benefit we have seen with Tysabri, we are encouraged by these safety findings," Soo added.

Benefit-Risk Profile

"The findings announced today are an important milestone in understanding the appropriate benefit-risk profile for Tysabri," said Lars Ekman, MD, PhD, executive vice president and president, Research and Development, Elan. "Patient safety remains our top priority."

Biogen Idec and Elan voluntarily suspended Tysabri from the US market and halted clinical trials of the drug on February 28, 2005, based on reports of PML, a rare and potentially fatal demyelinating disease of the central nervous system.

The companies are continuing their safety evaluation concerning Tysabri and any possible link to PML and will discuss their results with regulatory agencies to determine the appropriate path forward.