The Food and Drug Administration (FDA) on Monday issued a public health advisory to inform patients and health care providers about the suspensions, while the agency and the manufacturer evaluate the two serious adverse events reported with its use.

Shares in Biogen Idec and its Ireland-based partner Elan reportedly have been taking a beating on Wall Street, while doctors and patients scratch their heads over what to do next.

FDA: 'Offers Great Hope'

Tysabri (nataluzimab), which received accelerated approval from FDA in November 2004, is a new approach to treating MS patients that has been on the receiving end of a lot of positive buzz.

FDA now says it supports Biogen Idec's decision to suspend marketing, as well as use of the product in clinical trials. However, the agency "continues to believe Tysabri offers great hope to MS patients," says Dr. Steven Galson, Acting Director, FDA's Center for Drug Evaluation and Research (CDER).

"Patients being treated with Tysabri should contact their physician to discuss appropriate alternative treatments while these reports are being evaluated," added Dr. Galson.

No Known Effective Treatment

FDA received a report from Biogen Idec of one fatal case and one possible case of progressive multifocal leukoencephalopathy (PML) in patients receiving Tysabri for MS. FDA was given preliminary information about these cases by Biogen, Idec on February 18, 2005. Details became available to FDA the next week.

PML is a rare, serious progressive neurologic disease usually occurring in immunosuppressed patients. There is no known effective treatment for PML.

Although the relationship between Tysabri and PML is not known at this time, because of the serious and often fatal nature of PML, FDA concurred with the company that the drug be voluntarily withdrawn from marketing and that the use of Tysabri in clinical trials be suspended until more is known.

About 8,000 Patients Have Received Tysabri

During the review of Tysabri for marketing approval, FDA conducted an intensive analysis of possible adverse events that might be related to effects of the drug on the immune system. No cases of PML were seen in the clinical trials.

However, for any approved therapy, new and unexpected adverse events may occur that were not seen in clinical trials. In the case of Tysabri, required post-marketing studies facilitated the rapid reporting and response to this new information.

According to Biogen Idec, outside of the approximately 3,000 patients who received the drug in clinical trials, approximately 5,000 additional patients with MS have received Tysabri through their primary physician. Because Tysabri was just recently approved, these patients have only received, at most, a few doses of Tysabri.

Report Adverse Events

The FDA will maintain close contact with the company during the process of understanding the relationship between Tysabri and these two serious adverse events. The company is working on ways to get additional information soon about the possible risks of PML from the patients who have received Tysabri in the clinical trials.

The FDA urges health care providers and patients to report adverse event information to FDA via the MedWatch program by phone (1-800-FDA-1088), by fax (1-800-FDA-0178) or by the Internet at www.fda.gov/medwatch/index.html.