Biogen Idec and Elan Corporation, plc have voluntary suspended the marketing of Tysabri (natalizumab), a treatment for multiple sclerosis (MS).

The companies are suspending supply of Tysabri from commercial distribution and physicians should suspend dosing of Tysabri until further notification. In addition, the companies have suspended dosing in all clinical trials, the companies jointly announced here.

This decision is based on very recent reports of two serious adverse events that have occurred in patients treated with Tysabri in combination with Avonex (Interferon beta-1a) in clinical trials. These events involve one fatal, confirmed case and one suspected case of progressive multifocal leukoencephalopathy (PML), a rare and frequently fatal, demyelinating disease of the central nervous system. Both patients received more than two years of Tysabri therapy in combination with Avonex.

The companies' actions have been taken in consultation with US FDA. Worldwide regulatory agencies are being kept informed.

The companies will work with clinical investigators to evaluate Tysabri-treated patients and will consult with leading experts to better understand the possible risk of PML. The outcome of these evaluations will be used to determine possible re-initiation of dosing in clinical trials and future commercial availability, a Biogen release said.

In total, approximately 3,000 patients have been treated with Tysabri in clinical trials of MS, Crohn's disease, and rheumatoid arthritis. To date, the companies have received no reports of PML in MS patients receiving Tysabri monotherapy or in patients with Crohn's disease or rheumatoid arthritis in Tysabri clinical trials. Biogen Idec has received no reports of PML in patients treated with Avonex alone, a product that has been on the market since 1996, the release added.