Biogen Idec and Elan say they have dosed the first patients in a monotherapy safety extension study of their drug Tysabri, used in the treatment of multiple sclerosis.

Previously, the firms withdrew the drug from the US market and all clinical trials of Tysabri (natalizumab)after it was linked to the potentially-fatal brain disease progressive multifocal leukoencephalopathy, which caused the deaths of two patients.

Tysabri's withdrawal came as a severe blow to both firms, which were pinning their hopes on the drug for future growth, given that it had been tipped to achieve peak sales of $2 to $3 billion by 2007.

In the second half of 2005, Elan and Biogen, in collaboration with experts in MS and PML, completed a 3,000 patient safety evaluation of Tysabri which revealed no new cases of PML associated with the drug's use beyond the three which had been reported.

Then, on March 8 this year, a US Food and Drug Administration's advisory committee recommended the re-introduction of the product as a treatment for relapsing forms of the condition. Elan and Biogen say they expect a decision from the FDA before June 28 this year.