Elan Corp of Ireland and the USA¹s Biogen Idec have seen their shares fall again amid reports that their multiple sclerosis drug Tysabri (natalizumab), which was withdrawn from the market earlier this year [[01/03/05a]], may be linked to a fifth case of an often fatal virus of the brain.

Previously the firms had confirmed three cases of progressive multifocal leukoencephalopathy in patients taking Tysabri, two of whom died [[04/03/05a]] and the Boston Globe newspaper recently claimed that a fourth patient taking the drug may have contracted PML [[03/06/05a]]. However in the last case, Biogen Idec and Elan said that it had not been confirmed and have yet to comment on this fifth example.

Nevertheless, this is yet another kick in the teeth for the companies which had been hoping to bring Tysabri back to the market in the near future [[27/05/05c]] and had been encouraged by a report in the New England Journal of Medicine which said that it might be possible to make an earlier detection of the virus which would allow patients to recover [[10/06/05d]].

The companies are reviewing all patients¹ records for Tysabri and expect to present their findings to the US Food and Drug Administration by the end of the summer.