Once again Ireland’s Elan and US partner Biogen Idec have seen their share prices get hammered after reports were published claiming that their multiple sclerosis drug Tysabri (natalizumab), withdrawn earlier this year [[01/03/05a]], had caused a brain infection in yet another patient.

The Boston Globe newspaper says that a fourth patient taking the drug may have contracted progressive multifocal leukoencephalopathy – a rare and often fatal infection – and the US Food and Drug Administration has acknowledged that it is aware of the case. Elan and Biogen Idec suspended sales and clinical trials of Tysabri late in February this year after one patient died from the condition [[01/03/05a]]. Two more cases were identified in March [[31/03/05a]], [[04/03/05a]].

This is an enormous blow to Elan and Biogen as only last week the firms were talking about the possibility of getting Tysabri back on the market [[27/05/05c]]. Neither company has yet commented officially on the speculation.

What is particularly worrying for the firms is the time it has taken for the possible PML problem to be disclosed and Citigroup analyst Andrew Swanson issued a statement saying that Elan “has taken a significant credibility hit.” Elan’s chief executive, Kelly Martin, has said that an FDA investigation into the safety of Tysabri should be concluded by the end of the summer, but this latest news throws the idea of the drug returning to the market back up in the air.

Mike Booth, an analyst at Canaccord, told Bloomberg that “we just don’t know what’s going on behind the scenes,” adding that “the efficacy data is impressive, but the safety data is frankly worrying. If it is a compound that kills people, I don’t think the FDA will allow it back on the market.”