Biogen Idec and Irish partner Elan Corp have said that they have no intention of suspending sales of Tysabri despite being linked to two new cases of progressive multifocal leukoencephalopathy in patients with multiple sclerosis.

In a conference call, the companies said that there are no plans to pull Tysabri (natalizumab) off the market and that the stringent monitoring programme it has put in place to check for PML, a brain infection, is adequate. The two cases, reported by PharmaTimes World News on Friday, were the first to be recorded since the reintroduction of natalizumab in the USA and approval in Europe two years ago. Biogen and Elan had voluntarily withdrawn the drug a year earlier after three patients developed PML.

Biogen and Elan went to say that the risks associated with the drug are clearly labelled and all patients are not only notified but have to sign a waiver acknowledging the risks. PML almost always occurs in people with a severe immune deficiency, as is the case with most patients taking Tysabri. Multiple sclerosis is an autoimmune disorder that results in physical and neurological damage.

Stephen McGarry, an analyst at Goldman Sachs, issued a research note saying that "we do not believe that Tysabri will be withdrawn, based on this data, but it is likely to spook investors as the true rate of PML in Tysabri monotherapy is unknown". Biogen shares ended the day down 28.3% to $50.01 while Elan's stock has gone through the floor, having lost almost 74% of their value in a week, hit by the Tysabri news as well as potential safety concerns for its experimental Alzheimer's disease drug bapineuzumab which is being developing with Wyeth.

Ian Hunter at Goodbody stockbrokers noted that the number of PML cases (ie two in 30,000-plus patients) is well below the level acceptable to the US Food and Drug Administration (one in every 1,000). "However the fact that both cases occurred in patients who have been on the drug for over a year is cause for concern", he said, though "we believe that there is still value" in Tysabri.