THE rare fatal disease contracted by two patients involved in clinical trials of Tysabri, Elan’s multiple sclerosis (MS) drug, will dash any hopes that it will be allowed back to the market this year and may scupper its return altogether, according to experts.

Walter Atwood, an associate professor at Brown University on Rhode Island who specialises in progressive multifocal leukoencephalopathy (PML), the disease contracted by the two men, one of whom has died, said that even if it could be shown that Tysabri alone had not been responsible, it would take time to confirm this.

“Two cases out of 500 is pretty huge. Such a representative number in the order of 2-4% would be more typical in Aids patients,” he said. “The general incidence of the disease is between one in 100,000 and one in 500,000 and this is the first time PML has ever shown up in an MS patient to my knowledge.”

Both patients were taking Tysabri with Avonex, an established MS drug developed by Biogen Idec. Confirming the second case, the companies said in a joint statement that, for now, there was no evidence that either drug taken on its own was linked to PML. Atwood said this would make little difference for Tysabri.

“Even if it can be shown to be effective on its own then that will entail a long-term trial. If we had a treatment for PML that would be one thing but we don’t,” he said.

Other medical experts cast doubt on the prospects of a quick return for the drug that was expected to produce revenues of up to $4 billion (€3 billion) a year by 2008 and turn Elan profitable by the end of 2006.

The company has $1.5 billion in cash but $650m of a $2.5 billion debt falls due in February 2008. It was banking on Tysabri to pay the bills.

Kelly Martin, Elan’s president and chief executive officer, said on Friday that he saw no reason why Tysabri would not be back on the shelves in six months’ time. He said the American Food and Drug Administration (FDA) had insisted one of the trials would consist of patients taking Tysabri and Avonex.

“All of our energy is now going into trying to understand if there was anything in the combination therapy that would have caused or triggered this,” he said. “There is a real possibility Tysabri could go it alone. Assuming we don’t find significant issues when we review the data then there is no medical or scientific reason why this product can’t be restarted.”

Tysabri had been fast-tracked by the FDA and allowed on the market after a year of final trials rather than the usual two. The two cases had not been picked up until after the drug had been licensed. The drug was being used by about 3,000 MS sufferers in America and 22 people in Ireland before its withdrawal on Monday.

Researchers familiar with Tysabri and PML said they were not surprised that PML had not shown up in the one-year data. “It is a bit like mad cow disease. You could have eaten the offending item 20 years or more before,” said Abbey Meyers, the president of the US National Organisation for Rare Disorders.

“There will always be risk but I don’t think the FDA will allow it back quickly.”