LONDON (ShareCast) - Irish drug giant Elan said the risk of a fatal brain disease in patients treated with its multiple sclerosis drug Tysabri “cannot be precisely estimated.”

The group made the comments as it saw losses narrow for the half year on strong sales of the MS drug.

Shares in Elan more than halved last month after it confirmed two more cases of progressive multifocal leukoencephalopathy (PML) in patients treated with its multiple sclerosis drug Tysabri.

Tysabri was temporarily suspended in 2005 after three patients developed the potentially fatal viral disease.

But the group said today "the absolute risk for PML in patients treated with Tysabri cannot be precisely estimated."

“Patients and their physicians have chosen Tysabri because of its efficacy while being informed of the risk of PML,” it added.

Tysabri, which Elan developed in collaboration with US partner Biogen, recorded in-market sales in the first half of $359.7m, an increase of almost 200% over the $120.5m recorded in the same period of 2007.

At the end of June 2008, approximately 31,800 patients were on therapy worldwide, an increase of 127%.

It helped Elan narrow interim losses to $155.4m from $277.5m previously on revenue that increased 18% to $352.1m.

The group said Tysabri’s strong performance compensated for the reduced sales of Maxipime following the introduction of generic competition in June 2007.

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