SHARES IN biotech group Elan jumped almost 12 per cent yesterday following a bullish update on patient numbers for its multiple sclerosis drug, Tysabri.

Addressing the 60th American Academy of Neurology conference, Elan and its US partner Biogen said about 26,000 MS patients were now being treated with the drug.

They also reported that there had been no further cases of the fatal brain disease progressive multifocal leukoencephalopathy (PML) since the drug returned to the market in June 2006. Two cases of PML led to the drug being pulled from shelves shortly after it first went on sale in 2005.

The increase in patient numbers, which was ahead of market expectations, was driven by sales outside the United States.

In the first three months of the year, 208 new Tysabri patients were beginning therapy on average each week. That is a 35 per cent increase on the previous quarter and means that, for the first time, the take-up rate for the drug is faster in the rest of the world than in the US market.

Davy analyst Jack Gorman noted that, within the US, the company was adding new patients at the same rates as in previous quarters, despite a 10 per cent increase in the number of doctors prescribing the medicine.

Analysts now expect Elan's first-quarter financial figures next week to come in at the top end of forecasts.

The companies also released results of a number of trials monitoring patients over a two-year period, showing that a significant percentage of MS patients on the therapy were "disease free".

The Affirm and Sentinel trials measured the number of relapses experienced by patients on Tysabri against a group receiving a placebo. Elan and Biogen said that 36.7 per cent of Tysabri patients remained free of any MS activity over the two-year period compared with 7.2 per cent of those on a placebo. The Sentinel figures (31.7 per cent versus 10.9 per cent) were similarly significant.

"The ultimate goal of an MS treatment is to help patients remain symptom-free for as long as possible. These data show natalizumab [ Tysabri] may do just that as about one-third of patients were shown to have no relapses, no disability progression and no new MRI markers. This is further evidence that treatment with natalizumab can result in truly dramatic outcomes for a large group of patients," said the study's lead author, Steven Galetta, professor of neurology at the University of Pennsylvania school of medicine.

In a note to clients yesterday, Goodbody analyst Ian Hunter said: "The continued increase in patient numbers on Tysabri without any sign of PML or other untoward adverse reactions continues to shift the focus away from the safety profile towards the drug's superior efficacy."