Shares in Elan added nearly 3 per cent yesterday after the company said that Tysabri, its suspended multiple sclerosis treatment, had passed another safety hurdle in its bid to return to the market.

The latest safety study of Tysabri in patients with Crohn's disease and rheumatoid arthritis found no new confirmed cases of progressive multifocal leukoencephalopathy (PML), a potentially fatal brain disorder.

This completes the safety review of all 3,000 patients on Tysabri, which was withdrawn earlier this year after three confirmed cases of PML were reported, two of which proved fatal. The first safety study of the drug, in patients with multiple sclerosis, was completed in early August and also found no further cases of PML.

Shares in the company surged on the news, adding more than 10 per cent in early trading in Dublin before later giving up most of their gains to close at €6.80, a rise of 2.7 per cent.

In New York, they added 1.85 per cent to $8.25 (€6.86).

"This is a positive step forward for the return of Tysabri to the market," said NCB analyst Orla Hartford.

Ms Hartford said Tysabri would return to the market for use on its own as a monotherapy, while the PML risk will be mitigated by screening patients for immunosuppression prior to use of Tsyabri.

Analysts believe that the next step for Elan to get Tysabri back on the market is the US Food and Drug Administration's (FDA) response to Elan's request for a priority review of its licence application, submitted last month.

The company has requested a priority six-month review of the drug, rather than the standard 10-month review, and the FDA is due to decide by October 26th whether it will grant this request.

"Acceptance would be a positive signal for its return to the market," Goodbody analyst Ian Hunter said.

If the FDA agrees to the priority review, a decision on Tysabri's fate is expected by March next year, possibly allowing the drug to return to the market in the second quarter. Elan and Biogen Idec, its partner in developing Tysabri, said more than 1,500 patients with Crohn's disease or arthritis were eligible for the safety evaluation. Of these patients, 88 per cent participated in the evaluation, with 98 per cent of them examined by a neurologist and given an MRI scan.

Elan and Biogen also said that they had submitted data, similar to that submitted to the FDA, to the European Medicines Agency (EMEA) on Tysabri.