Irish biotech group Elan and US partner Biogen Idec last night won approval for the use of their multiple sclerosis (MS) medicine Tysabri as a treatment for Crohn's disease in the US.

The Food and Drug Administration (FDA) issued its ruling last night. The approval will allow the drug's use in adult patients with moderately to severely active Crohn's disease who fail to respond to existing therapies.

Elan expects the treatment will be available to patients from the end of February.

Chief executive Kelly Martin told a healthcare conference in the US last week that Elan would target a 40,000 subset of Crohn's patients currently on therapies such as Remicade and Humira.

The conference heard existing treatments fail in up to 60 per cent of such cases, creating a $600 million (€409 million) market opportunity for Elan.

Elan and Biogen are trying to expand the use of Tysabri, which brought in $93 million in the third quarter. The FDA ruling is a significant boost for the Irish company, which sees Tysabri as a potential blockbuster drug.

Tysabri is taken by about 21,000 MS patients under prescribing restrictions because the drug has been linked to a rare, fatal brain infection.

Crohn's disease is a chronic inflammation of the intestines that affects about one million people worldwide.

Elan discovered Tysabri, and is developing the medicine as a Crohn's treatment.

An advisory panel to the FDA voted 12-3 last July to recommend approval of the added use for Tysabri for Crohn's disease, with restrictions to reduce the risk of infection. But, in October, the FDA deferred a final decision on the treatment.

A month later, European regulators rejected an appeal by Elan and Biogen of an earlier decision that the product not be approved and marketed for Crohn's.