Elan and Biogen Idec have filed for approval to sell their drug Tysabri as a first-line treatment for multiple sclerosis.

The move that could boost sales of the drug.

Demand for Tysabri has been curtailed due to concerns over its association with a potentially fatal infection known as progressive multifocal leukoencephalopathy, or PML, which is caused by the JC virus.

Now, however, there is a test for the virus to predict if patients are at risk of developing PML.

This opens the possibility that Tysabri could be used more widely and at an earlier stage of treatment.

Elan and Biogen said today they had submitted applications to the US Food and Drug Administration and the European Medicines Agency seeking approval for first-line use in patients with relapsing forms of multiple sclerosis (MS) who have tested negative for antibodies to the JC virus.

The JC virus is generally harmless, but in people with weakened immune systems, such as those using immune system-suppressing drugs like Tysabri, it can lead to an increased chance of developing PML.

Tysabri use is currently limited to between 10-12% of treated MS patients, due to the risk of PML, and analysts said the hoped-for wider approval would improve uptake and send a positive signal to doctors.

Analysts said Tysabri's share of the MS market could increase to about 15% by 2015, representing sales of $2.9 billion, while today's share price for Elan implied peak sales of only some $2 billion.

Tysabri was briefly pulled from the market over PML concerns - but it was considered to be so effective, compared with other available treatments, that MS patients argued the risk was worth taking and demanded its return.

Health regulators bowed to the pressure and allowed the drug's relaunch with restrictions. Both the US and European regulators are expected to decide on the applications for first-line use later this year.