Biogen Idec and Elan have filed Tysabri on both sides of the Atlantic seeking approval to use the drug as a first-line therapy in a particular subset of patients with multiple sclerosis (MS).

Applications have been submitted in Europe and the US requesting that Tysabri's (natalizumab) labels be updated to include the first-line use of MS patients with relapsing forms of the condition who have tested negative for antibodies to the JC virus (JCV). 

This is because Tysabri is known to raise the risk of the viral brain infection progressive multifocal leukoencephalopathy (PML), which usually leads to death or severe disability, and infection with JCV puts patients at a greater risk of developing the condition.

If approval is given, a first-line label will allow all appropriate anti-JCV antibody negative patients the opportunity to get treatment with Tysabri at an earlier stage, regardless of the level of disease activity or prior treatment history, the firms said.

This, they stress, will give patients access to a "highly efficacious treatment that has been shown to slow disability progression by 42%-54% and reduce annualised relapse rates by 68%".

Treatment earlier in the course of the disease could potentially lead to better outcomes, said Alfred Sandrock, senior vice president, Development Sciences and chief medical officer at Biogen, and "this is an important consideration for people with MS who may want or need more efficacy", he stressed.

Because of the risk of PML, Tysabri's use is currently limited to up to around 12% of MS patients receiving treatment. But analysts at Berenberg believe the drug's share of the market could hit around 15% by 2015 if wider approval is granted, which equates to $2.9 billion, according to the Chicago Tribune.