Biogen Idec is testing an old malaria drug in the hope that it can treat the rare but deadly brain disease linked to its multiple sclerosis drug Tysabri.

Sales of Tysabri (natalizumab), which is partnered with Elan Corp of Ireland, have been strong but have been blighted by five reported cases of progressive multifocal leukoencephalopathy in patients taking the drug since July last year. Tysabri was pulled from the market in 2005 after three PML cases were reported and reintroduced a year later when the US Food and Drug Administration said the drug’s benefits outweigh the risks.

It has now emerged that Biogen has been looking at 2,000 compounds that have proved effective in treating brain infections. As a result, it is now testing mefloquine, a malaria drug marketed by Roche as Lariam, in 40 patients with PML.

Al Sandrock, Biogen’s head of neurology research, told Bloomberg that mefloquine has been used in thousands of patients for malaria and “if it can help, why wouldn’t you use it?” He added that “we knew we didn’t have time to develop a brand-new drug from scratch, but we thought there might be a drug that’s already been approved that might be effective against the virus. We chose mefloquine because we knew it penetrated the brain very well”.

Biogen is expected to reveal more details about its efforts to deal with the problem of PML associated with Tysabri at a meeting with analysts later today.

Meantime, Biogen has acquired an option to license the rights to cancer drugs being developed by Aveo Pharmaceuticals. The latter, a privately-held firm, has developed ErbB3-targeted antibodies, tyrosine kinase receptors belonging to the epidermal growth factor receptor family.

Biogen will have an option exercisable at proof of concept to development and commercialisation rights to the ErbB3 programme for territories outside of North America. Financial terms of the transaction were not disclosed.