PML risk test could boost Biogen's Tysabri—if it works

What if Biogen Idec could identify the multiple sclerosis patients who are most likely to develop Tysabri's worst side effect? No more blind worry about progressive multifocal leukoencephalopathy, or PML, the potentially deadly brain infection that has struck 42 Tysabri patients since the drug came back on the market in 2006.

Well, that's just what Biogen is trying to do. Later this year, clinical trials start on a diagnostic test for the JC virus that causes PML. If the test works, it's a "game changer," one MS expert tells Bloomberg. "If Biogen can validate it, that takes out the guessing game," says Patricia O'Looney, VP at the National Multiple Sclerosis Society.

And without that guessing game, more doctors would be likely to prescribe Tysabri, even as a first-line treatment. Some 61 percent of neurologists surveyed by RBC Capital Markets said the drug would be their first choice if they knew in advance about an individual's PML risk, Bloomberg reports. Analysts hesitated to put a dollar value on it, but did say that a diagnostic that could ensure Tysabri's safety in some patients would be a "longer-term positive."

Indeed, given that Tysabri is now officially a blockbuster, with $1.1 billion in sales in 2009, a year when Biogen made regular announcements of new PML diagnoses. But first, the test has to prove itself.

- read the Bloomberg story