* Applications to add anti-JC virus test to drug's label
* JC virus status potential factor in brain infection risk
* Elan shares up 1 percent
(Adds details, background)
LONDON, Dec 22 (Reuters) - Elan and Biogen Idec have asked regulators for a green light to change the label on multiple sclerosis drug Tysabri to recommend a virus test that may help predict the risk of serious brain infection.
The test can identify an antibody to the JC virus, which the two drugmakers said was a potential factor contributing to the risk of patients on the drug developing progressive multifocal leukoencephalopathy (PML).
"These filings are part of the companies' ongoing commitment to actively identify and address potential risk factors for PML in Tysabri-treated patients," the companies said in a statement on Wednesday.
Shares in Ireland's Elan were 1 percent higher in a flat European drugs sector by midsession following news of the submissions to the U.S. Food and Drug Administration and the European Medicines Agency.
Biogen last week reported four more cases of PML among patients on Tysabri, bringing the total number of confirmed cases as of Dec. 2 to 79, with 16 deaths.
Biogen, which markets the drug with Elan, said the rate of PML cases per 1,000 patients had edged slightly higher to 1.0 from 0.96 and was likely due to more patients being on the drug for more than two years.
The chances of contracting PML tends to increase the longer a patient is on the drug, which is considered the most important growth driver for both Biogen and Elan. The incidence of PML cases per 1,000 patients who have received between 1 to 12 infusions of Tysabri is just 0.01, but that rises to 2.05 at more than 24 infusions, according to Biogen's statistics.
Tysabri was briefly pulled from the U.S. market due to concerns over PML. But the drug was so effective compared to other available treatments that MS patients considered the risk worth taking and clamoured for its return.
Health regulators agreed to make it available with certain restrictions, including close monitoring and reporting of PML cases. (Reporting by Ben Hirschler; Editing by Jon Loades-Carter)