WASHINGTON (Reuters) - The brain infection risk from Biogen Idec's multiple sclerosis drug Tysabri appears highest during the third year of treatment, U.S. health officials said in an updated warning released on Friday.
The potentially fatal infection, known as progressive multiform leukoencephalopathy (PML), occurs in an estimated 1.5 per 1,000 patients treated with Tysabri during months 25 to 36, the Food and Drug Administration said. The drug is given through a monthly infusion.
Biogen withdrew the drug in 2005 after the first reports of the brain infection. Tysabri returned to the market with restrictions in 2006.
PML concerns have crimped use of the drug, which generates sales of more than $1 billion a year.
The FDA said on Friday the PML risk was 0.3 per 1,000 patients during the first two years of treatment. After three years, the rate was 0.9 per 1,000. Limited data is available beyond four years.
The estimates have been added to the prescribing instructions for Tysabri, which Biogen makes with Irish drugmaker Elan Corp Plc.
The FDA said it was the first time it had estimated the chances of PML for specific time intervals rather than providing a cumulative risk over years. The new estimates "will allow prescribers to better assess risk based on duration of treatment", the agency said.
Biogen spokeswoman Kate Weiss said the figures reinforced the company's view that PML risk increased after two years of treatment. The total number of cases worldwide stands at 111 as of April 1, she said.
The FDA "continues to believe that the benefits of taking Tysabri outweigh the potential risks", according to a notice posted on the agency website here
Annual sales of Tysabri, the brand name for the drug natalizumab, are expected to reach nearly $1.5 billion in 2015, according to Thomson Reuters consensus forecasts.
Reporting by Lisa Richwine; Editing by Leslie Adler and Dale Hudson