Tysabri was removed form the market following its original 2004 approval after it was shown to increase the risk of progressive multifocal leukoencephalopathy (PML), an opportunistic viral infection of the brain that usually leads to death or severe disability.
The FDA recently granted approval for reintroduction of the drug with enhanced safety warnings and a risk management plan designed to minimize potential risk of PML. Because of the increased risk of PML, Tysabri is generally recommended for patients who have had an inadequate response to, or are unable to tolerate, alternate MS therapies.
Two-year data from a trial which evaluated the drug as a monotherapy showed that treatment with Tysabri reduced the risk of disability progression by 42% and led to a 67% reduction in the annualized relapse rate compared to placebo.
Following the recent approval by the European Commission, the companies have also introduced Tysabri in several countries in Europe.