Tysabri was temporarily withdrawn from the US market in 2005 after three cases of progressive multifocal leukoencephalopathy, or PML, in patients taking it in clinical trials. Two of those patients died. However, the drug reduced the annualized relapse rate in patients with MS by 67% compared to placebo in trials, making it an attractive treatment option for sufferers despite the risk.
The FDA has since allowed prescription of the drug, provided patients enroll in a trial, which monitors the safety of the drug and imposes safety conditions. One condition of the trial is that patients do not take any other immunosuppressant drug while taking Tysabri, in case the combination of drugs triggered the original PML cases.
Paul O’Connor, of St. Michael’s Hospital in Toronto, lead investigator of the
Study, said: “The findings from the safety update combined with the data showing the sustained effect of Tysabri in patients treated for up to three years, contribute to our evolving understanding of the utilization of this therapy as an important treatment option for people living with the debilitating effects of MS.”