LOS ANGELES — There is limited risk of developing progressive multifocal leukoencephalopathy with the use of natalizumab, according to the results of a safety evaluation presented at the annual Digestive Disease Week.
Researchers from the Mayo Clinic in Rochester, Minn., and Cedars-Sinai Medical Center in Los Angeles evaluated patients who had taken the drug while participating in clinical trials of its use in treating Crohn's disease, multiple sclerosis, and rheumatoid arthritis.
Trials involving natalizumab (marketed as Tysabri) were halted in 2005 after there were two reports of patients who developed progressive multifocal leukoencephalopathy (PML) while taking combination therapy with natalizumab and interferon-β. A third report described a patient who was taking natalizumab alone and had previously taken the drug in combination with azathioprine.
Earlier this year, the FDA lifted is clinical hold on trials of the drug for multiple sclerosis. The agency has yet to announce a decision about wider marketing of the drug.
For the safety evaluation, the researchers screened participants in the two suspended studies and a completed 2004 study of multiple sclerosis. The majority of patients from the original studies participated in the safety evaluation.
No additional cases were found to have the JC virus, which has been associated with PML, according to the researchers. They found that the absolute risk of developing PML after taking natalizumab was about 0.1%.
Since only three patients developed PML, it was difficult for researchers to identify any risk factors for the rare disorder, said lead study author Dr. William Sandborn of the Mayo Clinic. Dr. Sandborn is a consultant for Elan, which jointly markets the drug with Biogen Idec. The companies funded the safety evaluation.
However, Dr. Sandborn noted that the patients who developed PML had taken natalizumab in combination with either interferon-β or azathioprine, and physicians were likely to use the drug as monotherapy until the risk factors were better understood.
It is unclear whether a screening strategy would be effective for PML, according to the researchers.