As of March 2008, more than 26,000 patients were on
Tysabri® worldwide. Since the re-launch of
Tysabri in July 2006, no new cases of progressive multifocal leukoencephalopathy (PML) have been reported. PML is an often-fatal viral infection of the brain. Data from the PLEX study suggest that plasma exchange may be an effective procedure for accelerating the removal of Tysabri from the blood if PML infection is suspected.