Use the advanced search for posts by biowatch for Tysabri in 2006. I went to the FDA advisory panel meeting about whether and how Tysabri could be reintroduced. (iHub service is intermittent at the moment or I would give you specific links.)
Basically, PML is a very rare event but very lethal if someone has a weakened immune system. It used to kill a number of people with HIV.
Tysabri does benefit many patients with fewer side effects than many of the drugs out there for MS, but checking for those with PML is not routine, and doing the brain scans needed to rule it out in advance is expensive and requires equipment that is not available everywhere.
Consequently, Tysabri may not be a big seller, but it does not mean it is not effective. However, the extra screening and tracking of the drug will slow sales. They had suggested that each vial of drug be assigned to a given patient, rather than being available on site for whichever patient made it to the clinic that day, but that was viewed as too stringent.
As I recall, most MS patients tend to switch drugs every six months or so as they search for side effects they can live with.
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