I'd like to take the time to sympathize with Elan Pharmaceuticals over what's happened to their Alzheimer's trial. They had the initiative (and the nerve) to pick up and run with an unusual discovery: that a protein that precipitates in the brains of Alzheimer's patients, beta-amyloid, can be attacked by an immune response.
I'm not going to take sides in the "does amyloid cause Alzheimer's, or does Alzheimer's give you amyloid" controversy. Most money is on the first choice, but there's a vocal minority for the second that makes some good points. At any rate, the idea of going after amyloid deposits by raising antibodies to them was pretty gutsy.
In general, you want to think twice about raising an immune response to one of your own proteins. It's like the old rule of black magic - don't call up anything that you don't know how to send back down. It seems, though, that amyloid, weird and insoluble stuff that it is, looks useless and foreign enough that it can be treated as an invader.
The brain is also considered an immune-privileged organ. You wouldn't have high hopes for a vaccine approach to work there. But they actually cleared amyloid deposits from the brains of rodents (in a special strain bred to have amyloid problems.)
Elan took this into human trials as fast as they could. Unfortunately, they've run into what some feared might be the downfall of the whole approach. Several patients are showing signs of central nervous system inflammation. The immune response appears to have gotten out of hand.
There may be a way to fix this, but it'll be a while before anyone is able to try this approach again. A lot of ground work will have to be done first. It's a pity, because this had the potential to be a home run against a disease that's consumed minds, lives, and a vast amount of research time, money, and talent.