The next 6 years brought striking advances in understanding the biology and pathology of AD, as well as technical advances in biomarker measurements. It became possible not only to measure AB and tau in cerebrospinal fluid but also to see these proteins in living brains with specialized PET ligands. It also became obvious that about a third of subjects in any given AD study didn’t have the disease-defining brain plaques and tangles – the therapeutic targets of all the largest drug studies to date. And while it’s clear that none of the interventions that have been through trials have exerted a significant benefit yet, “Treating people for a disease they don’t have can’t possibly help the results,” Dr. Jack said.
These research observations and revolutionary biomarker advances have reshaped the way researchers think about AD. To maximize research potential and to create a global classification standard that would unify studies as well, NIA and the Alzheimer’s Association convened several meetings to redefine Alzheimer’s disease biologically, by pathologic brain changes as measured by biomarkers. In this paradigm, cognitive dysfunction steps aside as the primary classification driver, becoming a symptom of AD rather than its definition.
“The way AD has historically been defined is by clinical symptoms: a progressive amnestic dementia was Alzheimer’s, and if there was no progressive amnestic dementia, it wasn’t,” Dr. Jack said. “Well, it turns out that both of those statements are wrong. About 30% of people with progressive amnestic dementia have other things causing it.”
It makes much more sense, he said, to define the disease based on its unique neuropathologic signature: amyloid beta plaques and tau neurofibrillary tangles in the brain.