Used in conjunction with a PET scan, the radioactive "tracer" is injected into patients, where it quickly binds to sticky plaques in the brain that have long been considered a hallmark of Alzheimer's disease.
The plaques appear brighter on the scan-an image of changes in the living brain once observable only under a microscope at autopsy.
Results presented by the PET tracer's maker, Avid Radiopharmaceuticals, demonstrated at least a 97 percent agreement between the labeled brain scans and pathology examination at autopsy in the diagnosis of Alzheimer's, according to Michael Weiner, M.D., director of the Center for Imaging of Neurodegenerative Diseases at the San Francisco VA Medical Center, who attended the talk.
In a companion study examining the brains of young people not expected to have Alzheimer's, none had a positive scan. "Overall the results are very good," says Weiner, who also is principal investigator of a major government-industry research initiative to determine the best methods for observing Alzheimer's in clinical trials. "The results confirm the view that scanning with an amyloid PET scanning agent is going to detect amyloid in the brain."
Being able to "see" the plaques, made up of a sticky protein fragment called beta-amyloid, gives researchers a new window into the disease process and helps them track the effects of experimental Alzheimer's drugs on the brain.
If the new tracer is approved for marketing by the U.S. Food and Drug Administration-Avid, expects to apply within months-it also will be available to doctors around the country. There's little doubt that Avid's tracer, and perhaps similar ones in development, are a boon to Alzheimer's research. But it will take some time before doctors know what role the scan can play in answering the more immediate questions from patients worried about memory loss or other symptoms of mental decline.
What the test tells us
"We don't exactly know what the clinical use of these scans will be," says Weiner. "We don't know their predictive value."
Indeed, perhaps the most pressing research question the scans can help to answer is the precise role of the amyloid plaques themselves. They are always present in the brains of people with Alzheimer's. But do they cause the dementia and other problems connected to the disease? Some researchers are convinced the culprit is a different kind of amyloid-floating clumps-rather than hardened plaques.
It's not clear, for example, that a 75-year-old person with emerging memory problems and a positive scan showing amyloid will go on to become severely demented; more than 30 percent of older people with normal mental functions show amyloid in the brain.
But doctors might use the new test to help confirm or rule out a diagnosis of Alzheimer's in patients exhibiting some symptoms of dementia.