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NEUROLOGY
Compound Traces Brain Plaques in Real TimeAmyloid Ligand May Give Clinicians Needed Tool to Diagnose, Monitor Alzheimer'sThe eyes may be the windows to the soul, but positron emission tomography (PET) may be a needed window to the brain for neurologists. In the Sept. 29 online edition of Proceedings of the National Academy of Sciences, assistant professor of neurology Brian Bacskai and the John B. Penney, Jr. professor of neurology Bradley Hyman, both at HMS and Massachusetts General Hospital, describe how an organic ligand, dubbed Pittsburgh compound B (PIB), which can be tracked by PET, may soon be used to diagnose and monitor Alzheimer's disease in humans.
Kinetics of plaque labeling in a mouse model of Alzheimer's. Fluorescence from three distinct time points is superimposed to build a four-dimensional image of PIB distribution. Green, yellow, and red pseudocolors correspond to images taken 0, 5, and 15 minutes after administration of the dye. The plaque, center, is about 75 microns wide and gets labeled, over time, from the outside in. (Image Courtesy of Brian Bacskai) PIB is a fluorescent derivative of thioflavin T, a dye that has been used for many years to visualize, postmortem, the amyloid plaques that pepper the brains of Alzheimer's victims. Hyman's collaborator and co-author on the PNAS paper, William Klunk of the University of Pittsburgh, designed PIB so that it would cross the blood-brain barrier yet retain its specificity and affinity for amyloid. If the molecule lives up to expectations, it could provide a valuable tool to image Alzheimer's plaques in vivo. To test this concept Bacskai and colleagues monitored transgenic mice using multiphoton microscopy. The mice express mutant forms of human amyloid-beta precursor protein and develop senile plaques as they age, one of the hallmarks of Alzheimer's. The multiphoton microscope, which can excite fluorescent dyes with infrared light instead of dangerous ultraviolet radiation, is ideal for in vivo work, and in addition, can track fluorescence with micrometer resolution. A Window on the BrainThrough a glass window embedded in the animals' skulls, Bacskai measured the flow and ebb of fluorescence as PIB entered and cleared the mouse brains. Injecting PIB through the tail vein, he discovered that it flooded the cranium in less than a second. In less than a minute, it could be seen crossing the small capillaries that make up the blood-brain barrier, and from there, it diffused into the neuropil. "At first the speed of the delivery was quite a pleasant surprise," said Bacskai, "but in hindsight, this is exactly what the compound was designed to do."In wild type mice, PIB was rapidly cleared from the brain so that within 30 minutes of injection, fluorescence was barely detectable. In the transgenic mice, however, some of the compound bound tightly to blood vessels and small diffuse hot spots that continued to absorb PIB for about 20 minutes after injection. "These are senile plaques," explained Bacskai, "which get more and more fluorescent as PIB diffuses from the surrounding tissue into the dense plaque core." In contrast, PIB trapped in the blood vessels is taken up rapidly and seems to bind to cerebral amyloid angiopathies, which are patches of amyloid-beta that stick to the walls of the vessels. An In Vivo ToolAlzheimer's disease is notoriously difficult to diagnose. Though sophisticated functional and cognitive tests can help, they often fail to distinguish between Alzheimer's and other non-amyloid-based dementias, particularly frontotemporal dementia. The ability to measure plaques in vivo would not only provide clinicians with an immediate and reliable diagnosis, but over time would allow them to gauge how the disease is progressing.Of course, adding windows to human crania is not practical, and even if it were, multiphoton microscopes cannot penetrate through much tissue. PET, on the other hand, can be used to peer through the skull deep into the brain, and PIB is well suited to PET imaging because it can easily be labeled with the positron-emitting carbon-11. In addition, as the current paper shows, the biodistribution kinetics are ideal since the compound is rapidly taken up and cleared from the brain. "You want the unbound probe to be cleared away quickly, otherwise it raises the background and makes the imaging less sensitive," said Bacskai. The PIB/PET combination is therefore poised to radically change how clinicians view Alzheimer's disease.
Brian Bacskai (front right) and Bradley Hyman (front left) together with research technicians Gregory Hickey and Jesse Skoch, showed that PIB is ideally suited for imaging amyloid plaques in living mice. (Photo by Steve Gilbert) In addition, both Bacskai and Hyman see PIB as a potentially powerful research tool. "I think that clinically making a firm diagnosis of Alzheimer's is important," said Hyman, "but even more important than that is finding a way of tracking whether or not various therapeutics are making an impact on the disease." While currently there is no cure for Alzheimer's, acetylcholinesterase inhibitors are already approved for use in mild to moderate disease, and there are many other drugs in the pipeline that may help relieve symptoms. Some of these, especially certain nonsteroidal anti-inflammatories, are thought to reduce the production of amyloid-beta. The PIB/PET combination could provide valuable data on the efficacy of these drugs in real time, rather than after data can be collected post mortem. "If we can contribute in some way to reaching a point where doctors can diagnose Alzheimer's, start patients on a medicine, and then track them to see if it is working, that would be fantastic," Hyman commented. One other tracer, called DDNP, has shown promise for PET imaging of amyloid plaques. However, this molecule is highly lipophilic and does not have the same specificity for amyloid as PIB does. It is also less well characterized. Bacskai and colleagues have also gone to great lengths to ensure PIB is a reliable and safe imaging agent for use in humans. Postmortem examination of mice injected with PIB shows that it binds to amyloid alone, while toxicological studies give it a clean bill of health. The compound is currently being evaluated in human volunteers in Sweden and Pittsburgh. --Tom Fagan |
