ARTICLE IN BRIEF
Proof-of-principle data show that an experimental MRI imaging agent may be able to detect oligomers, one of the earliest and most toxic molecules in Alzheimer's disease.
NEW ORLEANS—An experimental MRI agent may have the potential to image amyloid-beta (Abeta) oligomers in living brain, according to proof-of-principle data presented here in October at the Society for Neuroscience annual meeting. If, as an increasing number of Alzheimer's disease (AD) researchers believe, oligomers are the earliest and most toxic molecule in the disease, the new agent may provide a means to detect the disease at its earliest stages, and to track whether experimental therapies are clearing the protein doing the damage.
“A big problem with clinical trials is that they start too late,” said William L. Klein, PhD, professor of neurobiology at Northwestern University in Chicago. While the ability to image plaques — for instance, with the recently approved florbetapir — may aid the diagnosis of clinical AD, “what we really need to do is to try to find ways to image those toxins that actually initiate disease; plaques, although they can occur early, don't start the disease.”
Those toxins, he said, are Abeta oligomers, also known as “Abeta-derived diffusible ligands,” or ADDLs. They are composed of a small number of Abeta monomers, a property they share with Abeta fibrils. However, Dr. Klein emphasized, the two species are fundamentally different in their behavior. Fibrils quickly become insoluble, and form plaques. Oligomers, on the other hand, remain soluble, allowing them to act as diffusible ligands for their receptors at synapses. His previous work has shown that when Abeta oligomers bind to dendrites in the hippocampus, long-term potentiation — the molecular basis of learning and memory — is impaired. The changes induced by oligomers at these synapses, Dr. Klein said, provide a unifying mechanism for understanding Alzheimer's disease.
The concept that oligomers, not fibrils and plaques, are the key to AD pathogenesis is not universally accepted in the field, but certainly one critical tool in testing their importance is to be able to image them, especially in living tissue. In 2007, Dr. Klein's lab developed an antibody that binds to oligomers, and has since shown that when introduced into the CNS in a mouse model of AD, it labeled synapses where the oligomers bind, long before the development of plaques and tangles.
In the next step, they showed that the antibody could enter the CNS of a mouse brain when delivered intranasally. That step was critical for the potential development of a diagnostic, since delivery through lumbar puncture is unlikely to be an attractive option in the clinic. (Florbetapir is a small molecule that crosses the blood-brain barrier, and is administered peripherally.)
Six hours after intranasal delivery, the antibody, which was in that experiment fluorescently labeled, could be detected bound to oligomers at the dendrite in the hippocampus of transgenic mice that make Abeta, but was entirely absent from nontransgenic littermates. “So this is detecting the endogenously produced oligomers,” Dr. Klein said. “The data are amazing to me — I didn't think it would work this well.”
A POTENTIAL ADVANCE FOR THE CLINIC
In the latest iteration, and the one he hopes will be most relevant to the clinic, Dr. Klein's colleagues in nanochemistry at Northwestern University have covalently linked the antibody to an iron particle 16 nanometers in diameter, about the same size as the antibody itself. This probe shows up in an MRI scan, allowing a rapid and noninvasive means to image the presence of Abeta oligomers. So far, Dr. Klein said, the probe has been used to image oligomers in hippocampal neurons in cell culture, and can distinguish AD patients from controls in post-mortem human brain slices.
“We are a little bit past the beginning,” Dr. Klein said, with much development work remaining to be done. “The potential is ultimately to give us a probe that binds to toxin on synapses, to obtain images in living patients.”
“Our probe is exciting to us for two reasons. Ultimately we think it could play a role for confirming or establishing a diagnosis of Alzheimer's disease very early, because ADDLs show up before plaques show up,” Dr. Klein said. “I think it is also going to be very effective for development of disease-modifying drugs, because you will be able to show that your drug eliminates the toxins in the brain that actually cause dementia. I think that's a very exciting possibility.”
It may also facilitate research on the oligomer's receptor, whose identity and purpose is currently unknown. “Nobody has yet addressed the question of why ADDLs are binding to dendrites, and whether there is a normal role for them,” he said.
Perhaps most fundamentally, a probe for oligomers could help establish what their role is in Alzheimer's disease. According to Samuel E. Gandy, MD, PhD, professor of neurology and psychiatry at Mount Sinai Medical School in New York, “We can't know how important oligomers are until we have something like what Dr. Klein is trying to develop: a standard way of seeing the oligomer concentration in living brain, comparable with what we can do now for fibrillary Abeta. If the oligomer is the key molecule, watching fibrillary Abeta come and go may be the wrong thing to be looking at.”
The challenge, he said, is that there is currently no standard way among laboratories to image oligomers, and there is no method for imaging in vivo. That leaves some open questions about the relative significance of oligomers versus fibrillar forms. “I am totally prepared to be convinced, but I think we aren't there yet — we just don't have the tools. The concept could be right, but we can't prove it until we can see it.”
Neil Buckholtz, PhD, director of the Division of Neuroscience at the NIH National Institute on Aging, agrees on the potential utility of an imaging agent for oligomeric forms of Abeta. “Being able to measure the concentration of these species, especially in the human brain, is important,” he said. While the case for oligomers in pathogenesis “hasn't been absolutely nailed down,” there are a lot of data that support their importance. Having an imaging agent for them could help confirm their importance, he said, and could be critical in early intervention trials to determine if a potential therapeutic is reducing levels of oligomers.
If Dr. Klein is right, and if the antibody he has developed can penetrate the brain to image the oligomers, could it also be used to remove them? In fact, Dr. Klein said, the antibody was originally developed as a therapy, and when given chronically in AD mice, spared them from developing memory deficits. For now, that work has taken a back seat to the diagnostic developments, but ultimately, he said, it may become a “theragnostic” agent, a treatment strategy that combines therapeutics with diagnostics.
However, the hurdles would be even higher for that application, Dr. Gandy cautioned. Regarding results from recent antibody trials in AD, he said, “Bapineuzamab mostly failed. The results for solanezumab, though slightly more positive, were objectively equivocal.”
“The whole antibody concept has always been dependent on having the right antibodies, and there is always a risk with monoclonals, which are designed against one epitope, because you have to be right about the epitope,” Dr. Gandy said. And that, in a nutshell, is what Dr. Klein is counting on.