Two Ways to Attack Amyloid: Metal Chelator and Antibody
Using different approaches, two recent animal studies have reported some success toward achieving a goal common to several experimental AD therapies: both methods significantly lowered the Aβ burden in the brain.
In last month's Neuron, Ashley Bush at Massachusetts General Hospital, with Colin Masters and colleagues in Australia and elsewhere, described how the copper-zinc chelator clioquinol cut Aβ deposition in the brain by half without any apparent ill effects in AβPP2576 transgenic mice. Carrying a human familial AβPP mutation, these mice produce copious plaques, though they do not develop neuronal loss.
Also, in the July 2 early edition of PNAS, David Holtzman at Washington University School of Medicine in St. Louis, et al., introduced a new variation of AD immunotherapy. They found that intravenous injection of an Aβ antibody was able to interfere with the bidirectional transport of Aβ between the CNS and the periphery in such a way that the equilibrium was shifted toward Aβ's efflux from the CNS, thus reducing the amount of brain Aβ available for deposition.
First, consider the Neuron paper. Bush has worked out this approach systematically since the early 1990s, moving from analyzing the biochemical interaction of Aβ with copper and zinc to animal studies, and now, a clinical trial conducted in Australia, said Gunnar Gouras of Cornell University in New York, who cowrote a preview on Bush's paper in the same issue of Neuron. The approach is built around the concept that chelating copper and zinc ought to be able to break up plaques, since these metals are shown to potentiate Aβ aggregation and toxicity.
Metal chelation has for years faced some skepticism in the field, but the study addresses these concerns well, says Gouras. "This is rigorous, well-controlled work. It should be tested in the clinic and, in parallel, in further animal studies," he adds.
In this study, Bush et al. use clioquinol, an antibiotic that crosses the blood-brain barrier. They show that the drug dissolves Aβ deposits induced by copper or zinc ions in vitro, and that it liberates Aβ from postmortem brain samples of AD patients. They then found a 65 percent decrease in Aβ sediment in a pilot study of five 12-month-old mice-the age when plaques start to form in this model-and a 41 percent decrease in a larger cohort of 21-month-old mice. The authors conclude that clioquinol treatment inhibits and possibly reverses accumulation of Aβ deposits in the brains of these mice. Hydrophobic chelators "merit further investigation for their therapeutic utility, and the prevention and treatment of AD," they write.
A potential safety concern arises from evidence over the past few years suggesting that plaques may not be as toxic to neurons as is soluble Ab, possibly intracellular Aβ, or early aggregation stages such as Aβ protofibrils. This raises the question whether therapies releasing soluble Aβ from plaques may prove to be toxic. Bush addresses this concern by reporting that he found no change in synaptophysin levels (this indicator of synaptic loss correlates better with cognitive decline than do plaques), in GFAP levels (an indicator of astrocytosis), or in AβPP levels (declining AβPP levels would indicate neuronal dysfunction). His group also used a simple measurement of general well-being and survival to suggest that the treatment improved the health of the transgenic mice.
The pool of soluble Aβ did increase by 52 percent in treated mice, but the authors note that soluble Aβ constitutes only a tiny fraction of total Aβ, and that the soluble Aβ appearing after metal chelation may not be a toxic form.
The paper also recounts clioquinol's history as an antiamebic drug that was used for 20 years before being withdrawn worldwide in 1971 due to its likely association with cases of subacute myelo-optic neuropathy arising mostly in Japan. "Our current findings suggest that a reexamination of this drug and its side effects may be warranted," the authors write.
The second paper reduces Aβ by a completely different mechanism, and this approach is not yet advanced enough to include a detailed safety evaluation. In this study, Holtzman and Steven Paul at Eli Lilly and Co. in Indianapolis present a new twist to AD immunotherapies.
Searching for ways to improve Aβ clearance as a means to inhibit its deposition in the brain, they found that the monoclonal antibody m266, which binds Aβ with picomolar affinity, was able not only to sequester all Aβ present in plasma, but also to "draw" more Aβ out of the brain by removing it from the peripheral side of the central-peripheral transport equilibrium.
The study builds on previous work showing that receptor-mediated transport mechanisms at the blood-brain barrier efficiently transport Aβ from the CNS to plasma and back, and that such transport may regulate Aβ levels in brain (Shibata et al., and Zlokovic, et al.). This could, in theory, give them a handle for affecting brain Aβ from the outside, without the antibody having to enter the CNS.
After establishing m266's ability to act as an Aβ "sink" in an in-vitro dialysis system, the scientists tested the antibody in PDAβPP transgenic mice, in which a mutant human AβPP transgene results in production of human Aβ exclusively in the CNS. With an ELISA specific to human Aβ, they measured plasma and CSF Aβ before and after injecting the antibody intravenously. Several days after injection, treated mice contained 1,000 times more plasma Aβ than did controls that had not received m266. All of the Aβ was complexed with m266, suggesting that none was free to move back into the brain.
The scientists infused Aβ into the CSF of wildtype mice treated with intravenous m266, and monitored its appearance in the periphery. They detected it within minutes, and after three hours, plasma Aβ had accumulated 365-fold compared to controls. To determine whether increasing the efflux and decreasing the influx of brain Aβ could affect its deposition, the scientists treated PDAβPP mice with m266 from four months until nine months of age. One of the 14 treated mice had severe Aβ deposition compared to five of 13 and six of 14 in the two control groups.
The authors suggest that increasing Aβ clearance with a humanized parenteral antibody-or any other binding protein that could serve as a peripheral "sink"-could be useful in preventing and treating amyloid deposition in the brain.—Gabrielle Strobel
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