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Amyloid just won’t go away. Much as aggregates of the protein in the brains of people with Alzheimer’s disease stubbornly resist attempts at removal, the idea that such deposits are fundamental to Alzheimer pathogenesis has remarkable staying power.
The amyloid hypothesis has never been proven, nor has it borne notable therapeutic fruit. “There’s a long history of people trying to find aggregation inhibitors for amyloid beta 42 [Aβ42, the form of the protein believed to be critical], but none has been successfully translated to the clinic,” says Todd Golde, director of the Center for Translational Research in Neurodegenerative Disease at University of Florida.
But we keep trying. “Our basic understanding of the disease suggests it’s a target worth pursuing,” says Jeffrey Cummings, director of the Cleveland Clinic Lou Ruvo Center for Brain Health in Las Vegas. “There’s such a tremendous need for new treatments for Alzheimer’s that we’re always looking for new leads, always open to new amyloid-related interventions.”
A closer look
Findings by an international team of researchers, reported in Science Advances in February 2016, suggest why so much energy has gone for naught, and propose a way ahead that could prevent the disease altogether. The key: a more detailed understanding of how single molecules of Aβ42 combine to wreak brain havoc.
Using chemical kinetics to measure the rate of molecular reactions, the researchers dissected the subprocesses that build the amyloid clumps typical in Alzheimer’s brains. Individual molecules of Aβ42 first connect into small, soluble aggregates, oligomers, through primary nucleation. These catalyze their own replication at increasing speed—secondary nucleation— and eventually form large fibrils.
“It’s not the total amount of aggregates that matters, but the presence of the small ones, the oligomers,” says Michele Vendruscolo, professor of chemistry at Cambridge University and senior author of the paper. “These are what kill neurons.”
To his thinking, a drug might reduce the overall amount of aggregation, by shrinking large deposits, for example, but if it spurs oligomer proliferation, “it would increase toxicity.”
The researchers used this understanding of subprocesses to screen for molecules to inhibit the generation of toxic fragments. They identified a “library” of small molecules, consisting of 164 fragments of 88 compounds shown to interact with Aβ42, cross-referencing them with a database of small molecules.
They then examined how several of these compounds affected the aggregation of Aβ42—specifically, which steps in the process they inhibited.
Although secondary nucleation, the exponential growth in aggregates that takes off once oligomers are produced, is the truly dangerous step—it’s where the generation of toxic particles outstrips the body’s ability to dispose of them—”at this point it is difficult to block. We decided a more promising strategy was to stop secondary nucleation before it starts,” Vendruscolo said. “In this paper we were looking specifically at molecules that blocked primary nucleation.”
The researchers focused on two compounds shown in earlier research to reduce amyloid aggregation and also already had FDA approval to treat other ailments. One, tramiprosate, failed to inhibit aggregation altogether in their tests. But the other, bexarotene, appears to be a robust inhibitor of primary nucleation. Added to a solution containing Aβ42, it virtually stopped generation of oligomers.
Bexarotene, a drug used to treat non-Hodgkins lymphoma, has been a compound of interest to Alzheimer’s researchers since 2012, when a paper in Science reported that it cleared brain amyloid in animals genetically modified to accumulate it, and improved their cognitive performance.
Since then, results have been inconsistent. Early this year, the first small double-blind controlled trial of bexarotene for Alzheimer’s patients reported mixed outcomes.
The finding that bexarotene inhibits primary but not secondary nucleation suggests that contradictory results obtained earlier reflect subtleties of timing, Vendruscolo says; it predicts that “bexarotene is a good preventive molecule, not a good therapeutic molecule.”
Animal experiments reported in the same paper confirmed that hypothesis. Administered during the larval stage to nematode worms (C. elegans) genetically modified to overproduce amyloid, bexarotene blocked the formation of toxic aggregates that caused progressive muscle paralysis. But it was “totally ineffective” if given once symptoms were apparent, Vendruscolo says.
The case for prevention
This suggests chemoprevention might be a more feasible goal than drug treatment of Alzheimer’s disease, he says. What he calls “neurostatins”—an analogy to the statins given to lower heart attack risk—might essentially prevent Alzheimer’s before it starts.
Such drugs could enhance our natural ability to eliminate Aβ42 oligomers that becomes compromised with age. They would need to be taken early and long-term, and be far safer than bexarotene, which promotes increase of triglycerides in the body to dangerous levels.
“This study is a proof of concept that neurostatins can exist,” Vendruscolo says. “We know there is a class of molecules that can act for prevention, and one, we believe, will eventually become available.”
For such an approach to become viable, he adds, would require a quantum leap in our ability to identify people at sufficient risk of Alzheimer’s to merit lifelong preventive treatment. “This is an open area for research,” he says, “a counterpart to the search for a neurostatin.”
“I think this is a really elegant piece of work,” says Todd Golde. “They make a compelling case that you can alter aggregation by interfering with different steps in the process, with very different results in regard to toxicity.”
The notion that oligomers are toxic, while large clumps are not, is open to question, he says, “but yes, interfering with primary nucleation makes the most sense…seeding seems to be the critical event, and as a general concept, aggregation inhibition should be most effective for primary prevention.”
Drug trials nowadays “are actually secondary prevention—patients have a headful of amyloid, even if they aren’t showing cognitive deficits… The amyloid hypothesis predicts that preventing deposition would prevent or slow down AD; it doesn’t say that removing amyloid when you’re effectively in brain failure would do any good.”
That said, “it’s a long road” from findings like these to a useful drug in the clinic, with obstacles that include translation from highly approximate animal models and a regulatory climate that demands clinical rather than biomarker improvement, Golde says.
He would like to see analyses brought closer to real-life conditions, in which Aβ42 forms aggregates in a stew of other compounds that may interact with it. “It would be interesting for the researchers to try to incorporate other factors we know are important into their assays, to see if they still provide the same readout.”
And although prevention would be ideal, “we can’t just go after it; we have to continue to think of ways to intervene at later stages of the disease,” he says.
Having it both ways
That both aims might be served by the same agents was suggested by the clinical trial of bexarotene reported in Alzheimer’s Research & Therapy in January 2016. In what the authors describe as a “proof of concept trial,” 20 patients with Alzheimer’s disease were randomized to receive bexarotene (16) or placebo (4). Brain amyloid was measured by MRI, using the tracer florbetapin, initially and after four weeks.
Overall, there was no difference between the groups, but among the seven patients who did not carry the apolipoprotein (ApoE) E4 genotype, there was a significant reduction in brain amyloid in five of six brain regions.
“There is a lot of literature to suggest that the response to bexarotene is influenced by ApoE genotype, and we specified this as part of our primary analysis,” Cummings says. “We can’t say why the effect was limited to E4 non-carriers, and if we had carried the study further, perhaps we’d have seen it in carriers too.”
Cummings speculates that the reduction in brain amyloid may reflect the ability of bexarotene to increase lipidation of ApoE, enhancing amyloid clearance across the blood-brain barrier.
This is a completely different process from the amyloid aggregation that Vendruscolo and his team found inhibited by bexarotene. “Like many drugs, bexarotene may have multiple mechanisms,” Cummings said. “They have good experimental evidence for intervention in the seeding process; we have a different window, that led us down a different pathway. They’re not mutually exclusive.”
Like Vendruscolo, Cummings thinks that bexarotene itself is unlikely to be useful in Alzheimer’s, but a similar agent may.
“I think there’s enough of a signal here that we should pursue this pathway, if not necessarily this drug,” he says.