This is Part 2 of a seven-part series. See also Parts 1, 3, 4, 5, 6, 7.
30 December 2008. In a wide-ranging lecture at the 8th Eibsee Meeting on Cellular Mechanisms of Alzheimer’s Disease, held October 29 to November 1 in Germany, Todd Golde of the Mayo Clinic in Jacksonville, Florida, subsumed summaries of new data from three research areas in his laboratory under a broader theme that is coming to the fore across the field. That is, recent clinical data and some animal research are converging into a realization that anti-amyloid treatments may only succeed if given preventively, not as treatments once a person has received a diagnosis. As one example from his own ongoing research, Golde mentioned a treatment study in mice that tried to assess what might be the best window of amyloid removal.
Golde started by noting that the amyloid hypothesis remains the basis of much mechanistic and translational research in the field. Besides the much-cited genetic and subsequent animal and cell culture work, less appreciated support for the amyloid hypothesis comes from related brain amyloidoses such as the British and Danish familial dementias. In these diseases, a completely different protein, when mutated, can be cleaved to release a peptide that deposits as amyloid, leads to tau pathology, and causes dementia. No one truly knows why neurons die in Alzheimer’s or related diseases, and yet many people in academia and industry are confident that blocking Aβ aggregation will eventually have therapeutic value, Golde said.
Golde estimated that the total Aβ deposited in Alzheimer disease equals some 10 to 20 micromoles of Aβ. Based on measurements of daily turnover, this represents some two to five years’ worth of brain Aβ production. By comparison, about 10 nanomoles of Aβ accumulate in the brain of a 21-month-old Tg2576 mouse, and this reflects about a month’s worth of its brain Aβ production. In essence, APP transgenic mice without neuronal loss really model preclinical AD, not the symptomatic disease, Golde said, echoing a point Karen Ashe has been making for some time. That, then, is what scores of mouse therapeutic studies have been testing over the years, and it may also partly explain why mouse studies on a wide range of therapeutics succeed relatively easily.
One familiar treatment option is to slow Aβ production with γ-secretase inhibitors, of which one candidate is in Phase 3 trials, and a new generation of more selective inhibitors is entering Phase 1 (see related SfN/ICAD story). To ask when and for how long a patient would have to take such an inhibitor, postdoctoral fellow Pritam Das and colleagues administered the experimental γ-secretase inhibitor LY-411575 to Tg2576 mice at different ages and for different periods of time. This is relevant because amyloid deposition is thought to go through an initial, slow seeding phase, and then grow exponentially for some time before reaching a fairly stable plateau. This plateau is seen in Tg2576 mice from the age of 20 months onward, and in people for much of their years with clinical AD. Das and colleagues treated the mice for defined periods of time, either from four to seven months of age, or from seven to 10 months, or from 12 to 15 months. When examined at 15 months for brain Aβ levels and plaque load, the mice revealed that the only treatment that truly worked was the first. In other words, giving a γ-secretase inhibitor around the time of seeding held off subsequent amyloid deposition even though both the APP transgene and γ-secretase remained active after seven months of age. But once the deposition train had left the station, the break on γ-secretase had little effect. “This means that once the nucleation of amyloid formation has occurred, it is hard to stop further aggregation with secretase inhibition. And it really supports prophylactic rather than therapeutic targeting of Aβ,” Golde said. Newer γ-secretase inhibitors are entering the clinic, so far mostly for safety and pharmacokinetic/pharmacodynamic and biomarker studies (see SfN story).
γ-secretase modulators (GSMs) represent a separate therapeutic avenue, and it had a crisis this past summer. That endeavor draws on a broad consensus that a safer approach than all-out γ-secretase inhibition can be found in tweaking the enzyme complex such that it merely shifts its output toward more of the shorter forms of Aβ and fewer of the longer forms, particularly the highly fibrillogenic Aβ42. At the scientific level, this idea is widely accepted, and researchers led by Golde, Eddie Koo, Sascha Weggen, and other groups have elaborated it with the discovery of a range of compounds that either lower or increase the selective production of Aβ42. But at the clinical level, a drug identified early on in this line of research failed utterly this spring (see ICAD story). Scientists ascribed this downer to the chosen compound, to the lack of accompanying CSF biomarker data, and to having tried treatment rather than prevention. Flurizan, they decided, had really not tested the underlying notion of γ-secretase modulation in the first place, because too little of it had reached its target in the human brain. One setback later, follow-up research on second-generation GSMs, coupled with more detailed dissection of underlying mechanisms, is moving to center stage. “This is not the kiss of death for GSMs. New compounds up to 1,000 times more potent have been identified,” Golde said.
On their mechanism of action, this past June Golde, Boris Schmidt of the Technical University of Darmstadt, Germany, and a number of other groups raised eyebrows when they showed that some γ-secretase modulators do not bind the enzyme, as most drugs do, but stuck to the substrate APP. They did so, no less, smack in the middle of APP’s Aβ sequence, at amino acids 28-36. At the Eibsee conference, Golde discussed follow-up data on this work. For example, the group’s finding implied that any compound reported to interact with this part of Aβ could, in theory, be a GSM. Indeed, many substances tested, even some peptides, proved to be just that. This includes, for example, the amyloid dye X-34 and, more potently, the pharmaceutical compound GSM-1.
Curiously, when the researchers mutated the Aβ28-36 site to confirm that GSMs really bound there, they also discovered that one particular mutation of amino acid 28 shifted APP cleavage so dramatically that Aβ42 practically disappeared from the mix of cleavage products. “If you had this mutation, you would not get AD,” Golde speculated. It might be interesting to see if such APP variants exist in cognitively sharp nonagenarians or centenarians.
In all, this study to date suggests three possible mechanisms of action for GSMs. They are shown to lower Aβ42 generation, they increase the relative amount of the short Aβ forms that interfere with aggregation, and they appear to inhibit aggregation directly by getting in the way of Aβ-Aβ binding. Which of these might be most important in an AD therapy is unknown, Golde said.
Importantly, GSMs of the future, as well as other therapeutic candidates that are wending their way through preclinical testing, are facing the conundrum that they shut off an upstream cause of AD but are being tested in humans at its downstream end, years after much of the damage has been done.
Golde joined a growing chorus of investigators voicing this concern. These researchers draw an analogy with heart disease, where statins prevent catastrophic outcomes but cannot mend a heart once it is failing after years of atherosclerosis and infarctions. That, in essence, is what the current trial design asks AD drugs to do, Golde said, even though scientists now generally believe that AD has a preclinical period that includes the development of its hallmark pathologies and extensive neuronal death before patients ever see a doctor. Along a similar vein, epidemiology suggests a preventive effect for NSAIDs and indeed for estrogen and statins, yet all failed in subsequent treatment trials. “There is a paradox where we must stop the trigger, but are testing preventive drugs in treatment trials,” Golde said.
Golde brought to this audience of mostly basic scientists a discussion from the clinical world of Alzheimer disease, which struggles to get out of a Catch-22. No proven disease-modifying drugs exist to justify the expense and risk of prevention trials, yet with FDA-approvable treatment trial designs, candidate drugs (which are likely best as preventives) can’t easily be proven to modify disease (see also ACT-AD workshop; ICAD trial design story). At present, Golde noted, the pharmaceutical industry is trying to work its way out of this bind in steps: develop a safe medicine, hope to show a statistically significant effect in treatment trials even if it is very small, then retest at the MCI stage and then retest for prevention, hoping for bigger effects along the way. This could take 25 years. Initiatives such as ADNI may speed up the process by forging consensus around biomarkers for use in diagnosis and trials. Likewise, DIAN, an international network for the study of autosomal-dominant FAD mutation carriers, is gearing up to characterize their presymptomatic decade with the same longitudinal battery of biomarkers. Golde called on the audience to become engaged in these issues. One solution, he concluded, lies in public-private partnerships to lower the hurdles for prevention trials, another in developing treatments for downstream targets, i.e., targets other than amyloid.—Gabrielle Strobel.
This is Part 2 of a seven-part series. See also Parts 1, 3, 4, 5, 6, 7.