01 Feb 2008

Blocking amyloid fibrillization with small molecules is a widely studied strategy for tackling Alzheimer disease and other amyloidoses. But do some of the compounds fail to shape up? In the January 27 Nature Chemical Biology online, researchers at the University of San Francisco, California, suggest that many amyloid blockers, including three that inhibit Aβ fibrillization, work not because they specifically prevent amyloid growth, but because they form colloids themselves, which then interfere with protein-protein interactions in general. The finding will likely be heralded as both good and bad news, according to Brian Shoichet, who led the study: good because it finally clarifies how some of these compounds work, bad because it casts doubt on their suitability for drug development. “Hopefully, our study will make people more cautious and really look into the physical chemistry behind such drugs,” said Shoichet.

In addition to Alzheimer disease, amyloids represent the most prominent pathology of other fatal neurodegenerative disorders, including Parkinson’s, Huntington’s, and prion-based conditions such as Creutzfeldt-Jacob disease. Many small molecules have been discovered that prevent fibrillization of the proteins found in those amyloids. Baicalein, a flavinoid, prevents oligomerization of α-synuclein, for example (see ARF related news story), while Congo red and DAPH, or 4,5-dianilinophthalimide, block fibrillization of amyloid-β (see ARF related news story). Though these small molecules may not be structurally similar, they do share one feature—they all resemble molecules known to form “promiscuous chemical aggregates,” or colloidal particles, write Shoichet and colleagues. The similarities got the researchers wondering if these agents prevent amyloid formation by virtue of the general ability to sequester proteins, which colloids are apt to do.

To test this, first author Brian Feng and colleagues compared eight known colloidal molecules and three amyloid-β inhibitors for their ability to prevent the formation of prion amyloids. The three Aβ inhibitors tested were baicalein, DAPH, and clioquinol, a metal chelator that reduces amyloid deposition in animal models of AD and has shown some promise in a small pilot study (see ARF related news story). The UCSF researchers found that all eight known colloidal molecules, including Congo red, blocked polymerization of the yeast prion Sup35. Potency, at micromolar concentrations, ranged from about 34 percent inhibition to over 99 percent for rottlerin, Congo red, and a chemical called TIPT. Six of the compounds also inhibited amyloid formation by the mouse prion protein recMoPrP.

Given that these colloids inhibit formation of amyloids, the researchers then asked if amyloid inhibitors are, in fact, colloidal. The answer appears to be yes, at least for the three inhibitors tested. All three inhibited Sup35 polymerization, with DAPH being the most potent, followed by baicalein and clioquinol. Only the last did not prevent recMoPrP from forming amyloids. This lack of specificity alone would not indicate that colloids are at work; however, the researchers found other characteristics consistent with colloidal inhibition, as well. These include detergent, protein, and target concentration effects. The three amyloid inhibitors—and also Congo red and Direct Yellow 20, an inhibitor of huntingtin polymerization—blocked β-lactamase activity in the absence, but not in the presence of small amounts of detergent (0.01 percent Triton X-100); the addition of a secondary protein, in this case bovine serum albumin, substantially weakened or abolished inhibitory activity, and inhibition could be overcome by increasing the amount of seed in the polymerization reaction. All these effects are consistent with the presence of colloidal aggregates.

“A cautionary tale to emerge from these studies is that chemical aggregators may be common among inhibitors of amyloid fibrillization,” write the authors. In an interview with ARF, Shoichet said that this phenomenon is quite common in early drug discovery, not just for amyloid, and that people have learned over the last 5 or 6 years to look out for it. “I think the amyloid field, because it is not being dominated by traditional drug discovery, may be less aware of the problem,” he said.

The results in this paper pertain most directly to some compounds known to block Aβ fibrillization in research. Their impact on clinical drug candidates is less clear. For example, the UCSF researchers did not test tramiprosate (i.e., Alzhemed™), which appears to block formation of Aβ fibrils but failed in clinical trials (see ARF related news story), or PBT2, a second-generation experimental drug that behaves similarly to clioquinol (aka PBT1) and just completed a Phase 2a trial. The findings have no direct bearing on a range of other ongoing clinical approaches to tackling AD, such as secretase inhibitors, receptor agonists and antagonists, and antibodies for Aβ clearance.

Moreover, Shoichet made it clear that non-colloidal amyloid blockers can yet be found. In fact, Jeffery Kelly and colleagues at Scripps Research Institute, La Jolla, California, have developed 2-arylbenzoxazole derivatives that bind to monomers of transthyretin and prevent its fibrillization (see Johnson et al., 2008). The present findings also do not preclude the possibility that some compounds may be both colloids and have specific interactions with amyloidogenic proteins. But in that case, Shoichet considers it unlikely that the compounds could be successful in the clinic because of the pharmacodynamic complications associated with colloids.—Tom Fagan

Coll(o)iding With Physical Chemistry: Amyloid Inhibitors Questioned

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