Prionic attack: NMR characterises small molecule inhibitors

Prion screen

Alejandra Gallardo-Godoy, Joel Gever, Kimberly Fife, Michael Silber, Stanley Prusiner and Adam Renslo of the University of California, San Francisco explain how protein misfolding diseases are a family of debilitating neurological disorders associated with the misprocessing of cellular proteins into alternate non-native forms that are toxic and/or lead to deposits. Alzheimer's disease, Huntington's disease, Parkinson's disease, frontotemporal dementias and the prion diseases are all well-known examples of diseases in which protein misfolding occurs. Creutzfeldt-Jakob disease is an example of a human prion disease resulting from misfolding of the prion protein (PrPC) and it was Prusiner who won the "Nobel Prize in Physiology or Medicine" in 1997 for the discovery of these non-viral, non-microbial pathogenic agents.

Prions are converted by a still unknown biochemical or physical mechanism into a beta-sheet-rich form of the protein, which is resistant to the enzyme protease that would otherwise break down such misfolded proteins before they could cause harm in the body. According to Prusiner and colleagues the conversion process can occur spontaneously, result from inherited mutations, or be triggered by "infection" with prion proteins from exogenous sources. Prion diseases are usually fatal and no treatment is currently available. The UCSF team hopes to improve this situation.

Prion mechanistics

Indeed, no compounds have yet demonstrated animal efficacy against a broad range of prion strains, according to Prusiner and colleagues. The team suggests there might be two reasons for this, the first is that most of the compounds tested so far were developed to address other problems, such as malaria or hyperlipidema and so are not optimised to inhibit the very distinct process of prion propagation. Secondly, the majority cannot cross the blood-brain barrier and so would have no activity against a brain disease.

The team has, however, now described a class of small molecules that shows promise. The researchers tested a library of more than 10,000 of these compounds during the last year in the search for a drug lead. The initial screen pulled out a few dozen likely candidates and early positive signs of success in this mass screening approach came from research in laboratory mice and cell cultures, which led to a small group of target compounds that might be optimized for therapeutic efficacy and tested further. These aminothiazole-containing compounds appear not to diminish expression of the normal form of the prion protein, nor to denature the pathogenic prions once formed. This, the team says, suggests that this class of compounds likely works by inhibiting prion formation or enhancing clearance from the cultured neuroblastoma cells used in the studies.

By synthesizing new analogues of the original aminothiazole lead compounds, the UCSF team was able to identify improved analogues that reached high brain concentrations in mice after three days of dosing in a liquid rodent feed containing the aminothiazole compound, Renslo told SpectroscopyNOW. Structures of the newly synthesized aminothiazoles were determined using proton NMR spectroscopy, the team says. Structure-activity relationships revealed in the study suggest that that aminothiazoles act at a defined molecular target, although this has yet to be identified. Future work will examine the mode of action of the compounds and also attempt to discover whether they can reduce symptoms or extend lifespan in afflicted laboratory rodents.

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