New inhibitors of scrapie-associated prion protein formation in a library of 2,000 drugs and natural products

Transmissible spongiform encephalopathies (TSEs) are fatal, untreatable neurodegenerative diseases associated with the accumulation of a disease-specific form of prion protein (PrP) in the brain. One approach to TSE therapeutics is the inhibition of PrP accumulation. Indeed, many inhibitors of the accumulation of PrP associated with scrapie (PrPSc) in scrapie-infected mouse neuroblastoma cells (ScN(2)a) also have antiscrapie activity in rodents. To expedite the search for potential TSE therapeutic agents, we have developed a high-throughput screening assay for PrPSc inhibitors using ScN(2)a cells in a 96-well format. A library of 2,000 drugs and natural products was screened in ScN(2)a cells infected with scrapie strain RML (Chandler) or 22L. Forty compounds were found to have concentrations causing 50% inhibition (IC(50)s) of PrPSc accumulation of less than or equal to10 muM against both strains. Seventeen had IC(50)s of less than or equal to1 muM against both strains. Several classes of compounds were represented in the 17 most potent inhibitors, including naturally occurring polyphenols (e.g., tannic acid and tea extracts), phenothiazines, antihistamines, statins, and antimalarial compounds. These 17 compounds were also evaluated in a solid-phase cell-free hamster PrP conversion assay. Only the polyphenols inhibited the cell-free reaction, and their IC(50)s were near 100 nM. Several of the new PrPSc inhibitors cross the blood-brain barrier and thus have potential to be effective after TSE infection reaches the brain. The fact that many are either approved human drugs or edible natural products should facilitate their use in animal testing and clinical trials.