Prion diseases are invariably fatal neurodegenerative disorders that occur in sporadic, infectious, and inherited forms, and are caused by the conversion of either wild-type or mutant versions of the cellular prion protein (PrPC) into self-propagating, misfolded conformers (collectively termed PrPSc). There is currently no clinically effective treatment for any form of prion disease. Recently, chemical screens have identified different classes of oral drugs that can significantly decrease the rate of wild-type PrPSc formation, and thereby increase disease-free survival in prion-infected animals. However, in each case, drug treatment did not cure prion infection, which eventually overwhelmed the treated animals. In almost all cases, an alternative PrPSc conformation emerged during therapy, causing prion strain adaptation and, in some cases, drug resistance. Interestingly, prions from drug-treated animals can recover their original strain characteristics and drug susceptibility during serial passage in untreated hosts. In addition, our preliminary work shows that simultaneous co-administration of two different drugs to prion-infected mice failed to create a synergistic effect due to the emergence of an unorthodox new strain that is resistant to the two-drug combination yet susceptible to both drugs alone. Taken together, these observations show that wild-type prions are highly malleable, i.e. able to switch back and forth between different PrPSc conformations in response to changes in selective pressure caused by anti-prion drug therapy. The molecular mechanism responsible for the malleability of wild-type prions is currently unknown. It is also unknown whether mutant prions, which specifically cause the inherited forms of prion disease, are as malleable as wild-type prions. The overall objectives of this proposal are to evaluate novel therapeutic strategies that rationally target prion malleability, and to study the role of cofactor molecules in drug-induced prion strain adaptation. Specifically, we will: (1) evaluate the efficacy of alternating oral drug regimens in a wild-type prion infection model; (2) determine whether cofactor selection plays a role in the mechanism by which wild-type prions acquire drug resistance; and (3) test the efficacy of oral drug regimens in new knock-in mouse models of inherited prion diseases.