Based on in vitro observations in scrapie-infected neuroblastoma cells, quinacrine has recently been proposed as a treatment for Creutzfeldt-Jakob disease (CJD), including a new variant CJD which is linked to contamination of food by the bovine spongiform encephalopathy (BSE) agent. The present study investigated possible mechanisms of action of quinacrine on prions. The ability of quinacrine to interact with and to reduce the protease resistance of PrP peptide aggregates and PrPres of human and animal origin were analyzed, together with its ability to inhibit the in vitro conversion of the normal prion protein (PrPc) to the abnormal form (PrPres). Furthermore, the efficiencies of quinacrine and chlorpromazine, another tricyclic compound, were examined in different in vitro models and in an experimental murine model of BSE. Quinacrine efficiently hampered de novo generation of fibrillogenic prion protein and PrPres accumulation in ScN2a cells. However, it was unable to affect the protease resistance of preexisting PrP fibrils and PrPres from brain homogenates, and a "curing" effect was obtained in ScGT1 cells only after lengthy treatment. In vivo, no detectable effect was observed in the animal model used, consistent with other recent studies and preliminary observations in humans. Despite its ability to cross the blood-brain barrier, the use of quinacrine for the treatment of CJD is questionable, at least as a monotherapy. The multistep experimental approach employed here could be used to test new therapeutic regimes before their use in human trials.Transmissible spongiform encephalopathies are a group of neurodegenerative disorders including sporadic, genetic, and acquired forms of Creutzfeldt-Jakob disease (CJD) in humans, scrapie in sheep, and spongiform encephalopathy in cattle (bovine spongiform encephalopathy [BSE]). These diseases are characterized by the accumulation of a pathological form of the cellular prion protein (PrPc), called scrapie prion protein (PrPres), in the central nervous system and, in many instances, in the lymphoreticular system. PrPres shows several differences from PrPc: a high percentage of -sheet secondary structure, resistance to proteolysis, insolubility in detergents, and a propensity to polymerize into amyloid-like fibrils (4, 27, 28). The disease-related form of PrPc, PrPres, is the only specific molecular marker of the infection, and the inhibition of its accumulation is often used to evaluate the efficacy of therapeutic drugs.To date, several compounds have been described which decrease the PrPres concentration in different scrapie-infected cell lines or prolong the incubation period in animal models. These drugs belong to different classes, including sulfated