Prion diseases are transmissible, invariably fatal, neurodegenerative diseases which include Creutzfeldt-Jakob disease (CJD) in humans and bovine spongiform encephalopathy and scrapie in animals. A large number of putative treatments have been studied in experimental models over the past 30 years, with at best modest disease-modifying effects. The arrival of variant CJD in the UK in the 1990s has intensified the search for effective therapeutic agents, using an increasing number of animal, cellular and in vitro models with some recent promising proof of principle studies. Here, for the first time, we present a comprehensive systematic, rather than selective, review of published data on experimental approaches to prion therapeutics to provide a scientific resource for informing future therapeutics research, both in laboratory models and in clinical studies.

Carbon monoxide (CO)-releasing molecules (CORMs), mostly metal carbonyl compounds, are extensively used as experimental tools to deliver CO, a biological ‘gasotransmitter’, in mammalian systems. CORMs are also explored as potential novel antimicrobial drugs, effectively and rapidly killing bacteria in vitro and in animal models, but are reportedly benign towards mammalian cells. Ru-carbonyl CORMs, exemplified by CORM-3 (Ru(CO)3Cl(glycinate)), exhibit the most potent antimicrobial effects against Escherichia coli. We demonstrate that CORM-3 releases little CO in buffers and cell culture media and that the active antimicrobial agent is Ru(II), which binds tightly to thiols. Thus, thiols and amino acids in complex growth media – such as histidine, methionine and oxidised glutathione, but most pertinently cysteine and reduced glutathione (GSH) – protect both bacterial and mammalian cells against CORM-3 by binding and sequestering Ru(II). No other amino acids exert significant protective effects. NMR reveals that CORM-3 binds cysteine and GSH in a 1:1 stoichiometry with dissociation constants, Kd, of about 5 μM, while histidine, GSSG and methionine are bound less tightly, with Kd values ranging between 800 and 9000 μM. There is a direct positive correlation between protection and amino acid affinity for CORM-3. Intracellular targets of CORM-3 in both bacterial and mammalian cells are therefore expected to include GSH, free Cys, His and Met residues and any molecules that contain these surface-exposed amino acids. These results necessitate a major reappraisal of the biological effects of CORM-3 and related CORMs.

Prion infection is characterized by the conversion of host cellular prion protein (PrP C ) into disease-related conformers (PrP Sc ) and can be arrested in vivo by passive immunization with anti-PrP monoclonal antibodies. Here, we show that the ability of an antibody to cure prion-infected cells correlates with its binding affinity for PrP C rather than PrP Sc . We have visualized this interaction at the molecular level by determining the crystal structure of human PrP bound to the Fab fragment of monoclonal antibody ICSM 18, which has the highest affinity for PrP C and the highest therapeutic potency in vitro and in vivo. In this crystal structure, human PrP is observed in its native PrP C conformation. Interactions between neighboring PrP molecules in the crystal structure are mediated by close homotypic contacts between residues at position 129 that lead to the formation of a 4-strand intermolecular ␤-sheet. The importance of this residue in mediating protein-protein contact could explain the genetic susceptibility and prion strain selection determined by polymorphic residue 129 in human prion disease, one of the strongest common susceptibility polymorphisms known in any human disease.Creutzfeldt-Jakob disease ͉ PrP-Fab complex ͉ monoclonal antibody ͉ prion therapeutics