Improved 2,4-Diarylthiazole-Based Antiprion Agents: Switching the Sense of the Amide Group at C5 Leads to an Increase in Potency

Eschenmoser coupling reactions starting from primary thioamides. When do they work and when not?

Reactions of thiobenzamide or thioacetamide with 4-bromo-1,1-dimethyl-1,4-dihydroisoquinoline-3(2H)-one, 4-bromoisoquinoline-1,3(2H,4H)-dione and two α-bromo(phenyl)acetamides were examined under various conditions (base, solvent, thiophile, temperature) and structure/medium features that influence product distribution (Eschenmoser coupling reaction, Hantzsch thiazole synthesis and elimination to nitriles) were identified. The key factor that enables the successful Eschenmoser coupling reaction involves the optimum balance in acidity of nitrogen and carbon atoms of the intermediary α-thioiminium salts.

Small Molecules with Anti-Prion Activity

Prion pathologies are fatal neurodegenerative diseases caused by the misfolding of the physiological Prion Protein (PrP^C) into a β-structure-rich isoform called PrP^Sc. To date, there is no available cure for prion diseases and just a few clinical trials have been carried out. The initial approach in the search of anti-prion agents had PrP^Sc as a target, but the existence of different prion strains arising from alternative conformations of PrP^Sc, limited the efficacy of the ligands to a straindependent ability. That has shifted research to PrP^C ligands, which either act as chaperones, by stabilizing the native conformation, or inhibit its interaction with PrP^Sc. The role of transition-metal mediated oxidation processes in prion misfolding has also been investigated. Another promising approach is the indirect action via other cellular targets, like membrane domains or the Protein- Folding Activity of Ribosomes (PFAR). Also, new prion-specific high throughput screening techniques have been developed. However, so far no substance has been found to be able to extend satisfactorily survival time in animal models of prion diseases. This review describes the main features of the Structure-Activity Relationship (SAR) of the various chemical classes of anti-prion agents.

Pharmacological Agents Targeting the Cellular Prion Protein

Prion diseases are associated with the conversion of the cellular prion protein (PrP^C), a glycoprotein expressed at the surface of a wide variety of cell types, into a misfolded conformer (the scrapie form of PrP, or PrP^Sc) that accumulates in brain tissues of affected individuals. PrP^Sc is a self-catalytic protein assembly capable of recruiting native conformers of PrP^C, and causing their rearrangement into new PrP^Sc molecules. Several previous attempts to identify therapeutic agents against prion diseases have targeted PrP^Sc, and a number of compounds have shown potent anti-prion effects in experimental models. Unfortunately, so far, none of these molecules has successfully been translated into effective therapies for prion diseases. Moreover, mounting evidence suggests that PrP^Sc might be a difficult pharmacological target because of its poorly defined structure, heterogeneous composition, and ability to generate different structural conformers (known as prion strains) that can elude pharmacological intervention. In the last decade, a less intuitive strategy to overcome all these problems has emerged: targeting PrP^C, the common substrate of any prion strain replication. This alternative approach possesses several technical and theoretical advantages, including the possibility of providing therapeutic effects also for other neurodegenerative disorders, based on recent observations indicating a role for PrP^C in delivering neurotoxic signals of different misfolded proteins. Here, we provide an overview of compounds claimed to exert anti-prion effects by directly binding to PrP^C, discussing pharmacological properties and therapeutic potentials of each chemical class.

Discovery of a novel, monocationic, small-molecule inhibitor of scrapie prion accumulation in cultured sheep microglia and Rov cells

Prion diseases, including sheep scrapie, are neurodegenerative diseases with the fundamental pathogenesis involving conversion of normal cellular prion protein (PrP^C) to disease-associated prion protein (PrP^Sc). Chemical inhibition of prion accumulation is widely investigated, often using rodent-adapted prion cell culture models. Using a PrP^Sc-specific ELISA we discovered a monocationic phenyl-furan-benzimidazole (DB772), which has previously demonstrated anti-pestiviral activity and represents a chemical category previously untested for anti-prion activity, that inhibited PrP^Sc accumulation and prion infectivity in primary sheep microglial cell cultures (PRNP 136VV/154RR/171QQ) and Rov9 cultures (VRQ-ovinized RK13 cells). We investigated potential mechanisms of this anti-prion activity by evaluating PrP^C expression with quantitative RT-PCR and PrP ELISA, comparing the concentration-dependent anti-prion and anti-pestiviral effects of DB772, and determining the selectivity index. Results demonstrate at least an approximate two-log inhibition of PrP^Sc accumulation in the two cell systems and confirmed that the inhibition of PrP^Sc accumulation correlates with inhibition of prion infectivity. PRNP transcripts and total PrP protein concentrations within cell lysates were not decreased; thus, decreased PrP^C expression is not the mechanism of PrP^Sc inhibition. PrP^Sc accumulation was multiple logs more resistant than pestivirus to DB772, suggesting that the anti-PrP^Sc activity was independent of anti-pestivirus activity. The anti-PrP^Sc selectivity index in cell culture was approximately 4.6 in microglia and 5.5 in Rov9 cells. The results describe a new chemical category that inhibits ovine PrP^Sc accumulation in primary sheep microglia and Rov9 cells, and can be used for future studies into the treatment and mechanism of prion diseases.

CuBr-Catalysed Oxidative Desulfurisation of Thiobenzamides

An efficient oxidative desulfurisation of thioamides by CuBr/TBHP is reported. Thioamides containing alkyl or aryl on the nitrogen undergo desulfurisation and give amides with good yields. Thioamides not containing substituent on the nitrogen undergo desulfurisation and give 1,2,4-thiadiazoles in moderate to good yields.

Structure-activity relationship refinement and further assessment of indole-3-glyoxylamides as a lead series against prion disease

Structure-activity relationships within the indole-3-glyoxylamide series of antiprion agents have been explored further, resulting in discovery of several new compounds demonstrating excellent activity in a cell line model of prion disease (EC₅₀ <10 nM). After examining a range of substituents at the para-position of the N-phenylglyoxylamide moiety, five-membered heterocycles containing at least two heteroatoms were found to be optimal for the antiprion effect. A number of modifications were made to probe the importance of the glyoxylamide substructure, although none were well tolerated. The most potent compounds did, however, prove largely stable towards microsomal metabolism, and the most active library member cured scrapie-infected cells indefinitely on administration of a single treatment. The present results thereby confirm the indole-3-glyoxylamides as a promising lead series for continuing in vitro and in vivo evaluation against prion disease.

Anti-prion activities and drug-like potential of functionalized quinacrine analogs with basic phenyl residues at the 9-amino position

In this paper, we report the synthesis and cell-based anti-prion activity of quinacrine analogs derived by replacing the basic alkyl side chain of quinacrine with 4-(4-methylpiperazin-I-yl)phenyl, (1-benzylpiperidin-4-yl) and their structural variants. Several promising analogs were found that have a more favorable anti-prion profile than quinacrine in terms of potency and activity across different prion-infected murine cell models. They also exhibited greater binding affinities for a human prion protein fragment (hPrP(121-231)) than quinacrine, and had permeabilities on the PAMPA-BBB assay that fall within the range of CNS permeant candidates. When evaluated on bidirectional assays on a Pgp overexpressing cell line, one analog was less susceptible to Pgp efflux activity compared to quinacrine. Taken together, the results point to an important role for the substituted 9-amino side chain attached to the acridine, tetrahydroacridine and quinoline scaffolds. The nature of this side chain influenced cell-based potency, PAMPA permeability and binding affinity to hPrP(121-231).