Library synthesis and screening: 2,4-diphenylthiazoles and 2,4-diphenyloxazoles as potential novel prion disease therapeutics

N-Acylimino-λ3-iodanes from the Metathesis of Iodosoarenes and Nitriles for the Photoinduced C-H Perfluoroacylamination of (Hetero)Arenes

An efficient and scalable synthesis of N-perfluoroacylimino-λ³-iodanes was achieved via an unprecedented metathesis between iodosoarenes and perfluoroalkanenitriles. The perfluoroacylamino groups of the iodanes could be introduced to aromatic and heteroaromatic rings using photoirradiation.

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.

BF3·OEt2-TFAA Mediated Tetra-Functionalization of Amino Acids - Synthesis of Di- and Tri-Substituted 2-Trifluoromethyl Oxazoles in One Pot

A highly efficient, TFAA-BF₃·OEt₂ mediated multicomponent coupling of amino acid, TFAA, and aromatics provides a broad library of 2-trifluoromethyl equipped 2,5-disubstituted/2,4,5-trisubstituted oxazoles or N-(trifluoroacetyl)-β-aminoalkyl ketones. This amino acid tetra-functionalization approach involves amidation (C-N), anhydride (C-O), Friedel-Crafts acylation (C-C), and Robinson-Gabriel annulation (C-O) followed by dehydrative aromatization. This reaction takes place under operationally simple, mild, and metal-free conditions using readily available amino acids and aromatic compounds.

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.

Establishment of a simple cell-based ELISA for the direct detection of abnormal isoform of prion protein from prion-infected cells without cell lysis and proteinase K treatment

Prion-infected cells have been used for analyzing the effect of compounds on the formation of abnormal isoform of prion protein (PrP(Sc)). PrP(Sc) is usually detected using anti-prion protein (PrP) antibodies after the removal of the cellular isoform of prion protein (PrP(C)) by proteinase K (PK) treatment. However, it is expected that the PK-sensitive PrP(Sc) (PrP(Sc)-sen), which possesses higher infectivity and conversion activity than the PK-resistant PrP(Sc) (PrP(Sc)-res), is also digested through PK treatment. To overcome this problem, we established a novel cell-based ELISA in which PrP(Sc) can be directly detected from cells persistently infected with prions using anti-PrP monoclonal antibody (mAb) 132 that recognizes epitope consisting of mouse PrP amino acids 119-127. The novel cell-based ELISA could distinguish prion-infected cells from prion-uninfected cells without cell lysis and PK treatment. MAb 132 could detect both PrP(Sc)-sen and PrP(Sc)-res even if all PrP(Sc) molecules were not detected. The analytical dynamic range for PrP(Sc) detection was approximately 1 log. The coefficient of variation and signal-to-background ratio were 7%-11% and 2.5-3.3, respectively, demonstrating the reproducibility of this assay. The addition of a cytotoxicity assay immediately before PrP(Sc) detection did not affect the following PrP(Sc) detection. Thus, all the procedures including cell culture, cytotoxicity assay, and PrP(Sc) detection were completed in the same plate. The simplicity and non-requirement for cell lysis or PK treatment are advantages for the high throughput screening of anti-prion compounds.

Exploring Anti-Prion Glyco-Based and Aromatic Scaffolds: A Chemical Strategy for the Quality of Life

Prion diseases are fatal neurodegenerative disorders caused by protein misfolding and aggregation, affecting the brain progressively and consequently the quality of life. Alzheimer’s is also a protein misfolding disease, causing dementia in over 40 million people worldwide. There are no therapeutics able to cure these diseases. Cellular prion protein is a high-affinity binding partner of amyloid β (Aβ) oligomers, the most toxic species in Alzheimer’s pathology. These findings motivate the development of new chemicals for a better understanding of the events involved. Disease control is far from being reached by the presently known therapeutics. In this review we describe the synthesis and mode of action of molecular entities with intervention in prion diseases’ biological processes and, if known, their role in Alzheimer’s. A diversity of structures is covered, based on glycans, steroids and terpenes, heterocycles, polyphenols, most of them embodying aromatics and a structural complexity. These molecules may be regarded as chemical tools to foster the understanding of the complex mechanisms involved, and to encourage the scientific community towards further developments for the cure of these devastating diseases.

Decomposition of benzoylthioureas into benzamides and thiobenzamides under solvent-free conditions using iodine–alumina as the catalyst and its mechanistic study by density functional theory

Iodine–alumina catalyzed formation of benzamides and thiobenzamides through different paths by the microwave irradiation of benzoylthioureas.

Multitarget ligands and theranostics: sharpening the medicinal chemistry sword against prion diseases

Prion diseases (PrDs) are fatal neurodegenerative disorders, for which no effective therapeutic and diagnostic tools exist. The main pathogenic event has been identified as the misfolding of a disease-associated prion protein. Nevertheless, pathogenesis seems to involve an intricate array of concomitant processes. Thus, it may be unlikely that drugs acting on single targets can effectively control PrDs. In addition, diagnosis occurs late in the disease process, by which point it is difficult to determine a successful therapeutic intervention. In this context, multitarget ligands (MTLs) and theranostic ligands (TLs) emerge for their potential to effectively cure and diagnose PrDs. In this review, we discuss the medicinal chemistry challenges of identifying novel MTLs and TLs against PrDs, and envision their impact on prion drug discovery.

Isocyanide-based multicomponent reactions towards cyclic constrained peptidomimetics

In the recent past, the design and synthesis of peptide mimics (peptidomimetics) has received much attention. This because they have shown in many cases enhanced pharmacological properties over their natural peptide analogues. In particular, the incorporation of cyclic constructs into peptides is of high interest as they reduce the flexibility of the peptide enhancing often affinity for a certain receptor. Moreover, these cyclic mimics force the molecule into a well-defined secondary structure. Constraint structural and conformational features are often found in biological active peptides. For the synthesis of cyclic constrained peptidomimetics usually a sequence of multiple reactions has been applied, which makes it difficult to easily introduce structural diversity necessary for fine tuning the biological activity. A promising approach to tackle this problem is the use of multicomponent reactions (MCRs), because they can introduce both structural diversity and molecular complexity in only one step. Among the MCRs, the isocyanide-based multicomponent reactions (IMCRs) are most relevant for the synthesis of peptidomimetics because they provide peptide-like products. However, these IMCRs usually give linear products and in order to obtain cyclic constrained peptidomimetics, the acyclic products have to be cyclized via additional cyclization strategies. This is possible via incorporation of bifunctional substrates into the initial IMCR. Examples of such bifunctional groups are N-protected amino acids, convertible isocyanides or MCR-components that bear an additional alkene, alkyne or azide moiety and can be cyclized via either a deprotection-cyclization strategy, a ring-closing metathesis, a 1,3-dipolar cycloaddition or even via a sequence of multiple multicomponent reactions. The sequential IMCR-cyclization reactions can afford small cyclic peptide mimics (ranging from four- to seven-membered rings), medium-sized cyclic constructs or peptidic macrocycles (>12 membered rings). This review describes the developments since 2002 of IMCRs-cyclization strategies towards a wide variety of small cyclic mimics, medium sized cyclic constructs and macrocyclic peptidomimetics.

Optimization of Arylamides as Novel, Potent and Brain-penetrant Antiprion Lead Compounds

The prion diseases caused by PrP^Sc, an alternatively folded form of the cellular prion protein (PrP^C), are rapidly progressive, fatal, and untreatable neurodegenerative syndromes. We employed HTS ELISA assays to identify compounds that lower the level of PrP^Sc in prion-infected mouse neuroblastoma (ScN2a-cl3) cells and identified a series of arylamides. SAR studies indicated that small amides with one aromatic, or heteroaromatic ring, on each side of the amide bond are of modest potency. Of note, benzamide (7), with an EC₅₀ of 2200 nM, was one of only a few arylamide hits with a piperazine group on its aniline moiety. The basic piperazine nitrogen can be protonated at physiologic pH, improving solubility, and therefore we wanted to exploit this feature in our search for a drug candidate. An SAR campaign resulted in several key analogs, including a set with biaryl groups introduced on the carbonyl side for improved potency. Several of these biaryl analogs have submicromolar potency, with the most potent analog 17 having an EC₅₀ = 22 nM. More importantly, 17 and several biarylamides (20, 24, 26, 27) were able to traverse the BBB and displayed excellent drug levels in the brains of mice following oral dosing. These biarylamides may represent good starting points for further lead optimization for the identification of potential drug candidates for the treatment of prion diseases.

Unique drug screening approach for prion diseases identifies tacrolimus and astemizole as antiprion agents

Prion diseases such as Creutzfeldt-Jakob disease (CJD) are incurable and rapidly fatal neurodegenerative diseases. Because prion protein (PrP) is necessary for prion replication but dispensable for the host, we developed the PrP-FRET-enabled high throughput assay (PrP-FEHTA) to screen for compounds that decrease PrP expression. We screened a collection of drugs approved for human use and identified astemizole and tacrolimus, which reduced cell-surface PrP and inhibited prion replication in neuroblastoma cells. Tacrolimus reduced total cellular PrP levels by a nontranscriptional mechanism. Astemizole stimulated autophagy, a hitherto unreported mode of action for this pharmacophore. Astemizole, but not tacrolimus, prolonged the survival time of prion-infected mice. Astemizole is used in humans to treat seasonal allergic rhinitis in a chronic setting. Given the absence of any treatment option for CJD patients and the favorable drug characteristics of astemizole, including its ability to cross the blood-brain barrier, it may be considered as therapy for CJD patients and for prophylactic use in familial prion diseases. Importantly, our results validate PrP-FEHTA as a method to identify antiprion compounds and, more generally, FEHTA as a unique drug discovery platform.

2-Aminothiazoles with improved pharmacotherapeutic properties for treatment of prion disease

Recently, we described the aminothiazole lead (4-biphenyl-4-ylthiazol-2-yl)-(6-methylpyridin-2-yl)-amine (1), which exhibits many desirable properties, including excellent stability in liver microsomes, oral bioavailability of ∼40 %, and high exposure in the brains of mice. Despite its good pharmacokinetic properties, compound 1 exhibited only modest potency in mouse neuroblastoma cells overexpressing the disease-causing prion protein PrP(Sc) . Accordingly, we sought to identify analogues of 1 with improved antiprion potency in ScN2a-cl3 cells while retaining similar or superior properties. Herein we report the discovery of improved lead compounds such as (6-methylpyridin-2-yl)-[4-(4-pyridin-3-yl-phenyl)thiazol-2-yl]amine and cyclopropanecarboxylic acid (4-biphenylthiazol-2-yl)amide, which exhibit brain exposure/EC50 ratios at least tenfold greater than that of compound 1.

Characterization of 8-hydroxyquinoline derivatives containing aminobenzothiazole as inhibitors of dengue virus type 2 protease in vitro

Four serotypes of dengue virus (DENV1-4), mosquito-borne members of Flaviviridae family cause frequent epidemics causing considerable morbidity and mortality in humans throughout tropical regions of the world. There is no vaccine or antiviral therapeutics available for human use. In a previous study, we reported that compounds containing the 8-hydroxyquinoline (8-HQ) scaffold as inhibitors of West Nile virus serine protease. In this study, we analyzed potencies of some compounds with (8-HQ)-aminobenzothiazole derivatives for inhibition of DENV2 protease in vitro. We identified analogs 1-4 with 2-aminothiazole or 2-aminobenzothiazole scaffold with sub-micromolar potencies (IC(50)) in the in vitro protease assays. The kinetic constant (K(i)) for the most potent 8-HQ-aminobenzothiazole inhibitor (compound 1) with an IC(50) value of 0.91±0.05μM was determined to be 2.36±0.13μM. This compound inhibits the DENV2 NS2B/NS3pro by a competitive mode of inhibition.

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).

2-Aminothiazoles as therapeutic leads for prion diseases

2-Aminothiazoles are a new class of small molecules with antiprion activity in prion-infected neuroblastoma cell lines (J. Virol. 2010, 84, 3408). We report here structure-activity studies undertaken to improve the potency and physiochemical properties of 2-aminothiazoles, with a particular emphasis on achieving and sustaining high drug concentrations in the brain. The results of this effort include the generation of informative structure-activity relationships (SAR) and the identification of lead compounds that are orally absorbed and achieve high brain concentrations in animals. The new aminothiazole analogue (5-methylpyridin-2-yl)-[4-(3-phenylisoxazol-5-yl)-thiazol-2-yl]-amine (27), for example, exhibited an EC(50) of 0.94 μM in prion-infected neuroblastoma cells (ScN2a-cl3) and reached a concentration of ∼25 μM in the brains of mice following three days of oral administration in a rodent liquid diet. The studies described herein suggest 2-aminothiazoles as promising new leads in the search for effective therapeutics for prion diseases.

Design, synthesis, and structure-activity relationship of indole-3-glyoxylamide libraries possessing highly potent activity in a cell line model of prion disease

Transmissible spongiform encephalopathies (TSEs) are a family of invariably fatal neurodegenerative disorders for which no effective curative therapy currently exists. We report here the synthesis of a library of indole-3-glyoxylamides and their evaluation as potential antiprion agents. A number of compounds demonstrated submicromolar activity in a cell line model of prion disease together with a defined structure-activity relationship, permitting the design of more potent compounds that effected clearance of scrapie in the low nanomolar range. Thus, the indole-3-glyoxylamides described herein constitute ideal candidates to progress to further development as potential therapeutics for the family of human prion disorders.

Discovery of 2-aminothiazoles as potent antiprion compounds

Prion diseases are fatal, untreatable neurodegenerative diseases caused by the accumulation of the misfolded, infectious isoform of the prion protein (PrP), termed PrP(Sc). In an effort to identify novel inhibitors of prion formation, we utilized a high-throughput enzyme-linked immunosorbent assay (ELISA) to evaluate PrP(Sc) reduction in prion-infected neuroblastoma cell lines (ScN2a). We screened a library of approximately 10,000 diverse small molecules in 96-well format and identified 121 compounds that reduced PrP(Sc) levels at a concentration of 5 microM. Four chemical scaffolds were identified as potential candidates for chemical optimization based on the presence of preliminary structure-activity relationships (SAR) derived from the primary screening data. A follow-up analysis of a group of commercially available 2-aminothiazoles showed this class as generally active in ScN2a cells. Our results establish 2-aminothiazoles as promising candidates for efficacy studies of animals and validate our drug discovery platform as a viable strategy for the identification of novel lead compounds with antiprion properties.

New inhibitors of prion replication that target the amyloid precursor

At present, there is no effective therapy for any of the neurodegenerative amyloidoses, despite renewed efforts to identify compounds active against the various implicated pathogenetic molecules. We have screened a library of 2960 natural and synthetic compounds in two cell lines chronically infected with mouse prions, and have identified eight new inhibitors of prion replication in vitro. They belong to two distinct chemical families that have not previously been recognised as effective in the field of transmissible spongiform encephalopathies: seven are 3-aminosteroids and one is a derivative of erythromycin A with an oxime functionality. Our results suggest that these aminosteroids inhibit prion replication by triggering a common target, possibly implicated in the regulatory pathways of cellular prion protein metabolism. Furthermore, using a quantitative approach for the study of protein stability, it was shown that the erythromycin A derivative altered prion protein stability by direct interaction. Such direct targeting of this amyloid precursor might provide new clues for the understanding of prion diseases and, more importantly, help to define new molecules that are active against prion diseases.

Variety of antiprion compounds discovered through an in silico screen based on cellular-form prion protein structure: Correlation between antiprion activity and binding affinity

Transmissible spongiform encephalopathies are associated with the conformational conversion of the prion protein from the cellular form (PrP(C)) to the scrapie form. This process could be disrupted by stabilizing the PrP(C) conformation, using a specific ligand identified as a chemical chaperone. To discover such compounds, we employed an in silico screen that was based on the nuclear magnetic resonance structure of PrP(C). In combination, we performed ex vivo screening using the Fukuoka-1 strain-infected neuronal mouse cell line at a compound concentration of 10 microM and surface plasmon resonance. Initially, we selected 590 compounds according to the calculated docked energy and finally discovered 24 efficient antiprion compounds, whose chemical structures are quite diverse. Surface plasmon resonance studies showed that the binding affinities of compounds for PrP(C) roughly correlated with the compounds' antiprion activities, indicating that the identification of chemical chaperones that bind to the PrP(C) structure and stabilize it is one efficient strategy for antiprion drug discovery. However, some compounds possessed antiprion activities with low affinities for PrP(C), indicating a mechanism involving additional modulation factors. We classified the compounds roughly into five categories: (i) binding and effective, (ii) low binding and effective, (iii) binding and not effective, (iv) low binding and not effective, and (v) acceleration. In conclusion, we found a spectrum of compounds, many of which are able to modulate the pathogenic conversion reaction. The appropriate categorization of these diverse compounds would facilitate antiprion drug discovery and help to elucidate the pathogenic conversion mechanism.