Diaryl Ether: A Privileged Scaffold for Drug and Agrochemical Discovery

Diaryl ether (DE) is a functional scaffold existing widely both in natural products (NPs) and synthetic organic compounds. Statistically, DE is the second most popular and enduring scaffold within the numerous medicinal chemistry and agrochemical reports. Given its unique physicochemical properties and potential biological activities, DE nucleus is recognized as a fundamental element of medicinal and agrochemical agents aimed at different biological targets. Its drug-like derivatives have been extensively synthesized with interesting biological features including anticancer, anti-inflammatory, antiviral, antibacterial, antimalarial, herbicidal, fungicidal, insecticidal, and so on. In this review, we highlight the medicinal and agrochemical versatility of the DE motif according to the published information in the past decade and comprehensively give a summary of the target recognition, structure-activity relationship (SAR), and mechanism of action of its analogues. It is expected that this profile may provide valuable guidance for the discovery of new active ingredients both in drug and pesticide research.

Structural evidence for the critical role of the prion protein hydrophobic region in forming an infectious prion

Prion or PrPSc is the proteinaceous infectious agent causing prion diseases in various mammalian species. Despite decades of research, the structural basis for PrPSc formation and prion infectivity remains elusive. To understand the role of the hydrophobic region in forming infectious prion at the molecular level, we report X-ray crystal structures of mouse (Mo) prion protein (PrP) (residues 89-230) in complex with a nanobody (Nb484). Using the recombinant prion propagation system, we show that the binding of Nb484 to the hydrophobic region of MoPrP efficiently inhibits the propagation of proteinase K resistant PrPSc and prion infectivity. In addition, when added to cultured mouse brain slices in high concentrations, Nb484 exhibits no neurotoxicity, which is drastically different from other neurotoxic anti-PrP antibodies, suggesting that the Nb484 can be a potential therapeutic agent against prion disease. In summary, our data provides the first structure-function evidence supporting a crucial role of the hydrophobic region of PrP in forming an infectious prion.

Antioxidant and Metal Chelation-Based Therapies in the Treatment of Prion Disease

Many neurodegenerative disorders involve the accumulation of multimeric assemblies and amyloid derived from misfolded conformers of constitutively expressed proteins. In addition, the brains of patients and experimental animals afflicted with prion disease display evidence of heightened oxidative stress and damage, as well as disturbances to transition metal homeostasis. Utilising a variety of disease model paradigms, many laboratories have demonstrated that copper can act as a cofactor in the antioxidant activity displayed by the prion protein while manganese has been implicated in the generation and stabilisation of disease-associated conformers. This and other evidence has led several groups to test dietary and chelation therapy-based regimens to manipulate brain metal concentrations in attempts to influence the progression of prion disease in experimental mice. Results have been inconsistent. This review examines published data on transition metal dyshomeostasis, free radical generation and subsequent oxidative damage in the pathogenesis of prion disease. It also comments on the efficacy of trialed therapeutics chosen to combat such deleterious changes.

Immunotherapeutic approaches in prion disease: progress, challenges and potential directions

Therapeutic trials utilizing animal models of prion disease have explored a variety of compounds and a number of approaches with varying success, including several immunotherapeutic strategies, such as passive immunization through the delivery of viruses carrying nucleic acid inserts encoding prion protein-specific immunoglobulin. Targeted, organ-specific cellular production of therapeutic proteins is a relatively unexplored approach in the treatment of neurodegeneration despite many successful experimental outcomes in animal models and human trials of other diseases of the CNS. Emphasizing studies utilizing mouse models of disease, this review outlines developments and limitations of immunological approaches to the treatment of prion diseases. In addition, the authors discuss the potential of an experimental therapeutic strategy, utilizing hybridoma cells injected directly into the CNS to establish long-term production of anti-prion antibodies in vivo within the organ associated with the greatest pathogenic change in prion disease, the brain.

Allosteric function and dysfunction of the prion protein

Transmissible spongiform encephalopathies (TSEs) are neurodegenerative diseases associated with progressive oligo- and multimerization of the prion protein (PrP(C)), its conformational conversion, aggregation and precipitation. We recently proposed that PrP(C) serves as a cell surface scaffold protein for a variety of signaling modules, the effects of which translate into wide-range functional consequences. Here we review evidence for allosteric functions of PrP(C), which constitute a common property of scaffold proteins. The available data suggest that allosteric effects among PrP(C) and its partners are involved in the assembly of multi-component signaling modules at the cell surface, impose upon both physiological and pathological conformational responses of PrP(C), and that allosteric dysfunction of PrP(C) has the potential to entail progressive signal corruption. These properties may be germane both to physiological roles of PrP(C), as well as to the pathogenesis of the TSEs and other degenerative/non-communicable diseases.

Styryl-based and tricyclic compounds as potential anti-prion agents

Prion diseases currently have no effective therapy. These illnesses affect both animal and human populations, and are characterized by the conformational change of a normal self protein PrP^C (C for cellular) to a pathological and infectious conformer, PrP^Sc (Sc for scrapie). We used a well characterized tissue culture model of prion infection, where mouse neuroblastoma cells (N2a) were infected with 22L PrP^Sc, to screen compounds for anti-prion activity. In a prior study we designed a library of styryl based, potential imaging compounds which were selected for high affinity binding to Alzheimer's disease β-amyloid plaques and good blood-brain barrier permeability. In the current study we screened this library for activity in the N2a/22L tissue culture system. We also tested the anti-prion activity of two clinically used drugs, trimipramine and fluphenazine, in the N2a/22L system. These were selected based on their structural similarity to quinacrine, which was previously reported to have anti-prion activity. All the compounds were also screened for toxicity in tissue culture and their ability to disaggregate amyloid fibrils composed of PrP and β-amyloid synthetic peptides in vitro. Two of the imaging agents, 23I and 59, were found to be both effective at inhibiting prion infection in N2a/22L tissue culture and to be non-toxic. These two compounds, as well as trimipramine and fluphenazine were evaluated in vivo using wild-type CD-1 mice infected peripherally with 139A PrP^Sc. All four agents significantly prolonged the asymptomatic incubation period of prion infection (p<0.0001 log-rank test), as well as significantly reducing the degree of spongiform change, astrocytosis and PrP^Sc levels in the brains of treated mice. These four compounds can be considered, with further development, as candidates for prion therapy.

Discovery of 6-substituted indole-3-glyoxylamides as lead antiprion agents with enhanced cell line activity, improved microsomal stability and low toxicity

A series of highly potent indole-3-glyoxylamide based antiprion agents was previously characterized, focusing on optimization of structure-activity relationship (SAR) at positions 1-3 of the indole system. New libraries interrogating the SAR at indole C-4 to C-7 now demonstrate that introducing electron-withdrawing substituents at C-6 may improve biological activity by up to an order of magnitude, and additionally confer higher metabolic stability. For the present screening libraries, both the degree of potency and trends in SAR were consistent across two cell line models of prion disease, and the large majority of compounds showed no evidence of toxic effects in zebrafish. The foregoing observations thus make the indole-3-glyoxylamides an attractive lead series for continuing development as potential therapeutic agents against prion disease.

Immunomodulation for prion and prion-related diseases

Prion diseases are a unique category of illness, affecting both animals and humans, where the underlying pathogenesis is related to a conformational change of a normal self protein called cellular prion protein to a pathological and infectious conformer known as scrapie prion protein (PrP(Sc)). Currently, all prion diseases lack effective treatment and are universally fatal. Past experiences with bovine spongiform encephalopathy and variant Creutzfeldt-Jakob disease mainly in Europe, as well as the current epidemic of chronic wasting disease in North America, have highlighted the need to develop prophylactic and/or therapeutic approaches. In Alzheimer's disease that, like prion disease, is a conformational neurodegenerative disorder, both passive and active immunization has been shown to be highly effective in model animals at preventing disease and cognitive deficits, with emerging data from human trials suggesting that this approach is able to reduce amyloid-related pathology. However, any immunomodulatory approach aimed at a self-antigen has to finely balance an effective humoral immune response with potential autoimmune toxicity. The prion diseases most commonly acquired by infection typically have the alimentary tract as a portal of infectious agent entry. This makes mucosal immunization a potentially attractive method to produce a local immune response that partially or completely prevents prion entry across the gut barrier, while at the same time producing modulated systemic immunity that is unlikely to be associated with toxicity. Our results using an attenuated Salmonella vaccine strain expressing the prion protein showed that mucosal vaccination can protect against prion infection from a peripheral source, suggesting the feasibility of this approach. It is also possible to develop active and/or passive immunomodulatory approaches that more specifically target PrP(Sc) or target the shared pathological conformer found in numerous conformational disorders. Such approaches could have a significant impact on many of the common age-associated dementias.

Manganese chelation therapy extends survival in a mouse model of M1000 prion disease

Previous in vitro and in vivo investigations have suggested manganese (Mn(2+)) may play a role in pathogenesis through facilitating refolding of the normal cellular form of the prion protein into protease resistant, pathogenic isoforms (PrP(Sc)), as well as the subsequent promotion of higher order aggregation of these abnormal conformers. To further explore the role of Mn(2+) in pathogenesis, we undertook a number of studies, including an assessment of the disease modifying effects of chelation therapy in a well-characterized mouse model of prion disease. The di-sodium, calcium derivative of the chelator, cyclohexanediaminetetraacetic acid (Na(2)CaCDTA), was administered intraperitoneally to mice inoculated intra-cerebrally with either high or low-dose inocula, with treatment beginning early (shortly after inoculation) or late (at the usual mid-survival point of untreated mice). Analyses by inductively coupled plasma-mass spectrometry demonstrated brain Mn(2+) levels were selectively reduced by up to 50% in treated mice compared with untreated controls, with copper, iron, zinc and cobalt levels unchanged. In mice administered high-dose inocula, none of the treatment groups displayed an increase in survival although western blot analyses of early intensively treated mice showed reduced brain PrP(Sc) levels; mice infected using low-dose inocula however, showed a significant prolongation of survival (p = 0.002). Although our findings support a role for Mn(2+) in prion disease, further studies are required to more precisely delineate the extent of pathogenic involvement.