Potent inhibition of scrapie prion replication in cultured cells by bis-acridines

The concept of privileged structures in rational drug design: focus on acridine and quinoline scaffolds in neurodegenerative and protozoan diseases

INTRODUCTION

For nearly 20 years, privileged structures have offered an optimal source of core scaffolds and capping fragments for the design of combinatorial libraries directed at a broad spectrum of targets. From describing structures promiscuous within a given target family, the concept has evolved to include frameworks that can modulate proteins lacking a strict target class relation.

AREAS COVERED

Based on a literature search from 2000 to 2010, we discuss how two privileged motifs, quinolines and acridines, are particularly recurrent in compounds active against two quite different pathologies, neurodegenerative and protozoan diseases.

EXPERT OPINION

As privileged structures, quinolines and acridines could improve the productivity of drug discovery projects in the field of neurodegenerative and protozoan diseases. They could be particularly relevant for protozoan diseases because of the importance of cost-effective strategies and less stringent intellectual property concerns. Furthermore, because of their inherent affinity for various targets, privileged structures could offer a viable starting point in the search for novel multi-target ligands. Finally, from a broader perspective, they can serve as effective probes for investigating unknown but interrelated mechanisms of action.

Lysozyme: a model protein for amyloid research

Ever since lysozyme was discovered by Fleming in 1922, this protein has emerged as a model for investigations on protein structure and function. Over the years, several high-resolution structures have yielded a wealth of structural data on this protein. Extensive studies on folding of lysozyme have shown how different regions of this protein dynamically interact with one another. Data is also available from numerous biotechnological studies wherein lysozyme has been employed as a model protein for recovering active recombinant protein from inclusion bodies using small molecules like l-arginine. A variety of conditions have been developed in vitro to induce fibrillation in hen lysozyme. They include (a) acidic pH at elevated temperature, (b) concentrated solutions of ethanol, (c) moderate concentrations of guanidinium hydrochloride at moderate temperature, and (d) alkaline pH at room temperature. This review aims to bring together similarities and differences in aggregation mechanisms, morphology of aggregates, and related issues that arise using the different conditions mentioned above to improve our understanding. The alkaline pH condition (pH 12.2), discovered and studied extensively in our lab, shall receive special attention. More than a decade ago, it was revealed that mutations in human lysozyme can cause accumulation of large quantities of amyloid in liver, kidney, and other regions of gastrointestinal tract. Understanding the mechanism of lysozyme aggregation will probably have therapeutic implications for the treatment of systemic nonneuropathic amyloidosis. Numerous studies have begun to focus attention on inhibition of lysozyme aggregation using antibody or small molecules. The enzymatic activity of lysozyme presents a convenient handle to quantify the native population of lysozyme in a sample where aggregation has been inhibited. The rich information available on lysozyme coupled with the multiple conditions that have been successful in inducing/inhibiting its aggregation in vitro makes lysozyme an ideal model protein to investigate amyloidogenesis.

A novel class of potential prion drugs: preliminary in vitro and in vivo data for multilayer coated gold nanoparticles

Gold nanoparticles coated with oppositely charged polyelectrolytes, such as polyallylamine hydrochloride and polystyrenesulfonate, were examined for potential inhibition of prion protein aggregation and prion (PrPSc) conversion and replication. Different coatings, finishing with a positive or negative layer, were tested, and different numbers of layers were investigated for their ability to interact and reduce the accumulation of PrPSc in scrapie prion infected ScGT1 and ScN2a cells. The particles efficiently hampered the accumulation of PrPSc in ScN2a cells and showed curing effects on ScGT1 cells with a nanoparticle concentration in the picomolar range. Finally, incubation periods of prion-infected mice treated with nanomolar concentrations of gold nanoparticles were significantly longer compared to untreated controls.

Efficacy of novel acridine derivatives in the inhibition of hPrP90-231 prion protein fragment toxicity

Quinacrine is one of the few molecules tested to treat patients affected by prion diseases, although the clinical outcome is largely unsatisfactory. To identify novel derivatives with higher neuroprotective activity, we evaluated the effects of a small library of acridine derivatives. The 6-chloro-2-methoxyacridine derivatives bearing on position 9 a quinolizidin-1-ylamino (Q1, Q2) or a quinolizidin-1-ylalkylamino residue (Q3, Q4, Q6, Q7), the thio-bioisoster of Q3 (Q5), the 9-(N-lupinylthiopropyl)amino derivative (Q8) and simple acridines (Q9 and Q10) were considered. We compared the effects of quinacrine and these novel analogues in the inhibition of the cytotoxic activity and protease K (PK) resistance of the human prion protein fragment 90-231 (hPrP90-231). We demonstrate that quinacrine caused a significant reduction of hPrP90-231 toxicity due to its binding to the fragment and the prevention of its conversion in a toxic isoform. All acridine derivatives analyzed showed high affinity binding for hPrP90-231, but only Q3 and Q10, caused a significant reduction of hPrP90-231 cytotoxicity, with higher efficacy than quinacrine. We attempted to correlate the cytoprotective effects of the new compounds with some biochemical parameters (binding affinity to hPrP90-231, intrinsic fluorescence quenching, hydrophobic amino acid exposure), but a direct relationship occurred only with the reduction of PK resistance, likely due to the prevention of the acquisition of the β-sheet-rich toxic conformation. These data represent interesting leads for further modifications of the basic side chain and the substituent pattern of the acridine nucleus to develop novel compounds with improved antiprion activity to be tested in in vivo experimental setting.

A functional extracellular matrix biomaterial derived from ovine forestomach

Extracellular matrix (ECM) based biomaterials have an established place as medical devices for wound healing and tissue regeneration. In the search for biomaterials we have identified ovine forestomach matrix (OFM), a thick, large format ECM which is biochemically diverse and biologically functional. OFM was purified using an osmotic process that was shown to reduce the cellularity of the ECM and aid tissue delamination. OFM produced using this technique was shown to retain residual basement membrane components, as evidence by the presence of laminin and collagen IV. The collagenous microarchitecture of OFM retained many components of native ECM including fibronectin, glycosaminoglycans, elastin and fibroblast growth factor basic. OFM was non-toxic to mammalian cells and supported fibroblast and keratinocyte migration, differentiation and infiltration. OFM is a culturally acceptable alternative to current collagen-based biomaterials and has immediate clinical applications in wound healing and tissue regeneration.

Amyloid fibrillation and cytotoxicity of insulin are inhibited by the amphiphilic surfactants

Amyloid fibrils have been associated with at least 25 different degenerative diseases. The 51-residue polypeptide hormone insulin, which is associated with type II diabetes, has been shown to self-assemble to form amyloid fibrils in vitro. With bovine insulin as a model, the research presented here explores the effects of two amphiphilic surfactants (1,2-dihexanoyl-sn-glycero-3-phosphocholine (di-C7-PC) and 1,2-diheptanoyl-sn-glycero-3-phosphocholine (di-C7-PC)) on the in vitro fibrillation process of bovine insulin at pH 2.0 and 55 degrees C. We demonstrated that insulin fibrillation may be inhibited by both surfactants in a dose-dependent fashion. The best inhibition of fibril formation is observed when insulin is incubated with 4mM di-C7-PC. Moreover, the addition of either surfactant at the concentrations studied attenuated insulin fibril-induced cytotoxicity in both PC12 and SH-SY5Y cell lines. The results from this work may contribute to the understanding of the molecular factors affecting amyloid fibrillation and the molecular mechanism(s) of the interactions between the membrane and amyloid proteins.

Chemical induction of misfolded prion protein conformers in cell culture

Prion-infected cells accumulate a heterogeneous population of aberrantly folded PrP conformers, including the disease-causing isoform (PrP(Sc)). We found that specific chemicals can modulate the levels of various PrP conformers in cultured cells. Positively charged polyamidoamines (dendrimers) eliminated protease-resistant (r) PrP(Sc) from prion-infected cells and induced the formation of insoluble, protease-sensitive PrP aggregates (designated PrP(A)). Larger, positively charged polyamidoamines more efficaciously induced the formation of PrP(A) and cleared rPrP(Sc), whereas negatively charged polyamidoamines neither induced PrP(A) nor cleared rPrP(Sc). Although the biochemical properties of PrP(A) were shown to be similar to protease-sensitive (s) PrP(Sc), bioassays of PrP(A) indicated that it is not infectious. Our studies argue that PrP(A) represents an aggregated PrP species that is off-pathway relative to the formation of rPrP(Sc). It remains to be established whether the formation of PrP(A) inhibits the formation of rPrP(Sc) by sequestering PrP(C) in the form of benign, insoluble aggregates.

Continuous quinacrine treatment results in the formation of drug-resistant prions

Quinacrine is a potent antiprion compound in cell culture models of prion disease but has failed to show efficacy in animal bioassays and human clinical trials. Previous studies demonstrated that quinacrine inefficiently penetrates the blood-brain barrier (BBB), which could contribute to its lack of efficacy in vivo. As quinacrine is known to be a substrate for P-glycoprotein multi-drug resistance (MDR) transporters, we circumvented its poor BBB permeability by utilizing MDR(0/0) mice that are deficient in mdr1a and mdr1b genes. Mice treated with 40 mg/kg/day of quinacrine accumulated up to 100 microM of quinacrine in their brains without acute toxicity. PrP(Sc) levels in the brains of prion-inoculated MDR(0/0) mice diminished upon the initiation of quinacrine treatment. However, this reduction was transient and PrP(Sc) levels recovered despite the continuous administration of quinacrine. Treatment with quinacrine did not prolong the survival times of prion-inoculated, wild-type or MDR(0/0) mice compared to untreated mice. A similar phenomenon was observed in cultured differentiated prion-infected neuroblastoma cells: PrP(Sc) levels initially decreased after quinacrine treatment then rapidly recovered after 3 d of continuous treatment. Biochemical characterization of PrP(Sc) that persisted in the brains of quinacrine-treated mice had a lower conformational stability and different immunoaffinities compared to that found in the brains of untreated controls. These physical properties were not maintained upon passage in MDR(0/0) mice. From these data, we propose that quinacrine eliminates a specific subset of PrP(Sc) conformers, resulting in the survival of drug-resistant prion conformations. Transient accumulation of this drug-resistant prion population provides a possible explanation for the lack of in vivo efficacy of quinacrine and other antiprion drugs.

Lactoferrin induces cell surface retention of prion protein and inhibits prion accumulation

Prion diseases are fatal neurodegenerative disorders, and the conformational conversion of normal cellular prion protein (PrP(C)) into its pathogenic, amyloidogenic isoform (PrP(Sc)) is the essential event in the pathogenesis of these diseases. Lactoferrin (LF) is a cationic iron-binding glycoprotein belonging to the transferrin (TF) family, which accumulates in the amyloid deposits in the brain in neurodegenerative disorders, such as Alzheimer's disease and Pick's disease. In the present study, we have examined the effects of LF on PrP(Sc) formation by using cell culture models. Bovine LF inhibited PrP(Sc) accumulation in scrapie-infected cells in a time- and dose-dependent manner, whereas TF was not inhibitory. Bioassays of LF-treated cells demonstrated prolonged incubation periods compared with non-treated cells indicating a reduction of prion infectivity. LF mediated the cell surface retention of PrP(C) by diminishing its internalization and was capable of interacting with PrP(C) in addition to PrP(Sc). Furthermore, LF partially inhibited the formation of protease-resistant PrP as determined by the protein misfolding cyclic amplification assay. Our results suggest that LF has multifunctional antiprion activities.

Targeting prion proteins in neurodegenerative disease

BACKGROUND

Spongiform neurodegeneration is the pathological hallmark of individuals suffering from prion disease. These disorders, whose manifestation is sporadic, familial or acquired by infection, are caused by accumulation of the aberrantly folded isoform of the cellular prion protein (PrP(c)), termed PrP(Sc). Although usually rare, prion disorders are inevitably fatal and transferrable by infection.

OBJECTIVE

Pathology is restricted to the central nervous system and premortem diagnosis is usually not possible. Yet, promising approaches towards developing therapeutic regimens have been made recently.

METHODS

The biology of prion proteins and current models of neurotoxicity are discussed and prophylactic and therapeutic concepts are introduced.

RESULTS/CONCLUSIONS

Although various promising drug candidates with antiprion activity have been identified, this proof-of-concept cannot be transferred into translational medicine yet.

Tau-based treatment strategies in neurodegenerative diseases

Neurofibrillary tangles are a characteristic hallmark of Alzheimer's and other neurodegenerative diseases, such as Pick's disease (PiD), progressive supranuclear palsy (PSP), corticobasal degeneration (CBD), and frontotemporal dementia and parkinsonism linked to chromosome 17 (FTDP-17). These diseases are summarized as tauopathies, because neurofibrillary tangles are composed of intracellular aggregates of the microtubule-associated protein tau. The molecular mechanisms of tau-mediated neurotoxicity are not well understood; however, pathologic hyperphosphorylation and aggregation of tau play a central role in neurodegeneration and neuronal dysfunction. The present review, therefore, focuses on therapeutic approaches that aim to inhibit tau phosphorylation and aggregation or to dissolve preexisting tau aggregates. Further experimental therapy strategies include the enhancement of tau clearance by activation of proteolytic, proteasomal, or autophagosomal degradation pathways or anti-tau directed immunotherapy. Hyperphosphorylated tau does not bind microtubules, leading to microtubule instability and transport impairment. Pharmacological stabilization of microtubule networks might counteract this effect. In several tauopathies there is a shift toward four-repeat tau isoforms, and interference with the splicing machinery to decrease four-repeat splicing might be another therapeutic option.

A multimeric quinacrine conjugate as a potential inhibitor of Alzheimer's beta-amyloid fibril formation

Amyloid formation and accumulation of the amyloid beta-peptide (Abeta) in the brain is associated with Alzheimer's disease (AD) pathogenesis. Therefore, among the therapeutic approaches in development to fight the disease, the direct inhibition of the Abeta self-assembly process is currently widely investigated and is one of the most promising approaches. In this study we investigated the potential of a multimeric display of quinacrine derivatives, as compared to the monomer quinacrine, as a design principal for a novel class of inhibitors against Abeta fibril formation. The presented multimeric conjugate exhibits a cluster of four quinacrine derivatives on a rigid cyclopeptidic scaffold. Herein is reported the synthesis of the conjugate, together with the in vitro inhibitory evaluation of Abeta(1-40) fibrils using the thioflavin T fluorescence assay, and imaging with atomic force microscopy. Our data show that the multimeric compound inhibits Abeta(1-40) fibril formation with an IC(50) value of 20+/-10 microM, which contrasts with the nonactive monomeric analogue. This work suggests that assembling multiple copies of acridine moieties to a central scaffold, for multiple interactions, is a promising strategy for the engineering of inhibitors against Abeta fibril formation.

Design, synthesis, and biological evaluation of glycine-based molecular tongs as inhibitors of Abeta1-40 aggregation in vitro

A series of N-terminus benzamides of glycine-based symmetric peptides, linked to m-xylylenediamine and 3,4'-oxydianiline spacers, were prepared and tested as inhibitors of beta-amyloid peptide Abeta(1-40) aggregation in vitro. Compounds with good anti-aggregating activity were detected. Polyphenolic amides showed the highest anti-aggregating activity, with IC(50) values in the micromolar range. Structure-activity relationships suggested that pi-pi stacking and hydrogen-bonding interactions play a key role in the inhibition of Abeta(1-40) self-assembly leading to amyloid fibrils.

Chemically induced accumulation of GAGs delays PrP(Sc) clearance but prolongs prion disease incubation time

Prion diseases are a group of fatal neurodegenerative diseases affecting humans and animals. The only identified component of the infectious prion is PrP(Sc), an aberrantly folded isoform of PrP(C). Glycosaminoglycans, which constitute the main receptor for prions on cells, play a complex role in the pathogenesis of prion diseases. For example, while agents inducing aberrant lysosomal accumulation of GAGs such as Tilorone and Quinacrine significantly reduced PrP(Sc) content in scrapie-infected cells, administration of Quinacrine to prion-infected subjects did not improve their clinical status. In this study, we investigated the association of PrP(Sc )with cells cultured with Tilorone. We found that while the initial incorporation of PrP(Sc) was similar in the treated and untreated cells, clearance of PrP(Sc) from the Tilorone-treated cells was significantly impaired. Interestingly, prolonged administration of Tilorone to mice prior to prion infection resulted in a significant delay in disease onset, concomitantly with in vivo accumulation of lysosomal GAGs. We hypothesize that GAGs may complex with newly incorporated PrP(Sc) in lysosomes and further stabilize the prion protein conformation. Over-stabilized PrP(Sc) molecules have been shown to comprise reduced converting activity.

Mechanistic insights into the cure of prion disease by novel antiprion compounds

Prion diseases are fatal neurodegenerative disorders. Identification of possible therapeutic tools is important in the search for a potential treatment for these diseases. Congo red is an azo dye that has been used for many years to detect abnormal prion protein in the brains of diseased patients or animals. Congo red has little therapeutic potential for the treatment of these diseases due to toxicity and poor permeation of the blood-brain barrier. We have prepared two Congo red derivatives, designed without these liabilities, with potent activity in cellular models of prion disease. One of these compounds cured cells of the transmissible agent. The mechanism of action of these compounds is possibly multifactorial. The high affinity of Congo red derivatives, including compounds that are ineffective and are effective at the cure of prion disease, for abnormally folded prion protein suggests that the amyloidophylic property of these derivatives is not as critical to the mechanism of action as other effects. Congo red derivatives that are effective at the cure of prion disease increased the degradation of abnormal PrP by the proteasome. Therefore, the principal mechanism of action of the Congo red analogues was to prevent inhibition of proteasomal activity by PrPSc.

Potent inhibition of tau fibrillization with a multivalent ligand

Small-molecule inhibitors of tau fibrillization are under investigation as tools for interrogating the tau aggregation pathway and as potential therapeutic agents for Alzheimer's disease. Established inhibitors include thiacarbocyanine dyes, which can inhibit recombinant tau fibrillization in the presence of anionic surfactant aggregation inducers. In an effort to increase inhibitory potency, a cyclic bis-thiacarbocyanine molecule containing two thiacarbocyanine moieties was synthesized and characterized with respect to tau fibrillization inhibitory activity by electron microscopy and ligand aggregation state by absorbance spectroscopy. Results showed that the inhibitory activity of the bis-thiacarbocyanine was qualitatively similar to a monomeric cyanine dye, but was more potent with 50% inhibition achieved at approximately 80nM concentration. At all concentrations tested in aqueous solution, the bis-thiacarbocyanine collapsed to form a closed clamshell structure. However, the presence of tau protein selectively stabilized the open conformation. These results suggest that the inhibitory activity of bis-thiacarbocyanine results from multivalency, and reveal a route to more potent tau aggregation inhibitors.

Cyclodextrins inhibit replication of scrapie prion protein in cell culture

Prion diseases are fatal neurodegenerative disorders that are caused by the conversion of a normal host-encoded protein, PrP(C), to an abnormal, disease-causing form, PrP(Sc). This paper reports that cyclodextrins have the ability to reduce the pathogenic isoform of the prion protein PrP(Sc) to undetectable levels in scrapie-infected neuroblastoma cells. Beta-cyclodextrin removed PrP(Sc) from the cells at a concentration of 500 microM following 2 weeks of treatment. Structure activity studies revealed that antiprion activity was dependent on the size of the cyclodextrin. The half-maximal inhibitory concentration (IC(50)) for beta-cyclodextrin was 75 microM, whereas alpha-cyclodextrin, which possessed less antiprion activity, had an IC(50) of 750 microM. This report presents cyclodextrins as a new class of antiprion compound. For decades, the pharmaceutical industry has successfully used cyclodextrins for their complex-forming ability; this ability is due to the structural orientation of the glucopyranose units, which generate a hydrophobic cavity that can facilitate the encapsulation of hydrophobic moieties. Consequently, cyclodextrins could be ideal candidates for the treatment of prion diseases.

Prion biology relevant to bovine spongiform encephalopathy

Bovine spongiform encephalopathy (BSE) and chronic wasting disease (CWD) of deer and elk are a threat to agriculture and natural resources, as well as a human health concern. Both diseases are transmissible spongiform encephalopathies (TSE), or prion diseases, caused by autocatalytic conversion of endogenously encoded prion protein (PrP) to an abnormal, neurotoxic conformation designated PrPsc. Most mammalian species are susceptible to TSE, which, despite a range of species-linked names, is caused by a single highly conserved protein, with no apparent normal function. In the simplest sense, TSE transmission can occur because PrPsc is resistant to both endogenous and environmental proteinases, although many details remain unclear. Questions about the transmission of TSE are central to practical issues such as livestock testing, access to international livestock markets, and wildlife management strategies, as well as intangible issues such as consumer confidence in the safety of the meat supply. The majority of BSE cases seem to have been transmitted by feed containing meat and bone meal from infected animals. In the United Kingdom, there was a dramatic decrease in BSE cases after neural tissue and, later, all ruminant tissues were banned from ruminant feed. However, probably because of heightened awareness and widespread testing, there is growing evidence that new variants of BSE are arising "spontaneously," suggesting ongoing surveillance will continue to find infected animals. Interspecies transmission is inefficient and depends on exposure, sequence homology, TSE donor strain, genetic polymorphism of the host, and architecture of the visceral nerves if exposure is by an oral route. Considering the low probability of interspecies transmission, the low efficiency of oral transmission, and the low prion levels in nonnervous tissues, consumption of conventional animal products represents minimal risk. However, detection of rare events is challenging, and TSE literature is characterized by subsequently unsupported claims of species barriers or absolute tissue safety. This review presents an overview of TSE and summarizes recent research on pathogenesis and transmission.

Phosphorothioate oligonucleotides reduce PrP levels and prion infectivity in cultured cells

Prions are composed solely of the disease-causing prion protein (PrPSc) that is formed from the cellular isoform PrPC by a posttranslational process. Here we report that short phosphorothioate DNA (PS-DNA) oligonucleotides diminished the levels of both PrPC and PrPSc in prion-infected neuroblastoma (ScN2a) cells. The effect of PS-DNA on PrP levels was independent of the nucleotide sequence. The effective concentration (EC50) of PS-DNA required to achieve half-maximal diminution of PrPSc was approximately 70 nM, whereas the EC50 of PS-DNA for PrPC was more than 50-fold greater. This finding indicated that diminished levels of PrPSc after exposure to PS-DNA are unlikely to be due to decreased PrPC levels. Bioassays in transgenic mice demonstrated a substantial diminution in the prion infectivity after ScN2a cells were exposed to PS-DNAs. Whether PS-DNA will be useful in the treatment of prion disease in people or livestock remains to be established.

Hot spots in prion protein for pathogenic conversion

Prion proteins are key molecules in transmissible spongiform encephalopathies (TSEs), but the precise mechanism of the conversion from the cellular form (PrP(C)) to the scrapie form (PrP(Sc)) is still unknown. Here we discovered a chemical chaperone to stabilize the PrP(C) conformation and identified the hot spots to stop the pathogenic conversion. We conducted in silico screening to find compounds that fitted into a "pocket" created by residues undergoing the conformational rearrangements between the native and the sparsely populated high-energy states (PrP*) and that directly bind to those residues. Forty-four selected compounds were tested in a TSE-infected cell culture model, among which one, 2-pyrrolidin-1-yl-N-[4-[4-(2-pyrrolidin-1-yl-acetylamino)-benzyl]-phenyl]-acetamide, termed GN8, efficiently reduced PrP(Sc). Subsequently, administration of GN8 was found to prolong the survival of TSE-infected mice. Heteronuclear NMR and computer simulation showed that the specific binding sites are the A-S2 loop (N159) and the region from helix B (V189, T192, and K194) to B-C loop (E196), indicating that the intercalation of these distant regions (hot spots) hampers the pathogenic conversion process. Dynamics-based drug discovery strategy, demonstrated here focusing on the hot spots of PrP(C), will open the way to the development of novel anti-prion drugs.

Phosphorothioate oligonucleotides reduce PrP levels and prion infectivity in cultured cells

Prions are composed solely of the disease-causing prion protein (PrPSc) that is formed from the cellular isoform PrPC by a posttranslational process. Here we report that short phosphorothioate DNA (PS-DNA) oligonucleotides diminished the levels of both PrPC and PrPSc in prion-infected neuroblastoma (ScN2a) cells. The effect of PS-DNA on PrP levels was independent of the nucleotide sequence. The effective concentration (EC50) of PS-DNA required to achieve half-maximal diminution of PrPSc was approximately 70 nM, whereas the EC50 of PS-DNA for PrPC was more than 50-fold greater. This finding indicated that diminished levels of PrPSc after exposure to PS-DNA are unlikely to be due to decreased PrPC levels. Bioassays in transgenic mice demonstrated a substantial diminution in the prion infectivity after ScN2a cells were exposed to PS-DNAs. Whether PS-DNA will be useful in the treatment of prion disease in people or livestock remains to be established.

Synthesis and antitubercular activity of phenothiazines with reduced binding to dopamine and serotonin receptors

Analogs of the psychotropic phenothiazines were synthesized and examined as antitubercular agents against Mycobacterium tuberculosis H37Rv. The compounds were subsequently counter-screened for binding to the dopaminergic-receptor subtypes D1, D2, D3 and the serotonergic-receptor subtypes 5-HT(1A), 5-HT(2A), and 5-HT(2C). The most active compounds showed MICs from 2 to 4 microg/mL and had overall reduced binding to the dopamine and serotonin receptors compared to chlorpromazine and trifluoperazine.

Discriminating between cellular and misfolded prion protein by using affinity to 9-aminoacridine compounds

Quinacrine and related 9-aminoacridine compounds are effective in eliminating the alternatively folded prion protein, termed PrP(Sc), from scrapie-infected cultured cells. Clinical evaluations of quinacrine for the treatment of human prion diseases are progressing in the absence of a clear understanding of the molecular mechanism by which prion replication is blocked. Here, insight into the mode of action of 9-aminoacridine compounds was sought by using a chemical proteomics approach to target identification. Cellular macromolecules that bind 9-aminoacridine ligands were affinity-purified from tissue lysates by using a 9-aminoacridine-functionalized solid-phase matrix. Although the 9-aminoacridine matrix was conformationally selective for PrP(Sc), it was inefficient: approximately 5 % of PrP(Sc) was bound under conditions that did not support binding of the cellular isoform, PrP(C). Our findings suggest that 9-aminoacridine compounds may reduce the PrP(Sc) burden either by occluding epitopes necessary for templating on the surface of PrP(Sc) or by altering the stability of PrP(Sc) oligomers, where a one-to-one stoichiometry is not necessary.

Bis-acridines as lead antiparasitic agents: structure-activity analysis of a discrete compound library in vitro

Parasitic diseases are of enormous public health significance in developing countries-a situation compounded by the toxicity of and resistance to many current chemotherapeutics. We investigated a focused library of 18 structurally diverse bis-acridine compounds for in vitro bioactivity against seven protozoan and one helminth parasite species and compared the bioactivities and the cytotoxicities of these compounds toward various mammalian cell lines. Structure-activity relationships demonstrated the influence of both the bis-acridine linker structure and the terminal acridine heterocycle on potency and cytotoxicity. The bioactivity of polyamine-linked acridines required a minimum linker length of approximately 10 A. Increasing linker length resulted in bioactivity against most parasites but also cytotoxicity toward mammalian cells. N alkylation, but less so N acylation, of the polyamine linker ameliorated cytotoxicity while retaining bioactivity with 50% effective concentration (EC(50)) values similar to or better than those measured for standard drugs. Substitution of the polyamine for either an alkyl or a polyether linker maintained bioactivity and further alleviated cytotoxicity. Polyamine-linked compounds in which the terminal acridine heterocycle had been replaced with an aza-acridine also maintained acceptable therapeutic indices. The most potent compounds recorded low- to mid-nanomolar EC(50) values against Plasmodium falciparum and Trypanosoma brucei; otherwise, low-micromolar potencies were measured. Importantly, the bioactivity of the library was independent of P. falciparum resistance to chloroquine. Compound bioactivity was a function of neither the potential to bis-intercalate DNA nor the inhibition of trypanothione reductase, an important drug target in trypanosomatid parasites. Our approach illustrates the usefulness of screening focused compound libraries against multiple parasite targets. Some of the bis-acridines identified here may represent useful starting points for further lead optimization.

An overview of transmissible spongiform encephalopathies

Transmissible spongiform encephalopathies (TSEs) are fatal neurodegenerative disorders of humans and animals associated with an accumulation of abnormal isoforms of prion protein (PrP) in nerve cells. The pathogenesis of TSEs involves conformational conversions of normal cellular PrP (PrPc) to abnormal isoforms of PrP (PrPSc). While the protein-only hypothesis has been widely accepted as a causal mechanism of prion diseases, evidence from more recent research suggests a possible involvement of other cellular component(s) or as yet undefined infectious agent(s) in PrP pathogenesis. Although the underlying mechanisms of PrP strain variation and the determinants of interspecies transmissibility have not been fully elucidated, biochemical and molecular findings indicate that bovine spongiform encephalopathy in cattle and new-variant Creutzfeldt–Jakob disease in humans are caused by indistinguishable etiological agent(s). Cumulative evidence suggests that there may be risks of humans acquiring TSEs via a variety of exposures to infected material. The development of highly precise ligands is warranted to detect and differentiate strains, allelic variants and infectious isoforms of these PrPs. This article describes the general features of TSEs and PrP, the current understanding of their pathogenesis, recent advances in prion disease diagnostics, and PrP inactivation.

Does tetracycline bind helix 2 of prion? An integrated spectroscopical and computational study of the interaction between the antibiotic and alpha helix 2 human prion protein fragments

We demonstrate here that tetracycline (TC) can strongly interact (KD' = 189 +/- 7 nM) with model peptides derived from the C-terminal globular domain of the prion protein, hPrP [173-195], and that interaction concerns residues within the C-terminal half of the helix 2, a short region previously indicated as endowed with ambivalent conformational behavior and implicated in PrP conversion to the beta-sheet-rich, infective scrapie variant. Data have been confirmed by binding studies with the N-terminal truncated 180-195 variant that displays a dissociation constant of 483 +/- 30 nM. Remarkably, TC does not influence the structure of the N-terminally fluoresceinated peptides that both show alpha-helical conformations. Docking calculations and molecular dynamics simulations suggest a direct, strong interaction of the antibiotic with exposed side chain functional groups of threonines 190-193 on the solvent-exposed surface of helix 2.

Inhibitors of brain phospholipase A2 activity: their neuropharmacological effects and therapeutic importance for the treatment of neurologic disorders

The phospholipase A(2) family includes secretory phospholipase A(2), cytosolic phospholipase A(2), plasmalogen-selective phospholipase A(2), and calcium-independent phospholipase A(2). It is generally thought that the release of arachidonic acid by cytosolic phospholipase A(2) is the rate-limiting step in the generation of eicosanoids and platelet activating factor. These lipid mediators play critical roles in the initiation and modulation of inflammation and oxidative stress. Neurological disorders, such as ischemia, spinal cord injury, Alzheimer's disease, multiple sclerosis, prion diseases, and epilepsy are characterized by inflammatory reactions, oxidative stress, altered phospholipid metabolism, accumulation of lipid peroxides, and increased phospholipase A(2) activity. Increased activities of phospholipases A(2) and generation of lipid mediators may be involved in oxidative stress and neuroinflammation associated with the above neurological disorders. Several phospholipase A(2) inhibitors have been recently discovered and used for the treatment of ischemia and other neurological diseases in cell culture and animal models. At this time very little is known about in vivo neurochemical effects, mechanism of action, or toxicity of phospholipase A(2) inhibitors in human or animal models of neurological disorders. In kainic acid-mediated neurotoxicity, the activities of phospholipase A(2) isoforms and their immunoreactivities are markedly increased and phospholipase A(2) inhibitors, quinacrine and chloroquine, arachidonyl trifluoromethyl ketone, bromoenol lactone, cytidine 5-diphosphoamines, and vitamin E, not only inhibit phospholipase A(2) activity and immunoreactivity but also prevent neurodegeneration, suggesting that phospholipase A(2) is involved in the neurodegenerative process. This also suggests that phospholipase A(2) inhibitors can be used as neuroprotectants and anti-inflammatory agents against neurodegenerative processes in neurodegenerative diseases.

Searching for anti-prion compounds: cell-based high-throughput in vitro assays and animal testing strategies

The transmissible spongiform encephalopathies (TSEs) or prion diseases are infectious neurodegenerative diseases of mammals. Protease-resistant prion protein (PrP-res) is only associated with TSEs and thus has been a target for therapeutic intervention. The most effective compounds known against scrapie in vivo are inhibitors of PrP-res in infected cells. Mouse neuroblastoma (N2a) cells have been chronically infected with several strains of mouse scrapie including RML and 22L. Also, rabbit epithelial cells that produce sheep prion protein in the presence of doxycycline (Rov9) have been infected with sheep scrapie. Here a high-throughput 96-well plate PrP-res inhibition assay is described for each of these scrapie-infected cell lines. With this dot-blot assay, thousands of compounds can easily be screened for inhibition of PrP-res formation. This assay is designed to find new PrP-res inhibitors, which may make good candidates for in vivo anti-scrapie testing. However, an in vitro assay can only suggest that a given compound might have in vivo anti-scrapie activity, which is typically measured as increased survival times. Methods for in vivo testing of compounds for anti-scrapie activity in transgenic mice, a much more lengthy and expensive process, are also discussed.

Structure–activity relationship study of 9-aminoacridine compounds in scrapie-infected neuroblastoma cells

A focused library of variously substituted 9-aminoacridine compounds was screened for bioactivity against accumulation of the infectious prion protein isoform, denoted PrP(Sc), in a cell model of prion replication. The efficacy of compounds against PrP(Sc) accumulation was influenced by both substituents of the distal tertiary amine and acridine heterocycle, while cellular cytotoxicity was encoded in the acridine heterocycle substituents.

Protein-misfolding diseases and chaperone-based therapeutic approaches

A large number of neurodegenerative diseases in humans result from protein misfolding and aggregation. Protein misfolding is believed to be the primary cause of Alzheimer's disease, Parkinson's disease, Huntington's disease, Creutzfeldt-Jakob disease, cystic fibrosis, Gaucher's disease and many other degenerative and neurodegenerative disorders. Cellular molecular chaperones, which are ubiquitous, stress-induced proteins, and newly found chemical and pharmacological chaperones have been found to be effective in preventing misfolding of different disease-causing proteins, essentially reducing the severity of several neurodegenerative disorders and many other protein-misfolding diseases. In this review, we discuss the probable mechanisms of several protein-misfolding diseases in humans, as well as therapeutic approaches for countering them. The role of molecular, chemical and pharmacological chaperones in suppressing the effect of protein misfolding-induced consequences in humans is explained in detail. Functional aspects of the different types of chaperones suggest their uses as potential therapeutic agents against different types of degenerative diseases, including neurodegenerative disorders.

Tricyclic antidepressants, quinacrine and a novel, synthetic chimera thereof clear prions by destabilizing detergent-resistant membrane compartments

Prion diseases are invariably fatal, neurodegenerative diseases transmitted by an infectious agent, PrPSc, a pathogenic, conformational isoform of the normal prion protein (PrPC). Heterocyclic compounds such as acridine derivatives like quinacrine abolish prion infectivity in a cell culture model of prion disease. Here, we report that these compounds execute their antiprion activity by redistributing cholesterol from the plasma membrane to intracellular compartments, thereby destabilizing membrane domains. Our findings are supported by the fact that structurally unrelated compounds with known cholesterol-redistributing effects - U18666A, amiodarone, and progesterone - also possessed high antiprion potency. We show that tricyclic antidepressants (e.g. desipramine), another class of heterocyclic compounds, displayed structure-dependent antiprion effects and enhanced the antiprion effects of quinacrine, allowing lower doses of both drugs to be used in combination. Treatment of ScN2a cells with quinacrine or desipramine induced different ultrastructural and morphological changes in endosomal compartments. We synthesized a novel drug from quinacrine and desipramine, termed quinpramine, that led to a fivefold increase in antiprion activity compared to quinacrine with an EC50 of 85 nm. Furthermore, simvastatin, an inhibitor of cholesterol biosynthesis, acted synergistically with both heterocyclic compounds to clear PrPSc. Our data suggest that a cocktail of drugs targeting the lipid metabolism that controls PrP conversion may be the most efficient in treating Creutzfeldt-Jakob disease.

Transmissibility of mouse AApoAII amyloid fibrils: inactivation by physical and chemical methods

AApoAII amyloid fibrils have exhibited prion-like transmissibility in mouse senile amyloidosis. We have demonstrated that AApoAII is extremely active and can induce amyloidosis following doses less than 1 pg. We tested physical and chemical methods to disrupt AApoAII fibrils in vitro as determined by thioflavin T binding and electron microscopy (EM) as well as inactivating the transmissibility of AApoAII fibrils in vivo. Complete disruption of AApoAII fibrils was achieved by treatment with formic acid, 6 M guanidine hydrochloride, and autoclaving in an alkaline solution. Injection of these disrupted AApoAII fibrils did not induce amyloidosis in mice. Disaggregation with 6 M urea, autoclaving, and alkaline solution was incomplete, and injection of these AApoAII fibrils induced mild amyloidosis. Treatment with formalin, delipidation, freeze-thaw, and RNase did not have any major effect. A distinct correlation was obtained between the amounts of amyloid fibrils and the transmissibility of amyloid fibrils, thereby indicating the essential role of fibril conformation for transmission of amyloidosis. We also studied the inactivation of AApoAII fibrils by several organic compounds in vitro and in vivo. AApoAII amyloidosis provides a valuable system for studying factors that may prevent transmission of amyloid disease as well as potential novel therapies.

Suppressive effects of 4-phenylbutyrate on the aggregation of Pael receptors and endoplasmic reticulum stress

Endoplasmic reticulum (ER) stress is defined as an accumulation of unfolded proteins in the endoplasmic reticulum. 4-phenylbutyrate (4-PBA) has been demonstrated to promote the normal trafficking of the DeltaF508 cystic fibrosis transmembrane conductance regulator (CFTR) mutant from the ER to the plasma membrane and to restore activity. We have reported that 4-PBA protected against cerebral ischemic injury and ER stress-induced neuronal cell death. In this study, we revealed that 4-PBA possesses chemical chaperone activity in vitro, which prevents the aggregation of denatured alpha-lactalbumin and bovine serum albumin (BSA). Furthermore, we investigated the effects of 4-PBA on the accumulation of Parkin-associated endothelin receptor-like receptor (Pael-R) pathologically relevant to the loss of dopaminergic neurons in autosomal recessive juvenile parkinsonism (AR-JP). Interestingly, 4-PBA restored the normal expression of Pael-R protein and suppressed ER stress induced by the overexpression of Pael-R. In addition, we showed that 4-PBA attenuated the activation of ER stress-induced signal transduction pathways and subsequent neuronal cell death. Moreover, 4-PBA restored the viability of yeasts that fail to induce an ER stress response under ER stress conditions. These results suggest that 4-PBA suppresses ER stress by directly reducing the amount of misfolded protein, including Pael-R accumulated in the ER.

Mefloquine, an antimalaria drug with antiprion activity in vitro, lacks activity in vivo

In view of the effectiveness of antimalaria drugs inhibiting abnormal protease-resistant prion protein (PrP-res) formation in scrapie agent-infected cells, we tested other antimalarial compounds for similar activity. Mefloquine (MF), a quinoline antimalaria drug, was the most active compound tested against RML and 22L mouse scrapie agent-infected cells, with 50% inhibitory concentrations of approximately 0.5 and approximately 1.2 microM, respectively. However, MF administered to mice did not delay the onset of intraperitoneally inoculated scrapie agent, the result previously observed with quinacrine. While most anti-scrapie agent compounds inhibit PrP-res formation in vitro, many PrP-res inhibitors have no activity in vivo. This underscores the importance of testing promising candidates in vivo.

Rational design and synthesis of a double-stranded DNA–binder library

Described is a computer-assisted rational design of a DNA-bis-intercalator peptide library. The peptide library of 250 members was prepared and the most powerful binder identified. A value of the binding constant is almost two orders of magnitude higher than that of starting building block-9-aminoacridine. The binder affinity found toward calf thymus DNA is 30-fold of that of human prion peptide 106-126.

Acridin-9-yl exchange: A proposal for the action of some 9-aminoacridine drugs

The finding that several derivatives of 9-aminoacridine were deacridinylated in the presence of primary aliphatic amines during the solid phase synthesis of acridine-peptide conjugates prompted us to investigate the acridin-9-yl moiety transfer from a relatively low-molecular acridine source to a high-molecular carrier. The hydrophobic polymer was used as a model of hydrophobic core of biologically active proteins. While the alpha-amino group in the peptide was found to play the role of weak acridine acceptor, the epsilon-amino group of lysine appeared to serve as a moderate acceptor of acridine moiety. The covalent modification of the lysine residues side chain in the hydrophobic core of prion protein aggregates could thus explain the discrepancy between the ability of the acridine drug quinacrine to reduce efficiently the incidence of prion protein in cell culture and its weak prion binding affinity.

Using budding yeast to screen for anti-prion drugs

Prions are misfolded proteins capable of propagating their altered conformation which are commonly considered as the causative agent of transmissible spongiform encephalopathies, a class of fatal neurodegenerative diseases. Currently, no treatment for prion-based diseases is available. Recently we have developed a rapid, yeast-based, two-step assay to screen for anti-prion drugs [1]. This new method allowed us to identify several compounds that are effective in vivo against budding yeast [PSI+] and [URE3] prions but also able to promote mammalian prion clearance in three different cell culture-based assays. Taken together, these results validate our method as an economic and efficient high-throughput screening approach to identify novel prion inhibitors or to carry on comprehensive structure-activity studies for already isolated anti-mammalian prion drugs. These results suggest furthermore that biochemical pathways controlling prion formation and/or maintenance are conserved from yeast to human and thus amenable to pharmacological and genetic analysis. Finally, it would be very interesting to test active drugs isolated using the yeast-based assay in models for other diseases (neurodegenerative or not) involving amyloid fibers like Huntington's, Parkinson's or Alzheimer's diseases.

Comparison of protease-resistant prion protein inhibitors in cell cultures infected with two strains of mouse and sheep scrapie

The transmissible spongiform encephalopathies (TSEs) are fatal neurodegenerative diseases. A primary therapeutic target for TSE intervention has been a protease-resistant form of prion protein known as PrP(Sc) or PrP-res. In vitro testing of mouse scrapie-infected cell cultures has identified many PrP-res inhibitors that also have activity in vivo. Here we identify 32 new inhibitors of two strains of mouse scrapie PrP-res. Furthermore, to investigate the species-specificity of these and other PrP-res inhibitors, we have developed a high-throughput cell culture assay based on Rov9 cells chronically-infected with sheep scrapie. Of 32 inhibitors of murine PrP-res that were also tested in the Rov9 cells, only six showed inhibitory activity against sheep PrP-res. The three most potent inhibitors of both murine and ovine PrP-res formation (with 50% inhibition at < or =5 microM) were tannic acid, pentosan polysulfate and Fe(III) deuteroporphyrin 2,4-bisethyleneglycol. The latter two have anti-mouse scrapie activity in vivo. These results identify new inhibitors of murine and ovine PrP-res formation and reinforce the idea that compounds effective against PrP-res from one species or strain cannot be assumed to be active against others.

Parallel synthesis of 9-aminoacridines and their evaluation against chloroquine-resistant Plasmodium falciparum

A parallel synthetic strategy to the 9-aminoacridine scaffold of the classical anti-malarial drug quinacrine (2) is presented. The method features a new route to 9-chloroacridines that utilizes triflates of salicylic acid derivatives, which are commercially available in a variety of substitution patterns. The route allows ready variation of the two diversity elements present in this class of molecules: the tricyclic aromatic heterocyclic core, and the disubstituted diamine sidechain. In this study, a library of 175 compounds was designed, although only 93 of the final products had purities acceptable for screening. Impurity was generally due to incomplete removal of 9-acridones (18), a degradation product of the 9-chloroacridine synthetic intermediates. The library was screened against two strains of Plasmodium falciparum, including a model of the drug-resistant parasite, and six novel compounds were found to have IC(50) values in the low nanomolar range.

Systematic identification of antiprion drugs by high-throughput screening based on scanning for intensely fluorescent targets

Conformational changes and aggregation of specific proteins are hallmarks of a number of diseases, like Alzheimer's disease, Parkinson's disease, and prion diseases. In the case of prion diseases, the prion protein (PrP), a neuronal glycoprotein, undergoes a conformational change from the normal, mainly alpha-helical conformation to a disease-associated, mainly beta-sheeted scrapie isoform (PrP(Sc)), which forms amyloid aggregates. This conversion, which is crucial for disease progression, depends on direct PrP(C)/PrP(Sc) interaction. We developed a high-throughput assay based on scanning for intensely fluorescent targets (SIFT) for the identification of drugs which interfere with this interaction at the molecular level. Screening of a library of 10,000 drug-like compounds yielded 256 primary hits, 80 of which were confirmed by dose response curves with half-maximal inhibitory effects ranging from 0.3 to 60 microM. Among these, six compounds displayed an inhibitory effect on PrP(Sc) propagation in scrapie-infected N2a cells. Four of these candidate drugs share an N'-benzylidene-benzohydrazide core structure. Thus, the combination of high-throughput in vitro assay with the established cell culture system provides a rapid and efficient method to identify new antiprion drugs, which corroborates that interaction of PrP(C) and PrP(Sc) is a crucial molecular step in the propagation of prions. Moreover, SIFT-based screening may facilitate the search for drugs against other diseases linked to protein aggregation.