Inhibition of protein kinase C(alpha) by dequalinium analogues: dependence on linker length and geometry

The potential value of dequalinium chloride in the treatment of cancer: Focus on malignant glioma

Dequalinium chloride has been known as one kind of antibiotic that displays a broad antimicrobial spectrum and has been clinically proven to be very safe. In recent years, studies have shown that dequalinium chloride can inhibit the growth of malignant tumours, and reports were mainly used for solid tumours. Glioblastoma is the most common malignant neuroepithelial tumour of the central nervous system in adults, and the prognosis of glioblastoma is poor as it has a high resistance to apoptosis. This review summarizes the current understanding of dequalinium chloride-induced cancer cell apoptosis and its potential role in glioblastoma resistance and progression. Particularly, we focus on dequalinium chloride as it exerts a wide range of anti-cancer activity through its ability to target and accumulate in the mitochondria, and it effectively inhibits the growth of glioblastoma cells in vitro and vivo. Dequalinium chloride is an inhibitor of XIAP and can also act as a mitochondrial targeting agent, which gives it an interesting perspective regarding recent advances in the treatment of malignant glioma.

Medicinal applications and molecular targets of dequalinium chloride

For more than 60 years dequalinium chloride (DQ) has been used as anti-infective drug, mainly to treat local infections. It is a standard drug to treat bacterial vaginosis and an active ingredient of sore-throat lozenges. As a lipophilic bis-quaternary ammonium molecule, the drug displays membrane effects and selectively targets mitochondria to deplete DNA and to block energy production in cells. But beyond its mitochondriotropic property, DQ can interfere with the correct functioning of diverse proteins. A dozen of DQ protein targets have been identified and their implication in the antibacterial, antiviral, antifungal, antiparasitic and anticancer properties of the drug is discussed here. The anticancer effects of DQ combine a mitochondrial action, a selective inhibition of kinases (PKC-α/β, Cdc7/Dbf4), and a modulation of Ca²⁺-activated K⁺ channels. At the bacterial level, DQ interacts with different multidrug transporters (QacR, AcrB, EmrE) and with the transcriptional regulator RamR. Other proteins implicated in the antiviral (MPER domain of gp41 HIV-1) and antiparasitic (chitinase A from Vibrio harveyi) activities have been identified. DQ also targets α -synuclein oligomers to restrict protofibrils formation implicated in some neurodegenerative disorders. In addition, DQ is a typical bolaamphiphile molecule, well suited to form liposomes and nanoparticules useful for drug entrapment and delivery (DQAsomes and others). Altogether, the review highlights the many pharmacological properties and therapeutic benefits of this old 'multi-talented' drug, which may be exploited further. Its multiple sites of actions in cells should be kept in mind when using DQ in experimental research.

Inhibition of porcine reproductive and respiratory syndrome virus by PKC inhibitor dequalinium chloride in vitro

As a severe disease characterized by reproductive failure and respiratory distress, porcine reproductive and respiratory syndrome (PRRS) is one of the most leading threats to the swine industry worldwide. Highly evolving porcine reproductive and respiratory syndrome virus (PRRSV) strains with distinct genetic diversity make the current vaccination strategy much less cost-effective and thus urge alternative protective host directed therapeutic approaches. RACK1-PKC-NF-κB signalling axis was suggested as a potential therapeutic target for PRRS control, therefore we tested the inhibitory effect of PKC inhibitor dequalinium chloride (DECA) on the PRRSV infection in vitro. RT-qPCR, western blot, Co-IP and cytopathic effect (CPE) observations revealed that DECA suppressed PRRSV infection and protected Marc-145 cells and porcine alveolar macrophages (PAMs) from severe cytopathic effects, by repressing the PKCα expression, the interaction between RACK1 and PKCα, and subsequently the NF-κB activation. In conclusion, the data presented in this study shed more light on deeper understanding of the molecular pathogenesis upon PRRSV infection and more importantly suggested DECA as a potential promising drug candidate for PRRS control.

Dequalinium blocks macrophage-induced metastasis following local radiation

A major therapeutic obstacle in clinical oncology is intrinsic or acquired resistance to therapy, leading to subsequent relapse. We have previously shown that systemic administration of different cytotoxic drugs can induce a host response that contributes to tumor angiogenesis, regrowth and metastasis. Here we characterize the host response to a single dose of local radiation, and its contribution to tumor progression and metastasis. We show that plasma from locally irradiated mice increases the migratory and invasive properties of colon carcinoma cells. Furthermore, locally irradiated mice intravenously injected with CT26 colon carcinoma cells succumb to pulmonary metastasis earlier than their respective controls. Consequently, orthotopically implanted SW480 human colon carcinoma cells in mice that underwent radiation, exhibited increased metastasis to the lungs and liver compared to their control tumors. The irradiated tumors exhibited an increase in the colonization of macrophages compared to their respective controls; and macrophage depletion in irradiated tumor-bearing mice reduces the number of metastatic lesions. Finally, the anti-tumor agent, dequalinium-14, in addition to its anti-tumor effect, reduces macrophage motility, inhibits macrophage infiltration of irradiated tumors and reduces the extent of metastasis in locally irradiated mice. Overall, this study demonstrates the adverse effects of local radiation on the host that result in macrophage-induced metastasis.

Identification of novel telomeric G-quadruplex-targeting chemical scaffolds through screening of three NCI libraries

Thirteen compounds with diverse chemical structures have been identified as selective telomeric G-quadruplex-binding ligands through screening the NCI Diversity Set II, the NCI Natural Products Set II and the NCI Mechanistic Diversity Set libraries containing a total of 2307 members against a human telomeric G-quadruplex using a FRET-based DNA melting assay. These compounds show significant selectivity towards a telomeric G-quadruplex compared to duplex DNA, fall within a molecular weight range of 327-533, and are generally consistent with the Lipinski Rule of Five for drug-likeness. Thus they provide new chemical scaffolds for the development of novel classes of G-quadruplex-targeting agents.

Quaternary ammonium salts and their antimicrobial potential: targets or nonspecific interactions?

For more than 50 years dequalinium chloride has been used successfully as an antiseptic drug and disinfectant, particularly for clinical purposes. Given the success of dequalinium chloride, several series of mono- and bisquaternary ammonium compounds have been designed and reported to have improved antimicrobial activity. Furthermore, many of them exhibit high activity against mycobacteria and protozoa, especially against plasmodia. This review discusses the structure-activity relationships and the modes of action of the various series of (bis)quaternary ammonium compounds.

Characterization of dequalinium as a XIAP antagonist that targets the BIR2 domain

Inhibitor of apoptosis proteins (IAPs) regulate the activity of caspases in apoptosis. The human X chromosome-encoded IAP (XIAP) is one of the more potent members of the IAP family and it has been described as a central regulator of apoptosis. Thus, molecules that inhibit XIAP could offer therapeutic opportunities to treat unwanted apoptosis inhibition. In the present study we have applied the selective optimization of side activities (SOSA) approach to the discovery of XIAP inhibitors. In this sense, we have identified dequalinium hydrochloride (Dq) as an inhibitor of the XIAP/caspase-3 interaction both in vitro and in cellular assays.

Selective targeting of neuroblastoma tumour-initiating cells by compounds identified in stem cell-based small molecule screens

Neuroblastoma (NB) is the most deadly extra-cranial solid tumour in children necessitating an urgent need for effective and less toxic treatments. One reason for the lack of efficacious treatments may be the inability of existing drugs to target the tumour-initiating or cancer stem cell population responsible for sustaining tumour growth, metastases and relapse. Here, we describe a strategy to identify compounds that selectively target patient-derived cancer stem cell-like tumour-initiating cells (TICs) while sparing normal paediatric stem cells (skin-derived precursors, SKPs) and characterize two therapeutic candidates. DECA-14 and rapamycin were identified as NB TIC-selective agents. Both compounds induced TIC death at nanomolar concentrations in vitro, significantly reduced NB xenograft tumour weight in vivo, and dramatically decreased self-renewal or tumour-initiation capacity in treated tumours. These results demonstrate that differential drug sensitivities between TICs and normal paediatric stem cells can be exploited to identify novel, patient-specific and potentially less toxic therapies.

Staurosporine tethered peptide ligands that target cAMP-dependent protein kinase (PKA): optimization and selectivity profiling

We have recently developed a fragment based selection strategy for targeting kinases, where a small molecule warhead can be non-covalently tethered to a phage-displayed library of peptides. This approach was applied to the conversion of the promiscuous kinase inhibitor, staurosporine, into a potent bivalent ligand for cAMP-dependent protein kinase (PKA). Herein we report a systematic evaluation of this new bivalent ligand (BL); (a) Lineweaver-Burke analysis revealed that the BL, unlike substrate-based bivalent kinase inhibitors, displayed non-competitive inhibition with respect to the peptide substrate, suggesting an allosteric mechanism of action; (b) linker optimization of the BL, afforded one of the most potent, sub-nanomolar, inhibitors of PKA reported to date; (c) the BL was found to be modular, where attachment of active site targeted small molecule warheads in lieu of staurosporine could achieve similar gains in affinity; and (d) profiling studies of both the staurosporine derivative and the BL (amide isostere) against a panel of 90 kinases revealed almost unique enhancement in selectivity against PKA (>5-fold) compared to the starting staurosporine derivative. These combined results provide new insights for BL discovery, which has the potential to provide guidance toward the development of kinase selective reagents while uncovering new allosteric sites on kinases for therapeutic targeting.

Potential choline kinase inhibitors: a molecular modeling study of bis-quinolinium compounds

Choline kinase (ChoK) is reported to involve in cell signaling pathways and cell growth by regulating the intermediate, phosphocholine (PCho), which is the first step to biosynthesis a membrane phospholipid, phosphatidylcholine. The PCho levels are overexpressed due to elevated activation of the protein under carcinogenesis conditions. ChoK has thus evolved as a novel target for various cancers and a range of compounds has been reported in this course as potent ChoK inhibitors. However, not much information is known about the binding site of the inhibitors. Therefore, we ventured to unravel the possible binding site of 39 bis-quinolinium inhibitors from which the structural requirement for better protein-ligand complex was delved. Molecular docking and 3D-QSAR studies namely comparative molecular field analysis (CoMFA) and comparative molecular similarity indices analysis (CoMSIA) were performed on the series. The knowledge of the active site was obtained from the site id search and molcad surface calculations of Sybyl, which was further considered for docking studies. In 3D-QSAR, the best predictions were obtained from the model where 29 compounds were considered in the training set and remaining 10 in the test set. The best CoMFA statistics were obtained with r(2) of 0.99 and q(2) of 0.81 while, CoMSIA was resulted with r(2) of 0.98 and q(2) of 0.77. A comparative analysis was done with the resulted 3D-QSAR maps and the docked poses by overlaying the maps on the active site residues. Since, there is no reported ligand co-crystallized structure of ChoK the present study provides valuable clues on the binding conformation of the ligand and its interactions with the active site.

Inhibition of protein kinase C by dequalinium analogues: Structure–activity studies on head group variations

New dequalinium analogues and related heteroaromatic systems were synthesized and evaluated for inhibition of protein kinase Calpha. In vitro assays with recombinant human PKCalpha showed that the number of the aromatic ring head groups as well as their electron-richness, are critical factors that determine potency. The inhibitory strengths of the synthesized compounds are shown to correlate well with Mulliken charges on the head group ring nitrogen atoms making it possible to design likely candidate molecules having improved protein kinase Calpha inhibitory activity.

Synthetic multivalent ligands as probes of signal transduction

Cell-surface receptors acquire information from the extracellular environment and coordinate intracellular responses. Many receptors do not operate as individual entities, but rather as part of dimeric or oligomeric complexes. Coupling the functions of multiple receptors may endow signaling pathways with the sensitivity and malleability required to govern cellular responses. Moreover, multireceptor signaling complexes may provide a means of spatially segregating otherwise degenerate signaling cascades. Understanding the mechanisms, extent, and consequences of receptor co-localization and interreceptor communication is critical; chemical synthesis can provide compounds to address the role of receptor assembly in signal transduction. Multivalent ligands can be generated that possess a variety of sizes, shapes, valencies, orientations, and densities of binding elements. This Review focuses on the use of synthetic multivalent ligands to characterize receptor function.

Dequalinium-induced Protofibril Formation of α-Synuclein

alpha-Synuclein is the major constituent of Lewy bodies, a pathological signature of Parkinson disease, found in the degenerating dopaminergic neurons of the substantia nigra pars compacta. Amyloidosis generating the insoluble fibrillar protein deposition has been considered to be responsible for the cell death observed in the neurodegenerative disorder. In order to develop a controlling strategy toward the amyloid formation, 1,1'-(1,10-decanediyl)-bis-[4-a-mino-2-methylquinolinium] (dequalinium), was selected and examined in terms of its specific molecular interaction with alpha-synuclein. The protein was self-oligomerized by dequalinium, which gave rise to the ladder formation on N-[2-hydroxy-1,1-bis(hydroxymethyl)ethyl]glycine/SDS-PAGE in the presence of a coupling reagent of N-(ethoxycarbonyl)-2-ethoxy-1,2-dihydroquinoline. The double-headed structure of dequalinium with the two cationic 4-aminoquinaldinium rings was demonstrated to be critical for the protein self-oligomerization. The dequalinium-binding site was located on the acidic C-terminal region of the protein with an approximate dissociation constant of 5.5 mum. The protein self-oligomerization induced by the compound has resulted in the protofibril formation of alpha-synuclein before it has developed into amyloids. The protofibrils were demonstrated to affect the membrane intactness of liposomes, and they have also been shown to influence cell viability of human neuroblastoma cells. In addition, dequalinium treatment of the alpha-synuclein-overexpressing cells exerted a significant cell death. Therefore, it is pertinent to consider that dequalinium could be used as a molecular probe to assess toxic mechanisms related to the amyloid formation of alpha-synuclein. Ultimately, the compound could be employed to develop therapeutic and preventive strategies toward alpha-synucleinopathies including Parkinson disease.

Hitting multiple targets with multimeric ligands

Multimeric ligands consist of multiple monomeric ligands attached to a single backbone molecule, creating a multimer that can bind to multiple receptors or targets simultaneously. Numerous examples of multimeric binding exist within nature. Due to the multiple and simultaneous binding events, multimeric ligands bind with an increased affinity compared to their corresponding monomers. Multimeric ligands may provide opportunities in the field of drug discovery by providing enhanced selectivity and affinity of binding interactions, thus providing molecular-based targeted therapies. However, gaps in our knowledge currently exist regarding the quantitative measures for important design characteristics, such as flexibility, length and orientation of the inter-ligand linkers, receptor density and ligand sequence. In this review, multimeric ligand binding in two separate phases is examined. The prerecruitment phase describes the binding of one ligand of a multimer to its corresponding receptor, an event similar to monomeric ligand binding. This results in transient increases in the local concentration of the other ligands, leading to apparent cooperativity. The postrecruitment phase only occurs once all receptors have been aligned and bound by their corresponding ligand. This phase is analogous to DNA-DNA interactions in that the stability of the complex is derived from physical orientation. Multiple factors influence the kinetics and thermodynamics of multimeric binding, and these are discussed.

Fluorescence resonance energy transfer to probe human M1 muscarinic receptor structure and drug binding properties

Human M1 muscarinic receptor chimeras were designed (i) to allow detection of their interaction with the fluorescent antagonist pirenzepine labelled with Bodipy [558/568], through fluorescence resonance energy transfer, (ii) to investigate the structure of the N-terminal extracellular moiety of the receptor and (iii) to set up a fluorescence-based assay to identify new muscarinic ligands. Enhanced green (or yellow) fluorescent protein (EGFP or EYFP) was fused, through a linker, to a receptor N-terminus of variable length so that the GFP barrel was separated from the receptor first transmembrane domain by six to 33 amino-acids. Five fluorescent constructs exhibit high expression levels as well as pharmacological and functional properties superimposable on those of the native receptor. Bodipy-pirenzepine binds to the chimeras with similar kinetics and affinities, indicating a similar mode of interaction of the ligand with all of them. From the variation in energy transfer efficiencies determined for four different receptor-ligand complexes, relative donor (EGFP)-acceptor (Bodipy) distances were estimated. They suggest a compact architecture for the muscarinic M1 receptor amino-terminal domain which may fold in a manner similar to that of rhodopsin. Finally, this fluorescence-based assay, prone to miniaturization, allows reliable detection of unlabelled competitors.

Photo-induced inactivation of protein kinase calpha by dequalinium inhibits motility of murine melanoma cells

Dequalinium (DECA) is a potent antitumor agent and inhibitor of protein kinase C (PKC). Previously it was shown that PKCalpha activity in vitro could be irreversibly inhibited when treated with DECA at low micromolar concentrations and irradiated with 366 nm of light. This approach was used to probe the role of intracellular PKC activity in the motility of metastatic murine melanoma B16 F10 cells and as a target for DECA analogs with increasing PKC inhibitory potencies. Pretreatment of a monolayer of B16 F10 cells with 250 nM of a DECA analog in the presence of UV irradiation for 5 min resulted in 1) complete inhibition of cell motility for up to 4 h in a time-lapse motility assay and 40 to 60% inhibition of cell migration in a Boyden chamber, and 2) inhibition by 40 to 60% of intracellular phosphatidylserine/Ca(2+)-dependent PKC catalytic activity, signifying inactivation of a conventional PKC isoform. Because PKCalpha is the only conventional PKC isoform detected in B16 F10 cells, a stably transfected clone expressing a kinase-defective mutant of PKCalpha was developed that exhibited a substantial loss of adhesion and motility and was refractory to further inhibition by DECA. These findings identify PKCalpha catalytic activity both as a mechanistic component of cell motility and adhesion and as a critical intracellular target of DECA. These studies further suggest that the combined use of UV with nanomolar concentrations of DECA offers an effective chemotherapeutic approach to inhibit metastatic behavior of melanoma cells.