Synthesis and quantitative structure-activity relationship of a novel series of small conductance Ca(2+)-activated K+ channel blockers related to dequalinium

Development of Single-Stranded DNA Bisintercalating Inhibitors of Primase DnaG as Antibiotics

Many essential enzymes in bacteria remain promising potential targets of antibacterial agents. In this study, we discovered that dequalinium, a topical antibacterial agent, is an inhibitor of Staphylococcus aureus primase DnaG (SaDnaG) with low-micromolar minimum inhibitory concentrations against several S. aureus strains, including methicillin-resistant bacteria. Mechanistic studies of dequalinium and a series of nine of its synthesized analogues revealed that these compounds are single-stranded DNA bisintercalators that penetrate a bacterium by compromising its membrane. The best compound of this series likely interacts with DnaG directly, inhibits both staphylococcal cell growth and biofilm formation, and displays no significant hemolytic activity or toxicity to mammalian cells. This compound is an excellent lead for further development of a novel anti-staphylococcal therapeutic.

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.

Bis-Quinolinium Cyclophane Blockers of SK Potassium Channels Are Antagonists of M3 Muscarinic Acetylcholine Receptors

Dequalinium is used as an antimicrobial compound for oral health and other microbial infections. Derivatives of dequalinium, the bis-quinolinium cyclophanes UCL 1684 and UCL 1848, are high affinity SK potassium channel antagonists. Here we investigated these compounds as M3 muscarinic receptor (mACHR) antagonists. We used the R-CEPIAer endoplasmic reticulum calcium reporter to functionally assay for Gq-coupled receptor signaling, and investigated the bis-quinolinium cyclophanes as antagonists of M3 mACHR activation in transfected CHO cells. Given mACHR roles in airway smooth muscle (ASM) contractility, we also tested the ability of UCL 1684 to relax ASM. We find that these compounds antagonized M3 mACHRs with an IC₅₀ of 0.27 μM for dequalinium chloride, 1.5 μM for UCL 1684 and 1.0 μM for UCL 1848. UCL 1684 also antagonized M1 (IC₅₀ 0.12 μM) and M5 (IC₅₀ 0.52 μM) mACHR responses. UCL 1684 was determined to be a competitive antagonist at M3 receptors as it increased the EC₅₀ for carbachol without a reduction in the maximum response. The Ki for UCL1684 determined from competition binding experiments was 909 nM. UCL 1684 reduced carbachol-evoked ASM contractions (>90%, IC₅₀ 0.43 μM), and calcium mobilization in rodent and human lung ASM cells. We conclude that dequalinium and bis-quinolinium cyclophanes antagonized M3 mACHR activation at sub- to low micromolar concentrations, with UCL 1684 acting as an ASM relaxant. Caution should be taken when using these compounds to block SK potassium channels, as inhibition of mACHRs may be a side-effect if excessive concentrations are used.

Pharmacological modulation of mitochondrial ion channels

The field of mitochondrial ion channels has undergone a rapid development during the last three decades, due to the molecular identification of some of the channels residing in the outer and inner membranes. Relevant information about the function of these channels in physiological and pathological settings was gained thanks to genetic models for a few, mitochondria-specific channels. However, many ion channels have multiple localizations within the cell, hampering a clear-cut determination of their function by pharmacological means. The present review summarizes our current knowledge about the ins and outs of mitochondrial ion channels, with special focus on the channels that have received much attention in recent years, namely, the voltage-dependent anion channels, the permeability transition pore (also called mitochondrial megachannel), the mitochondrial calcium uniporter and some of the inner membrane-located potassium channels. In addition, possible strategies to overcome the difficulties of specifically targeting mitochondrial channels versus their counterparts active in other membranes are discussed, as well as the possibilities of modulating channel function by small peptides that compete for binding with protein interacting partners. Altogether, these promising tools along with large-scale chemical screenings set up to identify new, specific channel modulators will hopefully allow us to pinpoint the actual function of most mitochondrial ion channels in the near future and to pharmacologically affect important pathologies in which they are involved, such as neurodegeneration, ischaemic damage and cancer. LINKED ARTICLES: This article is part of a themed section on Mitochondrial Pharmacology: Featured Mechanisms and Approaches for Therapy Translation. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v176.22/issuetoc.

Time-dependent diaryl ether inhibitors of InhA: structure-activity relationship studies of enzyme inhibition, antibacterial activity, and in vivo efficacy

The diaryl ethers are a novel class of antituberculosis drug candidates that inhibit InhA, the enoyl-ACP reductase involved in the fatty acid biosynthesis (FASII) pathway, and have antibacterial activity against both drug-sensitive and drug-resistant strains of Mycobacterium tuberculosis. In the present work, we demonstrate that two time-dependent B-ring modified diaryl ether InhA inhibitors have antibacterial activity in a mouse model of TB infection when delivered by intraperitoneal injection. We propose that the efficacy of these compounds is related to their residence time on the enzyme, and to identify structural features that modulate drug-target residence time in this system, we have explored the inhibition of InhA by a series of B-ring modified analogues. Seven ortho-substituted compounds were found to be time-dependent inhibitors of InhA, where the slow step leading to the final enzyme-inhibitor complex (EI*) is thought to correlate with closure and ordering of the InhA substrate binding loop. A detailed mechanistic understanding of the molecular basis for residence time in this system will facilitate the development of InhA inhibitors with improved in vivo activity.

Further studies on bis-charged tetraazacyclophanes as potent inhibitors of small conductance Ca(2+)-activated K+ channels

Previously, quinolinium-based tetraazacyclophanes, such as UCL 1684 and UCL 1848, have been shown to be extraordinarily sensitive to changes in chemical structure (especially to the size of the cyclophane system) with respect to activity as potent non-peptidic blockers of the small conductance Ca(2+)-activated K(+) ion channels (SKCa). The present work has sought to optimize the structure of the linking chains in UCL 1848. We report the synthesis and SKCa channel-blocking activity of 29 analogues of UCL 1848 in which the central CH2 of UCL 1848 is replaced by other groups X or Y = O, S, CF2, CO, CHOH, CC, CHCH, CHMe to explore whether subtle changes in bond length or flexibility can improve potency still further. The possibility of improving potency by introducing ring substituents has also been explored by synthesizing and testing 25 analogues of UCL 1684 and UCL 1848 with substituents (NO2, NH2, CF3, F, Cl, CH3, OCH3, OCF3, OH) in the 5, 6 or 7 positions of the aminoquinolinium rings. As in our earlier work, each compound was assayed for inhibition of the afterhyperpolarization (AHP) in rat sympathetic neurons, an action mediated by the SK3 subtype of the SKCa channel. One of the new compounds (39, R(7) = Cl, UCL 2053) is twice as potent as UCL 1848 and UCL 1684: seven are comparable in activity.

Trafficking of intermediate (KCa3.1) and small (KCa2.x) conductance, Ca(2+)-activated K(+) channels: a novel target for medicinal chemistry efforts?

Ca(2+)-activated K(+) (KCa) channels play a pivotal role in the physiology of a wide variety of tissues and disease states, including vascular endothelia, secretory epithelia, certain cancers, red blood cells (RBC), neurons, and immune cells. Such widespread involvement has generated an intense interest in elucidating the function and regulation of these channels, with the goal of developing pharmacological strategies aimed at selective modulation of KCa channels in various disease states. Herein we give an overview of the molecular and functional properties of these channels and their therapeutic importance. We discuss the achievements made in designing pharmacological tools that control the function of KCa channels by modulating their gating properties. Moreover, this review discusses the recent advances in our understanding of KCa channel assembly and anterograde trafficking toward the plasma membrane, the micro-domains in which these channels are expressed within the cell, and finally the retrograde trafficking routes these channels take following endocytosis. As the regulation of intracellular trafficking by agonists as well as the protein-protein interactions that modify these events continue to be explored, we anticipate this will open new therapeutic avenues for the targeting of these channels based on the pharmacological modulation of KCa channel density at the plasma membrane.

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.

Synthesis and pharmacological testing of polyaminoquinolines as blockers of the apamin-sensitive Ca2+-activated K+ channel (SK(Ca))

The synthesis and pharmacological testing of a series of non-peptidic blockers of the SK(Ca) (SK-3) channel is described. Target compounds were designed to mimic the spatial relationships of selected key residues in the energy-minimised structure of the octadecapeptide apamin, which are a highly potent blocker of this channel. Structures consist of a central unit, either a fumaric acid or an aromatic ring, to which are attached two alkylguanidine or two to four alkylaminoquinoline substituents. Potency was tested by the ability to inhibit the SK(Ca) channel-mediated after-hyperpolarization (AHP) in cultured rat sympathetic neurones. It was found that bis-aminoquinoline derivatives are significantly more potent as channel blockers than are the corresponding guanidines. This adds to the earlier evidence that delocalisation of positive charge through the more extensive aminoquinolinium ring system is important for effective channel binding. It was also found that an increase in activity can be gained by the addition of a third aminoquinoline residue to give non-quaternized amines which have submicromolar potencies (IC(50)=0.13-0.36 microM). Extension to four aminoquinoline residues increased the potency to IC(50)=93 nM.

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.

Broad-Based Quantitative Structure−Activity Relationship Modeling of Potency and Selectivity of Farnesyltransferase Inhibitors Using a Bayesian Regularized Neural Network

Inhibitors of the enzyme farnesyltransferase show potential as novel anticancer agents. There are many known inhibitors, but efforts to build predictive SAR models have been hampered by the structural diversity and flexibility of inhibitors. We have undertaken for the first time a QSAR study of the potency and selectivity of a large, diverse data set of farnesyltransferase inhibitors. We used novel molecular descriptors based on binned atomic properties and invariants of molecular matrices and a robust, nonlinear QSAR mapping paradigm, the Bayesian regularized neural network. We have built robust QSAR models of farnesyltransferase inhibition, geranylgeranyltransferase inhibition, and in vivo data. We have derived a novel selectivity index that allows us to model potency and selectivity simultaneously and have built robust QSAR models using this index that have the potential to discover new potent and selective inhibitors.

Defining determinant molecular properties for the blockade of the apamin-sensitive SKCa channel in guinea-pig hepatocytes: the influence of polarizability and molecular geometry

QSAR studies of a series of blockers of the SK(Ca) channel in guinea-pig hepatocytes suggests that the polarizability of the blocker is an important factor controlling the binding to the channel. It is suggested that, upon binding, an ion-pair is formed, a process that is promoted by the reorganization of the water molecules. The polarizability is not adequate to describe the potency of the most potent blockers with a good stereochemical fit to the channel, presumably due to more specific interactions taking place.

Compounds that block both intermediate-conductance (IK(Ca)) and small-conductance (SK(Ca)) calcium-activated potassium channels

1. Nine bis-quinolinyl and bis-quinolinium compounds related to dequalinium, and previously shown to block apamin-sensitive small conductance Ca(2+)-activated K(+) channels (SK(Ca)), have been tested for their inhibitory effects on actions mediated by intermediate conductance Ca(2+)-activated K(+) channels (IK(Ca)) in rabbit blood cells. 2. In most experiments, a K(+)-sensitive electrode was employed to monitor the IK(Ca)-mediated net loss of cell K(+) that followed the addition of the Ca(2+) ionophore A23187 (2 microM) to red cells suspended at an haematocrit of 1% in a low K(+) (0.12 - 0.17 mM) solution. The remainder used an optical method based on measuring the reduction in light transmission that occurred on applying A23187 (0.4 or 2 microM) to a very dilute suspension of red cells (haematocrit 0.02%). 3. Of the compounds tested, the most potent IK(Ca) blocker was 1,12 bis[(2-methylquinolin-4-yl)amino]dodecane (UCL 1407) which had an IC(50) of 0.85+/-0.06 microM (mean+/-s.d. mean). 4. The inhibitory action of UCL 1407 and its three most active congeners was characterized by (i) a Hill slope greater than unity, (ii) sensitivity to an increase in external [K(+)], and (iii) a time course of onset that suggested use-dependence. Also, the potency of the nonquaternary compounds tested increased with their predicted lipophilicity. These findings suggested that the IK(Ca) blocking action resembles that of cetiedil rather than of clotrimazole. 5. Some quaternized members of the series were also active. The most potent was the monoquaternary UCL 1440 ((1-[N-[1-(3, 5-dimethoxybenzyl)-2-methylquinolinium-4-yl]amino]-10-[N'-(2-me thylqu inolinium-4yl)amino] decane (trifluoroacetate) which had an IC(50) of 1.8+/-0.1 microM. The corresponding bisquaternary UCL 1438 (1, 10-bis[N-[1-(3,5-dimethoxybenzyl)-2-methylquinolinium-4-yl]amino] decane bis(trifluoroacetate) was almost as active (IC(50) 2.7+/-0.3 microM). 6. A bis-aminoquinolium cyclophane (UCL 1684) had little IK(Ca) blocking action despite its great potency at SK(Ca) channels (IC(50) 4.1+/-0.2 nM). 7. The main outcome is the identification of new intermediate-conductance Ca(2+)-activated K(+) channel blockers with a wide range of IK(Ca)/SK(Ca) selectivities.

Dual-site binding of bivalent 4-aminopyridine- and 4-aminoquinoline-based AChE inhibitors: contribution of the hydrophobic alkylene tether to monomer and dimer affinities

Three series of 4-aminopyridine-and 4-aminoquinoline based symmetrical bivalent acetylcholinesterase (AChE) inhibitors were prepared and compared to previously synthesized dimers of 9-amino-1,2,3,4-tetrahydroacridine (tacrine). In each case significant, tether length-dependent increases in AChE inhibition potency and selectivity (up to 3000-fold) were observed relative to the corresponding monomer, indicating dual-site binding of these inhibitors to AChE. Assay of the corresponding alkylated monomers revealed that the alkylene tether played at least two complementary roles in the dimer series. In addition to reducing the entropy loss that occurs on binding both monomeric units of the dimer, the alkylene tether can also significantly improve potency through hydrophobic effects.

Evaluation of short-tether bis-THA AChE inhibitors. A further test of the dual binding site hypothesis

To provide a further test of the dual binding site hypothesis proposed for acetylcholinesterase (AChE) inhibitor heptylene-linked bis-(9-amino-1,2,3,4-tetrahydroacridine) A7A, short-tether (ethylene hexylene) homologs A2A-A6A were prepared. En route to these compounds, convenient and scaleable syntheses of useful pharmaceutical intermediate 9-chloro-1.2,3,4-tetrahydroacridine 3 and A7A were developed. AChE and butyrylcholinesterase (BChE) inhibition assays of A2A-A10A confirm that a seven methylene tether (A7A) optimizes AChE inhibition potency and AChE/BChE selectivity. Finally, these studies indicate that simultaneous binding of alkylene-linked 9-amino-1,2,3,4-tetrahydroacridine dimers to the catalytic and peripheral sites of AChE is possible with a tether length as short as 5 methylenes.

On the concept of a bivalent pharmacophore for SKCa channel blockers: synthesis, pharmacological testing, and radioligand binding studies on mono-, bis-, and tris-quinolinium compounds

The dissociation equilibrium constants (Kd values) of dequalinium (2) and the monoquinolinium compounds 1a and 1b have been determined from competition equilibrium radioligand binding with [125I]apamin on rat brain synaptic plasma membranes (SPMs). Dequalinium binds to the channel with 2 orders of magnitude higher affinity than 1a or 1b, suggesting that both quinolinium groups are needed for potent and selective SKCa channel blockade. The trisquinolinium compound 3 (1,1'-[5-[4-(4- aminoquinolinium-1-yl)but-1-yl]non-4-en-1,9-diyl]-bis-(4- aminoquinolinium)) has been synthesized and tested for inhibition of the afterhyperpolarization of rat sympathetic neurones and on the binding assay. Compound 3 shows approximately one order of magnitude higher potency than 2, being the most potent non-peptidic SKCa channel blocker reported so far (Kd approximately 30 nM). The higher affinity of 3 compared with 2 may be due to direct binding of the third quinolinium group to the channel or may arise from a reduction of the unfavorable entropy of binding via an increase of the "local concentration" of quinolinium groups.