Functional organisation and gain of activity: The case of the antibacterial tetra-para-guanidinoethyl-calix[4]arene

Enhancing Photothermal Energy Transduction through Inter- and Intramolecular Interactions of Multiple Two-Photon Dyes Appended onto Calix[4]arene

Organic dyes-based photothermal agents (OPTAs) have received increasing attention as alternative to inorganic materials due to their higher biocompatibility and extensive diversification. Maximizing nonradiative deexcitation channels is crucial to improve the photothermal conversion efficiency (PCE) of OPTAs. This is typically achieved through individual molecular design or collective enhancement using supramolecular strategies. Furthermore, photothermal therapy (PTT) generally relies on linear one-photon absorption of the light source by the OPTA, with less consideration given to nonlinear two-photon absorption (2PA) strategies, despite their potential benefits. Here, a synergistic strategy, which combines intramolecular and intermolecular quenching, is employed to maximize the photothermal efficiency of diphenylamino-substituted distyryl dicyanobenzene (DSB), an outstanding two-photon-absorbing chromophore. One to three DSB units have been introduced on the conic p-tert-butyl-calix[4]arene (CX), serving as a preorganizing platform to allow aggregate formation and promote intramolecular quenching within the multichromophoric systems. Importantly, the multichromophoric molecules had very high two-photon absorption capabilities with cross sections (δ2PA) reaching maximal values of 3290 GM at 810 nm. Experimental data accompanied by large-scale molecular dynamics simulations and time-dependent density functional theory calculations shed light onto the interaction mechanism in those multiple DSB-appended CX compounds to rationalize their optical properties. Then, the formulation with Pluronic F127 amphiphile yields water-dispersible nanoprecipitates (Nps), in which the PCE is further maximized and the photobleaching is reduced due to the combination of intra- and intermolecular quenching. The high two-photon absorption in the near-infrared (NIR) window associated with the high PCE of these nanosized OPTAs could serve as a basis to future in vivo 2P-PTT applications.

Functionalized Calixarenes as Promising Antibacterial Drugs to Face Antimicrobial Resistance

Since the discovery of polyphenolic resins 150 years ago, the study of polymeric compounds named calix[n]arene has continued to progress, and those skilled in the art perfectly know now how to modulate this phenolic ring. Consequently, calix[n]arenes are now used in a large range of applications and notably in therapeutic fields. In particular, the calix[4]arene exhibits multiple possibilities for regioselective polyfunctionalization on both of its rims and offers researchers the possibility of precisely tuning the geometry of their structures. Thus, in the crucial research of new antibacterial active ingredients, the design of calixarenes finds its place perfectly. This review provides an overview of the work carried out in this aim towards the development of intrinsically active prodrogues or metallic calixarene complexes. Out of all the work of the community, there are some excellent activities emerging that could potentially place these original structures in a very good position for the development of new active ingredients.

Synthesis of Ionizable Calix[4]arenes for Chelation of Selected Divalent Cations

Two sets of functionalised calix[4]arenes, either with a 1,3-crown ether bridge or with an open-chain oligo ether moiety in 1,3-position were prepared and further equipped with additional deprotonisable sulfonamide groups to establish chelating systems for selected cations Sr2+, Ba2+, and Pb2+ ions. To improve the complexation behaviour towards these cations, calix[4]arenes with oligo ether groups and modified crowns of different sizes were synthesized. Association constants were determined by UV/Vis titration in acetonitrile using the respective perchlorate salts and logK values between 3.2 and 8.0 were obtained. These findings were supported by the calculation of the binding energies exemplarily for selected complexes with Ba2+.

Thiacalixarene based quaternary ammonium salts as promising antibacterial agents

The search for new antibacterial and antiseptic drugs is an urgent problem due to the resistance of microorganisms to existing drugs. In this work, for the first time, the design of antibacterial and bactericidal agents based on quaternary ammonium compounds on thiacalixarene macrocyclic platform was proposed and implemented. A series of tetrasubstituted quaternary ammonium salts with different nature and length of the substituent (-N⁺(CH₃)₂R, R = CH₂Ph, C_nH_2n+1, n = 1, 4, 8, 10) based on p-tert-butylthiacalix[4]arene in cone and 1,3-alternate conformations was obtained with excellent yields. The obtained compounds have a high antibacterial effect against Gram-positive (S. aureus, S. epidermidis, B. subtilis) bacteria comparable with commercial antiseptics chlorhexidine, miramistin and benzalkonium chloride. It was found that quaternary ammonium derivatives of thiacalix[4]arene in 1,3-alternate conformation more effectively inhibit the growth of the tested bacterial strains in comparison with compounds in cone conformation. Cytotoxicity studies on human skin fibroblast (HSF) cells demonstrated that all compounds were less toxic compared to reference drugs. The different type of interaction of the studied compounds with model DPPC lipid membranes explains different antibacterial activity and cytotoxicity of compounds. The compounds in cone conformation are adsorbed on the DPPC vesicles membrane surface, while the incorporation of lipophilic alkyl fragments of macrocycles in 1,3-alternate conformation into the membrane leads to "clumping" of DPPC vesicles. It was shown the saving of antibacterial activity of thiacalixarene derivatives in 1,3-alternate conformation on Gram-positive clinical strains. The obtained results allow viewing the described thiacalixarene based quaternary ammonium compounds as promising molecules in the development of the new antibacterial agents.

Antimicrobial Activity of Calixarenes and Related Macrocycles

Calixarenes and related macrocycles have been shown to have antimicrobial effects since the 1950s. This review highlights the antimicrobial properties of almost 200 calixarenes, resorcinarenes, and pillararenes acting as prodrugs, drug delivery agents, and inhibitors of biofilm formation. A particularly important development in recent years has been the use of macrocycles with substituents terminating in sugars as biofilm inhibitors through their interactions with lectins. Although many examples exist where calixarenes encapsulate, or incorporate, antimicrobial drugs, one of the main factors to emerge is the ability of functionalized macrocycles to engage in multivalent interactions with proteins, and thus inhibit cellular aggregation.

The antibacterial activity of p-tert-butylcalix[6]arene and its effect on a membrane model: molecular dynamics and Langmuir film studies

The antibacterial activity of a calixarene derivative, p-tert-butylcalix[6]arene (Calix6), was assessed and was shown not to inhibit the growth of E. coli, S. aureus and B. subtilis bacteria. With the aim of gaining more insights into the absence of antibacterial activity of Calix6, the interaction of this derivative with DPPG, a bacterial cell membrane lipid, was studied. Langmuir monolayers were used as the model membrane. Pure DPPG and pure Calix6 monolayers, as well as binary DPPG:Calix6 mixtures were studied using surface pressure measurements, compressional modulus, Brewster angle and fluorescence microscopies, ellipsometry, polarization-modulation infrared reflection absorption spectroscopy and molecular dynamics simulations. Thermodynamic properties of the mixed monolayers were additionally calculated using thermodynamic parameters. The analysis of isotherms showed that Calix6 significantly affects the DPPG monolayers, modifying the isotherm profile and increasing the molecular area, in agreement with the molecular dynamics simulations. The presence of Calix6 in the mixed monolayers decreased the interfacial elasticity, indicating that calixarene disrupts the strong intermolecular interactions of DPPG hindering its organization into a compact arrangement. At low molar ratios of Calix6, the DPPG:Calix6 interactions are preferentially attractive, due to the interactions between the hydrophobic tails of DPPG and the tert-butyl groups of Calix6. Increasing the proportion of calixarene generates repulsive interactions. Calix6 significantly affects the hydrophobic tail organization, which was confirmed by PM-IRRAS measurements. Calix6 appears to be expelled from the mixed films at a biologically relevant surface pressure, π = 30 mN m-1, indicating a low interaction with the cell membrane model related to the absence of antibacterial activity.

Calix[4]API-s: fully functionalized calix[4]arene-based facial active pharmaceutical ingredients

This mini-review covers 25 fully functionalized facial calix[4]arene-based symmetrical and conical cyclic tetramers with significant (comparable to established therapeutic agents) anticancer and anti-infective activities. The main role of the calixarene scaffold in these calix[4]arene-based active pharmaceutical ingredients (calix[4]API-s) is to replicate embedded phenolic units in the cyclic tetramers. So, probably owing to the multivalency, facial, conical structures of calix[4]API-s and synergistic effect of their four replicated units, they can be considered as effective bioactive agents.

Synthesis and Antimicrobial Assessment of Fe3+ Inclusion Complex of p-tert-Butylcalix[4]arene Diamide Derivative

Present study deals with the synthesis of the p-tert-butylcalix[4]arene diamide derivative as ligand (L) and its Fe3+ complex, followed by its characterization using TLC and FT-IR, while UV-Vis and Job’s plot study were performed for complex formation. Antimicrobial activity of the derivative (L) and its metal complex was carried out by the disc diffusion method against bacteria (Escherichia coli and Staphylococcus albus) and fungi (R. stolonifer). Different concentrations of the derivative (L) (6, 3, 1.5, 0.75, and 0.37 μg/mL) and its Fe3+ complex were prepared, and Mueller–Hinton agar was used as the medium for the growth of microorganisms. Six successive dilutions of the derivative (L) and Fe3+ complex were used against microorganisms. Two successive dilutions (6 and 3 μg/mL) of the derivative (L) showed antibacterial action against both Gram-positive and Gram-negative bacteria. In addition, three successive dilutions (6, 3, and 1.5 μg/mL) of the derivative (L) showed antifungal activity. However, all of six dilutions of the Fe3+ complex showed antimicrobial activity. Derivative (L) showed 3 and 1.5 μg/mL minimum inhibitory concentrations (MIC) against bacteria and fungi, respectively. On the contrary, its Fe3+ complex showed 0.37 μg/mL value of MIC against bacteria and fungi. Hence, Fe3+ complex of the derivative (L) was found to be a more effective antimicrobial agent against selected bacteria and fungi than the diamide derivative (L).

Calixarenes: Generalities and Their Role in Improving the Solubility, Biocompatibility, Stability, Bioavailability, Detection, and Transport of Biomolecules

The properties and characteristics of calix[n]arenes are described, as well as their capacity to form amphiphilic assemblies by means of the design of synthetic macrocycles with a hydrophilic head and a hydrophobic tail. Their interaction with various substances of interest in pharmacy, engineering, and medicine is also described. In particular, the role of the calix[n]arenes in the detection of dopamine, the design of vesicles and liposomes employed in the manufacture of systems of controlled release drugs used in the treatment of cancer, and their role in improving the solubility of testosterone and anthelmintic drugs and the biocompatibility of biomaterials useful for the manufacture of synthetic organs is emphasized. The versatility of these macrocycles, able to vary in size, shape, functional groups, and hydrophobicity and to recognize various biomolecules and molecules with biological activity without causing cytotoxicity is highlighted.

Design, synthesis and antibacterial evaluation of a polycationic calix[4]arene derivative alone and in combination with antibiotics

The growing antibiotic resistance phenomenon continues to stimulate the search for new compounds and strategies to combat bacterial infections. In this study, we designed and synthesized a new polycationic macrocyclic compound (2) bearing four N-methyldiethanol ammonium groups clustered and circularly organized by a calix[4]arene scaffold. The in vitro activity of compound 2, alone and in combination with known antibiotics (ofloxacin, chloramphenicol or tetracycline), was assessed against strains of Staphylococcus aureus (ATCC 6538 and methicillin-resistant isolate 15), S. epidermidis (ATCC 35984 and methicillin-resistant isolate 57), and Pseudomonas aeruginosa (ATCC 9027 and antibiotic-resistant isolate 1). Calix[4]arene derivative 2 showed significant antibacterial activity against ATCC and methicillin-resistant Gram positive Staphylococci, improved the stability of tetracycline in water, and in combination with antibiotics enhanced the antibiotic efficacy against Gram negative P. aeruginosa by an additive effect.

The selective interactions of cationic tetra-p-guanidinoethylcalix[4]arene with lipid membranes: theoretical and experimental model studies

Behavior of cationic tetra-p-guanidinoethylcalix[4]arene (CX1) and its building block, p-guanidinoethylphenol (mCX1) in model monolayer lipid membranes was investigated using all atom molecular dynamics simulations and surface pressure measurements. Members of two classes of lipids were taken into account: zwitterionic 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) and anionic 1,2-dimyristoyl-sn-glycero-3-phospho-l-serine sodium salt (DMPS) as models of eukaryotic and bacterial cell membranes, respectively. It was demonstrated that CX1 and mCX1 accumulate near the negatively charged DMPS monolayers. The adsorption to neutral monolayers was negligible. In contrast to mCX1, CX1 penetrated into the hydrophobic part of the monolayer. The latter effect, which is possible due to a flip-flop inversion of the CX1 orientation in the lipid layer compared to the aqueous phase, may be responsible for its antibacterial activity.

Preparation and characterization of amphiphilic calixarene nanoparticles as delivery carriers for paclitaxel

Two types of amphoteric calix[n]arene carboxylic acid (CnCA) derivative, i.e., calix[6]arene hexa-carboxylic acid (C6HCA) and calix[8]arene octo-carboxylic acid (C8OCA), were synthesized by introducing acetoxyls into the hydroxyls of calix[n]arene (n=6, 8). C6HCA and C8OCA nanoparticles (NPs) were prepared successfully using the dialysis method. CnCA NPs had regular spherical shapes with an average diameter of 180-220 nm and possessed negative charges of greater than -30 mV. C6HCA and C8OCA NPs were stable in 4.5% bovine serum albumin solutions and buffers (pH 5-9), with a low critical aggregation concentration value of 5.7 mg·L(-1) and 4.0 mg·L(-1), respectively. C6HCA and C8OCA NPs exhibited good paclitaxel (PTX) loading capacity, with drug loading contents of 7.5% and 8.3%, respectively. The overall in vitro release behavior of PTX from the CnCA NPs was sustained, and C8OCA NPs had a slower release rate compared with C6HCA NPs. These favorable properties of CnCA NPs make them promising nanocarriers for tumor-targeted drug delivery.

The aqueous medium-dimethylsulfoxide conundrum in biological studies

A series of straight and branched chain pyrogallol[4]arenes was studied and found to be essentially nontoxic to two strains of E. coli.

Atomic Force Microscopy and pharmacology: from microbiology to cancerology

BACKGROUND

Atomic Force Microscopy (AFM) has been extensively used to study biological samples. Researchers take advantage of its ability to image living samples to increase our fundamental knowledge (biophysical properties/biochemical behavior) on living cell surface properties, at the nano-scale.

SCOPE OF REVIEW

AFM, in the imaging modes, can probe cells morphological modifications induced by drugs. In the force spectroscopy mode, it is possible to follow the nanomechanical properties of a cell and to probe the mechanical modifications induced by drugs. AFM can be used to map single molecule distribution at the cell surface. We will focus on a collection of results aiming at evaluating the nano-scale effects of drugs, by AFM. Studies on yeast, bacteria and mammal cells will illustrate our discussion. Especially, we will show how AFM can help in getting a better understanding of drug mechanism of action.

MAJOR CONCLUSIONS

This review demonstrates that AFM is a versatile tool, useful in pharmacology. In microbiology, it has been used to study the drugs fighting Candida albicans or Pseudomonas aeruginosa. The major conclusions are a better understanding of the microbes' cell wall and of the drugs mechanism of action. In cancerology, AFM has been used to explore the effects of cytotoxic drugs or as an innovative diagnostic technology. AFM has provided original results on cultured cells, cells extracted from patient and directly on patient biopsies.

GENERAL SIGNIFICANCE

This review enhances the interest of AFM technologies for pharmacology. The applications reviewed range from microbiology to cancerology.

Cationic quaternized aminocalix[4]arenes: cytotoxicity, haemolytic and antibacterial activities

This study reports the characterization of three cationic amphiphillic aminocalix[4]arenes as potential antimicrobial agents in vitro. In cytotoxicity tests on mouse macrophage RAW 264.7 cells aminocalix[4]arenes 1 and 3 showed no toxicity up to 200 and 100 μM concentrations, respectively, while 2 was non-toxic only up to 50 μM. With regard to the haemolytic activity on rabbit red blood cells, 1 was not active at concentrations up to 100 μM in contrast to the other two studied macrocycles. Compounds showed negligible ability to protect either mouse macrophage RAW 264.7 cells from anthrax lethal toxin of Bacillus anthracis (B. anthracis) or rabbit red blood cells from α-haemolysin of Staphylococcus aureus (S. aureus) in comparison to amino-β-cyclodextrins. However, all aminocalix[4]arenes showed potential as antimicrobials. Their minimum inhibitory concentrations (MIC) against Escherichia coli (E. coli) and S. aureus were in the 16-32 μg/ml concentration range, while minimum lethal concentrations (MLC) varied from 16 to 256 μg/ml depending on the bacteria and aminocalix[4]arene considered. Macrocycle 1 showed partial synergism against S. aureus in tandem with a model antibacterial drug, fusidic acid, at certain concentration combinations.

Calix[6]arene bypasses human pancreatic cancer aggressiveness: downregulation of receptor tyrosine kinases and induction of cell death by reticulum stress and autophagy

Pancreatic cancer ranks fourth among cancer-related causes of death in North America. Minimal progress has been made in the diagnosis and treatment of patients with late-stage tumors. Moreover, pancreatic cancer aggressiveness is closely related to high levels of pro-survival mediators, which can ultimately lead to rapid disease progression, resistance and metastasis. The main goal of this study was to define the mechanisms by which calix[6]arene, but not other calixarenes, efficiently decreases the aggressiveness of a drug resistant human pancreas carcinoma cell line (Panc-1). Calix[6]arene was more potent in reducing Panc-1 cell viability than gemcitabine and 5-fluorouracil. In relation to the underlying mechanisms of cytotoxic effects, it led to cell cycle arrest in the G0/G1 phase through downregulation of PIM1, CDK2, CDK4 and retinoblastoma proteins. Importantly, calix[6]arene abolished signal transduction of Mer and AXL tyrosine kinase receptors, both of which are usually overexpressed in pancreatic cancer. Accordingly, inhibition of PI3K and mTOR was also observed, and these proteins are positively modulated by Mer and AXL. Despite decreasing the phosphorylation of AKT at Thr308, calix[6]arene caused an increase in phosphorylation at Ser473. These findings in conjunction with increased BiP and IRE1-α provide a molecular basis explaining the capacity of calix[6]arene to trigger endoplasmic reticulum stress and autophagic cell death. Our findings highlight calix[6]arene as a potential candidate for overcoming pancreatic cancer aggressiveness. Importantly, we provide evidence that calix[6]arene affects a broad array of key targets that are usually dysfunctional in pancreatic cancer, a highly desirable characteristic for chemotherapeutics.

Intramolecular interactions versus hydration effects on p-guanidinoethyl-phenol structure and pKa values

We analyze the structure, hydration, and pK(a) values of p-guanidinoethyl-phenol through a combined experimental and theoretical study. These issues are relevant to understand the mechanism of action of the tetrameric form, the antibacterial compound tetra-p-guanidinoethyl-calix[4]arene (Cx1). The investigated system can also be useful to model other pharmaceutical drugs bearing a guanidine function in the vicinity of an ionizable group and the effect of arginine on the pK(a) of vicinal ionizable residues (in particular tyrosine) in peptides. The p-guanidinoethyl-phenol monomer (mCx1) has two ionizable groups. One important particularity of this system is that it exhibits high molecular flexibility that potentially leads to enhanced stabilization in folded structures by direct, strong Coulombic interactions between the ionizable groups. The first pK(a) corresponding to ionization of the -OH group has experimentally been shown to be only slightly different from usual values in substituted phenols. However, because of short-range Coulombic interactions, the role of intramolecular interactions and solvation effects on the acidities of this compound is expected to be important and it has been analyzed here on the basis of theoretical calculations. We use a discrete-continuum solvation model together with quantum-mechanical calculations at the B3LYP level of theory and the extended 6-311+G(2df,2p) basis set. Both intra- and intermolecular effects are very large (~70 kcal/mol) but exhibit an almost perfect compensation, thus explaining that the actual pK(a) of mCx1 is close to free phenol. The same compensation of environmental effects applies to the second pK(a) that concerns the guanidinium group. Such a pK(a) could not be determined experimentally with standard titration techniques and in fact the theoretical study predicts a value of 14.2, that is, one unit above the pK(a) of the parent ethyl-guanidinium molecule.

Nanoscale analysis of the effects of antibiotics and CX1 on a Pseudomonas aeruginosa multidrug-resistant strain

Drug resistance is a challenge that can be addressed using nanotechnology. We focused on the resistance of the bacteria Pseudomonas aeruginosa and investigated, using Atomic Force Microscopy (AFM), the behavior of a reference strain and of a multidrug resistant clinical strain, submitted to two antibiotics and to an innovative antibacterial drug (CX1). We measured the morphology, surface roughness and elasticity of the bacteria under physiological conditions and exposed to the antibacterial molecules. To go further in the molecules action mechanism, we explored the bacterial cell wall nanoscale organization using functionalized AFM tips. We have demonstrated that affected cells have a molecularly disorganized cell wall; surprisingly long molecules being pulled off from the cell wall by a lectin probe. Finally, we have elucidated the mechanism of action of CX1: it destroys the outer membrane of the bacteria as demonstrated by the results on artificial phospholipidic membranes and on the resistant strain.

Preparation and physicochemical characterization of a novel paclitaxel-loaded amphiphilic aminocalixarene nanoparticle platform for anticancer chemotherapy

OBJECTIVES

This paper describes the development and optimization of a nanoparticle delivery platform for the anticancer agent, paclitaxel, using a novel amphiphilic carrier, tetrahexyloxy-tetra-p-aminocalix[4]arene (A4C(6) ).

METHODS

Nanoparticles were successfully prepared at pH4 by an emulsion evaporation method whereby an organic phase containing paclitaxel: A4C(6) (molar ratio 1:10) was dispersed by probe sonication into an aqueous phase containing 0.5% w/v polyvinyl alcohol as stabilizer.

KEY FINDINGS

The drug-loaded nanoparticles had a mean size of 78.7±20.7nm, surface potential of 38.3±7.67mV, and paclitaxel loading and encapsulation efficiencies of 69.1±5.3µg drug/mg carrier and 50.4±3.2%, respectively. Transmission electron micrographs showed discrete particles with no evidence of agglomeration. In-vitro dissolution into phosphate buffered saline supplemented with 4% bovine serum albumin showed 32.7±3.9%, 82.6±5.3% and 91.0±6.0% of the encapsulated paclitaxel load was released at 5, 72 and 120h, respectively.

CONCLUSIONS

This is the first report on the use of amino-substituted amphiphilic calixarenes for the encapsulation of anticancer agents. The nanoparticles produced were significantly smaller than, but had comparable drug loads to the Abraxane nanoparticles, and have the potential to achieve targeted delivery of paclitaxel to tumour tissues.

Molecular drug-organiser: synthesis, characterization and biological evaluation of penicillin V and/or nalidixic acid calixarene-based podands

Two well-known antibiotic heterocycles, the 'quinolone' nalidixic acid and the β-lactam penicillin V, active at different levels of the bacterial growth process, have been attached via an ether-ester junction to the p-tert-butylcalix[4]arene lower rim, in alternate position. The resulting hydrophobic molecular drug-organisers were fully characterized, and evaluated over two Gram negative and three Gram positive reference strains, using disk diffusion assays with disks impregnated with solution of title compound in pure DMSO. An interesting activity was observed over Staphylococcus aureus ATCC 25923 with the dis-symmetrical podand incorporating one penicillin and one nalidixic ester moieties.

In vitro activity of para-guanidinoethylcalix[4]arene against susceptible and antibiotic-resistant Gram-negative and Gram-positive bacteria

OBJECTIVES

Emergence of multidrug-resistant bacteria has encouraged vigorous efforts to develop antimicrobial agents with new mechanisms of action. In this study, the in vitro antibacterial activity of para-guanidinoethylcalix[4]arene was evaluated and compared with that of its constitutive monomer, para-guanidinoethylphenol. Hexamidine, a widely used antiseptic, and synthalin A, an old antidiabetic and anti-trypanosomal compound, were chosen as references.

METHODS

MIC and MBC were determined for five reference strains (Escherichia coli ATCC 25922, Staphylococcus aureus ATCC 25923 and ATCC 29213, Enterococcus faecalis ATCC 29212 and Pseudomonas aeruginosa ATCC 27853), as well as five antibiotic-resistant clinical isolates. Toxicity on MRC-5 and HaCaT eukaryotic cell lines was also evaluated by MTT and Neutral Red assays.

RESULTS

No antibacterial activity was observed for para-guanidinoethylphenol (MIC >or= 512 mg/L) and synthalin A (MIC >or= 64 mg/L). Conversely, para-guanidinoethylcalix[4]arene and hexamidine: (i) showed a broad antibacterial spectrum, both on Gram-positive and on Gram-negative bacteria (MIC = 4 mg/L against E. coli and 8 mg/L against S. aureus for para-guanidinoethylcalix[4]arene), to a lesser degree against E. faecalis and P. aeruginosa (MIC = 32 mg/L); (ii) were bacteriostatic (MBC >or= 256 mg/L); and (iii) MICs and MBCs obtained for clinical isolates were similar to those obtained with reference strains. Both compounds, the monomer and the calixarene, showed no apparent cytotoxicity, whereas hexamidine and synthalin A had significant toxic effects that increased with time and concentration and in a range of 100-1000 times that for calixarene.

CONCLUSIONS

In conclusion, results confirm para-guanidinoethylcalix[4]arene as a broad-spectrum new agent or an auxiliary in antimicrobial chemotherapy.

Vers de nouvelles molécules antibactériennes. Intérêt du para-guanidinoéthylcalix[4]arène

We present here the results concerning the antibacterial properties evaluation of para-guanidinoethylcalix[4]arene, compared with its constitutive monomer, the para-guanidinoethylphenol, and hexamidine (Hexomédine), an antiseptic from the diamidine family widely used in therapeutic, chosen as a reference in this study for its resemblance in terms of functional groups. Antibacterial activities of those three compounds were evaluated by microdilution methods, in Mueller Hinton broth, onto 5 bacterial strains: Escherichia coli ATCC 25922, Pseudomonas aeruginosa ATCC 27853, Staphylococcus aureus ATCC 25923 & ATCC 29213 and Enterococcus faecalis ATCC 29212, according to CA-SFM and CLSI (formerly NCCLS) approved standards. In parallel, the effects of these three compounds on MRC-5 eukaryotic cell viability were evaluated with MTT assay. The results obtained here confirm a lack of activity for the monomer compound (MIC> or =512 mg/l) and a real antibacterial activity for the calixarene, comparable to hexamidine. This activity is expressed, both on Gram+and Gram- bacteria (MIC=4 mg/l for E. coli, 8 mg/l on both S. aureus strains) and at a lesser degree on E. faecalis and P. aeruginosa (MIC=32 mg/l). Similarly, both compounds, monomer and calixarene, slightly induce any modification on MRC-5 cells viability, and this until 168 h of treatment for concentrations reaching 10(-4) mol/L while hexamidine demonstrates a significant and increasing effect during the time of experiment and this for 100 to 1000 times lower concentrations. Thus, this study tends to confirm the significance of the organization of the para-guanidinoethylphenol monomer into its cyclic calixarenic tetramer for the gain of an antibacterial activity, similar to a widely used antiseptic one.