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.

Impact of Tetracationic Calix[4]arene Conformation-from Conic Structure to Expanded Bolaform-on Their Antibacterial and Antimycobacterial Activities.

The four possible conformers of a new tetrakisguanidino calix[4]arene thought to interact deleteriously with bacterial membranes have been synthesized, characterized, and evaluated for their in vitro cytotoxicity and antibacterial activity against various reference Gram-negative and Gram-positive bacteria, as well as Mycobacterium tuberculosis. It appears that reversal of at least one phenolic unit results in clear increases in their activities. This can be attributed to the evolution towards bolaform structures, which are able to interact more deeply with the bacterial membrane. Indeed, the 1,3-alternate conformer 16 exhibits the best antibacterial activity (MIC<1.0 µg mL-1 on Staphylococcus aureus). Moreover, 16 displays very good antibacterial activities against an isoniazid-resistant strain of M. tuberculosis (MIC=1.2 µg mL-1 ), associated with the lowest cytotoxicity, thus making it the most potent compound of the series; this could open new ways of research in the field of anti-infective drug development to meet the huge current demand.

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.

Guanidinium compounds with sub-micromolar activities against Mycobacterium tuberculosis. Synthesis, characterization and biological evaluations.

Seven polycharged species, incorporating 1, 2, 3, 4 and 6 guanidine arms organized around a benzene core were synthesized and assayed as anti-mycobacterial agents against Mycobacterium tuberculosis. They display MIC values comprised between 25 and 12.5 µM (close to ethambutol EMB) for the mono- and the hexa-substituted derivatives, and 0.8 µM (close to isoniazid and streptomycin) for the tri-substituted derivative. The three bi- and the tetra-substituted analogs displayed MIC values of ca. 6.5-3.0 µM. The latter were also evaluated against the isoniazid-resistant MYC5165 strain, resulting in highly interesting micromolar or sub-micromolar MIC, ca. 4-125 times more active than isoniazid. These preliminary results are attractive for the development of new anti-TB agents.

Molecular organization of nalidixate conjugated calixarenes in bacterial model membranes probed by molecular dynamics simulation and Langmuir monolayer studies.

Two p-tert-butylcalix[4]arene derivatives bearing one or two nalidixic acid groups connected to the lower rim of p-tert-butylcalix[4]arene through the propylenic spacer were studied upon interaction with model bacterial membranes. Indeed, these derivatives were developed recently as new macrocyclic antibiotic carriers for antibacterial therapy. To obtain molecular level information about the interaction between the calixarene conjugates and a membrane lipid, atomistic molecular dynamics simulation, as well as surface pressure, surface potential, polarization modulation infrared reflection-absorption spectroscopy, and Brewster angle microscopy studies of 1,2-dimyristoyl-sn-glycero-3-phosphoethanolamine (DMPE)-calixarene derivative films were performed. The results obtained indicate that the interaction between the calixarene derivatives and DMPE occurs via the phosphate and carbonyl groups present in the lipid. Although both calixarene derivatives increase the chain tilt and conformational disordering of the DMPE molecules, these effects are more important in the case of the monosubstituted derivative. Importantly, the two derivatives have an opposite impact on hydration of the phosphoglyceride polar head.

Poly-guanidinoethylphenylethers organised around a benzene ring: synthesis and evaluation of their antibacterial and cytotoxic properties.

A new family of poly-guanidinium species with a benzene core as the organising agent has been developed. Their antibacterial properties have been evaluated against reference Gram positive and Gram negative bacteria, and their cytotoxicity against eukaryotic cells. Most of these compounds exhibited Minimum Inhibitory Concentrations in the micromolar range.

Synthesis and evaluation of 1,ω-bis(1,2,3,5-thiatriazol-5-yl)alkanes as in vitro and in vivo α-amylase and lipase inhibitors.

Thionyl chloride reacts with 1,ω-bis-(1-tosylamidrazone)alkanes 1 to give a series of 1,ω-bis-(4-alkyl-2-tosyl-1,2,3,5-thiatriazol-5-yl)alkanes 2. All the newly synthesized compounds were characterized by IR, 1H NMR, 13C NMR, elemental analysis, and ESI-MS spectral data. All the new compounds were screened for their inhibitory effect on key enzymes related to diabetes and obesity, such as α-amylase and lipase. In vitro and in vivo studies revealed that these thiatriazole derivatives exert an inhibitory action against these key enzymes. Moreover the administration of these compounds to surviving diabetic rats induced a significant decrease in plasma glucose level. Additively 2d significantly protected the liver-kidney functions and modulated lipid metabolism, which were evidenced by the decrease in aspartate transaminase (AST), alanine transaminase (ALT), and gamma-glutamyl transpeptidase (GGT) activities and creatinine, urea albumin, LDL-cholesterol and triglycerides levels as well as an increase in the HDL-cholesterol level in surviving diabetic rats. Overall, the findings of the current study indicate that 2d exhibits attractive properties and can, therefore, be considered for future application in the development of anti-diabetic and hypolipidemic drugs.

Interaction of a ß-lactam calixarene derivative with a model eukaryotic membrane affects the activity of PLA2.

In this research, the interaction between a membrane phospholipid, 1,2-dilauroyl-sn-glycero-3-phosphocholine (DLPC), and a p-tert-butylcalix[4]arene derivative bearing 6-aminopenicillanic acid (Calix), conceived as a possible drug carrier, was studied. The Langmuir film balance technique was used to measure surface pressure and electrical surface potential of pure and mixed Calix/DLPC monolayers spread on water at different temperatures. Phospholipase A2 (PLA2) activity was used as well to detect the impact of the calixarene derivative on the monolayer properties. Interaction between the molecules in mixed monolayers has been described quantitatively using thermodynamic functions. Interestingly, low amounts of Calix introduce ordering in the lipid film. This effect may be analogous to that of cholesterol interacting with phospholipids. A lower activity of PLA2 observed with the Calix/DLPC films compared to pure DLPC may be related to structural modifications of the mixed systems.

Langmuir monolayers of an inclusion complex formed by a new calixarene derivative and fullerene.

The design of new molecules with directed interactions to functional molecules as complementary building blocks is one of the main goals of supramolecular chemistry. A new p-tert-butylcalix[6]arene monosubstituted derivative bearing only one alkyl chain with an acid group (C6A3C) has been synthesized. The C6A3C has been successfully used for building Langmuir monolayers at the air-water interface. The C6A3C molecule adopts a flatlike orientation with respect to the air-water interface. The molecular structure gives the molecule amphiphilic character, while allowing the control of both the dissociation degree and the molecular conformation at the air-water interface. The C63AC has been combined with pristine fullerene (C60) to form the supramolecular complex C6A3C:C60 in 2:1 molar ratio (CFC). The CFC complex retains the ability of C6A3C to form Langmuir monolayers at the air/water interface. The interfacial molecular arrangement of the CFC complex has been convincingly described by in situ UV-vis reflection spectroscopy and synchrotron X-ray reflectivity measurements. Computer simulations complement the experimental data, confirming a perpendicular orientation of the calixarene units of CFC with respect to the air-water interface. This orientation is stabilized by the formation of intermolecular H-bonds. The interfacial monolayer of the CFC supramolecular complex is proposed as a useful model for the well-defined self-assembly of recognition and functional building blocks.

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.

Anti-mycobacterial activities of some cationic and anionic calix[4]arene derivatives.

Various polycharged calix[4]arenes were assayed as anti-mycobacterial agents against Mycobacterium tuberculosis, H(37)Rv strain. The sulfonate, carboxylate and phosphonate anionic species displayed no activity. Cationic derivatives integrating four aminoethyl groups at the upper rim and two 6,6'-dimethyl-2,2'-bipyridyl- or 4,4'-dimethyl-2,2'-bithiazolyl subunits at the lower rim were also found inactive against M. tuberculosis, while the unsubstituded and the 5,5'-dimethyl-2,2'-bipyridyl-analogues exhibited MIC values of 3.2 and 0.8µM respectively. Introduction of guanidinoethyl groups at the upper rim resulted, except for the 6,6'-dimethyl-2,2'-bipyridyl-derivative, in high anti-mycobacterial activities for the unsubstituted, the 5,5'-dimethyl-2,2'-bipyridyl- and the 4,4'-dimethyl-2,2'-bithiazolyl analogues, with MIC values of 0.8, 0.8 and 1.6µM, respectively, similar to those of current commercial anti-tuberculosis agents. The five more active substances were also evaluated against the isoniazid-resistant strain MYC5165, resulting in highly interesting micromolar or sub-micromolar MIC and IC(50), ca. 4-125 times more active than isoniazid. These preliminary results are attractive for the development of new anti-TB agents.

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 ß-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.

The mechanism of metal cation binding in two nalidixate calixarene conjugates. A langmuir film and molecular modeling study.

The two new p-tert-butylcalix[4]arene derivatives described here bear one or two nalidixic acid arms linked to the lower calixarene rim via the quinolone carboxylate moiety. These derivatives were synthesized in order to investigate two important features of molecules conceived as potential antibiotics, namely, metal cation complexation and interfacial properties, and the way in which they interrelate. The properties of the calixarene derivatives were studied in monomolecular films spread on pure water and on aqueous subphases containing biologically relevant mono- and divalent metal cations. These systems were examined via surface pressure and surface electrical potential measurements, polarization modulation infrared reflection absorption spectroscopy, and molecular modeling. Molecular modeling shows that important differences exist, first, between the structure and stability of the complexes formed with the two derivatives and, second, between their mono- and dication complexes. Correlating the properties of the monolayers with those of the modeled molecules lets us propose that the derivatives bearing one or two nalidixic pending arms form preferentially inter- and intramolecular complexes, respectively. The results obtained in this study indicate that a possible biological role of the nalidixic arms grafted on the calixarene crown may be revealed upon cation complexation.

Synthesis and anti-HIV evaluation of water-soluble calixarene-based bithiazolyl podands.

Nine anionic water-soluble calix[4]arene species, incorporating sulfonate, carboxylate or phosphonate groups, six of them incorporating two 2,2'-bithiazole subunits in alternate position at the lower rim, have been synthesised and evaluated as anti-HIV agents on various HIV strains and cells of the lymphocytic lineage (HIV-1 III B/MT4, HIV-1 LAI/CEM-SS, HIV-1 Bal/PBMC), using AZT as reference compound. A toxicity was detected for a minority of compounds on PBMC whereas for the others no cellular toxicity was measured at concentrations up to 100 microM. Most of the compounds have an antiviral activity in a 10-50 microM range, and one of them, sulfonylated, displays its activity, whatever the tropism of the virus, at a micromolar concentration.

p-Guanidinoethyl calixarene and parent phenol derivatives exhibiting antibacterial activities. Synthesis and biological evaluation.

The tetra-para-guanidinoethyl-calix[4]arene, its distally-disubstituted ether derivatives involving 2,2'-bithiazolyl- or 2,2'-bipyridyl-methyl groups, as well as the para-guanidinoethylphenol and its analogous derivatives have been synthesized, fully characterized and evaluated as antibacterial agents towards both gram positive and gram negative reference bacteria. The simple phenolic species showed lower activity than their calixarene analogues, confirming the hypothesis that a synergistic effect should result from the spatial organization of guanidinium and heterocycles on a macrocyclic scaffold. Introduction of the bithiazole and bipyridine substituents enhanced the activity of simple phenol derivatives, reaching, for the two Staphylococcus aureus strains in particular, the values obtained for their calixarenic parents. MTT viability assays were carried out to determine selectivity indexes.

Towards calixarene-based prodrugs: Drug release and antibacterial behaviour of a water-soluble nalidixic acid/calix[4]arene ester adduct.

A water-soluble calixarene-based heterocyclic podand incorporating a quinolone antibiotic subunit, the nalidixic acid, was synthesised and fully characterised. Its prodrug behaviour was assessed in vitro by HPLC, demonstrating the release of the tethered quinolone in model biological conditions. Microbiological studies performed on various Gram-positive and Gram-negative reference strains showed very interesting antibacterial activities.

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.

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

The antibacterial activities of the para-guanidinoethylphenol and of its cyclic tetramer, the tetra-para-guanidinoethyl-calix[4]arene, have been evaluated on reference gram-positive and gram-negative bacteria. Antibiotic disk diffusion assays completed by micromethod technique were employed to determine if a synergistic effect could be expected from the spatial organisation of the monomer into its cyclic tetrameric analogue. Disk diffusion assays and microdilution experiments revealed better properties for the calixarene species, with a real and important gain of activity with regards to the monomer.