Durairaj Thiyagarajan

Synthesis, crystal structure and bio-macromolecular interaction studies of pyridine-based thiosemicarbazone and its Ni(II) and Cu(II) complexes

A new pyridine-based heterocyclic thiosemicarbazone ligand and its Ni(II) and Cu(II) complexes have been synthesized and characterized by structural, analytical and spectral methods. The mono-deprotonated anionic form of the ligand coordinates via NNS donor atoms to yield an octahedral Ni(II) complex and distorted square planar Cu(II) complex. UV-visible and fluorescence-based spectroscopic techniques revealed that both metal complexes interact with double stranded DNA via intercalation. A comparative assessment indicated that the Ni(II) complex displayed superior DNA binding. The interaction of these compounds with bovine serum albumin (BSA) suggested that the ligand and its Cu(II) complex quenched the intrinsic fluorescence of BSA in a static quenching process, whereas for the Ni(II) complex, fluorescence quenching of BSA was a combination of both static and collision/dynamic quenching processes. The quenching of the fluorescence of BSA is owing to energy transfer from the tryptophan residues of BSA to the compounds bound to BSA. Cytotoxicity tests based on the standard MTT assay revealed that the Cu(II) complex displayed prominent anti-proliferative activity against HeLa cells.

Biocompatible nanocarrier fortified with a dipyridinium-based amphiphile for eradication of biofilm.

Annihilation of bacterial biofilms is challenging owing to their formidable resistance to therapeutic antibiotics and thus there is a constant demand for development of potent antibiofilm agents that can abolish established biofilms. In the present study, the activity of a dipyridinium-based cationic amphiphile (compound 1) against established bacterial biofilms and the subsequent development of a compound 1-loaded nanocarrier for potential antibiofilm therapy are highlighted. Solution-based assays and microscopic analysis revealed the antagonistic effect of compound 1 on biofilms formed by Staphylococcus aureus MTCC 96 and Pseudomonas aeruginosa MTCC 2488. In combination studies, compound 1 could efficiently potentiate the action of tobramycin and gentamicin on P. aeruginosa and S. aureus biofilm, respectively. A human serum albumin (HSA)-based nanocarrier loaded with compound 1 was generated, which exhibited sustained release of compound 1 at physiological pH. The compound 1-loaded HSA nanocarrier (C1-HNC) displayed the signature membrane-directed activity of the amphiphile on target bacteria, efficiently eliminated established bacterial biofilms, and was observed to be nontoxic to a model human cell line. Interestingly, compound 1 as well as the amphiphile-loaded HSA nanocarrier could eradicate established S. aureus biofilm from the surface of a Foleys urinary catheter. On the basis of its biocompatibility and high antibiofilm activity, it is conceived that the amphiphile-loaded nanocarrier may hold potential in antibiofilm therapy.

Synthetic amphiphiles as therapeutic antibacterials: lessons on bactericidal efficacy and cytotoxicity and potential application as an adjuvant in antimicrobial chemotherapy

In this paper, we present a critical assessment of the therapeutic potential of low molecular weight pyridine-based synthetic amphiphiles based on structure-guided bactericidal activity and a rational evaluation of their cytotoxic potential. Fluorescence-based structure-function studies revealed that the amphiphiles were membrane-acting and displayed a hierarchical pattern of bactericidal activity, which could be correlated with their charge density and hydrophobicity. The membrane-targeting activity of the most potent cationic amphiphile (compound 6) was vindicated as it induced extensive membrane-disruption and dissipation of the transmembrane potential (ΔΨ) in pathogenic bacteria. At concentrations equivalent to the minimum inhibitory concentration (MIC) against the Gram-positive pathogen S. aureus MTCC 96, none of the amphiphiles exerted any cytotoxic effect on model human cell lines (HeLa, MCF-7 and HT-29). However, at elevated concentrations, a distinct gradation in the cytotoxic effect was manifested, which is probably accounted by the charge density and conformational flexibility of the amphiphiles. A viable therapeutic application of compound 6 is demonstrated in combinatorial assays, wherein the proclivity of the amphiphile to disrupt bacterial membranes at very low concentration is exploited to enhance the uptake and bactericidal efficacy of erythromycin against Gram-negative pathogenic bacteria.

A zinc complex of a neutral pyridine-based amphiphile: a highly efficient and potentially therapeutic bactericidal material

The alarming rise in antibiotic-resistant pathogenic bacteria demands a prudent approach in the generation of therapeutic antibacterials. The present study illustrates the development of a potent amphiphilic bactericidal material tailored to leverage interactions with metal-reactive groups (MRGs) present in the bacterial cell surface envelope. Complexation of Zn(ii) with a neutral pyridine-based synthetic amphiphile (C1) generated the cationic C1-Zn, which exhibited manyfold higher membrane-directed bactericidal activity compared to the neutral C1, or the cationic amphiphile bearing two pyridinium head groups (C2). The relevance of MRGs in C1-Zn-bacteria interactions was validated by amphiphile-bacteria binding studies and metal protection assays performed with Mg(ii). C1-Zn retained its bactericidal activity even in simulated gastric fluid (SGF) and the enhanced membrane-directed bactericidal activity of C1-Zn could be garnered in adjuvant applications to increase the efficacy of the therapeutic antibiotic erythromycin. Given the relevance of Zn(ii) in S. aureus biofilm formation, the antibiofilm potential of the amphiphile C1 realized through Zn(ii) complexation could be demonstrated. The lack of resistance in target bacteria coupled with a favorable therapeutic index (IC50/MIC) and non-toxic nature hold significant implications for C1-Zn as a potential antibacterial therapeutic material.

Amphiphile-mediated enhanced antibiotic efficacy and development of a payload nanocarrier for effective killing of pathogenic bacteria

Synthetic amphiphiles have emerged as potent bactericidal scaffolds owing to their high propensity to interact with bacterial cells and a membrane-directed mode of action, which is likely to overcome resistance development in pathogenic bacteria. In this study, we highlighted a membrane-acting quinolinium-based cationic amphiphile (compound 1) as an adjuvant for antibiotic-mediated eradication of pathogenic bacteria and demonstrated the generation of an amphiphile-loaded nanocarrier for potential antibacterial therapy. Treatment of Gram-negative pathogenic bacteria E. coli MTCC 433 and P. aeruginosa MTCC 2488 with 1 resulted in significant augmentation of the activity of erythromycin and a decrease in the minimum inhibitory concentration of the antibiotic. Interestingly, 1 promoted large-scale eradication of P. aeruginosa MTCC 2488 biofilm and could also enhance the anti-biofilm activity of tobramycin in combination. For potential therapeutic applications, a 1-loaded bovine serum albumin-based nanocarrier was developed, which exhibited sustained release of 1 both in physiological and acidic pH and the released amphiphile displayed antibacterial as well as anti-biofilm activities. Interestingly, the nanocarrier also displayed the signature membrane-directed activity of 1 against tested pathogenic bacteria. The high bactericidal and anti-biofilm activities in conjunction with a lack of cytotoxic effect on HT-29 human cell lines enhance the merit of the amphiphile-loaded nanocarrier as a potentially therapeutic antibacterial against clinically relevant drug-resistant pathogenic bacteria.

A near-infrared emissive Al3+ sensing platform for specific detection in solution, cells and probing DNase activity

A new tricarbocyanine-based chemosensor exhibited a dramatic Al(3+)-specific fluorescence turn-on response in the near-infrared (NIR) region. The receptor was found to be highly selective towards Al(3+) over other metal ions in physiological condition. The sensor was non-toxic and could thus be employed as an imaging probe for detecting intracellular Al(3+) in live cells. Interestingly, upon interaction with DNA in solution, the L-Al(3+) ensemble rendered tracking of DNase activity in solution through a systematic reduction in the fluorescence emission intensity.

Amphiphilic Cargo-loaded Nanocarrier Enhances Antibiotic Uptake and Perturbs Efflux: Effective Synergy for Mitigation of Methicillin-resistant Staphylococcus aureus

A pyridinium‐amphiphile‐loaded poly(lactic‐co‐glycolic acid) (PLGA) nanocarrier (C1‐PNC) was developed as an adjuvant in order to break the resistance and restore the susceptibility of methicillin‐resistant Staphylococcus aureus (MRSA) cells to therapeutic antibiotics. Notably, against a clinical MRSA strain, C1‐PNC was found to render 8‐ and 6‐fold decreases in the minimum biofilm eradication concentration (MBEC90) of gentamicin and ciprofloxacin, respectively. Mechanistic studies on MRSA planktonic cells revealed that in the case of gentamicin, C1‐PNC promotes enhanced cellular uptake of the antibiotic, whereas the propensity of C1‐PNC to inhibit efflux pump activity could be leveraged to enhance cellular accumulation of ciprofloxacin, leading to effective killing of MRSA cells. Interestingly, the combinatorial dosing regimen of C1‐PNC and the antibiotics was nontoxic to cultured HEK293 cells. This nontoxic amphiphile‐loaded nanomaterial holds considerable promise as an adjuvant for antibiotic‐mediated alleviation of MRSA biofilms.

A Nonbactericidal Zinc‐Complexing Ligand as a Biofilm Inhibitor: Structure‐Guided Contrasting Effects on Staphylococcus aureus Biofilm

Zinc‐complexing ligands are prospective anti‐biofilm agents because of the pivotal role of zinc in the formation of Staphylococcus aureus biofilm. Accordingly, the potential of a thiosemicarbazone (compound C1) and a benzothiazole‐based ligand (compound C4) in the prevention of S. aureus biofilm formation was assessed. Compound C1 displayed a bimodal activity, hindering biofilm formation only at low concentrations and promoting biofilm growth at higher concentrations. In the case of C4, a dose‐dependent inhibition of S. aureus biofilm growth was observed. Atomic force microscopy analysis suggested that at higher concentrations C1 formed globular aggregates, which perhaps formed a substratum that favored adhesion of cells and biofilm formation. In the case of C4, zinc supplementation experiments validated zinc complexation as a plausible mechanism of inhibition of S. aureus biofilm. Interestingly, C4 was nontoxic to cultured HeLa cells and thus has promise as a therapeutic anti‐biofilm agent. The essential understanding of the structure‐driven implications of zinc‐complexing ligands acquired in this study might assist future screening regimes for identification of potent anti‐biofilm agents.