Cationic Molecular Umbrellas as Antibacterial Agents with Remarkable Cell-Type Selectivity

Abstract

We synthesized a combinatorial library of dendrons that display a cluster of cationic charges juxtaposed with a hydrophobic alkyl chain, using the so-called “molecular umbrella” design approach. Systematically tuning the generation number and alkyl chain length enabled a detailed study of the structure–activity relationships in terms of both hydrophobic content and number of cationic charges. These discrete, unimolecular compounds display rapid and broad-spectrum bactericidal activity comparable to the activity of antibacterial peptides. Micellization was examined by pyrene emission and dynamic light scattering, which revealed that monomeric, individually solvated dendrons are present in aqueous media. The antibacterial mechanism of action is putatively driven by the membrane-disrupting nature of these cationic surfactants, which we confirmed by enzymatic assays on E. coli cells. The hemolytic activity of these dendritic macromolecules is sensitively dependent on the dendron generation and the alkyl chain length. Via structural optimization of these two key design features, we identified a leading candidate with potent broad-spectrum antibacterial activity (4–8 μg/mL) combined with outstanding hemocompatibility (up to 5000 μg/mL). This selected compound is >1000-fold more active against bacteria as compared to red blood cells, which represents one of the highest selectivity index values ever reported for a membrane-disrupting antibacterial agent. Thus, the leading candidate from this initial library screen holds great potential for future applications as a nontoxic, degradable disinfectant.

From Nanobiotechnology, Positively Charged Biomimetic Dendrimers as Novel Antibacterial Agents: A Review

“…Research in this field is, however, still very limited and, as far as our knowledge is concerned, the literature presents only two studies. A further study not dealing with Ds but only dendrons, reporting the synthesis of low-generation polyester dendrons, based on bis -HMPA, peripherally cationic for the presence of β-alanine, has been recently published by Chen and colleagues (2020) [ 96 ] ( Table 8 ).…”

“…A second study on dendrimer-like polyester-based molecules, prepared using bis-HMPA as a building block and peripherally functionalized with an amino acid (β-alanine) in order to confer the cationic character essential for exerting the antibacterial activity on contact, was recently reported by Chen et al (2020) [ 96 ] ( Table 8 ).…”

“…Among the compounds of each generation, the most active were those bearing the longest C14 chain ( Table 8 ), confirming that a greater hydrophobicity could help dendrons diffuse from OM toward the CM (in the case of Gram-negative bacteria) and/or through the CM destroying it and inactivating the bacterium. These compounds, tested for their hemolytic toxicity, showed that HC 50 values increased with the generation of the devices (from 10 to 5000 μg/mL), thus affirming the absence of hemolytic toxicity for G3/C14 (HC 50 = 5000 μg/mL), associated with a potent antimicrobial activity against both the bacteria species (MIC values = 7.8 and 3.9 μg/mL, respectively) [ 96 ]. The greater hemolytic activity of the lower-generation dendrons is probably due to their greater hydrophobicity, deriving from the presence of the same C14 chain, and a lower number of cationic charges, which helps such molecules diffuse through cells membranes, thus improving their non-specific lytic activity.…”

“…The results showed that the number of viable E. coli cells decreased immediately, and that within the first 30 min, the number of E. coli viable cells was reduced by approximately 2-logs (99.7% killing). Furthermore, after 1 and 2 h of incubation, reductions of 3.8 and 4.5-logs (99.9985 and 99.9997% killing) respectively were observed [ 96 ]. Considerably, the rate of bactericidal action was more rapid than the rate at which these polyester bis-HMPA-based dendrons typically are degraded by hydrolysis (37 °C, pH 7) [ 18 ].…”

“…Investigations into the mechanism of action on representative examples of dendrons from the library developed by Chen’s group, revealed that cationic dendrons indeed exerted a potent lytic activity against E. coli membranes, probably supported by the presence of the hydrophobic chain pendant from the dendron structure. Cytotoxicity essays on HeLa cell lines revealed that within the library, G2/C14 was cytotoxic (LC 50 = 32 μg/mL), G3/C14 moderately cytotoxic (LC 50 = 85 μg/mL), whereas G3/C2 was completely non-toxic, up to the highest concentration tested (LC 50 > 250 μg/mL), but also completely inactive against the pathogen [ 96 ].…”