The rise of multidrug-resistant Gram-negative bacteria poses a significant threat to global health. Polymyxins, particularly polymyxin B (PMB), are considered last-resort antibiotics for treating infections caused by these pathogens. However, the nephrotoxicity and emergence of resistance limit their clinical use. Therefore, strategies to enhance PMB efficacy while mitigating its toxicity are urgently needed. Nanocarriers, such as cubosomes, offer a promising approach for improving drug delivery and efficacy. Cubosomes are self-assembled, bicontinuous cubic liquid crystalline structures composed of lipids, providing a unique environment for drug encapsulation and controlled release. This research aimed to evaluate the potential of PMB-loaded cubosomes to improve the antimicrobial activity of PMB against Gram-negative bacteria. The study delved into the mechanism of action, exploring the interaction between PMB-loaded cubosomes and bacterial membranes, and ultimately aimed to provide a novel strategy for combating antibiotic resistance.

The literature extensively documents the increasing prevalence of multi-drug resistant Gram-negative bacteria and the consequent need for new therapeutic strategies. Polymyxins have re-emerged as a last-line defense against these pathogens, but concerns about toxicity and emerging resistance necessitate approaches to enhance their efficacy and reduce side effects. Numerous studies have investigated different drug delivery systems to improve antibiotic efficacy. Nanoparticle-based delivery systems, including liposomes and cubosomes, have shown promise in enhancing drug penetration and reducing toxicity. Cubosomes, in particular, are attractive due to their unique structural properties and biocompatibility. Previous research has explored the use of cubosomes for delivering various drugs, including other antimicrobial peptides, but their application with polymyxins has been less explored. This research builds upon the existing knowledge of both polymyxin limitations and the potential of cubosomes as a drug delivery system.

The researchers synthesized PMB-loaded cubosomes using phytantriol as the lipid phase. The physicochemical properties of the cubosomes were characterized using techniques such as dynamic light scattering (DLS), cryo-transmission electron microscopy (cryo-TEM), and small-angle neutron scattering (SANS). The interaction of the cubosomes with model bacterial membranes and live bacterial cells was investigated using QCM-D. This technique measures the changes in frequency and dissipation of a quartz crystal upon adsorption of molecules to its surface, providing information on the mass and viscoelastic properties of the adsorbed layer. Confocal microscopy was employed to visualize the distribution of PMB within the bacterial cells. The antimicrobial activity of PMB-loaded cubosomes was evaluated against various Gram-negative bacterial strains using minimum inhibitory concentration (MIC) assays. The release kinetics of PMB from the cubosomes was determined using a dialysis method. Neutron reflectometry (NR) experiments provided detailed information on the interaction of cubosomes with supported lipid bilayers (SLBs) mimicking bacterial membranes.

The study revealed that PMB-loaded cubosomes exhibited enhanced antimicrobial activity compared to free PMB. The MIC values for PMB-loaded cubosomes were significantly lower than those for free PMB against multiple Gram-negative bacterial strains. QCM-D measurements showed increased adsorption of PMB-loaded cubosomes to the bacterial membrane compared to free PMB. Confocal microscopy imaging confirmed the efficient delivery of PMB into the bacterial cells by the cubosomes. Neutron reflectometry data indicated a specific interaction between the cubosomes and the bacterial membrane, likely facilitating PMB penetration and action. The PMB release from the cubosomes was sustained, suggesting a potential for prolonged antimicrobial activity. The researchers found that the enhanced antimicrobial activity was not simply due to increased PMB concentration but rather due to the interaction of the cubosomes with the bacterial membrane and the consequent facilitation of PMB uptake.

The findings of this study demonstrate the potential of using cubosomes to enhance the efficacy of PMB against Gram-negative bacteria. The enhanced activity is attributed to improved membrane interaction, facilitated PMB delivery, and sustained drug release provided by the cubosomes. This novel approach could address some of the limitations of PMB, particularly the emergence of resistance and toxicity. The sustained release of PMB from the cubosomes could potentially reduce the required dosage, mitigating the risk of nephrotoxicity. The specific interaction between cubosomes and the bacterial membrane warrants further investigation, focusing on the underlying mechanisms and factors influencing this interaction. This work highlights the potential of using nanocarrier technology to combat antibiotic resistance and improve the treatment of infections caused by multi-drug resistant Gram-negative bacteria. Further research could focus on optimizing cubosome formulations for improved stability, targeted delivery, and reduced toxicity.

This research successfully demonstrated the potential of PMB-loaded cubosomes as a novel strategy to enhance the antimicrobial activity of PMB against Gram-negative bacteria. The improved efficacy is likely due to a combination of increased membrane interaction, facilitated PMB delivery, and sustained drug release. This approach offers a promising avenue for addressing the challenges associated with PMB use, particularly the emergence of resistance and toxicity. Future studies should focus on in vivo efficacy and safety evaluation to translate this promising approach to clinical settings.

The study primarily utilized in vitro models. While the results are promising, further in vivo studies are necessary to validate the findings and assess the efficacy and safety of PMB-loaded cubosomes in a living organism. The study also focused on a limited number of bacterial strains. Further research should expand the range of tested strains to assess the generality of the findings. Long-term stability and scalability of the cubosome formulation should also be addressed for potential clinical translation.