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pH-responsive drug delivery systems are yielding opportunities to directly deliver antibiotics to the site of infection. Therefore, this study aimed to develop and evaluate novel pH-responsive lipid−dendrimer hybrid nanoparticles (LDH-NPs) for the delivery of vancomycin (VCM) to the site of infection, by intracellular bacterial pathogens. The LDH-NPs were formulated using the emulsification solvent evaporation method and were characterized by various in vitro and molecular dynamic (MD) simulation techniques. LDH-NPs were 124.4 ± 2.01 nm in size, with a zeta-potential of −7.15 ± 2.98 mV and drug entrapment efficiency of 82.70 ± 4.09%, which exhibited pH-responsive behavior by shifting the surface charge from negative at physiological pH to positive in acidic pHs, with a size increase from 124.4 ± 2.01 to 173.9 ± 13.38 nm, and 252.7 ± 3.98 nm at pHs of 7.4, 6.0, and 4.5, respectively. Results indicated that the in vitro drug release of VCM from LDH-NPs occurred faster at pH 6.0 than at pH 7.4. The antibacterial activity of LDH-NPs against methicillin-resistance Staphylococcus aureus (MRSA) showed 8-fold lower MICs at pH 6.0 and 7.4, compared to treatment with VCM only. A bacterial cell viability study showed LDH-NPs had an 84.19% killing of MRSA, compared to VCM (49.26%) at the same MIC, further confirming its efficacy. Cell uptake studies showed that LDH-NPs intracellularly accumulated in HEK 293 cells, confirming significant clearance (p < 0.0001) of intracellular bacteria. MD simulation showed that interaction between the dendrimer and oleylamine was predominantly governed by van der Waals (VdW) interactions; whereas the interaction between the dendrimer and VCM was governed by both VdW and electrostatic interactions. Therefore, this study concludes that the pH-responsive release of VCM enhanced antibacterial efficacy against MRSA and intracellular delivery of an antibiotic. Thus, LDH-NPs is a promising nanocarrier system for antibiotics with the potential to improve the treatment outcomes of bacterial infections in patients with antibiotic resistant strains.

Globally, human beings continue to be at high risk of infectious diseases caused by methicillin-resistant Staphylococcus aureus (MRSA); and current treatments are being depleted due to antimicrobial resistance. Therefore, the synthesis and formulation of novel materials is essential for combating antimicrobial resistance. The study aimed to synthesize a quaternary bicephalic surfactant (StBAclm) and thereof to formulate pH-responsive vancomycin (VCM)-loaded quatsomes to enhance the activity of the antibiotic against MRSA. The surfactant structure was confirmed using 1H, 13C nuclear magnetic resonance (NMR), Fourier-transform infrared spectroscopy (FT-IR), and high-resolution mass spectrometry (HRMS). The quatsomes were prepared using a sonication/dispersion method and were characterized using various in vitro, in vivo, and in silico techniques. The in vitro cell biocompatibility studies of the surfactant and pH-responsive vancomycin-loaded quatsomes (VCM-StBAclm-Qt1) revealed that they are biosafe. The prepared quatsomes had a mean hydrodynamic diameter (MHD), polydispersity index (PDI), and drug encapsulation efficiency (DEE) of 122.9 ± 3.78 nm, 0.169 ± 0.02 mV, and 52.22 ± 8.4%, respectively, with surface charge switching from negative to positive at pH 7.4 and pH 6.0, respectively. High-resolution transmission electron microscopy (HR-TEM) characterization of the quatsomes showed spherical vesicles with MHD similar to the one obtained from the zeta-sizer. The in vitro drug release of VCM from the quatsomes was faster at pH 6.0 compared to pH 7.4. The minimum inhibitory concentration (MIC) of the drug loaded quatsomes against MRSA was 32-fold and 8-fold lower at pH 6.0 and pH 7.4, respectively, compared to bare VCM, demonstrating the pH-responsiveness of the quatsomes and the enhanced activity of VCM at acidic pH. The drug-loaded quatsomes demonstrated higher electrical conductivity and a decrease in protein and deoxyribonucleic acid (DNA) concentrations as compared to the bare drug. This confirmed greater MRSA membrane damage, compared to treatment with bare VCM. The flow cytometry study showed that the drug-loaded quatsomes had a similar bactericidal killing effect on MRSA despite a lower (8-fold) VCM concentration when compared to the bare VCM. Fluorescence microscopy revealed the ability of the drug-loaded quatsomes to eradicate MRSA biofilms. The in vivo studies in a skin infection mice model showed that groups treated with VCM-loaded quatsomes had a 13-fold decrease in MRSA CFUs when compared to the bare VCM treated groups. This study confirmed the potential of pH-responsive VCM-StBAclm quatsomes as an effective delivery system for targeted delivery and for enhancing the activity of antibiotics.

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