pH-responsive chitosan nanoparticles from a novel twin-chain anionic amphiphile for delivery of vancomycin

Antimicrobial resistance threatens the effective treatment of increasing range of bacterial infections and it is increasingly becoming a serious threat to public health. Without new effective antibiotic formulations, the success of major surgery and cancer chemotherapy would be compromised. For instance, patients with MRSA (methicillin-resistant Staphylococcus aureus) are estimated to be 64% more likely to die than people with a non-resistant form of the infection. Nano drug delivery systems like nanoparticles have earned enormous importance in biomedical research due to their ease of synthesis, versatile functionalization and advancing theranostics. They can also enhance antibiotic therapy and overcome bacterial resistance mechanisms by targeting the drug to the infection site. New generation nanoparticles are further being developed to be specifically responsive to the acidic conditions at infection sites to  improve targeting, reduce drug loss and increase expsoure of bacteria to the drug.

University of KwaZulu-Natal researchers in South Africa led by Professor Thirumala Govender and  Dr. Rahul Kalhapure successfully synthesized a novel gemini surfactant, AGS 7, based on oleic acid tails (hydrophobic portion) and anionic sodium carboxylate head groups (hydrophilic portion). They found AGS 7 a good solubilizer and stabilizer for colloidal formulations and therefore they used it in formulating vancomycin in chitosan nanoparticles. Their study was the first to report the use of gemini surfactant in developing a pH-responsive antibiotic delivery system. In the current studies, a series of experiments were conducted in vitro and in vivo for the charactrization of the new formulation and its antibacterial  activity.

When the authors tested their AGS 7 in an array of cell lines (MCF7 human breast adenocarcinoma, A549 human alveolar basal epithelial adenocarcinomia cells and HepG2 human liver hepatocellular carcinoma cell lines) no toxicity was observed. These results opened the way for exploring AGS 7 in preparation of pH-responsive vancomycin loaded chitosan nanoparticles for biological applications.

When the research team looked at the cumulative amount of vancomycin free base released from the formulation at acidic pH 6.5 and the physiological pH 7.4 they observed  3-fold greater cumulative drug release in acidic pH than physiological pH after just 30 min. The larger drug release at acidic pH continued at prolong time of 24 hour incubation. Vancomycin release was found to be concentation independent at both pH values with zero order kinetics suggesting a constant release rate.

The advantageous criteria of having pH-responsive drug release was possibily due to the decrease in ionization of their surfactant AGS 7 under mildly acidic conditions, resulting in a destabilization of the nanoparticles’ structure

When the resesarch team tested the formulation against S. aureus and their resistant counter (methicillin-resistant Staphylococus aureus, MRSA) at different pH conditions, they found a pronounced antibacterial activity. In acidic pH the vancomycin loaded chitosan nanoparticles stabilized by AGS 7 triggered the release of more vancomycin making it 4 -times potent against S. aureus at acidic pH than at physiological pH. Moreover, the effect was larger in the clinically relevant MRSA where the concentration released resulted in  8 times more activity at acidic pH compared to pH 7.4. The authors also conducted animal studies in skin infection mice disease model to confirm the in vitro results. The injected formulation showed 8 times less MRSA colonies and reduced tissue skin inflammation than control (vancomycin alone). The researchers explained the positive results of the enhanced therapeutic activity of their formulation to  the decrease in the ionization of AGS7 in acidic pH at the infection site. Moreover, the attachment of protonated amino groups to the negatively charged bacterial cell wall which will trigger vancomycin release from destablized nanoparticle structure.

Prof Govender acknowledged her research team for the succesful development of the  novel and safe ionic gemini surfactant, AGS 7, that can be used in formulating pH sensitive nanoantibiotics which can be used to overcome the challenges with  rising antimicrobial resistance.