Chitosan-EDTA conjugate: a novel polymer for topical gels

A recently developed chitosan-EDTA conjugate, neutralized with sodium hydroxide (NaChito-EDTA), has been tested for possible topical use. The technical properties and microbial stability of NaChito-EDTA have been compared with those of carmellose sodium (NaCMC), hydroxypropylmethylcellulose (HPMC), sodium polycarbophil (NaPCP) and sodium carbopol 980 (NaC980), well established gelatinizing agents. NaChito-EDTA forms stable, colourless, completely transparent hydrogels at a polymer concentration of 0.5%. Of the polymers tested the novel polymer had the lowest incompatibility with multivalent cations and with ethanol, and much the best swelling properties. After 28 days of incubation at room temperature the rates of growth of the complete bacterial spectrum occurring in demineralized water and of Escherichia coli, serving as model strain representative of gram-negative bacteria, were at least 2 log and 5.7 log, respectively, lower in NaChito-EDTA gels than in the other hydrogels. This antimicrobial activity of NaChito-EDTA can be explained by its highest binding affinity towards magnesium, which stabilizes the outer membrane of gram-negative bacteria. However, this antimicrobial effect is insufficient to guarantee microbial stability. Further results showed that the antimicrobially acting polypeptide nisin can be recommended as an alternative novel preservative for NaChito-EDTA gels, because its antimicrobial spectrum could also be increased towards gram-negative bacteria in combination with chelating excipients. NaChito-EDTA seems, therefore, to be a promising novel polymer for topically-used gels, with advantages over well established gelatinizing agents.