Biocompatible antimicrobial colistin loaded calcium phosphate nanoparticles for the counteraction of biofilm formation in cystic fibrosis related infections

The use of inhalable nanoparticles (NPs) for cystic fibrosis (CF) has been advocated as a promising tool to improve the efficacy of antimicrobials taking advantage of their ability to penetrate airway mucus and pathogen biofilm and to release the drug in or in proximity to the enclosed bacteria. Here, inhalable calcium phosphate (CaP) NPs were functionalized with colistin (Col) which is one of the most active antimicrobials against Gram-negative bacteria. The adsorption kinetic and isotherm of Col on CaP-NPs were investigated and fitted according to different mathematical models and revealed an electrostatic interaction between positively charged amine groups of Col and negatively charged surface of CaP-NPs. The maximum Col payload was of about 50 mg g of CaP-NPs. After functionalization, despite an increase of size (213 vs 95 nm), in citrate solution, CaP-NPs maintained a dimension and surface charge considered suitable for crossing mucus barrier. CaP-NPs do not interact with mucin and are able to permeate a layer of artificial mucus. In vitro tests on pulmonary cells demonstrated that CaP-NPs are not cytotoxic up to a concentration of 125 ?g mL. The antimicrobial and antibiofilm activity of Col loaded CaP-NPs tested on Pseudomonas aeruginosa RP73, a clinical strain isolated from a CF patient, was similar to that of free Col demonstrating that the therapeutic effect of Col adsorbed on CaP-NPs was retained. This work represents the first attempt to use CaP-NPs as delivery system for the CF treatment. The encouraging results open the way to further studies.