Release behavior and antibiofilm activity of usnic acid-loaded carboxylated poly(l-lactide) microparticles

•We synthesized low molecular weight carboxylated poly(l-lactide)s.•Usnic acid loaded microparticles are obtained without surfactant.•The double drug release mechanism was related to the microparticles structure.•Drug loaded microparticles show a higher antimicrobial activity than the free drug. The use of controlled drug delivery systems could give a significant contribution to the improvement of therapies against biofilm-based infections. The aim of this study was to develop polymer microparticles, based on carboxylated poly(l-lactide)s, to be employed as carriers for usnic acid (UA), a poorly soluble drug possessing antiviral, antiproliferative and wide spectrum antimicrobial activity. Thanks to polymer surfactant-like structure, 2.4μm-in-size microparticles were obtained by a surfactant-free oil-in-water emulsion/evaporation method. UA was encapsulated into these microparticles with a high loading efficiency (80%). The drug release kinetics was found to be temperature dependent (the released dose increasing with temperature) and showed bimodal release behavior. By polarized optical microscopy observations and the application of kinetics models, the initial burst effect was attributed to the delivery of the drug amorphous fraction while the slower release occurring for longer times to the crystalline one, both entrapped in the polymer amorphous phase. UA-loaded microparticles were able to promote the killing of a 24h-old Staphylococcus epidermidis biofilm more efficaciously than free UA.