Preparation of nanoparticles by solvent displacement for drug delivery: A shift in the “ouzo region” upon drug loading

As biodegradable nanoparticles meet with increasing interest for drug delivery applications, a series of investigations were carried out to understand the mechanism of the formation of drug-loaded nanoparticles using the solvent displacement method. Although previous explanations referred to Marangoni convection as the driving force for nanoprecipitation, recent publications describing the so-called “ouzo effect” sparked these current studies using a novel negatively charged polymer, poly(vinyl sulfonate-co-vinyl alcohol)-graft-poly(d,l-lactide-co-glycolide) (P(VS-VA)-g-PLGA), and a positively charged model drug, salbutamol. Interfacial tension did not influence the nanoparticle formation as would be expected if governed by Marangoni convection, but ternary phase diagrams outlined the so-called “ouzo regions” defining the polymer and solvent concentrations leading to stable nanoparticle suspensions for both this novel polymer and unmodified poly(d,l-lactide-co-glycolide) (PLGA). Physicochemical properties, morphology and drug loading of the nanoparticles were analyzed, and stable P(VS-VA)-g-PLGA nanoparticles with and without salbutamol ranged in size from 59–191 nm. The “ouzo region” phase diagram boundaries shifted considerably upon drug loading, which can be explained by the increased solubility of the polymer–drug complex. This behavior necessitated a substantial adjustment of polymer concentrations required to produce drug-loaded nanoparticles with characteristics comparable to blank nanoparticles. In conclusion, the use of “ouzo diagrams” is a beneficial tool to manufacture nanoparticles with specified physicochemical properties by the solvent displacement method.