Polymeric matrix hydrophobicity governs saponin packing-density on nanoparticle surface and the subsequent biological...

This study investigated the loading behavior of Quillaja saponin as a model surface-active cargo on (NP) nanoparticles prepared with various hydrophobic polymers and using different organic solvents through emulsification/solvent evaporation, and the impact of NP surface hydrophobicity upon the cytotoxic and hemolytic properties of the loaded entity. A superficial monolayered arrangement of saponins on NP was established (R² > 0.9) for all NP, as the saponin loading values complied with the Langmuir adsorption isotherm over the entire concentration range. Next, based on the measurement of interfacial tension between formulation phases, and the subsequent use of Gibb's adsorption isotherm, the packing density (Г_exc) and loading of saponins on various nanospheres could be predicted with good correlation with the actual values (R² > 0.95). The results demonstrated that the hydrophobicity of the polymeric matrix was the major determinant of saponin packing density on the nanospheres. Finally, the impact of NP surface properties upon saponin biological interactions was investigated, where a linear correlation was found between the NP surface hydrophobicity and their hemolytic properties (R² ≅ 0.79), and cytotoxicity against two cancer cell lines (R² > 0.76). The surface hydrophobicity of the polymeric NP seemingly governed the NP-cell membrane binding, which in turn determined the amount of membrane-bound saponins per unit NP surface area. As the saponins exert their cytotoxicity mainly through strong permeabilization of the cell membrane, a higher amount of NP-membrane association governed by a more hydrophobic matrix can lead to higher levels of cytotoxicity. These findings highlight the importance of a detailed characterization of NP surface properties, particularly in case of surface-active cargos, for these dictate the side effects and biological interactions of the delivery system.