Homogeneous and stable (+)-usnic acid loaded liposomes prepared by compressed CO2

The administration of hydrophobic actives and drugs for medical or cosmetical purposes generally requires a formulation that ensures adequate water solubility, which can be achieved through the encapsulation in liposomes. For the vehiculation of (+)-usnic acid (UA), a hydrophobic compound with antioxidant activity, we have prepared liposomes in a one-step process using compressed CO2. The investigated formulations are mainly composed of 1,2-dimyristoyl-sn-glycero-3-phosphocholine and cholesterol, but contain also a small molar fraction (10%) of a synthetic surfactant derived from L-prolinol. In previous investigations liposomes containing L-prolinol derivatives showed a higher efficacy as DNA or drug delivery systems with respect to liposomes of mere phospholipids. As a consequence, they were added to liposomes to make them more suitable UA delivery systems. By testing different surfactant chain lengths and headgroups, we studied how the chemical nature of the surfactant affects the physicochemical vesicle properties and their interaction with UA. Most formulations, especially those containing surfactants with longer alkyl chains (C14 and C16), show a good potentiality as UA delivery systems because they exhibit a higher stability, vesicle-to-vesicle homogeneity and bilayer compaction with respect to analog liposomes prepared by the conventional thin film hydration previously investigated. Our results confirm the advantages of DELOS-SUSP also in the case of mixed liposomes containing phospholipids and synthetic ionic surfactants. Moreover, this study demonstrates that liposomes composed of the same lipids can feature different properties if prepared according to different methodologies. In addition, this investigation points out that also the properties that a solute included in the bilayer show can be affected by the technique used for liposomes preparation.