Liposome Permeability to Essential Oil Components: A Focus on Cholesterol Content

Encapsulation in liposomes has been an efficient strategy to improve the stability of sensitive bioactive compounds such as essential oils (EOs). However, the stability of liposomal formulations remains a key parameter controlling the delivery of encapsulated ingredients. Cholesterol (Chol) modulates the membrane properties conferring stability to the lipid bilayer. Thus, the Chol content in the liposome formulations encapsulating EO components should be carefully chosen. In this work, various liposome formulations differing by Chol content (DPPC:Chol 100:10; 100:25; 100:50; 100:75; 100:100) were exposed to a series of 22 EO components at DPPC/EO 100/25. The formulations were characterized for their final composition and their permeability to the hydrophilic fluorophore, sulforhodamine B (SRB), was monitored. Results showed that the Chol content experimentally determined for the various formulations (above 10% Chol) was below the theoretical weighed Chol. Among the tested components, 13 molecules displayed a significant permeabilizing effect on 10% Chol membranes. Most of these possess a hydroxyl group. The EO induced permeability was dependent on the Chol content which affects the membrane phase: their effect was reduced upon increasing Chol content keeping five EOs components effective at 40% Chol. The EO's effect was also linked to the hydrophobicity of the molecule. Hence, the DPPC:Chol ratio of the formulation is chosen considering the structure of the compound, its hydrophobicity and its effect on the permeability at different Chol content: a formulation comprising 40% Chol is suggested for highly hydrophobic molecules whereas a formulation with higher Chol content could be selected for less hydrophobic compounds.