Influence of liposome bilayer fluidity on the transport of encapsulated substance into the skin as evaluated by EPR

Purpose. The influence of liposome composition on the bilayer fluidity and on the transport of encapsulated substance into the skin was investigated. Methods. Multilamellar vesicles (MLV) from dipalmitoylphosphatidylcholine (DPPC) or dimyristoylphosphatidylcholine (DMPC) with various amounts of cholesterol were prepared by the film method and characterised by photon correlation spectroscopy and electron paramagnetic resonance (EPR) methods. The transport of the hydrophilic spin probe encapsulated in MLV into pig ear skin was investigated by EPR imaging methods. The bilayer domain structure was studied by fitting the lineshape of the experimental EPR spectra with the spectra calculated by the model, which takes into account the heterogeneous structure of the bilayer with several coexisting domains. Results. Cholesterol strongly influences the entrapped volume of liposomes, the domain structure of the lipid bilayer, and the transport of hydrophilic spin probe into the skin. Transport was not observed for liposomes composed of phospholipid:cholesterol 1:0 or 9:1 (mol:mol), not even above the phase transition temperature from the gel to the liquid crystalline phase of DMPC. A significant delivery of hydrophilic spin probe was observed only if there was 30 or 50 mol% of cholesterol in the liposome bilayer. Conclusions, It can be concluded that the domain structure of the liposome bilayer is more important for the delivery of encapsulated substance into the skin than the liquid crystalline phase of the pure phospholipids bilayer.