Quantitative study of LUV-GUV interaction using fluorescence lifetime imaging microscopy and fluorescence correlation spectroscopy

Abstract Drug delivery systems are frequently used for targeted transport of pharmaceuticals and their controlled release at a destined target site. One of the most commonly used drug carriers are liposomes. Additionally, such drug-liposome system is used as model system for studying interaction processes at cellular or even molecular level. The aim of our work was to improve the understanding of drug carrier uptake mechanisms by applying fluorescence lifetime imaging microscopy (FLIM) and fluorescence correlation spectroscopy (FCS), both combined with two-photon (2P) excitation. We prepared giant unilamellar vesicles (GUVs) representing a simplified model system for cell membrane, labelled with the amphiphilic fluorescent dye 3,3'-dioctadecyloxacarbocyanine (DiOC 18 (3)). Furthermore, large unilamellar vesicles (LUVs) were used as a drug carrier system, containing the spectrally different fluorescent sulforhodamine 101 (SRh101) as drug imitate. Herein, we present results of the varying interaction between GUVs and LUVs depending on the used charged lipids. The exchange kinetics and structural changes of the liposome carriers during the fusion process were investigated. We also observed that the internalisation efficiency was mainly influenced by the vesicle´s lipid composition. We ultimately demonstrated that 2P-FLIM and FCS provide a unique methodological approach to study liposome interactions and use them as a versatile model system.