Measurement of Binding and Endocytotic/Fusional Uptake of Liposome Particles by Human Lung Cancer Cells

Quantification of the kinetics of liposome uptake by cells is a first step toward understanding the mechanism of uptake. It is also an essential prerequisite in the design of liposomes as drug carriers. The uptake of liposomes by cells is viewed as a sequence of two major steps: binding to sites on the plasma membrane, followed by entry into the cell. In application, the diffusion rate of the particles from the bulk media to cell surface, the association/dissociation rate of the particles to cell membranes, and the rate of endocytosis/fusion are important, and help to determine the overall rate of the particle uptake by targeted cells. In this study, roles of transport, diffusion and surface kinetic uptake of liposomes were determined simultaneously by human lung cancer cells (A549 cells). Experimentally, quantitative fluorescence spectroscopy was used to monitor time dependent particle uptake rates including low temperature (4 ºC) conditions where endocytosis is inhibited. By direct comparison of experimental data to model solutions, the adsorption constant, desorption constants and number of cell surface receptor sites were determined for liposome particles produced. Our quantitative analysis suggest both diffusion and adsorption accounts for the over all uptake of particles from the modified Thiele modulus. From the steady-state uptake data obtained by shutting off the endocytosis, the equilibrium constant, K was found. The rate of adsorption is much greater than the dissociation rate constant. The endocytotic rate constant was determined by pH sensitive probe Pyranine (1-hydroxypyrene-3,6,8-trisulfonic acid, HPTS).