Mechanism of Nile red transfer from o/w emulsions as carriers for passive drug targeting to peritoneal macrophages in vitro

Abstract

The uptake of the lipid-soluble fluorescent dye Nile red by plated mouse peritoneal macrophages at 37°C from 5% sesame oil emulsions containing various surface active agents was not significantly affected by sodium azide, cytochalasin B, and pretreatment of the macrophages by glutaraldehyde, albeit somewhat reduced at 4°C. These findings supported that the lipid transfer was not due to phagocytosis of oil droplets by the cells. The uptake rate was proportional to Nile red concentration when the emulsion concentration was maintained constant. However, when the concentration of emulsion as well as Nile red varied proportionally, the plot of uptake rate against the Nile red concentration resembled that of a saturable kinetic process. An inverse relationship was observed between the Nile red transfer rate and the partition coefficient of Nile red between the oil used in the emulsion preparations and a phosphate buffer. This observation was made using a series of binary mixtures of triacetin and trioctanoin as the oil phase of emulsions. Finally, the rate of Nile red was at least 10-fold smaller when the emulsion was physically separated from the macrophage monolayer by a polycarbonate membrane with 0.1 μm porosity. The average diameter of emulsion droplets was 0.2 μm when determined by photon correlation spectroscopy. These observations together with non-endocytic uptake described above support that the Nile red transfer to macrophage monolayers is collision-mediated rather than via a sequential desorption-diffusion-partition process.