Molecular Study of Ultrasound-Triggered Release of Fluorescein from Liposomes

Several investigations have suggested that ultrasound triggers the release of drugs encapsulated into liposomes at acoustic pressures low enough to avoid cavitation or high hyperthermia. However, the mechanism leading to this triggered release as well as the adequate composition of the liposome membrane remains unknown. Here, we investigate the ultrasound-triggered release of fluorescein disodium salt encapsulated into liposomes made of 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) or 1,2-distearoylphosphatidyl-ethanolamine (DSPC) lipids with various concentrations of cholesterol (from 0 to 44 mol %). The passive release of encapsulated fluorescein was first characterized. It was observed to be higher when the membrane is in a fluid phase and increased with temperature but decreased upon addition of cholesterol. Next, the release of fluorescein was measured at different acoustic frequencies (0.8, 1.1, and 3.3 MHz) and peak-to-peak pressures (0, 2, 2.5, 5, and 8 MPa). Measurements were performed at temperatures where DOPC and DSPC liposomes were, respectively, in the fluid or gel phase. We found that the release rate did not depend on the ultrasound frequency. For DOPC liposomes, the ultrasound-triggered release of fluorescein decreased with increasing concentration of cholesterol in liposomes, while the behavior was more complex for DSPC liposomes. Overall, the triggered release from DSPC liposomes was up to ten times less than DOPC liposomes. Molecular dynamics simulations performed on a pure DOPC membrane showed that a membrane experiences, under a directional pressure of +/- 2.4 MPa, various changes in properties such as the area per lipid (APL). An increase in the APL was notably observed when the simulation box was laterally stretched or perpendicularly compressed, which was accompanied by an increase in the number of water molecules crossing the membrane. This suggests that ultrasound most probably enhances the diffusion of encapsulated molecules at small acoustic pressures by increasing the distance between lipids.