Investigating the spatial extent of acoustically activated echogenic liposomes

The purpose of this work was to investigate the ability of bubbles entrapped within echogenic liposomes (ELIP) to serve as foci for cavitational events that would cause leakage in neighboring non-echogenic liposomes (NELIP). Previous studies have shown that entrapping bubbles into liposomes increases ultrasound-mediated leakage of hydrophilic components at ultrasound settings known to induce inertial cavitation, specifically 20kHz and 2.2W/cm². Using tone-burst approach and pulse repetition frequency of 10Hz would bring this intensity level to the one accepted (220mW/cm²) in clinical imaging. Mixed populations of ELIP and NELIP were simultaneously exposed to ultrasound at varying ratios to examine the effect of ELIP concentration on release of a hydrophilic dye, calcein, from NELIP. Calcein release from NELIP was observed to be independent of ELIP concentration, suggesting that the release enhancement from echogenicity is strictly a localized event. Additionally, it was observed that the release mechanisms independent of echogenicity were active for the duration of experiment whereas those associated with echogenicity were active for only the initial 1-2min.

Hydrophobic drug concentration affects the acoustic susceptibility of liposomes

The purpose of this study was to investigate the effect of encapsulated hydrophobic drug concentration on ultrasound-mediated leakage from liposomes. Studies have shown that membrane modifications affect the acoustic susceptibility of liposomes, likely because of changes in membrane packing. An advantage of liposome as drug carrier is its ability to encapsulate drugs of different chemistries. However, incorporation of hydrophobic molecules into the bilayer may cause changes in membrane packing, thereby affecting the release kinetics. Liposomes containing calcein and varying concentrations of papaverine, a hydrophobic drug, were exposed to 20 kHz, 2.2 Wcm(-2) ultrasound. Papaverine concentration was observed to affect calcein leakage although the effects varied widely based on liposome phase. For example, incorporation of 0.5mg/mL papaverine into Ld liposomes increased the leakage of hydrophilic encapsulants by 3× within the first minute (p=0.004) whereas the same amount of papaverine increased leakage by only 1.5× (p<0.0001). Papaverine was also encapsulated into echogenic liposomes and its concentration did not significantly affect calcein release rates, suggesting that burst release from echogenic liposomes is predictable regardless of encapsulants chemistry and concentration.