| Perfluorocarbon (PFC) microbubbles have been used for several decades as contrast agents in ultrasound imaging; over the last decade, microbubbles have attracted attention as potential drug/gene carriers. The microbubbles would have difficulty in carrying the drug to the tumor due to their sizes in the micron range that preclude effective extravasation. Drug encapsulation in perfluorocarbon nanoemulsions used as microbubble precursors will overcome this problem. Upon nanodroplet accumulation in the tumor tissue, the application of tumor-directed ultrasound triggers the droplet-to-bubble transition accompanied by localized drug release from the bubbles formed. We chose perfluoropentane (PFP) and perfluro-15-crown-5-ether (PFCE) emulsions for our drug delivery studies. PFCs were stabilized by polyethylene glycol (PEG) based amphiphilic block copolymers. For the drug delivery studies, doxorubicin (DOX), a widely used drug in chemotherapy, was loaded into the droplets. DOX has intrinsic fluorescence properties, which allowed studying DOX intracellular trafficking as well as cell damage through fluorescence imaging. Cell culture experiments showed that without ultrasound exposure, the intracellular DOX uptake was lower when DOX was delivered in nanodroplets compared to DOX encapsulated in polymeric micelles. This is advantageous for preventing DOX interaction with the healthy tissues. For drug uptake by cells, significant differences (F (2, 42) = 15.7; p << 0.01) were observed between different DOX encapsulated formulations.;The intracellular DOX uptake was enhanced by ultrasound application. For all sonicated groups, effect of ultrasound was statistically significant (p << 0.01). For the nuclear DOX penetration, the effect of ultrasound appeared stronger for the cells sonicated in the presence of nanodroplets; under the action of ultrasound, mean nuclear fluorescence increased 2.3-fold for micelles vs. 3.3-fold for nanodroplets. PFP has a low boiling point, and therefore was chosen to study the temperature response of stabilized PFP droplets. In-vitro studies with heated emulsion in a microchannel showed formation of microbubbles with a moderate change in temperature. The formation of new bubbles was attributed to nucleation sites inside the microchannel in the presence of PFP. The growth of the bubbles was attributed to changes in the surfactant structure at higher temperatures, leading to increase in coalescence of the bubbles and diffusion of dissolved gases from the solution into the bubble. |
