Liquid Nanodroplets for Ultrasound Therapy
Liquid perfluorocarbon nanodroplets have been recently investigated as a new type of ultrasound-responsive agent. In contrast to gas-filled microbubbles, these nanodroplets contain a liquid perfluorocarbon core surrounded by a lipid or polymer shell. The liquid perfluorocarbon is inert, non-toxic, and has a low boiling point such that it can be vaporized into gas using ultrasound, and the resulting bubbles can be detected with ultrasound imaging (Fig. 8.6). This phenomenon, known as
Figure 8.6 Liquid perfluorocarbon droplets are pumped through a dialysis tube. The droplets are incompressible and thus undetectable with ultrasound imaging. The droplets are vaporized within the tube by focused ultrasound, resulting in the formation of perfluorocarbon microbubbles that are compressible and detectable with ultrasound imaging.
"acoustic droplet vaporization," has been explored for multiple therapeutic ultrasound applications, including targeted delivery of drugs and genes [127]. The nanodroplets are generally 100-300 nm in diameter and have a much higher stability in circulation (several hours or more) compared to gas-filled microbubbles (typically less than 30 min) [128]. The stability and small size of the liquid nanodroplets allows them to accumulate in tumors through the EPR effect similar to other nanoparticles, in contrast to gas-filled microbubbles which are generally too large to extravasate from vessels. Furthermore, the fluorine content in perfluorocarbon nanodroplets can be detected with magnetic resonance imaging, thereby enabling their use as multimodality contrast agents [129]. Together, these properties have generated strong interest in developing perfluorocarbon nanodroplet emulsions as theranostic agents, and research efforts are actively growing in this area.
Nanodroplets for Ultrasound-Targeted Drug and Gene Delivery
Perfluorocarbon nanodroplets have been developed for ultrasound-targeted delivery of molecular therapeutics, including chemotherapeutics. Rapaport et al. developed perfluorocarbon nanodroplets with a block co-polymer shell to deliver doxorubicin and paclitaxel and successfully inhibit tumor growth in mice [130-133]. Fabiilli et al. produced double-emulsion perfluorocarbon droplets for ultrasound-mediated delivery of chlorambucil and thrombin in vitro [134, 135]. Another formulation recently developed for drug and gene delivery, known as "eLiposomes,” consists of perfluorocarbon nanodroplets encapsulated within the aqueous core of liposomes. These formulations have been used to deliver a fluorescent dye (calcein), a plasmid expressing green fluorescent protein (GFP), or doxorubicin to HeLa cells [136-138]. In another study, Burgess and Porter successfully delivered a GFP-targeted siRNA to GFP-expressing human breast carcinoma cells using ultrasound-activated perfluorocarbon nanodroplets emulsions [139].
Nanodroplets for Imaging and Tumor Ablation
Perfluorocarbon nanodroplets are also in development as contrast agents for ultrasound imaging [140-142]. Kripfgans et al. showed enhanced contrast in canine brain and kidneys following intravenous injection of perfluorocarbon droplets [143]. Sheeran et al. used a single ultrasound transducer to induce acoustic droplet vaporization and simultaneously image contrast generated by vaporized nanodroplets in rat kidneys [144]. In another study, Williams et al. demonstrated enhanced ultrasound contrast in mouse tumors using acoustic droplet vaporization 1 h after intravenous infusion of perfluorocarbon nanodroplets [145].
Tumor ablation therapy is another area where perfluorocarbon nanodroplets emulsions may have a positive impact. Clinical use of HIFU-mediated tumor ablation is currently limited by the long treatment times required (generally several hours per patient). After the perfluorocarbon nanodroplets accumulate in the tumor through the EPR effect, they can be activated using acoustic droplet vaporization to generate microbubbles inside the tumor, which can be utilized to accelerate HIFU-mediated heating and tumor ablation. Zhang et al. infused perfluorocarbon droplets intravenously to enhance HIFU-mediated ablation of canine liver [146]. In studies by Kopechek et al., perfluorocarbon nanodroplet emulsions ("phase-shift nanoemulsions”) were used to accelerate HIFU-mediated heating and ablation in rabbit VX2 tumors 2 h after intravenous injection [147-149]. Moyer et al. used perfluorocarbon nanodroplets to enhance HIFU-mediated heating in rat liver [150], and Zhou et al. demonstrated enhanced HIFU- mediated ablation of tumors in mice using perfluorocarbon nanoemulsions [151]. Together, these studies demonstrate that perfluorocarbon nanodroplets can potentially overcome the challenges that currently limit clinical applications of HIFU-mediated tumor ablation.
Future Outlook
The field of ultrasound-responsive nanomedicine is rapidly growing. In preclinical studies, ultrasound and microbubbles have been utilized to enhance drug and gene delivery with liposomes, polymeric nanoparticles, and perfluorocarbon nanodroplets. Emulsions of perfluorocarbon nanodroplets have been developed to accelerate high-intensity focused ultrasound ablation of tumors. Furthermore, ultrasound and nanoparticles are being utilized in new multimodal imaging applications involving MRI and photoacoustics.
Each year, new clinical trials are initiated to test novel therapeutic ultrasound applications. It is highly likely that the fusion of nanomedicine with ultrasound imaging technologies will transform medicine and significantly improve the treatment of many diseases. Ultrasound imaging is well established, but the field of ultrasound-responsive nanomedicine has only recently emerged, and the future outlook for this field is very promising.
