Ultrasound, optical and photoacoustic imaging of Acoustic Cluster Therapy enhanced delivery to human tumors in mice

Acoustic cluster therapy (ACT) is designed to overcome some of the limitations of conventional microbubbles for ultrasound-assisted delivery of drugs to tumors. ACT consists of clusters of oil microdroplets and microbubbles. Diagnostic ultrasound imaging can be used to visualize the microbubble component and to activate (vaporize) the droplets to produce large (>20 μm) bubbles which lodge in the tumor's microvessels. A subsequent low-frequency ultrasound field then gently pulsates the activated bubbles which are in direct contact with endothelium, enhancing vascular permeability. We present in-vivo pre-clinical imaging experiments designed to better understand ACT behavior. Conventional microbubbles were observed to wash out of tumors within 6 min whereas activated ACT failed to wash out during the observation time of 15 min, confirming activation. Stationary echoes in tumors uniquely accumulated in fundamental B-mode images after ACT activation (but not with a microbubble agent, nor in nonlinear contrast mode) providing a signature for identifying ACT activation in vivo. ACT significantly enhanced CW800 dye uptake in tumor relative to dye alone. Photoacoustic CW800 signals appeared in the tumor periphery, suggesting potential for high-resolution, three-dimensional dynamic monitoring although with lower sensitivity than fluorescence imaging.