Comparing nanodroplets and microbubbles for enhancing ultrasound-mediated gene transfection

Ultrasound is capable of transfecting cells with DNA through the process of sonoporation. Cell transfection by ultrasound alone is a relatively inefficient process but sonicating cells in the presence of microbubbles can significantly increase transfection efficiency. Unfortunately, microbubbles are relatively large (~3 μm diameter) and cannot leave the vasculature, preventing their direct interaction with cancer cells in solid tumors. Perfluorobutane microbubbles can be condensed into submicron nanodroplets that may be able to extravasate out of the tumor vasculature, potentially allowing these nanodroplets to accumulate in tumors. The current study compares microbubbles and nanodroplets for their abilities to enhance ultrasound-mediated transfection of cultured HEK293 cells. The nanodroplets in the current study were almost 6-fold smaller than their corresponding microbubbles and could be phase changed back into microbubbles with 400 cycles of 1.7 mechanical index ultrasound. Given enough cycles of ultrasound, low concentrations of nanodroplets (<; 1% v/v) were just as effective as microbubbles for enhancing ultrasound-mediated transfection of pEGFP-C1 DNA into HEK293 cells and high concentrations of nanodroplets (> 2 % v/v) were 2- to 3-fold more effective than microbubbles for transfecting HEK293 cells. The current study supports the use of nanodroplets to enhance ultrasound-mediated transfection due to their equivalent or greater transfection enhancing abilities as well as their smaller size which may allow nanodroplets to accumulate in tumors.