Microbubble Stability is a Major Determinant of the Efficiency of Ultrasound and Microbubble Mediated in vivo Gene Transfer

In the search for an efficient nonviral gene therapy approach for the treatment of genetic disorders of cardiac and skeletal muscle such as Duchenne muscular dystrophy, ultrasound in combination with contrast enhancing microbubbles has emerged as a promising tool for safe and site-specific enhancement of gene delivery. Indeed, microbubble-enhanced gene transfer (MBGT) has been investigated for a wide variety of target sites using both reporter and therapeutic genes. Although a range of different microbubbles have been used for MBGT studies, comparison of their efficiencies is difficult because microbubble concentration and the ultrasound settings used for the application vary considerably. Only two studies to date have attempted a direct comparison of commercially available microbubbles, and both concluded that not all microbubbles show the same efficiencies with MBGT. Thus far, the reason for this is unclear. Here, the efficiency of three commercially available microbubbles—Optison, SonoVue and Sonazoid—was analyzed to understand the microbubble properties that are important for their function as an effective enhancer for gene transfer in vivo. In this study, plasmid DNA or antisense oligonucleotides were delivered by systemic injection with MBGT, focused on the heart. Gene delivery to the heart with equalized concentrations of the three microbubbles showed that Optison and Sonazoid are more efficient in MBGT compared with SonoVue, which showed the weakest gene transfer to the myocardium. Investigations into the properties of these microbubbles showed that size and shell composition did not directly influence MBGT, whereas the microbubbles with increased stability in an ultrasound field showed better MBGT results than those degrading faster. Moreover, the microbubble concentration used for MBGT was also found to be an important factor influencing the efficiency of MBGT. In conclusion, the stability of a microbubble was shown to be a major influential factor for its performance in MBGT, as is the concentration of the microbubbles used. These findings emphasize the importance of detailed investigations into the properties of microbubbles to allow the production of a microbubble specifically designed for optimum performance with MBGT. (E-mail: d.wells@imperial.ac.uk)