Background: RNA interference (RNAi)-based therapy has gained attention because of its potent genesilencing
effect and high specificity. However, the efficient delivery of nucleic acids to the target site is a major
challenge to the clinical implementation. Recently, ultrasound-mediated gene delivery systems have been developed
and attracted interest due to its safety and site-specificity. By the combination with contrast agents, called
microbubbles, not only the delivery effects but also the imaging effects are significantly enhanced. We developed
lipid bubbles (LBs) entrapping an ultrasound contrast gas to enhance the efficacy of ultrasound-mediated delivery
and imaging. In this review, we summarize ultrasound-mediated nucleic acid delivery systems and discuss the
possibility of combining LBs and ultrasound for RNAi-based therapies.
Methods: We prepared polyethylene glycol-modified liposomes and entrapped an echo-contrast gas within the
liposomes. Small interfering RNA (siRNA) were transfected into cells and muscles using LBs and ultrasound.
Moreover, we also developed nucleic acid-loaded LBs using cholesterol-conjugated siRNA or positively-charged
lipid for an efficient systemic delivery of siRNA and microRNA. The usability of LBs for RNA delivery system
was evaluated by the silencing effects of target genes and the therapeutic effects on ischemia hind limb.
Results: A combination of LBs and therapeutic ultrasound was able to enhance the gene silencing effects by
siRNA. Nucleic acid-loaded LBs were able to efficiently deliver siRNA or microRNA by systemic administration.
A combination of LBs and diagnostic ultrasound also enhanced the imaging efficiency. Using a hindlimb
ischemia mouse model, microRNA-loaded LBs could lead to increased angiogenic factors and improved blood
Conclusion: Ultrasound technology is widely used in clinical settings not only for diagnosis but also for therapy.
Ultrasonic devices are being actively developed. Computer-controlled ultrasound systems can provide precise
exposure to the target site. The combination of precise ultrasound exposure and LBs might be useful for target
site-specific nucleic acids delivery, and holds potential to be developed into a beneficial therapeutic and diagnostic
system for various diseases.