Ultrasound-Mediated Gene Delivery

Human gene therapy holds great promise in treating not only hereditary genetic disorders, but also disease states such as cancer and viral infections, and contingencies such as stroke or myocardial infarctions. It can be achieved by delivery of a correct gene into target cells with genetic deficiency or mutations, or by transfer of a therapeutic agent such as agents targeting a cancer-causing oncogene, growth factor gene, antisense oligonucleotides (ODN), or small interfering RNA (siRNA) to correct the disease state using either viral or nonviral vectors. Viral gene therapy has succeeded in many animal disease models {Snyder 1999}, and has progressed to clinical trials {Hacein-Bey-Abina et al. 2002; Kay et al. 2000}. However, significant obstacles remain, including immune responses {Manno et al. 2006} or tumor genesis {Hacein-Bey-Abina et al. 2003}. A nonviral approach would provide a safer strategy. The potential for therapeutic ultrasound (US) to effect minimally invasive nonviral gene transfer has long been recognized, and a growing body of evidence indicates that significant enhancement of transgene expression can be achieved by using high frequency acoustic energy. In addition to its well-known role in providing inexpensive, real-time imaging capability, US has been used therapeutically for years {Herzog et al. 1999}. The most common therapeutic application involves low acoustic intensities and is intended to heat deep tissues; e.g., as used in sports medicine. At the other 'end' of the acoustic intensity spectrum is HIFU (high intensity focused ultrasound), which can be used to ablate {Fischer et al. 2010} or to liquefy tissues {Hall et al. 2009}. US of intermediate intensities has been applied to many systems, together with exogenous microbubbles [MBs], to use the acoustically-forced behavior of the MBs to generate desired bioeffects. The latter usually involves changing the permeability of endogenous barriers to otherwise impermeable materials (e.g., drugs or macromolecules). Many gene therapies have been attempted by direct intramuscular or intraparenchymal injection of gene vectors; these vectors gain immediate access to the interstitial space and must then traverse the plasma membrane of the targeted cells. US contrast agents are almost always administered intravascularly. When accompanied by a gene vector, the first barrier encountered is the vascular endothelium. The next are other vascular anatomical features (e.g., the basement membrane, smooth muscle layer, etc.) and then the outer cell membrane of the cells one hopes to target. Finally, DNA needs to be transferred across the nuclear membrane to enter the nucleus for efficient gene expression. This review will focus almost entirely on the use of ultrasound targeted microbubble destruction (UTMD) as a means by which to deliver foreign DNA (or drugs or photo