Ultrasound-Gene Delivery Applications in Cancer Research: An International Journey
By: Priscilla Chan
What do microbubbles—miniscule gas filled bubbles just three micrometres (μm) across—have to do with the future of non-invasive cancer treatment? Originally used as a contrast media to improve ultrasound images, microbubbles are now being harnessed for their novel therapeutic properties. Specifically, microbubbles may be used in targeted gene delivery to treat a variety of conditions, from cardiovascular diseases to cancer. I sat down with Dr. Howard Leong-Poi, a clinician-scientist at St. Michael’s Hospital, to talk about his work in non-invasive cancer therapy, specifically the application of ultrasound and microbubbles.
“Essentially, it’s the use of gas-filled microbubbles for a variety of purposes,” explains Dr. Leong-Poi. Microbubbles can be administered intravenously into the systemic circulation. Ultrasound waves cause these microbubbles to expand and contract, resonating strongly at high frequencies. Importantly, compared to soft tissue, microbubbles have a greater capacity to reflect ultrasound waves. In other words, microbubbles can be used to enhance the resolution of greyscale ultrasound images.1
This approach can be used for more than just diagnostics; it can also be applied for therapeutics. With sufficient excitation, microbubbles are ready to burst, opening up the vascular endothelium. As Dr. Leong-Poi describes it, “it’s a mechanical form of gene transfection.” Ultrasound-targeted microbubble destruction exploits this property to increase the efficiency of drug, protein, and gene delivery.2 Dr. Leong-Poi’s lab has used ultrasound and microbubbles for targeted gene delivery to treat peripheral artery disease, heart failure, and acute myocardial infarction. More recently, his team started looking into ultrasound-mediated gene therapies for cancer.
Currently, cancers are treated using a combination of surgery, chemotherapy, and radiation. Tumours can be partially or completely removed through surgery, but the procedure is invasive and mainly applicable to localized tumours that have not spread. Chemotherapy and radiation are also reasonably effective, but patients are left to deal with the many systemic side effects associated with the treatments. “They work on cancer for a reason,” says Dr. Leong-Poi, “they’re toxic.” In contrast, ultrasound-mediated gene therapy may provide a non-invasive alternative.
Dr. Leong-Poi and his colleagues have shown that microbubble delivery of a short hairpin (sh)RNA specific to VEGFR2, a gene contributing to neovascularization—the formation of new blood vessels—reduces angiogenesis (blood vessel formation) in an in vivo tumour model.3 His team also optimized the ultrasound pulse rate to achieve maximal knockdown of the gene. Their idea was to reduce angiogenesis in order to limit tumour growth. While this technique may not be able to make tumours disappear completely, Dr. Leong-Poi is hopeful that it might make tumour resection easier or be used in addition to other therapies.
Before ultrasound-mediated gene therapy can be implemented as a cancer treatment, organ accessibility needs to be considered. “Any organ or body system or disease state that is accessible to ultrasound could potentially be used for focused gene-delivery,” says Dr. Leong-Poi. However, there are a few natural barriers to ultrasound: air and bone. Tumours found in more superficial organs, such as the liver, may be treated with ultrasound-mediated gene therapy, while tumours located in the brain, spinal cord, or lungs pose a challenge. Some researchers have overcome these barriers by directly injecting microbubbles and administering ultrasound to specific organs, but Dr. Leong-Poi reminds us that “this takes away from the non-invasive nature” of the technique.
Perhaps an even greater challenge in translating ultrasound-mediated gene therapy to the clinic is the fact that it is a highly targeted technique. Although it may be able to limit growth of the primary tumour, ultrasound-mediated gene therapy does not address secondary tumours that have metastasized. Therefore, Dr. Leong-Poi believes that it may be hard to convince some oncologists of the technique’s clinical value. However, in his opinion, patients with hepatocellular carcinoma (liver cancer), head and neck cancers, and other “more locally invasive” cancers are more likely to benefit from this therapy.
After discussing his lab’s work, I was still left with one question: How does a cardiologist get involved in cancer research? “It was mainly through an IMS graduate student actually,” says Dr. Leong-Poi. Pratiek Matkar joined Dr. Leong-Poi’s lab as an international PhD student in 2012 from Pune, India. “He knew about some of the work we did on the cardiovascular side, but his particular interest was in cancer,” recalls Dr. Leong-Poi. “He asked: ‘Can [ultrasound] be applied to cancer?’ and [he] knew other investigators that have done that, but I hadn’t really thought about going down that route, in all honesty. Pratiek was a real catalyst for us and he’s been very successful.”
Pratiek’s work has largely focused on the mechanistic aspects of cancer. In his most recent publication, Pratiek used ultrasound-targeted microbubble destruction to study the role of neuropilin-1, an angiogenic protein, in endothelial-to-mesenchymal transition and fibrosis in pancreatic ductal adenocarcinoma (pancreatic cancer).4 Through Pratiek’s studies, Dr. Leong-Poi thinks that ultrasound therapeutics may be better suited to study cancer mechanisms than to treat cancer. Nevertheless, he points out that understanding the underlying mechanisms can inform how we approach the development of other cancer therapies.
What’s next for Dr. Leong-Poi’s lab? “We’re applying for grants to look at viral gene delivery.” He explains that although plasmids and non-viral vectors may be safer, viral vectors may be necessary to achieve more widespread and longer-lasting delivery. In terms of cancer research, Dr. Leong-Poi remarks, “It was a unique opportunity, and the road ahead depends entirely on future funding!” Whether or not another student will follow in Pratiek’s footsteps is unclear, but what is clear is that the work Dr. Leong-Poi’s lab has done thus far has made a meaningful contribution to our collective scientific knowledge. For this reason, Dr. Leong-Poi remains excited for the future of cancer research: “Even if you’re not the one to take it across the finish line, maybe someone else will be able to do that.”
As for Pratiek? We hear that he has successfully defended his PhD thesis. Congratulations Dr. Matkar and we wish you the best in your future endeavors!
- Blomley MJK, Cooke JC, Unger EC, Monaghan MJ, Cosgrove DO. Science, medicine, and the future: Microbubble contrast agents: a new era in ultrasound. BMJ [Internet]. 2001;322(7296):1222–5. Available from: http://www.bmj.com/cgi/doi/10.1136/bmj.322.7296.1222
- Mayer CR, Geis NA, Katus HA, Bekeredjian R. Ultrasound targeted microbubble destruction for drug and gene delivery. Expert Opin Drug Deliv. 2008;5(10):1121–38.
- Fujii H, Matkar P, Liao C, Rudenko D, Lee PJ, Kuliszewski MA, et al. Optimization of Ultrasound-mediated Anti-angiogenic Cancer Gene Therapy. Mol Ther – Nucleic Acids [Internet]. 2013;2(May):e94. Available from: http://linkinghub.elsevier.com/retrieve/pii/S2162253116301524
- Matkar PN, Singh KK, Rudenko D, Kim YJ, Kuliszewski A, Prud GJ, et al. Novel regulatory role of neuropilin-1 in endothelial-to-mesenchymal transition and fibrosis in pancreatic ductal adenocarcinoma. Oncotarget. 2016;7(43):69489–506.