Although stem-cell based therapy is moving closer to a clinical reality, questions remain about the safety of these viral-based gene strategies. One researcher has demonstrated that ultrasound-mediated gene delivery therapy offers a safer, noninvasive and more targeted method
of delivering therapeutic DNA to specific cells—a method he has dubbed the "Trojan Horse" approach.
Current cell-based therapeutic methods require the preactivation of a gene in stem or progenitor cells ex-vivo and then their insertion into diseased tissue directly or intravenously, creating an element of risk, said Sidhartha Tavri, M.B.B.S., resident physician, Department of Radiology, University of California San Diego (UCSD) School of Medicine.
"These approaches rely on the natural behavior of viruses to introduce genes into cells to infect them and activate the gene," Dr. Tavri said. "But delivering transfected cells or viruses into an organism raises the risk of these cells and viruses reaching unintended targets. In addition, viruses carry the potential of causing some serious side effects."
At UCSD, Dr. Tavri began working with Robert F. Mattrey, M.D., director of research and a professor of radiology at the school of medicine, to test the efficacy of ultrasound-guided therapy. "Microbubble cell labeling and tracking with ultrasound was a burgeoning project in my lab," Dr. Mattrey said.
Dr. Tavri began working on in vivo transfection of neuroprogenitor cells with ultrasound, an idea realized through a 2010-2011 Hitachi Medical Systems/RSNA Research Resident Grant for the project, "Stem Cells as a Trojan Horse for Ultrasound-Targeted Cell Therapy." Serving as primary investigator, Dr. Tavri worked with colleagues Wenjin Cui, Ph.D., and Boris Minev, M.D., on the project overseen by Dr. Mattrey.
"We came up with new ideas to try and achieve in vivo transfection of these cells using ultrasound," Dr. Tavri said. He hypothesized an approach that involved loading stem cell or progenitor cells ex vivo with DNA-carrying microbubbles, which became known as the "Trojan Horse" strategy.
"Since genes on internalized microbubbles remain inactive until exposed to ultrasound, our approach allows for systemic administration of microbubbleloaded cells and then gene activation under ultrasound guidance in only the desired locations," Dr. Tavri said. "Because diseased tissues naturally attract stem and progenitor cells, by pre-loading them with genes which remain silent until exposed to ultrasound, these cells metaphorically serve as 'Trojan Horses.'"
Before they could start in vivo experiments, team members needed to perform several in vitro studies. First, the researchers needed to show that they could attach DNA constructs on the microbubble shell and that these microbubbles could be internalized by the progenitor cells. Next, they worked to demonstrate in vitro that gene transfection of neuroprogenitor cells occurred in vitro only when the cells were exposed to ultrasound, proving that if the cells accumulated into undesirable areas they wouldn't be activated.
They then determined that these microbubbles would survive long enough to ensure they would still be alive when the pre-loaded cells reached targeted areas to allow for transfection. Simultaneously, the team performed sensitivity experiments to ensure that the microbubble-loaded cells would be detected by ultrasound. They showed that ultrasound was able to detect a single cell, which is "the highest sensitivity of any cell-labeling and tracking in vivo imaging technique."
Dr. Tavri and his colleagues demonstrated that these cells could be transfected in vivo in the skeletal muscle of mice. "Our preliminary results indicated that ultrasound not only mediates in vivo transfection of neuroprogenitor cells in skeletal muscle, but is also necessary," said Dr. Tavri. "We also showed that microbubble loading of cells is essential for gene expression and is better at transfecting the desired cells than an equivalent number of gene-loaded free microbubbles."
"The results of the study lay the foundation to achieve ultrasound-mediated extravascular delivery of gene therapy by using targeted cell-based therapy," Dr. Tavri said, adding that diagnostic radiology will play a key role in monitoring cell-based therapy. "Ultrasound offers real-time imaging of cells in vivo and has the potential to be more sensitive and less expensive than modalities like MR imaging or PET."
"Although we are still early in this process to know whether it will be possible to accumulate
enough cells in a region of interest, transfect the cells with ultrasound and induce the desired effect, data generated by Dr. Tavri so far continue to be promising," Dr. Mattrey added.
If results continue to be positive, the team plans to use the preliminary data for future applications, Dr. Mattrey said. "Should we succeed with our approach to deliver stem cells or progenitor cells loaded with gene-carrying microbubbles systemically, we would provide a new cell-based diagnostic and therapeutic platform," he said.
The RSNA Research Resident Grant not only allowed Dr. Tavri to collect data for this project but serves as a stepping-stone for his career in academic radiology, he said.
"I feel fortunate to have received the RSNA grant," Dr. Tavri said. "I understand it is competitive and getting more so every year. It was the first grant proposal I've submitted and putting it together was a great learning experience. It has also had a tremendous impact by giving me the opportunity to function as a 'mini-clinical scientist.'"
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Name: Sidhartha Tavri, M.B.B.S.
Grant Received: 2010-2011 Hitachi Medical Systems/RSNA Research Resident Grant
Study: "Stem Cells as a 'Trojan Horse' for Ultrasoundtargeted Cell Therapy"
Career Impact: The grant not only allowed Dr. Tavri to collect data to prove whether gene expression can be induced under ultrasound control, but serves as a stepping-stone for his career. "This project allowed me to not only nurture
my skills in molecular imaging but also as an academic radiologist," Dr. Tavri said. "My long-term objective is to add pioneering innovations to the advances in molecular imaging and hopefully translate it to human beings to make a positive impact in the lives of our patients."
Clinical Significance: "The results of the study lay the foundation to achieve ultrasoundmediated extravascular delivery of gene therapy by using targeted cellbased therapy," Dr. Tavri said, adding that diagnostic radiology will
play a key role in monitoring cell-based therapy. "Ultrasound offers real-time imaging of cells in vivo and has the potential to be more sensitive and less expensive than its counterparts like MR imaging or PET."
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