SEPTEMBER 22, 2008 -- Duke University (Durham, NC) bioengineers have demonstrated the use of tightly focused heat, optical coherence tomography (OCT),
and gold nanospheres for deep imaging of living tissue. The technique reveals molecular interaction, and if future animal studies prove fruitful, the researchers believe it can enable such clinical applications as visualizing the margins of a tumor as it is removed from the body, and assessing the effects of anti-cancer agents on blood vessels.
The Duke experiments represent the first time optical coherence tomography has been extended to the functional imaging of cells expressing particular molecular receptors. "This technique could possibly augment traditional methods of deep-tissue molecular imaging with a relatively high resolution," said Melissa Skala, a postdoctoral fellow working in the laboratory of Joseph Izatt, professor of biomedical engineering in Duke's Pratt School of Engineering. "Not only were we able to get better images, we were able to specifically target the types of cells we were looking for."
For their experiments, the Duke team attached nanospheres of gold to a targeting molecule known as a monoclonal antibody, which targets epidermal growth factor receptor (EGFR), a cell-surface receptor implicated in cancer. These "tagged" antibodies were then applied to the surface of a three-dimensional tissue model composed of human cells both cancerous and non-cancerous. Skala hoped that these antibodies would home in on cells that were overproducing EGFR on their surfaces, an indicator of cancerous activity. Then the photothermal OCT would be able to detect them by showing where the gold spheres were concentrated.
"When we directed the photothermal OCT at the tissue, we found that the cells that were overexpressing EGFR gave off a signal 300 percent higher than cells with low expressions of EGFR," Skala said. Adding heat to this form of microscopy technique created a phenomenon much like that seen on very hot days, when portions of the pavement far in the distance seem to float or hover above the road. "The heat causes a distortion in the way light is reflected off the gold nanospheres in a characteristic way," Skala explained. "As we changed the temperature, the light pathways would change in measurable ways."
In this manner, Skala explained, they were not only able to "see" cells within the tissue, but they were able to capture the molecular function of an antibody attaching to a receptor.
"The use of metal nanoparticles as contrast agents with photothermal OCT technology could lead to a host of potential clinical applications," Izatt said. "Organically-based contrast agents can cause damage or death to the targeted cells, while metal nanospheres are relatively safer."
"Also, given the wide range of nanoparticle shapes and sizes, coupled with the ability to 'tune" the optical wavelength of the OCT, we can customize our approach to many different target types," Izatt said.
The results of the Duke research were posted on line by Nano Letters, a journal published by the American Chemical Society. The research was supported by the National Institutes of Health.
Skala plans to expand the use of this approach in animal models to better understand the role of different cancer therapies. Tumors with elevated levels of EGFR are known to have a poor prognosis, and she plans to use photothermal OCT to measure how these tumor types react to different therapies.
Other members of the Duke team were Adam Wax, professor of biomedical engineering and his graduate student Matthew Crow. Skala also worked with Mark Dewhirst, a cancer researcher at Duke University Medical Center, and plans further collaborations with the Dewhirst laboratory to apply this technique better understand the fundamentals of cancer.
The results of the Duke research were posted on line by Nano Letters, a journal published by the American Chemical Society.