New technique sees into tissue at greater depth, resolution

By coupling a kicked-up version of microscopy with gold nanoparticles, investigators at Duke University have been able to peer so deeply into living tissue that they can see molecules interacting.

If future studies in animal models prove fruitful, the researchers believe that their new approach can have a wide spectrum of clinical applications, from studying the margins of a tumor as it is removed from the body to assessing the effects of anticancer agents on the blood vessels that nourish tumors.

The Duke bioengineers, led by Joseph Izatt, Ph.D., combined tightly focused heat with optical coherence tomography (OCT), which has often been called the optical equivalent of ultrasound. OCT is commonly used in medical clinics where imaging at the highest resolution is critical, such as in the retina. These experiments represent the first time the technique has been extended to the functional imaging of cells expressing particular molecular receptors. This work appears in the journal Nano Letters.

For its experiments, the Duke team attached nanospheres of gold to a monoclonal antibody that binds to the epidermal growth factor receptor (EGFR), a cell-surface receptor implicated in cancer. These targeted gold nanoparticles were then applied to the surface of a three-dimensional tissue model composed of human cells-both cancerous and noncancerous.

The investigators 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. And indeed, cells overexpressing EGFR gave off a signal 300 percent higher than cells with low expressions of EGFR.

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. As a result, the investigators were able to not only image cells within the tissue but also 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," Dr. Izatt said. "Organically based contrast agents can cause damage or death to the targeted cells, whereas 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."  

Dr. Izatt's team 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 the investigators plan to use photothermal OCT to measure how these tumor types react to different therapies.

This work is detailed in the paper "Photothermal Optical Coherence Tomography of Epidermal Growth Factor Receptor in Live Cells Using Immunotargeted Gold Nanospheres." An abstract of this paper is available at the journal's Web site. View abstract