In order to pinpoint tumors in the body and successfully remove them, surgeons rely heavily on medical imaging--x-ray, MRI, and CT scans--to light the way. From these images, a surgeon can tell a tumor's location and anatomy, or its size and shape. However, in many cases, medical images don't paint a clear enough picture. These images may light up healthy tissue surrounding a tumor, along with the tumor itself, and may leave smaller tumors, particularly at the millimeter scale, in the dark. Surgeons who depend on these images may end up leaving behind smaller tumors that could later grow and spread or removing healthy tissue that appeared to be cancerous.

Now a team of researchers at the National Cancer Institute (NCI) and the University of Tokyo have developed a new imaging probe that specifically targets and illuminates tumors, even at the submillimeter scale. The scientists designed the fluorescent probe to seek out and grab on to specific receptors on a tumor's surface, and activate, or light up, only when the probe has made it inside a cancer cell. The researchers reasoned that this targeted infiltration ensures that nothing but tumors are illuminated. The team injected the fluorescent probe into mice and was able to see live breast-cancer cells that had spread to their lungs.

"The first time we got this result, it was really exciting," says Hisataka Kobayashi, chief scientist of NCI's Molecular Imaging Program. "Only the tumor lit up, and nothing else, sort of like a sign in the dark sky, and it was really a beautiful view."

Kobayashi and his colleagues are part of a growing number of researchers who are taking a molecular approach to medical imaging. Termed molecular imaging, the concept is based on identifying specific molecules that signal disease and using them as beacons to identify early stages of, for example, cancer. Laboratories around the world are developing fluorescent compounds that seek out and bind to such molecules, from cancer-related enzymes to macrophages that signal inflammation.

One main hurdle in all these efforts is what's referred to in the field as target-to-background ratio. That is, scientists have found it difficult to illuminate just the target tissue or tumor, and not the background. Kobayashi says that's partly because the fluorescent probes used are "always on," meaning always lit, regardless of whether a probe is bound to its target. That could be a problem, particularly in identifying tumors.

"Cancer tissue is really leaky," says Kobayashi. "Some of these fluorescent probes can easily be leaked out into the tissue and stay on, and we can misread cancer in those tissues."

To get around this problem, the researchers picked a fluorescent agent that's only activated in highly acidic conditions, or environments with low pH. Kobayashi recognized that within a cancer cell is a highly acidic compartment called the lysosome, which digests large molecules. He predicted that the pH-sensitive probe would only light up once it reaches the lysosome, remaining dark in the less acidic environment outside cancer cells. This "activatable" probe would theoretically improve the target-to-background ratio.