Compound Helps Detect Cancer Cells in Mice

A breast cancer cell. National Cancer Institute.

Researchers have developed an imaging compound that allows them to visualize viable breast cancer cells that have spread to the lungs of mice. Similar compounds may one day help doctors monitor their patient's tumors and even aid in tumor removal.

Fluorescent compounds glow, or fluoresce, allowing scientists to pinpoint particular cells or molecules under the microscope. Researchers can activate these compounds once they’re in living cells.  However, the fluorescent compounds developed to date keep glowing once they’re turned on, no matter what happens to the cells they’re in. That makes it difficult to distinguish viable tumor cells from normal, dead or damaged tissue.

A research team led by Dr. Hisataka Kobayashi of the Center for Cancer Research at NIH’s National Cancer Institute (NCI), in collaboration with Dr. Yasuteru Urano at the University of Tokyo, set out to create an imaging compound that is turned on only when inside a living cell. To do this, they first developed a small compound that fluoresces only under acidic conditions. Once inside the cell, they reasoned, the compound would enter the acidic environment of a lysosome, which is a sac-like cell compartment containing enzymes to break down large molecules the cell doesn’t need. It would then become activated and fluoresce, but would stop fluorescing when it left the cell, such as when the cell died or became damaged.

To get the compound inside cancer cells, the researchers joined it to a drug called trastuzumab (Herceptin). Herceptin binds to a protein called HER2, which is found at high levels on the surface of some breast cancer cells. When this complex encounters a HER2-positive breast cancer cell, it binds to HER2 on the cell’s surface and is taken inside the cell.

The results appeared in the online edition of Nature Medicine on December 7, 2008. The researchers injected either the activatable compound, or a control that always fluoresces, into the tails of mice with HER2-positive breast cancer tumors that had spread to their lungs. One day later, the investigators found fluorescence from the activatable compound only in lung tumors. The “always on” control produced fluorescence in lung tumors, normal lung tissue and the heart.

Further experiments showed that the compound produced fluorescence only in HER2-positive tumors, not in other tumors that lacked the surface molecule. The team also confirmed that the compound detects only living cells. After tumor tissues were exposed to alcohol, which kills cells, the fluorescence significantly decreased.

By using a molecule that targets the surface of mouse ovarian cancer cells, the researchers demonstrated that similar compounds could be engineered to target other types of cancer cells. “Our design concept is very versatile,” Kobayashi said. “We can target the fluorescing agent to different types of cancer cells by using any antibody or molecule that is internalized by the targeted cells after it binds to the cell's surface proteins.”

These imaging compounds may one day allow doctors to monitor a patient's response to cancer therapy. The compounds might also be adapted for use in endoscopy procedures or as a surgical aid to improve removal of tumors.