New Fluorescent Imaging Compound Lights Up When Inside Viable Cells

A typical problem with fluorescent imaging compounds is that once activated, they fluoresce regardless of where they find themselves, smearing the image as they diffuse through the body.
Now a research team, led by Hisataka Kobayashi, M.D., Ph.D., at the Molecular Imaging Program of National Cancer Institute’s Center for Cancer Research (CCR), in collaboration with Yasuteru Urano, Ph.D., at the University of Tokyo, created an imaging compound that can target specific cancer cells and activate only once inside, while subsequently going dark when the cell dies.
The National Institutes of Health reports:

The team created the activatable, cancer-targeting compound by joining a drug called trastuzumab (Herceptin), which is an antibody that binds to HER2 and is used to treat HER2-positive breast cancer, to a modified version of a small fluorescent complex known as BODIPY. This complex fluoresces only under acidic conditions, such as those found inside cellular structures called lysosomes, which are sac-like compartments inside cells that contain enzymes that break down large molecules the cell does not need. When the activatable BODIPY-antibody compound encounters a HER2-positive breast cancer cell, the trastuzumab portion binds to HER2 proteins on the cell’s surface, and then the cell takes the HER2-activatable complex inside. When this complex is processed inside the cell and enters the acidic environment of a lysosome, BODIPY becomes activated and fluoresces.
“Our design concept is very versatile and can be used to detect many types of cancer,” said Kobayashi. “Unlike other activatable fluorescent compounds, our compound consists of a targeting agent and a fluorescing agent that act independently. 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.”

Using a mouse model, the Kobayashi team examined the potential of the activatable compound for detecting tumors within the body. They injected either the activatable compound, or a control that always fluoresces, into the tail of mice that had 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, whereas the “always on” control produced fluorescence in lung tumors, normal lung tissue, and the heart.
To confirm that the activatable compound was primarily processed by, or specific for, HER2-positive tumor cells, the researchers induced lung metastases in mice by intravenously injecting both HER2-positive tumor cells and tumor cells that carried the gene for red fluorescent protein (RFP) instead of the HER2 gene. After administering the fluorescence compounds, they found that the activatable compound produced fluorescence only in the HER2-positive tumors, whereas the “always on” control produced fluorescence in HER2-positive tumors and the surrounding tissues as well as the RFP-positive tumors.
Of the 472 HER2-positive tumors examined in mice with the activatable compound, only three showed fluorescence from both the activatable compound and the RFP (false positive), indicating that the activatable compound had a 99 percent tumor detection accuracy, or specificity, for HER2-positive tumors. The “always on” control had a specificity of less than 85 percent.
The team also confirmed that the activatable compound detects only living cells. Thirty minutes after they exposed tumor tissues to alcohol to kill the cells, they found that the fluorescence of the activatable compound significantly decreased in tumor tissue, whereas the fluorescence of the “always on” control showed minimal changes.
In another series of experiments, the researchers demonstrated the versatility of their design concept by linking a BODIPY complex to a molecule that targets the surface of mouse ovarian cancer cells. This compound allowed the researchers to detect clusters of live ovarian cancer cells that had spread to the peritoneum, or the tissue lining the walls of the abdomen, of mice.

Press release: New Targeted Fluorescent-Imaging Compound Allows Researchers to Detect Viable Cancer Cells in Mice …
Abstract: Selective molecular imaging of viable cancer cells with pH-activatable fluorescence probes