Researchers have developed a proof-of-concept nanosystem that dramatically improves the visualization of tumors. Published in Nature Communications, the platform achieves a five-fold increase over existing tumor-specific optical imaging methods. The novel approach generates bright tumor signals by delivering "quantum dots" to cancer cells without any toxic effects.
The new method utilizes quantum dots, QDs--tiny particles that emit intense fluorescent signals when exposed to light--and an "etchant" that eliminates background signals. The QDs are delivered intravenously, and some of them leave the bloodstream and cross membranes, entering cancer cells. Fluorescent signals emitted from excess QDs that remain in the bloodstream are then made invisible by injecting the etchant.
"The novelty of our nanosystem is how the etchant works," explains one of the authors. The etchant and the QDs undergo a "cation exchange" that occurs when zinc in the QDs is swapped for silver in the etchant. Silver-containing QDs lose their fluorescent capabilities, and because the etchant can't cross membranes to reach tumor cells, the QDs that have reached the tumor remain fluorescent. Thus, the entire process eliminates background fluorescence while preserving tumor-specific signals.
The method was developed using mice harboring human breast, prostate and gastric tumors. QDs were actively delivered to tumors using iRGD, a tumor penetrating peptide that activates a transport pathway that drives the peptide along with bystander molecules--in this case fluorescent QDs--into cancer cells.
To our knowledge, this is the first in vivo example of a background-destroying etchant being used to enhance the specificity of imaging," says the senior author. "We are encouraged that we were able to achieve a tumor-specific contrast index (CI) between five- and ten-fold greater than the general cut-off for optical imaging, which is 2.5."