3 August 2005
Disease Seeking Nanoprobes Get Glowing Report
by Kate Melville
Diagnostic techniques for serious diseases could improve dramatically now that researchers from Rice University's Center for Biological and Environmental Nanotechnology have developed a "smart" beacon hundreds of times smaller than a human cell. The nanoprobe has been designed to light up only when activated by defective proteases (enzymes which break the peptide bonds of proteins) - a common sign of cancer, atherosclerosis and many other diseases.
In Biochemical and Biophysical Research Communications, lead authors Jennifer West and Rebekah Drezek describe the development of the device for visualization of proteolytic activity in-vivo. "The idea is to develop a 'smart' nanostructure that is dark in its original state but lights up very brightly in the presence of enzymatic activity associated with a particular disease process," said West. "Other groups have used targeted nanostructures including quantum dots for molecular imaging, but they have never been able to adequately solve the problem of clearly distinguishing between the 'cancer is here' signal and the background light which arises from nanostructures not specifically bound to their molecular targets."
The Rice team has overcome the problem with quantum dots by using emissive nanoparticles that give off light in the near infrared (NIR), which emits no background component in biomedical imaging. NIR has the added bonus of being able to pass harmlessly through skin, muscle and cartilage, a feature that could alert doctors to tumors and other disease sites without the need for a biopsy or invasive surgery.
The nanoprobe uses a technique called "quenching" that involves tethering a gold nanoparticle to the quantum dot to inhibit luminescence until it reaches its target. The tether, a peptide sequence measuring only a few nanometers, holds the gold close enough to prevent the quantum dot from giving off its light. The researchers showed that the luminescence of the quantum dots was cut by more than 70 percent when they were attached to the gold particles. In experiments, the peptide sequence holding the gold nanoparticle to the probe was cut once it was exposed to the enzyme collagenase, after which the probe's luminescence would gradually return. Ultimately, the researchers hope to pair a series of quantum dots, each with a unique NIR optical signature, to an index of linker proteases.
"There is currently a critical need for methods to simultaneously image the activity of multiple proteases in-vivo," said Drezek. "This is important not only for early detection of several diseases, but perhaps more significantly, in understanding and monitoring the efficacy of therapeutic interventions, including the growing class of drugs that act as protease inhibitors. What is particularly powerful about the protease imaging probes described in this study is the combination of the contrast enhancement achievable through an activateable probe with the imaging advantages provided by the brightness, photostability, and tunability of quantum dots."