Scientists at Stanford University have developed a new laser therapy that destroys cancer cells but leaves healthy ones unharmed. The new, non-invasive treatment is described in a study published in the Aug. 1 online edition of the Proceedings of the National Academy of Sciences (PNAS).
"One of the longstanding problems in medicine is how to cure cancer without harming normal body tissue," says Hongjie Dai, an associate professor of chemistry at Stanford and co-author of the study. "Standard chemotherapy destroys cancer cells and normal cells alike. That's why patients often lose their hair and suffer numerous other side effects. For us, the Holy Grail would be finding a way to selectively kill cancer cells and not damage healthy ones."
For the PNAS experiment, Dai and his colleagues used a basic tool of nanotechnology--carbon nanotubes, synthetic rods that are only half the width of a DNA molecule. Thousands of nanotubes could easily fit inside a typical cell.
"An interesting property of carbon nanotubes is that they absorb near-infrared light waves, which are slightly longer than visible rays of light and pass harmlessly through our cells," Dai says. But shine a beam of near-infrared light on a carbon nanotube, and the results are dramatic. Electrons in the nanotube become excited and begin releasing excess energy in the form of heat.
In the experiment, Stanford researchers found that if they placed a solution of carbon nanotubes under a near-infrared laser beam, the solution would heat up to about 158 degrees F (70 C) in two minutes. When nanotubes were placed inside cells and radiated by the laser beam, the cells were quickly destroyed by the heat. However, cells without nanotubes showed no effects when placed under near-infrared light.
"It's actually quite simple and amazing," Dai observes. "We're using an intrinsic property of nanotubes to develop a weapon that kills cancer."
To assure that only diseased cells were destroyed in the experiment, the scientists had to find a way to selectively deliver carbon nanotubes into cancer cells and not into healthy ones. Dai and his co-workers achieved this by performing a bit of biochemical trickery. Unlike normal cells, the surface of a cancer cell contains numerous receptors for a vitamin known as folate. The researchers decided to coat the nanotubes with folate molecules, which would only be attracted to diseased cells with folate receptors.
The experiment worked as predicted. Most of the folate-coated nanotubes ended up inside cancer cells, bypassing the normal cells--like Trojan horses crossing the enemy line. Once the nanotubes were planted inside, the researchers shined the near-infrared laser on the cancer cells, which soon heated up and died.
"Folate is just an experimental model that we used," Dai says. "In reality, there are more interesting ways we can do this. For example, we can attach an antibody to a carbon nanotube to target a particular kind of cancer cell."
One example is lymphoma, or cancer of the lymphatic system. Like many cancers, lymphoma cells have well-defined surface receptors that recognize unique antibodies. When attached to a carbon nanotube, the antibody would play the role of a Trojan horse. Dai and Dean Felsher, a lymphoma researcher in the Stanford School of Medicine, have begun a collaboration using laboratory mice with lymphoma. The researchers want to determine if shining near-infrared light on the animal's skin will destroy lymphatic tumors, while leaving normal cells intact.
"It's a really interesting idea," says Felsher, an assistant professor of medicine and of pathology. "For a long time people have thought about ways to target cancer cells, and this is a very promising technique."