Current heat treatments for human tumors, such as radiofrequency, have shown promising results over the last couple of decades, even though they apply only a single-point of heat to the tumor. However, a new technique could prove much more effective by using nanotubes to apply heat throughout the tumor. Scientists found that by injecting the man-made, microscopic carbon tubes into tumors and heating them with a quick, 30-second zap of a laser, they were able to effectively kill kidney tumors in nearly 80 percent of mice.
For the study the researchers used multi-walled nanotubes (MWCNTs), which contain several nanotubes nested within each other. When the tubes are exposed to laser-generated near-infrared radiation, they respond by vibrating and creating heat. If enough heat is conducted, tumor cells near the tubes begin to shrink and die.
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Using mice as subjects, the researchers injected kidney tumors with different quantities of MWCNTs and exposed the area to a three-watt laser for 30 seconds. The mice that received no nanotubes died about 30 days into the study, as did those that received the nanotubes or the laser treatment alone.
However, for the mice who received both MWCNTs and a 30-second laser treatment, researchers found that the more nanotubes injected, the longer the mice lived and less tumor regrowth was seen. In fact, in the group that received the highest dose of MWCNTs, tumors completely disappeared in 80 percent of the mice. Many of those mice continued to live tumor-free through the completion of the study, which was about nine months later.
"You can actually watch the tumors shrinking until, one day, they are gone," said Suzy Torti, Ph.D., lead investigator for the study. "Not only did the mice survive, but they maintained their weight, didn't have any noticeable behavioral abnormalities and experienced no obvious problems with internal tissues. As far as we can tell, other than a transient burn on the skin that didn't seem to affect the animals and eventually went away, there were no real downsides – that's very encouraging."
In addition to the MWCNTs used in this study, other nanomaterials, such as single-walled carbon nanotubes and gold nanoshells, are also currently undergoing experimental investigation as cancer therapies at other institutions. However, Torti says, "MWCNTs are more effective at producing heat than other investigational nanomaterials."
Since the treatment is a heat therapy rather than a biological therapy, it works on all tumor types if the tubes can be made hot enough. The researchers are confident the results will be duplicated for people, but further study to test the long term safety and toxicity of the treatment needs to be completed before it can be tested on humans.
Conceptually, however, Torti says there is no barrier to applying the therapy to human patients, particularly in treating tumors close to the surface of the skin, such as the oral cavity and bladder wall.
The study, which is the result of a collaborative effort between Wake Forest University School of Medicine, the Wake Forest University Center for Nanotechnology and Molecular Materials, Rice University and Virginia Tech, appears in the August issue of Proceedings of the National Academy of Sciences (PNAS).