Carbon nanotubes—cylinders so tiny that it takes 50,000 lying side
by side to equal the width of a human hair—are packed with the
potential to be highly accurate vehicles for administering medicines
and other therapeutic agents to patients. But a dearth of data about
what happens to the tubes after they discharge their medical payloads
has been a major stumbling block to progress. Now, Stanford researchers, who spent months tracking the tiny tubes inside mice, have found some answers.
A detail of a carbon nanotube, composed of linked
hexagonal rings, with a representative molecule of branching
polyethylene glycol (PEG) attached.
Studies in mice already had shown that most nanomaterials tend to
accumulate in organs such as the liver and spleen, which was a concern
because no one knew how long they could linger. But fears that the tiny
tubes might be piling up in vital organs, like discarded refrigerators
at the bottom of a rural ravine, can now be put to rest, said Hongjie
Dai, the J. G. Jackson and C. J. Wood Professor of Chemistry at
Stanford, whose research team has demonstrated that the nanotubes exit
the organs.
Dai and his group found that the carbon nanotubes leave the body
primarily through the feces, with some by way of the urine. "That’s
nice to know," Dai said. "This now proves that they do get out of the
system."
The full extent of the news, which is scheduled to be published the week of Jan. 28 in Proceedings of the National Academy of Sciences Online Early Edition (PNAS),
is even better than that: The three-month-long study also allays
worries that the nanotubes, by simply remaining in the organs for a
long time, would prove toxic to the mouse.
"None of the mice died or showed any anomaly in the blood chemistry or in the main organs," said Dai, senior author on the PNAS
paper. "They appear very healthy, and they are gaining weight, just
like normal mice. There’s no obvious toxicity observed." The lack of
toxicity of nanotubes in mice is consistent with a previous pilot study
done by Sanjiv Gambhir, a professor of radiology at Stanford, and his
research group in collaboration with Dai’s group.
"This is the first time anyone has done a systematic circulation and
excretion study like this for nanotubes, and data on other nano
particles is also scarce," Dai said. "The excretion pathway may apply
to other nano materials and may need to be looked at closely like this
also."
Previous research published by Dai’s group has demonstrated the
potential for using nanotubes in treating cancerous cells and targeting
tumors in mice.
His group used Raman spectroscopy, a method of applying light from a
laser beam that effectively "illuminates" the presence of the target
molecules in the organs of the mice.
Being hit with light from the beam causes a detectable change in the
state of a molecule’s energy. Carbon nanotubes, composed entirely of
carbon atoms that are mostly arranged in linked hexagonal rings, give
off a strong signal in response to the beam. This allowed the
researchers to pinpoint the position of the chosen molecules, as well
as ascertain their abundance in the blood or organs.
Previous detection methods that relied on attaching fluorescent
labels or spectroscopic tags to the nanotubes had yielded unreliable
results. The attachments tended to either come loose from the tubes or
decay over time spans ranging from a few days to only a few hours—far
too short to reveal the ultimate fate of the nanotubes.
While knowing the carbon nanotubes will move through the digestive
system at a healthy pace is critical to future practical applications,
it is also crucial that the nanotubes not enter the digestive system
too soon after being injected; they need to spend enough time in the
circulatory system to find their way to their target location.
The key to fine-tuning the carbon nanotubes’ speed of circulation
turns on how the basic, bare-bones floor model is chemically
accessorized.
"You can make the nanotubes circulate a very long time in the blood,
if the chemistry is done right," Dai said. The researchers found that
coating their carbon nanotubes with polyethylene glycol (PEG), a common
ingredient in cosmetics, worked best.
They used a form of PEG with three little limbs sprouting off a
central trunk. "Those provide better shielding to the nanotube than
just a single branch. Therefore, they interact less with the biological
molecules around them," Dai said.
The team stuffed the PEG liberally into the linked hexagonal rings
that compose the nanotubes, prompting Dai to describe the end result as
resembling rolled-up chicken wire with feathers sticking out all over.
Though they may sound less than gorgeous visually, the feathery
nanotubes turned in a beautiful performance in practical terms, Dai
said. The coating of PEG made the nanotubes highly water soluble, which
helped them to stay in the blood instead of being absorbed.
"They circulate in the blood for about 10 hours or so in mice, which seems to be a good length of time," Dai said.
A representation of a carbon nanotube accessorized with a coating of branched PEG.
The right chemical coating on nanotubes also can help ease them out
of the mouse in a timely fashion, and the three-branched PEG was
effective there, too.
Dai’s earlier research demonstrated that nanotubes have promise for
treating cancer with two different approaches. Once they have zeroed in
on the target cells, shining light on the nanotubes causes them to
generate heat, which can kill cancer cells. The other method is to rig
the nanotubes to accumulate at targeted sites, where they can deliver
medication from within the tubes.
"[Carbon nanotubes] seem to be promising for biomedical applications
and for potentially treating cancer, either using drugs or using the
physical properties," Dai said. "This is the reason we carried out the
study of the fate of nanotubes in mice. I think this is really a very
fundamental issue."
The research was funded by the Cancer Center for Nanotechnology
Excellence, which is funded by the National Institutes of Health and
the National Cancer Institute. The first author of the PNAS
paper is Zhuang Liu, graduate student in chemistry. The paper’s other
authors, all affiliated with Stanford University, are Xiaoyuan "Shawn"
Chen, assistant professor in radiology; Dr. Corrine Davis of the
Veterinary Service Center in the Department of Comparative Medicine;
Weibo Cai, postdoctoral scholar in radiology; and Lina He, formerly a
technician in Chen’s research group.
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