Pilot toxicology study of intravenously injected carbon nanotubes

The toxicity issues surrounding carbon nanotubes (CNTs) are highly relevant for two reasons: Firstly, as more and more products containing CNTs come to market, there is a chance that free CNTs get released during their life cycles, most likely during production or disposal, and find their way through the environment into the body. Secondly, and much more pertinent with regard to potential health risks, is the use of CNTs in biological and medical settings. CNTs interesting structural, chemical, electrical, and optical properties are explored by numerous research groups around the world with the goal of drastically improving performance and efficacy of biological detection, imaging, and therapy applications. In many of these envisaged applications, CNTs would be deliberately injected or implanted in the body. For instance, CNT-based intercellular molecular delivery vehicles have been developed for intracellular gene and drug delivery in vitro.
What these CNTs do once inside the body and after they discharge their medical payloads is not well understood. Cell culture studies have shown evidence of cytotoxicity and oxidative stress induced by single-walled carbon nanotubes (SWCNTs), depending on whether and to what degree they are functionalized or oxidized. A recent report also found that inhaled single-walled CNTs can cause damage to the lungs in animal studies. On the other hand, another study (New nanotube findings give boost to potential biomedical applications) reported that the CNTs leave the body without accumulating in organs and without observable toxic effects (read more about this ongoing debate in The detection of carbon nanotubes and workplace safety).
So of course you need to take these results with a grain of salt (see Comparing apples with oranges - the problem of nanotubes risk assessment).
For most medical applications like drug delivery, the most relevant route into and through the body for CNTs would be in the circulatory system. However, close to nothing is known about the acute and chronic toxicity of SCWNTs when they enter the bloodstream. A new study at Stanford University tested non-covalently pegylated SWCNTs as a 'least toxic scenario', and oxidized, covalently functionalized nanotubes as a 'most toxic scenario' in a study on mice. It was found that SWCNTs injected intravenously into nude mice do not appear to have any significant toxicity during an observation period of four months following injection.
"Our study demonstrates the first systematic toxicity evaluation of functionalized SWCNTs following intravenous injection" Dr. Sanjiv Sam Gambhir tells Nanowerk. "Single administrations of high doses did not lead to acute or chronic toxicity, but we observed some changes in red blood cells. Because of the small number of animals used in the tests, our findings must be considered a pilot study. Although more extensive series are needed to confirm our results and show equivalence in other mouse strains, they do encourage further exploration of functionalized SWCNTs in biomedical applications in living animals."

Liver and spleen histology. a–f, Haematoxylin and eosin stains of liver (a–c) and spleen (d–f) tissues of mice injected with phosphate buffered saline (PBS) (a,d), non-covalently pegylated SWCNTs (SWCNT PEG) (b,e) or covalently functionalized nanotubes (SWCNT O PEG) (c,f). Finely granular brown-black pigments were seen in sinusoidal liver cells of SWCNT PEG (b, arrows) and SWCNT O PEG (c, arrows), as well as a golden-brown pigment in spleen macrophages of SWCNT PEG and SWCNT O PEG (e,f), without signs of cellular or tissue damage. (Reprinted with permission from Nature Publishing Group)
Gambhir, a professor in Stanford University's Departments of Radiology and Bioengineering, and Director, Molecular Imaging Program at Stanford (MIPS) as well as Head, Stanford Nuclear Medicine, collaborated on this project with Stanford researchers from MIPS, Hongjie Dai's group in the Department of Chemistry, and the Department of Comparative Medicine. The scientists published their findings in the March 30, 2008 online edition of Nature Nanotechnology ("A pilot toxicology study of single-walled carbon nanotubes in a small sample of mice"). This work was funded in part by the National Cancer Institute's (NCI) Center for Cancer Nanotechnology Excellence (CCNE).
All aspects of toxicity, including EKG, blood pressure, temperature, cell blood count, electrolytes, etc were monitored repeatedly during the 4-months study period. Gambhir says that, although minor changes occurred, no statistically significant changes occurred between mice given SWCNTs and those not given them.
"At the end of the monitoring period of 4 months all mice were sacrificed and a full tissue histology was done to look for signs of organ toxicity; but there was none." he says. "The Kupfer cells in the liver (a special type of cell that sits within the sinusoids, which are the phagocytes of the liver) had eaten up the nanotubes and they were found in these cells within the liver."
Gambhir and his group were motivated to do this study because they are developing imaging agents for cancer detection that rely on carbon nanotubes. Of course, before these technologies can be moved to a human trial stage the question of toxicity must be much better understood than it is today. Although preliminary, this recent study gives some hope in at least not finding any obvious toxicity problems.