New Genotoxicity Study on SWCNTs.

Eight scientists from NIOSH and two from Lockheed Martin recently published "Single-walled Carbon Nanotubes: Geno- and Cytotoxic Effects in Lung Fibroblast V79 Cells," in the Journal of Toxicology and Environmental Health, Part A, 70:2071-2079 (Dec. 15, 2007).  The authors claim the study is the first published research examining the potential genotoxic effects of cellular exposure to SWCNTs. The paper is summarized below.
 

The SWCNTs used in the study ranged from .4 nm to 1.2 nm in diameter, and were 1 to 3 microns in length. Importantly, there was only .23% residual iron in the tested material. Possible metal contaminants were tightly controlled because of criticism of prior nanotoxicology tests.

The scientists conducted three genotoxicity tests.

The first was a Comet Assay designed to test for breaks in double and single banded DNA potentially caused by exposure to SWCNTs.  The authors explained these type of DNA breaks are most likely to occur during cell interphase. The hamster lung cells in this test were subjected to four different concentrations of water-suspended SWCNTs for three and 24 hour periods. The scientists found exposure for three hours led to significant DNA damage/breaks only at the highest levels tested (96 µg/cm2).  Twenty-four hour exposure, on the other hand, increased DNA damage/breaks in a "concentration-dependent way."

The second test measured micronucleus formation in cells exposured to four different concentrations of SWCNTs for 24 hours. The test was designed to predict possible DNA damage during the cell division process.  Interestingly, this test did not produce significant micronucleus formation under any tested SWCNT concentration. 

The third test exposed two strains of salmonella bacteria to SWCNTs and looked for any mutagenic effects. Importantly, the authors found the mutation frequency of the bacteria exposed to SWCNTs was not substantially different from those in a control group not exposed to SWCNTs.

Beyond the three genotoxicity tests, the scientists also examined the general cytotoxicity of the SWCNTs.  Cells were exposed to varying concentrations of SWCNTs for three hour and 24 hour test intervals.  At the highest concentration level (96 µg/cm2), there was a 21% loss of cell viability after three hours and a 30% loss after 24 hours.  This last result did not surprise the authors given their prior findings in the area. However, they noted the cytotoxicity results "suggest SWCNT – induced loss of cells might interfere with accurate evaluation of genotoxicity responses to SWCNTs detectable in viable cells." 

The scientists concluded further research is necessary to reach more definitive conclusions on genotoxicity issues related to SWCNTs.