Absence of carcinogenic response to multiwall carbon nanotubes in a 2-year bioassay in the peritoneal cavity of the rat

Our plain-English summary of Absence of carcinogenic response to multiwall carbon nanotubes in a 2-year bioassay in the peritoneal cavity of the rat, by J. Muller, M. Delos, N. Panin, et al (Toxicological Sciences, 2009).

The study

• Authors: J. Muller, M. Delos, N. Panin, V. Rabolli, F. Huaux, D. Lison
• Journal: Toxicological Sciences
• Published: 2009;110(2):442-8
• Cited by: 172 articles, as counted by NIH iCite
• Read it: PubMed · Publisher

Abstract

Toxicological investigations of carbon nanotubes have shown that they can induce pulmonary toxicity, and similarities with asbestos fibers have been suggested. We previously reported that multiwall carbon nanotubes (MWCNT) induced lung inflammation, granulomas and fibrotic reactions. The same MWCNT also caused mutations in epithelial cells in vitro and in vivo. These inflammatory and genotoxic activities were related to the presence of defects in the structure of the nanotubes. In view of the strong links between inflammation, mutations and cancer, these observations prompted us to explore the carcinogenic potential of these MWCNT in the peritoneal cavity of rats. The incidence of mesothelioma and other tumors was recorded in three groups of 50 male Wistar rats injected intraperitoneally with a single dose of MWCNT with defects (2 or 20 mg/animal) and MWCNT without defects (20 mg/animal). Two additional groups of 26 rats were used as positive (2 mg UICC crocidolite/animal) and vehicle controls. After 24 months, although crocidolite induced a clear carcinogenic response (34.6% animals with mesothelioma vs. 3.8% in vehicle controls), MWCNT with or without structural defects did not induce mesothelioma in this bioassay (4, 0, or 6%, respectively). The incidence of tumors other than mesothelioma was not significantly increased across the groups. The initial hypothesis of a contrasting carcinogenic activity between MWCNT with and without defects could not be verified in this bioassay. We discuss the possible reasons for this absence of carcinogenic response, including the length of the MWCNT tested (< 1 mum on average), the absence of a sustained inflammatory reaction to MWCNT, and the capacity of these MWCNT to quench free radicals.

In plain English

In this animal experiment, the authors tested whether multiwall carbon nanotubes (MWCNT), tiny fiber-like carbon materials, cause tumors when injected into the abdominal (peritoneal) cavity of rats. Groups of 50 male Wistar rats received a single injection of 2 or 20 mg of nanotubes with structural defects, or 20 mg of heat-treated nanotubes without defects. Groups of 26 rats received 2 mg of crocidolite asbestos or a saline solution only. The nanotubes were about 0.7 micrometres long. After 24 months, 9 of 26 crocidolite-treated rats (34.6%) had mesothelioma, significantly more than the saline group, where 1 rat had one. In the nanotube groups, mesothelioma occurred in 2 of 50 (4%), 0 of 50, and 3 of 50 (6%) rats. Differences among the non-asbestos groups were not statistically significant. The authors concluded they found no evidence of carcinogenic activity for these nanotubes in this test. They said the negative result should be interpreted critically and suggested the test may mainly detect long fibers.

We wrote this plain-English summary from the full article with the help of AI, and a second AI model checked each sentence against the article. It is not the authors' wording; the abstract and the article are their own account.

What the study did and found

• Design: Animal experiment: a 24-month (2-year) intraperitoneal carcinogenicity bioassay in rats. It included a crocidolite asbestos positive control and a vehicle (saline) control, plus a 24-hour preliminary inflammation test.
• Who was studied: Male Wistar rats, aged 9 to 12 weeks at purchase, studied at the Catholic University of Louvain, Belgium. There were three nanotube groups of 50 rats each and two control groups of 26 rats each. Separate groups of 3 animals were used for the 24-hour inflammation test. Study dates are not stated.
• Exposure: A single intraperitoneal injection in 2 ml of phosphate buffered saline. Groups received 2 mg or 20 mg of MWCNT with structural defects (MWCNT+), or 20 mg of MWCNT without defects (MWCNT–, heated at 2400°C under argon). The positive control group received 2 mg of UICC crocidolite asbestos and the vehicle control group received saline only.
• Main results:
  • Crocidolite group: 9 of 26 rats (34.6%) developed mesothelioma, all before the end of the study. This was a significant increase compared with vehicle controls (p=0.005).
  • Vehicle control group: 1 of 26 rats had a mesothelioma, an epithelial (tubular) nodule on the diaphragm. The text gives this as 3.6% while the table gives 3.8%. No other peritoneal tumors were recorded in this group.
  • MWCNT+ 2 mg group: 2 of 50 rats (4%) had mesothelioma.
  • MWCNT+ 20 mg group: no mesothelioma among 50 rats. This group had one lipoma, one angiosarcoma and one liposarcoma.
  • MWCNT– 20 mg group: 3 of 50 rats (6%) had mesothelioma. One of these was in the scrotum, which the authors described as similar to tumors sometimes reported in old male rats. Three rats in this group had a lipoma.
  • Comparing all groups except crocidolite, there was no statistically significant difference in total peritoneal tumors (p=0.623), mesothelioma (p=0.427) or other tumors (p=0.569).
  • 24 hours after injection (3 animals per group), crocidolite and MWCNT+ 20 mg caused an early inflammatory reaction of similar size. The comparison with vehicle controls gave p=0.0008 for crocidolite and p=0.090 for MWCNT+ 20 mg. MWCNT– 20 mg did not cause such a response.
  • In nanotube-treated rats, granulomas with very limited inflammation formed around the nanotube deposits.
  • Survival did not differ among groups (p=0.1604), and body weight over the first 3 months did not differ significantly.
  • Table 1 gives the length of both nanotube materials as about 0.7 µm, compared with 2.5 µm for crocidolite.
• The authors' conclusion: The authors concluded that crocidolite produced the expected carcinogenic response, but they found no evidence of similar activity for nanotubes with or without defects in this bioassay. Their starting hypothesis, that nanotubes with defects would cause tumors and nanotubes without defects would not, was not verified.
• Limitations the authors state:
  • Injection into the abdominal cavity is not directly relevant to how humans are exposed.
  • Because the nanotubes clumped together, the authors could not reliably count them or measure their size distribution.
  • The test is reported to respond mainly to long fibers (longer than 5 µm), and the nanotube samples did not contain enough long nanotubes. The test may therefore not have been sensitive enough for these materials.
  • A single dose was given, and the inflammation may not have been sustained. Repeated smaller injections might have sustained inflammation and possibly revealed a carcinogenic response.
  • The authors said the negative result should be interpreted critically. They called for further studies with different nanotube preparations, different dosing regimens and/or other bioassays.

Topics

The National Library of Medicine indexes this study under subjects our site covers: Crocidolite, Mesothelioma.

Medical terms in this study, explained in our Lexicon: Carcinogenic, Peritoneal Cavity, Inflammation, Granuloma, Mesothelioma, UICC, Crocidolite.

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