Absence of Carcinogenic Response to Multiwall Carbon Nanotubes in a 2-Year Bioassay in the Peritoneal Cavity of the Rat

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.

Single-walled carbon nanotube-induced mitotic disruption

“…A more recent study by Kanno et al, observed significant increase of mesothelioma following intraperitoneal injection of mice with as little as 50 μg of long MWCNT [76]. By contrast, intraperitoneal injection of rats with 20 mg short MWCNT with a median diameter of less than 1 μm did not result in increased mesotheliomas [77]. Although the MWCNT exposure studies have been criticized due to the high dose and the route of exposure, the studies raise concerns about the potential of cancer due to occupational and environmental exposures to particles that may have physical properties similar to asbestos fibers.…”

Hazard Assessments of Manufactured Nanomaterials

“…On the other hand, also in an intraperitoneal injection study, Muller et al 60) obtained results which differed from those of Takagi et al 59) . After intraperitoneally administering 2 or 20 mg/rat of MWCNTs, Muller et al 60) observed the post-administration course for 2 yr at the longest, and reported no differences in the incidences of malignant mesothelioma between the MWCNT-exposed groups and the negative control.…”

“…The exposure time, t, was the daily exposure period of Morimoto's experiment 50) ; f r and f h are values calculated utilizing a multi-path particle dosimetry (MPPD) model; q r , S r and S h were taken and calculated from United States Environmental Protection Agency data (1994); and q h is the respiration rate for light work of 36 m 3 /day. Since the significant adverse effects of C 60 were not observed even in intratracheal instillation studies when the intrapulmonary amounts found were more than 40 times those seen in inhalation studies, and applying this as a correction factor, as well as an uncertainty factor of 6 for estimation of a long-term index from short-or medium-term indices, the provisional Threshold Level (TL) was estimated as implying an acceptable exposure concentration of 0.8 mg/m 3 in working environments for C 60 fullerene nanoparticles having a geometric mean diameter of 96 nm.…”

“…After intraperitoneally administering 2 or 20 mg/rat of MWCNTs, Muller et al 60) observed the post-administration course for 2 yr at the longest, and reported no differences in the incidences of malignant mesothelioma between the MWCNT-exposed groups and the negative control. In the study 2 mg/rat of UICC crocidolite was administered as a positive control, and the incidence of malignant mesothelioma increased significantly in the asbestos-exposed group, showing differences in the biological responses between the MWCNTs and asbestos-exposed groups.…”

Long‐term retention of pristine multi‐walled carbon nanotubes in rat lungs after intratracheal instillation

“…MWCNTs reportedly induced pulmonary symptoms, such as granulomatous inflammation and fibrotic responses, after inhalation exposure or intratracheal instillation in some studies (Muller et al ., 2005, Li et al ., 2007; Ma‐Hock et al ., 2009, Pauluhn, 2010), while no toxicity was reported by other studies (Mitchell et al ., 2007, Muller et al ., 2009, Kobayashi et al ., 2010, Morimoto et al ., 2012). These differences in toxicity were proposed to be as a result of the physicochemical characteristics of MWCNTs, such as length and diameter, impurities, components of their surface structure such as functional groups and dispersants (Donaldson et al ., 2010; Nagai et al ., 2011; Nakanishi 2011, ).…”