Two-year intermittent exposure of a multiwalled carbon nanotube by intratracheal instillation induces lung tumors and pleural mesotheliomas in F344 rats

Our plain-English summary of Two-year intermittent exposure of a multiwalled carbon nanotube by intratracheal instillation induces lung tumors and pleural mesotheliomas in F344 rats, by M. Hojo, A. Maeno, Y. Sakamoto, et al (Particle and Fibre Toxicology, 2022).

The study

• Authors: M. Hojo, A. Maeno, Y. Sakamoto, A. Ohnuki, Y. Tada, Y. Yamamoto, K. Ikushima, R. Inaba, J. Suzuki, Y. Taquahashi, S. Yokota, N. Kobayashi, M. Ohnishi, Y. Goto, T. Numano, H. Tsuda, D.B. Alexander, J. Kanno, A. Hirose, A. Inomata, D. Nakae
• Journal: Particle and Fibre Toxicology
• Published: 2022;19(1):38
• Cited by: 33 articles, as counted by NIH iCite
• Read it: PubMed · Publisher · Free full text

Abstract

Background: A mounting number of studies have been documenting the carcinogenic potential of multiwalled carbon nanotubes (MWCNTs); however, only a few studies have evaluated the pulmonary carcinogenicity of MWCNTs in vivo. A 2-year inhalation study demonstrated that MWNT-7, a widely used MWCNT, was a pulmonary carcinogen in rats. In another 2-year study, rats administered MWNT-7 by intratracheal instillation at the beginning of the experimental period developed pleural mesotheliomas but not lung tumors. To obtain data more comparable with rats exposed to MWNT-7 by inhalation, we administered MWNT-7 to F344 rats by intratracheal instillation once every 4-weeks over the course of 2 years at 0, 0.125, and 0.5 mg/kg body weight, allowing lung burdens of MWNT-7 to increase over the entire experimental period, similar to the inhalation study.

Results: Absolute and relative lung weights were significantly elevated in both MWNT-7-treated groups. Dose- and time-dependent toxic effects in the lung and pleura, such as inflammatory, fibrotic, and hyperplastic lesions, were found in both treated groups. The incidences of lung carcinomas, lung adenomas, and pleural mesotheliomas were significantly increased in the high-dose group compared with the control group. The pleural mesotheliomas developed mainly at the mediastinum. No MWNT-7-related neoplastic lesions were noted in the other organs. Cytological and biochemical parameters of the bronchoalveolar lavage fluid (BALF) were elevated in both treated groups. The lung burden of MWNT-7 was dose- and time-dependent, and at the terminal necropsy, the average value was 0.9 and 3.6 mg/lung in the low-dose and high-dose groups, respectively. The number of fibers in the pleural cavity was also dose- and time-dependent.

Conclusions: Repeated administration of MWNT-7 by intratracheal instillation over the 2 years indicates that MWNT-7 is carcinogenic to both the lung and pleura of rats, which differs from the results of the 2 carcinogenicity tests by inhalation or intratracheal instillation.

© 2022. The Author(s).

In plain English

This animal experiment tested MWNT-7, a manufactured multiwalled carbon nanotube fiber, not asbestos. Male F344 rats had an MWNT-7 suspension placed into their lungs through the windpipe once every 4 weeks, 26 times over 2 years. Doses were 0, 0.125 or 0.5 mg/kg body weight, and 28 to 30 rats per group were examined for tumors. Lung burden at 2 years was 0.9 mg (low dose) and 3.6 mg (high dose). In the high-dose group, 11 of 28 rats (39.3%) had lung tumors and 12 of 28 (42.9%) had malignant pleural mesothelioma. Both were significantly more than in controls, where 1 of 30 had a lung tumor and none had mesothelioma. In the low-dose group, 4 of 29 (13.8%) had pleural mesothelioma. This was not statistically significant, but the authors judged it very likely treatment-related because this tumor rarely arises spontaneously in these rats. The authors identified the rear (caudal) mediastinum as the probable site where mesotheliomas began. No treatment-related tumors were found in other organs. The authors concluded that repeated instillation caused dose-dependent lung tumors and pleural mesothelioma in these rats.

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: 2-year animal carcinogenicity experiment: repeated intratracheal instillation (26 times, every 4 weeks), with interim sacrifices at weeks 26 and 52 and terminal necropsy at week 104
• Who was studied: 150 male specific pathogen-free Fischer 344 rats, 5 weeks old on purchase, studied at the Tokyo Metropolitan Institute of Public Health, Japan. They were split into control, low-dose and high-dose groups of 40, 55 and 55 animals. 30 per group were assigned to the carcinogenicity assessment, and 30, 29 and 28 were examined. Study dates are not given.
• Exposure: MWNT-7, a multiwalled carbon nanotube made by Mitsui Chemicals and processed by the Taquann method to give dispersed single fibers (average length 5.11 µm, width 84.7 nm). It was given in saline with 0.1% Tween 80 at 0 (control), 0.125 (low dose) or 0.5 (high dose) mg/kg body weight per instillation. Cumulative administered doses were 1.2 and 4.8 mg/lung.
• Main results:
  • Lung burden of MWNT-7 at 2 years: 0.9 mg/lung in the low-dose group and 3.6 mg/lung in the high-dose group
  • Total lung tumors: 1 of 30 (3.3%) in controls, 3 of 29 (10.3%) in the low-dose group, and 11 of 28 (39.3%) in the high-dose group. Only the high-dose increase was statistically significant.
  • Bronchiolo-alveolar adenocarcinoma: 1 (3.3%) in controls, 2 (6.9%) low-dose, 6 (21.4%) high-dose, significant in the high-dose group. Bronchiolo-alveolar adenoma: 0, 1 (3.5%) and 4 (14.3%), significant in the high-dose group.
  • Malignant pleural mesothelioma: 0 in controls, 4 of 29 (13.8%) in the low-dose group, and 12 of 28 (42.9%) in the high-dose group. Only the high-dose increase was statistically significant.
  • The low-dose increase in pleural mesothelioma was not statistically significant. The authors judged it very likely treatment-related, given the very low spontaneous rate of pleural mesothelioma in F344 rats (0.03%).
  • The authors identified the mediastinum, especially its caudal (rear) region, as the possible or probable primary site of the mesotheliomas
  • Rats surviving to week 104: 25 (83.3%) controls, 22 (75.9%) low-dose, 12 (41.4%) high-dose. Average survival time was significantly shorter in the high-dose group than in controls.
  • In treated rats, non-cancerous lung and pleural lesions (inflammation, fibrosis, hyperplasia) tended to increase with dose and time
  • Tumors in other organs were not related to MWNT-7 treatment
  • The number of MWNT-7 fibers in fluid washed from the pleural cavity increased with time and dose. These fibers were significantly shorter on average than fibers in the lung (2.73 µm versus 4.23 µm).
• The authors' conclusion: The authors concluded that repeated intratracheal instillation of MWNT-7 in male F344 rats over 2 years caused dose-dependent lung tumors and pleural mesothelioma. They also suggested that whether mesothelioma appears is linked to the exposure method: instillation, not inhalation, produced it.
• Limitations the authors state:
  • The authors noted that early deaths from mesothelioma in the high-dose group likely prevented some lung tumors from developing
  • The authors stated that how the fibers move from the lung into the pleural cavity remains unclear

Topics

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

Medical terms in this study, explained in our Lexicon: Pleural Mesothelioma, Carcinogenic, Pleura, Carcinoma, Mediastinum, Bronchoalveolar Lavage (BAL), Autopsy, Pleural Cavity.

Related studies

• Dose-Response Relationships for Lung Tumor and Pleural Mesothelioma Induction by Repetitive Intratracheal Instillation of a Multiwalled Carbon Nanotube, MWCNT-7, in Rats: Comparison with Inhalation Carcinogenicity Based on Lung Burden — A. Maeno, M. Hojo, et al. (2026)
• Assessment of the toxicity and carcinogenicity of double-walled carbon nanotubes in the rat lung after intratracheal instillation: a two-year study — D.M. Saleh, S. Luo, et al. (2022)
• Multiwalled carbon nanotubes intratracheally instilled into the rat lung induce development of pleural malignant mesothelioma and lung tumors — M. Suzui, M. Futakuchi, et al. (2016)
• MWCNT-7 administered to the lung by intratracheal instillation induces development of pleural mesothelioma in F344 rats — T. Numano, H. Higuchi, et al. (2019)
• Multi-walled carbon nanotubes translocate into the pleural cavity and induce visceral mesothelial proliferation in rats — J. Xu, M. Futakuchi, et al. (2012)
• Intraperitoneal administration of tangled multiwalled carbon nanotubes of 15 nm in diameter does not induce mesothelial carcinogenesis in rats — H. Nagai, Y. Okazaki, et al. (2013)

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