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

Our plain-English summary of 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, by A. Maeno, M. Hojo, Y. Sakamoto, et al (Nanomaterials, 2026).

The study

• Authors: A. Maeno, M. Hojo, Y. Sakamoto, Y. Yamamoto, K. Ikushima, K. Igarashi, S. Yokota, Y. Taquahashi, N. Kobayashi, H. Tsuda, A. Naiki-Ito, T. Numano, J. Kanno, A. Hirose, A. Inomata, J. Kamiie, D. Nakae
• Journal: Nanomaterials
• Published: 2026;16(16)
• Read it: PubMed · Publisher · Free full text

Abstract

The multiwalled carbon nanotube, MWCNT-7, is carcinogenic to rat lungs when inhaled over the course of 2 years, but does not induce pleural mesothelioma. In contrast, intratracheal instillation of MWCNT-7 causes both lung tumors and pleural mesotheliomas. To explore the possibility of the use of the intratracheal instillation technique in risk characterization, we administered this widely used reference material using instillation to male F344 rats at doses of 0.0175, 0.07, 0.28, or 0.42 mg/kg once per week for 13 weeks, followed by an observation period of 91 weeks and compared the dose-response toxicological data with the 2-year inhalation study. Lung tumors and pleural mesotheliomas were induced in a dose-dependent manner (the 0.42 mg/kg group was excluded from analysis of lung tumor formation). The lung burden of MWCNT-7 at week 13 was dose-dependent: 0.0302, 0.191, 0.98, and 1.369 mg/lung in the 0.0175, 0.07, 0.28, and 0.42 mg/kg groups, respectively. The number of fibers in the pleural lavage fluid was also dose-dependent. The no-observed-adverse-effect-level (NOAEL) of MWCNT-7's carcinogenicity was 0.0302 mg/lung based on no significant lung tumor induction and no mesothelioma induction in the 0.0175 mg/kg group. When compared based on lung burden, the dose-response relationship and NOAEL of lung tumor induction resembled those in the 2-year inhalation study, offering the possibility of the application of this protocol for risk characterization.

In plain English

This was a two-year animal experiment on a carbon nanotube, not on asbestos. The authors instilled a multiwalled carbon nanotube called MWCNT-7 directly into the lungs of male F344 rats once a week for 13 weeks, at 0.0175, 0.07, 0.28 or 0.42 mg/kg body weight, and then observed the rats until week 104. Total lung tumors (benign and malignant combined) were significantly more frequent in the middle-dose group (7/40) and the high-dose group (7/40) than in controls (1/38), with a statistically significant dose-related trend. The highest-dose group was left out of the lung-tumor analysis because many of its rats died early from pleural mesothelioma. Malignant pleural mesothelioma did not develop in the lowest-dose group. It occurred in two middle-dose rats and was significantly increased in the high-dose group (11/40, 27.5%) and the highest-dose group (70%). Survival was significantly lower in the two highest-dose groups. The authors concluded that the lung-tumor dose-response, measured against the amount of material retained in the lung, was similar to that of an earlier two-year inhalation study. They also noted that instillation cannot fully reproduce inhalation.

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: 104-week carcinogenicity study using repeated intratracheal instillation (13 weekly doses followed by 91 weeks of observation)
• Who was studied: 220 male Fischer 344 (F344) rats, specific pathogen-free, five weeks old on purchase, studied at the Tokyo Metropolitan Institute of Public Health, Japan. Main groups: vehicle control 40, low-dose 40, middle-dose 40, high-dose 40, extra-high-dose 20, plus satellite rats used for lung-burden measurements.
• Exposure: MWCNT-7, a multiwalled carbon nanotube (average fiber length 4.8 µm, diameter 83.7 nm), given by intratracheal instillation 13 times (once a week) at 0.0175, 0.07, 0.28 or 0.42 mg/kg body weight
• Main results:
  • Total lung tumors (adenomas and/or carcinomas) were significantly increased in the middle-dose group (7/40) and the high-dose group (7/40) compared with the control group (1/38)
  • Across the four main groups, the survival-adjusted Poly-3 trend test showed a dose-dependent effect on the lung: p = 0.0267 for bronchiolo-alveolar adenoma and p = 0.0056 for total lung tumors
  • The extra-high-dose group (0.42 mg/kg) was excluded from the lung-tumor evaluation because of early deaths from pleural mesothelioma
  • Malignant pleural mesothelioma was significantly increased in the high-dose group (11/40, 27.5%) and the extra-high-dose group (70%) compared with controls
  • Pleural mesothelioma in the high-dose group, whose lung burden was 980 µg/lung, occurred in 11 of 40 rats (27.5%)
  • Pleural mesothelioma occurred in the extra-high-dose group at an incidence of 70%
  • Pleural mesothelioma occurred in two middle-dose rats and in none of the low-dose rats
  • Survival at the end of the study was significantly lower in the high-dose and extra-high-dose groups than in the control group
  • Lung burden at week 13 rose with dose: 30.2, 191.0, 980.4 and 1368.8 µg in the low-, middle-, high- and extra-high-dose groups
  • The number of MWCNT-7 fibers in the pleural lavage fluid increased with dose at both week 13 and week 104
  • The no-observed-adverse-effect level for lung tumor induction was the low dose: 0.064 mg/rat in total, with a peak lung burden of 30.2 µg/lung
  • Peritoneal mesothelioma was not significantly increased in any treated group compared with controls
  • At week 104, inflammatory cell counts in lung lavage fluid were not significantly increased in treated rats
  • Serum mesothelin was significantly higher in rats with mesothelioma than in control rats and in treated rats without mesothelioma
• The authors' conclusion: The authors concluded that, when compared by lung burden, the dose-response for lung tumors in this instillation study was similar to that of the earlier 2-year inhalation study by Kasai et al.
• Limitations the authors state:
  • Bolus exposure by intratracheal instillation cannot fully reproduce exposure by inhalation
  • Deposition of the nanotubes in the lung was somewhat uneven despite efforts to disperse the material
  • Pleural mesotheliomas in the high-dose group may have interfered with assessment of lung tumors
  • Lung burden was measured at only two time points
  • A different centrifuge setting from the inhalation study's may affect the comparison of pleural fiber counts
  • It is unknown whether the 13-week instillation protocol would work as an alternative to 2-year inhalation studies for other carbon nanotubes
  • The serum mesothelin data are biased toward advanced mesothelioma cases

Topics

Medical terms in this study, explained in our Lexicon: Dose-response relationship, Pleural Mesothelioma, Carcinogenic, NOAEL, Mesothelioma.

Related studies

• Two-year intermittent exposure of a multiwalled carbon nanotube by intratracheal instillation induces lung tumors and pleural mesotheliomas in F344 rats — M. Hojo, A. Maeno, et al. (2022)
• 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)
• MWCNT-7 administered to the lung by intratracheal instillation induces development of pleural mesothelioma in F344 rats — T. Numano, H. Higuchi, et al. (2019)
• Comparative Carcinogenicity of Double-Walled Carbon Nanotubes of Different Lengths Administered by Intratracheal Installation into Rat Lungs — O.H.M. Ahmed, D.M. Saleh, et al. (2025)
• Comparative carcinogenicity study of a thick, straight-type and a thin, tangled-type multi-walled carbon nanotube administered by intra-tracheal instillation in the rat — D.M. Saleh, W.T. Alexander, et al. (2020)
• Absence of in vivo mutagenicity of multi-walled carbon nanotubes in single intratracheal instillation study using F344 gpt delta rats — K. Horibata, A. Ukai, et al. (2017)

Browse more research in Nanomaterials, or return to medical research.