Tumorigenicity of cellulose fibers injected into the rat peritoneal cavity

Our plain-English summary of Tumorigenicity of cellulose fibers injected into the rat peritoneal cavity, by R.T. Cullen, B.G. Miller, S. Clark, et al (Inhalation Toxicology, 2002).

The study

• Authors: R.T. Cullen, B.G. Miller, S. Clark, J.M. Davis
• Journal: Inhalation Toxicology
• Published: 2002;14(7):685-703
• Cited by: 18 articles, as counted by NIH iCite
• Read it: PubMed · Publisher

Abstract

Cellulose fibers, along with many other organic fibers, are durable. Therefore, if inhaled, they have the potential to persist within the lung, and may then cause disease. Here we report the effects of injecting high-purity cellulose fibers into the abdominal cavity of rats. A respirable fraction of cellulose fiber was collected from an aerosol of a thermo-mechanically-processed wood pulp. A sample of respirable crocidolite asbestos, known to produce mesotheliomas in rats, was used as a positive control. Total doses of 10(6), 10(7), 10(8), or 10(9) WHO fibers were injected intraperitoneally as 3 weekly aliquots. A negative control was provided by phosphate-buffered saline used to suspend the fibers for injection. There were 50 rats per treatment group except for the 10(8) and 10(9) fibers crocidolite groups which were reduced to 26 rats because of the expectation of high tumor incidence in these groups. The two higher doses of crocidolite asbestos caused greatly reduced survival compared to the saline controls. With cellulose there was a much less marked effect on survival. In the highest dose cellulose group, multiple large nodules (granulomas) and widespread adhesions (bands of new tissue connecting organs to each other and to the abdominal wall) were present in all animals. Granulomas were not observed in the 10(9) fibers crocidolite group. More than 80% of animals in the 10(8) and 10(9) crocidolite asbestos groups had mesotheliomas, a type of tumor sometimes observed in people exposed to asbestos. In contrast, there were only 2 animals in the cellulose groups with mesothelioma tumors, 1 in the 10(7) and 1 in the 10(8) groups. However, 9 (18%) of the 10(9) cellulose group had malignant tumors that, in contrast to the usual pattern of mesothelioma development following treatment with mineral fibers in rats, showed no obvious involvement of mesothelial tissues, were not associated with blood-stained ascites fluid, and were thus classified as sarcomas. This study has demonstrated that a high dose of cellulose fibers is capable of producing tumors when injected into the abdominal cavity of rats.

In plain English

In this animal experiment, male Wistar rats received three weekly injections into the abdominal (peritoneal) cavity of one of three things: respirable cellulose fibers made from processed wood pulp, crocidolite asbestos (a comparison known to cause mesothelioma in rats), or saline alone. Total doses ranged from 10^6 (one million) to 10^9 (one billion) fibers longer than 5 µm. Most groups had 50 rats; the two highest crocidolite groups had 26. Crocidolite produced mesotheliomas at every dose, including 22 of 26 rats at the highest dose. Among cellulose-treated rats, one at 10^7 and one at 10^8 fibers had mesothelioma, and the authors could not say whether these were spontaneous. At the highest cellulose dose, 9 of 50 rats developed malignant tumors that the authors classified as sarcomas. All rats in that group had large granulomas and widespread adhesions. Saline-injected rats had no mesotheliomas. The authors concluded that high doses of cellulose fibers can produce tumors when injected into the rat abdominal cavity. They noted that this test delivers large doses nonphysiologically to an enclosed site.

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 (intraperitoneal injection carcinogenicity study in rats), with crocidolite asbestos as positive control and phosphate-buffered saline as negative control
• Who was studied: Specific-pathogen-free outbred male Wistar rats, about 9 to 12 weeks old at entry, studied at the Institute of Occupational Medicine, Edinburgh. There were 50 rats per group, except 26 in each of the 10^8 and 10^9 crocidolite groups (nine groups in total). The study was ended when the cellulose group with the most survivors fell to six rats; the final sacrifice was at 28 months after injection.
• Exposure: Respirable cellulose fibers collected from an aerosol of thermo-mechanically processed wood pulp (high-purity, from Sweden; the sample also contained non-fibrous particles at about 2 particles per fiber), or respirable crocidolite asbestos (from the TIMA fiber repository). Each was given at total doses of 10^6, 10^7, 10^8 or 10^9 fibers longer than 5 µm, in three weekly intraperitoneal injections. A dose of 10^9 fibers required 1.78 mg of crocidolite and 115.9 mg of cellulose. Controls received saline only.
• Main results:
  • Crocidolite produced typical mesotheliomas in all four dose groups, with a clear dose response. According to Table 1, mesothelioma occurred in 22 of 26 rats (10^9 fibers), 21 of 26 (10^8), 14 of 50 (10^7) and 4 of 50 (10^6).
  • In the highest-dose cellulose group (10^9 fibers), 9 of 50 rats developed large peritoneal tumors. These were usually of spindle-cell/fibrosarcomatous type, showed no obvious involvement of mesothelial tissues and were not associated with blood-stained ascites fluid, so they were classified as sarcomas. Mean time to death was 476 days. The authors noted that these tumors may form an extreme end of the mesothelioma range.
  • Only two cellulose-treated rats had tumors clearly classified as mesothelioma. One was in the 10^8 group (the rat died at 744 days) and one in the 10^7 group (the rat died at 400 days). The authors were not certain whether these were spontaneous or treatment related. No mesotheliomas were found in the 47 saline control animals examined.
  • All rats in the 10^9 cellulose group had multiple large granulomas and widespread adhesions. The 10^8 cellulose group usually had small granulomas with slight adhesions. None were seen in the 10^7 or 10^6 cellulose groups, in crocidolite-treated rats, or in saline controls. Cellulose fibers were still visible in granulomas at the final sacrifice.
  • For deaths from all causes, median survival was 457 days in the 10^9 crocidolite group, 555 days in the 10^8 crocidolite group and 716 days in saline controls. Medians in the cellulose groups ranged from 625 days (10^9) to 681 days (10^6). The differences among the cellulose and saline survival curves were not significant at the 5% level; the dose-trend test reached the 0.055 significance level.
  • For deaths with mesothelioma or sarcoma, the 10^9 cellulose group showed clearly poorer survival. Differences between cellulose and saline treatments were highly significant (p less than 0.001), and so was the dose-response regression (p less than 0.005).
• The authors' conclusion: The authors concluded that high doses of cellulose fibers are capable of producing tumors when injected into the abdominal cavity of rats. There were two mesotheliomas at 10^7 and 10^8 fibers, but the tumors at the highest dose (10^9 fibers) were identified as sarcomas rather than mesotheliomas. They suggested that such a massive dose may produce tumors by a mechanism different from classical mesothelioma, possibly related to the vigorous inflammatory response.
• Limitations the authors state:
  • The intraperitoneal injection assay gives large fiber doses nonphysiologically to an enclosed site with no clearance mechanism other than dissolution.
  • The cellulose sample contained many non-fibrous particles. Separate studies with fiber-free particulate would be needed to confirm that these particles have no carcinogenic effect.
  • The comparison of mesothelioma incidence in the mid-dose cellulose groups with controls relies on very few animals, and it is uncertain whether these two tumors were spontaneous.
  • The evidence of a dose-related increase in tumor risk for cellulose rested on combining sarcomas and mesotheliomas into one group.
  • In some rats that died spontaneously, delayed fixation and autolysis made detailed histological examination impossible, so a tumor could not be ruled out.

Topics

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

Medical terms in this study, explained in our Lexicon: Peritoneal Cavity, Respirable Fraction, Crocidolite, Mesothelioma, Granuloma, Mineral Fiber, Ascites, Sarcoma.

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• The long term fibrogenic effects of chrysotile and crocidolite asbestos dust injected into the pleural cavity of experimental animals — J.M. Davis (1970)
• In vivo pulmonary toxicity of cellulose in rats — E. Tátrai, M. Brozik, et al. (1996)
• Differential diagnosis of malignant tumours in the abdominal cavity of rats after intraperitoneal injection of crocidolite or benzo[a]pyrene — J. Friemann, M. Varnai, et al. (1996)
• Gender differences in murine pulmonary responses elicited by cellulose nanocrystals — A.A. Shvedova, E.R. Kisin, et al. (2016)
• Investigating the interaction of cellulose nanofibers derived from cotton with a sophisticated 3D human lung cell coculture — M.J. Clift, E.J. Foster, et al. (2011)

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