Evaluation of the tumorigenic potential of vermiculite by intrapleural injection in rats

Our plain-English summary of Evaluation of the tumorigenic potential of vermiculite by intrapleural injection in rats, by B. Hunter, C. Thomson (British Journal of Industrial Medicine, 1973).

The study

• Authors: B. Hunter, C. Thomson
• Journal: British Journal of Industrial Medicine
• Published: 1973;30(2):167-73
• Cited by: 6 articles, as counted by NIH iCite
• Read it: PubMed · Publisher · Free full text
• In J-M's library: Johns-Manville's company medical library held a copy (J-M no. 001305).

Abstract

Hunter, B., and Thomson, C. (1973).British Journal of Industrial Medicine,30, 167-173. Evaluation of the tumorigenic potential of vermiculite by intrapleural injection in rats. Vermiculite, the geological name for a group of hydrated laminar minerals, has industrial application in areas previously restricted to the use of asbestos. In this assessment of its tumorigenicity, Rhodesian chrysotile asbestos or vermiculite was injected into the pleural cavity of rats. Mesotheliomata developed in 48% of the rats treated with asbestos. No tumours occurred which were associated with the vermiculite injections.

In plain English

This was an animal experiment testing whether vermiculite, a mineral used to make asbestos-free boards and panels, could cause tumours. The authors gave one hundred and fifty young female rats a single injection into the right chest (pleural) cavity. Fifty received salt solution, 50 received 25 mg of ground, heat-expanded vermiculite from the Palabora deposit in South Africa, and 50 received 25 mg of Rhodesian chrysotile asbestos. The rats were kept for up to 104 weeks. In the chrysotile group, 48% developed mesotheliomata: 21 of 40 rats that died during the study and 2 of 8 killed at 104 weeks. The first appeared in a rat dying 399 days after injection. No mesotheliomata were recorded in the vermiculite or saline groups. One vermiculite-treated rat had a chest tumour tentatively diagnosed as undifferentiated adenocarcinoma. By week 80, vermiculite-treated rats had granulomata (small inflammatory nodules) with no sign of becoming cancerous. The authors noted that chest injection does not resemble human exposure. They concluded that vermiculite use is unlikely to carry the degree of hazard posed by asbestos.

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: single intrapleural injection in rats, compared with saline and chrysotile asbestos groups, followed for up to 104 weeks
• Who was studied: One hundred and fifty female CFY rats (a Sprague-Dawley-derived strain), 21 days old on arrival and injected at 5 weeks of age, randomly split into three groups of 50 (saline control, vermiculite, chrysotile asbestos)
• Exposure: 25 mg per rat of exfoliated, ground vermiculite from the Palabora deposit (N.E. Transvaal, South Africa), or 25 mg per rat of UICC standard Rhodesian chrysotile asbestos, each in 0.2 ml saline; controls received 0.2 ml saline
• Main results:
  • Chrysotile asbestos group: 48% had tumours classified microscopically as mesotheliomata (21 of 40 rats dying during the study and 2 of 8 killed at 104 weeks)
  • Chrysotile asbestos group: the first tumour was found in a rat dying 399 days after injection
  • Vermiculite group: the authors reported no tumours associated with the vermiculite injections, and Table 5 records no mesotheliomata in the vermiculite or saline groups
  • Vermiculite group: by week 80 all rats had granulomata, none showing evidence of neoplastic reaction
  • Vermiculite group: one intrathoracic tumour, in the mediastinum of a rat dying 672 days after injection, was tentatively diagnosed as undifferentiated adenocarcinoma
  • Fibrous pleural adhesions were found in 13 of 25 vermiculite-treated rats dying during the study, 16 of 25 vermiculite-treated rats killed at week 104, 31 of 40 chrysotile-treated rats dying during the study and 7 of 8 chrysotile-treated rats killed at week 104, and in no control rats. The authors attributed adhesions to accidental infection during injection, made worse by the injected material.
  • Vermiculite group: survival was equivalent to or slightly better than that of the saline controls; the higher deaths in the chrysotile group were attributed to treatment
  • All groups: mammary and pituitary tumours, common in this rat strain, showed no increase in incidence and no earlier onset in any group
• The authors' conclusion: The authors concluded that vermiculite use is unlikely to carry the degree of hazard posed by asbestos. They also considered it possible that vermiculite's lack of carcinogenic effect is linked to the animals walling it off in stable granulomata.
• Limitations the authors state:
  • Intrapleural injection is unrealistic compared with human exposure
  • Because few rats died before the first mesothelioma, the authors could not describe the sequence of events in tumour development

Topics

Medical terms in this study, explained in our Lexicon: vermiculite, Chrysotile, Pleural Cavity.

Related studies

• Mesothelioma in rats following intrapleural injection of chrysotile and phosphorylated chrysotile (chrysophosphate) — A. Van der Meeren, J. Fleury, et al. (1992)
• An electron microscope study of the response of mesothelial cells to the intrapleural injection of asbestos dust — J.M. Davis (1974)
• Incidence of various types of thoracic malignancy induced in rats by intrapleural injection of 2 mg of various mineral dusts after inhalation of 222Ra — J. Bignon, G. Monchaux, et al. (1983)
• Tumorigenesis by a ferroactinolite mineral — D.L. Coffin, L.D. Palekar, et al. (1982)
• P53 mutations in tumours induced by intraperitoneal injection of crocidolite asbestos and benzo[a]pyrene in rats — K. Unfried, N. Kociok, et al. (1997)
• 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)

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