Document pmMK8Q6X6E0OD37ovvV1ng7KX

:.Q'i ;c$-. this acted W o0?y VTOt H/n Cvxiut^ Environmental Health Perspectives Vol. 53, pp. 3-9,1983 Review of Published Studies of Orally Administered Asbestos by Lyman W. Condie* There hu been greet public concern about the adverse health effects resulting from the presence of asbestos fibers in municipal drinking water supplies. This article reviews and summarises the experimental findings of 11 published papers that have evaluated the carcinogenic potential of asbestos following its ingestion. The long-term, high-level ingestion of various types of asbestos fibers in more than one animal species failed to produce any definite, reproducible, organ-specific carcinogenic effect Introduction Prolonged industrial human exposure to asbes tos has been associated with an increase in the incidence of certain forms of cancer. The relation ship between inhaled asbestos and mesothe liomas of the pleura and peritoneum or pulmo nary carcinoma is particularly strong; other cancers have also been implicated from inhaled asbestos (2-3). In order to explain why cancer may occur at remote 3ites following the inhala tion of asbestos, it has been suggested that the asbestos fibers that are cleared from the lungs are swallowed and subsequently migrate through the gastrointestinal wall to the peritoneum where cancer may be initiated. Originally the question of pathogenicity from asbestos exposure had relevance only to people occupationally exposed, but the discovery of amphibole fibers in municipal water supplies (4) indicated that asbestos was more widely dis persed through the environment than once be lieved. Asbestos from natural sources, as well as from mining activities, has been shown to con taminate bodies of water that are used as sources of drinking water. Asbestos fibers have been de tected in commercial beverages, possibly result ing from the use of asbestos filters. The extensive use of asbestos cement pipe in municipal water systems has concerned officials of the U.S. Envi ronmental Protection Agency. This paper summa- Target Organ Ibxicology Branch. Ibxicology and Microbi ology Division, Health Effect* Research Laboratory, U.S. EPA, Cincinnati, OH 45268. rizes the various published asbestos ingestion studies that have attempted to answer the ques tion of whether or not ingested asbestos is a health hazard. Other relevant topics such as gas trointestinal penetration by asbestos, the pres ence of asbestos in municipal drinking water sup plies, and epidemiologic studies will be presented in subsequent papers of this workshop. The de tails of all but one of the cited experimental studies are summarized in Table 1. Results of Published Studies An abstract by Bonser and Clayson (5) initially reported experimental findings from an ingestion study. Asbestos was administered to SpragueDawley rats in their feed at a level of 0.15%. No malignant tumors were observed in the exposed animals, which may have been due to the low level of asbestos administered. The high mortal ity of the rats due to pulmonary infection seri ously compromised the study. Webster reported the only study conducted with primates (6). Because of lack of experimental detail, the findings of this article were omitted from Table 1. An unreported number of baboons were exposed to "heavy" concentrations of asbes tos in food and drinking water for up to 5 yr. There was no evidence of any peritoneal or gas trointestinal tumors. The 5-yr exposure time ap pears too short for the carcinogenesis process to occur if the time element of the baboon's reaction to asbestos is similar to that of a human. 4 L. W. C0ND1E Table 1. Summary of asbestos ingestion studies. Study Bonaar(5) Species Test material Rat Crocidolite Gross (7) Control Rat Chrysotile Control Chrysotile Crocidolite Crocidolite Control Gibel (8) Crocidolite (2 sources) Control Rat Chrysotile Cunningham (9) Rat Talc Control Chrysotile Control Chrysotile Control Dose 0.15% in diet ad libitum 5% in diet ad libitum 0 lOmg/wk 5 mg/wk 10 mg/wk 0 10 mg/wk 0 20 mg/day 20 mg/day 0 1% in diet ad libitum 0 1% in diet ad libitum 0 Exposure time To 78 wk Study duration To 78 wk Number of animals. Initial/ Examined 40/12 Malignant tumors ___ Number Location Type* 0 21 mo. To 86 wk 21 mo. 65/25 10/10 1 Liver 0 i n r o o o o c o r*o to o o c o o co cooc-oco o o o r o o r c o o 0 16 wk 16 wk 16 wk 0 21 mo. To 1.5 yr To 1.5 yr To 1.5 yr To 1.5 yr 5/5 31/31 33/33 34/34 24/24 18 wk 0 Life To 1.5 yr 63/63 To 1.5 yr 441* 24/24 50/42 Life 0 To 24 mo. 649* 702* To 24 mo. 50/45 50/49 10/7 0 To 24 mo. 10/8 To 24 mo. To 30 mo. 40/36 0 To 30 mo. 40/32 0 2 Breast 0 1 Node 5 3 Breast Thigh Node 0 0 12 Lung 4 Kidney 3 Node 4 Liver 3 Liver 2 Liver 6 Brain Pituitary Node 2 Kidney Peritoneum 1 Peritoneum 11 2 Thyroid Thyroid Liver Chemodectoma jugular body Colon Ileum Adrenal 2 Node Bone 11 Thyroid Liver 2 Adrenal Kidney Node 5 Fat 6113 21575 REVIEW OF ASBESTOS STUDIES Table 1. Summary of aabestoa ingestion studies (continued). 5 Study Species Wagner {10) Rat Test material Chrysotils Talc Control Dose 100 mg;day 100 mg/day 0 Exposure time 101 days/ 5 mo. 101 days/ 5 mo. 0 Study duration 819* 614* 641* Number of animals. Initial/ Examined 32/32 32/32 16/16 Malignant tumors Number 3 Location Node Stomach Uterus Type* L S S 3 2 Uterus Stomach s s 0 Smith ill) Hamster Amoaite Amosite 0.5 mg/L ad libitum 5 mg/L ad libitum Donham {12) Rat Amosite Taconite tailings Taconite tailings Taconite tailings Control Chrysotile Cellulose fiber Control 50 mg/L ad libitum 0.5 mg/L ad libitum 5 mg/L ad libitum 50 mg/L ad libitum 0 10% in diet ad libitum 10% in diet ad libitum 0 To 23 mo. To 23 mo. 60/60 To 23 mo. To 23 mo. 60/60 To 23 mo. To 23 mo. 60/60 To 23 mo. To 23 mo. 60/60 To 23 mo. To 23 mo. 60/60 To 23 mo. To 23 mo. 60/60 0 To 23 mo. 120/120 To 32 mo. To 32 mo. 240/189 To 32 mo. To 32 mo. 242/197 -- 0 To 32 mo. 121/115 1 Lung c 3 2 Stomach c Peritoneal mesothelioma 0 1 Uterus s 0 0 1 Node L 4 3 Colon C Abdominal mesothelioma 2 Colon C 3 Colon C Ward U3) Rat Azoxymethane* 7.4 mg/lcg wit 10 wk Azoxymethane 7.4 mg/kg wk plus amosite 10 mg 3/wk Azoxymethane 7.4 mg/kg wk plus chrysotile 10 mg 3/wk 10 wk 10 wk 34 wk 34 wk 34 wk 21/21 21/18 21/21 12 5 Ileum 7 Colon 10 3 Ileum 7 Colon 10 4 Ileum 6 Colon C C c c c c 6 Study_______ Specie* Ward (13) Rat Hiding (14) Rat L. W. CONDIE Table 1. Summary of aabeatoa ingestion studies (continued). Test material Amosite Chrysotile Saline Untreated Azoxymethane Azoxymethane plus amosite Saline plus amosite Exposure Dose time 10 mg 3/wk 10 wk 10 mg 3/wk 10 wk 1.0 mL 3/wk (gavage) 10 wk --0 7.4 mg/kg wk 10 wk 7.4 mg/kg wk 10 mg 3/wk 1/wk (SC) 10 mg 3/wk 10 wk 10 wk Study duration 34 wk 34 wk 34 wk Number of Initial/ Examined 21/21 21/21 21/21 34 wk To 95 wk 21/21 50/48 To 95 wk 50/48 To 95 wk 50/49 Malignant tumor* Number Location Type* 0 0 0 0 39 12 Ileum 27 Colon 44 15 Ileum 29 Colon 17 Ileum 16 Colon Filtered Duluth 1 mfl* 690* 690* 28/27 tapwater ad libitum Unfiltered Duluth 100 mfl 960* 960* 30/28 tapwater ad libitum Lake Superior 5,000 mfl 840* 840* 22/22 water ad libitum sediment Taconite tailings 100,000 mfl 870* 870* 30/30 ad libitum Chrysotile/ 20 mg/day 870* 870* 30/30 amosite Amosite 300 mg/day 750* 750* 20/20 Diatomaceous 20 mg/day 840* 840* 30/30 earth 3 Lung Ovary Forestomach 4 Salivary gland Skin Uterus Mediastinum 3 Lung Skin Uterus 3 Neck Chest wall Mediastinum 6 Breast 2 fibrous histio cytoma Skin Mediastinum Pleural meso thelioma 1 Leukemia 5 Salivary gland 2 Uterus Skin Peritoneal mesothelioma o too o o u u u n j o o to w m - i o u j r REVIEW OF ASBESTOS STUDIES Table 1. Summary of aabeatoa ingestion studies (continued). 7 Study Specie* Teat material Doa* Ezpoaur* time Study duration Number of animal*. Initial/ Examined Malignant tumor* Number Location Type* Bolton US) Ret Amoait* Crocidolite Chryaotile 250 mg/wk 250 mg/wk 250 mg/wk 25 mo. 25 mo. 25 mo. Life Life Ufa 24/24 22/22 22m Margarine control 0 0 Life 24/24 Control 0 0 Ufa 23/23 Type C " carcinoma; S 3 sarcoma; L 3 lymphoma. bMean survival time in days. 'Azoxymethane given subcutaneously, saline administered by oral gavage or subcutaneously. iw& 3 million amphibole fibers/L l Stomach S 1 Adrenal C 5 Fat s Pleural hiatio- cytoma. 2 Adrenal c Plaama cell tumor 4 2 Adrenal c Bladder c Peritoneum s 2 Fat Lymphoma s The results of a series of feeding experiments with different sources of chrysotile and crocidolite were reported by Gross et al. (7). This paper incorporated data from unpublished results of various studies conducted by three laboratories. Animals fed asbestos by gavage in butter or mar garine for up to 21 months failed to provide evi dence of a carcinogenic effect. The experiments were flawed for the following reasons: the number of rats in the experimental groups was small, the doses of asbestos were limited, significant infor mation on experimental protocol was missing, and systematic histologic examination was not performed on a significant number of rats. A study by Gibel et al. (8) was undertaken to feed asbestos filter material to rats because of the concern of the possible adverse health effects of erosion of asbestos from the filters used to purify commercial beverages. The filter material was composed of sulfated cellulose, a condensation resin and chrysotile asbestos (53%). The authors did not provide any information regarding the size and shape of the asbestos fibers that were incorporated into the filter material. Although 12 malignant tumors were noted in the asbestosexposed group of rats and the mean survival time was decreased in the asbestos-treated group, the authors stated that no conclusions could be made from their test results regarding the pathogenesis of the tumors caused by the oral intake of asbes tos material. The relationship of this study to asbestos carcinogenicity was also confounded by the presence of several substances in the filter material, which were not clearly identified. Cunningham and co-workers (9) conducted two limited feeding studies with male Wistar rats. Chrysotile asbestos (1% with 5% corn oil) was added to rat chow and fed to the animals for 24 months or 30 months. In the first study, 10 rats were exposed to asbestos. Six of the seven rats autopsied were found to have tumors, while only one malignancy was observed in the control ani mals. In the larger study of 80 animals, equal numbers of malignant tumors were noted in the exposed and the control groups. The authors stated that trace amounts of asbestos can pene trate the walls of the gastrointestinal tract, but evidence that asbestos causes cancer by the oral route of administration was inconclusive. Wagner et al. (10) fed 32 Wistar rats 100 mg/ day of chrysotile or talc in malted milk for 101 days over a 5-month period. A slight decrease in survival time was observed in the two experimen tal groups. One gastric leiomyosarcoma was de tected in each exposure group. Interpretation of the results of this experiment is difficult because of the small number of animals included in the study. A study in Smith's laboratory (21), which was the first study to utilize a large number of ani- ! ** $ 8 L. W. CONDIE mals, was designed to more closely simulate an environmental exposure to asbestos. Hamsters were exposed to either amosite fibers or taconite tailinp at three different dose levels in drinking water. The control animals received Lake Supe rior water that had been filtered by either a 0.45j.m or 0.1-p.m filter. A small number ofmalignant tumors was detected in the exposed groups, but these tumors were not specifically attributed to asbestos because no cancers were detected in the high dose groups. The particle size ofthe different fiber types was well characterized by the investi gators. Another large lifetime animal study by Donham et al. (12) was initiated to induce and to characterize colon lesions in F344 rats by feeding them high levels of asbestos (10% of feed). Be cause of the high level of asbestos in the feed, a nonnutritive cellulose fiber control group was in cluded. In this study only the colon and rectum were examined microscopically. Although differ ences in the number of colon tumors were not statistically significant between the asbestos-fed animals and control groups, the researchers pre sented the following observations, which suggest that ingested asbestos is not inert in the colon: evidence of increased probability of asbestos-fed rats to develop colon lesions generally, evidence for unique mesothelioma in rats fed asbestos, evidence for a colonic cell regulator defect, and evidence for asbestos fiber penetration of colon mucosa. Two experiments were designed by Ward et al. (13) to determine the promoter potential of oral exposure to asbestos. Could asbestos modify the response to azoxymethane, a known intestinal carcinogen? Rats were exposed to azoxymethane and/or asbestos for 10 weeks and were sacrificed 34 weeks later or observed throughout their life span. In the first experiment, intestinal carcino mas were detected only in the groups of animals receiving azoxymethane alone or in combination with asbestos. In the second experiment of longer duration, the incidence of intestinal tumors was only slightly greater in the amosite plus azoxy methane group as compared with the azoxyme thane group. Furthermore, the authors concluded that amosite alone caused a relatively high rate of intestinal neoplasia. However, there were r? control animals included in the second experi ment, which compromises the findings. The re searchers reported a 14% incidence of Zymbal gland tumors in the rats exposed only to amosite. The historical rate of Zymbal gland tumors in the National Cancer Institute Bioassay Program is 0.34%, indicating that it is a relatively rare tu mor. Since a single dose of azoxymethane has been shown to induce both Zymbal gland tumors and intestinal carcinomas (14), an inadvertent exposure to azoxymethane might have caused the high incidence of intestinal neoplasma and Zym bal gland tumors in the amosite-exposed animals. One has reason to doubt the authors' conclusion that oral asbestos exposure in F344 rats may have increased the incidence of intestinal tumors occurring naturally. Since amphibole fibers had been detected in Lake Superior and in the Duluth municipal water supply, a study was conducted by Hilding et al. (15) to investigate the potential carcinogenic ef fect of unfiltered Duluth tapwater, municipal wa ter reservoir sediments, taconite plant tailings, amosite, and diatomaceous earth. Under the ex perimental conditions of this study, no significant increases were detected in the incidence of malig nant tumors in any experimental group when compared to controls. The final study (16) considered in this review examined the effects of prolonged asbestos expo sure to rats. Animals were fed over 250 mg/week of amosite, crocidolite and chrysotile in marga rine for periods up to 25 months. No excess of malignant tumors were found in any experimen tal group, and no gastrointestinal mucosal abnor malities were detected. Bolton and co-workers concluded that there were no significant adverse health effects from prolonged asbestos ingestion in healthy laboratory rats. Conclusions Certain conclusions can be summarized from the various ingestion studies. The bulk of the experimental evidence indicates that the long term, high-level ingestion exposure to various types of asbestos fibers failed to produce any defi nite, reproducible, organ-specific carcinogenic ef fect. Although comparisons between studies are confounded by different rat strains utilized, by different dose levels or exposure conditions, and by different types of asbestos employed, the vast majority of the asbestos ingestion studies were either negative or equivocal. There was ap parently a carcinogenic response to amosite in one tudy (13), but the authors did not rule out that an inadvertent exposure to azoxymethane, an intestinal carcinogen, had occurred. Many of the studies suffered from an insufficient number of experimental animals and from an inadequate exposure time to asbestos. Another major draw back of many of the .studies was that they were not lifetime studies. C\\ 6113 REVIEW OF ASBESTOS STUDIES 9 One can question the suitability of the animal models employed in evaluating the human re sponse to oral exposure to asbestos, since suffi cient time may be lacking between exposure and the development of malignancies during the ani mal's lifetime. However, exposure to asbestos by other routes has induced cancer in rats. For ex ample, Wagner et al. (17) reported the develop ment of lung cancer and mesothelioma from brief to lengthy inhalation exposure to various types of asbestos, while Gross (18) reported asbestotic lung cancers in 25 of 72 rats that survived 16 months of exposure to chrysotile dust. Based on the carcinogenic effects of asbestos from nonoral exposure routes (2), one would expect to be able to produce a neoplastic response within the life time of conventional laboratory animals with massive doses of ingested asbestos such as those employed in some of the studies mentioned in this paper. These studies also cast some doubt on the hypothesis that peritoneal mesotheliomas and gastrointestinal cancers result from the ingestion ofasbestos fibers cleared from the lungs following inhalation exposure. The research described in this paper has been peer and administratively reviewed by the U.S. Environmental Protec tion Agency and approved for presentation and publication. Mention of trade names or commercial products does not constitute endorsement of recommendation for use. REFERENCES 1. Bogovski, P., Timbrell, V., Gilson, J. C., and Wagner, J. C. (Eds.). Biological Effects of Asbestos. IARC Scientific Publications, Lyon, 1973. 2. Selikoff, I. J., and Lee, D. H. K. Asbestos and Disease. Academic Press, New York, 1978. 3. Peters, G. A., and Peters, B. J. Sourcebook on Asbestos Diseases. Garland STPM Press, New York, 1980. 4. Cook, P. M., Glass, G. E., and TUcker, J. H. Asbestiform amphibole minerals: detection and measurement of high concentrations in municipal water supplies. Science 185: 853-855 (1974). 5. Bonser, G. M., and Clayson. D. B. Feeding of blue asbestos to rats. Brit. Erap. Cancer Campaign, Res. Ann. Rept. 45: 242(1967). 6. Webster, I. The ingestion of asbestos fibers. Environ. Health Perspect. 9: 199-202 (1974). 7. Gross, P., Harley, R. A., Swinburne, L. M., Davis, J. M. G., and Greene, W. B. Ingested mineral fibers. Arch. Environ. Health 29: 341-347 (1974). 8. Gibel, W,, Lohs, K,, Horn. K. H., Witdner, G. P., and Hoffman, F. Tierexperimentelle Untersuchungen uber eine Kanserogene Wirkung von Asbesfiltermaterial nach oraler Aufhahme (Experimental study on carcinogenic activity of asbestos filters following oral ingestion). Arch. Geschwulstforsch. 46: (6): 437-442 (1976). 9. Cunningham, H. M., Moodie, C. A., Lawrence. G. A., and Pontefract, R. D. Chronic effects of ingested asbestos in rata. Arch. Environ. Contam. Tbxicol. 6: 507-513(1977). 10. Wagner, J. C., Berry, C., Cook, T. J., Hill, R. J., Pooley, F. D., and Skidmore, J. W. Animal experiments with talc. In: Inhaled Particles IV (W. H. Walton, Ed.), Pergamon Press, New York, 1977, pp. 647-654. 11. Smith, W. E., Hubert, D. D., Sobel, H. J., Peters, E. T., and Doerfier, T. E. Health of experimental animals drinking water with and without amosite asbestos and other min eral particles. J. Environ. Pathol. Tbxicol. 3: 277-300 (1980). 12. Donham, K. J., Berg, J. W., Will, L. A., and Leininger, J. R. The effects of long-term ingestion of asbestos on the colon of F344 rats. Cancer 45:1073-1084 (1980). 13. Ward, J. M., Frank, A. L., Wenk, M,, Devor, D., and Throne, R. E. Ingested asbestos and intestinal carcinogen esis in F344 rata. J. Environ. Pathol. Tbxicol. 3: 301-312 (1980). 14. Ward, J. M. Dose response to a single injection of azoxymethane in rats. Vet. Pathol. 12:165-177 (1975). 15. Hilding, A. C., Hilding, D. A., and Larson, D. M. Biologi cal effects of ingested amosite asbestos taconite tailings, diatomaceous earth, and Lake Superior water in rats. Arch. Environ. Health 36: 298-303 (1981). 16. Bolton, R. E., Davis, J. M. G., and Lamb, D. The pathologi cal effects of prolonged asbestos ingestion in rats. Envi ron. Res. 29:134-150 (1982). 17. Wagner, J. C., Berry, G., Skidmore, J. W., and Timbrell, V. The effects of the inhalation of asbestos in rats. Brit. J. Cancer 29: 252-269 (1974). 18. Gross, P., deTVeville, R. T. P., Tblker, E. B., Kaschak. M., and Babyak, M. A. Experimental asbestosis: the develop ment of lung cancer in rats with pulmonary deposits of chrysotile asbestos dust. Arch. Environ. Health 15: 343355(1967).