Ingested asbestos and intestinal carcinogenesis in F344 rats.

The effect of oral exposure to amosite or chrysotile on azoxymethane-induced intestinal carcinogenesis was investigated in male F344 rats. In two separate experiments, F344 rats, 6 weeks of age, were given ten weekly subcutaneous injections of azoxymethane, and intragastric administration of amosite or chrysotile three times weekly during this period. In experiment 1, rats were sacrificed at 34 weeks. The incidence rates and mean number of intestinal tumors per rat were similar in all groups. However, a slightly higher incidence, not significant, of metastatic intestinal carcinomas was seen in rats exposed to asbestos. In the second experiment, rats were allowed to live out their life span. The incidence of intestinal tumors was similar in the rats receiving amosite with azoxymethane and azoxymethane alone. Thirty-two percent of the rats receiving amosite alone had colon tumors; these tumors are usually rare in F344 rats. It was concluded that the experimental evidence suggested but did not prove that oral asbestos exposure in F344 rats may have increased the incidence of intestinal tumors occurring naturally.