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.