Lack of effect of carcinogen treatment on development of tumors arising spontaneously in Fischer 344 rats.

The incidence of a set of neoplasms arising "spontaneously" in Fischer 344 (F344) rats was determined in control and carcinogen-treated animals. Data were obtained from approximately 9000 rats (4000 males and 5000 females) used to study the carcinogenicity of a variety of alkylating compounds, including N-nitroso compounds, azoxyalkanes, and triazenes. In these experiments treated rats and controls were allowed to die naturally and were necropsied, and the tissues were examined histopathologically. The spontaneous neoplasms of interest were mononuclear cell leukemia and neoplasms of the anterior pituitary, adrenal medulla, pancreas, thyroid gland, mammary gland, and testis. These tumors were generally absent from control animals that (rarely) died before 70 wk of age. Although many carcinogen-treated rats died early with treatment-related tumors, a substantial number (1700 males and 2300 females) survived as long as controls. The incidence of spontaneous neoplasms was determined among controls and chemically treated rats at 10-wk intervals from 0 to 140 wk. The incidence of spontaneous tumors was not higher and was frequently statistically lower among treated rats than the corresponding incidence in controls, with the exception of leukemia in female rats. The same result was obtained with the subset of carcinogens not requiring metabolic activation (mostly alkylnitrosoureas). These data indicate that in this rat tumor model system, the alkylating carcinogens, while capable collectively of tumor induction at more than 20 sites, did not accelerate the development of any of the six spontaneously arising solid tumors. This suggests that these spontaneous tumors might arise by a mechanism that is unresponsive to the actions of the alkylating carcinogens.