Mutagenic-carcinogenic relationships and the role of mutagenic screening tests for carcinogenicity

Mutagens in feces from vegetarians and non-vegetarians

Mutagens in water extracts from feces of persons in 3 different diet groups were measured with the fluctuation test for weak mutagens using Salmonella typhimurium TA100 and TA98 as tester strains. The 3 diet groups were ovo-lacto vegetarians (N = 6), strict vegetarians (N = 11) and non-vegetarians (N = 12). All subjects were from the urban area of Vancouver, British Columbia, Canada. On TA100 ovo-lacto vegetarians and strict vegetarians had significantly lower levels of fecal mutagens than non-vegetarians (P less than or equal to 0.025 and P less than 0.010, resp.). The same pattern, although less significant, was obtained with TA98. Correlation studies between mutagenicity on TA100 and TA98 and between the pH of the fecal homogenate and mutagenicity indicate the presence of 2 or more major fecal mutagens.

Point mutations at the thymidine kinase locus in L5178Y mouse lymphoma cells. II. Test validation and interpretation

The L5178Y Mouse Lymphoma TK assay was studied extensively to determine if this mammalian cell assay for gene mutations at the thymidine kinase (TK) locus could provide valid, interpretable determinations of mutagenic potential, and whether this information is of value in the safety evaluation of chemicals. We first determined that test-derived TFTR mutants were phenotypically stable, possessing little or no thymidine kinase activity as measured by labeled thymidine uptake, but demonstrating 100% cross resistance to bromodeoxyuridine. Common solvent vehicles such as acetone, dimethylsulfoxide and ethanol were shown to produce little cytotoxicity and no mutagenic activity when present at 1% levels. Out of a total of 10 noncarcinogens tested, all were negative when results were analyzed by a 2-sample loge t test on control and treated mutant count means. Of the 13 putative animal carcinogens tested, 10 were positive, 2 were negative (auramine O and sodium phenobarbital), and 1 showed sporadic activity (hydrazine sulfate) in the TK assay on the basis of test-derived t statistics. 2 compounds, 1,2-epoxybutane and ICR 191, which have been described as Ames positive non-carcinogens, were also positive in the TK assay. Although this sampling of a total of 29 compounds is insufficient for precise estimations of expected false-positive or false-negative frequencies, these data indicate the TK assay can be expected to detect a majority of carcinogens as mutagens including some missed by more established point-mutation assays.

Mutagens in the feces of 3 South-African populations at different levels of risk for colon cancer

The incidence of mutagens in the feces of 3 South-African populations at different risk levels for colon cancer has been determined. Lyophilized fecal samples were extracted with ether and the mutagenicity of the extracts determined using the Salmonella/mammalian microsome mutagenicity test. 19% of the samples from urban white South-Africans, a population at a high risk for colon cancer, were mutagenic using Salmonella typhimurium strain TA100. This incidence was significantly greater (p less than 0.001) than the incidence of mutagen excretion in the low-risk populations of urban blacks (2%) and rural blacks (0%). This pattern was also obtained using Salmonella typhimurium strain TA98. The incidence of mutagen excretion for urban whites was 10%, as compared to 5% and 2% for urban and rural blacks, respectively.

A possible short-term prediction of potential carcinogenicity of chemical compounds in vivo by means of a promoting activity test (PAT)

Carcinogens of chemically unrelated structure and different organotropism cause a mitotic stimulation of adrenocortical epithelia in vivo. A similar response is observed in liver regenerating rats following partial hepatectomy or intoxication by carbon tetrachloride. It is very probable from these and other results that the short-term effect (48 hours p. applic.) is brought about by the emergence of humoral growth stimulators which are evidenced in restorative regeneration and, therefore, accounts for the promoting activity of carcinogens. Special problems resulting from false positive (indomethacin, sodium nitrite) and false negative (7,12-dimethylbenz(a)anthracene, ethionine) findings as well as such of the role of solvents are discussed. The proof of the "contradictory" test result by long-term treatment in the case of trinitroso-trimethylene-triamine, nitroso-ethyltert-butylamine or nitroso-diallylamine has shown the accuracy of this short-term test.