Absence of effect in stochastic processes: its influence on test validation and test use

Classification of carcinogens: polemics, pedantics, or progress?

Rodent carcinogens may, for physiological or other reasons, induce cancer by a variety of mechanisms which vary in their ability to affect humans. While the current approach of some regulatory agencies to carcinogen risk assessment and regulation may possibly be justified with most genotoxic carcinogens, this is not true with all nongenotoxic carcinogens. Mechanisms attributable to high dose toxicity occasioned by misuse of the maximum tolerated dose concept, imbalancing of homeostasis, unphysiological conditions, and induced cellular proliferation are reviewed. The greatest present need for meaningful regulation of carcinogens is to obtain public acceptance of the fact that some carcinogens are species specific and probably will not exert their effects in humans.

International Commission for Protection Against Environmental Mutagens and Carcinogens. ICPEMC publication No. 19. The classification of carcinogens identified in the rodent bioassay as potential risks to humans: what type of substance should be tested next?

The relevance of rodent cancer bioassay data to humans is discussed in relation to the needs of regulatory agencies. The usefulness of in vivo and in vitro genotoxicity testing in this connection is also discussed. In the case of rodent carcinogens that do not elicit genotoxicity, it is suggested that homeostatic imbalance, cell proliferation, and other processes may play a major role in tumor development and its importance to the possible ability of the test agent to induce human cancer. These possibilities need to be evaluated on a case by case basis. The methods by which chemicals are selected for the rodent cancer bioassay are also discussed and it is pointed out that naturally-occurring constituents of human foods should in future receive greater priority as a consequence of anticipated changes resulting from biotechnology.

Genotoxicity of 1-nitronaphthalene in Chinese hamster V79 cells

1-Nitronaphthalene (1-NN) has been identified in the U.S. National Toxicology Program as a non-carcinogen showing some evidence of in vitro genotoxicity. We tested this compound in Chinese hamster V79 cells at 20-80 micrograms/ml with two endpoints: sister-chromatid exchange (SCE) and thioguanine resistance (TGR), with 5 repeat experiments. The SCE values in the presence of rat or hamster hepatocytes were consistently above the 95% and usually the 99% upper confidence limits for the corresponding control. Without hepatocyte activation, the control upper confidence limits were not exceeded except in one experiment in which the control SCE value was unusually low. TGR was scored both as proportion of plates with mutant colonies and as number of mutant colonies per plate. In 2 of 5 experiments, these values exceeded control 95% or 99% upper confidence limits; on the other hand, these values were substantially lower than those of the positive controls, dimethylbenz[a]anthracene (2.6 micrograms/ml) with activation and ethyl methanesulfonate (155 microgram/ml), which is direct-acting. For TGR, activation of 1-NN by either rat or hamster hepatocytes produced inconsistent results. Overall we would consider this compound to be a weak genotoxin, to which a cancer bioassay would be expected to be relatively insensitive.

International Commission for Protection Against Environmental Mutagens and Carcinogens. ICPEMC publication No. 17. Can a mechanistic rationale be provided for non-genotoxic carcinogens identified in rodent bioassays?

In a recent survey of the results of the National Cancer Institute/National Toxicology Program's Carcinogenesis Bioassay Program, Ashby and Tennant (1988) drew attention to the high proportion of carcinogens that were non-genotoxic insofar as their response to the Salmonella-microsome test was concerned. The present review contrasts these findings with what is known mechanistically about non-genotoxic carcinogens that affect the tissues which are considered to be particularly prone to non-genotoxic tumor induction. Excessive and often thresholded increases in cellular proliferation in the affected tissues appear to be one common feature in tumor induction by these agents, which act either through cytotoxicity followed by regeneration or through hormone-mimetic action. It is suggested that a weight of the evidence approach on a chemical by chemical basis is necessary to decide the relevance of these agents to the human situation.