RESPONSE OF THE KE TEST TO NCI NTP-SCREENED CHEMICALS .2. GENOTOXIC CARCINOGENS AND NONGENOTOXIC NONCARCINOGENS

A physico-chemical carcinogen-screening test was used to measure the rate constants of electron attachment, k(e)s, of 105 chemicals that had been screened in long-term rodent bioassays and short-term in vitro tests by the NCI/NTP. In the k(e) test, a pulse-conductivity technique is used to generate and monitor the decay of excess electrons that serve as nucleophilic surrogates for the target tissue of rodents. Of the 61 chemicals that had been found to be rodent carcinogens as well as Salmonella mutagens, 36 yield k(e)s that are equal to or greater than the diffusion-controlled k(e) of carbon tetrachloride and are considered to be positive k(e) test responses. In contrast, 29 of the remaining 44 chemicals that are putative non-carcinogens and non-mutagens yield k(e)s that are negative ke test responses. These results are combined with the ke responses of 46 non-mutagenic carcinogens and 20 mutagenic non-carcinogens that were reported earlier and are evaluated to determine the degree to which the measure of electron-accepting capacity that k(e) provides complements or overlaps the electrophilicity or DNA reactivity of chemicals that is indicated by positive mutagenicity responses in the Ames Salmonella tester strains or by positive structural alerts, S/As, of the chemicals. The combined k(e) test results indicate that the overall predictivity of the k(e) test is comparable to and complements the Ames Salmonella test and S/As in identifying rodent carcinogens. Moreover, the electrons serve as non-discriminate nucleophilic targets for both genotoxic and non-genotoxic electron-accepting molecules and appear to attach with equal efficiency to carcinogens that are active in various tissues of rodents. This property of excess electrons suggests that the predictivity of the k(e) test could be enhanced by combining the measured k(e) with an appropriate lipophilicity or pharmacokinetic parameter. A pre-chemical electron-transfer step that had been proposed to precede chemical interactions between the carcinogen and target tissue is discussed in light of recent developments in electron-donor/-acceptor chemistry and in the application of structure-activity relationships to identify carcinogens.