The NTP 2-year bioassay: Controversies in counting rodent tumors to predict human cancer

In late 2016, an examination of the 60 studies constituting the 2-year National Toxicology Program (NTP) inhalation study database conducted for the purpose of evaluating the clinical relevance to humans of chemical induction of bronchioloalveolar carcinomas in mice was initiated. From an anecdote of unknown origin that tumor concordance between rats and mice was reported to be 70%, therefore, the very high level of discordance in tumor formation and tumor site between rats and mice that the inhalation data actually showed was a surprising but interesting outcome. Further examination of the NTP inhalation database led to an initial publication in 2017. The inability to extrapolate the results from the inhalation route to the other routes of exposure (feed, gavage, drinking water, dermal, intraperitoneal injection) in the NTP database motivated Smith and Perfetti to analyze the studies on these other routes of exposure in terms of rat–mouse tumor incidence and site concordance and correlation of rodent tumorigenicity with Ames test results and, similarly, with other tests of genotoxicity as reported by the NTP. Over the last several years, Smith and his colleagues have closely followed the deliberations of the NTP expert panel regarding the potential genotoxicity and tumorigenicity of 1-bromopropane. From this experience, the emphasis placed by the NTP on historical data rather than on more recently conducted, state-of-the-art, GLP genotoxicity assays was striking. In the analysis of the feed, gavage, drinking water, dermal, and intraperitoneal injection 2-year NTP studies, Smith and Perfetti found a poor correlation between positive Ames test results and the development of rodent tumors and, concomitantly, a similarly poor correlation between negative Ames test results and the absence of rodent tumors. The inability to determine the quality of the Ames tests and other tests of genetic toxicity confounded the evaluation of whether the Ames test data were actually displaying poor predictive power or whether the Ames test data contained an inherently high error rate that interfered with the statistical analysis. Given the limitations of the data in the NTP database, Smith and Perfetti shifted the focus of the analysis to the molecular determinants of rodent tumorigenicity. Smith and Perfetti had a long-standing collaborative relationship with Dr Corwin Hansch, the originator of the QSAR concept and methodology, and following his passing, with his former postdoctoral fellow Dr Rajni Garg and her former postdoctoral fellow Dr Gene Ko. In collaboration with Drs Garg and Ko, Smith and Perfetti correlated the following for all the chemicals in the NTP database: Ames mutagenicity, structural alerts of carcinogenicity, Hansch QSAR parameters (ClogP, CMR, MgVol), tumor site concordance/multiplicity, and tumorigenicity rank. One of the conclusions was that since the clinical relevance of rodent tumors to human tumors cannot be determined from this data set, development of a comparative scale wherein the rodent tumorigenicity of a chemical can be compared under similar conditions to the rodent tumorigenicity of a similar chemical is the best that can be accomplished at this time, albeit of limited usefulness in a regulatory setting. Following completion of the collaboration with Drs Garg and Ko, a meeting was held with the USEPA Risk Assessment Division. At this meeting, EPA personnel expressed their confidence in EPA’s predictive program termed Oncologic. Following the meeting with EPA, Smith