Use of high-throughput in vitro toxicity screening data in cancer hazard evaluations by IARC Monograph Working Groups

Abstract

Evidence regarding carcinogenic mechanisms serves a critical role in International Agency for Research on Cancer (IARC) Monograph evaluations. Three recent IARC Working Groups pioneered inclusion of the US Environmental Protection Agency (EPA) ToxCast program high-throughput screening (HTS) data to supplement other mechanistic evidence. In Monograph V110, HTS profiles were compared between perfluorooctanoic acid (PFOA) and prototypical activators across multiple nuclear receptors. For Monograph V112-113, HTS assays were mapped to 10 key characteristics of carcinogens identified by an IARC expert group, and systematically considered as an additional mechanistic data stream. Both individual assay results and ToxPi-based rankings informed mechanistic evaluations. Activation of multiple nuclear receptors in HTS assays showed that PFOA targets not only peroxisome proliferator activated receptors, but also other receptors. ToxCast assays substantially covered 5 of 10 key characteristics, corroborating literature evidence of "induces oxidative stress" and "alters cell proliferation, cell death or nutrient supply" and filling gaps for "modulates receptor-mediated effects." Thus, ToxCast HTS data were useful both in evaluating specific mechanistic hypotheses and in contributing to the overall evaluation of mechanistic evidence. However, additional HTS assays are needed to provide more comprehensive coverage of the 10 key characteristics of carcinogens that form the basis of current IARC mechanistic evaluations.

Keywords: in vitro; high throughput screening; carcinogenicity; mechanisms.

Figure 1

Framework for IARC Monographs evaluations of carcinogenicity, integrating cancer studies in humans, cancer studies in experimental animals, and mechanistic and other relevant data. The arrows indicate the potential for mechanistic and other relevant data to modify the cancer classification based solely on human and experimental animal evidence. ESLC = Evidence Suggesting Lack of Carcinogenicity. From http://monographs.iarc.fr

Figure 2

Results of Case Study 1: Comparison of ToxCast AC50s (in microM) for nuclear-receptor-related assays between PFOA (including its ammonium salt) and selected “prototypical” compounds DEHP (including its metabolite MEHP), phenobarbital, and rifampicin. Longer bar indicates greater potency (lower AC50). Assays for which all compounds were negative are not displayed. See Supplemental Materials for the assay endpoints corresponding to the abbreviations shown.

Figure 3

ToxPi ranking of several pesticides against other IARC-evaluated chemicals screened in Tox21/ToxCast assays mapped to six key characteristics of carcinogens. ToxPi profiles of the individual compounds are shown as pie charts with each slice representing assays for each key characteristic. (A) Top 8 compounds in both comparisons. (B and D) Pesticides and their metabolites considered in volumes 112 and 113, respectively. (C and E) Global ranking of the chemicals according to their cumulative ToxPi score, i.e. each data point represents the sum of individual key characteristic scores shown in the inset to each chart.

Figure 4

ToxPi rankings using ToxCast assay endpoints mapped to the key characteristic of modulates receptor-mediated effects. ToxPi profiles of the individual compounds are shown as pie charts with each slice representing assays related to different nuclear receptor families. (A) Top 8 compounds in both comparisons. (B and D) Pesticides and their metabolites considered in volumes 112 and 113, respectively. (C and E) Global ranking of the chemicals according to their cumulative ToxPi score, i.e. each data point represents the sum of nuclear receptor family scores shown in the inset to each chart.