Developmental Effects of the ToxCast™ Phase I and Phase II Chemicals in Caenorhabditis elegans and Corresponding Responses in Zebrafish, Rats, and Rabbits

Abstract

Background:Modern toxicology is shifting from an observational to a mechanistic science. As part of this shift, high-throughput toxicity assays are being developed using alternative, nonmammalian species to prioritize chemicals and develop prediction models of human toxicity.Methods:The nematode Caenorhabditis elegans (C. elegans) was used to screen the U.S. Environmental Protection Agency’s (EPA’s) ToxCast™ Phase I and Phase II libraries, which contain 292 and 676 chemicals, respectively, for chemicals leading to decreased larval development and growth. Chemical toxicity was evaluated using three parameters: a biologically defined effect size threshold, half-maximal activity concentration (AC50), and lowest effective concentration (LEC).Results:Across both the Phase I and Phase II libraries, 62% of the chemicals were classified as active ≤ 200 μM in the C. elegans assay. Chemical activities and potencies in C. elegans were compared with those from two zebrafish embryonic development toxicity studies and developmental toxicity data for rats and rabbits. Concordance of chemical activity was higher between C. elegans and one zebrafish assay across Phase I chemicals (79%) than with a second zebrafish assay (59%). Using C. elegans or zebrafish to predict rat or rabbit developmental toxicity resulted in balanced accuracies (the average value of the sensitivity and specificity for an assay) ranging from 45% to 53%, slightly lower than the concordance between rat and rabbit (58%).Conclusions:Here, we present an assay that quantitatively and reliably describes the effects of chemical toxicants on C. elegans growth and development. We found significant overlap in the activity of chemicals in the ToxCast™ libraries between C. elegans and zebrafish developmental screens. Incorporating C. elegans toxicological assays as part of a battery of in vitro and in vivo assays provides additional information for the development of models to predict a chemical’s potential toxicity to humans.Citation:Boyd WA, Smith MV, Co CA, Pirone JR, Rice JR, Shockley KR, Freedman JH. 2016. Developmental effects of the ToxCast™ Phase I and II chemicals in Caenorhabditis elegans and corresponding responses in zebrafish, rats, and rabbits. Environ Health Perspect 124:586–593; http://dx.doi.org/10.1289/ehp.1409645

Use of alternative assays to identify and prioritize organophosphorus flame retardants for potential developmental and neurotoxicity

“…As 1% DMSO has previously been shown not to disrupt C. elegans development (Boyd et al 2015), the maximum concentration tested for any chemical was 1 mM. Three biological replicates were conducted for each assay.…”

Zebrafish in Toxicology and Environmental Health

“…That same year, the National Research Council published a report titled “Toxicity testing in the 21st Century: A Vision and Strategy” (National Research Council, 2007), which prompted rapid expansion of ToxCast, and in Phase II of the ToxCast program, the chemical library was expanded to 1878 compounds for which testing concluded in 2013 (Richard et al, 2016). ToxCast Phase I and II library compounds have been tested in model organisms including C. elegans and zebrafish (Boyd et al, 2016; Padilla et al, 2012; Sipes, Padilla, & Knudsen, 2011). Phase III of the ToxCast program contains greater than 3800 unique chemicals and compounds under evaluation (Richard et al, 2016).…”

Applying evolutionary genetics to developmental toxicology and risk assessment

“…Although humans have a greater biological complexity than most model organisms (Figure 3), the gene regulatory networks that connect stress response and developmental pathways in humans may be conserved in model organisms, such as C. elegans, Drosophila , and zebrafish. If so, these small model organisms can provide a useful HTS platform to assess potential hazard of developmental toxicity and teratogenesis in humans (Sipes et al 2011b; Rand et al 2014; Beekhuijzen et al 2015; Boyd et al 2016). …”

“…This in vitro bioactivity information covers chemical-biological interactions across a broad suite of experimental platforms and biological scales, from molecular lesions, subcellular events, and cellular disruption, to tissue dysfunction, and is generated from an array of in vitro experimental systems, including human cell lines, stem cells, small model organisms and, most recently, engineered microscale and microphysiological systems (Collins et al 2008; Tice et al 2013; Settivari et al 2015). Guideline testing for reproductive toxicity and prenatal developmental toxicity traditionally has relied on lower throughput approaches in whole-animal studies utilizing vertebrate species [clawed frogs, mice, rats, and rabbits (Carney et al 2008; Mouche et al 2011; Robinson et al 2012; Kalaskar et al 2014)] and higher throughput, more evolutionarily distant alternatives [zebrafish and invertebrate species, such as hydra, roundworms, water fleas, and fruit flies (Dang et al 2012; Padilla et al 2012; Glauber et al 2013; Liu et al 2014; Boyd et al 2016)]. The vast collections of in vitro data now available from 21st-century toxicology approaches, coupled with the diverse and distributed nature of these data and an ever-increasing knowledgebase for embryonic development across diverse species and biological systems, create a major challenge for data access, curation, integration, and interpretation.…”