Reference compounds for alternative test methods to indicate developmental neurotoxicity (DNT) potential of chemicals: example lists and criteria for their selection and use

Abstract

SummaryThere is a paucity of information concerning the developmental neurotoxicity (DNT) hazard posed by industrial and environmental chemicals. New testing approaches will most likely be based on batteries of alternative and complementary (non-animal) tests. As DNT is assumed to result from the modulation of fundamental neurodevelopmental processes (such as neuronal differentiation, precursor cell migration or neuronal network formation) by chemicals, the first generation of alternative DNT tests target these processes. The advantage of such types of assays is that they capture toxicants with multiple targets and modes-of-action. Moreover, the processes modelled by the assays can be linked to toxicity endophenotypes, i.e. alterations in neural connectivity that form the basis for neurofunctional deficits in man. The authors of this review convened in a workshop to define criteria for the selection of positive/negative controls, to prepare recommendations on their use, and to initiate the setup of a directory of reference chemicals. For initial technical optimization of tests, a set of >50 endpoint-specific control compounds was identified. For further test development, an additional “test” set of 33 chemicals considered to act directly as bona fide DNT toxicants is proposed, and each chemical is annotated to the extent it fulfills these criteria. A tabular compilation of the original literature used to select the test set chemicals provides information on statistical procedures, and toxic/non-toxic doses (both for pups and dams). Suggestions are provided on how to use the >100 compounds (including negative controls) compiled here to address specificity, adversity and use of alternative test systems.

Generation of a Triple-Transgenic Zebrafish Line for Assessment of Developmental Neurotoxicity during Neuronal Differentiation

“…SAC, APAP, and DFX did not significantly affect the fluorescent signals, which is consistent with the fact that these chemicals are considered to not be neurotoxicants. However, we were not able to detect the significant change of these fluorescent signals in zebrafish exposed to CPM, RA, MeHg, and DEX that have been reported to be developmental neurotoxicants [42,43]. Hierarchical clustering of the fluorescence parameters for each chemical ( whose average signal of YFP was relatively high, was not tightly clustered with other chemicals.…”

“…The measured NOEC for lethality of each chemical was designated the maximum tolerable concentration (MTC) and was used for DNT assessment. We employed 13 chemicals considered suitable for DNT test method validation [42,43], of which 10 were established developmental neurotoxicants: valproic acid (VPA), trichostatin A (TSA), carbamazepine (CBZ), nicotine (NCT), chlorpyrifos (CPF), cyclopamine (CPM), methyl mercury (MeHg), dexamethasone (DEX), retinoic acid (RA), and bisphenol A (BPA); and 3 were not neurotoxicants: deferoxamine (DFX), saccharin (SAC), and acetaminophen (APAP). We selected the 10 developmental neurotoxicants because they have been reported to affect neuronal differentiation [44][45][46][47][48][49][50][51][52][53][54][55][56].…”

Blood‐brain barrier development: Systems modeling and predictive toxicology

“…A manageable first step is to focus on the first part of the overall AOP by identifying MIEs that lead to BBB disruption; however, this is still a new area of research with not enough data on the effects of chemical exposures on BBB development. Although DNT compounds are known (Aschner et al, ; Corada et al, ; Kadereit et al, ; Mundy et al, ), there is little direct evidence in the literature that neurotoxicity follows BBB disruption. In this case, the paucity of evidence does not mean there is no association between DNT and BBB disruption, but rather the studies to determine this have not yet been conducted.…”

“…As pointed out by (Ek, Dziegielewska, Habgood, & Saunders, ), this mischaracterization derives partly because physical barrier characteristics (i.e., molecular influx) are commonly‐used indicators of BBB integrity, whereas physiological properties, such as metabolism and efflux, are potentially overlooked (Ek et al, ). A more complete understanding of BBB development would advance scientific understanding of developmental neurotoxicity (DNT) at a systems biology level, providing information on the neurodevelopmental impact of xenobiotic exposure (Aschner et al, ; Bal‐Price et al, ; Corada et al, ; Grandjean & Landrigan, ; Mundy et al, ; Tsuji & Crofton, ).…”

“…These interactions may take place at any location in the brain, including the BBB, whereby chemical‐induced disruptions of molecular events could lead to DNT. While reference lists of known developmental neurotoxicants have been established (Aschner et al, ; Kadereit, Zimmer, van, Hengstler, & Leist, ; Mundy et al, ), the role of the BBB in mediating DNT effects has not been adequately explored. Two broad routes by which developmental BBB disruption could lead to DNT are: (1) toxicokinetics (what the BBB does to a chemical, which may include transport and/or bioactivation to a more toxic metabolite); and (2) toxicodynamics (what the chemical does to the BBB (e.g., altering transport of key nutrients or promoting neuroinflammation).…”