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

Abstract

There 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.

Keywords: AOP; neurotoxicity; specificity; test development; validation.

Figure 1. Representation of the key events of neurodevelopment at the cellular level

Several fundamental neurodevelopmental processes are absolutely necessary for nervous system development, and therefore well-conserved across species. Moreover, the processes known from in vivo studies can be relatively faithfully modeled in vitro. It is assumend that DNT exert their toxicity, because they disturb at least one of these processes. Therefore, disturbances of the processes depicted here are KE of AOP relevant for DNT.

Figure 2. Toxicity endophenotypes

For development of relevant model systems, we need approaches for linking the observable DNT effect (= exophenotype; see red box) triggered by a xenobiotic to effects that this compound has in in vitro test systems (yellow circles). Toxicity endophenotypes (orange box) form the conceptual link between what is observed in man or experimental animals and on what test systems model. They are a description of the altered biological state of the nervous system (e.g. neuronal disarray in the frontal cortex) in vivo that causes the externally observable DNT phenotype (e.g. reduced IQ). Thus, ‘toxicity endophenotypes (TEP)’ describe the altered functional or structural connectivity or responsiveness of parts of the nervous system, triggered by xenobiotics. The TEP results from the disturbance of one or several fundamental biological processes (e.g. neurite growth). Notably, there may be a delay or lag of years between disturbance of a process by a chemical and the observation of DNT effects (dashed arrows linking processes and TEP). Both the setup of model systems and the characterization of tool compounds to validate such systems requires that we establish the following connections: (1) exophenotype to TEP (the exophenotype is the only robust and relevant starting point for identification of DNT compounds known at present); (2) association of TEP with disturbed biological process(es) that led to the TEP; (3) link of in vitro test system endpoint to prediction of a disturbed biological process in vivo. The fundamental biological processes as such (but not the TEP) may be modeled by alternative test systems. Thus, the test systems are inspired by the biological processes (green arrows), but the outcome of test systems predicts to some extent certain TEP (e.g. inhibited neuronal migration predicts neuronal disarray and/or a deficit in neuronal number in some brain region). In this sense, TEP represent the level of organisation that links in vitro test systems for fundamental biological processes to apical DNT endpoints (exophenotypes).