Reproductive Toxicology 55 (2015) 1–2

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Reproductive Toxicology journal homepage: www.elsevier.com/locate/reprotox

Editorial

Innovative testing in reproductive toxicology—The ChemScreen experience

The classical approach to chemical safety testing, based on protocolled animal studies, is still the benchmark for regulatory decision-making. Developments in mechanistic toxicology and the societal wish to reduce animal testing have provided strong incentives for innovation in this area. In silico tools for structure–activity relationships and dose–response modelling have been developed for non-testing predictions of toxicity. A wealth of in vitro alternative test systems have been developed covering distinct aspects of the biological and toxicological space and have been promoted as possible replacements for animal studies. The implementation of these in silico and in vitro models in regulatory toxicology lags behind. The causes for the slow implementation of alternative methods in regulatory toxicology are multiple and complex [1,2]. This situation is partly due to the relative simplicity of alternative assays as opposed to the complexity of the biological system. This necessitates the smart combination of complementary alternative assays in a testing strategy, in order to sufficiently cover all key events in toxic action. Moreover, classically in vitro assays have usually been validated 1:1 against in vivo data, using a limited set of chemicals. Statistically derived predictabilities calculated in such studies have been limited by uncertainties about the biological applicability domains of the assays and about the set of test compounds employed [3]. The advent of concepts such as integrated testing strategies, toxicity pathways and adverse outcome pathways [4–6] has broadened the view from the consideration of individual test systems to the need for combinations in tiered and battery approaches. This is especially relevant for multi-mechanism biological processes such as reproduction and (prenatal) development, which cannot be mimicked comprehensively in individual reductionistic alternative toxicity assays. Within adverse outcome pathways, the various critical events might be covered by a series of complementary assays, structured within a matrix of assays to be used in a stepwise iterative process to efficiently map toxicological properties of compounds at the mechanistic level. Given the well-known intimate connections among physiological pathways, likewise adverse outcome pathways can be expected to be heavily intertwined. This opens the way for defining a limited number of Adverse Outcome Pathways (AOPs) that would cover the essential mechanisms of action detecting all toxicants. If these AOPs can be tested in a limited number of alternative assays, these would constitute a test battery with which the mechanistic toxicity profile of a chemical could be efficiently mapped. Subsequent translation towards hazard and http://dx.doi.org/10.1016/j.reprotox.2014.10.025 0890-6238/© 2014 Elsevier Inc. All rights reserved.

risk should then be done considering this profile in the context of the integrated testing strategy, including, for example, exposure and kinetic modelling and application pattern of the chemical. The exact nature of apical adverse health effects caused by a specific exposure may vary extensively between species. At the basis of the mechanistic approach lies the assumption that the underlying physiological changes and mechanisms of action may show significant similarities between species, which may be employed to enhance the prediction of human hazard and risk. The successful application of the mechanistic approach using alternative methods in regulatory toxicology is dependent on an impressive host of factors. First, based on mechanistic knowledge (that could be defined as the zero requirement), assays pertinent to all critical nodes in the adverse outcome pathways essential for hazard identification should be selected such that there is sufficient confidence that all necessary aspects are covered by the group of assays. Second, the individual assays within the strategy should be standardized, reproducible and transferable. Third, their biological applicability domain requires detailed definition to allow meaningful interpretation of individual test results, and for optimized placing of individual tests within a test battery. Fourth, the battery should be placed in the context of existing and additional information pertinent to hazard identification. This may include in silico knowledge, such as structure-activity relationships (grouping, category approaches) and physicochemical properties, but also kinetic information on internal exposure characteristics, as well as any knowledge on toxicodynamic properties outside the area of reproductive and developmental toxicity. Finally, (concentration–response) findings in a test battery should be interpreted throughout in terms of adversity and translated to health hazard. The above listing, even though probably not comprehensive, indicates that there is an abundance of fundamental challenges ahead, which explains why the principally sound concept of mechanistic hazard identification is not yet ready to be applied in regulatory toxicology. Nevertheless, societal and political pressure continues to urge towards innovation in this area. The ChemScreen project, sponsored by the EU Framework 7 research programme, has taken a pragmatic approach to the issue [7]. Knowing that current knowledge and methods are generally not yet sufficient to warrant regulatory changes, the project aimed at a pragmatic approach, improving selected existing alternative methods, combining them in a battery, testing that battery in a proof of principle

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Editorial / Reproductive Toxicology 55 (2015) 1–2

approach, and placing it in a context of in silico information. Other studies that took a similar approach include ReProTect in Europe and, on a different scale, ToxCast in the USA [8,9]. These projects have proven useful learning experiences in various ways. They have illustrated the importance of a series of crucial factors to be addressed, such as those listed above. They also make us realize that we still need considerable effort to finalize a reliable alternative system of hazard and risk assessment. But perhaps most importantly in general terms, they have again drawn attention to this innovative field of knowledge that has the potential to revolutionize regulatory toxicology into an efficient, flexible, mechanistic, and animal-free methodology, that will improve chemical hazard and risk assessment for human health. In 2014, the annual prize for best manuscript in Reproductive Toxicology, awarded by Elsevier and juried by the European Teratology Society, was given for a publication presenting the ChemScreen battery approach, illustrating the appreciation for this line of research in the scientific community [10]. The present special issue of Reproductive Toxicology brings together a series of key original contributions of the ChemScreen output, adding to a growing list of scientific publications from the project [10–14]. Conflict of interest The author has no items to disclose. Transparency document The Transparency document associated with this article can be found in the online version. Acknowledgement This work was carried out with financial support from the Commission of the European Communities, the collaborative project ChemScreen (GA244236). References [1] Adler S, Basketter D, Creton S, Pelkonen O, van Benthem J, Zuang V, et al. Alternative (non-animal) methods for cosmetics testing: current status and future prospects-2010. Arch Toxicol 2011;85(5):367–485. [2] Piersma AH, Ezendam J, Luijten M, Muller JJA, Rorije E, van der Ven LTM, et al. A critical appraisal of the process of regulatory implementation of novel in vivo

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Aldert H. Piersma a,b,∗ Center for Health Protection, RIVM, National Institute for Public Health and the Environment, Bilthoven, The Netherlands b IRAS, Institute for Risk Assessment Sciences, Utrecht University, The Netherlands a

∗ Tel.: +31 030 2742526. E-mail address: [email protected]

Available online 15 November 2014