Case study on the use of Integrated approaches to testing and assessment for READ-ACROSS BASED FILLING OF DEVELOPMENTAL TOXICITY DATA GAP FOR METHYL HEXANOIC ACID This case study was developed by EU ToxRisk project (BIAC) for illustrating practical use of IATA and submitted to the 2019 review cycle of the IATA Case Studies Project. This case study was reviewed by the project team. The document was endorsed at the 4th meeting of the Working Party on Hazard Assessment in June 2020. 2-Methylhexanoic acid (MHA) is a compound for which developmental and reproductive toxicity test (DART) data is taken to be lacking. We have searched for structural analogues that have this data in order to explore the possibility to read across information of these source chemicals to MHA. The following structural related aliphatic carboxylic acids were selected that have in vivo developmental and/or reproductive toxicity data: 2-ethylhexanoic acid (EHA), 2-propylpentanoic acid (VPA), 2-propylheptanoic acid (PHA), 2-ethylbutanoic acid (EBA), 4-pentenoic acid (PA), 2-propyl-4-pentenoic acid (4-ene-VPA), and 2-dimethylpentanoic acid (DMPA). Some of these analogues proved to be clear developmental toxicants, i.e. VPA, PHA, EHA, and 4-ene-VPA, while others were identified as not being toxic to development, i.e. EBA, PA, and DMPA; i.e. they did or did not induce neural tube defects upon in vivo exposure. Thus, structural similarity alone doesn’t allow a conclusion on the developmental toxicity of MHA. Therefore, we have also tested MHA and all the selected source chemicals in a battery of in vitro tests with clear relevance to developmental toxicity, i.e. the Zebrafish Embryo Test (ZET), mouse Embryonic Stem cell Test (mEST), iPSC-based neurodevelopmental model (UKN1), and a series of CALUX Reporter assays, that we combined with toxicokinetic models to calculate effective cellular concentrations and associated in vivo exposure doses. With these new approach methodologies (NAM) we wanted to explore whether they could correctly predict the in vivo developmental toxic properties of these aliphatic carboxylic acids, and thus could be used to predict the in vivo developmental toxicity of MHA itself. This data would also allow to further explore the relationship between structure and developmental toxicity within this series of aliphatic carboxylic acids. For that reason, we have also tested 2-methylpentanoic acid (MPA) in these NAM, despite the absence of in vivo data. We have also investigated the potential to inhibit histone deacetylase in ZET, mEST, and UKN1 models, as this enzyme is postulated to be the molecular initiating target leading to neural tube defects observed with these analogues.The NAM results show that VPA, PHA, EHA, and 4-ene-VPA were correctly predicted as in vivo developmental toxicants, and EBA, and DMPA as non-developmental toxicants. The NAM results suggest that MHA may not be fully negative for developmental toxicity.