Evaluation of EndoSens: A reliable skin sensitization assessment model

The transition toward non-animal safety assessment has driven the development of in vitro models for skin sensitization, particularly those targeting key event 2 (KE2) within the adverse outcome pathway (AOP). While conventional KE2 assays often rely on randomly integrated reporter systems, the EndoSens model utilizes CRISPR/Cas9-mediated knock-in to precisely insert a luciferase reporter into the endogenous HMOX1 locus in HaCaT keratinocytes, providing a genomically anchored and physiologically relevant system. To enable reproducible adoption across laboratories, we established a standardized protocol with defined quality control (QC) criteria, incorporating cinnamyl alcohol as a quantitative positive control and a cytotoxicity pre-screen to identify non-cytotoxic test concentrations. Evaluation using ten OECD TG 442D reference chemicals and ten additional compounds demonstrated the assay’s robust performance in discriminating skin sensitizers with high inter-laboratory consistency, supporting its integration as a reliable in vitro component within defined approaches (DA) for animal-free skin sensitization assessment. Plain language summaryThe EndoSens assay is an in vitro alternative method for assessing whether chemicals may cause skin sensitization, addressing the Adverse Outcome Pathway Key Event on keratinocyte activation. It uses human HaCaT keratinocytes with a luciferase reporter gene precisely knocked into the endogenous HMOX1 locus via CRISPR/Cas9 and provides a physiologically relevant, genomically anchored model. This study including standardized quality controls and testing of 20 chemicals assessed the method’s reliability and relevance. The results demonstrate that the new method fulfills the requirements for identifying skin sensitizers like the models currently used in OECD TG 442D, supports the 3Rs principle by replacing animal testing, and provides a robust tool for animal‑free safety assessment.