How Optimization of Skin Sensitization and Photoirritation Assays are Advancing in vitro Photoallergy Assessment

The development of reliable in vitro methods to assess photoallergy is critical as global regulatory agencies move toward reducing, refining, and replacing animal testing. Currently, no validated OECD Test Guidelines exist for the evaluation of photoallergy; however, advances in New Approach Methodologies (NAMs) for skin sensitization and photoirritation provide a strong foundation for progress. This original research consists of case studies exploring optimization and evaluation of photo-adapted versions of three established skin sensitization assays (OECD Test Guideline series 442)—the photo-Direct Peptide Reactivity Assay (photo-DPRA), photo-KeratinoSens™, and photo-Human Cell Line Activation Test (photo-h-CLAT)—for their applicability in assessing photoallergic potential. Each assay was modified to include irradiation exposure based on existing literature. The case studies consist of assessing irradiation exposure on the three test systems in absence of reference chemicals, impacts of seeding density on controls and reference chemical prediction in the photo-KeratinoSens™ assay, and evaluation of cell surface marker expression in photo-h-CLAT. Results demonstrated irradiation conditions, reference chemical selection, evaluation of test system sensitivity, and evaluation of data are essential components for successful assay predictivity and reproducibility. Assessment of individual assay components prior to a validation study is critical to ensure reliable results. Historical experience performing traditional skin sensitization and photoirritation assays will facilitate transfer of the methods to laboratories seeking to develop or optimize these assays. Continued refinement and validation of these assays will be pivotal for establishing a comprehensive, tiered testing strategy for in vitro photoallergy evaluation.