Toward Animal-Free Phototoxicity Testing: A Keratinocyte-Based UVA Screening Assay to Distinguish Photoirritation from Photoallergy

Phototoxicity represents an increasing concern due to the presence of photoreactive chemicals in consumer products, including household products, pharmaceuticals, and cosmetic formulations. Upon exposure to solar radiation, these compounds may induce photoirritation, a non-immune-mediated inflammatory response, or photoallergy, an immune-mediated hypersensitivity reaction. However, the ability to discriminate between photoirritant and photoallergic responses remains an unmet need, as no validated in vitro assay is currently available for this purpose. Therefore, the development of reliable non-animal approaches is essential to address ethical concerns and comply with current regulatory restrictions on animal testing. Interleukin-18 (IL-18) has been proposed as a potential biomarker of allergic and photoallergic responses. In this study, we developed an in vitro assay adapted from OECD Test Guideline 432 using reference phototoxic compounds and demonstrated that exposure to the photoallergenic compound benzophenone induced IL-18 production. Furthermore, multiplex cytokine profiling identified two additional preliminary candidate biomarkers for discriminating between photoallergic and photoirritant responses. Matrix metalloproteinase-1 (MMP-1) was associated with photoallergic responses and was upregulated following exposure to chlorpromazine and benzophenone, whereas interleukin-6 (IL-6) was associated with photoirritant responses and was upregulated following exposure to 8-methoxypsoralen. Collectively, these findings provide proof-of-concept evidence supporting the future development of a novel non-animal strategy for predicting and discriminating between photoirritant and photoallergenic compounds.