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Development of a new photosafety test method based on singlet oxygen generation detected using electron spin resonance

Photosafety evaluations of chemicals used in consumer products, such as pharmaceuticals and cosmetics, are very important. Currently, two non‐animal tests for photosafety evaluations, the in vitro 3T3 neutral red uptake phototoxicity test (NRU PT) and the reactive oxygen species (ROS) assay, are used to detect photoreactive chemicals. However, these two tests are difficult to apply to hydrophobic chemicals. In the present study, we attempted to develop a new photosafety test method, named the electron spin resonance‐based photosafety test (ESR‐PT), that would be applicable even to hydrophobic chemicals based on the detection of singlet oxygen generation after irradiation using ESR spectroscopy with 4‐hydroxy‐2,2,6,6‐tetramethyl‐piperidine as a spin trap reagent. To achieve a quantitative evaluation, the singlet oxygen formation (SOF) value, which can be calculated as the increment in relative intensity after irradiation of the test mixture normalized by the increment in relative intensity after irradiation of the vehicle control solution, was calculated. The performance of the ESR‐PT was evaluated by testing all the proficiency chemicals of the ROS assay plus additional chemicals, including hydrophobic chemicals and chemicals that tested false negative in the 3T3‐NRU PT and ROS assay. SOF values were successfully calculated for all the chemicals tested including the hydrophobic chemicals, and the accuracy of the ESR‐PT using a tentative cutoff value of 2.8 against the photosafety information was 100%. Therefore, the SOF value could be an effective parameter for photosafety evaluations, suggesting that the newly developed ESR‐PT is a promising non‐animal test applicable even to hydrophobic chemicals.