Modified ESR‐based photosafety test (ESR‐PT) detecting singlet oxygen and free radical formation

Abstract The electron spin resonance‐based photosafety test (ESR‐PT) was modified using a new parameter, photoreactivity index (PRI), to detect singlet oxygen and free radical photoproducts simultaneously. With this modification, the modified ESR‐PT is expected to reduce the number of false negative results due to chemicals producing free radical photoproducts other than singlet oxygen. The assay performance of the modified ESR‐PT was evaluated using 56 chemicals, including hydrophobic chemicals. When using the PRI cutoff value of 2.0 in the modified ESR‐PT, the accuracy relative to photosafety reference data was 91.1%, and the applicability (100%) was better than the other non‐animal photosafety test. Among the chemicals producing positive results, bithionol, fenticlor, and doxycycline HCl were considered positive based on the detection of free radical photoproducts, suggesting that these three chemicals may have phototoxic or photoallergic potential via radical reactions. Additionally, this finding demonstrated the fundamental advantage of the modified ESR‐PT using ESR spectroscopy, which can detect radicals selectively and quantitatively. Accordingly, the new parameter PRI is effective for photosafety evaluations based on not only singlet oxygen but also free radical photoproducts generated from chemicals. Therefore, the modified ESR‐PT has a great potential for a photosafety test method applicable to various chemicals.