Low-dose-rate ionizing radiation increases singlet oxygen production by photosensitizers

Photosensitizers have significant potential as radiosensitizers in cancer treatment, yet the mechanism of ionizing-radiation-induced singlet oxygen ( 1 O 2 ) generation remains unclear. Here, we systematically investigated 1 O 2 production by the photosensitizer Chlorin e6 (Ce6) using the Singlet Oxygen Sensor Green probe and imidazole/ p -nitroso- N , N -dimethylaniline detection methods, evaluating the effects of photon energy (X-rays up to 310 kV and 60 Co gamma rays at 1.17 and 1.33 MeV), dose, and dose rate. Ce6 produced more 1 O 2 with increasing photon energy. At 5 Gy, the lowest dose rate (0.005 Gy/min) yielded significantly more 1 O 2 than higher dose rates (7–0.05 Gy/min). Scavenging experiments identified superoxide anions (·O 2 − ) as a key intermediate. We propose that, unlike classical triplet-state photosensitization, ionizing radiation induces Ce6 radical cations (Ce6 ⋅+ ), which react with radiation-induced ·O 2 − to generate 1 O 2 . These findings suggest potential for photosensitizer-radiation combinations in low-dose-rate therapies, although further biological validation and consideration of tumor redox status are required.