This article is a highlight of the paper by Huang et al. in this issue of Photochemistry and Photobiology. It describes shades of phototoxicity in fluorescent imaging agents that are not intended to be phototoxic. Phototoxicity was assessed using a modified neutral red uptake (NRU) in vitro assay with mean photo-effects (MPE) for the fluorescent agents IRdye800, indocyanine green (ICG), proflavine, and methylene blue (MB), with comparisons to known phototoxic agents benzoporphyrin derivative (BPD) and rose bengal (RB). The experimental conditions were aimed to mimic clinical settings, using not only visible light, but also near-infrared light for insight to photosafety and deep tissue damage.

Molecular mechanisms underlying the phototoxicities were not sought, but IRdye800 and ICG were mainly deemed to be safe, whereas proflavine and MB (Methylene Blue)  would require precautions since phototoxicity can overshadow their utility as fluorescent imaging agents.

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