The Endogenous Tryptophan-derived Photoproduct 6-formylindolo[3,2-b]carbazole (FICZ) Is a Nanomolar Photosensitizer That Can Be Harnessed for the Photodynamic Elimination of Skin Cancer Cells in Vitro and in Vivo.

UV-chromophores contained in human skin may act as endogenous sensitizers of photooxidative stress and can be employed therapeutically for the photodynamic elimination of malignant cells. Here we report that 6-formylindolo[3,2-b]carbazole (FICZ), a tryptophan-derived photoproduct and endogenous aryl hydrocarbon receptor agonist, displays activity as a nanomolar sensitizer of photooxidative stress, causing the photodynamic elimination of human melanoma and nonmelanoma skin cancer cells in vitro and in vivo. FICZ is an efficient UVA/Visible photosensitizer having absorbance maximum at 390 nm (ε = 9180 Lmol cm ), and fluorescence and singlet oxygen quantum yields of 0.15 and 0.5, respectively, in methanol. In a panel of cultured human squamous cell carcinoma and melanoma skin cancer cells (SCC-25, HaCaT-ras II-4, A375, G361, LOX), photodynamic induction of cell death was elicited by the combined action of solar simulated UVA (6.6 J/cm ) and FICZ (≥ 10 nM), preceded by the induction of oxidative stress as substantiated by MitoSOX Red fluorescence microscopy, comet detection of Fpg-sensitive oxidative genomic lesions, and upregulated stress response gene expression (HMOX1, HSPA1A, HSPA6). In SKH1 ‘high-risk’ mouse skin, an experimental FICZ/UVA photodynamic treatment regimen blocked the progression of UV-induced tumorigenesis suggesting feasibility of harnessing FICZ for the photooxidative elimination of malignant cells in vivo.
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