A New Naturally Derived Photosensitizer and Its Phototoxicity on Head and Neck Cancer Cells

In our screening for photosensitizers from natural resources, 15(1)-hydroxypurpurin-7-lactone ethyl methyl diester (compound 1) was isolated for the first time from an Araceae plant. To evaluate the efficacy of compound 1 as a photosensitizer for head and neck cancers, compound 1 was studied in reference to a known photosensitizer pheophorbide-a (Pha), in terms of photophysical properties, singlet oxygen generation and in in vitro experiments (intracellular uptake and phototoxicity assays) in two oral (HSC2 and HSC3) and two nasopharyngeal (HK1 and C666-1) cancer cell lines. In this study, compound 1 exhibited higher intracellular uptake over 24 h compared with Pha in both HSC3 and HK1 cells. When activated by ≥4.8 J cm(-2) of light, compound 1 was slightly more potent as a photosensitizer than Pha by consistently having marginally lower IC(50) values across different cell lines. In flow cytometry experiments to study the mechanism of photoactivated cell death in HSC3, compound 1 was observed to induce more pronounced apoptosis compared with Pha, which may have been driven by the transient G(2)/M cell cycle block which was also observed. These promising results on compound 1 warrant its further investigation as a clinically useful photodynamic therapy agent for head and neck cancer.