Dithionated Nucleobases as Effective Photodynamic Agents against Human Epidermoid Carcinoma Cells

Sulfur-substituted nucleobases (i.e., thiobases) are a prospective class of compounds for clinical and cosmetic topical phototherapies. Recent investigations of several thiobases have revealed the ultrafast and efficient population of reactive triplet states upon ultraviolet-A (UVA) irradiation and the subsequent generation of singlet oxygen in high yield. In this contribution, we examine the photosensitizing activities of three of the most promising thiobase derivatives discovered to date: 2,4-dithiothymine, 2,4-dithiouracil, and 2,6-dithiopurine. These derivatives are shown to decrease the proliferation of human epidermoid carcinoma cells by up to 63 % in vitro, only upon activation with a low dose of UVA radiation (5 J cm^-2 ). The generation of reactive oxygen species plays a minor role in the mode of action, suggesting these dithiobases may be effective within oxygen-deficient environments. Importantly, the photosensitized activity correlates with the magnitude of the triplet lifetime, which should guide the molecular design of next-generation photodynamic agents.