Harnessing the Excited States of 5-(5-Phenylthiophen-2-yl)-6-Azauridine as a Three-Pronged Agent for Skin Cancer Therapy: Photodynamic Action, Cell Imaging, and Cancer Cell Inhibition

Nucleoside analogs simultaneously exhibiting high fluorescence quantum yields and efficient triplet state population are rare. Such multifunctional nucleosides represent a crucial advancement for cell imaging-assisted photodynamic therapy relying on heavy-atom-free photosensitizers and hold additional promises as inhibitors of cancer cell proliferation. This study investigates the photophysical, electronic structure, excited state dynamics, and skin cancer cell photodynamic and inhibitory properties of 5-(5-phenylthiophen-2-yl)-6-azauridine (PTAU). PTAU absorbs up to 425 nm and demonstrates dual photophysical characteristics, with fluorescence and singlet oxygen quantum yields of 43 ± 1% and 52 ± 2% in acetonitrile and 12 ± 1% and 33 ± 2% in aqueous buffer, respectively. Time-resolved absorption and fluorescence spectroscopy, complemented by quantum chemical calculations, reveal the existence of two rotameric species of PTAU in solution. Both rotamers exhibit nanosecond-scale fluorescence and intersystem crossing lifetimes, as well as microsecond-scale triplet decay lifetimes. When applied to B16F10 murine melanoma cells, PTAU localizes mostly in the cytoplasm, primarily in mitochondria, and demonstrates moderate photodynamic activity, achieving IC₅₀ values of 125 ± 5 μM and 80 ± 3 μM at photoactivation doses of 7.5 J cm^-2 and 25 J cm^-2, respectively, while exhibiting no cytotoxicity in the dark. Notably, PTAU also inhibits the cell proliferation of B16F10 murine melanoma and A431 human epidermoid carcinoma by more than 95% at a concentration of 250 μM in the dark. Therefore, this proof-of-concept study reveals PTAU as the first example of a nucleoside analog with potential multifunctional applications, including photodynamic action, bioimaging, and the inhibition of skin cancer cell proliferation. These findings pave the way for further developing next-generation modified 6-azauridine analogs absorbing visible to near-infrared light for their use as cell imaging-assisted PDT agents and cancer cell inhibitors, targeting potential deep-tissue cancer treatment.