Decoding the molecular basis for the population mechanism of the triplet phototoxic precursors in UVA light-activated pyrimidine anticancer drugs

Photosensitization of DNA by thionucleosides is a promising photo-chemotherapeutic treatment option for a variety of malignancies. DNA metabolization of thiated prodrugs can lead to cell death upon exposure to a low dose of UVA light. The exact mechanisms of thionucleoside phototoxicity are still not fully understood. In this work, we have combined femtosecond broadband transient absorption experiments with state-of-the-art molecular simulations to provide mechanistic insights for the ultrafast and efficient population of the triplet-state in the UVA-activated pyrimidine anticancer drug: 4-thiothymine. The triplet state is thought to act as a precursor to the DNA lesions and the reactive oxygen species responsible for 4-thiothymine photocytotoxicity. The electronic-structure and mechanistic results presented in this contribution reveal key molecular design criteria that can assist in developing alternative chemotherapeutic agents that may overcome some of the primary deficiencies of classical photosensitizers.