Photochemotherapy—in which a photosensitizing drug is combined with ultraviolet or visible radiation—has proven therapeutic effectiveness. Existing approaches have drawbacks, however, and there is a clinical need to develop alternatives offering improved target cell selectivity. DNA substitution by 4-thiothymidine (S 4 TdR) sensitizes cells to killing by ultraviolet A (UVA) radiation. Here, we demonstrate that UVA photoactivation of DNA S 4 TdR does not generate reactive oxygen or cause direct DNA breakage and is only minimally mutagenic.
In an organotypic human skin model, UVA penetration is sufficiently robust to kill S 4 TdR-photosensitized epidermal cells. We have investigated the DNA lesions responsible for toxicity. Although thymidine is the predominant UVA photoproduct of S 4 TdR in dilute solution, more complex lesions are formed when S 4 TdR-containing oligonucleotides are irradiated. One of these, a thietane/S 5 -(6-4)T:T, is structurally related to the (6-4) pyrimidine:pyrimidone [(6-4) Py:Py] photoproducts induced by UVB/C radiation. These lesions are detectable in DNA from S 4 TdR/UVA-treated cells and are excised from DNA more efficiently by keratinocytes than by leukaemia cells.
UVA irradiation also induces DNA interstrand crosslinking of S 4 TdR-containing duplex oligonucleotides. Cells defective in repairing (6-4) Py:Py DNA adducts or processing DNA crosslinks are extremely sensitive to S 4 TdR/UVA indicating that these lesions contribute significantly to S 4 TdR/UVA cytotoxicity.
Reelfs, O., Macpherson, P., Ren, X., Xu, Y.Z., Karran, P. and Young, A.R., 2011. Identification of potentially cytotoxic lesions induced by UVA photoactivation of DNA 4-thiothymidine in human cells. Nucleic acids research, 39(22), pp.9620-9632.
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