UVA photosensitization of thiopurines and skin cancer in organ transplant recipients

Photochem Photobiol Sci. 2012 Jan;11(1):62-8. doi: 10.1039/c1pp05194f. Epub 2011 Aug 23.


The thiopurines azathioprine, 6-mercaptopurine and 6-thioguanine (6-TG) are important medications for cancer and inflammatory disorders. They are also widely prescribed as immunosuppressants in organ transplant patients. Their metabolism results in the incorporation of 6-TG into patients' DNA, and this increases skin sensitivity to incident UVA. Unlike the canonical DNA bases, which do not absorb UVA to a significant degree, DNA 6-TG is a strong UVA chromophore. It acts as a Type II UVA photosensitizer, and the combination of 6-TG and UVA treatment induces a synergistic toxicity in cultured human cells. Here, we review some of the damage that this interaction causes. Photochemical activation of DNA 6-TG triggers DNA and protein oxidation; it induces DNA breakage, DNA crosslinking, oxidation of DNA bases and the covalent attachment of proteins to DNA. Many of these photochemical DNA lesions are difficult for cells to deal with, and we review the evidence linking thiopurine immunosuppression with genome instability and the high incidence of skin cancer in organ transplant recipients.

Publication types

  • Research Support, Non-U.S. Gov't
  • Review

MeSH terms

  • Azathioprine / therapeutic use*
  • Humans
  • Immunosuppressive Agents / therapeutic use*
  • Mercaptopurine / therapeutic use*
  • Photosensitizing Agents / therapeutic use*
  • Skin Neoplasms / etiology*
  • Thioguanine / therapeutic use*
  • Ultraviolet Rays*


  • Immunosuppressive Agents
  • Photosensitizing Agents
  • Mercaptopurine
  • Thioguanine
  • Azathioprine