The thiopurines azathioprine, 6-thioguanine (6-TG), and 6-mercaptopurine (6-MP) are used as immunosuppressors in transplanted patients, and for treatment of inflammatory diseases, such as rheumatoid arthritis and systemic lupus. These drugs are metabolized by the purine salvage pathway, converted into thioguanine nucleotides and then incorporated in DNA as 6-TG. Recent studies reveal that thiopurine treatment is related to the appearance of skin cancer (squamous cells carcinoma) on those patients. This side effect arises due to the photosensitization of 6-TG by UVA radiation, giving rise to singlet oxygen. Singlet oxygen causes oxidative DNA damage, thus leading to the development of skin cancer in patients chronically exposed to sunlight. In order to understand the effects of 6-TG photosensitization and cellular oxidative stress, it is necessary to standardize the quantification methods for 6-TG in DNA. For this purpose, different lineages of epithelial cells will be cultivated, treated with 6-TG, have their DNA extracted, hydrolyzed, and submitted to high performance liquid chromatography (HPLC) analysis coupled to absorbance and fluorescence detectors.
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