Abstract 3048: Drug repurposing: Validation of sulfasalazine as a radiosensitizer in melanoma by blocking system Xc−

Proceedings: AACR 107th Annual Meeting 2016; April 16-20, 2016; New Orleans, LA Melanoma is a cancer that has become increasingly frequent over the last decades in the western world. Initial treatment for primary and locoregional melanoma is surgery. In metastatic disease, systematic treatment and recently immunotherapy has been the mainstay. At this stage however, the prognostic outlook is still bleak. Thus, new treatments are urgently needed. Melanoma is considered to be a radioresistant cancer. The mechanisms underlying radioresistance are multiple and incompletely characterized. In some tumors, radioresistance is mediated by increased synthesis of anti-oxidants that scavenge reactive oxygen species (ROS), induced by radiotherapy. Glutathione (GSH) is an anti-oxidant synthesized from cystine and constitutes a major defense system against oxidative stress in mammalian cells. Cystine is taken up through system Xc− (SXC), an anti-port transmembrane protein with catalytic subunit xCT. Sulfasalazine, a drug approved in the 1950s for treatment of rheumatoid arthritis and inflammatory bowel disease, has been shown to block SXC. Based on previous reports by others, and our own recent findings, we hypothesize that: I) xCT represents a mechanism for radioresistance and is expressed in radioresistant melanoma cancer and II) xCT inhibitors can act as radiosensitizers to potentiate the efficacy of radiotherapy. Expression of the catalytic subunit of SXC, xCT, was found in tissue micro array of primary and secondary melanoma biopsies. In addition, SAS treatment dramatically reduced cystine-uptake and GSH levels in melanoma cells in vitro, and markedly increased the levels of reactive oxygen species (ROS). Furthermore, SAS and radiation synergistically increased DNA double-strand breaks and increased glioma cell death, whereas adding the anti-oxidant N-acetyl-L-cysteine (NAC) reversed cell death. Moreover, SAS and irradiation synergistically reduced subcutaneous melanoma tumor growth in vivo, compared to controls or either treatment alone. Thus, future experimental studies are warranted to validate SAS as a radiosensitizer in the treatment of metastatic melanoma. Future studies will also be aimed at assessing the effect on pulmonary melanoma metastasis. Citation Format: Hilde Elise Forde, Linda Sleire, Heidi Espedal, Jan Ingemann Heggdal, Frode Selheim, Paal-Henning Pedersen, Per Oyvind Enger. Drug repurposing: Validation of sulfasalazine as a radiosensitizer in melanoma by blocking system Xc−. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 3048.