Abstract 1799: Mithramycin A as a radiation sensitizer

Abstract Introduction: Radiation therapy (RT) is used in the treatment of many cancers to reduce or eliminate tumor burden. In many cases RT is not curative and recurrence often occurs. Therefore agents that enhance the effectiveness of RT are necessary to improve cancer management. We investigated the use of the DNA binding antibiotic mithramycin A as a radiosensitizer of solid tumors. Mithramycin A binds to GC rich regions of double stranded DNA displacing transcription factors, such as SP1. Mithramycin A has also been shown to inhibit DNA double strand break repair by mechanisms unrelated to its effect on transcription. Methods: Human cancer cells (A549/lung, HT29/colon, DU145/prostate, and UM-UC3/bladder) were grown at 37°C and 5% CO2 in RPMI1640 media. Cells were treated with 25 nM mithramycin A 1 hour prior to radiation and then allowed to grow for 7-10 days after RT. Clonogenic colonies were counted to determine the effect of mithramyicn A on radiation sensitivity. The effects of mithramycin A treatment and RT on DNA damage repair was measured by counting nuclear gamma-H2AX foci and neutral Comet assay. Cell cycle changes induced by mithramycin A and RT were measured by flow cyctometry. Mitotic catastrophe was analyzed by nuclear fragmentation. In vivo radiosensitzation of mithramycin A (1mg/kg) was quantified using growth delay of A549 tumor xenografts in both single dose (4 Gy) and fractionated RT (4x 2Gy) dosing schedules. Results: Mithramycin A treatment of cell lines in vitro prior to RT increased radiosensitivity with a range of dose modifying factors between 1.17-1.48. DNA damage repair after RT was not affected by mithramycin A treatment. Cell cycle distribution was altered by mithramycin A treatment with fewer cells in S-phase after treatment. Mitotic catastrophe after combined treatment with mithramycin A and RT was significantly elevated above the level of either agent alone. In vivo A549 tumor xenografts treated with both mithramycin A and a single dose RT had more than an additive growth delay compared to either agent alone. Conclusion: Mithramycin A treatment sensitized human cancer cells to radiation therapy in in vitro assays and an in vivo model system through an increase in mitotic catastrophe. A more detailed analysis of the mechanism of action is ongoing. Citation Format: Bradley T. Scroggins, Jeffery F. Burkeen, Eun Joo Chung, Ayla O. White, Su I. Chung, Kathryn E. Hudak, Deborah E. Citrin. Mithramycin A as a radiation sensitizer. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 1799. doi:10.1158/1538-7445.AM2015-1799