Targeting Colorectal Cancer Stem/Progenitor Cells With A Combination Of Ionizing Radiation And Thymoquinone.

Abstract Introduction: Radiotherapy is a standard neo-adjuvant therapy for cancer treatment, the success of which is limited by the resistance of cancer stem cells (CSCs) to therapy in addition to the deleterious effects of ionizing radiation (IR) to surrounding normal tissues. Despite the anticancer efficacy of combining IR with standard chemotherapeutic drugs, such treatments have shown great toxicity to normal cells. Thus, recent therapeutic strategies have aimed to combine IR with natural compounds that could radiosensitize and target cancer cells while sparing healthy tissues. The plant-derived compound Thymoquinone (TQ) has limited toxicity and is known to exert a radio-potentiating role in breast cancer and head and neck squamous cell carcinomas. Yet, no studies have investigated the antitumor effect of TQ in combination with IR on CSCs. Here, we studied the effect of TQ alone or in combination with IR on CSCs derived from different human colorectal cancer cell lines. Methods: MTT and trypan blue exclusion assays were used to study the effect of TQ and IR on proliferation and viability of HCT116 p53+/+, HCT116 p53−/−, and HT-29 p53+/− human colorectal cancer cell lines. The effect of TQ and IR on the long-term survival of different CRC cell lines was investigated by colony formation assay. Three-dimensional (3D) spheroid models derived from radiation resistant cells were used to enrich for stem/progenitor cells and study effects on CSCs self-renewal capacity. Results: IR doses of 1 or 2 Gray (Gy) significantly reduced the proliferation of HCT116 p53+/+ cells at 48 hours post IR. The antiproliferative effect of IR (2 Gy) on HCT116 p53−/− and HCT116 p53+/+ cells was significantly enhanced at 48 hours when combined with TQ at doses as low as 20 µM and 40 µM, respectively. While treatment of HCT116 p53+/+, HCT116 p53−/−, and HT-29 cells with 2 Gy IR or 10 µM TQ alone showed no significant inhibition on cell viability at 48 hours, combining the same doses of TQ and IR showed an enhanced and significant effect on cell viability when compared to TQ alone. Interestingly, combination treatment induced a more pronounced reduction in colony formation ability than either treatment alone at doses as low as 10 µM TQ. Of relevance to 3D systems, the combination of 3 µM TQ and 2 Gy IR significantly inhibited the self-renewal capacity of radiation resistant colonospheres derived from HT-29 cells at generation 1 (G1). Conclusion: Our study shows that the combination of IR with TQ appears to be a promising therapeutic strategy for eradicating radioresistant colorectal cancer cells and stem/progenitor cells. Citation Format: Samar Al Bitar, Farah Ballout, Wassim Abou-Kheir, Hala Gali-Muhtasib. Targeting colorectal cancer stem/progenitor cells with a combination of ionizing radiation and thymoquinone [abstract]. In: Proceedings of the AACR Virtual Special Conference on Radiation Science and Medicine; 2021 Mar 2-3. Philadelphia (PA): AACR; Clin Cancer Res 2021;27(8_Suppl):Abstract nr PO-011.