Abstract 983: Metabolic radiosensitization: Overcoming the radioresistance of hypoxic tumors by targeting OXPHOS
Authors: Benej Martin , Hong Xiangqian , Yu Bing , Papandreou Ioanna , Denko Nicholas 
Published: August 2018

Abstract
It has been recognized decades ago that lower oxygen tension decreases the efficiency of radiation therapy (RT). Strategies to clinically overcome hypoxia have been disappointing. We have addressed this issue by targeting the tumor metabolism. “Metabolic radiosensitization” is a concept of achieving a temporary decrease in the oxygen consumption (OCR) of tumor cells and therefore reduce hypoxia prior to RT. Inhibiting mitochondrial respiration will cause a decrease in the demand for oxygen that will in turn lead to a decrease of tumor hypoxia providing a therapeutic window to deliver RT. A number of FDA-approved drugs have been shown to inhibit mitochondrial OCR as an off-target effect. Repurposing one of these drugs with well-established safety profiles for radiosensitization would thus provide an exciting opportunity for clinical translation. We found that papaverine (PPV) showed the most promising results by rapidly achieving up to 40% inhibition of OCR within minutes in clinically achievable doses in the low micromolar range. Using near infra-red optical spectroscopy (NIRS) with a mouse flank xenotransplantation model we confirmed that a single dose of the drug caused a 1.3-1.4x increase of tumor oxygenation in vivo in the first 30 minutes, while the normal tissue oxygenation remained unaffected. We then determined that combination of papaverine followed by RT after 30 minutes enhanced the tumor radiation growth delay significantly by 2.1-3.9x compared to RT alone. Papaverine has been traditionally used as a vasodilator and anti-spasmodic and is thought to act through its ability to inhibit phosphodiesterase 10A (PDE10A). However, we found that increased cAMP did not inhibit OCR in vitro, suggesting that PPV has dual functions as PDE10A and OCR inhibitor. To mechanistically establish that PPV is radiosensitizing through inhibition of mitochondrial complex I, we generated tumor cells with PPV-resistant mitochondria. We used CRISPR/Cas9 to knock out a gene essential for complex I core activity. We then rescued partial mitochondrial function in these cells by introducing papaverine-resistant yeast complex I encoded by the NDI1 gene. Analysis of these cells by Seahorse confirmed that they were resistant to the effects of PPV on OCR. These PPV-resistant tumor cells were used to grow tumors, and in radiation growth delay experiments, NDI1-expressing tumors were resistant to PPV radiosensitization. These findings confirm that PPV radiosensitizes hypoxic tumors though inhibition of complex I. In conclusion, our data indicate that repurposing FDA approved papaverine for metabolic radiosensitization has a strong potential for translation into the clinics.
Citation Format: Martin Benej, Xiangqian Hong, Bing Yu, Ioanna Papandreou, Nicholas Denko. Metabolic radiosensitization: Overcoming the radioresistance of hypoxic tumors by targeting OXPHOS [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 983.