ESTRO 2022 - Abstract Book

S372

Abstract book

ESTRO 2022

Results The POLQ inhibitors were effective radiosensitisers in a wide range of tumour cells but did not significantly affect normal cells, which were shown to express only low levels of POLQ. Increasing the number of radiation fractions led to an increase of POLQ inhibitor-mediated radiosensitisation in tumour cells. Radiosensitisation was most effective in cells in the S and G2 phase of the cell cycle and POLQ inhibition appeared to be particularly effective in cells with fast cell cycle progression. POLQ inhibition was associated with an increase in residual DNA damage foci after irradiation and increased G2/M arrest. Combination of POLQ inhibition with radiation in vivo led to a significant reduction in tumour growth and was very well- tolerated. Conclusion We showed that Artios’s potent and specific POLQ inhibitors are effective radiosensitisers in a wide range of tumour cells and also elicit tumour radiosensitisation in vivo. Our results will pave the way for testing POLQ inhibitors with radiotherapy in clinical trials.

OC-0429 Tamoxifen induces metabolic adaptations leading to radioresistance in breast cancer

F. Naumann 1 , G. Adema 1 , F. Sweep 2 , J. Bussink 1 , P. Span 1

1 Radboud University Medical Center, Radiotherapy, Nijmegen, The Netherlands; 2 Radboud University Medical Center, Laboratory Medicine, Nijmegen, The Netherlands Purpose or Objective Tamoxifen is a Selective Estrogen Receptor (ER) Modulator extensively used in the adjuvant and first line treatment of ER positive breast cancer. Recently, we reported that tamoxifen resistant breast cancer cells are cross-resistant to irradiation, possibly hampering optimal treatment of breast cancer patients. Tamoxifen has been shown to also exhibit ER-independent effects such as inhibition of mitochondrial oxidative phosphorylation, which might explain subsequent radioresistance. Here, we aim to investigate the mechanisms associated with tamoxifen induced radioresistance, and whether this depends on ER status. Materials and Methods To identify mechanisms underlying this cross-resistance, we induced tamoxifen resistance in ER positive MCF7 and ER negative MDA-MB-231 breast cancer cells by chronic treatment with increasing doses. Several metabolic characteristics were assessed in wild type (WT) cells and resistant (TAM) cells such as oxygen consumption and glycolysis, using the Seahorse metabolic analyzer. Additionally, we measured real-time ROS production in response to tamoxifen and H 2 O 2 as well as toxicity of these compounds to WT and TAM cells and assessed total antioxidant capacity. Attempting to increase the sensitivity of TAM cells to ROS, NRF2, the main activator for the antioxidant response element, was inhibited and the cells sensitivity to several ROS as well as irradiation was examined. Results Clonogenic survival reveals that also MDA-MB-231 cells lacking an ER can become resistant to irradiation after chronic tamoxifen treatment. We show that tamoxifen resistant cells exhibit a decreased oxygen consumption rate and have developed a more glycolytic phenotype compared to untreated wild type cells (Fig.1A), indicating mitochondrial dysfunction. Real time measurement of ROS demonstrates significantly lower overall ROS levels in resistant cells in response to tamoxifen (Fig.1), but also after H 2 O 2 treatment. Additionally, we found treatment with H 2 O 2 to be less toxic on tamoxifen resistant cells, suggesting ROS protective mechanisms as antioxidants to be more active in these cells. Indeed, we find higher antioxidant levels in tamoxifen resistant cells likely protecting cells from ROS induced DNA damage. By inhibiting NRF2, the activator of the antioxidant response element, tamoxifen resistant cells reestablished their sensitivity to ROS (Fig.1C).

Conclusion