Small-molecule Polθ inhibitors provide safe and effective tumor radiosensitization in preclinical models

DNA polymerase theta (Polθ) is a DNA repair enzyme critical for microhomology mediated end joining (MMEJ). Polθ has limited expression in normal tissues but is frequently overexpressed in cancer cells and, therefore, represents an ideal target for tumor-specific radiosensitization. Here, we show that ART558 and ART899, two novel and specific allosteric inhibitors of the Polθ DNA polymerase domain, potently radiosensitize tumor cells, particularly when combined with fractionated radiation. Importantly, normal fibroblasts are not radiosensitized by Polθ inhibition. Mechanistically, we show that the radiosensitization caused by Polθ inhibition is most effective in replicating cells and is due to impaired DNA damage repair. We also show that radiosensitization is still effective under hypoxia, suggesting that these inhibitors may help overcome hypoxia-induced radioresistance. In addition, we describe for the first time ART899 and characterize it as a potent and specific Polθ inhibitor with improved metabolic stability. In vivo , the combination of Polθ inhibition using ART899 with fractionated radiation is well tolerated and results in a significant reduction in tumor growth compared to radiation alone. These results pave the way for future clinical trials of Polθ inhibitors in combination with radiotherapy. ### Competing Interest Statement MR, MB, Aurora Cerutti, Alessandro Cicconi, AG, DG, VG, JM, ER, DP, MS, SJB, RH, GCMS and HR are all employees and shareholders of Artios Pharma Ltd. GCMS is a shareholder of AstraZeneca PLC. GSH has received consultancy fees from, and is a shareholder of Artios Pharma Ltd. All other authors declare no competing interests.