jiang, l.; Li, J.; Liu, L.; Li, M.; Li, W.; Li, Z.; Qian, S.; Shi, M.; Dai, J.; Ao, C.; Qu, Y.; Li, Z.; Zhou, L.; Yang, C.; Wang, X.; Tang, R.; Zhou, F.; Liu, G.; Xue, L.

Abstract The repair of DNA double strand break (DSB) is crucial for genome stability. Homologous recombination (HR) is a high-fidelity, error-free DSB repair pathway, and its dysfunction leads to genome instability and can result in tumorigenesis. In HR deficient contexts, microhomology-mediated end joining (MMEJ), dependent on DNA polymerase theta (Pol{theta}), is up-regulated to serve as a backup pathway for DSB repair. Several studies have proved that Pol{theta} inhibition causes synthetic lethality with HR deficiency, and Pol{theta} emerges as a potential DNA damage repair (DDR) target for the treatment of HR deficient tumors both as single agent or in combination with PARP inhibitors (PARPi). In addition, evidences show that the blockage of Pol{theta} can enhance the sensitivity to DNA damage agents. Herein, we characterized ZMS-6659, a highly potent small-molecule Pol{theta} inhibitor targeting the helicase domain. ZMS-6659 induces synthetic lethality in HR deficient models, elicits synergistic effects with even low dose of PARPi and delays PARPi resistance without additional hematotoxicity. Besides, ZMS-6659 also enhance the sensitivity to topoisomerase inhibitors in HR proficient cell lines. These findings support the clinical exploration of ZMS-6659 in combination with PARPi in HR deficient tumors, and suggest a potential application expansion beyond HR deficiency.