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Abstract Homologous recombination (HR) is a high-fidelity DNA double-strand break (DSB) repair pathway, and its dysfunction leads to cell genome instability and can result in tumorigenesis. In the situation of HR deficiency, DNA polymerase theta (Polθ) mediated microhomology-mediated end joining (MMEJ) is up-regulated to serve as a backup pathway for DSB repair. Several studies have proved that the inhibition of Polθ causes synthetic lethality with HR deficiency, and Polθ emerges as a potential DNA damage repair (DDR) target for the treatment of HR deficient tumors. In addition, there is evidence showing that the depletion of Polθ improves the sensitivity of some other DDR-related targeted therapies, chemotherapy, and radiotherapy. Here we identify a novel small molecular Polθ inhibitor, ZM-2311, which inhibits Polθ activity with an IC50 of 24 nM, and strongly inhibits cellular MMEJ pathway with a single nanomolar IC50. ZM-2311 illustrates a robust potency against tumor growth in a MDA-MB-436 SHLD2-/- xenograft model, suggesting potential monotherapy application in BRCA1 mutation patients with TP53BP1/Shieldin complex deficiency. Besides monotherapy, ZM-2311 elicits strong synergetic anti-proliferation activities in combination with PARP inhibitors on BRCA2-/- DLD-1 and endogenous HR deficient tumor cells in vitro and in vivo. Tumor regression is observed even at a low dosage, indicating a high potency of ZM-2311. As reversion mutation of HR genes such as BRCA1/2 mediated by the MMEJ pathway is reported to be a mechanism of PARP inhibitor resistance, ZM-2311 is evaluated and demonstrates its potential to overcome the resistance to PARP inhibitors. Furthermore, the combination of ZM-2311 with other DDR agents, such as ATR inhibitors, induces fatal DNA damage to HR deficient cells. Considering a low risk of hematotoxicity of Polθ inhibition, ZM-2311 may provide a new combination option for DDR agents. Besides, ZM-2311 was assessed in combination with different therapies respectively which functionally induce DNA damage, such as radiotherapy and chemotherapy. ZM-2311 renders tumor cells more sensitive to radiotherapy in both HR deficient cells and proficient cells in vitro, accompanying with the upregulation of Polθ expression after radiotherapy and an enhanced phosphorylation level of H2AX in the combination treatment. Similarly, ZM-2311 also achieves a robust synergetic effect in combination with chemotherapy in cell lines with different HR statuses. These results indicate a potential application expansion of ZM-2311 to HR-proficient tumors, which is expected to bring better efficacy and reduced hematotoxicity. Taken together, ZM-2311 has proved to be a highly potent Polθ inhibitor and has promising combination potential with multiple therapy strategies. Citation Format: Feng Zhou, Lu Liu, Lei Jiang, Shuo Qian, Liting Xue, Mengying Li, Zhengtao Li, Zhen Li, Jian Li, Renhong Tang. ZM-2311 a novel and highly potent Polθ inhibitor demonstrates synergistically anti-tumor activities in combination with different therapeutics abstract. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts) ; 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84 (6Suppl): Abstract nr 4539.
Zhou et al. (Fri,) studied this question.