Abstract 4264: A novel, potent DNA polymerase theta inhibitor, QX-001280, demonstrates anti-tumor activity in combination with a PARP1 inhibitor

Abstract Microhomology-mediated end joining (MMEJ) is a repair mechanism that resolves double strand DNA breaks. It is considered a secondary double strand break repair mechanism behind homologous recombination (HR) but is often utilized by tumors that have lost the ability to perform HR. Critical to this process is DNA polymerase theta (Pol θ). Pol θ is a heterobifunctional protein that consists of an N-terminal helicase domain, a disorder central region and a C-terminal polymerase domain. Following DNA resection by the MRE11-RAD50-NBS-1 (MRN) complex Pol θ is recruited to the site of the double strand break where it adds the missing bases. The bases are then ligated to resolve the double strand break. The central role of Pol θ in MMEJ is highlighted by the fact that inhibition of Pol θ can be synthetic lethal with BRCA mutation and PARP-1 inhibitor treatment, with loss of BRCA1 or 2 resulting in non-functional HR. This loss was exploited with the development of the novel Pol θ inhibitor, QX-001280, and combining with a PARP inhibitor to elicit anti-tumor effect. QX-001280 is a potent inhibitor which binds to the Pol θ Helicase domain. In vitro, QX-001280 had an IC50 for the inhibition of Polθ of 0.84 nM, indicating potent inhibition of the helicase domain of Pol θ. Potent inhibition was also observed in DLD-1 BRCA2 null cells with an IC50 of 1.03 nM. Combination with a PARP inhibitor, Olaparib, led to significant increase in phosphorylated H2AX indicating that double strand breaks are not being resolved inferring that Pol θ and MMEJ are being inhibited. In addition, cell viability was significantly reduced in DLD-1 BRCA2 null cells treated with increasing doses of PARP inhibitor in combination Pol θ inhibitor, with a reduction in IC50. In vivo, single agent PARP inhibitor or Pol θ inhibitor had a modest effect on tumor growth. However, when Olaparib and QX-001280 are combined robust tumor regressions were observed. QX-001280 was well tolerated in all the dose groups. In conclusion, QX-001280 is a potent and specific inhibitor of Polθ which exhibits anti-tumor activity both in vitro and in vivo. It is a high-quality candidate for clinical investigation in combination therapy with PARP inhibitors for cancer treatment. Citation Format: Dafydd Thomas, James Duan, Xianhai Huang, Vijay Reddy, Wayne Tang. A novel, potent DNA polymerase theta inhibitor, QX-001280, demonstrates anti-tumor activity in combination with a PARP1 inhibitor [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2025; Part 1 (Regular Abstracts); 2025 Apr 25-30; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2025;85(8_Suppl_1):Abstract nr 4264.