RTx-303, an Orally Bioavailable Polθ Polymerase Inhibitor That Potentiates PARP Inhibitors in BRCA Mutant Tumors

DNA polymerase θ (Polθ) is a polymerase-helicase
fusion
protein that is synthetically lethal with homologous recombination
(HR) factors, such as BRCA1/2, and confers resistance to PARP inhibitors
(PARPi) and other genotoxic cancer therapies. Previously developed
Polθ polymerase (Polθ-pol) inhibitors (Polθi) exhibited
limited pharmacological activity and metabolic stability, warranting
the development of a Polθi with improved drug-like properties.
Here, we developed RTx-303, a selective allosteric small-molecule
Polθ-pol inhibitor that exhibits 5.1 nM IC50, 88%
oral bioavailability, and a prolonged half-life along with its equipotent
metabolite. X-ray crystallography highlights the development of a
solvent-exposed side-chain that is essential for the optimal drug-like
properties of RTx-303. Notably, RTx-303 exhibits significantly higher
cellular potency than previously developed Polθ-pol inhibitors
and strongly potentiates PARPi in BRCA1/2 mutant cells and patient-derived
xenograft models. The superior potency, robust pharmacological activity,
and high tolerability of RTx-303 warrant further development as a
Polθ-pol inhibitor drug candidate.

DNA聚合酶θ（DNA polymerase θ，Polθ）是一类聚合酶-解旋酶融合蛋白，其与同源重组（homologous recombination，HR）因子（如BRCA1/2）存在合成致死关联，且可赋予肿瘤细胞对PARP抑制剂（PARP inhibitors，PARPi）及其他基因毒性癌症疗法的耐药性。此前开发的Polθ聚合酶（Polθ polymerase，Polθ-pol）抑制剂（Polθi）药理活性与代谢稳定性均较为有限，因此亟需开发具备更佳类药特性的Polθ抑制剂。本研究开发了RTx-303——一种选择性变构小分子Polθ聚合酶抑制剂，其IC₅₀值达5.1 nM，口服生物利用度为88%，半衰期持久，且其代谢产物亦具有同等药效。X射线晶体学（X-ray crystallography）分析显示，RTx-303分子中存在一处溶剂暴露侧链，该结构对其最优类药特性至关重要。值得注意的是，RTx-303的细胞活性显著优于此前开发的Polθ聚合酶抑制剂，且可在BRCA1/2突变细胞及患者来源异种移植模型中显著增强PARP抑制剂的抗肿瘤活性。RTx-303所具备的优异细胞活性、稳定的药理特性与良好的耐受性，使其有潜力作为Polθ聚合酶抑制剂候选药物开展进一步开发。