The Discovery of RP-2119: A Potent, Selective, and Orally Bioavailable Polθ ATPase Inhibitor

DNA polymerase theta (Polθ) plays a critical role in repairing DNA double-strand breaks through microhomology-mediated end joining (MMEJ) and has emerged as a key synthetic lethal drug target in cancers with homologous recombination (HR) deficiencies. Its inhibition has shown a strong potential to synergize with PARP inhibitors, particularly in tumors with deleterious <i>BRCA1</i> or <i>BRCA2</i> mutations. Here, we describe the discovery and preclinical development of RP-2119, a selective, potent, and bioavailable Polθ ATPase inhibitor. Starting from a high-throughput ATPase screen combined with literature insights, key vectors for enhancing potency were identified by structural studies using single-particle cryo-electron microscopy (cryo-EM) that revealed the inhibitor binding site. Further optimization of potency and ADME properties led to the identification of RP-2119 with robust <i>in vitro</i> cellular activity in a wide range of HR-deficient cancer cell lines. In HR-deficient cell line- and patient-derived mouse xenografts, RP-2119 demonstrated strong synergy with the PARP inhibitor, olaparib, without exacerbating its hematological toxicity.