Discovery of SY-589, a Highly Potent and Orally Bioavailable Polθ Helicase Inhibitor for the Treatment of HR-Deficient Tumors

DNA polymerase theta (Polθ), which mediates microhomology-mediated end joining (MMEJ) in homologous recombination-deficient (HRD) cancers, has recently emerged as a compelling synthetic lethal target. Combining Polθ inhibition with PARP inhibitors represents a promising strategy to overcome PARP inhibitor resistance. Here, we present the discovery of <b>SY-589</b>, a highly potent (ATPase IC₅₀ = 2.29 nM), selective (selectivity index >1800), and orally bioavailable (<i>F</i> = 107%) Polθ helicase inhibitor, which exhibits robust antitumor efficacy in HRD tumors in vitro (CTG IC₅₀ = 2.71 nM). Notably, <b>SY-589</b> synergized strongly with the PARP inhibitor Olaparib in vitro (Loewe score >20) and in vivo (TGI = 109%), enhancing antitumor effects while permitting reduced Olaparib dosing. Overall, <b>SY-589</b> is a promising candidate of Polθ inhibitor and has been positioned as a rational combination partner with PARP inhibitors, aiming to overcome PARP inhibitor resistance and mitigate their dose-limiting toxicities.