Discovery of an orally bioavailable selenium-containing Polθ inhibitor with a dual mechanism of DNA damage and immune activation for HR-deficient cancers

DNA Polymerase theta (Polθ)-mediated polymerase theta-mediated end joining (TMEJ) is a critical DNA repair pathway in homologous recombination (HR)-deficient cancers, making Polθ a promising synthetic-lethal target. Herein, employing multiple structure-based drug design strategies including cyclization and bioisosteric replacement, we present the discovery of compound XL-20, a potent and orally bioavailable (F = 137%) Polθ ATPase inhibitor. XL-20 exhibits low-nanomolar inhibition of Polθ ATPase (IC₅₀ = 4.3 nM), and demonstrates synergistic antitumor efficacy with PARP inhibition both in HR-deficient MDA-MB-436 cells and in vivo xenograft models. Notably, this selenium-containing compound XL-20 also significantly activates the cGAS-STING pathway and upregulates PD-L1, supporting its combination potential with immunotherapy. The promising dual-action efficacy and high oral bioavailability of XL-20 position it as a promising lead for further development.