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Therapeutic resistance remains a major challenge in cancer treatment, driven by compensatory signaling and stress response pathways that sustain tumor survival. Topoisomerase IIβ-binding protein 1 (TopBP1), a multifunctional scaffold protein with nine BRCT domains, integrates replication stress signaling with oncogenic networks and is frequently overexpressed in aggressive cancers. Its BRCT7/8 domains mediate critical interactions with E2F1, mutant p53, MIZ1, PLK1, and CIP2A, making TopBP1-BRCT7/8 an attractive therapeutic target. Using docking-guided screening and structure-activity relationship-driven optimization, we developed CS18 as a potent and selective BRCT7/8 inhibitor that disrupts oncogenic TopBP1 complexes without interfering with DNA replication. CS18 suppresses MYC transcriptional programs, restores E2F1-mediated apoptosis, and induces mitotic catastrophe. It exhibits broad-spectrum anticancer activity and synergizes with poly(ADP-ribose) polymerase (PARP) inhibitors in multiple cancer types and enhances osimertinib sensitivity in EGFR-mutated non-small cell lung cancer (NSCLC) cells. CS18 demonstrates efficacy in patient-derived breast cancer xenografts and overcomes osimertinib resistance in refractory NSCLC in vivo. These findings establish CS18 as a chemically distinct TopBP1 inhibitor with translational potential to overcome therapeutic resistance and advance precision oncology.