Embryonic ectoderm development (EED) inhibitor APG-5918 synergizes with topoisomerase I inhibitors in preclinical small-cell lung cancer (SCLC) models through epigenetic priming of chemosensitivity

SCLC initially responds to platinum-based chemotherapy but rapidly acquires resistance, resulting in poor prognosis. PRC2-mediated epigenetic silencing represses Schlafen 11 (SLFN11)-a biomarker of sensitivity to DNA-damaging therapies-contributing to resistance. EZH2, the PRC2 catalytic subunit, promotes chemoresistance via SLFN11 repression. EED, another core PRC2 subunit, stabilizes the complex and maintains its methyltransferase activity, making it an attractive target. Topoisomerase I inhibitors, such as topotecan and irinotecan, are used in relapsed SCLC, but efficacy is limited when SLFN11 is epigenetically suppressed. APG-5918, a selective and investigational EED inhibitor, disrupts PRC2 function. This study evaluated antitumor activity of APG-5918 combined with topoisomerase I inhibitors in preclinical SCLC models. In vitro antiproliferation was assessed using human SCLC cell lines (NCI-H446, NCI-H69, NCI-H889, DMS-114); cell viability via CellTiter-Glo® luminescent assays; apoptosis with Annexin V/PI flow cytometry; molecular mechanisms via western blot; and in vivo antitumor efficacy using a subcutaneous NCI-H446 cell-derived xenograft (CDX) model. APG-5918 synergized with topoisomerase I inhibitors (topotecan or SN-38, the active metabolite of irinotecan) to suppress proliferation and induce apoptosis in SCLC cell lines. In NCI-H446 CDX, single-agent APG-5918 (100 mg/kg) or irinotecan (5 mg/kg) at 27 days showed limited efficacy (T/C: 115.79% and 61.58%, respectively). The combination significantly enhanced antitumor activity, achieving a T/C value of 38.92% (p < 0.05 vs. vehicle) and a synergistic index of 1.83. No significant body-weight changes indicated favorable safety. Mechanistically, APG‑5918 reduced H3K27me3 confirming on-target epigenetic modulation, and upregulated SLFN11 and p21 (CDKN1A). This EED inhibition reverses epigenetic silencing of chemosensitivity genes and sensitizes tumor cells to topoisomerase I inhibitors. Notably, topotecan or SN-38 alone increased H3K27me3, indicating that compensatory epigenetic repression was countered by APG-5918. The combination further downregulated PRC2 components (EED, EZH1, EZH2, SUZ12), suppressed cell-cycle regulators (pRb, CDK4, CDK6), and induced DNA damage (gammaH2A.X) and apoptotic markers (cleaved PARP-1, cleaved caspase-3, BIM, Noxa), supporting a synergistic proapoptotic mechanism. APG-5918 synergistically enhances antitumor activity of topoisomerase I inhibitors in preclinical SCLC models by reversing PRC2-mediated epigenetic silencing of SLFN11 and amplifying DNA damage, cell-cycle arrest, and apoptosis. These findings support clinical investigation of APG-5918 combined with DNA-damaging agents as a promising strategy for SCLC.