Abstract 1264: Limited apoptotic response in RBM10 wild type/deficient EGFR-mutant lung cancer patient-derived models overcome by combined EGFR and BCL-2/BCL-xL inhibition.

Abstract Background: Loss of RBM10, an RNA splicing regulator, occurs in ∼8-10% of EGFR-mutant NSCLC and has been associated with reduced apoptosis and a poor response to EGFR-TKIs. RBM10 deficiency decreases the pro-apoptotic Bcl-xS/Bcl-xL ratio, thereby limiting osimertinib-induced cell death. We investigated whether combined inhibition of EGFR and BCL-2/BCL-xL overcomes this apoptotic defect using lung cancer organoids and PDXs harboring EGFR mutations with or without RBM10 deficiency. Methods: Patient-derived organoids from two EGFR-mutant PDX models—LG-0591 (RBM10-deficient/RBM10 wild type) and LG-0807 (RBM10-intact)—were treated with osimertinib, ABT-263 (navitoclax), or the combination—dose-response assays (CellTiter-Glo) quantified viability. Parallel in vivo studies were performed in NSG mice bearing LG-0591 PDXs treated for 28 days. Tumors harvested on treatment days 5 and 28 were analyzed by immunoblotting for EGFR, AKT, ERK, mTOR, 4EBP1, RBM10, Bcl-2 family proteins, Ki-67, cleaved PARP, and cleaved caspase-3. Results: RBM10-deficient, EGFR mutant organoids (LG-0591) showed reduced sensitivity to osimertinib monotherapy and minimal induction of cleaved PARP and caspase-3 relative to RBM10-normal organoids (LG-0807). RBM10 deficiency was associated with elevated Bcl-2/Bcl-xL and reduced Bcl-xS expression. ABT-263 alone showed modest activity; however, osimertinib + ABT-263 produced strong synergistic killing, restoring apoptosis and significantly lowering IC50 values in RBM10-deficient organoids. In vivo, osimertinib treatment suppressed EGFR/AKT/ERK signaling at day 5, but apoptosis remained limited in RBM10-deficient tumors, leading to tumor regrowth by day 28. ABT-263 alone modestly affected growth. The combination therapy achieved sustained suppression of downstream signaling, markedly increased cleaved PARP and cleaved caspase-3, and produced the most significant tumor regression with no observed toxicity. These effects were durable through day 28. Conclusions: RBM10 deficiency mediates intrinsic resistance to osimertinib by impairing mitochondrial apoptosis. Combined EGFR and BCL-2/BCL-xL inhibition fully restores apoptotic signaling and provides superior and durable antitumor activity in RBM10-deficient EGFR-mutant lung cancer, even if not RBM10 mutant. These findings support the clinical actionability of cotargeting EGFR and BCL-2/BCL-xL in patients with EGFR-mutant/RBM10-deficient NSCLC. Citation Format: Jonathan Wesley Riess, Hongyong Zhang, Jasmine Diaz Sezati, Peyton Apruzzese, Dongguang Wei, Tiffany Le, Luis G. Carvajal-Carmona. Limited apoptotic response in RBM10 wild type/deficient EGFR-mutant lung cancer patient-derived models overcome by combined EGFR and BCL-2/BCL-xL inhibition abstract. In: Proceedings of the American Association for Cancer Research Annual Meeting 2026; Part 1 (Regular Abstracts); 2026 Apr 17-22; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2026;86(7 Suppl):Abstract nr 1264.