Abstract 5332: Targeting CDK2 in estrogen receptor-positive/RB-deficient breast cancer

Abstract The estrogen receptor-positive (ER+) subtype continues to be the leading cause of breast cancer-related mortality worldwide. Although CDK4/6 inhibitors (CDK4/6i) have extended survival of patients with ER+ breast cancer, resistance to these therapies is inevitable. RB1 loss-of-function alterations are a well-established mechanism of resistance to CDK4/6i, occurring in approximately 10% of ER+ breast cancer patients who progress on CDK4/6i. With the broad use of CDK4/6i as the standard treatment, the population of patients with ER+/RB-deficient breast cancer is expected to rise, highlighting an urgent need for effective targeted therapies and therapeutic strategies for this drug-resistant subtype. Using a genome-wide CRISPR screen in ER+ RB1 knockout (RBKO) T47D breast cancer cells, we identified CDK2 as an essential gene. Additionally, analysis of the DepMap dataset further confirmed CDK2 as a top essential gene in cancer cells with low RB1 mRNA expression, suggesting an RB-independent role for CDK2 and its potential as a therapeutic target in cancers with RB deficiency. Supporting this hypothesis, siRNA-mediated silencing of CDK2 (siCDK2) in MCF7 RBKO cells and in lung (H596 and H1155), prostate (Du-145), and triple-negative breast (MDA-MB-468) cancer cells harboring somatic RB1 mutations or deletions led to the accumulation of cells in the G1 phase and a reduction of cells in the S phase, indicating an overall delay in cell cycle progression. Similarly, treatment of MCF7 RBKO cells with a CDK2-specific small molecule inhibitor, INX-315, also resulted in suppression of G1-to-S cell cycle progression. In line with the cell cycle analysis, gene set enrichment analysis (GSEA) of RNA-seq data revealed a significant downregulation of the E2F Hallmark gene signature (q<0.0001) in MCF7 RBKO cells transfected with siCDK2 vs. control siRNA. Notably, the therapeutic combination of INX-315 and fulvestrant (a selective ERα degrader) synergistically suppressed the growth of MCF7 RBKO xenografts and ER+/RB1-deleted patient-derived xenografts in mice. Treatment with this combination reduced Ki67 levels more potently than each monotherapy or vehicle. These findings collectively identify CDK2 as a promising therapeutic target for ER+/RB-deficient breast cancer and support clinical testing of CDK2- and ERα-tagerted combination therapies in this refractory population. Citation Format: Chang-Ching A. Lin, Pamela Luna, Khushi Ahuja, Dan Ye, Yisheng V. Fang, Alec Trub, John Bisi, Jay Strum, Patrick Roberts, Jeon Lee, Ariella B. Hanker, Carlos L. Arteaga. Targeting CDK2 in estrogen receptor-positive/RB-deficient breast cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2025; Part 1 (Regular Abstracts); 2025 Apr 25-30; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2025;85(8_Suppl_1):Abstract nr 5332.