Abstract 428: Low cellular pH promotes survival in CDK4/6 inhibitor resistant ER+ breast cancer

Abstract Cyclin dependent kinase 4 and 6 inhibitors (CDK4/6i) are part of standard of care therapy in combination with antiestrogens for the treatment of metastatic estrogen receptor positive (ER+) breast cancer. However, resistance to CDK4/6i is prevalent and advanced metastatic ER+ breast cancer remains incurable. Beside inhibiting the cell cycle, CDK4/6i are known to induce accumulation of lysosomes in sensitive cells but the role of lysosomes in CDK4/6i resistance remains elusive. Recently, we have shown that CDK4/6i resistant breast cancer cells gain metastatic potential relative to sensitive cells and this is potentially due to deregulated lysosomes. We have shown that all three CDK4/6i induce rapid cellular acidification in sensitive cells, which suggests a role of cellular pH in CDK4/6i-mediated cell death. Particularly, in abemaciclib resistant cells, intracellular pH is lower (pH=6.9) than that of sensitive cells (pH=7.4). Therefore, we hypothesize that a lower pH in CDK4/6i resistant cells promotes survival and this pathway can be leveraged to target resistance. Interestingly, protein levels of carbonic anhydrase IX (CA9), an important regulator of cellular pH, are significantly elevated in resistant cells. Increased CA9 levels correlate with impaired overall survival in breast and other cancers. Small molecule inhibitor of CA9, S4 or the clinical-grade drug SLC-0111, inhibits growth and invasive potential in resistant cells when combined with CDK4/6i plus an antiestrogen. CDK4/6-induced cellular acidity correlated with increased levels of hypoxia-inducible factor 2-alpha (HIF2α) protein in sensitive cells. Moreover, basal HIF2α was increased in resistant cells compared with sensitive cells. Furthermore, knockdown of HIF2α reduces CA9 protein expression and inhibits proliferation in resistant cells. Thus, our study highlights the role of the modulation of pH homeostasis as a mechanism for CDK4/6i resistant breast cancer and identifies CA9 and HIF2α as novel targets in treating advanced ER+ breast cancer. Citation Format: Erin R. Scheidemann, Ayesha N. Shajahan-Haq. Low cellular pH promotes survival in CDK4/6 inhibitor resistant ER+ 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 428.