Abstract 368: CDC20 drives acquired resistance to CDK4/6 inhibitors in estrogen receptor positive breast cancer.

Abstract Purpose: Cyclin-dependent kinase (CDK) 4/6 inhibitors, such as palbociclib, in combination with endocrine therapy are the standard of care for patients with metastatic Estrogen Receptor-positive (ER+) breast cancer. Despite significant clinical benefit, acquired resistance to CDK4/6 inhibitors is nearly universal, and therapeutic strategies to overcome such resistance remain limited. To identify novel molecular mediators of resistance, we employed an unbiased integrative approach combining transcriptomic profiles with palbociclib sensitivity data to uncover molecular drivers of CDK4/6 inhibitor resistance. Methods: We integrated RNA sequencing data from over 45 breast cancer cell lines with half-maximal inhibitory concentration (IC50) values for palbociclib. We performed correlation analyses and identified candidate genes whose expression was associated with palbociclib response. Top correlated gene was validated using western blotting in parental MCF-7 cells and multiple genetically engineered palbociclib-resistant derivatives. Functional relevance was assessed through siRNA-mediated knockdown and overexpression experiments. In vitro cell survival assays and in vivo xenograft studies using athymic nude mice were performed to determine the impact of CDC20 modulation on palbociclib response. Results: CDC20 expression showed a strong positive correlation with palbociclib IC50 values across multiple breast cancer cell lines, suggesting a potential role in mediating palbociclib response. Consistently, we observed a marked upregulation of CDC20 in palbociclib-resistant MCF-7 and T-47D derivative cells compared to their parental counterparts. Functional studies further supported this association; silencing CDC20 expression in resistant cells significantly restored their sensitivity to palbociclib. Conversely, overexpression of CDC20 in both MCF-7 and T-47D cells was sufficient to confer resistance to palbociclib. In vitro cell survival assays showed significantly reduced sensitivity to palbociclib in CDC20-overexpressing cells compared to parental controls (p0.001, two-tailed t-test). In vivo, palbociclib treatment (100 mg/kg by oral gavage, daily) was completely ineffective in inhibiting the growth of MCF-7-CDC20 xenografts (p0.5 for palbociclib vs. vehicle, unpaired two-tailed t-test). Conclusion: Together, these findings identify CDC20 as a key functional mediator of palbociclib resistance and highlight its potential as a therapeutic target to overcome resistance to CDK4/6 inhibitors in ER-positive breast cancer. Citation Format: Kamal Pandey, SM Nashir Udden, Jamsey Mathew, Prasanna G. Alluri. CDC20 drives acquired resistance to CDK4/6 inhibitors in estrogen receptor positive breast cancer 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 368.