Abstract PS4-02-26: Gper signaling as a biomarker for endocrine therapy sensitivity in pik3ca mutant hr+ advanced breast cancer

Abstract The INAVO 120 clinical trial confirmed the efficacy of first-line palbociclib combined with inavolisib in hormone receptor-positive (HR+) advanced breast cancer patients harboring PIK3CA mutations who developed resistance to adjuvant endocrine therapy. However, the comparative effectiveness of alternative CDK4/6 inhibitors (e.g., abemaciclib, ribociclib) in the first line setting, or the potential of crossline therapy (using abemaciclib or ribociclib combined with inavolisib) following palbociclib resistance in PIK3CA-mutant patients remains unclear. Our prior research identified the G protein-coupled estrogen receptor (GPER) as a pivotal initiator of endocrine resistance in HR+ advanced breast cancer, further driving activation of the PI3K/AKT/mTOR (PAM) signaling pathway. We infer that GPER signaling modulates endocrine therapy sensitivity in PIK3CA-mutant HR+ advanced breast cancer and could serve as a novel biomarker to guide personalized clinical treatment decisions. To demonstrate this hypothesis, we identified MCF-7 (E545K-mutant) and T47D (H1047R-mutant) breast cancer cell lines harboring PIK3CA mutations, and developed tamoxifen-resistant (MCF-7/TAM-R, T47D/TAM-R) and palbociclib-resistant (MCF-7/Palb-R, T47D/Palb-R) models to recapitulate first- and second-line endocrine resistance in clinic. Interestingly, GPER expression was significantly upregulated in all above resistant cell lines and promoting ABCG2 efflux pump expression via the GPER/PI3K/AKT pathway, as validated by quantitative PCR and Western blot. In GPERhigh tamoxifen-resistant cells, combinations of inavolisib with abemaciclib or ribociclib exhibited superior antiproliferative effects compared to palbociclib-based regimens, which assessed by MTT assays, flow cytometry for apoptosis, and cell cycle analysis. Knockdown of GPER/ABCG2 axis using siRNA normalized the efficacy of palbociclib, abemaciclib, and ribociclib in combination with inavolisib, underscoring GPER’s critical role in diverse CDK4/6 inhibitors sensitivity. Notably, in palbociclib-resistant cells, combinations of inavolisib with abemaciclib or ribociclib significantly outperformed inavolisib monotherapy. Furthermore, GPER/ABCG2 knockdown not only partially reversed palbociclib resistance, but also enhanced sensitivity to abemaciclib- and ribociclib-based regimens, demonstrating reduced tumor growth and improved therapeutic response upon GPER signaling inhibition. Therefore, assessing GPER signaling activation in tumor biopsies may enable precise identification of PIK3CA-mutant HR+ advanced breast cancer patients likely to benefit from specific endocrine and targeted therapy combinations. Integrating PI3K, CDK4/6, and GPER-targeted therapies represents a promising strategy to overcome endocrine resistance in the future, offering a path toward personalized treatment paradigms with improved clinical outcomes. Citation Format: X. ZengZ. SunQ. HuM. TangC. LiuZ. LiuT. Yu. Gper signaling as a biomarker for endocrine therapy sensitivity in pik3ca mutant hr+ advanced breast cancer [abstract]. In: Proceedings of the San Antonio Breast Cancer Symposium 2025; 2025 Dec 9-12; San Antonio, TX. Philadelphia (PA): AACR; Clin Cancer Res 2026;32(4 Suppl):Abstract nr PS4-02-26.