Abstract PS2-11-30: Functional analysis of gedatolisib combined with fulvestrant and/or palbociclib in breast cancer cell models adapted to estrogen receptor and/or CDK4/6 inhibitors

Abstract Background: The estrogen receptor (ER), cyclin-dependent kinase (CDK), and PI3K-AKT-mTOR (PAM) pathways are interconnected and drive hormone receptor (HR)+/HER2- advanced breast cancer (ABC). In the first line setting, endocrine therapy (ET) combined with a CDK4/6 inhibitor is the standard of care regimen for most patients with HR+/HER2- ABC. For patients in the second line setting, several inhibitors targeting components of the PAM pathway (e.g. PI3Kα, AKT, mTORC1) are approved in combination with ET. By only addressing a single target of the PAM pathway, the currently approved PI3Kα, AKT, mTORC1 inhibitors do not prevent adaptive resistance induced by the other uninhibited PAM targets. In a phase 1b clinical trial, gedatolisib, a multi-target PAM inhibitor, combined with ET and palbociclib showed promising efficacy and safety in patients with HR+/HER2- ABC, regardless of PIK3CA mutation status. Here we investigated the effects of gedatolisib and single-target PI3Kα and AKT inhibitors in combination with fulvestrant and/or palbociclib in breast cancer (BC) cell models with and without PAM pathway mutations and with varying levels of sensitivity to fulvestrant and palbociclib. Methods: Gedatolisib (pan-PI3K, mTORC1/2 inhibitor), inavolisib (PI3Kα inhibitor), capivasertib (AKT inhibitor) were tested as single agents or in combination with fulvestrant and/or palbociclib in treatment-naïve BC cell lines with or without PIK3CA mutations as well as in BC cell lines adapted to long-term treatment with fulvestrant and/or palbociclib. Cell viability, growth rate, cell cycle, DNA replication, apoptosis, and cell death were evaluated by luciferase-based and flow cytometry-based functional assays. Results: Gedatolisib reduced growth rates of treatment-naïve or fulvestrant/palbociclib-adapted BC cell lines, regardless of PIK3CA mutational status, and the addition of fulvestrant and/or palbociclib increased growth-inhibitory effects. From a functional standpoint, the gedatolisib/palbociclib/fulvestrant triplet inhibited DNA replication and/or induced apoptotic cell death more effectively than treatment with single drugs or drug doublets. The gedatolisib/palbociclib/fulvestrant triplet was more effective than PI3Kα and AKT inhibitors (inavolisib, capivasertib) combined with palbociclib and/or fulvestrant in the BC cell lines tested, regardless of PIK3CA mutational status or previous adaptation to palbociclib and/or fulvestrant. Conclusions: These results indicate that gedatolisib plus fulvestrant, with and without palbociclib, effectively controls the growth of treatment-naïve BC cells as well as BC cells adapted to palbociclib/fulvestrant treatment. Moreover, in contrast to the currently approved PI3Kα and AKT inhibitors, the gedatolisib/palbociclib/fulvestrant triplet is effective in BC cells with or without PIK3CA mutations. The combination of gedatolisib with ET and a CDK4/6 inhibitor is being evaluated in two ongoing Phase 3 clinical trials as first-line (VIKTORIA-2) or second-line (VIKTORIA-1) treatment of HR+/HER2- ABC with or without PIK3CA mutations. The non-clinical results presented here provide a strong mechanistic rationale for these clinical studies. Citation Format: S. Rossetti, A. Broege, L. Davis, M. Seibel, S. Stokke, J. Molden, I. Gorbatchevsky, B. Sullivan, L. Laing. Functional analysis of gedatolisib combined with fulvestrant and/or palbociclib in breast cancer cell models adapted to estrogen receptor and/or CDK4/6 inhibitors [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 PS2-11-30.