Abstract 3008: The interplay between FOXO3 and FOXM1 influences sensitivity to AKT inhibition in PIK3CA and PIK3CA/PTEN altered estrogen receptor positive breast cancer

Abstract Combining fulvestrant (a selective estrogen receptor (ER) degrader) with the PI3K-AKT signalling inhibitors such as alpelisib (PI3Kα inhibitor) or capivasertib (AKT inhibitor) gives benefit to patients with ER+ breast cancer (ER+ BC) harboring PIK3CA mutations (alpelisib) and PIK3CA, PTEN and AKT-1 altered tumors (capivasertib). While PIK3CA mutations are common, PI3K-AKT signalling activation can also occur due to other alterations, including in PTEN or AKT-1, which alter pathway function and dependency. Deletion of PTEN, leading to ablation of PTEN protein, attenuates the effectiveness of PI3Kα inhibition in PIK3CA altered tumors. Our study demonstrates that in ER+ BC tumors with PIK3CA activating mutations, co-occurring PTEN homozygous or hemizygous loss is common. This study evaluates the impact of reduced PTEN protein expression on the efficacy of AKT inhibition with capivasertib in the presence of PIK3CA mutations. In PIK3CA altered and intact PTEN protein, in vivo tumor and in vitro cell line models, PI3Kα and AKT inhibitors are equally effective. However, in in vivo and in vitro PIK3CA altered models, reduction in PTEN protein expression reduces the effectiveness of PI3Kα inhibition alone and in combination with fulvestrant, while AKT inhibition remains effective. The anti-tumor activity of both monotherapy and combination treatment with PI3Kα or AKT inhibitors is associated with a decrease in FOXM1 protein and FOXM1-mediated gene expression. The regulation of FOXM1 expression is FOXO3-dependent and is directly related to the ability of the PI3Kαi or AKTi to reduce AKT dependent downstream signalling. When FOXO3a is deleted from PTEN deleted - T47D and MCF7 cells, treatment with capivasertib did not reduce FOXM1 expression. Indeed, FOXO3a loss leads to an increase in FOXM1 expression, resulting in no impact on cell viability despite the presence of capivasertib. These findings indicate that inhibiting AKT signalling is a more effective strategy than targeting PI3Kα to address this crucial effector node across a heterogeneous array of PI3K-AKT tumor drivers. Furthermore, the study highlights the pivotal role of the AKT-FOXO3-FOXM1 axis in mediating responses to AKT inhibitors in ER+ breast cancer with PIK3CA mutations regardless of PTEN expression. In addition, loss of FOXO3a expression can mediate resistance, and FOXM1 downregulation serves as a crucial biomarker of response to the combination treatment. Citation Format: Valentina Cutano, Ming Li Chia,Eleanor M. Wigmore, Stuart C. Williamson, Amanda L. Christie, Brandon Willis, James Kerr, Sophie D’Arcy, Lauren Bradshaw, Cath Eberlein, Lambert Montava Garriga, Elza de Bruin, Susan E. Critchlow, Kevin M. Brindle, Simon T Barry, Susana Ros. The interplay between FOXO3 and FOXM1 influences sensitivity to AKT inhibition in PIK3CA and PIK3CA/PTEN altered estrogen receptor positive 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 3008.