Abstract 1415: Triple combination targeting ER, CDK4/6, and PI3K inhibits tumor growth in ER+ breast cancer resistant to combined fulvestrant and CDK4/6 or PI3K inhibitor
Authors: Karimi Leena , Alves Carla L. , Terp Mikkel G. , Tuttolomondo Martina , Ditzel Henrik J. 
Published: July 2021

Abstract
Despite the improved outcome of ER+ advanced breast cancer patients treated with combined CDK4/6 inhibitor (CDK4/6i) and endocrine therapy, the disease will eventually progress. Furthermore, the recently approved combined therapy with the α-specific PI3K inhibitor (PI3Ki) alpelisib and fulvestrant in PIK3CA-mutated ER+ metastatic breast cancer patients who had previously received endocrine therapy demonstrated a clinically meaningful benefit in clinical trials, but drug resistance is inevitable. Thus, novel and more effective treatment combinations are urgently needed, as well as defining the subsequent optimal sequence of treatment following progression on combined treatment with either CDK4/6i or Pi3Ki and endocrine therapy. We developed 4 PIK3CA-mutated ER+ breast cancer cell models resistant to combined fulvestrant and CDK4/6i or PI3Ki, derived from MCF-7 (MPFR and MPiFR) and T47D (TPFR and TPiFR) cells, and 2 orthotopic cell line xenograft models (MPFR and MPiFR). We used these models to investigate whether ER+ breast cancer cells resistant to combined fulvestrant and CDK4/6i or PI3Ki would benefit from switching between these combined therapies, or whether triple combination of these agents would be required. We observed that triple inhibition of CDK4/6, PI3K and ER significantly reduced growth of breast cancer cells and xenografts models resistant to combined fulvestrant and CDK4/6i or PI3Ki. The triple combination also induced apoptosis and cell cycle arrest and reduced the expression of key proteins of the ER, PI3K/AKT/mTOR and cyclin D-CDK4/6-Rb pathways, including ER, p-PRAS40 and p-S6, in cell lines resistant to combined fulvestrant and CDK4/6i or PI3Ki. Finally, the triple inhibition significantly prevented and/or delayed the acquisition of resistance in cells resistant to combined fulvestrant and CDK4/6i or PI3Ki. Importantly, switching between combined fulvestrant and CDK4/6i to fulvestrant and PI3Ki or vice versa upon resistance did not prevent disease progression, suggesting that simultaneous blockage of the 3 pathways is required in these tumors. Finally, we found high CDK2 levels in cells resistant to combined fulvestrant and PI3Ki, and targeting ER, PI3K, and CDK2 offered an additional treatment option for ER+ breast cancer resistant to combined fulvestrant and PI3Ki. Evaluation of the efficacy of these different targeted therapy combinations in patient-derived xenografts models is ongoing. Our data supports the use of combined therapy with either PI3Ki or CDK4/6i and fulvestrant following progression on or after an endocrine-based regime. However, following disease progression on combined therapy with fulvestrant and either CDK4/6i or PI3Ki, triple combination targeting ER, PI3K and either CDK4/6 or CDK2 is needed to inhibit tumor progression.
Citation Format: Leena Karimi, Carla L. Alves, Mikkel G. Terp, Martina Tuttolomondo, Henrik J. Ditzel. Triple combination targeting ER, CDK4/6, and PI3K inhibits tumor growth in ER+ breast cancer resistant to combined fulvestrant and CDK4/6 or PI3K inhibitor [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 1415.