Abstract P2-07-05: Activation of the AKT/mTOR signaling pathway is associated with response to the combination of endocrine therapy and CDK4/6 inhibitor in HR+/HER2- metastatic breast cancer

Abstract Background: As part of a multicenter study designed to identify markers of response or resistance to the combination of endocrine therapy (ET) and a cyclin dependent kinase 4/6 inhibitor (CDK 4/6i) as standard 1st-line therapy in patients with hormone receptor-positive, HER2-negative (HR+/HER2-), metastatic breast cancer (MBC), we assessed whether pretreatment levels of expression and phosphorylation of CDK 4/6 substrates and downstream molecules can predict response to this treatment. We then conducted an exploratory analysis to identify kinase-driven, pathway-centered mechanisms associated with response in pretreatment tumor samples. Methods: Tumor epithelia were isolated and enriched from the surrounding microenvironment using laser capture microdissection followed by downstream analysis using the Reverse Phase Protein Microarray (RPPA). Unmodified or post-transitionally modified residues were measured for 120 signaling proteins including 8 pre-specified qualifying proteins/phosphoproteins that are direct substrates of CDK 4/6 or whose activity is controlled by CDKI 4/6 activation as predictive markers of response to ET plus a CDK4/6i as 1st-line treatment in HR+/HER2- MBC. These 8 qualifying markers were: total Rb, phospho-Rb (S780), total Cyclin D1, phospho-Cyclin D1 (S286), total p16INK, total p27KIP, phospho-p27KIP (T187), and phospho-FOXM1 (T600). A total of 20 samples were available for the exploratory analysis. Specimens with expression or phosphorylation of the 120 biomarkers above or below the population median were classified as high or low, respectively. Chi-square analysis was used to compare proportion of patients with high and low expression between responders (CR, PR, or SD for a minimum of 12 months) and non-responders (PD within 12 months). Results: Pretreatment phosphorylation levels of Rb at the S780 residue were higher in non-responders compared to responders (p=0.025) and a similar trend was also detected for unmodified Rb, FoxM1 (T600), p27 Kip1 (T187), and Cyclin D1 (T289). Of the 120 proteins measured, there were statistically significant difference between responders and non-responders in 61. Non-responders were characterized by increased expression and phosphorylation of Rb regulators like CDK 4 (T172), as well as expression of CDK2, Cyclin E1, and Cyclin E2. Non-responders also presented with a broad activation of the PI3K/AKT/mTOR pathway, including phosphorylated PDK1 (S241) and AKT (S473 and T308), along with the AKT substrates FoxO1 (S256), FoxO1/FoxO3 (T24/T32), mTOR (S2448), and the mTOR regulator PRAS40 (T246). Finally, the mTORC1 complex downstream substrates p70S6K (T389) and 4EBP1 (S65 and T37/46) were also increased in non-responders. Conclusion: Taken together our data suggest that expression of Rb regulators along with the activation of the PI3K/AKT/mTOR signaling axis may modulate response to ET in combination with a CDK 4/6i. Targeting these pathways may be a novel therapeutic opportunity to enhance and prolong response to this treatment. Citation Format: Maysa Abu-Khalaf, Christos Hatzis, K. Alex Hodge, Elisa Baldelli, William Sikov, Monica Mita, Frances Valdes-Albini, Bryant Dunetz, Emanuel Petricoin, Mariaelena Pierobon. Activation of the AKT/mTOR signaling pathway is associated with response to the combination of endocrine therapy and CDK4/6 inhibitor in HR+/HER2- metastatic breast cancer [abstract]. In: Proceedings of the 2021 San Antonio Breast Cancer Symposium; 2021 Dec 7-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2022;82(4 Suppl):Abstract nr P2-07-05.