Abstract PO3-14-12: SIRT3 as a new potential predictive biomarker for response to CDK4/6 inhibition in ER+/HER2- metastatic breast cancer patients

Abstract Background: Several prospective, randomized clinical trials showed that the CDK4/6 inhibitor palbociclib in combination with either letrozole or fulvestrant significantly improves progression-free survival (PFS) in patients with ER+/HER2- metastatic breast cancer (MBC). However, in some cases, there is development of endocrine resistance and ultimate disease progression. Molecular analysis performed in tissue specimens collected in the PALOMA-3 study, comparing fulvestrant and Palbociclib versus fulvestrant placebo, demonstrated that one of the genes associated with an increased clinical benefit is SIRT3 (Sirtuin 3) which is thought to control mitochondrial integrity and metabolism (Cristofanilli M. et al., Lancet Oncol. 2016). This study aimed to evaluate in preclinical models the predictive role of SIRT3 in ER+/HER2- breast tumors. Methods: Using lentiviruses we generated MCF-7 and T47D cells stably expressing either control shRNA (sh ctr) or shRNA targeting SIRT3 (shSIRT3). With these models, we studied cell viability, tumor growth, apoptosis, and autophagy in MCF-7 cells treated with fulvestrant and palbociclib as a single treatment or in combination, these either alone or in combination with shSIRT3 (knockdown). Nude mice were used for our in vivo studies, and sh ctr or shSIRT3 MCF7 cells were injected. Results: We showed in vitro that the response to palbociclib was significantly associated with levels of SIRT3 expressionas high or low in ER+ cells (p< 0.0001). Furthermore, we found that irreversible cell growth inhibition mediates the beneficial effect of palbociclib in SIRT3 high expressing cellscompared to low expression (p< 0.001).Then, we evaluated the mechanistic activity of SIRT3. We showed thatSIRT3-low expression cells induced enhanced sensitivity to palbociclib by targeting the reactive oxygen species (ROS)/autophagy axis:(i) SIRT3 regulated the mitochondrial homeostasis (e.g. glucose, lactate, pyruvate, succinates), (ii) ROS levels where lower in sh crt comparing and shSIRT3 in treated and untreated cells (p< 0.001), (iii) in terms of senescence,a higher level of positive cells for β-gal activity was found (p≤0.05) and (iv) for autophagy, our western blot analysis showed cleaved LC3 proteins. Our in vivo studies showed that SIRT3 regulated treatment response to palbociclib with a significant decrease in tumor weight and tumor volume (p< 0.01) and importantly, after immunohistochemistry analysis, we validated the role of SIRT3 in regulating the proteins involved in the autophagy/senescence balance, was mainly in epithelial cells compared to stromal cells. Conclusion: Our preclinical studies demonstrated that elevated SIRT3 expression levels sensitizes ER+/HER2- breast tumors to palbociclib treatment. This study suggested that SIRT3 expression could represent a potential predictive biomarker in HR+ MBC patients treated with Palbociclib. Citation Format: Maroua Manai, Ghada Sahraoui, Raoudha Doghri, Lorenzo Gerratana, Paolo D’Amico, Youbin Zhang, Jeannine Donahue, Ami Shah, Carolina Reduzzi, Wenan Qiang, Massimo Cristofanilli. SIRT3 as a new potential predictive biomarker for response to CDK4/6 inhibition in ER+/HER2- metastatic breast cancer patients [abstract]. In: Proceedings of the 2023 San Antonio Breast Cancer Symposium; 2023 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2024;84(9 Suppl):Abstract nr PO3-14-12.