Breast cancer (BC) is the most common malignancy with 1.7 million new diagnoses/year and is responsible for more than 450,000 yearly deaths worldwide. Two thirds of BC express the estrogen receptor (ER) and/or progesterone receptor and are referred to as hormone receptor-positive (HR+) BC. Endocrine therapy (ET) is usually active in these tumors, although drug resistance and side effects limit its benefit. Growth factor signaling through the PI3K/AKT/mammalian target of rapamycin (mTOR) and MAP kinase axes enhances ER activity and is a key mechanism underlying endocrine resistance. Water-only fasting (fasting) or plant-based, low-calorie, carbohydrate- and protein-restricted fasting-mimicking diets (FMDs) reduce circulating growth factors, such as insulin and IGF1 Therefore, we hypothesized that these dietary interventions could be used to enhance the activity of ET and delay the occurrence of resistance. For our in vitro experiments we used the HR+ BC cell lines, MCF7, T47D, and ZR-75-1, as well as metastases-derived organoids from patients with HR+ BC. Our in vivo experiments in mouse xenografts of human BC cell lines, were conducted in six-to-eight-week old female NOD SCID or athymic Nude-FoxN1 mice treated with ET w/ or w/o 48-72 hours of fasting/FMD. We monitored tumor growth and mouse survival and collected tumor masses and blood to detect circulating levels of several growth factors, adipokines and cytokines. In vivo add back experiments with fasting-reduced factors were done with IGF1, insulin and leptin. Circulating growth factors and adipo-cytokines were also detected in blood samples from 36 patients with HR+ BC, who were enrolled in either one of two clinical trials (NCT03595540 and NCT03340935) assessing safety and feasibility of periodic FMD in cancer patients. Patient nutritional status and response to treatment were also monitored in our clinical trials.We found that in HR+ BC models, periodic fasting or FMD enhanced tamoxifen and fulvestrant activity by lowering circulating IGF1, insulin, and leptin levels and by blocking AKT-mTOR signaling via EGR1 and PTEN upregulation. When fulvestrant was combined with palbociclib (a cyclin-dependent kinase 4/6 inhibitor), adding periodic FMD cycles promoted long-lasting tumour regressions and reverted acquired resistance to this regime. Moreover, both fasting and FMD prevented tamoxifen-induced endometrial hyperplasia. In HR+ BC patients receiving ET, FMD cycles caused metabolic changes analogous to those observed in mice, including reduced leptin and IGF1 levels, which were found to remain low for extended periods. In mice, these long-lasting effects were associated with carryover anticancer activity. Overall, our results provide the rationale for conducting further clinical studies of fasting-based dietary strategies as an adjuvant to ET w/ or w/o CDK4/6 inhibitors in patients with HR+ BC.

Citation Format: Irene Caffa, Vanessa Spagnolo, Pamela Becherini, Francesca Valdemarin, Claudio Vernieri, Min Wei, Sebastian Brandhorst, Chiara Zucal, Else Driehuis, Lorenzo Ferrando, Luca Mastracci, Michele Cilli, Francesco Piacente, Anna Laura Cremonini, Mario Passalacqua, Valerio Vellone, Gabriele Zoppoli, Michele Cea, Giulia Salvadori, Salvatore Cortellino, Hans Clevers, Filippo De Braud, Alessandro Provenzani, Valter D. Longo, Alessio Nencioni. Fasting-mimicking diet and hormone therapy modulates metabolic factors to promote breast cancer regression and reduce side effects [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr CT075.