By the end of 2020, there were more than 8 million women alive who had received a breast cancer diagnosis in the previous 5 years, making it the most prevalent neoplasia in the world. About 70% of breast-cancer cases present positivity for estrogen and/or progesterone receptors and a lack of HER-2 overexpression. Endocrine therapy has traditionally been the standard of care for ER-positive and HER-2-negative metastatic breast cancer. In the last 8 years, the advent of CDK4/6 inhibitors has shown that adding them to endocrine therapy doubles PFS. As a result, this combination has become the gold standard in this setting. Three CDK4/6 inhibitors have been approved by the EMA and the FDA: abemaciclib, palbociclib, and ribociclib. They all have the same indications, and it is at each physician’s discretion to choose one or the other. The aim of our study was to perform a comparative efficacy analysis of the three CDK4/6i using real-world data. We selected patients diagnosed with endocrine-receptor-positive and HER2-negative breast cancer who were treated with all three CDK4/6i as first-line therapy at a reference center. After 42 months of retrospective follow up, abemaciclib was associated with a significant benefit in terms of progression-free survival in endocrine-resistant patients and in the population without visceral involvement. In our real-world cohort, we found no other statistically significant differences among the three CDK4/6 inhibitors.

Introduction and objectives Estrogen receptor positive (ER+) tumors are the most common form of breast cancer and are responsible for most of the deaths from the disease. Treatment of ER+ breast cancer comprises interventions that suppress estrogen production and/or target the ER directly. While endocrine therapy has considerably reduced recurrence and mortality from breast cancer, de novo and acquired resistance to this treatment remains a major challenge. In this context, patient-derived organoids (PDOs), 3D structures composed of epithelial cells, are changing our understanding of cancer heterogeneity and its implications for personalized medicine. Our aim was to establish and characterize a PDO platform of ER+ breast cancer as preclinical tool to decipher and target this heterogeneity in order to tailor effective treatments based on the precise molecular makeup of the tumor. Material and Methods Surgical resections were collected from patients diagnosed with ER+ breast cancer. Organoid generation from patient material were divided in three main steps: 1) mechanical fragmentation of tissue pieces; 2) digestion of fragments into single-cell suspension; and 3) plating of cell preparations into matrigel domes to mimic the extracellular matrix. Organoids were passaged every 7–21 days based on confluency. Each newly organoid line was cryopreserved for further expansion. Results We have successfully achieved an outgrowth efficiency of almost 50% for PDOs from patients diagnosed with breast cancer. Characterization of PDO cultures is essential to validate its predictive potential. To asses that PDOs represent the tumor of the patient, for each organoid line, expression levels of the main histological markers (ER, PR, HER2, Ki67) were evaluated and correlated with the tissue of origin. In parallel, we have performed immunofluorescence on paraffin-embedded organoid samples with routinely used cancer stem cell markers. Potential contamination with normal cells (present in the resection used to establish the culture) should be considered. To address the tumor purity of the culture we have performed targeted DNA sequencing in both the original tissue and established PDOs. Conclusions PDOs represent a superior preclinical system compared to previous models due to their inherent heterogeneity, long-term stability, applicability for high-throughput screens and enhanced capacity to capture tumor characteristics. Therefore, the implementation of a well-annotated patient-derived organoid biobank will be of great interest for drug discovery and personalized therapy. Understanding the sources and implications of tumor heterogeneity will undoubtedly improve our evolving definition of cancer and aid in the design of effective patient-specific treatment strategies. Citation Format: Maria A Dominguez-Cejudo, Raquel Chapresto, Ana Gil-Torralvo, Francisco Javier Salvador Bofill, Sonia Molina-Pinelo. Establishment of an in vitro preclinical platform: A living biobank of patient-derived organoids with ER+ breast cancer [abstract]. In: Proceedings of the 2022 San Antonio Breast Cancer Symposium; 2022 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2023;83(5 Suppl):Abstract nr P2-26-17.