Abstract PO5-18-04: Effect of Digoxin on clusters of circulating tumor cells in patients with metastatic breast cancer (DICCT)

Abstract Background: The presence of circulating tumor cell (CTC) clusters is associated with tumor progression and with a bad prognosis in various cancer types1,2. In breast cancer, a pre-clinical model demonstrated that Na/K-ATPase inhibitors such as digoxin could dissolve CTC clusters and reduce metastasis3. DICCT is a single-arm, therapeutic exploratory phase I study aimed to examine whether digoxin can disrupt CTC clusters in metastatic breast cancer patients (NCT03928210). Material and methods: Metastatic breast cancer patients in whom clusters of CTCs have been detected were eligible. Patients with concomitant heart disease were excluded. After progressive disease in any line of therapy, a window of opportunity before initiation of subsequent systemic therapy was used to administer digoxin treatment for seven (7) days. For CTC detection, blood samples were processed with the FDA-cleared Parsortix system (ANGLE plc, UK). Captured CTCs were stained for EPCAM, HER2, EGFR (positive CTC selection) and CD45 (exclusion marker). To gain insight into the cluster-dissolution ability of digoxin, the primary endpoint was size of CTC clusters (i.e. number of cells within each cluster) under digoxin therapy. The cumulative effect of digoxin treatment on the primary endpoint was assessed by a linear regression model. Results: Nine (9) patients with CTC clusters detected at baseline were enrolled. A target level of digoxin above 0.7 ng/ml could be confirmed at day 7 for all patients. For quantitative and qualitative analysis of CTC clusters, 5.5 to 27 ml of blood were analyzed at baseline, at day 0 pre-treatment, and post 2 hours, 3 days and 7 days of digoxin treatment. The average cluster size ranged between 2 and 11.8 cells (median 2.9) considering all samples from all patients. Digoxin treatment reduced cluster size in 8 patients (88.9%) to different extents, and the proportion of CTC cluster over single CTCs was also reduced in 6 patients (66.7%). The effect of digoxin was evident in both homotypic (cancer cells only) and heterotypic (aggregates of CTCs and white blood cells) clusters. The treatment was well tolerated, and no adverse events related to the study treatment occurred. All patients completed the treatment according to the protocol. Conclusion: DICCT provides the first-in-human proof-of-principle that digoxin induces a partial dissolution of CTC clusters in patients with metastatic breast cancer at drug levels that are safe and well tolerated. Conceptualization of follow-up trials including Na/K-ATPase inhibitors and patient outcome endpoints are underway. References: 1. Ring A, Nguyen-Strauli BD, Wicki A, Aceto N. Biology, vulnerabilities and clinical applications of circulating tumour cells. Nat Rev Cancer 2022:1-17. DOI: 10.1038/s41568-022-00536-4. 2. Schuster E, Taftaf R, Reduzzi C, Albert MK, Romero-Calvo I, Liu H. Better together: circulating tumor cell clustering in metastatic cancer. Trends Cancer 2021;7(11):1020-1032. DOI: 10.1016/j.trecan.2021.07.001. 3. Gkountela S, Castro-Giner F, Szczerba BM, et al. Circulating Tumor Cell Clustering Shapes DNA Methylation to Enable Metastasis Seeding. Cell 2019;176(1-2):98-112 e14. DOI: 10.1016/j.cell.2018.11.046. Citation Format: Marcus Vetter, Bich Doan Ngyuen-Sträuli, Ilona Krol, Alexander Ring, Angela Kohler, Francesc Castro-Giner, Maren Vogel, Cvetka Grasic Kuhar, Fabienne Schwab, Viola Heinzelmann-Schwarz, Gabriela Kuster Pfister, Walter Weber, Christian Kurzeder, Nicola Aceto. Effect of Digoxin on clusters of circulating tumor cells in patients with metastatic breast cancer (DICCT) [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 PO5-18-04.