Abstract 5837: Tolvaptan as a potential anticancer and immunomodulatory treatment for triple-negative breast cancer

Abstract Background: Triple-negative breast cancer (TNBC) is an aggressive breast cancer (BC) subtype characterized by rapid progression, high risk of recurrence and metastasis. Although immunotherapy has improved outcomes for some patients, treatment resistance remains a major challenge, highlighting the need for novel therapeutic targets and drug repurposing strategies. Hyponatraemia, defined by a serum sodium concentration 135 mEq/L, is the most common electrolyte disorder in cancer, affecting over one-third of BC patients. Tolvaptan, a G protein-coupled (GPC) vasopressin type 2 receptor (AVPR2) antagonist, is clinically used to treat hyponatremia in various settings, including cancer. Beyond its clinical use, in vitro and in vivo studies indicate that tolvaptan inhibits cancer cell proliferation, reduces invasiveness, and promotes apoptosis in several tumor types. Interestingly, database analysis revealed that TNBC has the highest expression of AVPR2 among all BC subtypes. Further, there is evidence of tolvaptan modulating T cells, and it is known that other GPC receptors can regulate T cell antitumor activity, although the underlying mechanism of tolvaptan immunoregulation remains unclear. This study aimed to investigate the potential anticancer and immunomodulatory effects of tolvaptan in TNBC models. Experimental Procedures and Results: Tolvaptan treatment significantly reduced TNBC cell viability in MTT assays in murine (C0321) and human (MDA-MB-231, MDA-MB-436) TNBC cell lines from the lowest concentration of 1 nM, while non-tumorigenic MCF10A cells remained unaffected up to 10 µM. In contrast, treatment with the AVPR2 agonist desmopressin had no effect on the viability at any concentration used. In agreement, tumor-derived TNBC organoids treated with tolvaptan showed a significant increase in cell death and a higher number of infiltrating CD8+ T cells compared to those treated with DMSO. Consistently, primary cultures of splenic murine CD8+ T cells treated with tolvaptan exhibited increased expression of the activation marker CD69, as determined by flow cytometry. In an orthotopic syngeneic TNBC mouse model (C0321, FVB mice), tolvaptan treatment (10 mg/kg for 27 days) resulted in smaller tumors, whereas desmopressin (2 µg/kg) promoted tumor growth. Immunohistochemical analysis demonstrated increased cleaved caspase-3 expression and CD8+ T cell infiltration in tolvaptan-treated tumors. Flow cytometry analysis confirmed a higher number of tumor-infiltrating CD8+ T cells and an upregulation of Notch-1 and IFN-γ, markers of an effective antitumor immune response. Conclusions: Overall, these findings indicate that tolvaptan exerts both direct cytotoxic and immune antitumor effects in TNBC models. The cytotoxicity toward TNBC cells, together with enhanced CD8+ T cell responses, supports the potential use of tolvaptan as a novel therapeutic approach for TNBC. Citation Format: Laura Naldi, Zhi Huang, Brionna King, Kimberly McCarter, Kelsey Gardiner, Alessandro Peri, Lucio Miele, Giulia Monticone. Tolvaptan as a potential anticancer and immunomodulatory treatment for triple-negative breast cancer abstract. In: Proceedings of the American Association for Cancer Research Annual Meeting 2026; Part 1 (Regular Abstracts); 2026 Apr 17-22; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2026;86(7 Suppl):Abstract nr 5837.