Monensin Inhibits Triple-Negative Breast Cancer in Mice by a Na⁺-Dependent Cytotoxic Action Unrelated to Cytostatic Effects

Triple-negative breast cancer (TNBC) represents the most aggressive breast cancer subtype, defined by its limited therapeutic options and poor outcomes. This study investigated the therapeutic potential of targeting Na⁺ homeostasis in TNBC cells to induce TNBC inhibition. For this purpose, BALB/c mice were inoculated with 4T1-Luc2 breast cancer cells and treated with the Na⁺ ionophore monensin (8 mg/kg) or vehicle alone. Tumor development and cellular Na⁺ content were assessed using vivo live imaging techniques, while intracellular Na⁺ variations and cytotoxicity were evaluated through live cell analysis. Monensin treatment increased Na⁺ levels in cancerous tissues and reduced TNBC mass (monensin: 0.146 ± 0.06; vehicle: 0.468 ± 0.2 cm³; p < 0.001). This treatment induced extensive necrosis in TNBC tumors while preserving the structural and functional integrity of healthy organs and maintaining the proliferative activity of both tumor and normal tissues. Monensin did not alter the expression of proliferating nuclear antigen (PCNA) in 4T1-Luc2 cells but triggered cytotoxicity preceded by intracellular Na⁺ accumulation. Na⁺-free conditions prevented both Na⁺ accumulation and 4T1-Luc2 cell death. Thus, monensin exerts its antitumor effects in TNBC through a Na⁺-dependent and tumor-specific cytotoxic mechanism, without inducing cytostatic effects on normal or transformed tissues. Collectively, these findings underscore the potential of Na⁺ ionophores as promising therapeutic agents for TNBC.