Fluoxetine suppresses triple-negative breast cancer progression by inducing apoptosis, inhibiting Src-mediated invasiveness, and enhancing anti-tumor immunity.

BACKGROUND AND PURPOSE: Triple-negative breast cancer (TNBC) is an aggressive subtype lacking ER, PR, and HER2 expression, with limited treatment options and poor prognosis. Recent evidence suggests the antidepressant fluoxetine may possess antitumor activity. This study evaluated the therapeutic potential and mechanisms of fluoxetine in TNBC. METHODS: MDA-MB-231 human and 4T1 murine TNBC cell lines were treated with fluoxetine. Cell viability, apoptosis, migration, invasion, and molecular pathway changes were assessed in vitro. A 4T1 orthotopic mouse model was used to evaluate in vivo antitumor efficacy, toxicity, and immune modulation. RESULTS: Fluoxetine reduced TNBC cell viability in a dose- and time-dependent manner, induced apoptosis via extrinsic (Fas/FasL, caspase-8) and intrinsic (mitochondrial membrane loss, caspase-9) pathways, and activated caspase-3. It inhibited migration and invasion by downregulating VEGF-A, MMP-2, MMP-9, and Cyclin D1, and suppressed the Src/JAK/NF-κB signaling axis and epithelial–mesenchymal transition markers. In vivo, fluoxetine significantly reduced tumor growth without systemic toxicity, enhanced cleaved caspase-3/8/9 expression, suppressed Src/JAK/NF-κB signaling, and increased cytotoxic T lymphocyte activity while reducing tumor-associated macrophages, myeloid-derived suppressor cells, and regulatory T cells. CONCLUSIONS AND IMPLICATIONS: Fluoxetine exerts potent antitumor effects against TNBC by inducing apoptosis, inhibiting metastatic signaling, and enhancing antitumor immunity. These findings support its potential as an adjunctive agent for use in parallel with existing TNBC therapies and warrant further clinical investigation.