Background <p>Breast cancer is a prevalent and complex disease with treatment limitations, including resistance, adverse side effects, and high recurrence rates. Tumor-associated macrophages (TAMs) are key players in breast cancer progression, with M2 TAMs promoting tumor growth and M1 TAMs inhibiting it. This study investigated the potential of polyaniline-coated iron oxide nanoparticles (Pani/γ-Fe₂O₃ NPs) to modulate the tumor microenvironment through macrophage reprogramming and broader immune effects in vitro and in vivo, offering a basis for innovative cancer immunotherapy approaches.</p> Results <p>In the in vitro study Pani/y-Fe₂O₃ demonstrated the ability to induce changes in the tumor microenvironment to an anti-tumoral profile leading to tumor cell apoptosis, enhance the production of pro-inflammatory cytokines (IL-12p70, TNF-α, and IL-6), and elevate hydrogen peroxide levels. In the 4T1 BALB/c model of metastatic breast cancer, Pani/γ-Fe₂O₃ treatment resulted in a 50% reduction in both tumor weight and volume. Tumor analysis showed a decrease in mitotic cell counts, an increase in natural killer (NK) cells and CD86⁺ macrophages, a reduction in neutrophil proportions, and suppression of MCP-1 cytokine production. Additionally, Pani/γ-Fe₂O₃ effectively inhibited 4T1 cell metastasis to the lungs.</p> Conclusions <p>This study highlights the potential of Pani/y-Fe₂O₃ as an immunostimulatory agent, capable of driving macrophage polarization and modulating the tumor microenvironment, inhibiting tumor progression and metastasis. These findings provide promising insights into the use of iron oxide nanoparticles as a novel tool for breast cancer immunotherapy.</p>