INTRODUCTION AND OBJECTIVES: Breast cancer has the highest rate of cancer mortality among women worldwide. Triple negative breast cancer exhibits greater resistance to chemotherapy and poor clinical outcomes. To obtain better results for this subgroup, it is necessary to optimize the cytotoxic effects of chemotherapeutics bypassing mechanisms of chemoresistance. In this context, cerium oxide (CeO2-x) and iron-cerium oxide (FexCe1-xO2-β) nanoparticles have been investigated for their cytotoxic and antitumor activity. It is known that CeO2-x nanoparticles have multi-enzyme mimetic activities - such as SOD – and, in cancer, CeO2-x can induce oxidative stress and dephosphorylation of substrates, which can induce autophagy or apoptosis. By adding iron to the structure, FexCe1-xO2-β should obtain an advantage over CeO2-x due to the increase in structural defects, increasing its ability to act on intracellular oxidative stress. However, little is known on their antineoplastic effects on breast cancer cells. The objective of this study is to synthesize the nanoparticles through a novel type of synthesis, colloidal stability, and evaluate them as direct agents for inducing cytotoxicity in triple negative breast cancer cells. MATERIAL AND METHODS: CeO2-x and FexCe1-xO2-β were synthesized by colloidal stability chemical synthesis and characterized via dynamic light scattering (DLS), scanning electron microscopy (SEM) and UV-Visible spectrometry (UV-Vis). MTT and clonogenic assays for MDA-MB-231 triple-negative cell line were used to assess nanoparticle-induced cytotoxicity. The nanoparticle concentrations ranged between 25-500 µg.mL-1. RESULTS AND CONCLUSION: Our results from UV-Vis, SEM and DLS revealed the synthesis of particles with nanometric size and spherical shape. For CeO2-x, the MTT assay showed more than 80% decrease in cell viability in MDA-MB-231 cells for the concentration of 500 µg.mL-1. On the other hand, CeO2-x concentrations ranging from 25-200 µg.mL-1 induced no changes in cell viability. Interestingly, FexCe1-xO2-β showed a dose-dependent decrease in cell viability at concentrations between 25-200 µg.mL-1. However, at the concentration of 500 µg.mL-1, there was a prominent 60% inhibition of cell viability. For the clonogenic assay, treatment with CeO2-x showed an increase in colony formation at lower concentrations but a decrease of around 80% at the concentration of 500 µg.mL-1. FexCe1-xO2-β also showed an increase in colony formation at lower concentrations, but less prominent than CeO2-x. Notably, 500 µg.mL-1 FexCe1-xO2-β supressed more than 85% of clonogenicity. These data might suggest that both CeO2-x and FexCe1-xO2-β confer greater cytotoxic activity in high concentrations for this cell model. Ongoing experiments will assess cytotoxicity in lymphocytes (non-neoplastic cells) at the same range of concentrations and further explore potential combinations between CeO2-x and FexCe1-xO2-β nanoparticles and chemotherapeutic agents.