Anti-bacterial and in vivo tumor treatment by reactive oxygen species generated by magnetic nanoparticles

Photodynamic therapy is widely used in clinics and for anti-bacterial applications. The major challenge is the limited depth of tissue penetration of light and poor targetability. In this study, magnetite nanoparticles were used as a highly sensitive T₂-weighted MR imaging contrast agents to target the tumor and mimic horseradish peroxidase (HRP), which could catalyze the decomposition of hydrogen peroxide to generate reactive oxygen species (ROS) to inhibit the tumor in vivo. In these experiments, MNPs were demonstrated to possess the enzyme-mimicking activity in different pH values, and the activity was dependent on the size of the MNPs: the smaller the size, the higher the activity. We demonstrated that MNPs showed highly efficient anti-bacterial (E. coli) activity in presence of H₂O₂. The E. coli inhibition ratio reached nearly 100% at optimal concentration. The anti-tumor activity was evaluated through HeLa cell viability under treatment with MNPs and H₂O₂. Consequently, the cell viability was significantly decreased and more than 80% of HeLa cells were dead after treatment with MNPs and H₂O₂ under different pH values. MR imaging was used to demonstrate the tumor targetability of 13 nm MNPs in vitro and in vivo. Consequently, the relaxivity of the 13 nm MNPs was determined to be r₂ = 104 s^-1 mM^-1. The MR signal was much more negative and the intensity was significantly diminished with the increase of the concentration of 13 nm MNPs in vitro. The tumor signal was clearly visualized and a 3-fold decrease of the MR signal intensity of the tumor site of the mice was observed after 24 h-post treatment with the 13 nm MNPs. Finally, the tumor inhibition in vivo was investigated using 6 nm MNPs in BALB/c nude female mice bearing subcutaneously implanted HeLa cells on the right flank. The results show statistically significant efficacy in delaying tumor growth from day 6, and an approximately 99% tumor inhibition ratio was shown by the combination of MNPs and H₂O₂ after treatment for 17 days. By leveraging the passive targeting and MR imaging properties, we expect that the enzyme-mimicking MNPs could be used for cancer theranostics and may open up a new avenue for the treatment of epidermal infections.