Nanozymes are a class of nanomaterials with intrinsic enzyme-like properties. They can catalyze the substrates of natural enzymes under mild conditions and follow the same kinetics and catalytic mechanism as natural enzymes. Moreover, nanozymes can serve as an enzyme substitute for the detection of diseases. Recently, researchers have discovered that the nanozymes with oxidoreductase activities can regulate reactive oxygen species in cells, for example, the peroxidase nanozymes can catalyze the hydrogen peroxide (H₂O₂) in the tumor microenvironment to produce hydroxyl radical to induce tumor cell apoptosis. However, due to the low level of H₂O₂ in the tumor microenvironment and the generally low affinity of nanozymes with H₂O₂, the insufficient amount of hydroxyl radical limited the tumor therapeutic effects. How to improve the affinity of nanozymes to substrate H₂O₂ and how to increase the concentration of H₂O₂ in tumor cells are two key questions that need to be addressed for the application of nanozymes in tumor therapy.

In 12^th March 2021, ACS Nano published online the latest research progress of the research team of Prof. YAN Xiyun in Institute of Biophysics, Chinese Academy of Sciences/CAS Engineering Laboratory for Nanozyme. To overcome the aforementioned challenges, the research team have designed a novel pyrite (FeS₂) nanozyme. They found that the pyrite nanozyme possessed ultra-high H₂O₂ affinity, resulting in a 4144- and 3086-fold increase of catalytic activity (k_cat/K_M) compared with that of classical Fe₃O₄ nanozyme and natural horseradish peroxidase, respectively. According to the first-principles theory calculation results, the coordinate covalent bonds formed by H₂O₂ and FeS₂ nanozymes were shorter than that of traditional Fe₃O₄ nanozymes. Moreover, the surface of FeS₂ nanozymes has the ravine-like structure, which increases the overlap of the electron cloud density between the H₂O₂molecule and the material surface. Thus, the absorption energy of H₂O₂ on the surface of FeS₂ nanozymes was stronger than that of Fe₃O₄ nanozymes, resulting in an efficient catalysis of limited H₂O₂ at tumor sites by pyrite nanozymes to produce abundant oOH and induce tumor cell apoptosis.

More interestingly, pyrite nanozymes not only exhibited ultra-high H₂O₂ affinity, but also increased the amount of H₂O₂ by themselves. This is because pyrite nanozymes possessed glutathione oxidase-like activity, which can oxidize glutathione (GSH) to oxidized glutathione and H₂O₂. The produced H₂O₂ can also serve as substrate of peroxidase. Therefore, pyrite nanozymes with dual-enzyme activity of glutathione oxidase and peroxidase functioned as a self-cascade platform to continuously produce hydroxyl radicals causing unceasing tumor cell apoptosis. Moreover, the glutathione oxidase activity of pyrite nanozymes also caused the depletion of GSH, which is also a cofactor of glutathione peroxidase 4, leading to the inactivation of the latter and hindering the removal of lipid hydroperoxides in cells to induce cell ferroptosis.

The ability of pyrite nanozymes to simultaneously induce apoptosis and ferroptosis in tumor cells endows them with efficient tumor-killing capability even on the apoptosis-resistant tumor cells harboring the KRAS mutation. Furthermore, the biosafety analyses of pyrite nanozymes showed that they exhibited tumor-specific cytotoxicity, which attributed to the fact that tumor cells are metabolically more active than normal cells and produce more H₂O₂, enabling nanozymes to catalyze the production of more hydroxyl radicals. Besides, tumor cells require more GSH to maintain redox balance, causing them to be more vulnerable to GSH depletion. Meanwhile, the rapid growth of tumor cells demands more iron ions, rendering them more sensitive to iron-regulated ferroptosis. In addition, pyrite nanozymes showed favorable biodegradability. The tumor-specific cytotoxicity and biodegradability ensured desirable safety of pyrite nanozymes for the in vivo application.

In summary, the high-performance pyrite nanozymes overcame the low affinity of traditional peroxidase nanozymes to H₂O₂ and efficiently catalyzed the limited H₂O₂ in the tumor microenvironment to produce sufficient cytotoxic hydroxyl radical for tumor treatment. Furthermore, the newly discovered glutathione oxidase activity of pyrite nanozymes provided a new idea for the nanozyme-based tumor therapy. It not only provides H₂O₂ for peroxidase-like activity, but also catalyzes the oxidation of glutathione to promote the ferroptosis of tumor cells, making it efficient even in apoptosis-resistant tumor cells. Importantly, pyrite nanozymes can achieve efficient and safe tumor therapy without the combination of other therapeutic interventions. The effectiveness of pyrite nanozyme as a standalone therapeutic indicates its great potential for future clinical translation.

This work was completed by the cooperation of Institute of Biophysics of Chinese Academy of Sciences, Yangzhou University and Shenzhen Second People's Hospital. Professor NIE Guohui at Shenzhen Second People's Hospital, academician YAN Xiyun at Institute of Biophysics of Chinese Academy of Sciences, Professor GAO Lizeng and FAN Kelong are the corresponding authors of this paper. MENG Xiangqin and Dr. LI Dandan are first authors of this paper. This research was financially supported by the Key Research Program of Frontier Sciences, CAS; the National Natural Science Foundation of China; CAS Interdisciplinary Innovation Team; Youth Innovation Promotion Association of Chinese Academy of Sciences; et al.

Scheme. High-performance self-cascade pyrite nanozymes for apoptosis-ferroptosis synergistic tumor therapy

(Image by Dr. FAN Kelong's group)

The article is available at https://pubs.acs.org/doi/10.1021/acsnano.1c01248

Contact: FAN Kelong

Institute of Biophysics, Chinese Academy of Sciences

Beijing 100101, China

Email: fankelong@ibp.ac.cn

(Reported by Dr. FAN Kelong's group)