Construction Of Iron/Cerium-based Oxide Nanozyme And Its Application In Tumor Diagnosis And Therapy

With increasing global morbidity and mortality,cancer still poses a serious threat to human life and health.Since conventional treatment modalities（e.g.chemotherapy,radiotherapy and surgical resection）have difficulty in achieving specific effects on malignant cells,these treatment modalities inevitably cause irreversible damage to normal cells and tissues in the body,resulting in poor patient prognosis.with the cross-vergence of nanomedicine and nanocatalysis,the utilization of nanozymes to trigger enzyme-catalyzed reactions under the unique physiological characteristics of the tumor microenvironment to achieve catalytic therapy with substrate specificity and low toxic side effects has become a novel strategy for tumor treatment.However,due to the complexity of the tumor microenvironment,the catalytic activity of nanozymes is often compromised under physiological conditions to cause incomplete tumor growth inhibition,which motivates researchers to amplify the tumor inhibition effect by modulating the tumor microenvironment and developing multi-modal synergistic therapies.In this dissertation,aiming at the special microenvironment of malignant tumors,we have explored novel efficient and tumor-specific nanocatalytic strategies for the diagnosis and treatment of malignant tumors based on the design and activity regulation of iron and cerium-based oxide nanozymes,and our main research is divided into four parts as follows:Aiming at the problem of insufficient intratumor H₂O₂ concentration and low enzymatic catalytic activity under physiological conditions,a magnetic cascade-catalytic platform was constructed by co-loading ultra-smallγ-Fe₂O₃ nanoparticles and glucose oxidase into dendritic mesoporous silica（DMSN）.The biocatalytic reaction of catalytic conversion of endogenous glucose to H₂O₂ using GOx elevated the intratumoral H₂O₂ level in situ and further acidified the tumor microenvironment.In a mildly acidic environment,γ-Fe₂O₃ nanoparticles could exert their peroxidase-like（POD-like）activity to catalyze the production of large amounts of strongly oxidative hydroxyl radicals from H₂O₂.Secondly,local heating generated from the photothermal properties ofγ-Fe₂O₃ under near-infrared light could significantly enhance the POD-like activity.Steady-state kinetic studies showed that the maximum reaction rate of its POD-like activity increased by 1.36-fold under heating conditions.Synergistic photothermal,starvation,and near-infrared light-enhanced chemodynamic therapy was validated at the cellular and animal levels to effectively inhibit tumor growth.Aiming at the problem of single enzymatic activity of Fe-based oxides in weakly acidic environment and combining with the properties that photothermal can improve the catalytic activity of nanozymes under physiological conditions,we developed a bacterial-like nanozyme（PEG/Ce-Bi@DMSN）with multi-enzymatic activity by coating DMSN onto Bi₂S₃ nanorods（Bi₂S₃@DMSN）in an oil-water biphase reaction system and then loading the ultrasmall Ce O₂nanozyme into the dendritic pore channel of Bi₂S₃@DMSN.The nanozyme exhibited dual enzyme-like catalytic activities including POD-like and catalase-like（CAT-like）activities under weakly acidic conditions.In the tumor microenvironment,this nanozyme could generate large amounts of reactive oxygen species through POD-like activity and meanwhile alleviate tumor hypoxia via CAT-like enzyme activity for sensitizing the oxidative DNA damage induced by reactive oxygen species.In addition,this nanozyme could effectively deplete the overexpressed glutathione in tumor cells,thus reducing the antioxidant capacity of tumor cells.Owing to the considerable absorption of this nanozyme in the second near-infrared light window,we further introduced photothermal to synergistically improve its dual-enzymatic catalytic activity and GSH depletion capacity.The anti-tumor experiments at the cellular and animal levels demonstrated a strategy of synergistic photothermal and reactive oxygen species-mediated nanocatalytic therapy significantly improved tumor suppression.Considering that the complex multienzyme activity of Ce O₂ nanozymes would inevitably lead to off-target depletion of POD-like substrate molecules and the lack of research on the catalytic mechanism of enzyme-like enzymes at the atomic/electronic level.Combining the unique microenvironmental characteristics of tumors,we constructed a hollow mesoporous Mn/Zr co-doped Ce O₂ cascade nanozyme（PHMZCO-AT）with regulable multienzyme activity,i.e.,enhancing both superoxide dismutase-like（SOD-like）and POD-like activities and inhibiting CAT-like activity.With the assistance of the endogenous CAT inhibitor 3-amino-1,2,4-triazole,PHMZCO-AT could significantly disrupt intracellular H₂O₂ homeostasis in tumor cells by promoting H₂O₂ production and inhibiting the off-target depletion of H₂O₂ to achieve enhanced chemodynamic therapy.The PHMZCO-AT nanozyme with SOD-like activity could catalyze the conversion of endogenous superoxide anion to H₂O₂ in the tumor region.The generated H₂O₂ could further be specifically catalyzed by the PHMZCO-AT nanozyme with POD-like activity to generate a large number of hydroxyl radicals,thereby inducing apoptosis in tumor cells.Under the designed treatment regimen,this nanozyme exhibited excellent tumor suppression（up to 81.9%inhibition）against 4T1 xenograft tumors.To overcome the heat resistance of tumor cells and non-specific heating in the aforementioned synergistic photothermal-nanocatalytic therapy process,a mitochondria-targeted,defect-engineered AFCT nanozyme constructed by loading 2,2′-azino-bis（3-ethylbenzothiazoline-6-sulfonic acid）（ABTS）into（4-carboxybutyl）triphenylphosphonium bromide modified hollow mesoporous Fe-Ce O_v nanozyme.Density functional theory calculations revealed the synergistic mechanism between multiple enzymatic active sites in the AFCT nanozyme.In the tumor microenvironment,the AFCT nanozyme could achieve an open-source supply of H₂O₂ through SOD-like activity.In response to the dual stimuli of H₂O₂ and mild acidity,the POD-like activity of AFCT nanozyme could not only catalyze the generation of large amounts of hydroxyl radicals from accumulated H₂O₂ but also convert the loaded ABTS into the oxidized form with strong near-infrared light absorption（ABTS·⁺）,thus specifically activating its photothermal and photoacoustic imaging properties.In addition,the electron transport chain function of mitochondria in tumor cells was severely disturbed due to the nicotinamide adenine dinucleotide peroxidase（NADH-like POD）activity-mediated NADH depletion of the AFCT nanozymes,thereby limiting ATP synthesis.The loss of the energy supply chain resulted in a significant reduction in the expression of heat shock proteins in the photothermal treatment and a considerable decrease in heat resistance of tumor cells to achieve enhanced mild photothermal therapy.Meanwhile,the accumulated hydroxyl radicals could facilitate both apoptosis and ferroptosis in tumor cells,resulting in synergistic therapeutic outcomes in combination with TME-activated mild PTT,with tumor suppression rate of 97.5%in mouse breast cancer model.