Functionalized nanozyme with drug loading for enhanced tumour combination treatment of catalytic therapy and chemotherapy†

Qian Song‡ ^a, Bin Chi‡ ^b, Haiqing Gao‡ ^a, Junke Wang ^a, Miaomiao Wu ^c, Yi Xu ^a, Yingxi Wang ^a, Zushun Xu ^a, Ling Li *^a, Jing Wang *^b and Run Zhang *^c
aMinistry-of-Education Key Laboratory for the Synthesis and Application of Organic Function Molecules, Hubei Collaborative Innovation Center for Advanced Organic Chemical Materials, Hubei University, Wuhan 430062, China. E-mail: lingli@hubu.edu.cn
^bDepartment of Radiology, Union Hospital, Tongji Medical College, Huazhong University of Science and Technology, Wuhan 430022, China. E-mail: jjwinflower@126.com
^cAustralian Institute for Bioengineering and Nanotechnology (AIBN), The University of Queensland, St Lucia, QLD 4072, Australia. E-mail: r.zhang@uq.edu.au

First published on 12th June 2023

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Introduction

Chemotherapy has been one of the most common cancer therapy procedures, but long-term administration of anti-cancer drugs inevitably causes drug resistance, thus negatively impacting the treatment efficacy.¹ To improve anti-cancer efficacy, modulation of the tumour microenvironment (TME) has recently emerged as an alternative treatment procedure.^2,3 Compared with normal physiological conditions, the TME is characterized by mild acidic conditions, a hypoxia microenvironment, and overexpressed glutathione (GSH) and hydrogen peroxide (H₂O₂).^2,4 As the internal environment of tumour origin and proliferation, the TME provides various new therapeutic targets, such as pH,^5,6 H₂O₂⁷ and GSH targets.^8,9 With the development of nanocatalytic medicine, great efforts are being made to develop TME responsive nanozyme-based tumour catalytic therapy.^2,10 Many TME responsive nanozyme-based medicines have been reported for tumour catalytic therapy.¹⁰ For example, TGF-b inhibitor (TI)-loaded PEGylated iron manganese silicate nanoparticles (IMSN) are used for TME-responsive tumour catalytic therapy.¹¹ Another TME-modulated nanozyme employing tin ferrite (SnFe₂O₄) is presented for tumour catalytic therapy.¹²

Due to tuneable catalytic activity, high stability, and easy preparation, metal organic frameworks (MOFs) with intrinsic enzyme-like characteristics are emerging as a promising platform in the biomedical field.^13,14 For example, iron-based MOFs have been widely studied as Fenton-like catalysts, in which iron can catalyse H₂O₂ in tumours to generate hydroxyl radicals (˙OH), showing a peroxidase-like (POD-like) performance.^15,16 However, the suitable pH for the Fenton reaction is pH 2–4,¹⁷ and the weak acidity (pH 5.8) of the TME cannot meet the catalytic requirements of iron-based MOF nanozymes, thus negatively impacting the effect of catalytic therapy.¹⁸ Therefore, the development of MOFs with POD-like properties at pH 5.8 is highly demanded to improve the efficiency of catalytic therapy.¹⁹ Recent studies have revealed that Zr-based MOFs having good stability, a large surface area, and a functional structure can be used for pH-responsive drug delivery.^20–22 However, the catalytic reaction of Zr-based MOFs is limited due to the low activity of Zr³⁺.^22,23 Most of the nanozymes with POD-like activity are found for the metal ions with variable valence because the interconversion between different valences could promote the generation of hydroxyl radicals.^24–26 Therefore, Ce³⁺ is introduced into Zr-based MOFs to form bimetal-active MOF nanozymes with pH-responsive release and POD-like performance,²⁷ which is expected to be applied in tumour catalytic therapy.

It is well known that the concentration of GSH in tumour is 4–10 times higher than that in normal cells.⁴ The GSH is able to capture the ˙OH, resulting in a poor catalytic performance of nanozymes.^28–30 Therefore, to achieve high efficacy of the catalytic treatment procedure, it is necessary to regulate the concentration of GSH in tumour cells.^31–34 Polydopamine (PDA) was proved to be suitable to respond the high GSH concentration based tumour environment.³⁵ The oxidation reaction occurs during the polymerization of dopamine, and reduced GSH can inhibit the oxidation process and trigger the degradation of polydopamine.³⁶ Variable valence metal ions with high oxidation states can also react with glutathione. For example, HKUST-1 is a Cu-based MOF that can respond to GSH, i.e. Cu²⁺-induced GSH depletion unlocked by acid dramatically enhances ˙OH generation.³⁷ Although these GSH depletion methods inhibited the scavenging of ˙OH and improve the catalytic efficiency, it remains challenging to track the therapeutic effect of tumours in real time. Compared with the above GSH regulation methods, MnO₂ coating showed unique advantages.³⁸ MnO₂ can oxidize GSH to produce glutathione oxidized (GSSG) and Mn²⁺ for an enzyme-like catalytic reaction, forming a self-enhanced catalytic reaction.^39,40 In addition, Mn²⁺ is also a T₁-MRI imaging contrast agent.^41,42 Compared with MnO₂ nanosheets, the reduced Mn²⁺ could enhance 48 and 120 times at the longitudinal relaxation rate r₁ and transverse relaxation rate r₂, respectively.⁴³ Therefore, in a weak acidic tumour environment, MnO₂ could contribute to not only enhancing T₁-weighted MRI, but also achieving on-demand drug release. The stimuli-responsive nanoparticles can thus provide more accurate diagnostic information, greatly improve the therapeutic effect and effectively reduce side effects.²

Inspired by the above Zr-MOF and MnO₂ studies, cerium-doped zirconium-based MOFs (Zr/Ce-MOFs) were designed and synthesized in this work to load anticancer drug doxorubicin (DOX) with following MnO₂ surface coating. The as-prepared Zr/Ce-MOFs/DOX/MnO₂ could be used as one kind of nanozyme for combination chemotherapy and catalytic treatment of tumours (Scheme 1). The DOX drug release of Zr/Ce-MOFs/DOX/MnO₂ nanozymes with pH/GSH dual response in the TME increases the local drug concentration to promote the chemotherapy. Zr/Ce-MOFs/DOX/MnO₂ nanozymes are able to produce ˙OH for cancer cell killing, and the consumption of GSH in the TME through the reaction with MnO₂ further enhances the catalytic treatment through promotion of ˙OH production. The combination treatment of chemotherapy and catalytic treatment with the guidance of imaging is then demonstrated in in vitro and in vivo cancer models.

Experimental section

Chemicals and reagents

All reagents and solvents are of analytical reagent grade and have been used without any further purification. Zirconium(IV) chloride (ZrCl₄, 98%), reduced L-glutathione (GSH, 98%) and Potassium permanganate (KMnO₄) were purchased from Shanghai Macklin Biochemical Co, Ltd. Biphenyl-4,4′-dicarboxylic acid (H₂BPDC, 98%) and doxorubicin hydrochloride (DOX, 98%) were purchased from Shanghai Aladdin Reagent Co., Ltd. Cerium(III) nitrate hexahydrate (Ce(NO₃)₃·6H₂O, 99.99%) was purchased from Alab (Shanghai) Chemical Technology Co., Ltd. Methylene blue (MB) was purchased from Sinopharm Chemical Reagent Co, Ltd. Acetic acid (HAc, 99.5%) and N,N-dimethylformamide (DMF, 99.5%) were obtained from Beijing Chemical Works. Ethanol and double distilled water were used throughout the experiment.

Synthesis method of Zr/Ce-MOFs/DOX/MnO₂

Instruments

The power X-ray diffraction (XRD) measurements were performed using a D8 Advance X-ray diffractometer (Bruker Company, USA). The UV-Vis spectra were recorded on a spectrophotometer (Shanghai INESA Instrument Co., Ltd). FT-IR spectra were recorded by using a single frequency infrared spectrophotometer (PerkinElmer, USA). Scanning electron microscopy (SEM) images were captured on a JSM6510LV scanning electron microscope (JEOL, Japan). The dynamic light scattering (DLS) was measured by Nanoseries (Malvern, UK).

Drug release

To study the pH and GSH response release of Zr/Ce-MOFs/DOX/MnO₂. Zr/Ce-MOFs/DOX/MnO₂ were placed in dialysis bag (Cat NO: MD10, MWCO: 14000 D, nominal flat width: 10 mm) and put it in 20 mL buffer solutions with different pH value and GSH concentration, respectively. At different time points, the absorbance of DOX (λ_abs = 480 nm) in buffer solution was measured on a UV-Vis spectrometer and the concentration of the released DOX was calculated.

Catalytic performance

To study the catalytic performance of the synthesized materials, MB was selected as an indicator of ˙OH production. Zr-MOFs were added into the buffer solution containing 10 mM H₂O₂, 10 μg mL⁻¹ MB and 25 mM NaHCO₃/5% CO₂. After incubation at room temperature for 30 min, the absorbance of MB (λ_abs = 665 nm) in buffer solution was measured by UV-Vis spectrometer. The degradation efficiency of MB was calculated based on the absorbance of MB in buffer solution. To compare the catalytic performance, above experiments were repeated using Ce-MOFs, Zr/Ce-MOFs, Zr/Ce-MOFs/DOX and Zr/Ce-MOFs/DOX/MnO₂ as the nanozymes. Then, the effect of GSH and TME on the catalytic reaction of Zr/Ce-MOFs/DOX/MnO₂ was investigated. All data were presented as mean ± sd (n = 3).

In vitro cellular toxicity

The 4T1 breast cancer cells (1 × 10³ cells per well) were seeded into 96-well plates. After 12–24 hours of incubation, the culture medium was replaced with fresh medium containing 25, 50, 100, 150, 200 μg mL⁻¹ of nanoparticles, including Zr/Ce-MOFs/DOX, Zr/Ce-MOFs/DOX and Zr/Ce-MOFs/DOX/MnO₂. After 24 hours incubation, the cell culture medium was removed, and the cells were washed with PBS three times. Then, 100 μL of 10% CCK-8 solution was added to each well, and further incubation at 37 °C for 4 h was carried out. Finally, the absorbance of each hole at 490 nm was measured via ELISA, and the cell viability was calculated.

In vivo therapy

The 4T1-tumour bearing mice were obtained by subcutaneous injection of 4T1 breast cancer cells. The tumour-bearing mice were randomly divided into 4 groups when the tumour volume reached about 150 mm.³ Group I was injected with PBS, Group II was injected with Zr/Ce-MOFs, Group III was injected with Zr/Ce-MOFs/DOX, Group VI was injected with Zr/Ce-MOFs/DOX/MnO₂. The changes in tumour volume and body weight of mice after treatment were recorded.

Animal experiments

To establish the in vivo tumour model, female mice (body weight 18–20 g) were subcutaneously injected with 4T1 cells (100 μL, 1 × 10⁷ cells per mL). After the tumour volume reached 100 mm³ (∼7 days post injection of 4T1 cells), 100 μL of Zr/Ce-MOFs/DOX/MnO₂ solution was intravenously injected into the mice through the tail vein. T₁-weighted MRI by a 3T MRI scanner (Siemens Magnetom Trio 3.0 T) was performed sequentially at 0, 1, 2, 3, and 4 h after intravenous injection. For evaluating the tumour treatment, the mice were randomly divided into four groups (five mice per group), including Group I: PBS, Group II: Zr/Ce-MOFs, Group III: Zr/Ce-MOFs/DOX, and Group IV: Zr/Ce-MOFs/DOX/MnO₂. The body weight and tumour volume (volume = length × width² × 0.5) of all mice were recorded every 2 days.

Results and discussion

Characterization of Zr/Ce-MOFs/DOX/MnO₂

The structures of Zr-MOFs, Ce-MOFs, Zr/Ce-MOFs, Zr/Ce-MOFs/DOX and Zr/Ce-MOFs/DOX/MnO₂ were confirmed by XRD analysis (Fig. 1). The characteristic diffraction peaks of Zr-MOFs appear at the positions of 2θ = 5.6°, 6.5°, 9°, 11.2°, 19.2°, 20.3°, 25–50°, and the diffraction peaks of Ce-MOFs appear at the positions of 2θ = 6°, 12.3°, 17.9°. Compared with Zr-MOFs, shifts of the diffraction peaks of Zr/Ce-MOFs at 2θ = 6.5°, 9°, 11.2°, 19.2° were observed, suggesting the increase of lattice constant. The shifts of diffraction peaks could be assigned to the fact that the ionic radius of Ce³⁺ is larger than that of Zr⁴⁺, confirming the successful preparation of Zr/Ce-MOFs. The crystal structure of Zr/Ce-MOFs/DOX changed in comparison with Zr/Ce-MOFs, suggesting successful DOX loading. Zr/Ce-MOFs/DOX/MnO₂ presented an amorphous structure. The diffraction peaks of Zr/Ce-MOFs/DOX at 2θ = 40–50° disappeared, confirming the successful synthesis of Zr/Ce-MOFs/DOX/MnO₂.

FT-IR of the H₂BPDC ligand, Zr-MOFs, Ce-MOFs, Zr/Ce-MOFs and corresponding drug loaded materials are shown in Fig. 2. The broad adsorption band of the H₂BPDC at 1643–1500 and 1394–1387 cm⁻¹ were attributed to the stretching and bending vibrations of the carboxyl group. The adsorption peaks at 768 cm⁻¹ of Zr-MOFs were the transverse vibration of the Zr–O bond. With the Ce³⁺ doping in MOFs, a small adsorption peak appeared at 1320 cm⁻¹, which was attributed to Ce-O stretching and Ce–O–C vibration. The data indicate the successful preparation of Zr/Ce-MOFs. The absorption peak belonging to DOX appeared at 2385 cm⁻¹ after Zr/Ce-MOFs were loaded with DOX. Compared with Zr/Ce-MOFs, the characteristic peaks of Zr/Ce-MOFs/DOX in the range of 1600–1490 cm⁻¹ were red shifted, which could be attributed to the hydrogen bonding between Zr/Ce-MOFs and DOX. After MnO₂ modification on the surface of Zr/Ce-MOFs/DOX, the adsorption bands in the range of 1000–500 cm⁻¹ changed in comparison with that of Zr/Ce-MOFs/DOX. The small peak at 530 cm⁻¹ was attributed to the Mn–O bond.

The morphology of Zr/Ce-MOFs, Zr/Ce-MOFs/DOX and Zr/Ce-MOFs/DOX/MnO₂ were observed by SEM images. As shown in Fig. 3a1 and a2, the morphology of Zr/Ce-MOFs is octahedral, and the size is about 100 nm. Fig. 3b1 and b2 show that the size increased to about 120 nm after DOX loading. As shown in Fig. 3c1 and c2, the size of Zr/Ce-MOFs/DOX/MnO₂ was further increased to about 280 nm after MnO₂ coating. The TEM images of Zr/Ce-MOFs, Zr/Ce-MOFs/DOX and Zr/Ce-MOFs/DOX/MnO₂ are shown in Fig. S1 (ESI†), and the size of the particles was consistent with the results of SEM. DLS measurements were employed to further evaluate the particle size (Fig. S2(a), ESI†). The particle size of Zr/Ce-MOFs, Zr/Ce-MOFs/DOX and Zr/Ce-MOFs/DOX/MnO₂ was about 105, 165 and 255 nm, respectively. The DLS hydrodynamic size distribution was then measured in PBS buffer with different pH (5.8 and 7.4) values. As shown in Fig. S3 (ESI†), there are no remarkable changes of the size distribution of Zr/Ce-MOFs/DOX/MnO₂, implying that the the Zr/Ce-MOFs/DOX/MnO₂ nanozyme would be stable at the mimic physiological environment. Besides, zeta potentials of Zr/Ce-MOFs, Zr/Ce-MOFs/DOX and Zr/Ce-MOFs/DOX/MnO₂ were measured and the data are presented in Fig. S2(b) (ESI†). Zr/Ce-MOFs/DOX/MnO₂ present negative charge in aqueous solution and can prevent protein adsorption and increase blood circulation time as a drug carrier.⁴⁴

Drug loading and drug release

The drug loading performance of Zr-MOFs, Ce-MOFs and Zr/Ce-MOFs were investigated using DOX as an anticancer drug model. The nitrogen adsorption–desorption isotherms were first determined to validate the porous structure of Zr/Ce-MOFs. As shown in Fig. S4 (ESI†), a characteristic mesoporous distribution was observed for Zr/Ce-MOFs, suggesting that porous Zr/Ce-MOFs could adsorb DOX for drug loading. As shown in Fig. S5 (ESI†), the drug loading capacity of Zr-MOFs and Zr/Ce-MOFs was higher than 30 mg g⁻¹ and 35 mg g⁻¹, respectively, while the Ce-MOFs exhibited much lower loading capability (about 5 mg g⁻¹). Interestingly, in comparison with Zr-MOFs, doping of Ce in Zr/Ce-MOFs slightly increased the loading capacity.

The drug release profile was evaluated at different pH values (pH = 7.4 and 5.8) and GSH concentrations (0 and 10 mM). As shown in Fig. S6 (ESI†), DOX release reached greater than 60% in the solution with a pH of 5.8 and a high concentration of GSH (10 mM). In comparison, under the other three conditions, the DOX release was less than 30%, indicating very slow release in the presence of only one pathological parameter trigger. Besides, only 8% DOX was released from Zr/Ce-MOFs/DOX/MnO₂ over 72 h in the buffer of pH 7.4. The greater DOX release profile in the solution with a pH of 5.8 and a high concentration of GSH is because of (1) the destruction of MnO₂ coating on surface of Zr/Ce-MOFs/DOX/MnO₂ by GSH resulting in the exposure of Zr/Ce-MOFs/DOX in solution;⁴⁵ and (2) the dissociation of hydrogen bonds as a result of the protonation of hydroxyl groups in acidic condition accelerated DOX release.⁴⁶

Analysis of the catalytic performance

Degradation of methylene blue (MB) is selected as the indicator to determine the production of ˙OH of this nanomedicine system.⁴⁷ The degradation efficiency of MB by different MOFs is shown in Fig. S7a (ESI†). MB degradation rates of Zr-MOFs, Ce-MOFs and Zr/Ce-MOFs are 13%, 38%, and 29%, respectively. Among these three MOFs, Ce-MOFs showed limited capability for loading DOX, despite its high efficiency in degrading MB (Fig. S5, ESI†). In comparison with Zr-MOFs, Zr/Ce-MOFs showed more than doubled MB degradation efficiency. As the MB degradation is correlated with the catalysis of H₂O₂ to ˙OH, the Zr/Ce-MOFs thus are expected to have more than double the catalytic performance of Zr-MOFs. The catalytic performance of these three materials can be further confirmed by electrochemical impedance analysis (Fig. S7b, ESI†). The semicircle of Zr/Ce-MOFs is smaller than that of Zr-MOFs, which indicated that doping Ce³⁺ can promote electron transfer of Zr/Ce-MOFs, thereby improving the catalytic efficiency.^48,49

The catalytic degradation efficiency of Zr/Ce-MOFs/DOX/MnO₂, Zr/Ce-MOFs and Zr/Ce-MOFs/DOX were then determined (Fig. 4a). Compared with Zr/Ce-MOFs, the MB degradation of Zr/Ce-MOFs/DOX slightly decreased. This may be due to the DOX loading into Zr/Ce-MOFs, which affected the active sites of Zr/Ce-MOFs and led to the decrease of catalytic efficiency. After the modification of MnO₂, the catalytic efficiency of Zr/Ce-MOFs/DOX/MnO₂ was about 1.5-fold increased. The improvement of catalytic efficiency can be explained by the promotion of electron transfer, which was confirmed by EIS Nyquist plots (Fig. 4b). The semicircle of Zr/Ce-MOFs is smaller than that of Zr-MOFs, suggesting the potential of electron transfer between Mn(IV)/Mn(II) after coating with MnO₂ on the surface.

To further clarify the catalytic activity of Zr/Ce-MOFs and Zr/Ce-MOFs/DOX/MnO₂, the typical steady-state Michaelis–Menten kinetics of reactions between Zr/Ce-MOFs, Zr/Ce-MOFs/DOX/MnO₂ and H₂O₂ were determined to investigate the enzyme-like catalytic functionalities and performance (Fig. 5). The maximum velocity (V_max) and the Michaelis–Menten constant (K_M) for Zr/Ce-MOFs/DOX/MnO₂ is calculated to be 2.21 × 10⁻⁴ mM min⁻¹ and 1.21, respectively. In sharp contrast, the V_max and K_M of Zr/Ce-MOFs are 1.89 × 10⁻⁴ mM min⁻¹ and 0.65. The larger V_max and K_M suggest a high level of ˙OH production for Zr/Ce-MOFs/DOX/MnO₂.⁵⁰

To understand the effect of GSH, the degradation efficiency of MB with Zr/Ce-MOFs was investigated. As shown in Fig. 6a, the degradation efficiency decreased in the presence of GSH because ˙OH can be captured by GSH. However, the cyclic conversion of Ce³⁺ and Ce⁴⁺ can be promoted with excessive GSH, thus improving the catalytic efficiency at high concentration of GSH. The surface modification of MnO₂ of Zr/Ce-MOFs/DOX can not only promote the production of ˙OH, but also deplete GSH to further promoting the production of ˙OH. The catalytic performance of Zr/Ce-MOFs/DOX/MnO₂ in the presence of GSH were investigated (Fig. 6a). With the increase in the concentration of GSH, increase of MB degradation efficiency was observed, which can be attributed to the depletion of GSH by reacting with Zr/Ce-MOFs/DOX/MnO₂. When the concentration of GSH was higher than 10 mM, all the MnO₂ on the surface of Zr/Ce-MOFs/DOX/MnO₂ could be consumed, resulting in a decrease in catalytic efficiency.

Considering the fact of acidic condition with a high level of GSH in TME, the catalytic degradation efficiency of Zr/Ce-MOFs and Zr/Ce-MOFs/DOX/MnO₂ were examined in a mimic environment of normal cells (pH = 7.4 + 1 mM GSH) and tumour cells (pH = 5.8 + 5 mM GSH), respectively. As shown in Fig. 6b, the catalytic efficiency of the Zr/Ce-MOFs/DOX/MnO₂ in the environment of pH 5.8 and 5 mM GSH was about two times higher than that of pH = 7.4 and 1 mM GSH, suggesting that higher catalytic activity of Zr/Ce-MOFs/DOX/MnO₂ could be obtained in tumour tissues. Compared with Zr/Ce-MOFs, the catalytic efficiency of Zr/Ce-MOFs/DOX/MnO₂ reached 83%, corroborating that an efficient catalytic performance of Zr/Ce-MOFs/DOX/MnO₂ could be achieved in a tumour environment.

Cytotoxicity analysis

The cytotoxicity of Zr/Ce-MOFs, Zr/Ce-MOFs/DOX and Zr/Ce-MOFs/DOX/MnO₂ was tested in 4T1 cells. As shown in Fig. S8 (ESI†), the cell viability of Zr/Ce-MOFs, Zr/Ce-MOFs/DOX and Zr/Ce-MOFs/DOX/MnO₂ gradually decreased with the increase of particle concentration. Compared with the catalytic therapy of Zr/Ce-MOFs, Zr/Ce-MOFs/DOX can release DOX in TME to achieve the combination of chemotherapy and catalytic therapy. In comparison, Zr/Ce-MOFs/DOX/MnO₂ can release DOX accurately with the stimulation of both pH and GSH, thus increasing the local content of DOX. The generated Mn²⁺ can catalyse H₂O₂ to produce more ˙OH through a Fenton reaction, promoting chemotherapy and catalytic therapy simultaneously. As a result, GSH in tumour cells would be consumed, and the combination therapy of catalytic therapy and chemotherapy would be promoted in this combination treatment.

In vivo therapy

In vivo MRI performance of Zr/Ce-MOFs/DOX/MnO₂ was also verified and the images are presented in Fig. S9 (ESI†). The T₁-weighted MRI images of the tumour sites were clearly observed after the injection of Zr/Ce-MOFs/DOX/MnO₂ indicating that Zr/Ce-MOFs/DOX/MnO₂ (1) could be used for T₁-weighted MRI imaging guided therapy; and (2) accumulated in the tumour section.

Encouraged by the promising data in in vitro experiments, the in vivo therapeutic effect of Zr/Ce-MOFs, Zr/Ce-MOFs/DOX and Zr/Ce-MOFs/DOX/MnO₂ was then evaluated in a tumour-bearing mouse model. The weight and tumour volume of mice in each group were measured in every 2 days within 14 days. As shown in Fig. 7a, the weight of the mice did not change significantly, indicating that the mice treated with different particles had no obvious side effects. As shown in Fig. 7b and c, the mice in the control group (Group I) showed a significant increase in the tumour volume. Upon treatment with Zr/Ce-MOFs (Group II) showed a slow increase in tumour volume, indicating that catalytic treatment of Zr/Ce-MOFs has a certain inhibitory effect. In comparison, mice treated with Zr/Ce-MOFs/DOX (Group III) showed a much slower increase in tumour volume, indicating that the combined treatment of catalytic therapy and chemotherapy has an improved inhibitory effect on tumours. In sharp contrast, the tumours of the mice treated with Zr/Ce-MOFs/DOX/MnO₂ (Group IV) were significantly reduced, indicating that the combination therapy of catalytic therapy and chemotherapy can be improved through 1) depletion of GSH level, 2) promoting the hydroxyl radical production, and 3) chemotherapy by DOX.

Conclusions

In summary, we reported the preparation and characterization of Zr/Ce-MOFs/DOX/MnO₂ as the nanozyme for tumour combination therapy. The Zr/Ce-MOFs/DOX/MnO₂ has been demonstrated to regulate a high GSH level in a tumour microenvironment; deliver DOX into the tumour site and release the DOX cargo through a pH/GSH dual-responsive approach; increase the local concentration of DOX in tumours to kill the cancer cells by chemotherapy; and produce large amount of free radicals for promoted cancer cell ablation. In this system, Mn²⁺ and GSSG can be generated during the reaction between GSH and Zr/Ce-MOFs/DOX/MnO₂, which promotes the catalytic efficiency of the enzyme-like activity of nanozymes through consumption of GSH. The Mn²⁺ production can also be used as a T₁-MRI imaging contrast agent for imaging guided therapy. It is expected that this combination of chemotherapy and catalytic therapy could provide a new approach to address the low catalytic efficiency of nanozymes in TME for tumour therapy.

Author contributions

Qian Song, Bin Chi, and Haiqing Gao: investigation, methodology, visualization, and writing-original draft. Junke Wang, Yi Xu, and Yingxi Wang: investigation, formal analysis, and visualization. Miaomiao Wu: methodology. Zushun Xu: methodology and investigation. Ling Li: supervision, funding acquisition, resources, project administration, and data curation. Jing Wang: methodology, investigation, and data curation. Run Zhang: formal analysis, methodology, and writing-review & editing.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the Ministry-of-Education Key Laboratory for the Synthesis and Application of Organic Function Molecules. All animal experiments were performed in accordance with the guidelines stated by the National Institutes Health and approved by the Institution Animal Care and Use Committee at Tongji Medical College, Huazhong University of Science and Technology (S2218). R. Zhang wishes to acknowledge the support of the National Health and Medical Research Council Investigator (APP1175808).

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Footnotes

† Electronic supplementary information (ESI) available: Experimental section and additional figures. See DOI: https://doi.org/10.1039/d3tb01002c

‡ These authors contributed equally to this work.

This journal is © The Royal Society of Chemistry 2023