Iron (II)-based metal-organic framework nanozyme for boosting tumor ferroptosis through inhibiting DNA damage repair and system Xc -

Development of ferroptosis-inducible nanoplatforms with high efficiency and specificity is highly needed and challenging in tumor ferrotherapy. Here, we demonstrate highly effective tumor ferrotherapy using iron (II)-based metal-organic framework (FessMOF) nanoparticles, assembled from disulfide bonds and ferrous ions. The as-prepared FessMOF nanoparticles exhibit peroxidase-like activity and pH/glutathione-dependent degradability, which enables tumor-responsive catalytic therapy and glutathione depletion by the thiol/disulfide exchange to suppress glutathione peroxidase 4, respectively. Upon PEGylation and Actinomycin D (ActD) loading, the resulting FessMOF/ActD-PEG nanoplatform induces marked DNA damage and lipid peroxidation. Concurrently, we found that ActD can inhibit Xc− system and elicit ferritinophagy, which further boosts the ferrotherapeutic efficacy of the FessMOF/ActD-PEG. In vivo experiments demonstrate that our fabricated nanoplatform presents excellent biocompatibility and a high tumor inhibition rate of 91.89%. Supplementary Information The online version contains supplementary material available at 10.1186/s12951-024-02508-2.


Introduction
Breast cancer has been considered as the first most common cause of death in women [1][2][3][4][5].Despite substantial advances in diagnostic techniques and combinatorial therapy, effective treatment of breast cancer with good prognosis and high survival remains a challenge, mainly enhance tumor treatments, tumoral antioxidants may debilitate ferroptosis by perishing the generated ROS and LPO [18,19].For instance, reductive glutathione (GSH) can remove ROS and serve as a cofactor for glutathione peroxidase 4 (GPX4) that inhibits ferroptosis [20,21].In this regard, the GPX4 has been considered a major ferroptotic suppression protein that can significantly detoxify lipid peroxidation [22,23].
Small molecule-based ferroptosis inducers have been used for ferrotherapy by inhibiting cystine/glutamate antiporter protein (system Xc − ) or GPX4 [24,25].However, most of those molecular inducers encounter dilemmas of poor water solubility and low tumor-targeting efficiency and the corresponding treatments are often accompanied by nephrotoxicity [26].Nanoparticles can improve the targeting and biocompatibility of the ferroptosis inducers [27][28][29].In addition, many kinds of nanoparticles hold enzyme-mimicking activities, which have been widely utilized in ferrotherapy [30].For example, Fe 3 O 4 nanoparticle has a peroxidase-like property that can catalyze intratumoral H 2 O 2 to • OH to induce cell damage and ferroptosis [31,32].Besides, GSH oxidase-mimic nanomaterials are capable trigger cellular ferroptotic death by GSH consumption [33].Nevertheless, cell damage can be repaired by cellular inherent repair mechanisms [34].And the GSH consumption will be recomposed by the continual cystine uptake through system Xc − and following GSH synthesis in tumor cells [35,36].It must also be mentioned that ferric ion (Fe 3+ )based nanoparticles have low Fenton activities compared to the ferrous ion (Fe 2+ )-contained materials [37].Therefore, designing powerful nanoplatforms that is consisted of Fe 2+ with properties of GSH oxidation, system Xc − inhibition, and cell damage repair suppression are highly needed in tumor ferrotherapy.
In this work, we fabricated an iron-based metalorganic framework nanoplatform (FessMOF/ActD-PEG) that was assembled from ferrous ions (Fe 2+ 96.63% and Fe 3+ 3.37%) and disulfide bonds followed by PEGylation and actinomycin D (ActD) loading for enhanced tumor ferrotherapy (Scheme 1).The FessMOF can be targeted to deliver and release ActD responsively in tumor sites due to its small size and GSH/pH-stimulated biodegradability, respectively.Functionally, FessMOF is amenable to deplete GSH by the thiol/disulfide exchange reaction to antagonize GPX4 [38].Besides, the peroxidase (POD)like activity enables FessMOF for tumor-specific catalytic therapy through • OH production and DNA damage in cancer cells.In the meanwhile, ActD can inhibit DNA damage repair to induce cellular apoptosis [39].Interestingly, we found that the ActD is capable of inhibiting SLC7A11 (Solute Carrier Family 7 Member 11) and inciting ferritinophagy, which further enhances cellular ferroptotic death by restraining GSH synthesis (indirectly inhibits GPX4 expression) and increasing labile iron pool, respectively.
Transmission electron microscopy (TEM) images showed the FessMOF nanoparticles hold uniform size and irregular shape (Fig. 1a).The elemental mapping (Fig. 1b) and energy dispersive x-ray spectroscopy (EDX) analysis (Fig. 1c) revealed the elements of S and Fe in FessMOF nanoparticles.As shown in Fig. 1d, the size of FessMOF nanoparticles is about 50 nm.Powder X-ray diffraction (PXRD) measurement indicated the synthesis of FessMOF nanoparticles (Fig. 1e).X-ray photoelectron spectroscopy (XPS) was further carried out to determine the elemental composition and valence.As depicted in Fig. 1f, there are S 2p (163.08 eV), O 1s (532.08 eV), C 1s (285.08 eV), and Fe 2p (711.08 eV) peaks in full XPS scan spectrum, showing that FessMOF nanoparticles consisted with elements of S, O, C, and Fe, which was in good consistent with EDX measurement in Fig. 1c.S 2p orbital analysis delineated the disulfide bonds (167.67 and 163.4 eV) in FessMOF nanoparticles (Fig. 1g).As can be seen from Fig. 1h, both Fe 3+ (727.1 and 713.8 eV) and Fe 2+ (711.6 and 724.9 eV) in the fabricated FessMOF nanoparticles.Semiquantitative analysis of Fe 2p spectra manifested the ratio of Fe 2+ in the iron composition is 96.63% while only 3.37% of Fe 3+ (Fig. 1i).For the evaluation of the successful PEGylation and drug loading of FessMOF nanoparticles, Fourier transform infrared spectroscopy (FTIR) spectrum and UV-vis absorption were investigated.As shown in Fig. 1j, the vibrational peak at 1648.2 cm − 1 provides further evidence that the FessMOF nanoparticles were assembled from S-S bonds.Moreover, the appearance of P = O (1107.74 cm − 1 ) and C = O (1701.74 cm − 1 ) vibrational peaks in FessMOF/ActD-PEG demonstrated the successful PEGylation and ActD loading by FessMOF nanoparticles, respectively.The absorption peak at 440 nm of FessMOF/ActD-PEG further informed the ActD was loaded (Fig. 1k).The zeta potential of FessMOF nanoparticles with various modifications was illustrated in Fig. S1.And the TEM image (inset of Fig. 1k) indicated the FessMOF/ActD-PEG nanoplatform retains an intact morphology compared to the FessMOF nanoparticle.Besides, FessMOF exhibited good stability within 5 days in PBS and DMEM medium with 10% FBS solutions (Fig. S2).
Next, the POD-mimic activity of FessMOF nanoparticles was evaluated by a 3,3' ,5,5'-tetramethylbenzidine (TMB) colorimetric assay (Fig. 2a), in which colorless TMB can be oxidized by • OH to bluish oxTMB.Fess-MOF-PEG nanoparticles (50 µg/mL) were incubated with 1% TMB solutions with different pH values (7.4,6.0, and 4.5).After 15 min, the absorbance of mixtures at 652 nm was measured (Fig. 2b).We found that FessMOF-PEG nanoparticles could generate • OH to oxidize TMB and the catalytic activity was enhanced in the presence of H 2 O 2 (50 mmol/L) and under acidic conditions (Fig. S3).The time-dependent absorbance changes of TMB solutions were shown in Fig. 2c, implying that the weak acidic environment contributes to the enhanced POD activity of the FessMOF-PEG nanoparticles.Furthermore, the steady-state kinetic curves were investigated by varying the H 2 O 2 concentration (0-50 mmol/L) under neutral conditions (pH 7.4) and measuring the change in absorbance of the reaction system at 652 nm from 0 to 30 min (Fig. 2d).The Michaelis-Menten curves of H 2 O 2 were obtained by plotting the initial reaction rate versus concentration (Fig. 2e).The Michaelis-Menten constant (K m ) and the maximum reaction rate (V max ) were calculated from the double inverse curve (Lineweaver-Burk plot) with K m of 1.023 mmol/L, and V max was 3.83 × 10 − 5 M/s, much higher than the V max of 8.71 × 10 − 8 M/s for HRP-catalyzed H 2 O 2 (Fig. S4, Table S1).In addition, the electron spin resonance (ESR) spectroscopy results using 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) as • OH trapping agent (Fig. 2f ) further demonstrated the POD activity of the FessMOF-PEG.Subsequently, the performance of GSH depletion by FessMOF-PEG nanoparticles was examined.As can be seen from Fig. 2g, FessMOF-PEG consumed 13.4% and 25.5% of GSH after 12 and 24 h, respectively, which confirmed the GSH depletion ability Scheme 1 Schematic showing the preparation of FessMOF/ActD-PEG and mechanism of ferritinophagy-activated synergistic ferrotherapy.Illustration of the manufacturing procedures of FessMOF/ActD-PEG, GSH/pH-stimulated responsive release of the drug, and the action of ActD to damage DNA and enhance ferritinophagy synergistically empowering the ferrotherapy of the disulfide bond-contained FessMOF-PEG nanoparticles.Because the exchange reaction between GSH and disulfides may destroy the integrity of disulfide bondconsisted materials, the responsive degradability of FessMOF-PEG nanoparticles was studied.We dispersed FessMOF-PEG nanoparticles in solutions with various pH values (7.4,6.0, and 4.5) and were shocked with 500 µg/mL GSH for 24 h.As illustrated in TEM images (Fig. 2h), FessMOF-PEG nanoparticles maintained a relatively intact morphology under physiological conditions (pH 7.4) in the absence of GSH.By contrast, the morphology of the nanoparticles disintegrated under acidic conditions (pH 6.0 and 4.5).In addition, GSH promoted the degradation of the FessMOF-PEG nanoparticles.All considered, these data suggest that our fabricated nanoparticles not only have POD-like activity and GSH consumption ability but hold pH/GSH-responsive biodegradability, suggesting the potential in tumor-specific catalytic therapy and drug delivery of the as-prepared FessMOF-PEG nanoparticles.

Cell uptake and cytotoxicity
Inspired by the catalytic properties of FessMOF-PEG, we closely followed with an experimental investigation of its synergistic anti-tumor properties in vitro in the murine breast tumor 4T1 cell line.We first examined the cellular uptake of FessMOF-PEG nanoparticles.Fluorescein isothiocyanate (FITC)-labeled FessMOF-PEG nanoparticles were incubated with 4T1 cells for different times and the cellular fluorescence intensity was analyzed by flow cytometry (Fig. 3a).The time-dependent increment of fluorescence signals demonstrated that 4T1 cells could effectively uptake FessMOF-PEG.Afterward, the cytotoxicity of FessMOF-PEG and ActD were investigated using CCK-8 experiments.According to Fig. S5-S9, a low concentration of ActD has a high cytotoxicity to 4T1 cells.Next, 4T1 cells were incubated with ActD (0.5 ng/mL), FessMOF-PEG (50 µg/mL) (Fig. S10), and ActDloaded FessMOF-PEG (ActD, 0.5 ng/mL; FessMOF-PEG, 50 µg/mL) for 24 h.As shown in Fig. 3b, FessMOF/ActD-PEG nanoplatform exhibited lower cell viability than the ActD or FessMOF-PEG, showing that ActD exhibits a synergistic effect on FessMOF-PEG nanoparticles for cell killing.Subsequently, the therapeutic mechanism of the FessMOF/ActD-PEG nanoplatform was studied by treatment with inhibitors including ferrostatin-1 (ferroptosis), 3-Methyladenine (autophagy), Z-VAD-FMK (apoptosis), and vitamin E (oxidant and ferroptosis).All inhibitors rescued FessMOF/ActD-PEG nanoplatforms-induced cell death (Fig. 3c), implying that cell-killing by FessMOF/ ActD-PEG involved ferroptosis, autophagy, apoptosis, and oxidative stress-mediated ferroptosis, respectively.Practically, clone-formation assay further evidenced the ferroptotic and autophagic cell killing mechanisms of the FessMOF/ActD-PEG nanoplatform (Fig. 3d).Subsequently, intracellular GSH levels were tested by a GSH detection kit (Fig. 3e).We found that FessMOF-PEG could also consume the cellular GSH.Interestingly, ActD with a low concentration (0.5 ng/mL) has a moderate GSH depletion capacity.And the combination of ActD and FessMOF-PEG significantly depleted GSH in 4T1 cells.

Ferroptosis and ferritinophagy evaluation
It is well-known that nanoparticles with POD-like activity are able to produce highly reactive • OH by Fenton reaction to bring about the accumulation of LPO and DNA damages in tumor cells [40].Accordingly, we evaluated the • OH generation in 4T1 cells using BBoxiProbe®O26 as the probe.As shown in Fig. 4a, the strong green fluorescence signals in cells that treated  1), ( 2), (3), and ( 4) indicate the groups of control, ActD, FessMOF-PEG, and FessMOF/ActD-PEG, respectively.All data were presented as mean ± standard deviation.Statistical differences were calculated using two-tailed Student's t test.Differences were considered significant when the p-value was less than or equal to 0.05.* p < 0.05, ** p < 0.01, *** p < 0.001, and **** p < 0.0001 with FessMOF/ActD-PEG and H 2 O 2 , confirming the efficient • OH generation by the nanoplatform in the presence of the H 2 O 2 .Besides, after the addition of deferoxamine (DFO) (Fig. S11), a classical ferroptosis inhibitor, the • OH generation of FessMOF/ActD-PEG + H 2 O 2 group significantly reduced, which implies that the iron ions have significant roles in • OH generation.Meanwhile, we found that ActD and FessMOF-PEG can trigger moderate LPO generation in 4T1 cells.By comparison, FessMOF/ActD-PEG induced significant higher LPO accumulation than ActD and FessMOF-PEG (Fig. 4b, c).Next, the lipid peroxidation product malondialdehyde (MDA) was determined (Fig. 4d).It was also found that both ActD and FessMOF-PEG nanoparticles elevated the exaggeration of MDA and the FessMOF/ActD-PEG induced more MDA accumulation than the ActD and FessMOF-PEG.This provides further evidence that both ActD and FessMOF-PEG nanoparticles are able to stimulate cell ferroptotic death and it suggests that ActD can promote ferrotherapeutic effects of the FessMOF-PEG nanoparticles.Oxidative stress can induce DNA damage and apoptosis while tumor cells tend to repair the damage through various mechanisms thus inhibiting cellular death [41,42].Whereas ActD is capable of inhibiting DNA damage repair by intercalating into DNA and interfering with RNA polymerases and DNA topoisomerases, thus inhibiting transcription and inducing various types of DNA damage [39], therefore we used the comet electrophoresis assay to detect DNA damage at the single-cell level.Results in Fig. 4e and f indicated that ActD provoked FessMOF-PEG-induced DNA damage in 4T1 cells.In addition, the elevation of the expression of γ-H2AX in FessMOF/ActD-PEG-treated cells further confirmed the DNA damage in cancer cells (Fig. 4g).Subsequently, the assay of Annexin V FITC and propidium iodide (PI) co-staining was used to study the cell apoptosis induced by nanoplatforms.As shown in Fig. 4h, the apoptosis ratio (Annexin V FITC + and PI + ) of FessMOF/ ActD-PEG-treated cells was 13.7% while the ratio of PBS, ActD, and FessMOF-PEG was only 4.03%, 6.45%, and 9.10%, respectively.Live/dead staining further confirmed the excellent therapeutic efficacy of the FessMOF/ActD-PEG nanoplatform (Fig. S12).In addition, cell cycle arrest analysis demonstrated that FessMOF/ActD-PEG lowered the count of cells in the S phase from 18 to 12.1% (Fig. 4i), suggesting FessMOF/ActD-PEG is able to inhibit cell DNA replication.These data imply that the as-prepared FessMOF/ActD-PEG nanoplatform can induce cell ferroptosis, apoptosis, DNA damage, and cell cycle arrest.
As a classical transcription inhibitor, ActD was found to momentously reduce the half-life of SLC7A11 mRNA, thus may sensitize cells to ferroptosis by repressing expression of SLC7A11 in the Xc − system [43].It also has been reported that ActD not only can activate cell autophagy and trigger cell apoptosis through inhibiting DNA damage repairment [39,44].In the meanwhile, ferroptosis has been identified as a cell death process associated with increased LPO [45].Therefore, the mechanism of ActD to enhance the ferrotherapeutic effects of the FessMOF/ActD-PEG nanoplatform may stem from the SLC7A11 defunctionalization.To validate our hypothesis, we analyzed the expression of the SLC7A11 protein of 4T1 cells treated with ActD, FessMOF-PEG, or FessMOF/ ActD-PEG nanoplatforms.As illustrated in Fig. 4j, ActD exactly reduced the expression of the SLC7A11 protein in cancer cells.And FessMOF/ActD-PEG had lower expression of SLC7A11 protein than ActD and FessMOF-PEG.Admittedly, the downregulation of the SLC7A11 protein will lead to the reduction of cysteine uptake.While the reduction of cystine uptake further inhibits GSH synthesis and thus suppresses the expression of GPX4 [46,47], which was evidenced by the western-blot analysis.Consequently, the as-prepared FessMOF/ActD-PEG nanoplatform induced pronounced 4-Hydroxynonenal (4-HNE) generation, one of the most important endproducts of lipid peroxidation [48].Furthermore, the elevated expression of LC3II/LC3I and NCOA4 (nuclear receptor coactivator 4) confirmed that ActD can stimulate the ferritinophagy of 4T1 cells (Fig. 4k).Specifically, the FessMOF/ActD-PEG nanoplatform induced ferritinophagy of cancer cells and resulted in the degradation of FTH1 (ferritin heavy chain 1), which may increase the labile iron pool and promote cellular ferroptotic death [49].All considered, we found that ActD inhibits the expression of SLC7A11 and GPX4 and induces cellular ferritinophagy.In the meanwhile, our designed Fess-MOF/ActD-PEG nanoplatform can inhibit DNA damage repair, initiate cell cycle arrest and apoptosis, and trigger robust cellular ferroptosis.

In vivo biodistribution
Next, the in vivo biodistribution of FessMOF-PEG was investigated.4T1 tumor-bearing mice were injected with ICG-labeled FessMOF-PEG (FessMOF-PEG@ICG, 40.21%ICG loading rate) (Fig. S13 and Fig. S14) and ICG molecule solutions through the tail vein, and the fluorescence signal was monitored by an imaging system.According to Fig. 5a and b, FessMOF-PEG@ICG accumulated at tumor sites.The higher fluorescence signals of FessMOF-PEG@ICG than free ICG demonstrated that the FessMOF-PEG nanoparticles can improve the tumortargeting efficiency of therapeutic molecules.After 24 h injection, mice were executed and the tumor and major organs (heart, liver, spleen, lung, kidney, and brain) were imaged.As shown in Fig. 5c and d, the fluorescence intensity of FessMOF-PEG@ICG was higher at the tumor site compared to free ICG, suggesting the preferable tumor-targeting property of FessMOF-PEG@ICG.

In vivo antitumor therapy efficacy
Given the outstanding in vitro synergistic therapeutic efficacy and tumor-targeting, the in vivo antitumor effects of the FessMOF/ActD-PEG nanoplatform were next examined (Fig. 6a).4T1 tumor-bearing mice were randomly divided into four groups and intravenously injected with PBS (control), ActD, FessMOF-PEG, and FessMOF/ActD-PEG, respectively.The body weight and tumor volume of the mice were recorded every other day.As can be seen from Fig. 6b and Fig. S15, the body weight of mice had no obvious changes, reflecting the favorable biocompatibility of nanoparticles.Tumors in group control grew rapidly while ActD had a slight effect the growth of the tumor (Fig. 6c, d).By contrast, Fess-MOF-PEG delayed the growth of tumors.Specifically, the group FessMOF/ActD-PEG showed a significant tumor inhibition effect and its tumor volume inhibition rate was 91.89% (Fig. S16a).Mice were executed after treatments for 14 days, and tumors from different groups were imaged and weighted (Fig. 6e, f ).It also found that the FessMOF/ActD-PEG nanoplatform had a higher inhibition efficiency than the other groups (Fig. 6g).H&E (hematoxylin and eosin) histologic staining (Fig. 6h) and TUNEL (terminal deoxynucleotidyl transferasemediated dUTP Nick-End labeling) immunofluorescent staining (Fig. 6i and Fig. S16b) of tumor slices illustrated that more cell death and damage was found in group FessMOF/ActD-PEG after treatments.All these results demonstrated that our designed FessMOF/ActD-PEG nanoplatform exhibited pronounced therapeutic effects on tumor therapy.

In vivo biosafety evaluation
The biosafety of nanomaterials is one of the major concerns in biomedical applications.To investigate the biosafety of the FessMOF-PEG nanoparticles, we first performed the hemolysis assay (Fig. 7a).We found that the hemolysis rate was less than 1% even though the concentration of FessMOF-PEG nanoparticles was up to 600 µg/mL, which demonstrates the excellent biocompatibility of the synthesized nanoparticles.After 14 days of treatments, blood and major organs from mice were collected.According to the blood and biochemical analysis, FessMOF/ActD-PEG has no noticeable fluctuation in the correlation index compared to other groups (Fig. 7b).Additionally, no significant cell necrosis or lesions were found in the major organs (heart, liver, spleen, lung, and kidney) of mice after treatments (Fig. 7c).These data suggest that the as-prepared FessMOF/ActD-PEG nanoplatform exhibits excellent in vivo biocompatibility.

Conclusion
In summary, we have demonstrated the construction of a ferrous-based MOF nanoplatform that is assembled from ferrous ions and disulfide bonds for tumor microenvironment-responsive ferrotherapy.The FessMOF nanoparticles hold POD and GSH oxidase dule enzymelike activities, which could alleviate cellular oxidative stress and suppress the GPX4.Combined with the ActD, the FessMOF/ActD-PEG nanoplatform induced more LPO generation, DNA damage, and cell cycle arrest than the FessMOF nanoparticle or free ActD.By taking full functions of ActD and FessMOF, the as-prepared nanoplatform results in a pronounced ferroptosis-mediated synergistic therapeutic efficacy both in vitro and in vivo.Additionally, FessMOF/ActD-PEG exhibits excellent biocompatibility and low side effects.In a nutshell, our work reported a smart ferroptosis nanoinducer for tumor-specific therapy, which may offer new insight into developing novel nanoplatforms for tumor ferrotherapy in the future.

Synthesis of FessMOF/ActD-PEG
FessMOF nanoparticles were synthesized at first according to a reported work with some modifications [27].In detail, 300 mg PVP (K40) was thoroughly ultrasonically dispersed in 3 mL DMF.Then, dithioglycolic acid (52 µL, 100 mg/mL), FeCl 2 •4H 2 O (7 mg), triethanolamine (600 µL), and 10 mL of a mixed solution of DMF and anhydrous ethanol (the volumetric ratio of DMF and anhydrous ethanol is 5: 3) were added into the above PVP solution under stirring.After thoroughly mixed, the mixture was transferred to a Teflon-lined stainless-steel autoclave and kept at 150 °C for 12 h.The sediments were centrifuged (11,000 rpm, 15 min) and washed several times with water.Next, FessMOF nanoparticles were absorbed with DSPE-mPEG.FessMOF nanoparticles (1 mg) were mixed with DSPE-mPEG (1 mg) in 1 mL distilled water for 6 h at 4 °C.Afterward, FessMOF-PEG nanoparticles were obtained by centrifugation.Finally, FessMOF-PEG nanoparticles (1 mg) were added into the 1 mL ActD solution (0.5 ng/mL) overnight at 4 °C, and the final precipitates were washed with distilled water, obtaining the FessMOF/ActD-PEG nanoplatform.The loading efficiency of ActD in the FessMOF-PEG was calculated as (input ActD quantity -supernatant ActD quantity) / input ActD quantity × 100%.The final load rate of ActD is about 100%.

Characterization
Transmission electron microscopy (TEM) images were obtained by HT7700 Exalens transmission electron microscope.Zeta potential and dynamic light scattering (DLS) were measured on the Malvern Zetasizer Nano ZS (Malvern Instruments, Ltd., Worcestershire, UK).X-ray photoelectron spectroscopy (XPS) spectra were acquired by the Thermo SCIENTIFIC Nexsa a K-Alpha 1063 instrument (Thermo Fisher Scientific, USA).Fourier transform infrared spectroscopy (FTIR) spectra were tested on an FTIR spectrometer (ThermoNicolet IS50).UV-vis spectra were measured by UV-8000 S spectrophotometer.X-ray Diffraction (XRD) analysis of samples was conducted by a Rigaku Miniflex600 X-ray diffractometer.

Hemolysis experiment
Fresh blood was obtained from Balb/c mice through eyeballs.Red blood cells were centrifuged and washed 3 times with PBS solution.Then the red blood cells were diluted with 5 mL PBS.Preparation of positive control solution: 50 µL diluted mice blood + 50 µL 0.5% Triton-100; preparation of negative control solution: 50 µL diluted mice blood + 50 µL PBS solution; preparation of test group solutions: 50 µL diluted mice blood + different volumes of 1 mg/mL FessMOF-PEG.The final concentrations of FessMOF-PEG were 10, 20, 50, 100, 150, 200, 400, and 600 µg/mL.After incubation for 60 min at 37 °C on a shaker, the mixture was centrifugated at 3000 rpm for 3 min.Then the supernatant was placed in a 96-well plate, and the absorbance values were measured by a microplate reader (adjusted to 576 nm).The hemolysis rate (%) was measured by the hemolysis rate formula: hemolysis rate (%) = [(absorbance value of the test group -absorbance value of the negative control group) / (absorbance value of the positive control group -absorbance value of the negative control group)] × 100%.

ESR detection
For ESR detections, the PBS solution (pH 6.0) containing FessMOF-PEG (100 µg/mL) and 50 mM H 2 O 2 were incubated for 15 min at room temperature.After that, the DMPO (100 mM) as the spin-trapping agent was added to the sbove mixture.

The POD-like activity of FessMOF-PEG
The POD-like activity of FessMOF-PEG was evaluated by the TMB colorimetric assay.In detail, 50 µg/mL FessMOF-PEG was dispersed in buffer solutions with different pH values (7.4,6.0, and 4.5).Then, H 2 O 2 solution (50 mmol/L) and 1% TMB solution were added.After 15 min, the absorbance values at 652 nm of various groups were immediately measured using a microplate reader (Thermo Fisher Scientific).

Degradation of FessMOF-PEG
Fifty µg/mL FessMOF-PEG nanoparticles were dispersed in buffer solutions with different pH values (7.4,6.0, and 4.5) with or without 500 µg/mL GSH solution.After slowly shaking for 24 h at 37 °C, the morphology of Fes-sMOF-PEG nanoparticles was observed using transmission electron microscopy.

Colony-forming assay
4T1 cells were seeded in 6-well plates at a density of 1000 cells per well and grew for 24 h.The cells were incubated with various treatment solutions for 24 h.The medium was changed every three days under the same treatment conditions.The co-incubation was terminated after 14 days when colonies were visible to the naked eye.Thereafter, the cells were washed three times with PBS, then fixed with 4% paraformaldehyde for 30 min, rinsed twice with running water, and stained for 30 min with crystal violet solution.Finally, the crystal violet solution was carefully washed away with running water, and the 6-well plates were inverted and naturally dried.Photographs were taken and videotapes were made.

• OH detection
4T1 cells were seeded in confocal dishes at a density of 10 4 cells per well and grew for 24 h.After 4T1 cells were incubated with various treatment solutions for 24 h.Diluted BBoxiProbe®O26 probe solution was added into confocal dishes.Then, the cells were washed twice with PBS and imaged by a confocal laser scanning microscopy (LSM780, Zeiss).

Liperfluo
For cellular LPO evaluation, 4T1 cells were incubated with various treatment solutions for 24 h.Afterward, the cell culture medium was replaced with 10 µmol/L liperfluo probe solution and cultured for 30 min at 37 °C.At last, the cells were imaged under a confocal laser scanning microscopy.Simultaneously, the fluorescence intensity was assessed by a flow cytometer.

Live/dead assay
After various treatments, 4T1 cells were co-incubated with Calcein-AM (2 µmol/L) and PI (5 µmol/L) staining reagents for 2 h and subsequently imaged by a confocal laser scanning microscopy.

Comet electrophoresis experiment
The experiment was conducted according to the kit instructions (KGI Bio).In detail, the layer of 1% normal melting point gel was configured on the slides.Then, 10 µL of cells solution (about 10 4 cells) and 75 µL of 0.7% low melting point gel were mixed well and spread on the pretreated slides at 4°C for 10 min to allow the agarose to solidify.Afterward, the glued slides were lysed for 90 min in a freshly prepared lysis buffer.Subsequently, the slides were rinsed in PBS solution before being placed in an electrophoresis buffer, denatured for 30 min, and electrophoresed for 20 min (25 V, 300 mA).And then, neutralization was performed three times for 5 min each with 400 mmol/L tris buffer (pH 7.5).At last, each slide was stained for 20 min with 20 µL of PI staining solution and observed under a Zeiss fluorescence microscope.

Apoptosis assay
4T1 cells were pre-seeded in 6-well plates at a density of 10 5 cells per well.The cells were incubated with various treatment solutions for 24 h.Afterward, the adherent cells in 6-well plates were digested with trypsin, washed twice with pre-cooled PBS solution, and finally collected 5 × 10 5 cells in tubes.Each tube was incubated with 5 µL Annexin V -FITC and 10 µL PI staining solutions for 10 min at 4 °C.Detection on flow cytometer: upper right quadrant (Annexin V + /PI + , late apoptotic cells); lower right quadrant (Annexin V + /PI -, early apoptotic cells), upper left quadrant (Annexin V -/PI + , dead cells), and lower left quadrant (Annexin V -/PI -, live cells).

Cell cycle assay
The cultivated 4T1 cells were tagged with EdU.Then, cells were fixed and permeabilized with a BeyoClick-TMEdU-555 kit.The results were analyzed by a flow cytometer.

Western blot
4T1 cells were pre-seeded in a 6-well culture plate.Cells were washed twice with PBS solution and then collected using cell lysate, PMSF, and protease inhibitor.The contents of protein were measured using the BCA protein concentration assay kit.According to the required sample volume, 5 × loading buffer was added into the protein solution at a ratio of 4: 1.Then the final protein solution was denatured at 100 ℃ for 10 min in a boiling water bath.The separation gel and concentrated gel were prepared, and the samples were loaded at 30 µg of protein volume, electrophoretically separated and transferred into the polyvinylidene fluoride (PVDF) membrane, and then closed by 5% skim milk solution at room temperature for 2 h.These samples were incubated with the corresponding primary antibody such as β-actin (1: 20,000, Proteintech), SLC7A11 (1: 1000, Origo), GPX4 (1: 1000, Proteintech), 4-HNE (1: 1000, Origo), LC3 (1: 1000, Proteintech), NCOA4 (1: 1000, Proteintech), FTH1 (1: 1000, Proteintech), and γ-H2AX (1: 1000, Proteintech) overnight at 4 °C.After that, these samples were cultivated with the secondary antibody (1: 10,000, Proteintech) for 2 h.At last, the membranes were visualized and processed with image J for grayscale values.

In vivo biodistribution
Female Balb/c mice (4 weeks) were purchased from Shanghai Slack Laboratory Animal Co., Ltd., and the experiments were implemented in accordance with protocols approved by the Animal Experimental Ethics Committee of Fujian Normal University.
4T1 tumor models were obtained by orthotopically injecting 5 × 10 6 4T1 cells in the breast fat pads of mice.When the tumor volume approached about 80 mm 3 , the eight tumor-bearing mice were randomly divided into two groups.Then, mice were intravenously injected with free ICG or FessMOF-PEG@ICG (containing 100 µg ICG per mouse).Subsequently, fluorescent signals of mice were acquired by an imaging spectrum system (PerkinElmer) at different time points (1, 2, 4, 8, 10, and 24 h).After injection for 24 h, mice were euthanized and tumors and major organs (heart, liver, spleen, lung, kidney, and brain) were collected.The ex vivo fluorescence images and intensities of tumors and major organs were recorded by the imaging system.
In vivo tumor therapy 4T1 cells (5 × 10 6 ) were orthotopically injected into the breast fat pads of female Balb/c mice.When the tumor volume reached approximately 100 mm 3 , these mice were injected intravenously with (a) PBS; (b) ActD (0.8 µg/ kg); (c) FessMOF-PEG (8 mg/kg); (d) FessMOF/ActD-PEG (8 mg/kg, containing 8 mg/kg FessMOF-PEG and 0.8 µg/kg ActD).Afterward, the tumor volumes and body weights were recorded every other day.The tumor volumes (mm 3 ) were measured according to V = ab 2 /2, in which a and b represent the length and width of the tumor, respectively.After 14 days, these mice were executed and their tumors and organs (heart, liver, spleen, lung, and kidney) were collected.Tumor slices were conducted with H&E staining and TUNEL staining.Slices of organs were stained with H&E for damage evaluation.In addition, blood solutions of treated mice were harvested for blood routine analysis including hemoglobin (HGB), red blood cells (RBC), lymphocytes (Lymph), and red blood cells (WBC).In the meanwhile, glutamic aminotransferase (ALT), aspartate aminotransferase (AST), creatinine (CREA), and UREA in plasmas were measured to evaluate the hepatic and renal functions.

Quantification and statistical analysis
Quantitative data were presented as mean ± standard error of the mean (SEM).Statistics were analyzed by oneway or two-way analysis of variance (ANOVA), followed by two-sided Student's t-test using GraphPad Prism 5.0.Differences were considered significant when the p-value

Fig. 5 Fig. 6
Fig. 5 In vivo biodistribution.(a) Fluorescence images of 4T1 tumor-bearing mice after intravenous injection of FessMOF-PEG@ICG and ICG.(b) Comparison of fluorescence intensity at tumor sites upon injection of FessMOF-PEG@ICG and ICG.(c) Representative fluorescence images of major organs and tumors after intravenous injection of FessMOF-PEG@ICG and ICG at 24 h (He, Li, Sp, Lu, Ki, Br, and Tu indicate the heart, liver, spleen, lung, kidney, brain, and tumor, respectively).(d) Quantification of fluorescence intensity in tumors and major organs at 24 h.All data were presented as mean ± standard deviation.Statistical differences were calculated using two-tailed Student's t test.Differences were considered significant when the p-value was less than or equal to 0.05.*** p < 0.001

Fig. 7
Fig. 7 Evaluation of biocompatibility.(a) Hemolysis rate of FessMOF-PEG with different concentrations.TritonX-100 (0.5%) as the positive control (P), inset is the photo of red blood cell solution after incubating with FessMOF-PEG nanoparticles.(b) Hematological indexes and biochemical data of 4T1 tumorbearing Balb/c mice after treatments for 14 days.HGB, RBC, Lymph, WBC, CREA, UREA, AST, and ALT indicate hemoglobin, red blood cell, lymphocyte, white blood cell, creatinine, urea, aspartate transaminase, and alanine transaminase, respectively.(c) H&E staining of major organs from treated mice.Scale bars are 50 μm