Redox dyshomeostasis strategy for tumor therapy based on nanomaterials chemistry

Redox homeostasis, as an innate cellular defense mechanism, not only contributes to malignant transformation and metastasis of tumors, but also seriously restricts reactive oxygen species (ROS)-mediated tumor therapies, such as chemotherapy, radiotherapy, photodynamic therapy (PDT), and chemodynamic therapy (CDT). Therefore, the development of the redox dyshomeostasis (RDH) strategy based on nanomaterials chemistry is of great significance for developing highly efficient tumor therapy. This review will firstly introduce the basic definition and function of cellular redox homeostasis and RDH. Subsequently, the current representative progress of the nanomaterial-based RDH strategy for tumor therapy is evaluated, summarized and discussed. This strategy can be categorized into three groups: (1) regulation of oxidizing species; (2) regulation of reducing species and (3) regulation of both of them. Furthermore, the current limitations and potential future directions for this field will be briefly discussed. We expect that this review could attract positive attention in the chemistry, materials science, and biomedicine fields and further promote their interdisciplinary integration.


Introduction
Cellular redox homeostasis, which refers to the dynamic balance between intracellular oxidizing species and reducing species, plays an important role in maintaining normal physiological processes such as cell growth, metabolism, differentiation, aging and apoptosis. 1 Oxidizing species are a class of chemical substances that tend to obtain electrons from reducing agents in redox reactions, including reactive oxygen species (ROS), 2 reactive nitrogen species (RNS), 3 and so on. A moderate concentration of intercellular oxidizing species such as ROS is required for various biological processes including cell signaling, biosynthetic processes, and host defense. 4 However, excessive amounts of ROS can cause oxidative damage to lipids, proteins, and DNA, which leads to oxidative stress. 5 Under this persistent intrinsic oxidative stress, reducing species, a class of oxidizing species-scavenging substances, are activated to protect cells from ROS-induced oxidative damage. Therefore, it is crucial to keep cellular redox homeostasis for normal cell survival and growth.
Compared with normal cells, numerous types of cancer cells tend to elevate levels of oxidizing species owing to the genetic instability, increased chronic inammation, and aberrant metabolism. 6 Oxidizing species in turn induce gene mutation and inammation, leading to the generation of more oxidizing species. 7 This cycle ultimately results in persistent oxidative stress. Cancer cells that survive oxidative stress mobilize a set of adaptive mechanisms through enhancing antioxidant capacity. 8,9 This adaptive system contributes to malignant transformation, metastasis, and resistance of tumor cells to ROS-mediated tumor therapies such as part of chemotherapeutic drugs, radiotherapy (RT), photodynamic therapy (PDT), and chemodynamic therapy (CDT). 4,10-12 Therefore, it is crucial to disrupt tumor redox homeostasis to make tumor cells sensitive to current tumor treatment modalities.
The redox dyshomeostasis (RDH) strategy for tumor therapy is rst proposed by our group, aiming to make tumor cells more sensitive to different tumor therapy patterns 13 through disrupting the cellular redox homeostasis using nanomaterials (Fig. 1). Unlike traditional chemotherapeutic drugs, nanomaterials have their special characteristics such as passive targeting, drug delivery, and tendency of losing or gaining electrons. Therefore, based on these characteristics, nanomaterials could be designed to modulate the redox homeostasis of tumor cells by delivering and reacting with oxidizing or reducing species, under endogenous stimuli such as low pH and hypoxia, or external stimuli such as light and ultrasound. 14 Here, this review summarizes the most recent progress of the nanomaterial-based RDH strategy for novel and efficient tumor therapies, mainly including the following regulations to realize RDH: (1) regulation of oxidizing species; (2) regulation of reducing species, and (3) regulation of both oxidizing species and reducing species.

Regulation of oxidizing species to realize RDH
Oxidizing species mainly include ROS and RNS. ROS are oxygencontaining molecules with higher chemical reactivity including superoxide anions (O 2 c À ), hydrogen peroxide (H 2 O 2 ), and hydroxyl radicals (cOH). RNS are nitrogen-containing oxidants, mainly including nitric oxide (NO) and peroxynitrite (ONOO À ). Among these oxidizing species, H 2 O 2 is a signicantly stable and diffusible form of ROS. 9 Furthermore, as an intermediate product of mammalian oxygen metabolism, H 2 O 2 is easily transformed into various active species in cells. 6 Therefore, H 2 O 2 is one of the most important oxidizing species, which is capable of being regulated to disrupt redox homeostasis. In the past few years, a series of nanomaterials have been constructed to increase H 2 O 2 production, realizing the RDH strategy to enhance the sensitivity of tumor therapy. On the basis of working principles, increasing the H 2 O 2 level can be realized through two distinctive strategies: (1) biosynthesis approaches; (2) chemical approaches. mitochondrial respiratory chain or endoplasmic reticulum (ER) stress can promote H 2 O 2 production. In addition, the production of H 2 O 2 is controlled by a series of biosynthesis-related enzymes, such as NADPH oxidase (NOX), glucose oxidase (GOx) and lactate oxidase (LOx) (Fig. 2). Therefore, enzymes, the function of mitochondria and ER, and the substrate (O 2 and O 2 c À ) have been regulated using a wide range of nanomaterials to increase the H 2 O 2 level, realizing RDH to enhance the sensitivity of tumor therapy. H 2 O 2 production is driven by both enzyme-dependent and -independent strategies.
2.1.1 Enzyme-dependent strategies. NOX, the main H 2 O 2generating enzymes in eukaryotic cells, are closely associated with redox homeostasis. 15 It has been reported that cisplatin induces the production of ROS via NADPH oxidase activation in human prostate cancer cells. 16    polymer dots. This nanosystem could be persistently immobilized into a tumor and catalyze glucose oxidation to generate H 2 O 2 , leading to RDH, which was benecial for phototherapy ( Fig. 3B). 19 4 (m 3 -OH) 4 clusters as centers. 23 This system could produce considerable O 2 c À under a hypoxic environment with 750 nm irradiation. The produced O 2 c À could not only kill tumor cells but transform to H 2 O 2 via SOD-mediated reactions. The robust H 2 O 2 induced the imbalance of redox homeostasis to realize RDH, which ultimately improved the PDT efficacy.
Altering lactic acid and energy metabolism can also trigger the production of H 2 O 2 . 24 The high rate of glycolysis in the tumor is associated with the excessive generation of lactic acid, which leads to the upregulation of MCT4, 25   from CAT@liposome, resulting in a long-lasting oxygen release and RDH, which further enhanced the therapeutic effects of radiotherapy and CTLA4 (cytotoxic T lymphocyte-associated antigen 4) antibody-mediated immunotherapy (Fig. 5B 30 Therefore, metal peroxides are capable of specically responding to the acidic tumor microenvironment to produce a large number of H 2 O 2 in tumor sites, which could disrupt the redox homeostasis of tumors and ultimately enhance the sensitivity of tumor cells to other treatments. For example, our group constructed a biodegradable nanoprodrug of transferrin-modied MgO 2 nanosheets. In response to the acidic and low catalase activity of the tumor microenvironment, MgO 2 produced abundant H 2 O 2 to disrupt redox homeostasis, ultimately enhancing transferrin-mediated CDT (Fig. 6A). 31 Similarly, Xiaoyuan Chen et al. reported a method of fabricating copper peroxide nanodots (CP). 32 The CP nanodots decomposed under the acidic environment of endo/lysosomes, allowing the simultaneous release of Fenton catalytic Cu 2+ and H 2 O 2 . The increase of H 2 O 2 induced RDH, sensitizing Cu 2+ -induced CDT. Altering the metal ion content balance could induce a series of intracellular responses, even cell death. This approach has been used to treat tumors and was dened as ion interference therapy (IIT). 33 For example, calcium ion (Ca 2+ ) accumulation is a cause of damage or cell death in various cell types. With this in mind, we constructed pHsensitive sodium-hyaluronate-modied calcium peroxide  nanoparticles (SH-CaO 2 NPs) to produce abnormal H 2 O 2 accumulation and calcium overload. 33 The increased level of H 2 O 2 disturbed cellular redox homeostasis and enhanced calcium overload-induced cell death (Fig. 6B). Taken together, these strategies combined RDH with CDT and IIT, opening a new door for further material design and corresponding tumor treatment.

Increasing other oxidizing species to realize RDH
Besides ROS, RNS including NO and ONOO À are another important oxidizing species. A few studies have been designed to produce RNS modulating the redox homeostasis of tumor cells. For example, our group constructed an intelligent X-raycontrolled NO-releasing upconversion nano-theranostic system PEG-USMSx-SNO. The increase of NO could induce RDH consequently sensitizing radiotherapy (Fig. 7A). 34 In order to achieve a better antitumor effect and inhibit tumor metastasis, Jun Lin et al. fabricated a multifunctional nanovaccine based on L-arginine (LA)-loaded black mesoporous titania (BMT). By using ultrasound, BMT and LA were triggered to produce NO which not only induced RDH but also induced cell apoptosis through DNA double-strand breaks. All of these eventually enhanced PD-L1 antibody-mediated immunotherapy, further inhibiting metastatic tumors (Fig. 7B). 35 3. Regulation of reducing species to realize RDH To protect tumor cells from damage by oxidative stress, the powerful antioxidant defense system of cells maintains the balance of intracellular redox by producing reducing species. 36 Reducing species, as another part of redox homeostasis, essentially transfer electrons to oxidizing species, which would decrease the sensitivity of tumor cells to ROS-medicated tumor therapy. Therefore, inhibiting the production of reducing species could effectively break the redox balance and also promote the efficacy of tumors. [37][38][39][40][41] It is well established that glutathione (GSH) is the primary reducing species for intracellular ROS scavenging, and its presence signicantly reduces intracellular ROS levels and weakens the effect of tumor therapy. Therefore, GSH is one of the most important reducing species capable of being regulated to disrupt redox homeostasis. Thus, how to reduce GSH to break the redox homeostasis of tumor cells is one of the cutting-edge research directions of the RDH strategy. In the past few years, numerous nanomaterials have been constructed to decrease GSH production, 42,43 adopting the RDH strategy to enhance the sensitivity of tumor therapy. Based on working principles, inhibition of GSH can be divided into two distinctive strategies: (1) suppressing GSH biosynthesis; (2) chemical approaches (Fig. 8).

Decreasing the GSH level of tumor cells to realize RDH via suppressing GSH biosynthesis
The biosynthesis of GSH consists of the production of GSH and the recycling of GSH. The process of GSH production includes the formation of g-glutamylcysteine from glutamate and  cysteine, and the formation of GSH from g-glutamylcysteine and glycine. 44 In this system, cysteine is an indispensable precursor for GSH. g-glutamine cysteine synthase (g-GCS) and GSH synthetase (GS) are two rate-limiting enzymes for the biosynthesis of GSH. Aer GSH production, GSH acts as an antioxidant to reduce hydrogen peroxide and lipid peroxide through GSH peroxidase (GPx)-catalyzed reactions. In this process, GSH is oxidized to oxidized-glutathione (GSSG). In turn, GSSG is reduced back to GSH by GSSG reductase (GR) at the expense of NADPH, thereby forming a redox cycle. Therefore, the biosynthesis level of GSH is dependent on synthesisrelated enzymes (g-GCS or GS), substances (cysteine, glutamate and glycine) and the recycling level (NADPH) (Fig. 8). Thus, with this knowledge, we conclude that nanomaterials have the potential to suppress GSH biosynthesis. The suppression of GSH is also driven by enzyme-dependent and -independent strategies.
3.1.1 Enzyme-dependent strategies. g-GCS and GS are two important enzymes for the biosynthesis of GSH. However, compared with the overexpression of GS, the overexpression of g-GCS could elevate higher levels of GSH. Therefore, g-GCS is used as a more popular target for the regulation of GSH synthesis. 45,46 The g-GCS inhibitor L-buthionine-(S,R)-sulfoximine (BSO) is the most widely used chemical compound for the inhibition of GSH biosynthesis. 47 Xiaolin Wang et al. developed near-infrared photothermal liposomal nano-antagonists (PLANs) containing a photosensitizer indocyanine (ICG) and BSO. BSO inhibited GSH synthesis to disrupt redox homeostasis, amplifying ICG-mediated cancer PDT. 48 Similarly, Kun Zhang et al. constructed a tumor metabolism-engineered composite nanoplatform Nb 2 C/TiO 2 /BSO-PVP. Nb 2 C was used to load TiO 2 sonosensitizers and BSO. The loaded BSO can block GSH synthesis to break redox homeostasis, which can maximally increase SDT (Fig. 9A). 49 3.1.2 Enzyme-independent strategies. In addition to enzyme-dependent strategies, the biosynthesis of GSH can be regulated by substrate availability and GSH recycling. Cysteine is the indispensable substrate for GSH biosynthesis. [50][51][52] However, cysteine is unstable in the cytosol of cells and easily oxidized to cystine. Therefore, uptake of stable cystine that is reduced to cysteine in cell cytosol is another rate-limiting step in GSH biosynthesis. Several transporters, such as cystine/ glutamate transporter System X C À (SLC7A11 and SLC3A2), Pglycoprotein (P-gp, ABCB1), and breast cancer resistance protein (BCRP, ABCG2), 14 have been conrmed to be involved in the control of cysteine concentration. Among them, System X C À and its inhibitors (salicylazosulfapyridine, 53 sorafenib, 54 erastin, 55 and sulfasalazine 56 ) have been broadly used to inhibit GSH biosynthesis. Ruijie Chen et al. reported a nanocomposite to inhibit the GSH synthesis, where ZnO nanoparticles (NPs) were used as a carrier, loaded with salicylazosulfapyridine (SASP) and stabilized with DSPE-PEG to form ultra-small NPs (SASP/ZnO NPs). 53 SASP was used to inhibit the transport function of SLC7A11, resulting in the decrease of cysteine for GSH synthesis. As a consequence, treatment of tumor cells with SASP/ZnO NPs resulted in a disruption of redox balance in tumor cells, enhancing ZnO-induced cell death (Fig. 9B). Similarly, sorafenib (SFB) could block the System X C À transport system to inhibit the biosynthesis of GSH. 57 60 In the process of GSH recycling, GSSG can be in turn reduced back to GSH under the catalysis of NADPH and the GSH reductase (GR), which forms a redox cycle. Therefore, decreasing the level of NADPH may inhibit GSH production by breaking the redox cycle. 43 Yanjun Zhao et al. constructed a polymer containing hydrophilic poly(ethyleneglycol) (PEG) that was linked by azobenzene with a nitroimidazole-conjugated polypeptide. 61 The nitroimidazole moiety was reduced by the overexpressed nitroreductase with reduced NADPH as the cofactor, resulting in transient depletion of NADPH. The lack of NADP impaired both the glutathione and thioredoxin redox cycle, leading to diminished intracellular glutathione and thioredoxin. As a result, tumor redox homeostasis was destroyed, which enhanced the sensitivity of RSL3induced ferroptosis (Fig. 10). This RDH strategy via blocking NADPH to decrease GSH is still less studied. Besides azobenzene, etomoxir, a carnitine palmitoyltransferase 1 inhibitor, can also decrease NADPH levels, thereby reducing the GSH content by inhibiting fatty acid oxidation. 62 Etomoxir has not been used for the construction of nanomaterials for RDH.

Decreasing the GSH level of tumor cells to realize RDH via chemical approaches
Decreasing GSH via biosynthesis approaches is an effective and specic strategy, but the underlying biological mechanisms and possible side effects require further study. Using chemical approaches to deplete GSH involves introducing some substances that can react with GSH. Compared with the suppression of GSH biosynthesis, chemical approaches are a more straightforward way. However, the function is temporary and short-lived, as tumor cells may increase the GSH level through alternative pathways. Currently, chemical approaches can be divided into two categories: (1) consumption of GSH by oxidizing agents, and (2) consumption of GSH by electrophiles.
3.2.1 Consumption of GSH by oxidizing agents. Converting GSH to GSSG using oxidizing agents is a rapid method to consume GSH. Oxidizing agents include transition metal (Mn, Cu, W, Mo, and Au) ions. Numerous research groups made extensive effort in this area. Weihong Tan et al. reported a smart chlorine 6 (Ce6)-manganese dioxide (MnO 2 ) nano drug delivery  system. 63 MnO 2 consumed GSH to induce RDH, thereby sensitizing photosensitizer Ce6-induced PDT effects (Fig. 11A). Mn 2+ could not only consume GSH but also display Fenton-like properties. [64][65][66] Based on this, Xiaoyuan Chen et al. constructed MnO 2 -based mesoporous silica nanoparticles (MS@MnO 2 NP). 67 Once taken up by cancer cells, the MnO 2 shell was used to oxidize GSH to accumulate H 2 O 2 , which led to RDH, ultimately enhancing Mn 2+ -mediated CDT. Compared with Mn ions, Cu + displays higher efficacy of CDT. [68][69][70] Our group developed self-assembled copper-amino acid mercaptide nanoparticles (Cu-Cys NPs). 71 Cu 2+ in Cu-Cys NPs was rst reduced by local GSH to generate Cu + and simultaneously realize GSH depletion, leading to RDH. Subsequently, this state of the cells was more sensitive to the cytotoxicity of Cu + -catalyzed toxic cOH (CDT). These cascade reactions can perfectly achieve "two birds with one stone" in cancer treatment (Fig. 11B). In addition, Yuliang Zhao et al. found that oxidative W(VI) also had the capacity to react with intracellular GSH in cancer cells. 72 They developed Gd-containing polyoxometalateconjugated chitosan (GdW 10 @CS nanosphere) as a radiosensitization system. The GdW 10 @CS nanospheres were able to promote the depletion of intracellular GSH by synergy W 6+triggered GSH oxidation. As a result, the as-synthesized GdW 10 @CS nanospheres disrupted the redox homeostasis of tumor cells, thereby facilitating the efficiency of radiotherapy (Fig. 11C). To achieve more specic radiotherapy, they further explored bismuth heteropolytungstate (BiP 5 W 30 ) nanoclusters as radiosensitizers. 73 In this system, BiP 5 W 30 had the capability of depleting GSH via redox reaction owing to its unique electron structure and multi-electron properties. GSH consumption induced RDH, sensitizing high-Z elements like Bi and Wenhanced radiotherapy. In recent years, it has been widely reported that gold (Au) as a radiosensitizer can exhaust GSH by the formation of an Au-S bond, showing great promise in preclinical studies. 74 Wu et al. prepared histidine-capped gold nanoclusters (Au NCs@His) for GSH depletion and RDH, thereby sensitizing Au-mediated cancer radiotherapy. 75 3.2.2 Consumption of GSH by electrophiles. Besides the use of oxidizing agents to deplete GSH, many electrophilic reagents that can directly react with thiol groups are also used to consume GSH, leading to the RDH of tumors. Compared with oxidizing agents, electrophilic reagents may non-selectively react with nucleophiles, leading to undesirable toxic effects. Therefore, moderately active electrophilic such as maleimide, isothiocyanate and sulforaphane were used to consume GSH. For example, Minjie Sun et al. developed maleimide liposome (ML) adjuvants to deplete GSH for augmenting the photothermal immunotherapy of breast cancer. 76 The maleimide group of ML can irreversibly react with thiols of GSH by Michael addition. The depletion of GSH caused RDH, enhancing photothermal immunotherapy of breast cancer through promoting immunogenic cell death (ICD) and dendritic cell maturation (Fig. 12). In addition, Xiangliang Yang et al. employed a natural compound, b-phenylethyl isothiocyanate (PEITC), found in cruciferous vegetables, to deplete intracellular GSH through the formation of the conjugate of PEITC with GSH. 77 They combined ICG-loaded hydroxyethylstarch-oleic acid conjugate (HES-OA) nanoparticles with PEITC for potent PDT. GSH depletion by PEITC led to the imbalance of redox homeostasis in tumor cells and signicantly enhanced ICG-based PDT. Furthermore, Guangjun Nie et al. utilized another naturally occurring compound, sulforaphane (SFN), stemming from broccoli, to deplete GSH via directly forming the GSH-SFN complex, which could be exported outside cells and result in disrupting tumor redox homeostasis. 78 Based on this, the authors constructed a codelivery system to simultaneously encapsulate two drugs, anticancer agent cisplatin (cis-dichlorodiammineplatinum(II), CDDP) and SFN. The use of SFN greatly improved the therapeutic efficacy of platinum (Pt)-based chemotherapy.

Decreasing other reducing species to realize RDH
Besides GSH, the thioredoxin/thioredoxin reductase (Trx/TrxR) system in cancer is another important antioxidant defense system. 79 TrxR, which is the major disulde reductase, can reduce the disulde of oxidized Trx (Trx-S 2 ) to a dithiol (Trx-(SH) 2 ). It has been reported that Au nanoclusters could effectively inhibit TrxR in the tumor cell cytoplasm. Xueyun Gao et al. constructed spatiotemporal controllable liposomal nanocomposites, co-loaded with Au nanoclusters and photosensitizer Chlorine 6 (Ce6). 80 Au nanoclusters effectively inhibited TrxR to induce RDH, which enhanced the Ce6mediated PDT efficacy. Similarly, Xianzheng Zhang et al. reported a porous MOF of Zirconium-metalloporphyrin PCN-22 (PCN), loaded with a TrxR inhibitor alkaloid piperlongumine (PL). 81 Inside tumor cells, the released PL could effectively inhibit TrxR-mediated H 2 O 2 elimination to disturb cellular redox homeostasis, improving the PDT efficacy (Fig. 13).

Regulation of both reducing and oxidizing species to realize RDH
In addition to single regulation of oxidizing species or reducing species in cells, simultaneous regulation of both oxidizing species and reducing species to realize RDH has also been widely developed. Compared with single regulation, simultaneous regulation of both of them may induce a faster and higher degree of RDH. Therefore, this "Two-way regulation" strategy has been widely used in various cancer therapeutic regimens including PDT, SDT, CDT, RT, starvation therapy, chemotherapy, immunotherapy, and ferroptosis. Currently, this section is divided into two groups: (1) two-way regulated RDH combined with monotherapy, and (2) two-way regulated RDH combined with combinational therapies.

Two-way regulated-RDH combined with monotherapy
To enhance the PDT efficiency, Xianzheng Zhang et al. constructed SRF@Fe(III)TA nanoparticles in which Fe 3+ and tannic acid (TA) spontaneously formed a network-like corona onto sorafenib (SRF) nanocores. 82 TA was arranged to chemically reduce Fe 3+ to Fe 2+ , inducing a Fenton-like reaction to produce cytotoxic oxidizing species cOH. Meanwhile, SRF can deactivate glutathione peroxidase 4 (GPX4) to block cellular antioxidant defense. Both of the above processes disrupted the redox homeostasis of tumor cells, enhancing photosensitizer methylene blue (MB)-induced PDT (Fig. 14A). To promote the SDT efficiency, Zhuang Liu et al. utilized CaCO 3 nanoparticles as the template to obtain pH-dissociable hollow metal-organic coordination nanostructures, where meso-tetra-(4-carboxyphenyl) porphine (TCPP) and ferric ions were chosen as the sonosensitizer and metallic center, respectively. 83 Upon endocytosis by tumor cells, the nanocomplex could fast dissociate to release Ca 2+ and BSO. The released Ca 2+ remarkably elevated intracellular Ca 2+ concentrations and resulted in mitochondria damage, which then induced severe ROS production and subsequently synergized with BSO-mediated GSH depletion to cause RDH. This strategy ultimately enhanced TCPP-mediated SDT (Fig. 14B) 93 Moreover, hemin (Fe 3+ ) acted as not only peroxidase producing cOH, but also glutathione peroxidase consuming antioxidant GSH, resulting in the destruction of redox homeostasis. Consequently, Fe 3+ would be converted to Fe 2+ , which sequentially triggered the drug DHA to burst C-centered free radicals. All the above processes amplied the immunogenic cell death effect and combated immunosuppressive tumors (Fig. 15B). Two-way regulated-RDH strategies have been widely developed to enhance the CDT efficiency. 94 Our group synthesized FeCysPW@ZIF-82@CAT Dz nanoparticles 13 in which the shell ZIF-82 can be degraded into Zn 2+ as a cofactor for catalase DNAzyme (CAT Dz) mediated CAT silencing and the electrophilic ligand for GSH depletion. This two-way regulation could induce RDH, sensitizing tumor cells to FeCysPW-induced CDT (Fig. 16A). To further enhance the efficacy of CDT, our group constructed DMON@Fe 0 /AT nanoparticles. 95 On one hand, S-S bond-rich dendritic mesoporous organic silica (DMON) could decrease the content of GSH in tumor cells. On the other hand, the catalase inhibitor (3-amino-1,2,4-triazole (AT)) could suppress the activity of catalase for H 2 O 2 accumulation. DMON@Fe 0 /AT can effectively inhibit the expression of ferroportin 1 to disrupt the cellular iron metabolism system, leading to the retention of iron in the cytoplasm for CDT. With DMONmediated GSH-depletion and AT-mediated H 2 O 2 production, DMON@Fe 0 /AT would dramatically induce RDH in tumor cells to enhance the CDT efficacy (Fig. 16B). This RDH-sensitized CDT strategy was also performed by other groups. For example, Xiaogang Qu et al. designed a nanosystem PZIF67-AT by modifying 3-AT and PEG on zeolitic imidazole framework-67 (ZIF-67) nanoparticles that were formed by bridging 2-methylimidazole (2-mim) and cobalt ions with a sodalite zeolite-type topology. 96 This system not only displayed SOD-like activity capable of converting O 2 c À to H 2 O 2 , but also could deplete GSH via catalase inhibitor AT, which led to an accumulation of H 2 O 2 . The above processes would ultimately cause RDH and intensify the Fenton-like reaction-based CDT. Furthermore, Guanbin Song et al. constructed b-lapachone (Lapa)-loaded iron oxide  nanocarriers (Fe 3 O 4 -HSA@Lapa). 97 The release of Lapa selectively increased the generation of H 2 O 2 via NADPH:NQO1 catalysis. Meanwhile, Lapa severely consumed intracellular NADPH, leading to the inhibition of GSSG to GSH catalyzed by glutathione reductase. Therefore, Lapa not only supplied sufficient H 2 O 2 but also inhibited GSH levels, causing RDH. Ultimately, this strategy boosted the efficiency of CDT.

Two-way regulated-RDH combined with combinational therapies
It is well established that monotherapy is less effective than combinational therapies. Therefore, the two-way regulated-RDH could be used to further strengthen the combinational therapy, which has received tremendous attention. Peng Huang et al. used PEG-modied GOx as a template to construct biodegradable copper-doped calcium phosphate nanoparticles, followed by the loading of doxorubicin (DOX), named PGC-DOX. GOx could effectively catalyze intracellular glucose to produce H 2 O 2 . 98 Meanwhile, the released Cu 2+ ions could react with GSH and induce GSH depletion. Cu 2+ could simultaneously be reduced to Fenton agent Cu + ions. The above processes would ultimately perturb redox homeostasis, resulting in enhanced Cu + -induced CDT and DOX-mediated chemotherapy (Fig. 17A). Similarly, Hangrong Chen et al. constructed a novel albumin-based multifunctional nanoagent, in which CuO and MnO x in situ grew inside the bovine serum albumin (BSA) through a facile biomineralization process, followed by the conjugation of the Pt(IV) prodrug to obtain the nal nanoagent. 99 In this system, copper species can catalyze H 2 O 2 to produce cOH by a Fenton-like reaction, while MnO x can react with GSH, leading to the GSH depletion. cOH accumulation and GSH depletion gave rise to a disruption of redox homeostasis, which promoted better chemotherapy and CDT efficacies (Fig. 17B).     100 Under laser irradiation, ICG simultaneously generated singlet oxygen and emitted heat to melt the phase-change material lauric acid (LA). The exposed CaO 2 reacted with water to rapidly generate H 2 O 2 and O 2 . The interaction between MSNs and GSH led to the release of Mn 2+ and GSH depletion. Thus, this system could accumulate H 2 O 2 and eliminate GSH to destroy redox homeostasis, which enhanced the efficacies of PDT and CDT (Fig. 18A). To further promote the efficiency of SDT-induced tumor therapy, Zhuang Liu et al. synthesized new nanomaterials to combine RDH with chemotherapy and radiotherapy. Gallic acid-ferrous (GA-Fe(II)) and BSO were co-encapsulated within liposome (BSO/GA-Fe(II)@liposome). 101 GA-Fe(II) nanocomplexes were used as the catalyst of Fenton reaction to enable persistent conversion of H 2 O 2 to highly cytotoxic hydroxyl radicals (cOH), leading to the increase of oxidizing species. Meanwhile, the released BSO could inhibit the biosynthesis of GSH, resulting in the decrease of reducing species. Thus, GA-Fe(II)-mediated cOH production and BSO-mediated GSH depletion would dramatically enhance RDH in tumor cells, which then resulted in remarkably improved therapeutic efficacies of concurrently applied chemotherapy or radiotherapy (Fig. 18B).

Conclusions and perspectives
This review summarizes the current progress of RDH for tumor therapy. This strategy focuses on the use of nanomaterials to disturb redox homeostasis by regulating intracellular oxidizing states, reducing states or both of them at the same time, which increases the sensitivity of tumor cells to various therapeutic modalities. Although nanomaterial-mediated RDH has been widely applied in the research of tumor therapy, many important scientic problems still need to be considered before it is ready for clinical transformation.
First, it is critically vital to deeply explore cellular biological mechanisms involved in redox homeostasis, which is benecial to further optimize the RDH strategy at the genetic and molecular levels. Second, redox homeostasis is ubiquitous in various cells. 102,103 Thus, it is necessary to make tumor cells more specic and sensitive for the RDH strategy, avoiding potential toxic and side effects on normal tissues and organs. Finally, the most important issue is to improve the efficacy of RDH strategy-mediated tumor treatment. At present, most of the tumor treatments are based on ROS-mediated oxidative stress to kill tumor cells. In view of the resistance of tumor cells to ROS-mediated therapy, some new tumor therapeutic modalities could be combined with RDH. On the one hand, there is an urgent need to explore novel active species other than ROS/RNS, such as alkyl radicals (Rc) 104,105 or chlorine radicals (cCl), 106 which may exhibit higher efficacy in tumor treatment. On the other hand, reductive stress attracts increasing attention, 107 which could break through the bottleneck of existing oxidative stress-mediated tumor treatment.
In summary, the RDH strategy, as a research hotspot in the eld of tumor therapy, has attracted increasing attention of worldwide researchers, despite the existence of some unresolved issues. Under the guidance of the RDH strategy, researchers are still exploring various novel nanomaterials for accurate regulation of redox homeostasis to enhance tumor therapy efficacy. We expect that the development of the RDH strategy will not only provide valuable insights for the design of various functional nanomaterials, but also bring more breakthroughs in the treatment of cancer or other diseases in the future.