Strategies for enhancing cancer chemodynamic therapy performance

Abstract Chemodynamic therapy (CDT) has emerged to be a frontrunner amongst reactive oxygen species‐based cancer treatment modalities. CDT utilizes endogenous H2O2 in tumor microenvironment (TME) to produce cytotoxic hydroxyl radicals (•OH) via Fenton or Fenton‐like reactions. While possessing advantages such as tumor specificity, no need of external stimuli, and low side effects, practical applications of CDT are still impeded owing to the heterogeneity, complexity, and reductive environment of TME. Over the past couple of years, strategies to enhance CDT for efficient tumor regression are in rapid development in synergy with the growth of nanomedicine. In this review, we initially outline the fundamental understanding of Fenton and Fenton‐like reactions and their relationship with CDT. Subsequently, the development in the design of nanosystems for CDT is highlighted in a general manner. Furthermore, recent advancement of the strategies to augment Fenton reactions in TME for enhanced CDT is discussed in detail. Finally, perspectives toward the future development of CDT for better therapeutic outcome are presented. This review is expected to draw attention for collaborative research on CDT in the best interest of its future clinical applications.


INTRODUCTION
Cancer mortality cases follow a rising curve owing to the low prognosis and rapid metastasis. [1] Current clinical treatment methods of cancer include chemotherapy, surgery, and radiotherapy. [2] However, complete eradication of tumor and nullifying the possibility of future metastasis through these methods are still difficult to achieve. Additionally, non-specificity and drug resistance of chemotherapeutic drugs, as well as post-surgery trauma from surgical removal of tumor and unavoidable side effects by radiotherapy limit efficient applications of current cancer treatment methodologies. [3] Urgent need of minimally invasive treatment strategies has compelled researchers to investigate alternative approaches. [4] Reactive oxygen species (ROS) in cancer cells are overexpressed as compared to that of the normal cells, thereby mak- ing cancer cells more prone to oxidative damages. [5] Even though current chemotherapy and radiotherapy modalities utilize ROS generation pathways for cancer treatment, there is still an urge of finding new modalities for ROS-based therapy by utilizing tumor associated nutrients. Tumor microenvironment (TME) consists of several defining characteristics, that is, hypoxia, overexpression of hydrogen peroxide (H 2 O 2 ) and glutathione (GSH), low pH, aberrant vessels, and elevated consumption of cell nutrients. [6] ROS related mechanisms such as photodynamic therapy (PDT), sonodynamic therapy (SDT), and enhanced radiotherapy have been in development based on the sensitive response activity of TME. [7] These strategies generally use external stimuli of light, ultrasound, or X-ray for the activation of nanoagents to amplify tumor oxidative stress. Utilization of such noninvasive or minimally invasive paradigms is at the frontier of the cancer therapy research. If the characteristics of TME are considered in synergy of ROS  of  generation approach, more efficient solutions toward tumor ablation would be achieved. [8] In 2016, Shi and coworkers reported chemodynamic therapy (CDT) by achieving the synergy of TME and Fenton reaction to generate tumor-specific and cytotoxic hydroxyl radical (•OH) in cancer cells. [9] CDT utilizes Fenton or Fentonlike reactions to convert endogenous H 2 O 2 to highly pernicious •OH, which in turn can induce tumor apoptosis by protein inactivation, phospholipid peroxidation, and DNA damage. [10] CDT is reliant on the overexpression of H 2 O 2 and mild acidity of TME, and highly specific toward cancerous tissues, while showing less or no toxicity to healthy cells. Several other factors like high level of catalytic ROS generation, less reliance on external stimuli, deep tissue treatment ability, and non-multidrug resistance make CDT highly promising over other ROS-based therapy methods. By avoiding the usage of O 2 in the catalytic process, CDT demonstrates a new paradigm of hypoxic tumor treatment. [11] When combined with stimuli to augment the Fenton or Fenton-like reactions, the efficacy and sensitivity of CDT could be enhanced manyfold. [12,13] Fenton chemistry is generally employed in water treatment applications and has garnered ample attention for research. [14] The development in the materials design and theoretical optimization has considerably enhanced Fenton reaction performance in recent years, and thereby such studies act as a guide toward enhancing the efficacy of CDT. Primarily being a catalytic reaction, Fenton chemistry is often amplified by external stimuli such as heat, [10] light, [15] and ultrasound. [16] the regression of tumor cell antioxidant mechanism and modulation of TME to favor Fenton or Fenton-like reactions can noticeably augment •OH generation. A range of chemodynamic agents, for example, Fe 2+ , [17] Cu + , [18] Mn 2+ , [19] Mo 4+ , [20] W 4+ , [21] and Ti 3+ -based nanomaterials, [22] have been designed with enhanced CDT efficacy over the years. On the other hand, the inability to eradicate tumor entirely by single therapeutic approach of CDT paved the path of research toward newer system design for multimodal therapy and enhanced CDT.
In this review, we focus on recent significant advancements in the structural designs of chemodynamic agents and the strategies to enhance the CDT performance in tumor for amplified therapeutic effect (Scheme 1). At the beginning, we discuss the basic information on Fenton/Fentonlike reaction, followed by a brief summary on different types of CDT nanosystems, including metal-based nanosystems, framework structures, and carbon-based nanosystems. Furthermore, the strategies to optimize the performance of Fenton reaction and CDT are emphasized. The strategies are focused on the modulation of TME, the utilization of external stimuli, the nanoparticle designs to incorporate chemical and biological stimuli, and others, aiming toward better understanding of the design principles of CDT agents for best possible therapeutic outcome. Finally, the main hurdles for the clinical applications of nanosystems for CDT are reviewed thoroughly, along with brief perspectives to tackle such challenges.

 FENTON REACTION IN CHEMODYNAMIC THERAPY
From the moment Fenton reaction was reported, it has been extensively used for water treatment research. Broadly, the reaction between Fe 2+ and H 2 O 2 could produce •OH, which in turn could degrade the pollutants of water. [14] Several factors like high demand of H 2 O 2 and maintaining narrow optimum pH window are important for suitable industrial performance of Fenton reaction. Researchers have noted that the Fenton reaction can impart oxidative damage toward the nutrients (such as, DNA, protein, or lipid) of cancer cells, leading to therapy. Overexpression of H 2 O 2 (∼100 μM) and mild acidity of TME create proper setting for Fenton reaction (Fe 2+ + H 2 O 2 → Fe 3+ + •OH + OH − ) to proceed. [23] Given the very short half-life of •OH (10 −9 s), it is rational to design Fe-based nanosystems for targeted intracellular Fenton reaction with noninvasive treatment efficacy. [24] To further alleviate the narrow window of acidic pH for efficient cancer CDT, several nanosystems and nanozymes have been designed by utilizing other transition metal ions, that is, Mo 4+ , [20] Ti 3+ , [22] Cu + , [25] Mn 2+ , [26] Ag + , [27] and V 2+ . [28] For example, Fenton-like reactions by Cu + can proceed ∼ 160 folds faster than that of Fe 2+ and reported to be more proficient in generating noxious •OH in TME (pH 6.5-6.9). [29] Additionally, the byproduct of such Fenton-like reaction, Cu 2+ , can be reduced back to Cu + ion by overexpressed endogenous GSH. Intracellular redox hemostasis is maintained by GSH and thus a depletion of the same can alleviate antioxidant barrier for enhanced Fenton or Fenton-like reactions. Such transition metal induced Fenton-like reactions offer several advantages, such as optimal performance in near-neutral conditions and high natural abundance of structurally different oxide compounds. [30] Although Fe-based nanocatalyts require low pH conditions and high catalyst doses, they possess the optimum activities with minimal H 2 O 2 concentration, and low energy of activation compared to other species. [31] Active redox cycle feasibility in the pH condition, catalyst loading, and stability of oxidation products are some factors to be considered before precise design of Fenton/Fenton-like reaction based nanomedicine. The complex nature of TME as well as the complexity of preparing an "all-in-one" chemodynamic agent often hinder the full potential of chemodynamic cancer therapy. Thereby, the design of suitable Fenton nanosystems and the modulation of TME in favor of CDT are of foremost concern for this research direction.

 DIFFERENT KINDS OF NANOSYSTEMS
Owing to the irreplaceable role of the catalysts in Fenton reactions, the selection of a suitable nanoplatform is of utmost importance. Structure-activity relationship of different nanosystems with CDT is a comprehensive research direction. In the following section, we discuss the recent development of CDT. [32] Similarly, Yang and coworkers loaded ultrasmall γ-Fe 2 O 3 nanoparticles and glucose oxidase (GOx) into dendritic mesoporous silica (DMSN) spheres, synthesizing magnetic targeting nanoplatform (γ-Fe 2 O 3 -GOx-DMSN). [33] In acidic TME, the γ-Fe 2 O 3 nanoparticles could generate •OH from H 2 O 2 that was produced as a side product of glucose consumption by GOx, through Fenton reaction. Like metal oxide nanoparticles, semiconductor-like metal chalcogenide nanoparticles have been in use for CDT. Semiconductor nanoparticles are actively explored owing to their distinctive electrical and optical properties, and their incremental use in biological and clinical research is fascinating. Recently, Fan et al. synthesized a pyrite (FeS 2 ) nanozyme with glutathione oxidase (GSHOx)-and peroxidase (POD)-like activities for the depletion of GSH and abundant generation of •OH toward cancer therapy ( Figure 1A). [34] Additionally, electron deficient copper chalcogenides (e.g., Cu 2-x S and Cu 2-x Se) have been of high interest of studies for their plasmonic properties and Cu(I)-mediated Fenton-like reactions. Moreover, metal-based nanoclusters and noble metalbased nanosystems are highly useful for CDT application due to their TME responsive activation and action. For example, Fe 0 nanoparticles tend to release Fenton active Fe 2+ ion more easily as compared to iron oxide nanoparticles. Shi and coworkers synthesized amorphous Fe 0 nanocrystals (AFeNPs), which released ferrous ion in mildly acidic TME for CDT. To bridge the gap between mildly acidic TME and pH required for ideal Fenton reaction (pH 3-4), Bu and coworkers fabricated carbonic anhydrase IX inhibitor (CAI) on the surface of AFeNPs (AFeNPs@CAI) to reconfigure tumor acidosis ( Figure 1B). [35] The inhibition of overexpressed carbonic anhydrase IX in cancer cells resulted in extracellular alkalinity increase and intracellular acidity decrease, which in turn was beneficial for augmenting the efficiency of Fenton reaction and hence •OH-mediated oxidative damage of tumor through self-enhanced CDT. In addition, noble metal (e.g., Au, Pt, and Pd)-based monometallic or multimetallic nanozymes have garnered much attention owing to their enzyme mimetic activities and have been employed in biosensing, antibacterial, and tumor treatment. Recently, we synthesized an ultrasmall trimetallic (Pd, Cu, and Fe) alloy nanozyme (PCF-a), possessing synergistic glutathione peroxidase (GSH-Px) and POD mimicking activities in TME. [36] PCF-a exhibited both photothermal and ultrasound augmented •OH generation from endogenous H 2 O 2 for cancer therapy.

. Framework structures and carbon-based nanosystems
Apart from using inorganic nanosystems, polymers and amino acids can also be utilized as the backbone of the nanostructures. Such organic nanostructures are preferable owing to the structural uniformity, physiological stability, and biocompatibility, compared to the other nanomaterials. Zhang et al. fabricated a hybrid semiconducting nanozyme (HSN) through Fe 2+ -chelation to amphiphilic semiconducting polymer PEGylated poly[(thiadiazoloquinoxaline- of  alt-benzodithiophene)-ran-(cyclopentadithiophene-altbenzodithiophene)] (pTBCB-PEG) ( Figure 1C,D). [37] HSN acted as an NIR-II photothermal transducer, which in turn potentiated Fe 2+ -mediated Fenton reaction to improve the outcome of therapy. Similarly, Li and coworkers synthesized self-assembled copper-amino acid mercaptide nanoparticles (Cu-Cys) for in situ GSH-activation and subsequent H 2 O 2 -reinforced CDT. [29] MOFs have been of ample interests in cancer therapy due to their porosity and easily modifiable structural composition along with enzyme-like properties in some cases. [38] Well-designed structural changes by utilizing Fenton-active metal nodes can provide the multifunctionality for practical applications. Based on such characteristics, Tang et al. synthesized a dihydroartemisinin (DHA) loaded and CaCO 3 coated Fe-TCPP [(4,4,4,4-(porphine-5,10,15,20-tetrayl) tetrakis(benzoic acid)] nMOF (nanoscale MOF) for multimodal therapy. [39] In acidic and reductive TME, DHA is released and TCPP is activated, facilitating the synergistic Fe 2+ -DHA-mediated CDT, TCPP-mediated PDT, and Ca 2+ -DHA-mediated oncosis therapy. Similarly, we developed a Prussian blue analogue-based MOF, namely PEG-modified copper hexacyanoferrate (Cu-HCF), having GSHOx-and POD-mimicking activities ( Figure 1E). [40] Cu-HCF singlesite nanozymes (SSNEs) could deplete intracellular GSH and promote the conversion of single-site Cu 2+ species into Cu + for augmented •OH generation through a Fenton-type Haber-Weiss reaction, showing the elimination of cancer cells in vivo.
At the same time, carbon-based nanomaterials are of interest because of their POD mimicking enzymatic activity to propel the decomposition of H 2 O 2 into •OH. Zhang et al. fabricated GOx-decorated N-doped carbon (NC) nanoparticles as a biomimetic nanozyme (NC@GOx). [41] GOx mediated starvation therapy could in turn reduce the expression of heat shock proteins, enhancing NIR-laser mediated photothermal therapy (PTT). H 2 O 2 , generated as the byproduct of starvation therapy, could further augment CDT by NC@GOx for efficient tumor therapy. Additionally, various MXene nanocomposites, displaying good biocompatibility and photothermal properties, have been studied for tumor therapy applications. Wu et al. doped Fe 2+ ion into the layers of Ti 3 C 2 nanosheets, giving rise to a multifunctional nanoshell of Fe(II)-Ti 3 C 2 (FTC). [42] FTC could show high photothermal conversion efficiency as well as Fenton reaction-mediated CDT by Fe 2+ ion.

 STRATEGIES TO ENHANCE CHEMODYNAMIC THERAPY
TME is recognized for several distinctive characteristics like low pH, hypoxia, and overexpression of H 2 O 2 and GSH. [8] The focus of enhanced Fenton/Fenton-like reaction in tumor is to enhance the generation of ROS by diminishing the possible blocking pathways. Such strategies can be roughly outlined into TME modulation, usage of external stimuli, utilization of chemical and biological stimuli, and design of the nanoplatforms (Table 1). [16,17,22,[34][35][36]40,

. Tumor microenvironment modulation
The modulation of TME properties in favor of Fenton/Fentonlike reactions can augment chemodynamic cancer therapy efficacy. Lowering pH, increasing H 2 O 2 concentration, and blocking the activity of antioxidants in TME are major research targets for CDT. Such modulations are often achieved by precise multifunctional naonsystem design.

pH modulation
The Fenton catalysis activity of Fe 2+ is strongly dependent on solution pH. Fe 2+ reaches a maximum catalytic activity at around pH 3 through the formation of Fe(OH) 2 . However, the as-generated Fe 3+ tends to form inactive hydrous oxyhydroxides at higher pH, diminishing the potential of efficient Fenton reactions. [106] The intracellular pH (5-7) of solid tumors poses a significant chemical barrier for efficient Fenton reaction. To overcome this issue, strategies to reduce the intracellular pH of tumors have been proposed. Song and coworkers developed an acidity-unlocked nanoplatform (FePt@FeO x @TAM-PEG) with loaded pHresponsive tamoxifen (TAM) drug ( Figure 2A). [43] The release of TAM drug can silence mitochondrial complex I in cancer cells, leading to intracellular H + accumulation through the upregulation of lactate content. The low pH can in turn result in the release of FePt@FeO x nanocatalyst to generate cytotoxic •OH from H 2 O 2 through Fenton-like reaction ( Figure 2B). FePt@FeO x @TAM-PEG treated cancer cells showed enhanced damage of DNA as compared to the control groups ( Figure 2C). The enhanced ROS production was proven to be effective for in vivo therapy. Bu et al. synthesized a nanocomposite (UMP) by coating NaYF 4 :Yb,Tm@NaYF 4 upconversion nanoparticles (UCNPs) with MIL-88B MOF along with the loading of photoacids. [44] Rapid proton dissociation from photoacids was observed when UMP was irradiated with 980 nm laser, leading to lowered pH in tumor cells.

4.1.3
Glutathione depletion Regular metabolism in cells is capable of self-protection by converting the produced ROS into O 2 or H 2 O via antioxidant mechanism. Such defense systems are mainly comprised of enzymes (GSH-Px, superoxide dismutase, and catalase), and reducing agents (cysteine, vitamin C, and GSH). [107] In cancer tissues, the antioxidant substances are highly overexpressed to counterattack the high level of ROS, endowing tumor cells with the resistance against ROS based therapy. Thereby, the downregulation of such antioxidants, like GSH, is of high impact toward the overall ROS generation for tumor therapy. Several studies have been persuaded to construct nanosystems with GSH depletion property in synergy to Fenton or Fenton-like reactions to augment the CDT efficiency. [109] One of the strategies is to inhibit the intracellular production of GSH by γ-glutamylcysteine synthetase inhibitor. Liu et al. used L-buthio-nine sulfoximine as such inhibitor along with ultrasmall gallic acid-ferrous (GA─Fe(II)) complex as Fenton reactor inside a liposome for GSH-depletion enhanced CDT. [110] Chen et al. reported that inorganic MnO 2 had GSH-depletion abilities and thereby could enhance the CDT performance through Mn 2+ -mediated OH generation and synergistic disruption of endogenous antioxidant defense mechanism. [111] Additionally, nanoparticles possessing disulfide (S─S) or diselenide bond (Se─Se) are useful in depleting Reproduced with permission. [58] Copyright 2020, Wiley-VCH GSH via competitive reactions. Bu and coworkers reported a biodegradable nanocarrier (DMON@Fe 0 /AT) by co-loading iron (Fe 0 ) dots and a catalase inhibitor (3-amino-1,2,4triazole (AT)) inside S─S bond-rich dendritic mesoporous organic silica nanoparticles (DMON). [55] In mild acidic TME, Fe 2+ is released to participate in CDT, while DMON persistently depletes endogenous GSH to elevate the oxidative damage.
The reduction from Cu 2+ to Cu + is possible by GSH in TME owing to the low redox potential (∼ 0.16 V) of Cu 2+ /Cu + redox pair, followed by the depletion of GSH concentration. Yang et al. synthesized PtCu 3 nanocages (PtCu 3 -PEG) to mimic both horseradish peroxidase (HRP) and GSH-Px for enhanced CDT ( Figure 3A). [58] PtCu 3 nanocages exhibited uniform size and morphology distribution and could act as an HRP-like nanozyme to decompose H 2 O 2 into •OH ( Figure 3B). Along with ultrasound mediated ROS generation, the GSH-Px-mimicking property allowed the nanocages to deplete GSH by the oxidation of H 2 O 2 . Upon ultrasound irradiation, PtCu 3 nanocages could significantly suppress tumor volumes in vivo ( Figure 3C). GSH and ROS staining of the treated tissue sections revealed a proportional relationship between ROS generation and GSH depletion, further confirming the enhancement of oxidative damage through GSH depletion ( Figure 3D). Similarly, dual enzyme-mimicking pyrite nanozymes were synthesized by Fan and coworkers. [34] The nanozymes exhibited GSHOx-like activity to deplete intracellular GSH concentration, which in turn complemented the •OH generation ability through POD-like activity.

. External stimuli
The rate limiting steps of Fenton/Fenton-like reactions are often governed by the surrounding conditions such as pH and temperature. The utilization of external stimuli, for example, light, ultrasound, and magnet, can provide point-of-care therapeutic performance along with the enhancement of reaction rate through promoting the mass transfer ratio.

Light
The induction of Fe 3+ /Fe 2+ redox cycle in Fenton-like reaction through photoenergy is a common practice to accelerate the disintegration of H 2 O 2 into free radicals. Under UV or visible light, Fe 3+ undergoes a photochemical reaction to form Fe 2+ along with the generation of •OH from H 2 O 2 . Such short wavelength mediated Fenton and Fenton-like process has been intensively used in water treatment applications. [112] The rate limiting step of Fenton reaction is the reduction of Fe 3+ /Fe 2+ by H 2 O 2 . The application of photoenergy to improve the efficacy of this step is the foremost motivation of the studies. However, photoinduced Fenton reaction is difficult to be utilized in CDT owing to the low tissue penetration depth of UV/vis light. To tackle this issue, Li and coworkers designed a nanolongan structure, possessing an UCNP core covered by gel particles made from 2,3-dimethylmaleic anhydride and Fe 3+ cross-linked polyethylenimine. [59] Upon NIR irradiation, the UV emission by UCNPs promoted the reduction of Fe 3+ /Fe 2+ , which in turn facilitated effective ferroptosis-apoptosis combined antitumor therapy. Along with the induction of Fe 3+ /Fe 2+ redox cycle by photons, the increase in temperature from adjacent photothermal materials can accelerate Fenton or Fenton-like reaction kinetics. Photothermal systems often absorb wavelengths of NIR-I window (700-900 nm) and NIR-II window (1000-1700 nm) for excitation, which can bypass the photon absorption and scattering effect restriction of the tissue components. [70,113] Photothermal component in a nanocomposite is utilized to either initiate or synergistically enhance Fenton reaction mediated CDT. Lin et al. reported an NIR light triggered F I G U R E  (A) Illustration of an NIR light-triggered Fe 2+ delivery agent (denoted as LET-6) for photothermal enhanced CDT. Reproduced with permission. [60] Copyright 2020, Wiley-VCH. (B) Schematic illustration of the preparation process of GSM nanotheranostic agent for TME-responsive photoacoustic (PA)/magnetic resonance (MR) dual imaging guided NIR-II photothermal-chemodynamic therapy. Reproduced with permission. [61] Copyright 2020, Wiley-VCH Fe 2+ delivery agent (LET-6), comprised of Fe 2+ chelated 4′-(amino-methyl phenyl)-2,2′:6′,2″-terpyridine modified cyanine ( Figure 4A). [60] LET-6 showed monodispersity with ∼ 50 nm average diameter along with an absorbance peak at ∼ 823 nm, indicating its potential as 808 nm photothermal transducer and photoacoustic contrast agent. The photothermal heating of LET-6 resulted in thermal expansion of the structure, exposing Fe 2+ into the TME and inducing augmented CDT through Fenton reaction. The enhanced •OH generation could be detected by the electron spin resonance (ESR) spectra. Additionally, laser irradiated LET-6 group exhibited significant tumor suppression compared to the control groups. By following a similar idea, Ju and coworkers constructed β-lapachone loaded metal-organic coordinated nanoparticles comprised of Cu 2+ as the node, 1,4,5,8tetrahydroxyanthraquinone and banoxantrone dihydrochloride as the organic ligands, and folic acid functionalized PEG as the stabilizing ligand. [65] Upon 1064 nm laser mediated photothermal heating and GSH reduction, the released βlapachone could trigger an intracellular cyclic reaction to generate abundant H 2 O 2 for further acceleration of Cu +mediated Fenton-like reaction, thus effectively enhancing the CDT efficacy. Recently, Yu and coworkers fabricated molybdenum diphosphide (MoP 2 ) nanorods for mild PTT enhanced CDT of oral cancer. [71] Moreover, the photothermal effect is often used in synergy with the Fenton reaction to augment CDT. Huang and coworkers constructed a nanocomposite GSM (GNR@SiO 2 @MnO 2 ) comprised of gold nanorods (GNR) with silica dioxide (SiO 2 ) and manganese dioxide (MnO 2 ) coating through a plasmonic modulation strategy ( Figure 4B). [61] The MnO 2 layer could be degraded into Mn 2+ ion upon exposure to endogenous acidity and the released Mn 2+ ion participated in Fenton-like reaction for CDT. NIR-II laser mediated PTT from GSM was utilized to ablate cancer cells in vitro, and the photothermal heating augmented the Fenton-like reaction. The combinational therapy could suppress tumor volume in vivo, showing the highest apoptotic signaling as compared to the control groups. For another work, Yin et al. fabricated a hollow Mn/Cu/Zn-MOF with ICG loading and MnO 2 coating for fluorescence and photothermal imaging guided multimodal therapy (PTT/PDT/CDT). The 808 nm laser mediated photothermal effect could induce local hyperthermia for accelerated •OH generation to enhance CDT. Additionally, CDT can often be used as a light induced therapy in synergism with PDT, and multiple ROS generation induces enhanced oxidative stress in TME. [114]

X-ray
Although researchers have made significant progress for using light as the external stimulus to augment CDT, the penetration depth of light is still less effective for deep tissue tumor treatment. X-ray possessing a wavelength range of 0.001-10 nm can produce a dose to deeply seated tumors. As a mild dose of X-ray can be tumor specific by sparing the adjacent healthy tissues, researchers are interested in utilizing X-ray as a viable physical excitation source. Studies revealed that mild X-ray dosage can activate the nanomaterial surface to elevate Fenton catalysis efficacy. Zhao and coworkers constructed a smart radiosensitizer based on Cu 2 (OH)PO 4 nanocrystals, which can generate Fenton active Cu + sites on the nanocrystals under low dose X-ray irradiation as a result of X-ray-induced photoelectron transfer process ( Figure 5A). [72] Terephthalic acid assay indicated that the nanocrystals could enhance the generation of •OH from Reproduced with permission. [72] Copyright 2019, American Chemical Society H 2 O 2 under X-ray induction as compared to the controls, which was further supported by the 2′,7′-dichlorofluorescin diacetate (DCFH-DA) assay ( Figure 5B,C). Additionally, the nanocrystals under X-ray irradiation exhibited a substantial number of late apoptotic/necrotic cells in comparison to the other groups, owing to the enhanced CDT ( Figure 5D). Zhao and coworkers synthesized Cu 3 BiS 3 nanocrystals functionalized with amphiphilic D-α-tocopherol polyethylene glycol 1000 succinate (TPGS-Cu 3 BiS 3 ) for both NIR-II light and X-ray mediated combinational radiotherapy and PTT, along with enhanced CDT. [74]

Ultrasound
Ultrasound could generate noninvasive sound waves (∼ 20 kHz), which has been in extensive clinical use for diagnosis and treatment. When travelling through liquid, ultrasound creates concentrated shock waves to produce cavitation bubbles and subsequent intense local vibration. [115] The mass diffusion resistance of the substrates can be diminished by ultrasound owing to the acoustic cavitation effect, augmenting the Fenton and Fenton-like catalytic reaction rates substantially. For instance, Yang and coworkers constructed one-dimensional ferrous phosphide nanorods for both photothermal and ultrasound assisted CDT. [16] Upon exposure to low intensity ultrasound, the conversion rate of Fe 3+ to Fe 2+ was accelerated for enhanced CDT, resulting in the tumor ablation in vivo. Cheng and coworkers synthesized ultrafine titanium monoxide (TiO 1+x ) nanorods to improve sono-sensitization and Fenton-like catalytic activity for ROS therapy of cancer. [22] Recently, we reported a one-pot synthesis of bismuth ferrite nanocatalysts (BFO) for ultrasound-augmented CDT against malignant tumors ( Figure 6A). [75] Under ultrasound application, the cavitation effect could enhance the generation of •OH from H 2 O 2 at TME mimicking conditions ( Figure 6B). BFO exhibited significant killing effect to cancer cells under ultrasound in vitro as compared to the control groups ( Figure 6C). Additionally, the introduction of low intensity focused ultrasound (LIFU) system could potentially minimize normal tissue damage by focusing on the tumor region through the minimum energy attenuation. Li and coworkers reported a magnetic nanoreactor (PLGA-SPIO&Vc) comprising of vitamin C and superparamagnetic iron oxide for LIFU-accelerated Fenton reaction with high tumor killing efficacy. [  (G) TUNEL staining images of tumor tissues from the mice after different treatments. Reproduced with permission. [80] Copyright 2021, Wiley-VCH simulate magnetically electronic catalysis through the induction of eddy currents in magnetic nanostructures. Bu and coworkers reported a Janus cubic-sphere FePt-FeC magnetic heterostructure as a nanoscale catalyst ( Figure 6D). [80] Under mild AMF, the electron transfer proceeded from FeC to FePt in the heterostructure, leading to the electron density modulation and increased intracellular NAD + reduction efficiency. The applied AMF could further accelerate the Fenton reaction to generate •OH radical ( Figure 6E). FePt-FeC heterostructure treated 4T1 tumor could be eradicated under AMF simulation as compared to the control groups ( Figure 6F,G).

. Chemical and biological stimuli
Use of external stimuli often focuses on the instantaneous acceleration of the Fenton reaction. Chemical and biological stimuli, such as gas molecules, immune adjuvants, gene silencing, and nutritional components, can be integrated to enhance the postreaction process stimulated by CDT to augment the whole reaction efficacy.

Gas
Gaseous signaling molecules in living systems regulate many physiological and pathological processes. For example, nitric oxide (NO) can be used to sensitize chemicals and radiation, carbon monoxide (CO) can activate caspase through the dysfunction of mitochondria, and hydrogen sulfide (H 2 S) can enhance blood flow in tumor region by reducing blood vessel tension. [116] For this aspect, Cai and coworkers synthesized amorphous ferrous sulfide-embedded bovine serum albumin (FeS@BSA) nanoclusters for H 2 S-amplified ROS therapy of Huh7 cancer. [81] The released H 2 S gas, in response to acidic TME, suppresses the activity of catalase in cancer cells, resulting in enhanced accumulation of H 2 O 2 and subsequent Fenton reaction by Fe 2+ . NO in cancer cells can rapidly react with ROS to form reactive nitrogen species (RNS), which is more potent than the individual components. Yang et al. fabricated a multifunctional nanocomposite (UMNOCC-PEG) for acidic TME responsive Cu 2+ , H 2 O 2 , chlorin e6, and NO release, which can enhance CDT and PDT and subsequently generate cytotoxic RNS in the presence of light. [82] Moreover, He and coworkers efficiently encapsulated iron carbonyl (FeCO) Reproduced with permission. [91] Copyright 2019, Elsevier and MnO 2 nanoparticles in mesoporous silica nanoparticles for endogenous acidity triggered sequential release of ROS and CO gas for synergistic CDT and chemodynamic gas therapy. [83]

Nutrition
Distinct from the respiratory metabolism of the normal cells, tumor cells rely on the anerobic glycolysis (Warburg effect) for energy production. [117] Glucose acts as a key nutritional support for the proliferation and growth of tumor. Thereby, impeding the path of glucose at the source can precipitously consume the nutritional supplement at the tumor region, making it vulnerable to additional therapeutic attack. GOx is identified as a crucial enzyme for tumor starvation therapy, where GOx catalyzes the glucose decomposition in TME to produce H 2 O 2 that in turn benefits CDT. [108] Recently, Xiang and coworkers prepared a nanocomposite (LipoCaO 2 /Fe(OH) 3 -GOx) by coloading GOx and CaO 2 /Fe(OH) 3 in a biocompatible liposome ( Figure 7A). [87] Acidic TME can decompose CaO 2 /Fe(OH) 3 nanostructure to generate H 2 O 2 and Fe 3+ for triggering the Fenton reaction. The evolved O 2 as a byproduct can enhance the catalytic efficacy of GOx for the consumption of glucose to produce gluconic acid and H 2 O 2 , contributing to the elevated •OH production in CDT. The integration of such cyclic catalytic reactions is beneficial for the downregulation of hypoxia-inducible factor-1α, promoting effective cell death of hypoxic tumors. An obvious reduction in tumor growth was observed in vivo for LipoCaO 2 /Fe(OH) 3 -GOx nanoparticle-treated group through starvation enhanced CDT. In another work, Yang and coworkers constructed fusiform-like nanoparticles (PCN-224(Cu)-GOx@MnO 2 ) for synergistic starvation therapy and CDT. [86] The MnO 2 layer catalytically transformed H 2 O 2 to O 2 , which was sequentially utilized for glucose oxidation through starvation therapy. The released Cu + can react with abundant H 2 O 2 to produce cytotoxic •OH for CDT. Interestingly, Wu et al. constructed a theranostic nanocomposite, that is, nanoselenium (nano-Se)coated manganese carbonate-deposited iron oxide nanoparticles (MCDION-Se), to mimic starvation therapy induced CDT without the use of GOx. [88] The nano-Se exhibited enzymatic activity to produce O 2 •− and H 2 O 2 , followed by transforming to cytotoxic •OH by Mn 2+ via Fenton-like reaction. Additionally, the nano-Se and Mn 2+ inhibited adenosine triphosphate generation, thus promoting tumor starvation and increasing vulnerability of tumor cells toward CDT.

Immunization
Immunotherapy exploits the congenital immune intervention mechanism to amplify antitumor responses to fight against cancer metastasis and subsequent tumor recurrence. Immunization strategies, such as immune cancer vaccines, cytokine therapy, checkpoint blockade therapy, and T cell therapy, have received substantial interest owing to their promising medical potential. [118] However, tumor cells can evade immune recognition via T cell signaling disruption, immune intervention, and tolerance induction. Thus, the induction of acute local inflammation through immunostimulatory treatments can augment tumor immunogenicity and enhance the T cell infiltration to elicit tumor immunity. ROS generated by CDT can promptly induce acute inflammation and immune reactions at the tumor region. For example, Lin and coworkers constructed a degradable nMOF (Cu-TBP) to mediate synergistic hormone-induced CDT and light induced PDT ( Figure 7B). [91] The biodegradable Cu-TBF nMOF released Cu 2+ ion in TME, which reacted with intratumoral estradiol to produce H 2 O 2 , •OH, and O 2 •− for CDT. The generated ROS could release tumor-associated antigens through cell apoptosis to induce immunogenicity, as indicated by higher calreticulin expression in vitro for light induced Cu-TBP group. Moreover, the synergy of PDT, CDT, and antiprogrammed death-ligand 1 (α-PD-L1) could exhibit both primary and distant tumor regression in vivo through abscopal effect ( Figure 7C,D). In another work, Dai and coworkers constructed immunogenic cell death (ICD) inducer nanoparticles (MDP) by the self-assembly of DOX, phenolic MnO 2 nanoreactor, Fe 3+ , and PEG-polyphenols through metal phenolic coordination. [89] ROS-dependent cell death via CDT could accelerate the ICD induction and enhance tumor infiltrating T cell population. Such ICD enhancement approach could effectively increase the response of tumor to PD-1 checkpoint blockade immunotherapy.

Gene silencing
Progress of cancer is often correlated with various gene alterations and disorders. Tumor targeted delivery of nucleic acids can modify such gene conditions to potentiate cancer therapy. [119] Several cancer gene therapy approaches with high efficacy and less side effects have been reported over the past few years. [120] Gene silencing methods have been utilized to customize TME for the enhancement of Fenton and Fenton-like reactions. Overexpression of monocarboxylate transporters (MCTs) in cancer cells associates themselves with metastasis, angiogenesis, and tumor recurrence, and maintains the intracellular pH homeostasis. Thereby, MCTrelated gene therapy can modulate the TME pH in favor of Fenton/Fenton-like reaction for enhanced CDT. Shi and coworkers reported an amorphous iron oxide (AIO) RNAi nanoparticle platform to modulate the glycolysis pathway by silencing MCT4 to induce tumor cell acidosis. [92] Blocking of intracellular lactate efflux by MCT4 silencing resulted in enhanced H 2 O 2 production in addition to tumor acidosis, combination of which could amplify intracellular ironmediated Fenton reaction and oxidative damage to tumor cells. Additionally, redox homeostasis is often a key factor for ROS tolerance in hypoxic tumor, and the disruption of the same can potentiate ROS-mediated tumor therapy. Bu and coworkers reported a redox dyshomeostasis (RDH) strategy to combat hypoxic tumor based on a nanoplatform, namely FeCysPW@ZIF-82@CATDz ( Figure 8A). [ Figure 8B). The in vivo tumor growth was significantly inhibited by the treatment of FeCysPW@ZIF-82@CATDz compared to the control groups owing to the RDH-enhanced CDT ( Figure 8C).

. Materials design
In addition to the TME modulation and application of stimuli, materials design to modulate the properties for the enhancement of electron transfer favoring Fenton reaction and CDT is a unique approach. Moreover, tumor targeting property of materials endows them with enhanced accumulation in tumor tissues for augmented •OH generation.

Electron rich nanosystems
The modulation in electron density distribution through materials functionalization can alter the chemical potential of reactive electrons, stimulating substantial influence on the reaction activation energy.  Figure 9A). [96]

Tumor targeting
One of the main issues of CDT is the short lifetime and diffusion distance of •OH in TME. Tumor targeting approach can deliver the therapeutic systems directly into the cells, shortening the diffusion distance and directing higher concentration of cytotoxic •OH toward vulnerable biomacromolecules for elevated cell death ratio. Recently, Qiao and coworkers fabricated traceable multistage targeting nanoparticles (BDT-LAG) with spatiotemporal CDT efficacy. [99] Triphenylphosphine (TPP) and biotin endowed the nanoparticles with mitochondria and tumor targeting ability, respectively. BDT-LAG could effectively deliver α-tocopheryl succinate and lonidamine for targeted CDT, showing the tumor ablation in vivo. Chen et al. also reported a mitochondria-specific nanocatalyst comprised of cisplatin prodrug and gallic acidferrous (GA-Fe(II)) for augmented CDT ( Figure 9B). [101] Additionally, some studies utilized cell membranes to camouflage nanomaterials in order to increase their biocompatibility and homologous targeting. Li and coworkers synthesized 4T1-tumor cell membrane-coated bismuth/manganese oxide nanoparticles with high ICG payload (mBMNI). [100] Upon specific tumor targeting, the mBMNI could release Mn 2+ for CDT in response to GSH, along with light induced PTT and PDT.

Single-atom nanosystems
Heterogeneous catalysis is considered as a surface phenomenon in general. Thus, increasing the surface-active sites of a catalyst may lead to accelerated catalytic performance. Single-atom nanoparticles can achieve the maximum catalytic efficiency by utilizing such surface-active sites in the atomic dimension, showing enhanced specific activity. [121] Fenton active atoms like Fe can be used to construct the active sites in the single-atom catalysts for enhanced CDT. Shi et al. synthesized Fe-N 4 based single-atom nanocatalysts (PSAF) for tumor therapy through local heterogeneous Fenton catalysis ( Figure 9C). [122] The pyridinic N species in the carbon structure could stabilize the active Fe site and aid to the enhancement of •OH generation via a proton-mediated homolytic H 2 O 2 dissociation. PSAF showed high anticancer efficacy through a sustained catalytic process with a tumor regression rate of 63.49% and 40.01% upon intratumoral and intravenous injections, respectively ( Figure 9D). Different elements such as Pd, [103] Mn, [104] and Cu [105] have been in use as a replacement for Fe in the single-atom catalyst for enhanced tumor therapy. Li and coworkers synthesized FeN 3 P-centered single-atom nanozyme (FeN 3 P-SAzyme) to mimic the POD activity of natural enzymes for cancer therapy. [102] Computational analysis revealed that the electron donation from the phosphorus atom as well as the less positive charge of the metal Fe center (Fe δ+ ) led to the elevated catalytic activity of FeN 3 P-SAzyme. In vitro cell viability studies indicated that

 CONCLUSIONS AND PERSPECTIVE
The unique feature of TME establishes an equilibrium state of elevated ROS and overexpressed antioxidant mechanism in comparison to the normal cells. In this review, we have introduced Fenton and Fenton-like reactions responsible for cancer therapy and discussed the current research progress of nanosystem design for enhanced CDT. Furthermore, we have outlined the factors for augmenting CDT and how those factors can be incorporated in the design principles of chemodynamic nanosystems. Most of the aspects for enhanced CDT are focused on enhancing the efficiency of Fenton or Fenton-like reactions through increasing intracellular H 2 O 2 concentration, decreasing the concentration of reductants, proper design of chemodynamic nanosystems, and applications of physical and chemical stimuli. Although many commendable studies have enriched the field of CDT over the past couples of years, several important matters need to be evaluated and unraveled.
(1) One of the main barriers toward clinical applications of CDT is less systematic research on the long-term biosafety of chemodynamic nanosystems. Metabolization of the administrated nanosystems is of high importance, and the detailed information about the systemic toxicity and side effects of such nanosystems should be investigated before further applications. (2) Solid tumor hypoxia is a major issue for cancer therapeutic approach. Being out of reach by the treatment agents, hypoxic tumors are resistant to the treatments and responsible for tumor recurrence and metastasis. [123] Although Fenton reactions do not depend on the availability of O 2 , the delivery of chemodynamic agents in hypoxic TME is still a major task. The integration of hypoxia specific treatment modalities with CDT may lead to promising therapeutic outcome. [124] (3) Large scale synthesis of nanosystems is of high necessity when considered for clinical applications. In laboratory research settings, only a few milligrams of nanosystems are often synthesized, which need to go through thorough optimization to maintain the morphology for large scale production. Designing straightforward synthetic procedures combined with microfluidic technology may be a way out for overcoming this issue. Computer science and artificial intelligence can be of great prospect for the previsualization of chemodynamic agent structures and morphology as well as predicting the structure-activity relationship. Such technology can be utilized to design and synthesize personalized CDT nanosystems in large scale and provide personalized treatment solution for different cancer patients. Additionally, computational approaches can be applied toward determining the catalytic efficiency and toxicity of CDT agents like single-atom nanomaterials. (4) In depth study of Fenton or Fenton-like reactions in TME is still required in order to certify such ROS-based therapy for clinical applications. Generally, studies are proceeded toward in vivo therapy based on the simplistic understanding of in vitro simulated conditions. However, TME is much more complicated owing to the interference of different oxidants, reductants, enzymes, biomacromolecules, etc. Structure-activity relationship between chemodynamic agents and TME needs to be established for future development of CDT nanosystems. (5) ROS is overexpressed in tumor tissues as compared to the normal counterparts, and such anomaly makes cancer cells more susceptible toward oxidative damage. Endogenous •OH produced from Fenton or Fenton-like reactions causes structural damage to cancer cell lipids, DNA, and proteins. Still, there are less trials performed on clinical stage to shed light on issues such as the rate of ROS generation or the extent of oxidative damage in the dynamic TME. Additional attention can be given to the modulation of gene regulation and protein expression to enhance the ROS therapy. The inhibition of glycolysis pathway in cancer cells as well as the induction of G2/M phase arrest may pave innovative ways to sensitize tumor cells to CDT. [125] In addition to the elaborate studies on the role of ROS in tumors, the role of ROS in tumor therapy needs thorough investigations in the best interest of future ROS-based therapeutic strategies.
(6) The design of CDT-based combinational therapy is another important approach in the cancer therapy, aiming to maximize the CDT efficiency. While CDT is selective for TME and can treat hypoxic tumors, the underwhelmed therapeutic performance still limits the proper utilization. By integrating other therapeutic modalities with CDT, multimodal therapy can be achieved in order to eradicate tumor more efficiently. [13,126] The synergy between different therapeutic agents can enhance the potential of individual therapy. However, the aim to construct a nanosystem for multimodal therapy can lead to the complexity in structure, which in turn imposes concerns like non-biodegradability, non-biocompatibility, and systemic biotoxicity. The idea for simplifying the composition of chemodynamic agents while achieving multimodal therapeutic outcome needs further research. Additionally, the structures and composition of such nanosystems should be tailored according to the nature of the tumor and the condition of patients for personalized therapy.
Indeed, CDT is a promising cancer therapeutic modality with minimal side effects. In depth investigations of Fenton or Fenton-like reactions and further modification of the catalytic path would bring along additional possibilities for enhanced CDT. Efforts to answer the unresolved issues of CDT will promote the clinical translation of this approach in coming years. Interdisciplinary research among chemistry, biology, medicine, and materials science can push the boundaries for achieving satisfactory CDT efficacy in cancer treatment, eventually benefiting the patients.

A C K N O W L E D G M E N T S
This research is supported by the Singapore Agency for Science, Technology and Research (A*STAR) AME IRG grant (A20E5c0081), and the Singapore National Research Foundation Investigatorship (NRF-NRFI2018-03).

C O N F L I C T O F I N T E R E S T
The authors declare no conflict of interest.