Apoptosis as a programmed cell death maintains the balance between cell death and cell proliferation, regulating the process of carcinogenesis. One of the characteristics of cancer is evasion of apoptosis, which makes cancer cells live with the ability to keep proliferating and escape cell death (Hanahan and Weinberg, 2011). Due to this characteristic of cancer, apoptosis induction becomes an effective approach for cancer therapy, which can kill malignant cells with no activation of an inflammatory response and little side effects to healthy cells (Chaudhry et al., 2022).

Nanozymes with multiple enzymatic activities can effectively produce reactive oxygen species in the tumor microenvironment through cascade catalytic reactions, thereby promoting the apoptosis of cancer cells (Zhao et al., 2021b; Yao et al., 2022b; Wang et al., 2022f). Ma et al. (2022) designed nanozymes with three nanoceria structures including nanoceria-cube, nanoceria-poly and nanoceria-rod. Among them, nanoceria-rod with highest concentration of surface oxygen vacancies has the highest POD-like and OXD-like enzyme activity. Due to the suitable isoelectric point, nanoceria-rod nanozyme can selectively enter the lysosomes and phagosomes of tumor cells in acidic environments. Then a large amount of ROS is produced and mitochondrial membrane potential is reduced, resulting in malignant cell apoptosis with no toxicity to normal cells.

Typically, the mechanism of nanozymes to induce apoptosis is mainly via mitochondrial apoptotic pathway. For example, PP-MnO(x) NPs, an OXD-like nanozyme, can catalyze O₂ to produce abundant ROS and induce the decrease of mitochondrial membrane potential. Meanwhile, PP-MnO(x) NPs activate BAX and Caspase-3, as well as decrease the expression of Bcl-2 (Xi et al., 2021). Moreover, Chu et al. (2020) Synthesized M@AAO@HFe–TA, a nanozyme system with amino acid oxidase (AAO) and POD activity. M@AAO@HFe–TA system induced apoptotic in tumor due to amino acid starvation and cascade Fenton reaction effect. The expression of Bcl-2 was suppressed and the level of BAX was increased, releasing the cytochrome C (cyt C) to the further active caspase 3. These suggest that nanozymes induce ROS generation and oxidative stress to initiate mitochondrial-related apoptosis.

Ferroptosis is a type of iron-dependent non-apoptotic cell death related to abnormal level of intracellular ferrous iron. Recently, ferroptosis was found to be widely present in many cancers with characteristics of mitochondrial damage and plasma membrane disrepair (Chen et al., 2021b; Tang et al., 2021). The induction of ferroptosis can potentiate the efficacy of chemotherapy, radiotherapy and immunotherapy in drug-resistant cancer (Friedmann Angeli et al., 2019; Ye et al., 2020; Fan et al., 2021), suggesting that targeting ferroptosis strategy is a viable method for cancer treatment (Mou et al., 2019).

Ferroptosis is mainly provoked by ROS accretion and unrestricted lipid peroxidation, which is induced by excessive endogenous Fenton reaction and the exhaustion of GSH (Wei et al., 2020; Yang et al., 2021). Thus, nanozymes with peroxidase-like and glutathione peroxidase-like enzyme activity are designed as ferroptosis inducer for killing cancer cells (Meng et al., 2021). Zhang et al. (2022b) fabricated gemcitabine (Gem)-loaded carbonaceous nanoparticles (MFC-Gem) as peroxidase-like and glutathione oxidase-like nanozymes. The dual-activity MFC-Gem nanozyme effectively catalyzes the Fenton reaction to generate huge ROS and deplete GSH for oxidative damage. Meanwhile, glutathione peroxidase 4 (GPX4), a lipid repair enzyme which can use GSH to detoxify lipid peroxidation, loss activities in MFC and MFC-Gem treated mice, making cancer cells more susceptible to ferroptosis. Similarly, boron and nitrogen cooped graphdiyne (BN-GDY) as a metal-free nanozyme possesses capability to produce •OH in the presence of H₂O₂ and boost GSH depletion, setting the onset of ferroptosis. Interestingly, unlike the most reported enzymes, the catalytic reaction of BN-GDY undergoes a symbolic sequence Bi−Bi mechanism instead of a ping-pong Bi−Bi mechanism (Zhang et al., 2022a). Moreover, Fenton reaction-independent ferroptosis catalyzed by photothermal nanozyme is a novel insight for ferroptosis-related therapy, due to its satisfactory therapeutic effects in tumor therapy (Xing et al., 2021).

Due to uncontrolled cell proliferation, aberrant cancer cell metabolism, and abnormal blood vessel development, tumor microenvironment (TME) is generally characterized by hypoxia (Semenza, 2000). The hypoxic TME make tumor cells exhibit increased drug efflux and lessen ROS-induced DNA damage, resulting in strong drug tolerance and radiation resistance (Brizel et al., 1999; Brown and Wilson, 2004; Gacche and Assaraf, 2018; Li et al., 2022a; Li et al., 2022c; Wang et al., 2022e). Nevertheless, the intrinsic hypoxia in TME suppresses the infiltration of anti-tumor immune cells and even induces the polarization of macrophages and T cells to pro-tumor subtype, such as M2 macrophages and regulatory T cells (Zhang et al., 2022c). Therefore, hypoxic TME hinders the efficacy of immunotherapy and augmenting O₂ supply becomes urgent to improve cancer treatment (Kopecka et al., 2021). Nanozymes with CAT-mimic activity can transfer H₂O₂ to O₂ and supply enough O₂ to reverse hypoxia tumor microenvironment. Combined with GOx-like nanozymes which consume glucose and produced H₂O₂, self-supply O₂ system is proposed by scientists (Yu et al., 2022).

Yu et al. (2022) modified carbon nitride (C₃N₄) with polydopamine before coating it with MIL-100, Gox, and hyaluronic acid to create a PCMGH nanozyme system. C₃N₄, as a water-splitting material, catalyzes H₂O to generate O₂, whereas MIL-100 acts like POD to raise the quantity of ROS. Upon 808 nm irradiation, polydopamine acts as a photothermal agent which convert light energy into hyperthermia to achieve photothermal therapy. The self-supplying O₂ and photothermal effect work synergically to improve the performance of chemotherapy and phototherapy.

The mesoporous silica nanorod was served as nanoplatform to integrate MnO₂ and Au nanoparticles, forming a new biomimetic nanozyme with dual enzyme activities. By decomposing H₂O₂, MnO₂ can catalyze the production of O₂, displaying enhanced CAT-like activity. Meanwhile, the Au nanoparticles which exhibit GOx-like activity effectively accelerate the oxidation of glucose and provide enough H₂O₂ for more O₂ production. The alleviation of hypoxic environment surrounding tumor sensitizing tumor cells to radiation therapy and photothermal therapy (Yang et al., 2020).

In addition to directly enhance the efficacy of antitumor treatment such as chemotherapy, phototherapy and radiotherapy, increasing O₂ level in vivo can change the expression of genes linked with tumor metabolism, proliferation and metastasis via regulating hypoxia-inducible factor 1 (HIF-1) pathway. Au-Pt nanozymes are reported to relieve the hypoxic TME and suppress the expression of HIF-1α, thus reducing lung metastasis (Shen et al., 2022).

Phototherapies including photothermal therapy (PTT) and photodynamic therapy (PDT) are considered as novel strategies employed widely in tumor treatment, due to their advantages of highly selective, negligible side effects, and minimally invasive (Vieyra-Garcia and Wolf, 2021). PDT provides oxidative damage on the tumor primarily through singlet oxygen, which is converted from triplet molecular oxygen by the photosensitizer as part of the photoconversion reaction (Zheng et al., 2022). While PTT initiates tumor cell death predominantly through hyperthermia via plasmonic dissipation, which is converted by the energy of photons absorbed on the photothermal agent under near infrared (NIR) laser irradiation (Figure 3). However, strict conditions such as oxygen leakage, acidic condition and overexpression of GSH in the tumor microenvironment (TME) constrain the photothermal conversion efficacy and ROS production of Phototherapies (Song et al., 2022). Photosensitive nanozymes which can remodel the restricted environment in tumor are intensively studied by scientists to achieve better phototherapeutic efficiency (Wang et al., 2022a; Tang et al., 2022).

In fact, the ROS generation in PDT often ceases with oxygen depletion. To solve this dilemma, an intelligent nanoplatform Ce₆/Ftn@MnO₂ with the ability of O₂ self-supply was constructed, which can reverse unfavorable hypoxia conditions and decrease the expression of hypoxia-inducible factor (HIF)-1α. With adequate O₂, Ce6/Ftn@MnO₂ produced a significantly higher cytotoxic ¹O₂ amount. The enhanced photodynamic effect of Ce₆/Ftn@MnO₂ caused reduction of mitochondrial membrane potential and disruption of lysosome integrity, resulting in tumor cell death (Zhu et al., 2022b). To achieve profound photothermal therapeutic efficacy and minimized side effects, a “dual lock-and-key” type tumor-specific nanozyme (ABTS@PAH-CNts) was designed with higher selectivity. Activated under both H₂O₂ and acidic pH, ABTS@PAH-CNts present remarkable photothermal effect, with negligible off-target hyperthermic damage to normal tissues (Chen et al., 2021c).

It is worth noting that PTT can be used to integrate with PDT to get better synergistic antitumor effect. For instance, Ni_0.5Fe_0.5S₂ efficiently converts near-infrared light energy into heat for realizing the photothermal therapy. Moreover, the peroxidase-like property of Ni_0.5Fe_0.5S₂ triggers formation of cytotoxic •OH and ¹O₂ to further enhance the efficiency of PDT (Zeng et al., 2022a). Similarly, Song et al. (2022) generated a PTT-PDT dual mode nanoplatform of RGD-BPNS@SMFN, which had self-synergetic behavior and enhanced photonic response than single PDT and PTT. The key self-synergetic mechanism is the PTT promoted temperature-dependent CAT-like activity, which alleviates the hypoxia microenvironment and further encourages the PDT behavior. Indeed, PTT induced hyperthermic temperature can accelerate enzyme-catalyzed ROS generation, which enhances the further PDT therapeutic performance (Zhou et al., 2022). On the other hand, ROS produced by PDT decreases the expression of heat shock protein 70 (HSP70), which was found to protect cells from the damage of heat, thus augmenting photothermal effect (Sun et al., 2021b). Taken together, the combination of PTT and PDT achieves strikingly therapeutic efficacy.

In recent decades, immunotherapy is a hot topic in cancer therapy which revolutionizes cancer treatment because of its striking tumor ablation effect. Multiple immunotherapeutic strategies, including cancer vaccines, immune checkpoint blockade (ICB), and adoptive cell transfer, have been developed by scientists (Rudd, 2019; He and Xu, 2020). However, the application of cancer immunotherapy is limited by low response rates and individual differences mainly due to immunosuppressive TME and heterogenicity. Nanozymes can act as a modulator to reverse immunosuppressive TME by degrading immunosuppressive molecules, inducing infiltration of anti-tumor immune cells and repolarizing pro-tumor cells (Wang et al., 2022b). Thus, nanozymes can be utilized as adjuvant therapeutics to boost immunotherapy efficacy.

The ChA CQDs nanozyme, formed by carbon quantum dots and chlorogenic acid, was reported with GSH oxidase-like activities and induction of cell ferroptosis. In addition to directly inducing cancer cell death through catalytic therapy, ChA CQDs activated the tumor immune microenvironment and converted the “cold tumor” to “hot tumor” by recruiting CD4⁺/CD8⁺ T cells, macrophages, and NK cells (Figure 4). The increase in immune cells provides an important basis for subsequent immunotherapy, changing the situation that immunotherapy is ineffective in several types of cancer (Yao et al., 2022a).

A novel nanozyme (CuPP) with trienzyme-like activities has been proposed to efficiently modulate immunosuppressive TME by releasing O₂ and •OH but consuming glutathione (GSH). With the reversion of hypoxic microenvironment, macrophages in tumor site were repolarized from pro-tumoral M2 to anti-tumoral M1 phenotype, accompanied by the upregulation of IL-12 and the downregulation of IL-10. Moreover, the ratio of CD4⁺ and CD8⁺ T cells was elevated by further combining with αPD-L1, exhibiting strong immune responses (Zeng et al., 2022b). Except for αPD-L1, the synergistic effect is also achieved by combining nanozymes with CTLA-4 blockade. CaO₂/DOX@SiO₂/DOX-MnO₂ (CDSDM) nanozyme reactor was reported with self-oxygenation and alleviation of hypoxia-adenosinergic signaling. Through the positive immune modulation of CDSDM NRs, the infiltration of cytotoxic T cells was facilitated and the population of immunosuppressive regulatory T cells was decreased. On the other hand, the DOX in CDSDM NRs promoted the maturation of antigen-presenting dendritic cells. Thus, the reversion of the immunosuppressive TME is expected to favor CTLA-4-mediated immunotherapy (Wang et al., 2019b).

HA@Cu₂−xS were synthesized by hyaluronic acid, CuCl₂ and Na₂S, which were further modified by PEG to form HA@Cu₂−xS-PEG nanozymes (PHCNs). The PHCNs nanosystem effectively triggers apoptosis of tumor cells due to its dual photothermal and peroxidase-like catalytic properties. Tumor-specific antigen released from dead tumor cells facilitates the infiltration and activation of chimeric antigen receptor (CAR) T cells, presenting a synergistic effect with CAR T-cell therapy (Zhu et al., 2021).

Radiotherapy occupies an essential position in cancer treatment and has been widely applied for solid tumors in the clinic. However, radioresistance, mainly caused by hypoxia and antioxidant environment around tumor, constrains the efficacy of radiotherapy (Dai et al., 2022). On the one hand, tumor cells overexpress the DNA repair proteins, making them more endurable than normal cells (Roos et al., 2016). On the other hand, the toxicities of the radiation to normal tissues can cause gastrointestinal damage, lung fibrosis, and cognitive impairment in patients, preventing them from being given at high enough doses (Stone et al., 2003). Thus, how to improve the radiation sensitivity of tumors and protect normal tissues from radiation damage has drawn considerable attention (Gong et al., 2022a; Gong et al., 2022b). Therefore, highly selective and multifunctional radiosensitizers are urgently needed for radiotherapy enhancement. To address this issue, nanozymes that can target the tumor and improve hypoxia are promising strategies (Li et al., 2021; Zeng et al., 2021).

The tumor ablation effect of radiotherapy mainly relies on the radiation-induced DNA damage and toxicity of radiation-induced ROS. By coating pyrite (FeS₂) with cancer cell-derived exosomes, a nanozyme system with homologous targeting abilities, GSH-OXD and POD-like properties was created. The dual enzyme activities of FeS₂ reduce intracellular GSH levels and generate huge •OH to disrupt redox homeostasis and mitochondria integrity, thus significantly reducing radiotherapy resistance of cancer cells and amplifying the radiotherapeutic efficacy (Huang et al., 2021a).

GDY–CeO₂ nanozyme are synthesized by 2D graphdiyne (GDY) anchoring to the Cerium oxide (CeO₂) nanoparticles. The sustainable catalase activity of GDY–CeO₂ can decompose H₂O₂ and elevate O₂ concentration in TME, while high-Z element cerium in GDY–CeO₂ act as a radiosensitizer to enhance the intracellular radiation energy deposition (Figure 5). Moreover, combining miR181a with the GDY–CeO₂ further regulates the Serine/threonine-protein kinase Chk2 pathway and destroy the DNA repair system in cancer cells, strikingly reinforcing the effect of radiotherapy (Zhou et al., 2021a).

A novel nanoprobes (Mn-doped Ag₂Se QDs) are engineered by doping Mn (II) ions into Ag₂Se quantum dot, using Mn (II) ions as CAT mimic to translate H₂O₂ into O₂ and Ag₂Se QDs as a radiosensitizer and NIR-II fluorescent materials to enhance radiation energy deposition and NIR-II imaging. By conjugating arginine-glycine-aspartate (RGD) tripeptides and polyethylene glycol (PEG) with Mn-doped Ag₂Se QDs, Mn-doped Ag₂Se-RGD-PEG nanoprobes were formed showing great biocompatibility and tumor specificity. The specific tumor-targeting and NIR-II-emitting abilities of the nanozymes enable precise NIR-II imaging-guided radiotherapy with negligible damage to normal tissues, which can be applied to boost the efficacy of RT (Wang et al., 2021).

Conventional tumor chemotherapy, which is most commonly applied as first-line therapy for cancer in the clinic, remains a formidable challenge due to the severe side effects and drug resistance (Alkhatib et al., 2022; Zhu et al., 2022a; Wang et al., 2022d; Kang et al., 2022). Through tremendous research by scientists, multifunctional nanozymes are proven to be a promising avenue to achieve higher tumor elimination efficacy in synergistic chemotherapy and catalytic therapy.

On the one hand, nanozymes have the advantage of their enzyme-mimic activities to catalase abundant ROS generation, which cause redox homeostasis disruption and damage to the cell membrane (Yuan et al., 2022). The direct effect of nanozymes can alleviate drug resistance and even kill cancer cells. On the other hand, nanozymes as nanoparticles can encapsulate chemotherapy drugs and release them in the tumor position (Ning et al., 2021). The specific targeting tumor property of nanozymes loaded drug system makes it possible to get better tumor elimination with a lower dose of chemotherapy drugs and less normal tissue injury (Han et al., 2022).

Zeolitic imidazolate framework (ZIF)-based nanoparticles co-loaded with glucose oxidase (GOX) and doxorubicin (DOX) were created to synergistically enhance chemotherapy. GOX activity of DOX/GOX-loaded ZIF (DGZ) effectively consumes glucose and generates adequate H₂O₂, restraining the mitochondrial energy metabolism and promoting ROS production. As degradation of DGZ, Zn²⁺ was released to break antioxidation homeostasis in cancer cells via inhibiting reductase systems such as glutathione reductase and thioredoxin reductase. In case of energy shortage and ROS accumulation, the therapeutic effects of DOX delivered by DGZ get greatly enhanced. More importantly, DGZ releases DOX drug only in the weak acid environment surrounding tumor rather than in normal tissue (Figure 6). This high degree of targeting makes it possible to use higher doses of chemotherapy drugs to get better therapeutic outcomes with fewer side effects (Huo et al., 2022).

The hypoxia-related drug resistance and the lack of potential uptake of drugs also hamper chemotherapy application in tumor treatment. Zheng et al. fabricated Fe₃O₄/Pt-FLU nanozymes by loading Pt nanozymes and 5-fluorouracil on the Fe₃O₄ nanospheres to achieve improvement in the treatment of breast cancer. The POD-liked and CAT-liked activities of Fe₃O₄/Pt-FLU alleviate the hypoxia environment in TME by increasing the level of ROS and O₂, which leads to the reduction of drug resistance of cancerous tissue. Furthermore, Fe₃O₄/Pt-FLU specifically delivers 5-fluorouracil to the tumor site in a pH-sensitive drug release manner. The drug carrier role of Fe₃O₄/Pt-FLU significantly increases the stability of 5-fluorouracil and reduces the cytotoxicity in off-target tissues. The catalytic/chemotherapy double-modality therapy shows great therapeutic outcome for tumor elimination (Nie et al., 2022).

Ultrasound (US)—triggered sonodynamic therapy (SDT) is mainly based on low-intensity US and sonosensitizers, which can induce the generation of cytotoxic ROS for tumor elimination (Chen et al., 2021a; Huang et al., 2021b). Compared with traditional therapy, SDT has the advantage of noninvasiveness, high tissue-penetrating capability, and safety, showing promising applications in new generation antitumor treatment (Pan et al., 2018; Son et al., 2020; Sun et al., 2021a). However, the ROS yield of sonosensitizers requires adequate O₂ assistance which is strongly hampered by hypoxia of TME. Thus, synergistic therapy with nanozyme-based O₂ supply and SDT may be a practical strategy (Gong and Dai, 2021).

AIMP, a multifunctional nanozyme, is formed by linking Angiopep-2 (Ang) with PLGA, in which IR780 and MnO₂ are encapsulated. Through the function of Ang and IR780, AIMP can target cancer cells and remain inside the mitochondria. Upon low-intensity US irradiation, IR780 behave as sonosensitizer to produce ROS and cause damage to cancer cells. On the other hand, enzyme-like activity of MnO₂ is responsible for O₂ release and GSH depletion, thereby boosting the production of ROS and enhancing SDT effect (Liu et al., 2021b).

The Pt-CuS Janus loaded with tetra-(4-aminophenyl) porphyrin (TAPP) as a sonosensitizer is designed to facilitate SDT-based synergistic therapeutic modality. Upon exposure to US, TAPP are released from the hollow Pt-CuS Janus and interact with O₂ to produce ¹O₂. Meanwhile, Pt possesses CAT-like activity for the degradation of H₂O₂ and production of O₂, which provides an O₂ source for SDT-induced ROS production (Figure 7). Importantly, the photothermal effect of Pt-CuS elevates the Pt catalytic activity for O₂ generation and facilitates SDT efficacy (Liang et al., 2019).

Chemodynamic therapy (CDT) is an emerging therapeutic strategy that can destroy cancer cells through catalyzing the endogenous H₂O₂ into lethal hydroxyl radical by Fenton reaction or Fenton-like reaction (Zhou et al., 2021b; Jia et al., 2022). CDT takes advantage of high selectivity, low side effects and no requirement for exogenous stimulus. However, the limited concentration of endogenous H₂O₂ and excessive antioxidant GSH hinder the application of CDT. Supplying sufficient intratumoral H₂O₂ and depleting endogenous GSH are necessary to improve the efficiency of CDT. Recently nanozyme cascade platforms have been extensively studied and wildly applied to enhance the antitumor effect of chemodynamic therapy (Zhang et al., 2021; Zhang et al., 2022e).

The cascade reaction systems ACD is fabricated by loading copper ion-doped ZIF-8 with ZIF-8 doped with Au nanozymes and doxorubicin hydrochloride (Zhao et al., 2021a). The POD-like activity of Au nanozymes can catalyze H₂O₂ to ·OH for CDT. While Au can also serve as an oxidase to deplete the GSH. Moreover, the Cu²⁺ ions released from ACD could consume the GSH via redox reactions and the generated Cu⁺ can produce ·OH by Fenton-like reaction. Thus, the increased ROS and exhaustion of GSH can effectively amplify the antitumor effect of CDT. On the other hand, doxorubicin hydrochloride not only act as chemotherapeutic drug to kill cancer cells but also supply sufficient H₂O₂ to boost the effect of CDT (Figure 8). Herein, the application of nanozymes in CDT can achieve better treatment effects and are promising for tumor elimination.

Apart from increasing ROS production and promoting GSH depletion, strengthening the tumor targeting ability of nanodrug is another way to augment the effect of CDT. Au-DNA-Fe, a microRNA-triggered nanozyme cascade platform is fabricated for enhanced tumor-specific chemodynamic therapy. Au nanozyme as glucose oxidase-like nanozyme catalyze Glucose into gluconic acid and H₂O₂, while the produced H₂O₂ is subsequently catalyzed by Fe₃O₄ nanoparticles to generate hydroxyl radicals for CDT. Interestingly, Au and Fe₃O₄ nanoparticles are modified by two different single-stranded DNA which are complementary to each part of miRNA-21. Due to the overexpression of miRNA-21 in cancer cells, the nanozyme Au-DNA-Fe can trigger the cascade reaction specifically in cancer cells with negligible side effects on normal tissues (Liu et al., 2021a).