Metal peroxides for cancer treatment

Abstract

In recent years, metal peroxide (MO₂) such as CaO₂ has received more and more attention in cancer treatment. MO₂ is readily decompose to release metal ions and hydrogen peroxide in the acidic tumor microenvironment (TME), resulting metal ions overloading, decreased acidity and elevated oxidative stress in TME. All of these changes making MO₂ an excellent tumor therapeutic agent. Moreover, by combining MO₂ with photosensitizers, enzymes or Fenton reagents, MO₂ can assist and promote various tumor therapies such as photodynamic therapy and chemodynamic therapy. In this review, the synthesis and modification methods of MO₂ are introduced, and the representative studies of MO₂-based tumor monotherapy and combination therapy are discussed in detail. Finally, the current challenges and prospects of MO₂ in the field of tumor therapy are emphasized to promote the development of MO₂-based cancer treatment.

Graphical abstract

In this review, the synthesis and modification methods of metal peroxide (MO₂) are introduced, and the representative studies of MO₂-based tumor monotherapy and combination therapy are discussed in detail. In addition, the current challenges and prospects of MO₂ in the field of tumor therapy are also emphasized to promote the development of MO₂-based cancer treatment.

Highlights

• •Metal peroxide (MO₂) has becoming a potential therapeutic agent for tumor treatment.
• •MO₂ can alleviate hypoxia and remodel tumor microenvironment by self-supplying O₂.
• •MO₂ can assist and promote various tumor therapies to enhance therapeutic efficacy.
• •The metal ions in MO₂ endow MO₂ with special functions and extend its applications.

1. Introduction

Cancer has always been one of the most threatening diseases for human survival. In the past ten years, the incidence of malignant tumors in China has maintained an annual increase of about 3.9%, even worse, estimated 9.6 million died of cancer worldwide in 2018 [1,2]. Therefore, the development of cancer therapeutic methods to fight against cancer is urgely needed.

With the progress of science and technology, in addition to traditional chemotherapy and radiotherapy, scientists have already developed various novel cancer therapies, including photodynamic therapy (PDT), chemodynamic therapy (CDT), photothermal therapy and so on [[3], [4], [5]]. However, the rapid proliferation of cancer cells resulted the supply and consumption of oxygen (O₂) unbalanced, combined with the abnormal structure and function of tumor blood vessels, made hypoxia the most prominent feature of tumor microenvironment (TME) [6,7]. As a result, many therapeutic resistance effects will occur. For example, insufficient O₂ sources lead to low reactive oxygen species (ROS) production and decrease cell sensitivity to ROS, which greatly reduce the effectiveness of O₂-dependent therapies such as PDT and radiation therapy [8,9]. Moreover, some chemotherapeutic drugs also failed to be activated in a hypoxic TME. Therefore, it is necessary to regulate the hypoxic TME to improve the antitumor effect.

To cope with the challenge of tumor hypoxia, the hyperbaric oxygen (HBO) therapy in which patients breathe pure O₂ or high-concentration O₂ in a hyperbaric chamber can used as an adjuvant therapy. HBO therapy is beneficial to increase the partial pressure of O₂ in the plasma and promote the O₂ transport to hypoxic tumor tissue [10]. However, HBO therapy has many side effects such as O₂ poisoning, barotrauma and decompression sickness [11]. The method by employing catalase (CAT) or MnO₂ to convert the intracellular hydrogen peroxide (H₂O₂) to O₂ has also been applied to overcome the hypoxia [[12], [13], [14]]. However, the O₂ supplied by MnO₂ degradation and CAT catalysis also face to be exhausted due to the limited intratumoral H₂O₂. Fortunately, metal peroxides (MO₂) has been explored better to alleviate hypoxia by a disproportionation reaction with H₂O in tumor tissue.

MO₂, including CaO₂, CuO₂, MgO₂, BaO₂, ZnO₂ etc., can be considered as the product of two hydrogen atoms in H₂O₂ replaced by metal ions. MO₂ can cause strong oxidation effect by its decomposition products (such as H₂O₂) under acidic condition, while it can also slowly release O₂ in water or under heating conditions [15]. These properties making it widely used in antibacterial [16], agricultural production [17], environmental protection [18], and other aspects [19]. Many of its other properties, if exploited, can also provide new ideas for cancer therapy: (I) Under acidic conditions, MO₂ can react with H₂O to produce H₂O₂, which causes increased oxidative stress; (II) The generated H₂O₂ reacts with Fenton or Fenton-like reagents (such as Fe²⁺, Mn²⁺, Cu⁺, Co²⁺, etc.) to produce hydroxyl radical (^⋅OH) and realize CDT; (III) The produced H₂O₂ can be decomposed by CAT or MnO₂ to produce O₂ and increase the efficacy of O₂-dependent cancer therapies such as PDT and radiation therapy; (IV) The released metal ions have some unexpected effects, for instance, calcium overloading caused by Ca²⁺ ions that released from CaO₂ makes mitochondria damage [20]; while the Ba²⁺ ions generated by the disproportionation reaction of BaO₂ act as a potassium ion channel inhibitor to inhibit tumor cell proliferation [21]. The application of MO₂ in cancer treatment is developed fastly, and there is no doubt that MO₂ is a promising candidate to regulate TME for various treatments.

In this review, recent advances in the preparation and application of MO₂ in cancer therapy will be discussed. As shown in Scheme 1, we starting from the preparation and surface modification of MO₂, and then elaborate the design ideas and application examples of MO₂ in CDT, PDT, ion interference therapy and various types of synergistic therapy. Finally, the current challenges and prospects of MO₂-based cancer treatment will be presented.

Scheme 1.

Preparations and applications of MO₂ in tumor therapy.

2. Synthesis and surface modification of MO₂

To date, several preparation methods of MO₂ are developed, including hydrolyzation-precipitation method [22,23], underwater Leidenfrost dynamic chemistry method [24], reversed-phase microemulsion method [25], gas diffusion method [26], and sonochemical method [27]. Due to the instability of MO₂, some surface modification is necessary to achieve better applications in biomedical fields, frequently used surface modifiers are polyethylene glycol (PEG), polyvinyl pyrrolidone (PVP), CO-520, and hyaluronic acid (HA) [23,25,28,29]. Proper surface modification not only increases the stability of MO₂, but also improves the dispersion of nanoparticles (NPs), even be possible to target the tumor. The preparation and surface modification of MO₂ are summarized in Table 1, and will be discussed in detail in this section.

Table 1.

The synthesis, surface modification and applications of MO₂-based biomaterials.

MO2	Preparation	Morphology and size	Surface modifier	Applications	Ref.
CaO2	Hydrolyzation-precipitation method	Particle, <5 nm	SH	Calcium-overload/Oxidation therapy	[20]
CaO2	Hydrolyzation-precipitation method	Particle, 15 ± 10 nm	PEG-200	PDT	[22]
CaO2	Hydrolyzation-precipitation method	Spherical structure, 5–15 nm	PEG-200	PDT	[23]
CaO2	Hydrolyzation-precipitation method	Particle	PVP	PDT	[28]
CaO2	Hydrolyzation-precipitation method	Particle, 107 ± 11 nm	HA	CDT	[29]
CaO2	Hydrolyzation-precipitation method	Clusters, 300 ± 20 nm	PEG-200	PDT	[30]
CaO2	Hydrolyzation-precipitation method	Spherical structure, 116.0 ± 7.6 nm	PEG-200	PDT	[31]
CaO2	Hydrolyzation-precipitation method	Particle, ＜200 nm	PEG-200	Chemotherapy/CDT	[15]
CaO2	Hydrolyzation-precipitation method	Particle, 20–30 nm	PEG-200	Magnetic hyperthermia therapy/CDT	[32]
CaO2	Hydrolyzation-precipitation method	Particle, ~20 nm	PEG-400	CDT/PDT	[33]
CaO2	Hydrolyzation-precipitation method	Particle, ~18 nm	PEG-200	PTT/CDT	[34]
CaO2	Hydrolyzation-precipitation method	Particle, ~8 nm	PVP	PTT/CDT	[35]
CaO2	Hydrolyzation-precipitation method	Particle, 200–240 nm	TA	–	[36]
CaO2	Gas diffusion method	Spherical structure, 90 nm	SiO2	Immunochemotherapy	[26]
CaO2	Reversed-phase microemulsion method	Particle, 273.4 ± 7.8 nm	CO-520	Chemotherapy	[25]
CuO2	Hydrolyzation-precipitation method	Nanodots, ~16.3 nm	PVP	CDT	[37]
ZnO2	Hydrolyzation-precipitation method	Particle, 66.1 nm	PVP	Oxidation therapy	[38]
ZnO2	Leidenfrost dynamic chemistry method	Particle, size-tailored	–	Oxidation therapy	[24]
ZnO2	Sonochemical method	Particle	–	–	[27]
MgO2	Reversed-phase microemulsion method	Nanosheets, 100–200 nm	CO-520	Molecular dynamic therapy	[39]
BaO2	Hydrolyzation-precipitation method	Bamboo-structure, <15 nm	GLDA	Radiation-assisted metal ion interference therapy	[21]

Notes: PEG, polyethylene glycol; HA, hyaluronic acid; SH, sodium hyaluronate; PVP, polyvinyl pyrrolidone; TA, tannic acid; GLDA, N, N-bis(carboxymethyl)-l-glutamic acid tetrasodium salt.

2.1. Hydrolyzation-precipitation method

Currently, the hydrolyzation-precipitation method is the most widely used method to prepare MO₂. This method usually used metal chloride, metal acetate or metal carbonate as precursors, through adding H₂O₂ to the alkaline aqueous solution of metal salt to precipitating the water-insoluble MO₂. This method has the advantages of simple, mild conditions, low cost, and the size of NPs can be adjusted to several nanometers for the further construction of nanostructures.

Take the synthesis of CaO₂ for example, the CaO₂ hydrate was obtained by Eq. (1), and then the reaction was promoted to precipitate in favor of the metal peroxide by adding ammonia to neutralize the HCl, Eq. (2) [40].

CaCl₂+ H₂O₂→ CaO₂ (hydrate) + 2HCl (1)

2HCl+2NH₃→ 2NH₄Cl (2)

Based on the above theory, Xia et al. reported an approach to synthesize CaO₂ nanocrystals and their spherical aggregates [41]. They chose ethanol as the solvent to reduce the hydrolysis of CaO₂ NPs. Typically, 2–15 nm primary nanocrystals were obtained through adding H₂O₂ into an ethanol solution with CaCl₂ and PVP, in which PVP cooperated with Ca²⁺ to affect the growth and assembly of the nanocrystalline, resulting in spherical aggregates of polycrystalline structure with uniform size (Fig. 1). Therefore, the size of NPs can be easily controlled by adjusting the concentration of CaCl₂ and/or PVP. The prepared CaO₂ could greatly improve the antibacterial activity by releasing H₂O₂ and O₂ at a faster rate after contact with H₂O, which provided a new way for CaO₂ to be used as a nano-drug. It is worth mentioning that PVP not only regulates particle size as mentioned above, but also acts as a stabilizer in hydrolyzed precipitation. Similarly, PEG can also be used as a stabilizer to modify the surface of MO₂ [22,33,37,38].

Fig. 1.

(A) Schematic illustration of the formation of spherical aggregates through PVP-directed aggregation of CaO₂ primary nanocrystals. (B) TEM images of CaO₂ spherical aggregates synthesized in the presence of CaCl₂ at different concentrations: a) 2.1, b) 4.2, c) 8.4, d) 25.2, e) 42, and f) 168 mM, respectively. (C) TEM images of CaO₂ spherical aggregates synthesized in the presence of PVP at different concentrations: a) 43.2, b) 30.9, c) 21.6, d) 6.17, e) 1.23, and f) 0 mg mL⁻¹, respectively. Reproduced with permission from Ref. [41]. Copyright 2019, Wiley-VCH.

Bu et al. reported the synthesis of BaO₂ by hydrolysis and precipitation reaction [21]. In short, sodium formate and BaCl₂ aqueous solution were mixed with ultrasonic and then added to anhydrous methanol. Excessive amounts of H₂O₂ were dropped into the mixture after vigorously stirring, choline hydroxide aqueous solution was followed to precipitate the BaO₂ NPs (Fig. 2A). They pointed out that the selection of organic ligands has a great influence on the growth rate and orientation of BaO₂ nanocrystal, so products of different particle sizes and morphology will be obtained. In their study, BaO₂ particle with sub-micron scale can be obtained by using free Ba²⁺ as the precursor, while BaO₂ nanocrystal with bamboo-structure will be formed by using formate coordinated Ba²⁺ as the precursor. The authors made a variety of attempts and found that the stronger the coordination ability with Ba²⁺, the smaller products will be obtained (Fig. 2B). Therefore, the introduction of a ligand with a certain coordination ability with Ba²⁺ into the reaction can effectively limit the growth of the crystal to get nanosized BaO₂ particles. Shape controllability gives nanomaterials different specific surface areas and functions, which is an important link in the construction of nanomaterials theranostics platform.

Fig. 2.

(A) Schematic diagram of the preparation procedure of GL-BaO₂ NPs. (B) TEM images of the formed BaO₂ particles with different precursors, including Ba²⁺, formate + Ba²⁺, and citric acid + Ba²⁺. Reproduced with permission from Ref. [21]. Copyright 2019, Royal Society of Chemistry.

In the example above, the authors modified BaO₂ with a biodegradable strong chelating agent l-glutamic acid (N-diacetic acid) to reduce the damage of free Ba²⁺ to normal tissues. When drugs entered the tumor, X-ray stimulation could separate Ba²⁺ and chelating agent, promoting the treatment of tumor. Which proved that MO₂ can be modified by chelating method and enhanced the performance of MO₂ performance.

In addition, hyaluronic acid (HA), sodium hyaluronate (SH), and tannic acid (TA) are also used for surface modification of MO₂. For example, after stabilization by HA, CaO₂ can remain stable in the humoral environment, and can only be degraded in the acidic TME to achieve material stabilization and tumor targeting [20,29,36]. To sum up, proper surface modification not only increases the stability of MO₂, but also improves the dispersion of NPs, even be possible to target tumor.

2.2. Leidenfrost dynamic chemistry method

As we all known, a drop of water on an iron plate at 100 °C will boil and evaporate quickly. But if the temperature of the iron continues to rise, the water droplets will roll around the iron plate and evaporate at a slower rate. Leidenfrost first found and put forward the phenomenon in 1756, and pointed out that when the iron temperature reached the Leidenfrost point, water droplets in contact with the iron part will quickly form a steam, other part will remain liquid, due to the heat transfer of vapor is much slower than liquid water. Vapor layer can form a protective layer to avoid liquid water contact with the iron plate directly, so as to reduce the evaporation rate of the water. This phenomenon is well known Leidenfrost phenomenon [42,43].

Inspired by the volcano-induced dynamic chemistry of the deep sea [44], Moheb Abdelaziz and co-authors found that Leidenfrost can occur underwater. By virtue of the research of underwater Leidenfrost phenomenon, they designed a new tool for customized creation of nanoclusters of ZnO₂ (Fig. 3A) [24]. In this method, the nucleation and growth of NPs are separated into two parts: firstly, at the overheated zone, nanochemistry occurs and the formed NPs assembled as nanoclusters; secondly, these nanoclusters will erupt into colder regions for further growth. Such a tendency could be harnessed to regulate the size of NPs. Specifically, zinc acetate solution was mixed with H₂O₂ and placed in a Petri dish, and then suddenly introduced to a superheated plate (300 °C), the solution can be observed to change from colorless to milky white, ZnO₂ NPs are thus formed.

Fig. 3.

(A) Scheme of Leidenfrost dynamic chemistry approach. (B) SEM images of the ZnO₂ NPs with different sizes synthesized by regulating the concentration of zinc acetate: a) 70 mM, ~70 nm; b) 50 mM, ~126 nm; c) 20 mM, ~220 nm; d) 5 mM, ~680 nm. Reproduced with permission from Ref. [24]. Copyright 2017, Nature Publishing Group.

All in all, the size of ZnO₂ NPs prepared by this method could be adjusted by changing the concentration of zinc acetate (Fig. 3B). Its synthesis path is very simple and takes a short time, and the NPs prepared are very uniform and excellent monodispersity, which is suitable for mass production. On the basis of ZnO₂, more MO₂ nanomaterials can be prepared.

2.3. Reversed-phase microemulsion method

As a new preparation method, reversed-phase microemulsion method has simple equipment and technology. It can provide a nano-scale microreactor and control the appearance of NPs more precisely. The prepared NPs have the advantages of small particle size, good dispersion, and no impurities, etc. Which is a promising preparation method and MO₂ can also be prepared by reversed microemulsion method.

CO-520 is always used as a nonionic surfactant in reversed-phase microemulsion. There had a report to synthesize MgO₂ nanosheets [39]. Cyclohexane and CO-520 were added into MgCl₂ solution to create a microemulsion system. After 30 min stirring, ammonium hydroxide was rapidly injected to forming the Mg(OH)₂ and keep stirring for 30 min. Then H₂O₂ was added to control the reaction process and anhydrous ethanol was used to destroy the reverse microemulsion system to obtain the MgO₂ nanosheets. Similarly, Xiang et al. fabricated CaO₂ nanoparticle and simultaneous incorporation of cisplatin and coating with negatively charged phospholipid through reverse microemulsion method [25].

In the reaction of microemulsion, the size of NPs can be adjusted by adjusting the water content and pH of reversed micelles. The organic solvent phase and surfactant membrane in the microemulsion system effectively isolated the precipitation particles and improved the dispersibility of the particles. Some chemotherapeutic drugs can also be directly added into the microemulsion system to prepare NPs and carry out drug loading at the same time. Thus, reversed-phase microemulsion is a convenient and rapid method for the preparation of MO₂.

2.4. Gas diffusion method

The gas diffusion method is often used as the preferred method for studying biomimetic synthesis of calcium carbonate (CaCO₃) minerals, which has the advantages of simple operation and easy observation. Deng and his colleagues designed a new synthesis approach of CaO₂ based on the gas diffusion synthesis of CaCO₃ [26]. Typically, the beaker containing the ethanol solution with CaCl₂ and H₂O₂ was covered by parafilm with a few pores, afterwards another beaker containing ammonia was placed in the same desiccator. The gas diffusion reaction will lasts for 2 h at 35 °C, the preparation of CaO₂ was completed. In fact, it is also an extension of the hydrolytic precipitation process described above.

This reaction is mild and simple, moreover, the prepared NPs presented monodispersed spherical morphology, the size was controlled below 100 nm and uniformed, it layed a foundation for the further construction of nanoplatform with it as the core.

2.5. Sonochemical method

Sonochemical method is a kind of “green” chemical synthesis method, the cavitation collapse of ultrasonic will produce chemical and physical effects so as to drive some reactions, the process of sonochemical synthesis is mild with improved yields and selectivities, can even replace some dangerous reagents, which gradually become a new method in the field of nanochemical synthesis [45].

Mahtab Pirouzmand and co-workers prepared ZnO₂ through sonochemical approach innovatively [27]. Their approach is so simple that ZnSO₄·7H₂O was dissolved in distilled water, and NaOH was added dropwise to adjust pH to 8.0. Followed by adding H₂O₂ and the mixture was irradiated under ultra-sound for half an hour, and then the ZnO₂ particles with great uniform size distribution and spherical shape were obtained. However, agglomeration of particles was observed. It remains to be explored how to adjust the particle size and improve the dispersion of NPs.

3. MO₂ based monotherapy

In recent years, MO₂ often be introduced as an O₂-generating material applied to the construction of tumor theranostics nanoplatform, which can regulate the TME to create a new work environment for those therapy whose efficacy is limited by the original TME. In addition, free metal ions can be involved in biological applications such as imaging and bone regeneration, made MO₂ a potential biological materials. This section will focus on MO₂-based monotherapy, including photodynamic therapy (PDT), chemodynamic therapy (CDT), and chemotherapy.

3.1. Oxidation therapy

Reactive oxygen species (ROS) including superoxide anion radical (^⋅O₂⁻), ¹O₂, H₂O₂, and ^⋅OH can damage lipids, proteins, and DNA, resulting in cell apoptosis and death [46]. The process of ROS levels exceeding the antioxidant capacity of cells and leading to cell death is called oxidative stress [47,48]. Oxidative stress has been widely used in tumor treatment in recent years, MO₂ is such a good material can lead oxidative stress in cells.

In the work of Chen, a PVP-modified ZnO₂ NPs was developed and doped with paramagnetic Mn²⁺ ions through cation exchange method [38]. In this system, ZnO₂ will be decomposed into Zn²⁺ and H₂O₂ in the weakly acidic TME. It is worth mentioning that Zn²⁺ has been reported to increase the mitochondria production of ROS by inhibiting the electron transfer chain [[49], [50], [51]], and the release of H₂O₂ increases the exogenous H₂O₂ to the cell. The endogenous generation combined with the exogenous release of ROS resulting in better tumor killing effect.

Bu et al. constructed transferrin-modified MgO₂ nanosheets (TMNSs), which have a corresponding response to the neutral acidity and low CAT activity of TME, MgO₂ reacts with H⁺ to generate H₂O₂ rapidly and damages the structure of transferrin on the surface of the nanosheets [39]. Then, transferrin releases the trapped Fe³⁺ and generates cytotoxic ^⋅OH through the Fenton reaction. The high concentration of H₂O₂ and the generated ^⋅OH destroyed tumor cells together, while TMNSs in weakly alkaline normal cells only generate a small amount of H₂O₂ that is enough to be decomposed by CAT. Thus, this nanosystem showed excellent tumor selectivity.

3.2. Photodynamic therapy

PDT utilizes photosensitizers (PS) to convert the local molecular oxygen into cytotoxic ROS, ROS can damage biological macromolecules and induce cell apoptosis [52,53]. However, the efficacy of PDT is extremely dependent on the O₂ concentration, thus the hypoxia of solid tumors has limited the efficacy of PDT [54], and the further O₂ consumption of PDT will aggravate the tumor's hypoxia and form a vicious circle [55]. In order to solve the above problems, MO₂ has been developed as an O₂ self-sufficient material to enhance the effect of PDT.

For instance, Zhang et al. designed a liposome-based nanoplatform for dual-stage light-driven PDT [22]. They encapsulated the hydrophilic PS (methylene blue, MB) and CaO₂ NPs into the aqueous cavity and hydrophobic layer, respectively. When LipoMB/CaO₂ reach the tumor tissue, CaO₂ inside liposomes would react with H₂O to generate O₂ in the mild acidic TME, which can alleviate tumor hypoxia. Then, a short time irradiation is applied in the first stage, the singlet oxygen (¹O₂) would activated by MB and break the liposome by oxidized the phospholipid bilayer, CaO₂ is further exposed to H₂O and generate more O₂. At last, a long time irradiation is given in the second stage, the PDT effect will improved a lot in such an O₂ sufficient TME. In a deeper analysis, after the alleviation of tumor hypoxia, the down-regulation of hypoxia-inducible factor-1α and vascular endothelial growth factor expression may also reduce the rate of tumor metastasis [56]. This two-stage light strategy based on CaO₂ is ingeniously designed to maximize the O₂ supply capacity of CaO₂. It is a nano-platform worth learning for alleviating tumor hypoxia and anti-tumor metastasis.

Similarly, hydrophobic aza-BODIPY dye (B1), oxygen-generating CaO₂ and hydrophilic ammonium bicarbonate (NH₄HCO₃) encapsulated in PEG shelled liposome to realize self-supplying O₂ PDT therapy also be reported [23]. In this study, NH₄HCO₃ acts as a thermoresponsive molecule. When the liposome system irradiated by near-infrared (NIR), B1 will cause the temperature to rise. Once the temperature reached 40 °C, NH₄CO₃ will be thermally decomposed to produce CO₂ [57], which will expand and destroy the liposomes, causing CaO₂ and CO₂ fully react to produce O₂ to improve the PDT effect of B1 (Fig. 4) [58]. Finally, they conducted tumor treatment experiments in vivo and find that enough O₂ generation from CaO₂/B1/NH₄HCO₃ liposome was favorable to produce ¹O₂ in the presence of photosensitizer B1 and inhibit tumor growth even induced tumor disappearance. In addition to the above examples of light-triggered O₂-generation, pH seems another feasible trigger. In the work of Callan et al., CaO₂ particles were coated with a pH-responsive methacrylate-based co-polymer. The tertiary amine unit of the copolymer will be ionized in an acidic environment, and CaO₂ will therefore exposed and producing O₂. The TME creates a favorable conditions for the dissolution of copolymers [59]. Finally, with the help of CaO₂ NPs, PDT from rose bengal (a kind of PS) achieved the best effect [31].

Fig. 4.

(A) Scheme illustration of O₂ self-supplying enhanced PDT of CaO₂/B1/NH₄HCO₃ lipo: a) NIR-regulated generation of O₂ and enhancement of ¹O₂; b) the mechanism of O₂ generation; c) O₂ self-sufficient CaO₂/B1/NH₄HCO₃ lipo enhanced PDT. (B) Photograph of tumors obtained from the mice after 14 days. (C) Tumor volume ratio of mice during the treatments. Reproduced with permission from Ref. [23]. Copyright 2019, Royal Society of Chemistry.

CaO₂ not only generates O₂ itself but also provides a reaction substrate for other O₂-generating materials. For example, CaO₂ reacts with H₂O to generate H₂O₂ (Eq (3)) which provide a raw material for manganese dioxide (MnO₂) for the generation of O₂ in a mild acid environment, Eq (4) [60]. By this way, Shi and his colleagues prepared a CaO₂/MnO₂@PDA-MB, system through MnO₂ nanosheet coated on the surface of CaO₂, in acidic TME, polydopamin (PDA) dissolution enables Mn²⁺ to fully react with H₂O₂ produced by CaO₂ and generate O₂, further promoting the PDT effects of MB that the singlet oxygen quantum yield reached 0.18 and realized switch-control fluorescence imaging [28].

CaO₂ +2H₂O → H₂O₂ + Ca(OH)₂ (3)

MnO₂ + H₂O₂ + 2H⁺ → Mn²⁺ +2H₂O + O₂ (4)

3.3. Chemodynamic therapy

CDT is an emerging nanotheranostic technology which catalyzes the conversion of H₂O₂ into ^⋅OH through an elaborately designed Fenton nano-catalyst [61,62]. However, limited by the concentration of endogenous H₂O₂ in the tumor, the effect of CDT is often unsatisfactory. Since MO₂ can generate H₂O₂ in the mild acidic TME, it can be designed to enhance CDT efficacy. Moreover, the metal ions like Cu²⁺, Co²⁺, Mn²⁺ that consisted in MO₂ possesses excellent Fenton catalytic activity, making MO₂ become a promising H₂O₂ self-supply CDT nanoagent [[63], [64], [65], [66]].

Based on above, Jiang et al. controlled the self-assembly of Fe₃O₄ on the surface of HA-stabled CaO₂ to form CaO₂–Fe₃O₄@HA NPs. It also realizes H₂O₂ self-supplying CDT treatment and demonstrated a desirable performance in tumor growth inhibition rate of 69.1%, moreover, by loading Cy7 the fluorescence imaging can be combined with therapy [29].

In another case, Chen and co-authors developed a Fenton-type copper peroxide (CP) nanodots that anchored by PVP with the aid of hydroxide ion [37]. The prepared CP nanodots could reversely decompose into Cu²⁺ and H₂O₂ in an acidic environment, thereby realizing the H₂O₂ self-supplying CDT (Fig. 5A). The pH-sensitive CP nanodots were internalized by tumor by enhanced permeability and retention effects [67], and generate large amounts of ^⋅OH through Fenton-like reaction in the acidic endo/lysosomal compartments, which can induced lysosomal membrane permeabilization-mediated tumor cell killing by lysosomal lipid peroxidation [68,69]. Finally, the authors evaluated the biological distribution of CP nanopods in U87MG tumor-bearing mice by inductively coupled plasma optical emission spectrometry found that the uptake of CP nanopods by tumors reached 5.96 ± 0.79% and showed excellent CDT anti-tumor effect with negligible weight loss (Fig. 5B–D).

Fig. 5.

(A) Formation of CP nanodots for H₂O₂ self-supplying CDT. (B) Biodistribution of Cu in major organs and tumor of U87MG tumor-bearing mice at 24 h post i.v. injection with CP nanodots. (C) Relative tumor volume and (D) variation of body weight of the mice after different treatments. Reproduced with permission from Ref. [37]. Copyright 2019, American Chemical Society.

3.4. Enhanced chemotherapy

Tumor cells have very strict mechanisms to deal with hypoxia and resist oxidation. These mechanisms are closely combined which made abnormal factors such as hypoxia, acidosis, and high glutathione (GSH) levels are simultaneously present in the TME, which promotes the drug resistance of tumor cells especially ROS-dependent drugs [[70], [71], [72], [73]]. Fortunately, MO₂ can generates O₂ or by acting as a reaction substrate to reverse tumor hypoxia and providing more O₂ for chemo-drugs to receive enhanced chemotherapy.

Xiang et al. reported a lipid-coated CaO₂/cisplatin NPs which used the O₂ production and oxidation capabilities of CaO₂ at the same time to overcome tumor hypoxia and reduce GSH levels [25]. More importantly, CaO₂ can significantly elevate the local pH and further accelerate GSH oxidation. After the TME was reversed, the binding of cisplatin to GSH was reduced, and the production of O₂ downregulated the hypoxia inducible factor 1 and resistance-associated protein 2, then the efflux pathway of cisplatin is blocked (Fig. 6A). Combined with the above process, an enhanced chemotherapy effect was achieved. As shown in Fig. 6B, fluorescent imaging showed the lipid-coated CaO₂/cisplatin NPs has a signal in the tumor for more than 48 h, indicating that it can achieve long circulation and efficient tumor accumulation, and the size of the tumor also showed that CaO₂ significantly enhanced the antitumor effect of cisplatin in vivo anti-tumor experiments intuitively (Fig. 6C and D). The work of Sung and his colleagues also proved that the O₂ production of CaO₂ also has an effect on the improvement of the efficacy of the chemotherapy drug doxorubicin (DOX) [74]. Deng et al. used CaO₂, MnO₂ and DOX to construct CaO₂/DOX@SiO₂/DOX-MnO₂ nanoreactor, and proved that the drug can effectively relieve hypoxia and reverse immunosuppressive TME to enhances anti-tumor immune responses from an immunological point of view [26]. What is most worth mentioning is the drug-loading method of this nano-platform. DOX is directly added when CaO₂ NPs were synthesized, which can form Ca-DOX complex so that DOX can directly combine with CaO₂ NPs to form the CaO₂/DOX core and greatly improved the drug-loading efficiency.

Fig. 6.

(A) Schematic illustration of LipoCaO₂/DPP for comprehensive TME modulation and cisplatin efflux pathway blockade: 1) produce Ca(OH)₂ to raise the local pH; 2) oxidize GSH under alkaline conditions and reduce cisplatin/GSH binding; 3) generate O₂ for inhibition of MRP2 by HIF-1 degradation, preventing the cisplatin–GSH adduct from pumping out of cells. (B) Real-time fluorescence imaging of free fluorescein (DIR) and LipoCaO₂/DDP loaded DIR treated mice. (C) Tumor photographs and (D) tumor volume after different treatments. Reproduced with permission from Ref. [25]. Copyright 2019, Royal Society of Chemistry.

4. MO₂ based combined therapy

The effect of monotherapy is limited, for most tumors, the treatment effect is unsatisfactory. Therefore, the combination of two or more therapeutic methods is particularly important. Each individual therapeutic agent has anti-tumor activity, and they can be built on a platform to achieve the effect of combined therapy. In addition, if the tumor killing mechanism of each therapeutic agent can complement each other, it can also realize the “1 + 1>2″ synergistic therapeutic effect [3]. After the introduction on the application of MO₂ to monotherapy above, combination therapies based on MO₂ will be discussed in this section.

4.1. CDT-based combination therapy

CDT can exert better curative effect by combining with other therapies [[75], [76], [77]]. Utilized the characteristics of CaO₂ that it can react with H₂O to generate H₂O₂ and O₂, Dong et el. constructed a H₂O₂/O₂ self-supplied thermoresponsive nanosystem (MSNs@CaO₂-ICG)@LA by using manganese silicate (MSNs) to load CaO₂ NPs and indocyanine green (ICG) and then modifying a layer of thermally dissolved lauric acid (LA, melting point: 44–46 °C). This nanosystem realized PDT/CDT synergistic cancer therapy and the tumors of MCF-7 bearing mice were completely eliminated [33].

In the work of Zhang and co-authors, they built a cobalt-based metal-organic framework (ZIF-67) on the surface of CaO₂@DOX [15]. The slightly acid in tumor decomposed ZIF-67 and quickly released Co²⁺ and DOX. The H₂O₂ produced by CaO₂ will be catalyzed by Co²⁺ and produced highly toxic ^⋅OH through the Fenton-like reaction, while the produced O₂ can improve the efficacy of DOX to enhance combined efficacy of CDT/chemotherapy. In another work, the Fe-GA/CaO₂@PCM NPs developed by Dong et al. used an organic phase change material (PCM) with a melting point of 46 °C as a protective layer, and co-encapsulate the hydrophilic iron-gallic acid (Fe-GA) and CaO₂ NPs [34]. After 808 nm laser irradiation, the temperature increase of Fe-GA causes the PCM to melt and exert the effect of photothermal therapy (PTT). H₂O₂ and Ca²⁺ produced by CaO₂ participate in the Fe-based Fenton reaction and induce mitochondrial damage, respectively. PTT can futher accelerate the generation of ^⋅OH. This nanoplatform can realized on-demand H₂O₂ self-supply for enhanced CDT/PTT treatment.

Moreover, MO₂-based materials can also be used in three dimensional (3D) printing technology. Among a multifunctional “all-in-one” biomaterial platform, CaO₂ and Fe₃O₄ NPs were co-loaded into a 3D printing akermanite (AKT) scaffold, named AKT-Fe₃O₄–CaO₂ (Fig. 7) [32]. To put it simply, CaO₂ reacts with H⁺ to produce H₂O₂ and Ca²⁺, H₂O₂ was catalyzed by Fe₃O₄ to produce ^⋅OH, while Ca²⁺ can also be used for bone regeneration. In addition, Fe₃O₄ will produce hyperthermia under the action of alternating magnetic field (AMF). Thus, the scaffold with magnetic hyperthermia-synergistic H₂O₂ self-sufficient CDT and bone-regeneration function can be used effectively in the treatment of osteosarcoma.

Fig. 7.

(A) Schematic of the cancer-therapeutic performance and bone-regeneration bioactivity of AKT-Fe₃O₄–CaO₂. (B) In vivo anti-tumor effect. (C) In vivo osteogenesis capability of AKT-Fe₃O₄–CaO₂ scaffolds: I) 3D reconstruction of micro-CT images of the cranium and scaffolds (red, newborn bone tissues; white, residual scaffolds). II) CLSM images of slices from the bone defect area (yellow, newborn bone stained by tetracycline hydrochloride injected at week 2; red, newborn bone stained by Alizarin Red S injected at week 4; green, newborn bone stained by calcein injected at week 6. III) Microscopy image of VG stained slices of cranium with two defects implanted with AKT or AKT-Fe₃O₄–CaO₂ scaffolds. Scale bar, 500 μm. Reproduced with permission from Ref. [32]. Copyright 2019, Wiley-VCH. F

4.2. Calcium-overload based combination therapy

Some harmful factors can cause dysfunction of calcium balance system and disorder of calcium distribution, leading to abnormal increase of intracellular calcium concentration, called calcium overload. Calcium overload can affect mitochondrial oxidative phosphorylation process, decrease the mitochondrial membrane potential, resulting in decrease of tissue ATP and activation of phospholipases and proteases in the cytoplasm, which cause irreversible cell damage [20,78]. In clinical treatment, internal calcification is frequently observed in certain tumors after radiotherapy or chemotherapy [79,80], so calcification is usually considered as a by-product of tumor treatment and it was found that calcified tumors often showed better treatment response. Given the importance role of Ca²⁺ in cell proliferation, metabolism, and death, the overloading process may be a destructive factor against tumor cells, and provide alternative drug-free method for cancer therapy.

Based on the above, Bu et al. developed a calcium-based nanomedicine, the sodium hyaluronate (SH)-modified CaO₂ (SH–CaO₂), to induce intracellular calcium overloading for cancer treatment (Fig. 8A) [20]. Results revealed that Ca²⁺ and H₂O₂ produced by CaO₂ caused intracellular calcium overload and oxidative stress, respectively. For the CAT down-regulated tumor cells, oxidative stress will change the function of the protein and hinder the accurate transmission of the calcium signal, thereby causing uncontrollable accumulation of Ca²⁺ and inducing cell death [81]. Computer tomography (CT) imaging and von Kossa staining further showed that SH-CaO₂ NPs can accelerate the process of tumor calcification (Fig. 8B and C). In addition, the in vivo experiments also proved that SH-CaO₂ NPs has significant anti-tumor effect whether intratumoral injection or intravenous injection, and the anti-tumor effect of intratumoral injection is better, which caused the tumor almost disappeared during the observation period of 14 days after the injection (Fig. 8D). Similarly, the nanosystem constructed by Yin et al. with CaO₂ as O₂ source and hematoporphyrin monomethyl either as photosensitizer also combined PDT and calcium overload [82].

Fig. 8.

(A) Schematic illustration of the functional pattern of SH-CaO₂ NPs. (B) CT images of mice after the following treatments: i) control, ii) 3 h after intratumor injection of SH-CaO₂ NPs, iii) 3 days after the injection of a single dose of SH-CaO₂ NPs for a small tumor, and iv) 12 days after the injection of multiple dose (4 times, injected every 2 days) for a larger tumor. (C) von Kossa staining of tumor tissue sections after multiple injections with SH-CaO₂ NPs. (D) Relative tumor volume changes during the treatments. Reproduced with permission from Ref. [20]. Copyright 2019, Elsevier.

4.3. Radiation-assisted metal ion interference therapy

High Z elements have a significant effect on radiosensitization [83], but most metal-based nanomedicines are hindered in clinical application due to the biological toxicity of heavy metals. Bu and his co-authors developed a BaO₂-based N,N-bis(carboxymethyl)-l-glutamic acid tetrasodium salt (GLDA) modified nanoplatform [21]. The chelation of Ba²⁺ with GLDA can reduce the toxic and side effects of the drug in normal tissues, then the ^⋅OH produced after X-ray radiation will destroy the chemical structure of GLDA and release Ba²⁺. Benefit from this point, they proposed an ion interference therapy. The free Ba²⁺ not only enhanced the radiosensitization effect but also targeted and competitively binded to potassium channels after entering the cell, then the potassium conducting pores would be blocked and the outflow of potassium ions (K⁺) would be prevented [84,85], which further affect cell membrane potential and the osmotic pressure in the cell, ultimately inhibit cell proliferation and induce cell death [86,87].

5. Conclusions

MO₂ has been developed in terms of self-supplying O₂ and self-supplying H₂O_2, and has become a very potential therapeutic agent for tumor. Under acidic conditions, the generated H₂O₂ through MO₂ reacting with H₂O not only leads to oxidative stress, but also produce more O₂ by acting as a reaction substrate for substances such as CAT or MnO₂, so as to alleviate tumor hypoxia and reverse TME. More importantly, the characteristics of MO₂ can be perfectly combined with photosensitizer, enzyme, metal NPs, Fenton reagent or chemotherapy drugs, etc., which can assist and promote various treatments such as PDT, CDT and chemotherapy. If combined multiple treatments, MO₂-based combination therapy will achieve more excellent anti-tumor effects.

Here, we have introduced the preparation and surface modification methods of MO₂-based nanomaterials and their application in tumor therapy in detail, including monotherapy and combination therapy, with emphasis on MO₂-based PDT, CDT, chemotherapy, oxidation therapy and ion therapy (Table 1). However, the application of MO₂-based nanomaterials in tumor therapy is still in the preliminary stage of research, and there are still many problems and challenges to be solved.

Firstly, morphology and size are key factors affecting the efficacy of nanomaterials, from previous studies, large NPs are more likely to retention in tumor tissue than small ones, while permeability is opposite, the smaller the size, the greater the ability to penetrate tumor tissue [88]. The size design of MO₂ should in accordance with the functional requirements to regulate. If MO₂ is used as the cargo, it is better design small size, which can not only implement efficient load but can accelerate the reaction rate of MO₂ in the TME. On the other side, if MO₂ is used as the carrier, large size may increase the rate of drug loading. In addition to particle size, morphology also plays an important role in tumor penetration. Most studies have shown that the tumor penetration ability of spherical NPs is lower than that of other shaped NPs [89]. At present, the morphology of MO₂ is basically spherical, it is necessary to design MO₂ with other morphology to increase the tumor permeability.

Secondly, there are few types of MO₂ reported for tumor therapy so far, and most of the reported researches are focused on CaO₂-based nanomaterials. Other MO₂-based nanomaterials such as MgO₂, BaO₂, ZnO₂, CuO₂-based materials have not been fully developed and their biological applications are also limited, developing them by modifying them appropriately or combining them with other therapeutic agents may be a trend of future research. In the abovementioned MO₂, we believe that CaO₂ has the most clinical translation value by far. Ca²⁺ is widely distributed in the body, endowing CaO₂ with good biocompatibility. In addition, Ca²⁺ is also distributed in tumor cells, so treatment strategies such as calcium overload have universality, and Ca²⁺ has the effect of accelerating osteogenesis, which can be well applied in the treatment of bone tissue related cancer such as osteosarcoma. However, the preparation and storage of CaO₂ even MO₂ are facing challenges, because of the instability, the morphology, size and dispersion of MO₂ are difficult to precise control, made it difficult to realize the mass production. Therefore, exploring some new methods for preparing MO₂ batch production is imperative, the Leidenfrost dynamic chemistry method is a good exploration [24]. For the clinical translation of MO₂, improve the stability of MO₂, extend drug life and ensure drug function is an urgent problem to be solved.

Thirdly, MO₂ has the potential to be used in a variety of therapeutic modalities, and its biological applications remained to be explored. In the reported studies, MO₂-based nanomaterials were mainly used in CDT, PDT and oxidation therapy, etc., but their application in magnetic heat, radiation, gas treatment and so on is rarely reported. Therefore, exploring the applications of MO₂ in a variety of therapies should be more innovative.

Fourthly, as summarized in this paper, different ions in MO₂ have additional functions. For example, Ba²⁺ not only plays the role of radiotherapy sensitization, but also leads to cell death and inhibit proliferation through K⁺ outflow from tissues [21]. Ca²⁺ not only induces calcium overload but also enhances CT imaging [20]. When CaO₂ is used for the treatment of osteosarcoma, it had the ability to accelerate bone regeneration [32]. In addition to Ba²⁺ and Ca²⁺ ions, the functions of other ions have yet to be developed. Hence, mining the special function of ions in MO₂ may futher broaden its application in the field of tumor therapy, even the imaging effects attached to some ions can be used for tumor theranostics [90].

Next, hypoxia TME will greatly weaken the effect of immunotherapy, hypoxia-A2-adenosinergic tumor biology is a barrier to be overcome in immunotherapy [78]. MO₂ can reverse TME by self-supplying O₂ and suppressing hypoxia-adenosinergic signaling, further reducing the expression of hypoxia-inducible factor 1 and CD39/CD73 in T cells to reduce the immunosuppression effect of tumor TME [91]. Therefore, the mechanism of MO₂ in immunotherapy and its combination with other therapeutic methods remains to be further studied and developed.

Finally, the application of MO₂ needs to pay attention to its biosafety and long-term toxicity. Although the reported MO₂-based nanodrugs caused no damage to the normal tissues and organs even at a high dose of 50 mg kg⁻¹, the biosafety should be systematically evaluated for a longer time and on larger animal models. Moreover, cytotoxicity induced by other parts except MO₂ of nanodrug should also be considered. Furthermore, how to modify MO₂ materials to ensure its anti-tumor effect and biosafety remains to be solved. Although there are still many problems to be solved, MO₂ has brought new approaches to tumor therapy and its application in biological fields is worth developing and expanding. We hope that MO₂-based nanodrugs can be applied in more anti-tumor methods and bring good news to patients.

Declaration of competing interest

The authors declare no conflict of interest.

Appendix A. Supplementary data

The following is the supplementary data to this article: