Novel gas‐based nanomedicines for cancer therapy

Gas therapy, an emerging cancer treatment method of inflammation‐related diseases, has recently received substantial attention. The rapid advances in nanomedicine and nanotechnology have made gas precision treatment possible through tumor targeted delivery and controlled release of therapeutic agents. Single therapeutic is often inevitably accompanied with limited therapy efficacy. Gas therapy combined with other treatment methods can sensitize different therapy modes to augment cancer therapy. Understanding the mechanism through which gas enhances other therapeutic modalities will enable the design of reasonable strategies for clinical cancer therapy. In this review, we summarize novel gas‐based nanomedicines, focusing on gas‐based nanomedicine carriers, along with the release of gas molecules and the mechanisms of gas enhanced therapy. We describe the design of novel gas‐releasing nanoplatforms and the underlying synergistic mechanisms against cancer. Moreover, we describe the current challenges and outlook for future prospects in novel gas‐based nanomedicines for gas therapy in cancer.

non-specific damage by reducing the side effects of traditional therapies through the virtue of the antiinflammation effect of gases. [3][4][5][6] Several types of gaseous molecules, such as nitric oxide (NO), hydrogen sulfide (H 2 S), carbon monoxide (CO), and H 2 have the function of regulating vasodilatation, neurotransmission, anti-inflammatory, and anti-oxidative reactions in the physiological and pathophysiological processes. 7,8 Importantly, several gas molecules showed specific therapeutic effects in cardiovascular diseases, Alzheimer's disease, infection, cancer, and neurotransmission owing to their physiological modulation functions. 9 Low concentration (<nM level) of NO/H 2 S/CO can protect cancer cells, thus favoring tumor cell proliferation, growth, and metastasis, whereas these gases with high concentration (≥nM level) have toxic to cancer cells. 10 However, because of the limits of low solubility in water and untargeted diffusion around the body, achieving highly efficient and sustainable gas delivery is difficult. Therefore, it is highly important to realize the accurate gas release and efficient gas delivery for the clinical translation of gas therapy. This review summarized the recent significant development in the elaborately designed gas nanomedicine. Given the great progress in nanomedicine, some welldesigned nanoplatforms, as gas-based nanomedicines, may achieve the aforementioned goal. In addition, these gas-transmitters can synergize other traditional cancer therapies, on the basis of the strength of nanomaterials, and provide broad and versatile opportunities in cancer therapy.
Given the important role of gas-based nanomedicines, this comprehensive review summarizes recent developments in novel gas-based nanomedicines for cancer therapy in the following four aspects (as shown in Figure 1): (1) gas carriers, (2) stimuli-responsive gas prodrug, (3) in situ gas-generating nanomedicines, and (4) gas-based nanomedicine for enhanced cancer therapy. In the gasbased nanomedicines, we introduce two gas-releasing strategies from the viewpoints of stimuli-responsive gas prodrug and in situ gas-generating nanomedicines. Gas therapy is a low-toxicity and high-efficacy therapy strategy that can sensitize cells to chemotherapeutic drugs, thus effectively killing cancer cells and realizing the combination between gas therapy and traditional therapy. Given the efficient therapeutic effect, we provide a detailed summary of the mechanisms of a series of gas-based nanomedicines in the enhancement of cancer therapy in Section 4. Finally, future challenges and potential inspirations for gas-based nanomedicines are also discussed to further promote their clinical applications.

Nanoliposomes
Liposomes are the first successful nano-drug delivery system that is translated into clinical applications. First described in the 1960s by Bangham et al., liposome has been shown to have benefits in the medical and cosmetic industries. 11 Liposomes are composed of phospholipids, which self enclose, forming spheres of lipid bilayers surrounding an aqueous core. 12 Modification of polyethylene glycol (PEG) to liposomes can prevent the reaction with plasma proteins and escape the capture of mononuclear F I G U R E 2 The category of gas-based nanomedicines carriers. Reproduced with permission. 74 Copyright 2017, American Chemical Society. Reproduced with permission. 88 Copyright 2018, Wiley-VCH. Reproduced with permission. 35 Copyright 2009, Wiley-VCH. Reproduced with permission. 84 Copyright 2008, American Chemical Society. Reproduced with permission. 66 Copyright 2011, Royal Society of Chemistry. Reproduced with permission. 80 Copyright 2018, Nature Publishing Group. Reproduced with permission. 4 Copyright 2019, Royal Society of Chemistry phagocytes. [13][14][15] Therefore, PEGylated liposomes endow NPs with ultra-long circulation compared with free drugs. Recent studies have shown that different types of liposomal encapsulation of various agents with different physicochemical properties can effectively deliver drugs to lesions and enhance cancer therapy effects. [16][17][18][19] H 2 is relatively safe, with no danger of blood poisoning at fairly high concentrations and without the tumor-promoting effects compared with those of NO, CO, and H 2 S gases. Liposomes can be used as nanoreactors for in situ photocatalytic hydrogen generation. In an in situ hydrogen therapy, the product H 2 diffuses across the lipid bilayer through a catalytic cycle of polymer dots and counteracts the reactive oxygen species (ROS) in diseased tissues. 20 However, the stability of liposomes is low, and the development of nanocarriers with excellent stability is urgently needed.

Polymeric NPs
Polymeric NPs have been investigated in therapeutic delivery applications because of their success in solving poorly soluble, rapid renal clearance, systemic toxicities, and improving delivery. 21 The most frequently used polymerbased NPs for decades have been PEG, polyethyleneimine, polylactic acid-co-glycolic acid (PLGA), and its related homopolymers. 22 Among these, PLGA and its related homopolymers as the US Food and Drug Administration approved nanocarriers have been wildly used in the biomedical field. 23 PLGA has the characteristic of degradation, and its degradation products are non-toxic H 2 O and CO 2 , which can be eliminated by the body. One important targeted strategy is surface modification, which plays a key role in biocompatibility and the treatment of disease. 24 Recently, polymer-drug conjugates, polymer micelles, and nanogels have been investigated to protect drugs against rapid clearance and enzymatic digestion, and enable controlled release under the surface modification. [25][26][27] Polymer-drug conjugates, one of the first classes of anti-cancer nanomedicines, could be developed as a single agent or a component of combination therapy in clinical cancer therapy. The first therapeutic polymer-drug conjugates have been shown to increase drug cytotoxicity and decrease cardiotoxicity by modifying biodistribution in preclinical studies, thus providing a promising way of circumventing multidrug resistance (MDR). 28 Polymeric micelles, as natural carriers, mimic the biological transport system in structure and function, fulfilling several tasks of selective delivery of ideal carriers at different levels. A hydrophilic shell helps the micelles remain unrecognized in the blood circulation, and a viruslike size (<100 nm) prevents the reticuloendothelial system from the uptake of them, resulting in some passive accumulation in the specific organization. [29][30][31][32] Compared with polymer-drug conjugates, polymeric micelles have relatively higher physicochemical stability. The size of typical polymeric nano-micelles is 20-100 nm, thus providing an advantage for polymers and nanomaterials. Foster et al. have reported the amphiphilic block copolymer micelles functionalized by S-aroylthiooxime, which can release H 2 S and are used to study the effect of H 2 S on the growth and proliferation of cancer cells. These H 2 S-releasing micelles, compared with other common H 2 S donors, significantly decrease the survival of HCT116 cells and provide a new strategy for studying the biological effects of H 2 S. 33 Nanogel is a sub-micron size cross-linked hydrogel particle with high water content, excellent colloidal stability, and flexibility to control drug release. [34][35][36][37] These characteristics provide broad prospects for the application of hydrogels in tissue engineering, biomedical implants, bio-nanotechnology, and drug delivery. In addition, some environmentally responsive groups introduced into the 3D cross-linking network of NPs have been found to endow nanogels with excellent stimuli-responsive properties. 36,38 Among them, degradable nanocarriers enabling both controlled drug release and high biosafety are crucial to their clinical translation. Our group has developed a new type of biodegradable zwitterionic nanogel based on poly(sulfobetaine methacrylate) with excellent sufficient drug release in a reductive environment. The equal cationic and anionic groups endow nanogel superior long circulation time, thereby increasing the tumor accumulation for cancer therapy. 39 With the development of gas therapy, an ultraviolet-visible responsive nanomedicine formula was proposed by Fan et al. 40 N, N'-di-sec-butyl-N,N'dinitroso-1,4-phenylenediamine (BNN6) and doxorubicin hydrochloride (DOX) were coloaded into monomethoxy (polyethylene glycol)-poly(lactic-co-glycolic acid) (mPEG-PLGA) and the nanomedicine shows good stability under physiological conditions but decomposes in response to ultraviolet-visible irradiation for NO gas release. The generated NO gas breaks the NP structure and facilitates the loaded DOX molecules releasing and realizing the gas/drug effect by reversing MDR under the functional of NO. Therefore, polymer NPs can serve as functional nanocarriers with degradable and long circulation time performance for cancer gas therapy.

Inorganic NPs
Inorganic nanomaterials have high chemical/ physiological stability and multifunctionality, and have shown remarkable potential in combating cancer. The incorporation of their unique properties has expanded alternative platforms for drug delivery. Among the most promising inorganic NPs being developed are iron oxide, silica, and carbon NPs.
Iron oxide NPs, the typical metallic material, have been successful, owing to their superparamagnetism effect, which can generate imaging contrast by magnetic resonance and be concentrated at specific target sites in diseased tissues by an external, high-gradient magnetic field. [41][42][43] Numerous studies have reported the development of superparamagnetic iron oxide NPs as a magnetic resonance imagining (MRI) agent in clinical applications in the past decade. 44,45 In addition, magnetic NPs, as a magnetic fluid hyperthermia donor, also increase the temperature of the tumor site to induce the cytotoxicity of cancer cells, which is a promising new technology for cancer treatment. [46][47][48][49] Current research on iron oxide NPs has opened up broad prospects for the application of diagnostic reagents in magnetic resonance imaging and drug delivery. Delivering anti-cancer drugs to targeted locations by coupling functionalized iron oxide NPs is one of the most interesting research areas as a kind of cancer treatment strategy. 50 Moreover, two different oxygen radicals were generated by the disproportionation of hydrogen peroxide (H 2 O 2 ) under the action of Fe 2+ /Fe 3+ . Therefore, the Fenton reaction has been proven to be a new platform for ROS generation. 51,52 Ding et al. developed pH-sensitive porous magnetite supra-particles, which allow loading of artemisinin and easy degradation to ferrous ions under acid environment to improve the cytotoxicity of cancer cells. 53 The recent emerging gas therapy is limited due to the low water solubility and strong tissue penetration. [54][55] For overcoming the rate of therapeutic gas releasing, magnetic iron oxide was selected as a carrier for covalent surface-bound CO-releasing molecules (CORMs), which trigger the release of CO under an alternating magnetic field based on its inherent magnetic hyperthermia. 56 In addition, superparamagnetic iron oxide is an excellent MRI contrast agent that can guide precision gas therapy. 57 Despite the potential biomedical applications of iron oxide NP-related nanocarriers, possible changes in iron homeostasis, oxidative stress, and cellular responses are the main critical issues that limit their clinical applications. 50 Silica and carbon are two classical inorganic nonmetallic NPs widely used in catalytic and biological fields. [58][59][60][61][62] Silicon is a necessary element with a role in metabolic processes as the second-largest element on earth. Most people approximately absorb about 20-50 mg of silicon per day and the ingested silicon is found in the form of silicic acid in the blood plasma. 63 Orthosilicic acid, which is mainly absorbed by humans, is present in many tissues including the bone, tendons, aorta, liver, and kidney. 59 Mesoporous silica NPs (MSNs) as a kind of silica-based NPs have been wildly used in biomedicine because of their high specific surface area, adjustable size, porosity parameters, high biocompatibility, and nontoxic degradation products in biorelevant media, thus providing a large reservoir for the packaging of guest goods. [64][65][66][67] He and co-authors effectively coated the hydrophobic manganese carbonyl (MnCO) prodrug on the advanced hollow MSN carrier to achieve a new H 2 O 2 responsive CO treatment nano-drugs. 68 Afterward, a nanomedicine-based strategy of a mitochondria-targeted and intramitochondrial microenvironment-responsive prodrug (iron carbonyl [FeCO]-TPP)-loaded mesoporous silica nanomedicine (FeCO-TPP@MSN@HA) was proposed for mitochondrial-targeted CO therapy. 69 The developed FeCO-TPP@MSN@HA nanomedicine could realize precise gas treatment by delivering carbon monoxide to the mitochondria of cancer cells. To improve cancer treatment outcomes, a CO-delivery nanomedicine (FeCO-MnO 2 @MSN) has been synthesized by introducing manganese dioxide (MnO 2 ) NPs and FeCO into MSNs to achieve the synergetic gas/chemodynamic therapy under the response of tumor acidic microenvironment. 70 Therefore, silica nanomaterials demonstrate excellent performance as drug carriers in biomedical applications and are expected to produce valuable pharmaceuticals in the future.
Carbon, a component present in millions of various compounds, is found in every living organism. Because a series of exciting carbon nanomaterials are being developed, carbon-based nanomaterials are wildly used in biomedical because of their favorable chemical and physical properties, including electrical, thermal, optical, and structural diversity, particularly in the biomedical imaging and cancer therapy areas. [71][72][73][74] Given the existence of diverse allotropes of carbon, there are many types of carbon-based materials, such as diamonds and the newly discovered and promising carbon nanotubes, graphene oxide (GO), graphene quantum dots, and fullerene. 75 Given the deeper appreciation and development of carbonbased nanomaterials, GO has been noted for its favorable properties of high near-infrared (NIR) photothermal conversion efficiency for applications in cancer diagnosis and treatment. A NIR-responsive CO nanomedicine (MnCO-GO) was constructed by He et al. through bind-ing MnCO CORMs into GO nanosheet. 76 They have found that the GO absorbs NIR light and transforms photons into active electrons. The electrons are transferred from GO to MnCO and then cause the detachment of CO from Mn. According to the photoelectronic effect of graphene, a sandwich structure of GO-BNN6 has been constructed based on GO nanosheets and the NO donor BNN6 through self-assembly for NIR light-responsive release of NO, thus exhibiting remarkable anti-cancer effects. 77

Metal-organic framework
MOFs are a new type of porous organic-inorganic crystal hybrid materials dominated by the self-assembly of metal atoms and organic units. They are widely used due to their various superior properties and functions. [78][79][80] The orderly arrangement between linkers and metal ions endows MOFs with different pores, tunnels, and cages, thus suggesting a high potential for loading different cargo molecules. Furthermore, MOFs can have their surfaces further modified to increase their functionality. 81 In addition, some lanthanide-containing metal oxides and/or light-emitting organic ligands can generate fluorescence or phosphorescence under ultraviolet light irradiation. Therefore, the choice of organic and inorganic components can enable modulation of the crystalline structure and chemical functionality of MOFs, thus supporting their use in a wide range of applications, including gas storage, catalysis, and drug loading. [82][83][84][85][86][87][88] More importantly, various MOFs can be triggered by external stimuli (magnetic field, acid, temperature, and light), thus enabling their use in bioimaging and drug delivery. [89][90][91] Molecular imaging has become an emerging field of medicine, which uses auxiliary contrast agents to evaluate the effect of drugs. Recently, MOFs have received extensive attention in the field of bioimaging due to their easy cellular internalization, good dispersion stability, and high bioavailability. 92 MOFs can be used as magnetic resonance contrast agents for MRI via coordinating superparamagnetic metal ions, and luminescent building blocks (ligand or metal ions) for optical imaging by incorporating fluorescein. 79 For instance, Taylor et al. have reported Mn-based nanoscale MOFs with controllable morphology and demonstrated its potential for MRI contrast with extraordinarily high MR enhancement. 93 In particular, the fluorescent ligands MOF provided play a key role in luminescent sensor and biological imaging, which improve the coordinated interactions of alterable fluorescence. Jin and coworkers have constructed a new Ti-based MOF (Ti-MOF) for loading CO prodrug (MnCO) to achieve CO releasing under the response of intratumoral H 2 O 2 and CO monitoring through fluorescence imaging. 94 The fluorescence annihilation effect after Ti-MOF loaded with MnCO and the fluorescence activation effect after Ti-MOF releases CO are used to realize real-time fluorescence monitoring of CO release. In addition, the high specific surface area and porosity enable loading and release of different cargoes, particularly therapeutic agents. In 2006, Férey and co-workers reported that MOFs could be used as a drug delivery system due to their remarkable capacity for drug loading and their controlled release behavior. 95 Smart materials including stimuli-responsive MOFs have received substantial attention, particularly in the biomedical field used for controllable drug release. 87 A nanoscale MOF based on porphyrin-palladium (Pd-MOF) was developed by Zhou et al. 4 The highly dispersed palladium atoms were used to absorb hydrogen gas for photoacoustic imaging-guided hydrogen-thermal therapy. Therefore, MOFs exhibit important advantages for outstanding applications in biomedicine, as compared with the traditional materials. More functional MOFs are expected to be designed and synthesized as superior gas-based nanocarriers for controlled gas release.

STRATEGIES FOR GENERATING GAS
The development of nanomedicines to achieve precise control of gas release plays an important role in improving gas therapy efficacy. In this section, we summarize two kinds of gas-based nanomedicines from the viewpoint of gas release. Stimuli-responsive gas prodrug molecules release gas molecules, depending on the properties of nanocarriers, under different stimuli, such as pH, glutathione (GSH), H 2 O 2, or NIR light ( Figure 3). In situ gas-generating nanomedicines can generate gas molecules through photocatalytic reaction, chemically catalytic reaction, and enzymocatalytic action ( Figure 4). The following two sections describe the strategies for devising gas-based nanomedicine and provide typical examples.

pH
Nanosystems that respond to the tumor microenvironment (TME) have drawn extensive attention in recent years, due to their potential applications in precise treatment of cancer. Weak acidity is an important feature of malignant tumors and the design of pH-responsive nanomedicines could achieve precise gas targeting to tumors ( Figure 3A). 96 Metastable γ-phase manganese sul-fide (MnS) NPs are promising for tumor pH-responsive H 2 S prodrugs. In light of this principle, He et al. have devised a nanomedicine based on metastable γ-MnS (MnS@BSA) for the combination of gas therapy and chemodynamic therapy based on the pH-responsive H 2 S prodrugs. 100 The acidic TME could trigger the dissociation of MnS@BSA and thus release Mn 2+ and H 2 S gas.
In the meantime, the production of •OH radicals and pH-responsive H 2 S gas release realize the combination of chemodynamic therapy (CDT) and gas therapy. Free ammonia borane (AB) rapidly decomposes into H 2 in the acidic phosphate buffer saline (PBS) and reflects high acid responsiveness. Recently, He et al. have used AB as a hydrogen prodrug and loaded it into mesoporous silica (AB@MSN) to achieve intratumoral high-payload delivery and in situ acid-triggered release of H 2 . 101 Later, Zhang et al. designed a nanosystem based on biomembranecoated polydopamine and AB was loaded into NPs under the interaction of hydrogen bonding for combined photothermal therapy and hydrogen therapy. 102 Sulfur dioxide (SO 2 ) is a double-faced gas molecule. As reported, SO 2 is another promising therapeutic gas in a number of diseases and conditions after CO and NO. [103][104][105] However, the limited gas penetration depth confines the in vivo and in situ delivery of SO 2 . Lu et al. have proposed a gas therapeutic based on SO 2 prodrug, which can precisely control SO 2 gas release under both photothermal and pH stimuli, thereby realizing the synergy of gas therapy and photothermal therapy. 106 Besides, the mechanism of SO 2 inducing cell apoptosis based on the nanoplatform has been found to involve the upregulation of intracellular ROS levels and the modulation of apoptosis-related proteins. Thus, this strategy may be worthy of further development for deep tumor therapy.

Glutathione
GSH is another endogenous stimulus source to control gas release and a significant intracellular anti-oxidant. The level of GSH in cancer cells is higher than that in normal cells. 107 Therefore, GSH-responsive gas release is possibly achieved by GSH-sensitive nanocarriers, such as MSNs, amphiphilic polymeric NPs, and organo-inorganic complexes. 108 111 The obtained amphiphilic polymeric pro-  123 Copyright 2020, American Chemical Society drug releases SO 2 rapidly in response to thiol compounds, as triggered by GSH, and the releasing rate of SO 2 is dependent on GSH concentration. SO 2 and DOX act in synergism to enhance anti-tumor effects against MCF-7 ADR cells both in vitro and in vivo. Therefore, it is promising to develop GSH-responsive gas-releasing nanomedicine.

Hydrogen peroxide
Hydrogen peroxide (H 2 O 2 ) is fairly higher in the TME than in normal cells/tissues, which is another characteristic of tumors. 10  further leads to oxidization of L-arginine (L-Arg) to NO. Therefore, a novel L-Arg and GOx co-loaded hollow mesoporous organosilica nanomedicine (L-Arg-HMON-GOx) has been constructed for combined cancer starvation-like/gas therapy without a need for external excitation. 112 In addition, the depletion of intratumoral H 2 O 2 could lead to the apoptosis of tumor cells, because ROS has a great impact on the development of potential cancer therapies through adjusting cellular redox levels. 113 Thereby, the design of dual pro-oxidation therapies is of great potential to be potent in selectively combating cancer cells.

Near-infrared light
In the past few years, many photo CORMs have been developed for precise temporal-spatial control of gas release, thanks to the non-invasive, inexpensive, and controllable characteristics of light. NIR light is a favorable choice, owing to its higher tissue penetrability and lower phototoxicity.  116 anti-mycobacterial characteristics, 117 and a combined treatment method for overcoming drug resistance in cancer chemotherapy. 109 In studies of the mechanism of SO 2 killing tumor cells, Li and co-workers have constructed SO 2 prodrug-loaded rattle-structured silica-coated upconversion NPs based on NIR light-triggered SO 2 generation. They have found that SO 2 could increase intracellular ROS levels to cause the nuclear DNA of cancer cell damage and induce cell apoptosis. This NIR light-triggered gas nanomedicine may provide an outlook of advancing synergistic cancer therapy platforms.

Others
X-ray and ultrasound (US) can also be used for controlled gas release because of their high tissue penetration ability. Recently, Shi and co-workers have described a smart X-ray-activated NO-releasing upconversion nanoplatform that was sensitive to X-rays for cleaving the S-N bond of S-nitrosothiol (SNO) and then causing NO release ( Figure 3E). 99 However, high-dose X-rays (>5 Gy) were required, which may result in serious adverse effects and inevitably cause harm to normal tissues. To develop an NO delivery system with low-dose X-ray stimulation, Xue et al. have designed a novel soft X-ray-activatable persistent luminescence nanotransducer for tunable and endurable NO release. Though this nanoplatform utilized an ultralow dosage (down to 0.9 mGy) soft X-rays, it has successfully achieved persistent luminescence and continuous NO release for approximately 40 min after stopping Xray irradiation. The tissue penetration depth of US reached 20 cm under a 1 MHz US wave and the US wave could facilely be focused on a small region of the body (Figure 3F). 10

In situ gas-generating nanomedicines
The aberrant metabolism of cancer cells results in a TME with unique characteristics, such as over-production of lactic acid, and fairly higher concentrations of GSH and H 2 O 2 . 119 In addition, the uncontrolled growth of cancer cells and structurally abnormal blood vessels in tumor tissues bring about hypoxic (oxygen-lacking) microenvironment in solid tumors. 120 Therefore, the design of NPs with diverse nanostructures could produce chemical catalysis reactions under either the endogenous TME or exogenous physical stimuli. A variety of new nanoplatforms have been devised for in situ stimuli-responsive gas generation. This tumor-specific gas generation, including photocatalytic, chemically catalytic, and enzymocatalytic generation, will be delineated in the following parts.

Photocatalysis
Photocatalysis excites the electrons in the valence band to the conduction band under light irradiation, leaving holes in the valence band, thereby generating negative-electron (e-) and positive-hole (h+) pairs. 124 The electrons and holes have reducing and oxidizing characteristics respectively, thus producing therapeutic gas under the TME to improve cancer therapy. HisAgCCN is a photoactivated nanomaterial which can cause the conversion of endogenous CO 2 to CO in vivo. Inspired by Ag 3 PO 4 doped carbon-dot-decorated C 3 N 4 NPs (AgCCN), an excellent Z-scheme system to transform CO 2 to CO, Zheng et al. have constructed HisAgCCN by modified AgCCN as photocatalysts to transform tumor endogenous CO 2 to CO for enhancing chemotherapy ( Figure 4A). 121 Interestingly, HisAgCCN promotes mitochondrial biogenesis and aggravates oxidative stress in tumor cells, while reducing the side effects of chemotherapy to normal cells. Hydrogen has been identified as a potentially safer gas without blood poisoning risk in the anti-cancer field, as compared with NO, CO, and H 2 S gases. To solve the problems of low water-soluble and poor hydrogen delivery, Sun

Enzymocatalysis
Enzymes are natural and sustainable catalysts. Enzymatic processes are environmentally friendly and cost-effective, with high rates and selectivity. In recent years, biocatalysis has emerged as an important green chemical reaction in the biomedical domain. However, long-term operational stability is a major drawback, and it is hard to recover and reuse enzymes. 126 Thanks to advances in nanotechnology, loading into nanocarriers is necessary to enhance the stability and recyclability of biocatalysts in comparison with free enzymes. Chandrawati

GAS-BASED NANOMEDICINES FOR ENHANCED CANCER THERAPY
Gas therapy is drawing attention in nanomedicine as a safe and enhanced therapeutically efficient technique. However, a single therapeutic effect is inevitably accompanied by limited efficacy. Often, tumors cannot be completely eliminated by individual gas therapy. Therefore, this therapy is often combined with other therapeutic methods, like chemotherapy, photothermal therapy, and PDT. Understanding the mechanism of gas enhancing other therapeutic modalities will aid in designing reasonable strategies for clinical cancer therapy. In the sections below, we generalize the mechanisms of gas-based nanomedicines enhancing cancer therapy in detail. These mechanisms are also listed and summarized in Table 1.

NO-based nanomedicines
NO, a free-radical gas, is endogenously generated from L-Arg by NOSs and has attracted considerable attention.
A recent study has shown that NO has a biphasic effect (pro-and anti-tumor) on cancer development. Low levels of NO are beneficial to cell growth and anti-apoptotic responses, and cell cycle arrest and apoptosis can be easily induced by high levels of NO conversely. [135][136][137] The positive effect at low doses of NO activates the cyclic guanosine monophosphate pathway, a critical mediator of the long-term proliferation response. 138 As NO concentrations increase (>1 μM), hypoxia-inducible factors (HIFs) are stabilized, and the accumulation of p53 is hindered effects associated with suppressed cell proliferation. 139,140 Hypoxia in tumor cells is considered the main reason for poor therapeutic effects. To relieve hypoxia in tumor cells, Zhang et al. have designed a Bi-SNO NP that served as a valid hypoxic radiosensitizer by triggering NO release via X-rays to enhance the efficacy of radiotherapy. 141 Additionally, HIF-1a expression in tumors declined after injection of Bi-SNO and treatment, thus demonstrating that NO alleviates the tumor cellular hypoxic conditions. Furthermore, NO mediates angiogenesis, epithelial-mesenchymal tran-sition, and metastasis. [142][143][144] Sung et al. first demonstrated that a NO-based nanomedicine can efficiently reprogram the tumor vasculature and immune microenvironment to overcome the resistance of cancer therapy by normalizing tumor vessels, ameliorating the immunosuppressive TME, suppressing tumor metastases, and improving the anti-cancer effectiveness of three treatment modeschemotherapy. 144 Our group has fabricated NIR lasertriggered NO nanogenerators for reversing MDR in cancer ( Figure 6A). 115 The generated NO molecules have been shown to have chemosensitizing effects by suppressing the expression of an efflux pump protein (Pglycol protein), thus successfully achieving MDR reversal. Recently, researchers have been inspired by this finding to develop delivery systems combined with NO and drugs. Moreover, Kim and co-workers have revealed the mechanism underlying the reversal of MDR for chemotherapy mediated by NO ( Figure 5A). 129 NO overcomes MDR by decreasing the DNA repair and detoxification, strengthening nuclear drug transport, inhibiting the expression of HIF and nuclear factor-κB (NF-κB), and/or inactivating drug efflux proteins. 129,[145][146][147][148] Therefore, a combination of NO and drug delivery systems is expected to augment chemotherapy for clinical applications.

CO-based nanomedicines
CO was first described by John Haldane for its physiological effects on the human body through its binding. 149 Its strong affinity for hemoglobin (>220-fold greater than that of oxygen) has caused CO to be viewed as a silent killer.
With in-depth studies on CO, potential clinical therapeutics have received greater attention in the form of inhaled gaseous therapy. Extensive preclinical evidence in large and small animals has suggested that CO and CORMs have beneficial effects on cardiovascular disease, sepsis and shock, kidney and liver injury, cancer, and acute lung. 150 CORMs provide a securer approach to control CO release as an advanced cancer therapy. Recent studies have found that CO can be used for anti-angiogenic therapy in triplenegative breast cancer by decreasing vascular endothelial growth factor (VEGF) expression and inhibiting phosphorylation of VGEF receptor 2 and downstream proteins to decrease the migration and tube formation capacity of the endothelial cells. 151 Hypoxia is usually lower in early-stage tumors than in advanced tumors, and CO increases mitochondrial biogenesis and accelerates mitochondrial oxygen consumption ( Figure 6B). 149,152,153 On this basis, Li et al. have developed a cooperative anti-cancer therapeutic approach involving bioreductive chemotherapy and CO-mediated pro-apoptotic gas therapy. 154 They have synthesized Prussian blue NPs as a photothermal TA B L E 1 Mechanism summary of different types of gases for cancer treatment

Gas Mechanism
Refs.
• Suppress the accumulation of p53.
• Reprogram the tumor vasculature to overcome resistance to cancer therapy.
• Downregulate the expression of P-gp protein to overcome MDR. 133,135,139,113,129 CO • Downregulate the expression of VEGF.
• Inhibit the phosphorylation of VEGFR2.
• Decrease the migration of downstream proteins and the tube formation ability of endothelial cells. • Increase mitochondrial biogenesis.
CO-generating reagent created the hypoxic environment in tumor cells and activated the hypoxia-bioreducible chemical of tirapazamine, which synergizes with CO-mediated pro-apoptotic effects for enhancing anti-tumor efficacy. In addition, CO-induced mitochondrial depletion also leads to ROS production and apoptosis of cancer cells. To our knowledge, mitochondrial damage activates mitophagy and autophagy induced by adenosine triphosphate (ATP) shortage for self-protection. 155 Wang and co-workers have designed a durable and biocompatible metal carbonyl complex delivery nanoplatform (Fe(CO) 5 @Au) by placing iron pentacarbonyl (Fe(CO) 5 ) inside an Au nanocage to achieve CO-induced autophagy for improving cancer therapy ( Figure 5B). 130 The released CO damages mitochondria and subsequently initiates autophagy. The nanomaterial accumulates in autolysosomes and results in their destruction during autophagy, thus achieving synergistic effects in cancer cells. In addition, Zhang et al. have developed a US-driven biomimetic nanosystem with excellent synergistic antitumor effects, which has shown excellent effective suppression of tumor growth through US/H 2 O 2 -generated 1 O 2 and CO-induced cell apoptosis and mitochondrial dysfunction. 156 Recently, our group has reported a versatile CO/thermo/chemotherapy nanoplatform (FeCO-DOX@MCN) for the combined treatment of CO-induced ferroptosis. 157 FeCO-DOX@MCN nanomedicine efficaciously kills cancer cells, and the released CO extremely increases the sensitivity of cells to chemotherapeutics. Importantly, cell viability is obviously developed when a typical inhibitor of ferroptosis (ferrostatin-1) is added, thus indicating that ferroptosis was an important factor for CO to increase the sensitivity of cancer cells to chemotherapeutic agents. Therefore, we conclude that CO influenced cellular behavior by increasing mitochondrial biogenesis and driving mitochondria, thus increasing ATP and ROS production.   thus resulting in high metabolic acid production, which in turn results in intense intracellular acidification and subsequently cancer cell death. 131 As can be seen in Figure 5C, Lee

H 2 -based nanomedicines
Hydrogen gas (H 2 ) has higher biosafety than NO, CO, and H 2 S, as well as cancer-selective effects while protecting normal cells. A well-known mechanism has been attributed to the selective anti-oxidation property of H 2 through selective reduction of hydroxyl radicals. 171 Typically, in the TME, redox homeostasis can be perturbed and which results in redox stress, thus leading to cell damage and apoptosis. However, in normal cells, H 2 plays a protective role by removing excess ROS and preventing oxidative damage. 172 Therefore, the minimal adverse effects of their byproducts make H 2 therapy secure and efficient for clinical applications. Prevalent pathways for H 2 therapy in clinical settings are inhalation of hydrogencontaining air, oral intake of hydrogen-rich water, and injection of hydrogen-rich physiological solutions, the concentration of which is limited due to their very low water solubility. [173][174][175] Therefore, It is necessary to find strategies for targeting the delivery of H 2 and precise treatment. For targeted hydrogen therapy, He and coworkers have fabricated an AB-loaded mesoporous silica nanomedicine (AB@MSN) and carboxymethyl cellulose (CMC)-stabilized Fe (Fe@CMC) NPs to achieve in situ acid-controlled release of H 2 . 101,176 Interestingly, they have found that hydrogen molecules attenuate the toxic adverse effects of chemotherapy and enhance its effectiveness by developing a novel hydrogen-gas producing prodrug. 177 Many synergistic treatment strategies have been developed. Sun et al. have clarified the mechanism of hydrogen chemotherapy by investigating the metabolic behavior of intracellular drugs ( Figure 5D). 132 They have found that the combination of hydrogen with chemotherapeutic drugs enhances the phosphorylation of voltage-dependent anion channel 1 (VDAC1), and decreases mitochondrial membrane potential, inhibits mitochondrial function, and hinders ATP synthesis, thus leading to down-regulation of P-gp proteins for enhancing the drug transport capacity. Zhao et al. have used the anti-inflammatory effect of hydrogen to decrease the adverse effects of single thermal therapy on normal cells/tissues and enhance photothermal therapy. 98 Recently, Zhu and co-workers have designed a nanoscale porphyrin MOF integrated with nanopalladium crystals for synergistic hydrogen/PDT ( Figure 6D). 159 Hence, H 2 is particularly prospective for tumor therapy as a tumor-specific therapeutic gas. ) nano-platform for co-loading chlorine and doxorubicin to achieve oxygen sensitization for combined chemo-PDT. 178 Zhang and coworkers have found that MB activates 1 O 2 and induced lipid peroxidation, thus breaking liposomes, enlarging the contact area between CaO 2 and H 2 O, and resulting in accelerated production of O 2 after short irradiation periods. Therefore, the authors designed a liposome-based NP with O 2 self-sufficient properties (LipoMB/CaO 2 ) for PDT against hypoxic tumors ( Figure 5F). 134 However, MDR occurs during several courses of chemotherapy, thus significantly decreasing the effectiveness of cancer treatment. SO 2 , produced through the body's metabolic processes, can be used for combating MDR in cancer therapy. Shen and co-workers have designed a GSH-responsive polymeric prodrug of SO 2 to combat MDR in MCF-7 ADR cancer cells. 109 The released SO 2 increases ROS in tumor cells and then causes oxidative damage, thus sensitizing MCF-7 ADR cells to DOX. Our group has prepared a high SO 2loading nanosystem for overcoming the MDR to augment tumor accumulation and treatment efficacy through suppressing the expression of P-glycol protein ( Figure 5E). 133 Therefore, gas therapy may provide a new strategy for cancer therapy.

CONCLUSION AND OUTLOOK
Gas-based therapeutics is an emerging method for selective cancer killing while protecting normal cells. Many gasreleasing nanomedicines using multifunctional nanoplatforms have been designed for cancer treatment. In this review, we introduced the recent advances in gas therapy in detail. The designed stimuli-responsive nanocarriers are encouraging in healing malignant tumors with reduced risk of gas poisoning. Furthermore, some gas molecules have been combined with other therapeutic approaches. For example, NO molecules have been used to reverse MDR, and oxygen has been used to enhance radiotherapy and PDT. However, many challenges for gas-related applications remain, which require in-depth exploration. For example, most gas releasing platforms have short release times, thus weakening the therapeutic effect. In addition, developing nanomedicines with the on-demand gas release is important. With the rapid development of gas therapy-related basic research, increasing novel gas-based nanomedicines have been designed. However, many unknown areas exist regarding the biological effects of nanomaterials on cells or the body. The nanomaterials used in the clinic are usually very simple and differ from the described functional nanomaterials. Therefore, a long path to the use of novel gas-based nanomedicines in clinical settings remains, and treatment strategies that are simple and easy to implement should be exploited to achieve clinical gas therapy. The mechanisms of gas therapeutic effects are incompletely understood, thus limiting the application of gas therapy to some extent. The detailed mechanisms of gas therapy require further investigation and exploitation. In addition, the inhibition of cancer-related inflammation has shown great potential in preventing or suppressing cancer. The concentration-dependent influence of gasotransmitters (NO, CO, H 2 S, H 2 , etc.) on inflammation, either pro-or anti-inflammatory effect, remains a complex problem, while these gases often display anti-inflammatory effects at certain concentrations. [179][180][181][182] Overall, gas therapy is a fast-growing field emerging in nanomedicine, and efforts should be made to understand the mechanisms and develop applications to accelerate the adoption of gas therapy in clinical settings.

A C K N O W L E D G M E N T
This work was supported by the National Natural Science Foundation of China (Nos. 51933002 and 51872188) and the Program of Shanghai Academic Research Leader (20XD1400400).

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.