Gas Therapy: An Emerging “Green” Strategy for Anticancer Therapeutics

Nano-medicine therapy reprogramming metabolic network of tumour microenvironment: new opportunity for cancer therapies

Metabolic heterogeneity is one of the characteristics of tumour cells. In order to adapt to the tumour microenvironment of hypoxia, acidity and nutritional deficiency, tumour cells have undergone extensive metabolic reprogramming. Metabolites involved in tumour cell metabolism are also very different from normal cells, such as a large number of lactate and adenosine. Metabolites play an important role in regulating the whole tumour microenvironment. Taking metabolites as the target, it aims to change the metabolic pattern of tumour cells again, destroy the energy balance it maintains, activate the immune system, and finally kill tumour cells. In this paper, the regulatory effects of metabolites such as lactate, glutamine, arginine, tryptophan, fatty acids and adenosine were reviewed, and the related targeting strategies of nano-medicines were summarised, and the future therapeutic strategies of nano-drugs were discussed. The abnormality of tumour metabolites caused by tumour metabolic remodelling not only changes the energy and material supply of tumour, but also participates in the regulation of tumour-related signal pathways, which plays an important role in the survival, proliferation, invasion and metastasis of tumour cells. Regulating the availability of local metabolites is a new aspect that affects tumour progress. (The graphical abstract is by Figdraw).

Multifunctional gold nanoparticles for cancer theranostics

The diagnosis and treatment of cancer can often be challenging requiring more attractive options. Some types of cancers are more aggressive than others and symptoms for many cancers are subtle, especially in the early stages. Nanotechnology provides high sensitivity, specificity and multimodal capability for cancer detection, treatment and monitoring. In particular, metal nanoparticles (NPs) such as gold nanoparticles (AuNPs) are attractive nanosystems for researchers interested in bioimaging and therapy. The size, shape and surface of AuNPs can be modified for improving targeting and accumulation in cancer cells, for example through introduction of ligands and surface charge. The interactions of AuNPs with electromagnetic radiation (e.g., visible-near-infrared, X-rays) can be used for photothermal therapy and radiation therapy, through heat generated from light absorption and emission of Auger electrons, respectively. The subsequent expansion and high X-ray attenuation from AuNPs can be used for enhancing contrast for tumor detection (e.g., using photoacoustic, computed tomography imaging). Multi-functionality can be further extended through covalent/non-covalent functionalization, for loading additional imaging/therapeutic molecules for combination therapy and multimodal imaging. In order to cover the important aspects for designing and using AuNPs for cancer theranostics, this review focuses on the synthesis, functionalization and characterization methods that are important for AuNPs, and presents their unique properties and different applications in cancer theranostics.

Tumor-Targeted CO Nanodelivery System Design and Therapy for Hepatocellular Carcinoma

In recent years, carbon monoxide (CO) has garnered increased attention as a novel green therapy for hepatocellular carcinoma (HCC) treatment. However, the CO donor is still limited in clinical application due to its lack of targeted ability and unstable release rate. Here, self-assembled amphiphilic nanomicelles glucose-polyethylene glycol (PEG)-lipoic acid (LA)-Fe2(CO)6 (Glu-Fe2(CO)6) are first designed as a CO donor and synthesized via a chemical method, combining glucose with Fe2(CO)6 through PEG-LA. Some advantages of this tumor-targeted Glu-Fe2(CO)6 delivery system include (I) good water-solubility, (II) the glutathione responsive CO slow release, (III) the active tumor-targeted ability of glucose as targeted ligands, and (IV) outstanding efficacy of antitumor and safety of CO therapy of HCC both in vitro and in vivo. These findings suggest that Glu-Fe2(CO)6 nanomicelles hold promise for enhancing antitumor therapeutic capabilities, presenting a novel tumor-targeted delivery strategy in gas therapy for HCC treatment.

A chemically engineered water-soluble block copolymer for redox responsive SO2 release in antibacterial therapy

Sulfur dioxide (SO2) has emerged as a promising gasotransmitter for various therapeutic applications, including antibacterial activities. However, the potential of polymeric SO2 donors for antimicrobial activities remains largely unexplored. Herein, we report a water-soluble, redox-responsive, SO2-releasing amphiphilic block copolymer poly(polyethylene glycol methyl ether methacrylate) (PPEGMA)-b-poly(2-((2,4-dinitrophenyl)sulfonamido)ethyl methacrylate (PM)) (BCPx) to investigate their antibacterial properties. BCPx contains hydrophilic polyethylene glycol (PEG) pendants and a hydrophobic SO2-releasing PM block, facilitating the formation of self-assembled nanoparticles (BCPxNp) in an aqueous medium, studied by critical aggregation concentration (CAC) measurements, dynamic light scattering (DLS), field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM). BCPxNp exhibits sustained SO2 release up to 12 h in the presence of glutathione (GSH), with a yield of 30-80% of theoretical SO2 release. In vitro antibacterial studies unveil the outstanding antibacterial activity of BCP3Np against Gram-positive bacteria Bacillus subtilis, as evidenced by FESEM and live/dead cell fluorescence assay. We further elucidate the antibacterial mechanism through reactive oxygen species (ROS) generation studies. Overall, the polymer exhibits excellent biocompatibility at effective antimicrobial concentrations and provides insights into the design of a new class of SO2-releasing polymeric antibacterial agents.

Gas Therapy: Generating, Delivery, and Biomedical Applications

Oxygen (O2), nitric oxide (NO), carbon monoxide (CO), hydrogen sulfide (H2S), and hydrogen (H2) with direct effects, and carbon dioxide (CO2) with complementary effects on the condition of various diseases are known as therapeutic gases. The targeted delivery and in situ generation of these therapeutic gases with controllable release at the site of disease has attracted attention to avoid the risk of gas poisoning and improve their performance in treating various diseases such as cancer therapy, cardiovascular therapy, bone tissue engineering, and wound healing. Stimuli-responsive gas-generating sources and delivery systems based on biomaterials that enable on-demand and controllable release are promising approaches for precise gas therapy. This work highlights current advances in the design and development of new approaches and systems to generate and deliver therapeutic gases at the site of disease with on-demand release behavior. The performance of the delivered gases in various biomedical applications is then discussed.

X-Ray-Triggered CO-Release from Gold Nanocluster: All-in-One Nanoplatforms for Cancer Targeted Gas and Radio Synergistic Therapy

Glycolysis-dominant metabolic pathway in cancer cells can promote their therapeutic resistance against radiotherapy (RT). Carbon monoxide (CO) as a glycolysis inhibitor can enhance the efficiency of RT. Herein, an X-ray responsive CO-releasing nanocomposite (HA@AuNC@CO) based on strong host-guest interactions between the radiosensitizer and CO donor for enhanced RT is developed. The encapsulated gold nanoclusters (CD-AuNCs) can effectively generate cytotoxic reactive oxygen species (ROS) under X-ray radiation, which not only directly inactivate cancer cells but also induce in situ CO gas generation from adamantane modified metal carbonyl (Ada-CO) for glycolysis inhibition. Both in vitro and in vivo results demonstrate that HA@AuNC@CO exhibits active targeting toward CD44 overexpressed cancer cells, along with excellent inhibition of glycolysis and efficient RT against cancer. This study offers a new strategy for the combination of gas therapy and RT in tumor treatment.

Dual-Responsive Supramolecular Polymeric Nanomedicine for Self-Cascade Amplified Cancer Immunotherapy

Insufficient tumor immunogenicity and immune escape from tumors remain common problems in all tumor immunotherapies. Recent studies have shown that pyroptosis, a form of programmed cell death that is accompanied by immune checkpoint inhibitors, can induce effective immunogenic cell death and long-term immune activation. Therapeutic strategies to jointly induce pyroptosis and reverse immunosuppressive tumor microenvironments are promising for cancer immunotherapy. In this regard, a dual-responsive supramolecular polymeric nanomedicine (NCSNPs) to self-cascade amplify the benefits of cancer immunotherapy is designed. The NCSNPs are formulated by β-cyclodextrin coupling nitric oxide (NO) donor, a pyroptosis activator, and NLG919, an indoleamine 2,3-dioxygenase (IDO) inhibitor, and self-assembled through host-guest molecular recognition and hydrophobic interaction to obtain nanoparticles. NCSNPs possess excellent tumor accumulation and bioavailability attributed to ingenious supramolecular engineering. The study not only confirms the occurrence of NO-triggered pyroptosis in tumors for the first time but also reverses the immunosuppressive microenvironment in tumor sites via an IDO inhibitor by enhancing the infiltration of cytotoxic T lymphocytes, to achieve remarkable inhibition of tumor proliferation. Thus, this study provides a novel strategy for cancer immunotherapy.

Single-Cell Analysis Reveals Cxcl14+ Fibroblast Accumulation in Regenerating Diabetic Wounds Treated by Hydrogel-Delivering Carbon Monoxide

Nonhealing skin wounds are a problematic complication associated with diabetes. Therapeutic gases delivered by biomaterials have demonstrated powerful wound healing capabilities. However, the cellular responses and heterogeneity in the skin regeneration process after gas therapy remain elusive. Here, we display the benefit of the carbon monoxide (CO)-releasing hyaluronan hydrogel (CO@HAG) in promoting diabetic wound healing and investigate the cellular responses through single-cell transcriptomic analysis. The presented CO@HAG demonstrates wound microenvironment responsive gas releasing properties and accelerates the diabetic wound healing process in vivo. It is found that a new cluster of Cxcl14+ fibroblasts with progenitor property is accumulated in the CO@HAG-treated wound. This cluster of Cxcl14+ fibroblasts is yet unreported in the skin regeneration process. CO@HAG-treated wound macrophages feature a decrease in pro-inflammatory property, while their anti-inflammatory property increases. Moreover, the TGF-β signal between the pro-inflammatory (M1) macrophage and the Cxcl14+ fibroblast in the CO@HAG-treated wound is attenuated based on cell-cell interaction analysis. Our study provides a useful hydrogel-mediated gas therapy method for diabetic wounds and new insights into cellular events in the skin regeneration process after gas-releasing biomaterials therapy.

Sulfur signaling pathway in cardiovascular disease

Hydrogen sulfide (H2S) and sulfur dioxide (SO2), recognized as endogenous sulfur-containing gas signaling molecules, were the third and fourth molecules to be identified subsequent to nitric oxide and carbon monoxide (CO), and exerted diverse biological effects on the cardiovascular system. However, the exact mechanisms underlying the actions of H2S and SO2 have remained elusive until now. Recently, novel post-translational modifications known as S-sulfhydration and S-sulfenylation, induced by H2S and SO2 respectively, have been proposed. These modifications involve the chemical alteration of specific cysteine residues in target proteins through S-sulfhydration and S-sulfenylation, respectively. H2S induced S-sulfhydrylation can have a significant impact on various cellular processes such as cell survival, apoptosis, cell proliferation, metabolism, mitochondrial function, endoplasmic reticulum stress, vasodilation, anti-inflammatory response and oxidative stress in the cardiovascular system. Alternatively, S-sulfenylation caused by SO2 serves primarily to maintain vascular homeostasis. Additional research is warranted to explore the physiological function of proteins with specific cysteine sites, despite the considerable advancements in comprehending the role of H2S-induced S-sulfhydration and SO2-induced S-sulfenylation in the cardiovascular system. The primary objective of this review is to present a comprehensive examination of the function and potential mechanism of S-sulfhydration and S-sulfenylation in the cardiovascular system. Proteins that undergo S-sulfhydration and S-sulfenylation may serve as promising targets for therapeutic intervention and drug development in the cardiovascular system. This could potentially expedite the future development and utilization of drugs related to H2S and SO2.

Immunogenic Radiation Therapy for Enhanced Antitumor Immunity via a Core-Shell Nanosensitizer-Mediated Immunosuppressive Tumor Microenvironment Modulation

Due to the immunosuppressive tumor microenvironment (TME) and weak radiation absorption, the immune response triggered by radiation therapy (RT) is limited. Herein, a core-shell nanosensitizer UiO@MnS (denoted as UM) was genuinely constructed for the amplification of RT efficacy and induction of immunogenicity via integrating MnS-reprogrammed TME with Hf-based UiO-sensitized RT. The acid-sensitive MnS would produce H2S under acidic TME to improve oxygenation through inhibition mitochondrial respiration and reducing metabolic oxygen consumption, leading to decreased HIF-1α expression and enhanced radiosensitization. In addition, the generated H2S inhibited the catalase activity to increase the H2O2 level, which subsequently enhanced the Mn2+-mediated Fenton-like reaction, resulting in G2/M cell cycle arrest to improve the cellular sensitivity for radiation. This impressive tumor oxygenation, cell cycle arrest, and radiosensitization procedure boosted RT efficacy and resulted in strong antitumor immunogenicity. Taken together, combining the immunosuppressive TME modulation with a sensitizing radiation strategy shows great promise for magnifying immunogenic RT outputs.

GSH-Triggered/Photothermal-Enhanced H2S Signaling Molecule Release for Gas Therapy

Traditional treatment methods for tumors are inefficient and have severe side effects. At present, new therapeutic methods such as phototherapy, chemodynamic therapy, and gasodynamic therapy have been innovatively developed. High concentrations of hydrogen sulfide (H2S) gas exhibit cancer-suppressive effects. Herein, a Prussian blue-loaded tetra-sulfide modified dendritic mesoporous organosilica (PB@DMOS) was rationally constructed with glutathione (GSH)-triggered/photothermal-enhanced H2S signaling molecule release properties for gas therapy. The as-synthesized nanoplatform confined PB nanoparticles in the mesoporous structure of organosilica silica due to electrostatic adsorption. In the case of a GSH overexpressed tumor microenvironment, H2S gas was controllably released. And the temperature increases due to the photothermal effects of PB nanoparticles, further enhancing H2S release. At the same time, PB nanoparticles with excellent hydrogen peroxide catalytic performance also amplified the efficiency of tumor therapy. Thus, a collective nanoplatform with gas therapy/photothermal therapy/catalytic therapy functionalities shows potential promise in terms of efficient tumor therapy.

Mechanisms and Application of Gas-Based Anticancer Therapies

Cancer is still one of the major factors threatening public health, with morbidity and mortality rates at the forefront of the world. Clinical drawbacks, such as high toxicity and side effects of drug therapy, and easy recurrence after surgery affect its therapeutic effect. Gas signaling molecules are essential in maintaining biological homeostasis and physiological functions as specific chemical substances for biological information transfer. In recent years, the physiological regulatory functions of gas molecules in the cancer process have been gradually revealed and have shown broad application prospects in tumor therapy. In this paper, standard gas therapies are classified and introduced. Taking H2, CO2, NO, CO, H2S, and SO2 gases as examples, the research progress and application of gas therapies in malignant tumors are mainly introduced in terms of biological characteristics, anticancer mechanisms, and treatment strategies. Finally, the problems and prospects for developing gases as anticancer drugs are outlined.

Thiol-Responsive Polypeptide Sulfur Dioxide Prodrug Nanoparticles for Effective Tumor Inhibition

Sulfur dioxide (SO2) based gas therapy has emerged as a novel anticancer therapeutic strategy because of its high therapeutic efficacy and biosafety. To precisely adjust the SO2 content and control gas release, herein, a thiol-responsive polypeptide SO2 prodrug mPEG-block-poly(2-amino-6-(2,4-dinitrophenylsulfonamido)hexanoic acid) (PEG-b-PLys-DNs) was designed and facilely synthesized by polymerization of a novel N-carboxyanhydride SO2-NCA. The anticancer potential of the self-assembled nanoparticles (SO2-NPs) was investigated in detail. First, PEG-b-PLys-DNs were synthesized by ring-opening polymerization of SO2-NCA, which self-assembled into NPs sized 88.4 nm in aqueous. Subsequently, SO2-NPs were endocytosed into 4T1 cells and quickly released SO2 under a high concentration of glutathione in tumor cells. This process depleted cellular glutathione, generated reactive oxygen species, and dramatically increased oxidative stress, which led to cancer cell apoptosis. Finally, the in vivo anticancer efficacy of SO2-NPs was verified in 4T1-tumor-bearing mice. Our results indicated that this novel SO2 polymeric prodrug has great potential in eradicating tumors.

Recent applications of phase-change materials in tumor therapy and theranostics

Phase-change materials (PCMs) are a type of special material which can store and release a large amount of thermal energy without any significant temperature change. They are emerging in recent years as a promising functional material in tumor therapy and theranostics due to their accurate responses to the temperature variations, biocompatibility and low toxicity. In this review, we will introduce the main types of PCMs and their desirable physiochemical properties for biomedical applications, and highlight the recent progress of PCM's applications in the modulated release of antitumor drugs, with special attentions paid to various ways to initiate temperature-dependent phase change, the concomitant thermal therapy and its combination with or activation of other therapies, particularly unconventional therapies. We will also summarize PCM's recent applications in tumor theranostics, where both drugs and imaging probes are delivered by PCMs for controlled drug release and imaging-guided therapy. Finally, the future perspectives and potential limitations of harnessing PCMs in tumor therapy will be discussed.

Carbon monoxide therapy: a promising strategy for cancer

Cancer is one of the acute life-threatening diseases endangering the whole of humanity. The treatment modalities for cancer are various. However, in most cases, a single treatment choice provides multiple side effects, poor targeting, and ineffective treatment. In recent years, the physiological regulatory function of carbon monoxide (CO) in the cancer process has been reported gradually, and CO-related nano-drugs have been explored. It shows better application prospects in cancer treatment and provides new ideas for treatment. The present review introduces the pathophysiological role of CO. The recent advances in cancer therapy, such as CO-mediated gas therapy, combined application of CO chemotherapy, photodynamic therapy (PDT), photothermal therapy (PTT), and immunotherapy, are described. Current challenges and future developments in CO-based treatment are also discussed. This review provides comprehensive information on recent advances in CO therapy and also some valuable guidance for promoting the progress of gas therapy nanomedicine.

Photoresponsive polymeric microneedles: An innovative way to monitor and treat diseases

Microneedles (MN) technology is an emerging technology for the transdermal delivery of therapeutics. When combined with photoresponsive (PR) materials, MNs can deliver therapeutics precisely and effectively with enhanced efficacy or synergistic effects. This review systematically summarizes the therapeutic applications of PRMNs in cancer therapy, wound healing, diabetes treatment, and diagnostics. Different PR approaches to activate and control the release of therapeutic agents from MNs are also discussed. Overall, PRMNs are a powerful tool for stimuli-responsive controlled-release therapeutic delivery to treat various diseases.

Recent advances in near-infrared-II hollow nanoplatforms for photothermal-based cancer treatment

Near-infrared-II (NIR-II, 1000–1700 nm) light-triggered photothermal therapy (PTT) has been regarded as a promising candidate for cancer treatment, but PTT alone often fails to achieve satisfactory curative outcomes. Hollow nanoplatforms prove to be attractive in the biomedical field owing to the merits including good biocompatibility, intrinsic physical-chemical nature and unique hollow structures, etc. On one hand, hollow nanoplatforms themselves can be NIR-II photothermal agents (PTAs), the cavities of which are able to carry diverse therapeutic units to realize multi-modal therapies. On the other hand, NIR-II PTAs are capable of decorating on the surface to combine with the functions of components encapsulated inside the hollow nanoplatforms for synergistic cancer treatment. Notably, PTAs generally can serve as good photoacoustic imaging (PAI) contrast agents (CAs), which means such kind of hollow nanoplatforms are also expected to be multifunctional all-in-one nanotheranostics. In this review, the recent advances of NIR-II hollow nanoplatforms for single-modal PTT, dual-modal PTT/photodynamic therapy (PDT), PTT/chemotherapy, PTT/catalytic therapy and PTT/gas therapy as well as multi-modal PTT/chemodynamic therapy (CDT)/chemotherapy, PTT/chemo/gene therapy and PTT/PDT/CDT/starvation therapy (ST)/immunotherapy are summarized for the first time. Before these, the typical synthetic strategies for hollow structures are presented, and lastly, potential challenges and perspectives related to these novel paradigms for future research and clinical translation are discussed.

Multifunctional nanotheranostics for near infrared optical imaging-guided treatment of brain tumors

Malignant brain tumors, a heterogeneous group of primary and metastatic neoplasms in the central nervous system (CNS), are notorious for their highly invasive and devastating characteristics, dismal prognosis and low survival rate. Recently, near-infrared (NIR) optical imaging modalities including fluorescence imaging (FLI) and photoacoustic imaging (PAI) have displayed bright prospect in innovation of brain tumor diagnoses, due to their merits, like noninvasiveness, high spatiotemporal resolution, good sensitivity and large penetration depth. Importantly, these imaging techniques have been widely used to vividly guide diverse brain tumor therapies in a real-time manner with high accuracy and efficiency. Herein, we provide a systematic summary of the state-of-the-art NIR contrast agents (CAs) for brain tumors single-modal imaging (e.g., FLI and PAI), dual-modal imaging (e.g., FLI/PAI, FLI/magnetic resonance imaging (MRI) and PAI/MRI) and triple-modal imaging (e.g., MRI/FLI/PAI and MRI/PAI/computed tomography (CT) imaging). In addition, we update the most recent progress on the NIR optical imaging-guided therapies, like single-modal (e.g., photothermal therapy (PTT), chemotherapy, surgery, photodynamic therapy (PDT), gene therapy and gas therapy), dual-modal (e.g., PTT/chemotherapy, PTT/surgery, PTT/PDT, PDT/chemotherapy, PTT/chemodynamic therapy (CDT) and PTT/gene therapy) and triple-modal (e.g., PTT/PDT/chemotherapy, PTT/PDT/surgery, PTT/PDT/gene therapy and PTT/gene/chemotherapy). Finally, we discuss the opportunities and challenges of the CAs and nanotheranostics for future clinic translation.

Enhancing the Efficiency of Mild-Temperature Photothermal Therapy for Cancer Assisting with Various Strategies

Conventional photothermal therapy (PTT) irradiates the tumor tissues by elevating the temperature above 48 °C to exert thermal ablation, killing tumor cells. However, thermal ablation during PTT harmfully damages the surrounding normal tissues, post-treatment inflammatory responses, rapid metastasis due to the short-term mass release of tumor-cellular contents, or other side effects. To circumvent this limitation, mild-temperature photothermal therapy (MTPTT) was introduced to replace PTT as it exerts its activity at a therapeutic temperature of 42-45 °C. However, the significantly low therapeutic effect comes due to the thermoresistance of cancer cells as MTPTT figures out some of the side-effects issues. Herein, our current review suggested the mechanism and various strategies for improving the efficacy of MTPTT. Especially, heat shock proteins (HSPs) are molecular chaperones overexpressed in tumor cells and implicated in several cellular heat shock responses. Therefore, we introduced some methods to inhibit activity, reduce expression levels, and hinder the function of HSPs during MTPTT treatment. Moreover, other strategies also were emphasized, including nucleus damage, energy inhibition, and autophagy mediation. In addition, some therapies, like radiotherapy, chemotherapy, photodynamic therapy, and immunotherapy, exhibited a significant synergistic effect to assist MTPTT. Our current review provides a basis for further studies and a new approach for the clinical application of MTPTT.

External stimuli-responsive gasotransmitter prodrugs: Chemistry and spatiotemporal release

Gasotransmitters like nitric oxide, carbon monoxide, and hydrogen sulfide with unique pleiotropic pharmacological effects in mammals are an emerging therapeutic modality for different human diseases including cancer, infection, ischemia-reperfusion injuries, and inflammation; however, their clinical translation is hampered by the lack of a reliable delivery form, which delivers such gasotransmitters to the action site with precisely controlled dosage. The external stimuli-responsive prodrug strategy has shown tremendous potential in developing gasotransmitter prodrugs, which affords precise temporospatial control and better dose control compared with endogenous stimuli-sensitive prodrugs. The promising external stimuli employed for gasotransmitter activation range from photo, ultrasound, and bioorthogonal click chemistry to exogenous enzymes. Herein, we highlight the recent development of external stimuli-mediated decaging chemistry for the temporospatial delivery of gasotransmitters including nitric oxide, carbon monoxide, hydrogen sulfide and sulfur dioxide, and discuss the pros and cons of different designs.

Enhancing the therapeutic efficacy of nanoparticles for cancer treatment using versatile targeted strategies

Poor targeting of therapeutics leading to severe adverse effects on normal tissues is considered one of the obstacles in cancer therapy. To help overcome this, nanoscale drug delivery systems have provided an alternative avenue for improving the therapeutic potential of various agents and bioactive molecules through the enhanced permeability and retention (EPR) effect. Nanosystems with cancer-targeted ligands can achieve effective delivery to the tumor cells utilizing cell surface-specific receptors, the tumor vasculature and antigens with high accuracy and affinity. Additionally, stimuli-responsive nanoplatforms have also been considered as a promising and effective targeting strategy against tumors, as these nanoplatforms maintain their stealth feature under normal conditions, but upon homing in on cancerous lesions or their microenvironment, are responsive and release their cargoes. In this review, we comprehensively summarize the field of active targeting drug delivery systems and a number of stimuli-responsive release studies in the context of emerging nanoplatform development, and also discuss how this knowledge can contribute to further improvements in clinical practice.

X-ray-Triggered CO Release Based on GdW10/MnBr(CO)5 Nanomicelles for Synergistic Radiotherapy and Gas Therapy

Carbon monoxide (CO) therapy has become a hot topic in the field of gas therapy because of its application prospect in the treatment of various diseases. Due to the high affinity for human hemoglobin, the main challenge of CO-loaded nanomedicine is the lack of selectivity and toxicity in the delivery process. Although many commercial CO-releasing molecules (CORMs) have been widely developed because of their ability to deliver CO, CORMs still have some disadvantages, including difficult on-demand controlled CO release, poor solubility, and potential toxicity, which are limiting their further application. Herein, an X-ray-triggered CO-releasing nanomicelle system (GW/MnCO@PLGA) based on GdW₁₀ nanoparticles (NPs) (GW) and MnBr(CO)₅ (MnCO) encapsulating in the poly(lactic-co-glycolic acid) (PLGA) polymer was constructed for synergistic CO radiotherapy (RT). The production of strongly oxidative superoxide anion (O₂^-•) active species can lead to cell apoptosis under the X-ray sensitization of GW. Moreover, strongly oxidative O₂^-• radicals further oxidize and compete with the Mn center, resulting in the on-demand release of CO. The radio/gas therapy synergy to enhance the efficient tumor inhibition of the nanomicelles was investigated in vivo and in vitro. Therefore, the establishment of an X-ray-triggered controlled CO release system has great application potential for further synergistic RT CO therapy in deep tumor sites.

Near-infrared light-triggered nano-prodrug for cancer gas therapy

Gas therapy (GT) has attracted increasing attention in recent years as a new cancer treatment method with favorable therapeutic efficacy and reduced side effects. Several gas molecules, such as nitric oxide (NO), carbon monoxide (CO), hydrogen (H2), hydrogen sulfide (H2S) and sulfur dioxide (SO2), have been employed to treat cancers by directly killing tumor cells, enhancing drug accumulation in tumors or sensitizing tumor cells to chemotherapy, photodynamic therapy or radiotherapy. Despite the great progress of gas therapy, most gas molecules are prone to nonspecific distribution when administered systemically, resulting in strong toxicity to normal tissues. Therefore, how to deliver and release gas molecules to targeted tissues on demand is the main issue to be considered before clinical applications of gas therapy. As a specific and noninvasive stimulus with deep penetration, near-infrared (NIR) light has been widely used to trigger the cleavage and release of gas from nano-prodrugs via photothermal or photodynamic effects, achieving the on-demand release of gas molecules with high controllability. In this review, we will summarize the recent progress in cancer gas therapy triggered by NIR light. Furthermore, the prospects and challenges in this field are presented, with the hope for ongoing development.

Self-Propelled Asymmetrical Nanomotor for Self-Reported Gas Therapy

Gas therapy has emerged as a new therapeutic strategy in combating cancer owing to its high therapeutic efficacy and biosafety. However, the clinical translation of gas therapy remains challenging due to the rapid diffusion and limited tissue penetration of therapeutic gases. Herein, a self-propelled, asymmetrical Au@MnO₂ nanomotor for efficient delivery of therapeutic gas to deep-seated cancer tissue for enhanced efficacy of gas therapy, is reported. The Au@MnO₂ nanoparticles (NPs) catalyze endogenous H₂ O₂ into O₂ that propels NPs into deep solid tumors, where SO₂ prodrug is released from the hollow NPs owing to the degradation of MnO₂ shells. Fluorescein isothiocyanate (FITC) is conjugated onto the surface of Au via caspase-3 responsive peptide (DEVD) and the therapeutic process of gas therapy can be optically self-reported by the fluorescence of FITC that is turned on in the presence of overexpressed caspase-3 as an apoptosis indicator. Au@MnO₂ nanomotors show self-reported therapeutic efficacy and high biocompatibility both in vitro and in vivo, offering important new insights to the design and development of novel nanomotors for efficient payload delivery into deep tumor tissue and in situ monitoring of the therapeutic process.

Tailoring Supramolecular Prodrug Nanoassemblies for Reactive Nitrogen Species-Potentiated Chemotherapy of Liver Cancer

The development of a controllable reactive nitrogen species (RNS) generation system for cancer treatment has remained elusive. Herein, a supramolecular prodrug nanoassemblies (SPNA) strategy that co-delivers a nitric oxide (NO) donor and a superoxide anion (O₂^•-) inducing chemotherapeutic agent was reported for RNS-potentiated chemotherapy. The mole ratio of platinum(IV) prodrug and NO donor could be precisely tailored in SPNA_Pt/NO. Platinum(II) and NO would be released intracellularly to produce a highly toxic RNS, peroxynitrite anion (ONOO^-). The levels of glutathione reductase (GR) and xeroderma pigmentosum group A (XPA) were down-regulated by ONOO^-, thus synergistically decreasing detoxification and blocking DNA damage repair of Pt-based chemotherapy. The RNS-potentiated efficacy of SPNA_Pt/NO was validated on subcutaneous hepatoma xenograft models and an orthotopic cisplatin-resistant hepatoma model. This co-delivery strategy of NO donor and O₂^•- inducing chemotherapeutic agents for RNS-mediated therapy provides an insightful direction for cancer treatment.

Multifunctional Carbon Monoxide Prodrug-Loaded Nanoplatforms for Effective Photoacoustic Imaging-Guided Photothermal/Gas Synergistic Therapy

Multifunctional cancer treatments based on gas therapy combined with other cancer treatments have gained tremendous attention and hold great promise in biomedical applications. In this study, a carbon monoxide-releasing nanoplatform combined with near-infrared (NIR) laser-triggered photothermal therapy (PTT) was constructed. The nanoplatform was composed of manganese pentacarbonyl bromide (MnCO)-loaded g-carbon nitride/polypyrrole (CNPpy) nanomaterials (MnCO@CNPpy). MnCO can be triggered to produce CO under H₂O₂ conditions. Upon exogenous NIR light stimulation and tumor microenvironment-overexpressed H₂O₂, MnCO@CNPpy exhibited excellent CO generation performance and photothermal effect. The generation of CO induced intracellular oxidative stress and caused cell apoptosis. Additionally, photoacoustic (PA) imaging was performed to track the delivery and accumulation of the nanomaterial in tumor sites because of the great photothermal conversion of CNPpy. The presented MnCO@CNPpy nanoplatform displayed desirable PTT and CO therapy in the inhibition of tumor growth and may provide a promising strategy for multifunctional antitumor synergistic treatments.

Acidity-Triggered Tumor-Targeted Nanosystem for Synergistic Therapy via a Cascade of ROS Generation and NO Release

Nitric oxide (NO) gas therapy has aroused intense interest in recent years. l-Arginine (l-Arg) reacts with reactive oxygen species (ROS) in tumor cells to generate NO. This phenomenon represents an effective method for tumor therapy. However, endogenous ROS levels in most types of tumor cells cannot enable an effective reaction. β-Lapachone is generally used to increase H₂O₂, which can oxidize guanidine derivatives to form nitric oxide in tumor cells. In addition, based on the ferrocene (Fc)-catalyzed Fenton reaction, ·OH is generated from H₂O₂, and the ONOO^- could be generated from an interaction between ·O₂^- (generated through the Haber-Weiss reaction) and NO. Arg-rich poly(ε-caprolactone) (PCL)-b-PArg, a macromolecular NO donor, was accurately synthesized to avoid premature l-Arg leakage during in vivo transport. In this design, the self-assembled PCL-b-PArg nanoparticles were dressed with the tumor-shreddable masking (PEG-b-PDMA, a negatively charged pH-sensitive hydrophilic diblock polymer), to prepare P-lapa-Fc nanoparticles and hide penetrative capability in the circulation. The experimental results confirmed that this synergistic therapy based on ROS and NO had a significant inhibitory effect on cancer cells, thereby providing new inspiration for NO gas treatment.

Zinc Oxide Particles Catalytically Generate Nitric Oxide from Endogenous and Exogenous Prodrugs

Nitric oxide (NO) is a potent biological molecule that contributes to a wide spectrum of physiological processes. However, the full potential of NO as a therapeutic agent is significantly complicated by its short half-life and limited diffusion distance in human tissues. Current strategies for NO delivery focus on encapsulation of NO donors into prefabricated scaffolds or an enzyme-prodrug therapy approach. The former is limited by the finite pool of NO donors available, while the latter is challenged by the inherent low stability of natural enzymes. Zinc oxide (ZnO) particles with innate glutathione peroxidase and glycosidase activities, a combination that allows to catalytically decompose both endogenous (S-nitrosoglutathione) and exogenous (β-gal-NONOate) donors to generate NO at physiological conditions are reported. By tuning the concentration of ZnO particles and NO prodrugs, physiologically relevant NO levels are achieved. ZnO preserves its catalytic property for at least 6 months and the activity of ZnO in generating NO from prodrugs in human serum is demonstrated. The ZnO catalytic activity will be beneficial toward generating stable NO release for long-term biomedical applications.

A continuous stimuli-responsive system for NIR-II fluorescence/photoacoustic imaging guided photothermal/gas synergistic therapy

Nanosystems responsive to a tumor microenvironment (TME) have recently attracted great attention due to their potential in precision cancer theranostics. However, theranostic nanosystems with a TME-activated consecutive cascade for the accurate diagnosis and treatment of cancer have rarely been exploited. Herein, an activatable theranostic nanosystem (Bi2S3-Ag2S-DATS@BSA-N3 NYs) is designed and constructed on the basis of a one-pot biomineralization method and surface functional modification to improve second near-infrared (NIR-II) fluorescence/photoacoustic (PA) imaging-guided photothermal therapy (PTT)/gas therapy (GT). Based on enhanced penetration and retention (EPR) effect-mediated tumor accumulation, the tumor-overexpressed glutathione (GSH) can accelerate hydrogen sulfide (H2S) generation from the nanoparticles by reacting with the encapsulated diallyl trisulfide (DATS). Meanwhile, the in situ released H2S can be used not only for gas therapy, but also to start the reduction of -N3(-) to -NH2(+), thereby enhancing the tumor-specific aggregation of NYs. As a result, the activatable nanosystems with excellent tumor accumulation and biodistribution could achieve an accurate NIR-II/PA dual-modality imaging for guiding the synergistic anticancer efficacy (PTT/GT). Thus, this work provides a promising TME-mediated continuously responsive strategy for efficient anticancer therapy.

Surface Charge Switchable Supramolecular Nanocarriers for Nitric Oxide Synergistic Photodynamic Eradication of Biofilms

Biofilm has resulted in numerous obstinate clinical infections, posing severe threats to public health. It is urgent to develop original antibacterial strategies for eradicating biofilms. Herein, we develop a surface charge switchable supramolecular nanocarrier exhibiting pH-responsive penetration into an acidic biofilm for nitric oxide (NO) synergistic photodynamic eradication of the methicillin-resistant Staphylococcus aureus (MRSA) biofilm with negligible damage to healthy tissues under laser irradiation. Originally, by integrating the glutathione (GSH)-sensitive α-cyclodextrin (α-CD) conjugated nitric oxide (NO) prodrug (α-CD-NO) and chlorin e6 (Ce6) prodrug (α-CD-Ce6) into the pH-sensitive poly(ethylene glycol) (PEG) block polypeptide copolymer (PEG-(KLAKLAK)₂-DA) via host-guest interaction, the supramolecular nanocarrier α-CD-Ce6-NO-DA was finely prepared. The supramolecular nanocarrier shows complete surface charge reversal from negative charge at physiological pH (7.4) to positive charge at acidic biofilm pH (5.5), promoting efficient penetration into the biofilm. Once infiltrated into the biofilm, the nanocarrier exhibits rapid NO release triggered by the overexpressed GSH in the biofilm, which not only produces abundant NO for killing bacteria but also reduces the biofilm GSH level to improve photodynamic therapy (PDT) efficiency. On the other hand, NO can react with reactive oxygen species (ROS) to produce reactive nitrogen species (RNS), further improving the PDT efficiency. Due to the effective penetration into the biofilm and depletion of biofilm GSH, the surface charge switchable GSH-sensitive NO nanocarrier can greatly improve the PDT efficiency at a low photosensitizer dose and laser intensity and cause negligible side effect to healthy tissues. Considering the above advantages, the strategy developed in this work may offer great possibilities to fight against biofilm infections.

Recent advances in pH-responsive nanomaterials for anti-infective therapy

The design and synthesis of pH-responsive antibacterial nanomaterials and their applications in anti-infective therapy.

Emerging Delivery Strategies of Carbon Monoxide for Therapeutic Applications: from CO Gas to CO Releasing Nanomaterials

Carbon monoxide (CO) therapy has emerged as a hot topic under exploration in the field of gas therapy as it shows the promise of treating various diseases. Due to the gaseous property and the high affinity for human hemoglobin, the main challenges of administrating medicinal CO are the lack of target selectivity as well as the toxic profile at relatively high concentrations. Although abundant CO releasing molecules (CORMs) with the capacity to deliver CO in biological systems have been developed, several disadvantages related to CORMs, including random diffusion, poor solubility, potential toxicity, and lack of on-demand CO release in deep tissue, still confine their practical use. Recently, the advent of versatile nanomedicine has provided a promising chance for improving the properties of naked CORMs and simultaneously realizing the therapeutic applications of CO. This review presents a brief summarization of the emerging delivery strategies of CO based on nanomaterials for therapeutic application. First, an introduction covering the therapeutic roles of CO and several frequently used CORMs is provided. Then, recent advancements in the synthesis and application of versatile CO releasing nanomaterials are elaborated. Finally, the current challenges and future directions of these important delivery strategies are proposed.

Gas-Mediated Cancer Bioimaging and Therapy

Gas-involving cancer theranostics have attracted considerable attention in recent years due to their high therapeutic efficacy and biosafety. We have reviewed the recent significant advances in the development of stimuli-responsive gas releasing molecules (GRMs) and gas nanogenerators for cancer bioimaging, targeted and controlled gas therapy, and gas-sensitized synergistic therapy. We have focused on gases with known anticancer effects, such as oxygen (O₂), carbon monoxide (CO), nitric oxide (NO), hydrogen sulfide (H₂S), hydrogen (H₂), sulfur dioxide (SO₂), carbon dioxide (CO₂), and heavy gases that act via the gas-generating process. The GRMs and gas nanogenerators for each gas have been described in terms of the stimulation method, followed by their applications in ultrasound and multimodal imaging, and finally their primary and synergistic actions with other cancer therapeutic modalities. The current challenges and future possibilities of gas therapy and imaging vis-à-vis clinical translation have also been discussed.

Starvation-amplified CO generation for enhanced cancer therapy via an erythrocyte membrane-biomimetic gas nanofactory

Carbon monoxide (CO)-based gas therapy has emerged as an attractive therapeutic strategy for cancer therapy. However, the main challenges are the in situ-triggered and efficient delivery of CO in tumors, which limit its further clinical application. Herein, we developed an erythrocyte membrane-biomimetic gas nanofactory (MGP@RBC) to amplify the in situ generation of CO for combined energy starvation of cancer cells and gas therapy. This nanofactory was constructed by encapsulating glucose oxidase (GOx) and Mn₂(CO)₁₀ (CO-donor) into the biocompatible polymer poly(lactic-co-glycolic acid), obtaining MGP nanoparticles, which are further covered by red blood cell (RBC) membrane. Because of the presence of proteins on RBC membranes, the nanoparticles could effectively avoid immune clearance in macrophages (Raw264.7) and significantly prolong their blood circulation time, thereby achieving higher accumulation at the tumor site. After that, the GOx in GMP@RBC could effectively catalyze the conversion of endogenous glucose to hydrogen peroxide (H₂O₂) in the presence of oxygen. The concomitant generation of H₂O₂ could efficiently trigger CO release to cause dysfunction of mitochondria and activate caspase, thereby resulting in apoptosis of the cancer cells. In addition, the depletion of intratumoral glucose could starve tumor cells by shutting down the energy supply. Altogether, the in vitro and in vivo studies of our synthesized biomimetic gas nanofactory exhibited an augmentative synergistic efficacy of CO gas therapy and energy starvation to inhibit tumor growth. It provides an attractive strategy to amplify CO generation for enhanced cancer therapy in an accurate and more efficient manner. STATEMENT OF SIGNIFICANCE: Carbon monoxide (CO) based gas therapy has emerged as an attractive therapeutic strategy for cancer therapy. In this study, we developed an erythrocyte membrane biomimetic gas nanofactory to amplify the in-situ generation of CO for combined cancer starvation and gas therapy. It is constructed by coating glucose oxidase (GOx) and CO donor-loaded nanoparticles with erythrocyte membrane. Due to the erythrocyte membrane, it can effectively prolong blood circulation time and achieve higher tumor accumulation. After accumulated in tumor, endogenous glucose can be effectively catalyzed to hydrogen peroxide, in-situ amplified CO release to induce the apoptosis of cancer cells. In addition, depleting glucose can also starve tumor cells by shutting down the energy supply. Overall, our biomimetic gas nanofactory exhibits an augmentative synergistic efficacy of CO gas therapy and starvation to increased tumor inhibition. It provide a novel strategy to deliver CO in an accurate and more efficient manner, promising for combined cancer therapy in future clinical application.

3-Hydroxyflavones and 3-Hydroxy-4-oxoquinolines as Carbon Monoxide-Releasing Molecules

Carbon monoxide-releasing molecules (CORMs) that enable the delivery of controlled amounts of CO are of strong current interest for applications in biological systems. In this review, we examine the various conditions under which CO is released from 3-hydroxyflavones and 3-hydroxy-4-oxoquinolines to advance the understanding of how these molecules, or derivatives thereof, may be developed as CORMs. Enzymatic pathways from quercetin dioxygenases and 3-hydroxy-4-oxoquinoline dioxygenases leading to CO release are examined, along with model systems for these enzymes. Base-catalyzed and non-redox-metal promoted CO release, as well as UV and visible light-driven CO release from 3-hydroxyflavones and 3-hydroxy-4-oxoquinolines, are summarized. The visible light-induced CO release reactivity of recently developed extended 3-hydroxyflavones and a 3-hydroxybenzo[g]quinolone, and their uses as intracellular CORMs, are discussed. Overall, this review provides insight into the chemical factors that affect the thermal and photochemical dioxygenase-type CO release reactions of these heterocyclic compounds.