Yaping Li1,2,3, Lu An1,2,3, Jiaomin Lin1,2,3, Qiwei Tian1,2,3, Shiping Yang1,2,3. 1. Key Laboratory of Resource Chemistry of the Ministry of Education. 2. The Shanghai Key Laboratory of Rare Earth Functional Materials. 3. The Shanghai Municipal Education Committee Key Laboratory of Molecular Imaging Probes and Sensors, Shanghai Normal University, Shanghai 200234, People's Republic of China.
Abstract
Effective tumor diagnosis and therapy have always been a significant but challenging issue. Although nanomedicine has shown great potential for improving the outcomes of tumor diagnosis and therapy, the nonspecial targeted distribution of nanomedicine agents in the whole body causes a low diagnosis signal-to-noise ratio and a potential risk of systemic toxicity. Recently, the development of smart nanomedicine agents with diagnosis and therapy functions that can only be activated by the tumor microenvironment (TME) is regarded as an effective strategy to improve the theranostic sensitivity and selectivity, as well as reduce the potential side effects during treatment. This article will introduce and summarize the latest achievements in the design and fabrication of TME-responsive smart nanomedicine agents, and highlight their prospects for enhancing tumor diagnosis and therapy.
Effective tumor diagnosis and therapy have always been a significant but challenging issue. Although nanomedicine has shown great potential for improving the outcomes of tumor diagnosis and therapy, the nonspecial targeted distribution of nanomedicine agents in the whole body causes a low diagnosis signal-to-noise ratio and a potential risk of systemic toxicity. Recently, the development of smart nanomedicine agents with diagnosis and therapy functions that can only be activated by the tumor microenvironment (TME) is regarded as an effective strategy to improve the theranostic sensitivity and selectivity, as well as reduce the potential side effects during treatment. This article will introduce and summarize the latest achievements in the design and fabrication of TME-responsive smart nanomedicine agents, and highlight their prospects for enhancing tumor diagnosis and therapy.
Malignant tumor is one of the key diseases leading to mortality around the word. Owing to the limited outcomes and undesirable side effects of conventional therapy (such as surgery and chemotherapy), many efforts from various fields have been devoted to exploring effective and safe therapeutic modalities and agents.1–3 In the past two decades, a number of imaging technology and therapeutic modalities of minimally invasive nature have shown great promise toward this goal.4–7 For example, photodynamic therapy, which employed a photosensitizer to generate cytotoxic singlet oxygen to kill tumor cells in the specified position irradiated by excitation light, displays high treat selectivity and leaves little or no scarring.8,9 These promising imaging technologies and therapeutic modalities are boosted by the unceasing emergence of nanomedicine agents that possess versatile physiochemical properties, such as fluorescence,10 magnetism,11 near-infrared (NIR) absorption,12 and porous structures.13 For instance, gold nanoparticles with strong NIR absorption can be utilized for photoacoustic imaging and photothermal therapy.14 Porous silicon and metal–organic frameworks with high porosity and large surface area can be used as carriers for delivering anticancer drugs.15,16Paradigms of smart nanomedicine agents for cancerAbbreviations: CAP, cellulose acetate phthalate; CDT, chemodynamic therapy; FL, fluorescence imaging; GSH, glutathione; MOF, metal–organic framework; MRI, magnetic resonance imaging; NP, nanoparticle; PAI, photoacoustic imaging; PDT, photodynamic therapy; PEG, polyethylene glycol; PTT, photothermal therapy; PU, polyurethane; DPP, diketopyrrolopyrrole; ZIF, Zeolitic imidazole frameworks; BPOx, benzo[ a ]phenoxazine; HAS, human serum albumin; DOX, doxorubicin; iNRG, CRNGRGPDC; Ara, Cytarabine; HA, hyaluronic acid; PU, polyurethane; AA, amino acid; Azo, azobenezene; EG6, oligoehtyleneglycol; PBLA, poly(β-benzyl-Laspartate); Ce6, chlorin e6; SPB@PON, semiconducting polymer brush and polyoxometalate cluster; CYA, cyanine; RGD, a tumor-targeting unit; OEG, oligo(ethylene glycol); SN38, 7-ethyl-10-hydroxyl-camptothecin; HMSNs, hollow mesoporoussilica nanoparticals; MSNs, mesoporoussilica nanoparticals; HAOP NP, H2O2-activatable and O2-evolving PDT nanoparticle; PCN-224, porous coordination network-224; MS, mesoporoussilica; BODIPY, boron dipyrromethene; NIR‐II, the second near‐infrared window; BODPA, semi-cyanine-BODIPY hybrid dyes.One of the major concerns for nanomedicine agents in practical application is their nonspecial targeted distribution in the body.17 Although nanoparticles are preferred to accumulate in the tumor area (because of the EPR effect) and the accumulated benefits can be further improved through decorating tumor-specific targeting moieties (eg, peptides, aptamers, and antibodies) on the surface of the nanoparticles, still only a very small amount (about 0.7%) of administered materials can reach the tumor.18 Indeed, most of the nanoparticles are sequestered by the reticuloendothelial system. As a result, the diagnosis signals and therapeutic functions appear in the whole body, especially in the reticuloendothelial-system-rich organs (eg, liver and kidney), leading to a low diagnosis signal-to-noise ratio and risk of systemic toxicity.19,20To overcome these challenge, great research interest has recently been focused on exploring stimulus-responsive/smart nanomedicine agents, whose diagnosis and therapy functions can only be activated at the target site by special exogenous stimuli (eg, light, magnetism, ultrasound) or endogenous stimuli (eg, pH, redox, enzyme).17,21–24 Because of the abnormal growth and metabolism of the tumor cells, the tumor tissues are usually involved in a variety of unique physicochemical microenvironments, including acidic pH, hypoxia, high level of glutathione (GSH) and H2O2, as well as overexpressed enzymes and proteins, etc.25 These unique microenvironments are undesirable because they are usually beneficial for tumor proliferation, invasion, adhesion, and antitherapy,26,27 while, on the other hand, they can be regarded as endogenous stimuli for designing tumor-specific smart nanomedicine agents.17,28 Typically, the theranostic functions of these smart nanomedicine agents are in “closed” state in normal tissues, but become “on” state when taken up by tumor cells, giving high theranostic sensitive and selectivity, as well as low side effects.29 Furthermore, the diagnosis signals activated by the tumor microenvironment (TME) may in turn reflect the change of the physiological parameters of the tumor cells/tissues, providing valuable information for doctors to alter the theranostic strategy in real time.30Up to now, a great number of TME-responsive smart nanomedicine agents have been explored, and many of them showed great potential for application in tumor diagnosis and treatment.1,31 Based on the functions of these smart nanomedicine agents, they can be mainly divided into three types: 1) smart nanoprobes for specific tumor imaging and detection; 2) smart nanocarriers for antitumor drug delivery and controlling release; and 3) smart therapy/theranostic agents that possess functions of treatment or combine both functions of diagnosis and treatment. In this review article, we will introduce and discuss recent developments in the design and fabrication of smart nanomedicine agents for enhancing tumor diagnosis and treatment by exploiting the TME, including acidic pH and overexpressed GSH, H2O2, and H2S (Table 1).
In tumor tissues, because the growth rate of tumor cells is usually much faster than that of normal cells, the existing nutrients and blood oxygen content cannot meet the growth needs.32 As a result, tumor cells produce energy for survival through anaerobic glycolysis, which is different from that of oxidative phosphorylation for normal cells. With such metabolisms, tumor cells would generate a large amount of lactic acid and adenosine triphosphate hydrolysate, as well as some excess carbon dioxide and protons, which results in increased acidity of the tumorsite and lower pH value than that of normal tissue.33 Generally, the pH value in the normal human tissue cells and normal cell lysosomes is about 7.4 and 5.0–6.5, respectively, while that of the tumor tissue and tumor cell lysosomes is about 6.0–7.0 and 4.0–5.0, respectively.34 To explore this special acidic TME for improving tumor diagnosis and treatment, a number of pH-sensitive nanomedicine agents have been developed.35
pH-responsive smart nanoprobes
Many researchers have utilized the difference pH between tumor tissue and normal tissue to design smart nanoprobes, which display significantly different/varying signals in these two tissues, giving a high diagnosis signal-to-noise ratio.36,37 To date, a number of pH-responsive smart nanoprobes have been explored on the basis of various imaging techniques, including fluorescence imaging,38 photoacoustic imaging,39 and magnetic resonance imaging (MRI).40 For instance, Liu et al41 designed a pH-responsive nanoassembly based on DPP-thiophene-4 (diketopyrrolopyrrole) for fluorescent images of numbers of different malignant tumors (Figure 1A). With pH>7.0, the fluorescence molecules of DPP-thiophene-4 (diketopyrrolopyrrole) self-assemble into nanoassemblies with very weak fluorescent emission, while when the pH is lower than 6.8 the assemblies disassemble into individual fluorescence molecules, associating with strong fluorescent emission, as shown in Figure 1B. Besides, with every 0.2 pH unit change, the signal of fluorescent emission increased by about 10-fold, which makes this pH-responsive nanoassembly a promising probe for precisely imaging different malignant tumors in vivo (Figure 1C).
Because traditional molecule antitumor drugs have significant side effects for normal organs, enormous interest has been focused on the development of a smart nanocarrier that can deliver and control the release of molecular drug.44 The low pH value of the TME makes it possible to design a pH-responsive smart nanocarrier for tumor-specific chemotherapy. Theoretically, a pH-sensitive nanocarrier would deliver and control the release of the antitumor drug upon encountering the acid microenvironment of tumor, while exhibiting very low or zero drug release in the normal tissue, thus reducing the damage to normal tissue during treatment.45 Ye et al46 designed a pH-sensitive lipid-polypeptide hybrid nanoparticle (iNGR-lPNs) loaded with the antitumor drug doxorubicin (DOX) to address cellular uptake and intracellular drug release for tumor treatment. Likely a pH-sensitive switch, this smart nanoparticle undergoes a first phase transition at pH 7.0–6.5 with the surface potential transformed from negative charge to neutral charge for increasing cellular uptake, and a second phase transition at pH 6.5–4.5 with disassembly of the skeleton to induce endolysosome escape and release the DOX into the cytoplasm. In vitro and in vivo studies demonstrated that this two-step pH-responsive delivery can promote cell uptake and control the release of drug in the acidic environment, consequently leading to more potent antitumor efficacy and less systemic toxicity.For the design of smart nanocarriers, metal–organic frameworks have attracted great attention because of their designable structures and unique porous frameworks for high drug loading. Zhou et al reported one-pot synthesis of a metal–organic framework (ZIF-8) with high encapsulation of DOX (Figure 2A).47 Because the ZIF-8 is stable in the neutral condition, but decomposes in the acid environment, the release of DOX molecules that loaded in the ZIF-8 matrix can be controlled by pH (Figure 2B and C). Zhang et al48 developed a versatile prodrug strategy to further increase the amount of drug loading within the pH-responsive metal–organic framework carrier (ZIF-8) (Figure 2D). As a proof of concept, a drug molecule (cytarabine, Ara) was bonded to a fluorescence molecule (indocyanine green, Ara-IR820) to form a prodrug Ara-IR820 (Figure 2E), which was then embedded into the ZIF-8 matrix (Ara-IR820@ZIF-8) with high loading owing to the strong interaction between sulfonic groups (from IR820) and ZIF-8. At the same time, a tumor targeting molecular HA was bound to the ZIF-8 to improve the tumor targeting ability. Upon entering the tumor tissues/cells, the low pH triggered the HA/Ara-IR820@ZIF-8 to disassemble and release Ara-IR820, which subsequently hydrolyzed (the amide bond) to form the individual molecule of IR820 for fluorescence imaging and Ara for chemotherapy. In vitro and in vivo experiments demonstrated that this pH-sensitive HA/Ara-IR820@ZIF-8 with good tumor targeting capability exhibited excellent pH-triggered fluorescence imaging-guided chemotherapy and photodynamic dual treatment against cancers (Figure 2F).
By utilizing the low pH in the TME, it is possible to design smart nanodiagnostic agents with diagnosis and treat functions simultaneously activated by the change of pH values.53 For example, Ling et al54 developed a pH-responsive magnetic nanotherapeutic agent (termed pH-sensitive magnetic nanogrenades, PMNs) for MRI imaging and fluorescence imaging guiding photodynamic therapy of resistant heterogenous tumors (Figure 3A). This nanotherapeutic agent was built by self-assembly of Ce6-grafted-poly(ethylene glycol)-poly(β-benzyl-L-aspartate) (PEG-PBLA-Ce6) and ultra-small iron oxide nanoparticles. In a neutral environment (pH 7.4), the surface charge of the PMN is negative, and the Ce6 encapsulated in the PMN loses its fluorescence because of the fluorescence resonance energy transfer. Once it reaches a slightly acidic environment, the whole body of the PMN becomes positively charged and expands to promote cell uptake. When the pH is below 6.5, excessive H+ in the solution causes the monomers in the PMN to repel each other, leading to cracking of the PMN and the release of iron oxide nanoparticles for T1-weighted contrast and Ce6 for fluorescent imaging and generation of 1O2. In vivo experiments with colon cancer tumorshave shown that PMNs exhibited excellent activated dual-mode imaging and photodynamic therapy effects (Figure 3B), demonstrating that such pH-responsive PMNs have great potential for early tumor detection and specific treatment.
Due to the different potential between the internal and external environments of tumor cells, tumor cells can overproduce some reducing substances.57 One of the typical overexpressed reducing substances is GSH.58 Generally, the concentration of GSH in tumor cytoplasm can reach 2–10 mmol/L,59 which is 100–1000 times higher than that of the extracellular fluid and blood. Therefore, GSHhas been identified as an ideal stimulating element for designing tumor-specific smart nanomedicine agents.
GSH-responsive smart nanoprobes
For the design of a GSH-responsive organic molecule probe or nanoprobe, several reducible bonds, including the disulfide bond,60 diselenium bond,61 and nitroazo-aryl-ether,62 have attracted great attention. In the presence of GSH, these reducible bonds can be cleaved, thus leading to the activation of the probe. For example, Yuan et al63 reported a GSH turn-on NIR fluorescent probe (CyA-cRGD), composed of a NIR fluorescence unit (CyA) binding with a fluorescence quenching unit (nitroazo aryl ether group) and a tumor-targeting unit (cRGD) (Figure 4A). With the presence of GSH, the nitroazo aryl ether group connecting the fluorescence unit and the fluorescence quenching unit will be cleaved, leading to the turn-on of the fluorescence (Figure 4B). Competition experiments revealed that this GSH-responsive CyA-cRGD probe has high selectivity along different amino acids and metal ions (Figure 4C and D). Moreover, with excellent tumor targeting capability, this probe displays a high fluorescence signal-to-noise ratio for distinguishing the tumor tissue and normal tissue, making it highly promising for application in early tumor diagnosis.
For the design of GSH-responsive drug nanocarriers, two common strategies have been widely used. The first strategy is directly connecting the drug molecule to another molecule or polymer through clearable bonds (such as a disulfide bond), and then self-assembly into a nanoparticle or lipidosome.65 The second strategy is loading the drug molecule into GSH-responsive or non-GSH-responsive porous matrixes such as mesoporoussilicon and metal–organic frameworks.66 For the porous matrixes without GSH-responsive ability, their aperture can be sealed after the adsorption of the drug using small molecules that contain clearable bonds or using nanoparticles that can be degraded by GSH.67 For example, Wang et al68 designed a GSH and ROS heterogeneity-responsive prodrug nanocapsule (OEG-2S-SN38) through self-assembly of a polymer, composed of a chemotherapy drug SN38 and an oligo(ethylene glycol) (OEG) chain linked by a thioether chain with ester groups (Figure 5A). Upon encountering GSH/ROS, the nanoparticle would be disassembled owing to the thiolysis triggered by GSH (Figure 5B) and enhanced hydrolysis of the linker triggered by the ROX oxidation, leading to the release of the parent drug SN38 for anticancer therapy (Figure 5C).
Besides smart probes and drug carriers, the design of GSH-responsive smart nanotheranostic agents that combine both functions of diagnosis and therapy is also a hot research topic in the field of nanomaterial and nanomedicine.70 To date, many kinds of nanomaterials, including organic polymer,71 metal oxide,72 gold nanoparticles,73 and polyoxometalate clusters,74 have been utilized to design GSH-responsive nanotheranostic agents. For example, Gong et al75 successfully synthesized a bimetallic oxide MnMoOX nanorod as a GSH-responsive smart nanotheranostic agent (Figure 6A). The original PEG-modified MnMoOX nanorods exhibited almost no NIR absorption. However, once interacted with GSH, the MoVI ions in MnMoOx were reduced to MoV ions, making the nanorods possess strong NIR absorption that can be utilized for photoacoustic imaging and photothermal therapy (Figure 6B and C). Besides, the change of the charge of Mn ions leads to increased r1 relaxivity with improved MRI (Figure 6D). In vivo experiments demonstrated that this MnMoOX nanorod possessed good biodegradability and excellent GSH-triggered photoacoustic imaging and MRI for guiding photothermal therapy (Figure 6E–G).
Hydrogen peroxide (H2O2) is another overproducing metabolite in most common tumors.77 Accumulating evidence suggests that H2O2 in normal tissues is usually at a low level, while that in the tumor tissues is much higher at 100 μM–1 mM,78 which may attributed to the overproduction of oxide dismutase (SOD) for catalyzing the conversion of superoxide anion radicals to H2O2 and O2.79–81 An increased level of H2O2 may play a significant role, directly or indirectly, in the development of the cancer cells, but can also induce apoptosis of cancer cells when increased to a much higher level. For chemists, the characteristic high level of H2O2 for the tumor environment can be explored to design H2O2-responsive drug carriers,77 endogenous O2 producers,82 and chemodynamic therapy agents83 for tumor-specific diagnosis and treatment.84
H2O2-responsive smart nanocarriers
For the design of H2O2-responsive drug nanocarriers, oxidation-responsive polymershave attracted considerable attention. For example, poly(propylene sulfide) is a hydrophobic polymer, but it can transform to a hydrophilic polymer (poly(propylene sulfphone)) upon oxidative conversion.85 The hydrophobic poly(propylene sulfide) can self-assemble into nanoparticles with a hydrophilic block such as poly(ethylene glycol) and a hydrophobic drug molecule such as DOX. Upon encountering H2O2, the hydrophobic poly(propylene sulfide) would be oxidized into hydrophilic poly(propylene sulfphone), resulting in disassembly of the nanoparticles and release of the DOX molecule for chemotherapy.Ma et al86 reported a redox dual-responsive assembly containing diselenide block copolymers as a potential drug carrier (Figure 7A). The diselenide bonds (Se–Se) in the block copolymers can be cleaved and oxidized into seleninic acid in the oxidation environment and reduced into selenol in the presence of reductants. Therefore, the micelles formed by such diselenide block copolymers will disassemble upon encountering oxidants such as H2O2 or reductants such as GSH, and simultaneously release the cargo loading in the micelles (Figure 7B–D).
In situ O2 producer for improving photodynamic therapy
Hypoxia is a state referring to the low level of oxygen, which is a typical characteristic of most solid tumors.88 The origins of tumor hypoxia can be mainly traced to abnormal vascularization raised by the fast growth of the tumor. Compared with the normal cells that have an oxygen concentration of about 2–9%, the oxygen concentration of the TME is usually down to about 0.02–2%.89 The hypoxia not only provides an environment that is beneficial for the spread of the cancer stem cells, but also increases the multidrug-resistant proteins and decreases the therapeutic efficacy of many anticancer drugs and oxygen-dependent invasive therapy, such as photodynamic therapy.81 To date, a number of strategies have been used to increase the concentration of oxygen in the tumor tissue, including directly delivering oxygen by nanocarriers, and in situ catalysis of the decomposition of H2O2 to generate O2.With the presence of a catalyst, such as natural catalase,83 MnO2,90 or Pt and Au nanoparticles,91 the overproduced H2O2 in the tumorsite can be utilized as an in situ O2 generator for improving the efficacy of photodynamic therapy. For example, Chen et al92 reported a PDT nanoparticle that activated H2O2 and continuously generated O2 for efficient hypoxic tumor therapy (Figure 8A). This nanoparticle was self-assembled by PLGA, combined with methylene blue as photosensitizer, catalase as H2O2 catalyst, black hole quencher-3 as quencher of the photosensitizer, and RGDfK as tumor targeting ligand. Upon being taken up by the tumor cells, the H2O2 would penetrate into the core of the nanoparticle and self-decompose to generate O2 under catalysis by catalase. The generated O2 would trigger the crack of the nanoparticles, followed by the release of the photosensitizer (Figure 8B). Upon irradiation by a 635-nm laser, the released photosensitizer can generate sufficient 1O2 to effectively destroy the cancer cells owing to the self-sufficiency of O2 in the hypoxia tumorsite (Figure 8C and D). Therefore, this design not only uses H2O2 as a trigger for activating the generation of 1O2 with high region selectivity, but also as an O2 generator for improving the efficacy of photodynamic therapy in the hypoxia tumor.
Besides as an O2 producer, utilizing the overproduction of H2O2 in the tumorsite to trigger chemodynamic therapy has also recently received tremendous interest. Chemodynamic therapy is an emerging therapeutic strategy using the hydrogen radical (OH), generated through the Fenton reaction or a Fenton-like reaction in the presence of Fenton agents and H2O2, as a toxic reactive oxygen species to kill tumor cells.94 This therapeutic strategy was recently proposed by Bu et al.45 Because it is activated via two endogenous stimulating elements, including sufficient H2O2 and mildly acidic conditions (to dissolve ferrous ions from the nanomaterials), chemodynamic therapy has advantages of high logicality and selectivity, as compared with many other therapy methods such as chemotherapy, photodynamic therapy, and radiotherapy. Up to now, a number of inorganic and inorganic–organic hybrid nanomaterials, including Fe3O4,95 FeS2,96 and Cu/Fe complex nanoparticles,97 have been explored as H2O2 catalysts for chemodynamic therapy on the basic principles of the Fenton reaction.Utilizing the high content of H2O2 in the tumor, Tang et al96 designed an antiferromagnetic pyrite nanocube decorated with polyethylene glycol (FeS2-PEG) for self-enhanced MRI and chemodynamic therapy. In the tumorsite, the FeS2-PEG catalyzed the endogenous H2O2 to generate OH effectively through the Fenton reaction (Figure 9A). Besides, the localized heat from the photothermal properties of the pyrite can accelerate the Fenton reaction, making it more effective for chemodynamic therapy (Figure 9B and C). Furthermore, upon surface oxidation by H2O2, the valence state of the ferrous ion was changed, leading to enhancement of the T1 and T2 MRI signals for guiding chemodynamic therapy (Figure 9D).
Hydrogen sulfide (H2S) is a key signal molecule in the human body and plays an important role in health and disease.100,101 Accordingly, in mammalian systems, endogenous H2S is primarily synthesized from cysteine or cysteine derivatives in the presence of enzyme catalyst, such as cystathionine-β-synthase (CBS), cystathionine-γ-lyase (CSE), and 3-mercaptopyruvate thiotransferase (3-MST).102–104 It has been suggested that many diseases such as Down syndrome, Alzheimer’s disease, cirrhosis, diabetes, and cancer are associated with an abnormal concentration of endogenous H2S.104 Therefore, research on endogenous H2S, including the detection of H2S and utilizing H2S to develop specific nanotheranostic agents, has attracted considerable interest in the areas of medical, nanomaterial, and chemical science.
H2S-responsive smart nanoprobes
Given the key role of the H2S molecule in vivo, detection of this signaling molecule accurately is of great important.ance To date, many interests have been focused on the design of intelligent nanoprobes owing to their high sensitivity, high signal-to-noise ratio, real-time imaging, and simple operation features.105,106 Particularly, smart fluorescent probes with high sensitivity have attracted great attention. Generally, intelligent fluorescent probes were designed based on fluorescent molecules that can react with the H2S molecule, leading to the change of the fluorescent emission.107,108 Nevertheless, because of the low concentration of endogenous H2S and large amounts of interference molecules such as GSH and cysteine (Cys) in the complex biological systems, the design of H2S-responsive fluorescent probes with high sensitivity and chemical selectivity still remains a formidable challenge.Zhao et al109 developed a boron dipyrromethene (BODIPY)-based fluorescence micelle as an H2S-responsive probe for detecting H2S. This probe micelle contained a semi-cyanine-BODIPY dye (BODInD-Cl) as the H2S interaction molecule, and BODIPY as a complementary energy donor of BODInD-Cl. The main feature of this nanoprobe is that the absorption of the energy acceptor BODInD-Cl will shift from 540 nm to 738 nm after the H2S trigger to reduce the efficiency of Förster resonance energy transfer, leading to the simultaneous “turn-on” of the fluorescent signal of energy donorBODIPY1 and “turn-off” of the fluorescent signal of energy acceptor BODInD-Cl. As a result, this probe can be used to quickly detect and track H2S using a fluorescence ratio. Besides, competition experiments showed that the red shift of the absorption peak of BODInD-Cl can only be mainly triggered by H2S, while the influence of other small molecules is very weak, demonstrating the high detecting selectivity of this probe. Zhang et al93 also designed a sulfoxide-functionalized BODIPY-based fluorescent probe for selectively detecting endogenous H2S by confining sulfoxide-functionalized BODIPY within the interior of porous silica matrix. The other influencing molecules with size larger than the aperture of the porous silica are unable to react with sulfoxide-functionalized BODIPY. Therefore, this common fluorescence molecule can only react with the small H2S molecule with a substantial red shift in absorption and emission, giving high chemical selectivity and sensitivity.Given that NIR fluorescent probes have high resolution with deep-tissue penetration, Xu et al110 developed an H2S-activated NIR-II nanoprobe (NIR-II@Si) for visualizing colorectal cancers (Figure 10A). NIR-II@Si is composed of a covalently cross-linked silica shell with two organic chromophores in its cavity, in which a boron-dipyrromethene (ZX-NIR) dye serves as an H2S-responsive chromophore to generate NIR-II emission, and an H2S-inert aza-BODIPY (aza-BOD) dye with strong emission at 700 nm emission serves as internal reference. Upon reaction with H2S, ZX-NIR was transformed to NIRII-HS accompanied by maximum emission shifting from 600 to 900 nm, while aza-BOD keeps maximum emission at 700 nm with similar intensity, forming ratiometric fluorescence with high signal-to-background ratios (Figure 10B and C). This H2S-responsive ratiometric fluorescence nanoprobe with excellent targeting capability exhibits excellent performance for selectively identifying the H2S-rich colon cancer cells. Moreover, the merits of NIR-II imaging at depth and spatial resolution enable this H2S-responsive probe to accurately identify colorectal tumors in animal models (Figure 10D).
H2S is also an overproduced molecule in some cancer cells, such as colon cancer.112 Currently, the maindiagnosis and therapy methods for colon cancer are colonoscopy diagnosis and surgical treatment, but there still remain some serious problems, such as missed diagnosis, misdiagnosis, recurrence, and metastasis.113 Using endogenous H2S to activate the diagnosis and therapy functions of smart theranostic agents has been considered an effectively strategy to reduce the rate of misdiagnosis and improve the treatment efficacy of colon cancer. To date, several nanomaterials, including a Cu-based metal–organic framework114 and CuO,115 have been explored for designing H2S-responsive theranostic agents.Ma et al prepared a colon cancer antitumor agent based on H2S-responsive photodynamic diagnosis (Figure 11A). They synthesized a copper-zinc mixed metal organic skeleton nanoparticle (NP-1), constructed of zinc metalated porphyrin (ZnTcpp) as ligand and Cu2+ ions as building blocks.116 In NP-1, ZnTcpp served as photosensitizer, while Cu2+ served as fluorescence quencher. Before activation, the fluorescence of ZnTcpp was quenched by Cu2+ ions, resulting in a low yield of singlet oxygen. Upon encountering H2S, the Cu2+ ions would react with H2S, followed by recovering the original fluorescence and singlet oxygen generation functions of NP-1 (Figure 11B and C). Cell and mousetumor model treatment experiments demonstrated that NP-1has excellent photodynamic efficiency upon triggering by endogenous H2S (Figure 11D).
In summary, this article presents recent advances in the design and fabrication of pH, GSH, H2O2, and H2S-responsive nanomedicine agents, and their application in tumor diagnosis and treatment. Because the diagnosis and treatment functions of smart nanomedicine were designed to be silenced before activation, while turned “on” upon triggering by the TME, they exhibited higher theranostic sensitive and selectivity with lower harmful side effects, as compared with traditional nanomedicine agents. These merits make them highly promising for improving tumor diagnosis and therapy. In fact, in addition to pH, H2O2, GSH, and H2S, there are many other TME-stimulating elements (eg, hypoxia, immune, enzyme, and protein) and exogenous elements (eg, light, magnetism, and ultrasound) that can be utilized to design smart nanomedicine agents.45 Furthermore, these elements can be merged together to explore multiresponsive nanomedicine agents.23 In this case, it is possible to activate the synergistic theranostic functions (eg, control the release of different drugs) at expected time points, further improving the tumor theranostic efficacies and mitigating the side effects. Besides, the permeable barriers for nanodrugs to effectively enter the tumor cells are also possible to overcome through multiresponsive steps. Despite these promising results, there are still many challenges to overcome for TME-responsive nanomedicine agents toward the clinical translation. Firstly, the safety issues of nanomedicine agents need to be thoroughly investigated. Secondly, the triggering efficiency of nanomedicine agents needs to be improved since the concentration of overproduced substances and the accumulation of nanoparticles in the tumorsite are very limited. Finally, but not the least, the activated selectivity of nanomedicine agents needs to be further improved because most of the tumor-overexpressed substances also exist in the normal organs/tissues. Nevertheless, it is believed that with the continuous development of science and technology, these problems will be overcome, and these TME-responsive nanomedicine agents will facilitate the improvement of tumor diagnosis and therapy.
Authors: Marina Martín-Contreras; Saúl A Navarro-Marchal; José Manuel Peula-García; Ana Belén Jódar-Reyes Journal: Pharmaceutics Date: 2022-02-10 Impact factor: 6.321
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