Erythrocyte-Membrane-Enveloped Perfluorocarbon as Nanoscale Artificial Red Blood Cells to Relieve Tumor Hypoxia and Enhance Cancer Radiotherapy

Perfluorocarbon nanodroplets stabilized with cisplatin-prodrug-constructed lipids enable efficient tumor oxygenation and chemo-radiotherapy of cancer

Concurrent chemo-radiotherapy has been widely applied for the treatment of a wide range of cancers, but its therapeutic efficacy against most solid tumors is severely impaired by their intrinsic hypoxic microenvironments. Utilizing the high oxygen loading capacity of perfluoro-15-crown-5-ether (PFCE), herein, we prepare PFCE nanodroplets with cisplatin prodrug (cisPt(iv)) conjugated phospholipids and other commercial lipids as the stabilizer to enable tumor targeted oxygen shuttling. The obtained PFCE@cisPt(iv)-Lip shows high physiological stability and efficient oxygen loading capacity. As vividly visualized under an in vivo photoacoustic imaging system, tumors on the mice with intravenous injection of such PFCE@cisPt(iv)-Lip show effective tumor oxygenation. Together with X-ray exposure, such PFCE@cisPt(iv)-Lip upon intravenous injection could induce severe DNA damage of cells, thereby remarkably suppressing the tumor growth and significantly prolonging their survival time without causing obvious toxic side effects. This work highlights PFCE@cisPt(iv)-Lip as an adjuvant nanomedicine for enhanced chemo-radiotherapy of tumors by attenuating hostile tumor hypoxia, indicating its promising potential for future clinical translation ascribed to its straightforward synthesis and notable tumor growth inhibition at a safe dose.

Irradiation pretreatment enhances the therapeutic efficacy of platelet-membrane-camouflaged antitumor nanoparticles

BACKGROUND

Cell membrane-based nanocarriers are promising candidates for delivering antitumor agents. The employment of a simple and feasible method to improve the tumor-targeting abilities of these systems is appealing for further application. Herein, we prepared a platelet membrane (PM)-camouflaged antitumor nanoparticle. The effects of irradiation pretreatment on tumor targeting of the nanomaterial and on its antitumor action were evaluated.

RESULTS

The biomimetic nanomaterial constructed by indocyanine green, poly(d,l-lactide-co-glycolide), and PM is termed PINPs@PM. A 4-Gy X-ray irradiation increased the proportions of G2/M phase and Caveolin-1 content in 4T1 breast cancer cells, contributing to an endocytic enhancement of PINPs@PM. PINPs@PM produced hyperthermia and reactive oxygen species upon excitation by near-infrared irradiation, which were detrimental to the cytoplasmic lysosome and resulted in cell death. Irradiation pretreatment thus strengthened the antitumor activity of PINPs@PM in vitro. Mice experiments revealed that irradiation enhanced the tumor targeting capability of PINPs@PM in vivo. When the same dose of PINPs@PM was intravenously administered, irradiated mice had a better outcome than did mice without X-ray pretreatment.

CONCLUSION

The study demonstrates an effective strategy combining irradiation pretreatment and PM camouflage to deliver antitumor nanoparticles, which may be instrumental for targeted tumor therapy.

Relieving immunosuppression during long-term anti-angiogenesis therapy using photodynamic therapy and oxygen delivery

Angiogenesis is an irreplaceable therapeutic cancer target, where anti-angiogenesis are drugs that are limited by their hydrophobicity and low therapeutic effects. What is more, the long-term shutdown of tumor blood vessel density also aggravates hypoxia and causes immunosuppression in the tumor microenvironment (TME). In order to solve these shortcomings, we developed a single therapeutic agent based on a bovine serum albumin nanocarrier that can co-deliver the anti-angiogenic drug Sorafenib ("S") and the photosensitizer Ce6 ("C") along with a molecular oxygen supply based on MnO2 ("M") as a convenient one-pot formulated nanoscale agent (SCM@BSA). Compared with anti-angiogenesis monotherapy, SCM@BSA can not only improve upon the solubility and therapeutic effects of anti-angiogenesis agents, but it also reshapes the immunosuppressive TME during anti-angiogenic therapy. Together, these results point out that SCM@BSA synthesized via a very simple method can solve the shortcomings usually experienced during long-term anti-angiogenic therapy.

Advances in nanomaterials for treatment of hypoxic tumor

The hypoxic tumor microenvironment is characterized by disordered vasculature and rapid proliferation of tumors, resulting from tumor invasion, progression and metastasis. The hypoxic conditions restrict efficiency of tumor therapies, such as chemotherapy, radiotherapy, phototherapy and immunotherapy, leading to serious results of tumor recurrence and high mortality. Recently, research has concentrated on developing functional nanomaterials to treat hypoxic tumors. In this review, we categorize such nanomaterials into (i) nanomaterials that elevate oxygen levels in tumors for enhanced oxygen-dependent tumor therapy and (ii) nanomaterials with diminished oxygen dependence for hypoxic tumor therapy. To elevate oxygen levels in tumors, oxygen-carrying nanomaterials, oxygen-generating nanomaterials and oxygen-economizing nanomaterials can be used. To diminish oxygen dependence of nanomaterials for hypoxic tumor therapy, therapeutic gas-generating nanomaterials and radical-generating nanomaterials can be used. The biocompatibility and therapeutic efficacy of these nanomaterials are discussed.

On-demand drug release nanoplatform based on fluorinated aza-BODIPY for imaging-guided chemo-phototherapy

Intelligent drug delivery systems (DDS), integrating with multi-modal imaging guidance and controlled drug release, have practical significance in enhancing the therapeutic efficiency of tumors. Herein, fluorinated aza-boron-dipyrromethene (NBF) with high near-infrared absorption is synthesized by introducing nonadecafluorodecanoic acid into aza-BODIPY via the amide bond. Through the co-precipitation methods, nanoparticles (NPs) based on NBF are fabricated and the obtained NBF NPs can not only load with DOX with a high loading efficiency (25%, DNBF NPs), but also absorb PFC droplets (1H-perfluoropentane) with bp of 42 °C because of the fluorinated chains inside NBF NPs (PDNBF NPs). Under 808-nm laser irradiation, the hyperthermia effect of NBF could induce the liquid-gas phase transition of PFC droplets, triggering the burst release of DOX and enhancing echo signals for ultrasound imaging as well. With efficient enrichment of PDNBF NPs at tumor site as revealed by in vivo ultrasound imaging and photoacoustic imaging, significant improvement in inhibiting tumor growth is achieved with PDNBF NPs under laser irradiation without noticeable side effects. The work presents a multifunctional organic DDS with great biocompatibility, high drug loading efficiency and light-stimuli-responsive drug release, which provides a new strategy for the manufacture of intelligent composite theranostic nanoplatform.

Nanomaterials for Combinational Radio–Immuno Oncotherapy

Radiotherapy, a clinically used local treatment modality of cancers, is regarded as a promising candidate to promote current immunotherapy through initiating an in situ vaccination effect and reprogramming the immunosuppressive microenvironment. The combination of radiotherapy and immunotherapy, referred to as combinational radio–immuno oncotherapy (CRIOT), elicits a synergistic antitumor effect based on the immunomodulatory properties of radiation. Unfortunately, current CRIOT accompanies low response rate and severe toxicity in clinical trials, thus limiting its application. To this end, various nanomaterials are being developed to sensitize radiotherapy or deliver immune agents, or both, to improve the unsatisfactory outcomes of CRIOT. Herein, enhanced antitumor efficacy of CRIOT with nanomaterials through the possible mechanisms of rejuvenation and activation of T cells, increased presentation of tumor‐related antigens, and inhibition of suppressive macrophages is presented, and the prospect of CRIOT in clinical practice is proposed.

Protein-Based Artificial Nanosystems in Cancer Therapy

Proteins, like actors, play different roles in specific applications. In the past decade, significant achievements have been made in protein-engineered biomedicine for cancer therapy. Certain proteins such as human serum albumin, working as carriers for drug/photosensitizer delivery, have entered clinical use due to their long half-life, biocompatibility, biodegradability, and inherent nonimmunogenicity. Proteins with catalytic abilities are promising as adjuvant agents for other therapeutic modalities or as anticancer drugs themselves. These catalytic proteins are usually defined as enzymes with high biological activity and substrate specificity. However, clinical applications of these kinds of proteins remain rare due to protease-induced denaturation and weak cellular permeability. Based on the characteristics of different proteins, tailor-made protein-based nanosystems could make up for their individual deficiencies. Therefore, elaborately designed protein-based nanosystems, where proteins serve as drug carriers, adjuvant agents, or therapeutic drugs to make full use of their intrinsic advantages in cancer therapy, are reviewed. Up-to-date progress on research in the field of protein-based nanomedicine is provided.

Relief of Biofilm Hypoxia Using an Oxygen Nanocarrier: A New Paradigm for Enhanced Antibiotic Therapy

Biofilms are chief culprits of most intractable infections and pose great threats to human health. Conventional antibiotic therapies are hypodynamic to biofilms due to their strong drug resistance, closely related with biofilm hypoxia. A new strategy for enhanced antibiotic therapy by relieving biofilm hypoxia is reported here. A two-step sequential delivery strategy is fabricated using perfluorohexane (PFH)-loaded liposomes (lip) as oxygen (O2) carriers (denoted as lip@PFH@O2) and commercial antibiotics. The results indicate that the two-step sequential treatment exhibits much lower minimum bactericidal concentrations than the antibiotic treatment alone. In this design, the lip@PFH@O2 holds positively charged surface for better biofilm penetration. After penetrating into biofilm, oxygen can be released from lip@PFH@O2 by inches, which greatly relieves biofilm hypoxia. With the relief of hypoxia, the quorum sensing and the drug efflux pumps of bacteria are suppressed by restraining related gene expression, leading to the reduced antibiotic resistance. Furthermore, the in vivo experimental results also demonstrate that lip@PFH@O2 can effectively relieve biofilm hypoxia and enhance therapeutic efficacy of antibiotics. As a proof-of-concept, this research provides an innovative strategy for enhanced antibiotic therapy by relieving hypoxia, which may hold a bright future in combating biofilm-associated infections.

Glutathione-Depleting Nanoenzyme and Glucose Oxidase Combination for Hypoxia Modulation and Radiotherapy Enhancement

Nanoenzymes perceive the properties of enzyme-like catalytic activity, thereby offering significant cancer therapy potential. In this study, Fe₃ O₄ @MnO₂ , a magnetic field (MF) targeting nanoenzyme with a core-shell structure, is synthesized and applied to radiation enhancement with using glucose oxidase (GOX) for combination therapy. The glucose is oxidized by the GOX to produce excess H₂ O₂ in an acidic extracellular microenvironment, following which the MnO₂ shell reacts with H₂ O₂ to generate O₂ and overcome hypoxia. Concurrently, intracellular glutathione (GSH)-which limits the effects of radiotherapy (RT)-can be oxidized by the MnO₂ shell while the latter is reduced to Mn²⁺ for T₁ -weighed MRI. The core Fe₃ O₄ , with its good magnetic targeting ability, can be utilized for T₂ -weighed MRI. In summary, the work demonstrates that Fe₃ O₄ @MnO₂ , as a dual T₁ - and T₂ -weighed MRI contrast agent with strong biocompatibility, exhibits striking potential for radiation enhancement under magnetic targeting.

Current developments in nanotechnology for improved cancer treatment, focusing on tumor hypoxia

Hypoxia is a common feature of the tumor microenvironment, which is characterized by tissue oxygen deficiency due to an aggressive proliferation of cancer cells. Hypoxia activates hypoxia-inducible factor-dependent signaling, which in turn regulates metabolic reprogramming, immune suppression, resistance to apoptosis, angiogenesis, metastasis, and invasion to secondary sites. In this review, we provide an overview of the use of nanotechnology to harmonize intra-tumoral oxygen or suppress hypoxia-related signaling for an improved efficacy of cancer treatment. The biological background was followed by conducting a literature review on the (1) nanoparticles responsible for enhancing oxygen levels within the tumor, (2) nanoparticles sensitizing hypoxia, (3) nanoparticles suppressing hypoxia-inducing factor, (4) nanoparticles that relieve tumor hypoxia for enhancement of chemotherapy, photodynamic therapy, and immunotherapy, either individually or in combination. Lastly, the heterogeneity of cancer and limitations of nanotechnology are discussed to facilitate translational therapeutic treatment.

Ultrasound-Activated Sensitizers and Applications

Modalities for photo-triggered anticancer therapy are usually limited by their low penetrative depth. Sonotheranostics especially sonodynamic therapy (SDT), which is different from photodynamic therapy (PDT) by the use of highly penetrating acoustic waves to activate a class of sound-responsive materials called sonosensitizers, has gained significant interest in recent years. The effect of SDT is closely related to the structural and physicochemical properties of the sonosensitizers, which has led to the development of new sound-activated materials as sonosensitizers for various biomedical applications. This Review provides a summary and discussion of the types of novel sonosensitizers developed in the last few years and outlines their specific designs and the potential challenges. The applications of sonosensitizers with various functions such as for imaging and drug delivery as well as in combination with other treatment modalities would provide new strategies for disease therapy.

Homotypic Targeting Delivery of siRNA with Artificial Cancer Cells

The camouflage with cell membrane bestows nanoparticles with cell-like functions, such as specific recognition, long blood circulation, and immune escaping. For cancer therapy, the nanoparticles camouflaged with cancer cell membrane (CCM) from homologous cells show homotypic targeting delivery of small molecule compounds, photosensitizers, or enzymes to the tumors. However, effective gene therapy encounters difficulties by this approach due to the properties of nucleic acids. Herein, a cancer cell-like gene delivery system is developed using an excellent polymer poly(β-amino ester) (PBAE) to condense small interfering RNA (siRNA) (targeting to Plk1 gene) into nanoparticles (PBAE/siPlk1) as the core, which is further camouflaged with CCM. These novel biomimetic nanoparticles CCM/PBAE/siPlk1 (CCMPP) demonstrate highly specific targeting to homotypic cancer cells, effective downregulation of PLK1 level, and inducing apoptosis of cancer cells. Based on the homotypic binding adhesion molecules on the CCM, the cellular internalization and homotypic-targeting accumulation to the tumors are clearly improved. CCMPP induces highly efficient apoptosis of cancer cells both in vitro and in vivo and results in significant tumor inhibition. The artificial cancer cells with homotypic properties can serve as a biomimetic delivery system for cancer-targeted gene therapy.

Overcoming the biological barriers in the tumor microenvironment for improving drug delivery and efficacy

The delivery of drugs to tumors by nanoparticles is a rapidly growing field. However, the complex tumor microenvironment (TME) barriers greatly hinder drug delivery to tumors. In this study, we first summarized the barriers in TME, including anomalous vasculature, rigid extracellular matrix, hypoxia, acidic pH, irregular enzyme level, altered metabolism pathway and immunosuppressive conditions. To overcome these barriers, many strategies have been developed, such as modulating TME, active targeting by ligand modification and biomimetic strategies, and TME-responsive drug delivery strategies to improve nanoparticle penetration, cellular uptake and drug release. Although extensive progress has been achieved, there are still many challenges, which are discussed in the last section. Overall, we carefully discuss the landscape of TME, development for improving drug delivery, and challenges that need to be further addressed.

RBC Membrane Camouflaged Semiconducting Polymer Nanoparticles for Near-Infrared Photoacoustic Imaging and Photothermal Therapy

Semiconducting conjugated polymer nanoparticles (SPNs) represent an emerging class of phototheranostic materials with great promise for cancer treatment. In this report, low-bandgap electron donor-acceptor (D-A)-conjugated SPNs with surface cloaked by red blood cell membrane (RBCM) are developed for highly effective photoacoustic imaging and photothermal therapy. The resulting RBCM-coated SPN (SPN@RBCM) displays remarkable near-infrared light absorption and good photostability, as well as high photothermal conversion efficiency for photoacoustic imaging and photothermal therapy. Particularly, due to the small size (< 5 nm), SPN@RBCM has the advantages of deep tumor penetration and rapid clearance from the body with no appreciable toxicity. The RBCM endows the SPNs with prolonged systematic circulation time, less reticuloendothelial system uptake and reduced immune-recognition, hence improving tumor accumulation after intravenous injection, which provides strong photoacoustic signals and exerts excellent photothermal therapeutic effects. Thus, this work provides a valuable paradigm for safe and highly efficient tumor photoacoustic imaging and photothermal therapy for further clinical translation.

Folic Acid-Modified Nanoerythrocyte for Codelivery of Paclitaxel and Tariquidar to Overcome Breast Cancer Multidrug Resistance

The efflux of anticancer agents mediated by P-glycoprotein (P-gp) is one of the main causes of multidrug resistance (MDR) and eventually leads to chemotherapy failure. To overcome this problem, the delivery of anticancer agents in combination with a P-gp inhibitor using nanocarrier systems is considered an effective strategy. On the basis of the physiological compatibility and excellent drug loading ability of erythrocytes, we hypothesized that nanoerythrocytes could be used for the codelivery of an anticancer agent and a P-gp inhibitor to overcome MDR in breast cancer. Herein, a folic acid-modified nanoerythrocyte system (PTX/TQR NPs@NanoRBC-PEG/FA) was prepared to simultaneously transport paclitaxel and tariquidar, and the in vitro and in vivo characteristics of this delivery system were evaluated through several experiments. The results indicated that the average diameter and surface potential of this nanocarrier system were 159.8 ± 1.4 nm and -10.98 mV, respectively. Within 120 h, sustained release of paclitaxel was observed in both pH 6.5 media and pH 7.4 media. Tariquidar release from this nanocarrier suppressed the P-gp function of MCF-7/Taxol cells and significantly increased the intracellular paclitaxel level (p < 0.01 versus the PTX group). The results of the MTT assay indicated that the simultaneous transportation of paclitaxel and tariquidar could significantly inhibit the growth of MCF-7 cells or MCF-7/Taxol cells. After 48 h of incubation with PTX/TQR NPs@NanoRBC-PEG/FA, the viability of MCF-7 cells and MCF-7/Taxol cells decreased to 7.37% and 30.2%, respectively, and the IC50 values were 2.49 μM and 6.30 μM. Pharmacokinetic results illustrated that, compared with free paclitaxel, all test paclitaxel nanoformulations prolonged the drug release time and showed similar plasma concentration-time profiles. The peak concentration (Cmax), area under the curve (AUC0-∞), and half-life (t1/2) of PTX/TQR NPs@NanoRBC-PEG/FA were 3.33 mg/L, 6.02 mg/L·h, and 5.84 h, respectively. Moreover, this active targeting nanocarrier dramatically increased the paclitaxel level in tumor tissues. Furthermore, compared with those of the other paclitaxel formulations, the cellular reactive oxygen species (ROS) and malondialdehyde (MDA) levels of the PTX/TQR NPs@NanoRBC-PEG/FA group increased by 1.38-fold (p < 0.01) and 1.36-fold (p < 0.01), respectively, and the activities of superoxide dismutase (SOD) and catalase (CAT) decreased to 67.8% (p < 0.01) and 65.4% (p < 0.001), respectively. More importantly, in vivo antitumor efficacy results proved that the PTX/TQR NPs@NanoRBC-PEG/FA group exerted an outstanding tumor inhibition effect with no marked body weight loss and fewer adverse effects. In conclusion, by utilizing the inherent and advantageous properties of erythrocytes and surface modification strategies, this biomimetic targeted drug delivery system provides a promising platform for the codelivery of an anticancer agent and a P-gp inhibitor to treat MDR in breast cancer.

Self-Delivery Nanomedicine for O2-Economized Photodynamic Tumor Therapy

Tumor hypoxia is the Achilles heel of oxygen-dependent photodynamic therapy (PDT), and tremendous challenges are confronted to reverse the tumor hypoxia. In this work, an oxidative phosphorylation inhibitor of atovaquone (ATO) and a photosensitizer of chlorine e6 (Ce6)-based self-delivery nanomedicine (designated as ACSN) were prepared via π-π stacking and hydrophobic interaction for O₂-economized PDT against hypoxic tumors. Specifically, carrier-free ACSN exhibited an extremely high drug loading rate and avoided the excipient-induced systemic toxicity. Moreover, ACSN not only dramatically improved the solubility and stability of ATO and Ce6 but also enhanced the cellular internalization and intratumoral permeability. Abundant investigations confirmed that ACSN effectively suppressed the oxygen consumption to reverse the tumor hypoxia by inhibiting mitochondrial respiration. Benefiting from the synergistic mechanism, an enhanced PDT effect of ACSN was observed on the inhibition of tumor growth. This self-delivery system for oxygen-economized PDT might be a potential appealing clinical strategy for tumor eradication.

Unimolecular Photodynamic O2-Economizer To Overcome Hypoxia Resistance in Phototherapeutics

Tumor hypoxia has proven to be the major bottleneck of photodynamic therapy (PDT) to clinical transformation. Different from traditional O₂ delivery approaches, here we describe an innovative binary photodynamic O₂-economizer (PDOE) tactic to reverse hypoxia-driven resistance by designing a superoxide radical (O₂^•-) generator targeting mitochondria respiration, termed SORgenTAM. This PDOE system is able to block intracellular O₂ consumption and down-regulate HIF-1α expression, which successfully rescues cancer cells from becoming hypoxic and relieves the intrinsic hypoxia burden of tumors in vivo, thereby sparing sufficient endogenous O₂ for the PDT process. Photosensitization mechanism studies demonstrate that SORgenTAM has an ideal intersystem crossing rate and triplet excited state lifetime for generating O₂^•- through type-I photochemistry, and the generated O₂^•- can further trigger a biocascade to reduce the PDT's demand for O₂ in an O₂-recycble manner. Furthermore, SORgenTAM also serves to activate the AMPK metabolism signaling pathway to inhibit cell repair and promote cell death. Consequently, using this two-step O₂-economical strategy, under relatively low light dose irradiation, excellent therapeutic responses toward hypoxic tumors are achieved. This study offers a conceptual while practical paradigm for overcoming the pitfalls of phototherapeutics.

Preparation of polyethylene glycol-polyacrylic acid block copolymer micelles with pH/hypoxic dual-responsive for tumor chemoradiotherapy

Block copolymers of poly(ethylene glycol)-poly(acrylic acid) linked with metronidazole (MN-PAA-PEG) were prepared via carbodiimide and esterification methods, and self-assembled into core-shell micelles as nano radiosensitizers and carriers of doxorubicin (DOX) delivery. These DOX/MN-PAA-PEG micelles exhibited good pH value and hypoxia dual-responsive properties via analyzing the change of micelle size and drug‒release behavior under hypoxia humor condition. The results of the cell test indicated that DOX was efficiently delivered by DOX/MN-PAA-PEG micelles into the cell nuclei. Compared to 22.4 % of their DOX release under pH 7.4, the rate of DOX release from DOX/MN-PAA-PEG micelles under reducing condition (pH 5.0) was up to 55.9 %. DOX-loaded micelles under 600 MU electron radiation and hypoxia induced the rapidest apoptosis of the tumor-cells, indicating the synergistic effect of their radiotherapy and chemotherapy from the prepared micelles. In vivo investigation and fluorescence imaging revealed that MN-PAA-PEG possessed no toxicity on main organs, and DOX/MN-PAA-PEG micelles were mainly accumulated in the tumor site at 10 h of post-injection, suggesting their good passive tumor-targeted effect. These results suggested that DOX/MN-PAA-PEG micelles were promising candidates for chemoradiotherapy on tumor.

Platelet-Like Gold Nanostars for Cancer Therapy: The Ability to Treat Cancer and Evade Immune Reactions

The cell membrane-coating strategy has opened new opportunities for the development of biomimetic and multifunctional drug delivery platforms. Recently, a variety of gold nanoparticles, which can combine with blood cell membranes, have been shown to provide an effective approach for cancer therapy. Meanwhile, this class of hybrid nanostructures can deceive the immunological system to exhibit synergistic therapeutic effects. Here, we synthesized red blood cell (RBC) and platelet membrane-coated gold nanostars containing curcumin (R/P-cGNS) and evaluated whether R/P-cGNS had improved anticancer efficacy. We also validated a controlled release profile under near-infrared irradiation for the ability to target melanoma cells and to have an immunomodulatory effect on macrophages. RBC membrane coating provided self-antigens; therefore, it could evade clearance by macrophages, while platelet membrane coating provided targetability to cancer cells. Additionally, the nutraceutical curcumin provided anticancer and anti-inflammatory effects. In conclusion, the results presented in this study demonstrated that R/P-cGNS can deliver drugs to the target region and enhance anticancer effects while avoiding macrophage phagocytosis. We believe that R/P-cGNS can be a new design of the cell-based hybrid system for effective cancer therapy.

Ultra-sensitive Nanoprobe Modified with Tumor Cell Membrane for UCL/MRI/PET Multimodality Precise Imaging of Triple-Negative Breast Cancer

Triple-negative breast cancer (TNBC) is a subtype of breast cancer in which the estrogen receptor and progesterone receptor are not expressed, and human epidermal growth factor receptor 2 is not amplified or overexpressed either, which make the clinical diagnosis and treatment very challenging. Molecular imaging can provide an effective way to diagnose TNBC. Upconversion nanoparticles (UCNPs), are a promising new generation of molecular imaging probes. However, UCNPs still need to be improved for tumor-targeting ability and biocompatibility. This study describes a novel probe based on cancer cell membrane-coated upconversion nanoparticles (CCm-UCNPs), owing to the low immunogenicity and homologous-targeting ability of cancer cell membranes, and modified multifunctional UCNPs. This probe exhibits excellent performance in breast cancer molecular classification and TNBC diagnosis through UCL/MRI/PET tri-modality imaging in vivo. By using this probe, MDA-MB-231 was successfully differentiated between MCF-7 tumor models in vivo. Based on the tumor imaging and molecular classification results, the probe is also expected to be modified for drug delivery in the future, contributing to the treatment of TNBC. The combination of nanoparticles with biomimetic cell membranes has the potential for multiple clinical applications.

Advanced nanotechnology for hypoxia-associated antitumor therapy

Hypoxia is a hallmark of the tumor microenvironment, which promotes the proliferation, metastasis and invasion of tumors and stimulates the resistance of cancer treatments, leading to the serious consequence of tumor recurrence. Many nanotechnology-based studies have been conducted to improve the efficacy of cancer treatments using a hypoxia strategy. This is usually achieved by (i) activating bioreductive prodrugs in the tumor hypoxic/exacerbated hypoxic microenvironment, or (ii) delivering therapeutic agents to hypoxic tumor tissue using targeting molecules. Normally, a good therapeutic effect can be expected upon modulating the hypoxic microenvironment for tumor treatments. To achieve this, various nanotechnology strategies based on overcoming hypoxia have been exploited to alleviate tumor hypoxia and enhance the therapeutic efficacy of tumor therapy, including (i) reducing oxygen consumption by inhibiting cell respiration, (ii) normalizing tumor vessels to promote blood flow in the tumor, (iii) carrying exogenous oxygen into the tumor, and (iv) generating oxygen in situ. The strategy of in situ oxygen production is refined, and the scope of this strategy is further expanded. Finally, the inspiration of using advanced nanotechnology in hypoxia-associated antitumor therapy guides the study of tumor hypoxia for clinical use.

Cell-Based Nanoparticles Delivery Systems for Targeted Cancer Therapy: Lessons from Anti-Angiogenesis Treatments

The main strategy of cancer treatment has focused on attacking the tumor cells. Some cancers initially responsive to chemotherapy become treatment-resistant. Another strategy is to block the formation of tumor vessels. However, tumors also become resistant to anti-angiogenic treatments, mostly due to other cells and factors present in the tumor microenvironment, and hypoxia in the central part of the tumor. The need for new cancer therapies is significant. The use of nanoparticle-based therapy will improve therapeutic efficacy and targeting, while reducing toxicity. However, due to inefficient accumulation in tumor sites, clearance by reticuloendothelial organs and toxicity, internalization or conjugation of drug-loaded nanoparticles (NPs) into mesenchymal stem cells (MSCs) can increase efficacy by actively delivering them into the tumor microenvironment. Nanoengineering MSCs with drug-loaded NPs can increase the drug payload delivered to tumor sites due to the migratory and homing abilities of MSCs. However, MSCs have some disadvantages, and exosomes and membranes from different cell types can be used to transport drug-loaded NPs actively to tumors. This review gives an overview of different cancer approaches, with a focus on hypoxia and the emergence of NPs as drug-delivery systems and MSCs as cellular vehicles for targeted delivery due to their tumor-homing potential.

Cancer Nano-Immunotherapy from the Injection to the Target: The Role of Protein Corona

Immunotherapy has become a promising cancer therapy, improving the prognosis of patients with many different types of cancer and offering the possibility for long-term cancer remission. Nevertheless, some patients do not respond to these treatments and immunotherapy has shown some limitations, such as immune system resistance or limited bioavailability of the drug. Therefore, new strategies that include the use of nanoparticles (NPs) are emerging to enhance the efficacy of immunotherapies. NPs present very different pharmacokinetic and pharmacodynamic properties compared with free drugs and enable the use of lower doses of immune-stimulating molecules, minimizing their side effects. However, NPs face issues concerning stability in physiological conditions, protein corona (PC) formation, and accumulation in the target tissue. PC formation changes the physicochemical and biological properties of the NPs and in consequence their therapeutic effect. This review summarizes the recent advances in the study of the effects of PC formation in NP-based immunotherapy. PC formation has complex effects on immunotherapy since it can diminish ("immune blinding") or enhance the immune response in an uncontrolled manner ("immune reactivity"). Here, future perspectives of the field including the latest advances towards the use of personalized protein corona in cancer immunotherapy are also discussed.

Self-activated in vivo therapeutic cascade of erythrocyte membrane-cloaked iron-mineralized enzymes

Biomineralization of enzymes for in vivo diagnosis and treatment of diseases remain a considerable challenge, due to their severe reaction conditions and complicated physiological environment. Herein, we reported a biomimetic enzyme cascade delivery nanosystem, tumor-targeted erythrocyte membrane (EM)-cloaked iron-mineralized glucose oxidases (GOx-Fe⁰@EM-A) for enhancing anticancer efficacy by self-activated in vivo cascade to generate sufficient high toxic •OH at tumor site.

Methods: An ultra-small Fe⁰ nanoparticle (Fe⁰NP) was anchored in the inner cavity of glucose oxidase (GOx) to form iron-mineralized glucose oxidase (GOx-Fe⁰) as a potential tumor therapeutic nanocatalyst. Moreover, erythrocyte membrane cloaking delivery of GOx-Fe⁰in vivo was designed to effectively accumulate ultra-small GOx-Fe⁰ at tumor site.

Results: GOx-Fe⁰@EM-A had satisfactory biocompatibility and light-trigged release efficiency. Erythrocyte membrane cloaking of GOx-Fe⁰@EM-A not only prolongs blood circulation but also protects in vivo enzyme activity of GOx-Fe⁰; Tumor targeting of GOx-Fe⁰@EM-A endowed preferential accumulation at tumor site. After NIR light irradiation at tumor site, erythrocyte membrane of GOx-Fe⁰@EM-A was ruptured to achieve light-driven release and tumor deep penetration of ultra-small nanosize GOx-Fe⁰ by the photothermal effect of ICG. Then, GOx-Fe⁰ occurred self-activated in vivo cascade to effectively eradicate tumor by producing the highly cumulative and deeply penetrating •OH at tumor site.

Conclusion: Tumor-targeted erythrocyte membrane-cloaked iron-mineralized glucose oxidase (GOx-Fe0@EM-A) exhibits a promising strategy for striking antitumor efficacy by light-driven tumor deep penetration and self-activated therapeutic cascade.

NIR-triggered thermo-responsive biodegradable hydrogel with combination of photothermal and thermodynamic therapy for hypoxic tumor

Hypoxia is a typical feature of solid tumors, which highly limits the application of the oxygen-dependent therapy. Also, the dense and hyperbaric tumor tissues impede the penetration of nanoparticles into the deep tumor. Thereby, we designed a novel localized injectable hydrogel combining the photothermal therapy (PTT) and the thermodynamic therapy (TDT), which is based on the generation of free radicals even in the absence of oxygen for hypoxic tumor therapy. In our study, gold nanorods (AuNRs) and 2,2'-Azobis[2-(2-imidazalin-2-yl)propane] dihydrochlaride (AIPH) were incorporated into the hydrogel networks, which were formed by the copolymerization of hydrophobic N-isopropyl acrylamide (NIPAM) and hydrophilic glycidyl methacrylate modified hyaluronic acid (HA-GMA) to fabricate an injectable and near-infrared (NIR) responsive hydrogel. The crosslinked in situ forming hydrogel could not only realize PTT upon the NIR laser irradiation, but also generate free radicals even in hypoxic condition. Meanwhile the shrink of hydrogels upon thermal could accelerate the generation of free radicals to further damage the tumors, achieving the controlled drug release on demand. The designed hydrogel with a sufficient loading capacity, excellent biocompatibility and negligible systemic toxicity could serve as a long-acting implant for NIR-triggered thermo-responsive free radical generation. The in vitro cytotoxicity result and the in vivo antitumor activity illustrated the excellent therapeutic effect of hydrogels even in the absence of oxygen. Therefore, this innovative oxygen-independent platform combining the antitumor effects of PTT and TDT would bring a new insight into hypoxic tumor therapy by the application of alkyl free radical.

Cell-mediated and cell membrane-coated nanoparticles for drug delivery and cancer therapy

Nanotechnology-based drug delivery platforms have been developed over the last two decades because of their favorable features in terms of improved drug bioavailability and stability. Despite recent advancement in nanotechnology platforms, this approach still falls short to meet the complexity of biological systems and diseases, such as avoiding systemic side effects, manipulating biological interactions and overcoming drug resistance, which hinders the therapeutic outcomes of the NP-based drug delivery systems. To address these issues, various strategies have been developed including the use of engineered cells and/or cell membrane-coated nanocarriers. Cell membrane receptor profiles and characteristics are vital in performing therapeutic functions, targeting, and homing of either engineered cells or cell membrane-coated nanocarriers to the sites of interest. In this context, we comprehensively discuss various cell- and cell membrane-based drug delivery approaches towards cancer therapy, the therapeutic potential of these strategies, and the limitations associated with engineered cells as drug carriers and cell membrane-associated drug nanocarriers. Finally, we review various cell types and cell membrane receptors for their potential in targeting, immunomodulation and overcoming drug resistance in cancer.

Biomimetic Decoy Inhibits Tumor Growth and Lung Metastasis by Reversing the Drawbacks of Sonodynamic Therapy

Sonodynamic therapy (SDT) shows tremendous potential to induce immunogenic cell death (ICD) and activate antitumor immunity. However, it can aggravate hypoxia and release platelet (PLT)-associated danger-associated molecular patterns (DAMPs), which impede therapeutic efficacy and promote tumor metastasis. In order to solve these problems, a biomimetic decoy (designated as Lipo-Ce6/TPZ@M_H ) is constructed to reverse the drawbacks of SDT by loading sonosensitizer chlorin e6 (Ce6) and hypoxia-activated tirapazamine (TPZ) in the red blood cells-PLTs hybrid membrane (M_H )-camouflaged pH-sensitive liposome. After administration, the decoy exhibits enhanced cancer accumulation and retention abilities due to the immune escape and specific targeting behaviors by biomimetic surface coating. Upon local ultrasound, Ce6 produces toxic reactive oxygen species for SDT, and the resulting hypoxia microenvironment activates TPZ, which can realize a high-effective synergistic therapy. Meanwhile, DAMPs-mediated tumor metastasis is significantly inhibited, because the decoy retains platelet binding functions but is incapable of platelet-mediated metastasis. In addition, ICD-mediated strong antitumor immunities further prevent the growth and metastasis of the residual tumors left behind after synergistic treatment. Taken together, this study highlights the potential of using this cascade therapeutic therapy plus biomemitic decoy in one nanosystem to both eliminate melanoma in situ and suppress lung metastasis.

Nanomedicine-Enabled Modulation of Tumor Hypoxic Microenvironment for Enhanced Cancer Therapy

Hypoxia is a common condition of solid tumors that is mainly caused by enhanced tumor proliferative activity and dysfunctional vasculature. In the treatment of hypoxic human solid tumors, many conventional therapeutic approaches (e.g., oxygen-dependent photodynamic therapy, anticancer drug-based chemotherapy or X-ray induced radiotherapy) become considerably less effective or ineffective. In recent years, various strategies have been explored to deliver or generate oxygen inside solid tumors to overcome tumorous hypoxia and show promising evidence to improve the antitumor efficiency. In this review, the extrinsic regulation of tumor hypoxia via nanomaterial delivery is discussed followed by a summary of the mechanisms through which the modulated tumor hypoxic microenvironment improves therapeutic efficacy. The review concludes with future perspectives, to specifically address the translation of nanomaterial-based therapeutic strategies for clinical applications.

Responsive agarose hydrogel incorporated with natural humic acid and MnO2nanoparticles for effective relief of tumor hypoxia and enhanced photo-induced tumor therapy

In spite of widespread applications of nano-photosensitizers, poor tumor penetration and severe hypoxia in the tumor microenvironment (TME) always result in an undesirable therapeutic outcome of photodynamic therapy (PDT).

Cell membrane-engineered hybrid soft nanocomposites for biomedical applications

An overview of various cell membrane-engineered hybrid soft nanocomposites for medical applications.

Red blood cell membrane-enveloped O2 self-supplementing biomimetic nanoparticles for tumor imaging-guided enhanced sonodynamic therapy

Non-invasive sonodynamic therapy (SDT) was developed because of its advantages of high penetration depth and low side effects; however, tumor hypoxia greatly restricts its therapeutic effect. In this study, we aimed to develop ideal O₂ self-supplementing nanoparticles for imaging-guided enhanced sonodynamic therapy of tumors with the adept coalescence of biology with nanotechnology. Methods: Based on the natural enzyme system of red blood cells (RBC), biomimetic nanoparticles (QD@P)Rs were fabricated by encapsulating Ag₂S quantum dots (QD) in RBC vesicle membranes. The anti-tumor drug PEITC was employed to increase the intracellular H₂O₂ concentration in tumor cells. Results: In vitro and in vivo experiments demonstrated excellent biocompatibility and prolonged blood circulation of (QD@P)Rs. Following oral administration of PEITC in mice to improve the H₂O₂ concentration, the enzyme in the nanoprobe catalyzed endogenous H₂O₂ to increase O₂ content and effectively alleviate tumor hypoxia. Triggered by ultrasound under the guidance of fluorescence imaging, (QD@P)Rs generated reactive oxygen species (ROS) to induce tumor cell death, and the increased content of O₂ significantly enhanced the effect of SDT. Conclusion: Ag₂S QDs were used, for the first time, as a sonosensitizer in the SDT field. In this study, we integrated the advantages of the natural enzyme system and SDT to develop a novel approach for effective non-invasive treatment of cancer.

Defeating relapsed and refractory malignancies through a nano-enabled mitochondria-mediated respiratory inhibition and damage pathway

Hypoxia, which frequently reduces the sensitivity to many therapeutic interventions, including chemotherapy, radiotherapy and phototherapy, has been acknowledged as an important reason for poor prognosis. Burgeoning evidences have proved that the tumor hypoxia microenvironment can reduce the therapeutic effect on tumor through inhibiting the drug efficacy, limiting immune cell infiltration of tumors and accelerating tumor recurrence and metastasis. However, the relationship between oxygen supply and the proliferation of cancer cells is still ambiguous and argued. Different from the current commonly used oxygen supply strategies, this study concentrated on the reduction of endogenous oxygen consumption. Specifically, a novel photosensitizers (IR780) and metformin are packaged in PEG-PCL liposomes. Once such nanoparticles accumulated in tumor tissues, the tumor foci were irradiated through 808 nm laser, generated ROS to further release metformin and IR780. Metformin can directly inhibit the activity of complex Ⅰ in the mitochondrial electron transport chain, thus performed a potent inhibitor of cell respiration. After overcoming tumor hypoxia, the combination of mitochondria-targeted photodynamic therapy (PDT) and photothermic therapy (PTT) via IR780 may achieve superior synergistically therapeutic efficacy. Benefit from excellent characteristics of IR780, such synergistic PDT PTT with the inhibition of mitochondrial respiration can be monitored through near-infrared/photoacoustic dual-modal imaging. Such a conception of reducing endogenous oxygen consumption may offer a novel way to solve the important puzzles of hypoxia-induced tumor resistance to therapeutic interventions, not limited to phototherapy.

Recent trends in cell membrane-cloaked nanoparticles for therapeutic applications

Synthetic nanoparticles are extensively utilized in various biomedical engineering fields because of their unique physicochemical properties. However, their exogenous characteristics result in synthetic nanosystem invaders that easily induce the passive immune clearance mechanism, thereby increasing the retention effect caused by reticuloendothelial system (RES), resulting in low therapeutic efficacy and toxic effects. Recently, a cell membrane cloaking has been emerging technique as a novel interfacing approach from the biological/immunological perspective. This has been considered as useful technique for improving the performance of synthetic nanocarriers in vivo. By cell membrane cloaking, nanoparticles acquire the biological functions of natural cell membranes due to the presence of membrane-anchored proteins, antigens, and immunological moieties as well as physicochemical property of natural cell membrane. Due to cell membrane cloaking, the derived biological properties and functions of nanoparticles such as their immunosuppressive capability, long circulation time, and disease targeting ability have enhanced their future potential in biomedicine. Here, we review the cell membrane-cloaked nanosystems, highlight their novelty, introduce the preparation and characterization methods with relevant biomedical applications, and describe the prospects for using this novel biomimetic system that was developed from a combination of cell membranes and synthetic nanomaterials.

Bacteria-Responsive Biomimetic Selenium Nanosystem for Multidrug-Resistant Bacterial Infection Detection and Inhibition

Multidrug-resistant (MDR) bacterial infections are a severe threat to public health owing to their high risk of fatality. Noticeably, the premature degradation and undeveloped imaging ability of antibiotics still remain challenging. Herein, a selenium nanosystem in response to a bacteria-infected microenvironment is proposed as an antibiotic substitute to detect and inhibit methicillin-resistant Staphylococcus aureus (MRSA) with a combined strategy. Using natural red blood cell membrane (RBCM) and bacteria-responsive gelatin nanoparticles (GNPs), the Ru-Se@GNP-RBCM nanosystem was constructed for effective delivery of Ru-complex-modified selenium nanoparticles (Ru-Se NPs). Taking advantage of natural RBCM, the immune system clearance was reduced and exotoxins were neutralized efficiently. GNPs could be degraded by gelatinase in pathogen-infected areas in situ; therefore, Ru-Se NPs were released to destroy the bacteria cells. Ru-Se NPs with intense fluorescence imaging capability could accurately monitor the infection treatment process. Moreover, excellent in vivo bacteria elimination and a facilitated wound healing process were confirmed by two kinds of MRSA-infected mice models. Overall, the above advantages proved that the prepared nanosystem is a promising antibiotic alternative to combat the ever-threatening multidrug-resistant bacteria.

Cancer Cell Membrane-Camouflaged Nanorods with Endoplasmic Reticulum Targeting for Improved Antitumor Therapy

Cell membrane-coated nanocarriers have been developed for drug delivery due to their enhanced blood circulation and tissue targeting capacities; however, previous works have generally focused on spherical nanoparticles and extracellular barriers. Many living organisms with different shapes, such as rod-shaped bacilli and rhabdovirus, display different functionalities regarding tissue penetration, cellular uptake, and intracellular distribution. Herein, we developed cancer cell membrane (CCM)-coated nanoparticles with spherical and rod shapes. CCM-coated nanorods (CRs) showed superior endocytosis efficiency compared with their spherical counterparts (CCM-coated nanospheres, CSs) due to the caveolin-mediated pathway. Moreover, CRs can effectively accumulate in the endoplasmic reticulum (ER) region and ship the loaded DOX to the nucleus at a considerable concentration, resulting in ER stress and subsequent apoptosis. After intravenous injection into human pancreatic adenocarcinoma cell (BxPC-3) and pancreatic stellate cell (HPSC) hybrid tumor-bearing nude mice, CRs exhibited improved immune escape ability, rapid extracellular matrix (ECM) penetration (8.2-fold higher than CSs), and enhanced tumor accumulation, further contributing to the enhanced antitumor efficacy. These findings may actually suggest the significance of shape design in improving current cell membrane-based drug delivery systems for effective subcellular targets and tumor therapy.

Extremely Effective Chemoradiotherapy by Inducing Immunogenic Cell Death and Radio-Triggered Drug Release under Hypoxia Alleviation

Local hypoxia in solid malignancies often results in resistance to radiotherapy (RT) and chemotherapy (CT), which may be one of the main reasons for their failure in clinical application. Especially, oxygen is an essential element for enhancing DNA damage caused by ionizing radiation in radiotherapy. Here, two biomimetic oxygen delivery systems were designed by encapsulating hemoglobin (Hb) alone into a liposome (Hb-Lipo) or co-encapsulating Hb and doxorubicin (DOX) into a liposome (DOX-Hb-Lipo). Our data indicated that both Hb-Lipo and DOX-Hb-Lipo could effectively alleviate hypoxia in tumors. We demonstrated that RT plus tumor-targeting delivery of oxygen mediated by Hb-Lipo could significantly overcome the tolerance of hypoxic cancer cells to RT, showing significantly enhanced cancer-cell killing and tumor growth inhibition ability, mainly attributing to hypoxia alleviation and increased reactive oxygen species production under RT in cancer cells. Furthermore, a melanoma model that was quite insensitive to both RT and CT was used to test the efficacy of chemoradiotherapy combined with hypoxia alleviation. RT plus Hb-Lipo only caused a limited increase in antitumor activity. However, extremely strong tumor inhibition could be obtained by RT combined with DOX-Hb-Lipo-mediated CT, attributed to radio-triggered DOX release and enhanced immunogenic cell death induced by RT under an oxygen supplement. Our study provided a valuable reference for overcoming hypoxia-induced radioresistance and a useful therapeutic strategy for cancers that are extremely insensitive to chemo- or radiotherapy.

Nanoparticles-mediated reoxygenation strategy relieves tumor hypoxia for enhanced cancer therapy

Tumor hypoxia is a characteristic hallmark of malignant solid tumors, which remains an essential impediment to many current treatments like chemotherapy, radiotherapy, photodynamic therapy and immunotherapy, thereby leading to poor clinical prognosis after therapy. Rationally, modulating tumor hypoxia can be of great interest to augment the therapeutic efficacy of these treatments. In this review, we focus our discussion on current advances in nanoparticles-mediated tumor reoxygenation strategy for relieving tumor hypoxia to improve the therapeutic efficacy of versatile therapies. These nanoparticles can improve tumor oxygen levels via nanoparticles-mediated oxygen-carrying or oxygen-generating tactics to synergize the effectiveness of many current therapeutic modalities. Based on these considerable summaries and analyses, we propose some feasible perspectives on nanoparticles-based tumor reoxygenations to ameliorate the therapeutic outcomes.

"All-in-One" Theranostic Agent with Seven Functions Based on Bi-Doped Metal Chalcogenide Nanoflowers

Most of the existing single-component nanostructures cannot provide comprehensive diagnostic information, and their treatment strategies always have to combine other therapeutics as a complementary for effective biomedical application. Here, we adopted a facile approach to design a theranostic nanoflower (NF) with robust efficacy for comprehensive tumor diagnosis and quadruple synergistic cancer therapy. The NF is equipped with a metallic hybrid of several functional elements and flower-like superstructures and thus shows excellent in vitro and in vivo theranostic performance. It shows high X-ray attenuation coefficiency for the Bi element, strong near-infrared (NIR) plasmon absorbance and singlet oxygen (¹O₂) generation ability for the Mo element, and great photothermal conversion efficiency (54.7%) because of enhanced photoabsorption of the petal structure. Moreover, the NF realizes a very high doxorubicin-loading efficiency (90.0%) and bimodal pH/NIR-responsive drug release, posing a promise as a controlled drug carrier. The NF also shows excellent performance at trimodal magnetic resonance/X-ray computed tomography/photoacoustic imaging for comprehensive tumor diagnosis. To our best knowledge, it is the first time that integrating at least seven functions into one biomedical nanomaterial for well-rounded tumor theranostics has been reported. This "all-in-one" NF opens a new perspective in developing novel and efficient multifunctional nanotheranostics.

Mn-Porphyrin-Based Metal-Organic Framework with High Longitudinal Relaxivity for Magnetic Resonance Imaging Guidance and Oxygen Self-Supplementing Photodynamic Therapy

A nanoplatform for magnetic resonance imaging guidance and oxygen self-supplementing photodynamic therapy (PDT) was constructed on the basis of a porous metal-organic framework (PCN-222(Mn)), which was built by simple Mn-porphyrin ligands and biocompatible Zr⁴⁺ ions. Because of the good dispersibility of Mn³⁺ in the open framework and the high water affinity of the channel, PCN-222(Mn) exhibits a high longitudinal relaxivity of ∼35.3 mM^-1 s^-1 (1.0 T). In addition, it shows good catalytic activity for the conversion of endogenous hydrogen peroxide into oxygen, thereby improving tumor hypoxia during photodynamic therapy. The intravenous injection of PCN-222(Mn) into tumor-bearing mice mode provided good T₁-weighted contrast of the tumor site and effectively inhibited tumor growth upon a single-laser irradiation. The findings provide insights for the development of multifunctional theranostic nanoplatforms based on simple components.

Improving cancer therapy through the nanomaterials-assisted alleviation of hypoxia

Hypoxia, resulting from the imbalance between oxygen supply and consumption is a critical component of the tumor microenvironment. It has a paramount impact on cancer growth, metastasis and has long been known as a major obstacle for cancer therapy. However, none of the clinically approved anticancer therapeutics currently available for human use directly tackles this problem. Previous clinical trials of targeting tumor hypoxia with bioreductive prodrugs have failed to demonstrate satisfactory results. Therefore, new ideas are needed to overcome the hypoxia barrier. The method of modulating hypoxia to improve the therapeutic activity is of great interest but remains a considerable challenge. One of the emerging concepts is to supply or generate oxygen at the tumor site to increase the partial oxygen pressure and thereby reverse the hypoxia and its effects. In this review, we present an overview of the recent progress in the development of novel nanomaterials for the alleviation of hypoxic microenvironment. Two main strategies for hypoxia augmentation, i) direct delivery of O₂ into the tumor, and ii) in situ O₂ generations in the tumor microenvironment through different methods such as catalytic decomposition of endogenous hydrogen peroxide (H₂O₂) and light-triggered water splitting are discussed in detail. At present, these emerging nanomaterials are in their early phase and expected to grow rapidly in the coming years. Despite the promising start, there are several challenges needed to overcome for successful clinical translation.

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.