Hypoxia-specific ultrasensitive detection of tumours and cancer cells in vivo

Hypoxia-targeted drug delivery

Hypoxia is a state of low oxygen tension found in numerous solid tumours. It is typically associated with abnormal vasculature, which results in a reduced supply of oxygen and nutrients, as well as impaired delivery of drugs. The hypoxic nature of tumours often leads to the development of localized heterogeneous environments characterized by variable oxygen concentrations, relatively low pH, and increased levels of reactive oxygen species (ROS). The hypoxic heterogeneity promotes tumour invasiveness, metastasis, angiogenesis, and an increase in multidrug-resistant proteins. These factors decrease the therapeutic efficacy of anticancer drugs and can provide a barrier to advancing drug leads beyond the early stages of preclinical development. This review highlights various hypoxia-targeted and activated design strategies for the formulation of drugs or prodrugs and their mechanism of action for tumour diagnosis and treatment.

Multifunctional hypoxia imaging nanoparticles: multifunctional tumor imaging and related guided tumor therapy

Hypoxia is a common feature of most solid tumors. Having a comprehensive understanding of tumor hypoxia condition is a key to tumor therapy. Many hypoxia imaging nanoparticles have been used for tumor detection. However, simple optical hypoxia imaging is not enough for tumor diagnosis. Also, the tumor therapy process needs the information about the tumor hypoxia condition. Recently, researchers developed multimodal hypoxia tumor imaging nanoparticles and multifunctional hypoxia imaging-guided tumor therapy nanoparticles. The multimodal hypoxia imaging could produce more tumor region information and engage in functional tumor imaging to better understand the tumor condition. The multifunctional hypoxia imaging-guided tumor therapy could monitor the tumor therapy process and evaluate tumor therapeutic effect. Meanwhile, many challenges and limitations are still remaining in the application of multifunctional hypoxia nanoparticles. In this review, we first introduce the types of multifunctional hypoxia imaging nanoparticles. Then we focus on multimodal hypoxia imaging nanoparticles and hypoxia imaging-guided tumor therapy nanoparticles. We also discuss the challenges and future perspectives of this field. There has not been many studies in this field for now. We hope this review would bring more researchers' attention to this field so that it would substantially contribute to tumor precise therapy.

Self-Assembly of a Monochromophore-Based Polymer Enables Unprecedented Ratiometric Tracing of Hypoxia

The accuracy of traditional bischromophore-based ratiometric probes is always compromised by undesirable energy/charge transferring interactions between the internal reference moiety and the sensing chromophore. In this regard, ratiometric sensing with a monochromophore system is highly desirable. Herein, an unprecedented monochromophore-based ratiometric probe, which consists of a hydrophilic backbone poly(N-vinylpyrrolidone) (PVP) and single chromophore of platinum(II) tetraphenylporphyrin (Pt-TPP) is reported. Combination of the specific assembled clustering-triggered fluorescent emission (oxygen-insensitive) with the original Pt-TPP phosphorescence (oxygen-sensitive) enables successful construction of a monochromophore-based ratiometric nanosensor for directly tracing hypoxia in vivo, along with the preferable facilitation of enhanced permeation and retention effect and long excitation wavelength. The unique ratiometric signals enable the direct observation from normoxic to hypoxic environment in both living A549 cells and a tumor-bearing mice model, providing a significant paradigm of a monochromophore-based dual-emissive system with the specific assembled cluster emission. The work satisfactorily demonstrates a valuable strategy for designing monochromophore-based dual-emissive materials, and validates its utility for in vivo ratiometric biological sensing without the common energy/charge interference in bischromophore-based system.

Ultrafast labeling and high-fidelity imaging of mitochondria in cancer cells using an aggregation-enhanced emission fluorescent probe

An aggregation-enhanced emission probe was developed for ultrafast labeling and high-fidelity imaging of mitochondria in cancer cells with a high signal-to-noise ratio.

Hypoxia-triggered single molecule probe for high-contrast NIR II/PA tumor imaging and robust photothermal therapy

Hypoxia is a common characteristic of solid tumors. This important feature is associated with resistance to radio-chemotherapy, which results in poor prognosis and probability of tumor recurrence. Taking advantage of background-free NIR II fluorescence imaging and deeper-penetrating photoacoustic (PA) imaging, we developed a hypoxia-triggered and nitroreductase (NTR) enzyme-responsive single molecule probe for high-contrast NIR II/PA tumor imaging and hypoxia-activated photothermal therapy (PTT), which will overcome cellular resistance during hypoxia. Methods: The single molecule probe IR1048-MZ was synthesized by conjugating a nitro imidazole group as a specific hypoxia trigger with an IR-1048 dye as a NIR II/PA signal reporter. We investigated the NIR II fluorescence, NIR absorbance and photothermal effect in different hypoxia conditions in vitro, and performed NIR II/PA tumor imaging and hypoxia-activated photothermal therapy in mice. Results: This versatile molecular probe IR1048-MZ not only realized high-contrast tumor visualization with a clear boundary by NIR II fluorescence imaging, but also afforded deep-tissue penetration at the centimeter level by 3D PA imaging. Moreover, after being activated by NTR that is overexpressed in hypoxic tumors, the probe exhibited a significant photothermal effect for curative tumor ablation with no recurrence. Conclusions: We have developed the first hypoxia-triggered and NTR enzyme-responsive single molecule probe for high-contrast NIR II/PA tumor imaging and hypoxia-activated photothermal therapy. By tracing the activity of NTR, IR1048-MZ may be a promising contrast agent and theranostic formulation for other hypoxia-related diseases (such as cancer, inflammation, stroke, and cardiac ischemia).

Engineering of Electrochromic Materials as Activatable Probes for Molecular Imaging and Photodynamic Therapy

Electrochromic materials (EMs) are widely used color-switchable materials, but their applications as stimuli-responsive biomaterials to monitor and control biological processes remain unexplored. This study reports the engineering of an organic π-electron structure-based EM (dicationic 1,1,4,4-tetraarylbutadiene, 1²⁺) as a unique hydrogen sulfide (H₂S)-responsive chromophore amenable to build H₂S-activatable fluorescent probes (1²⁺-semiconducting polymer nanoparticles, 1²⁺-SNPs) for in vivo H₂S detection. We demonstrate that EM 1²⁺, with a strong absorption (500-850 nm), efficiently quenches the fluorescence (580, 700, or 830 nm) of different fluorophores within 1²⁺-SNPs, while the selective conversion into colorless diene 2 via H₂S-mediated two-electron reduction significantly recovers fluorescence, allowing for non-invasive imaging of hepatic and tumor H₂S in mice in real time. Strikingly, EM 1²⁺ is further applied to design a near-infrared photosensitizer with tumor-targeting and H₂S-activatable ability for effective photodynamic therapy (PDT) of H₂S-related tumors in mice. This study demonstrates promise for applying EMs to build activatable probes for molecular imaging of H₂S and selective PDT of tumors, which may lead to the development of new EMs capable of detecting and regulating essential biological processes in vivo.

Erythrocyte Membrane Cloaked Metal-Organic Framework Nanoparticle as Biomimetic Nanoreactor for Starvation-Activated Colon Cancer Therapy

Shutting down glucose supply by glucose oxidase (GOx) to starve tumors has been considered to be an attractive strategy in cancerous starvation therapy. Nevertheless, the in vivo applications of GOx-based starvation therapy are severely restricted by the poor GOx delivery efficiency and the self-limiting therapeutic effect. Herein, a biomimetic nanoreactor has been fabricated for starvation-activated cancer therapy by encapsulating GOx and prodrug tirapazamine (TPZ) in an erythrocyte membrane cloaked metal-organic framework (MOF) nanoparticle (TGZ@eM). The fabricated TGZ@eM nanoreactor can assist the delivery of GOx to tumor cells and then exhaust endogenous glucose and O₂ to starve tumors efficiently. Importantly, the resulting tumor hypoxia by GOx-based starvation therapy further initiates the activation of TPZ, which is released from the nanoreactor in the acid lyso/endosome environment, for enhanced colon cancer therapy. More importantly, by integrating the biomimetic surface modification, the immunity-escaping and prolonged blood circulation characteristics endow our nanoreactor dramatically improved cancer targeting ability. The in vitro and in vivo outcomes indicate our biomimetic nanoreactor exhibits a strong synergistic cascade effect for colon cancer therapy in an accurate and facile manner.

"Smart" Nanoprobes for Visualization of Tumor Microenvironments

Physiological parameters in tumor microenvironments, including hypoxia, low extracellular pH, enzymes, reducing conditions, and so on, are closely associated with the proliferation, angiogenesis, invasion, and metastasis of cancer, and impact the therapeutic administrations. Therefore, monitoring the tumor microenvironment is significant for diagnosing tumors, predicting the invasion potential, evaluating therapeutic efficacy, planning the treatment, and cancer prognostics. Noninvasive molecular imaging technologies combined with novel "smart" nanoparticle-based activatable probes provide a feasible approach to visualize tumor-associated microenvironment factors. This review summarizes recent achievements in the designs of "smart" molecular imaging nanoprobes responding to the tumor microenvironment-related features, and highlights the state of the art in tumor heterogeneity imaging.

Hypoxia-Activated Anticancer Prodrug for Bioimaging, Tracking Drug Release, and Anticancer Application

A novel anticancer theranostic prodrug, FDU-DB-NO₂, specifically activated by hypoxia for selective two-photon imaging hypoxia status, real-time tracking drug release, and solid tumor therapy was designed. The devised prodrug consists of an anticancer drug floxuridine (FDU), a fluorescence dye precursor 4'-(diethylamino)-1,1'-biphenyl-2-carboxylate (DB), and a hypoxic trigger 4-nitrobenzyl group. In normal cells, FDU-DB-NO₂ is "locked". Whereas in tumor cells, the prodrug is "unlocked" by hypoxia and results in fluorescent dye 7-(diethylamino)coumarin (CM) generation along with FDU release. The amounts and rates of CM formation and FDU release were controlled by hypoxic status and increased with the decreasing of the O₂ concentration. The hypoxic status, distribution of oxygen, and amount of FDU release in tumor cells, spheroids, and tumor tissue could be visualized by fluorescence. FDU-DB-NO₂ showed high cytotoxicity against hypoxic MCF-7 and MCG-803 cell lines and no cytotoxicity against normoxic BRL-3A cells and exhibited effective inhibition on tumor growth of MCF-7-cell-inoculated xenograft nude mice. This strategy may provide a promising platform for selective two-photon imaging hypoxia status, real-time tracking drug release, and personalized solid tumor treatment.

Simultaneous Assay of Oxygen-Dependent Cytotoxicity and Genotoxicity of Anticancer Drugs on an Integrated Microchip

Oxygen deprivation is a common feature in a variety of cancer tissues and associated with tumor progression, acquisition of antiapoptotic potential, and clinical therapeutic resistance. Thus, great interest has been aroused to develop new platforms or approaches of activity assays to impact on the hypoxic microenvironment and oxygen-dependent drug responses to improve the productivity of new drug discovery. In this study, an integrated microsystem is established to combine the cytotoxic and genotoxic tests together for continuous multiple measurements under mimicking hypoxic tumor microenvironment. We fabricated a double-layer chip device by combining a single-cell-arrayed agarose layer with a microfluidics-based oxygen gradient-generating layer using a PDMS membrane. Using tirapazamine (TPZ) and blemycin (BLM) as model anticancer drugs, we demonstrated its application and performance in single cell loading, cell cultivation, and subsequent drug treatment as well as in situ analysis of oxygen-dependent cytotoxicity and genotoxicity of anticancer drugs. The results demonstrated the opposite oxygen-dependent toxicity of TPZ and BLM, which also indicated that the formation of DNA breaks is related with cell apoptosis. Compared with the traditional assays, this device takes advantage of microfluidic phenomena to generate various oxygen concentrations while exhibiting the combinatorial diversities achieved by the single cell microarray, offering a powerful tool to study single cell behaviors and responses under different oxygen conditions with desired high-content and high-throughput capabilities.

Near Infrared Boron Dipyrromethene Nanoparticles for Optotheranostics

Boron dipyrromethene (BODIPY) is a class of important emerging fluorescent dyes. Due to their unique chemical and optical properties, near infrared (NIR)-emitting BODIPY dyes containing nanoparticles have recently been developed for a wide array of cutting-edge cancer optotheranostic applications. These nanoparticles not only have robust photostability and tunable photophysical properties, but they can also be flexibly tailored to a multitude of functional uses. Based on these outstanding characteristics, such nanoparticles have shown great promise in diagnosis as biological sensors, as well as in their utilization in advanced imaging and photomedicine for cancer treatment. In particular, here, this study first discusses their use as photoswitchable fluorescence probes toward in vitro single-molecule imaging. Second, this study takes a look at their opportunities for photoacoustic imaging utilization. Third, approaches are discussed to construct new NIR-absorbing BODIPY nanoparticles for photodynamic therapy (PDT). Fourth, this study delves into the new approach to use such nanoparticles as an emerging version of triplet-triplet annihilation upconversion (TTA-UC) and their biological uses, such as their photoactivation prodrug therapy (PAPT) for cancer. Finally, new biological sensors based on NIR BODIPY nanoparticles are introduced.

Recent advancements in biocompatible inorganic nanoparticles towards biomedical applications

Due to their intrinsic physical properties potentially useful for imaging and therapy as well as their highly engineerable surface, biocompatible inorganic nanoparticles offer novel platforms to develop advanced diagnostic and therapeutic agents for improved detection and more efficacious treatment of major diseases. The in vivo application of inorganic nanoparticles was demonstrated more than two decades ago, however it turns out to be very complicated as nanomaterials exhibit much more sophisticated pharmacokinetic properties than conventional drugs. In this review, we first discuss the in vivo behavior of inorganic nanoparticles after systematic administration, including the basic requirements for nanoparticles to be used in vivo, the impact of the particles' physicochemical properties on their pharmacokinetics, and the effects of the protein corona formed across the nano-bio interface. Next, we summarize the state-of-the-art of the preparation of biocompatible inorganic nanoparticles and bioconjugation strategies for obtaining target-specific nanoprobes. Then, the advancements in sensitive tumor imaging towards diagnosis and visualization of the abnormal signatures in the tumor microenvironment, together with recent studies on atherosclerosis imaging are highlighted. Finally, the future challenges and the potential for inorganic nanoparticles to be translated into clinical applications are discussed.

Biomimetic nanoflowers by self-assembly of nanozymes to induce intracellular oxidative damage against hypoxic tumors

Reactive oxygen species (ROS)-induced apoptosis is a promising treatment strategy for malignant neoplasms. However, current systems are highly dependent on oxygen status and/or external stimuli to generate ROS, which greatly limit their therapeutic efficacy particularly in hypoxic tumors. Herein, we develop a biomimetic nanoflower based on self-assembly of nanozymes that can catalyze a cascade of intracellular biochemical reactions to produce ROS in both normoxic and hypoxic conditions without any external stimuli. In our formulation, PtCo nanoparticles are firstly synthesized and used to direct the growth of MnO2. By adjusting the ratio of reactants, highly-ordered MnO2@PtCo nanoflowers with excellent catalytic efficiency are obtained, where PtCo behaves as oxidase mimic and MnO2 functions as catalase mimic. In this way, the well-defined MnO2@PtCo nanoflowers not only can relieve hypoxic condition but also induce cell apoptosis significantly through ROS-mediated mechanism, thereby resulting in remarkable and specific inhibition of tumor growth.

Real-Time Multimodal Bioimaging of Cancer Cells and Exosomes through Biosynthesized Iridium and Iron Nanoclusters

Multimodal bioimaging is a powerful tool for visualizing the abnormal state at the target site of the related disease. In this study, we used multimodal imaging techniques such as computed tomography, fluorescence, and magnetic resonance imaging to improve early and precise diagnosis of tumor. Herein, we reported the facile in situ biosynthesis of iridium and iron oxide nanoclusters (NCs) in cancer cells or tumor tissue. These NCs are used as a multimodal bioimaging probe to improve the image sensitivity and specificity toward the tumor. These NCs are applied for the in vivo multimodal imaging in the form of an imaging probe capable of enhancing the sensitivity of the image and specificity toward the tumor tissue. Our observation demonstrates that highly luminescent and magnetic NCs are not only biocompatible but also tumor-targeted because NC formation does not take place in normal cells and tissues. In addition, we isolated exosomes and the biosynthesized NCs internalized within exosomes, and these exosomes can be used as cancer biomarkers.

In Vivo and in Situ Activated Aggregation-Induced Emission Probes for Sensitive Tumor Imaging Using Tetraphenylethene-Functionalized Trimethincyanines-Encapsulated Liposomes

The design and exploration of fluorescent probes with high-sensitivity and low-background are essential for noninvasive optical molecular imaging. The in vivo and in situ activated aggregation-induced emission (AIE) probes were found to be ideal for achieving higher signal-to-background ratios for tumor detections. We herein developed novel tetraphenylethene-encapsulated liposomes (TPE-LPs) constructed by loading TPE-trimethincyanine into liposomes for the first time, and the probes were applied to tumor bioimaging in vivo. TPE-functionalized trimethincyanines were synthesized with a new and efficient one-pot reaction. In TPE-LPs, TPE-functionalized bicarboxylic acids benzoindole trimethinecyanine (TPE-BICOOH) fluorophores were found to be well dispersed in lipid bilayers (with non-restricted rotation) during the blood circulation, and then aggregated (with restriction of intramolecular rotation) upon liposome rupture in the tumor tissue, achieving a low-background and high-target signal for tumor imaging. The in situ activated AIE probes not only had great accumulation at the tumor site after intravenous injection in 4T1 tumor-bearing mice but also demonstrated excellent signal-to-background ratios, as well as low cytotoxicity and excellent biocompatibility. The proposed strategy is believed to be a simple and powerful tool for the sensitive detection of tumors.

Hyperbaric Oxygen Potentiates Doxil Antitumor Efficacy by Promoting Tumor Penetration and Sensitizing Cancer Cells

Hypoxia is a fundamental hallmark of solid tumors and helps contribute to chemotherapy resistance. Hyperbaric oxygen (HBO) therapy can overcome tumor hypoxia and promote chemotherapy antitumor efficacy; however, the simultaneous administration of some conventional chemotherapies, including doxorubicin (DOX), with HBO is considered an absolute contraindication. Here, DOX-loaded liposome (Doxil) is coadministered with HBO to assess the safety and efficacy of this combination treatment. By overcoming tumor hypoxia, HBO not only improves Doxil tumor penetration by decreasing the collagen deposition but also sensitizes tumor cells to Doxil. As a result, the combination treatment synergistically inhibits H22 tumor growth, with a tumor inhibition rate of 91.5%. The combination of HBO with Doxil shows neither extra side effects nor promotion of tumor metastasis. These results collectively reveal that the combination of HBO with Doxil is an effective and safe treatment modality. As both HBO and Doxil are routinely used, their combination could quickly translate to clinical trials for patients with hypoxic solid tumors.

A facile strategy for realizing room temperature phosphorescence and single molecule white light emission

Research on materials with pure organic room temperature phosphorescence (RTP) and their application as organic single-molecule white light emitters is a hot area and relies on the design of highly efficient pure organic RTP luminogens. Herein, a facile strategy of heavy-atom-participated anion-π+ interactions is proposed to construct RTP-active organic salt compounds (1,2,3,4-tetraphenyloxazoliums with different counterions). Those compounds with heavy-atom counterions (bromide and iodide ions) exhibit outstanding RTP due to the external heavy atom effect via anion-π+ interactions, evidently supported by the single-crystal X-ray diffraction analysis and theoretical calculation. Their single-molecule white light emission is realized by tuning the degree of crystallization. Such white light emission also performs well in polymer matrices and their use in 3D printing is demonstrated by white light lampshades.

Photophysical Tuning of N-Oxide-Based Probes Enables Ratiometric Photoacoustic Imaging of Tumor Hypoxia

Hypoxia results when the oxygen supply to rapidly growing tumors becomes inadequate to support various physiological processes. This plays a role in tumor metastasis and treatment resistance. Therefore, identifying tumor hypoxia can guide treatment planning and predict patient responses. However, hypoxic volumes are heterogeneously dispersed throughout a tumor, making it a challenge to pinpoint them with any degree of accuracy. Herein, we report the development of ratiometric hypoxia probe 1 (rHyP-1), which is a hypoxia-responsive small-molecule probe designed for reliable hypoxia detection using photoacoustic imaging. Photoacoustic imaging utilizes near-infrared (NIR) light to induce the production of ultrasound signals, enabling high-resolution image acquisition at centimeter depths. Together with the ratiometric capability of rHyP-1, reliable hypoxia detection with unprecedented spatial resolution is possible while minimizing error associated with concentration dependence and tissue heterogeneity.

Dual-Phosphorescent Iridium(III) Complexes Extending Oxygen Sensing from Hypoxia to Hyperoxia

Hypoxia and hyperoxia, referring to states of biological tissues in which oxygen supply is in sufficient and excessive, respectively, are often pathological conditions. Many luminescent oxygen probes have been developed for imaging intracellular and in vivo hypoxia, but their sensitivity toward hyperoxia becomes very low. Here we report a series of iridium(III) complexes in which limited internal conversion between two excited states results in dual phosphorescence from two different excited states upon excitation at a single wavelength. Structural manipulation of the complexes allows rational tuning of the dual-phosphorescence properties and the spectral profile response of the complexes toward oxygen. By manipulating the efficiency of internal conversion between the two emissive states, we obtained a complex exhibiting naked-eye distinguishable green, orange, and red emission in aqueous buffer solution under an atmosphere of N₂, air, and O₂, respectively. This complex is used for intracellular and in vivo oxygen sensing not only in the hypoxic region but also in normoxic and hyperoxic intervals. To the best of our knowledge, this is the first example of using a molecular probe for simultaneous bioimaging of hypoxia and hyperoxia.

Light-Induced Hypoxia-Triggered Living Nanocarriers for Synergistic Cancer Therapy

Living drug delivery system has been proposed as new concept materials because it is able to communicate with biological system, sense subtle changes in body microenvironment caused by disease, and then make rapid response to cure in the early stage of disease. Herein, taking full advantage of the tumor hypoxia physiology and successive effects of photodynamic therapy (PDT), we designed a new living delivery system via combining the PDT and hypoxia-responsive chemotherapy, abbreviated as Ce6-PEG-Azo-PCL. Then, according to the fact that oxygen can be converted into reactive oxygen species during irradiation of the photosensitizer, tumor cells could be killed after the poly(ethylene glycol) (PEG) conjugated photosensitizer chlorine e6 was irradiated at the tumor site. What is more, the continuous consumption of oxygen could further amplify the hypoxia condition of tumor and trigger the disassembly of hypoxia-responsive azobenzene bridges at the tumor site to release loaded chemotherapeutics drugs doxorubicin. The ongoing collaboration with PDT and hypoxia-responsive chemotherapy provided an integrated therapeutic effect in vitro and in vivo to suppress tumor growth.

Intravital Imaging to Monitor Therapeutic Response in Moving Hypoxic Regions Resistant to PI3K Pathway Targeting in Pancreatic Cancer

Application of advanced intravital imaging facilitates dynamic monitoring of pathway activity upon therapeutic inhibition. Here, we assess resistance to therapeutic inhibition of the PI3K pathway within the hypoxic microenvironment of pancreatic ductal adenocarcinoma (PDAC) and identify a phenomenon whereby pronounced hypoxia-induced resistance is observed for three clinically relevant inhibitors. To address this clinical problem, we have mapped tumor hypoxia by both immunofluorescence and phosphorescence lifetime imaging of oxygen-sensitive nanoparticles and demonstrate that these hypoxic regions move transiently around the tumor. To overlay this microenvironmental information with drug response, we applied a FRET biosensor for Akt activity, which is a key effector of the PI3K pathway. Performing dual intravital imaging of drug response in different tumor compartments, we demonstrate an improved drug response to a combination therapy using the dual mTORC1/2 inhibitor AZD2014 with the hypoxia-activated pro-drug TH-302.

A multifunctional luminogen with aggregation-induced emission characteristics for selective imaging and photodynamic killing of both cancer cells and Gram-positive bacteria

The increasing impact of bacteria on cancer progression and treatments has been witnessed in recent years. Insufficient attention to cancer-related bacteria may lead to distant metastasis, poor therapeutic efficiency and low survival rates for cancers. Exploiting new approaches that enable selective imaging and effective killing of cancer cells and bacteria are thus of great value for the battle against cancers. Herein, we report an aggregation-induced emission (AIE) luminogen, namely TPPCN, with intense emission and efficient reactive oxygen species production for fluorescence imaging and killing cancer cells and Gram-positive bacteria. This work not only demonstrates the potential of AIE luminogens in comprehensive cancer treatments but also stimulates the enthusiasm of scientists to design more multifunctional AIE systems for both cancer and bacteria theranostics.

Excitation-Dependent Theranostic Nanosheet for Cancer Treatment

In this work, a novel ruthenium complex loaded monolayer layered double hydroxide (LDH) (denoted as Ru(C-bpy)₂ /mLDH) as supramolecular nanosensor is synthesized, which is greatly exclusive to the hypoxic tumor microenvironment. The Ru(C-bpy)₂ /mLDH ultrathin sheet displays not only enhanced luminescence lifetime compared to the parent Ru(C-bpy)₂ alone, but also improved oxygen responsibility under an excitation of 488 or 800 nm. Moreover, the Ru(C-bpy)₂ /mLDH is possessed of two-photon fluorescence imaging ability under the 800 nm irradiation. In addition, the Ru(C-bpy)₂ /mLDH can generate singlet oxygen with a high yield (φ_∆ ) of 0.28 under the 520 nm irradiation, while the φ_∆ of Ru(C-bpy)₂ is 0.19. Therefore, the Ru(C-bpy)₂ /mLDH can be applied as a supramolecular theranostic agent with light-switchable cancer imaging and photodynamic therapy properties.

An upconverting nanotheranostic agent activated by hypoxia combined with NIR irradiation for selective hypoxia imaging and tumour therapy

A novel upconverting nanotheranostic agent, UCNP-CA_E-FDU/NO₂, activated by both hypoxia (internal stimuli) and NIR irradiation (external stimuli) was designed and synthesized for simultaneous imaging and chemotherapy of solid tumours. The devised theranostic agent consists of an active drug, floxuridine (FDU), upconverting nanoparticles (UCNP: NaYF₄:Yb³⁺/Tm³⁺, multifunctional carriers for upconverting 980 nm NIR light to 365 nm UV light and tumour-targeted drug delivery), (E)-o-hydroxycinnamic acid (CA_E, a UV-photo trigger and a fluorescence dye precursor), and a 4-nitrobenzyl group (a hypoxic trigger). In addition, FDU was modified by CA_E, and CA_E was modified by the 4-nitrobenzyl group; moreover, CA_E was conjugated to UCNPs by covalent bonds to form a novel UCNP-CA_E-FDU/NO₂ platform. In normal cells, the platform is "locked", whereas in tumour cells, hypoxia combined with NIR illumination (980 nm) "unlocks" the platform, based on a series of reactions including the reduction of UCNP-CA_E-FDU/NO₂ catalyzed by over-expression of nitroreductase (NTR), 1,6-rearrangement-elimination, the photo-isomerization of UCNP-CA_E-FDU caused by absorption of NIR irradiation and emission at 365 nm of UCNP-CA_E-FDU/NO₂, and intramolecular esterification, which initiate the fluorescent dye in conjugation with UCNP (UCNP-CM) formation and FDU release with high spatio-temporal control. The amounts of FDU and UCNP-CM released can be accurately tuned by controlling the NIR illumination time. UCNP-CA_E-FDU/NO₂ showed excellent selectivity for hypoxic cells, exhibited high cytotoxicity against cancer cells and almost no cytotoxicity to normal cells, presented significant inhibition of tumour growth in vivo, and displayed sensitive detection of the hypoxic status and the amount of FDU released. The excellent properties of UCNP-CA_E-FDU/NO₂ endow it with great potential applications for precise imaging of tumour cells and personalized solid tumour treatment.

Two-photon fluorescent probe for detection of nitroreductase and hypoxia-specific microenvironment of cancer stem cell

Hypoxia plays a crucial role in cancer progression, and it has great significance for monitoring hypoxic level in biosystems. Cancer stem cells (CSCs) represent a small population of tumour cells that regard as the key to seed tumours. The survival of CSCs depend on the tumour microenvironment, which is distinct region has the hypoxic property. Therefore, the detection of the hypoxic CSC niche plays a pivotal role in the destructing the 'soil' of CSCs, and eliminating CSCs population. Numerous one-photon excited fluorescent probes have been developed to indicate the hypoxic status in tumours through the detection of nitroreductase (NTR) level. However, the biomedical application of one-photon fluorescent probes is limited due to the poor tissue penetration. In the present work, we reported a two-photon fluorescent probe to detect the NTR in CSCs and monitor the hypoxic microenvironment in vivo. The two-photon fluorescent molecular probe with a hypoxic specific response group can be reduced by NTR under hypoxic conditions. We used the two-photon probe to detect the hypoxia status of 3D cultured-CSCs in vitro and in vivo CSCs' microenvironment in tumour. The two-photon absorption cross section extends fluorescent excitation spectra to the near infrared region, which dramatically promotes the tissue penetration for hypoxic microenvironment detection of CSC in vivo.

Optical molecular imaging for tumor detection and image-guided surgery

We have witnessed rapid development of fluorescence molecular imaging of solid tumors for cancer diagnosis and image-guided surgery in the past decade. Many biomarkers unique to cancer cells or tumor microenvironment, such as cell surface receptors, hypoxia, secreted proteases and extracellular acidosis have been characterized, and can be used to distinguish cancer from normal tissue. A variety of optical imaging probes have been developed to target these biomarkers to improve tumor contrast over the background tissue. Unlike conventional anatomical and molecular imaging technologies, fluorescent imaging method benefits from its safety, high-spatial resolution and real-time capability, and therefore, has become a highly adoptable imaging method for tumor detection and image-guided surgery in clinics. In this review, we summarize recent progress in 'always-ON' and stimuli-activatable fluorescent imaging probes, and discuss their potentials in tumor detection and image-guided surgery.

A Magnetofluorescent Carbon Dot Assembly as an Acidic H2 O2 -Driven Oxygenerator to Regulate Tumor Hypoxia for Simultaneous Bimodal Imaging and Enhanced Photodynamic Therapy

Recent studies indicate that carbon dots (CDs) can efficiently generate singlet oxygen (¹ O₂ ) for photodynamic therapy (PDT) of cancer. However, the hypoxic tumor microenvironment and rapid consumption of oxygen in the PDT process will severely limit therapeutic effects of CDs due to the oxygen-dependent PDT. Thus, it is becoming particularly important to develop a novel CD as an in situ tumor oxygenerator for overcoming hypoxia and substantially enhancing the PDT efficacy. Herein, for the first time, magnetofluorescent Mn-CDs are successfully prepared using manganese(II) phthalocyanine as a precursor. After cooperative self-assembly with DSPE-PEG, the obtained Mn-CD assembly can be applied as a smart contrast agent for both near-infrared fluorescence (FL) (maximum peak at 745 nm) and T₁ -weighted magnetic resonance (MR) (relaxivity value of 6.97 mM^-1 s^-1 ) imaging. More interestingly, the Mn-CD assembly can not only effectively produce ¹ O₂ (quantum yield of 0.40) but also highly catalyze H₂ O₂ to generate oxygen. These collective properties of the Mn-CD assembly enable it to be utilized as an acidic H₂ O₂ -driven oxygenerator to increase the oxygen concentration in hypoxic solid tumors for simultaneous bimodal FL/MR imaging and enhanced PDT. This work explores a new biomedical use of CDs and provides a versatile carbon nanomaterial candidate for multifunctional nanotheranostic applications.

Growth and Immune Evasion of Lymph Node Metastasis

Cancer patients with lymph node (LN) metastases have a worse prognosis than those without nodal disease. However, why LN metastases correlate with reduced patient survival is poorly understood. Recent findings provide insight into mechanisms underlying tumor growth in LNs. Tumor cells and their secreted molecules engage stromal, myeloid, and lymphoid cells within primary tumors and in the lymphatic system, decreasing antitumor immunity and promoting tumor growth. Understanding the mechanisms of cancer survival and growth in LNs is key to designing effective therapy for the eradication of LN metastases. In addition, uncovering the implications of LN metastasis for systemic tumor burden will inform treatment decisions. In this review, we discuss the current knowledge of the seeding, growth, and further dissemination of LN metastases.

Nanobubble-embedded inorganic 808 nm excited upconversion nanocomposites for tumor multiple imaging and treatment

Contrast reagents for ultrasound imaging are widely used in clinical medical diagnosis because ultrasound resolution is limited. Contrast agents must be utilized to enhance the image resolution. At present, microbubbles (MBs) and nanobubbles (NBs) are the main contrast reagent candidates for improving the signal resolution. Fluorescence upconversion nanoparticles provide high sensitivity and also function as nanocarriers. This can label tumor cells in a specific organ under irradiation of near-infrared (NIR) light. However, despite the use of NIR light, the penetration depth of NIR is only approximately 15 mm. Thus, we combine fluorescence with ultrasonic imaging to achieve the effect of multiple imaging and solve the low penetration depth of fluorescence imaging and the poor resolution of ultrasound imaging. The dual imaging modalities achieved higher resolution or signal to noise ratios. In this study, Nd³⁺-sensitized upconversion nanoparticles (UCNPs) are combined with graphitic carbon nitride quantum dots (CNs) and embedded in NBs (UCNP-CN@NBs). The UCNPs are excited by 808 nm light and emit visible and ultraviolet light. Then, the energy of the ultraviolet light is transferred to the CNs to produce reactive oxygen species (ROS) for photodynamic therapy. Ultrasonic waves are also used to promote NB bursting and the release of ROS molecules in photodynamic therapy, leading to cancer cell apoptosis.

Polyacrylic Acid Functionalized Co0.85Se Nanoparticles: An Ultrasmall pH-Responsive Nanocarrier for Synergistic Photothermal-Chemo Treatment of Cancer

To surmount the challenges of limited drug penetration and therapeutic resistance in solid tumors, stimuli-responsive nanocarrier-based drug delivery systems (DDSs) with relatively small sizes are inherently favorable for combined treatment of cancerous cells. In this work, poly(acrylic acid) (PAA) functionalized Co_0.85Se nanoparticles (PAA-Co_0.85Se NPs) were synthesized through an ambient aqueous precipitating approach for synergistic photothermal-chemo treatment of cancer. The obtained PAA-Co_0.85Se NPs possess ultrasmall size (8.2 ± 2.6 nm), considerable near-infrared (NIR) light absorption, high photothermal transforming efficiency (45.2%) and low cytotoxicity, all of which are beneficial for localized photothermal ablation of cancer cells. Doxorubicin hydrochloride (DOX·HCl) was then successfully loaded on PAA-Co_0.85Se NPs with a loading capacity up to 8.3% to form PAA-Co_0.85Se-DOX composites, which exhibited an exciting acidic pH-responsive drug release property due to the protonation of amino groups in DOX and carboxyl groups in PAA molecules. As expected, when HeLa cells were treated with PAA-Co_0.85Se-DOX NPs as well as NIR laser irradiation, a significant synergistic cell-killing effect was observed, greatly improving the treatment efficiency. Thus, this work presents novel insight into the design of ultrasmall stimuli-responsive nanocarrier-based DDSs for synergistic photothermal-chemo treatment of cancer cells.

Quantitatively monitoring oxygen variation in endoplasmic reticulum with a fluorophore–phosphor energy transfer cassette

EET cassette Ir-Np-OH has been constructed by connecting an Ir(iii) complex as acceptor to a naphthalimide dye as donor.

Construction of a hypoxia responsive upconversion nanosensor for tumor imaging by fluorescence resonance energy transfer from carbon dots to ruthenium complex

Hypoxia responsive upconversion nano-aggregates are synthesized which can be excited by NIR light to give oxygen dependent phosphorescence emission via the FRET process.

A nitroreductase and acidity detecting dual functional ratiometric fluorescent probe for selectively imaging tumor cells

A dual functional ratiometric fluorescent probe can obviously distinguish acidity, nitroreductase, and nitroreductase in an acidic environment. Confocal fluorescence imaging of A549 cells indicates the probe can detect acidity and expressed nitroreductase in living cells.

Water-soluble cyclometalated platinum(ii) and iridium(iii) complexes: synthesis, tuning of the photophysical properties, and in vitro and in vivo phosphorescence lifetime imaging

This paper presents synthesis and photophysical investigation of cyclometalated water-soluble Pt(ii) and Ir(iii) complexes containing auxiliary sulfonated diphosphine (bis(diphenylphosphino)benzene (dppb), P^P*) ligand. The complexes demonstrate considerable variations in excitation (extending up to 450 nm) and emission bands (with maxima ranging from ca. 450 to ca. 650 nm), as well as in the sensitivity of excited state lifetimes to molecular oxygen (from almost negligible to more than 4-fold increase in degassed solution). Moreover, all the complexes possess high two-photon absorption cross sections (400–500 GM for Pt complexes, and 600–700 GM for Ir complexes). Despite their negative net charge, all the complexes demonstrate good uptake by HeLa cells and low cytotoxicity within the concentration and time ranges suitable for two-photon phosphorescence lifetime (PLIM) microscopy. The most promising complex, [(ppy)₂Ir(sulfo-dppb)] (Ir1*), upon incubation in HeLa cells demonstrates two-fold lifetime variations under normal and nitrogen atmosphere, correspondingly. Moreover, its in vivo evaluation in athymic nude mice bearing HeLa tumors did not reveal acute toxicity upon both intravenous and topical injections. Finally, Ir1* demonstrated statistically significant difference in lifetimes between normal tissue (muscle) and tumor in macroscopic in vivo PLIM imaging.

Novel water-soluble iridium complexes with sulfonated diphosphine allow in vitro and in vivo lifetime hypoxia imaging.

A luminescent bimetallic iridium(iii) complex for ratiometric tracking intracellular viscosity

The luminescent bimetallic iridium probe C10 could distinguish cancer cells from normal cells and track viscosity changes during cell apoptosis.

Value of combining dynamic contrast enhanced ultrasound and optoacoustic tomography for hypoxia imaging

Optoacoustic imaging (OAI) can detect haemoglobin and assess its oxygenation. However, the lack of a haemoglobin signal need not indicate a lack of perfusion. This study uses a novel method to assist the co-registration of optoacoustic images with dynamic contrast enhanced ultrasound (DCE-US) images to demonstrate, in preclinical tumour models, the value of combining haemoglobin imaging with a perfusion imaging method, showing that a lack of a haemoglobin signal does not necessarily indicate an absence of perfusion. DCE-US was chosen for this particular experiment because US is extremely sensitive to microbubble contrast agents and because microbubbles, like red blood cells but unlike currently available optical contrast agents, do not extravasate. Significant spatial correlations were revealed between the DCE-US properties and tumour blood-oxygen saturation and haemoglobin, as estimated using OAI. It is speculated that DCE-US properties could be applied as surrogate biomarkers for hypoxia when planning clinical radiotherapy or chemotherapy.

A bioreducible N-oxide-based probe for photoacoustic imaging of hypoxia

Hypoxia occurs when limited oxygen supply impairs physiological functions and is a pathological hallmark of many diseases including cancer and ischemia. Thus, detection of hypoxia can guide treatment planning and serve as a predictor of patient prognosis. Unfortunately, current methods suffer from invasiveness, poor resolution and low specificity. To address these limitations, we present Hypoxia Probe 1 (HyP-1), a hypoxia-responsive agent for photoacoustic imaging. This emerging modality converts safe, non-ionizing light to ultrasound waves, enabling acquisition of high-resolution 3D images in deep tissue. HyP-1 features an N-oxide trigger that is reduced in the absence of oxygen by heme proteins such as CYP450 enzymes. Reduction of HyP-1 produces a spectrally distinct product, facilitating identification via photoacoustic imaging. HyP-1 exhibits selectivity for hypoxic activation in vitro, in living cells, and in multiple disease models in vivo. HyP-1 is also compatible with NIR fluorescence imaging, establishing its versatility as a multimodal imaging agent.

Hypoxia is a hallmark of many diseases including cancer and ischemia, and detection can be invasive and of low resolution and specificity. Here the authors show a hypoxia probe that converts non-ionizing light to ultrasound, which enables the acquisition of high-resolution 3D images in deep tissue.

Luminescent Probe Based Techniques for Hypoxia Imaging

Hypoxia is a condition of tissue environments wherein a lower than normal level of oxygen is available, and it serves as the root cause and indicator of various diseases. Detection of hypoxia in tumors is imperative for furthering our understanding of the pathological effects and the development of proper treatments, as it is well established that hypoxic tumors are able to impede the cancer treatment process by being resistant to many therapies. It is important therefore to be able to detect hypoxia in tissues and tumors through in vivo imaging methods. A growing area for detection of hypoxia in vivo is the use of fluorescent/luminescent probes which has accelerated in recent years. The continued quest for improvements in selectivity and sensitivity has inspired researchers to pursue new strategies for fluorescence/luminescent probe design. This review will discuss various luminescent probes based on small molecules, dyes, macromolecules, and nanoparticles for sensitive and specific detection of oxygen levels directly or by indirect mechanisms such as the presence of enzymes or related factors that arise in a hypoxic environment. Following the particular mechanism of detection, each probe has specific structural and photophysical properties which permit its selectivity and sensitivity. These probes show promise in terms of low toxicity and high specificity among other merits discussed, and in providing new dimensions for hypoxia detection, these works contribute to future potential methods for clinical diagnosis of hypoxic tissues and tumors.