Development of hypoxia-sensitive Gd3+-based MRI contrast agents

Hypoxia-selective prodrug restrains tumor cells through triggering mitophagy and inducing apoptosis

Hypoxia is a common feature of various solid tumors, which reduces the sensitivity of tumor cells to both radiotherapy and chemotherapy. However, hypoxia also presents an opportunity for tumor-selective therapy. The prodrug strategy, leveraging the hypoxic nature of the tumor microenvironment, shows significant potential for clinical application. Here we present CHD-1, a hypoxia-activated antitumor prodrug that activates in hypoxic environments, effectively inhibiting hypoxic tumor cells while exhibiting no toxicity to normoxic cells. CHD-1 impairs mitochondrial morphology and membrane potential of hypoxic tumor cells, further triggers excessive mitophagy and induces apoptosis. Moreover, prodrug CHD-1 significantly inhibits HeLa xenograft growth in vivo, and shows lower toxicity than parent molecule in an acute toxicity assessment in animal models. This study introduces a promising hypoxia-activated antitumor prodrug with strong potential for further development in hypoxic tumor therapy.

A hypoxia-targeting and hypoxia-responsive nano-probe for tumor detection and early diagnosis

Accurate imaging of tumor hypoxia in vivo is critical for early cancer diagnosis and clinical outcomes, highlighting the great need for its detection specificity and sensitivity. In this report, we propose a probe (HTRNP) that simultaneously has hypoxia-targeting and hypoxia-responsive capabilities to enhance the tumor hypoxia imaging efficiency. HTRNP was successfully prepared through the encapsulation of Pt(II)-tetrakis(pentafluorophenyl)porphyrin (PtPFPP), which exhibits hypoxia-dependent phosphorescence, within the amphiphilic block copolymer OPDMA-PF, which has hypoxia-targeting tertiary amine N-oxide moieties and hydrophobic perfluorobenzene ring structures, which highly improved the loading content and water solubility of PtPFPP. By combining targeting and response abilities toward hypoxic conditions, the HTRNP micelles efficiently accumulate in the tumor tissues and emit intense phosphorescence, thus enabling ultrasensitive detection of various tumor models, even of hundreds of cancer cells, indicating its promising potential for early cancer detection and phenotypic characterization.

The role of responsive MRI probes in the past and the future of molecular imaging

Magnetic resonance imaging (MRI) has become an indispensable tool in biomedical research and clinical radiology today. It enables the tracking of physiological changes noninvasively and allows imaging of specific biological processes at the molecular or cellular level. To this end, bioresponsive MRI probes can greatly contribute to improving the specificity of MRI, as well as significantly expanding the scope of its application. A large number of these sensor probes has been reported in the past two decades. Importantly, their development was done hand in hand with the ongoing advances in MRI, including emerging methodologies such as chemical exchange saturation transfer (CEST) or hyperpolarised MRI. Consequently, several approaches on successfully using these probes in functional imaging studies have been reported recently, giving new momentum to the field of molecular imaging, also the chemistry of MRI probes. This Perspective summarizes the major strategies in the development of bioresponsive MRI probes, highlights the major research directions within an individual group of probes (T 1- and T 2-weighted, CEST, fluorinated, hyperpolarised) and discusses the practical aspects that should be considered in designing the MRI sensors, up to their intended application in vivo.

Recent Advances in Cardiac Magnetic Resonance for Imaging of Acute Myocardial Infarction

Acute myocardial infarction (AMI) is one of the primary causes of death worldwide, with a high incidence and mortality rate. Assessment of the infarcted and surviving myocardium, along with microvascular obstruction, is crucial for risk stratification, treatment, and prognosis in patients with AMI. Nonionizing radiation, excellent soft tissue contrast resolution, a large field of view, and multiplane imaging make cardiac magnetic resonance (CMR) a "one-stop" method for assessing cardiac structure, function, perfusion, and metabolism. Hence, this imaging technology is considered the "gold standard" for evaluating myocardial function and viability in AMI. This review critically compares the advantages and disadvantages of CMR with other cardiac imaging technologies, and relates the imaging findings to the underlying pathophysiological processes in AMI. A more thorough understanding of CMR technology will clarify their advanced clinical diagnosis and prognostic assessment applications, and assess the future approaches and challenges of CMR in the setting of AMI.

Phosphonated mesoporous silica nanoparticles bearing ruthenium complexes used as molecular probes for tracking oxygen levels in cells and tissues

Molecular oxygen plays an important role in living organisms. Its concentration and fluctuation in cells or tissues are related to many diseases. Therefore, there is a need for molecular systems that can be used to detect and quantify oxygen levels in vitro and in vivo. In this study, we synthesized phosphonated mesoporous silica nanoparticles bearing ruthenium complexes in their pores (pM-Rus) and evaluated their photophysical and biological properties. The pM-Rus were highly soluble in water and showed robust phosphorescence under hypoxic conditions, while the addition of oxygen suppressed this emission. Cellular experiments revealed that pM-Rus with a size of 100 nm showed efficient cellular uptake to emit phosphorescence in hypoxic cells. In addition, pM-Rus have negligible toxicity to cells due to the blockage of direct contact between ruthenium complexes and intracellular biomolecules and the deactivation of singlet oxygen (1O2) generated by photoexcitation of ruthenium complexes before leaking out of the pores. Animal experiments confirmed that pM-Rus showed robust emission at hypoxic regions in mice. Thus, pM-Rus are promising oxygen probes for living systems.

Synthesis of adenine dinucleosides SAM analogs as specific inhibitors of SARS-CoV nsp14 RNA cap guanine-N7-methyltransferase

The spreading of new viruses is known to provoke global human health threat. The current COVID-19 pandemic caused by the recently emerged coronavirus SARS-CoV-2 is one significant and unfortunate example of what the world will have to face in the future with emerging viruses in absence of appropriate treatment. The discovery of potent and specific antiviral inhibitors and/or vaccines to fight these massive outbreaks is an urgent research priority. Enzymes involved in the capping pathway of viruses and more specifically RNA N7- or 2′O-methyltransferases (MTases) are now admitted as potential targets for antiviral chemotherapy. We designed bisubstrate inhibitors by mimicking the transition state of the 2′-O-methylation of the cap RNA in order to block viral 2′-O MTases. This work resulted in the synthesis of 16 adenine dinucleosides with both adenosines connected by various nitrogen-containing linkers. Unexpectedly, all the bisubstrate compounds were barely active against 2′-O MTases of several flaviviruses or SARS-CoV but surprisingly, seven of them showed efficient and specific inhibition against SARS-CoV N7-MTase (nsp14) in the micromolar to submicromolar range. The most active nsp14 inhibitor identified is as potent as but particularly more specific than the broad-spectrum MTase inhibitor, sinefungin. Molecular docking suggests that the inhibitor binds to a pocket formed by the S-adenosyl methionine (SAM) and cap RNA binding sites, conserved among SARS-CoV nsp14. These dinucleoside SAM analogs will serve as starting points for the development of next inhibitors for SARS-CoV-2 nsp14 N7-MTase.

Nanoconfinement-mediated cancer theranostics

Despite various therapeutic or diagnostic developments, cancer is still one of the most lethal diseases due to insufficiently adequate treatments and the delay of the early stage of disease detection. An image-guided drug delivery system (IGDDS), as a real-time noninvasive imaging assessment of therapeutic response, has the strong potential to improve the diagnosis and treatment of cancer because its imaging property offers the quantification of nanomedicine at the intended disease sites, the possible assurance of adequate treatment and elimination of undesirable delay of early-stage diagnosis due to low resolution. One of potential modality that overcomes these challenges could be the nanoconfinement of gold (Au) nanoparticles within other nanoparticles called "Particle-in-Particle (PIP)", which is a strong candidate of cancer treatment because of its "theranostic (therapy + diagnostics)" advantages including imaging (e.g., CT) and therapeutic hyperthermia application. In this review, we will elaborate on the current application of theranostic by nanoconfinement. Then, we will narrow down the gold nanoparticle-mediated theranostic application and its nanoconfinement advantages. Finally, the future direction for maximum nanoconfinement mediated cancer therapy will be included.

A fluorescent naphthalimide NADH mimic for continuous and reversible sensing of cellular redox state

A new naphthalimide based NADH mimic that functions as a fully reversible fluorescent “on off” probe for redox state has been synthesised and evaluated.

Biological reduction of nitroimidazole-functionalized gold nanorods for photoacoustic imaging of tumor hypoxia

Tumor-selective accumulation of gold nanorods (GNR) has been demonstrated for visualization of tumor hypoxia by photoacoustic imaging. We prepared GNRs with hypoxia-targeting nitroimidazole units (G-NI) on their surface. Biological experiments revealed that G-NI produced a strong photoacoustic signal in hypoxic tumor cells and tissues.

Tumor-selective accumulation of gold nanorods (GNR) has been demonstrated for visualization of tumor hypoxia by photoacoustic imaging.

Naphthalimides in fluorescent imaging of tumor hypoxia - An up-to-date review

Hypoxia is a distinctive characteristic of advanced solid malignancies that results from a disparity between oxygen supply and its consumption. The degree of hypoxia is believed to have adverse prognostic significance. Therefore detecting cellular hypoxia can potentially offer insights into the grade of tumour as well as its evolution towards a progressive malignant phenotype, which clinically translates to greater metastatic potential and treatment resistance. Fluorescence imaging to visualize hypoxia in biological systems is a minimally-invasive method. Recently there are several reports on interdisciplinary research that aims at developing functional probes that can be efficiently used for non-invasive imaging of hypoxic tumours. Upregulated levels of nitroreductase (NTR) is detected in hypoxic solid malignancies, and this characteristic feature is increasingly utilized in the development of NTR-targeted fluorescent molecules to selectively sense hypoxia in vivo. The present review summarizes various reports published on the design concepts of nitro naphthalimide-based bio-reductive fluorescent sensors that can be applied noninvasively to image hypoxia in cancer.

Bioinspired Small-Molecule Tools for the Imaging of Redox Biology

The availability of electrons to biological systems underpins the mitochondrial electron transport chain (ETC) that powers living cells. It is little wonder, therefore, that the sufficiency of electron supply is critical to cellular health. Considering mitochondrial redox activity alone, a lack of oxygen (hypoxia) leads to impaired production of adenosine triphosphate (ATP), the major energy currency of the cell, whereas excess oxygen (hyperoxia) is associated with elevated production of reactive oxygen species (ROS) from the interaction of oxygen with electrons that have leaked from the ETC. Furthermore, the redox proteome, which describes the reversible and irreversible redox modifications of proteins, controls many aspects of biological structure and function. Indeed, many major diseases, including cancer and diabetes, are now termed "redox diseases", spurring much interest in the measurement and monitoring of redox states and redox-active species within biological systems. In this Account, we describe recent efforts to develop magnetic resonance (MR) and fluorescence imaging probes for studying redox biology. These two classes of molecular imaging tools have proved to be invaluable in supplementing the structural information that is traditionally provided by MRI and fluorescence microscopy, respectively, with highly sensitive chemical information. Importantly, the study of biological redox processes requires sensors that operate at biologically relevant reduction potentials, which can be achieved by the use of bioinspired redox-sensitive groups. Since oxidation-reduction reactions are so crucial to modulating cellular function and yet also have the potential to damage cellular structures, biological systems have developed highly sophisticated ways to regulate and sense redox changes. There is therefore a plethora of diverse chemical structures in cells with biologically relevant reduction potentials, from transition metals to organic molecules to proteins. These chemical groups can be harnessed in the development of exogenous molecular imaging agents that are well-tuned to biological redox events. To date, small-molecule redox-sensitive tools for oxidative stress and hypoxia have been inspired from four classes of cellular regulators. The redox-sensitive groups found in redox cofactors, such as flavins and nicotinamides, can be used as reversible switches in both fluorescent and MR probes. Enzyme substrates that undergo redox processing within the cell can be modified to provide fluorescence or MR readout while maintaining their selectivity. Redox-active first-row transition metals are central to biological homeostasis, and their marked electronic and magnetic changes upon oxidation/reduction have been used to develop MR sensors. Finally, redox-sensitive amino acids, particularly cysteine, can be utilized in both fluorescent and MR sensors.

Chemistry of MRI Contrast Agents: Current Challenges and New Frontiers

Tens of millions of contrast-enhanced magnetic resonance imaging (MRI) exams are performed annually around the world. The contrast agents, which improve diagnostic accuracy, are almost exclusively small, hydrophilic gadolinium(III) based chelates. In recent years concerns have arisen surrounding the long-term safety of these compounds, and this has spurred research into alternatives. There has also been a push to develop new molecularly targeted contrast agents or agents that can sense pathological changes in the local environment. This comprehensive review describes the state of the art of clinically approved contrast agents, their mechanism of action, and factors influencing their safety. From there we describe different mechanisms of generating MR image contrast such as relaxation, chemical exchange saturation transfer, and direct detection and the types of molecules that are effective for these purposes. Next we describe efforts to make safer contrast agents either by increasing relaxivity, increasing resistance to metal ion release, or by moving to gadolinium(III)-free alternatives. Finally we survey approaches to make contrast agents more specific for pathology either by direct biochemical targeting or by the design of responsive or activatable contrast agents.

A novel nitroreductase-enhanced MRI contrast agent and its potential application in bacterial imaging

Nitroreductases (NTRs) are known to be able to metabolize nitro-substituted compounds in the presence of reduced nicotinamide adenine dinucleotide (NADH) as an electron donor. NTRs are present in a wide range of bacterial genera and, to a lesser extent, in eukaryotes hypoxic tumour cells and tumorous tissues, which makes it an appropriate biomarker for an imaging target to detect the hypoxic status of cancer cells and potential bacterial infections. To evaluate the specific activation level of NTR, great efforts have been devoted to the development of fluorescent probes to detect NTR activities using fluorogenic methods to probe its behaviour in a cellular context; however, NTR-responsive MRI contrast agents are still by far underexplored. In this study, para-nitrobenzyl substituted T1-weighted magnetic resonance imaging (MRI) contrast agent Gd-DOTA-PNB (probe 1) has been designed and explored for the possible detection of NTR. Our experimental results show that probe 1 could serve as an MRI-enhanced contrast agent for monitoring NTR activity. The in vitro response and mechanism of the NTR catalysed reduction of probe 1 have been investigated through LC-MS and MRI. Para-nitrobenzyl substituted probe 1 was catalytically reduced by NTR to the intermediate para-aminobenzyl substituted probe which then underwent a rearrangement elimination reaction to Gd-DOTA, generating the enhanced T1-weighted MR imaging. Further, LC-MS and MRI studies of living Escherichia coli have confirmed the NTR activity detection ability of probe 1 at a cellular level. This method may potentially be used for the diagnosis of bacterial infections.

Synthesis and evaluation of MR probes for targeted-reporter imaging

Visualizing disease heterogeneity remains a challenging task since most imaging agents are targeted to a single receptor. We describe the development of an MR platform able to report on multiple molecular events. Enzyme activation and enhanced cellular uptake of this modular probe make it suitable for subsequent targeted-reporter imaging applications.

Hypoxia-Responsive 19F MRI Probes with Improved Redox Properties and Biocompatibility

¹⁹F magnetic resonance imaging (MRI), an emerging modality in biomedical imaging, has shown promise for in vitro and in vivo preclinical studies. Here we present a series of fluorinated Cu(II)ATSM derivatives for potential use as ¹⁹F magnetic resonance agents for sensing cellular hypoxia. The synthesized complexes feature a hypoxia-targeting Cu²⁺ coordination core, nine equivalent fluorine atoms connected via a variable-length poly(ethylene glycol) linker. Introduction of the fluorine moiety maintains the planar coordination geometry of the Cu²⁺ center, while the linker length modulates the Cu^2+/+ reduction potential, ¹⁹F NMR relaxation properties, and lipophilicity. In particular, the ¹⁹F NMR relaxation properties were quantitatively evaluated by the Solomon-Bloembergen model, revealing a regular pattern of relaxation enhancement tuned by the distance between Cu²⁺ and F atoms. Finally, the potential utility of these complexes for sensing reductive environments was demonstrated using both ¹⁹F MR phantom imaging and ¹⁹F NMR, including experiments in intact live cells.

Aptamer-PEG-modified Fe3O4@Mn as a novel T1- and T2- dual-model MRI contrast agent targeting hypoxia-induced cancer stem cells

Hypoxia-induced cancer stem cells have been known to be involved in tumour metastasis, resistance to chemo/radio therapy and tumour recurrence. Magnetic Resonance Imaging is a widely used imaging tool for cancers in clinics and research. To develop T1-positive and T2-negative dual mode MRI agents for more comprehensive and accurate diagnostic information under hypoxic conditions, a hypoxia-inducible factor-1α based aptamer and Mn(II)-modified nanoparticles D-Fe₃O₄@PMn were synthesized and characterized. In vitro and in vivo studies show that D-Fe₃O₄@PMn NPs are biocompatible and less cytotoxic and can produce significant contrast enhancement in T1- and T2-weighted MR imaging. Furthermore, the D-Fe₃O₄@PMn NPs enable targeted dual-contrast T1- and T2-weighted MR imaging of cancer cells expressing high levels of HIF-1α and cancer stem cell-related proteins under hypoxic condition. In conclusion, NPs with HIF-1α and Mn(II) are promising diagnostic agents for dual-mode T1 and T2 imaging by targeting cancer stem cells as they are non-toxic and biocompatible.

Factors Predicting Effectiveness of Neoadjuvant Therapy for Esophageal Squamous Cell Carcinoma

The aim of the study was to elucidate pretreatment factors that can predict the outcome of neoadjuvant chemoradiotherapy or chemotherapy (NAC(R)T) and help us choose treatment strategies appropriate for individual patients. Few studies have investigated whether clinical data obtainable before the treatment can predict the efficacy of NAC(R)T. Of 1540 patients treated for esophageal squamous cell carcinoma (ESCC) at our department between January 2000 and June 2014, those who underwent surgical resection of cStage II or more advanced ESCC after NAC(R)T (113 NACRT and 146 NACT patients) were enrolled in this study. Information all available before the treatment was analyzed to extract factors that can predict the effectiveness of NAC(R)T. NAC(R)T was considered effective when Grade 2 or greater treatment efficacy was achieved based on the histological grading system. NACRT was effective in 51 (45%) of 113 patients. The analysis of 35 pretreatment factors showed that female sex (hazard ratio [HR] = 3.650; 1.181-11.236), absence of dyslipidemia (HR = 3.284; 1.341-8.041), and histologically poorly differentiated tumor (HR = 2.431; 1.052-5.619) were factors predicting NACRT effectiveness. On the other hand, NACT was effective in 21 (14%) of 146 patients. The analysis of pretreatment factors showed that absence of dyslipidemia (HR = 10.204; 1.302-83.33) and therapy with docetaxel, cisplatin, and 5-fluorouracil (HR = 2.097; 1.027-4.280) were factors predicting NACT effectiveness. The findings of this study investigating factors that could predict the outcome of NAC(R)T suggest that the prevalence of dyslipidemia influences the outcome of NAC(R)T for ESCC.

Copper complexes as a source of redox active MRI contrast agents

The study reports an advance in designing copper-based redox sensing MRI contrast agents. Although the data demonstrate that copper(II) complexes are not able to compete with lanthanoids species in terms of contrast, the redox-dependent switch between diamagnetic copper(I) and paramagnetic copper(II) yields a novel redox-sensitive contrast moiety with potential for reversibility.

A review of responsive MRI contrast agents: 2005-2014

This review focuses on MRI contrast agents that are responsive to a change in a physiological biomarker. The response mechanisms are dependent on six physicochemical characteristics, including the accessibility of water to the agent, tumbling time, proton exchange rate, electron spin state, MR frequency or superparamagnetism of the agent. These characteristics can be affected by changes in concentrations or activities of enzymes, proteins, nucleic acids, metabolites, or metal ions, or changes in redox state, pH, temperature, or light. A total of 117 examples are presented, including ones that employ nuclei other than (1) H, which attests to the creativity of multidisciplinary research efforts to develop responsive MRI contrast agents.

Optical imaging of targeted β-galactosidase in brain tumors to detect EGFR levels

A current limitation in molecular imaging is that it often requires genetic manipulation of cancer cells for noninvasive imaging. Other methods to detect tumor cells in vivo using exogenously delivered and functionally active reporters, such as β-gal, are required. We report the development of a platform system for linking β-gal to any number of different ligands or antibodies for in vivo targeting to tissue or cells, without the requirement for genetic engineering of the target cells prior to imaging. Our studies demonstrate significant uptake in vitro and in vivo of an EGFR-targeted β-gal complex. We were then able to image orthotopic brain tumor accumulation and localization of the targeted enzyme when a fluorophore was added to the complex, as well as validate the internalization of the intravenously administered β-gal reporter complex ex vivo. After fluorescence imaging localized the β-gal complexes to the brain tumor, we topically applied a bioluminescent β-gal substrate to serial sections of the brain to evaluate the delivery and integrity of the enzyme. Finally, robust bioluminescence of the EGFR-targeted β-gal complex was captured within the tumor during noninvasive in vivo imaging.

Fluorescent/phosphorescent dual-emissive conjugated polymer dots for hypoxia bioimaging

Fluorescent/phosphorescent dual-emissive conjugated polymer dots were designed and synthesized, ere used for tumor hypoxia sensing via ratiometric imaging and photoluminescence lifetime imaging.

A kind of fluorescent/phosphorescent dual-emissive conjugated polyelectrolyte has been prepared by introducing phosphorescent platinum(ii) porphyrin (O₂-sensitive) into a fluorene-based conjugated polyelectrolyte (O₂-insensitive), which can form ultrasmall conjugated polymer dots (FP-Pdots) in the phosphate buffer solution (PBS) via self-assembly caused by their amphiphilic structures with hydrophobic backbones and hydrophilic side chains. These FP-Pdots can exhibit an excellent ratiometric luminescence response to O₂ content with high reliability and full reversibility for measuring oxygen levels, and the excellent intracellular ratiometric O₂ sensing properties of the FP-Pdots nanoprobe have also been confirmed by the evident change in the I_red/I_blue ratio values in living cells cultured at different O₂ concentrations. To confirm the reliability of the O₂ sensing measurements of the FP-Pdots nanoprobe, O₂ quenching experiments based on lifetime measurements of phosphorescence from Pt(ii) porphyrin moieties have also been carried out. Utilizing the sensitivity of the long phosphorescence lifetime from Pt(ii) porphyrins to oxygen, the FP-Pdots have been successfully applied in time-resolved luminescence imaging of intracellular O₂ levels, including photoluminescence lifetime imaging and time-gated luminescence imaging, which will evidently improve the sensing sensitivity and reliability. Finally, in vivo oxygen sensing experiments were successfully performed by luminescence imaging of tumor hypoxia in nude mice.

Synthesis and characterization of a porphyrazine-Gd(III) MRI contrast agent and in vivo imaging of a breast cancer xenograft model

Porphyrazines (Pz), or tetraazaporphyrins, are being studied for their potential use in detection and treatment of cancer. Here, an amphiphilic Cu-Pz-Gd(III) conjugate has been prepared via azide-alkyne Huisgen cycloaddition or 'click' chemistry between an azide functionalized Pz and alkyne functionalized DOTA-Gd(III) analog for use as an MRI contrast agent. This agent, Cu-Pz-Gd(III), is synthesized in good yield and exhibits solution-phase ionic relaxivity (r1  = 11.5 mM(-1) s(-1)) that is approximately four times higher than that of a clinically used monomeric Gd(III) contrast agent, DOTA-Gd(III). Breast tumor cells (MDA-MB-231) associate with Cu-Pz-Gd(III) in vitro, where significant contrast enhancement (9.336 ± 0.335 contrast-to-noise ratio) is observed in phantom cell pellet MR images. This novel contrast agent was administered in vivo to an orthotopic breast tumor model in athymic nude mice and MR images were collected. The average T1 of tumor regions in mice treated with 50 mg kg(-1) Cu-Pz-Gd(III) decreased relative to saline-treated controls. Furthermore, the decrease in T1 was persistent relative to mice treated with the monomeric Gd(III) contrast agent. An ex vivo biodistribution study confirmed that Cu-Pz-Gd(III) accumulates in the tumors and is rapidly cleared, primarily through the kidneys. Differential accumulation and T1 enhancement by Cu-Pz-Gd(III) in the tumor's core relative to the periphery offer preliminary evidence that this agent would find application in the imaging of necrotic tissue.

Ultrasmall particles for Gd-MRI and (68) Ga-PET dual imaging

Nanoparticles made of a polysiloxane matrix and surrounded by 1,4,7,10-tetraazacyclododecane-1-glutaric anhydride-4,7,10-triacetic acid (DOTAGA)[Gd(3+) ] and 2,2'-(7-(1-carboxy-4-((2,5-dioxopyrrolidin-1-yl)oxy)-4-oxobutyl)-1,4,7-triazonane-1,4-diyl)diacetic acid) NODAGA[(68) Ga(3+) ] have been synthesized for positron emission tomography/magnetic resonance (PET/MRI) dual imaging. Characterizations were carried out in order to determine the nature of the ligands available for radiolabelling and to quantify them. High radiolabelling purity (>95%) after (68) Ga labelling was obtained. The MR and PET images demonstrate the possibility of using the nanoparticles for a combined PET/MR imaging scanner. The images show fast renal elimination of the nanoparticles after intravenous injection.

Ultrasensitive nanosensors based on upconversion nanoparticles for selective hypoxia imaging in vivo upon near-infrared excitation

Hypoxia is a distinct feature of malignant solid tumors, which is a possible causative factor for the serious resistance to chemo- and radiotherapy or the development of invasion and metastasis. The exploration of nanosensors with the capabilities like the accurate diagnosis of hypoxic level will be helpful to estimate the malignant degree of tumor and subsequently implement more effective personalized treatment. Here, we report the design and synthesis of nanosensors that can selectively and reversibly detect the level of hypoxia both in vitro and in vivo. The designed nanosensor is composed of two main moieties: oxygen indicator [Ru(dpp)3](2+)Cl2 for detection of hypoxia and upconversion nanoparticles for offering the excitation light of [Ru(dpp)3](2+)Cl2 by upconversion process under 980 nm exposure. The results show that the nanosensors can reversibly become quenched or luminescent under hyperoxic or hypoxic conditions, respectively. Compared with free [Ru(dpp)3](2+)Cl2, the designed nanosensors exhibit enhanced sensitivity for the detection of oxygen in hypoxic regions. More attractively, the nanosensors can image hypoxic regions with high penetration depth because the absorption and emission wavelength are within the NIR and far-red region, respectively. Most importantly, nanosensors display a high selectivity for detection of relevant oxygen changes in cells and zebrafish.

Redox- and hypoxia-responsive MRI contrast agents

The development of responsive or "smart" magnetic resonance imaging (MRI) contrast agents that can report specific biomarker or biological events has been the focus of MRI contrast agent research over the past 20 years. Among various biological hallmarks of interest, tissue redox and hypoxia are particularly important owing to their roles in disease states and metabolic consequences. Herein we review the development of redox-/hypoxia-sensitive T1 shortening and paramagnetic chemical exchange saturation transfer (PARACEST) MRI contrast agents. Traditionally, the relaxivity of redox-sensitive Gd(3+) -based complexes is modulated through changes in the ligand structure or molecular rotation, while PARACEST sensors exploit the sensitivity of the metal-bound water exchange rate to electronic effects of the ligand-pendant arms and alterations in the coordination geometry. Newer designs involve complexes of redox-active metal ions in which the oxidation states have different magnetic properties. The challenges of translating redox- and hypoxia-sensitive agents in vivo are also addressed.

Magnetic resonance imaging (MRI) contrast agents for tumor diagnosis

This review focuses on MRI contrast agents for tumor diagnosis. Several types of low molecular weight Gd3+-based complexes and dextran-coated superparamagnetic iron oxide (SPIO) nanoparticles have been used for clinical tumor diagnosis as longitudinal relaxation time (T1) and transverse relaxation time (T2) MRI contrast agents, respectively. To further improve the sensitivity of MRI, new types of chelates for T1 MRI contrast agents and combination of low molecular weight T1 MRI contrast agents with different types of carriers have been investigated. Different types of materials for forming secure coating layers of SPIO and novel superparamagnetic particles with higher relaxivity values have been explored. Various types of ligands were applied to improve the capability to target tumor for both T1 and T2 contrast agents. Furthermore, MRI contrast agents for detection of tumor metabolism were also pursued.

Preparation of Gd Complex-Immobilized Silica Particles and Their Application to MRI

A preparation method for Gd-ethylenediaminetetraacetic acid disodium salt dihydrate (ETDA) complex-immobilized silica particles (Gd-EDTA/SiO2) is proposed. Preparation of spherical silica particles was performed by a sol-gel method at 35°C using 0.2 M tetraethylorthosilicate, 25 M H2O, and 0.01 M NaOH in ethanol, which produced silica particles with an average size of  nm. Immobilization of Gd-EDTA on the silica particles was conducted at 35°C by introducing amino groups on the silica particles with (3-aminopropyl)trimethoxysilane at pH 3 (NH2/SiO2) and then making Gd-EDTA act on the NH2/SiO2 particles at pH 5. The as-prepared Gd-EDTA/SiO2 particle colloid solution was concentrated up to a Gd concentration of 0.347 mM by centrifugation. The sphere structure of Gd-EDTA/SiO2 particles was undamaged, and the colloid solution was still colloidally stable, even after the concentrating process. The concentrated Gd-EDTA/SiO2 colloid solution revealed good MRI properties. A relaxivity value for T1-weighted imaging was as high as 5.15 mM−1 s−1, that was comparable to that for a commercial Gd complex contrast agent.

Redox-activated manganese-based MR contrast agent

Here we report a simple Mn coordination complex with utility as a redox-sensitive MR probe. The HBET ligand stabilizes both the Mn(2+) and Mn(3+) oxidation states. In the presence of glutathione (GSH), low relaxivity Mn(III)-HBET is converted to high relaxivity Mn(II)-HBET with a 3-fold increase in relaxivity, and concomitant increase in MR signal. Alternately, hydrogen peroxide can convert Mn(II)-HBET to Mn(III)-HBET with a reduction in MR signal.

Reversible off-on fluorescence probe for hypoxia and imaging of hypoxia-normoxia cycles in live cells

We report a fully reversible off-on fluorescence probe for hypoxia. The design employs QSY-21 as a Förster resonance energy transfer (FRET) acceptor and cyanine dye Cy5 as a FRET donor, based on our finding that QSY-21 undergoes one-electron bioreduction to the radical under hypoxia, with an absorbance decrease at 660 nm. At that point, FRET can no longer occur, and the dye becomes strongly fluorescent. Upon recovery of normoxia, the radical is immediately reoxidized to QSY-21, with loss of fluorescence due to restoration of FRET. We show that this probe, RHyCy5, can monitor repeated hypoxia-normoxia cycles in live cells.

Promising strategies for Gd-based responsive magnetic resonance imaging contrast agents

Magnetic resonance imaging is a powerful imaging modality that is often coupled with paramagnetic contrast agents based on gadolinium to enhance sensitivity and image quality. Responsive contrast agents are key to furthering the diagnostic potential of MRI, both to provide anatomical information and to discern biochemical activity. Recent design of responsive gadolinium-based T₁ agents has made interesting progress, with the development of novel complexes which sense their chemical environment through changes in the coordination of water molecules, the molecular tumbling time or the number of metal centres. Particular promising design strategies include the use of multimeric systems, and the development of dual imaging probes.