Synthesis and characterization of a new bioactivated paramagnetic gadolinium(III) complex [Gd(DOTA-FPG)(H2O)] for tracing gene expression

MRI Contrast Agents in Glycobiology

Molecular recognition involving glycoprotein-mediated interactions is ubiquitous in both normal and pathological natural processes. Therefore, visualization of these interactions and the extent of expression of the sugars is a challenge in medical diagnosis, monitoring of therapy, and drug design. Here, we review the literature on the development and validation of probes for magnetic resonance imaging using carbohydrates either as targeting vectors or as a target. Lectins are important targeting vectors for carbohydrate end groups, whereas selectins, the asialoglycoprotein receptor, sialic acid end groups, hyaluronic acid, and glycated serum and hemoglobin are interesting carbohydrate targets.

In Situ Generated Novel 1H MRI Reporter for β-Galactosidase Activity Detection and Visualization in Living Tumor Cells

For wide applications of the lacZ gene in cellular/molecular biology, small animal investigations, and clinical assessments, the improvement of noninvasive imaging approaches to precisely assay gene expression has garnered much attention. In this study, we investigate a novel molecular platform in which alizarin 2-O-β-d-galactopyranoside AZ-1 acts as a lacZ gene/β-gal responsive 1H-MRI probe to induce significant 1H-MRI contrast changes in relaxation times T 1 and T 2 in situ as a concerted effect for the discovery of β-gal activity with the exposure of Fe3+. We also demonstrate the capability of this strategy for detecting β-gal activity with lacZ-transfected human MCF7 breast and PC3 prostate cancer cells by reaction-enhanced 1H-MRI T 1 and T 2 relaxation mapping.

A self-immobilizing near-infrared fluorogenic probe for sensitive imaging of extracellular enzyme activity in vivo

Reported herein is a self-immobilizing near-infrared fluorogenic probe that can be used to image extracellular enzyme activity in vivo. Using a fluorophore as a quinone methide precursor, this probe covalently anchors at sites of activation and greatly enhances the fluorescence intensity at 710 nm upon enzymatic stimulus, significantly boosting detection sensitivity in a highly dynamic in vivo system.

Self-Immolative Activation of β-Galactosidase-Responsive Probes for In Vivo MR Imaging in Mouse Models

Our lab has developed a new series of self-immolative MR agents for the rapid detection of enzyme activity in mouse models expressing β-galactosidase (β-gal). We investigated two molecular architectures to create agents that detect β-gal activity by modulating the coordination of water to GdIII . The first is an intermolecular approach, wherein we designed several structural isomers to maximize coordination of endogenous carbonate ions. The second involves an intramolecular mechanism for q modulation. We incorporated a pendant coordinating carboxylate ligand with a 2, 4, 6, or 8 carbon linker to saturate ligand coordination to the GdIII ion. This renders the agent ineffective. We show that one agent in particular (6-C pendant carboxylate) is an extremely effective MR reporter for the detection of enzyme activity in a mouse model expressing β-gal.

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.

Glycosylated lanthanide cyclen complexes as luminescent probes for monitoring glycosidase enzyme activity

The development of synthetic chemical probes for the detection of enzymes is extremely important for biological, medicinal, and industrial applications. Here we report the synthesis of an array of novel glycosylated Tb(iii) complexes, their photophysical properties in solution, and their ability to function as luminescent probes for observing glycosidase enzyme activity in real time. Our initial studies into the application of these complexes for the detection of the Concanavalin A (ConA) lectin is also reported, highlighting the broad scope of these novel chemical probes.

Advances in Monitoring Cell-Based Therapies with Magnetic Resonance Imaging: Future Perspectives

Cell-based therapies are currently being developed for applications in both regenerative medicine and in oncology. Preclinical, translational, and clinical research on cell-based therapies will benefit tremendously from novel imaging approaches that enable the effective monitoring of the delivery, survival, migration, biodistribution, and integration of transplanted cells. Magnetic resonance imaging (MRI) offers several advantages over other imaging modalities for elucidating the fate of transplanted cells both preclinically and clinically. These advantages include the ability to image transplanted cells longitudinally at high spatial resolution without exposure to ionizing radiation, and the possibility to co-register anatomical structures with molecular processes and functional changes. However, since cellular MRI is still in its infancy, it currently faces a number of challenges, which provide avenues for future research and development. In this review, we describe the basic principle of cell-tracking with MRI; explain the different approaches currently used to monitor cell-based therapies; describe currently available MRI contrast generation mechanisms and strategies for monitoring transplanted cells; discuss some of the challenges in tracking transplanted cells; and suggest future research directions.

Synthesis and evaluation of a series of 6-chloro-4-methylumbelliferyl glycosides as fluorogenic reagents for screening metagenomic libraries for glycosidase activity

Screening of large enzyme libraries such as those derived from metagenomic sources requires sensitive substrates. Fluorogenic glycosides typically offer the best sensitivity but typically must be used in a stopped format to generate good signal. Use of fluorescent phenols of pKa < 7, such as halogenated coumarins, allows direct screening at neutral pH. The synthesis and characterisation of a set of nine different glycosides of 6-chloro-4-methylumbelliferone are described. The use of these substrates in a pooled format for screening of expressed metagenomic libraries yielded a "hit rate" of 1 in 60. Hits were then readily deconvoluted with the individual substrates in a single plate to identify specific activities within each clone. The use of such a collection of substrates greatly accelerates the screening process.

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.

Recent advances in the development of synthetic chemical probes for glycosidase enzymes

The emergence of synthetic glycoconjugates as chemical probes for the detection of glycosidase enzymes has resulted in the development of a range of useful chemical tools with applications in glycobiology, biotechnology, medical and industrial research. Critical to the function of these probes is the preparation of substrates containing a glycosidic linkage that when activated by a specific enzyme or group of enzymes, irreversibly releases a reporter molecule that can be detected. Starting from the earliest examples of colourimetric probes, increasingly sensitive and sophisticated substrates have been reported. In this review we present an overview of the recent advances in this field, covering an array of strategies including chromogenic and fluorogenic substrates, lanthanide complexes, gels and nanoparticles. The applications of these substrates for the detection of various glycosidases and the scope and limitations for each approach are discussed.

Activatable Dendritic 19F Probes for Enzyme Detection

We describe a novel activatable probe for fluorine-19 NMR based on self-assembling amphiphilic dendrons. The dendron probe has been designed to be spectroscopically silent due to the formation of large aggregates. Upon exposure to the specific target enzyme, the aggregates disassemble to give rise to a sharp ¹⁹F NMR signal. The probe is capable of detecting enzyme concentrations in the low nanomolar range. Response time of the probe was found to be affected by the hydrophilic-lipophilic balance of dendrons. Understanding the structural factors that underlie this design principle provides the pathway for using this strategy for a broad range of enzyme-based imaging.

Bioresponsive probes for molecular imaging: concepts and in vivo applications

Molecular imaging is a powerful tool to visualize and characterize biological processes at the cellular and molecular level in vivo. In most molecular imaging approaches, probes are used to bind to disease-specific biomarkers highlighting disease target sites. In recent years, a new subset of molecular imaging probes, known as bioresponsive molecular probes, has been developed. These probes generally benefit from signal enhancement at the site of interaction with its target. There are mainly two classes of bioresponsive imaging probes. The first class consists of probes that show direct activation of the imaging label (from "off" to "on" state) and have been applied in optical imaging and magnetic resonance imaging (MRI). The other class consists of probes that show specific retention of the imaging label at the site of target interaction and these probes have found application in all different imaging modalities, including photoacoustic imaging and nuclear imaging. In this review, we present a comprehensive overview of bioresponsive imaging probes in order to discuss the various molecular imaging strategies. The focus of the present article is the rationale behind the design of bioresponsive molecular imaging probes and their potential in vivo application for the detection of endogenous molecular targets in pathologies such as cancer and cardiovascular disease.

Detecting enzyme activities with exogenous MRI contrast agents

This review focuses on exogenous magnetic resonance imaging (MRI) contrast agents that are responsive to enzyme activity. Enzymes can catalyze a change in water access, rotational tumbling time, the proximity of a (19)F-labeled ligand, the aggregation state, the proton chemical-exchange rate between the agent and water, or the chemical shift of (19)F, (31)P, (13)C or a labile (1)H of an agent, all of which can be used to detect enzyme activity. The variety of agents attests to the creativity in developing enzyme-responsive MRI contrast agents.

Specific lipase-responsive polymer-coated gadolinium nanoparticles for MR imaging of early acute pancreatitis

Currently, available methods for diagnosis of acute pancreatitis (AP) are mainly dependent on serum enzyme analysis and imaging techniques that are too low in sensitivity and specificity to accurately and promptly diagnose AP. The lack of early diagnostic tools highlights the need to search for a highly effective and specific diagnostic method. In this study, we synthesized a conditionally activated, gadolinium-containing, nanoparticle-based MRI nanoprobe as a diagnostic tool for the early identification of AP. Gadolinium diethylenetriaminepentaacetic fatty acid (Gd-DTPA-FA) nanoparticles were synthesized by conjugation of DTPA-FA ligand and gadolinium acetate. Gd-DTPA-FA exhibited low cytotoxicity and excellent biocompatibility when characterized in vitro and in vivo studies. L-arginine induced a gradual increase in the intensity of the T1-weighted MRI signal from 1 h to 36 h in AP rat models. The increase in signal intensity was most significant at 1 h, 6 h and 12 h. These results suggest that the Gd-DTPA-FA as an MRI contrast agent is highly efficient and specific to detect early AP.

In vivo magnetic resonance imaging of mice liver tumors using a new gadolinium-based contrast agent

We compared the enhancement effect between a newly synthesized tissue-specific contrast agent, [Gd-DOTA-FPβG], and a commercially available agent, [Gd(DOTA)](-), in a murine model of liver tumor using a clinical magnetic resonance imaging scanner. The colon cancer cell lines with and without β-glucuronidase (βG) expression were implanted into the liver of mice. Self-synthesized gadolinium-based magnetic resonance contrast agent, [Gd(DOTA-FPβG)], was administered to measure enhancement on magnetic resonance images using a commercially available agent, [Gd(DOTA)](-), as control in a clinical 3.0 tesla (T) magnetic resonance scanner. In vivo fluorescence imaging and histopathology of the liver were also performed to compare and correlate with the magnetic resonance studies. The in vivo fluorescence imaging failed to depict a sufficiently intense signal for liver or liver tumor of mice without exposure of the liver following an incision on the abdominal wall. The tissue-specific magnetic resonance agent, [Gd(DOTA-FPβG)], caused significantly stronger enhancement in tumors expressing βG (CT26/mβG-eB7) than in tumors not expressing βG (CT26) (p < 0.05). In the magnetic resonance imaging studies using control agent [Gd(DOTA)](-), the tumors with and without βG expression depicted no significant difference in enhancement on the T1-weighted images. The [Gd(DOTA-FPβG)] also provided significantly more contrast uptake in the CT26/mβG-eB7 tumor than in the normal liver parenchyma, whereas the [Gd(DOTA)](-) did not. This study confirms that better contrast enhancement can be readily detected in vivo by the use of a tissue-specific magnetic resonance contrast agent to target tumor cells with specific biomarkers in a clinical magnetic resonance imaging scanner.

Novel S-Gal(®) analogs as (1)H MRI reporters for in vivo detection of β-galactosidase

The quantitative assessment of gene expression and related enzyme activity in vivo could be important for the characterization of gene altering diseases and therapy. The development of imaging techniques, based on specific reporter molecules may enable routine non-invasive assessment of enzyme activity and gene expression in vivo. We recently reported the use of commercially available S-Gal(®) as a β-galactosidase reporter for (1)H MRI, and the synthesis of several S-Gal(®) analogs with enhanced response to β-galactosidase activity. We have now compared these analogs in vitro and have identified the optimal analog, C3-GD, based on strong T1 and T2 response to enzyme presence (ΔR1 and ΔR2~1.8 times S-Gal(®)). Moreover, application is demonstrated in vivo in human breast tumor xenografts. MRI studies in MCF7-lacZ tumors implanted subcutaneously in athymic nude mice (n=6), showed significant reduction in T1 and T2 values (each~13%) 2h after intra-tumoral injection of C3-GD, whereas the MCF7 (wild type) tumors showed slight increase. Thus, C3-GD successfully detects β-galactosidase activity in vivo and shows promise as a lacZ gene (1)H MR reporter molecule.

Novel molecular platform integrated iron chelation therapy for 1H-MRI detection of β-galactosidase activity

Targeting the increased Fe(3+) content in tumors, we propose a novel molecular platform integrated cancer iron chelation therapy for (1)H-magnetic resonance imaging (MRI) detection of β-galactosidase (β-gal) activity. Following this idea, we have designed, synthesized, and characterized a series of β-d-galactosides conjugated with various chelators and demonstrated the feasibility of this concept for assessing β-gal activity in solution by (1)H-MRI T1 and T2 relaxation mapping.

A smart (19) F and (1) H MRI probe with self-immolative linker as a versatile tool for detection of enzymes

Here we report on a dual-modal (19) F and (1) H MRI paramagnetic probe with a self-immolative linker, Gd-DOMF-Gal. The enzymatic conversion of this probe by β-galactosidase resulted in a simultaneous turning on of the fluorine signal and changed ability of the Gd(3+) complex to modulate the (1) H MR signal intensity of the surrounding water molecules. A versatile imaging platform for monitoring a variety of enzymes by (19) F and (1) H MRI using this molecular design is proposed.

Development of a Gd(III)-based receptor-induced magnetization enhancement (RIME) contrast agent for β-glucuronidase activity profiling

β-Glucuronidase is a key lysosomal enzyme and is often overexpressed in necrotic tumor masses. We report here the synthesis of a pro receptor-induced magnetization enhancement (pro-RIME) magnetic resonance imaging (MRI) contrast agent ([Gd(DOTA-FPβGu)]) for molecular imaging of β-glucuronidase activity in tumor tissues. The contrast agent consists of two parts, a gadolinium complex and a β-glucuronidase substrate (β-d-glucopyranuronic acid). The binding association constant (KA) of [Gd(DOTA-FPβGu)] is 7.42 × 10(2), which is significantly lower than that of a commercially available MS-325 (KA = 3.0 × 10(4)) RIME contrast agent. The low KA value of [Gd(DOTA-FPβGu)] is due to the pendant β-d-glucopyranuronic acid moiety. Therefore, [Gd(DOTA-FPβGu)] can be used for detection of β-glucuronidase through RIME modulation. The detail mechanism of enzymatic activation of [Gd(DOTA-FPβGu)] was elucidated by LC-MS. The kinetics of β-glucuronidase catalyzed hydrolysis of [Eu(DOTA-FPβGu)] at pH 7.4 best fit the Miechalis-Menten kinetic mode with Km = 1.38 mM, kcat = 3.76 × 10(3), and kcat/Km = 2.72 × 10(3) M(-1) s(-1). The low Km value indicates high affinity of β-glucuronidase for [Gd(DOTA-FPβGu)] at physiological pH. Relaxometric studies revealed that T1 relaxivity of [Gd(DOTA-FPβGu)] changes in response to the concentration of β-glucuronidase. Consistent with the relaxometric studies, [Gd(DOTA-FPβGu)] showed significant change in MR image signal in the presence of β-glucuronidase and HSA. In vitro and in vivo MR images demonstrated appreciable differences in signal enhancement in the cell lines and tumor xenografts in accordance to their expression levels of β-glucuronidase.

Simple and real-time colorimetric assay for glycosidases activity using functionalized gold nanoparticles and its application for inhibitor screening

The development of real-time assays for enzymes has been receiving a great deal of attention in biomedical research recently. Self-immolative elimination is the spontaneous and irreversible disassembly of a multicomponent construct into its constituent fragments through a cascade of elimination processes, in response to external stimuli. Here, we report a simple and real-time colorimetric assay for glycosidases (β-galactosidase and β-glucosidase). Self-immolative elimination was utilized to release amines to give rise to aggregation and color change by electrostatic attraction after cleavage of the trigger by enzymes displayed on functionalized gold nanoparticles (Gal-Lip-AuNPs and Glc-Lip-AuNPs, where AuNPs denotes gold nanoparticles). The detection limits for β-galactosidase and β-glucosidase were as low as 9.2 and 22.3 nM at 20 min, and they improved slightly over time. Thus, glycosidase activity was detected successfully in real time, and this technique could be used for glycosidase inhibitor screening, based on real-time colorimetric variation.

Novel Fe3+-Based 1H MRI β-Galactosidase Reporter Molecules**

There is increasing interest in the development of reporter agents to reveal enzyme activity in vivo using small animal imaging. We have previously demonstrated the feasibility of detecting lacZ gene activity using the commercially available 3,4-cyclohexenoesculetin-β-D-galactopyranoside (S-Gal™) as a 1H MRI reporter. Specifically, β-galactosidase (β-gal) releases the aglycone, which forms an MR contrast-inducing paramagnetic precipitate in the presence of Fe3+. Contrast was primarily T2-weighted signal loss, but T1 effects were also observed. Since T1-contrast generally provides signal enhancement as opposed to loss, it appeared attractive to explore whether analogues could be generated with enhanced characteristics. We now report the design and successful synthesis of novel analogues together with characterization of 1H MRI contrast based on both T1 and T2 response to β-gal activity in vitro for the lead agent.

A cell-permeable gadolinium contrast agent for magnetic resonance imaging of copper in a Menkes disease model

We present the synthesis and characterization of octaarginine-conjugated Copper-Gad-2 (Arg₈CG2), a new copper-responsive magnetic resonance imaging (MRI) contrast agent that combines a Gd³⁺-DO3A scaffold with a thioether-rich receptor for copper recognition. The inclusion of a polyarginine appendage leads to a marked increase in cellular uptake compared to previously reported MRI-based copper sensors of the CG family. Arg₈CG2 exhibits a 220% increase in relaxivity (r₁ = 3.9 to 12.5 mM^-1 s^-1) upon 1 : 1 binding with Cu⁺, with a highly selective response to Cu⁺ over other biologically relevant metal ions. Moreover, Arg₈CG2 accumulates in cells at nine-fold greater concentrations than the parent CG2 lacking the polyarginine functionality and is retained well in the cell after washing. In cellulo relaxivity measurements and T₁-weighted phantom images using a Menkes disease model cell line demonstrate the utility of Arg₈CG2 to report on biological perturbations of exchangeable copper pools.

Is there a path beyond BOLD? Molecular imaging of brain function

The dependence of BOLD on neuro-vascular coupling leaves it many biological steps removed from direct monitoring of neural function. MRI based approaches have been developed aimed at reporting more directly on brain function. These include: manganese enhanced MRI as a surrogate for calcium ion influx; agents responsive to calcium concentrations; approaches to measure membrane potential; agents to measure neurotransmitters; and strategies to measure gene expression. This work has led to clever design of molecular imaging tools and many contributions to studies of brain function in animal models. However, a robust approach that has potential to get MRI closer to neurons in the human brain has not yet emerged.

Dual 19F/1H MR gene reporter molecules for in vivo detection of β-galactosidase

Increased emphasis on personalized medicine and novel therapies requires the development of noninvasive strategies for assessing biochemistry in vivo. The detection of enzyme activity and gene expression in vivo is potentially important for the characterization of diseases and gene therapy. Magnetic resonance imaging (MRI) is a particularly promising tool, since it is noninvasive and has no associated radioactivity, yet penetrates deep tissue. We now demonstrate a novel class of dual (1)H/(19)F nuclear magnetic resonance (NMR) lacZ gene reporter molecule to specifically reveal enzyme activity in human tumor xenografts growing in mice. We report the design, synthesis, and characterization of six novel molecules and evaluation of the most effective reporter in mice in vivo. Substrates show a single (19)F NMR signal and exposure to β-galactosidase induces a large (19)F NMR chemical shift response. In the presence of ferric ions, the liberated aglycone generates intense proton MRI T(2) contrast. The dual modality approach allows both the detection of substrate and the imaging of product enhancing the confidence in enzyme detection.

Lanthanide(III) complexes that contain a self-immolative arm: potential enzyme responsive contrast agents for magnetic resonance imaging

Enzyme-responsive MRI-contrast agents containing a "self-immolative" benzylcarbamate moiety that links the MRI-reporter lanthanide complex to a specific enzyme substrate have been developed. The enzymatic cleavage initiates an electronic cascade reaction that leads to a structural change in the Ln(III) complex, with a concomitant response in its MRI-contrast-enhancing properties. We synthesized and investigated a series of Gd(3+) and Yb(3+) complexes, including those bearing a self-immolative arm and a sugar unit as selective substrates for β-galactosidase; we synthesized complex LnL(1), its NH(2) amine derivatives formed after enzymatic cleavage, LnL(2), and two model compounds, LnL(3) and LnL(4). All of the Gd(3+) complexes synthesized have a single inner-sphere water molecule. The relaxivity change upon enzymatic cleavage is limited (3.68 vs. 3.15 mM(-1) s(-1) for complexes GdL(1) and GdL(2), respectively; 37 °C, 60 MHz), which prevents application of this system as an enzyme-responsive T(1) relaxation agent. Variable-temperature (17)O NMR spectroscopy and (1)H NMRD (nuclear magnetic relaxation dispersion) analysis were used to assess the parameters that determine proton relaxivity for the Gd(3+) complexes, including the water-exchange rate (k(ex)(298), varies in the range 1.5-3.9×10(6) s(-1)). Following the enzymatic reaction, the chelates contain an exocyclic amine that is not protonated at physiological pH, as deduced from pH-potentiometric measurements (log K(H)=5.12(±0.01) and 5.99(±0.01) for GdL(2) and GdL(3), respectively). The Yb(3+) analogues show a PARACEST effect after enzymatic cleavage that can be exploited for the specific detection of enzymatic activity. The proton-exchange rates were determined at various pH values for the amine derivatives by using the dependency of the CEST effect on concentration, saturation time, and saturation power. A concentration-independent analysis of the saturation-power-dependency data was also applied. All these different methods showed that the exchange rate of the amine protons of the Yb(III) complexes decreases with increasing pH value (for YbL(3), k(ex)=1300 s(-1) at pH 8.4 vs. 6000 s(-1) at pH 6.4), thereby resulting in a diminution of the observed CEST effect.

β-Gal gene expression MRI reporter in melanoma tumor cells. Design, synthesis, and in vitro and in vivo testing of a Gd(III) containing probe forming a high relaxivity, melanin-like structure upon β-Gal enzymatic activation

The aim of this work is to design and test an MRI probe (Gd-DOTAtyr-gal) able to report on the gene expression of β-galactosidase (β-Gal) in melanoma cells. The probe consists of a Gd-DOTA reporter bearing on its surface a tyrosine-galactose-pyranose functionality that, upon the release of the sugar moiety, readily transforms, in the presence of tyrosinase, into melanin oligomeric/polymeric mixture. The formation of Gd-DOTA-containing melanin oligomers and polymers is accompanied by a marked increase of the water proton relaxation rate. The steps involving the release of the galactose-pyranose group and the formation of the melanin-like structure have been carefully investigated in vitro by relaxometric and UV-vis measurements. Cellular uptake studies of Gd-DOTAtyr-gal by melanoma cells have shown that the probe enters the cells, and it appears not to be confined in endosomal vesicles. Using B16-F10LacZ transfected cells, the fast formation of paramagnetic melanin-Gd(III)-containing species has been assessed by the measurement of increased longitudinal relaxation rates of the cellular pellets suspensions. The in vitro results have been confirmed in in vivo MRI investigations on murine melanoma tumor bearing mice. Upon direct injection of Gd-DOTAtyr-gal, a good contrast is observed after 5 h post injection in B16-F10LacZ tumors, but not in B16-F10 tumors lacking the β-Gal enzyme. Gd-DOTAtyr-gal in combination with tyrosinase introduces a novel approach for the detection of β-Gal expression by MRI in vivo.

Activatable T₁ and T₂ magnetic resonance imaging contrast agents

Magnetic resonance imaging (MRI) has become one of the most important diagnosis tools available in medicine. Typically MRI is not capable of sensing biochemical activities. However, recently emerged activatable MRI contrast agents (CAs), whose relaxivity is variable in response to a specific parameter change in the surrounding physiological microenvironment, potentially allow for MRI to indicate biological processes. Among the various factors influencing the relaxivity of a CA, the number of inner-sphere water molecules (q) directly coordinated to the metal center, the residence time of the coordinated water molecule (τ (m)), and the rotational correlation time representing the molecular tumbling time of a complex (τ (R)) contribute strongly to the relaxivity of an activatable CA. Tuning the ligand structure and properties has been the subject of intensive research for activatable MR CA designs. This review summarizes a variety of activatable MRI CAs sensitive to common variables in microenvironment in vivo, i.e., pH, luminescence, metal ions, redox, and enzymes, etc., with emphasis on the influence of ligand design on parameters q, τ (m), and τ (R).

Imaging beta-galactosidase activity in human tumor xenografts and transgenic mice using a chemiluminescent substrate

Background

Detection of enzyme activity or transgene expression offers potential insight into developmental biology, disease progression, and potentially personalized medicine. Historically, the lacZ gene encoding the enzyme β-galactosidase has been the most common reporter gene and many chromogenic and fluorogenic substrates are well established, but limited to histology or in vitro assays. We now present a novel approach for in vivo detection of β-galactosidase using optical imaging to detect light emission following administration of the chemiluminescent 1,2-dioxetane substrate Galacto-Light PlusTM.

Methodology and Principal Findings

B-gal activity was visualized in stably transfected human MCF7-lacZ tumors growing in mice. LacZ tumors were identified versus contralateral wild type tumors as controls, based on two- to tenfold greater light emission following direct intra tumoral or intravenous administration of reporter substrate. The 1,2-dioxetane substrate is commercially available as a kit for microplate-based assays for β-gal detection, and we have adapted it for in vivo application. Typically, 100 µl substrate mixture was administered intravenously and light emission was detected from the lacZ tumor immediately with gradual decrease over the next 20 mins. Imaging was also undertaken in transgenic ROSA26 mice following subcutaneous or intravenous injection of substrate mixture.

Conclusion and Significance

Light emission was detectable using standard instrumentation designed for more traditional bioluminescent imaging. Use of 1,2-dioxetane substrates to detect enzyme activity offers a new paradigm for non-invasive biochemistry in vivo.

S-Gal, a novel 1H MRI reporter for beta-galactosidase

Reporter genes and associated enzyme activity are becoming increasingly significant for research in vivo. The lacZ gene and beta-galactosidase (beta-gal) expression have long been exploited as reporters of biologic manipulation at the molecular level, and a noninvasive detection strategy based on proton MRI is particularly attractive. 3,4-Cyclohexenoesculetin beta-D-galactopyranoside (S-Gal) is a commercial histologic stain, which forms a black precipitate in the presence of beta-gal and ferric ions, suggesting potential detectability by MRI. Generation of the precipitate is now shown to cause strong T(2)* relaxation, revealing beta-gal activity. A series of tests with the enzyme in vitro and with tumor cells shows that this approach can be used as an assay for beta-gal activity. Proof of principle is shown in human breast tumor xenografts in mice. Upon direct injection of a mixture of 3,4-cyclohexenoesculetin beta-D-galactopyranoside and ferric ammonium citrate, intense contrast was observed immediately in MCF7-lacZ tumors, but not in wild-type tumors. 3,4-Cyclohexenoesculetin beta-D-galactopyranoside activation in combination with ferric ions introduces a novel approach for assaying enzyme activity by MRI in vivo.

Molecular imaging of in vivo gene expression

Molecular imaging provides spatial and temporal information on cellular changes that occur during development and in disease. MRI and optical imaging of reporter genes allows for the visualization of promoter activity, protein-protein interactions, protein stability and the tracking of individual proteins and cells. Reporter genes can be genetically encoded in transgenic animals or detected through the administration of an exogenous contrast agent. Advances in molecular imaging of reporter genes have led to the development of imaging probes that detect changes in endogenous cellular changes. The ability to use contrast agents coupled with functional information on cellular events will allow for sensitive assessment of individual patient therapies, leading to an accurately tailored treatment regimen.

Imaging the tissue distribution of glucose in livers using a PARACEST sensor

Noninvasive imaging of glucose in tissues could provide important insights about glucose gradients in tissue, the origins of gluconeogenesis, or perhaps differences in tissue glucose utilization in vivo. Direct spectral detection of glucose in vivo by (1)H NMR is complicated by interfering signals from other metabolites and the much larger water signal. One potential way to overcome these problems is to use an exogenous glucose sensor that reports glucose concentrations indirectly through the water signal by chemical exchange saturation transfer (CEST). Such a method is demonstrated here in mouse liver perfused with a Eu(3+)-based glucose sensor containing two phenylboronate moieties as the recognition site. Activation of the sensor by applying a frequency-selective presaturation pulse at 42 ppm resulted in a 17% decrease in water signal in livers perfused with 10 mM sensor and 10 mM glucose compared with livers with the same amount of sensor but without glucose. It was shown that livers perfused with 5 mM sensor but no glucose can detect glucose exported from hepatocytes after hormonal stimulation of glycogenolysis. CEST images of livers perfused in the magnet responded to changes in glucose concentrations demonstrating that the method has potential for imaging the tissue distribution of glucose in vivo.