Detection of enzymatic activity by PARACEST MRI: a general approach to target a large variety of enzymes

Lanthanide-Based Probes for Imaging Detection of Enzyme Activities by NIR Luminescence, T1- and ParaCEST MRI

Applying a single molecular probe to monitor enzymatic activities in multiple, complementary imaging modalities is highly desirable to ascertain detection and to avoid the complexity associated with the use of agents of different chemical entities. We demonstrate here the versatility of lanthanide (Ln3+) complexes with respect to their optical and magnetic properties and their potential for enzymatic detection in NIR luminescence, CEST and T1 MR imaging, controlled by the nature of the Ln3+ ion, while using a unique chelator. Based on X-ray structural, photophysical, and solution NMR investigations of a family of Ln3+ DO3A-pyridine model complexes, we could rationalize the luminescence (Eu3+, Yb3+), CEST (Yb3+) and relaxation (Gd3+) properties and their variations between carbamate and amine derivatives. This allowed the design ofL n L G a l 5 ${{{\bf L n L}}_{{\bf G a l}}^{5}}$ probes which undergo enzyme-mediated changes detectable in NIR luminescence, CEST and T1-weighted MRI, respectively governed by variations in their absorption energy, in their exchanging proton pool and in their size, thus relaxation efficacy. We demonstrate that these properties can be exploited for the visualization of β-galactosidase activity in phantom samples by different imaging modalities: NIR optical imaging, CEST and T1-weighted MRI.

A near-infrared fluorescent probe with a substantial Stokes shift designed for the detection and imaging of β-galactosidase within living cells and animals

β-Galactosidase serves as a pivotal biomarker for both cancer and cellular aging. The advancement of fluorescent sensors for tracking β-galactosidase activity is imperative in the realm of cancer diagnosis. We have designed a near-infrared fluorescent probe (PTA-gal) for the detection of β-galactosidase in living systems with large Stokes shifts. PTA-gal exhibits remarkable sensitivity and selectivity in detecting β-galactosidase, producing near-infrared fluorescent signals with a remarkably low detection limit (2.2 × 10-5 U/mL) and a high quantum yield (0.30). Moreover, PTA-gal demonstrates biocompatibility and can effectively detect β-galactosidase in cancer cells as well as within living animals.

The Development of a Smart Magnetic Resonance Imaging and Chemical Exchange Saturation Transfer Contrast Agent for the Imaging of Sulfatase Activity

The molecular imaging of biomarkers plays an increasing role in medical diagnostics. In particular, the imaging of enzyme activity is a promising approach, as it enables the use of its inherent catalytic activity for the amplification of an imaging signal. The increased activity of a sulfatase enzyme has been observed in several types of cancers. We describe the development and in vitro evaluation of molecular imaging agents that allow for the detection of sulfatase activity using the whole-body, non-invasive MRI and CEST imaging methods. This approach relies on a responsive ligand that features a sulfate ester moiety, which upon sulfatase-catalyzed hydrolysis undergoes an elimination process that changes the functional group, coordinating with the metal ion. When Gd3+ is used as the metal, the complex can be used for MRI, showing a 25% decrease at 0.23T and a 42% decrease at 4.7T in magnetic relaxivity after enzymatic conversion, thus providing a "switch-off" contrast agent. Conversely, the use of Yb3+ as the metal leads to a "switch-on" effect in the CEST imaging of sulfatase activity. Altogether, the results presented here provide a molecular basis and a proof-of-principle for the magnetic imaging of the activity of a key cancer biomarker.

Protein and peptide engineering for chemical exchange saturation transfer imaging in the age of synthetic biology

At the beginning of the millennium, the first chemical exchange saturation transfer (CEST) contrast agents were bio-organic molecules. However, later, metal-based CEST agents (paraCEST agents) took center stage. This did not last too long as paraCEST agents showed limited translational potential. By contrast, the CEST field gradually became dominated by metal-free CEST agents. One branch of research stemming from the original work by van Zijl and colleagues is the development of CEST agents based on polypeptides. Indeed, in the last 2 decades, tremendous progress has been achieved in this field. This includes the design of novel peptides as biosensors, genetically encoded recombinant as well as synthetic reporters. This was a result of extensive characterization and elucidation of the theoretical requirements for rational designing and engineering of such agents. Here, we provide an extensive overview of the evolution of more precise protein-based CEST agents, review the rationalization of enzyme-substrate pairs as CEST contrast enhancers, discuss the theoretical considerations to improve peptide selectivity, specificity and enhance CEST contrast. Moreover, we discuss the strong influence of synthetic biology on the development of the next generation of protein-based CEST contrast agents.

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.

Theoretical Study of Complexes of Tetravalent Actinides with DOTA

1,4,7,10-Tetraazacyclododecane-N,N′,N″,N‴-tetraacetic acid (H4DOTA) is a prominent chelating ligand with potential applications in various fields, from radiotherapy to the separation of fission products. The present study explores the stability, structure, and bonding properties of its complexes with tetravalent actinides (An = Th, U, Np, Pu) using density functional theory and relativistic multireference calculations. Neutral complexes prefer to form symmetric (C4) structures with DOTA. The first coordination sphere of the actinide ions is readily saturated by a weakly bonded H2O ligand. The latter ligand reduces the molecular symmetry while exerting only marginal effects on the properties of the parent complex. An-ligand bonding is mainly electrostatic, but there are also significant charge-transfer contributions from DOTA to the An 6d/5f orbitals. The charge-transfer interactions and the covalent character of bonding increase gradually in the order of Th < U < Np < Pu, as indicated by analysis of the electron density distribution using the Quantum Theory of Atoms in Molecules.

Simultaneous Evaluations of pH and Enzyme Activity with a CEST MRI Contrast Agent

The extracellular tumor microenvironment of many solid tumors has high acidosis and high protease activity. Simultaneously assessing both characteristics may improve diagnostic evaluations of aggressive tumors and the effects of anticancer treatments. Noninvasive imaging methods have previously been developed that measure extracellular pH or can detect enzyme activity using chemical exchange saturation transfer (CEST) magnetic resonance imaging (MRI). Herein, we developed a single-hybrid CEST agent that can simultaneously measure pH and evaluate protease activity using a combination of dual-power acidoCEST MRI and catalyCEST MRI. Our agent showed CEST signals at 9.2 ppm from a salicylic acid moiety and at 5.0 ppm from an aryl amide. The CEST signal at 9.2 ppm could be measured after selective saturation was applied at 1 and 4 μT, and these measurements could be used with a ratiometric analysis to determine pH. The CEST signal at 5.0 ppm from the aryl amide disappeared after the agent was treated with cathepsin B, while the CEST signal at 9.2 ppm remained, indicating that the agent could detect protease activity through the amide bond cleavage. Michaelis-Menten kinetics studies with catalyCEST MRI demonstrated that the binding affinity (as shown with the Michaelis constant K_M), the catalytic turnover rate (k_cat), and catalytic efficiency (k_cat/K_M) were each higher for cathepsin B at lower pH. The k_cat rates measured with catalyCEST MRI were lower than the comparable rates measured with liquid chromatography-mass spectrometry (LC-MS), which reflected a limitation of inherently noisy and relatively insensitive CEST MRI analyses. Although this level of precision limited catalyCEST MRI to semiquantitative evaluations, these semiquantitative assessments of high and low protease activity still had value by demonstrating that high acidosis and high protease activity can be used as synergistic, multiparametric biomarkers.

Theoretical Study of Actinide(III)-DOTA Complexes

1,4,7,10-Tetraazacyclododecane-N,N',N″,N‴-tetraacetic acid (DOTA) is a prominent chelating ligand used in imaging contrast agents and radiopharmaceuticals. The present study explores the stabilities, structures, and bonding properties of its complexes with trivalent actinides (Ac, U, Np, Pu, Am, Cm, Cf) using density functional theory and relativistic multireference calculations. For reference purposes, the La- and Lu-DOTA complexes are also included. Similar to La3+, the large An3+ ions prefer the TSAP conformer of the ligand. The An-ligand bonding is mainly electrostatic, with minor charge transfer contributions to the An 6d orbitals. For the assessment of the thermodynamic stabilities in aqueous solution, PCM radii to use in conjunction with the SMD solvation model were developed. Basically, the thermodynamic stability of the DOTA complexes increases along the An row but with notable counteracting of spin-orbit coupling.

Lanthanide DO3A-Complexes Bearing Peptide Substrates: The Effect of Peptidic Side Chains on Metal Coordination and Relaxivity

Two DO3A-type ligands conjugated to substrates of urokinase (L3) and caspase-3 (L4) via a propyl-amide linker were synthesized and their lanthanide(III) (Ln3+) complexes studied. A model compound without peptide substrate (L2) and an amine derivative ligand mimicking the state after enzymatic cleavage (L1) were also prepared. Proton Nuclear Magnetic Relaxation Dispersion (NMRD) profiles recorded on the gadolinium(III) (Gd3+) complexes, complemented with the assessment of hydration numbers via luminescence lifetime measurements on the Eu3+ analogues, allowed us to characterize the lanthanide coordination sphere in the chelates. These data suggest that the potential donor groups of the peptide side chains (carboxylate, amine) interfere in metal coordination, leading to non-hydrated LnL3 and LnL4 complexes. Nevertheless, GdL3 and GdL4 retain a relatively high relaxivity due to an important second-sphere contribution generated by the strongly hydrophilic peptide chain. Weak PARACEST effects are detected for the amine-derivative EuL1 and NdL1 chelates. Unfortunately, the GdL3 and GdL4 complexes are not significantly converted by the enzymes. The lack of enzymatic recognition of these complexes can likely be explained by the participation of donor groups from the peptide side chain in metal coordination.

Evaluation of a similarity anisotropic diffusion denoising approach for improving in vivo CEST-MRI tumor pH imaging

PURPOSE

Chemical exchange saturation transfer MRI provides new approaches for investigating tumor microenvironment, including tumor acidosis that plays a key role in tumor progression and resistance to therapy. Following iopamidol injection, the detection of the contrast agent inside the tumor tissue allows measurements of tumor extracellular pH. However, accurate tumor pH quantifications are hampered by the low contrast efficiency of the CEST technique and by the low SNR of the acquired CEST images, hence in a reduced detectability of the injected agent. This work aims to investigate a novel denoising method for improving both tumor pH quantification and accuracy of CEST-MRI pH imaging.

METHODS

An hybrid denoising approach was investigated for CEST-MRI pH imaging based on the combination of the nonlocal mean filter and the anisotropic diffusion tensor method. The denoising approach was tested in simulated and in vitro data and compared with previously reported methods for CEST imaging and with established denoising approaches. Finally, it was validated with in vivo data to improve the accuracy of tumor pH maps.

RESULTS

The proposed method outperforms current denoising methods in CEST contrast quantification and detection of the administered contrast agent at several increasing noise levels with simulated data. In addition, it achieved a better pH quantification in in vitro data and demonstrated a marked improvement in contrast detection and a substantial improvement in tumor pH accuracy in in vivo data.

CONCLUSION

The proposed approach effectively reduces the noise in CEST images and increases the sensitivity detection in CEST-MRI pH imaging.

Preclinical Applications of Magnetic Resonance Imaging in Oncology

The evolving possibilities of molecular imaging (MI) are fundamentally changing the way we look at cancer, with imaging paradigms now shifting away from basic morphological measures toward the longitudinal assessment of functional, metabolic, cellular, and molecular information in vivo. Recent developments of imaging methodology and probe molecules utilizing the vast number of novel animal models of human cancers have enhanced our ability to non-invasively characterize neoplastic tissue and follow anticancer treatments. While preclinical molecular imaging offers a whole palette of excellent methodology to choose from, we will focus on magnetic resonance imaging (MRI) techniques, since they provide excellent molecular imaging capabilities and bear high potential for clinical translation. Prerequisites and consequences of using animal models as surrogates of human cancers in preclinical molecular imaging are outlined. We present physical principles, values, and limitations of MRI as molecular imaging modality and comment on its high potential to non-invasively assess information on metabolism, hypoxia, angiogenesis, and cell trafficking in preclinical cancer research.

Macrocyclic Pyclen-Based Gd3+ Complex with High Relaxivity and pH Response

We report the synthesis and characterization of the macrocyclic ligand 2,2'-((2-(3,9-bis(carboxymethyl)-3,6,9-triaza-1(2,6)-pyridinacyclodecaphane-6-yl)ethyl)azanediyl)diacetic acid (H₄L) and several of its complexes with lanthanide ions. The structure of the free ligand was determined using X-ray diffraction measurements. Two N atoms of the pyclen moiety in the trans position are protonated in the solid state, together with the exocyclic N atom and one of the carboxylate groups of the ligand. The relaxivity of the Gd³⁺ complex was found to increase from 6.7 mM^-1 s^-1 at pH 8.6 to 8.5 mM^-1 s^-1 below pH ≈ 6.0. Luminescence lifetime measurements recorded from H₂O and D₂O solutions of the Eu³⁺ complex evidence the presence of a single complex species in solution at low pH (∼5.0) that contains two inner-sphere water molecules. DFT calculations suggest that the coordination environment of the Ln³⁺ ion is fulfilled by the four N atoms of the pyclen unit, two oxygen atoms of the macrocyclic acetate groups, and an oxygen atom of an exocyclic carboxylate group. The two inner-sphere water molecules complete coordination number nine around the metal ion. At high pH (∼9.3), the lifetime of the excited ⁵D₀ level of Eu³⁺ displays a biexponential behavior that can be attributed to the presence of two species in solution with hydration numbers of q = 0 and q = 1. The ¹H NMR and DOSY spectra recorded from solutions of the Eu³⁺ and Y³⁺ complexes reveal a structural change triggered by pH and the formation of small aggregates at high pH values.

Applications for Transition-Metal Chemistry in Contrast-Enhanced Magnetic Resonance Imaging

Contrast-enhanced magnetic resonance imaging (MRI) is an indispensable tool for diagnostic medicine. However, safety concerns related to gadolinium in commercial MRI contrast agents have emerged in recent years. For patients suffering from severe renal impairment, there is an important unmet medical need to perform contrast-enhanced MRI without gadolinium. There are also concerns over the long-term effects of retained gadolinium within the general patient population. Demand for gadolinium-free MRI contrast agents is driving a new wave of inorganic chemistry innovation as researchers explore paramagnetic transition-metal complexes as potential alternatives. Furthermore, advances in personalized care making use of molecular-level information have motivated inorganic chemists to develop MRI contrast agents that can detect pathologic changes at the molecular level. Recent studies have highlighted how reaction-based modulation of transition-metal paramagnetism offers a highly effective mechanism to achieve MRI contrast enhancement that is specific to biochemical processes. This Viewpoint highlights how recent advances in transition-metal chemistry are leading the way for a new generation of MRI contrast agents.

Inert macrocyclic Eu3+ complex with affirmative paraCEST features

Highly kinetically inert paramagnetic platform shows outstanding CEST properties suitable for advantageous MRI applications.

Assessments of tumor metabolism with CEST MRI

Chemical exchange saturation transfer (CEST) is a relatively new contrast mechanism for MRI. CEST MRI exploits a specific MR frequency (chemical shift) of a molecule while generating an image with good spatial resolution using standard MRI techniques, combining the specificity of MRS with the spatial resolution of MRI. Many CEST MRI acquisition methods have been developed to improve analyses of tumor metabolism. GluCEST, CrCEST, and LATEST can map glutamate, creatine, and lactate, which are important metabolites involved in tumor metabolism. GlucoCEST MRI tracks the pharmacokinetics of glucose transport and cell internalization within tumors. CatalyCEST MRI detects enzyme catalysis that changes a substrate CEST agent. AcidoCEST MRI measures extracellular pH of the tumor microenvironment by exploiting a ratio of two pH-dependent CEST signals. This review describes each technique, the technical issues involved with CEST MRI and each specific technique, and the merits and challenges associated with applying each CEST MRI technique to study tumor metabolism.

Responsive ParaCEST Contrast Agents

This article aimed at reviewing the advances on the development of paramagnetic complexes used as chemical exchange saturation transfer agents in magnetic resonance imaging. This relatively new type of contrast opens new avenues in the development of MRI probes for molecular imaging, and coordination chemistry lies at the center of such advances. Strategies to detect important biomarkers such as pH, cations, anions, metabolites, enzyme, and O2 were described. The current challenges, limitations, and opportunities in this field of research were discussed.

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.

Dramatic enhancement in pH sensitivity and signal intensity through ligand modification of a dicobalt PARACEST probe

A ratiometric dicobalt PARACEST pH probe that exhibits dramatic enhancements in sensitivity and signal intensity over analogous probes is reported.

Water exchange in lanthanide complexes for MRI applications. Lessons learned over the last 25 years

Coordination chemistry offers convenient strategies to modulate the exchange of coordinated water molecules in lanthanide-based contrast agents.

An intracellular diamine oxidase triggered hyperpolarized 129Xe magnetic resonance biosensor

Here, a novel method was developed for suppressing 129Xe signals in cucurbit[6]uril (CB6) until the trigger is activated by a specific enzyme. Due to its noncovalent interactions with amino-groups and CB6, putrescine dihydrochloride (Put) was chosen for blocking interactions between 129Xe and CB6. Upon adding diamine oxidase (DAO), Put was released from CB6 and a 129Xe@CB6 Hyper-CEST signal emerged. This proposed 129Xe biosensor was then tested in small intestinal villus epithelial cells.

Protonation of carboxyl groups in EuDOTA-tetraamide complexes results in catalytic prototropic exchange and quenching of the CEST signal

The CEST properties of EuDOTA-tetraamide complexes bearing pendant carboxylate and carboxyl ethyl esters were measured as a function of pH. The CEST signal from the Eu3+-bound water molecule decreased in intensity between pH 8.5 and 4.5 while the proton exchange rates (kex) increased over this same pH range. In comparison, the CEST signal in the corresponding carboxyl ester derivatives was nearly constant. Both observations are consistent with stepwise protonation of the four carboxylic acid groups over this same pH range. This indicates that negative charges on the carboxyl groups above pH 6 facilitate the formation of a strong hydrogen-bonding network in the coordination second sphere above the single Eu3+-bound water molecule, thereby decreasing prototropic exchange of protons on the bound water molecule with bulk water protons. The percentage of square antiprismatic versus twisted square antiprism coordination isomers also decreased as the appended carboxylic acid groups were positioned further away from the amide. The net effect of lowering the pH was an overall increase in kex and a quenching of the CEST signal.This article is part of the themed issue 'Challenges for chemistry in molecular imaging'.

Diamagnetic Imaging Agents with a Modular Chemical Design for Quantitative Detection of β-Galactosidase and β-Glucuronidase Activities with CatalyCEST MRI

Imaging agents for the noninvasive in vivo detection of enzyme activity in preclinical and clinical settings could have fundamental implications in the field of drug discovery. Furthermore, a new class of targeted prodrug treatments takes advantage of high enzyme activity to tailor therapy and improve treatment outcomes. Herein, we report the design and synthesis of new magnetic resonance imaging (MRI) agents that quantitatively detect β-galactosidase and β-glucuronidase activities by measuring changes in chemical exchange saturation transfer (CEST). Based on a modular approach, we incorporated the enzymes' respective substrates to a salicylate moiety with a chromogenic spacer via a carbamate linkage. This furnished highly selective diamagnetic CEST agents that detected and quantified enzyme activities of glycoside hydrolase enzymes. Michaelis-Menten enzyme kinetics studies were performed by monitoring catalyCEST MRI signals, which were validated with UV-vis assays.

Preliminary evaluation of PARACEST MRI agents for the detection of nitric oxide synthase

Several paramagnetic chemical exchange saturation transfer magnetic resonance imaging (PARACEST MRI) agents for the potential detection of nitric oxide synthase (NOS) have been synthesized and evaluated. These agents are based on an amino acid- or dipeptide-decorated DOTAM (1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid amide) chelator possessing either Tm3+ or Dy3+. The amino acid and dipeptide decorated DOTAMs were designed such that the terminal amino acid pendant group was l-arginine, which may be converted to l-citrulline by NOS. Preliminary evaluation has revealed that some of the l-arginine-decorated complexes are recognized and metabolized by the NOS. Differences in the CEST properties between Dy3+-metallated l-arginine- and l-citrulline-modified complexes suggest that these might be suitable for imaging of the NOS enzymatic activity.

Advances in Magnetic Resonance Imaging Contrast Agents for Biomarker Detection

Recent advances in magnetic resonance imaging (MRI) contrast agents have provided new capabilities for biomarker detection through molecular imaging. MRI contrast agents based on the T2 exchange mechanism have more recently expanded the armamentarium of agents for molecular imaging. Compared with T1 and T2* agents, T2 exchange agents have a slower chemical exchange rate, which improves the ability to design these MRI contrast agents with greater specificity for detecting the intended biomarker. MRI contrast agents that are detected through chemical exchange saturation transfer (CEST) have even slower chemical exchange rates. Another emerging class of MRI contrast agents uses hyperpolarized (13)C to detect the agent with outstanding sensitivity. These hyperpolarized (13)C agents can be used to track metabolism and monitor characteristics of the tissue microenvironment. Together, these various MRI contrast agents provide excellent opportunities to develop molecular imaging for biomarker detection.

Noninvasive detection of enzyme activity in tumor models of human ovarian cancer using catalyCEST MRI

PURPOSE

We proposed to detect the in vivo enzyme activity of γ-glutamyl transferase (GGT) within mouse models of human ovarian cancers using catalyCEST MRI with a diamagnetic CEST agent.

METHODS

A CEST-FISP MRI protocol and a diamagnetic CEST agent were developed to detect GGT enzyme activity in biochemical solution. A quantitative Michaelis-Menten enzyme kinetics study was performed to confirm that catalyCEST MRI can measure enzyme activity. In vivo catalyCEST MRI studies generated pixel-wise activity maps of GGT activities. Ex vivo fluorescence imaging was performed for validation.

RESULTS

CatalyCEST MRI selectively detected two CEST signals from a single CEST agent, whereby one CEST signal was responsive to GGT enzyme activity and the other CEST signal was an unresponsive control signal. The comparison of these CEST signals facilitated in vivo catalyCEST MRI studies that detected high GGT activity in OVCAR-8 tumors, low GGT activity in OVCAR-3 tumors, and low or no GGT activity in muscle tissues.

CONCLUSION

CatalyCEST MRI with a diamagnetic CEST agent can detect the level of GGT enzyme activity within in vivo tumor models of human ovarian cancers. Magn Reson Med 77:2005-2014, 2017. © 2016 International Society for Magnetic Resonance in Medicine.

Detection of Sulfatase Enzyme Activity with a CatalyCEST MRI Contrast Agent

A chemical exchange saturation transfer (CEST) MRI contrast agent has been developed that detects sulfatase enzyme activity. The agent produces a CEST signal at δ=5.0 ppm before enzyme activity, and a second CEST signal appears at δ=9.0 ppm after the enzyme cleaves a sulfate group from the agent. The comparison of the two signals improved detection of sulfatase activity.

An enzyme-activatable and cell-permeable MnIII-porphyrin as a highly efficient T1 MRI contrast agent for cell labeling

MnAMP, a cell-trappable pro-contrast agent gets enzymatically activated and accumulated intracellularly to provide a strong MRI signal for cell labeling.

Magnetic resonance imaging (MRI) is a preferred technique for noninvasively monitoring the fate of implanted cells, such as stem cells and immune cells in vivo. Cellular MRI requires contrast agents (CAs) to label the cells of interest. Despite promising progress made in this emerging field, highly sensitive, stable and biocompatible T₁ CAs with high cell permeability and specificity remains an unmet challenge. To address this need, a novel Mn^III-porphyrin, MnAMP was designed and synthesized based on the modification of Mn^IIItetra(carboxy-porphyrin) (MnTCP), a small and highly stable non-Gd extracellular CA with good biocompatibility and high T₁ relaxivity (r₁ = 7.9 mM^–1 s^–1) at clinical field of 3 Tesla (T). Cell permeability was achieved by masking the polar carboxylates of MnTCP with acetoxymethyl-ester (AM) groups, which are susceptible to hydrolysis by intracellular esterases. The enzymatic cleavage of AM groups led to disaggregation of the hydrophobic MnAMP, releasing activated MnTCP with significant increase in T₁ relaxivity. Cell uptake of MnAMP is highly efficient as tested on two non-phagocytic human cell lines with no side effects observed on cell viability. MRI of labeled cells exhibited significant contrast enhancement with a short T₁ of 161 ms at 3 T, even though a relatively low concentration of MnAMP and short incubation time was applied for cell labeling. Overall, MnAMP is among the most efficient T₁ cell labeling agents developed for cellular MRI.

A CatalyCEST MRI Contrast Agent that Can Simultaneously Detect Two Enzyme Activities

The simultaneous detection of multiple enzyme activities can improve the specificity of disease diagnoses. We therefore synthesized and characterized a diamagnetic chemical exchange saturation transfer (CEST) MRI contrast agent that can simultaneously detect two enzyme activities. Sulfatase and esterase enzymes cleave the ligands of the CEST agent, releasing salicylic acid that can be detected with CEST MRI. Importantly, both enzymes are required to activate the agent to produce a CEST MRI contrast, and the CEST agent was stable without enzyme treatment. These results established that this diamagnetic CEST MRI contrast agent is a platform technology with a modular design that can be potentially exploited to detect other combinations of enzyme activities, which can expand the armamentarium of contrast agents for molecular imaging.

A single diamagnetic catalyCEST MRI contrast agent that detects cathepsin B enzyme activity by using a ratio of two CEST signals

CatalyCEST MRI can detect enzyme activity by monitoring the change in chemical exchange with water after a contrast agent is cleaved by an enzyme. Often these molecules use paramagnetic metals and are delivered with an additional non-responsive reference molecule. To improve this approach for molecular imaging, a single diamagnetic agent with enzyme-responsive and enzyme-unresponsive CEST signals was synthesized and characterized. The CEST signal from the aryl amide disappeared after cleavage of a dipeptidyl ligand with cathepsin B, while a salicylic acid moiety was largely unresponsive to enzyme activity. The ratiometric comparison of the two CEST signals from the same agent allowed for concentration independent measurements of enzyme activity. The chemical exchange rate of the salicylic acid moiety was unchanged after enzyme catalysis, which further validated that this moiety was enzyme-unresponsive. The temperature dependence of the chemical exchange rate of the salicylic acid moiety was non-Arrhenius, suggesting a two-step chemical exchange mechanism for salicylic acid. The good detection sensitivity at low saturation power facilitates clinical translation, along with the potentially low toxicity of a non-metallic MRI contrast agent. The modular design of the agent constitutes a platform technology that expands the variety of agents that may be employed by catalyCEST MRI for molecular imaging.

Prototypes of Lanthanide(III) Agents Responsive to Enzymatic Activities in Three Complementary Imaging Modalities: Visible/Near-Infrared Luminescence, PARACEST-, and T1-MRI

We report first prototypes of responsive lanthanide(III) complexes that can be monitored independently in three complementary imaging modalities. Through the appropriate choice of lanthanide(III) cations, the same reactive ligand can be used to form complexes providing detection by (i) visible (Tb(3+)) and near-infrared (Yb(3+)) luminescence, (ii) PARACEST- (Tb(3+), Yb(3+)), or (iii) T1-weighted (Gd(3+)) MRI. The use of lanthanide(III) ions of different natures for these imaging modalities induces only a minor change in the structure of complexes that are therefore expected to have a single biodistribution and cytotoxicity.

Double agents and secret agents: the emerging fields of exogenous chemical exchange saturation transfer and T2-exchange magnetic resonance imaging contrast agents for molecular imaging

Two relatively new types of exogenous magnetic resonance imaging contrast agents may provide greater impact for molecular imaging by providing greater specificity for detecting molecular imaging biomarkers. Exogenous chemical exchange saturation transfer (CEST) agents rely on the selective saturation of the magnetization of a proton on an agent, followed by chemical exchange of a proton from the agent to water. The selective detection of a biomarker-responsive CEST signal and an unresponsive CEST signal, followed by the ratiometric comparison of these signals, can improve biomarker specificity. We refer to this improvement as a "double-agent" approach to molecular imaging. Exogenous T₂-exchange agents also rely on chemical exchange of protons between the agent and water, especially with an intermediate rate that lies between the slow exchange rates of CEST agents and the fast exchange rates of traditional T₁ and T₂ agents. Because of this intermediate exchange rate, these agents have been relatively unknown and have acted as "secret agents" in the contrast agent research field. This review exposes these secret agents and describes the merits of double agents through examples of exogenous agents that detect enzyme activity, nucleic acids and gene expression, metabolites, ions, redox state, temperature, and pH. Future directions are also provided for improving both types of contrast agents for improved molecular imaging and clinical translation. Therefore, this review provides an overview of two new types of exogenous contrast agents that are becoming useful tools within the armamentarium of molecular imaging.

Gd(3+)-Based Magnetic Resonance Imaging Contrast Agent Responsive to Zn(2+)

We report the heteroditopic ligand H5L, which contains a DO3A unit for Gd(3+) complexation connected to an NO2A moiety through a N-propylacetamide linker. The synthesis of the ligand followed a convergent route that involved the preparation of 1,4-bis(tert-butoxycarbonylmethyl)-1,4,7-triazacyclononane following the orthoamide strategy. The luminescence lifetimes of the Tb((5)D4) excited state measured for the TbL complex point to the absence of coordinated water molecules. Density functional theory calculations and (1)H NMR studies indicate that the EuL complex presents a square antiprismatic coordination in aqueous solution, where eight coordination is provided by the seven donor atoms of the DO3A unit and the amide oxygen atom of the N-propylacetamide linker. Addition of Zn(2+) to aqueous solutions of the TbL complex provokes a decrease of the emission intensity as the emission lifetime becomes shorter, which is a consequence of the coordination of a water molecule to the Tb(3+) ion upon Zn(2+) binding to the NO2A moiety. The relaxivity of the GdL complex recorded at 7 T (25 °C) increases by almost 150% in the presence of 1 equiv of Zn(2+), while Ca(2+) and Mg(2+) induced very small relaxivity changes. In vitro magnetic resonance imaging experiments confirmed the ability of GdL to provide response to the presence of Zn(2+).

Detection of in vivo enzyme activity with CatalyCEST MRI

PURPOSE

CatalyCEST MRI compares the detection of an enzyme-responsive chemical exchange saturation transfer (CEST) agent with the detection of an unresponsive "control" CEST agent that accounts for other conditions that influence CEST. The purpose of this study was to investigate the feasibility of in vivo catalyCEST MRI.

METHODS

CEST agents that were responsive and unresponsive to the activity of urokinase plasminogen activator were shown to have negligible interaction with each other. A CEST-fast imaging with steady state precession (FISP) MRI protocol was used to acquire MR CEST spectroscopic images with a Capan-2 pancreatic tumor model after intravenous injection of the CEST agents. A function of (super)-Lorentzian line shapes was fit to CEST spectra of a region-of-interest that represented the tumor.

RESULTS

The CEST effects from each agent showed the same initial uptake into tumor tissues, indicating that both agents had the same pharmacokinetic transport rates. Starting 5 min after injection, CEST from the enzyme-responsive agent disappeared more quickly than CEST from the unresponsive agent, indicating that the enzyme responsive agent was being catalyzed by urokinase plasminogen activator, while both agents also experienced net pharmacokinetic washout from the tumor.

CONCLUSION

CatalyCEST MRI demonstrates that dynamic tracking of enzyme-responsive and unresponsive CEST agents during the same in vivo MRI study is feasible.

Functional MR Imaging Techniques in Oncology in the Era of Personalized Medicine

DW and DCE MR imaging contribute significantly to diagnosis, treatment planning, response assessment, and prognosis in personalized cancer medicine. Nevertheless, the need for further standardization of these techniques needs to be addressed. Whole-body DW MR imaging is an exciting field; however, future studies need to investigate in more depth the biologic significance of the findings depicted, their prognostic relevance, and cost-effectiveness in comparison with MDCT and PET/CT. New MR imaging probes, such as targeted or activatable contrast agents and dynamic nuclear hyperpolarization, show great promise to further improve the care of patients with cancer in the near future.

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.