Detection of Alkaline Phosphatase Enzyme Activity with a CatalyCEST MRI Biosensor

Intramolecular Hydrogen Bonding Based CEST MRI Contrast Agents As an Emerging Design Strategy: A Mini-Review

Intramolecular hydrogen bonding-based chemical exchange saturation transfer magnetic resonance imaging (CEST MRI) contrast agents represent an innovative design strategy aiming to overcome limitations in diamagnetic CEST (diaCEST) MRI contrast agent specificity and also those associated with traditional metal-based MRI contrast agents. Ward and Balaban's proposal of small diamagnetic compounds marked a paradigm shift in contrast-based radiologic research, inspiring extensive investigations since 2000. These contrast agents leverage labile hydrogen bonds, serving as chemical exchange sites to induce saturation of water. The selective manipulation of radiofrequency (RF) allows for optimized signal contrast in soft tissue, with a significant signal amplification even at low probe concentrations, mitigating concerns about dose-dependent toxicities. This mini-review delves into the evolution of CEST MRI, its classification, and the strategic design principles of synthetic small molecules containing intramolecular hydrogen bonds. With a focus on applications and potential clinical relevance, the authors highlight the promising role of intramolecular hydrogen bonding-based CEST MRI in diverse medical contexts, especially renal imaging and pH mapping, paving the way for enhanced molecular imaging capabilities. Ongoing research endeavors aim to further optimize and expand the utility of these contrast agents, underscoring their transformative potential in clinical diagnostics and imaging.

A snapshot of the vast array of diamagnetic CEST MRI contrast agents

Since the inception of CEST MRI in the 1990s, a number of compounds have been identified as suitable for generating contrast, including paramagnetic lanthanide complexes, hyperpolarized atom cages and, most interesting, diamagnetic compounds. In the past two decades, there has been a major emphasis in this field on the identification and application of diamagnetic compounds that have suitable biosafety profiles for usage in medical applications. Even in the past five years there has been a tremendous growth in their numbers, with more and more emphasis being placed on finding those that can be ultimately used for patient studies on clinical 3 T scanners. At this point, a number of endogenous compounds present in tissue have been identified, and also natural and synthetic organic compounds that can be administered to highlight pathology via CEST imaging. Here we will provide a very extensive snapshot of the types of diamagnetic compound that can generate CEST MRI contrast, together with guidance on their utility on typical preclinical and clinical scanners and a review of the applications that might benefit the most from this new technology.

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.

An alkaline phosphatase-induced immunosensor for SARS-CoV-2 N protein and cardiac troponin I based on the in situ fluorogenic self-assembly between N-heterocyclic boronic acids and alizarin red S

Alkaline phosphatase (ALP)-induced in situ fluorescent immunosensor is less investigated and reported. Herein, a high-performance ALP-labeled in situ fluorescent immunoassay platform was constructed. The developed platform was based on a fluorogenic self-assembly reaction between pyridineboronic acid (PyB(OH)₂) and alizarin red S (ARS). We first used density functional theory (DFT) to theoretically calculate the changes of Gibbs free energy of the used chemicals before and after the combination and simulated the electrostatic potential on its' surfaces. The free ARS and PyB(OH)₂ exist alone, neither emits no fluorescence. However, the ARS/PyB(OH)₂ complex emits strong fluorescence, which could be effectively quenched by PPi based on the stronger affinity between PPi and PyB(OH)₂ than that of ARS and PyB(OH)₂. PyB(OH)₂ coordinated with ARS again in the presence of ALP due to the ALP-catalyzed hydrolysis of PPi, and correspondingly, the fluorescence was restored. We chose cTnI and SARS-CoV-2 N protein as the model antigen to construct ALP-induced immunosensor, which exhibited a wide dynamic range of 0-175 ng/mL for cTnI and SARS-CoV-2 N protein with a low limit of detection (LOD) of 0.03 ng/mL and 0.17 ng/mL, respectively. Moreover, the proposed immunosensor was used to evaluate cTnI and SARS-CoV-2 N protein level in serum with satisfactory results. Consequently, the method laid the foundation for developing novel fluorescence-based ALP-labeled ELISA technologies in the early diagnosis of diseases.

Enzyme-triggered click chemistry combined with surface-enhanced Raman spectroscopy for the simple and sensitive detection of alkaline phosphatase activity from complex biological samples

Schematic of the enzyme-triggered click chemistry combined with the SERS technique for ALP detection.

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.

Ceria Nanozyme and Phosphate Prodrugs: Drug Synthesis through Enzyme Mimicry

Nanozymes can mimic the activities of diverse enzymes, and this ability finds applications in analytical sciences and industrial chemistry, as well as in biomedical applications. Among the latter, prodrug conversion mediated by nanozymes is investigated as a step toward site-specific drug synthesis, to achieve localized therapeutic effects. In this work, we investigated a ceria nanozyme as a mimic to phosphatase, to mediate conversion of phosphate prodrugs into corresponding therapeutics. To this end, the substrate scope of ceria as a phosphatase mimic was analyzed using a broad range of natural phosphor(di)esters and pyrophosphates. Knowledge of this scope guided the selection of existing phosphate prodrugs that can be converted by ceria into the corresponding therapeutics. "Extended scaffold phosphates" were engineered using self-immolative linkers to accommodate a prodrug design for amine-containing drugs, such as monomethyl auristatin E. Phosphate prodrugs masked activity of the toxin, whereas prodrug conversion mediated by the nanozyme restored drug toxicity, which was validated in mammalian cell culture. The main novelty of this work lies in the rational pairing of the ceria nanozyme with the existing and the de novo designed "extended scaffold" phosphate prodrugs toward their use in nanozyme-prodrug therapy based on the defined nanozyme substrate scope.

The Design of Abnormal Microenvironment Responsive MRI Nanoprobe and Its Application

Magnetic resonance imaging (MRI) is often used to diagnose diseases due to its high spatial, temporal and soft tissue resolution. Frequently, probes or contrast agents are used to enhance the contrast in MRI to improve diagnostic accuracy. With the development of molecular imaging techniques, molecular MRI can be used to obtain 3D anatomical structure, physiology, pathology, and other relevant information regarding the lesion, which can provide an important reference for the accurate diagnosis and treatment of the disease in the early stages. Among existing contrast agents, smart or activatable nanoprobes can respond to selective stimuli, such as proving the presence of acidic pH, active enzymes, or reducing environments. The recently developed environment-responsive or smart MRI nanoprobes can specifically target cells based on differences in the cellular environment and improve the contrast between diseased tissues and normal tissues. Here, we review the design and application of these environment-responsive MRI nanoprobes.

A novel aggregation-induced dual emission probe for in situ light-up detection of endogenous alkaline phosphatase

Abnormal level of alkaline phosphatase (ALP) activity has been linked to many diseases in human. The development of fluorescent molecular probes that can report the expression and activity of ALP in various biological systems will be extremely valuable. However, the in vivo monitoring for ALP in living cells and more complex biological systems remains a great challenge. The excited-state intramolecular proton transfer (ESIPT) probe with proportional fluorescence has low background noise, while the aggregation induced emission (AIE) probe has the advantages of signal amplification and good light stability. Herein, an "AIE + ESIPT" fluorescent probe 2-(benzo[d]thiazol-2-yl)-4-(1,4,5-triphenyl-1H-imidazole-2-yl)phenyl dihydrogen phosphate (THP) was constructed for the highly selective and sensitive detection of ALP. By introducing a phosphate ester at the hydroxyl position of the solid fluorophore 2-(benzo[d]thiazol-2-yl)-4-(1,4,5-triphenyl-1H-imidazole-2-yl)phenol, ESIPT was hindered and the probe present a faint blue fluorescence in DMSO solution. While ALP was introduced, causing the phosphate in THP hydrolyzed, and the ESIPT process was restored to yield a yellow fluorescence at 550 nm, thereby achieving proportionality detection. THP exhibited high selectivity and sensitively to ALP with low limit of detection (1.21228 U/L), and the reaction completed within 20 min. In addition, with its outstanding advantages of low biological toxicity and enzyme conversion characteristics, THP has been successfully applied to ALP imaging in living cells (Hela cells, A549 cells and Hek293 cells), and can provide in situ information on the reaction site. Therefore, THP has the potential for detecting ALP activity in biomedical application.

Repurposing Clinical Agents for Chemical Exchange Saturation Transfer Magnetic Resonance Imaging: Current Status and Future Perspectives

Molecular imaging is becoming an indispensable tool to pursue precision medicine. However, quickly translating newly developed magnetic resonance imaging (MRI) agents into clinical use remains a formidable challenge. Recently, Chemical Exchange Saturation Transfer (CEST) MRI is emerging as an attractive approach with the capability of directly using low concentration, exchangeable protons-containing agents for generating quantitative MRI contrast. The ability to utilize diamagnetic compounds has been extensively exploited to detect many clinical compounds, such as FDA approved drugs, X-ray/CT contrast agents, nutrients, supplements, and biopolymers. The ability to directly off-label use clinical compounds permits CEST MRI to be rapidly translated to clinical settings. In this review, the current status of CEST MRI based on clinically available compounds will be briefly introduced. The advancements and limitations of these studies are reviewed in the context of their pre-clinical or clinical applications. Finally, future directions will be briefly discussed.

N-Aryl Amides as Chemical Exchange Saturation Transfer Magnetic Resonance Imaging Contrast Agents

Chemical exchange saturation transfer (CEST) MRI has recently emerged as a versatile molecular imaging approach in which diamagnetic compounds can be utilized to generate an MRI signal. To expand the scope of CEST MRI applications, herein, we systematically investigated the CEST properties of N-aryl amides with different N-aromatic substitution, revealing their chemical shifts (4.6-5.8 ppm) and exchange rates (up to thousands s^-1 ) are favorable to be used as CEST agents as compared to alkyl amides. As the first proof-of-concept study, we used CEST MRI to detect the enzymatic metabolism of the drug acebutolol directly by its intrinsic CEST signal without any chemical labeling. Our study implies that N-aryl amides may enable the label-free CEST MRI detection of the metabolism of many N-aryl amide-containing drugs and a variety of enzymes that act on N-aryl amides, greatly expanding the scope of CEST MR molecular imaging.

Synthesis and Comparative Evaluation of Photoswitchable Magnetic Resonance Imaging Contrast Agents

A series of spiropyran (SP)-based magnetic resonance imaging (MRI) contrast agents have been synthesized and evaluated for changes in relaxivity resulting from irradiation with visible light. Both electron-donating and electron-withdrawing substituents were appended to the SP ring in order to study the electronic effects on the photochromic and relaxivity properties of these photoswitchable MRI contrast agents. Photoswitches lacking an electron-withdrawing substituent isomerize readily between the merocyanine and SP forms, while the addition of a nitro group prevents this process. Complexes capable of isomerizing were demonstrated to effect a change in the relaxivity of the appended gadolinium complex.

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.

Detecting acid phosphatase enzymatic activity with phenol as a chemical exchange saturation transfer magnetic resonance imaging contrast agent (PhenolCEST MRI)

Phenol contains an exchangeable hydroxyl proton resonant at 4.8 ppm from the resonance frequency of water in the 1H nuclear magnetic resonance (1H NMR) spectrum, enabling itself to be detected at sub-mM concentration by either chemical exchange saturation transfer magnetic resonance imaging (CEST MRI) or exchange-based T2 relaxation enhancement (T2ex) effect under acidic and basic conditions, respectively. We recently investigated the T2ex effects of phenol and its derivatives, but the CEST characteristics of phenols are unknown in detail, and no study on using the natural CEST MRI effects of phenol for detecting enzymatic activity has been conducted. Herein, on the basis of the inherent CEST MR property of phenol, namely phenolCEST, we developed the first MRI approach to detect acid phosphatase (AcP) enzymatic activity. Upon the activity of AcP at pH = 5.0, non-CEST-detectable enzyme substrate phenyl phosphate was converted to CEST-detectable phenol, providing a simple way to quantify AcP activity directly without the need for a second signalling probe. We showed the application of this phenolCEST biosensor for measuring AcP activity in both enzyme solutions and cell lysates of prostate cells. This work opens a door for the utilization of phenolCEST MRI technique in sensor design and development.

Ratiometric Fluorescent Strategy for Localizing Alkaline Phosphatase Activity in Mitochondria Based on the ESIPT Process

Fluorescent probes are powerful tools for detecting and mapping the species of interest in vitro and in vivo. Although the probes always show high selectivity and sensitivity, they are usually affected by some factors, such as detecting conditions and the probe concentrations. Ratiometric fluorescent strategies, possessing advantage of low background noise, would solve the problem effectively and lead to a higher sensing performance. Thus, an ESIPT-based ratiometric probe (HBTP-mito) was developed on the basis of a phosphorylated 2-(2'-hydroxyphenyl)-benzothiazole derivative for the determination of ALP activity. HBTP-mito is water soluble and emits green fluorescence in TBS buffer due to the blockage of ESIPT. Upon the introduction of ALP, the phosphate ester of HBTP-mito was hydrolyzed and the ESIPT process was restored. Accordingly, the fluorescence at 514 nm decreases, while emission at 650 nm shows a "turn-on" response. The ratio of intensity (I_514nm/I_650nm) decreases linearly with ALP activity increasing from 0 to 60 mU/mL, obtained an LOD of 0.072 mU/mL. The favorable performance of the probe enables its application not only in the detection of ALP activity in biological samples, but also in the localization of the ALP levels in living cells and in vivo.

Recent progress in the imaging detection of enzyme activities in vivo

Enzymatic activities are important for normal physiological processes and are also critical regulatory mechanisms for many pathologies. Identifying the enzyme activities in vivo has considerable importance in disease diagnoses and monitoring of the physiological metabolism. In the past few years, great strides have been made towards the imaging detection of enzyme activity in vivo based on optical modality, MRI modality, nuclear modality, photoacoustic modality and multifunctional modality. This review summarizes the latest advances in the imaging detection of enzyme activities in vivo reported within the past years, mainly concentrating on the probe design, imaging strategies and demonstration of enzyme activities in vivo. This review also highlights the potential challenges and the further directions of this field.

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.

Hydrazo-CEST: Hydrazone-Dependent Chemical Exchange Saturation Transfer Magnetic Resonance Imaging Contrast Agents

The rapid formation of hydrazones under physiological conditions was exploited for the detection of aldehydes through chemical exchange saturation transfer magnetic resonance imaging (CEST-MRI). A metal-free, diamagnetic contrast agent derived from N-amino anthranilic acid was introduced, which selectively "turned-on" upon hydrazone formation through an effect termed Hydrazo-CEST. While the hydrazine form of the probe produced no CEST-MRI signal enhancement, the formation of the aryl hydrazone resulted in >20 % intensity decrease in the bulk water signal through the CEST effect, as measured by 300 MHz ¹ H NMR, 3 T and 7 T MRI. Both the electronic contributions of the N-amino anthranilate and the aldehyde binding partner were shown to directly impact the exchange rate of the proton on the ring-proximal nitrogen, and thus the imaging signal. Additionally, the presence of the carboxylic acid moiety ortho to the hydrazine was necessary not only for contrast production, but also for rapid hydrazone formation and prolonged hydrazone product stability under physiological conditions. This work provided the first example of an MRI-based contrast agent capable of a "turn on" response upon reaction with bioactive aldehydes, and outlined both the structural and electronic requirements to expand on Hydrazo-CEST, a novel, hydrazone-dependent subtype of diamagnetic CEST-MRI.

Salicylic Acid-Based Polymeric Contrast Agents for Molecular Magnetic Resonance Imaging of Prostate Cancer

Chemical exchange saturation transfer (CEST) magnetic resonance imaging (MRI) is an innovative molecular imaging technique in which contrast agents are labeled by saturating their exchangeable proton spins by radio-frequency irradiation. Salicylic acid and its analogues are a promising class of highly sensitive, diamagnetic CEST agents. Herein, polymeric agents grafted with salicylic acid moieties and a known high-affinity ligand targeting prostate-specific membrane antigen in approximately 10:1 molar ratio were synthesized to provide sufficient MRI sensitivity and receptor specificity. The proton-exchange properties of the contrast agent in solution and in an experimental murine model are reported to demonstrate the feasibility of receptor-targeted CEST MRI of prostate cancer. Furthermore, the CEST imaging data were validated with an 111 In-labeled analogue of the agent by in vivo single photon emission computed tomographic imaging and tissue biodistribution studies.

Two decades of dendrimers as versatile MRI agents: a tale with and without metals

Dendrimers or dendritic polymers are a class of compounds with great potential for nanomedical use. Some of their properties, including their rigidity, low polydispersity and the ease with which their surfaces can be modified make them particularly well suited for use as MRI diagnostic or theranostic agents. For the past 20 years, researchers have recognized this potential and refined dendrimer formulations to optimize these nanocarriers for a host of MRI applications, including blood pool imaging agents, lymph node imaging agents, tumor-targeted theranostic agents and cell tracking agents. This review summarizes the various types of dendrimers according to the type of MR contrast they can provide. This includes the metallic T1 , T2 and paraCEST imaging agents, and the non-metallic diaCEST and fluorinated (19 F) heteronuclear imaging agents. This article is categorized under: Diagnostic Tools > In Vivo Nanodiagnostics and Imaging Implantable Materials and Surgical Technologies > Nanomaterials and Implants.

Alkaline Phosphatase-Instructed Self-Assembly of Gadolinium Nanofibers for Enhanced T2-Weighted Magnetic Resonance Imaging of Tumor

Alkaline phosphatase (ALP) is an important enzyme but using ALP-instructed self-assembly of gadolinium nanofibers for enhanced T₂-weighted magnetic resonance imaging (MRI) of tumor has not been reported. In this work, we rationally designed a hydrogelator Nap-FFFYp-EDA-DOTA(Gd) (1P) which, under the catalysis of ALP, was able to self-assemble into gadolinium nanofibers to form hydrogel Gel I for enhanced T₂-weighted MR imaging of ALP activity in vitro and in tumor. T₂ phantom MR imaging indicated that the transverse relaxivity (r₂) value of Gel I was 33.9% higher than that of 1P and both of them were 1 order of magnitude higher than that of Gd-DTPA. In vivo T₂-weighted MR imaging showed that, at 9.4 T, ALP-overexpressing HeLa tumors of 1P-injected mice showed obviously enhanced T₂ contrast. We anticipate that, by replacing ALP with other enzymes, our approach could be applied for MR diagnosis of other diseases in the future.

Activatable probes for diagnosis and biomarker detection by MRI

Magnetic resonance imaging (MRI) is a non-invasive imaging technique with widespread use in diagnosis. Frequently, contrast in MRI is enhanced with the aid of a contrast agent, among which smart, responsive, OFF/ON or activatable probes are of particular interest. These kinds of probes elicit a response to selective stimuli, evidencing the presence of enzymes or acidic pH, for instance. In this review, we will focus on smart probes that are detectable by both ¹H and ¹⁹F MRI, frequently based on nanomaterials. We will discuss the triggering factors and the strategies employed thus far to activate each probe.