CEST theranostics: label-free MR imaging of anticancer drugs

Preclinical Application of CEST MRI to Detect Early and Regional Tumor Response to Local Brain Tumor Treatment

Treating glioblastoma and monitoring treatment response non-invasively remain challenging. Here, we developed a robust approach using a drug-loaded liposomal hydrogel that is mechanically compatible with the brain, and, simultaneously, we successfully monitored early tumor response using Chemical Exchange Saturation Transfer (CEST) MRI. This CEST-detectable liposomal hydrogel was optimized based on a sustainable drug release and a soft hydrogel for the brain tumor, which is unfavorable for tumor cell proliferation. After injecting the hydrogel next to the tumor, three distinctive CEST contrasts enabled the monitoring of tumor response and drug release longitudinally at 3T. As a result, a continuous tumor volume decrease was observed in the treatment group along with a significant decrease in CEST contrasts relating to the tumor response at 3.5 ppm (Amide Proton Transfer; APT) and at -3.5 ppm (relayed Nuclear Overhauser Effect; rNOE) when compared to the control group (p < 0.05). Interestingly, the molecular change at 3.5 ppm on day 3 (p < 0.05) was found to be prior to the significant decrease in tumor volume on day 5. An APT signal also showed a strong correlation with the number of proliferating cells in the tumors. This demonstrated that APT detected a distinctive decrease in mobile proteins and peptides in tumors before the change in tumor morphology. Moreover, the APT signal showed a regional response to the treatment, associated with proliferating and apoptotic cells, which allowed an in-depth evaluation and prediction of the tumor treatment response. This newly developed liposomal hydrogel allows image-guided brain tumor treatment to address clinical needs using CEST MRI.

Contrasting Properties of Polymeric Nanocarriers for MRI-Guided Drug Delivery

Poor pharmacokinetics and low aqueous solubility combined with rapid clearance from the circulation of drugs result in their limited effectiveness and generally high therapeutic doses. The use of nanocarriers for drug delivery can prevent the rapid degradation of the drug, leading to its increased half-life. It can also improve the solubility and stability of drugs, advance their distribution and targeting, ensure a sustained release, and reduce drug resistance by delivering multiple therapeutic agents simultaneously. Furthermore, nanotechnology enables the combination of therapeutics with biomedical imaging agents and other treatment modalities to overcome the challenges of disease diagnosis and therapy. Such an approach is referred to as "theranostics" and aims to offer a more patient-specific approach through the observation of the distribution of contrast agents that are linked to therapeutics. The purpose of this paper is to present the recent scientific reports on polymeric nanocarriers for MRI-guided drug delivery. Polymeric nanocarriers are a very broad and versatile group of materials for drug delivery, providing high loading capacities, improved pharmacokinetics, and biocompatibility. The main focus was on the contrasting properties of proposed polymeric nanocarriers, which can be categorized into three main groups: polymeric nanocarriers (1) with relaxation-type contrast agents, (2) with chemical exchange saturation transfer (CEST) properties, and (3) with direct detection contrast agents based on fluorinated compounds. The importance of this aspect tends to be downplayed, despite its being essential for the successful design of applicable theranostic nanocarriers for image-guided drug delivery. If available, cytotoxicity and therapeutic effects were also summarized.

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.

Imaging of sugar-based contrast agents using their hydroxyl proton exchange properties

The ability of CEST MRI to detect the presence of millimolar concentrations of non-metallic contrast agents has made it possible to study, non-invasively, important biological molecules such as proteins and sugars, as well as drugs already approved for clinical use. Here, we review efforts to use sugar and sugar polymers as exogenous contrast agents, which is possible based on the exchange of their hydroxyl protons with water protons. While this capability has raised early enthusiasm, for instance about the possibility of imaging D-glucose metabolism with MRI in a way analogous to PET, experience over the past decade has shown that this is not trivial. On the other hand, many studies have confirmed the possibility of imaging a large variety of sugar analogues, each with potentially interesting applications to assess tissue physiology. Some promising applications are the study of (i) sugar delivery and transport to assess blood-brain barrier integrity and (ii) sugar uptake by cells for their characterization (e.g., cancer versus healthy), as well as (iii) clearance of sugars to assess tissue drainage-for instance, through the glymphatic system. To judge these opportunities and their challenges, especially in the clinic, it is necessary to understand the technical aspects of detecting the presence of rapidly exchanging protons through the water signal in MRI, especially as a function of magnetic field strength. We expect that novel approaches in terms of MRI detection (both saturation transfer and relaxation based), MRI data analysis, and sugar design will push this young field forward in the next decade.

Compartmentalized agents: A powerful strategy for enhancing the detection sensitivity of chemical exchange saturation transfer contrast

Since the very beginnings of the chemical exchange saturation transfer (CEST) technique, poor overall sensitivity has appeared to be one of its strongest limitations for future applications. Research has therefore focused on designing systems, such as supramolecular and nanosized agents, that contain a high number of magnetically equivalent mobile spins. However, the number of mobile spins offered by these systems is still limited by their composition and surface/volume ratio. The design of compartmentalized agents, that is, systems where an aqueous inner core is separated from the MRI-detected bulk pool via a semipermeable barrier/membrane, is very much a step forward for the technique. These vesicular systems can (i) act as biocompatible and versatile carriers for dia-, para-, and hetero-nuclear CEST probes, thus offering new application options; and (ii) act as CEST probes themselves via the encapsulation of a suitable agent (e.g., a paramagnetic shift reagent) that can change the resonance frequency of the spin pool in the inner compartment only. LipoCEST agents were the pioneers in the latter category, as they are able to grant picomolar sensitivity (in terms of nanoparticle concentration), and paved the way for new applications for CEST agents, especially in the theranostic research area. The use of larger, natural vesicular systems, such as yeasts and cells, in which the huge number of intravesicular spins lowers the detection threshold to a femtomolar limit, is a further step forward in the development of compartmentalized CEST agents. Finally, interesting combinations of nanovesicular and cellular compartmentalized systems have been proposed, thus highlighting how the approach has the potential to drive CEST agents towards completing their journey to mature clinical translation.

Label-Free Chemically and Molecularly Selective Magnetic Resonance Imaging

Biomedical imaging, especially molecular imaging, has been a driving force in scientific discovery, technological innovation, and precision medicine in the past two decades. While substantial advances and discoveries in chemical biology have been made to develop molecular imaging probes and tracers, translating these exogenous agents to clinical application in precision medicine is a major challenge. Among the clinically accepted imaging modalities, magnetic resonance imaging (MRI) and magnetic resonance spectroscopy (MRS) exemplify the most effective and robust biomedical imaging tools. Both MRI and MRS enable a broad range of chemical, biological and clinical applications from determining molecular structures in biochemical analysis to imaging diagnosis and characterization of many diseases and image-guided interventions. Using chemical, biological, and nuclear magnetic resonance properties of specific endogenous metabolites and native MRI contrast-enhancing biomolecules, label-free molecular and cellular imaging with MRI can be achieved in biomedical research and clinical management of patients with various diseases. This review article outlines the chemical and biological bases of several label-free chemically and molecularly selective MRI and MRS methods that have been applied in imaging biomarker discovery, preclinical investigation, and image-guided clinical management. Examples are provided to demonstrate strategies for using endogenous probes to report the molecular, metabolic, physiological, and functional events and processes in living systems, including patients. Future perspectives on label-free molecular MRI and its challenges as well as potential solutions, including the use of rational design and engineered approaches to develop chemical and biological imaging probes to facilitate or combine with label-free molecular MRI, are discussed.

Theranostic Cancer Treatment Using Lentinan-Coated Selenium Nanoparticles and Label-Free CEST MRI

Selenium nanoparticle (SeNP)-based nanotherapeutics have become an emerging cancer therapy, while effective drug delivery remains a technical hurdle. A theranostic approach, through which imaging companions are integrated with SeNPs, will allow image-guided drug delivery and, therefore, is highly desirable. Traditional methods require the chemical conjugation of imaging agents to the surface of nanoparticles, which may impede the later clinical translation. In this study, we developed a label-free strategy in which lentinan-functionalized SeNPs (LNT-SeNPs) are detected using MRI by the hydroxyl protons carried on LNT molecules. The in vitro phantom study showed that LNT and LNT-SeNPs have a strong CEST signal at 1.0 ppm apart from the water resonance, suggesting an in vivo detectability in the µM concentration range. Demonstrated on CT26 colon tumor cells, LNT-SeNPs exert a strong anticancer effect (IC50 = 4.8 μM), prominently attributed to the ability to generate intracellular reactive oxygen species. However, when testing in a mouse model of CT26 tumors, administration of LNT-SeNPs alone was found unable to deliver sufficient drugs to the tumor, leading to poor treatment responses. To improve the drug delivery, we co-injected LNT-SeNPs and TNF-α, a previously reported drug that could effectively damage the endothelial cells in the tumor vasculature, thereby increasing drug delivery to the tumor. Our results revealed a 75% increase in the intratumoral CEST MRI signal, indicating a markedly increased delivery efficiency of LNT-SeNPs when combined with TNF-α. The combination therapy also resulted in a significantly enhanced treatment outcome, as revealed by the tumor growth study. Taken together, our study demonstrates the first label-free, SeNP-based theranostic system, in which LNT was used for both functional surface coating and CEST MRI signal generating. Such a theranostic LNT-SeNP system is advantageous because it requires chemical labeling and, therefore, has high biocompatibility and low translatable barriers.

Label-Free Assessment of Mannitol Accumulation Following Osmotic Blood-Brain Barrier Opening Using Chemical Exchange Saturation Transfer Magnetic Resonance Imaging

PURPOSE

Mannitol is a hyperosmolar agent for reducing intracranial pressure and inducing osmotic blood-brain barrier opening (OBBBO). There is a great clinical need for a non-invasive method to optimize the safety of mannitol dosing. The aim of this study was to develop a label-free Chemical Exchange Saturation Transfer (CEST)-based MRI approach for detecting intracranial accumulation of mannitol following OBBBO.

METHODS

In vitro MRI was conducted to measure the CEST properties of D-mannitol of different concentrations and pH. In vivo MRI and MRS measurements were conducted on Sprague-Dawley rats using a Biospec 11.7T horizontal MRI scanner. Rats were catheterized at the internal carotid artery (ICA) and randomly grouped to receive either 1 mL or 3 mL D-mannitol. CEST MR images were acquired before and at 20 min after the infusion.

RESULTS

In vitro MRI showed that mannitol has a strong, broad CEST contrast at around 0.8 ppm with a mM CEST MRI detectability. In vivo studies showed that CEST MRI could effectively detect mannitol in the brain. The low dose mannitol treatment led to OBBBO but no significant mannitol accumulation, whereas the high dose regimen resulted in both OBBBO and mannitol accumulation. The CEST MRI findings were consistent with ¹H-MRS and Gd-enhanced MRI assessments.

CONCLUSION

We demonstrated that CEST MRI can be used for non-invasive, label-free detection of mannitol accumulation in the brain following BBBO treatment. This method may be useful as a rapid imaging tool to optimize the dosing of mannitol-based OBBBO and improve its safety and efficacy.

Hypoxia-Responsive Luminescent CEST MRI Agent for In Vitro and In Vivo Tumor Detection and Imaging

Hypoxia is a feature of most solid tumors and a key determinant of cancer growth and propagation. Sensing hypoxia effectively could lead to more favorable clinical outcomes. Here, we report a molecular antenna-based bimodal probe designed to exploit the complementary advantages of magnetic resonance (MR)- and optical-based imaging. Specifically, we describe the synthesis and evaluation of a dual-action probe (NO₂-Eu) that permits hypoxia-activated chemical exchange saturation transfer (CEST) MR and optical imaging. In CT26 cells, this NO₂-Eu probe not only provides an enhanced CEST MRI signal but also turns "on" the optical signal under hypoxic conditions. Time-dependent in vivo CEST imaging in a hypoxic CT26 tumor xenograft mouse model revealed probe-dependent tumor detection by CEST MRI contrast in the tumor area. We thus suggest that dual-action hypoxia probes, like that reported here, could have a role to play in solid tumor diagnosis and monitoring.

Nanotheranostics for Image-Guided Cancer Treatment

Image-guided nanotheranostics have the potential to represent a new paradigm in the treatment of cancer. Recent developments in modern imaging and nanoparticle design offer an answer to many of the issues associated with conventional chemotherapy, including their indiscriminate side effects and susceptibility to drug resistance. Imaging is one of the tools best poised to enable tailoring of cancer therapies. The field of image-guided nanotheranostics has the potential to harness the precision of modern imaging techniques and use this to direct, dictate, and follow site-specific drug delivery, all of which can be used to further tailor cancer therapies on both the individual and population level. The use of image-guided drug delivery has exploded in preclinical and clinical trials although the clinical translation is incipient. This review will focus on traditional mechanisms of targeted drug delivery in cancer, including the use of molecular targeting, as well as the foundations of designing nanotheranostics, with a focus on current clinical applications of nanotheranostics in cancer. A variety of specially engineered and targeted drug carriers, along with strategies of labeling nanoparticles to endow detectability in different imaging modalities will be reviewed. It will also introduce newer concepts of image-guided drug delivery, which may circumvent many of the issues seen with other techniques. Finally, we will review the current barriers to clinical translation of image-guided nanotheranostics and how these may be overcome.

Molecular Imaging of Brain Tumors and Drug Delivery Using CEST MRI: Promises and Challenges

Chemical exchange saturation transfer (CEST) magnetic resonance imaging (MRI) detects molecules in their natural forms in a sensitive and non-invasive manner. This makes it a robust approach to assess brain tumors and related molecular alterations using endogenous molecules, such as proteins/peptides, and drugs approved for clinical use. In this review, we will discuss the promises of CEST MRI in the identification of tumors, tumor grading, detecting molecular alterations related to isocitrate dehydrogenase (IDH) and O-6-methylguanine-DNA methyltransferase (MGMT), assessment of treatment effects, and using multiple contrasts of CEST to develop theranostic approaches for cancer treatments. Promising applications include (i) using the CEST contrast of amide protons of proteins/peptides to detect brain tumors, such as glioblastoma multiforme (GBM) and low-grade gliomas; (ii) using multiple CEST contrasts for tumor stratification, and (iii) evaluation of the efficacy of drug delivery without the need of metallic or radioactive labels. These promising applications have raised enthusiasm, however, the use of CEST MRI is not trivial. CEST contrast depends on the pulse sequences, saturation parameters, methods used to analyze the CEST spectrum (i.e., Z-spectrum), and, importantly, how to interpret changes in CEST contrast and related molecular alterations in the brain. Emerging pulse sequence designs and data analysis approaches, including those assisted with deep learning, have enhanced the capability of CEST MRI in detecting molecules in brain tumors. CEST has become a specific marker for tumor grading and has the potential for prognosis and theranostics in brain tumors. With increasing understanding of the technical aspects and associated molecular alterations detected by CEST MRI, this young field is expected to have wide clinical applications in the near future.

Chemical exchange saturation transfer for detection of antiretroviral drugs in brain tissue

OBJECTIVE

Antiretroviral drug theranostics facilitates the monitoring of biodistribution and efficacy of therapies designed to target HIV type-1 (HIV-1) reservoirs. To this end, we have now deployed intrinsic drug chemical exchange saturation transfer (CEST) contrasts to detect antiretroviral drugs within the central nervous system (CNS).

DESIGN AND METHODS

CEST effects for lamivudine (3TC) and emtricitabine (FTC) were measured by asymmetric magnetization transfer ratio analyses. The biodistribution of 3TC in different brain sub-regions of C57BL/6 mice treated with lipopolysaccharides was determined using MRI. CEST effects of 3TC protons were quantitated by Lorentzian fitting analysis. 3TC levels in plasma and brain regions were measured using ultraperformance liquid chromatography tandem mass spectrometry to affirm the CEST test results.

RESULTS

CEST effects of the hydroxyl and amino protons in 3TC and FTC linearly correlated to drug concentrations. 3TC was successfully detected in vivo in brain sub-regions by MRI. The imaging results were validated by measurements of CNS drug concentrations.

CONCLUSION

CEST contrasts can be used to detect antiretroviral drugs using MRI. Such detection can be used to assess spatial--temporal drug biodistribution. This is most notable within the CNS where drug biodistribution may be more limited with the final goal of better understanding antiretroviral drug-associated efficacy and potential toxicity.

Imaging Self-Healing Hydrogels and Chemotherapeutics Using CEST MRI at 3 T

Imaging hydrogel-based local drug delivery to the brain after tumor resection has implications for refining treatments, especially for brain tumors with poor prognosis and high recurrence rate. Here, we developed a series of self-healing chitosan-dextran (CD)-based hydrogels for drug delivery to the brain. These hydrogels are injectable, self-healing, mechanically compatible, and detectable by chemical exchange saturation transfer magnetic resonance imaging (CEST MRI). CD hydrogels have an inherent CEST contrast at 1.1 ppm, which decreases as the stiffness increases. We further examined the rheological properties and CEST contrast of various chemotherapeutic-loaded CD hydrogels, including gemcitabine (Gem), doxorubicin, and procarbazine. Among these formulations, Gem presented the best compatibility with the rheological (G': 215.3 ± 4.5 Pa) and CEST properties of CD hydrogels. More importantly, the Gem-loaded CD hydrogel generated another CEST readout at 2.2 ppm (11.6 ± 0.1%) for monitoring Gem. This enabled independent and simultaneous imaging of the drug and hydrogel integrity using a clinically relevant 3 T MRI scanner. In addition, the Gem-loaded CD hydrogel exhibited a longitudinal antitumor efficacy of Gem over a week in vitro. Furthermore, the CD hydrogel could be visualized by CEST after brain injection with a contrast of 7.38 ± 2.31%. These natural labels on both the chemotherapeutics and hydrogels demonstrate unique image-guided local drug delivery for brain applications.

CEST MRI trackable nanoparticle drug delivery systems

Mounting evidence shows the great promise of nanoparticle drug delivery systems (nano-DDSs) to improve delivery efficiency and reduce off-target adverse effects. By tracking drug delivery and distribution, monitoring nanoparticle degradation and drug release, aiding and optimizing treatment planning, and directing the design of more robust nano-DDSs, image guidance has become a vital component of nanomedicine. Recently, chemical exchange saturation transfer (CEST) magnetic resonance imaging (MRI) has emerged as an attempting imaging method for achieving image-guided drug delivery. One of the unbeatable advantages of CEST MRI is its ability to detect diamagnetic compounds that cannot be detected using conventional MRI methods, making a broad spectrum of bioorganic agents, natural compounds, even nano-carriers directly MRI detectable in a high-spatial-resolution manner. To date, CEST MRI has become a versatile and powerful imaging technology for non-invasive in vivo tracking of nanoparticles and their loaded drugs. In this review, we will provide a concise overview of different forms of recently developed, CEST MRI trackable nano-DDSs, including liposomes, polymeric nanoparticles, self-assembled drug-based nanoparticles, and carbon dots. The potential applications and future perspectives will also be discussed.

Renal pH Imaging Using Chemical Exchange Saturation Transfer (CEST) MRI: Basic Concept

Magnetic Resonance Imaging (MRI) has been actively explored in the last several decades for assessing renal function by providing several physiological information, including glomerular filtration rate, renal plasma flow, tissue oxygenation and water diffusion. Within MRI, the developing field of chemical exchange saturation transfer (CEST) has potential to provide further functional information for diagnosing kidney diseases. Both endogenous produced molecules as well as exogenously administered CEST agents have been exploited for providing functional information related to kidney diseases in preclinical studies. In particular, CEST MRI has been exploited for assessing the acid-base homeostasis in the kidney and for monitoring pH changes in several disease models. This review summarizes several CEST MRI procedures for assessing kidney functionality and pH, for monitoring renal pH changes in different kidney injury models and for evaluating renal allograft rejection.This chapter is based upon work from the COST Action PARENCHIMA, a community-driven network funded by the European Cooperation in Science and Technology (COST) program of the European Union, which aims to improve the reproducibility and standardization of renal MRI biomarkers. This introduction chapter is complemented by two separate chapters describing the experimental procedure and data analysis.

In vivo detection of teriflunomide-derived fluorine signal during neuroinflammation using fluorine MR spectroscopy

Background: Magnetic resonance imaging (MRI) is indispensable for diagnosing neurological conditions such as multiple sclerosis (MS). MRI also supports decisions regarding the choice of disease-modifying drugs (DMDs). Determining in vivo tissue concentrations of DMDs has the potential to become an essential clinical tool for therapeutic drug monitoring (TDM). The aim here was to examine the feasibility of fluorine-19 (¹⁹F) MR methods to detect the fluorinated DMD teriflunomide (TF) during normal and pathological conditions.

Methods: We used ¹⁹F MR spectroscopy to detect TF in the experimental autoimmune encephalomyelitis (EAE) mouse model of multiple sclerosis (MS) in vivo. Prior to the in vivo investigations we characterized the MR properties of TF in vitro. We studied the impact of pH and protein binding as well as MR contrast agents.

Results: We could detect TF in vivo and could follow the ¹⁹F MR signal over different time points of disease. We quantified TF concentrations in different tissues using HPLC/MS and showed a significant correlation between ex vivo TF levels in serum and the ex vivo¹⁹F MR signal.

Conclusion: This study demonstrates the feasibility of ¹⁹F MR methods to detect TF during neuroinflammation in vivo. It also highlights the need for further technological developments in this field. The ultimate goal is to add ¹⁹F MR protocols to conventional ¹H MRI protocols in clinical practice to guide therapy decisions.

On the interference from agar in chemical exchange saturation transfer MRI parameter optimization in model solutions

Chemical exchange saturation transfer (CEST) MRI is currently set to become part of clinical routine as it enables indirect detection of low concentrated molecules and proteins. Recently, intermediate to fast exchanging functional groups of glucose and its derivatives, glutamate and dextran, have gained attention as promising CEST contrast agents. To increase the specificity of CEST MRI for certain functional groups, the presaturation module is commonly optimized. At an early stage, this is performed in well-defined model solutions, in which, for instance, the relaxation times are adjusted to mimic in vivo conditions. This often involves agar, assuming the substance would not yield significant CEST effects by itself, which the current study proves to be an invalid assumption. Model solutions at different pH values and concentrations of agar were investigated at different temperatures at a 9.4 T human whole body MR scanner. High power presaturation of around 4 μT, optimal for investigating intermediate to fast exchanging groups, was applied. Postprocessing included spatiotemporal corrections for B₀ and spatial corrections for B₁⁺ . CEST effects of up to 3 % of the bulk water signal were observed. From pH, concentration and temperature dependency, it was concluded that the observed behavior reflects a CEST effect of agar. It was also shown how to remove this undesirable contribution from CEST MRI data. It was concluded that if agar is involved in the CEST MRI parameter optimization process, its contribution to the observed effects has to be taken into account. CEST agent concentration must be sufficiently high to be able to neglect the contribution of agar, or a control sample at matched pH is necessary for correction. Experiments on pure agarose showed reduced CEST effects compared with agar but did not provide a neutral baseline either.

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.

Nanomedicine Particles Associated With Chemical Exchange Saturation Transfer Contrast Agents in Biomedical Applications

Theranostic agents are particles containing both diagnostic and medicinal agents in a single platform. Theranostic approaches often employ nanomedicine because loading both imaging probes and medicinal drugs onto nanomedicine particles is relatively straightforward, which can simultaneously provide diagnostic and medicinal capabilities within a single agent. Such systems have recently been described as nanotheranostic. Currently, nanotheranostic particles incorporating medicinal drugs are being widely explored with multiple imaging methods, including computed tomography, positron emission tomography, single-photon emission computed tomography, magnetic resonance imaging, and fluorescence imaging. However, most of these particles are metal-based multifunctional nanotheranostic agents, which pose potential toxicity or radiation risks. Hence, alternative non-metallic and biocompatible nanotheranostic agents are urgently needed. Recently, nanotheranostic agents that combine medicinal drugs and chemical exchange saturated transfer (CEST) contrast agents have shown good promise because CEST imaging technology can utilize the frequency-selective radiofrequency pulse from exchangeable protons to indirectly image without requiring metals or radioactive agents. In this review, we mainly describe the fundamental principles of CEST imaging, features of nanomedicine particles, potential applications of nanotheranostic agents, and the opportunities and challenges associated with clinical transformations.

CEST MRI detectable liposomal hydrogels for multiparametric monitoring in the brain at 3T

Adjuvant treatment using local drug delivery is applied in treating glioblastoma multiforme (GBM) after tumor resection. However, there are no non-invasive imaging techniques available for tracking the compositional changes of hydrogel-based drug treatment.

Methods: We developed Chemical Exchange Saturation Transfer Magnetic Resonance Imaging (CEST MRI) detectable and injectable liposomal hydrogel to monitor these events in vivo at 3T clinical field. Mechanical attributes of these hydrogels and their in vitro and in vivo CEST imaging properties were systematically studied.

Results: The MRI detectable hydrogels were capable of generating multiparametric readouts for monitoring specific components of the hydrogel matrix simultaneously and independently. Herein, we report, for the first time, CEST contrast at -3.4 ppm provides an estimated number of liposomes and CEST contrast at 5 ppm provides an estimated amount of encapsulated drug. CEST contrast decreased by 1.57% at 5 ppm, while the contrast at -3.4 ppm remained constant over 3 d in vivo, demonstrating different release kinetics of these components from the hydrogel matrix. Furthermore, histology analysis confirmed that the CEST contrast at -3.4 ppm was associated with liposome concentrations.

Conclusion: This multiparametric CEST imaging of individual compositional changes in liposomal hydrogels, formulated with clinical-grade materials at 3T and described in this study, has the potential to facilitate the refinement of adjuvant treatment for GBM.

Furin-mediated intracellular self-assembly of olsalazine nanoparticles for enhanced magnetic resonance imaging and tumour therapy

Among the strategies used for enhancement of tumour retention of imaging agents or anticancer drugs is the rational design of probes that undergo a tumour-specific enzymatic reaction preventing them from being pumped out of the cell. Here, the anticancer agent olsalazine (Olsa) was conjugated to the cell-penetrating peptide RVRR. Taking advantage of a biologically compatible condensation reaction, single Olsa-RVRR molecules were self-assembled into large intracellular nanoparticles by the tumour-associated enzyme furin. Both Olsa-RVRR and Olsa nanoparticles were readily detected with chemical exchange saturation transfer magnetic resonance imaging by virtue of exchangeable Olsa hydroxyl protons. In vivo studies using HCT116 and LoVo murine xenografts showed that the OlsaCEST signal and anti-tumour therapeutic effect were 6.5- and 5.2-fold increased, respectively, compared to Olsa without RVRR, with an excellent 'theranostic correlation' (R² = 0.97) between the imaging signal and therapeutic response (normalized tumour size). This furin-targeted, magnetic resonance imaging-detectable platform has potential for imaging tumour aggressiveness, drug accumulation and therapeutic response.

CT and CEST MRI bimodal imaging of the intratumoral distribution of iodinated liposomes

Background

To develop liposomes loaded with iodinated agents as nanosized CT/MRI bimodal contrast agents for monitoring liposome-mediated drug delivery.

Methods

Rhodamine-labeled iodixanol (Visipaque^TM)-loaded liposomes (IX-lipo) were prepared and tested for their properties as a diamagnetic CEST contrast agent in vitro. Mice bearing subcutaneous CT26 colon tumors were injected i.v. with 1 g/kg (535 mg iodine/kg) IX-lipo, and in vivo CT and CEST MR images were acquired on day 3. CT and CEST MR images were also acquired for tumor-bearing mice co-injected with IX-lipo and tumor necrosis factor (TNF-α).

Results

In addition to CT contrast, IX-lipo exhibited a strong CEST contrast similar to its non-liposomal form, with a detectability of ~2 nM per liposome. Both CT imaging and CEST MRI showed that i.v. injection of IX-lipo resulted in a rim enhancement of CT26 tumors with a heterogeneous central distribution. In contrast, co-injection of TNF-α caused a significantly augmented CT/MRI contrast in the tumor center. The intratumoral biodistribution of IX-lipo correlated well to the rhodamine patterns observed with fluorescence microscopy.

Conclusions

We have developed a CT/MRI bimodal imaging approach for monitoring the delivery and biodistribution of liposomes by loading them with the clinically approved X-ray/CT contrast agent iodixanol. Our approach may be easily adapted for other-FDA approved iodinated agents and thus has great translational potential.

Carbon Dots as a New Class of Diamagnetic Chemical Exchange Saturation Transfer (diaCEST) MRI Contrast Agents

While carbon dots (C-dots) have been extensively investigated pertaining to their fluorescent, phosphorescent, electrochemiluminescent, optoelectronic, and catalytic features, their inherent chemical exchange saturation transfer magnetic resonance imaging (CEST MRI) properties are unknown. By virtue of their hydrophilicity and abundant exchangeable protons of hydroxyl, amine, and amide anchored on the surface, we report here that C-dots can be adapted as effective diamagnetic CEST (diaCEST) MRI contrast agents. As a proof-of-concept demonstration, human glioma cells were labeled with liposomes with or without encapsulated C-dots and implanted in mouse brain. In vivo CEST MRI was able to clearly differentiate labeled cells from non-labeled cells. The present findings may encourage new applications of C-dots for in vivo imaging in deep tissues, which is currently not possible using conventional fluorescent (near-infrared) C-dots.

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 (T_2ex) effect under acidic and basic conditions, respectively. We recently investigated the T_2ex 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.

Molecular Imaging of Deoxycytidine Kinase Activity Using Deoxycytidine-Enhanced CEST MRI

Deoxycytidine kinase (DCK) is a key enzyme for the activation of a broad spectrum of nucleoside-based chemotherapy drugs (e.g., gemcitabine); low DCK activity is one of the most important causes of cancer drug-resistance. Noninvasive imaging methods that can quantify DCK activity are invaluable for assessing tumor resistance and predicting treatment efficacy. Here we developed a "natural" MRI approach to detect DCK activity using its natural substrate deoxycytidine (dC) as the imaging probe, which can be detected directly by chemical exchange saturation transfer (CEST) MRI without any synthetic labeling. CEST MRI contrast of dC and its phosphorylated form, dCTP, successfully discriminated DCK activity in two mouse leukemia cell lines with different DCK expression. This dC-enhanced CEST MRI in xenograft leukemic cancer mouse models demonstrated that DCK(+) tumors have a distinctive dynamic CEST contrast enhancement and a significantly higher CEST contrast than DCK(-) tumors (AUC^{0-60 min} = 0.47 ± 0.25 and 0.20 ± 0.13, respectively; P = 0.026, paired Student t test, n = 4) at 1 hour after the injection of dC. dC-enhanced CEST contrast also correlated well with tumor responses to gemcitabine treatment. This study demonstrates a novel MR molecular imaging approach for predicting cancer resistance using natural, nonradioactive, nonmetallic, and clinically available agents. This method has great potential for pursuing personalized chemotherapy by stratifying patients with different DCK activity. SIGNIFICANCE: A new molecular MRI method that detects deoxycytidine kinase activity using its natural substrate deoxycytidine has great translational potential for clinical assessment of tumor resistance and prediction of treatment efficacy.

Extradomain-B Fibronectin-Targeted Dextran-Based Chemical Exchange Saturation Transfer Magnetic Resonance Imaging Probe for Detecting Pancreatic Cancer

A dextran-peptide conjugate was developed for magnetic resonance (MR) molecular imaging of pancreatic ductal adenocarcinoma (PDAC) through its overexpressed microenvironment biomarker, extradomain-B fibronectin (EDB-FN). This new agent consists of diamagnetic and biocompatible dextran and a targeting peptide. Dextrans can be directly detected by chemical exchange saturation transfer magnetic resonance imaging (CEST MRI) without the need for radionuclide or metallic labeling. In addition, large molecular weight dextran, dextran 10 (MW ∼ 10 kDa), provides an approximately 50 times higher sensitivity per molecule than a single glucose unit. The potential of this highly biocompatible diamagnetic probe is demonstrated in a murine syngeneic allograft PDAC tumor model. The biocompatibility and sensitivity of this new agent clearly show potential for a path to clinical translation.

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.

Sugar-based biopolymers as novel imaging agents for molecular magnetic resonance imaging

Sugar-based biopolymers have been recognized as attractive materials to develop macromolecule- and nanoparticle-based cancer imaging and therapy. However, traditional biopolymer-based imaging approaches rely on the use of synthetic or isotopic labeling, and because of it, clinical translation often is hindered. Recently, a novel magnetic resonance imaging (MRI) technology, chemical exchange saturation transfer (CEST), has emerged, which allows the exploitation of sugar-based biopolymers as MRI agents by their hydroxyl protons-rich nature. In the study, we reviewed recent studies on the topic of CEST MRI detection of sugar-based biopolymers. The CEST MRI property of each biopolymer was briefly introduced, followed by the pre-clinical and clinical applications. The findings of these preliminary studies imply the enormous potential of CEST detectable sugar-based biopolymers in developing highly sensitive and translatable molecular imaging agents and constructing image-guided biopolymer-based drug delivery systems. This article is categorized under: Diagnostic Tools > in vivo Nanodiagnostics and Imaging Therapeutic Approaches and Drug Discovery > Emerging Technologies.

Precise delivery of a multifunctional nanosystem for MRI-guided cancer therapy and monitoring of tumor response by functional diffusion-weighted MRI

Herein we synthesize a cRGD peptide-conjugated PLGA nanosystem which is a high-efficiency drug-delivery platform for MR imaging-guided cancer theranostics.

Triazoles as T2 -Exchange Magnetic Resonance Imaging Contrast Agents for the Detection of Nitrilase Activity

We characterized the T₂ -exchange (T_2ex ) magnetic resonance imaging (MRI) contrast of azole protons that have large chemical shifts from the water proton resonance as a function of pH, temperature, and chemical modification. Our results showed that 1,2,4-triazoles could be tuned into excellent diamagnetic T_2ex contrast agents, with an optimal exchange-based relaxivity r_2ex of 0.10 s^-1  mm^-1 at physiological pH and B₀ =9.4 T. A fit of r_2ex data to the Swift-Connick equation indicated that imino proton exchange of triazoles is dominated by a base-catalyzed process at higher pH values and an acid-catalyzed process at lower pH. The magnitude of r_2ex was also found to be heavily dependent on chemical modifications, that is, enhanced by electron-donating groups, such as amines and methyls, or by intramolecular hydrogen bonding between the imino proton and the carboxyl, and weakened by electron-withdrawing groups like bromo, cyano, and nitro. In light of these findings, we applied T_2ex MRI to assess the activity of nitrilase, an enzyme catalyzing the hydrolysis of 1,2,4-triazole-3-carbonitrile to 1,2,4-triazole-3-carboxylic acid, resulting in the enhancement of R_2ex . Our findings suggest that 1,2,4-triazoles have potential to provide sensitive and tunable diagnostic probes for MRI.

In vivo drug tracking with 19F MRI at therapeutic dose

Tracking drugs with 19F MRI would be beneficial for developing theranostics and optimizing drug therapy. To this end, a fluorinated dendritic amphiphile with high 19F MRI sensitivity and biocompatibility has been developed for 19F MRI tracking of doxorubicin (DOX)-loaded liposomes in mice, which may provide an effective platform to in vivo trace various drugs with 19F MRI.

Theranostics and metabolotheranostics for precision medicine in oncology

Most diseases, especially cancer, would significantly benefit from precision medicine where treatment is shaped for the individual. The concept of theragnostics or theranostics emerged around 2002 to describe the incorporation of diagnostic assays into the selection of therapy for this purpose. Increasingly, theranostics has been used for strategies that combine noninvasive imaging-based diagnostics with therapy. Within the past decade theranostic imaging has transformed into a rapidly expanding field that is located at the interface of diagnosis and therapy. A critical need in cancer treatment is to minimize damage to normal tissue. Molecular imaging can be applied to identify targets specific to cancer with imaging, design agents against these targets to visualize their delivery, and monitor response to treatment, with the overall purpose of minimizing collateral damage. Genomic and proteomic profiling can provide an extensive 'fingerprint' of each tumor. With this cancer fingerprint, theranostic agents can be designed to personalize treatment for precision medicine of cancer, and minimize damage to normal tissue. Here, for the first time, we have introduced the term 'metabolotheranostics' to describe strategies where disease-based alterations in metabolic pathways detected by MRS are specifically targeted with image-guided delivery platforms to achieve disease-specific therapy. The versatility of MRI and MRS in molecular and functional imaging makes these technologies especially important in theranostic MRI and 'metabolotheranostics'. Our purpose here is to provide insights into the capabilities and applications of this exciting new field in cancer treatment with a focus on MRI and MRS.

Assessment of in vivo degradation profiles of hyaluronic acid hydrogels using temporal evolution of chemical exchange saturation transfer (CEST) MRI

Hyaluronic acid (HA) hydrogels have found a wide range of applications in biomedicine: regenerative medicine to drug delivery applications. In vivo quantitative assessment of these hydrogels using magnetic resonance imaging (MRI) provides an effective, accurate, safe, and non-invasive translational approach to assess the biodegradability of HA hydrogels. Chemical exchange saturation transfer (CEST) is an MRI contrast enhancement technique that overcomes the concentration limitation of other techniques like magnetic resonance spectroscopy (MRS) by detecting metabolites at up to two orders of magnitude or higher. In this study, HA hydrogels were synthesized based on different crosslinking agents and assessed using CEST MRI to investigate the in vivo degradation profiles of these gels in a mouse subcutaneous injection model over a three-month period. Nature of crosslinking agents was found to influence their degradation profiles. Since CEST MRI provides a unique chemical signature to visualize HA hydrogels, our studies proved that this technique could be used as a guide in the hydrogel optimization process for drug delivery and regenerative medicine applications.

Phenols as Diamagnetic T2 -Exchange Magnetic Resonance Imaging Contrast Agents

Although T2 -exchange (T2ex ) NMR phenomena have been known for decades, there has been a resurgence of interest to develop T2ex MRI contrast agents. One indispensable advantage of T2ex MR agents is the possibility of using non-toxic and/or bio-compatible diamagnetic compounds with intermediate exchangeable protons. Herein a library of phenol-based compounds is screened and their T2ex contrast (exchange relaxivity, r2ex ) at 9.4 T determined. The T2ex contrast of phenol protons allows direct detection by MRI at a millimolar concentration level. The effect of chemical modification of the phenol on the T2ex MRI contrast through modulation of exchange rate and chemical shift was also studied and provides a guideline for use of endogenous and exogenous phenols for T2ex MRI contrast. As a proof-of-principle application, phenol T2ex contrast can be used to detect enzyme activity in a tyrosinase-catalyzed catechol oxidation reaction.

A dextran-based probe for the targeted magnetic resonance imaging of tumours expressing prostate-specific membrane antigen

Safe imaging agents able to render the expression and distribution of cancer receptors, enzymes or other biomarkers would facilitate clinical screening of the disease. Here, we show that diamagnetic dextran particles coordinated to a urea-based targeting ligand for prostate-specific membrane antigen (PSMA) enable targeted magnetic resonance imaging (MRI) of the PSMA receptor. In a xenograft model of prostate cancer, micromolar concentrations of the dextran –ligand probe provided sufficient signal to specifically detect PSMA-expressing tumours via chemical exchange saturation transfer MRI. The dextran-based probe could be detected via the contrast originating from dextran hydroxyl protons, thereby avoiding the need of chemical substitution for radioactive or metallic labelling. Because dextrans are currently used clinically, dextran-based contrast agents may help extend receptor-targeted imaging to clinical MRI.

Characterization of tumor vascular permeability using natural dextrans and CEST MRI

PURPOSE

To investigate the use of natural dextrans as nano-sized chemical exchange saturation transfer (CEST) MRI probes for characterizing size-dependent tumor vascular permeability.

METHODS

Dextrans of different molecular weight (10, 70, 150, and 2000 kD) were characterized for their CEST contrast. Mice (N = 5) bearing CT26 subcutaneous colon tumors were injected intravenously with 10 kD (D10, 6 nm) and 70 kD (D70, 12 nm) dextran at a dose of 375 mg/kg. The CEST-MRI signal in the tumors was assessed before and approximately 40 min after each injection using a dynamic CEST imaging scheme.

RESULTS

All dextrans of different molecular weights have a strong CEST signal with an apparent maximum of approximately 0.9 ppm. The detectability and effects of pH and saturation conditions (B₁ and T_sat ) were investigated. When applied to CT26 tumors, the injection of D10 could produce a significant "dexCEST" enhancement in the majority of the tumor area, whereas the injection of D70 only resulted in an increase in the tumor periphery. Quantitative analysis revealed the differential permeability of CT26 tumors to different size particles, which was validated by fluorescence imaging and immunohistochemistry.

CONCLUSIONS

As a first application, we used 10- and 70-kD dextrans to visualize the spatially variable, size-dependent permeability in the tumor, indicating that nano-sized dextrans can be used for characterizing tumor vascular permeability with dexCEST MRI and, potentially, for developing dextran-based theranostic drug delivery systems. Magn Reson Med 79:1001-1009, 2018. © 2017 International Society for Magnetic Resonance in Medicine.

Clinical applications of chemical exchange saturation transfer (CEST) MRI

Chemical exchange saturation transfer (CEST) magnetic resonance imaging (MRI) has been developed and employed in multiple clinical imaging research centers worldwide. Selective radiofrequency (RF) saturation pulses with standard 2D and 3D MRI acquisition schemes are now routinely performed, and CEST MRI can produce semiquantitative results using magnetization transfer ratio asymmetry (MTR_asym ) analysis while accounting for B₀ inhomogeneity. Faster clinical CEST MRI acquisition methods and more quantitative acquisition and analysis routines are under development. Endogenous biomolecules with amide, amine, and hydroxyl groups have been detected during clinical CEST MRI studies, and exogenous CEST agents have also been administered to patients. These CEST MRI tools show promise for contributing to assessments of cerebral ischemia, neurological disorders, lymphedema, osteoarthritis, muscle physiology, and solid tumors. This review summarizes the salient features of clinical CEST MRI protocols and critically evaluates the utility of CEST MRI for these clinical imaging applications.

LEVEL OF EVIDENCE

5 Technical Efficacy: Stage 1 J. Magn. Reson. Imaging 2018;47:11-27.

Detection and Quantification of Hydrogen Peroxide in Aqueous Solutions Using Chemical Exchange Saturation Transfer

The development of new analytical methods to accurately quantify hydrogen peroxide is of great interest. In the current study, we developed a new magnetic resonance (MR) method for noninvasively quantifying hydrogen peroxide (H2O2) in aqueous solutions based on chemical exchange saturation transfer (CEST), an emerging MRI contrast mechanism. Our method can detect H2O2 by its specific CEST signal at ∼6.2 ppm downfield from water resonance, with more than 1000 times signal amplification compared to the direct NMR detection. To improve the accuracy of quantification, we comprehensively investigated the effects of sample properties on CEST detection, including pH, temperature, and relaxation times. To accelerate the NMR measurement, we implemented an ultrafast Z-spectroscopic (UFZ) CEST method to boost the acquisition speed to 2 s per CEST spectrum. To accurately quantify H2O2 in unknown samples, we also implemented a standard addition method, which eliminated the need for predetermined calibration curves. Our results clearly demonstrate that the presented CEST-based technique is a simple, noninvasive, quick, and accurate method for quantifying H2O2 in aqueous solutions.

One-Component Supramolecular Filament Hydrogels as Theranostic Label-Free Magnetic Resonance Imaging Agents

Gadolinium (Gd)-based compounds and materials are the most commonly used magnetic resonance imaging (MRI) contrast agents in the clinic; however, safety concerns associated with their toxicities in the free ionic form have promoted the development of new generations of metal-free contrast agents. Here we report a supramolecular strategy to convert an FDA-approved anticancer drug, Pemetrexed (Pem), to a molecular hydrogelator with inherent chemical exchange saturation transfer (CEST) MRI signals. The rationally designed drug-peptide conjugate can spontaneously associate into filamentous assemblies under physiological conditions and consequently form theranostic supramolecular hydrogels for injectable delivery. We demonstrated that the local delivery and distribution of Pem-peptide nanofiber hydrogels can be directly assessed using CEST MRI in a mouse glioma model. Our work lays out the foundation for the development of drug-constructed theranostic supramolecular materials with an inherent CEST MRI signal that enables noninvasive monitoring of their in vivo distribution and drug release.

Label-free CEST MRI Detection of Citicoline-Liposome Drug Delivery in Ischemic Stroke

Citicoline (CDPC) is a natural supplement with well-documented neuroprotective effects in the treatment of neurodegenerative diseases. In the present study, we sought to exploit citicoline as a theranostic agent with its inherent chemical exchange saturation transfer (CEST) MRI signal, which can be directly used as an MRI guidance in the citicoline drug delivery. Our in vitro CEST MRI results showed citicoline has two inherent CEST signals at +1 and +2 ppm, attributed to exchangeable hydroxyl and amine protons, respectively. To facilitate the targeted drug delivery of citicoline to ischemic regions, we prepared liposomes encapsulating citicoline (CDPC-lipo) and characterized the particle properties and CEST MRI properties. The in vivo CEST MRI detection of liposomal citicoline was then examined in a rat brain model of unilateral transient ischemia induced by a two-hour middle cerebral artery occlusion. The results showed that the delivery of CPDC-lipo to the brain ischemic areas could be monitored and quantified by CEST MRI. When administered intra-arterially, CDPC-lipo clearly demonstrated a detectable CEST MRI contrast at 2 ppm. CEST MRI revealed that liposomes preferentially accumulated in the areas of ischemia with a disrupted blood-brain-barrier. We furthermore used CEST MRI to detect the improvement in drug delivery using CDPC-lipo targeted against vascular cell adhesion molecule (VCAM)-1 in the same animal model. The MRI findings were validated using fluorescence microscopy. Hence, liposomal citicoline represents a prototype theranostic system, where the therapeutic agent can be detected directly by CEST MRI in a label-free fashion.