Lanthanide ion (III) complexes of 1,4,7,10‐tetraazacyclododecane‐1,4,7,10‐tetraaminophosphonate for dual biosensing of pH with chemical exchange saturation transfer (CEST) and biosensor imaging of redundant deviation in shifts (BIRDS)

Relaxivity‐based magnetic resonance of phosphonated ligands chelated with gadolinium (Gd³⁺) shows promise for pH imaging. However instead of monitoring the paramagnetic effect of lanthanide complexes on the relaxivity of water protons, biosensor (or molecular) imaging with magnetic resonance is also possible by detecting either the nonexchangeable or the exchangeable protons on the lanthanide complexes themselves. The nonexchangeable protons (e.g. –CH_x, where 3 ≥ x ≥ 1) are detected using a three‐dimensional chemical shift imaging method called biosensor imaging of redundant deviation in shifts (BIRDS), whereas the exchangeable protons (e.g. –OH or –NH_y, where 2 ≥ y ≥ 1) are measured with chemical exchange saturation transfer (CEST) contrast. Here we tested the feasibility of BIRDS and CEST for pH imaging of 1,4,7,10‐tetraazacyclododecane‐1,4,7,10‐tetraaminophosphonate (DOTA‐4AmP^8?) chelated with thulium (Tm³⁺) and ytterbium (Yb³⁺). BIRDS and CEST experiments show that both complexes are responsive to pH and temperature changes. Higher pH and temperature sensitivities are obtained with BIRDS for either complex when using the chemical shift difference between two proton resonances vs using the chemical shift of a single proton resonance, thereby eliminating the need to use water resonance as reference. While CEST contrast for both agents is linearly dependent on pH within a relatively large range (i.e. 6.3–7.9), much stronger CEST contrast is obtained with YbDOTA‐4AmP^5? than with TmDOTA‐4AmP^5?. In addition, we demonstrate the prospect of using BIRDS to calibrate CEST as new platform for quantitative pH imaging. Copyright © 2014 John Wiley & Sons, Ltd.     

Modulation of water exchange in Eu(III) DOTA–tetraamide complexes: considerations for in vivo imaging of PARACEST agents

Modulation of water exchange in lanthanide(III)–DOTA type complexes has drawn considerable attention over the past two decades, particularly because of their application as contrast agents for magnetic resonance imaging. LnDOTA–tetraamide complexes display unusually slow water exchange kinetics and this chemical property offers an opportunity to use these complexes as a new type of contrast agent based upon the chemical exchange saturation transfer (CEST) mechanism. Six new DOTA–tetraamide ligands having side‐chain amide arms with varying hydrophobicity and polarity were prepared and the water exchange characteristic of complexes formed with europium(III) complexes were investigated. The results show that introduction of steric bulk into the amide side‐chain arms of the europium(III) complexes not only favors formation of the mono‐capped twisted square antiprism coordination isomers, the isomer that is generally less favourable for CEST, but also accelerates water exchange in the mono‐capped square antiprism isomers. However, converting single methyl groups on these bulky arms to carboxyl or carboxyl ethyl esters results in a rather dramatic decrease in water exchange rates, about 50‐fold. Thus, steric bulk, polarity and hydrophobicity of the amide side‐chains each contribute to organization of water molecules in the second hydration sphere of the europium(III) ion and this in turn controls water exchange in these complexes. Copyright © 2009 John Wiley & Sons, Ltd.     

Entrapment of a neutral Tm(III)‐based complex with two inner‐sphere coordinated water molecules into PEG‐stabilized vesicles: towards an alternative strategy to develop high‐performance LipoCEST contrast agents for MR imaging

Chemical exchange saturation transfer (CEST) probes issued from the encapsulation of a water proton paramagnetic shift reagent into the inner aqueous volume of lipid vesicles provide an emerging class of frequency‐selective contrast agents with huge potential in the field of molecular magnetic resonance imaging (MRI). This work deals with the generation of such LipoCEST agents properly designed to optimize, under isotonic conditions, the chemical shift offset of the intra‐liposomal water protons as well as the number of exchangeable protons under reasonably low radiofrequency (RF) fields of saturation. The strategy lies in the loading of poly(ethylene glycol)‐stabilized nanosized liposomes with uncharged lanthanide chelates, binding more than one water molecule in the first hydration sphere, exemplified here by [Tm(III)–DO3A (H₂O)₂] complex. The key properties of the probes are demonstrated by complementary NMR investigations. The residence lifetime of the water molecules coordinated to the lanthanide center was outstandingly short (9.5 ± 0.2 ns from ¹⁷O NMR), and indeed relevant for effective LipoCEST responsiveness. The ¹H NMR CEST spectra (7.01 T magnetic field) prove that the theoretically expected optimal sensitivity can be approximated in the nanomolar concentration range, at reasonably low RF presaturation pulses (6.7–12 μT) and saturation frequency offsets of the intra‐liposomal water protons beyond 10 ppm, making possible selective irradiation in biological environment. CEST‐MRI images (7.01 T magnetic field and 10–12 μT RF pulse) explicitly confirm the interest of these newly conceived LipoCEST agents, indeed among the most efficient ones developed so far under isosmotic conditions. Copyright © 2014 John Wiley & Sons, Ltd.     

Nanoparticle‐based PARACEST agents: the quenching effect of silica nanoparticles on the CEST signal from surface‐conjugated chelates

Silica nanoparticles of average diameter 53 ± 3 nm were prepared using standard water‐in‐oil microemulsion methods. After conversion of the surface Si–OH groups to amino groups for further conjugation, the PARACEST agent, EuDOTA–(gly)₄^? was coupled to the amines via one or more side‐chain carboxyl groups in an attempt to trap water molecules in the inner‐sphere of the complex. Fluorescence and ICP analyses showed that ~1200 Eu³⁺ complexes were attached to each silica nanoparticle, leaving behind excess protonated amino groups. CEST spectra of the modified silica nanoparticles showed that attachment of the EuDOTA–(gly)₄^? to the surface of the nanoparticles did not result in a decrease in water exchange kinetics as anticipated, but rather resulted in a complete elimination of the normal Eu³⁺‐bound water exchange peak and broadening of the bulk water signal. This observation was traced to catalysis of proton exchange from the Eu³⁺‐bound water molecule by excess positively charged amino groups on the surface of the nanoparticles. Copyright © 2012 John Wiley & Sons, Ltd.     

Dendritic PARACEST contrast agents for magnetic resonance imaging

MRI contrast agents based on chemical exchange‐dependent saturation transfer (CEST), such as Yb(III)DOTAM complexes, are highly suitable for pH mapping. In this paper, the synthesis of Yb(III)DOTAM‐functionalized poly(propylene imine) dendrimers is described. The applicability of these dendritic PARACEST MRI agents for pH mapping has been evaluated on a 7 T NMR spectrometer and on a 3 T clinical MRI scanner. As expected, based on the different numbers of exchangeable amide protons, the lowest detectable concentration of the first and third generation dendritic PARACEST agents is by a respective factor of about 4 and 16 lower than that of a mononuclear reference complex. The pH dependence of the CEST effect observed for these compounds depends on the generation of the poly(propylene imine) dendrimer. Upon going to higher generations of the Yb(III)DOTAM‐terminated dendrimer, a shift of the maximum CEST effect towards lower pH values was observed. This allows for a fine‐tuning of the responsive pH region by varying the dendritic framework. Copyright © 2007 John Wiley & Sons, Ltd.     

Spectral properties of a bifunctional PARACEST europium chelate: an intermediate for targeted imaging applications

This report describes a preliminary study to investigate the effect of structural variations in a bifunctional ligand being developed for targeted PARACEST applications. Utilizing Eu‐DOTA‐4AmCE as the functional PARACEST core, a nitro‐aromatic functionality was incorporated to provide the point for conjugation. This intermediate possesses a coordinated water proton signal at 57 ppm which upon saturation produces a 22% decrease in bulk water signal intensity using 10 mM agent. Curve fitting analysis of the CEST spectrum to an exchange model based upon modified Bloch equations gave an estimate of 687 ± 100 µs for the bound water lifetime (τ_m). Although substantially longer than the 382 µs observed for the parent chelate structure, this water exchange rate remains in the necessary window to produce a significant CEST effect. These findings demonstrate the need to recognize the influence of bifunctional linkers on water exchange rates in lanthanide complexes of this type and the potential for using this functionality as a means of fine‐tuning PARACEST performance. Copyright © 2007 John Wiley & Sons, Ltd.     

Improved pH measurements with a single PARACEST MRI contrast agent

The measurement of extracellular pH has potential utility for assessing the therapeutic effects of pH‐dependent and pH‐altering therapies. A PARAmagnetic chemical exchange saturation transfer (PARACEST) MRI contrast agent, Yb–DO3A–oAA, has two CEST effects that are dependent on pH. A ratio derived from these CEST effects was linearly correlated with pH throughout the physiological pH range. The pH can be measured with a precision of 0.21 pH units and an accuracy of 0.09 pH units. The pH measurement is independent of concentration and T₁ relaxation times, but is dependent on temperature. Although MR coalescence affects the CEST measurements, especially at high pH, the ratiometric analysis of the CEST effects can account for incomplete saturation of the agent's amide and amine that results from MR coalescence. Provided that an empirical calibration is determined with saturation conditions, magnetic field strength and temperature that can be used for subsequent studies, these results demonstrate that this single PARACEST MRI contrast agent can accurately measure pH. Copyright © 2012 John Wiley & Sons, Ltd.     

Synthesis and evaluation of a polydisulfide with Gd–DOTA monoamide side chains as a biodegradable macromolecular contrast agent for MR blood pool imaging

Macromolecular Gd(III)‐based contrast agents are effective for contrast‐enhanced blood pool and cancer MRI in preclinical studies. However, their clinical applications are impeded by potential safety concerns associated with slow excretion and prolonged retention of these agents in the body. To minimize the safety concerns of macromolecular Gd contrast agents, we have developed biodegradable macromolecular Gd contrast agents based on polydisulfide Gd(III) complexes. In this study, we designed and synthesized a new generation of the polydisulfide Gd(III) complexes containing a macrocyclic Gd(III) chelate, Gd–DOTA monoamide, to improve the in vivo kinetic inertness of the Gd(III) chelates. (N₆‐Lysyl)lysine‐(Gd–DOTA) monoamide and 3‐(2‐carboxyethyldisulfanyl)propanoic acid copolymers (GODC) were synthesized by copolymerization of (N₆‐lysyl)lysine DOTA monoamide and dithiobis(succinimidylpropionate), followed by complexation with Gd(OAc)₃. The GODC had an apparent molecular weight of 26.4 kDa and T₁ relaxivity of 8.25 m m ^?1 s^?1 per Gd at 1.5 T. The polymer chains of GODC were readily cleaved by l ‐cysteine and the chelates had high kinetic stability against transmetallation in the presence of an endogenous metal ion Zn²⁺. In vivo MRI study showed that GODC produced strong and prolonged contrast enhancement in the vasculature and tumor periphery of mice with breast tumor xenografts. GODC is a promising biodegradable macromolecular MRI contrast agent with high kinetic stability for MR blood pool imaging. Copyright © 2013 John Wiley & Sons, Ltd.     

Simultaneous experimental determination of labile proton fraction ratio and exchange rate with irradiation radio frequency power‐dependent quantitative CEST MRI analysis

Chemical exchange saturation transfer (CEST) imaging is sensitive to dilute proteins/peptides and microenvironmental properties, and has been increasingly evaluated for molecular imaging and in vivo applications. However, the experimentally measured CEST effect depends on the CEST agent concentration, exchange rate and relaxation time. In addition, there may be non‐negligible direct radio‐frequency (RF) saturation effects, particularly severe for diamagnetic CEST (DIACEST) agents owing to their relatively small chemical shift difference from that of the bulk water resonance. As such, the commonly used asymmetry analysis only provides CEST‐weighted information. Recently, it has been shown with numerical simulation that both labile proton concentration and exchange rate can be determined by evaluating the RF power dependence of DIACEST effect. To validate the simulation results, we prepared and imaged two CEST phantoms: a pH phantom of serially titrated pH at a fixed creatine concentration and a concentration phantom of serially varied creatine concentration titrated to the same pH, and solved the labile proton fraction ratio and exchange rate per‐pixel. For the concentration phantom, we showed that the labile proton fraction ratio is proportional to the CEST agent concentration with negligible change in the exchange rate. Additionally, we found the exchange rate of the pH phantom is dominantly base‐catalyzed with little difference in the labile proton fraction ratio. In summary, our study demonstrated quantitative DIACEST MRI, which remains promising to augment the conventional CEST‐weighted MRI analysis. Copyright © 2013 John Wiley & Sons, Ltd.     

Bifunctional Gd(III) and Tb(III) chelates based on a pyridine–bis(iminodiacetate) platform,suitable optical probes and contrast agents for magnetic resonance imaging

To study the physicochemical properties of lanthanide complexes derived from a bifunctional chelating agent based on a PMN‐tetraacetic acid moiety {PMN‐tetraacetic acid (1): [2,6‐pyridinediylbis(methylene nitrilo)‐tetraacetic acid]}, 4‐carboxylic acid substituted pyridine derivative (2) was synthesized. This ligand forms heptadentate (N₃O₄) Ln(III) complexes (Ln = Gd, Eu, Tb), with two water molecules completing the inner coordination sphere of the metal. The parameters that govern the relaxivity of the Gd(III) complex and the luminescence of Eu(III) and Tb(III) complexes were obtained by ¹⁷O and ¹H NMR studies and time‐resolved fluorescence experiments, respectively. The gadolinium and terbium complexes show interesting properties either for MRI or FOR optical imaging; that is, for the Gd complex, a high proton relaxivity (r₁ = 6.4 s^?1 mm ^?1 at 20 MHz) with short water residence time (τ_M = 38.5 ns); for the Tb complex, a luminescence lifetime of 1.22 ms at room temperature and a luminescence quantum yield of 10%. The kinetic stability of these complexes toward blood protein, cation or bioactive oxyanion was also examined. The Gd(2)(H₂O)₂ complex does not interact with human serum albumin, but undergoes a transmetalation reaction with Zn(II) in a phosphate buffer solution (pH 7.4), rather similar to that of Gd–DTPA–BMA(H₂O). On the other hand, as observed for Eu and Tb complexes, these chelates do not form ternary complexes with bidentate anions such as l ‐lactate, citrate or carbonate. Finally, a phosphatidylserine‐specific hexapeptide (TLVSSL) was grafted on Gd or Tb chelates, and the Gd–peptide conjugate was used in vitro for targeting apoptotic cells. Copyright © 2014 John Wiley & Sons, Ltd.     

A gadolinium triacetic monoamide DOTA derivative with a methanethiosulfonate anchor group. Relaxivity properties and conjugation with albumin and thiolated particles

The gadolinium(III) complex with a new DOTA‐based ligand bearing a methanethiosulfonate group (MTS) was synthesized and its relaxivity properties were investigated. MTS‐ADO3A is a triacid DOTA derivative with an amide arm substituted by an ethylmethanethiosulfonate function. This ligand was obtained in two steps: tri‐tert‐butyl 2,2′,2″‐(1,4,7,10‐tetraazacyclododecane‐1,4,7‐triyl)triacetate was reacted with S‐(2‐aminoethyl)methanesulfonothioate and the tert‐butyl groups were removed with trifluoroacetic acid. The Gd(III) MTS–ADO3A complex readily formed disulfide bonds with albumin (BSA) in its native and reduced forms and with thiolated silica particles. Four‐ to five‐fold relaxivity increases at 20 MHz were measured on the isolated adducts. The EuMTS‐ADO3A chelate was found to be monohydrated by fluorescence and the relaxivity parameters of the Gd(III) complex were obtained by ¹⁷O NMR and by measuring the nuclear magnetic relaxation dispersion between 0.01 and 80 MHz. The water exchange time τ_m is increased upon forming disulfide bonds with macromolecules and particles and the relaxivity gains of all the complexes are limited by the τ_m factor. Forming covalent or hydrophobic/electrostatic bonds with BSA seems to bring about similar relaxivity changes but the covalent BSA adducts can be isolated and their properties can be directly studied. The addition of dithiothreitol or glutathione leads to the removal of the metal chelates from the macromolecules, as indicated by the relaxation times reverting to their values before binding. It is thus expected that the chelate will stay in the body long enough for imaging but will still be excreted through the kidneys. Copyright © 2007 John Wiley & Sons, Ltd.     

The effects of intramolecular H‐bond formation on the stability constant and water exchange rate of the Gd(III)‐diethylenetriamine‐N′‐(3‐amino‐1,1‐propylenephosphonic)‐N,N,N″,N″‐tetraacetate complex

The binding interaction of metal chelates to biological macromolecules, though driven by properly devoted recognition synthons, may cause dramatic changes in some property associated with the coordination cage such as the thermodynamic stability or the exchange rate of the metal coordinated water. Such changes are due to electrostatic and H‐bonding interactions involving atoms of the coordination cage and atoms of the biological molecule at the binding site. To mimic this type of H‐bonding interactions, lanthanide(III) complexes with a DTPA–monophosphonate ligand bearing a propylamino moiety (H₆NP–DTPA) were synthesized. Their thermodynamic stabilities and the exchange lifetime of the coordinated water molecule (for the Gd‐complex) were compared with those of the analog complexes with DTPA and the parent DTPA–monophosphonate derivative (H₆P–DTPA). It was found that the intramolecular H‐bond between the ε‐amino group and the phosphonate moiety in NP–DTPA complexes causes displacements of electric charges in their coordination cage that are markedly pH dependent. In turn, this affects the characteristic properties of the coordination cage. In particular it results in a marked elongation of the exchange lifetime of the coordinated water molecule. Copyright © 2007 John Wiley & Sons, Ltd.     

In vitro study of endogenous CEST agents at 3 T and 7 T

Chemical exchange saturation transfer (CEST) has been an intensive research area in MRI, providing contrast mechanisms for the amplified detection and monitoring of biomarkers and physiologically active molecules. In biological tissues and organs, many endogenous CEST agents coexist, and their CEST effects may overlap. The interpretation of such overlapped CEST effects can be addressed when the individual CEST effects originating from various metabolites are characterized. In this work, we present the in vitro measurements of the CEST effects from endogenous CEST agents that are commonly found in biological tissues and organs, at the external magnetic fields of 3 T and 7 T and under various pH conditions. Together with the proton NMR spectra measured at 11.7 T, these CEST effects have been evaluated in consideration of the chemical exchange rates, chemical shifts, and acidities of the labile protons. Amine protons of small metabolites might not be visible at 3 T, but some of them can be probed at 7 T, wherein their CEST effects may overlap with those from coexisting amide and hydroxyl protons.     

DOTAM‐type ligands possessing arginine pendant groups for use in PARACEST MRI

A synthetic methodology was developed for the preparation of metal‐chelating ligands that possess arginine pendant groups relying on the alkylation of 1,4,7,10‐tetraazacyclododecane (cyclen) with arginine‐containing electrophiles. Conditions for the selective trialkylation or peralkylation of cyclen are described, the outcome being dependent on the nature of the arginine‐derived electrophile and the solvent used for the reaction. Lanthanide metal complexes of the ligands prepared by the described route were evaluated for their suitability as PARACEST contrast agents for use in magnetic resonance imaging. The Dy³⁺ and Tm³⁺ complexes display CEST effects that are associated with the amide protons proximate to the metal center. These signals exhibit pH dependence in the range of 6.0–8.0 and thus may have the potential for pH measurement in physiological range. Copyright © 2012 John Wiley & Sons, Ltd.     

Towards MRI contrast agents responsive to Ca(II) and Mg(II) ions: metal‐induced oligomerization of dota–bisphosphonate conjugates

In magnetic resonance imaging (MRI), paramagnetic complexes are utilized as contrast agents. Much attention has been paid to the development of new contrast agents responsive to pH, temperature or concentration of various components of body liquids. We report a new type of MRI probe sensing the concentrations of calcium and magnesium in biological media. The ligand do3ap^BP combines a dota‐like chelator with a bisphosphonate group. In the complex, the Gd(III ) ion is entrapped in the macrocyclic cavity whereas the bisphosphonate group is not coordinated and therefore is available for coordination with endogenous metal ions. In the presence of metal ions, Gd–do3ap^BP appears to show formation of coordination oligomers leading to an unprecedented increase in r₁ up to 200–500%. The extremely high relaxivity response makes this type of compound interesting for further studies as MRI ion‐responsive probes for biomedical research. Copyright © 2010 John Wiley & Sons, Ltd.     

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

Porphyrazines (Pz), or tetraazaporphyrins, are being studied for their potential use in detection and treatment of cancer. Here, an amphiphilic Cu–Pz–Gd(III) conjugate has been prepared via azide–alkyne Huisgen cycloaddition or ‘click’ chemistry between an azide functionalized Pz and alkyne functionalized DOTA–Gd(III) analog for use as an MRI contrast agent. This agent, Cu–Pz–Gd(III), is synthesized in good yield and exhibits solution‐phase ionic relaxivity (r₁ = 11.5 mm ^?1 s^?1) that is approximately four times higher than that of a clinically used monomeric Gd(III) contrast agent, DOTA–Gd(III). Breast tumor cells (MDA‐MB‐231) associate with Cu–Pz–Gd(III) in vitro, where significant contrast enhancement (9.336 ± 0.335 contrast‐to‐noise ratio) is observed in phantom cell pellet MR images. This novel contrast agent was administered in vivo to an orthotopic breast tumor model in athymic nude mice and MR images were collected. The average T₁ of tumor regions in mice treated with 50 mg kg^?1 Cu–Pz–Gd(III) decreased relative to saline‐treated controls. Furthermore, the decrease in T₁ was persistent relative to mice treated with the monomeric Gd(III) contrast agent. An ex vivo biodistribution study confirmed that Cu–Pz–Gd(III) accumulates in the tumors and is rapidly cleared, primarily through the kidneys. Differential accumulation and T₁ enhancement by Cu–Pz–Gd(III) in the tumor's core relative to the periphery offer preliminary evidence that this agent would find application in the imaging of necrotic tissue. Copyright © 2014 John Wiley & Sons, Ltd.     

Improved synthesis of DOTA tetraamide ligands for lanthanide(III) ions: A tool for increasing the repertoire of potential PARACEST contrast agents for MRI and/or fluorescent sensors

The synthesis of new DOTA tetraamide (DOTAMR₄) compounds is of great interest given their application in the formation of Ln(III) complexes as potential PARACEST contrast agents in MRI or fluorescent molecular probes. In this context amino acid and peptide DOTAMR₄ derivatives are particularly attractive since the amino‐acid and/or peptide moiety can show responsive properties dependent on a given stimuli which might translate to changes in water exchange rates of the corresponding Ln(III) complex. Current synthesis of DOTAMR₄ derivatives is typically carried out by reacting haloacetamide intermediates with cyclen. However, this method fails to generate the tetra‐substituted products when bulky substituents are present in the haloacetamide and in some cases this intermediate cannot be prepared by conventional acylation procedures limiting the number of DOTAMR₄ compounds available for study. As a solution to these limitations, an improved methodology for the synthesis of DOTAMR₄ by coupling DOTA to an appropriate amine containing reagent (i.e. protected amino‐acids with the α‐amino group free) is presented in this work. Several DOTAMR₄ derivatives which are difficult or impossible to prepare with the traditional methodologies were easily obtained starting with DOTA. A new protocol was derived using this methodology for the solution‐phase synthesis of DOTA peptide derivatives. With this methodology, many other DOTAMR₄ peptide and non‐peptide derivatives have been prepared in our laboratories with several of these new compounds showing interesting properties for molecular imaging. Copyright © 2010 John Wiley & Sons, Ltd.     

Sensitivity‐enhanced chemical exchange saturation transfer (CEST) MRI with least squares optimization of Carr Purcell Meiboom Gill multi‐echo echo planar imaging

Chemical exchange saturation transfer (CEST) imaging is a novel MRI technique that is sensitive to biomolecules, local pH and temperature, and offers considerable advantages for in vivo applications. However, the magnitude of CEST effect for dilute CEST agents undergoing slow or intermediate chemical exchange is typically small, requiring the use of signal averaging to enhance its sensitivity. Given that T₂‐induced signal loss can be normalized by asymmetry analysis, the magnitude of CEST effect is independent of echo time. Therefore, CEST MRI with multi‐echo echo planar imaging (EPI) readout should yield the same CEST effect as conventional single echo acquisition. Importantly, CEST multi‐echo (CESTme) EPI images can be averaged to enhance CEST MRI sensitivity. The goal of this study was to validate CESTme EPI using a creatine–agarose gel CEST phantom with similar T₂ as biological tissue. Using least‐squares optimization, we found that the sensitivity of CESTme sequence was significantly higher than that obtained by conventional single echo CEST‐EPI acquisition. Specifically, signal‐to‐noise ratio and contrast‐to‐noise ratio from the proposed CESTme EPI were approximately equivalent to that obtained by doubling the number of signal averages of the standard single echo CEST MRI sequence. In summary, our results demonstrated CESTme EPI for sensitivity‐enhanced CEST imaging. Copyright © 2014 John Wiley & Sons, Ltd.     

Anthranilic acid analogs as diamagnetic CEST MRI contrast agents that feature an intramolecular‐bond shifted hydrogen

Diamagnetic chemical exchange saturation transfer (diaCEST) agents are a new class of imaging agents, which have unique magnetic resonance (MR) properties similar to agents used for optical imaging. Here we present a series of anthranilic acid analogs as examples of diaCEST agents that feature an exchangeable proton shifted downfield, namely, an intramolecular‐bond shifted hydrogen (IM‐SHY), which produces significant and tunable contrast at frequencies of 4.8–9.3 ppm from water. Five analogs of N‐sulfonyl anthranilic acids are all highly soluble and produced similar CEST contrast at ~6–8 ppm. We also discovered that flufenamic acid, a commercial nonsteroidal anti‐inflammatory drug, displayed CEST contrast at 4.8 ppm. For these N–H IM‐SHY agents, the contrast produced was insensitive to pH, making them complementary to existing diaCEST probes. This initial IM‐SHY library includes the largest reported shifts for N–H protons on small organic diaCEST agents, and should find use as multifrequency MR agents for in vivo applications. Copyright © 2014 John Wiley & Sons, Ltd.     

Improved measurement of labile proton concentration‐weighted chemical exchange rate (kws) with experimental factor‐compensated and T1‐normalized quantitative chemical exchange saturation transfer (CEST) MRI

Chemical exchange saturation transfer (CEST) MRI enables measurement of dilute CEST agents and microenvironment properties such as pH and temperature, holding great promise for in vivo applications. However, because of confounding concomitant radio frequency (RF) irradiation and relaxation effects, the CEST‐weighted MRI contrast may not fully characterize the underlying CEST phenomenon. We postulated that the accuracy of quantitative CEST MRI could be improved if the experimental factors (labeling efficiency and RF spillover effect) were estimated and taken into account. Specifically, the experimental factor was evaluated as a function of exchange rate and CEST agent concentration ratio, which remained relatively constant for intermediate RF irradiation power levels. Hence, the experimental factors can be calculated based on the reasonably estimated exchange rate and labile proton concentration ratio, which significantly improved quantification. The simulation was confirmed with creatine phantoms of serially varied concentration titrated to the same pH, whose reverse exchange rate (k_ws) was found to be linearly correlated with the concentration. In summary, the proposed solution provides simplified yet reasonably accurate quantification of the underlying CEST system, which may help guide the ongoing development of quantitative CEST MRI. Copyright © 2012 John Wiley & Sons, Ltd.