S. James Ratnakar

Alternatives to Gadolinium-Based Metal Chelates for Magnetic Resonance Imaging†

Magnetic Resonance Imaging (MRI) has been immensely valuable in diagnostic clinical imaging over the last few decades owing to its exceptional spatial and anatomical resolution. The signal in MRI is generated by relaxation of the transverse component of the net magnetization of protons present in the body, predominantly from bulk water. Thus, any agent or process that affects the net magnetization of the water protons in body tissues will also influence image contrast. Gd3+-based contrast agents shorten both the longitudinal and transverse relaxation times (T1 and T2) of water protons to approximately the same extent, in essence by relaxing all nearby proton spins. This effect is detected as increased signal intensity in T1 weighted MRI images when the appropriate pulse sequence is applied. Over the past 25 years Gd3+-complexes have been spectacularly successful as extracellular or blood pool T1 agents but their relative insensitivity to changes in environment coupled with the fact that they are never completely silent limits their applicability in the design of responsive MRI agents. A conceptually different approach to contrast enhancement is based on chemical exchange saturation transfer (CEST). This technique relies on dynamic chemical exchange processes inherent in biological tissues to transfer saturated 1H spins into the bulk water proton pool, which leads to a decrease of net magnetization and is detected as a negative contrast (darkening of the image) in MRI. Originally exchangeable -NH and -OH protons of various biomolecules were used to generate CEST contrast (DIACEST). However, these agents suffer from a few drawbacks, particularly in association with the small, usually less than 6 ppm, chemical shift difference between the two exchanging pools. The great benefit of using paramagnetic hyperfine shifting lanthanide complexes as CEST agents (PARACEST) is that the chemical shift difference between the two exchanging pools can potentially be much larger, up to several hundred ppm, facilitating easy saturation of one of the exchangeable spin pools without partial saturation of the bulk water pool. Another advantage of PARACEST is that the exchangeable sites are not limited to -NH or -OH protons but sites with faster exchange rates such as a Ln3+-bound H2O molecule, in particular, can also be considered. Since the water exchange rate on lanthanide complexes is extremely sensitive to the chemical environment, this has created unprecedented opportunities in the design of responsive PARACEST agents. In addition, multi-frequency MRI imaging is inherent to PARACEST: multiple agents present in the body can be imaged in one experiment by selectively turning on and off each agent by applying the appropriate saturation frequency.

BDPA: an efficient polarizing agent for fast dissolution dynamic nuclear polarization NMR spectroscopy.

Keywords: dynamic nuclear polarization ; hyperpolarization ; NMR spectroscopy ; radicals ; relaxation ; Signal Reference EPFL-ARTICLE-169192doi:10.1002/chem.201102037View record in Web of Science Record created on 2011-10-03, modified on 2017-05-12

A Multislice Gradient Echo Pulse Sequence for CEST Imaging

Chemical exchange–dependent saturation transfer and paramagnetic chemical exchange–dependent saturation transfer are agent‐mediated contrast mechanisms that depend on saturating spins at the resonant frequency of the exchangeable protons on the agent, thereby indirectly saturating the bulk water. In general, longer saturating pulses produce stronger chemical and paramagnetic exchange–dependent saturation transfer effects, with returns diminishing for pulses longer than T1. This could make imaging slow, so one approach to chemical exchange–dependent saturation transfer imaging has been to follow a long, frequency‐selective saturation period by a fast imaging method. A new approach is to insert a short frequency‐selective saturation pulse before each spatially selective observation pulse in a standard, two‐dimensional, gradient‐echo pulse sequence. Being much less than T1 apart, the saturation pulses have a cumulative effect. Interleaved, multislice imaging is straightforward. Observation pulses directed at one slice did not produce observable, unintended chemical exchange–dependent saturation transfer effects in another slice. Pulse repetition time and signal‐to noise ratio increase in the normal way as more slices are imaged simultaneously. Magn Reson Med, 2010.

Multi-Frequency PARACEST Agents Based on Europium(III)-DOTA-Tetraamide Ligands†

Magnetic resonance imaging (MRI) is one of the most versatile and powerful diagnostic tools in modern medicine. Recently, a conceptually different approach to contrast enhancement based on chemical exchange saturation transfer (CEST) has emerged that takes advantage of slow-to-intermediate exchange conditions between two or more pools of protons (kex ≤ Δω).[1] While the first reported CEST agents were diamagnetic molecules containing exchangeable NH and OH groups (Δω ≤ 5 ppm), it was later shown that the slow water exchange characteristics of certain paramagnetic Ln3+ complexes of DOTA-tetraamide ligands allows selective saturation of a hyperfine shifted Ln3+-bound water pool (Δω > 50 ppm) for creating CEST contrast.[2] Radio frequency (RF) saturation of highly shifted exchange resonances in paramagnetic systems offer significant advantages over diamagnetic CESTagents with small Δω values.[3]

Production and NMR Characterization of Hyperpolarized 107,109Ag Complexes

Both isotopes of silver, (107)Ag and (109)Ag, were simultaneously polarized by dynamic nuclear polarization (DNP), thus allowing large signal enhancements and the NMR characterization of Ag complexes in the millimolar concentration range. Since both isotopes have long relaxation times T(1), the hyperpolarized NMR signal of one isotope could still be observed even after the magnetization of the other isotope had been destroyed by radio-frequency pulses.

Activation of a PARACEST agent for MRI through selective outersphere interactions with phosphate diesters

Ln(S-THP)(3+) complexes are paramagnetic chemical exchange saturation transfer (PARACEST) agents for magnetic resonance imaging (MRI; S-THP = (1S,4S,7S,10S)-1,4,7,10-tetrakis(2-hydroxypropyl)-1,4,7,10-tetraazacyclododecane, Ln(III) = Ce(III), Eu(III), Yb(III)). CEST spectra at 11.7 T show that the PARACEST effect of these complexes is enhanced at neutral pH in buffered solutions containing 100 mM NaCl upon the addition of 1-2 equiv of diethylphosphate (DEP). CEST images of phantoms at 4.7 T confirm that DEP enhances the properties of Yb(S-THP)(3+) as a PARACEST MRI agent in buffered solutions at neutral pH and 100 mM NaCl. Studies using (1)H NMR, direct excitation Eu(III) luminescence spectroscopy, and UV-visible spectroscopy show that DEP is an outersphere ligand. Dissociation constants for [Ln(S-THP)(OH(2))](DEP) are 1.9 mM and 2.8 mM for Ln(III) = Yb(III) at pH 7.0 and Eu(III) at pH 7.4. Related ligands including phosphorothioic acid, O,O-diethylester, ethyl methylphosphonate, O-(4-nitrophenylphosphoryl)choline, and cyclic 3,5-adenosine monophosphate do not activate PARACEST. BNPP (bis(4-nitrophenyl phosphate) activates PARACEST of Ln(S-THP)(3+) (Ln(III) = Eu(III), Yb(III)), albeit less effectively than does DEP. These data show that binding through second coordination sphere interactions is selective for phosphate diesters with two terminal oxygens and two identical ester groups. A crystal structure of [Eu(S-THP)(OH(2))]((O(2)NPhO)(2)PO(2))(2)(CF(3)SO(3)) x 2 H(2)O x iPrOH has two outersphere BNPP anions that form hydrogen bonds to the alcohol groups of the macrocycle and the bound water ligand. This structure supports (1)H NMR spectroscopy studies showing that outersphere interactions of the phosphate diester with the alcohol protons modulate the rate of alcohol proton exchange to influence the PARACEST properties of the complex. Further, DEP interacts only with the nonionized form of the complex, Ln(S-THP)(OH(2))(3+) contributing to the pH dependence of the PARACEST effect.

Maximizing T2‐exchange in Dy3+DOTA‐(amide)X chelates: Fine‐tuning the water molecule exchange rate for enhanced T2 contrast in MRI

The water molecule exchange rates in a series of DyDOTA‐(amide)X chelates were fine‐tuned to maximize the effects of T2‐exchange line broadening and improve T2 contrast.

Oxidative Conversion of a Europium(II)‐Based T1 Agent to a Europium(III)‐Based paraCEST Agent that can be Detected In Vivo by Magnetic Resonance Imaging

The Eu(II) complex of 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA) tetra(glycinate) has a higher reduction potential than most Eu(II) chelates reported to date. The reduced Eu(II) form acts as an efficient water proton T1 relaxation reagent, while the Eu(III) form acts as a water-based chemical exchange saturation transfer (CEST) agent. The complex has extremely fast water exchange rate. Oxidation to the corresponding Eu(III) complex yields a well-defined signal from the paraCEST agent. The time course of oxidation was studied in vitro and in vivo by T1-weighted and CEST imaging.

Electrochemical Investigation of the Eu3+/2+ Redox Couple in Complexes with Variable Numbers of Glycinamide and Acetate Pendant Arms

The Eu3+/2+ redox couple provides a convenient design platform for responsive pO2 sensors for magnetic resonance imaging (MRI). Specifically the Eu2+ ion provides T1w contrast enhancement under hypoxic conditions in tissues, whereas, under normoxia, the Eu3+ ion can produce contrast from chemical exchange saturation transfer in MRI. The oxidative stability of the Eu3+/2+ redox couple for a series of tetraaza macrocyclic complexes was investigated in this work using cyclic voltammetry. A series of Eu-containing cyclen-based macrocyclic complexes revealed positive shifts in the Eu3+/2+ redox potentials with each replacement of a carboxylate coordinating arm of the ligand scaffold with glycinamide pendant arms. The data obtained reveal that the complex containing four glycinamide coordinating pendant arms has the highest oxidative stability of the series investigated.

Crystallographic Characterization and Non-Innocent Redox Activity of the Glycine Modified DOTA Scaffold and Its Impact on EuIII Electrochemistry: Crystallographic Characterization and Non-Innocent Redox Activity of the Glycine Modified DOTA Scaffold and Its Impact on EuIII Electrochemistry

EuDOTA-glycine derivatives have been explored as alternatives to typical gadolinium-containing complexes for MRI agents used in diagnostic imaging. Different imaging modalities can be accessed (T 1 or PARACEST) dependent on the oxidation state of the europium ion. Throughout the past 30 years, there have been significant manipulations and additions made to the DOTA scaffold; yet, characterizations related to electrochemistry and structure determined through XRD analysis have not been fully analyzed. In this work, electrochemical analysis using cyclic voltammetry was carried out on EuDOTA derivatives, including the free ligand DOTAGly4 (4) and the complexes. Effects of glycinate substitution on the DOTA scaffold, specifically, ligand interactions with the glassy carbon electrode were observed. A range of electrochemical investigations were carried out to show that increased glycinate substitution led to increased interaction with the electrode surface, thus implicating a new factor to consider when evaluating the electrochemistry of glycinate substituted ligands. In addition, the solid-state structure of EuDOTAGly4 (Eu4) was determined by X-ray diffraction and a brief analysis is presented compared to known Ln3+ structures found within literature. The Eu4 complex crystalizes in a rare polymer type arrangement via bridging side-arms between adjacent complexes.