Simultaneous steady-state and dynamic 13C NMR can differentiate alternative routes of pyruvate metabolism in living cancer cells

Metabolic reprogramming facilitates cancer cell growth, so quantitative metabolic flux measurements could produce useful biomarkers. However, current methods to analyze flux in vivo provide either a steady-state overview of relative activities (infusion of (13)C and analysis of extracted metabolites) or a dynamic view of a few reactions (hyperpolarized (13)C spectroscopy). Moreover, although hyperpolarization has successfully quantified pyruvate-lactate exchanges, its ability to assess mitochondrial pyruvate metabolism is unproven in cancer. Here, we combined (13)C hyperpolarization and isotopomer analysis to quantify multiple fates of pyruvate simultaneously. Two cancer cell lines with divergent pyruvate metabolism were incubated with thermally polarized [3-(13)C]pyruvate for several hours, then briefly exposed to hyperpolarized [1-(13)C]pyruvate during acquisition of NMR spectra using selective excitation to maximize detection of H[(13)C]O3(-) and [1-(13)C]lactate. Metabolites were then extracted and subjected to isotopomer analysis to determine relative rates of pathways involving [3-(13)C]pyruvate. Quantitation of hyperpolarized H[(13)C]O3(-) provided a single definitive metabolic rate, which was then used to convert relative rates derived from isotopomer analysis into quantitative fluxes. This revealed that H[(13)C]O3(-) appearance reflects activity of pyruvate dehydrogenase rather than pyruvate carboxylation followed by subsequent decarboxylation reactions. Glucose substantially altered [1-(13)C]pyruvate metabolism, enhancing exchanges with [1-(13)C]lactate and suppressing H[(13)C]O3(-) formation. Furthermore, inhibiting Akt, an oncogenic kinase that stimulates glycolysis, reversed these effects, indicating that metabolism of pyruvate by both LDH and pyruvate dehydrogenase is subject to the acute effects of oncogenic signaling on glycolysis. The data suggest that combining (13)C isotopomer analyses and dynamic hyperpolarized (13)C spectroscopy may enable quantitative flux measurements in living tumors.

A novel class of polymeric pH-responsive MRI CEST agents

In this communication, we report that ionizable, tertiary amine-based block copolymers can be used as pH-responsive contrast agents for magnetic resonance imaging (MRI) through the chemical exchange saturation transfer (CEST) mechanism. The CEST signal is essentially "off" when the polymers form micelles near physiological pH but is activated to the "on" state when the micelles dissociate in an acidic environment.

GdDO3NI, a nitroimidazole-based T1 MRI contrast agent for imaging tumor hypoxia in vivo

Tumor hypoxia is known to affect sensitivity to radiotherapy and promote development of metastases; therefore, the ability to image tumor hypoxia in vivo could provide useful prognostic information and help tailor therapy. We previously demonstrated in vitro evidence for selective accumulation of a gadolinium tetraazacyclododecanetetraacetic acid monoamide conjugate of 2-nitroimidazole (GdDO3NI), a magnetic resonance imaging T1-shortening agent, in hypoxic cells grown in tissue culture. We now report evidence for accumulation of GdDO3NI in hypoxic tumor tissue in vivo. Our data show that GdDO3NI accumulated significantly (p < 0.05) in the central, poorly perfused regions of rat prostate adenocarcinoma AT1 tumors (threefold higher concentration than for the control agent) and showed better clearance from well-perfused regions and complete clearance from the surrounding muscle tissue. Inductively coupled plasma mass spectroscopy confirmed that more GdDO3NI than control agent was retained in the central region and that more GdDO3NI was retained in the central region than at the periphery. These results show the utility of GdDO3NI to image tumor hypoxia and highlight the potential of GdDO3NI for application to image-guided interventions for radiation therapy or hypoxia-activated chemotherapy.

Modulation of CEST images in vivo by T1 relaxation: a new approach in the design of responsive PARACEST agents

A novel approach for the design of responsive paramagnetic chemical exchange saturation transfer (PARACEST) magnetic resonance imaging (MRI) agents has been developed where the signal is "turned on" by altering the longitudinal relaxation time (T1) of bulk water protons. To demonstrate this approach, a model Eu(DOTA-tetraamide) complex (DOTA = 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid) containing two nitroxide free radical units was synthesized. The nitroxide groups substantially shortened the T1 of the bulk water protons which, in turn, resulted in quenching of the CEST signal. Reduction of paramagnetic nitroxide moieties to a diamagnetic species resulted in the appearance of CEST. The modulation of CEST by T1 relaxation provides a new platform for designing biologically responsive MRI agents.

Advantages of paramagnetic chemical exchange saturation transfer (CEST) complexes having slow to intermediate water exchange properties as responsive MRI agents

Paramagnetic chemical exchange saturation transfer (PARACEST) complexes are exogenous contrast agents that have great potential to further extend the functional and molecular imaging capabilities of magnetic resonance. As a result of the presence of a central paramagnetic lanthanide ion (Ln(3+) ≠ La(3+) , Gd(3+) , Lu(3+) ) within the chelate, the resonance frequencies of exchangeable protons bound to the PARACEST agent are shifted far away from the bulk water frequency. This large chemical shift, combined with an extreme sensitivity to the chemical exchange rate, make PARACEST agents ideally suited for the reporting of significant biological metrics, such as temperature, pH and the presence of metabolites. In addition, the ability to turn PARACEST agents 'off' and 'on' using a frequency-selective saturation pulse gives them a distinct advantage over Gd(3+) -based contrast agents. A current challenge for PARACEST research is the translation of the promising in vitro results into in vivo systems. This short review article first describes the basic theory behind PARACEST contrast agents, their benefits over other contrast agents and their applications to MRI. It then describes some of the recent PARACEST research results: specifically, pH measurements using water molecule exchange rate modulation, T2 exchange contrast caused by water molecule exchange, the use of ultrashort TEs (TE < 10 µs) to overcome T2 exchange line broadening and the potential application of T2 exchange as a new contrast mechanism for MRI.

Electron spin resonance studies of trityl OX063 at a concentration optimal for DNP

We have performed temperature-dependent electron spin resonance (ESR) measurements of the stable free radical trityl OX063, an efficient polarizing agent for dissolution dynamic nuclear polarization (DNP), at the optimum DNP concentration (15 mM). We have found that (i) when compared to the W-band electron spin-lattice relaxation rate T1e(-1) of other free radicals used in DNP at the same concentration, trityl OX063 has slower T1e(-1) than BDPA and 4-oxo-TEMPO. At T > 20 K, the T1e(-1)vs. T data of trityl OX063 appears to follow a power law dependence close to the Raman process prediction whereas at T < 10 K, electronic relaxation slows and approaches the direct process behaviour. (ii) Gd(3+) doping, a factor known to enhance DNP, of trityl OX063 samples measured at W-band resulted in monotonic increases of T1e(-1) especially at temperatures below 20-40 K while the ESR lineshapes remained essentially unchanged. (iii) The high frequency ESR spectrum can be fitted with an axial g-tensor with a slight g-anisotropy: g(x) = g(y) = 2.00319(3) and g(z) = 2.00258(3). Although the ESR linewidth D monotonically increases with field, the temperature-dependent T1e(-1) is almost unchanged as the ESR frequency is increased from 9.5 GHz to 95 GHz, but becomes faster at 240 GHz and 336 GHz. The ESR properties of trityl OX063 reported here may provide insights into the efficiency of DNP of low-γ nuclei performed at various magnetic fields, from 0.35 T to 12 T.

A europium(III)-based PARACEST agent for sensing singlet oxygen by MRI

A europium(III) DOTA-tetraamide complex was designed as a MRI sensor of singlet oxygen ((1)O2). The water soluble, thermodynamically stable complex reacts rapidly with (1)O2 to form an endoperoxide derivative that results in an ∼3 ppm shift in the position of the Eu(III)-bound water chemical exchange saturation transfer (CEST) peak. The potential of using this probe to detect accumulation of the endoperoxide derivative in biological media by ratiometric CEST imaging was demonstrated.

Metabolism of glycerol, glucose, and lactate in the citric acid cycle prior to incorporation into hepatic acylglycerols

During hepatic lipogenesis, the glycerol backbone of acylglycerols originates from one of three sources: glucose, glycerol, or substrates passing through the citric acid cycle via glyceroneogenesis. The relative contribution of each substrate source to glycerol in rat liver acylglycerols was determined using (13)C-enriched substrates and NMR. Animals received a fixed mixture of glucose, glycerol, and lactate; one group received [U-(13)C6]glucose, another received [U-(13)C3]glycerol, and the third received [U-(13)C3]lactate. After 3 h, the livers were harvested to extract fats, and the glycerol moiety from hydrolyzed acylglycerols was analyzed by (13)C NMR. In either fed or fasted animals, glucose and glycerol provided the majority of the glycerol backbone carbons, whereas the contribution of lactate was small. In fed animals, glucose contributed >50% of the total newly synthesized glycerol backbone, and 35% of this contribution occurred after glucose had passed through the citric acid cycle. By comparison, the glycerol contribution was ~40%, and of this, 17% of the exogenous glycerol passed first through the cycle. In fasted animals, exogenous glycerol became the major contributor to acylglycerols. The contribution from exogenous lactate did increase in fasted animals, but its overall contribution remained small. The contributions of glucose and glycerol that had passed through the citric acid cycle first increased in fasted animals from 35 to 71% for glucose and from 17 to 24% for glycerol. Thus, a substantial fraction from both substrate sources passed through the cycle prior to incorporation into the glycerol moiety of acylglycerols in the liver.

CEST: from basic principles to applications, challenges and opportunities

Chemical Exchange Saturation Transfer (CEST) offers a new type of contrast for MRI that is molecule specific. In this approach, a slowly exchanging NMR active nucleus, typically a proton, possessing a chemical shift distinct from water is selectively saturated and the saturated spin is transferred to the bulk water via chemical exchange. Many molecules can act as CEST agents, both naturally occurring endogenous molecules and new types of exogenous agents. A large variety of molecules have been demonstrated as potential agents, including small diamagnetic molecules, complexes of paramagnetic ions, endogenous macromolecules, dendrimers and liposomes. In this review we described the basic principles of the CEST experiment, with emphasis on the similarity to earlier saturation transfer experiments described in the literature. Interest in quantitative CEST has also resulted in the development of new exchange-sensitive detection schemes. Some emerging clinical applications of CEST are described and the challenges and opportunities associated with translation of these methods to the clinical environment are discussed.

In vivo imaging of paraCEST agents using frequency labeled exchange transfer MRI

PURPOSE

A main obstacle to in vivo applications of paramagnetic chemical exchange saturation transfer (paraCEST) is interference from endogenous tissue magnetization transfer contrast (MTC). The suitability of excitation-based frequency labeled exchange transfer (FLEX) to separate out such MTC effects in vivo was tested.

METHODS

The FLEX sequence measures modulation of the water signal based on the chemical shift evolution of solute proton magnetization as a function of evolution time. Time-domain analysis of this water signal allows identification of different solute components and provides a mechanism to separate out the rapidly decaying MTC components with short effective transverse relaxation time ( T2*) values.

RESULTS

FLEX imaging of paraCEST agents was possible in vitro in phantoms and in vivo in mouse kidneys and bladder. The results demonstrated that FLEX is capable of separating out the MTC signal from tissues in vivo while providing a quantitative exchange rate for the rapidly exchanging paraCEST water protons by fitting the FLEX time-domain signal to FLEX theory.

CONCLUSIONS

The first in vivo FLEX images of a paraCEST agent were acquired, which allowed separation of the tissue MTC components. These results show that FLEX imaging has potential for imaging the distribution of functional paraCEST agents in biological tissues.

Dissolution DNP-NMR spectroscopy using galvinoxyl as a polarizing agent

The goal of this work was to test feasibility of using galvinoxyl (2,6-di-tert-butyl-α-(3,5-di-tert-butyl-4-oxo-2,5-cyclohexadien-1-ylidene)-p-tolyloxy) as a polarizing agent for dissolution dynamic nuclear polarization (DNP) NMR spectroscopy. We have found that galvinoxyl is reasonably soluble in ethyl acetate, chloroform, or acetone and the solutions formed good glasses when mixed together or with other solvents such as dimethyl sulfoxide. W-band electron spin resonance (ESR) measurements revealed that galvinoxyl has an ESR linewidth D intermediate between that of carbon-centered free radical trityl OX063 and the nitroxide-based 4-oxo-TEMPO, thus the DNP with galvinoxyl for nuclei with low gyromagnetic ratio γ such as (13)C and (15)N is expected to proceed predominantly via the thermal mixing process. The optimum radical concentration that would afford the highest (13)C nuclear polarization (approximately 6% for [1-(13)C]ethyl acetate) at 3.35 T and 1.4 K was found to be around 40 mM. After dissolution, large liquid-state NMR enhancements were achieved for a number of (13)C and (15)N compounds with long spin-lattice relaxation time T(1). In addition, the hydrophobic galvinoxyl free radical can be easily filtered out from the dissolution liquid when water is used as the solvent. These results indicate that galvinoxyl can be considered as an easily available free radical polarizing agent for routine dissolution DNP-NMR spectroscopy.

The importance of water exchange rates in the design of responsive agents for MRI

The rate of water exchange in lanthanide complexes is often overlooked as an important parameter in the design of responsive MR imaging agents. Most often, the number of inner-sphere water coordination sites or the rotational mobility of the complex is considered as the central theme while water exchange is either assumed to be "fast enough" or entirely ignored. On the contrary, relaxation and shift theories predict that water exchange rates may indeed be the key parameter one should consider in any new molecular design. In this short review, the impact of water exchange rates on three classes of lanthanide-based MRI contrast agents, T₁-based relaxation agents, T₂ exchange line-broadening agents and chemical exchange saturation transfer (CEST) agents, is illustrated and discussed.

Evidence for transaldolase activity in the isolated heart supplied with [U-13C3]glycerol

Studies of glycerol metabolism in the heart have largely emphasized its role in triglyceride synthesis. However, glycerol may also be oxidized in the citric acid cycle, and glycogen synthesis from glycerol has been reported in the nonmammalian myocardium. The intent of this study was to test the hypothesis that glycerol may be metabolized to glycogen in mammalian heart. Isolated rat hearts were supplied with a mixture of substrates including glucose, lactate, pyruvate, octanoate, [U-(13)C(3)]glycerol, and (2)H(2)O to probe various metabolic pathways including glycerol oxidation, glycolysis, the pentose phosphate pathway, and carbon sources of stored glycogen. NMR analysis confirmed that glycogen production from the level of the citric acid cycle did not occur and that the glycerol contribution to oxidation in the citric acid cycle was negligible in the presence of alternative substrates. Quite unexpectedly, (13)C from [U-(13)C(3)]glycerol appeared in glycogen in carbon positions 4-6 of glucosyl units but none in positions 1-3. The extent of [4,5,6-(13)C(3)]glucosyl unit enrichment in glycogen was enhanced by insulin but decreased by H(2)O(2). Given that triose phosphate isomerase is generally assumed to fully equilibrate carbon tracers in the triose pool, the marked (13)C asymmetry in glycogen can only be attributed to conversion of [U-(13)C(3)]glycerol to [U-(13)C(3)]dihydroxyacetone phosphate and [U-(13)C(3)]glyceraldehyde 3-phosphate followed by rearrangements in the nonoxidative branch of the pentose phosphate pathway involving transaldolase that places this (13)C-enriched 3-carbon unit only in the bottom half of hexose phosphate molecules contributing to glycogen.

Imaging free zinc levels in vivo - what can be learned?

Our ever-expanding knowledge about the role of zinc in biology includes its role in redox modulation, immune response, neurotransmission, reproduction, diabetes, cancer, and Alzheimers disease is galvanizing interest in detecting and monitoring the various forms of Zn(II) in biological systems. This paper reviews reported strategies for detecting and tracking of labile or "free" unchelated Zn(II) in tissues. While different bound structural forms of Zn(II) have been identified and studied in vitro by multiple techniques, very few molecular imaging methods have successfully tracked the ion in vivo. A number of MRI and optical strategies have now been reported for detection of free Zn(II) in cells and tissues but only a few have been applied successfully in vivo. A recent report of a MRI sensor for in vivo tracking of Zn(II) released from pancreatic β-cells during insulin secretion exemplifies the promise of rational design of new Zn(II) sensors for tracking this biologically important ion in vivo. Such studies promise to provide new insights into zinc trafficking in vivo and the critical role of this ion in many human diseases.

Noninvasive monitoring of lactate dynamics in human forearm muscle after exhaustive exercise by (1)H-magnetic resonance spectroscopy at 7 tesla

Despite its importance in energy metabolism, lactate in human skeletal muscle has been difficult to detect by noninvasive (1)H-magnetic resonance spectroscopy mainly due to interference from large water and lipid signals. Long echo-time acquisitions at 7 T effectively attenuates the water and lipid signals in forearm muscle allowing direct observation of both lactate resonances, the methine at 4.09 ppm and the methyl at 1.31 ppm. Using this approach, we were able to monitor lactate dynamics at a temporal resolution of 32 s. While lactate was not detectable at rest, immediately after an acute period of exercise to fatigue the forearm muscle, lactate rose to a level comparable to that of creatine (∼30 mmol/kg wet weight). In a typical (1)H-magnetic resonance spectrum collected using a echo-time of 140 ms, the lactate methine and methyl resonances both appear as doublets with an unusually large splitting of ∼20 Hz due to residual dipolar coupling. During muscle recovery following exercise, the lactate signals decay rapidly with a time constant of t½ = 2.0 ± 0.6 min (n = 12 subjects). This fast and simple lactate detection method may prove valuable for monitoring lactate metabolism in cancer and in sports medicine applications.

Comparison of kinetic models for analysis of pyruvate-to-lactate exchange by hyperpolarized 13 C NMR

The activity of specific enzyme-catalyzed reactions may be detected in vivo by (13) C NMR of hyperpolarized (HP) substrates. The signals from HP substrates and products, acquired over time, have been fitted to a number of different mathematical models to determine fluxes, but these models have not been critically compared. In this study, two-pool and three-pool first-order models were constructed to measure flux through lactate dehydrogenase in isolated glioblastoma cells by NMR detection of lactate and pyruvate following the addition of HP [1-(13) C]pyruvate. Mass spectrometry (MS) was used to independently monitor (13) C enrichment in intra- and extracellular lactate. Six models were evaluated using time-dependent pyruvate C2 and lactate C1 HP NMR data acquired by the use of selective excitation pulses, plus (13) C enrichment data from intracellular and extracellular lactate measured by MS. A three-pool bidirectional model provided the most accurate description of pyruvate metabolism in these cells. With computed values for T(1) of pyruvate and lactate, as well as the effect of pulsing, the initial flux through lactate dehydrogenase was well determined by both the two-pool bidirectional and unidirectional models when only HP data were available. The three-pool model was necessary to fit the combined data from both MS and HP, but the simpler two-pool exchange model was sufficient to determine the (13) C lactate concentration when the lactate appearance was measured only by HP.

Synthesis and characterization of a hypoxia-sensitive MRI probe

Tissue hypoxia occurs in pathologic conditions, such as cancer, ischemic heart disease and stroke when oxygen demand is greater than oxygen supply. An imaging method that can differentiate hypoxic versus normoxic tissue could have an immediate impact on therapy choices. In this work, the gadolinium(III) complex of 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA) with a 2-nitroimidazole attached to one carboxyl group via an amide linkage was prepared, characterized and tested as a hypoxia-sensitive MRI agent. A control complex, Gd(DO3A-monobutylamide), was also prepared in order to test whether the nitroimidazole side-chain alters either the water proton T(1) relaxivity or the thermodynamic stability of the complex. The stabilities of these complexes were lower than that of Gd(DOTA)(-) as expected for mono-amide derivatives. The water proton T(1) relaxivity (r(1)), bound water residence lifetime (τ(M)) and rotational correlation time (τ(R)) of both complexes was determined by relaxivity measurements, variable temperature (17) O NMR spectroscopy and proton nuclear magnetic relaxation dispersion (NMRD) studies. The resulting parameters (r(1) =6.38 mM(-1)  s(-1) at 20 MHz, τ(M) =0.71 μs, τ(R) =141 ps) determined for the nitroimidazole derivative closely parallel to those of other Gd(DO3A-monoamide) complexes of similar molecular size. In vitro MR imaging experiments with 9L rat glioma cells maintained under nitrogen (hypoxic) versus oxygen (normoxic) gas showed that both agents enter cells but only the nitroimidazole derivative was trapped in cells maintained under N(2) as evidenced by an approximately twofold decrease in T(1) measured for hypoxic cells versus normoxic cells exposed to this agent. These results suggest that the nitroimidazole derivative might serve as a molecular reporter for discriminating hypoxic versus normoxic tissues by MRI.

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 approximately 1200 Eu(3+) 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 Eu(3+) -bound water exchange peak and broadening of the bulk water signal. This observation was traced to catalysis of proton exchange from the Eu(3+) -bound water molecule by excess positively charged amino groups on the surface of the nanoparticles.

Impact of Gd3+ on DNP of [1-13C]pyruvate doped with trityl OX063, BDPA, or 4-oxo-TEMPO

Hyperpolarized [1-(13)C]pyruvate has become an important diagnostic tracer of normal and aberrant cellular metabolism for in vitro and in vivo NMR spectroscopy (MRS) and imaging (MRI). In pursuit of achieving high NMR signal enhancements in dynamic nuclear polarization (DNP) experiments, we have performed an extensive investigation of the influence of Gd(3+) doping, a parameter previously reported to improve hyperpolarized NMR signals, on the DNP of this compound. [1-(13)C]Pyruvate samples were doped with varying amounts of Gd(3+) and fixed optimal concentrations of free radical polarizing agents commonly used in fast dissolution DNP: trityl OX063 (15 mM), 4-oxo-TEMPO (40 mM), and BDPA (40 mM). In general, we have observed three regions of interest, namely, (i) a monotonic increase in DNP-enhanced nuclear polarization P(dnp) upon increasing the Gd(3+) concentration until a certain threshold concentration c(1) (1-2 mM) is reached, (ii) a region of roughly constant maximum P(dnp) from c(1) until a concentration threshold c(2) (4-5 mM), and (iii) a monotonic decrease in P(dnp) at Gd(3+) concentration c > c(2). Of the three free radical polarizing agents used, trityl OX063 gave the best response to Gd(3+) doping, with a 300% increase in the solid-state nuclear polarization, whereas addition of the optimum Gd(3+) concentration on BDPA and 4-oxo-TEMPO-doped samples only yielded a relatively modest 5-20% increase in the base DNP-enhanced polarization. The increase in P(dnp) due to Gd(3+) doping is ascribed to the decrease in the electronic spin-lattice relaxation T(1e) of the free radical electrons, which plays a role in achieving lower spin temperature T(s) of the nuclear Zeeman system. These results are discussed qualitatively in terms of the spin temperature model of DNP.

The pH sensitivity of -NH exchange in LnDOTA-tetraamide complexes varies with amide substituent

The amide proton exchange rates in various lanthanide(III) DOTA-tetraamide complexes were investigated by CEST as a function of variable chemical structures and charges on the amide substituents. Comparisons were made between YbDOTA-(gly)(4)(-) (Yb-1), YbDOTA-(NHCH(2)PO(3))(4 (5-) (Yb-2) and YbDOTA-(NHCH(2)PO(3)Et(2))(4)(3+) (Yb-3). The general shapes of the CEST vs pH profiles were similar for the three complexes but they showed maximum CEST intensities at different pH values, pH 8.3, 8.8 and 6.9 for Yb-1, Yb-2 and Yb-3, respectively. This indicates that a more negatively charged substituent on the amide helps stabilize the partial positive charge on the amide nitrogen and consequently more base is required to catalyze proton exchange. The chemical shifts of the -NH protons in Yb-1 and Yb-2 were similar (-17 ppm) while the -NH proton in Yb-3 was at -13 ppm. This shows that the crystal field produced by the amide oxygen donor atoms in Yb-3 is substantially weaker than that in the other two complexes. In an effort to expand the useful range of pH values that might be measured using these complexes as CEST agents, the shapes of the CEST vs pH curves were also determined for two thulium(III) complexes with much larger hyperfine shifted -NH proton resonances. The ratio of CEST from -NH exchange in Tm-1 compared with CEST from -NH exchange in Tm-3 was found to be linear over an extended pH range, from 6.3 to 7.4. This demonstrates a potential advantage of using mixtures of lanthanide(III) DOTA-tetraamides for mapping tissue pH by use of ratiometric CEST imaging.

Dynamic monitoring of carnitine and acetylcarnitine in the trimethylamine signal after exercise in human skeletal muscle by 7T 1H-MRS

A trimethylamine (TMA) moiety is present in carnitine and acetylcarnitine, and both molecules play critical roles in muscle metabolism. At 7 T, the chemical shift dispersion was sufficient to routinely resolve the TMA signals from carnitine at 3.20 and from acetylcarnitine at 3.17 ppm in the (1)H-MRS (Magnetic Resonance Spectroscopy) of human soleus muscle with a temporal resolution of about 2 min. In healthy, sedentary adults, the concentration of acetylcarnitine increased nearly 10-fold, to 4.1 ± 1.0 mmol/kg, in soleus muscle after 5 min of calf-raise exercise and recovered to a baseline concentration of 0.5 ± 0.3 mmol/kg. While the half-time for decay of acetylcarnitine was the same whether measured from the TMA signal (18.8 ± 5.6 min) or measured from the methyl signal (19.4 ± 6.1 min), the detection of acetylcarnitine by its TMA signal in soleus has the advantage of higher sensitivity and without overlapping from lipid signals. Although the activity of carnitine acetyltransferase is sufficient to allow equilibrium between carnitine and coenzyme-A pools, the exchange in TMA signal between carnitine and acetylcarnitine is slow in soleus following exercise on 7T (1)H-NMR time scale. The TMA signal provides a simple and direct measure of the relative amounts of carnitine and acetylcarnitine.

Amphiphilic EuDOTA-tetraamide complexes form micelles with enhanced CEST sensitivity

The synthesis and characterization of four new DOTA-tetraamide ligands having variable alkyl chain lengths (C(1), C(12), C(14), and C(16)) and their respective europium (III) complexes are reported. The three EuL complexes having long alkyl chains spontaneously form micelles of variable size. The critical micelle concentration differed for each complex (lower for the C(16) complex than the C(12) complex) while micelle size increased with increasing alkyl chain length. Chemical exchange saturation transfer (CEST) experiments showed that all four Eu(III) complexes display slow-to-intermediate water exchange kinetics. As expected, the CEST signals in these complexes decreased with increasing temperatures due to faster water exchange but, interestingly, the CEST signals for the C(14) and C(16) complexes approached a maximum near 25°C consistent with exchange limited CEST at or near room temperature. The water residence lifetimes obtained by fitting the CEST spectra to the Bloch equations increased in parallel with an increase in alkyl carbon chain-length. By comparisons with the monomethylamide complex, which served as control, the data illustrate that micelle formation serves to slow the rate of water exchange in these systems. The complex having the largest CEST effect per unit Eu(III) concentration (the C(16) analog) had a detection limit of 5.3 μM. This represents an approximate 250-fold increase in sensitivity relative to the monomethylamide control (detection limit ~1.3 mM). These features highlight the potential of using micelle-based systems such as these as paramagnetic chemical exchange saturation transfer (PARACEST) agents for molecular imaging by MRI.

Europium(III) DOTA-tetraamide complexes as redox-active MRI sensors

PARACEST redox sensors containing the NAD(+)/NADH mimic N-methylquinolinium moiety as a redox-active functional group have been designed and synthesized. The Eu(3+) complex with two quinolinium moieties was nearly completely CEST-silent in the oxidized form but was "turned on" upon reduction with β-NADH. The CEST effect of the Eu(3+) complex containing only one quinolinium group was much less redox-responsive but showed an unexpected sensitivity to pH in the physiologically relevant pH range.