Expeditious dissolution dynamic nuclear polarization without glassing agents

The hyperpolarization of metabolic substrates at low temperature using dynamic nuclear polarization (DNP), followed by rapid dissolution and injection into an MRSI or NMR system, allows in vitro or in vivo observation and tracking of biochemical reactions and metabolites in real time. This article describes an elegant approach to sample preparation which is broadly applicable for the rapid polarization of aqueous small‐molecule substrate solutions and obviates the need for glassing agents. We demonstrate its utility for solutions of sodium acetate, pyruvate and butyrate. The polarization behavior of substrates prepared using rapid freezing without glassing agents enabled a 1.5–3‐fold time savings in polarization buildup, whilst removing the need for toxic glassing agents used as standard for dissolution DNP. The achievable polarization with fully aqueous substrate solutions was equal to that observed using standard approaches and glassing agents. Copyright © 2016 John Wiley & Sons, Ltd.     

Simultaneous investigation of cardiac pyruvate dehydrogenase flux, Krebs cycle metabolism and pH, using hyperpolarized [1,2-(13)C2]pyruvate in vivo

(13)C MR spectroscopy studies performed on hearts ex vivo and in vivo following perfusion of prepolarized [1-(13)C]pyruvate have shown that changes in pyruvate dehydrogenase (PDH) flux may be monitored non-invasively. However, to allow investigation of Krebs cycle metabolism, the (13)C label must be placed on the C2 position of pyruvate. Thus, the utilization of either C1 or C2 labeled prepolarized pyruvate as a tracer can only afford a partial view of cardiac pyruvate metabolism in health and disease. If the prepolarized pyruvate molecules were labeled at both C1 and C2 positions, then it would be possible to observe the downstream metabolites that were the results of both PDH flux ((13)CO(2) and H(13)CO(3)(-)) and Krebs cycle flux ([5-(13)C]glutamate) with a single dose of the agent. Cardiac pH could also be monitored in the same experiment, but adequate SNR of the (13)CO(2) resonance may be difficult to obtain in vivo. Using an interleaved selective RF pulse acquisition scheme to improve (13)CO(2) detection, the feasibility of using dual-labeled hyperpolarized [1,2-(13)C(2)]pyruvate as a substrate for dynamic cardiac metabolic MRS studies to allow simultaneous investigation of PDH flux, Krebs cycle flux and pH, was demonstrated in vivo.     

In vivo measurement of apparent diffusion coefficients of hyperpolarized 13C‐labeled metabolites

The combination of hyperpolarized MRS with diffusion weighting (dw) allows for determination of the apparent diffusion coefficient (ADC), which is indicative of the intra‐ or extracellular localization of the metabolite. Here, a slice‐selective pulsed‐gradient spin echo sequence was implemented to acquire a series of dw spectra from rat muscle in vivo to determine the ADCs of multiple metabolites after a single injection of hyperpolarized [1‐¹³C]pyruvate. An optimal control optimized universal‐rotation pulse was used for refocusing to minimize signal loss caused by B₁ imperfections. Non‐dw spectra were acquired interleaved with the dw spectra and these were used to correct for signal decay during the acquisition as a result of T₁ decay, pulse imperfections, flow etc. The data showed that the ADC values for [1‐¹³C]lactate (0.4–0.7 µm²/ms) and [1‐¹³C]alanine (0.4–0.9 µm²/ms) were about a factor of two lower than the ADC of [1‐¹³C]pyruvate (1.1–1.5 µm²/ms). This indicates a more restricted diffusion space for the former two metabolites consistent with lactate and alanine being intracellular. The higher ADC for pyruvate (similar to the proton ADC) reflected that the injected substance was not confined inside the muscle cells but also present extracellular. Copyright © 2014 John Wiley & Sons, Ltd.     

In vivo detection of intermediate metabolic products of [1-(13) C]ethanol in the brain using (13) C MRS

In this study, in vivo ¹³C MRS was used to investigate the labeling of brain metabolites after intravenous administration of [1‐¹³C]ethanol. After [1‐¹³C]ethanol had been administered systemically to rats, ¹³C labels were detected in glutamate, glutamine and aspartate in the carboxylic and amide carbon spectral region. ¹³C‐labeled bicarbonate HCO (161.0 ppm) was also detected. Saturating acetaldehyde C1 at 207.0 ppm was found to have no effect on the ethanol C1 (57.7 ppm) signal intensity after extensive signal averaging, providing direct in vivo evidence that direct metabolism of alcohol by brain tissue is minimal. To compare the labeling of brain metabolites by ethanol with labeling by glucose, in vivo time course data were acquired during intravenous co‐infusion of [1‐¹³C]ethanol and [¹³C₆]‐D ‐glucose. In contrast with labeling by [¹³C₆]‐D ‐glucose, which produced doublets of carboxylic/amide carbons with a J coupling constant of 51 Hz, the simultaneously detected glutamate and glutamine singlets were labeled by [1‐¹³C]ethanol. As ¹³C labels originating from ethanol enter the brain after being converted into [1‐¹³C]acetate in the liver, and the direct metabolism of ethanol by brain tissue is negligible, it is suggested that orally or intragastrically administered ¹³C‐labeled ethanol may be used to study brain metabolism and glutamatergic neurotransmission in investigations involving alcohol administration. In vivo ¹³C MRS of rat brain following intragastric administration of ¹³C‐labeled ethanol is demonstrated. Published in 2011 by John Wiley & Sons, Ltd.     

Enhancing the [13C]bicarbonate signal in cardiac hyperpolarized [1‐13C]pyruvate MRS studies by infusion of glucose,insulin and potassium

A change in myocardial metabolism is a known effect of several diseases. MRS with hyperpolarized ¹³C‐labelled pyruvate is a technique capable of detecting changes in myocardial pyruvate metabolism, and has proven to be useful for the evaluation of myocardial ischaemia in vivo. However, during fasting, the myocardial glucose oxidation is low and the fatty acid oxidation (β‐oxidation) is high, which complicates the interpretation of pyruvate metabolism with the technique. The aim of this study was to investigate whether the infusion of glucose, insulin and potassium (GIK) could increase the myocardial glucose oxidation in the citric acid cycle, reflected as an increase in the [¹³C]bicarbonate signal in cardiac hyperpolarized [1‐¹³C]pyruvate MRS measurements in fasted rats. Two groups of rats were infused with two different doses of GIK and investigated by MRS after injection of hyperpolarized [1‐¹³C]pyruvate. No [¹³C]bicarbonate signal could be detected in the fasted state. However, a significant increase in the [¹³C]bicarbonate signal was observed by the infusion of a high dose of GIK. This study demonstrates that a high [¹³C]bicarbonate signal can be achieved by GIK infusion in fasted rats. The increased [¹³C]bicarbonate signal indicates an increased flux of pyruvate through the pyruvate dehydrogenase enzyme complex and an increase in myocardial glucose oxidation through the citric acid cycle. Copyright © 2013 John Wiley & Sons, Ltd.     

An indirect method for in vivo T2 mapping of [1‐13C] pyruvate using hyperpolarized 13C CSI

An indirect method for in vivo T₂ mapping of ¹³C–labeled metabolites using T₂ and T₂* information of water protons obtained a priori is proposed. The T₂ values of ¹³C metabolites are inferred using the relationship to T₂′ of coexisting ¹H and the T₂* of ¹³C metabolites, which is measured using routine hyperpolarized ¹³C CSI data. The concept is verified with phantom studies. Simulations were performed to evaluate the extent of T₂ estimation accuracy due to errors in the other measurements. Also, bias in the ¹³C T₂* estimation from the ¹³C CSI data was studied. In vivo experiments were performed from the brains of normal rats and a rat with C6 glioma. Simulation results indicate that the proposed method provides accurate and unbiased ¹³C T₂ values within typical experimental settings. The in vivo studies found that the estimated T₂ of [1‐¹³C] pyruvate using the indirect method was longer in tumor than in normal tissues and gave values similar to previous reports. This method can estimate localized T₂ relaxation times from multiple voxels using conventional hyperpolarized ¹³C CSI and can potentially be used with time resolved fast CSI.     

How the signal-to-noise ratio influences hyperpolarized 13C dynamic MRS data fitting and parameter estimation

MRS of hyperpolarized (13) C-labeled compounds represents a promising technique for in vivo metabolic studies. However, robust quantification and metabolic modeling are still important areas of investigation. In particular, time and spatial resolution constraints may lead to the analysis of MRS signals with low signal-to-noise ratio (SNR). The relationship between SNR and the precision of quantitative analysis for the evaluation of the in vivo kinetic behavior of metabolites is unknown. In this article, this topic is addressed by Monte Carlo simulations, covering the problem of MRS signal model parameter estimation, with strong emphasis on the peak amplitude and kinetic model parameters. The results of Monte Carlo simulation were confirmed by in vivo experiments on medium-sized animals injected with hyperpolarized [1-(13) C]pyruvate. The results of this study may be useful for the establishment of experimental planning and for the optimization of kinetic model estimation as a function of the SNR value.     

Altered metabolites of the rat hippocampus after mild and moderate traumatic brain injury – a combined in vivo and in vitro 1H–MRS study

Traumatic brain injury (TBI) has been shown to affect hippocampus‐associated learning, memory and higher cognitive functions, which may be a consequence of metabolic alterations. Hippocampus‐associated disorders may vary depending on the severity of injury [mild TBI (miTBI) and moderate TBI (moTBI)] and time since injury. The underlying hippocampal metabolic irregularities may provide an insight into the pathological process following TBI. In this study, in vivo and in vitro proton magnetic resonance spectroscopy (¹H–MRS) data were acquired from the hippocampus region of controls and TBI groups (miTBI and moTBI) at D0 (pre‐injury), 4 h, Day 1 and Day 5 post‐injury (PI). In vitro MRS results indicated trauma‐induced changes in both miTBI and moTBI; however, in vivo MRS showed metabolic alterations in moTBI only. miTBI and moTBI showed elevated levels of osmolytes indicating injury‐induced edema. Altered levels of citric acid cycle intermediates, glutamine/glutamate and amino acid metabolism indicated injury‐induced aberrant bioenergetics, excitotoxicity and oxidative stress. An overall similar pattern of pathological process was observed in both miTBI and moTBI, with the distinction of depleted N‐acetylaspartate levels (indicating neuronal loss) at 4 h and Day 1 and enhanced lactate production (indicating heightened energy depletion leading to the commencement of the anaerobic pathway) at Day 5 in moTBI. To the best of our knowledge, this is the first study to investigate the hippocampus metabolic profile in miTBI and moTBI simultaneously using in vivo and in vitro MRS.     

Tricarboxylic acid cycle of glia in the in vivo human brain

In the brain, acetate is exclusively oxidized by glia. To determine the contribution of glial metabolism to the tricarboxylic acid cycle (TCA), 1-(13)C-acetate was infused in six studies in three normal adult subjects and -one epileptic receiving valproic acid for seizure control. Ten grams of 99% 1-(13)C labeled acetate were infused intravenously as a 3.3% w/v solution over 60 min, during which in vivo 13C MR spectra of the brain were acquired. As expected, 13C label rapidly enriched cerebral bicarbonate, glutamate and glutamine C5. The mean rate of acetate oxidation calculated from steady-state 13C enrichment of bicarbonate in fasted normal subjects was 0.13 +/- 0.03 micromol/g/min (n=4), approximately 20% of the total cerebral TCA cycle rate.     

T2 relaxation times of 13C metabolites in a rat hepatocellular carcinoma model measured in vivo using 13C‐MRS of hyperpolarized [1‐13C]pyruvate

A single‐voxel Carr‐Purcell‐Meibloom‐Gill sequence was developed to measure localized T₂ relaxation times of ¹³C‐labeled metabolites in vivo for the first time. Following hyperpolarized [1‐¹³C]pyruvate injections, pyruvate and its metabolic products, alanine and lactate, were observed in the liver of five rats with hepatocellular carcinoma and five healthy control rats. The T₂ relaxation times of alanine and lactate were both significantly longer in HCC tumors than in normal livers (p < 0.002). The HCC tumors also showed significantly higher alanine signal relative to the total ¹³C signal than normal livers (p < 0.006). The intra‐ and inter‐subject variations of the alanine T₂ relaxation time were 11% and 13%, respectively. The intra‐ and inter‐subject variations of the lactate T₂ relaxation time were 6% and 7%, respectively. The intra‐subject variability of alanine to total carbon ratio was 16% and the inter‐subject variability 28%. The intra‐subject variability of lactate to total carbon ratio was 14% and the inter‐subject variability 20%. The study results show that the signal level and relaxivity of [1‐¹³C]alanine may be promising biomarkers for HCC tumors. Its diagnostic values in HCC staging and treatment monitoring are yet to be explored. Copyright © 2010 John Wiley & Sons, Ltd.     

Pyruvate‐lactate exchange and glucose uptake in human prostate cancer cell models. A study in xenografts and suspensions by hyperpolarized [1‐13C]pyruvate MRS and [18F]FDG‐PET

Reprogramming of energy metabolism in the development of prostate cancer can be exploited for a better diagnosis and treatment of the disease. The goal of this study was to determine whether differences in glucose and pyruvate metabolism of human prostate cancer cells with dissimilar aggressivenesses can be detected using hyperpolarized [1‐¹³C]pyruvate MRS and [¹⁸F]FDG‐PET imaging, and to evaluate whether these measures correlate. For this purpose, we compared murine xenografts of human prostate cancer LNCaP cells with those of more aggressive PC3 cells. [1‐¹³C]pyruvate was hyperpolarized by dissolution dynamic nuclear polarization (dDNP) and [1‐¹³C]pyruvate to lactate conversion was followed by ¹³C MRS. Subsequently [¹⁸F]FDG uptake was investigated by static and dynamic PET measurements. Standard uptake values (SUVs) for [¹⁸F]FDG were significantly higher for xenografts of PC3 compared with those of LNCaP. However, we did not observe a difference in the average apparent rate constant k_pl of ¹³C label exchange from pyruvate to lactate between the tumor variants. A significant negative correlation was found between SUVs from [¹⁸F]FDG PET measurements and k_pl values for the xenografts of both tumor types. The k_pl rate constant may be influenced by various factors, and studies with a range of prostate cancer cells in suspension suggest that LDH inhibition by pyruvate may be one of these. Our results indicate that glucose and pyruvate metabolism in the prostate cancer cell models differs from that in other tumor models and that [¹⁸F]FDG‐PET can serve as a valuable complementary tool in dDNP studies of aggressive prostate cancer with [1‐¹³C]pyruvate.     

Detection of myocardial medium‐chain fatty acid oxidation and tricarboxylic acid cycle activity with hyperpolarized [1–13C]octanoate

Under normal conditions, the heart mainly relies on fatty acid oxidation to meet its energy needs. Changes in myocardial fuel preference are noted in the diseased and failing heart. The magnetic resonance signal enhancement provided by spin hyperpolarization allows the metabolism of substrates labeled with carbon‐13 to be followed in real time in vivo. Although the low water solubility of long‐chain fatty acids abrogates their hyperpolarization by dissolution dynamic nuclear polarization, medium‐chain fatty acids have sufficient solubility to be efficiently polarized and dissolved. In this study, we investigated the applicability of hyperpolarized [1–¹³C]octanoate to measure myocardial medium‐chain fatty acid metabolism in vivo. Scanning rats infused with a bolus of hyperpolarized [1–¹³C]octanoate, the primary metabolite observed in the heart was identified as [1–¹³C]acetylcarnitine. Additionally, [5‐¹³C]glutamate and [5‐¹³C]citrate could be respectively resolved in seven and five of 31 experiments, demonstrating the incorporation of oxidation products of octanoate into the tricarboxylic acid cycle. A variable drop in blood pressure was observed immediately following the bolus injection, and this drop correlated with a decrease in normalized acetylcarnitine signal (acetylcarnitine/octanoate). Increasing the delay before infusion moderated the decrease in blood pressure, which was attributed to the presence of residual gas bubbles in the octanoate solution. No significant difference in normalized acetylcarnitine signal was apparent between fed and 12‐hour fasted rats. Compared with a solution in buffer, the longitudinal relaxation of [1–¹³C]octanoate was accelerated ~3‐fold in blood and by the addition of serum albumin. These results demonstrate the potential of hyperpolarized [1–¹³C]octanoate to probe myocardial medium‐chain fatty acid metabolism as well as some of the limitations that may accompany its use.     

1H and 13C HR-MAS spectroscopy of intact biopsy samples ex vivo and in vivo 1H MRS study of human high grade gliomas

High-resolution magic angle spinning (HR-MAS) one- and two-dimensional 1H and 13C nuclear magnetic resonance (NMR) spectroscopy has been used to study intact glioblastoma (GBM) brain tumour tissue. The results were compared with in vitro chemical extract and in vivo spectra. The resolution of 1H one-dimensional, 1H TOCSY and 13C HSQC HR-MAS spectra is comparable to that obtained on perchloric extracts. 13C HSQC HR-MAS spectra have been particularly useful for the identification of 37 different metabolites in intact biopsy tumours, excluding water and DSS components. To our knowledge, this is the most detailed assignment of biochemical compounds obtained in intact human tissue, in particular in brain tumour tissue. Tissue degradation during the recording of the NMR experiment was avoided by keeping the sample at a temperature of 4 degrees C. Detailed metabolical compositions of 10 GBM (six primary, two secondary and two unclassified) were obtained. A good correlation between ex vivo and in vivo MRS has been found.     

Measuring mitochondrial metabolism in rat brain in vivo using MR Spectroscopy of hyperpolarized [2‐13C]pyruvate

Hyperpolarized [1‐¹³C]pyruvate ([1‐¹³C]Pyr) has been used to assess metabolism in healthy and diseased states, focusing on the downstream labeling of lactate (Lac), bicarbonate and alanine. Although hyperpolarized [2‐¹³C]Pyr, which retains the labeled carbon when Pyr is converted to acetyl‐coenzyme A, has been used successfully to assess mitochondrial metabolism in the heart, the application of [2‐¹³C]Pyr in the study of brain metabolism has been limited to date, with Lac being the only downstream metabolic product reported previously. In this study, single‐time‐point chemical shift imaging data were acquired from rat brain in vivo. [5‐¹³C]Glutamate, [1‐¹³C]acetylcarnitine and [1‐¹³C]citrate were detected in addition to resonances from [2‐¹³C]Pyr and [2‐¹³C]Lac. Brain metabolism was further investigated by infusing dichloroacetate, which upregulates Pyr flux to acetyl‐coenzyme A. After dichloroacetate administration, a 40% increase in [5‐¹³C]glutamate from 0.014 ± 0.004 to 0.020 ± 0.006 (p = 0.02), primarily from brain, and a trend to higher citrate (0.002 ± 0.001 to 0.004 ± 0.002) were detected, whereas [1‐¹³C]acetylcarnitine was increased in peripheral tissues. This study demonstrates, for the first time, that hyperpolarized [2‐¹³C]Pyr can be used for the in vivo investigation of mitochondrial function and tricarboxylic acid cycle metabolism in brain. Copyright © 2013 John Wiley & Sons, Ltd.     

Real‐time measurement of hyperpolarized lactate production and efflux as a biomarker of tumor aggressiveness in an MR compatible 3D cell culture bioreactor

We have developed a 3D cell/tissue culture bioreactor compatible with hyperpolarized (HP) ¹³C MR and interrogated HP [1‐¹³C]lactate production and efflux in human renal cell carcinoma (RCC) cells. This platform is capable of resolving intracellular and extracellular HP lactate pools, allowing the kinetic measurement of lactate production and efflux in the context of cancer aggressiveness and response to therapy. HP ¹³C MR studies were performed on three immortalized human renal cell lines: HK2, a normal renal proximal tubule cell line from which a majority of RCCs arise, UMRC6, a cell line derived from a localized RCC, and UOK262, an aggressive and metastatic RCC. The intra‐ (Lac_in) and extracellular (Lac_ex) HP lactate signals were robustly resolved in dynamic ¹³C spectra of the cell lines due to a very small but reproducible chemical shift difference (0.031 ± 0.0005 ppm). Following HP [1‐¹³C]pyruvate delivery, the ratio of HP Lac_in/Lac_ex was significantly lower for UOK262 cells compared with both UMRC6 and HK2 cells due to a significant (p < 0.05) increase in the Lac_ex pool size. Lac_in/Lac_ex correlated with the MCT4 mRNA expression of the cell lines, and inhibition of MCT4 transport using DIDS resulted in a significant reduction in the HP Lac_ex pool size. The extension of these studies to living patient‐derived RCC tissue slices using HP [1,2‐¹³C₂]pyruvate demonstrated a similarly split lactate doublet with a high Lac_ex pool fraction; in contrast, only a single NMR resonance is noted for HP [5‐¹³C]glutamate, consistent with intracellular localization. These studies support the importance of lactate efflux as a biomarker of cancer aggressiveness and metastatic potential, and the utility of the MR compatible 3D cell/tissue culture bioreactor to study not only cellular metabolism but also transport. Additionally, this platform offers a sophisticated way to follow therapeutic interventions and screen novel therapies that target lactate export. Copyright © 2015 John Wiley & Sons, Ltd.     

A calibration‐based approach to real‐time in vivo monitoring of pyruvate C1 and C2 polarization using the JCC spectral asymmetry

A calibration‐based technique for real‐time measurement of pyruvate polarization by partial integral analysis of the doublet from the neighbouring J‐coupled carbon is presented. In vitro calibration data relating the C₂ and C₁ asymmetries to the instantaneous C₁ and C₂ polarizations, respectively, were acquired in blood. The feasibility of using the in vitro calibration data to determine the instantaneous in vivo C₁ and C₂ polarizations was demonstrated in the analysis of rat kidney and pig heart spectral data. An approach for incorporating this technique into in vivo protocols is proposed. Copyright © 2013 John Wiley & Sons, Ltd.     

Dynamic 2D and 3D mapping of hyperpolarized pyruvate to lactate conversion in vivo with efficient multi‐echo balanced steady‐state free precession at 3 T

The aim of this study was to acquire the transient MRI signal of hyperpolarized tracers and their metabolites efficiently, for which specialized imaging sequences are required. In this work, a multi‐echo balanced steady‐state free precession (me‐bSSFP) sequence with Iterative Decomposition with Echo Asymmetry and Least squares estimation (IDEAL) reconstruction was implemented on a clinical 3 T positron‐emission tomography/MRI system for fast 2D and 3D metabolic imaging. Simulations were conducted to obtain signal‐efficient sequence protocols for the metabolic imaging of hyperpolarized biomolecules. The sequence was applied in vitro and in vivo for probing the enzymatic exchange of hyperpolarized [1–¹³C]pyruvate and [1–¹³C]lactate. Chemical shift resolution was achieved using a least‐square, iterative chemical species separation algorithm in the reconstruction. In vitro, metabolic conversion rate measurements from me‐bSSFP were compared with NMR spectroscopy and free induction decay‐chemical shift imaging (FID‐CSI). In vivo, a rat MAT‐B‐III tumor model was imaged with me‐bSSFP and FID‐CSI. 2D metabolite maps of [1–¹³C]pyruvate and [1–¹³C]lactate acquired with me‐bSSFP showed the same spatial distributions as FID‐CSI. The pyruvate‐lactate conversion kinetics measured with me‐bSSFP and NMR corresponded well. Dynamic 2D metabolite mapping with me‐bSSFP enabled the acquisition of up to 420 time frames (scan time: 180‐350 ms/frame) before the hyperpolarized [1–¹³C]pyruvate was relaxed below noise level. 3D metabolite mapping with a large field of view (180 × 180 × 48 mm³) and high spatial resolution (5.6 × 5.6 × 2 mm³) was conducted with me‐bSSFP in a scan time of 8.2 seconds. It was concluded that Me‐bSSFP improves the spatial and temporal resolution for metabolic imaging of hyperpolarized [1–¹³C]pyruvate and [1–¹³C]lactate compared with either of the FID‐CSI or EPSI methods reported at 3 T, providing new possibilities for clinical and preclinical applications.     

Effects of acute and chronic administration of mitomycin C on the induction of sister chromatid exchanges in vivo

Frequencies of sister chromatid exchanges (SCE) were analyzed in bone marrow cells of mice injected with mitomycin C (MMC) both before and during infusion with bromodeoxyuridine (BrdU). Administration of MMC at 1, 6.5, and 13 hours after the onset of BrdU infusion resulted in the induction of approximately 45 SCE/cell, independent of time of administration. When MMC was injected 26 hours prior to BrdU infusion, only baseline levels of SCE were noted. The effects of multiple doses of MMC (chronic administration) were examined in mice treated with 1–5 mg/kg on a weekly or bimonthly basis. SCE analysis was performed one week after the final injection. At all doses and with all treatment regimes, SCE frequencies did not differ from control levels. The results indicate that most or all MMC-induced DNA damage that results in SCE formation is removed in a single cell cycle after its administration.     

In vivo measurement of aldehyde dehydrogenase‐2 activity in rat liver ethanol model using dynamic MRSI of hyperpolarized [1‐13C]pyruvate

To date, measurements of the activity of aldehyde dehydrogenase‐2 (ALDH2), a critical mitochondrial enzyme for the elimination of certain cytotoxic aldehydes in the body and a promising target for drug development, have been largely limited to in vitro methods. Recent advancements in MRS of hyperpolarized ¹³C‐labeled substrates have provided a method to detect and image in vivo metabolic pathways with signal‐to‐noise ratio gains greater than 10 000‐fold over conventional MRS techniques. However aldehydes, because of their toxicity and short T₁ relaxation times, are generally poor targets for such ¹³C‐labeled studies. In this work, we show that dynamic MRSI of hyperpolarized [1‐¹³C]pyruvate and its conversion to [1‐¹³C]lactate can provide an indirect in vivo measurement of ALDH2 activity via the concentration of NADH (nicotinamide adenine dinucleotide, reduced form), a co‐factor common to both the reduction of pyruvate to lactate and the oxidation of acetaldehyde to acetate. Results from a rat liver ethanol model (n = 9) show that changes in ¹³C‐lactate labeling following the bolus injection of hyperpolarized pyruvate are highly correlated with changes in ALDH2 activity (R² = 0.76). Copyright © 2012 John Wiley & Sons, Ltd.