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The role of pyruvate dehydrogenase in mediating lipid-induced insulin resistance stands as a central question in the pathogenesis of type 2 diabetes mellitus. Many researchers have invoked the Randle hypothesis to explain the reduced glucose disposal in skeletal muscle by envisioning an elevated acetyl CoA pool arising from increased oxidation of fatty acids. Over the years, in vivo NMR studies have challenged that monolithic view. The advent of the dissolution dynamic nuclear polarization NMR technique and a unique type 2 diabetic rat model provides an opportunity to clarify. Dynamic nuclear polarization enhances dramatically the NMR signal sensitivity and allows the measurement of metabolic kinetics in vivo. Diabetic muscle has much lower pyruvate dehydrogenase activity than control muscle, as evidenced in the conversion of [1-13C]lactate and [2-13C]pyruvate to HCO3- and acetyl carnitine. The pyruvate dehydrogenase kinase inhibitor, dichloroacetate, restores rapidly the diabetic pyruvate dehydrogenase activity to control level. However, diabetic muscle has a much larger dynamic change in pyruvate dehydrogenase flux than control. The dichloroacetate-induced surge in pyruvate dehydrogenase activity produces a differential amount of acetyl carnitine but does not affect the tricarboxylic acid flux. Further studies can now proceed with the dynamic nuclear polarization approach and a unique rat model to interrogate closely the biochemical mechanism interfacing oxidative metabolism with insulin resistance and metabolic inflexibility.

Background Pyruvate dehydrogenase (PDH) and lactate dehydrogenase are essential for adenosine triphosphate production in skeletal muscle. At the onset of exercise, oxidation of glucose and glycogen is quickly enabled by dephosphorylation of PDH. However, direct measurement of PDH flux in exercising human muscle is daunting, and the net effect of covalent modification and other control mechanisms on PDH flux has not been assessed. Purpose To demonstrate the feasibility of assessing PDH activation and changes in pyruvate metabolism in human skeletal muscle after the onset of exercise using carbon 13 (13C) MRI with hyperpolarized (HP) [1-13C]-pyruvate. Materials and Methods For this prospective study, sedentary adults in good general health (mean age, 42 years ± 18 [standard deviation]; six men) were recruited from August 2019 to September 2020. Subgroups of the participants were injected with HP [1-13C]-pyruvate at resting, during plantar flexion exercise, or 5 minutes after exercise during recovery. In parallel, hydrogen 1 arterial spin labeling MRI was performed to estimate muscle tissue perfusion. An unpaired t test was used for comparing 13C data among the states. Results At rest, HP [1-13C]-lactate and [1-13C]-alanine were detected in calf muscle, but [13C]-bicarbonate was negligible. During moderate flexion-extension exercise, total HP 13C signals (tC) increased 2.8-fold because of increased muscle perfusion (P = .005), and HP [1-13C]-lactate-to-tC ratio increased 1.7-fold (P = .04). HP [13C]-bicarbonate-to-tC ratio increased 8.4-fold (P = .002) and returned to the resting level 5 minutes after exercise, whereas the lactate-to-tC ratio continued to increase to 2.3-fold as compared with resting (P = .008). Conclusion Lactate and bicarbonate production from hyperpolarized (HP) [1-carbon 13 {13C}]-pyruvate in skeletal muscle rapidly reflected the onset and the termination of exercise. These results demonstrate the feasibility of imaging skeletal muscle metabolism using HP [1-13C]-pyruvate MRI and the sensitivity of in vivo pyruvate metabolism to exercise states. © RSNA, 2021 Online supplemental material is available for this article.

Purpose: Pyruvate, produced from either glucose, glycogen, or lactate, is the dominant precursor of cerebral oxidative metabolism. Pyruvate dehydrogenase (PDH) flux is a direct measure of cerebral mitochondrial function and metabolism. Detection of [13C]bicarbonate in the brain from hyperpolarized [1‐13C]pyruvate using carbon‐13 (13C) MRI provides a unique opportunity for assessing PDH flux in vivo. This study is to assess changes in cerebral PDH flux in response to visual stimuli using in vivo 13C MRS with hyperpolarized [1‐13C]pyruvate. Methods: From seven sedentary adults in good general health, time‐resolved [13C]bicarbonate production was measured in the brain using 90° flip angles with minimal perturbation of its precursors, [1‐13C]pyruvate and [1‐13C]lactate, to test the hypothesis that the appearance of [13C]bicarbonate signals in the brain reflects the metabolic changes associated with neuronal activation. With a separate group of healthy participants (n = 3), the likelihood of the bolus‐injected [1‐13C]pyruvate being converted to [1‐13C]lactate prior to decarboxylation was investigated by measuring [13C]bicarbonate production with and without [1‐13C]lactate saturation. Results: In the course of visual stimulation, the measured [13C]bicarbonate signal normalized to the total 13C signal in the visual cortex increased by 17.1% ± 15.9% (p = 0.017), whereas no significant change was detected in [1‐13C]lactate. Proton BOLD fMRI confirmed the regional activation in the visual cortex with the stimuli. Lactate saturation decreased bicarbonate‐to‐pyruvate ratio by 44.4% ± 9.3% (p < 0.01). Conclusion: We demonstrated the utility of 13C MRS with hyperpolarized [1‐13C]pyruvate for assessing the activation of cerebral PDH flux via the detection of [13C]bicarbonate production. [ABSTRACT FROM AUTHOR]

The goal of this study was to investigate the origin of brain lactate (Lac) signal in the healthy anesthetized rat after injection of hyperpolarized (HP) [1‐13C]pyruvate (Pyr). Dynamic two‐dimensional spiral chemical shift imaging with flow‐sensitizing gradients revealed reduction in both vascular and brain Pyr, while no significant dependence on the level of flow suppression was detected for Lac. These results support the hypothesis that the HP metabolites predominantly reside in different compartments in the brain (i.e., Pyr in the blood and Lac in the parenchyma). Data from high‐resolution metabolic imaging of [1‐13C]Pyr further demonstrated that Lac detected in the brain was not from contributions of vascular signal attributable to partial volume effects. Additionally, metabolite distributions and kinetics measured with dynamic imaging after injection of HP [1‐13C]Lac were similar to Pyr data when Pyr was used as the substrate. These data do not support the hypothesis that Lac observed in the brain after Pyr injection was generated in other organs and then transported across the blood–brain barrier (BBB). Together, the presented results provide further evidence that even in healthy anesthetized rats, the transport of HP Pyr across the BBB is sufficiently fast to permit detection of its metabolic conversion to Lac within the brain. [ABSTRACT FROM AUTHOR]

Purpose: [13C]Bicarbonate formation from hyperpolarized [1‐13C]pyruvate via pyruvate dehydrogenase, a key regulatory enzyme, represents the cerebral oxidation of pyruvate and the integrity of mitochondrial function. The present study is to characterize the chronology of cerebral mitochondrial metabolism during secondary injury associated with acute traumatic brain injury (TBI) by longitudinally monitoring [13C]bicarbonate production from hyperpolarized [1‐13C]pyruvate in rodents. Methods: Male Wistar rats were randomly assigned to undergo a controlled‐cortical impact (CCI, n = 31) or sham surgery (n = 22). Seventeen of the CCI and 9 of the sham rats longitudinally underwent a 1H/13C‐integrated MR protocol that includes a bolus injection of hyperpolarized [1‐13C]pyruvate at 0 (2 h), 1, 2, 5, and 10 days post‐surgery. Separate CCI and sham rats were used for histological validation and enzyme assays. Results: In addition to elevated lactate, we observed significantly reduced bicarbonate production in the injured site. Unlike the immediate appearance of hyperintensity on T2‐weighted MRI, the contrast of bicarbonate signals between the injured region and the contralateral brain peaked at 24 h post‐injury, then fully recovered to the normal level at day 10. A subset of TBI rats demonstrated markedly increased bicarbonate in normal‐appearing contralateral brain regions post‐injury. Conclusion: This study demonstrates that aberrant mitochondrial metabolism occurring in acute TBI can be monitored by detecting [13C]bicarbonate production from hyperpolarized [1‐13C]pyruvate, suggesting that [13C]bicarbonate is a sensitive in‐vivo biomarker of the secondary injury processes. [ABSTRACT FROM AUTHOR]

Purpose: This study is to investigate time‐resolved 13C MR spectroscopy (MRS) as an alternative to imaging for assessing pyruvate metabolism using hyperpolarized (HP) [1‐13C]pyruvate in the human brain. Methods: Time‐resolved 13C spectra were acquired from four axial brain slices of healthy human participants (n = 4) after a bolus injection of HP [1‐13C]pyruvate. 13C MRS with low flip‐angle excitations and a multichannel 13C/1H dual‐frequency radiofrequency (RF) coil were exploited for reliable and unperturbed assessment of HP pyruvate metabolism. Slice‐wise areas under the curve (AUCs) of 13C‐metabolites were measured and kinetic analysis was performed to estimate the production rates of lactate and HCO3‐. Linear regression analysis between brain volumes and HP signals was performed. Region‐focused pyruvate metabolism was estimated using coil‐wise 13C reconstruction. Reproducibility of HP pyruvate exams was presented by performing two consecutive injections with a 45‐minutes interval. Results: [1‐13C]Lactate relative to the total 13C signal (tC) was 0.21–0.24 in all slices. [13C]HCO3‐/tC was 0.065–0.091. Apparent conversion rate constants from pyruvate to lactate and HCO3‐ were calculated as 0.014–0.018 s−1 and 0.0043–0.0056 s−1, respectively. Pyruvate/tC and lactate/tC were in moderate linear relationships with fractional gray matter volume within each slice. White matter presented poor linear regression fit with HP signals, and moderate correlations of the fractional cerebrospinal fluid volume with pyruvate/tC and lactate/tC were measured. Measured HP signals were comparable between two consecutive exams with HP [1‐13C]pyruvate. Conclusions: Dynamic MRS in combination with multichannel RF coils is an affordable and reliable alternative to imaging methods in investigating cerebral metabolism using HP [1‐13C]pyruvate. [ABSTRACT FROM AUTHOR]

Purpose: Previous cardiac imaging studies using hyperpolarized (HP) [1‐13C]pyruvate were acquired at end‐diastole (ED). Little is known about the interaction between cardiac cycle and metabolite content in the myocardium. In this study, we compared images of HP pyruvate and products at end‐systole (ES) and ED. Methods: A dual‐phase 13C MRI sequence was implemented to acquire two sequential HP images within a single cardiac cycle at ES and ED during successive R‐R intervals in an interleaved manner. Each healthy volunteer (N = 3) received two injections of HP [1‐13C]pyruvate for the dual‐phase imaging on the short‐axis and the vertical long‐axis planes. Spatial distribution of HP 13C metabolites at each cardiac phase was correlated to multiphase 1H MRI to confirm the mechanical changes. Ratios of myocardial HP metabolites were compared between ES and ED. Segmental analysis was performed on the midcavity short‐axis plane. Results: In addition to mechanical changes, metabolic profiles of the heart detected by HP [1‐13C]pyruvate differed between ES and ED. The myocardial signal of [13C]bicarbonate relative to [1‐13C]lactate was significantly smaller at ED than the ratio at ES (p <.05), particularly in mid‐anterior and mid‐inferoseptal segments. The distinct metabolic profiles in the myocardium likely reflect the technical aspects of the imaging approach such as the coronary flow in addition to the cyclical changes in metabolism. Conclusion: The study demonstrates that metabolic profiles of the heart, measured by HP [1‐13C]pyruvate, are affected by the cardiac cycle in which that the data are acquired. [ABSTRACT FROM AUTHOR]

Purpose: Noninvasive imaging with hyperpolarized (HP) pyruvate can capture in vivo cardiac metabolism. For proper quantification of the metabolites and optimization of imaging parameters, understanding MR characteristics such as T2∗s of the HP signals is critical. This study is to measure in vivo cardiac T2∗s of HP [1‐13C]pyruvate and the products in rodents and humans. Methods: A dynamic 13C multi‐echo spiral imaging sequence that acquires [13C]bicarbonate, [1‐13C]lactate, and [1‐13C]pyruvate images in an interleaved manner was implemented for a clinical 3 Tesla system. T2∗ of each metabolite was calculated from the multi‐echo images by fitting the signal decay of each region of interest mono‐exponentially. The performance of measuring T2∗ using the sequence was first validated using a 13C phantom and then with rodents following a bolus injection of HP [1‐13C]pyruvate. In humans, T2∗ of each metabolite was calculated for left ventricle, right ventricle, and myocardium. Results: Cardiac T2∗s of HP [1‐13C]pyruvate, [1‐13C]lactate, and [13C]bicarbonate in rodents were measured as 24.9 ± 5.0, 16.4 ± 4.7, and 16.9 ± 3.4 ms, respectively. In humans, T2∗ of [1‐13C]pyruvate was 108.7 ± 22.6 ms in left ventricle and 129.4 ± 8.9 ms in right ventricle. T2∗ of [1‐13C]lactate was 40.9 ± 8.3, 44.2 ± 5.5, and 43.7 ± 9.0 ms in left ventricle, right ventricle, and myocardium, respectively. T2∗ of [13C]bicarbonate in myocardium was 64.4 ± 2.5 ms. The measurements were reproducible and consistent over time after the pyruvate injection. Conclusion: The proposed metabolite‐selective multi‐echo spiral imaging sequence reliably measures in vivo cardiac T2∗s of HP [1‐13C]pyruvate and products. [ABSTRACT FROM AUTHOR]

Purpose: To evaluate the utility of hyperpolarized [1‐13C]‐l‐lactate to detect hepatic pyruvate carboxylase activity in vivo under fed and fasted conditions. Methods: [1‐13C]‐labeled sodium L‐lactate was polarized using a dynamic nuclear polarizer. Polarization level and the T1 were measured in vitro in a 3 Telsa MR scanner. Two groups of healthy rats (fasted vs. fed) were prepared for in vivo studies. Each rat was anesthetized and intravenously injected with 60‐mM hyperpolarized [1‐13C]‐l‐lactate, immediately followed by dynamic acquisition of 13C (carbon‐13) MR spectra from the liver at 3 Tesla. The dosage‐dependence of the 13C‐products was also investigated by performing another injection of an equal volume of 30‐mM hyperpolarized [1‐13C]‐l‐lactate. Results: T1 and liquid polarization level of [1‐13C]‐l‐lactate were estimated as 67.8 s and 40.0%, respectively. [1‐13C]pyruvate and [1‐13C]alanine, [13C]bicarbonate (HCO3‐) and [1‐13C]aspartate were produced from hyperpolarized [1‐13C]‐l‐lactate in rat liver. Smaller HCO3‐ and larger aspartate were measured in the fed group compared to the fasted group. Pyruvate and alanine production were increased in proportion to the lactate concentration, whereas the amount of HCO3‐ and aspartate production was consistent between 30‐mM and 60‐mM lactate injections. Conclusion: This study demonstrates that a unique biomarker of pyruvate carboxylase flux, the appearance of [1‐13C]aspartate from [1‐13C]‐l‐lactate, is sensitive to nutritional state and may be monitored in vivo at 3 Tesla. Because [13C]HCO3‐ is largely produced by pyruvate dehydrogenase flux, these results suggest that the ratio of [1‐13C]aspartate and [13C]HCO3‐ (aspartate/HCO3‐) reflects the saturable pyruvate carboxylase/pyruvate dehydrogenase enzyme activities. [ABSTRACT FROM AUTHOR]

Carbon‐13 magnetic resonance spectroscopy (MRS) following oral intake of 13C‐labeled glucose is the gold standard for imaging glycogen metabolism in humans. However, the temporal resolution of previous studies has been >13 minutes. Here, we describe a high‐sensitivity 13C MRS method for imaging hepatic glycogen synthesis with a temporal resolution of 1 minute or less. Nuclear magnetic resonance spectra were acquired from the liver of 3 healthy volunteers, using a 13C clamshell radiofrequency transmit and paddle‐shaped array receive coils in a 3 Tesla magnetic resonance imaging system. Following a 15‐minute baseline 13C MRS scan of the liver, [1‐13C]‐glucose was ingested and 13C MRS data were acquired for an additional 1‐3 hours. Dynamic change of the hepatic glycogen synthesis level was analyzed by reconstructing the acquired MRS data with temporal resolutions of 30 seconds to 15 minutes. Plasma levels of 13C‐labeled glucose and lactate were measured using gas chromatography–mass spectrometry. While not detected at baseline 13C MRS, [1‐13C]‐labeled α‐glucose and β‐glucose and glycogen peaks accumulated rapidly, beginning as early as ~2 minutes after oral administration of [1‐13C]‐glucose. The [1‐13C]‐glucose signals peaked at ~5 minutes, whereas [1‐13C]‐glycogen peaked at ~25 minutes after [1‐13C]‐glucose ingestion; both signals declined toward baseline levels over the next 1‐3 hours. Plasma levels of 13C‐glucose and 13C‐lactate rose gradually, and approximately 20% of all plasma glucose and 5% of plasma lactate were 13C‐labeled by 2 hours after ingestion. Conclusion: We observed rapid accumulation of hepatic [1‐13C]‐glycogen following orally administered [1‐13C]‐glucose, using a dynamic 13C MRS method with a temporal resolution of 1 minute or less. Commercially available technology allows high temporal resolution studies of glycogen metabolism in the human liver. [ABSTRACT FROM AUTHOR]

Hyperpolarized 13C magnetic resonance spectroscopy (MRS) provides unprecedented opportunities to obtain clinical diagnostic information through in vivo monitoring of metabolic pathways. The continuing advancement of this field relies on the identification of molecular probes that can effectively interrogate pathways critical to disease. In this report, we describe the synthesis, development, and in vivo application of sodium [1-13C]-glycerate ([13C]-Glyc) as a novel probe for evaluating glycolysis using hyperpolarized 13C MRS. This agent was prepared by a concise synthetic route and formulated for dynamic nuclear polarization. [13C]-Glyc displayed a high level of polarization and long spin-lattice relaxation timeboth of which are necessary for future clinical investigations. In vivo spectroscopic studies with hyperpolarized [13C]-Glyc in rat liver furnished metabolic products, [13C]-labeled pyruvate and lactate, originating from glycolysis. The levels of production and relative intensities of these metabolites were directly correlated with the induced glycolytic state (fasted versus fed groups). This work establishes hyperpolarized [13C]-Glyc as a novel agent for clinically relevant 13C MRS studies of energy metabolism and further provides opportunities for evaluating intracellular redox states in biochemical investigations. [ABSTRACT FROM AUTHOR]

Purpose The intracellular lactate to pyruvate concentration ratio is a commonly used tissue assay biomarker of redox, being proportional to free cytosolic [NADH]/[NAD+]. In this study, we assessed the use of hyperpolarized [1-13C]alanine and the subsequent detection of the intracellular products of [1-13C]pyruvate and [1-13C]lactate as a useful substrate for assessing redox levels in the liver in vivo. Methods Animal experiments were conducted to measure in vivo metabolism at baseline and after ethanol infusion. A solution of 80-mM hyperpolarized [1-13C]alanine was injected intravenously at baseline (n = 8) and 45 min after ethanol infusion (n = 4), immediately followed by the dynamic acquisition of 13C MRS spectra. Results In vivo rat liver spectra showed peaks from [1-13C] alanine and the products of [1-13C]lactate, [1-13C]pyruvate, and 13C-bicarbonate. A significantly increased 13C-lactate/13C-pyruvate ratio was observed after ethanol infusion (8.46 ± 0.58 at baseline versus 13.58 ± 0.69 after ethanol infusion; P < 0.001) consistent with the increased NADH produced by liver metabolism of ethanol to acetaldehyde and then acetate. A decrease in 13C-bicarbonate production was also noted, potentially reflecting ethanol-induced mitochondrial redox changes. Conclusion A method to measure in vivo tissue redox using hyperpolarized [1-13C]alanine is presented, with the validity of the proposed 13C-pyruvate/13C-lactate metric tested using an ethanol challenge to alter liver redox state. Magn Reson Med 77:1741-1748, 2017. © 2017 International Society for Magnetic Resonance in Medicine. [ABSTRACT FROM AUTHOR]

Hyperpolarized [1-13C]pyruvate MRS provides a unique imaging opportunity to study the reaction kinetics and enzyme activities of in vivo metabolism because of its favorable imaging characteristics and critical position in the cellular metabolic pathway, where it can either be reduced to lactate (reflecting glycolysis) or converted to acetyl-coenzyme A and bicarbonate (reflecting oxidative phosphorylation). Cancer tissue metabolism is altered in such a way as to result in a relative preponderance of glycolysis relative to oxidative phosphorylation (i.e. Warburg effect). Although there is a strong theoretical basis for presuming that readjustment of the metabolic balance towards normal could alter tumor growth, a robust noninvasive in vivo tool with which to measure the balance between these two metabolic processes has yet to be developed. Until recently, hyperpolarized 13C-pyruvate imaging studies had focused solely on [1-13C]lactate production because of its strong signal. However, without a concomitant measure of pyruvate entry into the mitochondria, the lactate signal provides no information on the balance between the glycolytic and oxidative metabolic pathways. Consistent measurement of 13C-bicarbonate in cancer tissue, which does provide such information, has proven difficult, however. In this study, we report the reliable measurement of 13C-bicarbonate production in both the healthy brain and a highly glycolytic experimental glioblastoma model using an optimized 13C MRS imaging protocol. With the capacity to obtain signal in all tumors, we also confirm for the first time that the ratio of 13C-lactate to 13C-bicarbonate provides a more robust metric relative to 13C-lactate for the assessment of the metabolic effects of anti-angiogenic therapy. Our data suggest a potential application of this ratio as an early biomarker to assess therapeutic effectiveness. Furthermore, although further study is needed, the results suggest that anti-angiogenic treatment results in a rapid normalization in the relative tissue utilization of glycolytic and oxidative phosphorylation by tumor tissue. Copyright © 2016 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Purpose MRS of hyperpolarized [2-13C]pyruvate can be used to assess multiple metabolic pathways within mitochondria as the 13C label is not lost with the conversion of pyruvate to acetyl-CoA. This study presents the first MR spectroscopic imaging of hyperpolarized [2-13C]pyruvate in glioma-bearing brain. Methods Spiral chemical shift imaging with spectrally undersampling scheme (1042 Hz) and a hard-pulse excitation was exploited to simultaneously image [2-13C]pyruvate, [2-13C]lactate, and [5-13C]glutamate, the metabolites known to be produced in brain after an injection of hyperpolarized [2-13C]pyruvate, without chemical shift displacement artifacts. A separate undersampling scheme (890 Hz) was also used to image [1-13C]acetyl-carnitine. Healthy and C6 glioma-implanted rat brains were imaged at baseline and after dichloroacetate administration, a drug that modulates pyruvate dehydrogenase kinase activity. Results The baseline metabolite maps showed higher lactate and lower glutamate in tumor as compared to normal-appearing brain. Dichloroacetate led to an increase in glutamate in both tumor and normal-appearing brain. Dichloroacetate-induced %-decrease of lactate/glutamate was comparable to the lactate/bicarbonate decrease from hyperpolarized [1-13C]pyruvate studies. Acetyl-carnitine was observed in the muscle/fat tissue surrounding the brain. Conclusion Robust volumetric imaging with hyperpolarized [2-13C]pyruvate and downstream products was performed in glioma-bearing rat brains, demonstrating changes in mitochondrial metabolism with dichloroacetate. Magn Reson Med 75:973-984, 2016. © 2015 Wiley Periodicals, Inc. [ABSTRACT FROM AUTHOR]

To facilitate diagnosis and staging of liver disease, sensitive and non-invasive methods for the measurement of liver metabolism are needed. This study used hyperpolarized 13C-pyruvate to assess metabolic parameters in a CCl4 model of liver damage in rats. Dynamic 3D 13C chemical shift imaging data from a volume covering kidney and liver were acquired from 8 control and 10 CCl4-treated rats. At 12 time points at 5 s temporal resolution, we quantified the signal intensities and established time courses for pyruvate, alanine, and lactate. These measurements were compared with standard liver histology and an alanine transaminase (ALT) enzyme assay using liver tissue from the same animals. All CCl4-treated but none of the control animals showed histological liver damage and elevated ALT enzyme levels. In agreement with these results, metabolic imaging revealed an increased alanine/pyruvate ratio in liver of CCl4-treated rats, which is indicative of elevated ALT activity. Similarly, lactate/pyruvate ratios were higher in CCl4-treated compared with control animals, demonstrating the presence of inflammation. No significant differences in metabolite ratios were observed in kidney or vasculature. Thus this work shows that metabolic imaging using 13C-pyruvate can be a successful tool to non-invasively assess liver damage in vivo. [ABSTRACT FROM AUTHOR]

Purpose In contrast to [1-13C]pyruvate, hyperpolarized [2-13C]pyruvate permits the ability to follow the 13C label beyond flux through pyruvate dehydrogenase complex and investigate the incorporation of acetyl-coenzyme A into different metabolic pathways. However, chemical shift imaging (CSI) with [2-13C]pyruvate is challenging owing to the large spectral dispersion of the resonances, which also leads to severe chemical shift displacement artifacts for slice-selective acquisitions. Methods This study introduces a sequence for three-dimensional CSI of [2-13C]pyruvate using spectrally selective excitation of limited frequency bands containing a subset of metabolites. Dynamic CSI data were acquired alternately from multiple frequency bands in phantoms for sequence testing and in vivo in rat heart. Results Phantom experiments verified the radiofrequency pulse design and demonstrated that the signal behavior of each group of resonances was unaffected by excitation of the other frequency bands. Dynamic three-dimensional 13C CSI data demonstrated the sequence capability to image pyruvate, lactate, acetylcarnitine, glutamate, and acetoacetate, enabling the analysis of organ-specific spectra and metabolite time courses. Conclusions The presented method allows CSI of widely separated resonances without chemical shift displacement artifact, acquiring multiple frequency bands alternately to obtain dynamic time-course information. This approach enables robust imaging of downstream metabolic products of acetyl-coenzyme A with hyperpolarized [2-13C]pyruvate. Magn Reson Med 71:2051-2058, 2014. © 2013 Wiley Periodicals, Inc. [ABSTRACT FROM AUTHOR]

The tricarboxylic acid (TCA) cycle performs an essential role in the regulation of energy and metabolism, and deficiencies in this pathway are commonly correlated with various diseases. However, the development of non-invasive techniques for the assessment of the cycle in vivo has remained challenging. In this work, the applicability of a novel imaging agent, [1,4-13C]-diethylsuccinate, for hyperpolarized 13C metabolic imaging of the TCA cycle was explored. In vivo spectroscopic studies were conducted in conjunction with in vitro analyses to determine the metabolic fate of the imaging agent. Contrary to previous reports (Zacharias NM et al. J. Am. Chem. Soc. 2012; 134: 934-943), [13C]-labeled diethylsuccinate was primarily metabolized to succinate-derived products not originating from TCA cycle metabolism. These results illustrate potential issues of utilizing dialkyl ester analogs of TCA cycle intermediates as molecular probes for hyperpolarized 13C metabolic imaging. Copyright © 2014 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Hyperpolarized 13C MRS allows the in vivo assessment of pyruvate dehydrogenase complex (PDC) flux, which converts pyruvate to acetyl-coenzyme A (acetyl-CoA). [1-13C]pyruvate has been used to measure changes in cardiac PDC flux, with demonstrated increase in 13C-bicarbonate production after dichloroacetate (DCA) administration. With [1-13C]pyruvate, the 13C label is released as 13CO2/13C-bicarbonate, and, hence, does not allow us to follow the fate of acetyl-CoA. Pyruvate labeled in the C2 position has been used to track the 13C label into the TCA (tricarboxylic acid) cycle and measure [5-13C]glutamate as well as study changes in [1-13C]acetylcarnitine with DCA and dobutamine. This work investigates changes in the metabolic fate of acetyl-CoA in response to metabolic interventions of DCA-induced increased PDC flux in the fed and fasted state, and increased cardiac workload with dobutamine in vivo in rat heart at two different pyruvate doses. DCA led to a modest increase in the 13C labeling of [5-13C]glutamate, and a considerable increase in [1-13C]acetylcarnitine and [1,3-13C]acetoacetate peaks. Dobutamine resulted in an increased labeling of [2-13C]lactate, [2-13C]alanine and [5-13C]glutamate. The change in glutamate with dobutamine was observed using a high pyruvate dose but not with a low dose. The relative changes in the different metabolic products provide information about the relationship between PDC-mediated oxidation of pyruvate and its subsequent incorporation into the TCA cycle compared with other metabolic pathways. Using a high dose of pyruvate may provide an improved ability to observe changes in glutamate. Copyright © 2013 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Purpose Commonly used anesthetic agents such as isoflurane are known to be potent cerebral vasodilators, with reported dose-dependent increase in cerebral blood flow and cerebral blood volume. Despite the widespread use of isoflurane in hyperpolarized 13C preclinical research studies, a quantitative assessment of its effect on metabolic measurements is limited. This work investigates the effect of isoflurane anesthesia dose on hyperpolarized 13C MR metabolic measurements in rat brain for [1-13C]pyruvate and 2-keto[1-13C]isocaproate. Methods Dynamic 2D and 3D spiral chemical shift imaging was used to acquire metabolic images of rat brain as well as kidney and liver following bolus injections of hyperpolarized [1-13C]pyruvate or 2-keto[1-13C]isocaproate. The impact of a 'low dose' vs. a 'high dose' of isoflurane on cerebral metabolite levels and apparent conversion rates was examined. Results The cerebral substrate signal levels, and hence the metabolite-to-substrate ratios and apparent conversion rates, were found to depend markedly on isoflurane dose, while signal levels of metabolic products and their ratios, e.g. bicarbonate/lactate, were largely insensitive to isoflurane levels. No obvious dependence on isoflurane was observed in kidney or liver for pyruvate. Conclusion This study highlights the importance of careful attention to the effects of anesthesia on the metabolic measures for hyperpolarized 13C metabolic imaging in brain. Magn Reson Med, 70:1117-1124, 2013. © 2012 Wiley Periodicals, Inc. [ABSTRACT FROM AUTHOR]

In addition to an increased lactate-to-pyruvate ratio, altered metabolism of a malignant glioma can be further characterized by its kinetics. Spatially resolved dynamic data of pyruvate and lactate from C6-implanted female Sprague-Dawley rat brain were acquired using a spiral chemical shift imaging sequence after a bolus injection of a hyperpolarized [1-13C]pyruvate. Apparent rate constants for the conversion of pyruvate to lactate in three different regions (glioma, normal appearing brain, and vasculature) were estimated based on a two-site exchange model. The apparent conversion rate constant was 0.018 ± 0.004 s−1 (mean ± standard deviation, n = 6) for glioma, 0.009 ± 0.003 s−1 for normal brain, and 0.005 ± 0.001 s−1 for vasculature, whereas the lactate-to-pyruvate ratio, the metabolic marker used to date to identify tumor regions, was 0.36 ± 0.07 (mean ± SD), 0.24 ± 0.07, and 0.12 ± 0.02 for glioma, normal brain, and vasculature, respectively. The data suggest that the apparent conversion rate better differentiate glioma from normal brain ( P = 0.001, n = 6) than the lactate-to-pyruvate ratio ( P = 0.02). Magn Reson Med, 2012. © 2012 Wiley Periodicals, Inc. [ABSTRACT FROM AUTHOR]

In addition to cancer imaging, 13C-MRS of hyperpolarized pyruvate has also demonstrated utility for the investigation of cardiac metabolism and ischemic heart disease. Although no adverse effects have yet been reported for doses commonly used in vivo, high substrate concentrations have lead to supraphysiological pyruvate levels that can affect the underlying metabolism and should be considered when interpreting results. With lactate serving as an important energy source for the heart and physiological lactate levels one to two orders of magnitude higher than for pyruvate, hyperpolarized lactate could potentially be used as an alternative to pyruvate for probing cardiac metabolism. In this study, hyperpolarized [1-13C]lactate was used to acquire time-resolved spectra from the healthy rat heart in vivo and to measure dichloroacetate (DCA)-modulated changes in flux through pyruvate dehydrogenase (PDH). Both primary oxidation of lactate to pyruvate and subsequent conversion of pyruvate to alanine and bicarbonate could reliably be detected. Since DCA stimulates the activity of PDH through inhibition of PDH kinase, a more than 2.5-fold increase in bicarbonate-to-substrate ratio was found after administration of DCA, similar to the effect when using [1-13C]pyruvate as the substrate. Copyright © 2012 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

The cover image, by Jae Mo Park et al., is based on the Research Article Hyperpolarized 13C‐lactate to 13C‐bicarbonate ratio as a biomarker for monitoring the acute response of anti‐vascular endothelial growth factor (anti‐VEGF) treatment, DOI: 10.1002/nbm.3509. [ABSTRACT FROM AUTHOR]

Hyperpolarized 13 C-labeled fumarate probes tissue necrosis via the production of 13 C-malate. Despite its promises in detecting tumor necrosis and kidney injuries, its clinical translation has been limited, primarily due to the low solubility in conventional glassing solvents. In this study, we introduce a new formulation of fumarate for dissolution dynamic nuclear polarization (DNP) by using meglumine as a counterion, a nonmetabolizable derivative of sorbitol. We have found that meglumine fumarate vitrifies by itself with enhanced water solubility (4.8 M), which is expected to overcome the solubility-restricted maximum concentration of hyperpolarized fumarate after dissolution. The achievable liquid-state polarization level of meglumine-fumarate is more than doubled (29.4 ± 1.3%) as compared to conventional dimethyl sulfoxide (DMSO)-mixed fumarate (13.5 ± 2.4%). In vivo comparison of DMSO- and meglumine-prepared 50-mM hyperpolarized [1,4- 13 C 2 ]fumarate shows that the signal sensitivity in rat kidneys increases by 10-fold. As a result, [1,4- 13 C 2 ]aspartate and [ 13 C]bicarbonate in addition to [1,4- 13 C 2 ]malate can be detected in healthy rat kidneys in vivo using hyperpolarized meglumine [1,4- 13 C 2 ]fumarate. In particular, the appearance of [ 13 C]bicarbonate indicates that hyperpolarized meglumine [1,4- 13 C 2 ]fumarate can be used to investigate phosphoenolpyruvate carboxykinase, a key regulatory enzyme in gluconeogenesis.

The feasibility of hyperpolarized [2- 13 C, 3- 2 H 3 ]pyruvate for probing gluconeogenesis in vivo was investigated in this study. Whereas hyperpolarized [1- 13 C]pyruvate has clear access to metabolic pathways that convert pyruvate to lactate, alanine, and bicarbonate, its utility for assessing pyruvate carboxylation and gluconeogenesis has been limited by technical challenges, including spectral overlap and an obscure enzymatic step that decarboxylates the labeled carbon. To achieve unambiguous detection of gluconeogenic products, the carbonyl carbon in pyruvate was labeled with 13 C. To prolong the T 1 relaxation time, [2- 13 C, 3- 2 H 3 ]pyruvate was synthesized and dissolved with D 2 O after dynamic nuclear polarization. The T 1 of [2- 13 C, 3- 2 H 3 ]pyruvate in D 2 O could be improved by 76.9% (79.6 s at 1 T and 74.5 s at 3 T) as compared to [2- 13 C]pyruvate in water. Hyperpolarized [2- 13 C, 3- 2 H 3 ]pyruvate with D 2 O dissolution was applied to rat livers in vivo under normal feeding and fasting conditions. A gluconeogenic product, [2- 13 C]phosphoenolpyruvate, was observed at 149.9 ppm from fasted rats only, highlighting the utility of [2- 13 C, 3- 2 H 3 ]pyruvate in detecting key gluconeogenic enzyme activities such as pyruvate carboxylase and phosphoenolpyruvate carboxykinase in vivo .

Purpose: Pyruvate, produced from either glucose, glycogen, or lactate, is the dominant precursor of cerebral oxidative metabolism. Pyruvate dehydrogenase (PDH) flux is a direct measure of cerebral mitochondrial function and metabolism. Detection of [ 13 C]bicarbonate in the brain from hyperpolarized [1- 13 C]pyruvate using carbon-13 ( 13 C) MRI provides a unique opportunity for assessing PDH flux in vivo. This study is to assess changes in cerebral PDH flux in response to visual stimuli using in vivo 13 C MRS with hyperpolarized [1- 13 C]pyruvate., Methods: From seven sedentary adults in good general health, time-resolved [ 13 C]bicarbonate production was measured in the brain using 90° flip angles with minimal perturbation of its precursors, [1- 13 C]pyruvate and [1- 13 C]lactate, to test the hypothesis that the appearance of [ 13 C]bicarbonate signals in the brain reflects the metabolic changes associated with neuronal activation. With a separate group of healthy participants (n = 3), the likelihood of the bolus-injected [1- 13 C]pyruvate being converted to [1- 13 C]lactate prior to decarboxylation was investigated by measuring [ 13 C]bicarbonate production with and without [1- 13 C]lactate saturation., Results: In the course of visual stimulation, the measured [ 13 C]bicarbonate signal normalized to the total 13 C signal in the visual cortex increased by 17.1% ± 15.9% (p = 0.017), whereas no significant change was detected in [1- 13 C]lactate. Proton BOLD fMRI confirmed the regional activation in the visual cortex with the stimuli. Lactate saturation decreased bicarbonate-to-pyruvate ratio by 44.4% ± 9.3% (p < 0.01)., Conclusion: We demonstrated the utility of 13 C MRS with hyperpolarized [1- 13 C]pyruvate for assessing the activation of cerebral PDH flux via the detection of [ 13 C]bicarbonate production., (© 2024 The Authors. Magnetic Resonance in Medicine published by Wiley Periodicals LLC on behalf of International Society for Magnetic Resonance in Medicine.)

The goal of this study was to investigate the origin of brain lactate (Lac) signal in the healthy anesthetized rat after injection of hyperpolarized (HP) [1- 13 C]pyruvate (Pyr). Dynamic two-dimensional spiral chemical shift imaging with flow-sensitizing gradients revealed reduction in both vascular and brain Pyr, while no significant dependence on the level of flow suppression was detected for Lac. These results support the hypothesis that the HP metabolites predominantly reside in different compartments in the brain (i.e., Pyr in the blood and Lac in the parenchyma). Data from high-resolution metabolic imaging of [1- 13 C]Pyr further demonstrated that Lac detected in the brain was not from contributions of vascular signal attributable to partial volume effects. Additionally, metabolite distributions and kinetics measured with dynamic imaging after injection of HP [1- 13 C]Lac were similar to Pyr data when Pyr was used as the substrate. These data do not support the hypothesis that Lac observed in the brain after Pyr injection was generated in other organs and then transported across the blood-brain barrier (BBB). Together, the presented results provide further evidence that even in healthy anesthetized rats, the transport of HP Pyr across the BBB is sufficiently fast to permit detection of its metabolic conversion to Lac within the brain., (© 2023 The Authors. NMR in Biomedicine published by John Wiley & Sons Ltd.)

Purpose: [ 13 C]Bicarbonate formation from hyperpolarized [1- 13 C]pyruvate via pyruvate dehydrogenase, a key regulatory enzyme, represents the cerebral oxidation of pyruvate and the integrity of mitochondrial function. The present study is to characterize the chronology of cerebral mitochondrial metabolism during secondary injury associated with acute traumatic brain injury (TBI) by longitudinally monitoring [ 13 C]bicarbonate production from hyperpolarized [1- 13 C]pyruvate in rodents., Methods: Male Wistar rats were randomly assigned to undergo a controlled-cortical impact (CCI, n = 31) or sham surgery (n = 22). Seventeen of the CCI and 9 of the sham rats longitudinally underwent a 1 H/ 13 C-integrated MR protocol that includes a bolus injection of hyperpolarized [1- 13 C]pyruvate at 0 (2 h), 1, 2, 5, and 10 days post-surgery. Separate CCI and sham rats were used for histological validation and enzyme assays., Results: In addition to elevated lactate, we observed significantly reduced bicarbonate production in the injured site. Unlike the immediate appearance of hyperintensity on T 2 -weighted MRI, the contrast of bicarbonate signals between the injured region and the contralateral brain peaked at 24 h post-injury, then fully recovered to the normal level at day 10. A subset of TBI rats demonstrated markedly increased bicarbonate in normal-appearing contralateral brain regions post-injury., Conclusion: This study demonstrates that aberrant mitochondrial metabolism occurring in acute TBI can be monitored by detecting [ 13 C]bicarbonate production from hyperpolarized [1- 13 C]pyruvate, suggesting that [ 13 C]bicarbonate is a sensitive in-vivo biomarker of the secondary injury processes., (© 2023 The Authors. Magnetic Resonance in Medicine published by Wiley Periodicals LLC on behalf of International Society for Magnetic Resonance in Medicine.)

Mitochondrial oxidative phosphorylation (OXPHOS) is sensitive to a variety of biological factors, and dysregulated OXPHOS is observed during the development of numerous pathological conditions. ATP production via OXPHOS is intrinsically dependent on the availability of acetyl-coenzyme A (CoA), which can enter the tricarboxylic acid (TCA) cycle to drive the oxidative pathway. Acetyl-l-carnitine (ALCAR) is an interchangeable endogenous source of acetyl-CoA, and therefore, ALCAR-derived probes are uniquely positioned for the assessment of OXPHOS. In this report, we develop hyperpolarized (HP) [1- 13 C]ALCAR as a noninvasive probe to investigate cardiac TCA cycle activity in vivo . We initially synthesized the isotopically labeled substrate and demonstrated that the 13 C nucleus maintained a suitable T 1 value (50.1 ± 0.8 s at 3 T) and polarization levels (21.3 ± 5.3%) to execute in vivo metabolic measurements. HP [1- 13 C]ALCAR was employed for cardiac analyses of OXPHOS in rats under fed and fasted conditions. [5- 13 C]Glutamate was successfully detected, and the metabolite was used to analyze the TCA cycle activity in both nutritional states. These assessments were compared to analogous experiments with the HP [1- 13 C]pyruvate. Our report represents the first study to demonstrate that HP methods using [1- 13 C]ALCAR enable direct analyses of mitochondrial function and TCA cycle activity, which are fundamental to cardiac cell homeostasis.

Purpose: This study is to investigate time-resolved 13 C MR spectroscopy (MRS) as an alternative to imaging for assessing pyruvate metabolism using hyperpolarized (HP) [1- 13 C]pyruvate in the human brain., Methods: Time-resolved 13 C spectra were acquired from four axial brain slices of healthy human participants (n = 4) after a bolus injection of HP [1- 13 C]pyruvate. 13 C MRS with low flip-angle excitations and a multichannel 13 C/ 1 H dual-frequency radiofrequency (RF) coil were exploited for reliable and unperturbed assessment of HP pyruvate metabolism. Slice-wise areas under the curve (AUCs) of 13 C-metabolites were measured and kinetic analysis was performed to estimate the production rates of lactate and HCO 3 - . Linear regression analysis between brain volumes and HP signals was performed. Region-focused pyruvate metabolism was estimated using coil-wise 13 C reconstruction. Reproducibility of HP pyruvate exams was presented by performing two consecutive injections with a 45-minutes interval., Results: [1- 13 C]Lactate relative to the total 13 C signal (tC) was 0.21-0.24 in all slices. [ 13 C] HCO 3 - /tC was 0.065-0.091. Apparent conversion rate constants from pyruvate to lactate and HCO 3 - were calculated as 0.014-0.018 s -1 and 0.0043-0.0056 s -1 , respectively. Pyruvate/tC and lactate/tC were in moderate linear relationships with fractional gray matter volume within each slice. White matter presented poor linear regression fit with HP signals, and moderate correlations of the fractional cerebrospinal fluid volume with pyruvate/tC and lactate/tC were measured. Measured HP signals were comparable between two consecutive exams with HP [1- 13 C]pyruvate., Conclusions: Dynamic MRS in combination with multichannel RF coils is an affordable and reliable alternative to imaging methods in investigating cerebral metabolism using HP [1- 13 C]pyruvate., (© 2021 International Society for Magnetic Resonance in Medicine.)