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

Brain metabolism after therapeutic hypothermia for murine hypoxia-ischemia using hyperpolarized [1-13C] pyruvate magnetic resonance spectroscopy

Hypoxic-ischemic encephalopathy (HIE) is a common neurological syndrome in newborns with high mortality and morbidity. Therapeutic hypothermia (TH), which is standard of care for HIE, mitigates brain injury by suppressing anaerobic metabolism. However, more than 40% of HIE neonates have a poor outcome, even after TH. This study aims to provide metabolic biomarkers for predicting the outcomes of hypoxia-ischemia (HI) after TH using hyperpolarized [1-13C] pyruvate magnetic resonance spectroscopy. Postnatal day 10 (P10) mice with HI underwent TH at 1 h and were scanned at 6-8 h (P10), 24 h (P11), 7 days (P17), and 21 days (P31) post-HI on a 14.1-T NMR spectrometer. The metabolic images were collected, and the conversion rate from pyruvate to lactate and the ratio of lactate to pyruvate in the injured left hemisphere (kPL(L) and Lac/Pyr(L), respectively) were calculated at each timepoint. The outcomes of TH were determined by the assessments of brain injury on T2-weighted images and behavioral tests at later timepoint. kPL(L) and Lac/Pyr(L) over time between the good-outcome and poor-outcome groups and across timepoints within groups were analyzed. We found significant differences in temporal trends of kPL(L) and Lac/Pyr(L) between groups. In the good-outcome group, kPL(L) increased until P31 with a significantly higher value at P31 compared with that at P10, while the level of Lac/Pyr(L) at P31 was notably higher than those at all other timepoints. In the poor-outcome group, kPL(L) and Lac/Pyr(L) increased within 24 h. The kPL(L) value at P11 was considerably higher compared with P10. Discrete temporal changes of kPL(L) and Lac/Pyr(L) after TH between the good-outcome and poor-outcome groups were seen as early as 24 h after HI, reflecting various TH effects on brain anaerobic metabolism, which may provide insights for early screening for response to TH.

Enhanced Solubility and Polarization of 13C-Fumarate with Meglumine Allows for In Vivo Detection of Gluconeogenic Metabolism in Kidneys

Hyperpolarized 13C-labeled fumarate probes tissue necrosis via the production of 13C-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 [1,4-13C2]fumarate shows that the signal sensitivity in rat kidneys increases by 10-fold. As a result, [1,4-13C2]aspartate and [13C]bicarbonate in addition to [1,4-13C2]malate can be detected in healthy rat kidneys in vivo using hyperpolarized meglumine [1,4-13C2]fumarate. In particular, the appearance of [13C]bicarbonate indicates that hyperpolarized meglumine [1,4-13C2]fumarate can be used to investigate phosphoenolpyruvate carboxykinase, a key regulatory enzyme in gluconeogenesis.

Hyperpolarized [2-13C, 3-2H3]Pyruvate Detects Hepatic Gluconeogenesis In Vivo

The feasibility of hyperpolarized [2-13C, 3-2H3]pyruvate for probing gluconeogenesis in vivo was investigated in this study. Whereas hyperpolarized [1-13C]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 13C. To prolong the T1 relaxation time, [2-13C, 3-2H3]pyruvate was synthesized and dissolved with D2O after dynamic nuclear polarization. The T1 of [2-13C, 3-2H3]pyruvate in D2O could be improved by 76.9% (79.6 s at 1 T and 74.5 s at 3 T) as compared to [2-13C]pyruvate in water. Hyperpolarized [2-13C, 3-2H3]pyruvate with D2O dissolution was applied to rat livers in vivo under normal feeding and fasting conditions. A gluconeogenic product, [2-13C]phosphoenolpyruvate, was observed at 149.9 ppm from fasted rats only, highlighting the utility of [2-13C, 3-2H3]pyruvate in detecting key gluconeogenic enzyme activities such as pyruvate carboxylase and phosphoenolpyruvate carboxykinase in vivo.

Probing human heart TCA cycle metabolism and response to glucose load using hyperpolarized [2-13 C]pyruvate MRS

INTRODUCTION

The healthy heart has remarkable metabolic flexibility that permits rapid switching between mitochondrial glucose oxidation and fatty acid oxidation to generate ATP. Loss of metabolic flexibility has been implicated in the genesis of contractile dysfunction seen in cardiomyopathy. Metabolic flexibility has been imaged in experimental models, using hyperpolarized (HP) [2-13 C]pyruvate MRI, which enables interrogation of metabolites that reflect tricarboxylic acid (TCA) cycle flux in cardiac myocytes. This study aimed to develop methods, demonstrate feasibility for [2-13 C]pyruvate MRI in the human heart for the first time, and assess cardiac metabolic flexibility.

METHODS

Good manufacturing practice [2-13 C]pyruvic acid was polarized in a 5 T polarizer for 2.5-3 h. Following dissolution, quality control parameters of HP pyruvate met all safety and sterility criteria for pharmacy release, prior to administration to study subjects. Three healthy subjects each received two HP injections and MR scans, first under fasting conditions, followed by oral glucose load. A 5 cm axial slab-selective spectroscopy approach was prescribed over the left ventricle and acquired at 3 s intervals on a 3 T clinical MRI scanner.

RESULTS

The study protocol, which included HP substrate injection, MR scanning, and oral glucose load, was performed safely without adverse events. Key downstream metabolites of [2-13 C]pyruvate metabolism in cardiac myocytes include the glycolytic derivative [2-13 C]lactate, TCA-associated metabolite [5-13 C]glutamate, and [1-13 C]acetylcarnitine, catalyzed by carnitine acetyltransferase (CAT). After glucose load, 13 C-labeling of lactate, glutamate, and acetylcarnitine from 13 C-pyruvate increased by an average of 39.3%, 29.5%, and 114% respectively in the three subjects, which could result from increases in lactate dehydrogenase, pyruvate dehydrogenase, and CAT enzyme activity as well as TCA cycle flux (glucose oxidation).

CONCLUSIONS

HP [2-13 C]pyruvate imaging is safe and permits noninvasive assessment of TCA cycle intermediates and the acetyl buffer, acetylcarnitine, which is not possible using HP [1-13 C]pyruvate. Cardiac metabolite measurement in the fasting/fed states provides information on cardiac metabolic flexibility and the acetylcarnitine pool.

Investigating the origin of the 13 C lactate signal in the anesthetized healthy rat brain in vivo after hyperpolarized [1-13 C]pyruvate injection

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.

Longitudinal assessment of mitochondrial dysfunction in acute traumatic brain injury using hyperpolarized [1-13 C]pyruvate

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 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 [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.

Probing Human Heart TCA Cycle Metabolism and Response to Glucose Load using Hyperpolarized [2-13C]Pyruvate MR Spectroscopy

Introduction

The normal heart has remarkable metabolic flexibility that permits rapid switching between mitochondrial glucose oxidation and fatty acid (FA) oxidation to generate ATP. Loss of metabolic flexibility has been implicated in the genesis of contractile dysfunction seen in cardiomyopathy. Metabolic flexibility has been imaged in experimental models, using hyperpolarized (HP) [2-13C]pyruvate MRI, which enables interrogation of metabolites that reflect tricarboxylic acid (TCA) cycle flux in cardiac myocytes. This study aimed to develop methods, demonstrate feasibility for [2-13C]pyruvate MRI in the human heart for the first time, and assess cardiac metabolic flexibility.

Methods

Good Manufacturing Practice [2-13C]pyruvic acid was polarized in a 5T polarizer for 2.5-3 hours. Following dissolution, QC parameters of HP pyruvate met all safety and sterility criteria for pharmacy release, prior to administration to study subjects. Three healthy subjects each received two HP injections and MR scans, first under fasting conditions, followed by oral glucose load. A 5cm axial slab-selective spectroscopy approach was prescribed over the left ventricle and acquired at 3s intervals on a 3T clinical MRI scanner.

Results

The study protocol which included HP substrate injection, MR scanning and oral glucose load, was performed safely without adverse events. Key downstream metabolites of [2-13C]pyruvate metabolism in cardiac myocytes include the glycolytic derivative [2-13C]lactate, TCA-associated metabolite [5-13C]glutamate, and [1-13C]acetylcarnitine, catalyzed by carnitine acetyltransferase (CAT). After glucose load, 13C-labeling of lactate, glutamate, and acetylcarnitine from 13C-pyruvate increased by 39.3%, 29.5%, and 114%, respectively in the three subjects, that could result from increases in lactate dehydrogenase (LDH), pyruvate dehydrogenase (PDH), and CAT enzyme activity as well as TCA cycle flux (glucose oxidation).

Conclusions

HP [2-13C]pyruvate imaging is safe and permits non-invasive assessment of TCA cycle intermediates and the acetyl buffer, acetylcarnitine, which is not possible using HP [1-13C]pyruvate. Cardiac metabolite measurement in the fasting/fed states provides information on cardiac metabolic flexibility and the acetylcarnitine pool.

Hyperpolarized [2-13C]pyruvate MR molecular imaging with whole brain coverage

Hyperpolarized (HP) 13C Magnetic Resonance Imaging (MRI) was applied for the first time to image and quantify the uptake and metabolism of [2-13C]pyruvate in the human brain to provide new metabolic information on cerebral energy metabolism. HP [2-13C]pyruvate was injected intravenously and imaged in 5 healthy human volunteer exams with whole brain coverage in a 1-minute acquisition using a specialized spectral-spatial multi-slice echoplanar imaging (EPI) pulse sequence to acquire 13C-labeled volumetric and dynamic images of [2-13C]pyruvate and downstream metabolites [5-13C]glutamate and [2-13C]lactate. Metabolic ratios and apparent conversion rates of pyruvate-to-lactate (kPL) and pyruvate-to-glutamate (kPG) were quantified to investigate simultaneously glycolytic and oxidative metabolism in a single injection.

In-vivo magnetic resonance spectroscopy of lactate as a non-invasive biomarker of dichloroacetate activity in cancer and non-cancer central nervous system disorders

The adverse effects of lactic acidosis in the cancer microenvironment have been increasingly recognized. Dichloroacetate (DCA) is an orally bioavailable, blood brain barrier penetrable drug that has been extensively studied in the treatment of mitochondrial neurologic conditions to reduce lactate production. Due to its effect reversing aerobic glycolysis (i.e., Warburg-effect) and thus lactic acidosis, DCA became a drug of interest in cancer as well. Magnetic resonance spectroscopy (MRS) is a well-established, non-invasive technique that allows detection of prominent metabolic changes, such as shifts in lactate or glutamate levels. Thus, MRS is a potential radiographic biomarker to allow spatial and temporal mapping of DCA treatment. In this systematic literature review, we gathered the available evidence on the use of various MRS techniques to track metabolic changes after DCA administration in neurologic and oncologic disorders. We included in vitro, animal, and human studies. Evidence confirms that DCA has substantial effects on lactate and glutamate levels in neurologic and oncologic disease, which are detectable by both experimental and routine clinical MRS approaches. Data from mitochondrial diseases show slower lactate changes in the central nervous system (CNS) that correlate better with clinical function compared to blood. This difference is most striking in focal impairments of lactate metabolism suggesting that MRS might provide data not captured by solely monitoring blood. In summary, our findings corroborate the feasibility of MRS as a pharmacokinetic/pharmacodynamic biomarker of DCA delivery in the CNS, that is ready to be integrated into currently ongoing and future human clinical trials using DCA.

Assessing the effect of anesthetic gas mixtures on hyperpolarized 13 C pyruvate metabolism in the rat brain

PURPOSE

To determine the effect of altering anesthetic oxygen protocols on measurements of cerebral perfusion and metabolism in the rodent brain.

METHODS

Seven rats were anesthetized and underwent serial MRI scans with hyperpolarized [1-¹³ C]pyruvate and perfusion weighted imaging. The anesthetic carrier gas protocol used varied from 100:0% to 90:10% to 60:40% O₂ :N₂ O. Spectra were quantified with AMARES and perfusion imaging was processed using model-free deconvolution. A 1-way ANOVA was used to compare results across groups, with pairwise t tests performed with correction for multiple comparisons. Spearman's correlation analysis was performed between O₂ % and MR measurements.

RESULTS

There was a significant increase in bicarbonate:total ¹³ C carbon and bicarbonate:¹³ C pyruvate when moving between 100:0 to 90:10 and 100:0 to 60:40 O₂ :N₂ O % (0.02 ± 0.01 vs. 0.019 ± 0.005 and 0.02 ± 0.01 vs. 0.05 ± 0.02, respectively) and (0.04 ± 0.01 vs. 0.03 ± 0.01 and 0.04 ± 0.01 vs. 0.08 ± 0.02, respectively). There was a significant difference in ¹³ C pyruvate time to peak when moving between 100:0 to 90:10 and 100:0 to 60:40 O₂ :N₂ O % (13 ± 2 vs. 10 ± 1 and 13 ± 2 vs. 7.5 ± 0.5 s, respectively) as well as significant differences in cerebral blood flow (CBF) between gas protocols. Significant correlations between bicarbonate:¹³ C pyruvate and gas protocol (ρ = -0.47), mean transit time and gas protocol (ρ = 0.41) and ¹³ C pyruvate time-to-peak and cerebral blood flow (ρ = -0.54) were also observed.

CONCLUSIONS

These results demonstrate that the detection and quantification of cerebral metabolism and perfusion is dependent on the oxygen protocol used in the anesthetized rodent brain.

In vivo Human MR Spectroscopy Using a Clinical Scanner: Development, Applications, and Future Prospects

MR spectroscopy (MRS) is a unique and useful method for noninvasively evaluating biochemical metabolism in human organs and tissues, but its clinical dissemination has been slow and often limited to specialized institutions or hospitals with experts in MRS technology. The number of 3-T clinical MR scanners is now increasing, representing a major opportunity to promote the use of clinical MRS. In this review, we summarize the theoretical background and basic knowledge required to understand the results obtained with MRS and introduce the general consensus on the clinical utility of proton MRS in routine clinical practice. In addition, we present updates to the consensus guidelines on proton MRS published by the members of a working committee of the Japan Society of Magnetic Resonance in Medicine in 2013. Recent research into multinuclear MRS equipped in clinical MR scanners is explained with an eye toward future development. This article seeks to provide an overview of the current status of clinical MRS and to promote the understanding of when it can be useful. In the coming years, MRS-mediated biochemical evaluation is expected to become available for even routine clinical practice.

Acquisition and quantification pipeline for in vivo hyperpolarized 13 C MR spectroscopy

PURPOSE

The goal of this study was to combine a specialized acquisition method with a new quantification pipeline to accurately and efficiently probe the metabolism of hyperpolarized ¹³ C-labeled compounds in vivo. In this study, we tested our approach on [2-¹³ C]pyruvate and [1-¹³ C]α-ketoglutarate data in rat orthotopic brain tumor models at 3T.

METHODS

We used a multiband metabolite-specific radiofrequency (RF) excitation in combination with a variable flip angle scheme to minimize substrate polarization loss and measure fast metabolic processes. We then applied spectral-temporal denoising using singular value decomposition to enhance spectral quality. This was combined with LCModel-based automatic ¹³ C spectral fitting and flip angle correction to separate overlapping signals and rapidly quantify the different metabolites.

RESULTS

Denoising improved the metabolite signal-to-noise ratio (SNR) by approximately 5. It also improved the accuracy of metabolite quantification as evidenced by a significant reduction of the Cramer Rao lower bounds. Furthermore, the use of the automated and user-independent LCModel-based quantification approach could be performed rapidly, with the kinetic quantification of eight metabolite peaks in a 12-spectrum array achieved in less than 1 minute.

CONCLUSION

The specialized acquisition method combined with denoising and a new quantification pipeline using LCModel for the first time for hyperpolarized ¹³ C data enhanced our ability to monitor the metabolism of [2-¹³ C]pyruvate and [1-¹³ C]α-ketoglutarate in rat orthotopic brain tumor models in vivo. This approach could be broadly applicable to other hyperpolarized agents both preclinically and in the clinical setting.

13C-Labeled Diethyl Ketoglutarate Derivatives as Hyperpolarized Probes of 2-Ketoglutarate Dehydrogenase Activity

The TCA cycle is a central metabolic pathway for energy production and biosynthesis. A major control point of metabolic flux through the cycle is the decarboxylation of 2-ketoglutarate by the TCA cycle enzyme 2-ketoglutarate dehydrogenase (2-KGDH). In this project, we developed 13C labeled 2-ketoglutarate derivatives to monitor 2-KGDH activity in vivo. 13C NMR analysis of liver extracts revealed that uniformly 13C labeled 2-ketogutarate, in its cell permeable ester form, was rapidly taken up and hydrolyzed in liver and underwent extensive metabolism to produce labeled glutamate, succinate, lactate and other metabolites. Diethyl [1,2-13C2]-2-ketoglutarate was successfully polarized by dynamic nuclear polarization and within seconds after injection into rats, the probe produced hyperpolarized [13C]bicarbonate in the liver reflecting flux through the TCA cycle. These experiments demonstrate that this tracer offers the possibility of directly monitoring flux through 2-KGDH in vivo.

Placental mitochondrial function as a driver of angiogenesis and placental dysfunction

The placenta is a highly vascularized and complex foetal organ that performs various tasks, crucial to a healthy pregnancy. Its dysfunction leads to complications such as stillbirth, preeclampsia, and intrauterine growth restriction. The specific cause of placental dysfunction remains unknown. Recently, the role of mitochondrial function and mitochondrial adaptations in the context of angiogenesis and placental dysfunction is getting more attention. The required energy for placental remodelling, nutrient transport, hormone synthesis, and the reactive oxygen species leads to oxidative stress, stemming from mitochondria. Mitochondria adapt to environmental changes and have been shown to adjust their oxygen and nutrient use to best support placental angiogenesis and foetal development. Angiogenesis is the process by which blood vessels form and is essential for the delivery of nutrients to the body. This process is regulated by different factors, pro-angiogenic factors and anti-angiogenic factors, such as sFlt-1. Increased circulating sFlt-1 levels have been linked to different preeclamptic phenotypes. One of many effects of increased sFlt-1 levels, is the dysregulation of mitochondrial function. This review covers mitochondrial adaptations during placentation, the importance of the anti-angiogenic factor sFlt-1in placental dysfunction and its role in the dysregulation of mitochondrial function.

Denoising of hyperpolarized 13 C MR images of the human brain using patch-based higher-order singular value decomposition

PURPOSE

To improve hyperpolarized ¹³ C (HP-¹³ C) MRI by image denoising with a new approach, patch-based higher-order singular value decomposition (HOSVD).

METHODS

The benefit of using a patch-based HOSVD method to denoise dynamic HP-¹³ C MR imaging data was investigated. Image quality and the accuracy of quantitative analyses following denoising were evaluated first using simulated data of [1-¹³ C]pyruvate and its metabolic product, [1-¹³ C]lactate, and compared the results to a global HOSVD method. The patch-based HOSVD method was then applied to healthy volunteer HP [1-¹³ C]pyruvate EPI studies. Voxel-wise kinetic modeling was performed on both non-denoised and denoised data to compare the number of voxels quantifiable based on SNR criteria and fitting error.

RESULTS

Simulation results demonstrated an 8-fold increase in the calculated SNR of [1-¹³ C]pyruvate and [1-¹³ C]lactate with the patch-based HOSVD denoising. The voxel-wise quantification of k_PL (pyruvate-to-lactate conversion rate) showed a 9-fold decrease in standard errors for the fitted k_PL after denoising. The patch-based denoising performed superior to the global denoising in recovering k_PL information. In volunteer data sets, [1-¹³ C]lactate and [¹³ C]bicarbonate signals became distinguishable from noise across captured time points with over a 5-fold apparent SNR gain. This resulted in >3-fold increase in the number of voxels quantifiable for mapping k_PB (pyruvate-to-bicarbonate conversion rate) and whole brain coverage for mapping k_PL .

CONCLUSIONS

Sensitivity enhancement provided by this denoising significantly improved quantification of metabolite dynamics and could benefit future studies by improving image quality, enabling higher spatial resolution, and facilitating the extraction of metabolic information for clinical research.

In Vivo Assessment of Metabolic Abnormality in Alport Syndrome Using Hyperpolarized [1-13C] Pyruvate MR Spectroscopic Imaging

Alport Syndrome (AS) is a genetic disorder characterized by impaired kidney function. The development of a noninvasive tool for early diagnosis and monitoring of renal function during disease progression is of clinical importance. Hyperpolarized ¹³C MRI is an emerging technique that enables non-invasive, real-time measurement of in vivo metabolism. This study aimed to investigate the feasibility of using this technique for assessing changes in renal metabolism in the mouse model of AS. Mice with AS demonstrated a significant reduction in the level of lactate from 4- to 7-week-old, while the levels of lactate were unchanged in the control mice over time. This reduction in lactate production in the AS group accompanied a significant increase of PEPCK expression levels, indicating that the disease progression in AS triggered the gluconeogenic pathway and might have resulted in a decreased lactate pool size and a subsequent reduction in pyruvate-to-lactate conversion. Additional metabolic imaging parameters, including the level of lactate and pyruvate, were found to be different between the AS and control groups. These preliminary results suggest that hyperpolarized ¹³C MRI might provide a potential noninvasive tool for the characterization of disease progression in AS.

Biomedical Applications of the Dynamic Nuclear Polarization and Parahydrogen Induced Polarization Techniques for Hyperpolarized 13C MR Imaging

Since the first pioneering report of hyperpolarized [1-13C]pyruvate magnetic resonance imaging (MRI) of the Warburg effect in prostate cancer patients, clinical dissemination of the technique has been rapid; close to 10 sites worldwide now possess a polarizer fit for the clinic, and more than 30 clinical trials, predominantly for oncological applications, are already registered on the US and European clinical trials databases. Hyperpolarized 13C probes to study pathophysiological processes beyond the Warburg effect, including tricarboxylic acid cycle metabolism, intra-cellular pH and cellular necrosis have also been demonstrated in the preclinical arena and are pending clinical translation, and the simultaneous injection of multiple co-polarized agents is opening the door to high-sensitivity, multi-functional molecular MRI with a single dose. Here, we review the biomedical applications to date of the two polarization methods that have been used for in vivo hyperpolarized 13C molecular MRI; namely, dissolution dynamic nuclear polarization and parahydrogen-induced polarization. The basic concept of hyperpolarization and the fundamental theory underpinning these two key 13C hyperpolarization methods, along with recent technological advances that have facilitated biomedical realization, are also covered.

Hyperpolarized 13C Spectroscopy with Simple Slice-and-Frequency-Selective Excitation

Hyperpolarized ¹³C nuclear magnetic resonance spectroscopy can characterize in vivo tissue metabolism, including preclinical models of cancer and inflammatory disease. Broad bandwidth radiofrequency excitation is often paired with free induction decay readout for spectral separation, but quantification of low-signal downstream metabolites using this method can be impeded by spectral peak overlap or when frequency separation of the detected peaks exceeds the excitation bandwidth. In this work, alternating frequency narrow bandwidth (250 Hz) slice-selective excitation was used for ¹³C spectroscopy at 7 T in a subcutaneous xenograft rat model of human pancreatic cancer (PSN1) to improve quantification while measuring the dynamics of injected hyperpolarized [1-¹³C]lactate and its metabolite [1-¹³C]pyruvate. This method does not require sophisticated pulse sequences or specialized radiofrequency and gradient pulses, but rather uses nominally spatially offset slices to produce alternating frequency excitation with simpler slice-selective radiofrequency pulses. Additionally, point-resolved spectroscopy was used to calibrate the ¹³C frequency from the thermal proton signal in the target region. This excitation scheme isolates the small [1-¹³C]pyruvate peak from the similar-magnitude tail of the much larger injected [1-¹³C]lactate peak, facilitates quantification of the [1-¹³C]pyruvate signal, simplifies data processing, and could be employed for other substrates and preclinical models.

Signal-enhanced real-time magnetic resonance of enzymatic reactions at millitesla fields

The phenomenon of nuclear magnetic resonance (NMR) is widely applied in biomedical and biological science to study structures and dynamics of proteins and their reactions. Despite its impact, NMR is an inherently insensitive phenomenon and has driven the field to construct spectrometers with increasingly higher magnetic fields leading to more detection sensitivity. Here, we are demonstrating that enzymatic reactions can be followed in real-time at millitesla fields, three orders of magnitude lower than the field of state-of-the-art NMR spectrometers. This requires signal-enhancing samples via hyperpolarization. Within seconds, we have enhanced the signals of 2-¹³C-pyruvate, an important metabolite to probe cancer metabolism, in 22 mM concentrations (up to 10.1% ± 0.1% polarization) and show that such a large signal allows for the real-time detection of enzymatic conversion of pyruvate to lactate at 24 mT. This development paves the pathways for biological studies in portable and affordable NMR systems with a potential for medical diagnostics.

We demonstrate that metabolism can be monitored in real-time with magnetic resonance at milli-tesla fields that are 1000 fold lower than state-of-the-art high field spectrometers.

MRI of [2-13 C]Lactate without J-coupling artifacts

PURPOSE

Imaging of [2-¹³ C]lactate, a metabolic product of [2-¹³ C]pyruvate, is over considerable interest in hyperpolarized ¹³ C studies. However, artifact-free imaging of a J-coupled nuclear spin species can be challenging due to the peak-splitting induced by the spin-spin interactions. In this work, two new techniques resolving these J-modulated artifacts are presented.

THEORY AND METHODS

The Product Operator Formalism (POF) of density matrix theory is used to both numerically and analytically derive the coherences arising during radiofrequency excitation and readout of a J-coupled spin system. A combination of computer simulations and experiments with [2-¹³ C]lactate and ¹³ C-formate phantoms are then used to verify the performance of two imaging methods. In the first approach, a quadrature imaging technique is used to eliminate scalar coupling artifacts via the combination of in-phase and quadrature images acquired at echo times differing by 1/2J with an echoplanar readout. The second approach employs a highly narrowband RF excitation pulse to image a single peak from the J-coupled doublet.

RESULTS

Simulations using a numerical Shepp-Logan phantom, in vitro experiments using thermally polarized [2-¹³ C]lactate, thermally and hyperpolarized ¹³ C-formate phantoms, and in vivo imaging of [2-¹³ C]lactate produced in rat brain following injection of hyperpolarized [2-¹³ C]pyruvate show artifact-free images and demonstrate potential utility of these methods.

CONCLUSION

The quadrature imaging and the narrowband excitation techniques resolve the J-coupling induced ghosting and blurring artifacts present with conventional MRI of J-coupled signals such as [2-¹³ C]lactate.

Hyperpolarized 13C MRI: A novel approach for probing cerebral metabolism in health and neurological disease

Cerebral metabolism is tightly regulated and fundamental for healthy neurological function. There is increasing evidence that alterations in this metabolism may be a precursor and early biomarker of later stage disease processes. Proton magnetic resonance spectroscopy (1H-MRS) is a powerful tool to non-invasively assess tissue metabolites and has many applications for studying the normal and diseased brain. However, the technique has limitations including low spatial and temporal resolution, difficulties in discriminating overlapping peaks, and challenges in assessing metabolic flux rather than steady-state concentrations. Hyperpolarized carbon-13 magnetic resonance imaging is an emerging clinical technique that may overcome some of these spatial and temporal limitations, providing novel insights into neurometabolism in both health and in pathological processes such as glioma, stroke and multiple sclerosis. This review will explore the growing body of pre-clinical data that demonstrates a potential role for the technique in assessing metabolism in the central nervous system. There are now a number of clinical studies being undertaken in this area and this review will present the emerging clinical data as well as the potential future applications of hyperpolarized 13C magnetic resonance imaging in the brain, in both clinical and pre-clinical studies.

Imaging Brain Metabolism Using Hyperpolarized 13C Magnetic Resonance Spectroscopy

Aberrant metabolism is a key factor in many neurological disorders. The ability to measure such metabolic impairment could lead to improved detection of disease progression, and development and monitoring of new therapeutic approaches. Hyperpolarized ¹³C magnetic resonance spectroscopy (MRS) is a developing imaging technique that enables non-invasive measurement of enzymatic activity in real time in living organisms. Primarily applied in the fields of cancer and cardiac disease so far, this metabolic imaging method has recently been used to investigate neurological disorders. In this review, we summarize the preclinical research developments in this emerging field, and discuss future prospects for this exciting technology, which has the potential to change the clinical paradigm for patients with neurological disorders.

Acquisition strategies for spatially resolved magnetic resonance detection of hyperpolarized nuclei

Hyperpolarization is an emerging method in magnetic resonance imaging that allows nuclear spin polarization of gases or liquids to be temporarily enhanced by up to five or six orders of magnitude at clinically relevant field strengths and administered at high concentration to a subject at the time of measurement. This transient gain in signal has enabled the non-invasive detection and imaging of gas ventilation and diffusion in the lungs, perfusion in blood vessels and tissues, and metabolic conversion in cells, animals, and patients. The rapid development of this method is based on advances in polarizer technology, the availability of suitable probe isotopes and molecules, improved MRI hardware and pulse sequence development. Acquisition strategies for hyperpolarized nuclei are not yet standardized and are set up individually at most sites depending on the specific requirements of the probe, the object of interest, and the MRI hardware. This review provides a detailed introduction to spatially resolved detection of hyperpolarized nuclei and summarizes novel and previously established acquisition strategies for different key areas of application.

First hyperpolarized [2-13C]pyruvate MR studies of human brain metabolism

We developed methods for the preparation of hyperpolarized (HP) sterile [2-¹³C]pyruvate to test its feasibility in first-ever human NMR studies following FDA-IND & IRB approval. Spectral results using this MR stable-isotope imaging approach demonstrated the feasibility of investigating human cerebral energy metabolism by measuring the dynamic conversion of HP [2-¹³C]pyruvate to [2-¹³C]lactate and [5-¹³C]glutamate in the brain of four healthy volunteers. Metabolite kinetics, signal-to-noise (SNR) and area-under-curve (AUC) ratios, and calculated [2-¹³C]pyruvate to [2-¹³C]lactate conversion rates (k_PL) were measured and showed similar but not identical inter-subject values. The k_PL measurements were equivalent with prior human HP [1-¹³C]pyruvate measurements.

Hyperpolarized 13 C magnetic resonance spectroscopy detects toxin-induced neuroinflammation in mice

Lipopolysaccharide (LPS) is a commonly used agent for induction of neuroinflammation in preclinical studies. Upon injection, LPS causes activation of microglia and astrocytes, whose metabolism alters to favor glycolysis. Assessing in vivo neuroinflammation and its modulation following therapy remains challenging, and new noninvasive methods allowing for longitudinal monitoring would be highly valuable. Hyperpolarized (HP) ¹³ C magnetic resonance spectroscopy (MRS) is a promising technique for assessing in vivo metabolism. In addition to applications in oncology, the most commonly used probe of [1-¹³ C] pyruvate has shown potential in assessing neuroinflammation-linked metabolism in mouse models of multiple sclerosis and traumatic brain injury. Here, we aimed to investigate LPS-induced neuroinflammatory changes using HP [1-¹³ C] pyruvate and HP ¹³ C urea. 2D chemical shift imaging following simultaneous intravenous injection of HP [1-¹³ C] pyruvate and HP ¹³ C urea was performed at baseline (day 0) and at days 3 and 7 post-intracranial injection of LPS (n = 6) or saline (n = 5). Immunofluorescence (IF) analyses were performed for Iba1 (resting and activated microglia/macrophages), GFAP (resting and reactive astrocytes) and CD68 (activated microglia/macrophages). A significant increase in HP [1-¹³ C] lactate production was observed at days 3 and 7 following injection, in the injected (ipsilateral) side of the LPS-treated mouse brain, but not in either the contralateral side or saline-injected animals. HP ¹³ C lactate/pyruvate ratio, without and with normalization to urea, was also significantly increased in the ipsilateral LPS-injected brain at 7 days compared with baseline. IF analyses showed a significant increase in CD68 and GFAP staining at 3 days, followed by increased numbers of Iba1 and GFAP positive cells at 7 days post-LPS injection. In conclusion, we can detect LPS-induced changes in the mouse brain using HP ¹³ C MRS, in alignment with increased numbers of microglia/macrophages and astrocytes. This study demonstrates that HP ¹³ C spectroscopy has substantial potential for providing noninvasive information on neuroinflammation.

Production of highly polarized [1-13 C]acetate by rapid decarboxylation of [2-13 C]pyruvate - application to hyperpolarized cardiac spectroscopy and imaging

PURPOSE

The objective of the present work was to develop and implement an efficient approach to hyperpolarize [1-¹³ C]acetate and apply it to in vivo cardiac spectroscopy and imaging.

METHODS

Rapid hydrogen peroxide induced decarboxylation was used to convert hyperpolarized [2-¹³ C]pyruvate into highly polarized [1-¹³ C]acetate employing an additional step following rapid dissolution of [2-¹³ C]pyruvate in a home-built multi-sample dissolution dynamic nuclear polarization system. Phantom dissolution experiments were conducted to determine optimal parameters of the decarboxylation reaction, retaining polarization and T₁ of [1-¹³ C]acetate. In vivo feasibility of detecting [1-¹³ C]acetate metabolism is demonstrated using slice-selective spectroscopy and multi-echo imaging of [1-¹³ C]acetate and [1-¹³ C]acetylcarnitine in the healthy rat heart.

RESULTS

The first in vivo signal was observed ~23 s after dissolution. At the corresponding time point in the phantom experiments, 97.9 ± 0.4% of [2-¹³ C]pyruvate were converted into [1-¹³ C]acetate by the decarboxylation reaction. T₁ and polarization of [1-¹³ C]acetate was determined to be 29.7 ± 1.9% and a 47.7 ± 0.5 s. Polarization levels of [2-¹³ C]pyruvate and [1-¹³ C]acetate were not significantly different after transfer to the scanner. In vivo, [1-¹³ C]acetate and [1-¹³ C]acetylcarnitine could be detected using spectroscopy and imaging.

CONCLUSION

Decarboxylation of hyperpolarized [2-¹³ C]pyruvate enables the efficient production of highly polarized [1-¹³ C]acetate that is applicable to study short-chain fatty acid metabolism in the in vivo heart.

Assessing Cerebral Metabolism in the Immature Rodent: From Extracts to Real-Time Assessments

Brain development is an energy-expensive process. Although glucose is irreplaceable, the developing brain utilizes a variety of substrates such as lactate and the ketone bodies, β-hydroxybutyrate and acetoacetate, to produce energy and synthesize the structural components necessary for cerebral maturation. When oxygen and nutrient supplies to the brain are restricted, as in neonatal hypoxia-ischemia (HI), cerebral energy metabolism undergoes alterations in substrate use to preserve the production of adenosine triphosphate. These changes have been studied by in situ biochemical methods that yielded valuable quantitative information about high-energy and glycolytic metabolites and established a temporal profile of the cerebral metabolic response to hypoxia and HI. However, these analyses relied on terminal experiments and averaging values from several animals at each time point as well as challenging requirements for accurate tissue processing.More recent methodologies have focused on in vivo longitudinal analyses in individual animals. The emerging field of metabolomics provides a new investigative tool for studying cerebral metabolism. Magnetic resonance spectroscopy (MRS) has enabled the acquisition of a snapshot of the metabolic status of the brain as quantifiable spectra of various intracellular metabolites. Proton (1H) MRS has been used extensively as an experimental and diagnostic tool of HI in the pursuit of markers of long-term neurodevelopmental outcomes. Still, the interpretation of the metabolite spectra acquired with 1H MRS has proven challenging, due to discrepancies among studies, regarding calculations and timing of measurements. As a result, the predictive utility of such studies is not clear. 13C MRS is methodologically more challenging, but it provides a unique window on living tissue metabolism via measurements of the incorporation of 13C label from substrates into brain metabolites and the localized determination of various metabolic fluxes. The newly developed hyperpolarized 13C MRS is an exciting method for assessing cerebral metabolism in vivo, that bears the advantages of conventional 13C MRS but with a huge gain in signal intensity and much shorter acquisition times. The first part of this review article provides a brief description of the findings of biochemical and imaging methods over the years as well as a discussion of their associated strengths and pitfalls. The second part summarizes the current knowledge on cerebral metabolism during development and HI brain injury.

Hyperpolarized 13C MRI: State of the Art and Future Directions

Hyperpolarized (HP) carbon 13 (¹³C) MRI is an emerging molecular imaging method that allows rapid, noninvasive, and pathway-specific investigation of dynamic metabolic and physiologic processes that were previously inaccessible to imaging. This technique has enabled real-time in vivo investigations of metabolism that are central to a variety of diseases, including cancer, cardiovascular disease, and metabolic diseases of the liver and kidney. This review provides an overview of the methods of hyperpolarization and ¹³C probes investigated to date in preclinical models of disease. The article then discusses the progress that has been made in translating this technology for clinical investigation. In particular, the potential roles and emerging clinical applications of HP [1-¹³C]pyruvate MRI will be highlighted. The future directions to enable the adoption of this technology to advance the basic understanding of metabolism, to improve disease diagnosis, and to accelerate treatment assessment are also detailed.

Brain metabolism under different anesthetic conditions using hyperpolarized [1-13 C]pyruvate and [2-13 C]pyruvate

Carbon-13 NMR spectroscopy (13 C MRS) offers the unique capability to measure brain metabolic rates in vivo. Hyperpolarized 13 C reduces the time required to assess brain metabolism from hours to minutes when compared with conventional 13 C MRS. This study investigates metabolism of hyperpolarized [1-13 C]pyruvate and [2-13 C]pyruvate in the rat brain in vivo under various anesthetics: pentobarbital, isoflurane, α-chloralose, and morphine. The apparent metabolic rate from pyruvate to lactate modeled using time courses obtained after injection of hyperpolarized [1-13 C]pyruvate was significantly greater for isoflurane than for all other anesthetic conditions, and significantly greater for morphine than for α-chloralose. The apparent metabolic rate from pyruvate to bicarbonate was significantly greater for morphine than for all other anesthetic conditions, and significantly lower for pentobarbital than for α-chloralose. Results show that relative TCA cycle rates determined from hyperpolarized 13 C data are consistent with rates previously measured using conventional 13 C MRS under similar anesthetic conditions, and that using morphine for sedation greatly improves detection of downstream metabolic products compared with other anesthetics.

Brain metabolism under different anesthetic conditions using hyperpolarized [1-13 C]pyruvate and [2-13 C]pyruvate

Carbon-13 NMR spectroscopy (¹³ C MRS) offers the unique capability to measure brain metabolic rates in vivo. Hyperpolarized ¹³ C reduces the time required to assess brain metabolism from hours to minutes when compared with conventional ¹³ C MRS. This study investigates metabolism of hyperpolarized [1-¹³ C]pyruvate and [2-¹³ C]pyruvate in the rat brain in vivo under various anesthetics: pentobarbital, isoflurane, α-chloralose, and morphine. The apparent metabolic rate from pyruvate to lactate modeled using time courses obtained after injection of hyperpolarized [1-¹³ C]pyruvate was significantly greater for isoflurane than for all other anesthetic conditions, and significantly greater for morphine than for α-chloralose. The apparent metabolic rate from pyruvate to bicarbonate was significantly greater for morphine than for all other anesthetic conditions, and significantly lower for pentobarbital than for α-chloralose. Results show that relative TCA cycle rates determined from hyperpolarized ¹³ C data are consistent with rates previously measured using conventional ¹³ C MRS under similar anesthetic conditions, and that using morphine for sedation greatly improves detection of downstream metabolic products compared with other anesthetics.

Observation of acetyl phosphate formation in mammalian mitochondria using real-time in-organelle NMR metabolomics

Recent studies point out the link between altered mitochondrial metabolism and cancer, and detailed understanding of mitochondrial metabolism requires real-time detection of its metabolites. Employing heteronuclear 2D NMR spectroscopy and ¹³C₃-pyruvate, we propose in-organelle metabolomics that allows for the monitoring of mitochondrial metabolic changes in real time. The approach identified acetyl phosphate from human mitochondria, whose production has been largely neglected in eukaryotic metabolism since its first description about 70 years ago in bacteria. The kinetic profile of acetyl phosphate formation was biphasic, and its transient nature suggested its role as a metabolic intermediate. The method also allowed for the estimation of pyruvate dehydrogenase (PDH) enzyme activity through monitoring of the acetyl-CoA formation, independent of competing cytosolic metabolism. The results confirmed the positive regulation of mitochondrial PDH activity by p53, a well-known tumor suppressor. Our approach can easily be applied to other organelle-specific metabolic studies.

Multi-modality imaging to assess metabolic response to dichloroacetate treatment in tumor models

Reverting glycolytic metabolism is an attractive strategy for cancer therapy as upregulated glycolysis is a hallmark in various cancers. Dichloroacetate (DCA), long used to treat lactic acidosis in various pathologies, has emerged as a promising anti-cancer drug. By inhibiting the pyruvate dehydrogenase kinase, DCA reactivates the mitochondrial function and decreases the glycolytic flux in tumor cells resulting in cell cycle arrest and apoptosis. We recently documented that DCA was able to induce a metabolic switch preferentially in glycolytic cancer cells, leading to a more oxidative phenotype and decreasing proliferation, while oxidative cells remained less sensitive to DCA treatment. To evaluate the relevance of this observation in vivo, the aim of the present study was to characterize the effect of DCA in glycolytic MDA-MB-231 tumors and in oxidative SiHa tumors using advanced pharmacodynamic metabolic biomarkers. Oxygen consumption, studied by ¹⁷O magnetic resonance spectroscopy, glucose uptake, evaluated by ¹⁸F-FDG PET and pyruvate transformation into lactate, measured using hyperpolarized ¹³C-magnetic resonance spectroscopy, were monitored before and 24 hours after DCA treatment in tumor bearing mice. In both tumor models, no clear metabolic shift was observed. Surprisingly, all these imaging parameters concur to the conclusion that both glycolytic tumors and oxidative tumors presented a similar response to DCA. These results highlight a major discordance in metabolic cancer cell bioenergetics between in vitro and in vivo setups, indicating critical role of the local microenvironment in tumor metabolic behaviors.

Probing the cardiac malate-aspartate shuttle non-invasively using hyperpolarized [1,2-13 C2 ]pyruvate

Previous studies have demonstrated that using hyperpolarized [2-¹³ C]pyruvate as a contrast agent can reveal ¹³ C signals from metabolites associated with the tricarboxylic acid (TCA) cycle. However, the metabolites detectable from TCA cycle-mediated oxidation of [2-¹³ C]pyruvate are the result of several metabolic steps. In the instance of the [5-¹³ C]glutamate signal, the amplitude can be modulated by changes to the rates of pyruvate dehydrogenase (PDH) flux, TCA cycle flux and metabolite pool size. Also key is the malate-aspartate shuttle, which facilitates the transport of cytosolic reducing equivalents into the mitochondria for oxidation via the malate-α-ketoglutarate transporter, a process coupled to the exchange of cytosolic malate for mitochondrial α-ketoglutarate. In this study, we investigated the mechanism driving the observed changes to hyperpolarized [2-¹³ C]pyruvate metabolism. Using hyperpolarized [1,2-¹³ C]pyruvate with magnetic resonance spectroscopy (MRS) in the porcine heart with different workloads, it was possible to probe ¹³ C-glutamate labeling relative to rates of cytosolic metabolism, PDH flux and TCA cycle turnover in a single experiment non-invasively. Via the [1-¹³ C]pyruvate label, we observed more than a five-fold increase in the cytosolic conversion of pyruvate to [1-¹³ C]lactate and [1-¹³ C]alanine with higher workload. ¹³ C-Bicarbonate production by PDH was increased by a factor of 2.2. Cardiac cine imaging measured a two-fold increase in cardiac output, which is known to couple to TCA cycle turnover. Via the [2-¹³ C]pyruvate label, we observed that ¹³ C-acetylcarnitine production increased 2.5-fold in proportion to the ¹³ C-bicarbonate signal, whereas the ¹³ C-glutamate metabolic flux remained constant on adrenergic activation. Thus, the ¹³ C-glutamate signal relative to the amount of ¹³ C-labeled acetyl-coenzyme A (acetyl-CoA) entering the TCA cycle was decreased by 40%. The data strongly suggest that NADH (reduced form of nicotinamide adenine dinucleotide) shuttling from the cytosol to the mitochondria via the malate-aspartate shuttle is limited on adrenergic activation. Changes in [5-¹³ C]glutamate production from [2-¹³ C]pyruvate may play an important future role in non-invasive myocardial assessment in patients with cardiovascular diseases, but careful interpretation of the results is required.

Guidelines on experimental methods to assess mitochondrial dysfunction in cellular models of neurodegenerative diseases

Neurodegenerative diseases are a spectrum of chronic, debilitating disorders characterised by the progressive degeneration and death of neurons. Mitochondrial dysfunction has been implicated in most neurodegenerative diseases, but in many instances it is unclear whether such dysfunction is a cause or an effect of the underlying pathology, and whether it represents a viable therapeutic target. It is therefore imperative to utilise and optimise cellular models and experimental techniques appropriate to determine the contribution of mitochondrial dysfunction to neurodegenerative disease phenotypes. In this consensus article, we collate details on and discuss pitfalls of existing experimental approaches to assess mitochondrial function in in vitro cellular models of neurodegenerative diseases, including specific protocols for the measurement of oxygen consumption rate in primary neuron cultures, and single-neuron, time-lapse fluorescence imaging of the mitochondrial membrane potential and mitochondrial NAD(P)H. As part of the Cellular Bioenergetics of Neurodegenerative Diseases (CeBioND) consortium (www.cebiond.org), we are performing cross-disease analyses to identify common and distinct molecular mechanisms involved in mitochondrial bioenergetic dysfunction in cellular models of Alzheimer’s, Parkinson’s, and Huntington’s diseases. Here we provide detailed guidelines and protocols as standardised across the five collaborating laboratories of the CeBioND consortium, with additional contributions from other experts in the field.

A Metabolic Therapy for Malignant Glioma Requires a Clinical Measure

Cancers are "reprogrammed" to use a much higher rate of glycolysis (GLY) relative to oxidative phosphorylation (OXPHOS), even in the presence of adequate amounts of oxygenation. Originally identified by Nobel Laureate Otto Warburg, this hallmark of cancer has recently been termed metabolic reprogramming and represents a way for the cancer tissue to divert carbon skeletons to produce biomass. Understanding the mechanisms that underlie this metabolic shift should lead to better strategies for cancer treatments. Malignant gliomas, cancers that are very resistant to conventional treatments, are highly glycolytic and seem particularly suited to approaches that can subvert this phenotype.

Measuring glucose cerebral metabolism in the healthy mouse using hyperpolarized 13C magnetic resonance

The mammalian brain relies primarily on glucose as a fuel to meet its high metabolic demand. Among the various techniques used to study cerebral metabolism, ¹³C magnetic resonance spectroscopy (MRS) allows following the fate of ¹³C-enriched substrates through metabolic pathways. We herein demonstrate that it is possible to measure cerebral glucose metabolism in vivo with sub-second time resolution using hyperpolarized ¹³C MRS. In particular, the dynamic ¹³C-labeling of pyruvate and lactate formed from ¹³C-glucose was observed in real time. An ad-hoc synthesis to produce [2,3,4,6,6-²H₅, 3,4-¹³C₂]-D-glucose was developed to improve the ¹³C signal-to-noise ratio as compared to experiments performed following [U-²H₇, U-¹³C]-D-glucose injections. The main advantage of only labeling C3 and C4 positions is the absence of ¹³C-¹³C coupling in all downstream metabolic products after glucose is split into 3-carbon intermediates by aldolase. This unique method allows direct detection of glycolysis in vivo in the healthy brain in a noninvasive manner.

Real-Time in Vivo Detection of H2O2 Using Hyperpolarized 13C-Thiourea

Reactive oxygen species (ROS) are essential cellular metabolites widely implicated in many diseases including cancer, inflammation, and cardiovascular and neurodegenerative disorders. Yet, ROS signaling remains poorly understood, and their measurements are a challenge due to high reactivity and instability. Here, we report the development of ¹³C-thiourea as a probe to detect and measure H₂O₂ dynamics with high sensitivity and spatiotemporal resolution using hyperpolarized ¹³C magnetic resonance spectroscopic imaging. In particular, we show ¹³C-thiourea to be highly polarizable and to possess a long spin-lattice relaxation time (T₁), which enables real-time monitoring of ROS-mediated transformation. We also demonstrate that ¹³C-thiourea reacts readily with H₂O₂ to give chemically distinguishable products in vitro and validate their detection in vivo in a mouse liver. This study suggests that ¹³C-thiourea is a promising agent for noninvasive detection of H₂O₂ in vivo. More broadly, our findings outline a viable clinical application for H₂O₂ detection in patients with a range of diseases.

The use of hyperpolarized carbon-13 magnetic resonance for molecular imaging

Until recently, molecular imaging using magnetic resonance (MR) has been limited by the modality's low sensitivity, especially with non-proton nuclei. The advent of hyperpolarized (HP) MR overcomes this limitation by substantially enhancing the signal of certain biologically important probes through a process known as external nuclear polarization, enabling real-time assessment of tissue function and metabolism. The metabolic information obtained by HP MR imaging holds significant promise in the clinic, where it could play a critical role in disease diagnosis and therapeutic monitoring. This review will provide a comprehensive overview of the developments made in the field of hyperpolarized MR, including advancements in polarization techniques and delivery, probe development, pulse sequence optimization, characterization of healthy and diseased tissues, and the steps made towards clinical translation.

The tricarboxylic acid cycle activity in cultured primary astrocytes is strongly accelerated by the protein tyrosine kinase inhibitor tyrphostin 23

Tyrphostin 23 (T23) is a well-known inhibitor of protein tyrosine kinases and has been considered as potential anti-cancer drug. T23 was recently reported to acutely stimulate the glycolytic flux in primary cultured astrocytes. To investigate whether T23 also affects the tricarboxylic acid (TCA) cycle, we incubated primary rat astrocyte cultures with [U-¹³C]glucose in the absence or the presence of 100 μM T23 for 2 h and analyzed the ¹³C metabolite pattern. These incubation conditions did not compromise cell viability and confirmed that the presence of T23 doubled glycolytic lactate production. In addition, T23-treatment strongly increased the molecular carbon labeling of the TCA cycle intermediates citrate, succinate, fumarate and malate, and significantly increased the incorporation of ¹³C-labelling into the amino acids glutamate, glutamine and aspartate. These results clearly demonstrate that, in addition to glycolysis, also the mitochondrial TCA cycle is strongly accelerated after exposure of astrocytes to T23, suggesting that a protein tyrosine kinase may be involved in the regulation of the TCA cycle in astrocytes.

Intracerebral synthesis of glutamine from hyperpolarized glutamate

PURPOSE

Changes in glutamate (Glu) levels occur in a number of neurodegenerative diseases. We proposed the use of ¹³ C spectroscopy and the highly amplified signal generated by hyperpolarization to achieve spatial and temporal resolutions adequate for in vivo studies of Glu metabolism in the healthy rat brain. Thus, we investigated uptake of hyperpolarized [1^-13C ]Glu after a temporary blood-brain barrier (BBB) disruption protocol and its conversion to glutamine (Gln) in the brain.

METHODS

[1-¹³ C]Glu was hyperpolarized using the dynamic nuclear polarization process. A temporary BBB disruption using mannitol allowed hyperpolarized [1-¹³ C]Glu to reach the brain. Then, hyperpolarized [1-¹³ C]Glu brain metabolism was observed in vivo by MR spectroscopy experiments at 3T. Products synthesized from [1-¹³ C]Glu were assigned via liquid chromatography-mass spectrometry.

RESULTS

Hyperpolarized [1-¹³ C]Glu reached 20% ± 2.3% polarization after 90 min. After validation of the BBB disruption protocol, hyperpolarized [1-¹³ C]Glu (175.4 ppm) was detected inside the rat brain, and the formation of [1-¹³ C]Gln at 174.9 ppm was also observed.

CONCLUSION

The Gln synthesis from hyperpolarized [1-¹³ C]Glu can be monitored in vivo in the healthy rat brain after opening the BBB. Magn Reson Med 78:1296-1305, 2017. © 2016 International Society for Magnetic Resonance in Medicine.

Hyperpolarization MRI: Preclinical Models and Potential Applications in Neuroradiology

Hyperpolarization is a novel technology that can dramatically increase signal to noise in magnetic resonance. The method is being applied to small injectable endogenous molecules, which can be used to monitor transient in vivo metabolic events, in real time. The emergence of hyperpolarized C-labeled probes, specifically C pyruvate, has enabled monitoring of core cellular metabolic events. Neuro-oncological applications have been demonstrated in preclinical models. Many more applications of this technology are envisioned, with transformative potential in magnetic resonance imaging.

Hyperpolarized MRS: New tool to study real-time brain function and metabolism

The advent of dissolution dynamic nuclear polarization (DNP) led to the emergence of a new kind of magnetic resonance (MR) measurements providing the opportunity to probe metabolism in vivo in real time. It has been shown that, following the injection of hyperpolarized substrates prepared using dissolution DNP, specific metabolic bioprobes that can be used to differentiate between healthy and pathological tissue in preclinical and clinical studies can be readily detected by MR thanks to the tremendous signal enhancement. The present article aims at reviewing the studies of cerebral function and metabolism based on the use of hyperpolarized MR. The constraints and future opportunities that this technology could offer are discussed.

Accelerated high-bandwidth MR spectroscopic imaging using compressed sensing

PURPOSE

To develop a compressed sensing (CS) acceleration method with a high spectral bandwidth exploiting the spatial-spectral sparsity of MR spectroscopic imaging (MRSI).

METHODS

Accelerations were achieved using blip gradients during the readout to perform nonoverlapped and stochastically delayed random walks in kx -ky -t space, combined with block-Hankel matrix completion for efficient reconstruction. Both retrospective and prospective CS accelerations were applied to (13) C MRSI experiments, including in vivo rodent brain and liver studies with administrations of hyperpolarized [1-(13) C] pyruvate at 7.0 Tesla (T) and [2-(13) C] dihydroxyacetone at 3.0 T, respectively.

RESULTS

In retrospective undersampling experiments using in vivo 7.0 T data, the proposed method preserved spectral, spatial, and dynamic fidelities with R(2) ≥ 0.96 and ≥ 0.87 for pyruvate and lactate signals, respectively, 750-Hz spectral separation, and up to 6.6-fold accelerations. In prospective in vivo experiments, with 3.8-fold acceleration, the proposed method exhibited excellent spatial localization of metabolites and peak recovery for pyruvate and lactate at 7.0 T as well as for dihydroxyacetone and its metabolic products with a 4.5-kHz spectral span (140 ppm at 3.0 T).

CONCLUSIONS

This study demonstrated the feasibility of a new CS approach to accelerate high spectral bandwidth MRSI experiments. Magn Reson Med 76:369-379, 2016. © 2016 Wiley Periodicals, Inc.

Hyperpolarized (13)C-lactate to (13)C-bicarbonate ratio as a biomarker for monitoring the acute response of anti-vascular endothelial growth factor (anti-VEGF) treatment

Hyperpolarized [1-(13)C]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 (13)C-pyruvate imaging studies had focused solely on [1-(13)C]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 (13)C-bicarbonate in cancer tissue, which does provide such information, has proven difficult, however. In this study, we report the reliable measurement of (13)C-bicarbonate production in both the healthy brain and a highly glycolytic experimental glioblastoma model using an optimized (13)C MRS imaging protocol. With the capacity to obtain signal in all tumors, we also confirm for the first time that the ratio of (13)C-lactate to (13)C-bicarbonate provides a more robust metric relative to (13)C-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.

Molecular Imaging of Metabolic Reprograming in Mutant IDH Cells

Mutations in the metabolic enzyme isocitrate dehydrogenase (IDH) have recently been identified as drivers in the development of several tumor types. Most notably, cytosolic IDH1 is mutated in 70-90% of low-grade gliomas and upgraded glioblastomas, and mitochondrial IDH2 is mutated in ~20% of acute myeloid leukemia cases. Wild-type IDH catalyzes the interconversion of isocitrate to α-ketoglutarate (α-KG). Mutations in the enzyme lead to loss of wild-type enzymatic activity and a neomorphic activity that converts α-KG to 2-hydroxyglutarate (2-HG). In turn, 2-HG, which has been termed an "oncometabolite," inhibits key α-KG-dependent enzymes, resulting in alterations of the cellular epigenetic profile and, subsequently, inhibition of differentiation and initiation of tumorigenesis. In addition, it is now clear that the IDH mutation also induces a broad metabolic reprograming that extends beyond 2-HG production, and this reprograming often differs from what has been previously reported in other cancer types. In this review, we will discuss in detail what is known to date about the metabolic reprograming of mutant IDH cells, and how this reprograming has been investigated using molecular metabolic imaging. We will describe how metabolic imaging has helped shed light on the basic biology of mutant IDH cells, and how this information can be leveraged to identify new therapeutic targets and to develop new clinically translatable imaging methods to detect and monitor mutant IDH tumors in vivo.

Hyperpolarized 13C NMR observation of lactate kinetics in skeletal muscle

The production of glycolytic end products, such as lactate, usually evokes a cellular shift from aerobic to anaerobic ATP generation and O2 insufficiency. In the classical view, muscle lactate must be exported to the liver for clearance. However, lactate also forms under well-oxygenated conditions, and this has led investigators to postulate lactate shuttling from non-oxidative to oxidative muscle fiber, where it can serve as a precursor. Indeed, the intracellular lactate shuttle and the glycogen shunt hypotheses expand the vision to include a dynamic mobilization and utilization of lactate during a muscle contraction cycle. Testing the tenability of these provocative ideas during a rapid contraction cycle has posed a technical challenge. The present study reports the use of hyperpolarized [1-(13)C]lactate and [2-(13)C]pyruvate in dynamic nuclear polarization (DNP) NMR experiments to measure the rapid pyruvate and lactate kinetics in rat muscle. With a 3 s temporal resolution, (13)C DNP NMR detects both [1-(13)C]lactate and [2-(13)C]pyruvate kinetics in muscle. Infusion of dichloroacetate stimulates pyruvate dehydrogenase activity and shifts the kinetics toward oxidative metabolism. Bicarbonate formation from [1-(13)C]lactate increases sharply and acetyl-l-carnitine, acetoacetate and glutamate levels also rise. Such a quick mobilization of pyruvate and lactate toward oxidative metabolism supports the postulated role of lactate in the glycogen shunt and the intracellular lactate shuttle models. The study thus introduces an innovative DNP approach to measure metabolite transients, which will help delineate the cellular and physiological role of lactate and glycolytic end products.

Studies of Metabolism Using (13)C MRS of Hyperpolarized Probes

First described in 2003, the dissolution dynamic nuclear polarization (DNP) technique, combined with (13)C magnetic resonance spectroscopy (MRS), has since been used in numerous metabolic studies and has become a valuable metabolic imaging method. DNP dramatically increases the level of polarization of (13)C-labeled compounds resulting in an increase in the signal-to-noise ratio (SNR) of over 50,000 fold for the MRS spectrum of hyperpolarized compounds. The high SNR enables rapid real-time detection of metabolism in cells, tissues, and in vivo. This chapter will present a comprehensive review of the DNP approaches that have been used to monitor metabolism in living systems. First, the list of (13)C DNP probes developed to date will be presented, with a particular focus on the most commonly used probe, namely [1-(13)C] pyruvate. In the next four sections, we will then describe the different factors that need to be considered when designing (13)C DNP probes for metabolic studies, conducting in vitro or in vivo hyperpolarized experiments, as well as acquiring, analyzing, and modeling hyperpolarized (13)C data.

IDH1 Mutation Induces Reprogramming of Pyruvate Metabolism

Mutant isocitrate dehydrogenase 1 (IDH1) catalyzes the production of 2-hydroxyglutarate but also elicits additional metabolic changes. Levels of both glutamate and pyruvate dehydrogenase (PDH) activity have been shown to be affected in U87 glioblastoma cells or normal human astrocyte (NHA) cells expressing mutant IDH1, as compared with cells expressing wild-type IDH1. In this study, we show how these phenomena are linked through the effects of IDH1 mutation, which also reprograms pyruvate metabolism. Reduced PDH activity in U87 glioblastoma and NHA IDH1 mutant cells was associated with relative increases in PDH inhibitory phosphorylation, expression of pyruvate dehydrogenase kinase-3, and levels of hypoxia inducible factor-1α. PDH activity was monitored in these cells by hyperpolarized (13)C-magnetic resonance spectroscopy ((13)C-MRS), which revealed a reduction in metabolism of hyperpolarized 2-(13)C-pyruvate to 5-(13)C-glutamate, relative to cells expressing wild-type IDH1. (13)C-MRS also revealed a reduction in glucose flux to glutamate in IDH1 mutant cells. Notably, pharmacological activation of PDH by cell exposure to dichloroacetate (DCA) increased production of hyperpolarized 5-(13)C-glutamate in IDH1 mutant cells. Furthermore, DCA treatment also abrogated the clonogenic advantage conferred by IDH1 mutation. Using patient-derived mutant IDH1 neurosphere models, we showed that PDH activity was essential for cell proliferation. Taken together, our results established that the IDH1 mutation induces an MRS-detectable reprogramming of pyruvate metabolism, which is essential for cell proliferation and clonogenicity, with immediate therapeutic implications.

Volumetric spiral chemical shift imaging of hyperpolarized [2-(13) c]pyruvate in a rat c6 glioma model

PURPOSE

MRS of hyperpolarized [2-(13)C]pyruvate can be used to assess multiple metabolic pathways within mitochondria as the (13)C label is not lost with the conversion of pyruvate to acetyl-CoA. This study presents the first MR spectroscopic imaging of hyperpolarized [2-(13)C]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-(13)C]pyruvate, [2-(13)C]lactate, and [5-(13)C]glutamate, the metabolites known to be produced in brain after an injection of hyperpolarized [2-(13)C]pyruvate, without chemical shift displacement artifacts. A separate undersampling scheme (890 Hz) was also used to image [1-(13)C]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-(13)C]pyruvate studies. Acetyl-carnitine was observed in the muscle/fat tissue surrounding the brain.

CONCLUSION

Robust volumetric imaging with hyperpolarized [2-(13)C]pyruvate and downstream products was performed in glioma-bearing rat brains, demonstrating changes in mitochondrial metabolism with dichloroacetate.