Assessment of real-time myocardial uptake and enzymatic conversion of hyperpolarized [1-¹³C]pyruvate in pigs using slice selective magnetic resonance spectroscopy

Current methods for hyperpolarized [1-13C]pyruvate MRI human studies

MRI with hyperpolarized (HP) 13C agents, also known as HP 13C MRI, can measure processes such as localized metabolism that is altered in numerous cancers, liver, heart, kidney diseases, and more. It has been translated into human studies during the past 10 years, with recent rapid growth in studies largely based on increasing availability of HP agent preparation methods suitable for use in humans. This paper aims to capture the current successful practices for HP MRI human studies with [1-13C]pyruvate-by far the most commonly used agent, which sits at a key metabolic junction in glycolysis. The paper is divided into four major topic areas: (1) HP 13C-pyruvate preparation; (2) MRI system setup and calibrations; (3) data acquisition and image reconstruction; and (4) data analysis and quantification. In each area, we identified the key components for a successful study, summarized both published studies and current practices, and discuss evidence gaps, strengths, and limitations. This paper is the output of the "HP 13C MRI Consensus Group" as well as the ISMRM Hyperpolarized Media MR and Hyperpolarized Methods and Equipment study groups. It further aims to provide a comprehensive reference for future consensus, building as the field continues to advance human studies with this metabolic imaging modality.

Concentration-dependent effects of dichloroacetate in type 2 diabetic hearts assessed by hyperpolarized [1-13 C]-pyruvate magnetic resonance imaging

Personalized medicine or individualized therapy promises a paradigm shift in healthcare. This is particularly true in complex and multifactorial diseases such as diabetes and the multitude of related pathophysiological complications. Diabetic cardiomyopathy represents an emerging condition that could be effectively treated if better diagnostic and, in particular, better therapeutic monitoring tools were available. In this study, we investigate the ability to differentiate low and high doses of metabolically targeted therapy in an obese type 2 diabetic rat model. Low-dose dichloroacetate (DCA) treatment was associated with increased lactate production, while no or little change was seen in bicarbonate production. High-dose DCA treatment was associated with a significant metabolic switch towards increased bicarbonate production. These findings support further studies using hyperpolarized [1-¹³ C]-pyruvate magnetic resonance imaging to differentiate treatment effects and thus allow for personalized titration of therapeutics.

Cardiac metabolic imaging using hyperpolarized [1-13 C]lactate as a substrate

Hyperpolarized (HP) [1-¹³ C]lactate is an attractive alternative to [1-¹³ C]pyruvate as a substrate to investigate cardiac metabolism in vivo: it can be administered safely at a higher dose and can be polarized to a degree similar to pyruvate via dynamic nuclear polarization. While ¹³ C cardiac experiments using HP lactate have been performed in small animal models, they have not been demonstrated in large animal models or humans. Utilizing the same hardware and data acquisition methods as the first human HP ¹³ C cardiac study, ¹³ C metabolic images were acquired following injections of HP [1-¹³ C]lactate in porcine hearts. Data were also acquired using HP [1-¹³ C]pyruvate for comparison. The ¹³ C bicarbonate signal was localized to the myocardium and had a similar appearance with both substrates for all animals. No ¹³ C pyruvate signal was detected in the experiments following injection of HP ¹³ C lactate. The signal-to-noise ratio (SNR) of injected lactate was 88 ± 4% of the SNR of injected pyruvate, and the SNR of bicarbonate in the experiments using lactate as the substrate was 52 ± 19% of the SNR in the experiments using pyruvate as the substrate. The lower SNR was likely due to the shorter T₁ of [1-¹³ C]lactate as compared with [1-¹³ C]pyruvate and the additional enzyme-catalyzed metabolic conversion step before the ¹³ C nuclei from [1-¹³ C]lactate were detected as ¹³ C bicarbonate. While challenges remain, the potential of HP lactate as a substrate for clinical metabolic imaging of human heart has been demonstrated.

Optimizing SNR for multi-metabolite hyperpolarized carbon-13 MRI using a hybrid flip-angle scheme

PURPOSE

To improve the SNR of hyperpolarized carbon-13 MRI of [1-13 C]pyruvate using a multispectral variable flip angle (msVFA) scheme in which the spectral profile and flip angle vary dynamically with time.

METHODS

Each image acquisition in a time-resolved imaging experiment used a unique spectrally varying RF pulse shape for msVFA. Therefore, the flip angle for every acquisition was optimized for pyruvate and each of its metabolites to yield the highest SNR across the acquisition. Multispectral VFA was compared with a spectrally varying constant flip-angle excitation model through simulations and in vivo. A modified broadband chemical shift-encoded gradient-echo sequence was used for in vivo experiments on six pregnant guinea pigs. Regions of interest placed in the placentae, maternal liver, and maternal kidneys were used as areas for SNR measurement.

RESULTS

In vivo experiments showed significant increases in SNR for msVFA relative to constant flip angle of up to 250% for multiple metabolites.

CONCLUSION

Hyperpolarized carbon-13 imaging with msVFA excitation produces improved SNR for all metabolites in organs of interest.

Cardiac metabolic modulation upon low-carbohydrate low-protein ketogenic diet in diabetic rats studied in vivo using hyperpolarized 13 C pyruvate, butyrate and acetoacetate probes

AIM

To investigate the effects of long-term low-carbohydrate low-protein ketogenic diet (KD) on cardiac metabolism and diabetic cardiomyopathy status in lean diabetic Goto-Kakizaki (GK) rats.

MATERIALS AND METHODS

Diabetic GK rats were fed with KD for 62 weeks. Cardiac function and metabolism were assessed using magnetic resonance imaging and ¹³ C magnetic resonance spectroscopy (¹³ C-MRS), at rest and under dobutamine stress. ¹³ C-MRS was performed following injection of hyperpolarized [3-¹³ C]acetoacetate, [1-¹³ C]butyrate or [1-¹³ C]pyruvate to assess ketone body, short-chain fatty acid or glucose utilization, respectively. Protein expression and cardiomyocyte structure were determined via Western blotting and histology, respectively.

RESULTS

KD lowered blood glucose, triglyceride and insulin levels while increasing blood ketone body levels. In KD-fed diabetic rats, myocardial ketone body and glucose oxidation were lower than in chow-fed diabetic rats, while myocardial glycolysis and short-chain fatty acid oxidation were unaltered. Dobutamine stress revealed an increased cardiac preload and reduced cardiac compliance in KD-fed diabetic rats. Dobutamine-induced stimulation of myocardial glycolysis was more enhanced in KD-fed diabetic rats than in chow-fed diabetic rats, which was potentially facilitated via an upregulation in basal expression of proteins involved in glucose transport and glycolysis in the hearts of KD-fed rats. The metabolic profile induced by KD was accompanied by cardiac hypertrophy, a trend for increased myocardial lipid and collagen content, and an increased marker of oxidative stress.

CONCLUSION

KD seems to exacerbate diabetic cardiomyopathy in GK rats, which may be associated with maladaptive cardiac metabolic modulation and lipotoxicity.

Metabolic and Molecular Imaging with Hyperpolarised Tracers

Since reaching the clinic, magnetic resonance imaging (MRI) has become an irreplaceable radiological tool because of the macroscopic information it provides across almost all organs and soft tissues within the human body, all without the need for ionising radiation. The sensitivity of MR, however, is too low to take full advantage of the rich chemical information contained in the MR signal. Hyperpolarisation techniques have recently emerged as methods to overcome the sensitivity limitations by enhancing the MR signal by many orders of magnitude compared to the thermal equilibrium, enabling a new class of metabolic and molecular X-nuclei based MR tracers capable of reporting on metabolic processes at the cellular level. These hyperpolarised (HP) tracers have the potential to elucidate the complex metabolic processes of many organs and pathologies, with studies so far focusing on the fields of oncology and cardiology. This review presents an overview of hyperpolarisation techniques that appear most promising for clinical use today, such as dissolution dynamic nuclear polarisation (d-DNP), parahydrogen-induced hyperpolarisation (PHIP), Brute force hyperpolarisation and spin-exchange optical pumping (SEOP), before discussing methods for tracer detection, emerging metabolic tracers and applications and progress in preclinical and clinical application.

Hyperpolarized [1-13 C]pyruvate MRI can image the metabolic shift in cardiac metabolism between the fasted and fed state in a porcine model

PURPOSE

Owing to its noninvasive nature, hyperpolarized MRI may improve delineation of myocardial metabolic derangement in heart disease. However, consistency may depend on the changeable nature of cardiac metabolism in relation to whole-body metabolic state. This study investigates the impact of feeding status on cardiac hyperpolarized MRI in a large animal model resembling human physiology.

METHODS

Thirteen 30-kg pigs were subjected to an overnight fast, and 5 pigs were fed a carbohydrate-rich meal on the morning of the experiments. Vital parameters and blood samples were registered. All pigs were then scanned by hyperpolarized [1-¹³ C]pyruvate cardiac MRI, and results were compared between the 2 groups and correlated with circulating substrates and hormones.

RESULTS

The fed group had higher blood glucose concentration and mean arterial pressure than the fasted group. Plasma concentrations of free fatty acids (FFAs) were decreased in the fed group, whereas plasma insulin concentrations were similar between groups. Hyperpolarized MRI showed that fed animals had increased lactate/pyruvate, alanine/pyruvate, and bicarbonate/pyruvate ratios. Metabolic ratios correlated negatively with FFA levels.

CONCLUSION

Hyperpolarized MR can identify the effects of different metabolic states on cardiac metabolism in a large animal model. Unlike previous rodent studies, all metabolic derivatives of pyruvate increased in the myocardium of fed pigs. Carbohydrate-rich feeding seems to be a feasible model for standardized, large animal hyperpolarized MRI studies of myocardial carbohydrate metabolism.

13C Pyruvate Transport Across the Blood-Brain Barrier in Preclinical Hyperpolarised MRI

Hyperpolarised MRI with Dynamic Nuclear Polarisation overcomes the fundamental thermodynamic limitations of conventional magnetic resonance, and is translating to human studies with several early-phase clinical trials in progress including early reports that demonstrate the utility of the technique to observe lactate production in human brain cancer patients. Owing to the fundamental coupling of metabolism and tissue function, metabolic neuroimaging with hyperpolarised [1-13C]pyruvate has the potential to be revolutionary in numerous neurological disorders (e.g. brain tumour, ischemic stroke, and multiple sclerosis). Through the use of [1-13C]pyruvate and ethyl-[1-13C]pyruvate in naïve brain, a rodent model of metastasis to the brain, or porcine brain subjected to mannitol osmotic shock, we show that pyruvate transport across the blood-brain barrier of anaesthetised animals is rate-limiting. We show through use of a well-characterised rat model of brain metastasis that the appearance of hyperpolarized [1-13C]lactate production corresponds to the point of blood-brain barrier breakdown in the disease. With the more lipophilic ethyl-[1-13C]pyruvate, we observe pyruvate production endogenously throughout the entire brain and lactate production only in the region of disease. In the in vivo porcine brain we show that mannitol shock permeabilises the blood-brain barrier sufficiently for a dramatic 90-fold increase in pyruvate transport and conversion to lactate in the brain, which is otherwise not resolvable. This suggests that earlier reports of whole-brain metabolism in anaesthetised animals may be confounded by partial volume effects and not informative enough for translational studies. Issues relating to pyruvate transport and partial volume effects must therefore be considered in pre-clinical studies investigating neuro-metabolism in anaesthetised animals, and we additionally note that these same techniques may provide a distinct biomarker of blood-brain barrier permeability in future studies.

Biomolecular imaging of 13C-butyrate with dissolution-DNP: Polarization enhancement and formulation for in vivo studies

Magnetic Resonance Spectroscopy of hyperpolarized isotopically enriched molecules facilitates the non-invasive real-time investigation of in vivo tissue metabolism in the time-frame of a few minutes; this opens up a new avenue in the development of biomolecular probes. Dissolution Dynamic Nuclear Polarization is a hyperpolarization technique yielding a more than four orders of magnitude increase in the ¹³C polarization for in vivo Magnetic Resonance Spectroscopy studies. As reported in several studies, the dissolution Dynamic Nuclear Polarization polarization performance relies on the chemico-physical properties of the sample. In this study, we describe and quantify the effects of the different sample components on the dissolution Dynamic Nuclear Polarization performance of [1-¹³C]butyrate. In particular, we focus on the polarization enhancement provided by the incremental addition of the glassy agent dimethyl sulfoxide and gadolinium chelate to the formulation. Finally, preliminary results obtained after injection in healthy rats are also reported, showing the feasibility of an in vivo Magnetic Resonance Spectroscopy study with hyperpolarized [1-¹³C]butyrate using a 3T clinical set-up.

Cardiac applications of hyperpolarised magnetic resonance

Cardiovascular disease is the leading cause of death world-wide. It is increasingly recognised that cardiac pathologies show, or may even be caused by, changes in metabolism, leading to impaired cardiac energetics. The heart turns over 15 times its own weight in ATP every day and thus relies heavily on the availability of substrates and on efficient oxidation to generate this ATP. A number of old and emerging drugs that target different aspects of metabolism are showing promising results with regard to improved cardiac outcomes in patients. A non-invasive imaging technique that could assess the role of different aspects of metabolism in heart disease, as well as measure changes in cardiac energetics due to treatment, would be valuable in the routine clinical care of cardiac patients. Hyperpolarised magnetic resonance spectroscopy and imaging have revolutionised metabolic imaging, allowing real-time metabolic flux assessment in vivo for the first time. In this review we summarise metabolism in the healthy and diseased heart, give an introduction to the hyperpolarisation technique, 'dynamic nuclear polarisation' (DNP), and review the preclinical studies that have thus far explored healthy cardiac metabolism and different models of human heart disease. We furthermore show what advances have been made to translate this technique into the clinic, what technical challenges still remain and what unmet clinical needs and unexplored metabolic substrates still need to be assessed by researchers in this exciting and fast-moving field.

Metabolic Measurements of Nonpermeating Compounds in Live Cells Using Hyperpolarized NMR

Hyperpolarization by dissolution dynamic nuclear polarization (D-DNP) has emerged as a technique for enhancing NMR signals by several orders of magnitude, thereby facilitating the characterization of metabolic pathways both in vivo and in vitro. Following the introduction of an externally hyperpolarized compound, real-time NMR enables the measurement of metabolic flux in the corresponding pathway. Spin relaxation however limits the maximum experimental time and prevents the use of this method with compounds exhibiting slow membrane transport rates. Here, we demonstrate that on-line electroporation can serve as a method for membrane permeabilization for use with D-DNP in cell cultures. An electroporation apparatus hyphenated with stopped-flow sample injection permits the introduction of the hyperpolarized metabolite within 3 s after the electrical pulse. In yeast cells that do not readily take up pyruvate, the addition of the electroporation pulse to the D-DNP experiment increases the signals of the downstream metabolic products CO2 and HCO3-, which otherwise are near the detection limit, by 8.2- and 8.6-fold. Modeling of the time dependence of these signals then permits the determination of the respective kinetic rate constants. The observed conversion rate from pyruvate to CO2 normalized for cell density was found to increase by a factor of 12 due to the alleviation of the membrane transport limitation. The use of electroporation therefore extends the applicability of D-DNP to in vitro studies with a wider range of metabolites and at the same time reduces the influence of membrane transport on the observed conversion rates.

Mapping of intracellular pH in the in vivo rodent heart using hyperpolarized [1-13C]pyruvate

Purpose

To demonstrate the feasibility of mapping intracellular pH within the in vivo rodent heart. Alterations in cardiac acid‐base balance can lead to acute contractile depression and alterations in Ca²⁺ signaling. The transient reduction in adenosine triphosphate (ATP) consumption and cardiac contractility may be initially beneficial; however, sustained pH changes can be maladaptive, leading to myocardial damage and electrical arrhythmias.

Methods

Spectrally selective radiofrequency (RF) pulses were used to excite the HCO3− and CO₂ resonances individually while preserving signal from the injected hyperpolarized [1‐¹³C]pyruvate. The large flip angle pulses were placed within a three‐dimensional (3D) imaging acquisition, which exploited CA‐mediated label exchange between HCO3− and CO₂. Images at 4.5 × 4.5 × 5 mm³ resolution were obtained in the in vivo rodent heart. The technique was evaluated in healthy rodents scanned at baseline and during high cardiac workload induced by dobutamine infusion.

Results

The intracellular pH was measured to be 7.15 ± 0.04 at baseline, and decreased to 6.90 ± 0.06 following 15 min of continuous β‐adrenergic stimulation.

Conclusions

Volumetric maps of intracellular pH can be obtained following an injection of hyperpolarized [1‐¹³C]pyruvate. The new method is anticipated to enable assessment of stress‐inducible ischemia and potential ventricular arrythmogenic substrates within the ischemic heart. Magn Reson Med 77:1810–1817, 2017. © 2016 The Authors Magnetic Resonance in Medicine published by Wiley Periodicals, Inc. on behalf of International Society for Magnetic Resonance in Medicine. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

Modeling non-linear kinetics of hyperpolarized [1-(13)C] pyruvate in the crystalloid-perfused rat heart

Hyperpolarized (13)C MR measurements have the potential to display non-linear kinetics. We have developed an approach to describe possible non-first-order kinetics of hyperpolarized [1-(13)C] pyruvate employing a system of differential equations that agrees with the principle of conservation of mass of the hyperpolarized signal. Simultaneous fitting to a second-order model for conversion of [1-(13)C] pyruvate to bicarbonate, lactate and alanine was well described in the isolated rat heart perfused with Krebs buffer containing glucose as sole energy substrate, or glucose supplemented with pyruvate. Second-order modeling yielded significantly improved fits of pyruvate-bicarbonate kinetics compared with the more traditionally used first-order model and suggested time-dependent decreases in pyruvate-bicarbonate flux. Second-order modeling gave time-dependent changes in forward and reverse reaction kinetics of pyruvate-lactate exchange and pyruvate-alanine exchange in both groups of hearts during the infusion of pyruvate; however, the fits were not significantly improved with respect to a traditional first-order model. The mechanism giving rise to second-order pyruvate dehydrogenase (PDH) kinetics was explored experimentally using surface fluorescence measurements of nicotinamide adenine dinucleotide reduced form (NADH) performed under the same conditions, demonstrating a significant increase of NADH during pyruvate infusion. This suggests a simultaneous depletion of available mitochondrial NAD(+) (the cofactor for PDH), consistent with the non-linear nature of the kinetics. NADH levels returned to baseline following cessation of the pyruvate infusion, suggesting this to be a transient effect.

MRS: a noninvasive window into cardiac metabolism

A well-functioning heart requires a constant supply of a balanced mixture of nutrients to be used for the production of adequate amounts of adenosine triphosphate, which is the main energy source for most cellular functions. Defects in cardiac energy metabolism are linked to several myocardial disorders. MRS can be used to study in vivo changes in cardiac metabolism noninvasively. MR techniques allow repeated measurements, so that disease progression and the response to treatment or to a lifestyle intervention can be monitored. It has also been shown that MRS can predict clinical heart failure and death. This article focuses on in vivo MRS to assess cardiac metabolism in humans and experimental animals, as experimental animals are often used to investigate the mechanisms underlying the development of metabolic diseases. Various MR techniques, such as cardiac (31) P-MRS, (1) H-MRS, hyperpolarized (13) C-MRS and Dixon MRI, are described. A short overview of current and emerging applications is given. Cardiac MRS is a promising technique for the investigation of the relationship between cardiac metabolism and cardiac disease. However, further optimization of scan time and signal-to-noise ratio is required before broad clinical application. In this respect, the ongoing development of advanced shimming algorithms, radiofrequency pulses, pulse sequences, (multichannel) detection coils, the use of hyperpolarized nuclei and scanning at higher magnetic field strengths offer future perspective for clinical applications of MRS.

Kinetic analysis of hyperpolarized data with minimum a priori knowledge: Hybrid maximum entropy and nonlinear least squares method (MEM/NLS)

PURPOSE

To assess the feasibility of using a hybrid Maximum-Entropy/Nonlinear Least Squares (MEM/NLS) method for analyzing the kinetics of hyperpolarized dynamic data with minimum a priori knowledge.

THEORY AND METHODS

A continuous distribution of rates obtained through the Laplace inversion of the data is used as a constraint on the NLS fitting to derive a discrete spectrum of rates. Performance of the MEM/NLS algorithm was assessed through Monte Carlo simulations and validated by fitting the longitudinal relaxation time curves of hyperpolarized [1-(13) C] pyruvate acquired at 9.4 Tesla and at three different flip angles. The method was further used to assess the kinetics of hyperpolarized pyruvate-lactate exchange acquired in vitro in whole blood and to re-analyze the previously published in vitro reaction of hyperpolarized (15) N choline with choline kinase.

RESULTS

The MEM/NLS method was found to be adequate for the kinetic characterization of hyperpolarized in vitro time-series. Additional insights were obtained from experimental data in blood as well as from previously published (15) N choline experimental data.

CONCLUSION

The proposed method informs on the compartmental model that best approximate the biological system observed using hyperpolarized (13) C MR especially when the metabolic pathway assessed is complex or a new hyperpolarized probe is used.

Dissolution dynamic nuclear polarization of non-self-glassing agents: spectroscopy and relaxation of hyperpolarized [1-13C]acetate

The intrinsic physicochemical properties of the sample formulation are the key factors for efficient hyperpolarization through dissolution dynamic nuclear polarization (dissolution-DNP). We provide a comprehensive characterization of the DNP process for Na-[1-(13)C]acetate selected as a model for non-self-glassing agents: the solid-state polarization dynamics of different formulations and the effect of the paramagnetic agent (trityl radical) on the pattern of polarization and the relaxation profile were extensively analyzed. We quantified the effects of the glassing agent and Gd(3+)-chelate on DNP performance. The results reported here describe the constraints of the acetate formulation useful for future studies in this field with non-self-glassing enriched molecules.

Real-time cardiac metabolism assessed with hyperpolarized [1-(13) C]acetate in a large-animal model

Dissolution-dynamic nuclear polarization (dissolution-DNP) for magnetic resonance (MR) spectroscopic imaging has recently emerged as a novel technique for noninvasive studies of the metabolic fate of biomolecules in vivo. Since acetate is the most abundant extra- and intracellular short-chain fatty acid, we focused on [1-(13) C]acetate as a promising candidate for a chemical probe to study the myocardial metabolism of a beating heart. The dissolution-DNP procedure of Na[1-(13) C]acetate for in vivo cardiac applications with a 3 T MR scanner was optimized in pigs during bolus injection of doses of up to 3 mmol. The Na[1-(13) C]acetate formulation was characterized by a liquid-state polarization of 14.2% and a T1Eff in vivo of 17.6 ± 1.7 s. In vivo Na[1-(13) C]acetate kinetics displayed a bimodal shape: [1-(13) C]acetyl carnitine (AcC) was detected in a slice covering the cardiac volume, and the signal of (13) C-acetate and (13) C-AcC was modeled using the total area under the curve (AUC) for kinetic analysis. A good correlation was found between the ratio AUC(AcC)/AUC(acetate) and the apparent kinetic constant of metabolic conversion, from [1-(13) C]acetate to [1-(13) C]AcC (kAcC ), divided by the AcC longitudinal relaxation rate (r1 ). Our study proved the feasibility and the limitations of administration of large doses of hyperpolarized [1-(13) C]acetate to study the myocardial conversion of [1-(13) C]acetate in [1-(13) C]acetyl-carnitine generated by acetyltransferase in healthy pigs.

Structured errors in reconstruction methods for Non-Cartesian MR data

BACKGROUND

Reconstruction methods for Non-Cartesian magnetic resonance imaging have often been analyzed using the root mean square error (RMSE). However, RMSE is not able to measure the level of structured error associated with the reconstruction process.

METHODS

An index for geometric information loss was presented using the 2D autocorrelation function. The performances of Least Squares Non Uniform Fast Fourier Transform (LS-NUFFT) and gridding reconstruction (GR) methods were compared. The Direct Summation method (DS) was used as reference. For both methods, RMSE and the loss in geometric information were calculated using a digital phantom and a hyperpolarized (13)C dataset.

RESULTS

The performance of the geometric information loss index was analyzed in the presence of noise. Comparing to GR, LS-NUFFT obtained a lower RMSE, but its error image appeared more structured. This was observed in both phantom and in vivo experiments.

DISCUSSION

The evaluation of geometric information loss together with the reconstruction error was important for an appropriate performance analysis of the reconstruction methods. The use of geometric information loss was helpful to determine that LS-NUFFT loses relevant information in the reconstruction process, despite the low RMSE.

In vivo investigation of cardiac metabolism in the rat using MRS of hyperpolarized [1-13C] and [2-13C]pyruvate

Hyperpolarized (13)C MRS allows the in vivo assessment of pyruvate dehydrogenase complex (PDC) flux, which converts pyruvate to acetyl-coenzyme A (acetyl-CoA). [1-(13)C]pyruvate has been used to measure changes in cardiac PDC flux, with demonstrated increase in (13)C-bicarbonate production after dichloroacetate (DCA) administration. With [1-(13)C]pyruvate, the (13)C label is released as (13 CO2 /(13)C-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 (13)C label into the TCA (tricarboxylic acid) cycle and measure [5-(13)C]glutamate as well as study changes in [1-(13)C]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 (13)C labeling of [5-(13)C]glutamate, and a considerable increase in [1-(13)C]acetylcarnitine and [1,3-(13)C]acetoacetate peaks. Dobutamine resulted in an increased labeling of [2-(13)C]lactate, [2-(13)C]alanine and [5-(13)C]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.

Metabolic imaging of acute and chronic infarction in the perfused rat heart using hyperpolarised [1-13C]pyruvate

Hyperpolarised (13)C MRI can be used to generate metabolic images of the heart in vivo. However, there have been no similar studies performed in the isolated perfused heart. Therefore, the aim of this study was to develop a method for the creation of (13)C metabolite maps of the perfused rat heart and to demonstrate the technique in a study of acute and chronic myocardial infarction. Male Wistar rat hearts were isolated, perfused and imaged before and after occlusion of the left anterior descending (LAD) coronary artery, creating an acute infarct group. In addition, a chronic infarct group was generated from hearts which had their LAD coronary artery occluded in vivo. Four weeks later, hearts were excised, perfused and imaged to generate metabolic maps of infused pyruvate and its metabolites lactate and bicarbonate. Myocardial perfusion and energetics were assessed by first-pass perfusion imaging and (31)P MRS, respectively. In both acute and chronically infarcted hearts, perfusion was reduced to the infarct region, as revealed by reduced gadolinium influx and lower signal intensity in the hyperpolarised pyruvate images. In the acute infarct region, there were significant alterations in the lactate (increased) and bicarbonate (decreased) signal ratios. In the chronically infarcted region, there was a significant reduction in both bicarbonate and lactate signals. (31)P-derived energetics revealed a significant decrease between control and chronic infarcted hearts. Significant decreases in contractile function between control and both acute and chronic infracted hearts were also seen. In conclusion, we have demonstrated that hyperpolarised pyruvate can detect reduced perfusion in the rat heart following both acute and chronic infarction. Changes in lactate and bicarbonate ratios indicate increased anaerobic metabolism in the acute infarct, which is not observed in the chronic infarct. Thus, this study has successfully demonstrated a novel imaging approach to assess altered metabolism in the isolated perfused rat heart.

Model free approach to kinetic analysis of real-time hyperpolarized 13C magnetic resonance spectroscopy data

Real-time detection of the rates of metabolic flux, or exchange rates of endogenous enzymatic reactions, is now feasible in biological systems using Dynamic Nuclear Polarization Magnetic Resonance. Derivation of reaction rate kinetics from this technique typically requires multi-compartmental modeling of dynamic data, and results are therefore model-dependent and prone to misinterpretation. We present a model-free formulism based on the ratio of total areas under the curve (AUC) of the injected and product metabolite, for example pyruvate and lactate. A theoretical framework to support this novel analysis approach is described, and demonstrates that the AUC ratio is proportional to the forward rate constant k. We show that the model-free approach strongly correlates with k for whole cell in vitro experiments across a range of cancer cell lines, and detects response in cells treated with the pan-class I PI3K inhibitor GDC-0941 with comparable or greater sensitivity. The same result is seen in vivo with tumor xenograft-bearing mice, in control tumors and following drug treatment with dichloroacetate. An important finding is that the area under the curve is independent of both the input function and of any other metabolic pathways arising from the injected metabolite. This model-free approach provides a robust and clinically relevant alternative to kinetic model-based rate measurements in the clinical translation of hyperpolarized (13)C metabolic imaging in humans, where measurement of the input function can be problematic.

Noninvasive identification and assessment of functional brown adipose tissue in rodents using hyperpolarized ¹³C imaging

OBJECTIVE

The recent identification of functional depots of brown adipose tissue (BAT) in adult humans has potential implications for the treatment of obesity. In order to evaluate new therapies aimed at inducing the production of more BAT or activating BAT in humans, it will be important to develop noninvasive methods to assess the functional state of the tissue in vivo. In this study, we investigate the feasibility of using hyperpolarized (13)C imaging to noninvasively identify functional, activated BAT in an in vivo rodent model, in less than 1 min, following an infusion of pre-polarized [1-(13)C] pyruvate.

DESIGN

Hyperpolarized (13)C imaging was used to monitor BAT metabolic conversion of pre-polarized [1-(13)C] pyruvate in rats during baseline and norepinephrine (NE)-stimulated conditions.

RESULTS

Activated BAT, stimulated by NE injection, can be detected in rats by increased conversion of pre-polarized [1-(13)C] pyruvate into its downstream products (13)C bicarbonate and [1-(13)C] lactate. The colocalization of the (13)C signal to interscapular BAT was validated using hematoxylin-eosin histological staining.

CONCLUSION

The radiation-free nature and recent translation into the clinic of the hyperpolarized (13)C-imaging test may potentially facilitate trials of therapeutics targeting BAT activation in humans.

Quantitative measurement of cancer metabolism using stimulated echo hyperpolarized carbon-13 MRS

PURPOSE

Magnetic resonance spectroscopy of hyperpolarized substrates allows for the observation of label exchange catalyzed by enzymes providing a powerful tool to investigate tissue metabolism and potentially kinetics in vivo. However, the accuracy of current methods to calculate kinetic parameters has been limited by T1 relaxation effects, extracellular signal contributions, and reduced precision at lower signal-to-noise ratio.

THEORY AND METHODS

To address these challenges, we investigated a new modeling technique using metabolic activity decomposition-stimulated echo acquisition mode. The metabolic activity decomposition-stimulated echo acquisition mode technique separates exchanging from nonexchanging metabolites providing twice the information as conventional techniques.

RESULTS

This allowed for accurate measurements of rates of conversion and of multiple T1 values simultaneously using a single acquisition.

CONCLUSION

The additional measurement of T1 values for the reaction metabolites provides further biological information about the cellular environment of the metabolites. The new technique was investigated through simulations and in vivo studies of transgenic mouse models of cancer demonstrating improved assessments of kinetic rate constants and new T1 relaxation value measurements for hyperpolarized (13) C-pyruvate, (13) C-lactate, and (13) C-alanine.