Effect of Doxorubicin on Myocardial Bicarbonate Production From Pyruvate Dehydrogenase in Women With Breast Cancer

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.

Hyperpolarized Carbon-13 Magnetic Resonance Imaging: Technical Considerations and Clinical Applications

Hyperpolarized (HP) carbon-13 (13C) MRI represents an innovative approach for noninvasive, real-time assessment of dynamic metabolic flux, with potential integration into routine clinical MRI. The use of [1-13C]pyruvate as a probe and its conversion to [1-13C]lactate constitute an extensively explored metabolic pathway. This review comprehensively outlines the establishment of HP 13C-MRI, covering multidisciplinary team collaboration, hardware prerequisites, probe preparation, hyperpolarization techniques, imaging acquisition, and data analysis. This article discusses the clinical applications of HP 13C-MRI across various anatomical domains, including the brain, heart, skeletal muscle, breast, liver, kidney, pancreas, and prostate. Each section highlights the specific applications and findings pertinent to these regions, emphasizing the potential versatility of HP 13C-MRI in diverse clinical contexts. This review serves as a comprehensive update, bridging technical aspects with clinical applications and offering insights into the ongoing advancements in HP 13C-MRI.

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.

Dynamic T 2 * relaxometry of hyperpolarized [1-13 C]pyruvate MRI in the human brain and kidneys

PURPOSE

This study aimed to quantifyT 2 * $$ {T}_2^{\ast } $$ for hyperpolarized [1-13 C]pyruvate and metabolites in the healthy human brain and renal cell carcinoma (RCC) patients at 3 T.

METHODS

DynamicT 2 * $$ {T}_2^{\ast } $$ values were measured with a metabolite-specific multi-echo spiral sequence. The dynamicT 2 * $$ {T}_2^{\ast } $$ of [1-13 C]pyruvate, [1-13 C]lactate, and 13 C-bicarbonate was estimated in regions of interest in the whole brain, sinus vein, gray matter, and white matter in healthy volunteers, as well as in kidney tumors and the contralateral healthy kidneys in a separate group of RCC patients.T 2 * $$ {T}_2^{\ast } $$ was fit using a mono-exponential function; and metabolism was quantified using pyruvate-to-lactate conversion rate maps and lactate-to-pyruvate ratio maps, which were compared with and without an estimatedT 2 * $$ {T}_2^{\ast } $$ correction.

RESULTS

TheT 2 * $$ {T}_2^{\ast } $$ of pyruvate was shown to vary during the acquisition, whereas theT 2 * $$ {T}_2^{\ast } $$ of lactate and bicarbonate were relatively constant through time and across the organs studied. TheT 2 * $$ {T}_2^{\ast } $$ of lactate was similar in gray matter (29.75 ± 1.04 ms), white matter (32.89 ± 0.9 ms), healthy kidney (34.61 ± 4.07 ms), and kidney tumor (33.01 ± 2.31 ms); and theT 2 * $$ {T}_2^{\ast } $$ of bicarbonate was different between whole-brain (108.17 ± 14.05 ms) and healthy kidney (58.45 ± 6.63 ms). TheT 2 * $$ {T}_2^{\ast } $$ of pyruvate had similar trends in both brain and RCC studies, reducing from 75.56 ± 2.23 ms to 22.24 ± 1.24 ms in the brain and reducing from 122.72 ± 9.86 ms to 57.38 ± 7.65 ms in the kidneys.

CONCLUSION

Multi-echo dynamic imaging can quantifyT 2 * $$ {T}_2^{\ast } $$ and metabolism in a single integrated acquisition. Clear differences were observed in theT 2 * $$ {T}_2^{\ast } $$ of metabolites and in their behavior throughout the timecourse.

Quo Vadis Hyperpolarized 13C MRI?

Over the last two decades, hyperpolarized 13C MRI has gained significance in both preclinical and clinical studies, hereby relying on technologies like PHIP-SAH (ParaHydrogen-Induced Polarization-Side Arm Hydrogenation), SABRE (Signal Amplification by Reversible Exchange), and dDNP (dissolution Dynamic Nuclear Polarization), with dDNP being applied in humans. A clinical dDNP polarizer has enabled studies across 24 sites, despite challenges like high cost and slow polarization. Parahydrogen-based techniques like SABRE and PHIP offer faster, more cost-efficient alternatives but require molecule-specific optimization. The focus has been on imaging metabolism of hyperpolarized probes, which requires long T1, high polarization and rapid contrast generation. Efforts to establish novel probes, improve acquisition techniques and enhance data analysis methods including artificial intelligence are ongoing. Potential clinical value of hyperpolarized 13C MRI was demonstrated primarily for treatment response assessment in oncology, but also in cardiology, nephrology, hepatology and CNS characterization. In this review on biomedical hyperpolarized 13C MRI, we summarize important and recent advances in polarization techniques, probe development, acquisition and analysis methods as well as clinical trials. Starting from those we try to sketch a trajectory where the field of biomedical hyperpolarized 13C MRI might go.

Regional quantification of cardiac metabolism with hyperpolarized [1-13C]-pyruvate CMR evaluated in an oral glucose challenge

BACKGROUND

The heart has metabolic flexibility, which is influenced by fed/fasting states, and pathologies such as myocardial ischemia and hypertrophic cardiomyopathy (HCM). Hyperpolarized (HP) 13C-pyruvate MRI is a promising new tool for non-invasive quantification of myocardial glycolytic and Krebs cycle flux. However, human studies of HP 13C-MRI have yet to demonstrate regional quantification of metabolism, which is important in regional ischemia and HCM patients with asymmetric septal/apical hypertrophy.

METHODS

We developed and applied methods for whole-heart imaging of 13C-pyruvate, 13C-lactate and 13C-bicarbonate, following intravenous administration of [1-13C]-pyruvate. The image acquisition used an autonomous scanning method including bolus tracking, real-time magnetic field calibrations and metabolite-specific imaging. For quantification of metabolism, we evaluated 13C metabolite images, ratio metrics, and pharmacokinetic modeling to provide measurements of myocardial lactate dehydrogenase (LDH) and pyruvate dehydrogenase (PDH) mediated metabolic conversion in 5 healthy volunteers (fasting & 30 min following oral glucose load).

RESULTS

We demonstrate whole heart coverage for dynamic measurement of pyruvate-to-lactate conversion via LDH and pyruvate-to-bicarbonate conversion via PDH at a resolution of 6 × 6 × 21 mm3 (13C-pyruvate) and 12 × 12 × 21 mm3 (13C-lactate, 13C-bicarbonate). 13C-pyruvate and 13C-lactate were detected simultaneously in the RV blood pool, immediately after intravenous injection, reflecting LDH activity in blood. In healthy volunteers, myocardial 13C-pyruvate-SNR, 13C-lactate-SNR, 13C-bicarbonate-SNR, 13C-lactate/pyruvate ratio, 13C-pyruvate-to-lactate conversion rate, kPL, and 13C-pyruvate-to-bicarbonate conversion rate, kPB, all had statistically significant increases following oral glucose challenge. kPB, reflecting PDH activity and pyruvate entering the Krebs Cycle, had the highest correlation with blood glucose levels and was statistically significant.

CONCLUSIONS

We demonstrate first-in-human regional quantifications of cardiac metabolism by HP 13C-pyruvate MRI that aims to reflect LDH and PDH activity.

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 hyperpolarized 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 & 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.

Regional quantification of cardiac metabolism with hyperpolarized [1-13C]-pyruvate MRI evaluated in an oral glucose challenge

Background

The heart has metabolic flexibility, which is influenced by fed/fasting states, and pathologies such as myocardial ischemia and hypertrophic cardiomyopathy (HCM). Hyperpolarized (HP) 13C-pyruvate MRI is a promising new tool for non-invasive quantification of myocardial glycolytic and Krebs cycle flux. However, human studies of HP 13C-MRI have yet to demonstrate regional quantification of metabolism, which is important in regional ischemia and HCM patients with asymmetric septal/apical hypertrophy.

Methods

We developed and applied methods for whole-heart imaging of 13C-pyruvate, 13C-lactate and 13C-bicarbonate, following intravenous administration of [1-13C]-pyruvate. The image acquisition used an autonomous scanning method including bolus tracking, real-time magnetic field calibrations and metabolite-specific imaging. For quantification of metabolism, we evaluated 13C metabolite images, ratio metrics, and pharmacokinetic modeling to provide measurements of myocardial lactate dehydrogenase (LDH) and pyruvate dehydrogenase (PDH) mediated metabolic conversion in 5 healthy volunteers (fasting & 30 min following oral glucose load).

Results

We demonstrate whole heart coverage for dynamic measurement of pyruvate-to-lactate conversion via LDH and pyruvate-to-bicarbonate conversion via PDH at a resolution of 6×6×21 mm3 (13C-pyruvate) and 12×12×21 mm3 (13C-lactate, 13C-bicarbonate) . 13C-pyruvate and 13C-lactate were detected simultaneously in the RV blood pool, immediately after intravenous injection, reflecting LDH activity in blood. In healthy volunteers, myocardial 13C-pyruvate-SNR, 13C-lactate-SNR, 13C-bicarbonate-SNR, 13C-lactate/pyruvate ratio, 13C-pyruvate-to-lactate conversion rate, kPL, and 13C-pyruvate-to-bicarbonate conversion rate, kPB, all had statistically significant increases following oral glucose challenge. kPB, reflecting PDH activity and pyruvate entering the Krebs Cycle, had the highest correlation with blood glucose levels and was statistically significant.

Conclusions

We demonstrate first-in-human regional quantifications of cardiac metabolism by HP 13C-pyruvate MRI that aims to reflect LDH and PDH activity.

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.

A metabolite specific 3D stack-of-spirals bSSFP sequence for improved bicarbonate imaging in hyperpolarized [1-13C]Pyruvate MRI

13C-bicarbonate is a crucial measure of pyruvate oxidation and TCA cycle flux, but is challenging to measure due to its relatively low concentration and thus will greatly benefit from improved signal-to-noise ratio (SNR). To address this, we developed and investigated the feasibility of a 3D stack-of-spirals metabolite-specific balanced steady-state free precession (MS-bSSFP) sequence for improving the SNR and spatial resolution of dynamic 13C-bicarbonate imaging in hyperpolarized [1-13C]pyruvate studies. The bicarbonate MS-bSSFP sequence was evaluated by simulations, phantoms studies, preclinical studies on five rats, brain studies on two healthy volunteers and renal study on one renal cell carcinoma patient. The simulations and phantom results showed that the bicarbonate-specific pulse had minimal perturbation of other metabolites (<1%). In the animal studies, the MS-bSSFP sequence provided an approximately 2.6-3 × improvement in 13C-bicarbonate SNR compared to a metabolite-specific gradient echo (MS-GRE) sequence without altering the bicarbonate or pyruvate kinetics, and the shorter spiral readout in the MS-bSSFP approach reduced blurring. Using the SNR ratio between MS-bSSFP and MS-GRE, the T2 values of bicarbonate and lactate in the rat kidneys were estimated as 0.5 s and 1.1 s, respectively. The in-vivo feasibility of bicarbonate MS-bSSFP sequence was demonstrated in two human brain studies and one renal study. These studies demonstrate the potential of the sequence for in-vivo applications, laying the foundation for future studies to observe this relatively low concentration metabolite with high-quality images and improve measurements of pyruvate oxidation.

Detection of increased pyruvate dehydrogenase flux in the human heart during adenosine stress test using hyperpolarized [1-13C]pyruvate cardiovascular magnetic resonance imaging

BACKGROUND

Hyperpolarized (HP) [1-13C]pyruvate cardiovascular magnetic resonance (CMR) imaging can visualize the uptake and intracellular conversion of [1-13C]pyruvate to either [1-13C]lactate or 13C-bicarbonate depending on the prevailing metabolic state. The aim of the present study was to combine an adenosine stress test with HP [1-13C]pyruvate CMR to detect cardiac metabolism in the healthy human heart at rest and during moderate stress.

METHODS

A prospective descriptive study was performed between October 2019 and August 2020. Healthy human subjects underwent cine CMR and HP [1-13C]pyruvate CMR at rest and during adenosine stress. HP [1-13C]pyruvate CMR images were acquired at the mid-left-ventricle (LV) level. Semi-quantitative assessment of first-pass myocardial [1-13C]pyruvate perfusion and metabolism were assessed. Paired t-tests were used to compare mean values at rest and during stress.

RESULTS

Six healthy subjects (two female), age 29 ± 7 years were studied and no adverse reactions occurred. Myocardial [1-13C]pyruvate perfusion was significantly increased during stress with a reduction in time-to-peak from 6.2 ± 2.8 to 2.7 ± 1.3 s, p = 0.02. This higher perfusion was accompanied by an overall increased myocardial uptake and metabolism. The conversion rate constant (kPL) for lactate increased from 11 ± 9 *10-3 to 20 ± 10 * 10-3 s-1, p = 0.04. The pyruvate oxidation rate (kPB) increased from 4 ± 4 *10-3 to 12 ± 7 *10-3 s-1, p = 0.008. This increase in carbohydrate metabolism was positively correlated with heart rate (R2 = 0.44, p = 0.02).

CONCLUSIONS

Adenosine stress testing combined with HP [1-13C]pyruvate CMR is feasible and well-tolerated in healthy subjects. We observed an increased pyruvate oxidation during cardiac stress. The present study is an important step in the translation of HP [1-13C]pyruvate CMR into clinical cardiac imaging. Trial registration EUDRACT, 2018-003533-15. Registered 4th of December 2018, https://www.clinicaltrialsregister.eu/ctr-search/search?query=2018-003533-15.

Dual-phase imaging of cardiac metabolism using hyperpolarized pyruvate

PURPOSE

Previous cardiac imaging studies using hyperpolarized (HP) [1-¹³ C]pyruvate were acquired at end-diastole (ED). Little is known about the interaction between cardiac cycle and metabolite content in the myocardium. In this study, we compared images of HP pyruvate and products at end-systole (ES) and ED.

METHODS

A dual-phase ¹³ C MRI sequence was implemented to acquire two sequential HP images within a single cardiac cycle at ES and ED during successive R-R intervals in an interleaved manner. Each healthy volunteer (N = 3) received two injections of HP [1-¹³ C]pyruvate for the dual-phase imaging on the short-axis and the vertical long-axis planes. Spatial distribution of HP ¹³ C metabolites at each cardiac phase was correlated to multiphase ¹ H MRI to confirm the mechanical changes. Ratios of myocardial HP metabolites were compared between ES and ED. Segmental analysis was performed on the midcavity short-axis plane.

RESULTS

In addition to mechanical changes, metabolic profiles of the heart detected by HP [1-¹³ C]pyruvate differed between ES and ED. The myocardial signal of [¹³ C]bicarbonate relative to [1-¹³ C]lactate was significantly smaller at ED than the ratio at ES (p < .05), particularly in mid-anterior and mid-inferoseptal segments. The distinct metabolic profiles in the myocardium likely reflect the technical aspects of the imaging approach such as the coronary flow in addition to the cyclical changes in metabolism.

CONCLUSION

The study demonstrates that metabolic profiles of the heart, measured by HP [1-¹³ C]pyruvate, are affected by the cardiac cycle in which that the data are acquired.

Cancer Therapy-Induced Cardiotoxicity-A Metabolic Perspective on Pathogenesis, Diagnosis and Therapy

Long-term cardiovascular complications of cancer therapy are becoming ever more prevalent due to increased numbers of cancer survivors. Cancer therapy-induced cardiotoxicity (CTIC) is an incompletely understood consequence of various chemotherapies, targeted anti-cancer agents and radiation therapy. It is typically detected clinically by a reduction in cardiac left ventricular ejection fraction, assessed by echocardiography. However, once cardiac functional decline is apparent, this indicates irreversible cardiac damage, highlighting a need for the development of diagnostics which can detect CTIC prior to the onset of functional decline. There is increasing evidence to suggest that pathological alterations to cardiac metabolism play a crucial role in the development of CTIC. This review discusses the metabolic alterations and mechanisms which occur in the development of CTIC, with a focus on doxorubicin, trastuzumab, imatinib, ponatinib, sunitinib and radiotherapy. Potential methods to diagnose and predict CTIC prior to functional cardiac decline in the clinic are evaluated, with a view to both biomarker and imaging-based approaches. Finally, the therapeutic potential of therapies which manipulate cardiac metabolism in the context of adjuvant cardioprotection against CTIC is examined. Together, an integrated view of the role of metabolism in pathogenesis, diagnosis and treatment is presented.

Dihydrolipoamide dehydrogenase, pyruvate oxidation, and acetylation-dependent mechanisms intersecting drug iatrogenesis

In human metabolism, pyruvate dehydrogenase complex (PDC) is one of the most intricate and large multimeric protein systems representing a central hub for cellular homeostasis. The worldwide used antiepileptic drug valproic acid (VPA) may potentially induce teratogenicity or a mild to severe hepatic toxicity, where the underlying mechanisms are not completely understood. This work aims to clarify the mechanisms that intersect VPA-related iatrogenic effects to PDC-associated dihydrolipoamide dehydrogenase (DLD; E3) activity. DLD is also a key enzyme of α-ketoglutarate dehydrogenase, branched-chain α-keto acid dehydrogenase, α-ketoadipate dehydrogenase, and the glycine decarboxylase complexes. The molecular effects of VPA will be reviewed underlining the data that sustain a potential interaction with DLD. The drug-associated effects on lipoic acid-related complexes activity may induce alterations on the flux of metabolites through tricarboxylic acid cycle, branched-chain amino acid oxidation, glycine metabolism and other cellular acetyl-CoA-connected reactions. The biotransformation of VPA involves its complete β-oxidation in mitochondria causing an imbalance on energy homeostasis. The drug consequences as histone deacetylase inhibitor and thus gene expression modulator have also been recognized. The mitochondrial localization of PDC is unequivocal, but its presence and function in the nucleus were also demonstrated, generating acetyl-CoA, crucial for histone acetylation. Bridging metabolism and epigenetics, this review gathers the evidence of VPA-induced interference with DLD or PDC functions, mainly in animal and cellular models, and highlights the uncharted in human. The consequences of this interaction may have significant impact either in mitochondrial or in nuclear acetyl-CoA-dependent processes.

Co-Polarized [1-13C]Pyruvate and [1,3-13C2]Acetoacetate Provide a Simultaneous View of Cytosolic and Mitochondrial Redox in a Single Experiment

Cellular redox is intricately linked to energy production and normal cell function. Although the redox states of mitochondria and cytosol are connected by shuttle mechanisms, the redox state of mitochondria may differ from redox in the cytosol in response to stress. However, detecting these differences in functioning tissues is difficult. Here, we employed ¹³C magnetic resonance spectroscopy (MRS) and co-polarized [1-¹³C]pyruvate and [1,3-¹³C₂]acetoacetate ([1,3-¹³C₂]AcAc) to monitor production of hyperpolarized (HP) lactate and β-hydroxybutyrate as indicators of cytosolic and mitochondrial redox, respectively. Isolated rat hearts were examined under normoxic conditions, during low-flow ischemia, and after pretreatment with either aminooxyacetate (AOA) or rotenone. All interventions were associated with an increase in [P_i]/[ATP] measured by ³¹P NMR. In well-oxygenated untreated hearts, rapid conversion of HP [1-¹³C]pyruvate to [1-¹³C]lactate and [1,3-¹³C₂]AcAc to [1,3-¹³C₂]β-hydroxybutyrate ([1,3-¹³C₂]β-HB) was readily detected. A significant increase in HP [1,3-¹³C₂]β-HB but not [1-¹³C]lactate was observed in rotenone-treated and ischemic hearts, consistent with an increase in mitochondrial NADH but not cytosolic NADH. AOA treatments did not alter the productions of HP [1-¹³C]lactate or [1,3-¹³C₂]β-HB. This study demonstrates that biomarkers of mitochondrial and cytosolic redox may be detected simultaneously in functioning tissues using co-polarized [1-¹³C]pyruvate and [1,3-¹³C₂]AcAc and ¹³C MRS and that changes in mitochondrial redox may precede changes in cytosolic redox.

Future Perspectives of Cardiovascular Biomarker Utilization in Cancer Survivors: A Scientific Statement From the American Heart Association

Improving cancer survival represents the most significant effect of precision medicine and personalized molecular and immunologic therapeutics. Cardiovascular health becomes henceforth a key determinant for the direction of overall outcomes after cancer. Comprehensive tissue diagnostic studies undoubtedly have been and continue to be at the core of the fight against cancer. Will a systemic approach integrating circulating blood-derived biomarkers, multimodality imaging technologies, strategic panomics, and real-time streams of digitized physiological data overcome the elusive cardiovascular tissue diagnosis in cardio-oncology? How can such a systemic approach be personalized for application in day-to-day clinical work, with diverse patient populations, cancer diagnoses, and therapies? To address such questions, this scientific statement approaches a broad definition of the biomarker concept. It summarizes the current literature on the utilization of a multitude of established cardiovascular biomarkers at the intersection with cancer. It identifies limitations and gaps of knowledge in the application of biomarkers to stratify the cardiovascular risk before cancer treatment, monitor cardiovascular health during cancer therapy, and detect latent cardiovascular damage in cancer survivors. Last, it highlights areas in biomarker discovery, validation, and clinical application for concerted efforts from funding agencies, scientists, and clinicians at the cardio-oncology nexus.

Preoperative imaging of glioblastoma patients using hyperpolarized 13C pyruvate: Potential role in clinical decision making

Background

Glioblastoma remains incurable despite treatment with surgery, radiation therapy, and cytotoxic chemotherapy, prompting the search for a metabolic pathway unique to glioblastoma cells.¹³C MR spectroscopic imaging with hyperpolarized pyruvate can demonstrate alterations in pyruvate metabolism in these tumors.

Methods

Three patients with diagnostic MRI suggestive of a glioblastoma were scanned at 3 T 1–2 days prior to tumor resection using a ¹³C/¹H dual-frequency RF coil and a ¹³C/¹H-integrated MR protocol, which consists of a series of ¹H MR sequences (T₂ FLAIR, arterial spin labeling and contrast-enhanced [CE] T₁) and ¹³C spectroscopic imaging with hyperpolarized [1-¹³C]pyruvate. Dynamic spiral chemical shift imaging was used for ¹³C data acquisition. Surgical navigation was used to correlate the locations of tissue samples submitted for histology with the changes seen on the diagnostic MR scans and the ¹³C spectroscopic images.

Results

Each tumor was histologically confirmed to be a WHO grade IV glioblastoma with isocitrate dehydrogenase wild type. Total hyperpolarized ¹³C signals detected near the tumor mass reflected altered tissue perfusion near the tumor. For each tumor, a hyperintense [1-¹³C]lactate signal was detected both within CE and T₂-FLAIR regions on the ¹H diagnostic images (P = .008). [¹³C]bicarbonate signal was maintained or decreased in the lesion but the observation was not significant (P = .3).

Conclusions

Prior to surgical resection, ¹³C MR spectroscopic imaging with hyperpolarized pyruvate reveals increased lactate production in regions of histologically confirmed glioblastoma.

Hyperpolarized 13C MR Spectroscopy Depicts in Vivo Effect of Exercise on Pyruvate Metabolism in Human Skeletal Muscle

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

Cardiac T 2 ∗ measurement of hyperpolarized 13 C metabolites using metabolite-selective multi-echo spiral imaging

PURPOSE

Noninvasive imaging with hyperpolarized (HP) pyruvate can capture in vivo cardiac metabolism. For proper quantification of the metabolites and optimization of imaging parameters, understanding MR characteristics such as T 2 ∗ s of the HP signals is critical. This study is to measure in vivo cardiac T 2 ∗ s of HP [1-¹³ C]pyruvate and the products in rodents and humans.

METHODS

A dynamic ¹³ C multi-echo spiral imaging sequence that acquires [¹³ C]bicarbonate, [1-¹³ C]lactate, and [1-¹³ C]pyruvate images in an interleaved manner was implemented for a clinical 3 Tesla system. T 2 ∗ of each metabolite was calculated from the multi-echo images by fitting the signal decay of each region of interest mono-exponentially. The performance of measuring T 2 ∗ using the sequence was first validated using a ¹³ C phantom and then with rodents following a bolus injection of HP [1-¹³ C]pyruvate. In humans, T 2 ∗ of each metabolite was calculated for left ventricle, right ventricle, and myocardium.

RESULTS

Cardiac T 2 ∗ s of HP [1-¹³ C]pyruvate, [1-¹³ C]lactate, and [¹³ C]bicarbonate in rodents were measured as 24.9 ± 5.0, 16.4 ± 4.7, and 16.9 ± 3.4 ms, respectively. In humans, T 2 ∗ of [1-¹³ C]pyruvate was 108.7 ± 22.6 ms in left ventricle and 129.4 ± 8.9 ms in right ventricle. T 2 ∗ of [1-¹³ C]lactate was 40.9 ± 8.3, 44.2 ± 5.5, and 43.7 ± 9.0 ms in left ventricle, right ventricle, and myocardium, respectively. T 2 ∗ of [¹³ C]bicarbonate in myocardium was 64.4 ± 2.5 ms. The measurements were reproducible and consistent over time after the pyruvate injection.

CONCLUSION

The proposed metabolite-selective multi-echo spiral imaging sequence reliably measures in vivo cardiac T 2 ∗ s of HP [1-¹³ C]pyruvate and products.

Characterization and compensation of f 0 inhomogeneity artifact in spiral hyperpolarized 13 C imaging of the human heart

PURPOSE

This study aimed to investigate the role of regional f 0 inhomogeneity in spiral hyperpolarized ¹³ C image quality and to develop measures to alleviate these effects.

METHODS

Field map correction of hyperpolarized ¹³ C cardiac imaging using spiral readouts was evaluated in healthy subjects. Spiral readouts with differing duration (26 and 45 ms) but similar resolution were compared with respect to off-resonance performance and image quality. An f 0 map-based image correction based on the multifrequency interpolation (MFI) method was implemented and compared to correction using a global frequency shift alone. Estimation of an unknown frequency shift was performed by maximizing a sharpness objective based on the Sobel variance. The apparent full width half at maximum (FWHM) of the myocardial wall on [¹³ C]bicarbonate was used to estimate blur.

RESULTS

Mean myocardial wall FWHM measurements were unchanged with the short readout pre-correction (14.1 ± 2.9 mm) and post-MFI correction (14.1 ± 3.4 mm), but significantly decreased in the long waveform (20.6 ± 6.6 mm uncorrected, 17.7 ± 7.0 corrected, P = .007). Bicarbonate signal-to-noise ratio (SNR) of the images acquired with the long waveform were increased by 1.4 ± 0.3 compared to those acquired with the short waveform (predicted 1.32). Improvement of image quality was observed for all metabolites with f 0 correction.

CONCLUSIONS

f 0 -map correction reduced blur and recovered signal from dropouts, particularly along the posterior myocardial wall. The low image SNR of [¹³ C]bicarbonate can be compensated with longer duration readouts but at the expense of increased f 0 artifacts, which can be partially corrected for with the proposed methods.