Non-invasive in vivo assessment of IDH1 mutational status in glioma

Real-Time Monitoring of Hydrolysis Reactions of Pyrophosphates with Dissolution Dynamic Nuclear Polarization

Dissolution dynamic nuclear polarization (d-DNP) has enabled applications such as the real-time monitoring of chemical reactions. Such applications are mainly for 13C and 15N spins with long spin-lattice relaxation times in the molecules of interest. However, the only applications for phosphorus using d-DNP are pH imaging and nucleation during crystallization due to the short relaxation times. Here we show that it is possible to observe enzyme reactions using d-DNP with phosphorus. Hyperpolarized 31P spins in pyrophosphate were obtained using bullet-DNP, which requires less dilution of highly polarized solid samples. Real-time monitoring of the hydrolysis reaction of pyrophosphate by inorganic pyrophosphatase from baker's yeast at physiological pH and was successfully achieved and the reaction rate was determined. This is an important reaction for a wide range of applications related to medicine, agriculture, and quantum life science.

Data Format Standardization and DICOM Integration for Hyperpolarized 13C MRI

Hyperpolarized (HP) 13C MRI has shown promise as a valuable modality for in vivo measurements of metabolism and is currently in human trials at 15 research sites worldwide. With this growth it is important to adopt standardized data storage practices as it will allow sites to meaningfully compare data. In this paper we (1) describe data that we believe should be stored and (2) demonstrate pipelines and methods that utilize the Digital Imaging and Communications in Medicine (DICOM) standard. This includes proposing a set of minimum set of information that is specific to HP 13C MRI studies. We then show where the majority of these can be fit into existing DICOM Attributes, primarily via the "Contrast/Bolus" module. We also demonstrate pipelines for utilizing DICOM for HP 13C MRI. DICOM is the most common standard for clinical medical image storage and provides the flexibility to accommodate the unique aspects of HP 13C MRI, including the HP agent information but also spectroscopic and metabolite dimensions. The pipelines shown include creating DICOM objects for studies on human and animal imaging systems with various pulse sequences. We also show a python-based method to efficiently modify DICOM objects to incorporate the unique HP 13C MRI information that is not captured by existing pipelines. Moreover, we propose best practices for HP 13C MRI data storage that will support future multi-site trials, research studies and technical developments of this imaging technique.

Data Format Standardization and DICOM Integration for Hyperpolarized 13C MRI

Hyperpolarized (HP) 13C MRI has shown promise as a valuable modality for in vivo measurements of metabolism and is currently in human trials at 15 research sites worldwide. With this growth, it is important to adopt standardized data storage practices as it will allow sites to meaningfully compare data. In this paper, we (1) describe data that we believe should be stored and (2) demonstrate pipelines and methods that utilize the Digital Imaging and Communications in Medicine (DICOM) standard. This includes proposing a set of minimum set of information that is specific to HP 13C MRI studies. We then show where the majority of these can be fit into existing DICOM attributes, primarily via the "Contrast/Bolus" module. We also demonstrate pipelines for utilizing DICOM for HP 13C MRI. DICOM is the most common standard for clinical medical image storage and provides the flexibility to accommodate the unique aspects of HP 13C MRI, including the HP agent information but also spectroscopic and metabolite dimensions. The pipelines shown include creating DICOM objects for studies on human and animal imaging systems with various pulse sequences. We also show a python-based method to efficiently modify DICOM objects to incorporate the unique HP 13C MRI information that is not captured by existing pipelines. Moreover, we propose best practices for HP 13C MRI data storage that will support future multi-site trials, research studies, and technical developments of this imaging technique.

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.

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.

Exceptionally Mild and High-Yielding Synthesis of Vinyl Esters of Alpha-Ketocarboxylic Acids, Including Vinyl Pyruvate, for Parahydrogen-Enhanced Metabolic Spectroscopy and Imaging

Metabolic changes often occur long before pathologies manifest and treatment becomes challenging. As key elements of energy metabolism, α-ketocarboxylic acids (α-KCA) are particularly interesting, e.g., as the upregulation of pyruvate to lactate conversion is a hallmark of cancer (Warburg effect). Magnetic resonance imaging with hyperpolarized metabolites has enabled imaging of this effect non-invasively and in vivo, allowing the early detection of cancerous tissue and its treatment. Hyperpolarization by means of dynamic nuclear polarization, however, is complex, slow, and expensive, while available precursors often limit parahydrogen-based alternatives. Here, we report the synthesis for novel 13C, deuterated ketocarboxylic acids, and a much-improved synthesis of 1-13C-vinyl pruvate-d6, arguably the most promising tracer for hyperpolarizing pyruvate using parahydrogen-induced hyperpolarization by side arm hydrogenation. The new synthesis is scalable and provides a high yield of 52%. We elucidated the mechanism of our Pd-catalyzed trans-vinylation reaction. Hydrogenation with parahydrogen allowed us to monitor the addition, which was found to depend on the electron demand of the vinyl ester. Electron-poor α-keto vinyl esters react slower than "normal" alkyl vinyl esters. This synthesis of 13C, deuterated α-ketocarboxylic acids opens up an entirely new class of biomolecules for fast and cost-efficient hyperpolarization with parahydrogen and their use for metabolic imaging.

Fingerprinting and profiling in metabolomics of biosamples

This review focuses on metabolomics from an NMR point of view. It attempts to cover the broad scope of metabolomics and describes the NMR experiments that are most suitable for each sample type. It is addressed not only to NMR specialists, but to all researchers who wish to approach metabolomics with a clear idea of what they wish to achieve but not necessarily with a deep knowledge of NMR. For this reason, some technical parts may seem a bit naïve to the experts. The review starts by describing standard metabolomics procedures, which imply the use of a dedicated 600 MHz instrument and of four properly standardized 1D experiments. Standardization is a must if one wants to directly compare NMR results obtained in different labs. A brief mention is also made of standardized pre-analytical procedures, which are even more essential. Attention is paid to the distinction between fingerprinting and profiling, and the advantages and disadvantages of fingerprinting are clarified. This aspect is often not fully appreciated. Then profiling, and the associated problems of signal assignment and quantitation, are discussed. We also describe less conventional approaches, such as the use of different magnetic fields, the use of signal enhancement techniques to increase sensitivity, and the potential of field-shuttling NMR. A few examples of biomedical applications are also given, again with the focus on NMR techniques that are most suitable to achieve each particular goal, including a description of the most common heteronuclear experiments. Finally, the growing applications of metabolomics to foodstuffs are described.

Recent advances in understanding brain cancer metabolomics: a review

Regardless of the significant progress made in surgical techniques and adjuvant therapies, brain tumors are a major contributor to cancer-related morbidity and mortality in both pediatric and adult populations. Gliomas represent a significant proportion of cerebral neoplasms, exhibiting diverse levels of malignancy. The etiology and mechanisms of resistance of this malignancy are inadequately comprehended, and the optimization of patient diagnosis and prognosis is a challenge due to the diversity of the disease and the restricted availability of therapeutic options. Metabolomics refers to the comprehensive analysis of endogenous and exogenous small molecules, both in a targeted and untargeted manner, that enables the characterization of an individual's phenotype and offers valuable insights into cellular activity, particularly in the context of cancer biology, including brain tumor biology. Metabolomics has garnered attention in current years due to its potential to facilitate comprehension of the dynamic spatiotemporal regulatory network of enzymes and metabolites that enables cancer cells to adapt to their environment and foster the development of tumors. Metabolic changes are widely acknowledged as a significant characteristic for tracking the advancement of diseases, treatment efficacy, and identifying novel molecular targets for successful medical management. Metabolomics has emerged as an exciting area for personalized medicine and drug discovery, utilizing advanced analytical techniques such as nuclear magnetic resonance spectroscopy (MRS) and mass spectrometry (MS) to achieve high-throughput analysis. This review examines and highlights the latest developments in MRS, MS, and other technologies in studying human brain tumor metabolomics.

A Toolbox for Glutamine Use in Dissolution Dynamic Nuclear Polarization: from Enzymatic Reaction Monitoring to the Study of Cellular Metabolic Pathways and Imaging

Glutamine is under scrutiny regarding its metabolic deregulation linked to energetic reprogramming in cancer cells. Many analytical techniques have been used to better understand the impact of the metabolism of amino acids on biological processes, however only a few are suited to work with complex samples. Here, we report the use of a general dissolution dynamic nuclear polarization (D-DNP) formulation using an unexpensive radical as a multipurpose tool to study glutamine, with insights from enzymatic modelling to complex metabolic networks and fast imaging. First, hyperpolarized [5-13 C] glutamine is used as molecular probe to study the kinetic action of two enzymes: L-asparaginase that has been used as an anti-metabolic treatment for cancer, and glutaminase. These results are also compared with those acquired with another hyperpolarized amino acid, [1,4-13 C] asparagine. Second, we explored the use of hyperpolarized (HP) substrates to probe metabolic pathways by monitoring metabolic profiles arising from hyperpolarized glutamine in E. coli extracts. Finally, a highly concentrated sample formulation is proposed for the purpose of fast imaging applications. We think that this approach can be extended to formulate other amino acids as well as other metabolites and provide complementary insights into the analysis of metabolic networks.

Electrochemical Biosensors for Whole Blood Analysis: Recent Progress, Challenges, and Future Perspectives

Whole blood, as one of the most significant biological fluids, provides critical information for health management and disease monitoring. Over the past 10 years, advances in nanotechnology, microfluidics, and biomarker research have spurred the development of powerful miniaturized diagnostic systems for whole blood testing toward the goal of disease monitoring and treatment. Among the techniques employed for whole-blood diagnostics, electrochemical biosensors, as known to be rapid, sensitive, capable of miniaturization, reagentless and washing free, become a class of emerging technology to achieve the target detection specifically and directly in complex media, e.g., whole blood or even in the living body. Here we are aiming to provide a comprehensive review to summarize advances over the past decade in the development of electrochemical sensors for whole blood analysis. Further, we address the remaining challenges and opportunities to integrate electrochemical sensing platforms.

Advanced MR Techniques for Preoperative Glioma Characterization: Part 2

Preoperative clinical MRI protocols for gliomas, brain tumors with dismal outcomes due to their infiltrative properties, still rely on conventional structural MRI, which does not deliver information on tumor genotype and is limited in the delineation of diffuse gliomas. The GliMR COST action wants to raise awareness about the state of the art of advanced MRI techniques in gliomas and their possible clinical translation. This review describes current methods, limits, and applications of advanced MRI for the preoperative assessment of glioma, summarizing the level of clinical validation of different techniques. In this second part, we review magnetic resonance spectroscopy (MRS), chemical exchange saturation transfer (CEST), susceptibility-weighted imaging (SWI), MRI-PET, MR elastography (MRE), and MR-based radiomics applications. The first part of this review addresses dynamic susceptibility contrast (DSC) and dynamic contrast-enhanced (DCE) MRI, arterial spin labeling (ASL), diffusion-weighted MRI, vessel imaging, and magnetic resonance fingerprinting (MRF). EVIDENCE LEVEL: 3. TECHNICAL EFFICACY: Stage 2.

Monitoring response to a clinically relevant IDH inhibitor in glioma-Hyperpolarized 13C magnetic resonance spectroscopy approaches

Background

Mutant isocitrate dehydrogenase (IDHmut) catalyzes 2-hydroxyglutarate (2HG) production and is considered a therapeutic target for IDHmut tumors. However, response is mostly associated with inhibition of tumor growth. Response assessment via anatomic imaging is therefore challenging. Our goal was to directly detect IDHmut inhibition using a new hyperpolarized (HP) 13C magnetic resonance spectroscopy-based approach to noninvasively assess α-ketoglutarate (αKG) metabolism to 2HG and glutamate.

Methods

We studied IDHmut-expressing normal human astrocyte (NHAIDH1mut) cells and rats with BT257 tumors, and assessed response to the IDHmut inhibitor BAY-1436032 (n ≥ 4). We developed a new 13C Echo Planar Spectroscopic Imaging sequence with an optimized RF pulse to monitor the fate of HP [1-13C]αKG and [5-12C,1-13C]αKG with a 2.5 × 2.5 × 8 mm3 spatial resolution.

Results

Cell studies confirmed that BAY-1436032-treatment leads to a drop in HP 2HG and an increase in HP glutamate detectable with both HP substrates. Data using HP [5-12C,1-13C]αKG also demonstrated that its conversion to 2HG is detectable without the proximal 1.1% natural abundance [5-13C]αKG signal. In vivo studies showed that glutamate is produced in normal brains but no 2HG is detectable. In tumor-bearing rats, we detected the production of both 2HG and glutamate, and BAY-1436032-treatment led to a drop in 2HG and an increase in glutamate. Using HP [5-12C,1-13C]αKG we detected metabolism with an signal-to-noise ratio of 23 for 2HG and 17 for glutamate.

Conclusions

Our findings point to the clinical potential of HP αKG, which recently received FDA investigational new drug approval for research, for noninvasive localized imaging of IDHmut status.

Rapid 13C Hyperpolarization of the TCA Cycle Intermediate α-Ketoglutarate via SABRE-SHEATH

α-Ketoglutarate is a key biomolecule involved in a number of metabolic pathways─most notably the TCA cycle. Abnormal α-ketoglutarate metabolism has also been linked with cancer. Here, isotopic labeling was employed to synthesize [1-13C,5-12C,D4]α-ketoglutarate with the future goal of utilizing its [1-13C]-hyperpolarized state for real-time metabolic imaging of α-ketoglutarate analytes and its downstream metabolites in vivo. The signal amplification by reversible exchange in shield enables alignment transfer to heteronuclei (SABRE-SHEATH) hyperpolarization technique was used to create 9.7% [1-13C] polarization in 1 minute in this isotopologue. The efficient 13C hyperpolarization, which utilizes parahydrogen as the source of nuclear spin order, is also supported by favorable relaxation dynamics at 0.4 μT field (the optimal polarization transfer field): the exponential 13C polarization buildup constant Tb is 11.0 ± 0.4 s whereas the 13C polarization decay constant T1 is 18.5 ± 0.7 s. An even higher 13C polarization value of 17.3% was achieved using natural-abundance α-ketoglutarate disodium salt, with overall similar relaxation dynamics at 0.4 μT field, indicating that substrate deuteration leads only to a slight increase (∼1.2-fold) in the relaxation rates for 13C nuclei separated by three chemical bonds. Instead, the gain in polarization (natural abundance versus [1-13C]-labeled) is rationalized through the smaller heat capacity of the "spin bath" comprising available 13C spins that must be hyperpolarized by the same number of parahydrogen present in each sample, in line with previous 15N SABRE-SHEATH studies. Remarkably, the C-2 carbon was not hyperpolarized in both α-ketoglutarate isotopologues studied; this observation is in sharp contrast with previously reported SABRE-SHEATH pyruvate studies, indicating that the catalyst-binding dynamics of C-2 in α-ketoglutarate differ from that in pyruvate. We also demonstrate that 13C spectroscopic characterization of α-ketoglutarate and pyruvate analytes can be performed at natural 13C abundance with an estimated detection limit of 80 micromolar concentration × *%P13C. All in all, the fundamental studies reported here enable a wide range of research communities with a new hyperpolarized contrast agent potentially useful for metabolic imaging of brain function, cancer, and other metabolically challenging diseases.

Application of 7T MRS to High-Grade Gliomas

MRS, including single-voxel spectroscopy and MR spectroscopic imaging, captures metabolites in high-grade gliomas. Emerging evidence indicates that 7T MRS may be more sensitive to aberrant metabolic activity than lower-field strength MRS. However, the literature on the use of 7T MRS to visualize high-grade gliomas has not been summarized. We aimed to identify metabolic information provided by 7T MRS, optimal spectroscopic sequences, and areas for improvement in and new applications for 7T MRS. Literature was found on PubMed using "high-grade glioma," "malignant glioma," "glioblastoma," "anaplastic astrocytoma," "7T," "MR spectroscopy," and "MR spectroscopic imaging." 7T MRS offers higher SNR, modestly improved spatial resolution, and better resolution of overlapping resonances. 7T MRS also yields reduced Cramér-Rao lower bound values. These features help to quantify D-2-hydroxyglutarate in isocitrate dehydrogenase 1 and 2 gliomas and to isolate variable glutamate, increased glutamine, and increased glycine with higher sensitivity and specificity. 7T MRS may better characterize tumor infiltration and treatment effect in high-grade gliomas, though further study is necessary. 7T MRS will benefit from increased sample size; reductions in field inhomogeneity, specific absorption rate, and acquisition time; and advanced editing techniques. These findings suggest that 7T MRS may advance understanding of high-grade glioma metabolism, with reduced Cramér-Rao lower bound values and better measurement of smaller metabolite signals. Nevertheless, 7T is not widely used clinically, and technical improvements are necessary. 7T MRS isolates metabolites that may be valuable therapeutic targets in high-grade gliomas, potentially resulting in wider ranging neuro-oncologic applications.

In-cell NMR: Why and how?

NMR spectroscopy has been applied to cells and tissues analysis since its beginnings, as early as 1950. We have attempted to gather here in a didactic fashion the broad diversity of data and ideas that emerged from NMR investigations on living cells. Covering a large proportion of the periodic table, NMR spectroscopy permits scrutiny of a great variety of atomic nuclei in all living organisms non-invasively. It has thus provided quantitative information on cellular atoms and their chemical environment, dynamics, or interactions. We will show that NMR studies have generated valuable knowledge on a vast array of cellular molecules and events, from water, salts, metabolites, cell walls, proteins, nucleic acids, drugs and drug targets, to pH, redox equilibria and chemical reactions. The characterization of such a multitude of objects at the atomic scale has thus shaped our mental representation of cellular life at multiple levels, together with major techniques like mass-spectrometry or microscopies. NMR studies on cells has accompanied the developments of MRI and metabolomics, and various subfields have flourished, coined with appealing names: fluxomics, foodomics, MRI and MRS (i.e. imaging and localized spectroscopy of living tissues, respectively), whole-cell NMR, on-cell ligand-based NMR, systems NMR, cellular structural biology, in-cell NMR… All these have not grown separately, but rather by reinforcing each other like a braided trunk. Hence, we try here to provide an analytical account of a large ensemble of intricately linked approaches, whose integration has been and will be key to their success. We present extensive overviews, firstly on the various types of information provided by NMR in a cellular environment (the "why", oriented towards a broad readership), and secondly on the employed NMR techniques and setups (the "how", where we discuss the past, current and future methods). Each subsection is constructed as a historical anthology, showing how the intrinsic properties of NMR spectroscopy and its developments structured the accessible knowledge on cellular phenomena. Using this systematic approach, we sought i) to make this review accessible to the broadest audience and ii) to highlight some early techniques that may find renewed interest. Finally, we present a brief discussion on what may be potential and desirable developments in the context of integrative studies in biology.

CXCL14 Promotes a Robust Brain Tumor-Associated Immune Response in Glioma

PURPOSE

The immunosuppressive tumor microenvironment present in the majority of diffuse glioma limits therapeutic response to immunotherapy. As the determinants of the glioma-associated immune response are relatively poorly understood, the study of glioma with more robust tumor-associated immune responses may be particularly useful to identify novel immunomodulatory factors that can promote T-cell effector function in glioma.

EXPERIMENTAL DESIGN

We used multiplex immune-profiling, proteomic profiling, and gene expression analysis to define the tumor-associated immune response in two molecular subtypes of glioma and identify factors that may modulate this response. We then used patient-derived glioma cultures and an immunocompetent murine model for malignant glioma to analyze the ability of tumor-intrinsic factors to promote a CD8+ T-cell response.

RESULTS

As compared with isocitrate dehydrogenase (IDH)-mutant astrocytoma, MAPK-activated pleomorphic xanthoastrocytoma (PXA) harbored increased numbers of activated cytotoxic CD8+ T cells and Iba1+ microglia/macrophages, increased MHC class I expression, enrichment of genes associated with antigen presentation and processing, and increased tumor cell secretion of the chemokine CXCL14. CXCL14 promoted activated CD8+ T-cell chemotaxis in vitro, recruited tumor-infiltrating CD8+ T cells in vivo, and prolonged overall survival in a cytotoxic T-cell-dependent manner. The immunomodulatory molecule B7-H3 was also highly expressed in PXA.

CONCLUSIONS

We identify the MAPK-activated lower grade astrocytoma PXA as having an immune-rich tumor microenvironment and suggest this tumor may be particularly vulnerable to immunotherapeutic modulation. We also identify CXCL14 as an important determinant of the glioma-associated immune microenvironment, sufficient to promote an antitumor CD8+ T-cell response.

Multi-tracer and multiparametric PET imaging to detect the IDH mutation in glioma: a preclinical translational in vitro, in vivo, and ex vivo study

Background

This translational study explores multi-tracer PET imaging for the non-invasive detection of the IDH1 mutation which is a positive prognostic factor in glioma.

Methods

U87 human high-grade glioma (HGG) isogenic cell lines with or without the IDH1 mutation (CRISP/Cas9 method) were stereotactically grafted into rat brains, and examined, in vitro, in vivo and ex vivo. PET imaging sessions, with radiotracers specific for glycolytic metabolism ([¹⁸F]FDG), amino acid metabolism ([¹⁸F]FDopa), and inflammation ([¹⁸F]DPA-714), were performed sequentially during 3–4 days. The in vitro radiotracer uptake was expressed as percent per million cells. For each radiotracer examined in vivo, static analyses included the maximal and mean tumor-to-background ratio (TBR_max and TBR_mean) and metabolic tumor volume (MTV). Dynamic analyses included the distribution volume ratio (DVR) and the relative residence time (RRT) extracted from a reference Logan model. Ex vivo analyses consisted of immunological analyses.

Results

In vitro, IDH1+ cells (i.e. cells expressing the IDH1 mutation) showed lower levels of [¹⁸F]DPA-714 uptake compared to IDH1- cells (p < 0.01). These results were confirmed in vivo with lower [¹⁸F]DPA-714 uptake in IDH+ tumors (3.90 versus 5.52 for TBR_max, p = 0.03). Different values of [¹⁸F]DPA-714 and [¹⁸F] FDopa RRT (respectively 11.07 versus 22.33 and 2.69 versus − 1.81 for IDH+ and IDH- tumors, p < 0.02) were also observed between the two types of tumors. RRT [¹⁸F]DPA-714 provided the best diagnostic performance to discriminate between the two cell lines (AUC of 100%, p < 0.01). Immuno-histological analyses revealed lower expression of Iba-1 and TSPO antibodies in IDH1+ tumors.

Conclusions

[18F]DPA-714 and [18F] FDopa both correlate with the presence of the IDH1 mutation in HGG. These radiotracers are therefore good candidates for translational studies investigating their clinical applications in patients.

Supplementary Information

The online version contains supplementary material available at 10.1186/s40644-022-00454-6.

Metabolic response to radiation therapy in cancer

Tumor metabolism has emerged as a hallmark of cancer and is involved in carcinogenesis and tumor growth. Reprogramming of tumor metabolism is necessary for cancer cells to sustain high proliferation rates and enhanced demands for nutrients. Recent studies suggest that metabolic plasticity in cancer cells can decrease the efficacy of anticancer therapies by enhancing antioxidant defenses and DNA repair mechanisms. Studying radiation-induced metabolic changes will lead to a better understanding of radiation response mechanisms as well as the identification of new therapeutic targets, but there are few robust studies characterizing the metabolic changes induced by radiation therapy in cancer. In this review, we will highlight studies that provide information on the metabolic changes induced by radiation and oxidative stress in cancer cells and the associated underlying mechanisms.

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.

Synthesis of [1-13 C-5-12 C]-alpha-ketoglutarate enables noninvasive detection of 2-hydroxyglutarate

Isocitrate dehydrogenase 1 (IDH1) mutations that generate the oncometabolite 2-hydroxyglutarate (2-HG) from α-ketoglutarate (α-KG) have been identified in many types of tumors and are an important prognostic factor in gliomas. 2-HG production can be determined by hyperpolarized carbon-13 magnetic resonance spectroscopy (HP-¹³ C-MRS) using [1-¹³ C]-α-KG as a probe, but peak contamination from naturally occurring [5-¹³ C]-α-KG overlaps with the [1-¹³ C]-2-HG peak. Via a newly developed oxidative-Stetter reaction, [1-¹³ C-5-¹² C]-α-KG was synthesized. α-KG metabolism was measured via HP-¹³ C-MRS using [1-¹³ C-5-¹² C]-α-KG as a probe. [1-¹³ C-5-¹² C]-α-KG was synthesized in high yields, and successfully eliminated the signal from C5 of α-KG in the HP-¹³ C-MRS spectra. In HCT116 IDH1 R132H cells, [1-¹³ C-5-¹² C]-α-KG allowed for unimpeded detection of [1-¹³ C]-2-HG. ¹² C-enrichment represents a novel method to circumvent spectral overlap, and [1-¹³ C-5-¹² C]-α-KG shows promise as a probe to study IDH1 mutant tumors and α-KG metabolism.

Monitoring Early Glycolytic Flux Alterations Following Radiotherapy in Cancer and Immune Cells: Hyperpolarized Carbon-13 Magnetic Resonance Imaging Study

Alterations in metabolism following radiotherapy affect therapeutic efficacy, although the mechanism underlying such alterations is unclear. A new imaging technique-named dynamic nuclear polarization (DNP) carbon-13 magnetic resonance imaging (MRI)-probes the glycolytic flux in a real-time, dynamic manner. The [1-¹³C]pyruvate is transported by the monocarboxylate transporter (MCT) into cells and converted into [1-¹³C]lactate by lactate dehydrogenase (LDH). To capture the early glycolytic alterations in the irradiated cancer and immune cells, we designed a preliminary DNP ¹³C-MRI study by using hyperpolarized [1-¹³C]pyruvate to study human FaDu squamous carcinoma cells, HMC3 microglial cells, and THP-1 monocytes before and after irradiation. The pyruvate-to-lactate conversion rate (k_PL [Pyr.]) calculated by kinetic modeling was used to evaluate the metabolic alterations. Western blotting was performed to assess the expressions of LDHA, LDHB, MCT1, and MCT4 proteins. Following irradiation, the pyruvate-to-lactate conversion rates on DNP ¹³C-MRI were significantly decreased in the FaDu and the HMC3 cells but increased in the THP-1 cells. Western blot analysis confirmed the similar trends in LDHA and LDHB expression levels. In conclusion, DNP ¹³C-MRI non-invasively captured the different glycolytic alterations among cancer and immune systems in response to irradiation, implying its potential for clinical use in the future.

Tumor Microenvironment-Derived Metabolites: A Guide to Find New Metabolic Therapeutic Targets and Biomarkers

Metabolic reprogramming is a hallmark of cancer that enables cancer cells to grow, proliferate and survive. This metabolic rewiring is intrinsically regulated by mutations in oncogenes and tumor suppressors, but also extrinsically by tumor microenvironment factors (nutrient and oxygen availability, cell-to-cell interactions, cytokines, hormones, etc.). Intriguingly, only a few cancers are driven by mutations in metabolic genes, which lead metabolites with oncogenic properties (i.e., oncometabolites) to accumulate. In the last decade, there has been rekindled interest in understanding how dysregulated metabolism and its crosstalk with various cell types in the tumor microenvironment not only sustains biosynthesis and energy production for cancer cells, but also contributes to immune escape. An assessment of dysregulated intratumor metabolism has long since been exploited for cancer diagnosis, monitoring and therapy, as exemplified by 18F-2-deoxyglucose positron emission tomography imaging. However, the efficient delivery of precision medicine demands less invasive, cheaper and faster technologies to precisely predict and monitor therapy response. The metabolomic analysis of tumor and/or microenvironment-derived metabolites in readily accessible biological samples is likely to play an important role in this sense. Here, we review altered cancer metabolism and its crosstalk with the tumor microenvironment to focus on energy and biomass sources, oncometabolites and the production of immunosuppressive metabolites. We provide an overview of current pharmacological approaches targeting such dysregulated metabolic landscapes and noninvasive approaches to characterize cancer metabolism for diagnosis, therapy and efficacy assessment.

Non-Invasive Differentiation of M1 and M2 Activation in Macrophages Using Hyperpolarized 13C MRS of Pyruvate and DHA at 1.47 Tesla

Macrophage activation, first generalized to the M1/M2 dichotomy, is a complex and central process of the innate immune response. Simply, M1 describes the classical proinflammatory activation, leading to tissue damage, and M2 the alternative activation promoting tissue repair. Given the central role of macrophages in multiple diseases, the ability to noninvasively differentiate between M1 and M2 activation states would be highly valuable for monitoring disease progression and therapeutic responses. Since M1/M2 activation patterns are associated with differential metabolic reprogramming, we hypothesized that hyperpolarized 13C magnetic resonance spectroscopy (HP 13C MRS), an innovative metabolic imaging approach, could distinguish between macrophage activation states noninvasively. The metabolic conversions of HP [1-13C]pyruvate to HP [1-13C]lactate, and HP [1-13C]dehydroascorbic acid to HP [1-13C]ascorbic acid were monitored in live M1 and M2 activated J774a.1 macrophages noninvasively by HP 13C MRS on a 1.47 Tesla NMR system. Our results show that both metabolic conversions were significantly increased in M1 macrophages compared to M2 and nonactivated cells. Biochemical assays and high resolution 1H MRS were also performed to investigate the underlying changes in enzymatic activities and metabolite levels linked to M1/M2 activation. Altogether, our results demonstrate the potential of HP 13C MRS for monitoring macrophage activation states noninvasively.

Hyperpolarized Metabolic MRI-Acquisition, Reconstruction, and Analysis Methods

Hyperpolarized metabolic MRI with 13C-labeled agents has emerged as a powerful technique for in vivo assessments of real-time metabolism that can be used across scales of cells, tissue slices, animal models, and human subjects. Hyperpolarized contrast agents have unique properties compared to conventional MRI scanning and MRI contrast agents that require specialized imaging methods. Hyperpolarized contrast agents have a limited amount of available signal, irreversible decay back to thermal equilibrium, bolus injection and perfusion kinetics, cellular uptake and metabolic conversion kinetics, and frequency shifts between metabolites. This article describes state-of-the-art methods for hyperpolarized metabolic MRI, summarizing data acquisition, reconstruction, and analysis methods in order to guide the design and execution of studies.

In vivo characterization of physiological and metabolic changes related to isocitrate dehydrogenase 1 mutation expcression by multiparametric MRI and MRS in a rat model with orthotopically grafted human-derived glioblastoma cell lines

The physiological mechanism induced by the isocitrate dehydrogenase 1 (IDH1) mutation, associated with better treatment response in gliomas, remains unknown. The aim of this preclinical study was to characterize the IDH1 mutation through in vivo multiparametric MRI and MRS. Multiparametric MRI, including the measurement of blood flow, vascularity, oxygenation, permeability, and in vivo MRS, was performed on a 4.7 T animal MRI system in rat brains grafted with human-derived glioblastoma U87 cell lines expressing or not the IDH1 mutation by the CRISPR/Cas9 method, and secondarily characterized with additional ex vivo HR-MAS and histological analyses. In univariate analyses, compared with IDH1-, IDH1+ tumors exhibited higher vascular density (p < 0.01) and better perfusion (p = 0.02 for cerebral blood flow), but lower vessel permeability (p < 0.01 for time to peak (TTP), p = 0.04 for contrast enhancement) and decreased T1 map values (p = 0.02). Using linear discriminant analysis, vascular density and TTP values were found to be independent MRI parameters for characterizing the IDH1 mutation (p < 0.01). In vivo MRS and ex vivo HR-MAS analysis showed lower metabolites of tumor aggressiveness for IDH1+ tumors (p < 0.01). Overall, the IDH1 mutation exhibited a higher vascularity on MRI, a lower permeability, and a less aggressive metabolic profile. These MRI features may prove helpful to better pinpoint the physiological mechanisms induced by this mutation.

Early Noninvasive Metabolic Biomarkers of Mutant IDH Inhibition in Glioma

Approximately 80% of low-grade glioma (LGGs) harbor mutant isocitrate dehydrogenase 1/2 (IDH1/2) driver mutations leading to accumulation of the oncometabolite 2-hydroxyglutarate (2-HG). Thus, inhibition of mutant IDH is considered a potential therapeutic target. Several mutant IDH inhibitors are currently in clinical trials, including AG-881 and BAY-1436032. However, to date, early detection of response remains a challenge. In this study we used high resolution ¹H magnetic resonance spectroscopy (¹H-MRS) to identify early noninvasive MR (Magnetic Resonance)-detectable metabolic biomarkers of response to mutant IDH inhibition. In vivo ¹H-MRS was performed on mice orthotopically-implanted with either genetically engineered (U87IDHmut) or patient-derived (BT257 and SF10417) mutant IDH1 cells. Treatment with either AG-881 or BAY-1436032 induced a significant reduction in 2-HG. Moreover, both inhibitors led to a significant early and sustained increase in glutamate and the sum of glutamate and glutamine (GLX) in all three models. A transient early increase in N-acetylaspartate (NAA) was also observed. Importantly, all models demonstrated enhanced animal survival following both treatments and the metabolic alterations were observed prior to any detectable differences in tumor volume between control and treated tumors. Our study therefore identifies potential translatable early metabolic biomarkers of drug delivery, mutant IDH inhibition and glioma response to treatment with emerging clinically relevant therapies.

Magnetic resonance spectroscopy for the study of cns malignancies

Despite intensive research, brain tumors are amongst the malignancies with the worst prognosis; therefore, a prompt diagnosis and thoughtful assessment of the disease is required. The resistance of brain tumors to most forms of conventional therapy has led researchers to explore the underlying biology in search of new vulnerabilities and biomarkers. The unique metabolism of brain tumors represents one potential vulnerability and the basis for a system of classification. Profiling this aberrant metabolism requires a method to accurately measure and report differences in metabolite concentrations. Magnetic resonance-based techniques provide a framework for examining tumor tissue and the evolution of disease. Nuclear Magnetic Resonance (NMR) analysis of biofluids collected from patients suffering from brain cancer can provide biological information about disease status. In particular, urine and plasma can serve to monitor the evolution of disease through the changes observed in the metabolic profiles. Moreover, cerebrospinal fluid can be utilized as a direct reporter of cerebral activity since it carries the chemicals exchanged with the brain tissue and the tumor mass. Metabolic reprogramming has recently been included as one of the hallmarks of cancer. Accordingly, the metabolic rewiring experienced by these tumors to sustain rapid growth and proliferation can also serve as a potential therapeutic target. The combination of 13C tracing approaches with the utilization of different NMR spectral modalities has allowed investigations of the upregulation of glycolysis in the aggressive forms of brain tumors, including glioblastomas, and the discovery of the utilization of acetate as an alternative cellular fuel in brain metastasis and gliomas. One of the major contributions of magnetic resonance to the assessment of brain tumors has been the non-invasive determination of 2-hydroxyglutarate (2HG) in tumors harboring a mutation in isocitrate dehydrogenase 1 (IDH1). The mutational status of this enzyme already serves as a key feature in the clinical classification of brain neoplasia in routine clinical practice and pilot studies have established the use of in vivo magnetic resonance spectroscopy (MRS) for monitoring disease progression and treatment response in IDH mutant gliomas. However, the development of bespoke methods for 2HG detection by MRS has been required, and this has prevented the wider implementation of MRS methodology into the clinic. One of the main challenges for improving the management of the disease is to obtain an accurate insight into the response to treatment, so that the patient can be promptly diverted into a new therapy if resistant or maintained on the original therapy if responsive. The implementation of 13C hyperpolarized magnetic resonance spectroscopic imaging (MRSI) has allowed detection of changes in tumor metabolism associated with a treatment, and as such has been revealed as a remarkable tool for monitoring response to therapeutic strategies. In summary, the application of magnetic resonance-based methodologies to the diagnosis and management of brain tumor patients, in addition to its utilization in the investigation of its tumor-associated metabolic rewiring, is helping to unravel the biological basis of malignancies of the central nervous system.

Glutamate Is a Noninvasive Metabolic Biomarker of IDH1-Mutant Glioma Response to Temozolomide Treatment

Although lower grade gliomas are driven by mutations in the isocitrate dehydrogenase 1 (IDH1) gene and are less aggressive than primary glioblastoma, they nonetheless generally recur. IDH1-mutant patients are increasingly being treated with temozolomide, but early detection of response remains a challenge and there is a need for complementary imaging methods to assess response to therapy prior to tumor shrinkage. The goal of this study was to determine the value of magnetic resonance spectroscopy (MRS)-based metabolic changes for detection of response to temozolomide in both genetically engineered and patient-derived mutant IDH1 models. Using 1H MRS in combination with chemometrics identified several metabolic alterations in temozolomide-treated cells, including a significant increase in steady-state glutamate levels. This was confirmed in vivo, where the observed 1H MRS increase in glutamate/glutamine occurred prior to tumor shrinkage. Cells labeled with [1-13C]glucose and [3-13C]glutamine, the principal sources of cellular glutamate, showed that flux to glutamate both from glucose via the tricarboxylic acid cycle and from glutamine were increased following temozolomide treatment. In line with these results, hyperpolarized [5-13C]glutamate produced from [2-13C]pyruvate and hyperpolarized [1-13C]glutamate produced from [1-13C]α-ketoglutarate were significantly higher in temozolomide-treated cells compared with controls. Collectively, our findings identify 1H MRS-detectable elevation of glutamate and hyperpolarized 13C MRS-detectable glutamate production from either pyruvate or α-ketoglutarate as potential translatable metabolic biomarkers of response to temozolomide treatment in mutant IDH1 glioma. SIGNIFICANCE: These findings show that glutamate can be used as a noninvasive, imageable metabolic marker for early assessment of tumor response to temozolomide, with the potential to improve treatment strategies for mutant IDH1 patients.

Emerging Techniques in Imaging of Glioma Microenvironment

Magnetic resonance imaging (MRI) has been the cornerstone of imaging of brain tumors in the past 4 decades. Conventional MRI remains the workhorse for neuro-oncologic imaging, not only for basic information such as location, extent, and navigation but also able to provide information regarding proliferation and infiltration, angiogenesis, hemorrhage, and more. More sophisticated MRI sequences have extended the ability to assess and quantify these features; for example, permeability and perfusion acquisitions can assess blood-brain barrier disruption and angiogenesis, diffusion techniques can assess cellularity and infiltration, and spectroscopy can address metabolism. Techniques such as fMRI and diffusion fiber tracking can be helpful in diagnostic planning for resection and radiation therapy, and more sophisticated iterations of these techniques can extend our understanding of neurocognitive effects of these tumors and associated treatment responses and effects. More recently, MRI has been used to go beyond such morphological, physiological, and functional characteristics to assess the tumor microenvironment. The current review highlights multiple recent and emerging approaches in MRI to characterize the tumor microenvironment.

MR-detectable metabolic biomarkers of response to mutant IDH inhibition in low-grade glioma

Mutations in isocitrate dehydrogenase 1 (IDH1mut) are reported in 70-90% of low-grade gliomas and secondary glioblastomas. IDH1mut catalyzes the reduction of α-ketoglutarate (α-KG) to 2-hydroxyglutarate (2-HG), an oncometabolite which drives tumorigenesis. Inhibition of IDH1mut is therefore an emerging therapeutic approach, and inhibitors such as AG-120 and AG-881 have shown promising results in phase 1 and 2 clinical studies. However, detection of response to these therapies prior to changes in tumor growth can be challenging. The goal of this study was to identify non-invasive clinically translatable metabolic imaging biomarkers of IDH1mut inhibition that can serve to assess response. Methods: IDH1mut inhibition was confirmed using an enzyme assay and 1H- and 13C- magnetic resonance spectroscopy (MRS) were used to investigate the metabolic effects of AG-120 and AG-881 on two genetically engineered IDH1mut-expressing cell lines, NHAIDH1mut and U87IDH1mut. Results:1H-MRS indicated a significant decrease in steady-state 2-HG following treatment, as expected. This was accompanied by a significant 1H-MRS-detectable increase in glutamate. However, other metabolites previously linked to 2-HG were not altered. 13C-MRS also showed that the steady-state changes in glutamate were associated with a modulation in the flux of glutamine to both glutamate and 2-HG. Finally, hyperpolarized 13C-MRS was used to show that the flux of α-KG to both glutamate and 2-HG was modulated by treatment. Conclusion: In this study, we identified potential 1H- and 13C-MRS-detectable biomarkers of response to IDH1mut inhibition in gliomas. Although further studies are needed to evaluate the utility of these biomarkers in vivo, we expect that in addition to a 1H-MRS-detectable drop in 2-HG, a 1H-MRS-detectable increase in glutamate, as well as a hyperpolarized 13C-MRS-detectable change in [1-13C] α-KG flux, could serve as metabolic imaging biomarkers of response to treatment.

Robust detection of oncometabolic aberrations by 1H-13C heteronuclear single quantum correlation in intact biological specimens

Magnetic resonance (MR) spectroscopy has potential to non-invasively detect metabolites of diagnostic significance for precision oncology. Yet, many metabolites have similar chemical shifts, yielding highly convoluted ¹H spectra of intact biological material and limiting diagnostic utility. Here, we show that hydrogen–carbon heteronuclear single quantum correlation (¹H–¹³C HSQC) offers dramatic improvements in sensitivity compared to one-dimensional (1D) ¹³C NMR and significant signal deconvolution compared to 1D ¹H spectra in intact biological settings. Using a standard NMR spectroscope with a cryoprobe but without specialized signal enhancing features such as magic angle spinning, metabolite extractions or ¹³C-isotopic enrichment, we obtain well-resolved 2D ¹H–¹³C HSQC spectra in live cancer cells, in ex vivo freshly dissected xenografted tumors and resected primary tumors. This method can identify tumors with specific oncometabolite alterations such as IDH mutations by 2-hydroxyglutarate and PGD-deleted tumors by gluconate. Results suggest potential of ¹H–¹³C HSQC as a non-invasive diagnostic in precision oncology.

Barekatain et al. demonstrate that hydrogen–carbon heteronuclear single quantum correlation (HSQC) spectra, obtained using a standard NMR spectroscope, can detect tumours with specific oncometabolite alterations including IDH1 mutant glioblastoma, suggesting the feasibility of this method as a diagnostic tool.

Functional Genetic Screening Enables Theranostic Molecular Imaging in Cancer

PURPOSE

Targeted therapies for cancer have accelerated the need for functional imaging strategies that inform therapeutic efficacy. This study assesses the potential of functional genetic screening to integrate therapeutic target identification with imaging probe selection through a proof-of-principle characterization of a therapy-probe pair using dynamic nuclear polarization (DNP)-enhanced magnetic resonance spectroscopic imaging (MRSI).

EXPERIMENTAL DESIGN

CRISPR-negative selection screens from a public dataset were used to identify the relative dependence of 625 cancer cell lines on 18,333 genes. Follow-up screening was performed in hepatocellular carcinoma with a focused CRISPR library targeting imaging-related genes. Hyperpolarized [1-¹³C]-pyruvate was injected before and after lactate dehydrogenase inhibitor (LDHi) administration in male Wistar rats with autochthonous hepatocellular carcinoma. MRSI evaluated intratumoral pyruvate metabolism, while T₂-weighted segmentations quantified tumor growth.

RESULTS

Genetic screening data identified differential metabolic vulnerabilities in 17 unique cancer types that could be imaged with existing probes. Among these, hepatocellular carcinoma required lactate dehydrogenase (LDH) for growth more than the 29 other cancer types in this database. LDH inhibition led to a decrease in lactate generation (P < 0.001) and precipitated dose-dependent growth inhibition (P < 0.01 overall, P < 0.05 for dose dependence). Intratumoral alanine production after inhibition predicted the degree of growth reduction (P < 0.001).

CONCLUSIONS

These findings demonstrate that DNP-MRSI of LDH activity using hyperpolarized [1-¹³C]-pyruvate is a theranostic strategy for hepatocellular carcinoma, enabling quantification of intratumoral LDHi pharmacodynamics and therapeutic efficacy prediction. This work lays the foundation for a novel theranostic platform wherein functional genetic screening informs imaging probe selection to quantify therapeutic efficacy on a cancer-by-cancer basis.

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.

Fast Imaging for Hyperpolarized MR Metabolic Imaging

MRI with hyperpolarized carbon-13 agents has created a new type of noninvasive, in vivo metabolic imaging that can be applied in cell, animal, and human studies. The use of ¹³ C-labeled agents, primarily [1-¹³ C]pyruvate, enables monitoring of key metabolic pathways with the ability to image substrate and products based on their chemical shift. Over 10 sites worldwide are now performing human studies with this new approach for studies of cancer, heart disease, liver disease, and kidney disease. Hyperpolarized metabolic imaging studies must be performed within several minutes following creation of the hyperpolarized agent due to irreversible decay of the net magnetization back to equilibrium, so fast imaging methods are critical. The imaging methods must include multiple metabolites, separated based on their chemical shift, which are also undergoing rapid metabolic conversion (via label exchange), further exacerbating the challenges of fast imaging. This review describes the state-of-the-art in fast imaging methods for hyperpolarized metabolic imaging. This includes the approach and tradeoffs between three major categories of fast imaging methods-fast spectroscopic imaging, model-based strategies, and metabolite specific imaging-as well additional options of parallel imaging, compressed sensing, tailored RF flip angles, refocused imaging methods, and calibration methods that can improve the scan coverage, speed, signal-to-noise ratio (SNR), resolution, and/or robustness of these studies. To date, these approaches have produced extremely promising initial human imaging results. Improvements to fast hyperpolarized metabolic imaging methods will provide better coverage, SNR, resolution, and reproducibility for future human imaging studies. LEVEL OF EVIDENCE: 5 TECHNICAL EFFICACY STAGE: 1.

Exploring Cancer Metabolism: Applications of Metabolomics and Metabolic Phenotyping in Cancer Research and Diagnostics

Altered metabolism is one of the key hallmarks of cancer. The development of sensitive, reproducible and robust bioanalytical tools such as Nuclear Magnetic Resonance Spectroscopy and Mass Spectrometry techniques offers numerous opportunities for cancer metabolism research, and provides additional and exciting avenues in cancer diagnosis, prognosis and for the development of more effective and personalized treatments. In this chapter, we introduce the current state of the art of metabolomics and metabolic phenotyping approaches in cancer research and clinical diagnostics.

Oncogenic R132 IDH1 Mutations Limit NADPH for De Novo Lipogenesis through (D)2-Hydroxyglutarate Production in Fibrosarcoma Sells

Neomorphic mutations in NADP-dependent isocitrate dehydrogenases (IDH1 and IDH2) contribute to tumorigenesis in several cancers. Although significant research has focused on the hypermethylation phenotypes associated with (D)2-hydroxyglutarate (D2HG) accumulation, the metabolic consequences of these mutations may also provide therapeutic opportunities. Here we apply flux-based approaches to genetically engineered cell lines with an endogenous IDH1 mutation to examine the metabolic impacts of increased D2HG production and altered IDH flux as a function of IDH1 mutation or expression. D2HG synthesis in IDH1-mutant cells consumes NADPH at rates similar to de novo lipogenesis. IDH1-mutant cells exhibit increased dependence on exogenous lipid sources for in vitro growth, as removal of medium lipids slows growth more dramatically in IDH1-mutant cells compared with those expressing wild-type or enzymatically inactive alleles. NADPH regeneration may be limiting for lipogenesis and potentially redox homeostasis in IDH1-mutant cells, highlighting critical links between cellular biosynthesis and redox metabolism.

Badur et al. apply metabolic flux analysis to understand how oncogenic mutations in IDH1 alter redox metabolism. Production of (D)2-hydroxyglutarate (D2HG) consumes NADPH at levels similar to de novo lipogenesis, and removal of lipids compromises in vitro growth of IDH1-mutant cells.

Modeling the diffusion of D-2-hydroxyglutarate from IDH1 mutant gliomas in the central nervous system

Background

Among diffusely infiltrative gliomas in adults, 20%-30% contain a point mutation in isocitrate dehydrogenase 1 (IDH1mut), which increases production of D-2-hydroxyglutarate (D2HG). This is so efficient that D2HG often reaches 30 mM within IDH1mut gliomas. Yet, while up to 100 µM D2HG can be detected in the circulating cerebrospinal fluid of IDH1mut glioma patients, the exposure of nonneoplastic cells within and surrounding an IDH1mut glioma to D2HG is unknown and difficult to measure directly.

Methods

Conditioned medium from patient-derived wild type IDH1 (IDH1wt) and IDH1mut glioma cells was analyzed for D2HG by liquid chromatography-mass spectrometry (LC-MS). Mathematical models of D2HG release and diffusion around an IDH1mut glioma were independently generated based on fluid dynamics within the brain and on previously reported intratumoral and cerebrospinal D2HG concentrations.

Results

LC-MS analysis indicates that patient-derived IDH1mut glioma cells release 3.7-97.0 pg D2HG per cell per week. Extrapolating this to an average-sized tumor (30 mL glioma volume and 1 × 108 cells/mL tumor), the rate of D2HG release by an IDH1mut glioma (SA) is estimated at 3.2-83.0 × 10-12 mol/mL/sec. Mathematical models estimate an SA of 2.9-12.9 × 10-12 mol/mL/sec, within the range of the in vitro LC-MS data. In even the most conservative of these models, the extracellular concentration of D2HG exceeds 3 mM within a 2 cm radius from the center of an IDH1mut glioma.

Conclusions

The microenvironment of an IDH1mut glioma is likely being exposed to high concentrations of D2HG, in the low millimolar range. This has implications for understanding how D2HG affects nonneoplastic cells in an IDH1mut glioma.

Cancer in the crosshairs: targeting cancer metabolism with hyperpolarized carbon-13 MRI technology

Magnetic resonance (MR)-based hyperpolarized (HP) 13 C metabolic imaging is under active pursuit as a new clinical diagnostic method for cancer detection, grading, and monitoring of therapeutic response. Following the tremendous success of metabolic imaging by positron emission tomography, which already plays major roles in clinical oncology, the added value of HP 13 C MRI is emerging. Aberrant glycolysis and central carbon metabolism is a hallmark of many forms of cancer. The chemical transformations associated with these pathways produce metabolites ranging in general from three to six carbons, and are dependent on the redox state and energy charge of the tissue. The significant changes in chemistry associated with flux through these pathways imply that HP imaging can take advantage of the underlying chemical shift information encoded into an MR experiment to produce images of the injected substrate as well as its metabolites. However, imaging of HP metabolites poses unique constraints on pulse sequence design related to detection of X-nuclei, decay of the HP magnetization due to T1 , and the consumption of HP signal by the inspection pulses. Advancements in the field continue to depend critically on customization of MRI systems and pulse sequences for optimized detection of HP 13 C signals, focused largely on extracting the maximum amount of information during the short lifetime of the HP magnetization. From a clinical perspective, the success of HP 13 C MRI of cancer will largely depend upon the utility of HP pyruvate for the detection of lactate pools associated with the Warburg effect, though several other agents are also under investigation, with novel agents continually being formulated. In this review, the salient aspects of HP 13 C imaging will be highlighted, with an emphasis on both technological challenges and the biochemical aspects of HP experimental design.

PI3K/mTOR inhibition of IDH1 mutant glioma leads to reduced 2HG production that is associated with increased survival

70-90% of low-grade gliomas and secondary glioblastomas are characterized by mutations in isocitrate dehydrogenase 1 (IDHmut). IDHmut produces the oncometabolite 2-hydroxyglutarate (2HG), which drives tumorigenesis in these tumors. The phosphoinositide-3-kinase (PI3K)/mammalian target of rapamycin (mTOR) pathway represents an attractive therapeutic target for IDHmut gliomas, but noninvasive indicators of drug target modulation are lacking. The goal of this study was therefore to identify magnetic resonance spectroscopy (MRS)-detectable metabolic biomarkers associated with IDHmut glioma response to the dual PI3K/(mTOR) inhibitor XL765. ¹H-MRS of two cell lines genetically modified to express IDHmut showed that XL765 induced a significant reduction in several intracellular metabolites including 2HG. Importantly, examination of an orthotopic IDHmut tumor model showed that enhanced animal survival following XL765 treatment was associated with a significant in vivo ¹H-MRS detectable reduction in 2HG but not with significant inhibition in tumor growth. Further validation is required, but our results indicate that 2HG could serve as a potential noninvasive MRS-detectable metabolic biomarker of IDHmut glioma response to PI3K/mTOR inhibition.

The Continuing Evolution of Molecular Functional Imaging in Clinical Oncology: The Road to Precision Medicine and Radiogenomics (Part II)

The present era of precision medicine sees "cancer" as a consequence of molecular derangements occurring at the commencement of the disease process, with morphological changes happening much later in the process of tumourigenesis. Conventional imaging techniques, such as computed tomography (CT), ultrasound (US) and magnetic resonance imaging (MRI) play an integral role in the detection of disease at the macroscopic level. However, molecular functional imaging (MFI) techniques entail the visualisation and quantification of biochemical and physiological processes occurring during tumourigenesis. MFI has the potential to play a key role in heralding the transition from the concept of "one-size-fits-all" treatment to "precision medicine". Integration of MFI with other fields of tumour biology such as genomics has spawned a novel concept called "radiogenomics", which could serve as an indispensable tool in translational cancer research. With recent advances in medical image processing, such as texture analysis, deep learning and artificial intelligence, the future seems promising; however, their clinical utility remains unproven at present. Despite the emergence of novel imaging biomarkers, the majority of these require validation before clinical translation is possible. In this two part review, we discuss the systematic collaboration across structural, anatomical and molecular imaging techniques that constitute MFI. Part I reviews positron emission tomography, radiogenomics, AI, and optical imaging, while part II reviews MRI, CT and ultrasound, their current status, and recent advances in the field of precision oncology.

Hyperpolarized [6-13C,15N3]-Arginine as a Probe for in Vivo Arginase Activity

Alterations in arginase enzyme expression are linked with various diseases and have been shown to support disease progression, thus motivating the development of an imaging probe for this enzymatic target. ¹³C-enriched arginine can be used as a hyperpolarized (HP) magnetic resonance (MR) probe for arginase flux since the arginine carbon-6 resonance (157 ppm) is converted to urea (163 ppm) following arginase-catalyzed hydrolysis. However, scalar relaxation from adjacent ¹⁴N-nuclei shortens cabon-6 T ₁ and T ₂ times, yielding poor spectral properties. To address these limitations, we report the synthesis of [6-¹³C,¹⁵N₃]-arginine and demonstrate that ¹⁵N-enrichment increases carbon-6 relaxation times, thereby improving signal-to-noise ratio and spectral resolution. By overcoming these limitations with this novel isotope-labeling scheme, we were able to perform in vitro and in vivo arginase activity measurements with HP MR. We present HP [6-¹³C,¹⁵N₃]-arginine as a noninvasive arginase imaging agent for preclinical studies, with the potential for future clinical diagnostic use.

Assessing Therapeutic Efficacy in Real-time by Hyperpolarized Magnetic Resonance Metabolic Imaging

Precisely measuring tumor-associated alterations in metabolism clinically will enable the efficient assessment of therapeutic responses. Advances in imaging technologies can exploit the differences in cancer-associated cell metabolism as compared to normal tissue metabolism, linking changes in target metabolism to therapeutic efficacy. Metabolic imaging by Positron Emission Tomography (PET) employing 2-fluoro-deoxy-glucose ([18F]FDG) has been used as a routine diagnostic tool in the clinic. Recently developed hyperpolarized Magnetic Resonance (HP-MR), which radically increases the sensitivity of conventional MRI, has created a renewed interest in functional and metabolic imaging. The successful translation of this technique to the clinic was achieved recently with measurements of 13C-pyruvate metabolism. Here, we review the potential clinical roles for metabolic imaging with hyperpolarized MRI as applied in assessing therapeutic intervention in different cancer systems.

Emerging Technologies to Image Tissue Metabolism

Due to the implication of altered metabolism in a large spectrum of tissue function and disease, assessment of metabolic processes becomes essential in managing health. In this regard, imaging can play a critical role in allowing observation of biochemical and physiological processes. Nuclear imaging methods, in particular positron emission tomography, have been widely employed for imaging metabolism but are mainly limited by the use of ionizing radiation and the sensing of only one parameter at each scanning session. Observations in healthy individuals or longitudinal studies of disease could markedly benefit from non-ionizing, multi-parameter imaging methods. We therefore focus this review on progress with the non-ionizing radiation methods of MRI, hyperpolarized magnetic resonance and magnetic resonance spectroscopy, chemical exchange saturation transfer, and emerging optoacoustic (photoacoustic) imaging. We also briefly discuss the role of nuclear and optical imaging methods for research and clinical protocols.

Hyperpolarized 13C MRI: Path to Clinical Translation in Oncology

This white paper discusses prospects for advancing hyperpolarization technology to better understand cancer metabolism, identify current obstacles to HP (hyperpolarized) 13C magnetic resonance imaging's (MRI's) widespread clinical use, and provide recommendations for overcoming them. Since the publication of the first NIH white paper on hyperpolarized 13C MRI in 2011, preclinical studies involving [1-13C]pyruvate as well a number of other 13C labeled metabolic substrates have demonstrated this technology's capacity to provide unique metabolic information. A dose-ranging study of HP [1-13C]pyruvate in patients with prostate cancer established safety and feasibility of this technique. Additional studies are ongoing in prostate, brain, breast, liver, cervical, and ovarian cancer. Technology for generating and delivering hyperpolarized agents has evolved, and new MR data acquisition sequences and improved MRI hardware have been developed. It will be important to continue investigation and development of existing and new probes in animal models. Improved polarization technology, efficient radiofrequency coils, and reliable pulse sequences are all important objectives to enable exploration of the technology in healthy control subjects and patient populations. It will be critical to determine how HP 13C MRI might fill existing needs in current clinical research and practice, and complement existing metabolic imaging modalities. Financial sponsorship and integration of academia, industry, and government efforts will be important factors in translating the technology for clinical research in oncology. This white paper is intended to provide recommendations with this goal in mind.

Noninvasively evaluating the grading and IDH1 mutation status of diffuse gliomas by three-dimensional pseudo-continuous arterial spin labeling and diffusion-weighted imaging

PURPOSE

To noninvasively evaluate the value of three-dimensional pseudo-continuous arterial spin labeling (3D pCASL) and diffusion-weighted imaging (DWI) in diffuse gliomas grading as well as isocitrate dehydrogenase (IDH) 1 mutation status.

METHODS

Fifty-six patients with pathologically confirmed diffuse gliomas with preoperative 3D pCASL and DWI were enrolled in this study. The Student's t test and Mann-Whitney U test were used to evaluate differences in parameters of DWI and 3D pCASL between low and high grade as well as between mutant and wild-type IDH1 diffuse gliomas; receiver operator characteristic (ROC) analysis was used to assess the diagnostic performance. Subsequently, a multivariate stepwise logistic regression analysis was used to identify the independent parameters. Besides, Kruskal-Wallis H test was used to examine the differences among grades II, III, and IV diffuse gliomas.

RESULTS

All parameters but CBF_mean showed significant differences between low- and high-grade diffuse gliomas. In ROC analysis, the AUC of CBF_max, rCBF_mean, rCBF_max, ADC_mean, and ADC_min were 0.701, 0.730, 0.746, 0.810, and 0.856 respectively. Only the value of ADC_min was identified as the independent parameter in the differentiation of low- from high-grade diffuse gliomas. All parameters but CBF_mean showed significant differences among the three grades. And the values of CBF_mean, CBF_max, rCBF_mean, and ADC_mean showed significant differences between mutant and wild-type IDH1 in grade II-III astrocytoma.

CONCLUSION

Both 3D pCASL and DWI could be useful tools for distinguishing low- from high-grade diffuse gliomas and have the potential to differentiate different IDH1 mutation statuses of astrocytoma.

In Vivo Imaging of Glutamine Metabolism to the Oncometabolite 2-Hydroxyglutarate in IDH1/2 Mutant Tumors

The oncometabolite 2-hydroxyglutarate (2-HG) is a signature biomarker in various cancers, where it accumulates as a result of mutations in isocitrate dehydrogenase (IDH). The metabolic source of 2-HG, in a wide variety of cancers, dictates both its generation and also potential therapeutic strategies, but this remains difficult to access in vivo. Here, utilizing patient-derived chondrosarcoma cells harboring endogenous mutations in IDH1 and IDH2, we report that 2-HG can be rapidly generated from glutamine in vitro. Then, using hyperpolarized magnetic resonance imaging (HP-MRI), we demonstrate that in vivo HP [1-¹³C] glutamine can be used to non-invasively measure glutamine-derived HP 2-HG production. This can be readily modulated utilizing a selective IDH1 inhibitor, opening the door to targeting glutamine-derived 2-HG therapeutically. Rapid rates of HP 2-HG generation in vivo further demonstrate that, in a context-dependent manner, glutamine can be a primary carbon source for 2-HG production in mutant IDH tumors.