A New Horizon of DNP technology: Application to In-vivo 13C Magnetic Resonance Spectroscopy and Imaging

Cocrystalline Matrices for Hyperpolarization at Room Temperature Using Photoexcited Electrons

We propose using cocrystals as effective polarization matrices for triplet dynamic nuclear polarization (DNP) at room temperature. The polarization source can be uniformly doped into cocrystals formed through acid-acid, amide-amide, and acid-amide synthons. The dense-packing crystal structures, facilitated by multiple hydrogen bonding and π-π interactions, result in extended T1 relaxation times, enabling efficient polarization diffusion within the crystals. Our study demonstrates the successful polarization of a DNP-magnetic resonance imaging molecular probe, such as urea, within a cocrystal matrix at room temperature using triplet-DNP.

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.

Heterogeneous Overhauser-DNP on 1 H dominated by scalar coupling in aqueous solution

The Overhauser Dynamic Nuclear Polarization (O-DNP) of ¹ H nuclei usually involves a dipolar coupling with the polarizing agent, whereas scalar coupling via hyperfine interactions are more common with ¹³ C nuclei. Here, we show a scalar-coupling dominated ¹ H O-DNP, using polyaniline as a heterogeneous polarizing agent in an aqueous solution.

Metabolic Imaging Using Hyperpolarization for Assessment of Premalignancy

There is an unmet need for noninvasive surrogate markers that can help identify premalignant lesions across different tumor types. Here we describe the methodology and technical details of protocols employed for in vivo 13C pyruvate metabolic imaging experiments. The goal of the method described is to identify and understand metabolic changes, to enable detection of pancreatic premalignant lesions, as a proof of concept of the high sensitivity of this imaging modality.

Current Trends in Non-Invasive Imaging of Interactions in the Liver Tumor Microenvironment Mediated by Tumor Metabolism

Simple Summary

Deregulated tumor metabolism is known to shape the tumor microenvironment and directly affect the local immune response, promoting tumor growth, metastasis, and resistance to treatment. However, the metabolic profile or tumor cells, and therefore, the composition of their microenvironment, are highly variable among patients and even within the same tumor, resulting in heterogeneous response rates to oncologic therapies, making patient selection a key issue. This review article focuses on non-invasive imaging techniques that aim to visualize the crosstalk between tumor cells and their microenvironment in liver cancer mediated by tumor metabolism. In addition to improved tumor detection, such imaging tools may be able to provide a more accurate characterization of the individual tumor and ultimately improve understanding, as well as guide personalized treatment regimens for patients with liver cancer.

Abstract

With the increasing understanding of resistance mechanisms mediated by the metabolic reprogramming in cancer cells, there is a growing clinical interest in imaging technologies that allow for the non-invasive characterization of tumor metabolism and the interactions of cancer cells with the tumor microenvironment (TME) mediated through tumor metabolism. Specifically, tumor glycolysis and subsequent tissue acidosis in the realms of the Warburg effect may promote an immunosuppressive TME, causing a substantial barrier to the clinical efficacy of numerous immuno-oncologic treatments. Thus, imaging the varying individual compositions of the TME may provide a more accurate characterization of the individual tumor. This approach can help to identify the most suitable therapy for each individual patient and design new targeted treatment strategies that disable resistance mechanisms in liver cancer. This review article focuses on non-invasive positron-emission tomography (PET)- and MR-based imaging techniques that aim to visualize the crosstalk between tumor cells and their microenvironment in liver cancer mediated by tumor metabolism.

Early Detection of Pancreatic Intraepithelial Neoplasias (PanINs) in Transgenic Mouse Model by Hyperpolarized 13C Metabolic Magnetic Resonance Spectroscopy

While pancreatic cancer (PC) survival rates have recently shown modest improvement, the disease remains largely incurable. Early detection of pancreatic cancer may result in improved outcomes and therefore, methods for early detection of cancer, even premalignant lesions, may provide more favorable outcomes. Pancreatic intraepithelial neoplasias (PanINs) have been identified as premalignant precursor lesions to pancreatic cancer. However, conventional imaging methods used for screening high-risk populations do not have the sensitivity to detect PanINs. Here, we have employed hyperpolarized metabolic imaging in vivo and nuclear magnetic resonance (¹H-NMR) metabolomics ex vivo to identify and understand metabolic changes, towards enabling detection of early PanINs and progression to advanced PanINs lesions that precede pancreatic cancer formation. Progression of disease from tissue containing predominantly low-grade PanINs to tissue with high-grade PanINs showed a decreasing alanine/lactate ratio from high-resolution NMR metabolomics ex vivo. Hyperpolarized magnetic resonance spectroscopy (HP-MRS) allows over 10,000-fold sensitivity enhancement relative to conventional magnetic resonance. Real-time HP-MRS was employed to measure non-invasively changes of alanine and lactate metabolites with disease progression and in control mice in vivo, following injection of hyperpolarized [1-¹³C] pyruvate. The alanine-to-lactate signal intensity ratio was found to decrease as the disease progressed from low-grade PanINs to high-grade PanINs. The biochemical changes of alanine transaminase (ALT) and lactate dehydrogenase (LDH) enzyme activity were assessed. These results demonstrate that there are significant alterations of ALT and LDH activities during the transformation from early to advanced PanINs lesions. Furthermore, we demonstrate that real-time conversion kinetic rate constants (k_PA and k_PL) can be used as metabolic imaging biomarkers of pancreatic premalignant lesions. Findings from this emerging HP-MRS technique can be translated to the clinic for detection of pancreatic premalignant lesion in high-risk populations.

Probing carbohydrate metabolism using hyperpolarized 13 C-labeled molecules

Glycolysis is a fundamental metabolic process in all organisms. Anomalies in glucose metabolism are linked to various pathological conditions. In particular, elevated aerobic glycolysis is a characteristic feature of rapidly growing cells. Glycolysis and the closely related pentose phosphate pathway can be monitored in real time by hyperpolarized 13 C-labeled metabolic substrates such as 13 C-enriched, deuterated D-glucose derivatives, [2-13 C]-D-fructose, [2-13 C] dihydroxyacetone, [1-13 C]-D-glycerate, [1-13 C]-D-glucono-δ-lactone and [1-13 C] pyruvate in healthy and diseased tissues. Elevated glycolysis in tumors (the Warburg effect) was also successfully imaged using hyperpolarized [U-13 C6 , U-2 H7 ]-D-glucose, while the size of the preexisting lactate pool can be measured by 13 C MRS and/or MRI with hyperpolarized [1-13 C]pyruvate. This review summarizes the application of various hyperpolarized 13 C-labeled metabolites to the real-time monitoring of glycolysis and related metabolic processes in normal and diseased tissues.

Combining Hyperpolarized Real-Time Metabolic Imaging and NMR Spectroscopy To Identify Metabolic Biomarkers in Pancreatic Cancer

Pancreatic ductal adenocarcinoma (PDAC) is a deadly cancer that progresses without any symptom, and oftentimes, it is detected at an advanced stage. The lack of prior symptoms and effective treatments have created a knowledge gap in the management of this lethal disease. This issue can be addressed by developing novel noninvasive imaging-based biomarkers in PDAC. We explored in vivo hyperpolarized (HP) ¹³C MRS of pyruvate to lactate conversion and ex vivo ¹H NMR spectroscopy in a panel of well-annotated patient-derived PDAC xenograft (PDXs) model and investigated the correlation between aberrant glycolytic metabolism and aggressiveness of the tumor. Real-time metabolic imaging data demonstrate the immediate intracellular conversion of HP ¹³C pyruvate to lactate after intravenous injection interrogating upregulated lactate dehydrogenase (LDH) activity in aggressive PDXs. Total ex vivo lactate measurement by ¹H NMR spectroscopy showed a direct correlation with in vivo dynamic pyruvate-to-lactate conversion and demonstrated the potential of dynamic metabolic flux as a biomarker of total lactate concentration and aggressiveness of the tumor. Furthermore, the metabolite concentrations were very distinct among all four tumor types analyzed in this study. Overexpression of LDH-A and hypoxia-inducible factor (HIF-1α) plays a significant role in the conversion kinetics of HP pyruvate-to-lactate in tumors. Collectively, these data identified aberrant metabolic characteristics of pancreatic cancer PDXs and could potentially delineate metabolic targets for therapeutic intervention. Metabolic imaging with HP pyruvate and NMR metabolomics may enable identification and classification of aggressive subtypes of patient-derived xenografts. Translation of this real-time metabolic technique to the clinic may have the potential to improve the management of patients at high risk of developing pancreatic diseases.

Prostate Cancer Energetics and Biosynthesis

Cancers must alter their metabolism to satisfy the increased demand for energy and to produce building blocks that are required to create a rapidly growing tumor. Further, for cancer cells to thrive, they must also adapt to an often changing tumor microenvironment, which can present new metabolic challenges (ex. hypoxia) that are unfavorable for most other cells. As such, altered metabolism is now considered an emerging hallmark of cancer. Like many other malignancies, the metabolism of prostate cancer is considerably different compared to matched benign tissue. However, prostate cancers exhibit distinct metabolic characteristics that set them apart from many other tumor types. In this chapter, we will describe the known alterations in prostate cancer metabolism that occur during initial tumorigenesis and throughout disease progression. In addition, we will highlight upstream regulators that control these metabolic changes. Finally, we will discuss how this new knowledge is being leveraged to improve patient care through the development of novel biomarkers and metabolically targeted therapies.

Metabolic Differences in Glutamine Utilization Lead to Metabolic Vulnerabilities in Prostate Cancer

The new oncologic paradigm of precision medicine is focused on identifying metabolic, proteomic, transcriptomic and genomic variabilities in tumors that can be exploited to tailor treatments and improve patient outcomes. Metabolic changes are a hallmark of cancer, and inhibition of metabolic pathways is now a major strategy in medicinal chemistry for targeting cancers. However, non-invasive biomarkers to categorize metabolic subtypes are in short supply. The purpose of this study was to characterize the intracellular and extracellular metabolic profiles of four prostate cancer cell lines with varying degrees of aggressiveness. We observed metabolic differences between the aggressive prostate cancer cell line PC3 and the even more aggressive, metastatic subline PC3M assessed by hyperpolarized in vivo pyruvate studies, nuclear magnetic resonance spectroscopy, and carbon-13 feeding studies. On further examination of the differences between these two cell lines, we found increased glutamine utilization in the metastatic PC3M subline that led directly to sensitivity to glutaminase inhibitor CB-839. Our study supports the theory that metastatic progression increases glutamine utilization and the inhibition of glutaminolysis could have clinical implications.

New frontiers in metabolomics: from measurement to insight

Metabolomics is the newest addition to the "omics" disciplines and has shown rapid growth in its application to human health research because of fundamental advancements in measurement and analysis techniques. Metabolomics has unique and proven advantages in systems biology and biomarker discovery. The next generation of analysis techniques promises even richer and more complete analysis capabilities that will enable earlier clinical diagnosis, drug refinement, and personalized medicine. A review of current advancements in methodologies and statistical analysis that are enhancing and improving the performance of metabolomics is presented along with highlights of some recent successful applications.

Liquid-state carbon-13 hyperpolarization generated in an MRI system for fast imaging

Hyperpolarized (HP) tracers dramatically increase the sensitivity of magnetic resonance imaging (MRI) to monitor metabolism non-invasively and in vivo. Their production, however, requires an extra polarizing device (polarizer) whose complexity, operation and cost can exceed that of an MRI system itself. Furthermore, the lifetime of HP tracers is short and some of the enhancement is lost during transfer to the application site. Here, we present the production of HP tracers in water without an external polarizer: by Synthesis Amid the Magnet Bore, A Dramatically Enhanced Nuclear Alignment (SAMBADENA) is achieved within seconds, corresponding to a hyperpolarization of ∼20%. As transfer of the tracer is no longer required, SAMBADENA may permit a higher polarization at the time of detection at a fraction of the cost and complexity of external polarizers. This development is particularly promising in light of the recently extended portfolio of biomedically relevant para-hydrogen-tracers and may lead to new diagnostic applications.

Hyperpolarized MRI uses molecules with a nuclear spin polarization beyond the thermodynamic equilibrium to enhance imaging contrast. Here, Schmidt et al. enable a single MRI system to both generate a hyperpolarized tracer and perform imaging, eliminating the need for an external polarizer.

Nuclear magnetic resonance studies of DNP-ready trehalose obtained by solid state mechanochemical amorphization

(1)H nuclear spin-lattice relaxation and Dynamic Nuclear Polarization (DNP) have been studied in amorphous samples of trehalose sugar doped with TEMPO radicals by means of mechanical milling, in the 1.6-4.2 K temperature range. The radical concentration was varied between 0.34 and 0.81%. The highest polarization of 15% at 1.6 K, observed in the sample with concentration 0.50%, is of the same order of magnitude of that reported in standard frozen solutions with TEMPO. The temperature and concentration dependence of the spin-lattice relaxation rate 1/T1, dominated by the coupling with the electron spins, were found to follow power laws with an exponent close to 3 in all samples. The observed proportionality between 1/T1 and the polarization rate 1/Tpol, with a coefficient related to the electron polarization, is consistent with the presence of Thermal Mixing (TM) and a good contact between the nuclear and the electron spins. At high electron concentration additional relaxation channels causing a decrease in the nuclear polarization must be considered. These results provide further support for a more extensive use of amorphous DNP-ready samples, obtained by means of comilling, in dissolution DNP experiments and possibly for in vivo metabolic imaging.

The role of the glassy dynamics and thermal mixing in the dynamic nuclear polarization and relaxation mechanisms of pyruvic acid

The temperature dependence of (1)H and (13)C nuclear spin-lattice relaxation rate 1/T1 has been studied in the 1.6-4.2 K temperature range in pure pyruvic acid and in pyruvic acid containing trityl radicals at a concentration of 15 mM. The temperature dependence of 1/T1 is found to follow a quadratic power law for both nuclei in the two samples. Remarkably the same temperature dependence is displayed also by the electron spin-lattice relaxation rate 1/T1e in the sample containing radicals. These results are explained by considering the effect of the structural dynamics on the relaxation rates in pyruvic acid. Dynamic nuclear polarization experiments show that below 4 K the (13)C build up rate scales with 1/T1e, in analogy to (13)C 1/T1 and consistently with a thermal mixing scenario where all the electrons are collectively involved in the dynamic nuclear polarization process and the nuclear spin reservoir is in good thermal contact with the electron spin system.

Nanodiamond as a New Hyperpolarizing Agent and Its (13)C MRS

In this work, we have hyperpolarized carbonaceous nanoparticles (D ≈ 10 nm), that is, "nanodiamonds", with 1.1% (13)C (natural abundance) using dynamic nuclear polarization (DNP). The polarization buildup curve showed a signal enhancement with relative intensity up to 4700 at 1.4 K and 100 mW microwave power. (13)C magnetic resonance spectra (MRS) were obtained from the sample at 7 T, and the signal decayed with a T1 of 55 ± 3s. Notably, polarization was possible in the absence of added radical, consistent with previous results showing endogenous unpaired electrons in natural nanodiamonds. These likely contribute to the shorter T1's compared to those of highly pure diamond. Despite the relatively short T1, these observations suggest that natural nanodiamonds may be useful for in vivo applications.