Imaging pH with hyperpolarized 13C

Parahydrogen-enhanced pH measurements using [1-13C]bicarbonate derived from non-enzymatic decarboxylation of [1-13C]pyruvate-d3

Alterations in pH are a hallmark in several pathologies including cancer, ischemia, and inflammation. Non-invasive magnetic resonance methods to measure pH offer a new approach for early diagnosis of diseases characterized by acid-base imbalances. The hyperpolarization with parahydrogen-induced polarization (PHIP) enhances inherently low signals in magnetic resonance experiments by several orders of magnitude and offers a suitable platform to obtain biocompatible markers in less than one minute. Here, we present an optimized preparation of an hyperpolarized H13CO3-/13CO2 pH sensor via non-enzymatic decarboxylation with H2O2 of [1-13C]pyruvate-d3 obtained by PHIP at 7 T. An improved 13C polarization of purified [1-13C]pyruvate-d3 in water with 36.65 ± 0.06% polarization was obtained starting from 50 mM precursor. Subsequent decarboxylation, H13CO3-/13CO2 exhibited 12.46 ± 0.01% of polarization at physiological pH, 45 seconds after the reaction start. Considering the dilution factor that [1-13C]pyruvate-d3 exhibits in vivo, we optimized our methodology to test the accuracy of the pH sensor at single digit millimolar concentration. In vitro pH estimations on phantoms and cell culture media demonstrated accurate pH calculations with uncertainties of less than 0.08 units. These promising results highlight the efficiency of a pH sensor generated via PHIP in less than one minute, with remarkable polarization, and biocompatibility suitable for future in vivo studies.

Clinically Translatable Hyperpolarized 13C Bicarbonate pH Imaging Method for Use in Prostate Cancer

Solid tumors such as prostate cancer (PCa) commonly develop an acidic microenvironment with pH 6.5-7.2, owing to heterogeneous perfusion, high metabolic activity, and rapid cell proliferation. In preclinical prostate cancer models, disease progression is associated with a decrease in tumor extracellular pH, suggesting that pH imaging may reflect an imaging biomarker to detect aggressive and high-risk disease. Therefore, we developed a hyperpolarized carbon-13 MRI method to image the tumor extracellular pH (pHe) and prepared it for clinical translation for detection and risk stratification of PCa. This method relies on the rapid breakdown of hyperpolarized (HP) 1,2-glycerol carbonate (carbonyl-13C) via base-catalyzed hydrolysis to produce HP 13CO32-, which is neutralized and converted to HP H13CO3-. After injection, HP H13CO3- equilibrates with HP 13CO2 in vivo and enables the imaging of pHe. Using insights gleaned from mechanistic studies performed in the hyperpolarized state, we solved issues of polarization loss during preparation in a clinical polarizer system. We successfully customized a reaction apparatus suitable for clinical application, developed clinical standard operating procedures, and validated the radiofrequency pulse sequence and imaging data acquisition with a wide range of animal models. The results demonstrated that we can routinely produce a highly polarized and safe HP H13CO3- contrast agent suitable for human injection. Preclinical imaging studies validated the reliability and accuracy of measuring acidification in healthy kidney and prostate tumor tissue. These methods were used to support an Investigational New Drug application to the U.S. Food and Drug Administration. This methodology is now ready to be implemented in human trials, with the ultimate goal of improving the management of PCa.

The crosstalking of lactate-Histone lactylation and tumor

Lactate was once considered to be a by-product of energy metabolism, but its unique biological value was only gradually explored with the advent of the Warburg effect. As an end product of glycolysis, lactate can act as a substrate for energy metabolism, a signal transduction molecule, a regulator of the tumor microenvironment and immune cells, and a regulator of the deubiquitination of specific enzymes, and is involved in various biological aspects of tumor regulation, including energy shuttling, growth and invasion, angiogenesis and immune escape. Furthermore, we describe a novel lactate-dependent epigenetic modification, namely histone lactylation modification, and review the progress of its study in tumors, mainly involving the reprogramming of tumor phenotypes, regulation of related gene expression, mediation of the glycolytic process in tumor stem cells (CSCs) and influence on the tumor immune microenvironment. The study of epigenetic regulation of tumor genes by histone modification is still in its infancy, and we expect that by summarizing the effects of lactate and histone modification on tumor and related gene regulation, we will clarify the scientific significance of future histone modification studies and the problems to be solved, and open up new fields for targeted tumor therapy.

Development of Dissolution Dynamic Nuclear Polarization of [15 N3 ]Metronidazole: A Clinically Approved Antibiotic

We report dissolution Dynamic Nuclear Polarization (d-DNP) of [15 N3 ]metronidazole ([15 N3 ]MNZ) for the first time. Metronidazole is a clinically approved antibiotic, which can be potentially employed as a hypoxia-sensing molecular probe using 15 N hyperpolarized (HP) nucleus. The DNP process is very efficient for [15 N3 ]MNZ with an exponential build-up constant of 13.8 min using trityl radical. After dissolution and sample transfer to a nearby 4.7 T Magnetic Resonance Imaging scanner, HP [15 N3 ]MNZ lasted remarkably long with T1 values up to 343 s and 15 N polarizations up to 6.4 %. A time series of HP [15 N3 ]MNZ images was acquired in vitro using a steady state free precession sequence on the 15 NO2 peak. The signal lasted over 13 min with notably long T2 of 20.5 s. HP [15 N3 ]MNZ was injected in the tail vein of a healthy rat, and dynamic spectroscopy was performed over the rat brain. The in vivo HP 15 N signals persisted over 70 s, demonstrating an unprecedented opportunity for in vivo studies.

Nanomaterials for hyperpolarized nuclear magnetic resonance and magnetic resonance imaging

Nanomaterials play an important role in the development and application of hyperpolarized materials for magnetic resonance imaging (MRI). In this context they can not only act as hyperpolarized materials which are directly imaged but also play a role as carriers for hyperpolarized gases and catalysts for para-hydrogen induced polarization (PHIP) to generate hyperpolarized substrates for metabolic imaging. Those three application possibilities are discussed, focusing on carbon-based materials for the directly imaged particles. An overview over recent developments in all three fields is given, including the early developments in each field as well as important steps towards applications in MRI, such as making the initially developed methods more biocompatible and first imaging experiments with spatial resolution in either phantoms or in vivo studies. Focusing on the important features nanomaterials need to display to be applicable in the MRI context, a wide range of different approaches to that extent is covered, giving the reader a general idea of different possibilities as well as recent developments in those different fields of hyperpolarized magnetic resonance. This article is categorized under: Therapeutic Approaches and Drug Discovery > Emerging Technologies Diagnostic Tools > In Vivo Nanodiagnostics and Imaging.

Intraperitoneal Delivery of Iopamidol to Assess Extracellular pH of Orthotopic Pancreatic Tumor Model by CEST-MRI

The extracellular pH (pHe) of solid tumors is often acidic, as a consequence of the Warburg effect, and an altered metabolic state is often associated with malignancy. It has been shown that acidosis can promote tumor progression; thus, many therapeutic strategies have been adopted against tumor metabolism; one of these involves alkalinization therapies to raise tumor pH to inhibit tumor progression, improve immune surveillance, and overcome resistance to chemotherapies. Chemical exchange saturation transfer-magnetic resonance imaging (CEST-MRI) is a noninvasive technique that can measure pH in vivo using pH-sensitive contrast agents. Iopamidol, an iodinated contrast agent, clinically used for computed tomography (CT), contains amide group protons with pH-dependent exchange rates that can reveal the pHe of the tumor microenvironment. In this study, we optimized intraperitoneal (IP) delivery of iopamidol to facilitate longitudinal assessments of orthotopic pancreatic tumor pHe by CEST-MRI. Following IV-infusion and IP-bolus injections, we compared the two protocols for assessing tumor pH. Time-resolved CT imaging was used to evaluate the uptake of iopamidol in the tumor, revealing that IP-bolus delivered a high amount of contrast agent 40 min postinjection, which was similar to the amounts reached with the IV-infusion protocol. As expected, both IP and IV injection protocols produced comparable measurements of tumor pHe, showing no statistically significant difference between groups (p=0.16). In addition, we showed the ability to conduct longitudinal monitoring of tumor pHe using CEST-MRI with the IP injection protocol, revealing a statistically significant increase in tumor pHe following bicarbonate administration (p < 0.001). In conclusion, this study shows the capability to measure pHe using an IP delivery of iopamidol into orthotopic pancreatic tumors, which is important to conduct longitudinal studies.

Imaging and treatment of brain tumors through molecular targeting: Recent clinical advances

Molecular imaging techniques have rapidly progressed over recent decades providing unprecedented in vivo characterization of metabolic pathways and molecular biomarkers. Many of these new techniques have been successfully applied in the field of neuro-oncological imaging to probe tumor biology. Targeting specific signaling or metabolic pathways could help to address several unmet clinical needs that hamper the management of patients with brain tumors. This review aims to provide an overview of the recent advances in brain tumor imaging using molecular targeting with positron emission tomography and magnetic resonance imaging, as well as the role in patient management and possible therapeutic implications.

Schulte R, Laustsen C. Comprehensive Literature Review of Hyperpolarized Carbon-13 MRI: The Road to Clinical Application

This review provides a comprehensive assessment of the development of hyperpolarized (HP) carbon-13 metabolic MRI from the early days to the present with a focus on clinical applications. The status and upcoming challenges of translating HP carbon-13 into clinical application are reviewed, along with the complexity, technical advancements, and future directions. The road to clinical application is discussed regarding clinical needs and technological advancements, highlighting the most recent successes of metabolic imaging with hyperpolarized carbon-13 MRI. Given the current state of hyperpolarized carbon-13 MRI, the conclusion of this review is that the workflow for hyperpolarized carbon-13 MRI is the limiting factor.

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

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

Design of Nuclear Magnetic Resonance Molecular Probes for Hyperpolarized Bioimaging

Nuclear hyperpolarization has emerged as a method to dramatically enhance the sensitivity of NMR spectroscopy. By application of this powerful tool, small molecules with stable isotopes have been used for highly sensitive biomedical molecular imaging. The recent development of molecular probes for hyperpolarized in vivo analysis has demonstrated the ability of this technique to provide unique metabolic and physiological information. This review presents a brief introduction of hyperpolarization technology, approaches to the rational design of molecular probes for hyperpolarized analysis, and examples of molecules that have met with success in vitro or in vivo.

Detailed evaluation of pyruvate dehydrogenase complex inhibition in simulated exercise conditions

The mammalian pyruvate dehydrogenase complex (PDC) is a mitochondrial multienzyme complex that connects glycolysis to the tricarboxylic acid cycle by catalyzing pyruvate oxidation to produce acetyl-CoA, NADH, and CO₂. This reaction is required to aerobically utilize glucose, a preferred metabolic fuel, and is composed of three core enzymes: pyruvate dehydrogenase (E1), dihydrolipoyl transacetylase (E2), and dihydrolipoyl dehydrogenase (E3). The pyruvate-dehydrogenase-specific kinase (PDK) and pyruvate-dehydrogenase-specific phosphatase (PDP) are considered the main control mechanism of mammalian PDC activity. However, PDK and PDP activity are allosterically regulated by several effectors fully overlapping PDC substrates and products. This collection of positive and negative feedback mechanisms confounds simple predictions of relative PDC flux, especially when all effectors are dynamically modulated during metabolic states that exist in physiologically realistic conditions, such as exercise. Here, we provide, to our knowledge, the first globally fitted, pH-dependent kinetic model of the PDC accounting for the PDC core reaction because it is regulated by PDK, PDP, metal binding equilibria, and numerous allosteric effectors. The model was used to compute PDH regulatory complex flux as a function of previously determined metabolic conditions used to simulate exercise and demonstrates increased flux with exercise. Our model reveals that PDC flux in physiological conditions is primarily inhibited by product inhibition (∼60%), mostly NADH inhibition (∼30-50%), rather than phosphorylation cycle inhibition (∼40%), but the degree to which depends on the metabolic state and PDC tissue source.

Tumor Microenvironment Biosensors for Hyperpolarized Carbon-13 Magnetic Resonance Spectroscopy

Hyperpolarization (HP) of a carbon-13 molecule via dynamic nuclear polarization (DNP) involves polarization at low temperature, followed by a dissolution procedure producing a solution with highly polarized spins at room temperature. This dissolution DNP method significantly increases the signal-to-noise ratio (SNR) of nuclear magnetic resonance (NMR) over 10,000-fold and facilitates the use of magnetic resonance spectroscopy (MRS) to image not only metabolism but also the extracellular microenvironment. The extracellular tumor microenvironment (TME) closely interacts with tumor cells and stimulates their growth and metastasis. Thus, the ability to detect pathological changes in the TME is pivotal for the detection and study of cancers. This review highlights the potential use of MRS to study features of the TME-elevated export of lactate, reduced interstitial pH, imbalanced redox equilibrium, and altered metal homeostasis. The promising outcomes of both in vitro and in vivo assays suggest that DNP-MRS may be a useful technique to study aspects of the TME. With continued improvements, this tool has the potential to study the TME and provide guidance for accurate patient stratification and precise personal therapy. Graphical Abstract.

Preclinical Applications of Magnetic Resonance Imaging in Oncology

The evolving possibilities of molecular imaging (MI) are fundamentally changing the way we look at cancer, with imaging paradigms now shifting away from basic morphological measures toward the longitudinal assessment of functional, metabolic, cellular, and molecular information in vivo. Recent developments of imaging methodology and probe molecules utilizing the vast number of novel animal models of human cancers have enhanced our ability to non-invasively characterize neoplastic tissue and follow anticancer treatments. While preclinical molecular imaging offers a whole palette of excellent methodology to choose from, we will focus on magnetic resonance imaging (MRI) techniques, since they provide excellent molecular imaging capabilities and bear high potential for clinical translation. Prerequisites and consequences of using animal models as surrogates of human cancers in preclinical molecular imaging are outlined. We present physical principles, values, and limitations of MRI as molecular imaging modality and comment on its high potential to non-invasively assess information on metabolism, hypoxia, angiogenesis, and cell trafficking in preclinical cancer research.

Recent advances in fluorescent probes for extracellular pH detection and imaging

Extracellular pH plays vital roles in physiological and pathological processes including tumor metastasis and chemotherapy resistance. Abnormal extracellular pH is known to be associated with various pathological states, such as those in tumors, ischemic stroke, infection, and inflammation. Specifically, dysregulated pH is regarded as a hallmark of cancer because enhanced glycolysis and poor perfusion in most solid malignant tumors create an acidic extracellular environment, which enhances tumor growth, invasion, and metastasis. Close connection between the cell functions with extracellular pH means that precise and real-time measurement of the dynamic change of extracellular pH can provide critical information for not only studying physiological and pathological processes but also diagnosis of cancer and other diseases. This review highlights the recent development of based fluorescent probes for extracellular pH measurement, including design strategies, reaction mechanism and applications for the detection and imaging of extracellular pH.

Hemicyanine based naked-eye ratiometric fluorescent probe for monitoring lysosomal pH and its application

Lysosome as the typical acidic organelle involved in many biological processes, the dysfunction of its pH would alter many diseases. Small-molecule fluorescent probe has been considered as vital tool for monitoring pH fluctuation in living cells. Herein, a hemicyanine based ratiometric fluorescent probe was synthesized, namely, 2,3-trimethy-3-[2-(dimethyl-amino-4-yl) vinyl]-3H-benzo[e]indole (BiDL), for rapidly detection pH under acidic conditions. BiDL exhibited ratio fluorescence emission (F_534nm/F_622nm) characteristic with pK_a 4.25. BiDL showed good linearly response in pH 3.4-4.82, indicating that the probe can be used for quantitative detection pH. The probe also displayed large Stokes shift (112 nm/201 nm) under neutral and acidic conditions, respectively, which could effectively reduce the excitation interference. BiDL had excellent cell membrane permeability, good photo-stability and low toxicity in living cells. The dual-channel confocal fluorescent microscopic ratiometric imaging application of pH in HeLa cells and lysosome were achieved successfully, indicating that BiDL has good potential for investigating lysosome-relevant pathological and physiological processes.

Demarcation of Sepsis-Induced Peripheral and Central Acidosis with pH (Low) Insertion Cycle Peptide

Acidosis is a key driver for many diseases, including cancer, sepsis, and stroke. The spatiotemporal dynamics of dysregulated pH across disease remain elusive, and current diagnostic strategies do not provide localization of pH alterations. We sought to explore if PET imaging using hydrophobic cyclic peptides that partition into the cellular membrane at low extracellular pH (denoted as pH [low] insertion cycles, or pHLIC) can permit accurate in vivo visualization of acidosis. Methods: Acid-sensitive cyclic peptide c[E₄W₅C] pHLIC was conjugated to bifunctional maleimide-NO2A and radiolabeled with ⁶⁴Cu (half-life, 12.7 h). C57BL/6J mice were administered lipopolysaccharide (15 mg/kg) or saline (vehicle) and serially imaged with [⁶⁴Cu]Cu-c[E₄W₅C] over 24 h. Ex vivo autoradiography was performed on resected brain slices and subsequently stained with cresyl violet to enable high-resolution spatial analysis of tracer accumulation. A non-pH-sensitive cell-penetrating control peptide (c[R₄W₅C]) was used to confirm specificity of [⁶⁴Cu]Cu-c[E₄W₅C]. CD11b (macrophage/microglia) and TMEM119 (microglia) immunostaining was performed to correlate extent of neuroinflammation with [⁶⁴Cu]Cu-c[E₄W₅C] PET signal. Results: [⁶⁴Cu]Cu-c[E₄W₅C] radiochemical yield and purity were more than 95% and more than 99%, respectively, with molar activity of more than 0.925 MBq/nmol. Significantly increased [⁶⁴Cu]Cu-c[E₄W₅C] uptake was observed in lipopolysaccharide-treated mice (vs. vehicle) within peripheral tissues, including blood, lungs, liver, and small intestines (P < 0.001-0.05). Additionally, there was significantly increased [⁶⁴Cu]Cu-c[E₄W₅C] uptake in the brains of lipopolysaccharide-treated animals. Autoradiography confirmed increased uptake in the cerebellum, cortex, hippocampus, striatum, and hypothalamus of lipopolysaccharide-treated mice (vs. vehicle). Immunohistochemical analysis revealed microglial or macrophage infiltration, suggesting activation in brain regions containing increased tracer uptake. [⁶⁴Cu]Cu-c[R₄W₅C] demonstrated significantly reduced uptake in the brain and periphery of lipopolysaccharide mice compared with the acid-mediated [⁶⁴Cu]Cu-c[E₄W₅C] tracer. Conclusion: Here, we demonstrate that a pH-sensitive PET tracer specifically detects acidosis in regions associated with sepsis-driven proinflammatory responses. This study suggests that [⁶⁴Cu]Cu-pHLIC is a valuable tool to noninvasively assess acidosis associated with both central and peripheral innate immune activation.

Metabolic alterations in acute myocardial ischemia-reperfusion injury and necrosis using in vivo hyperpolarized [1-13C] pyruvate MR spectroscopy

This study aimed to investigate real-time early detection of metabolic alteration in a rat model with acute myocardial ischemia-reperfusion (AMI/R) injury and myocardial necrosis, as well as its correlation with intracellular pH level using in vivo hyperpolarized [1-¹³C] pyruvate magnetic resonance spectroscopy (MRS). Hyperpolarized ¹³C MRS was performed on the myocardium of 8 sham-operated control rats and 8 rats with AMI/R injury, and 8 sham-operated control rats and 8 rats with AMI-induced necrosis. Also, the correlations of levels of [1-¹³C] metabolites with pH were analyzed by Spearman’s correlation test. The AMI/R and necrosis groups showed significantly higher ratios of [1-¹³C] lactate (Lac)/bicarbonate (Bicar) and [1-¹³C] Lac/total carbon (tC), and lower ratios of ¹³C Bicar/Lac + alanine (Ala), and ¹³C Bicar/tC than those of the sham-operated control group. Moreover, the necrosis group showed significantly higher ratios of [1-¹³C] Lac/Bicar and [1-¹³C] Lac/tC, and lower ratios of ¹³C Bicar/Lac + Ala and ¹³C Bicar/tC than those of the AMI/R group. These results were consistent with the pattern for in vivo the area under the curve (AUC) ratios. In addition, levels of [1-¹³C] Lac/Bicar and [1-¹³C] Lac/tC were negatively correlated with pH levels, whereas ¹³C Bicar/Lac + Ala and ¹³C Bicar/tC levels were positively correlated with pH levels. The levels of [1-¹³C] Lac and ¹³C Bicar will be helpful for non-invasively evaluating the early stage of AMI/R and necrosis in conjunction with reperfusion injury of the heart. These findings have potential application to real-time evaluation of cardiac malfunction accompanied by changes in intracellular pH level and enzymatic activity.

Using bidirectional chemical exchange for improved hyperpolarized [13 C]bicarbonate pH imaging

PURPOSE

Rapid chemical exchange can affect SNR and pH measurement accuracy for hyperpolarized pH imaging with [¹³ C]bicarbonate. The purpose of this work was to investigate chemical exchange effects on hyperpolarized imaging sequences to identify optimal sequence parameters for high SNR and pH accuracy.

METHODS

Simulations were performed under varying rates of bicarbonate-CO₂ chemical exchange to analyze exchange effects on pH quantification accuracy and SNR under different sampling schemes. Four pulse sequences, including 1 new technique, a multiple-excitation 2D EPI (multi-EPI) sequence, were compared in phantoms using hyperpolarized [¹³ C]bicarbonate, varying parameters such as tip angles, repetition time, order of metabolite excitation, and refocusing pulse design. In vivo hyperpolarized bicarbonate-CO₂ exchange measurements were made in transgenic murine prostate tumors to select in vivo imaging parameters.

RESULTS

Modeling of bicarbonate-CO₂ exchange identified a multiple-excitation scheme for increasing CO₂ SNR by up to a factor of 2.7. When implemented in phantom imaging experiments, these sampling schemes were confirmed to yield high pH accuracy and SNR gains. Based on measured bicarbonate-CO₂ exchange in vivo, a 47% CO₂ SNR gain is predicted.

CONCLUSION

The novel multi-EPI pulse sequence can boost CO₂ imaging signal in hyperpolarized ¹³ C bicarbonate imaging while introducing minimal pH bias, helping to surmount a major hurdle in hyperpolarized pH imaging.

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.

Bioconjugation strategy for cell surface labelling with gold nanostructures designed for highly localized pH measurement

Regulation of intracellular pH is critically important for many cellular functions. The quantification of proton extrusion in different types of cells and physiological conditions is pivotal to fully elucidate the mechanisms of pH homeostasis. Here we show the use of gold nanoparticles (AuNP) to create a high spatial resolution sensor for measuring extracellular pH in proximity of the cell membrane. We test the sensor on HepG2 liver cancer cells and MKN28 gastric cancer cells before and after inhibition of Na+/H+ exchanger. The gold surface conjugation strategy is conceived with a twofold purpose: i) to anchor the AuNP to the membrane proteins and ii) to quantify the local pH from AuNP using surface enhanced Raman spectroscopy (SERS). The nanometer size of the cell membrane anchored sensor and the use of SERS enable us to visualize highly localized variation of pH induced by H+ extrusion, which is particularly upregulated in cancer cells.

Metabolic and Molecular Imaging with Hyperpolarised Tracers

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

Esterase-Catalyzed Production of Hyperpolarized 13C-Enriched Carbon Dioxide in Tissues for Measuring pH

¹³C Magnetic resonance imaging of hyperpolarized (HP) ¹³C-enriched bicarbonate (H¹³CO₃^-) and carbon dioxide (¹³CO₂) is a novel and sensitive technique for tissue pH mapping in vivo. Administration of the HP physiological buffer pair is attractive, but poor polarization and the short T₁ of ¹³C-enriched inorganic bicarbonate salts are major drawbacks for this approach. Here, we report a new class of mixed anhydrides for esterase-catalyzed production of highly polarized ¹³CO₂ and H¹³CO₃^- in tissue. A series of precursors with different alkoxy and acyl groups were synthesized and tested for chemical stability and T₁. ¹³C-enriched ethyl acetyl carbonate (¹³C-EAC) was found to be the most suitable candidate due to the relatively long T₁ and good chemical stability. Our results showed that ¹³C-EAC can be efficiently and rapidly polarized using BDPA. HP ¹³C-EAC was rapidly hydrolyzed by esterase to ¹³C-enriched monoacetyl carbonate (¹³C-MAC), which then decomposed to HP ¹³CO₂. Equilibrium between the newly produced ¹³CO₂ and H¹³CO₃^- was quickly established by carbonic anhydrase, producing a physiological buffer pair with ¹³C NMR signals that can be quantified for pH measurements. Finally, in vivo tissue pH measurements using HP ¹³C-EAC was successfully demonstrated in the liver of healthy rats. These results suggest that HP ¹³C-EAC is a novel imaging probe for in vivo pH measurements.

Positron Emission Tomography Imaging of Lesions pH Using 11C-Labeled Bicarbonate

OBJECTIVES

As acid-base imbalance is involved in many pathological processes, the capability to image tissue pH alterations in the clinic could offer new ways to detect disease and respond to treatment. In this study, the authors show that tissue pH can be imaged in vivo with ¹¹C-labeled bicarbonate (H¹¹CO₃^-) buffer and positron emission tomography (PET).

METHODS

H¹¹CO₃^- was produced by on-column NaOH adsorption. Biodistribution of H¹¹CO₃^- in normal mice was determined. In addition, uptake studies and inhibition experiments of H¹¹CO₃^- in the S180 fibrosarcoma-bearing mice and the inflammatory mice were investigated with PET imaging. The tumor and inflammatory interstitial pH was measured by a needle pH microelectrode.

RESULTS

PET imaging demonstrated the high uptake of H¹¹CO₃^- in mice tumor tissues and inflammatory tissues, which showed that the average tumor or inflammatory interstitial pH was significantly lower than the surrounding tissue. Administration of sodium bicarbonate in the drinking water increased the measured tumor pH, while the uptake of H¹¹CO₃^- in mice model tissues had no change. Similarly, administration with ammonium chloride (NH₄Cl) decreased the pH, whereas the unchanged uptake of H¹¹CO₃^- in mice model tissues was also found. However, after administration of acetazolamide, the low uptake of H¹¹CO₃^- in mice model tissues was observed.

CONCLUSIONS

H¹¹CO₃^- solution is an endogenous bicarbonate buffer tracer that can be injected into patients without toxicity. H¹¹CO₃^- PET can be used clinically to image pathological processes that are associated with acid-base imbalance, such as cancer and inflammation.

Cardiac applications of hyperpolarised magnetic resonance

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

Unsupervised Segmentation of 5D Hyperpolarized Carbon-13 MRI Data Using a Fuzzy Markov Random Field Model

Hyperpolarized MRI with 13C-labelled compounds is an emerging clinical technique allowing in vivo metabolic processes to be characterized non-invasively. Accurate quantification of 13C data, both for clinical and research purposes, typically relies on the use of region-of-interest analysis to detect and compare regions of altered metabolism. However, it is not clear how this should be determined from the five-dimensional data produced and most standard methodologies are unable to exploit the multidimensional nature of the data. Here we propose a solution to the novel problem of 13C image segmentation using a hybrid Markov random field model with continuous fuzzy logic. The algorithm fully utilizes the multi-dimensional data format in order to classify each voxel into one of six distinct classes based on its metabolic characteristics. Bayesian priors fully incorporate spatial, temporal and ratiometric contextual information whilst image contrast from multiple spectral dimensions are considered concurrently by using an analogy from color image segmentation. Performance of the algorithm is demonstrated on in silico data, where the superiority of the approach over a reference thresholding method is consistently observed. Application to in vivo animal data from a pre-clinical subcutaneous tumor model illustrates the ability of the MRF algorithm to successfully detect tumor location whilst avoiding image artifacts. This work has the potential to assist the analysis of human hyperpolarized 13C data in the future.

Placental physiology monitored by hyperpolarized dynamic 13C magnetic resonance

Placental functions, including transport and metabolism, play essential roles in pregnancy. This study assesses such processes in vivo, from a hyperpolarized MRI perspective. Hyperpolarized urea, bicarbonate, and pyruvate were administered to near-term pregnant rats, and all metabolites displayed distinctive behaviors. Little evidence of placental barrier crossing was observed for bicarbonate, at least within the timescales allowed by ¹³C relaxation. By contrast, urea was observed to cross the placental barrier, with signatures visible from certain fetal organs including the liver. This was further evidenced by the slower decay times observed for urea in placentas vis-à-vis other maternal compartments and validated by mass spectrometric analyses. A clear placental localization, as well as concurrent generation of hyperpolarized lactate, could also be detected for [1-¹³C]pyruvate. These metabolites also exhibited longer lifetimes in the placentas than in maternal arteries, consistent with a metabolic activity occurring past the trophoblastic interface. When extended to a model involving the administration of a preeclampsia-causing chemical, hyperpolarized MR revealed changes in urea's transport, as well as decreases in placental glycolysis vs. the naïve animals. These distinct behaviors highlight the potential of hyperpolarized MR for the early, minimally invasive detection of aberrant placental metabolism.

Acid-Sensing Ion Channels as Potential Therapeutic Targets in Neurodegeneration and Neuroinflammation

Acid-sensing ion channels (ASICs) are a family of proton-sensing channels that are voltage insensitive, cation selective (mostly permeable to Na⁺), and nonspecifically blocked by amiloride. Derived from 5 genes (ACCN1-5), 7 subunits have been identified, 1a, 1b, 2a, 2b, 3, 4, and 5, that are widely expressed in the peripheral and central nervous system as well as other tissues. Over the years, different studies have shown that activation of these channels is linked to various physiological and pathological processes, such as memory, learning, fear, anxiety, ischemia, and multiple sclerosis to name a few, so their potential as therapeutic targets is increasing. This review focuses on recent advances that have helped us to better understand the role played by ASICs in different pathologies related to neurodegenerative diseases, inflammatory processes, and pain.

T1 nuclear magnetic relaxation dispersion of hyperpolarized sodium and cesium hydrogencarbonate-13 C

In vivo pH mapping in tissue using hyperpolarized hydrogencarbonate-¹³ C has been proposed as a method to study tumor growth and treatment and other pathological conditions related to pH changes. The finite spin-lattice relaxation times (T₁ ) of hyperpolarized media are a significant limiting factor for in vivo imaging. Relaxation times can be measured at standard magnetic fields (1.5 T, 3.0 T etc.), but no such data are available at low fields, where T₁ values can be significantly shorter. This information is required to determine the potential loss of polarization as the agent is dispensed and transported from the polarizer to the MRI scanner. The purpose of this study is to measure T₁ dispersion from low to clinical magnetic fields (0.4 mT to 3.0 T) of different hyperpolarized hydrogencarbonate formulations previously proposed in the literature for in vivo pH measurements. ¹³ C-enriched cesium and sodium hydrogencarbonate preparations were hyperpolarized using dynamic nuclear polarization, and the T₁ values of different samples were measured at different magnetic field strengths using a fast field-cycling relaxometer and a 3.0 T clinical MRI system. The effects of deuterium oxide as a dissolution medium for sodium hydrogencarbonate were also analyzed. This study finds that the cesium formulation has slightly shorter T₁ values compared with the sodium preparation. However, the higher solubility of cesium hydrogencarbonate-¹³ C means it can be polarized at greater concentration, using less trityl radical than sodium hydrogencarbonate-¹³ C. This study also establishes that the preparation and handling of sodium hydrogencarbonate formulations in relation to cesium hydrogencarbonate is more difficult, due to the higher viscosity and lower achievable concentrations, and that deuterium oxide significantly increases the T₁ of sodium hydrogencarbonate solutions. Finally, this work also investigates the influence of pH on the spin-lattice relaxation of cesium hydrogencarbonate-¹³ C measured over a pH range of 7 to 9 at 0.47 T.

Biochemical phosphates observed using hyperpolarized 31P in physiological aqueous solutions

The dissolution-dynamic nuclear polarization technology had previously enabled nuclear magnetic resonance detection of various nuclei in a hyperpolarized state. Here, we show the hyperpolarization of ³¹P nuclei in important biological phosphates (inorganic phosphate and phosphocreatine) in aqueous solutions. The hyperpolarized inorganic phosphate showed an enhancement factor >11,000 (at 5.8 T, 9.3% polarization) in D₂O (T₁ 29.4 s). Deuteration and the solution composition and pH all affected the lifetime of the hyperpolarized state. This capability opens up avenues for real-time monitoring of phosphate metabolism, distribution, and pH sensing in the live body without ionizing radiation. Immediate changes in the microenvironment pH have been detected here in a cell-free system via the chemical shift of hyperpolarized inorganic phosphate. Because the ³¹P nucleus is 100% naturally abundant, future studies on hyperpolarized phosphates will not require expensive isotope labeling as is usually required for hyperpolarization of other substrates.

Real-time monitoring of phosphate metabolism and distribution in the live body without ionizing radiation is highly desirable. Here, the authors show dissolution-dynamic nuclear polarization technology can enable nuclear magnetic resonance detection of hyperpolarized ³¹P of important biological phosphates in aqueous solutions.

Imaging of pH in vivo using hyperpolarized 13C-labelled zymonic acid

Natural pH regulatory mechanisms can be overruled during several pathologies such as cancer, inflammation and ischaemia, leading to local pH changes in the human body. Here we demonstrate that ¹³C-labelled zymonic acid (ZA) can be used as hyperpolarized magnetic resonance pH imaging sensor. ZA is synthesized from [1-¹³C]pyruvic acid and its ¹³C resonance frequencies shift up to 3.0 p.p.m. per pH unit in the physiological pH range. The long lifetime of the hyperpolarized signal enhancement enables monitoring of pH, independent of concentration, temperature, ionic strength and protein concentration. We show in vivo pH maps within rat kidneys and subcutaneously inoculated tumours derived from a mammary adenocarcinoma cell line and characterize ZA as non-toxic compound predominantly present in the extracellular space. We suggest that ZA represents a reliable and non-invasive extracellular imaging sensor to localize and quantify pH, with the potential to improve understanding, diagnosis and therapy of diseases characterized by aberrant acid-base balance.

Local pH alterations can be manifestations of pathologies such as cancer, inflammation and ischaemia. Here Düwel et al. show hyperpolarized ¹³C-labelled zymonic acid can be used as a non-invasive probe to map and measure pH in vivo, suggesting it as a candidate for clinical imaging and a diagnostic tool.

Dicarboxylic acids as pH sensors for hyperpolarized 13C magnetic resonance spectroscopic imaging

Imaging tumoral pH may help to characterize aggressiveness, metastasis, and therapeutic response. We report the development of hyperpolarized [2-13C,D10]diethylmalonic acid, which exhibits a large pH-dependent 13C chemical shift over the physiological range. We demonstrate that co-polarization with [1-13C,D9]tert-butanol accurately measures pH via13C NMR and magnetic resonance spectroscopic imaging in phantoms.

Magnetic resonance imaging with hyperpolarized agents: methods and applications

In the past decade, hyperpolarized (HP) contrast agents have been under active development for MRI applications to address the twin challenges of functional and quantitative imaging. Both HP helium (³He) and xenon (¹²⁹Xe) gases have reached the stage where they are under study in clinical research. HP ¹²⁹Xe, in particular, is poised for larger scale clinical research to investigate asthma, chronic obstructive pulmonary disease, and fibrotic lung diseases. With advances in polarizer technology and unique capabilities for imaging of ¹²⁹Xe gas exchange into lung tissue and blood, HP ¹²⁹Xe MRI is attracting new attention. In parallel, HP ¹³C and ¹⁵N MRI methods have steadily advanced in a wide range of pre-clinical research applications for imaging metabolism in various cancers and cardiac disease. The HP [1-¹³C] pyruvate MRI technique, in particular, has undergone phase I trials in prostate cancer and is poised for investigational new drug trials at multiple institutions in cancer and cardiac applications. This review treats the methodology behind both HP gases and HP ¹³C and ¹⁵N liquid state agents. Gas and liquid phase HP agents share similar technologies for achieving non-equilibrium polarization outside the field of the MRI scanner, strategies for image data acquisition, and translational challenges in moving from pre-clinical to clinical research. To cover the wide array of methods and applications, this review is organized by numerical section into (1) a brief introduction, (2) the physical and biological properties of the most common polarized agents with a brief summary of applications and methods of polarization, (3) methods for image acquisition and reconstruction specific to improving data acquisition efficiency for HP MRI, (4) the main physical properties that enable unique measures of physiology or metabolic pathways, followed by a more detailed review of the literature describing the use of HP agents to study: (5) metabolic pathways in cancer and cardiac disease and (6) lung function in both pre-clinical and clinical research studies, concluding with (7) some future directions and challenges, and (8) an overall summary.

NMR-based Stable Isotope Resolved Metabolomics in systems biochemistry

Metabolism is the basic activity of live cells, and monitoring the metabolic state provides a dynamic picture of the cells or tissues, and how they respond to external changes, for in disease or treatment with drugs. NMR is an extremely versatile analytical tool that can be applied to a wide range of biochemical problems. Despite its modest sensitivity its versatility make it an ideal tool for analyzing biochemical dynamics both in vitro and in vivo, especially when coupled with its isotope editing capabilities, from which isotope distributions can be readily determined. These are critical for any analyses of flux in live organisms. This review focuses on the utility of NMR spectroscopy in metabolomics, with an emphasis on NMR applications in stable isotope-enriched tracer research for elucidating biochemical pathways and networks with examples from nucleotide biochemistry. The knowledge gained from this area of research provides a ready link to genomic, epigenomic, transcriptomic, and proteomic information to achieve systems biochemical understanding of living cells and organisms.

Imaging biomarker roadmap for cancer studies

Imaging biomarkers (IBs) are integral to the routine management of patients with cancer. IBs used daily in oncology include clinical TNM stage, objective response and left ventricular ejection fraction. Other CT, MRI, PET and ultrasonography biomarkers are used extensively in cancer research and drug development. New IBs need to be established either as useful tools for testing research hypotheses in clinical trials and research studies, or as clinical decision-making tools for use in healthcare, by crossing 'translational gaps' through validation and qualification. Important differences exist between IBs and biospecimen-derived biomarkers and, therefore, the development of IBs requires a tailored 'roadmap'. Recognizing this need, Cancer Research UK (CRUK) and the European Organisation for Research and Treatment of Cancer (EORTC) assembled experts to review, debate and summarize the challenges of IB validation and qualification. This consensus group has produced 14 key recommendations for accelerating the clinical translation of IBs, which highlight the role of parallel (rather than sequential) tracks of technical (assay) validation, biological/clinical validation and assessment of cost-effectiveness; the need for IB standardization and accreditation systems; the need to continually revisit IB precision; an alternative framework for biological/clinical validation of IBs; and the essential requirements for multicentre studies to qualify IBs for clinical use.

Characterization of the Tumor Microenvironment and Tumor-Stroma Interaction by Non-invasive Preclinical Imaging

Tumors are often characterized by hypoxia, vascular abnormalities, low extracellular pH, increased interstitial fluid pressure, altered choline-phospholipid metabolism, and aerobic glycolysis (Warburg effect). The impact of these tumor characteristics has been investigated extensively in the context of tumor development, progression, and treatment response, resulting in a number of non-invasive imaging biomarkers. More recent evidence suggests that cancer cells undergo metabolic reprograming, beyond aerobic glycolysis, in the course of tumor development and progression. The resulting altered metabolic content in tumors has the ability to affect cell signaling and block cellular differentiation. Additional emerging evidence reveals that the interaction between tumor and stroma cells can alter tumor metabolism (leading to metabolic reprograming) as well as tumor growth and vascular features. This review will summarize previous and current preclinical, non-invasive, multimodal imaging efforts to characterize the tumor microenvironment, including its stromal components and understand tumor-stroma interaction in cancer development, progression, and treatment response.

Imaging the Tumor Microenvironment

The tumor microenvironment consists of tumor, stromal, and immune cells, as well as extracellular milieu. Changes in numbers of these cell types and their environments have an impact on cancer growth and metastasis. Non-invasive imaging of aspects of the tumor microenvironment can provide important information on the aggressiveness of the cancer, whether or not it is metastatic, and can also help to determine early response to treatment. This chapter provides an overview on non-invasive in vivo imaging in humans and mouse models of various cell types and physiological parameters that are unique to the tumor microenvironment. Current clinical imaging and research investigation are in the areas of nuclear imaging (positron emission tomography (PET) and single photon emission computed tomography (SPECT)), magnetic resonance imaging (MRI) and optical (near infrared (NIR) fluorescence) imaging. Aspects of the tumor microenvironment that have been imaged by PET, MRI and/or optical imaging are tumor associated inflammation (primarily macrophages and T cells), hypoxia, pH changes, as well as enzymes and integrins that are highly prevalent in tumors, stroma and immune cells. Many imaging agents and strategies are currently available for cancer patients; however, the investigation of novel avenues for targeting aspects of the tumor microenvironment in pre-clinical models of cancer provides the cancer researcher with a means to monitor changes and evaluate novel treatments that can be translated into the clinic.

Dissolution Dynamic Nuclear Polarization capability study with fluid path

Signal enhancement by hyperpolarization is a way of overcoming the low sensitivity in magnetic resonance; MRI in particular. One of the most well-known methods, dissolution Dynamic Nuclear Polarization, has been used clinically in cancer patients. One way of ensuring a low bioburden of the hyperpolarized product is by use of a closed fluid path that constitutes a barrier to contamination. The fluid path can be filled with the pharmaceuticals, i.e. imaging agent and solvents, in a clean room, and then stored or immediately used at the polarizer. In this study, we present a method of filling the fluid path that allows it to be reused. The filling method has been investigated in terms of reproducibility at two extrema, high dose for patient use and low dose for rodent studies, using [1-13C]pyruvate as example. We demonstrate that the filling method allows high reproducibility of six quality control parameters with standard deviations 3-10 times smaller than the acceptance criteria intervals in clinical studies.

Hyperpolarized Multi-Metal 13C-Sensors for Magnetic Resonance Imaging

We introduce hyperpolarizable ¹³C-labeled probes that identify multiple biologically important divalent metals via metal-specific chemical shifts. These features enable NMR measurements of calcium concentrations in human serum in the presence of magnesium. In addition, signal enhancement through dynamic nuclear polarization (DNP) increases the sensitivity of metal detection to afford measuring micromolar concentrations of calcium as well as simultaneous multi-metal detection by chemical shift imaging. The hyperpolarizable ¹³C-MRI sensors presented here enable sensitive NMR measurements and MR imaging of multiple divalent metals in opaque biological samples.

The Potential of Metabolic Imaging

Metabolic imaging is a field of molecular imaging that focuses and targets changes in metabolic pathways for the evaluation of different clinical conditions. Targeting and quantifying metabolic changes noninvasively is a powerful approach to facilitate diagnosis and evaluate therapeutic response. This review addresses only techniques targeting metabolic pathways. Other molecular imaging strategies, such as affinity or receptor imaging or microenvironment-dependent methods are beyond the scope of this review. Here we describe the current state of the art in clinically translatable metabolic imaging modalities. Specifically, we focus on PET and MR spectroscopy, including conventional (1)H- and (13)C-MR spectroscopy at thermal equilibrium and hyperpolarized MRI. In this article, we first provide an overview of metabolic pathways that are altered in many pathologic conditions and the corresponding probes and techniques used to study those alterations. We then describe the application of metabolic imaging to several common diseases, including cancer, neurodegeneration, cardiac ischemia, and infection or inflammation.

(13) C magnetic resonance spectroscopy measurements with hyperpolarized [1-(13) C] pyruvate can be used to detect the expression of transgenic pyruvate decarboxylase activity in vivo

PURPOSE

Dissolution dynamic nuclear polarization can increase the sensitivity of the (13) C magnetic resonance spectroscopy experiment by at least four orders of magnitude and offers a novel approach to the development of MRI gene reporters based on enzymes that metabolize (13) C-labeled tracers. We describe here a gene reporter based on the enzyme pyruvate decarboxylase (EC 4.1.1.1), which catalyzes the decarboxylation of pyruvate to produce acetaldehyde and carbon dioxide.

METHODS

Pyruvate decarboxylase from Zymomonas mobilis (zmPDC) and a mutant that lacked enzyme activity were expressed using an inducible promoter in human embryonic kidney (HEK293T) cells. Enzyme activity was measured in the cells and in xenografts derived from the cells using (13) C MRS measurements of the conversion of hyperpolarized [1-(13) C] pyruvate to H(13) CO3-.

RESULTS

Induction of zmPDC expression in the cells and in the xenografts derived from them resulted in an approximately two-fold increase in the H(13) CO3-/[1-(13) C] pyruvate signal ratio following intravenous injection of hyperpolarized [1-(13) C] pyruvate.

CONCLUSION

We have demonstrated the feasibility of using zmPDC as an in vivo reporter gene for use with hyperpolarized (13) C MRS. Magn Reson Med 76:391-401, 2016. © 2015 The Authors. Magnetic Resonance in Medicine published by Wiley Periodicals, Inc. on behalf of International Society for Magnetic Resonance in Medicine. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

Probe for the measurement of cell surface pH in vivo and ex vivo

We have developed a way to measure cell surface pH by positioning a pH-sensitive fluorescent dye, seminaphtharhodafluor (SNARF), conjugated to the pH low insertion peptide (pHLIP). It has been observed that many diseased tissues are acidic and that tumors are especially so. A combination of effects acidifies tumor cell interiors, and cells pump out lactic acid and protons to maintain intracellular pH, acidifying the extracellular space. Overexpression of carbonic anhydrases on cell surfaces further contributes to acidification. Thus, the pH near tumor cell surfaces is expected to be low and to increase with distance from the membrane, so bulk pH measurements will not report surface acidity. Our new surface pH-measurement tool was validated in cancer cells grown in spheroids, in mouse tumor models in vivo, and in excised tumors. We found that the surface pH is sensitive to cell glycolytic activity: the pH decreases in high glucose and increases if glucose is replaced with nonmetabolized deoxyglucose. For highly metastatic cancer cells, the pH measured at the surface was 6.7-6.8, when the surrounding external pH was 7.4. The approach is sensitive enough to detect 0.2-0.3 pH unit changes in vivo in tumors induced by i.p. injection of glucose. The pH at the surfaces of highly metastatic cells within tumors was found to be about 6.1-6.4, whereas in nonmetastatic tumors, it was 6.7-6.9, possibly creating a way to distinguish more aggressive from less aggressive tumors. Other biological roles of surface acidity may be found, now that targeted measurements are possible.

Advances in Magnetic Resonance Imaging Contrast Agents for Biomarker Detection

Recent advances in magnetic resonance imaging (MRI) contrast agents have provided new capabilities for biomarker detection through molecular imaging. MRI contrast agents based on the T2 exchange mechanism have more recently expanded the armamentarium of agents for molecular imaging. Compared with T1 and T2* agents, T2 exchange agents have a slower chemical exchange rate, which improves the ability to design these MRI contrast agents with greater specificity for detecting the intended biomarker. MRI contrast agents that are detected through chemical exchange saturation transfer (CEST) have even slower chemical exchange rates. Another emerging class of MRI contrast agents uses hyperpolarized (13)C to detect the agent with outstanding sensitivity. These hyperpolarized (13)C agents can be used to track metabolism and monitor characteristics of the tissue microenvironment. Together, these various MRI contrast agents provide excellent opportunities to develop molecular imaging for biomarker detection.

Applications of NMR spectroscopy to systems biochemistry

The past decades of advancements in NMR have made it a very powerful tool for metabolic research. Despite its limitations in sensitivity relative to mass spectrometric techniques, NMR has a number of unparalleled advantages for metabolic studies, most notably the rigor and versatility in structure elucidation, isotope-filtered selection of molecules, and analysis of positional isotopomer distributions in complex mixtures afforded by multinuclear and multidimensional experiments. In addition, NMR has the capacity for spatially selective in vivo imaging and dynamical analysis of metabolism in tissues of living organisms. In conjunction with the use of stable isotope tracers, NMR is a method of choice for exploring the dynamics and compartmentation of metabolic pathways and networks, for which our current understanding is grossly insufficient. In this review, we describe how various direct and isotope-edited 1D and 2D NMR methods can be employed to profile metabolites and their isotopomer distributions by stable isotope-resolved metabolomic (SIRM) analysis. We also highlight the importance of sample preparation methods including rapid cryoquenching, efficient extraction, and chemoselective derivatization to facilitate robust and reproducible NMR-based metabolomic analysis. We further illustrate how NMR has been applied in vitro, ex vivo, or in vivo in various stable isotope tracer-based metabolic studies, to gain systematic and novel metabolic insights in different biological systems, including human subjects. The pathway and network knowledge generated from NMR- and MS-based tracing of isotopically enriched substrates will be invaluable for directing functional analysis of other 'omics data to achieve understanding of regulation of biochemical systems, as demonstrated in a case study. Future developments in NMR technologies and reagents to enhance both detection sensitivity and resolution should further empower NMR in systems biochemical research.

A ratiometric NMR pH sensing strategy based on a slow-proton-exchange (SPE) mechanism

Real time and non-invasive detection of pH in live biological systems is crucial for understanding the physiological role of acid-base homeostasis and for detecting pathological conditions associated with pH imbalance. One method to achieve in vivo pH monitoring is NMR. Conventional NMR methods, however, mainly utilize molecular sensors displaying pH-dependent chemical shift changes, which are vulnerable to multiple pH-independent factors. Here, we present a novel ratiometric strategy for sensitive and accurate pH sensing based on a small synthetic molecule, SPE1, which exhibits exceptionally slow proton exchange on the NMR time scale. Each protonation state of the sensor displays distinct NMR signals and the ratio of these signals affords precise pH values. In contrast to standard NMR methods, this ratiometric mechanism is not based on a chemical shift change, and SPE1 binds protons with high selectivity, resulting in accurate measurements. SPE1 was used to measure the pH in a single oocyte as well as in bacterial cultures, demonstrating the versatility of this method and establishing the foundation for broad biological applications.