Hyperpolarized Amino Acid Derivatives as Multivalent Magnetic Resonance pH Sensor Molecules

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.

Detecting biomarkers by dynamic nuclear polarization enhanced magnetic resonance

Hyperpolarization stands out as a technique capable of significantly enhancing the sensitivity of nuclear magnetic resonance (NMR) and magnetic resonance imaging (MRI). Dynamic nuclear polarization (DNP), among various hyperpolarization methods, has gained prominence for its efficacy in real-time monitoring of metabolism and physiology. By administering a hyperpolarized substrate through dissolution DNP (dDNP), the biodistribution and metabolic changes of the DNP agent can be visualized spatiotemporally. This approach proves to be a distinctive and invaluable tool for non-invasively studying cellular metabolism in vivo, particularly in animal models. Biomarkers play a pivotal role in influencing the growth and metastasis of tumor cells by closely interacting with them, and accordingly detecting pathological alterations of these biomarkers is crucial for disease diagnosis and therapy. In recent years, a range of hyperpolarized DNP molecular bioresponsive agents utilizing various nuclei, such as 13C, 15N, 31P, 89Y, etc., have been developed. In this context, we explore how these magnetic resonance signals of nuclear spins enhanced by DNP respond to biomarkers, including pH, metal ions, enzymes, or redox processes. This review aims to offer insights into the design principles of responsive DNP agents, target selection, and the mechanisms of action for imaging. Such discussions aim to propel the future development and application of DNP-based biomedical imaging agents.

Designing of multifunctional graphene quantum dot-polyvinyl alcohol-polyglycerol luminescent film for fluorescence detection of pH in sweat

BACKGROUND

Wound infection, skin disease, renal failure, cancer, cystic fibrosis, and other pathologies may induce obvious pH changes in sweat. Thus, tracking skin pH changes can help monitor human health in a convenient manner. Owing to their biocompatibility, easy preparation, and sensitive response to pH changes, graphene quantum dots (GQDs) have received increased attention in the optical detection of pH changes. However, their poor luminescent efficiency under visible light excitation and lack of functional diversification limit their application in skin pH monitoring. Therefore, the development of GQDs with excellent ultraviolet protection ability and antibacterial and luminescence performance is essential.

RESULTS

Folic acid-, histidine-, and serine-functionalized boron-doped graphene quantum dots (FHSB-GQDs) were designed and synthesized via thermal treatment. The resulting FHSB-GQDs exhibit strong yellow fluorescence emission under excitation with 490-nm visible light and sensitive pH responsiveness. The peak fluorescence intensity linearly decreases with increasing pH from 4 to 9. Furthermore, the FHSB-GQDs were integrated with polyvinyl alcohol and polyglycerol to form a luminescent film via hydrogen bond interactions. The film exhibits high transparency, mechanical flexibility, ultraviolet protection ability, and antibacterial activity. The presence of polyvinyl alcohol and polyglycerol restricts the free movement of the FHSB-GQDs and improves fluorescence behavior. The film was successfully applied in an intelligent pH-sensing system for monitoring pH changes in human sweat.

SIGNIFICANCE

The graphene quantum dot-polyvinyl alcohol-polyglycerol luminescent film offers excellent transparency, mechanical flexibility, ultraviolet protection ability, antibacterial activity, and luminescence performance. It was successfully applied in an intelligent pH sensing system for the detection of pH changes in human sweat. This study provides a new strategy for the design and construction of wearable sensing systems for health monitoring, facial masks, and medical dressings.

Opportunities and challenges with hyperpolarized bioresponsive probes for functional imaging using magnetic resonance

The development of hyperpolarized bioresponsive probes for magnetic resonance imaging (MRI) applications is an emerging and rapidly growing topic in chemistry. A wide range of hyperpolarized molecular biosensors for functional MRI have been developed in recent years. These probes comprise many different types of small-molecule reporters that can be hyperpolarized using dissolution dynamic nuclear polarization and parahydrogen-induced polarization or xenon-chelated macromolecular conjugates hyperpolarized using spin-exchange optical pumping. In this Perspective, we discuss how the amplified magnetic resonance signals of these agents are responsive to biologically relevant stimuli such as target proteins, reactive oxygen species, pH or metal ions. We examine how functional MRI using these systems allows a great number of biological processes to be monitored rapidly. Consequently, hyperpolarized bioresponsive probes may play a critical role in functional molecular imaging for observing physiology and pathology in real time.

Hyperpolarized (1-13C)Alaninamide Is a Multifunctional In Vivo Sensor of Aminopeptidase N Activity, pH, and CO2

Spin hyperpolarization enables real-time metabolic imaging of carbon-13-labeled substrates. While hyperpolarized l-(1-¹³C)alaninamide is a probe of the cell-surface tumor marker aminopeptidase-N (APN, CD13), its activity in vivo has not been described. Scanning the kidneys of rats infused with hyperpolarized alaninamide shows both conversion to [1-¹³C]alanine and several additional spectral peaks with distinct temporal dynamics. The (1-¹³C)alaninamide chemical shift is pH-sensitive, with a pK_a of 7.9 at 37 °C, and the peaks correspond to at least three different compartments of pH 7.46 ± 0.02 (1), 7.21 ± 0.02 (2), and 6.58 ± 0.05 (3). An additional peak was assigned to the carboxyamino adduct formed by reaction with dissolved CO₂. Spectroscopic imaging showed nonuniform distribution, with the low-pH signal more concentrated in the inner medulla. Treatment with the diuretic acetazolamide resulted in significant pH shifts in compartment 1 to 7.38 ± 0.03 (p = 0.0057) and compartment 3 to 6.80 ± 0.05 (p = 0.0019). While the pH of compartment 1 correlates with blood pH, the pH of compartment 3 did not correspond to the pH of urine. In vitro experiments show that alaninamide readily enters blood cells and can detect intracellular pH. While carbamate formation depends on pH and pCO₂, the carbamate-to-alaninamide ratio did not correlate with either arterial blood pH or pCO₂, suggesting that it may reflect variations in tissue pH and pCO₂. This study demonstrates the feasibility of using hyperpolarized sensors to simultaneously image enzyme activity, pCO₂, and pH in vivo.

Performance and reproducibility of 13C and 15N hyperpolarization using a cryogen-free DNP polarizer

The setup, operational procedures and performance of a cryogen-free device for producing hyperpolarized contrast agents using dissolution dynamic nuclear polarization (dDNP) in a preclinical imaging center is described. The polarization was optimized using the solid-state, DNP-enhanced NMR signal to calibrate the sample position, microwave and NMR frequency and power and flip angle. The polarization of a standard formulation to yield ~ 4 mL, 60 mM 1-13C-pyruvic acid in an aqueous solution was quantified in five experiments to P(13C) = (38 ± 6) % (19 ± 1) s after dissolution. The mono-exponential time constant of the build-up of the solid-state polarization was quantified to (1032 ± 22) s. We achieved a duty cycle of 1.5 h that includes sample loading, monitoring the polarization build-up, dissolution and preparation for the next run. After injection of the contrast agent in vivo, pyruvate, pyruvate hydrate, lactate, and alanine were observed, by measuring metabolite maps. Based on this work sequence, hyperpolarized 15N urea was obtained (P(15N) = (5.6 ± 0.8) % (30 ± 3) s after dissolution).

In Vivo Monitoring of pH in Subacute PD Mouse Brains with a Ratiometric Electrochemical Microsensor Based on Poly(melamine) Films

In vivo monitoring of cerebral pH is of great significance because its disturbance is related to some pathological processes such as neurodegenerative diseases, for example, Parkinson's disease (PD). In this study, we developed an electrochemical microsensor based on poly(melamine) (P_Mel) films for ratiometric monitoring of pH in subacute PD mouse brains. In this microsensor, P_Mel films were prepared from a simple electropolymerization approach in a melamine-containing solution, serving as the selective pH recognition membrane undergoing a 2H⁺/2e^- process. Meanwhile, electrochemically oxidized graphene oxide (EOGO) produced a built-in correction signal which helped avoid the environmental interference of the complicated brain systems. The potential difference between the peaks generated from EOGO and P_Mel gradually decreased with the aqueous pH increasing from 4.0 to 9.0, constituting the detection foundation of the ratiometric electrochemical microsensor (REM). The in vitro studies demonstrated that this proposed method exhibited a high sensitivity (a Nernstian response of -61.35 mV/pH) and remarkable selectivity against amino acids, anions, cations, and biochemical and reactive oxygen species coexisting in the brain. Coupled with its excellent stability and reproducibility and good antibiofouling based on short-term detection, the developed REM could serve as a disposable sensor for the determination of cerebral pH in vivo. Its following successful application in the real-time measurement of pH in the striatum, hippocampus, and cortex of rat brains in the events of global cerebral ischemia/reperfusion verified the reliability of this method. Finally, we adopted this robust REM to systematically analyze and compare the average pH in different regions of normal and subacute PD mouse brains.

15N-Azides as practical and effective tags for developing long-lived hyperpolarized agents

Azide moieties, unique linear species containing three nitrogen atoms, represent an attractive class of molecular tag for hyperpolarized magnetic resonance imaging (HP-MRI). Here we demonstrate (¹⁵N)₃-azide-containing molecules exhibit long-lasting hyperpolarization lifetimes up to 9.8 min at 1 T with remarkably high polarization levels up to 11.6% in water, thus establishing (¹⁵N)₃-azide as a powerful spin storage for hyperpolarization. A single (¹⁵N)-labeled azide has also been examined as an effective alternative tag with long-lived hyperpolarization. A variety of biologically important molecules are studied in this work, including choline, glucose, amino acid, and drug derivatives, demonstrating great potential of ¹⁵N-labeled azides as universal hyperpolarized tags for nuclear magnetic resonance imaging applications.

Parahydrogen-Induced Polarization of Amino Acids

Nuclear magnetic resonance (NMR) has become a universal method for biochemical and biomedical studies, including metabolomics, proteomics, and magnetic resonance imaging (MRI). By increasing the signal of selected molecules, the hyperpolarization of nuclear spin has expanded the reach of NMR and MRI even further (e.g. hyperpolarized solid-state NMR and metabolic imaging in vivo). Parahydrogen (pH2 ) offers a fast and cost-efficient way to achieve hyperpolarization, and the last decade has seen extensive advances, including the synthesis of new tracers, catalysts, and transfer methods. The portfolio of hyperpolarized molecules now includes amino acids, which are of great interest for many applications. Here, we provide an overview of the current literature and developments in the hyperpolarization of amino acids and peptides.

Correlation between lactate dehydrogenase/pyruvate dehydrogenase activities ratio and tissue pH in the perfused mouse heart: A potential noninvasive indicator of cardiac pH provided by hyperpolarized magnetic resonance

Cardiovascular diseases account for more than 30% of all deaths worldwide and many could be ameliorated with early diagnosis. Current cardiac imaging modalities can assess blood flow, heart anatomy and mechanical function. However, for early diagnosis and improved treatment, further functional biomarkers are needed. One such functional biomarker could be the myocardium pH. Although tissue pH is already determinable via MR techniques, and has been since the early 1990s, it remains elusive to use practically. The objective of this study was to explore the possibility to evaluate cardiac pH noninvasively, using in-cell enzymatic rates of hyperpolarized [1-¹³ C]pyruvate metabolism (ie, moles of product produced per unit time) determined directly in real time using magnetic resonance spectroscopy in a perfused mouse heart model. As a gold standard for tissue pH we used ³¹ P spectroscopy and the chemical shift of the inorganic phosphate (Pi) signal. The nonhomogenous pH distribution of the perfused heart was analyzed using a multi-parametric analysis of this signal, thus taking into account the heterogeneous nature of this characteristic. As opposed to the signal ratio of hyperpolarized [¹³ C]bicarbonate to [¹³ CO₂ ], which has shown correlation to pH in other studies, we investigated here the ratio of two intracellular enzymatic rates: lactate dehydrogenase (LDH) and pyruvate dehydrogenase (PDH), by way of determining the production rates of [1-¹³ C]lactate and [¹³ C]bicarbonate, respectively. The enzyme activities determined here are intracellular, while the pH determined using the Pi signal may contain an extracellular component, which could not be ruled out. Nevertheless, we report a strong correlation between the tissue pH and the LDH/PDH activities ratio. This work may pave the way for using the LDH/PDH activities ratio as an indicator of cardiac intracellular pH in vivo, in an MRI examination.

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.

Cardiac pH-Imaging With Hyperpolarized MRI

Regardless of the importance of acid-base disturbances in cardiac disease, there are currently no methods for clinical detection of pH in the heart. Several magnetic resonance imaging techniques hold translational promise and may enable in-vivo mapping of pH. We provide a brief overview of these emerging techniques. A particular focus is on the promising advance of magnetic resonance spectroscopy and imaging with hyperpolarized ¹³C-subtrates as biomarkers of myocardial pH. Hyperpolarization allows quantification of key metabolic substrates and their metabolites. Hereby, pH-sensitive reactions can be probed to provide a measure of acid-base alterations. To date, the most used substrates are [1-¹³C]pyruvate and ¹³C-labeled bicarbonate; however, others have been suggested. In cardiovascular medicine, hyperpolarized magnetic resonance spectroscopy has been used to probe acid-base disturbances following pharmacological stress, ischemia and heart failure in animals. In addition to pH-estimation, the technique can quantify fluxes such as the pivotal conversion of pyruvate to lactate via lactate dehydrogenase. This capability, a good safety profile and the fact that the technique is employable in clinical scanners have led to recent translation in early clinical trials. Thus, magnetic resonance spectroscopy and imaging may provide clinical pH-imaging in the near future.

Hyperpolarized 89Y-EDTMP complex as a chemical shift-based NMR sensor for pH at the physiological range

Yttrium (III) complexes are interesting due to the similarity of their chemistry with gadolinium complexes that are used as contrast agents in nuclear magnetic resonance (NMR) spectroscopy or imaging (MRI). While most of the paramagnetic Gd³⁺-based MRI contrast agents are T₁ or T₂ relaxation-based sensors such as Gd³⁺-complexes for zinc or pH detection, a number of diamagnetic Y³⁺-complexes rely on changes in the chemical shift for potential quantitative MRI in biological milieu. ⁸⁹Y, however, is a challenging nucleus to work with in conventional NMR or MRI due to its inherently low sensitivity and relatively long T₁ relaxation time. This insensitivity problem in ⁸⁹Y-based complexes can be circumvented with the use of dissolution dynamic nuclear polarization (DNP) which allows for several thousand-fold enhancement of the NMR or MRI signal relative to thermal equilibrium signal. Herein, we report on the feasibility of using hyperpolarized ⁸⁹Y-complexes with phosphonated open-chain ligands, ⁸⁹Y-EDTMP and ⁸⁹Y-DTPMP, as potential chemical shift-based pH NMR sensors. Our DNP-NMR data show that hyperpolarized ⁸⁹Y-DTPMP has an apparent pK_a ~ 7.01 with a 4 ppm-wide chemical shift dispersion with the signal disappearing at pH below 6.2. On the other hand, pH titration data on hyperpolarized ⁸⁹Y-EDTMP show that it has an apparent pK_a of pH 6.7 and a 16-ppm wide chemical shift dispersion at pH 5-9 range. In comparison, the previously reported hyperpolarized pH NMR sensor ⁸⁹Y-DOTP has a pK_a of 7.64 and ~ 10-ppm wide chemical shift dispersion at pH 4-9 range. Overall, our data suggest that hyperpolarized ⁸⁹Y-EDTMP is better than hyperpolarized ⁸⁹Y-DOTP in terms of pH sensing capability at the physiological range.

Simultaneous Assessment of Intracellular and Extracellular pH Using Hyperpolarized [1-13C]Alanine Ethyl Ester

Tissue pH is tightly regulated in vivo, being a sensitive physiological biomarker. Advent of dissolution dynamic nuclear polarization (DNP) and its translation to humans stimulated development of pH-sensitive agents. However, requirements of DNP probes such as biocompatibility, signal sensitivity, and spin-lattice relaxation time (T₁) complicate in vivo translation of the agents. Here, we developed a ¹³C-labeled alanine derivative, [1-¹³C]-l-alanine ethyl ester, as a viable DNP probe whose chemical shift is sensitive to the physiological pH range, and demonstrated the feasibility in phantoms and rat livers in vivo. Alanine ethyl ester readily crosses cell membrane while simultaneously assessing extracellular and intracellular pH in vivo. Following cell transport, [1-¹³C]-l-alanine ethyl ester is instantaneously hydrolyzed to [1-¹³C]-l-alanine, and subsequently metabolized to [1-¹³C]lactate and [¹³C]bicarbonate. The pH-insensitive alanine resonance was used as a reference.

How and Why Are Cancers Acidic? Carbonic Anhydrase IX and the Homeostatic Control of Tumour Extracellular pH

The acidic tumour microenvironment is now recognized as a tumour phenotype that drives cancer somatic evolution and disease progression, causing cancer cells to become more invasive and to metastasise. This property of solid tumours reflects a complex interplay between cellular carbon metabolism and acid removal that is mediated by cell membrane carbonic anhydrases and various transport proteins, interstitial fluid buffering, and abnormal tumour-associated vessels. In the past two decades, a convergence of advances in the experimental and mathematical modelling of human cancers, as well as non-invasive pH-imaging techniques, has yielded new insights into the physiological mechanisms that govern tumour extracellular pH (pH_e). In this review, we examine the mechanisms by which solid tumours maintain a low pH_e, with a focus on carbonic anhydrase IX (CAIX), a cancer-associated cell surface enzyme. We also review the accumulating evidence that suggest a role for CAIX as a biological pH-stat by which solid tumours stabilize their pH_e. Finally, we highlight the prospects for the clinical translation of CAIX-targeted therapies in oncology.

Application of 15N2-Diazirines as a Versatile Platform for Hyperpolarization of Biological Molecules by d-DNP

¹⁵N₂-Diazirines represent an attractive class of imaging tags for hyperpolarized magnetic resonance imaging (HP-MRI), offering desirable biocompatibility, ease of incorporation into a variety of molecules, and ability to deliver long-lasting polarization. We have recently established hyperpolarization of ¹⁵N₂-diazirines in organic solvents using SABRE-Shield Enables Alignment Transfer to Heteronuclei (SABRE-SHEATH). Yet, the current challenge of SABRE-SHEATH in water, specifically poor polarization efficiency, presents a barrier in examining the practical use of ¹⁵N₂-diazirines for HP-MRI. Herein, we show that efficient polarization of diverse ¹⁵N₂-diazirine-labeled molecules in water can be readily achieved by dissolution dynamic nuclear polarization (d-DNP), a hyperpolarization technique used in clinical practice. Hyperpolarization by d-DNP also demonstrates greater enhancement for long-lasting ¹⁵N signals, in comparison with SABRE-SHEATH. Various biologically important molecules are studied in this work, including amino acid, sugar, and drug compounds, demonstrating the great potential of ¹⁵N₂-diazirines as molecular tags in broad biomedical and clinical applications.

Multifunctional gold nanoparticles as smart nanovehicles with enhanced tumour-targeting abilities for intracellular pH mapping and in vivo MR/fluorescence imaging

A number of multimodal agents have been developed for tumour imaging and diagnosis, but most of them cannot be used to study the detailed physiological or pathological changes in living cells at the same time. Herein, a series of pH-responsive magnetic resonance and fluorescence imaging (MRI/FI) dual-modal "nanovehicles" are developed and tested. These new dual-modal materials allow for intercellular pH sensing, and those with units that are dually sensitive towards both acidic and basic environments have the ability for intracellular pH mapping and can be used to quantify pH at the cellular level. In addition, detailed pH changes in organelles (including lysosomes and mitochondria) can be investigated at the same time. On the other hand, with the tumour-targeting peptide (cRGD)-modified dual-modal nanovehicles, in vivo tumour MR and fluorescence imaging, which is suitable for cancer diagnosis, can be achieved. Moreover, it has been proved that these materials can pass through the blood brain barrier (BBB). By combining the above mentioned promising properties, these novel multifunctional "nanovehicles" may provide a new method for studying the role of pH during cancer diagnosis and treatment.

Hyperpolarized in vivo pH imaging reveals grade-dependent acidification in prostate cancer

There is an unmet clinical need for new and robust imaging biomarkers to distinguish indolent from aggressive prostate cancer. Hallmarks of aggressive tumors such as a decrease in extracellular pH (pH_e) can potentially be used to identify aggressive phenotypes. In this study, we employ an optimized, high signal-to-noise ratio hyperpolarized (HP) ¹³C pH_e imaging method to discriminate between indolent and aggressive disease in a murine model of prostate cancer. Transgenic adenocarcinoma of the mouse prostate (TRAMP) mice underwent a multiparametric MR imaging exam, including HP [¹³C] bicarbonate MRI for pH_e, with ¹H apparent diffusion coefficient (ADC) mapping and HP [1-¹³C] pyruvate MRI to study lactate metabolism. Tumor tissue was excised for histological staining and qRT-PCR to quantify mRNA expression for relevant glycolytic enzymes and transporters. We observed good separation in pH_e between low- and high-grade tumor regions, with high-grade tumors demonstrating a lower pH_e. The pH_e also correlated strongly with monocarboxylate transporter Mct4 gene expression across all tumors, suggesting that lactate export via MCT4 is associated with acidification in this model. Our results implicate extracellular acidification as an indicator of indolent-to-aggressive transition in prostate cancer and suggest feasibility of HP pH_e imaging to detect high-grade, clinically significant disease in men as part of a multiparametric MRI examination.

Spatiotemporal pH Heterogeneity as a Promoter of Cancer Progression and Therapeutic Resistance

Dysregulation of pH in solid tumors is a hallmark of cancer. In recent years, the role of altered pH heterogeneity in space, between benign and aggressive tissues, between individual cancer cells, and between subcellular compartments, has been steadily elucidated. Changes in temporal pH-related processes on both fast and slow time scales, including altered kinetics of bicarbonate-CO₂ exchange and its effects on pH buffering and gradual, progressive changes driven by changes in metabolism, are further implicated in phenotypic changes observed in cancers. These discoveries have been driven by advances in imaging technologies. This review provides an overview of intra- and extracellular pH alterations in time and space reflected in cancer cells, as well as the available technology to study pH spatiotemporal heterogeneity.

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 [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.

Simulating Non-linear Chemical and Physical (CAP) Dynamics of Signal Amplification By Reversible Exchange (SABRE)

The hyperpolarization of nuclear spins by using parahydrogen (pH₂ ) is a fascinating technique that allows spin polarization and thus the magnetic resonance signal to be increased by several orders of magnitude. Entirely new applications have become available. Signal amplification by reversible exchange (SABRE) is a relatively new method that is based on the reversible exchange of a substrate, catalyst and parahydrogen. SABRE is particularly interesting for in vivo medical and industrial applications, such as fast and low-cost trace analysis or continuous signal enhancement. Ever since its discovery, many attempts have been made to model and understand SABRE, with various degrees of simplifications. In this work, we reduced the simplifications further, taking into account non-linear chemical and physical (CAP) dynamics of several multi-spin systems. A master equation was derived and realized using the MOIN open-source software. The effects of different parameters (exchange rates, concentrations, spin-spin couplings) on relaxation and the polarization level have been evaluated and the results provide interesting insights into the mechanism of SABRE.

Progress in low-field benchtop NMR spectroscopy in chemical and biochemical analysis

The employment of spectroscopically-resolved NMR techniques as analytical probes have previously been both prohibitively expensive and logistically challenging in view of the large sizes of high-field facilities. However, with recent advances in the miniaturisation of magnetic resonance technology, low-field, cryogen-free "benchtop" NMR instruments are seeing wider use. Indeed, these miniaturised spectrometers are utilised in areas ranging from food and agricultural analyses, through to human biofluid assays and disease monitoring. Therefore, it is both intrinsically timely and important to highlight current applications of this analytical strategy, and also provide an outlook for the future, where this approach may be applied to a wider range of analytical problems, both qualitatively and quantitatively.

pH Dependence of T1 for 13 C-Labelled Small Molecules Commonly Used for Hyperpolarized Magnetic Resonance Imaging

Hyperpolarization is a method to enhance the nuclear magnetic resonance signal by up to five orders of magnitude. However, the hyperpolarized (HP) state is transient and decays with the spin-lattice relaxation time (T₁ ), which is on the order of a few tens of seconds. Here, we analyzed the pH-dependence of T₁ for commonly used HP ¹³ C-labelled small molecules such as acetate, alanine, fumarate, lactate, pyruvate, urea and zymonic acid. For instance, the T₁ of HP pyruvate is about 2.5 fold smaller at acidic pH (25 s, pH 1.7, B₀ =1 T) compared to pH close to physiological conditions (66 s, pH 7.3, B₀ =1 T). Our data shows that increasing hydronium ion concentrations shorten the T₁ of protonated carboxylic acids of most of the analyzed molecules except lactate. Furthermore it suggests that intermolecular hydrogen bonding at low pH can contribute to this T₁ shortening. In addition, enhanced proton exchange and chemical reactions at the pK_a appear to be detrimental for the HP-state.

A non-synthetic approach to extending the lifetime of hyperpolarized molecules using D2O solvation

Although dissolution dynamic nuclear polarization is a robust technique to significantly increase magnetic resonance signal, the short T₁ relaxation time of most ¹³C-nuclei limits the timescale of hyperpolarized experiments. To address this issue, we have characterized a non-synthetic approach to extend the hyperpolarized lifetime of ¹³C-nuclei in close proximity to solvent-exchangeable protons. Protons exhibit stronger dipolar relaxation than deuterium, so dissolving these compounds in D₂O to exchange labile protons with solvating deuterons results in longer-lived hyperpolarization of the ¹³C-nucleus 2-bonds away. ¹³C T₁ and T₂ times were longer in D₂O versus H₂O for all molecules in this study. This phenomenon can be utilized to improve hyperpolarized signal-to-noise ratio as a function of longer T₁, and enhanced spectral and imaging resolution via longer T₂.

Simultaneous characterization of tumor cellularity and the Warburg effect with PET, MRI and hyperpolarized 13C-MRSI

Modern oncology aims at patient-specific therapy approaches, which triggered the development of biomedical imaging techniques to synergistically address tumor biology at the cellular and molecular level. PET/MR is a new hybrid modality that allows acquisition of high-resolution anatomic images and quantification of functional and metabolic information at the same time. Key steps of the Warburg effect-one of the hallmarks of tumors-can be measured non-invasively with this emerging technique. The aim of this study was to quantify and compare simultaneously imaged augmented glucose uptake and LDH activity in a subcutaneous breast cancer model in rats (MAT-B-III) and to study the effect of varying tumor cellularity on image-derived metabolic information.

Methods: For this purpose, we established and validated a multimodal imaging workflow for a clinical PET/MR system including proton magnetic resonance (MR) imaging to acquire accurate morphologic information and diffusion-weighted imaging (DWI) to address tumor cellularity. Metabolic data were measured with dynamic [¹⁸F]FDG-PET and hyperpolarized (HP) ¹³C-pyruvate MR spectroscopic imaging (MRSI). We applied our workflow in a longitudinal study and analyzed the effect of growth dependent variations of cellular density on glycolytic parameters.

Results: Tumors of similar cellularity with similar apparent diffusion coefficients (ADC) showed a significant positive correlation of FDG uptake and pyruvate-to-lactate exchange. Longitudinal DWI data indicated a decreasing tumor cellularity with tumor growth, while ADCs exhibited a significant inverse correlation with PET standard uptake values (SUV). Similar but not significant trends were observed with HP-¹³C-MRSI, but we found that partial volume effects and point spread function artifacts are major confounders for the quantification of ¹³C-data when the spatial resolution is limited and major blood vessels are close to the tumor. Nevertheless, analysis of longitudinal data with varying tumor cellularity further detected a positive correlation between quantitative PET and ¹³C-data.

Conclusions: Our workflow allows the quantification of simultaneously acquired PET, MRSI and DWI data in rodents on a clinical PET/MR scanner. The correlations and findings suggest that a major portion of consumed glucose is metabolized by aerobic glycolysis in the investigated tumor model. Furthermore, we conclude that variations in cell density affect PET and ¹³C-data in a similar manner and correlations of longitudinal metabolic data appear to reflect both biochemical processes and tumor cellularity.

Application of Flexible Four-In-One Microsensor to Internal Real-Time Monitoring of Proton Exchange Membrane Fuel Cell

In recent years, the development of green energy sources, such as fuel cell, biomass energy, solar energy, and tidal energy, has become a popular research subject. This study aims at a flexible four-in-one microsensor, which can be embedded in the proton exchange membrane fuel cell (PEMFC) for real-time microscopic diagnosis so as to assist in developing and improving the technology of the fuel cell. Therefore, this study uses micro-electro-mechanical systems (MEMS) technology to integrate a micro humidity sensor, micro pH sensor, micro temperature sensor, and micro voltage sensor into a flexible four-in-one microsensor. This flexible four-in-one microsensor has four functions and is favorably characterized by small size, good acid resistance and temperature resistance, quick response, and real-time measurement. The goal was to be able to put the four-in-one microsensor in any place for measurement without affecting the performance of the fuel cell.