Hyperpolarized 13 C metabolic imaging of the human abdomen with spatiotemporal denoising

PURPOSE

Improving the quality and maintaining the fidelity of large coverage abdominal hyperpolarized (HP) 13 C MRI studies with a patch based global-local higher-order singular value decomposition (GL-HOVSD) spatiotemporal denoising approach.

METHODS

Denoising performance was first evaluated using the simulated [1-13 C]pyruvate dynamics at different noise levels to determine optimal kglobal and klocal parameters. The GL-HOSVD spatiotemporal denoising method with the optimized parameters was then applied to two HP [1-13 C]pyruvate EPI abdominal human cohorts (n = 7 healthy volunteers and n = 8 pancreatic cancer patients).

RESULTS

The parameterization of kglobal  = 0.2 and klocal  = 0.9 denoises abdominal HP data while retaining image fidelity when evaluated by RMSE. The kPX (conversion rate of pyruvate-to-metabolite, X = lactate or alanine) difference was shown to be <20% with respect to ground-truth metabolic conversion rates when there is adequate SNR (SNRAUC  > 5) for downstream metabolites. In both human cohorts, there was a greater than nine-fold gain in peak [1-13 C]pyruvate, [1-13 C]lactate, and [1-13 C]alanine apparent SNRAUC . The improvement in metabolite SNR enabled a more robust quantification of kPL and kPA . After denoising, we observed a 2.1 ± 0.4 and 4.8 ± 2.5-fold increase in the number of voxels reliably fit across abdominal FOVs for kPL and kPA quantification maps.

CONCLUSION

Spatiotemporal denoising greatly improves visualization of low SNR metabolites particularly [1-13 C]alanine and quantification of [1-13 C]pyruvate metabolism in large FOV HP 13 C MRI studies of the human abdomen.

A continuous flow bioreactor system for high-throughput hyperpolarized metabolic flux analysis

Hyperpolarized carbon-13 labeled compounds are increasingly being used in medical MR imaging (MRI) and MR imaging (MRI) and spectroscopy (MRS) research, due to its ability to monitor tissue and cell metabolism in real-time. Although radiological biomarkers are increasingly being considered as clinical indicators, biopsies are still considered the gold standard for a large variety of indications. Bioreactor systems can play an important role in biopsy examinations because of their ability to provide a physiochemical environment that is conducive for therapeutic response monitoring ex vivo. We demonstrate here a proof-of-concept bioreactor and microcoil receive array setup that allows for ex vivo preservation and metabolic NMR spectroscopy on up to three biopsy samples simultaneously, creating an easy-to-use and robust way to simultaneously run multisample carbon-13 hyperpolarization experiments. Experiments using hyperpolarized [1-13C]pyruvate on ML-1 leukemic cells in the bioreactor setup were performed and the kinetic pyruvate-to-lactate rate constants ( k PL ) extracted. The coefficient of variation of the experimentally found k PL s for five repeated experiments was C V = 35 % . With this statistical power, treatment effects of 30%-40% change in lactate production could be easily differentiable with only a few hyperpolarization dissolutions on this setup. Furthermore, longitudinal experiments showed preservation of ML-1 cells in the bioreactor setup for at least 6 h. Rat brain tissue slices were also seen to be preserved within the bioreactor for at least 1 h. This validation serves as the basis for further optimization and upscaling of the setup, which undoubtedly has huge potential in high-throughput studies with various biomarkers and tissue types.

Iridium Pyridylpyrrolides Hyperpolarised by Para-Hydrogen

Chemical activation of the nuclear singlet state of dihydrogen, para-hydrogen, can dramatically increase the sensitivity of magnetic resonance spectroscopy and imaging. Here, we show that the highly reversible activation of para-hydrogen by an iridium pyridylpyrrolide complex is capable of producing this hyperpolarisation effect. Bound alkene ligands exhibit signal enhancement without reduction to alkanes, which is in contrast to the most widely used hyperpolarisation catalysts. The complex is recoverable due to the highly reversible binding and release of H2, and result in enhanced hydride signals in a wide range of coordinating and non-coordinating deuterated solvents. Synthetic modification of the ligand substituents and the addition of co-ligands show a strong dependence of chemical structure on reactivity, which reveals an untapped potential to exploit pyridylpyrrolides as ligands in the development of tunable para-hydrogen induced hyperpolarisation catalysts or molecular probes.

Multi-axis fields boost SABRE hyperpolarization

The inherently low signal-to-noise ratio of NMR and MRI is now being addressed by hyperpolarization methods. For example, iridium-based catalysts that reversibly bind both parahydrogen and ligands in solution can hyperpolarize protons (SABRE) or heteronuclei (X-SABRE) on a wide variety of ligands, using a complex interplay of spin dynamics and chemical exchange processes, with common signal enhancements between 103 and 104. This does not approach obvious theoretical limits, and further enhancement would be valuable in many applications (such as imaging mM concentration species in vivo). Most SABRE/X-SABRE implementations require far lower fields (μT-mT) than standard magnetic resonance (>1T), and this gives an additional degree of freedom: the ability to fully modulate fields in three dimensions. However, this has been underexplored because the standard simplifying theoretical assumptions in magnetic resonance need to be revisited. Here, we take a different approach, an evolutionary strategy algorithm for numerical optimization, multi-axis computer-aided heteronuclear transfer enhancement for SABRE (MACHETE-SABRE). We find nonintuitive but highly efficient multiaxial pulse sequences which experimentally can produce a sevenfold improvement in polarization over continuous excitation. This approach optimizes polarization differently than traditional methods, thus gaining extra efficiency.

Parahydrogen-Induced Polarization of 14N Nuclei

Hyperpolarization techniques provide a dramatic increase in sensitivity of nuclear magnetic resonance spectroscopy and imaging. In spite of the outstanding progress in solution-state hyperpolarization of spin-1/2 nuclei, hyperpolarization of quadrupolar nuclei remains challenging. Here, hyperpolarization of quadrupolar 14N nuclei with natural isotopic abundance of >99 % is demonstrated. This is achieved via pairwise addition of parahydrogen to tetraalkylammonium salts with vinyl or allyl unsaturated moieties followed by a subsequent polarization transfer from 1H to 14N nuclei at high magnetic field using PH-INEPT or PH-INEPT+ radiofrequency pulse sequence. Catalyst screening identified water-soluble rhodium complex [Rh(P(m-C6H4SO3Na)3)3Cl] as the most efficient catalyst for hyperpolarization of the substrates under study, providing up to 1.3 % and up to 6.6 % 1H polarization in the cases of vinyl and allyl precursors, respectively. The performance of PH-INEPT and PH-INEPT+ pulse sequences was optimized with respect to interpulse delays, and the resultant experimental dependences were in good agreement with simulations. As a result, 14N NMR signal enhancement of up to 760-fold at 7.05 T (corresponding to 0.15 % 14N polarization) was obtained.

Carbon-13 Radiofrequency Amplification by Stimulated Emission of Radiation of the Hyperpolarized Ketone and Hemiketal Forms of Allyl [1-13C]Pyruvate

13C hyperpolarized pyruvate is an emerging MRI contrast agent for sensing molecular events in cancer and other diseases with aberrant metabolic pathways. This metabolic contrast agent can be produced via several hyperpolarization techniques. Despite remarkable success in research settings, widespread clinical adoption faces substantial roadblocks because the current sensing technology utilized to sense this contrast agent requires the excitation of 13C nuclear spins that also need to be synchronized with MRI field gradient pulses. Here, we demonstrate sensing of hyperpolarized allyl [1-13C]pyruvate via the stimulated emission of radiation that mitigates the requirements currently blocking broader adoption. Specifically, 13C Radiofrequency Amplification by Stimulated Emission of Radiation (13C RASER) was obtained after pairwise addition of parahydrogen to a pyruvate precursor, detected in a commercial inductive detector with a quality factor (Q) of 32 for sample concentrations as low as 0.125 M with 13C polarization of 4%. Moreover, parahydrogen-induced polarization allowed for the preparation of a mixture of ketone and hemiketal forms of hyperpolarized allyl [1-13C]pyruvate, which are separated by 10 ppm in 13C NMR spectra. This is a good model system to study the simultaneous 13C RASER signals of multiple 13C species. This system models the metabolic production of hyperpolarized [1-13C]lactate from hyperpolarized [1-13C]pyruvate, which has a similar chemical shift difference. Our results show that 13C RASER signals can be obtained from both species simultaneously when the emission threshold is exceeded for both species. On the other hand, when the emission threshold is exceeded only for one of the hyperpolarized species, 13C stimulated emission is confined to this species only, therefore enabling the background-free detection of individual hyperpolarized 13C signals. The reported results pave the way to novel sensing approaches of 13C hyperpolarized pyruvate, potentially unlocking hyperpolarized 13C MRI on virtually any MRI system─an attractive vision for the future molecular imaging and diagnostics.

Toward Lung Ventilation Imaging Using Hyperpolarized Diethyl Ether Gas Contrast Agent

Hyperpolarized 129 Xe gas was FDA-approved as an inhalable contrast agent for magnetic resonance imaging of a wide range of pulmonary diseases in December 2022. Despite the remarkable success in clinical research settings, the widespread clinical translation of HP 129 Xe gas faces two critical challenges: the high cost of the relatively low-throughput hyperpolarization equipment and the lack of 129 Xe imaging capability on clinical MRI scanners, which have narrow-bandwidth electronics designed only for proton (1 H) imaging. To solve this translational grand challenge of gaseous hyperpolarized MRI contrast agents, here we demonstrate the utility of batch-mode production of proton-hyperpolarized diethyl ether gas via heterogeneous pairwise addition of parahydrogen to ethyl vinyl ether. An approximately 0.1-liter bolus of hyperpolarized diethyl ether gas was produced in 1 second and injected in excised rabbit lungs. Lung ventilation imaging was performed using sub-second 2D MRI with up to 2×2 mm2 in-plane resolution using a clinical 0.35 T MRI scanner without any modifications. This feasibility demonstration paves the way for the use of inhalable diethyl ether as a gaseous contrast agent for pulmonary MRI applications using any clinical MRI scanner.

Temperature lowering of liquid nitrogen via injection of helium gas bubbles improves the generation of parahydrogen-enriched gas

The para spin isomer of hydrogen gas possesses high nuclear spin order that can enhance the NMR signals of a variety of molecular species. Hydrogen is routinely enriched in the para spin state by lowering the gas temperature while flowing through a catalyst. Although parahydrogen enrichments approaching 100% are achievable near the H2 liquefaction temperature of 20 K, many experimentalists operate at liquid nitrogen temperatures (77 K) due to the lower associated costs and overall simplicity of the parahydrogen generator. Parahydrogen that is generated at 77 K provides an enrichment value of ~51% of the para spin isomer; while useful, there are many applications that can benefit from low-cost access to higher parahydrogen enrichments. Here, we introduce a method of improving parahydrogen enrichment values using a liquid nitrogen-cooled generator that operates at temperatures less than 77 K. The boiling temperature of liquid nitrogen is lowered through internal evaporation into helium gas bubbles that are injected into the liquid. Changes to liquid nitrogen temperatures and parahydrogen enrichment values were monitored as a function of helium gas flow rate. The injected helium bubbles lowered the liquid nitrogen temperature to ~65.5 K, and parahydrogen enrichments of up to ~59% were achieved; this represents an ~16% improvement compared with the expected parahydrogen fraction at 77 K. This technique is simple to implement in standard liquid nitrogen-cooled parahydrogen generators and may be of interest to a wide range of scientists that require a cost-effective approach to improving parahydrogen enrichment values.

Metal-Mediated Catalytic Polarization Transfer from para Hydrogen to 3,5-Dihalogenated Pyridines

The neutral catalysts [IrCl(H)2(NHC)(substrate)2] or [IrCl(H)2(NHC)(substrate)(sulfoxide)] are used to transfer polarization from para hydrogen (pH2) to 3,5-dichloropyridine and 3,5-dibromopyridine substrates. This is achieved in a rapid, reversible, and low-cost process that relies on ligand exchange within the active catalyst. Notably, the sulfoxide-containing catalyst systems produced NMR signal enhancements between 1 and 2 orders of magnitude larger than its unmodified counterpart. Consequently, this signal amplification by reversible exchange hyperpolarization method can boost the 1H, 13C, and 15N nuclear magnetic resonance (NMR) signal intensities by factors up to 4350, 1550, and 46,600, respectively (14.0, 1.3, and 15.4% polarization). In this paper, NMR and X-ray crystallography are used to map the evolution of catalytically important species and provide mechanistic rational for catalytic efficiency. Furthermore, applications in spontaneous radiofrequency amplification by stimulated emission and NMR reaction monitoring are also shown.

Shining a Spotlight on Methyl Groups: Photochemically Induced Dynamic Nuclear Polarization Spectroscopy of 5-Deazariboflavin and Its Nor Analogs

5-Deazaflavins are analogs of naturally occurring flavin cofactors. They serve as substitutes for natural flavin cofactors to investigate and modify the reaction pathways of flavoproteins. Demethylated 5-deazaflavins are potential candidates for artificial cofactors, allowing us to fine-tune the reaction kinetics and absorption characteristics of flavoproteins. In this contribution, demethylated 5-deazariboflavin radicals are investigated (1) to assess the influence of the methyl groups on the electronic structure of the 5-deazaflavin radical and (2) to explore their photophysical properties with regard to their potential as artificial cofactors. We determined the proton hyperfine structure of demethylated 5-deazariboflavins using photochemically induced dynamic nuclear polarization (photo-CIDNP) spectroscopy, as well as density functional theory (DFT). To provide context, we compare our findings to a study of flavin mononucleotide (FMN) derivatives. We found a significant influence of the methylation pattern on the absorption properties, as well as on the proton hyperfine coupling ratios of the xylene moiety, which appears to be solvent-dependent. This effect is enhanced by the replacement of N5 by C5-H in 5-deazaflavin derivatives compared to their respective flavin counterparts.

Hyperpolarized 13 C Metabolic MRI of Patients with Pancreatic Ductal Adenocarcinoma

BACKGROUND

Pancreatic ductal adenocarcinoma (PDA) is the third leading cause of cancer-related death in the United States. However, early response assessment using the current approach of measuring changes in tumor size on computed tomography (CT) or MRI is challenging.

PURPOSE

To investigate the feasibility of hyperpolarized (HP) [1-13 C]pyruvate MRI to quantify metabolism in the normal appearing pancreas and PDA, and to assess changes in PDA metabolism following systemic chemotherapy.

STUDY TYPE

Prospective.

SUBJECTS

Six patients (65.0 ± 7.6 years, 2 females) with locally advanced or metastatic PDA enrolled prior to starting a new line of systemic chemotherapy.

FIELD STRENGTH/SEQUENCE

3-T, T1-weighted gradient echo, metabolite-selective 13 C echoplanar imaging.

ASSESSMENT

Time-resolved HP [1-13 C]pyruvate data were acquired before (N = 6) and 4-weeks after (N = 3) treatment initiation. Pyruvate metabolism, as quantified by pharmacokinetic modeling and metabolite area-under-the-curve ratios, was assessed in manually segmented PDA and normal appearing pancreas ROIs (N = 5). The change in tumor metabolism before and 4-weeks after treatment initiation was assessed in primary PDA (N = 2) and liver metastases (N = 1), and was compared to objective tumor response defined by response evaluation criteria in solid tumors (RECIST) on subsequent CTs.

STATISTICAL TESTS

Descriptive tests (mean ± standard deviation), model fit error for pharmacokinetic rate constants.

RESULTS

Primary PDA showed reduced alanine-to-lactate ratios when compared to normal pancreas, due to increased lactate-to-pyruvate or reduced alanine-to-pyruvate ratios. Of the three patients who received HP [1-13 C]pyruvate MRI before and 4-weeks after treatment initiation, one patient had a primary tumor with early metabolic response (increase in alanine-to-lactate) and subsequent partial response according to RECIST, one patient had a primary tumor with relatively stable metabolism and subsequent stable disease by RECIST, and one patient had metastatic PDA with increase in lactate-to-pyruvate of the liver metastases and corresponding progressive disease according to RECIST.

DATA CONCLUSION

Altered pyruvate metabolism with increased lactate or reduced alanine was observed in the primary tumor. Early metabolic response assessed at 4-weeks after treatment initiation correlated with subsequent objective tumor response assessed using RECIST.

LEVEL OF EVIDENCE

2 TECHNICAL EFFICACY: Stage 2.

Longitudinal assessment of mitochondrial dysfunction in acute traumatic brain injury using hyperpolarized [1-13 C]pyruvate

PURPOSE

[13 C]Bicarbonate formation from hyperpolarized [1-13 C]pyruvate via pyruvate dehydrogenase, a key regulatory enzyme, represents the cerebral oxidation of pyruvate and the integrity of mitochondrial function. The present study is to characterize the chronology of cerebral mitochondrial metabolism during secondary injury associated with acute traumatic brain injury (TBI) by longitudinally monitoring [13 C]bicarbonate production from hyperpolarized [1-13 C]pyruvate in rodents.

METHODS

Male Wistar rats were randomly assigned to undergo a controlled-cortical impact (CCI, n = 31) or sham surgery (n = 22). Seventeen of the CCI and 9 of the sham rats longitudinally underwent a 1 H/13 C-integrated MR protocol that includes a bolus injection of hyperpolarized [1-13 C]pyruvate at 0 (2 h), 1, 2, 5, and 10 days post-surgery. Separate CCI and sham rats were used for histological validation and enzyme assays.

RESULTS

In addition to elevated lactate, we observed significantly reduced bicarbonate production in the injured site. Unlike the immediate appearance of hyperintensity on T2 -weighted MRI, the contrast of bicarbonate signals between the injured region and the contralateral brain peaked at 24 h post-injury, then fully recovered to the normal level at day 10. A subset of TBI rats demonstrated markedly increased bicarbonate in normal-appearing contralateral brain regions post-injury.

CONCLUSION

This study demonstrates that aberrant mitochondrial metabolism occurring in acute TBI can be monitored by detecting [13 C]bicarbonate production from hyperpolarized [1-13 C]pyruvate, suggesting that [13 C]bicarbonate is a sensitive in-vivo biomarker of the secondary injury processes.

Molecular precursors to produce para-hydrogen enhanced metabolites at any field

Enhancing magnetic resonance signal via hyperpolarization techniques enables the real-time detection of metabolic transformations even in vivo. The use of para-hydrogen to enhance 13 C-enriched metabolites has opened a rapid pathway for the production of hyperpolarized metabolites, which usually requires specialized equipment. Metabolite precursors that can be hyperpolarized and converted into metabolites at any given field would open up opportunities for many labs to make use of this technology because already existing hardware could be used. We report here on the complete synthesis and hyperpolarization of suitable precursor molecules of the side-arm hydrogenation approach. The better accessibility to such side-arms promises that the para-hydrogen approach can be implemented in every lab with existing two channel NMR spectrometers for 1 H and 13 C independent of the magnetic field.

Investigating cerebral perfusion with high resolution hyperpolarized [1-13 C]pyruvate MRI

PURPOSE

To investigate high-resolution hyperpolarized (HP) 13 C pyruvate MRI for measuring cerebral perfusion in the human brain.

METHODS

HP [1-13 C]pyruvate MRI was acquired in five healthy volunteers with a multi-resolution EPI sequence with 7.5 × 7.5 mm2 resolution for pyruvate. Perfusion parameters were calculated from pyruvate MRI using block-circulant singular value decomposition and compared to relative cerebral blood flow calculated from arterial spin labeling (ASL). To examine regional perfusion patterns, correlations between pyruvate and ASL perfusion were performed for whole brain, gray matter, and white matter voxels.

RESULTS

High resolution 7.5 × 7.5 mm2 pyruvate images were used to obtain relative cerebral blood flow (rCBF) values that were significantly positively correlated with ASL rCBF values (r = 0.48, 0.20, 0.28 for whole brain, gray matter, and white matter voxels respectively). Whole brain voxels exhibited the highest correlation between pyruvate and ASL perfusion, and there were distinct regional patterns of relatively high ASL and low pyruvate normalized rCBF found across subjects.

CONCLUSION

Acquiring HP 13 C pyruvate metabolic images at higher resolution allows for finer spatial delineation of brain structures and can be used to obtain cerebral perfusion parameters. Pyruvate perfusion parameters were positively correlated to proton ASL perfusion values, indicating a relationship between the two perfusion measures. This HP 13 C study demonstrated that hyperpolarized pyruvate MRI can assess cerebral metabolism and perfusion within the same study.

Delivering Robust Proton-Only Sensing of Hyperpolarized [1,2-13C2]-Pyruvate Using Broad-Spectral-Range Nuclear Magnetic Resonance Pulse Sequences

Hyperpolarized [1-13C]pyruvate is the leading hyperpolarized injectable contrast agent and is currently under evaluation in clinical trials for molecular imaging of metabolic diseases, including cardiovascular disease and cancer. One aspect limiting broad scalability of the technique is that hyperpolarized 13C MRI requires specialized 13C hardware and software that are not generally available on clinical MRI scanners, which employ proton-only detection. Here, we present an approach that uses pulse sequences to transfer 13C hyperpolarization to methyl protons for detection of the 13C-13C pyruvate singlet, employing proton-only excitation and detection only. The new pulse sequences are robust to the B1 and B0 magnetic field inhomogeneities. The work focuses on singlet-to-magnetization (S2M) and rotor-synchronized (R) pulses, both relying on trains of hard pulses with broad spectral width coverage designed to effectively transform hyperpolarized 13C2-singlet hyperpolarization to 1H polarization on the CH3 group of [1,2-13C2]pyruvate. This approach may enable a broader adoption of hyperpolarized MRI as a molecular imaging technique.

Terpenes in Cannabis sativa Inhibit Capsaicin Responses in Rat DRG Neurons via Na+/K+ ATPase Activation

Terpenes in Cannabis sativa exert analgesic effects, but the mechanisms are uncertain. We examined the effects of 10 terpenes on capsaicin responses in an established model of neuronal hypersensitivity. Adult rat DRG neurons cultured with neurotrophic factors NGF and GDNF were loaded with Fura2AM for calcium imaging, and treated with individual terpenes or vehicle for 5 min, followed by 1 µMol capsaicin. In vehicle treated control experiments, capsaicin elicited immediate and sustained calcium influx. Most neurons treated with terpenes responded to capsaicin after 6-8 min. Few neurons showed immediate capsaicin responses that were transient or normal. The delayed responses were found to be due to calcium released from the endoplasmic reticulum, as they were maintained in calcium/magnesium free media, but not after thapsigargin pre-treatment. Terpene inhibition of calcium influx was reversed after washout of medium, in the absence of terpenes, and in the presence of the Na+/K+ ATPase inhibitor ouabain, but not CB1 or CB2 receptor antagonists. Thus, terpenes inhibit capsaicin evoked calcium influx by Na+/K+ ATPase activation. Immunofluorescence showed TRPV1 co-expression with α1β1 Na+/K+ ATPase in most neurons while others were either TRPV1 or α1β1 Na+/K+ ATPase positive.

Ultra-Low-Cost Disposable Hand-Held Clinical-Scale Propane Gas Hyperpolarizer for Pulmonary Magnetic Resonance Imaging Sensing

Hyperpolarized magnetic resonance imaging (MRI) contrast agents are revolutionizing the field of biomedical imaging. Hyperpolarized Xe-129 was recently FDA approved as an inhalable MRI contrast agent for functional lung imaging sensing. Despite success in research settings, modern Xe-129 hyperpolarizers are expensive (up to $1M), large, and complex to site and operate. Moreover, Xe-129 sensing requires specialized MRI hardware that is not commonly available on clinical MRI scanners. Here, we demonstrate that proton-hyperpolarized propane gas can be produced on demand using a disposable, hand-held, clinical-scale hyperpolarizer via parahydrogen-induced polarization, which relies on parahydrogen as a source of hyperpolarization. The device consists of a heterogeneous catalytic reactor connected to a gas mixture storage can containing pressurized hyperpolarization precursors: propylene and parahydrogen (10 bar total pressure). Once the built-in flow valve of the storage can is actuated, the precursors are ejected from the can into a reactor, and a stream of hyperpolarized propane gas is ejected from the reactor. Robust operation of the device is demonstrated for producing proton sensing polarization of 1.2% in a wide range of operational pressures and gas flow rates. We demonstrate that the propylene/parahydrogen gas mixture can retain potency for days in the storage can with a monoexponential decay time constant of 6.0 ± 0.5 days, which is limited by the lifetime of the parahydrogen singlet spin state in the storage container. The utility of the produced sensing agent is demonstrated for phantom imaging on a 3 T clinical MRI scanner located 100 miles from the agent/device preparation site and also for ventilation imaging of excised pig lungs using a 0.35 T clinical MRI scanner. The cost of the device components is less than $35, which we envision can be reduced to less than $5 for mass-scale production. The hyperpolarizer device can be reused, recycled, or disposed.

Electrical Signals at the Plasma Membrane and Their Influence on Chlorophyll Fluorescence of Chara Chloroplasts in vivo

Action potentials of plant cells are engaged in the regulation of many cell processes, including photosynthesis and cytoplasmic streaming. Excitable cells of characean algae submerged in a medium with an elevated K+ content are capable of generating hyperpolarizing electrical responses. These active responses of plasma membrane originate upon the passage of inward electric current comparable in strength to natural currents circulating in illuminated Chara internodes. So far, it remained unknown whether the hyperpolarizing electrical signals in Chara affect the photosynthetic activity. Here, we showed that the negative shift of cell membrane potential, which drives K+ influx into the cytoplasm, is accompanied by a delayed decrease in the actual yield of chlorophyll fluorescence F' and the maximal fluorescence yield Fm' under low background light (12.5 µmol m-2 s-1). The transient changes in F' and Fm' were evident only under illumination, which suggests their close relation to the photosynthetic energy conversion in chloroplasts. Passing the inward current caused an increase in pH at the cell surface (pHo), which reflected high H+/OH- conductance of the plasmalemma and indicated a decrease in cytoplasmic pH due to the H+ entry into the cell. The shifts in pHo arising in response to the first hyperpolarizing pulse disappeared upon repeated stimulation, thus indicating the long-term inactivation of plasmalemmal H+/OH- conductance. Suppression of plasmalemmal H+ fluxes did not abolish the hyperpolarizing responses and the analyzed changes in chlorophyll fluorescence. These results suggest that K+ fluxes between the extracellular medium, cytoplasm, and stroma are involved in the functional changes of chloroplasts reflected by transients of F' and Fm'.

Parahydrogen-Polarized [1-13 C]Pyruvate for Reliable and Fast Preclinical Metabolic Magnetic Resonance Imaging

Hyperpolarization techniques increase nuclear spin polarization by more than four orders of magnitude, enabling metabolic MRI. Even though hyperpolarization has shown clear value in clinical studies, the complexity, cost and slowness of current equipment limits its widespread use. Here, a polarization procedure of [1-13 C]pyruvate based on parahydrogen-induced polarization by side-arm hydrogenation (PHIP-SAH) in an automated polarizer is demonstrated. It is benchmarked in a study with 48 animals against a commercial dissolution dynamic nuclear polarization (d-DNP) device. Purified, concentrated (≈70-160 mM) and highly hyperpolarized (≈18%) solutions of pyruvate are obtained at physiological pH for volumes up to 2 mL within 85 s in an automated process. The safety profile, image quality, as well as the quantitative perfusion and lactate-to-pyruvate ratios, are equivalent for PHIP and d-DNP, rendering PHIP a viable alternative to established hyperpolarization techniques.

A Simulation of the Effects of Diffusion on Hyperpolarized [1-13C]-Pyruvate Signal Evolution

OBJECTIVE

Hyperpolarized [1-13C]-pyruvate magnetic resonance imaging is an emerging metabolic imaging method that offers unprecedented spatiotemporal resolution for monitoring tumor metabolism in vivo. To establish robust imaging biomarkers of metabolism, we must characterize phenomena that may modulate the apparent pyruvate-to-lactate conversion rate (kPL). Here, we investigate the potential effect of diffusion on pyruvate-to-lactate conversion, as failure to account for diffusion in pharmacokinetic analysis may obscure true intracellular chemical conversion rates.

METHODS

Changes in hyperpolarized pyruvate and lactate signal were calculated using a finite-difference time domain simulation of a two-dimensional tissue model. Signal evolution curves with intracellular kPL values from 0.02 to 1.00 s-1 were analyzed using spatially invariant one-compartment and two-compartment pharmacokinetic models. A second spatially variant simulation incorporating compartmental instantaneous mixing was fit with the same one-compartment model.

RESULTS

When fitting with the one-compartment model, apparent kPL underestimated intracellular kPL by approximately 50% at an intracellular kPL of 0.02 s-1. This underestimation increased for larger kPL values. However, fitting the instantaneous mixing curves showed that diffusion accounted for only a small part of this underestimation. Fitting with the two-compartment model yielded more accurate intracellular kPL values.

SIGNIFICANCE

This work suggests diffusion is not a significant rate-limiting factor in pyruvate-to-lactate conversion given that our model assumptions hold true. In higher order models, diffusion effects may be accounted for by a term characterizing metabolite transport. Pharmacokinetic models used to analyze hyperpolarized pyruvate signal evolution should focus on carefully selecting the analytical model for fitting rather than accounting for diffusion effects.

Computational modeling of the variation of the transmembrane potential of the endothelial cells of the blood-brain-barrier subject to an external electric field

The electromechanical properties of the membrane of endothelial cells forming the blood-brain barrier play a vital role in the function of this barrier. The mechanical effect exerted by external electric fields on the membrane could change its electrical properties. In this study the effect of extremely low frequency (ELF) external electric fields on the electrical activity of these cells has been studied by considering the mechanical effect of these fields on the capacitance of the membrane. The effect of time-dependent capacitance of the membrane is incorporated in the current components of the parallel conductance model for the electrical activity of the cells. The results show that the application of ELF electric fields induces hyperpolarization, having an indirect effect on the release of nitric oxide from the endothelial cell and the polymerization of actin filaments. Accordingly, this could play an important role in the permeability of the barrier. Our finding can have possible consequences in the field of drug delivery into the central nervous system.

Signaling Roleplay between Ion Channels during Mammalian Sperm Capacitation

In order to accomplish their primary goal, mammalian spermatozoa must undergo a series of physiological, biochemical, and functional changes crucial for the acquisition of fertilization ability. Spermatozoa are highly polarized cells, which must swiftly respond to ionic changes on their passage through the female reproductive tract, and which are necessary for male gametes to acquire their functional competence. This review summarizes the current knowledge about specific ion channels and transporters located in the mammalian sperm plasma membrane, which are intricately involved in the initiation of changes within the ionic milieu of the sperm cell, leading to variations in the sperm membrane potential, membrane depolarization and hyperpolarization, changes in sperm motility and capacitation to further lead to the acrosome reaction and sperm-egg fusion. We also discuss the functionality of selected ion channels in male reproductive health and/or disease since these may become promising targets for clinical management of infertility in the future.

Hyperpolarized [1-13C]Acetyl-l-Carnitine Probes Tricarboxylic Acid Cycle Activity In Vivo

Mitochondrial oxidative phosphorylation (OXPHOS) is sensitive to a variety of biological factors, and dysregulated OXPHOS is observed during the development of numerous pathological conditions. ATP production via OXPHOS is intrinsically dependent on the availability of acetyl-coenzyme A (CoA), which can enter the tricarboxylic acid (TCA) cycle to drive the oxidative pathway. Acetyl-l-carnitine (ALCAR) is an interchangeable endogenous source of acetyl-CoA, and therefore, ALCAR-derived probes are uniquely positioned for the assessment of OXPHOS. In this report, we develop hyperpolarized (HP) [1-13C]ALCAR as a noninvasive probe to investigate cardiac TCA cycle activity in vivo. We initially synthesized the isotopically labeled substrate and demonstrated that the 13C nucleus maintained a suitable T1 value (50.1 ± 0.8 s at 3 T) and polarization levels (21.3 ± 5.3%) to execute in vivo metabolic measurements. HP [1-13C]ALCAR was employed for cardiac analyses of OXPHOS in rats under fed and fasted conditions. [5-13C]Glutamate was successfully detected, and the metabolite was used to analyze the TCA cycle activity in both nutritional states. These assessments were compared to analogous experiments with the HP [1-13C]pyruvate. Our report represents the first study to demonstrate that HP methods using [1-13C]ALCAR enable direct analyses of mitochondrial function and TCA cycle activity, which are fundamental to cardiac cell homeostasis.

Reversible Parahydrogen Induced Hyperpolarization of 15 N in Unmodified Amino Acids Unraveled at High Magnetic Field

Amino acids (AAs) and ammonia are metabolic markers essential for nitrogen metabolism and cell regulation in both plants and humans. NMR provides interesting opportunities to investigate these metabolic pathways, yet lacks sensitivity, especially in case of 15 N. In this study, spin order embedded in p-H2 is used to produce on-demand reversible hyperpolarization in 15 N of pristine alanine and ammonia under ambient protic conditions directly in the NMR spectrometer. This is made possible by designing a mixed-ligand Ir-catalyst, selectively ligating the amino group of AA by exploiting ammonia as a strongly competitive co-ligand and preventing deactivation of Ir by bidentate ligation of AA. The stereoisomerism of the catalyst complexes is determined by hydride fingerprinting using 1 H/D scrambling of the associated N-functional groups on the catalyst (i.e., isotopological fingerprinting), and unravelled by 2D-ZQ-NMR. Monitoring the transfer of spin order from p-H2 to 15 N nuclei of ligated and free alanine and ammonia targets using SABRE-INEPT with variable exchange delays pinpoints the monodentate elucidated catalyst complexes to be most SABRE active. Also RF-spin locking (SABRE-SLIC) enables transfer of hyperpolarization to 15 N. The presented high-field approach can be a valuable alternative to SABRE-SHEATH techniques since the obtained catalytic insights (stereochemistry and kinetics) will remain valid at ultra-low magnetic fields.

Hyperpolarized Carbon-13 MRI in Breast Cancer

One of the hallmarks of cancer is metabolic reprogramming, including high levels of aerobic glycolysis (the Warburg effect). Pyruvate is a product of glucose metabolism, and ¹³C-MR imaging of the metabolism of hyperpolarized (HP) [1-¹³C]pyruvate (HP ¹³C-MRI) has been shown to be a potentially versatile tool for the clinical evaluation of tumor metabolism. Hyperpolarization of the ¹³C nuclear spin can increase the sensitivity of detection by 4-5 orders of magnitude. Therefore, following intravenous injection, the location of hyperpolarized ¹³C-labeled pyruvate in the body and its subsequent metabolism can be tracked using ¹³C-MRI. Hyperpolarized [¹³C]urea and [1,4-¹³C₂]fumarate are also likely to translate to the clinic in the near future as tools for imaging tissue perfusion and post-treatment tumor cell death, respectively. For clinical breast imaging, HP ¹³C-MRI can be combined with ¹H-MRI to address the need for detailed anatomical imaging combined with improved functional tumor phenotyping and very early identification of patients not responding to standard and novel neoadjuvant treatments. If the technical complexity of the hyperpolarization process and the relatively high associated costs can be reduced, then hyperpolarized ¹³C-MRI has the potential to become more widely available for large-scale clinical trials.

RNA interference reveals the escape response mechanism of Paramecium to mechanical stimulation

In Paramecium, a mechanical stimulus applied to the posterior portion of the cell causes a transient increase in membrane permeability to potassium ions, transiently rendering the membrane in a hyperpolarized state. Hyperpolarization causes a transient increase in Cyclic adenosine monophosphate (cAMP) concentration in the cilia, resulting in a transient fast-forward swimming of the cell. Schultz and coworkers (1992) reported that a unique adenylate cyclase (AC)-coupled potassium channel is involved in the reaction underlying this response, which is known as the "escape response." However, the AC responsible for this reaction remains to be identified. Moreover, the molecular linkage between mechanoreception and AC activation has not been elucidated adequately. Currently, we can perform an efficient and simple gene-knockdown technique in Paramecium using RNA interference (RNAi). Paramecium is one of the several model organisms for which whole-genome sequences have been elucidated. The RNAi technique can be applied to whole genome sequences derived from the Paramecium database (ParameciumDB) to investigate the types of proteins that elicit specific biological responses and compare them with those of other model organisms. In this review, we describe the applications of the RNAi technique in elucidating the molecular mechanism underlying the escape response and identifying the AC involved in this reaction. The findings of this study highlight the advantages of the RNAi technique and ParameciumDB.

Deuterated TEKPol Biradicals and the Spin-Diffusion Barrier in MAS DNP

The sensitivity of NMR spectroscopy is considerably enhanced by dynamic nuclear polarization (DNP). In DNP polarization is transferred from unpaired electrons of a polarizing agent to nearby proton spins. In solids, this transfer is followed by the transport of hyperpolarization to the bulk via 1H-1H spin diffusion. The efficiency of these steps is critical to obtain high sensitivity gains, but the pathways for polarization transfer in the region near the unpaired electron spins are unclear. Here we report a series of seven deuterated and one fluorinated TEKPol biradicals to probe the effect of deprotonation on MAS DNP at 9.4 T. The experimental results are interpreted with numerical simulations, and our findings support that strong hyperfine couplings to nearby protons determine high transfer rates across the spin diffusion barrier to achieve short build-up times and high enhancements. Specifically, 1H DNP build-up times increase substantially with TEKPol isotopologues that have fewer hydrogen atoms in the phenyl rings, suggesting that these protons play a crucial role transferring the polarization to the bulk. Based on this new understanding, we have designed a new biradical, NaphPol, which yields significantly increased NMR sensitivity, making it the best performing DNP polarizing agent in organic solvents to date.

The Steady-State ALTADENA RASER generates Continuous NMR Signals

A RASER (Radio Amplification by Stimulated Emission of Radiation) facilitates the study of nonlinear phenomena, as well as the determination of NMR parameters with high-precision. To achieve maximum sensitivity in the desired operating mode, it is crucial to control the RASER over long periods of time. So far, this was only possible at ultra-low magnetic fields. Here, we introduce a way to control the operating regime of a RASER at a magnetic field of 1.45 T. We employ a continuous-flow RASER, pumped by PHIP (ParaHydrogen Induced Polarization). The hydrogenation of vinyl acetate (VA) with parahydrogen provides the required negative polarization on the methyl group of the product ethyl acetate (EA). The protons within the methyl group, separated by a 7 Hz J-coupling, are RASER active.  This system demonstrates five RASER phenomena: inequivalent and equivalent amplitudes in the "normal NMR mode", period doublings, frequency combs, and chaos. The experiments match with simulations based on a theoretical model of two nonlinear-coupled RASER modes. We predict the RASER regime at set conditions, and visualize the prediction in a bifurcation diagram.

Considering whole-body metabolism in hyperpolarized MRI through 13 C breath analysis-An alternative way to quantification and normalization?

PURPOSE

Hyperpolarized [1-¹³ C]pyruvate MRI is an emerging clinical tool for metabolic imaging. It has the potential for absolute quantitative metabolic imaging. However, the method itself is not quantitative, limiting comparison of images across both time and between individuals. Here, we propose a simple signal normalization to the whole-body oxidative metabolism to overcome this limitation.

THEORY AND METHODS

A simple extension of the model-free ratiometric analysis of hyperpolarized [1-¹³ C]pyruvate MRI is presented, using the expired ¹³ CO₂ in breath for normalization. The proposed framework was investigated in two porcine cohorts (N = 11) subjected to local renal hypoperfusion defects and subsequent [1-¹³ C]pyruvate MRI. A breath sample was taken before the [1-¹³ C]pyruvate injection and 5 min after. The raw MR signal from both the healthy and intervened kidney in the two cohorts was normalized using the ¹³ CO₂ in the expired air.

RESULTS

¹³ CO₂ content in the expired air was significantly different between the two cohorts. Normalization to this reduced the coefficients of variance in the aerobic metabolic sensitive pathways by 25% for the alanine/pyruvate ratio, and numerical changes were observed in the bicarbonate/pyruvate ratio. The lactate/pyruvate ratio was largely unaltered (<2%).

CONCLUSION

Our results indicate that normalizing the hyperpolarized ¹³ C-signal ratios by the ¹³ CO₂ content in expired air can reduce variation as well as improve specificity of the method by normalizing the metabolic readout to the overall metabolic status of the individual. The method is a simple and cheap extension to the hyperpolarized ¹³ C exam.

Real-Time Reaction Monitoring of Azide-Alkyne Cycloadditions Using Benchtop NMR-Based Signal Amplification by Reversible Exchange (SABRE)

Rufinamide, possessing a triazole ring, is a new antiepileptic drug (AED) relatively well-absorbed in the lower dose range (10 mg/kg per day) and is currently being used in antiepileptic medications. Triazole derivatives can interact with various enzymes and receptors in biological systems via diverse non-covalent interactions, thus inducing versatile biological effects. Strain-promoted azide-alkyne cycloaddition (SPAAC) is a significant method for obtaining triazoles, even under physiological conditions, in the absence of a copper catalyst. To confirm the progress of chemical reactions under biological conditions, research on reaction monitoring at low concentrations is essential. This promising strategy is gaining acceptance for applications in fields such as drug development and nanoscience. We investigated the optimum Ir catalyst and magnetic field for achieving maximum proton hyperpolarization transfer in triazole derivatives. These reactions were analyzed using signal amplification by reversible exchange (SABRE) to overcome the limitations of low sensitivity in nuclear magnetic resonance spectroscopy, when monitoring copper-free click reactions in real time. Finally, a more versatile copper-catalyzed click reaction was monitored in real time, using a 60 MHz benchtop NMR system, in order to analyze the reaction mechanism.

Missing Pieces in Structure Puzzles: How Hyperpolarized NMR Spectroscopy Can Complement Structural Biology and Biochemistry

Structure determination lies at the heart of many biochemical research programs. However, the "giants": X-ray diffraction, electron microscopy, molecular dynamics simulations, and nuclear magnetic resonance, among others, leave quite a few dark spots on the structural pictures drawn of proteins, nucleic acids, membranes, and other biomacromolecules. For example, structural models under physiological conditions or of short-lived intermediates often remain out of reach of the established experimental methods. This account frames the possibility of including hyperpolarized, that is, dramatically signal-enhanced NMR in existing workflows to fill these spots with detailed depictions. We highlight how integrating methods based on dissolution dynamic nuclear polarization can provide valuable complementary information about formerly inaccessible conformational spaces for many systems. A particular focus will be on hyperpolarized buffers to facilitate the NMR structure determination of challenging systems.

Enhanced Contraction of Arterial Smooth Muscle Cell in Skin Artery Is Sensitive to Hyperpolarization Mediated by BKCa Channel in Chronic Constriction Injury Model Rat

Previous our study found that improvement of skin blood flow associated with neuropathic pain using vasodilators is useful for alleviation of neuropathic pain. In this study, we aimed to elucidate the mechanism underlying enhanced vasorelaxation induced by vasodilators, which increase cAMP and cyclic guanosine monophosphate (cGMP), in chronic constriction injury model rat. We assessed vasorelaxation effect of vasodilators by measurement of isometric contraction in isolated plantar artery from chronic constriction injury of sciatic nerve model rats. Nifedipine, a voltage-dependent Ca²⁺ channel inhibitor, NS1619, Ca²⁺-activated K⁺ (BK_Ca) channel opener, and diazoxide, an ATP-sensitive potassium channel opener, -induced vasorelaxation in ipsilateral plantar artery was enhanced compared to the these in contralateral plantar artery. Sodium nitroprusside (SNP), a nitric oxide (NO) donor, and substance P, a NK₁ receptor agonist, caused vasorelaxation in both ipsilateral and contralateral artery. The vasorelaxation induced by SNP and substance P in ipsilateral artery is enhanced compared to the these in contralateral artery. Isoprenaline, a β adrenoceptor agonist, and salbutamol, a β₂ adrenoceptor agonist, caused strong vasorelaxation in ipsilateral artery but not in contralateral artery. Iberiotoxin, a BK_Ca channel inhibitor, prominently suppressed the enhanced vasorelaxation induced by SNP, substance P, isoprenaline and salbutamol. In summary, the enhanced contraction of arterial smooth muscle cell in skin artery is sensitive to hyperpolarization in chronic constriction injury model rat. Furthermore, β adrenoceptor agonist would be a good drug to improve the decreased skin blood flow because it has selective vasorelaxation to ipsilateral plantar artery.

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.

Efficient SABRE-SHEATH Hyperpolarization of Potent Branched-Chain-Amino-Acid Metabolic Probe [1-13C]ketoisocaproate

Efficient 13C hyperpolarization of ketoisocaproate is demonstrated in natural isotopic abundance and [1-13C]enriched forms via SABRE-SHEATH (Signal Amplification By Reversible Exchange in SHield Enables Alignment Transfer to Heteronuclei). Parahydrogen, as the source of nuclear spin order, and ketoisocaproate undergo simultaneous chemical exchange with an Ir-IMes-based hexacoordinate complex in CD3OD. SABRE-SHEATH enables spontaneous polarization transfer from parahydrogen-derived hydrides to the 13C nucleus of transiently bound ketoisocaproate. 13C polarization values of up to 18% are achieved at the 1-13C site in 1 min in the liquid state at 30 mM substrate concentration. The efficient polarization build-up becomes possible due to favorable relaxation dynamics. Specifically, the exponential build-up time constant (14.3 ± 0.6 s) is substantially lower than the corresponding polarization decay time constant (22.8 ± 1.2 s) at the optimum polarization transfer field (0.4 microtesla) and temperature (10 °C). The experiments with natural abundance ketoisocaproate revealed polarization level on the 13C-2 site of less than 1%-i.e., one order of magnitude lower than that of the 1-13C site-which is only partially due to more-efficient relaxation dynamics in sub-microtesla fields. We rationalize the overall much lower 13C-2 polarization efficiency in part by less favorable catalyst-binding dynamics of the C-2 site. Pilot SABRE experiments at pH 4.0 (acidified sample) versus pH 6.1 (unaltered sodium [1-13C]ketoisocaproate) reveal substantial modulation of SABRE-SHEATH processes by pH, warranting future systematic pH titration studies of ketoisocaproate, as well as other structurally similar ketocarboxylate motifs including pyruvate and alpha-ketoglutarate, with the overarching goal of maximizing 13C polarization levels in these potent molecular probes. Finally, we also report on the pilot post-mortem use of HP [1-13C]ketoisocaproate in a euthanized mouse, demonstrating that SABRE-hyperpolarized 13C contrast agents hold promise for future metabolic studies.

SABRE Hyperpolarization with up to 200 bar Parahydrogen in Standard and Quickly Removable Solvents

Parahydrogen (p-H₂)-based techniques are known to drastically enhance NMR signals but are usually limited by p-H₂ supply. This work reports p-H₂-based SABRE hyperpolarization at p-H₂ pressures of hundreds of bar, far beyond the typical ten bar currently reported in the literature. A recently designed high-pressure setup was utilized to compress p-H₂ gas up to 200 bar. The measurements were conducted using a sapphire high-pressure NMR tube and a 43 MHz benchtop NMR spectrometer. In standard methanol solutions, it could be shown that the signal intensities increased with pressure until they eventually reached a plateau. A polarization of about 2%, equal to a molar polarization of 1.2 mmol L^-1, could be achieved for the sample with the highest substrate concentration. While the signal plateaued, the H₂ solubility increased linearly with pressure from 1 to 200 bar, indicating that p-H₂ availability is not the limiting factor in signal enhancement beyond a certain pressure, depending on sample composition. Furthermore, the possibility of using liquefied ethane and compressed CO₂ as removable solvents for hyperpolarization was demonstrated. The use of high pressures together with quickly removable organic/non-organic solvents represents an important breakthrough in the field of hyperpolarization, advancing SABRE as a promising tool for materials science, biophysics, and molecular imaging.

Hyperpolarizing DNA Nucleobases via NMR Signal Amplification by Reversible Exchange

The present work investigates the potential for enhancing the NMR signals of DNA nucleobases by parahydrogen-based hyperpolarization. Signal amplification by reversible exchange (SABRE) and SABRE in Shield Enables Alignment Transfer to Heteronuclei (SABRE-SHEATH) of selected DNA nucleobases is demonstrated with the enhancement (ε) of ¹H, ¹⁵N, and/or ¹³C spins in 3-methyladenine, cytosine, and 6-O-guanine. Solutions of the standard SABRE homogenous catalyst Ir(1,5-cyclooctadeine)(1,3-bis(2,4,6-trimethylphenyl)imidazolium)Cl ("IrIMes") and a given nucleobase in deuterated ethanol/water solutions yielded low ¹H ε values (≤10), likely reflecting weak catalyst binding. However, we achieved natural-abundance enhancement of ¹⁵N signals for 3-methyladenine of ~3300 and ~1900 for the imidazole ring nitrogen atoms. ¹H and ¹⁵N 3-methyladenine studies revealed that methylation of adenine affords preferential binding of the imidazole ring over the pyrimidine ring. Interestingly, signal enhancements (ε~240) of both ¹⁵N atoms for doubly labelled cytosine reveal the preferential binding of specific tautomer(s), thus giving insight into the matching of polarization-transfer and tautomerization time scales. ¹³C enhancements of up to nearly 50-fold were also obtained for this cytosine isotopomer. These efforts may enable the future investigation of processes underlying cellular function and/or dysfunction, including how DNA nucleobase tautomerization influences mismatching in base-pairing.

Metabolic Tumor Imaging with Rapidly Signal-Enhanced 1-13 C-Pyruvate-d3

The metabolism of malignant cells differs significantly from that of healthy cells and thus, it is possible to perform metabolic imaging to reveal not only the exact location of a tumor, but also intratumoral areas of high metabolic activity. Herein, we demonstrate the feasibility of metabolic tumor imaging using signal-enhanced 1-¹³ C-pyruvate-d₃ , which is rapidly enhanced via para-hydrogen, and thus, the signal is amplified by several orders of magnitudes in less than a minute. Using as a model, human melanoma xenografts injected with signal-enhanced 1-¹³ C-pyruvate-d3, we show that the conversion of pyruvate into lactate can be monitored along with its kinetics, which could pave the way for rapidly detecting and monitoring changes in tumor metabolism.

EPR and Related Magnetic Resonance Imaging Techniques in Cancer Research

Imaging tumor microenvironments such as hypoxia, oxygenation, redox status, and/or glycolytic metabolism in tissues/cells is useful for diagnostic and prognostic purposes. New imaging modalities are under development for imaging various aspects of tumor microenvironments. Electron Paramagnetic Resonance Imaging (EPRI) though similar to NMR/MRI is unique in its ability to provide quantitative images of pO₂ in vivo. The short electron spin relaxation times have been posing formidable challenge to the technology development for clinical application. With the availability of the narrow line width trityl compounds, pulsed EPR imaging techniques were developed for pO₂ imaging. EPRI visualizes the exogenously administered spin probes/contrast agents and hence lacks the complementary morphological information. Dynamic nuclear polarization (DNP), a phenomenon that transfers the high electron spin polarization to the surrounding nuclear spins (¹H and ¹³C) opened new capabilities in molecular imaging. DNP of ¹³C nuclei is utilized in metabolic imaging of 13C-labeled compounds by imaging specific enzyme kinetics. In this article, imaging strategies mapping physiologic and metabolic aspects in vivo are reviewed within the framework of their application in cancer research, highlighting the potential and challenges of each of them.

Parahydrogen-Induced Hyperpolarization of Unsaturated Phosphoric Acid Derivatives

Parahydrogen-induced nuclear polarization offers a significant increase in the sensitivity of NMR spectroscopy to create new probes for medical diagnostics by magnetic resonance imaging. As precursors of the biocompatible hyperpolarized probes, unsaturated derivatives of phosphoric acid, propargyl and allyl phosphates, are proposed. The polarization transfer to ¹H and ³¹P nuclei of the products of their hydrogenation by parahydrogen under the ALTADENA and PASADENA conditions, and by the PH-ECHO-INEPT+ pulse sequence of NMR spectroscopy, resulted in a very high signal amplification, which is among the largest for parahydrogen-induced nuclear polarization transfer to the ³¹P nucleus.

Proton-Only Sensing of Hyperpolarized [1,2-13C2]Pyruvate

Hyperpolarized MRI is emerging as a next-generation molecular imaging modality that can detect metabolic transformations in real time deep inside tissue and organs. 13C-hyperpolarized pyruvate is the leading hyperpolarized contrast agent that can probe cellular energetics in real time. Currently, hyperpolarized MRI requires specialized "multinuclear" MRI scanners that have the ability to excite and detect 13C signals. The objective of this work is the development of an approach that works on conventional (i.e., proton-only) MRI systems while taking advantage of long-lived 13C hyperpolarization. The long-lived singlet state of [1,2-13C2]pyruvate is hyperpolarized with parahydrogen in reversible exchange, and subsequently, the polarization is transferred from the 13C2 spin pair to the methyl protons of pyruvate for detection. This polarization transfer is accomplished with spin-lock induced crossing pulses that are only applied to the methyl protons yet access the hyperpolarization stored in the 13C2 singlet state. Theory and first experimental demonstrations are provided for our method, which obviates 13C excitation and detection for proton sensing of 13C-hyperpolarized pyruvate with an overall experimental-polarization transfer efficiency of ∼22% versus a theoretically predicted polarization transfer efficiency of 25%.

Micro-Slab Coil Design for Hyperpolarized Metabolic Flux Analysis in Multiple Samples

Abnormal metabolism is a hallmark of cancer cells. Accumulating evidence suggests that metabolic changes are likely to occur before other cellular responses in cancer cells upon drug treatment. Therefore, the metabolic activity or flux in cancer cells could be a potent biomarker for cancer detection and treatment monitoring. Magnetic resonance (MR)-based sensing technologies have been developed with hyperpolarized molecules for real-time flux analysis, but they still suffer from low sensitivity and throughput. To address this limitation, we have developed an innovative miniaturized MR coil, termed micro-slab MR coil, for simultaneous analysis of metabolic flux in multiple samples. Combining this approach with hyperpolarized probes, we were able to quantify the pyruvate-to-lactate flux in two different leukemic cell lines in a non-destructive manner, simultaneously. Further, we were able to rapidly assess flux changes with drug treatment in a single hyperpolarization experiment. This new multi-sample system has the potential to transform our ability to assess metabolic dynamics at scale.

Investigating Rubidium Density and Temperature Distributions in a High-Throughput (129)Xe-Rb Spin-Exchange Optical Pumping Polarizer

Accurate knowledge of the rubidium (Rb) vapor density, [Rb], is necessary to correctly model the spin dynamics of 129Xe-Rb spin-exchange optical pumping (SEOP). Here we present a systematic evaluation of [Rb] within a high-throughput 129Xe-Rb hyperpolarizer during continuous-flow SEOP. Near-infrared (52S1/2→52P1/2 (D1)/52P3/2 (D2)) and violet (52S1/2→62P1/2/62P3/2) atomic absorption spectroscopy was used to measure [Rb] within 3.5 L cylindrical SEOP cells containing different spatial distributions and amounts of Rb metal. We were able to quantify deviation from the Beer-Lambert law at high optical depth for D2 and 62P3/2 absorption by comparison with measurements of the D1 and 62P1/2 absorption lines, respectively. D2 absorption deviates from the Beer-Lambert law at [Rb]D2>4×1017 m−3 whilst 52S1/2→62P3/2 absorption deviates from the Beer-Lambert law at [Rb]6P3/2>(4.16±0.01)×1019 m−3. The measured [Rb] was used to estimate a 129Xe-Rb spin exchange cross section of γ′=(1.2±0.1)×10−21 m3 s−1, consistent with spin-exchange cross sections from the literature. Significant [Rb] heterogeneity was observed in a SEOP cell containing 1 g of Rb localized at the back of the cell. While [Rb] homogeneity was improved for a greater surface area of the Rb source distribution in the cell, or by using a Rb presaturator, the measured [Rb] was consistently lower than that predicted by saturation Rb vapor density curves. Efforts to optimize [Rb] and thermal management within spin polarizer systems are necessary to maximize potential future enhancements of this technology.

Observing Nearby Nuclei on Paramagnetic Trityls and MOFs via DNP and Electron Decoupling

Dynamic nuclear polarization (DNP) is an NMR sensitivity enhancement technique that mediates polarization transfer from unpaired electrons to NMR-active nuclei. Despite its success in elucidating important structural information on biological and inorganic materials, the detailed polarization-transfer pathway from the electrons to the nearby and then the bulk solvent nuclei, and finally to the molecules of interest-remains unclear. In particular, the nuclei in the paramagnetic polarizing agent play significant roles in relaying the enhanced NMR polarizations to more remote nuclei. Despite their importance, the direct NMR observation of these nuclei is challenging because of poor sensitivity. Here, we show that a combined DNP and electron decoupling approach can facilitate direct NMR detection of these nuclei. We achieved an ∼80 % improvement in NMR intensity via electron decoupling at 0.35 T and 80 K on trityl radicals. Moreover, we recorded a DNP enhancement factor of ϵ ${\varepsilon{} }$ ∼90 and ∼11 % higher NMR intensity using electron decoupling on paramagnetic metal-organic framework, magnesium hexaoxytriphenylene (MgHOTP MOF).

Magnetic Resonance Imaging and Spectroscopy Methods to Study Hepatic Glucose Metabolism and Their Applications in the Healthy and Diabetic Liver

The liver plays an important role in whole-body glucose homeostasis by taking up glucose from and releasing glucose into the blood circulation. In the postprandial state, excess glucose in the blood circulation is stored in hepatocytes as glycogen. In the postabsorptive state, the liver produces glucose by breaking down glycogen and from noncarbohydrate precursors such as lactate. In metabolic diseases such as diabetes, these processes are dysregulated, resulting in abnormal blood glucose levels. Magnetic resonance imaging (MRI) and magnetic resonance spectroscopy (MRS) are noninvasive techniques that give unique insight into different aspects of glucose metabolism, such as glycogenesis, glycogenolysis, and gluconeogenesis, in the liver in vivo. Using these techniques, liver glucose metabolism has been studied in regard to a variety of interventions, such as fasting, meal intake, and exercise. Moreover, deviations from normal hepatic glucose metabolism have been investigated in both patients with type 1 and 2 diabetes, as well as the effects of antidiabetic medications. This review provides an overview of current MR techniques to measure hepatic glucose metabolism and the insights obtained by the application of these techniques in the healthy and diabetic liver.

Catalyst-Free Aqueous Hyperpolarized [1-13C]Pyruvate Obtained by Re-Dissolution Signal Amplification by Reversible Exchange

Despite great successes in oncology, patient outcomes are often still discouraging, and hence the diagnostic imaging paradigm is increasingly shifting toward functional imaging of the pathology to better understand individual disease biology and to personalize therapies. The dissolution Dynamic Nuclear Polarization (d-DNP) hyperpolarization method has enabled unprecedented real-time MRI sensing of metabolism and tissue pH using hyperpolarized [1-13C]pyruvate as a biosensor with great potential for diagnosis and monitoring of cancer patients. However, current d-DNP is expensive and suffers from long hyperpolarization times, posing a substantial translational roadblock. Here, we report the development of Re-Dissolution Signal Amplification By Reversible Exchange (Re-D SABRE), which relies on fast and low-cost hyperpolarization of [1-13C]pyruvate by chemical exchange with parahydrogen at microtesla magnetic fields. [1-13C]pyruvate is precipitated from catalyst-containing methanol using ethyl acetate and rapidly reconstituted in aqueous media. 13C polarization of 9 ± 1% is demonstrated after redissolution in water with residual iridium mass fraction of 8.5 ± 1.5 ppm; further improvement is anticipated via process automation. Re-D SABRE makes hyperpolarized [1-13C]pyruvate biosensor available at a fraction of the cost (<$10,000) and production time (≈1 min) of currently used techniques and makes aqueous hyperpolarized [1-13C]pyruvate "ready" for in vivo applications.

Subsecond Three-Dimensional Nitrogen-15 Magnetic Resonance Imaging Facilitated by Parahydrogen-Based Hyperpolarization

Magnetic resonance imaging (MRI) provides unique information about the internal structure and function of living organisms in a non-invasive way. The use of conventional proton MRI for the observation of real-time metabolism is hampered by the dominant signals of water and fat, which are abundant in living organisms. Heteronuclear MRI in conjunction with the hyperpolarization methods does not encounter this issue. In this work, we polarized 15N nuclei of [15N1]fampridine (a drug used for the treatment of multiple sclerosis) to the level of 4% in nuclear magnetic resonance (NMR) experiments and 0.7% in MRI studies using spin-lock-induced crossing combined with signal amplification by reversible exchange. Consequently, three-dimensional 15N MRI of the hyperpolarized 15N-labeled drug was acquired in 0.1 s with a signal-to-noise ratio of 70. In addition, the NMR signal enhancements for 15N-enriched fampridine and fampridine with a natural abundance of 15N nuclei were compared and an explanation for their difference was proposed.

Kinetic analysis of multi-resolution hyperpolarized 13 C human brain MRI to study cerebral metabolism

PURPOSE

To investigate multi-resolution hyperpolarized (HP) 13 C pyruvate MRI for measuring kinetic conversion rates in the human brain.

METHODS

HP [1-13 C]pyruvate MRI was acquired in 6 subjects with a multi-resolution EPI sequence at 7.5 × 7.5 mm2 resolution for pyruvate and 15 × 15 mm2 resolution for lactate and bicarbonate. With the same lactate data, 2 quantitative maps of pyruvate-to-lactate conversion (kPL ) maps were generated: 1 using 7.5 × 7.5 mm2 resolution pyruvate data and the other using synthetic 15 × 15 mm2 resolution pyruvate data to simulate a standard constant resolution acquisition. To examine local kPL values, 4 voxels were manually selected in each study representing brain tissue near arteries, brain tissue near veins, white matter, and gray matter.

RESULTS

High resolution 7.5 × 7.5 mm2 pyruvate images increased the spatial delineation of brain structures and decreased partial volume effects compared to coarser resolution 15 × 15 mm2 pyruvate images. Voxels near arteries, veins and in white matter exhibited higher calculated kPL for multi-resolution images.

CONCLUSION

Acquiring HP 13 C pyruvate metabolic data with a multi-resolution approach minimized partial volume effects from vascular pyruvate signals while maintaining the SNR of downstream metabolites. Higher resolution pyruvate images for kinetic fitting resulted in increased kinetic rate values, particularly around the superior sagittal sinus and cerebral arteries, by reducing extracellular pyruvate signal contributions from adjacent blood vessels. This HP 13 C study showed that acquiring pyruvate with finer resolution improved the quantification of kinetic rates throughout the human brain.

Synthesis of 13 C and 2 H Labeled Vinyl Pyruvate and Hyperpolarization of Pyruvate

The hyperpolarization of nuclear spins has enabled unique applications in chemistry, biophysics, and particularly metabolic imaging. Parahydrogen-induced polarization (PHIP) offers a fast and cost-efficient way of hyperpolarization. Nevertheless, PHIP lags behind dynamic nuclear polarization (DNP), which is already being evaluated in clinical studies. This shortcoming is mainly due to problems in the synthesis of the corresponding PHIP precursor molecules. The most widely used DNP tracer in clinical studies, particularly for the detection of prostate cancer, is 1-¹³ C-pyruvate. The ideal derivative for PHIP is the deuterated vinyl ester because the spin physics allows for 100 % polarization. Unfortunately, there is no efficient synthesis for vinyl esters of β-ketocarboxylic acids in general and pyruvate in particular. Here, we present an efficient new method for the preparation of vinyl esters, including ¹³ C labeled, fully deuterated vinyl pyruvate using a palladium-catalyzed procedure. Using 50 % enriched parahydrogen and mild reaction conditions, a ¹³ C polarization of 12 % was readily achieved; 36 % are expected with 100 % pH₂ . Higher polarization values can be potentially achieved with optimized reaction conditions.

RF coil design for accurate parallel imaging on 13 C MRSI using 23 Na sensitivity profiles

PURPOSE

To develop a coil-based method to obtain accurate sensitivity profiles in ¹³ C MRI at 3T from the endogenous ²³ Na. An eight-channel array is designed for ¹³ C MR acquisitions. As application examples, the array is used for two-fold accelerated acquisitions of both hyperpolarized ¹³ C metabolic imaging of pig kidneys and the human brain.

METHODS

A flexible coil array was tuned optimally for ¹³ C at 3T (32.1 MHz), with the coil coupling coefficients matched to be nearly identical at the resonance frequency of ²³ Na (33.8 MHz). This is done by enforcing a high decoupling (obtained through highly mismatched preamplifiers) and adjusting the coupling frequency response. The SNR performance is compared to reference coils.

RESULTS

The measured sensitivity profiles on a phantom showed high spatial similarity for ¹³ C and ²³ Na resonances, with average noise correlation of 9 and 11%, respectively. For acceleration factors 2, 3, and 4, the obtained maximum g-factors were 1.0, 1.1, and 2.6, respectively. The ²³ Na profiles obtained in vivo could be used successfully to perform two-fold acceleration of hyperpolarized ¹³ C 3D acquisitions of both pig kidneys and a healthy human brain.

CONCLUSION

A receive array has been developed in such a way that the ¹³ C sensitivity profiles could be accurately obtained from measurements at the ²³ Na frequency. This technique facilitates accelerated acquisitions for hyperpolarized ¹³ C imaging. The SNR performance obtained at the ¹³ C frequency, compares well to other state-of-the-art coils for the same purpose, showing slightly better superficial and central SNR.

Concentration-dependent effects of dichloroacetate in type 2 diabetic hearts assessed by hyperpolarized [1-13 C]-pyruvate magnetic resonance imaging

Personalized medicine or individualized therapy promises a paradigm shift in healthcare. This is particularly true in complex and multifactorial diseases such as diabetes and the multitude of related pathophysiological complications. Diabetic cardiomyopathy represents an emerging condition that could be effectively treated if better diagnostic and, in particular, better therapeutic monitoring tools were available. In this study, we investigate the ability to differentiate low and high doses of metabolically targeted therapy in an obese type 2 diabetic rat model. Low-dose dichloroacetate (DCA) treatment was associated with increased lactate production, while no or little change was seen in bicarbonate production. High-dose DCA treatment was associated with a significant metabolic switch towards increased bicarbonate production. These findings support further studies using hyperpolarized [1-¹³ C]-pyruvate magnetic resonance imaging to differentiate treatment effects and thus allow for personalized titration of therapeutics.