Selective excitation of hydrogen doubles the yield and improves the robustness of parahydrogen-induced polarization of low-γ nuclei

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.

J Coupling Constants of <1 Hz Enable 13C Hyperpolarization of Pyruvate via Reversible Exchange of Parahydrogen

Observing pyruvate metabolism in vivo has become a focal point of molecular magnetic resonance imaging. Signal amplification by reversible exchange (SABRE) has recently emerged as a versatile hyperpolarization technique. Tuning of the spin order transfer (SOT) in SABRE is challenging as the small 1H-13C J couplings, in the 13C-pyruvate case, result in SOT being not readily discernible. We demonstrate an experimental method using frequency-selective excitation of parahydrogen-derived polarization SOT sequence (SEPP-SPINEPT); its application led to up to 5700-fold 13C signal gain. In this way, we estimated the lifetime of two Ir-pyruvate SABRE complexes alongside the individual probing of eight small 1H-13C J couplings that connect the hydride protons in these complexes to 1- and 2-13C pyruvate spins, affording values between 0 and 2.69 Hz. Using electronic structure calculations, we define the low-energy structure of the corresponding complexes. Hence, this study demonstrates a novel approach to analyzing the spin topology of short-lived organometallic complexes.

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.

Spying on parahydrogen-induced polarization transfer using a half-tesla benchtop MRI and hyperpolarized imaging enabled by automation

Nuclear spin hyperpolarization is a quantum effect that enhances the nuclear magnetic resonance signal by several orders of magnitude and has enabled real-time metabolic imaging in humans. However, the translation of hyperpolarization technology into routine use in laboratories and medical centers is hampered by the lack of portable, cost-effective polarizers that are not commercially available. Here, we present a portable, automated polarizer based on parahydrogen-induced hyperpolarization (PHIP) at an intermediate magnetic field of 0.5 T (achieved by permanent magnets). With a footprint of 1 m2, we demonstrate semi-continuous, fully automated 1H hyperpolarization of ethyl acetate-d6 and ethyl pyruvate-d6 to P = 14.4% and 16.2%, respectively, and a 13C polarization of 1-13C-ethyl pyruvate-d6 of P = 7%. The duty cycle for preparing a dose is no more than 1 min. To reveal the full potential of 1H hyperpolarization in an inhomogeneous magnetic field, we convert the anti-phase PHIP signals into in-phase peaks, thereby increasing the SNR by a factor of 5. Using a spin-echo approach allowed us to observe the evolution of spin order distribution in real time while conserving the expensive reagents for reaction monitoring, imaging and potential in vivo usage. This compact polarizer will allow us to pursue the translation of hyperpolarized MRI towards in vivo applications further.

Modern Manufacturing Enables Magnetic Field Cycling Experiments and Parahydrogen-Induced Hyperpolarization with a Benchtop NMR

Benchtop NMR (btNMR) spectrometers are revolutionizing the way we use NMR and lowering the cost drastically. Magnetic field cycling (MFC) experiments with precise timing and control over the magnetic field, however, were hitherto not available on btNMRs, although some systems exist for high-field, high-resolution NMR spectrometers. Still, the need and potential for btNMR MFC is great─e.g., to perform and analyze parahydrogen-induced hyperpolarization, another method that has affected analytical chemistry and NMR beyond expectations. Here, we describe a setup that enables MFC on btNMRs for chemical analysis and hyperpolarization. Taking full advantage of the power of modern manufacturing, including computer-aided design, three-dimensional printing, and microcontrollers, the setup is easy to reproduce, highly reliable, and easy to adjust and operate. Within 380 ms, the NMR tube was shuttled reliably from the electromagnet to the NMR isocenter (using a stepper motor and gear rod). We demonstrated the power of this setup by hyperpolarizing nicotinamide using signal amplification by reversible exchange (SABRE), a versatile method to hyperpolarize a broad variety of molecules including metabolites and drugs. Here, the standard deviation of SABRE hyperpolarization was between 0.2 and 3.3%. The setup also allowed us to investigate the field dependency of the polarization and the effect of different sample preparation protocols. We found that redissolution of the activated and dried Ir catalyst always reduced the polarization. We anticipate that this design will greatly accelerate the ascension of MFC experiments for chemical analysis with btNMR─adding yet another application to this rapidly developing field.

Radio-Frequency Sweeps at Microtesla Fields for Parahydrogen-Induced Polarization of Biomolecules

Magnetic resonance imaging of ¹³C-labeled metabolites enhanced by parahydrogen-induced polarization (PHIP) enables real-time monitoring of processes within the body. We introduce a robust, easily implementable technique for transferring parahydrogen-derived singlet order into ¹³C magnetization using adiabatic radio frequency sweeps at microtesla fields. We experimentally demonstrate the applicability of this technique to several molecules, including some molecules relevant for metabolic imaging, where we show significant improvements in the achievable polarization, in some cases reaching above 60% nuclear spin polarization. Furthermore, we introduce a site-selective deuteration scheme, where deuterium is included in the coupling network of a pyruvate ester to enhance the efficiency of the polarization transfer. These improvements are enabled by the fact that the transfer protocol avoids relaxation induced by strongly coupled quadrupolar nuclei.

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.

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.

High-resolution NMR spectroscopy for measuring complex samples based on chemical-shift-difference selection

NMR spectroscopy serves as an immensely powerful tool for component assignments and molecular structure elucidations. However, proton NMR spectra are generally trapped with spectral congestion caused by limited frequency differences and complex multiplets. 2D NMR can effectively relieve spectral congestion, but its resolution and acquisition efficiency are restricted by the broad spectral bandwidth. Herein, we introduce an NMR method based on chemical-shift-difference selection by chirp excitation to record high-resolution 2D NMR spectra for extracting coupling correlation networks and multiplet structures, suitable for measurements on complex samples. The performance of the proposed method is illustrated in determining diastereotopic methylene protons, small frequency-difference coupled proton pairs of furanose, pyranose and benzene rings. This study is expected to benefit molecular structure elucidation and composition analysis of complex samples in chemistry, biochemistry and metabonomics.

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.

Parahydrogen-induced polarization and spin order transfer in ethyl pyruvate at high magnetic fields

Nuclear magnetic resonance has experienced great advances in developing and translating hyperpolarization methods into procedures for fundamental and clinical studies. Here, we propose the use of a wide-bore NMR for large-scale (volume- and concentration-wise) production of hyperpolarized media using parahydrogen-induced polarization. We discuss the benefits of radio frequency-induced parahydrogen spin order transfer, we show that 100% polarization is theoretically expected for homogeneous B₀ and B₁ magnetic fields for a three-spin system. Moreover, we estimated that the efficiency of spin order transfer is not significantly reduced when the B₁ inhomogeneity is below ± 5%; recommendations for the sample size and RF coils are also given. With the latest breakthrough in the high-yield synthesis of 1-¹³C-vinyl pyruvate and its deuterated isotopologues, the high-field PHIP-SAH will gain increased attention. Some remaining challenges will be addressed shortly.

Parahydrogen-induced polarization and spin order transfer in ethyl pyruvate at high magnetic fields

Nuclear magnetic resonance has experienced great advances in developing and translating hyperpolarization methods into procedures for fundamental and clinical studies. Here, we propose the use of a wide-bore NMR for large-scale (volume- and concentration-wise) production of hyperpolarized media using parahydrogen-induced polarization. We discuss the benefits of radio frequency-induced parahydrogen spin order transfer, we show that 100% polarization is theoretically expected for homogeneous B₀ and B₁ magnetic fields for a three-spin system. Moreover, we estimated that the efficiency of spin order transfer is not significantly reduced when the B₁ inhomogeneity is below ± 5%; recommendations for the sample size and RF coils are also given. With the latest breakthrough in the high-yield synthesis of 1-¹³C-vinyl pyruvate and its deuterated isotopologues, the high-field PHIP-SAH will gain increased attention. Some remaining challenges will be addressed shortly.

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.

Symmetry Constraints on Spin Order Transfer in Parahydrogen-Induced Polarization (PHIP)

It is well known that the association of parahydrogen (pH2) with an unsaturated molecule or a transient metalorganic complex can enhance the intensity of NMR signals; the effect is known as parahydrogen-induced polarization (PHIP). During recent decades, numerous methods were proposed for converting pH2-derived nuclear spin order to the observable magnetization of protons or other nuclei of interest, usually 13C or 15N. Here, we analyze the constraints imposed by the topological symmetry of the spin systems on the amplitude of transferred polarization. We find that in asymmetric systems, heteronuclei can be polarized to 100%. However, the amplitude drops to 75% in A2BX systems and further to 50% in A3B2X systems. The latter case is of primary importance for biological applications of PHIP using sidearm hydrogenation (PHIP-SAH). If the polarization is transferred to the same type of nuclei, i.e., 1H, symmetry constraints impose significant boundaries on the spin-order distribution. For AB, A2B, A3B, A2B2, AA’(AA’) systems, the maximum average polarization for each spin is 100%, 50%, 33.3%, 25%, and 0, respectively, (where A and B (or A’) came from pH2). Remarkably, if the polarization of all spins in a molecule is summed up, the total polarization grows asymptotically with ~1.27 and can exceed 2 in the absence of symmetry constraints (where is the number of spins). We also discuss the effect of dipole–dipole-induced pH2 spin-order distribution in heterogeneous catalysis or nematic liquid crystals. Practical examples from the literature illustrate our theoretical analysis.

Quasi-continuous production of highly hyperpolarized carbon-13 contrast agents every 15 seconds within an MRI system

Hyperpolarized contrast agents (HyCAs) have enabled unprecedented magnetic resonance imaging (MRI) of metabolism and pH in vivo. Producing HyCAs with currently available methods, however, is typically time and cost intensive. Here, we show virtually-continuous production of HyCAs using parahydrogen-induced polarization (PHIP), without stand-alone polarizer, but using a system integrated in an MRI instead. Polarization of ≈2% for [1-¹³C]succinate-d₂ or ≈19% for hydroxyethyl-[1-¹³C]propionate-d₃ was created every 15 s, for which fast, effective, and well-synchronized cycling of chemicals and reactions in conjunction with efficient spin-order transfer was key. We addressed these challenges using a dedicated, high-pressure, high-temperature reactor with integrated water-based heating and a setup operated via the MRI pulse program. As PHIP of several biologically relevant HyCAs has recently been described, this Rapid-PHIP technique promises fast preclinical studies, repeated administration or continuous infusion within a single lifetime of the agent, as well as a prolonged window for observation with signal averaging and dynamic monitoring of metabolic alterations.