Robust Imidazole‐ 15 N 2 Synthesis for High‐Resolution Low‐Field (0.05 T) 15 N Hyperpolarized NMR Spectroscopy

Enhancing 19F Benchtop NMR Spectroscopy by Combining para-Hydrogen Hyperpolarization and Multiplet Refocusing

Benchtop NMR spectrometers provide a promising alternative to high-field NMR for applications that are limited by instrument size and/or cost. ¹⁹F benchtop NMR is attractive due to the larger chemical shift range of ¹⁹F relative to ¹H and the lack of background signal in most applications. However, practical applications of benchtop ¹⁹F NMR are limited by its low sensitivity due to the relatively weak field strengths of benchtop NMR spectrometers. Here we present a sensitivity-enhancement strategy that combines SABRE (Signal Amplification By Reversible Exchange) hyperpolarization with the multiplet refocusing method SHARPER (Sensitive, Homogeneous, And Resolved PEaks in Real time). When applied to a range of fluoropyridines, SABRE-SHARPER achieves overall signal enhancements of up to 5700-fold through the combined effects of hyperpolarization and line-narrowing. This approach can be generalized to the analysis of mixtures through the use of a selective variant of the SHARPER sequence, selSHARPER. The ability of SABRE-selSHARPER to simultaneously boost sensitivity and discriminate between two components of a mixture is demonstrated, where selectivity is achieved through a combination of selective excitation and the choice of polarization transfer field during the SABRE step.

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.

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.

Real-Time High-Sensitivity Reaction Monitoring of Important Nitrogen-Cycle Synthons by 15N Hyperpolarized Nuclear Magnetic Resonance

Here, we show how signal amplification by reversible exchange hyperpolarization of a range of ¹⁵N-containing synthons can be used to enable studies of their reactivity by ¹⁵N nuclear magnetic resonance (NO₂^– (28% polarization), ND₃ (3%), PhCH₂NH₂ (5%), NaN₃ (3%), and NO₃^– (0.1%)). A range of iridium-based spin-polarization transfer catalysts are used, which for NO₂^– work optimally as an amino-derived carbene-containing complex with a DMAP-d₂ coligand. We harness long ¹⁵N spin-order lifetimes to probe in situ reactivity out to 3 × T₁. In the case of NO₂^– (T₁ 17.7 s at 9.4 T), we monitor PhNH₂ diazotization in acidic solution. The resulting diazonium salt (¹⁵N-T₁ 38 s) forms within 30 s, and its subsequent reaction with NaN₃ leads to the detection of hyperpolarized PhN₃ (T₁ 192 s) in a second step via the formation of an identified cyclic pentazole intermediate. The role of PhN₃ and NaN₃ in copper-free click chemistry is exemplified for hyperpolarized triazole (T₁ < 10 s) formation when they react with a strained alkyne. We also demonstrate simple routes to hyperpolarized N₂ in addition to showing how utilization of ¹⁵N-polarized PhCH₂NH₂ enables the probing of amidation, sulfonamidation, and imine formation. Hyperpolarized ND₃ is used to probe imine and ND₄⁺ (T₁ 33.6 s) formation. Furthermore, for NO₂^–, we also demonstrate how the ¹⁵N-magnetic resonance imaging monitoring of biphasic catalysis confirms the successful preparation of an aqueous bolus of hyperpolarized ¹⁵NO₂^– in seconds with 8% polarization. Hence, we create a versatile tool to probe organic transformations that has significant relevance for the synthesis of future hyperpolarized pharmaceuticals.

Hyperpolarization of common antifungal agents with SABRE

Signal amplification by reversible exchange (SABRE) is a robust and inexpensive hyperpolarization (HP) technique to enhance nuclear magnetic resonance (NMR) spectroscopy and magnetic resonance imaging (MRI) signals using parahydrogen (pH2 ). The substrate scope of SABRE is continually expanding. Here, we present the polarization of three antifungal drugs (voriconazole, clotrimazole, and fluconazole) and elicit the detailed HP mechanisms for 1 H and 15 N nuclei. In this exploratory work, 15 N polarization values of ~1% were achieved using 50% pH2 in solution of 3-mM catalyst and 60-mM substrate in perdeuterated methanol. All hyperpolarized 15 N sites exhibited long T1 in excess of 1 min at a clinically relevant field of 1 T. Hyperpolarizing common drugs is of interest due to their potential biomedical applications as MRI contrast agents or to enable studies on protein dynamics at physiological concentrations. We optimize the polarization with respect to temperature and the polarization transfer field (PTF) for 1 H nuclei in the millitesla regime and for 15 N nuclei in the microtesla regime, which provides detailed insights into exchange kinetics and spin evolution. This work broadens the SABRE substrate scope and provides mechanistic and kinetic insights into the HP process.

15 N hyperpolarisation of the antiprotozoal drug ornidazole by Signal Amplification By Reversible Exchange in aqueous medium

Signal amplification by reversible exchange (SABRE) offers a cost-effective route to boost nuclear magnetic resonance (NMR) signal by several orders of magnitude by employing readily available para-hydrogen as a source of hyperpolarisation. Although ¹ H spins have been the natural choice of SABRE hyperpolarisation since its inception due to its simplicity and accessibility, limited spin lifetimes of ¹ H makes it harder to employ them in a range of time-dependent NMR experiments. Heteronuclear spins, for example, ¹³ C and ¹⁵ N, in general have much longer T₁ lifetimes and thereby are found to be more suitable for hyperpolarised biological applications as demonstrated previously by para-hydrogen induced polarisation (PHIP) and dynamic nuclear polarisation (DNP). In this study we demonstrate a simple procedure to enhance ¹⁵ N signal of an antibiotic drug ornidazole by up to 71,000-folds with net ¹⁵ N polarisation reaching ~23%. Further, the effect of co-ligand strategy is studied in conjunction with the optimum field transfer protocols and consequently achieving ¹⁵ N hyperpolarised spin lifetime of >3 min at low field. Finally, we present a convenient route to harness the hyperpolarised solution in aqueous medium free from catalyst contamination leading to a strong ¹⁵ N signal detection for an extended duration of time.

Synthetic Approaches for 15 N-Labeled Hyperpolarized Heterocyclic Molecular Imaging Agents for 15 N NMR Signal Amplification by Reversible Exchange in Microtesla Magnetic Fields

NMR hyperpolarization techniques enhance nuclear spin polarization by several orders of magnitude resulting in corresponding sensitivity gains. This enormous sensitivity gain enables new applications ranging from studies of small molecules by using high-resolution NMR spectroscopy to real-time metabolic imaging in vivo. Several hyperpolarization techniques exist for hyperpolarization of a large repertoire of nuclear spins, although the 13 C and 15 N sites of biocompatible agents are the key targets due to their widespread use in biochemical pathways. Moreover, their long T1 allows hyperpolarized states to be retained for up to tens of minutes. Signal amplification by reversible exchange (SABRE) is a low-cost and ultrafast hyperpolarization technique that has been shown to be versatile for the hyperpolarization of 15 N nuclei. Although large sensitivity gains are enabled by hyperpolarization, 15 N natural abundance is only ∼0.4 %, so isotopic labeling of the molecules to be hyperpolarized is required in order to take full advantage of the hyperpolarized state. Herein, we describe selected advances in the preparation of 15 N-labeled compounds with the primary emphasis on using these compounds for SABRE polarization in microtesla magnetic fields through spontaneous polarization transfer from parahydrogen. Also, these principles can certainly be applied for hyperpolarization of these emerging contrast agents using dynamic nuclear polarization and other techniques.

15 N NMR Hyperpolarization of Radiosensitizing Antibiotic Nimorazole by Reversible Parahydrogen Exchange in Microtesla Magnetic Fields

Nimorazole belongs to the imidazole-based family of antibiotics to fight against anaerobic bacteria. Moreover, nimorazole is now in Phase 3 clinical trial in Europe for potential use as a hypoxia radiosensitizer for treatment of head and neck cancers. We envision the use of [15 N3 ]nimorazole as a theragnostic hypoxia contrast agent that can be potentially deployed in the next-generation MRI-LINAC systems. Herein, we report the first steps to create long-lasting (for tens of minutes) hyperpolarized state on three 15 N sites of [15 N3 ]nimorazole with T1 of up to ca. 6 minutes. The nuclear spin polarization was boosted by ca. 67000-fold at 1.4 T (corresponding to P15N of 3.2 %) by 15 N-15 N spin-relayed SABRE-SHEATH hyperpolarization technique, relying on simultaneous exchange of [15 N3 ]nimorazole and parahydrogen on polarization transfer Ir-IMes catalyst. The presented results pave the way to efficient spin-relayed SABRE-SHEATH hyperpolarization of a wide range of imidazole-based antibiotics and chemotherapeutics.

SABRE polarized low field rare-spin spectroscopy

High-field nuclear magnetic resonance (NMR) spectroscopy is an indispensable technique for identification and characterization of chemicals and biomolecular structures. In the vast majority of NMR experiments, nuclear spin polarization arises from thermalization in multi-Tesla magnetic fields produced by superconducting magnets. In contrast, NMR instruments operating at low magnetic fields are emerging as a compact, inexpensive, and highly accessible alternative but suffer from low thermal polarization at a low field strength and consequently a low signal. However, certain hyperpolarization techniques create high polarization levels on target molecules independent of magnetic fields, giving low-field NMR a significant sensitivity boost. In this study, SABRE (Signal Amplification By Reversible Exchange) was combined with high homogeneity electromagnets operating at mT fields, enabling high resolution ¹H, ¹³C, ¹⁵N, and ¹⁹F spectra to be detected with a single scan at magnetic fields between 1 mT and 10 mT. Chemical specificity is attained at mT magnetic fields with complex, highly resolved spectra. Most spectra are in the strong coupling regime where J-couplings are on the order of chemical shift differences. The spectra and the hyperpolarization spin dynamics are simulated with SPINACH. The simulations start from the parahydrogen singlet in the bound complex and include both chemical exchange and spin evolution at these mT fields. The simulations qualitatively match the experimental spectra and are used to identify the spin order terms formed during mT SABRE. The combination of low field NMR instruments with SABRE polarization results in sensitive measurements, even for rare spins with low gyromagnetic ratios at low magnetic fields.

A versatile synthetic route to the preparation of 15 N heterocycles

A robust medium-scale (approximately 3 g) synthetic method for 15 N labeling of pyridine (15 N-Py) is reported based on the Zincke reaction. 15 N enrichment in excess of 81% was achieved with approximately 33% yield. 15 N-Py serves as a standard substrate in a wide range of studies employing a hyperpolarization technique for efficient polarization transfer from parahydrogen to heteronuclei; this technique, called SABRE (signal amplification by reversible exchange), employs a simultaneous chemical exchange of parahydrogen and a to-be-hyperpolarized substrate (e.g., pyridine) on metal centers. In studies aimed at the development of hyperpolarized contrast agents for in vivo molecular imaging, pyridine is often employed either as a model substrate (for hyperpolarization technique development, quality assurance, and phantom imaging studies) or as a co-substrate to facilitate more efficient hyperpolarization of a wide range of emerging contrast agents (e.g., nicotinamide). Here, the produced 15 N-Py was used for the feasibility study of spontaneous 15 N hyperpolarization at high magnetic (HF) fields (7 T and 9.4 T) of an NMR spectrometer and an MRI scanner. SABRE hyperpolarization enabled acquisition of 2D MRI imaging of catalyst-bound 15 N-pyridine with 75 × 75 mm2 field of view (FOV), 32 × 32 matrix size, demonstrating the feasibility of 15 N HF-SABRE molecular imaging with 2.4 × 2.4 mm2 spatial resolution.

SABRE: Chemical kinetics and spin dynamics of the formation of hyperpolarization

In this review, we present the physical principles of the SABRE (Signal Amplification By Reversible Exchange) method. SABRE is a promising hyperpolarization technique that enhances NMR signals by transferring spin order from parahydrogen (an isomer of the H₂ molecule that is in a singlet nuclear spin state) to a substrate that is to be polarized. Spin order transfer takes place in a transient organometallic complex which binds both parahydrogen and substrate molecules; after dissociation of the SABRE complex, free hyperpolarized substrate molecules are accumulated in solution. An advantage of this method is that the substrate is not modified chemically, and its polarization can be regenerated multiple times by bubbling fresh parahydrogen through the solution. Thus, SABRE requires two key ingredients: (i) polarization transfer and (ii) chemical exchange of both parahydrogen and substrate. While there are several excellent reviews on applications of SABRE, the background of the method is discussed less frequently. In this review we aim to explain in detail how SABRE hyperpolarization is formed, focusing on key aspects of both spin dynamics and chemical kinetics, as well as on the interplay between them. Hence, we first cover the known spin order transfer methods applicable to SABRE - cross-relaxation, coherent spin mixing at avoided level crossings, and coherence transfer - and discuss their practical implementation for obtaining SABRE polarization in the most efficient way. Second, we introduce and explain the principle of SABRE hyperpolarization techniques that operate at ultralow (<1 μT), at low (1μT to 0.1 T) and at high (>0.1 T) magnetic fields. Finally, chemical aspects of SABRE are discussed in detail, including chemical systems that are amenable to SABRE and the exchange processes that are required for polarization formation. A theoretical treatment of the spin dynamics and their interplay with chemical kinetics is also presented. This review outlines known aspects of SABRE and provides guidelines for the design of new SABRE experiments, with the goal of solving practical problems of enhancing weak NMR signals.

Quasi-Resonance Fluorine-19 Signal Amplification by Reversible Exchange

We report on an extension of the quasi-resonance (QUASR) pulse sequence used for signal amplification by reversible exchange (SABRE), showing that we may target distantly J-coupled 19F-spins. Polarization transfer from the parahydrogen-derived hydrides to the 19F nucleus is accomplished via weak five-bond J-couplings using a shaped QUASR radio frequency pulse at a 0.05 T magnetic field. The net result is the direct generation of hyperpolarized 19F z-magnetization, derived from the parahydrogen singlet order. An accumulation of 19F polarization on the free ligand is achieved with subsequent repetition of this pulse sequence. The hyperpolarized 19F signal exhibits clear dependence on the pulse length, irradiation frequency, and delay time in a manner similar to that reported for 15N QUASR-SABRE. Moreover, the hyperpolarized 19F signals of 3-19F-14N-pyridine and 3-19F-15N-pyridine isotopologues are similar, suggesting that (i) polarization transfer via QUASR-SABRE is irrespective of the nitrogen isotopologue and (ii) the presence or absence of the spin-1/2 15N nucleus has no impact on the efficiency of QUASR-SABRE polarization transfer. Although optimization of polarization transfer efficiency to 19F (P19F ≈ 0.1%) was not the goal of this study, we show that high-field SABRE can be efficient and broadly applicable for direct hyperpolarization of 19F spins.

Hyperpolarizing Concentrated Metronidazole 15 NO2 Group over Six Chemical Bonds with More than 15 % Polarization and a 20 Minute Lifetime

The NMR hyperpolarization of uniformly 15 N-labeled [15 N3 ]metronidazole is demonstrated by using SABRE-SHEATH. In this antibiotic, the 15 NO2 group is hyperpolarized through spin relays created by 15 N spins in [15 N3 ]metronidazole, and the polarization is transferred from parahydrogen-derived hydrides over six chemical bonds. In less than a minute of parahydrogen bubbling at approximately 0.4 μT, a high level of nuclear spin polarization (P15N ) of around 16 % is achieved on all three 15 N sites. This product of 15 N polarization and concentration of 15 N spins is around six-fold better than any previous value determined for 15 N SABRE-derived hyperpolarization. At 1.4 T, the hyperpolarized state persists for tens of minutes (relaxation time, T1 ≈10 min). A novel synthesis of uniformly 15 N-enriched metronidazole is reported with a yield of 15 %. This approach can potentially be used for synthesis of a wide variety of in vivo metabolic probes with potential uses ranging from hypoxia sensing to theranostic imaging.

NMR Spectroscopy for Metabolomics Research

Over the past two decades, nuclear magnetic resonance (NMR) has emerged as one of the three principal analytical techniques used in metabolomics (the other two being gas chromatography coupled to mass spectrometry (GC-MS) and liquid chromatography coupled with single-stage mass spectrometry (LC-MS)). The relative ease of sample preparation, the ability to quantify metabolite levels, the high level of experimental reproducibility, and the inherently nondestructive nature of NMR spectroscopy have made it the preferred platform for long-term or large-scale clinical metabolomic studies. These advantages, however, are often outweighed by the fact that most other analytical techniques, including both LC-MS and GC-MS, are inherently more sensitive than NMR, with lower limits of detection typically being 10 to 100 times better. This review is intended to introduce readers to the field of NMR-based metabolomics and to highlight both the advantages and disadvantages of NMR spectroscopy for metabolomic studies. It will also explore some of the unique strengths of NMR-based metabolomics, particularly with regard to isotope selection/detection, mixture deconvolution via 2D spectroscopy, automation, and the ability to noninvasively analyze native tissue specimens. Finally, this review will highlight a number of emerging NMR techniques and technologies that are being used to strengthen its utility and overcome its inherent limitations in metabolomic applications.

Detecting low concentrations of unsaturated C-C bonds by parahydrogen-induced polarization using an efficient home-built parahydrogen generator

Parahydrogen is a potentially significant source of hyperpolarization. However, a heat exchanger at an ultralow temperature, which is normally sustained wastefully using liquid nitrogen, is essential for the generation of hyperpolarized parahydrogen. In order to cut down on the use of liquid nitrogen, we employed a cryogenic storage dewar as the key component of our home-built parahydrogen generator, which lasted over 20 days with a single filling. Small concentrations of an unsaturated compound in a mixture were identified by hydrogenation in a principle-based experiment involving the use of hyperpolarization and phase difference. Less than 1 μl of styrene in 1 ml of chloroform was identified in a single scan with a 43 MHz benchtop nuclear magnetic resonance (NMR) spectrometer following hydrogenation with 50% parahydrogen. This method can potentially undergo a significant development through the use of high-field NMR techniques, higher parahydrogen concentrations, and increased scan times for data collection, among others. Because hydrogenation with parahydrogen induces a phase reversal during attachment to unsaturated CC bonds, it may be possible to detect many other unsaturated bonds in organic molecules. All in all, this study not only broadens the research on parahydrogen-based unsaturated-bond detection, but also facilitates the use of hyperpolarization by a broader range of researchers through the introduction of a long-lasting home-built parahydrogen generator.

Quasi-Resonance Signal Amplification by Reversible Exchange

Here we present the feasibility of NMR signal amplification by reversible exchange (SABRE) using radio frequency irradiation at low magnetic field (0.05 T) in the regime where the chemical shifts of free and catalyst-bound species are similar. In SABRE, the 15N-containing substrate and parahydrogen perform simultaneous chemical exchange on an iridium hexacoordinate complex. A shaped spin-lock induced crossing (SLIC) radio frequency pulse sequence followed by a delay is applied at quasi-resonance (QUASR) conditions of 15N spins of a 15N-enriched substrate. As a result of this pulse sequence application, 15N z-magnetization is created from the spin order of parahydrogen-derived hyperpolarized hydrides. The repetition of the pulse sequence block consisting of a shaped radio frequency pulse and the delay leads to the buildup of 15N magnetization. The modulation of this effect by the irradiation frequency, pulse duration and amplitude, delay duration, and number of pumping cycles was demonstrated. Pyridine-15N, acetonitrile-15N, and metronidazole-15N2-13C2 substrates were studied representing three classes of compounds (five- and six-membered heterocycles and nitrile), showing the wide applicability of the technique. Metronidazole-15N2-13C2 is an FDA-approved antibiotic that can be injected in large quantities, promising noninvasive and accurate hypoxia sensing. The 15N hyperpolarization levels attained with QUASR-SABRE on metronidazole-15N2-13C2 were more than 2-fold greater than those with SABRE-SHEATH (SABRE in shield enables alignment transfer to heteronuclei), demonstrating that QUASR-SABRE can deliver significantly more efficient means of SABRE hyperpolarization.

19F Hyperpolarization of 15N-3-19F-Pyridine Via Signal Amplification by Reversible Exchange

We report synthesis of ¹⁵N-3-¹⁹F-pyridine via Zincke salt formation with the overall 35% yield and 84% ¹⁵N isotopic purity. Hyperpolarization studies of Signal Amplification by Reversible Exchange (SABRE) and SABRE in SHield Enables Alignment Transfer to Heteronuclei (SABRE-SHEATH) were performed to investigate the mechanism of polarization transfer from parahydrogen-derived hydride protons to ¹⁹F nucleus in milli-Tesla and micro-Tesla magnetic field regimes in ¹⁵N-3-¹⁹F-pyridine and ¹⁴N-3-¹⁹F-pyridine. We found the mismatch between ¹⁵N and ¹⁹F magnetic field hyperpolarization profiles in the micro-Tesla regime indicating that the spontaneous hyperpolarization process likely happens directly from parahydrogen-derived hydride protons to ¹⁹F nucleus without spin-relaying via ¹⁵N site. In case of SABRE magnetic field regime (milli-Tesla magnetic field range), we found that magnetic field profiles for ¹H and ¹⁹F hyperpolarization are very similar, and ¹⁹F polarization levels are significantly lower than ¹H SABRE polarization levels and lower than ¹⁹F SABRE-SHEATH (i.e. obtained at micro-Tesla magnetic field) polarization levels. Our findings support the hypothesis that in milli-Tesla magnetic field regime, the process of ¹⁹F nuclei hyperpolarization is relayed via protons of substrate, and therefore is very inefficient. These findings are important in the context of improvement of the hyperpolarization hardware and rational design of the hyperpolarized molecular probes.

Parahydrogen-Based Hyperpolarization for Biomedicine

Magnetic resonance (MR) is one of the most versatile and useful physical effects used for human imaging, chemical analysis, and the elucidation of molecular structures. However, its full potential is rarely used, because only a small fraction of the nuclear spin ensemble is polarized, that is, aligned with the applied static magnetic field. Hyperpolarization methods seek other means to increase the polarization and thus the MR signal. A unique source of pure spin order is the entangled singlet spin state of dihydrogen, parahydrogen (pH2 ), which is inherently stable and long-lived. When brought into contact with another molecule, this "spin order on demand" allows the MR signal to be enhanced by several orders of magnitude. Considerable progress has been made in the past decade in the area of pH2 -based hyperpolarization techniques for biomedical applications. It is the goal of this Review to provide a selective overview of these developments, covering the areas of spin physics, catalysis, instrumentation, preparation of the contrast agents, and applications.

Toward Cleavable Metabolic/pH Sensing "Double Agents" Hyperpolarized by NMR Signal Amplification by Reversible Exchange

We show the simultaneous generation of hyperpolarized 13 C-labeled acetate and 15 N-labeled imidazole following spin-relay of hyperpolarization and hydrolysis of the acetyl moiety on 1-13 C-15 N2 -acetylimidazole. Using SABRE-SHEATH (Signal Amplification by Reversible Exchange in SHield Enables Alignment Transfer to Heteronuclei), transfer of spin order occurs from parahydrogen to acetylimidazole 15 N atoms and the acetyl 13 C site (≈263-fold enhancement), giving rise to relatively long hyperpolarization lifetimes at 0.3 T (T1 ≈52 s and ≈149 s for 13 C and 15 N, respectively). Immediately following polarization transfer, the 13 C-labeled acetyl group is hydrolytically cleaved to produce hyperpolarized 13 C-acetate/acetic acid (≈140-fold enhancement) and 15 N-imidazole (≈180-fold enhancement), the former with a 13 C T1 of ≈14 s at 0.3 T. Straightforward synthetic routes, efficient spin-relay of SABRE hyperpolarization, and facile bond cleavage open a door to the cheap and rapid generation of long-lived hyperpolarized states within a wide range of molecular targets, including biologically relevant carboxylic acid derivatives, for metabolic and pH imaging.

SABRE hyperpolarization enables high-sensitivity 1H and 13C benchtop NMR spectroscopy

Benchtop NMR spectrometers operating with low magnetic fields of 1-2 T at sub-ppm resolution show great promise as analytical platforms that can be used outside the traditional laboratory environment for industrial process monitoring. One current limitation that reduces the uptake of benchtop NMR is associated with the detection fields' reduced sensitivity. Here we demonstrate how para-hydrogen (p-H2) based signal amplification by reversible exchange (SABRE), a simple to achieve hyperpolarization technique, enhances agent detectability within the environment of a benchtop (1 T) NMR spectrometer so that informative 1H and 13C NMR spectra can be readily recorded for low-concentration analytes. SABRE-derived 1H NMR signal enhancements of up to 17 000-fold, corresponding to 1H polarization levels of P = 5.9%, were achieved for 26 mM pyridine in d4-methanol in a matter of seconds. Comparable enhancement levels can be achieved in both deuterated and protio solvents but now the SABRE-enhanced analyte signals dominate due to the comparatively weak thermally-polarized solvent response. The SABRE approach also enables the acquisition of 13C NMR spectra of analytes at natural isotopic abundance in a single scan as evidenced by hyperpolarized 13C NMR spectra of tens of millimolar concentrations of 4-methylpyridine. Now the associated signal enhancement factors are up to 45 500 fold (P = 4.0%) and achieved in just 15 s. Integration of an automated SABRE polarization system with the benchtop NMR spectrometer framework produces renewable and reproducible NMR signal enhancements that can be exploited for the collection of multi-dimensional NMR spectra, exemplified here by a SABRE-enhanced 2D COSY NMR spectrum.

Spin-Lattice Relaxation of Hyperpolarized Metronidazole in Signal Amplification by Reversible Exchange in Micro-Tesla Fields

Simultaneous reversible chemical exchange of parahydrogen and to-be-hyperpolarized substrate on metal centers enables spontaneous transfer of spin order from parahydrogen singlet to nuclear spins of the substrate. When performed at sub-micro-Tesla magnetic field, this technique of NMR Signal Amplification by Reversible Exchange in SHield Enables Alignment Transfer to Heteronuclei (SABRE-SHEATH). SABRE-SHEATH has been shown to hyperpolarize nitrogen-15 sites of a wide range of biologically interesting molecules to a high polarization level (P > 20%) in one minute. Here, we report on a systematic study of 1H, 13C and 15N spin-lattice relaxation (T1) of metronidazole-13C2-15N2 in SABRE-SHEATH hyperpolarization process. In micro-Tesla range, we find that all 1H, 13C and 15N spins studied share approximately the same T1 values (ca. 4 s at the conditions studied) due to mixing of their Zeeman levels, which is consistent with the model of relayed SABRE-SHEATH effect. These T1 values are significantly lower than those at higher magnetic (i.e. the Earth's magnetic field and above), which exceed 3 minutes in some cases. Moreover, these relatively short T1 values observed below 1 micro-Tesla limit the polarization build-up process of SABRE-SHEATH- thereby, limiting maximum attainable 15N polarization. The relatively short nature of T1 values observed below 1 micro-Tesla is primarily caused by intermolecular interactions with quadrupolar iridium centers or dihydride protons of the employed polarization transfer catalyst, whereas intramolecular spin-spin interactions with 14N quadrupolar centers have significantly smaller contribution. The presented experimental results and their analysis will be beneficial for more rational design of SABRE-SHEATH (i) polarization transfer catalyst, and (ii) hyperpolarized molecular probes in the context of biomedical imaging and other applications.

Spin Relays Enable Efficient Long-Range Heteronuclear Signal Amplification By Reversible Exchange

A systematic experimental study is reported on the polarization transfer to distant spins, which do not directly bind to the polarization transfer complexes employed in Signal Amplification By Reversible Exchange (SABRE) experiments. Both, long-range transfer to protons and long-range transfer to heteronuclei i.e. 13C and 15N are examined. Selective destruction of hyperpolarization on 1H, 13C, and 15N sites is employed, followed by their re-hyperpolarization from neighboring spins within the molecules of interest (pyridine for 1H studies and metronidazole-15N2-13C2 for 13C and 15N studies). We conclude that long-range sites can be efficiently hyperpolarized when a network of spin-½ nuclei enables relayed polarization transfer (i.e. via short-range interactions between sites). In case of proton SABRE in the milli-Tesla regime, a relay network consisting of protons only is sufficient. However, in case 13C and 15N are targeted (i.e. via SABRE in SHield Enables Alignment Transfer to Heteronuclei or SABRE-SHEATH experiment), the presence of a heteronuclear network (e.g. consisting of 15N) enables a relay mechanism that is significantly more efficient than the direct transfer of spin order from para-H2-derived hydrides.

High-resolution hyperpolarized in vivo metabolic 13C spectroscopy at low magnetic field (48.7mT) following murine tail-vein injection

High-resolution ¹³C NMR spectroscopy of hyperpolarized succinate-1-¹³C-2,3-d₂ is reported in vitro and in vivo using a clinical-scale, biplanar (80cm-gap) 48.7mT permanent magnet with a high homogeneity magnetic field. Non-localized ¹³C NMR spectra were recorded at 0.52MHz resonance frequency over the torso of a tumor-bearing mouse every 2s. Hyperpolarized ¹³C NMR signals with linewidths of ∼3Hz (corresponding to ∼6ppm) were recorded in vitro (2mL in a syringe) and in vivo (over a mouse torso). Comparison of the full width at half maximum (FWHM) for ¹³C NMR spectra acquired at 48.7mT and at 4.7T in a small-animal MRI scanner demonstrates a factor of ∼12 improvement for the ¹³C resonance linewidth attainable at 48.7mT compared to that at 4.7T in vitro. ¹³C hyperpolarized succinate-1-¹³C resonance linewidths in vivo are at least one order of magnitude narrower at 48.7mT compared to those observed in high-field (≥3T) studies employing HP contrast agents. The demonstrated high-resolution ¹³C in vivo spectroscopy could be useful for high-sensitivity spectroscopic studies involving monitoring HP agent uptake or detecting metabolism using HP contrast agents with sufficiently large ¹³C chemical shift differences.

Toward Hyperpolarized 19 F Molecular Imaging via Reversible Exchange with Parahydrogen

Fluorine-19 has high NMR detection sensitivity-similar to that of protons-owing to its large gyromagnetic ratio and high natural abundance (100 %). Unlike protons, however, fluorine-19 (¹⁹ F) has a negligible occurrence in biological objects, as well as a more sensitive chemical shift. As a result, in vivo ¹⁹ F NMR spectroscopy and MR imaging offer advantages of negligible background signal and sensitive reporting of the local molecular environment. Here we report on NMR hyperpolarization of ¹⁹ F nuclei using reversible exchange reactions with parahydrogen gas as the source of nuclear spin order. NMR signals of 3-fluoropyridine were enhanced by ≈100 fold, corresponding to 0.3 % ¹⁹ F nuclear spin polarization (at 9.4 T), using about 50 % parahydrogen. While future optimization efforts will likely significantly increase the hyperpolarization levels, we already demonstrate the utility of ¹⁹ F hyperpolarization for high-resolution hyperpolarized ¹⁹ F imaging and hyperpolarized ¹⁹ F pH sensing.