Current concepts on hyperpolarized molecules in MRI

Exceptionally Mild and High-Yielding Synthesis of Vinyl Esters of Alpha-Ketocarboxylic Acids, Including Vinyl Pyruvate, for Parahydrogen-Enhanced Metabolic Spectroscopy and Imaging

Metabolic changes often occur long before pathologies manifest and treatment becomes challenging. As key elements of energy metabolism, α-ketocarboxylic acids (α-KCA) are particularly interesting, e.g., as the upregulation of pyruvate to lactate conversion is a hallmark of cancer (Warburg effect). Magnetic resonance imaging with hyperpolarized metabolites has enabled imaging of this effect non-invasively and in vivo, allowing the early detection of cancerous tissue and its treatment. Hyperpolarization by means of dynamic nuclear polarization, however, is complex, slow, and expensive, while available precursors often limit parahydrogen-based alternatives. Here, we report the synthesis for novel 13C, deuterated ketocarboxylic acids, and a much-improved synthesis of 1-13C-vinyl pruvate-d6, arguably the most promising tracer for hyperpolarizing pyruvate using parahydrogen-induced hyperpolarization by side arm hydrogenation. The new synthesis is scalable and provides a high yield of 52%. We elucidated the mechanism of our Pd-catalyzed trans-vinylation reaction. Hydrogenation with parahydrogen allowed us to monitor the addition, which was found to depend on the electron demand of the vinyl ester. Electron-poor α-keto vinyl esters react slower than "normal" alkyl vinyl esters. This synthesis of 13C, deuterated α-ketocarboxylic acids opens up an entirely new class of biomolecules for fast and cost-efficient hyperpolarization with parahydrogen and their use for metabolic imaging.

Design, synthesis and evaluation of 15N- and 13C-labeled molecular probes as hyperpolarized nitric oxide sensors

Nitric oxide (NO) is an important signaling molecule involved in a wide range of biological processes. Development of non-invasive, real-time detection of NO is greatly desired yet remains challenging. Here we report the design and development of novel 15N- and 13C-labeled NO-sensing probes for hyperpolarized nuclear magnetic resonance (HP-NMR) studies. These probes undergo selective and rapid reaction with NO to generate in situ AZO-products that can be monitored with distinguishable NMR signals as a read-out. This study also allows for a direct comparison of the 15N and 13C nuclei performances in hyperpolarized reaction-based probes. The simple and general SABRE-SHEATH hyperpolarization method works on the 15N- and 13C-NO-sensing probes. Measured long spin-lattice relaxation (T1) values, especially for 15N-NO probes, will allow for real-time reaction-based imaging of NO.

Enabling Clinical Technologies for Hyperpolarized 129 Xenon Magnetic Resonance Imaging and Spectroscopy

Hyperpolarization is a technique that can increase nuclear spin polarization with the corresponding gains in nuclear magnetic resonance (NMR) signals by 4-8 orders of magnitude. When this process is applied to biologically relevant samples, the hyperpolarized molecules can be used as exogenous magnetic resonance imaging (MRI) contrast agents. A technique called spin-exchange optical pumping (SEOP) can be applied to hyperpolarize noble gases such as 129 Xe. Techniques based on hyperpolarized 129 Xe are poised to revolutionize clinical lung imaging, offering a non-ionizing, high-contrast alternative to computed tomography (CT) imaging and conventional proton MRI. Moreover, CT and conventional proton MRI report on lung tissue structure but provide little functional information. On the other hand, when a subject breathes hyperpolarized 129 Xe gas, functional lung images reporting on lung ventilation, perfusion and diffusion with 3D readout can be obtained in seconds. In this Review, the physics of SEOP is discussed and the different production modalities are explained in the context of their clinical application. We also briefly compare SEOP to other hyperpolarization methods and conclude this paper with the outlook for biomedical applications of hyperpolarized 129 Xe to lung imaging and beyond.

Dynamic Nuclear Polarization of Silicon Carbide Micro- and Nanoparticles

Two dominant crystalline phases of silicon carbide (SiC): α-SiC and β-SiC, differing in size and chemical composition, were investigated regarding their potential for dynamic nuclear polarization (DNP). ²⁹Si nuclei in α-SiC micro- and nanoparticles with sizes ranging from 650 nm to 2.2 μm and minimal oxidation were successfully hyperpolarized without the use of free radicals, while β-SiC samples did not display appreciable degrees of polarization under the same polarization conditions. Long T₁ relaxation times in α-SiC of up to 1600 s (∼27 min) were recorded for the ²⁹Si nuclei after 1 h of polarization at a temperature of 4 K. Interestingly, these promising α-SiC particles allowed for direct hyperpolarization of both ²⁹Si and ¹³C nuclei, resulting in comparably strong signal amplifications. Moreover, the T₁ relaxation time of ¹³C nuclei in 750 nm-sized α-SiC particles was over 33 min, which far exceeds T₁ times of conventional ¹³C DNP probes with values in the order of 1-2 min. The present work demonstrates the feasibility of DNP on SiC micro- and nanoparticles and highlights their potential as hyperpolarized magnetic resonance imaging agents.

Probing carbohydrate metabolism using hyperpolarized 13 C-labeled molecules

Glycolysis is a fundamental metabolic process in all organisms. Anomalies in glucose metabolism are linked to various pathological conditions. In particular, elevated aerobic glycolysis is a characteristic feature of rapidly growing cells. Glycolysis and the closely related pentose phosphate pathway can be monitored in real time by hyperpolarized 13 C-labeled metabolic substrates such as 13 C-enriched, deuterated D-glucose derivatives, [2-13 C]-D-fructose, [2-13 C] dihydroxyacetone, [1-13 C]-D-glycerate, [1-13 C]-D-glucono-δ-lactone and [1-13 C] pyruvate in healthy and diseased tissues. Elevated glycolysis in tumors (the Warburg effect) was also successfully imaged using hyperpolarized [U-13 C6 , U-2 H7 ]-D-glucose, while the size of the preexisting lactate pool can be measured by 13 C MRS and/or MRI with hyperpolarized [1-13 C]pyruvate. This review summarizes the application of various hyperpolarized 13 C-labeled metabolites to the real-time monitoring of glycolysis and related metabolic processes in normal and diseased tissues.

Pharmacokinetics of the SABRE agent 4,6-d2-nicotinamide and also nicotinamide in rats following oral and intravenous administration

To prepare the way for using the isotopically labelled SABRE hyperpolarized 4,6-d₂-nicotinamide as an MRI agent in humans we have performed an in-vivo study to measure its pharmacokinetics in the plasma of healthy rats after intravenous and oral administration. Male Han Wistar rats were dosed with either 4,6-d₂-nicotinamide or the corresponding control, non-labelled nicotinamide, and plasma samples were obtained at eight time points for up to 24 h after administration. Pharmacokinetic parameters were determined from agent concentration-versus-time data for both 4,6-d₂-nicotinamide and nicotinamide. 4,6-d₂-Nicotinamide proved to be well tolerated regardless of route of administration at the concentrations used (20, 80 and 120 mg/kg). Pharmacokinetic parameters were similar after oral and intravenous administration and similar to those obtained for nicotinamide. Analysis of nicotinamide plasma concentrations after dosing 4,6-d₂-nicotinamide intravenously demonstrates a reversible exchange of endogenous nicotinamide by this labelled agent over the time-course of our assays. Supported by a large body of evidence for the safety of nicotinamide when dosed orally in humans, we conclude that 4,6-d₂-nicotinamide can also be safely administered intravenously, which will provide significant benefit when using this agent for planned imaging studies in humans.

Design strategy for serine hydroxymethyltransferase probes based on retro-aldol-type reaction

Serine hydroxymethyltransferase (SHMT) is an enzyme that catalyzes the reaction that converts serine to glycine. It plays an important role in one-carbon metabolism. Recently, SHMT has been shown to be associated with various diseases. Therefore, SHMT has attracted attention as a biomarker and drug target. However, the development of molecular probes responsive to SHMT has not yet been realized. This is because SHMT catalyzes an essential yet simple reaction; thus, the substrates that can be accepted into the active site of SHMT are limited. Here, we focus on the SHMT-catalyzed retro-aldol reaction rather than the canonical serine-glycine conversion and succeed in developing fluorescent and 19F NMR molecular probes. Taking advantage of the facile and direct detection of SHMT, the developed fluorescent probe is used in the high-throughput screening for human SHMT inhibitors, and two hit compounds are obtained.

Structural exploration of rhodium catalysts and their kinetic studies for efficient parahydrogen-induced polarization by side arm hydrogenation

New rhodium catalysts for parahydrogen-induced polarization.

Polymeric 1 H MRI Probes for Visualizing Tumor In Vivo

Magnetic resonance imaging (MRI) has become a prominent non- or low-invasive imaging technique, providing high-resolution, three-dimensional images as well as physiological information about tissues. Low-molecular-weight Gd-MRI contrast agents (CAs), such as Gd-DTPA (DTPA: diethylenetriaminepentaacetic acid), are commonly used in the clinical diagnosis, while macromolecular Gd-MRI CAs have several advantages over low-molecular-weight Gd-MRI CAs, which help minimize the dose of CAs and the risk of side effects. Accordingly, we developed chiral dendrimer Gd-MRI CAs, which showed high r₁ values. The association constant values (K_a ) of S-isomeric dendrimer CAs to bovine serum albumin (BSA) were higher than those of R-isomeric dendrimer CAs. Besides, based on a totally new concept, we developed ¹³ C/¹⁵ N-enriched multiple-resonance NMR/MRI probes, which realized highly selective observation of the probes and analysis of metabolic reactions of interest. This account summarizes our recent study on developing both chiral dendrimer Gd-MRI CAs, and self-traceable ¹³ C/¹⁵ N-enriched phosphorylcholine polymer probes for early detection of tumors.

Molecular imaging with engineered physiology

In vivo imaging techniques are powerful tools for evaluating biological systems. Relating image signals to precise molecular phenomena can be challenging, however, due to limitations of the existing optical, magnetic and radioactive imaging probe mechanisms. Here we demonstrate a concept for molecular imaging which bypasses the need for conventional imaging agents by perturbing the endogenous multimodal contrast provided by the vasculature. Variants of the calcitonin gene-related peptide artificially activate vasodilation pathways in rat brain and induce contrast changes that are readily measured by optical and magnetic resonance imaging. CGRP-based agents induce effects at nanomolar concentrations in deep tissue and can be engineered into switchable analyte-dependent forms and genetically encoded reporters suitable for molecular imaging or cell tracking. Such artificially engineered physiological changes, therefore, provide a highly versatile means for sensitive analysis of molecular events in living organisms.

(13)C/(15)N-Enriched l-Dopa as a Triple-Resonance NMR Probe to Monitor Neurotransmitter Dopamine in the Brain and Liver Extracts of Mice

In an attempt to monitor μm-level trace constituents, we applied here (1)H-{(13)C-(15)N} triple-resonance nuclear magnetic resonance (NMR) to (13)C/(15)N-enriched l-Dopa as the inevitable precursor of the neurotransmitter dopamine in the brain. The perfect selectivity (to render endogenous components silent) and μm-level sensitivity (700 MHz spectrometer equipped with a cryogenic probe) of triple-resonance allowed the unambiguous and quantitative metabolic and pharmacokinetic analyses of administered l-Dopa/dopamine in the brain and liver of mice. The level of dopamine generated in the brain (within the range 7-76 μm, which covers the typical stimulated level of ∼30 μm) could be clearly monitored ex vivo, but was slightly short of the detection limit of a 7 T MR machine for small animals. This work suggests that μm-level trace constituents are potential targets of ex vivo monitoring as long as they contain N atom(s) and their appropriate (13)C/(15)N-enrichment is synthetically accessible.

The Role of the Amino Protecting Group during Parahydrogenation of Protected Dehydroamino Acids

A series of dehydroamino acids endowed with different protective groups at the amino and carboxylate moieties and with different substituents at the double bond have been reacted with parahydrogen. The observed ParaHydrogen Induced Polarization (PHIP) effects in the (1)H NMR spectra are strongly dependent on the amino protecting group. DFT calculations allowed us to establish a relationship between the structures of the reaction intermediates (whose energies depend on the amido substitution) and the observed PHIP patterns.

Achieving 1% NMR polarization in water in less than 1min using SABRE

The development of biocompatible hyperpolarized media is a crucial step towards application of hyperpolarization in vivo. This article describes the achievement of 1% hyperpolarization of 3-amino-1,2,4-triazine protons in water using the parahydrogen induced polarization technique based on signal amplification by reversible exchange (SABRE). Polarization was achieved in less than 1 min.

Dual-contrast molecular imaging allows noninvasive characterization of myocardial ischemia/reperfusion injury after coronary vessel occlusion in mice by magnetic resonance imaging

BACKGROUND

Inflammation and myocardial necrosis play important roles in ischemia/reperfusion injury after coronary artery occlusion and recanalization. The detection of inflammatory activity and the extent of myocardial necrosis itself are of great clinical and prognostic interest. We developed a dual, noninvasive imaging approach using molecular magnetic resonance imaging in an in vivo mouse model of myocardial ischemia and reperfusion.

METHODS AND RESULTS

Ischemia/reperfusion injury was induced in 10-week-old C57BL/6N mice by temporary ligation of the left anterior descending coronary artery. Activated platelets were targeted with a contrast agent consisting of microparticles of iron oxide (MPIOs) conjugated to a single-chain antibody directed against a ligand-induced binding site (LIBS) on activated glycoprotein IIb/IIIa (LIBS-MPIOs). After injection and imaging of LIBS-MPIOs, late gadolinium enhancement was used to depict myocardial necrosis; these imaging experiments were also performed in P2Y12 (-/-) mice. All imaging results were correlated to immunohistochemistry findings. Activated platelets were detectable by magnetic resonance imaging via a significant signal effect caused by LIBS-MPIOs in the area of left anterior descending coronary artery occlusion 2 hours after reperfusion. In parallel, late gadolinium enhancement identified the extent of myocardial necrosis. Immunohistochemistry confirmed that LIBS-MPIOs bound significantly to microthrombi in reperfused myocardium. Only background binding was found in P2Y12 (-/-) mice.

CONCLUSIONS

Dual molecular imaging of myocardial ischemia/reperfusion injury allows characterization of platelet-driven inflammation by LIBS-MPIOs and myocardial necrosis by late gadolinium enhancement. This noninvasive imaging strategy is of clinical interest for both diagnostic and prognostic purposes and highlights the potential of molecular magnetic resonance imaging for characterizing ischemia/reperfusion injury.

A platform for designing hyperpolarized magnetic resonance chemical probes

Hyperpolarization is a highly promising technique for improving the sensitivity of magnetic resonance chemical probes. Here we report [¹⁵N, D₉]trimethylphenylammonium as a platform for designing a variety of hyperpolarized magnetic resonance chemical probes. The platform structure shows a remarkably long ¹⁵N spin–lattice relaxation value (816 s, 14.1 T) for retaining its hyperpolarized spin state. The extended lifetime enables the detection of the hyperpolarized ¹⁵N signal of the platform for several tens of minutes and thus overcomes the intrinsic short analysis time of hyperpolarized probes. Versatility of the platform is demonstrated by applying it to three types of hyperpolarized chemical probes: one each for sensing calcium ions, reactive oxygen species (hydrogen peroxide) and enzyme activity (carboxyl esterase). All of the designed probes achieve high sensitivity with rapid reactions and chemical shift changes, which are sufficient to allow sensitive and real-time monitoring of target molecules by ¹⁵N magnetic resonance.

Hyperpolarization of chemical nuclei is known to greatly increase sensitivity to characterization by magnetic resonance imaging. Here a new platform that allows for the design of a number of hyperpolarized probes for chemical sensing applications is demonstrated.

LabVIEW-based control software for para-hydrogen induced polarization instrumentation

The elucidation of cell metabolic mechanisms is the modern underpinning of the diagnosis, treatment, and in some cases the prevention of disease. Para-Hydrogen induced polarization (PHIP) enhances magnetic resonance imaging (MRI) signals over 10,000 fold, allowing for the MRI of cell metabolic mechanisms. This signal enhancement is the result of hyperpolarizing endogenous substances used as contrast agents during imaging. PHIP instrumentation hyperpolarizes Carbon-13 ((13)C) based substances using a process requiring control of a number of factors: chemical reaction timing, gas flow, monitoring of a static magnetic field (Bo), radio frequency (RF) irradiation timing, reaction temperature, and gas pressures. Current PHIP instruments manually control the hyperpolarization process resulting in the lack of the precise control of factors listed above, resulting in non-reproducible results. We discuss the design and implementation of a LabVIEW based computer program that automatically and precisely controls the delivery and manipulation of gases and samples, monitoring gas pressures, environmental temperature, and RF sample irradiation. We show that the automated control over the hyperpolarization process results in the hyperpolarization of hydroxyethylpropionate. The implementation of this software provides the fast prototyping of PHIP instrumentation for the evaluation of a myriad of (13)C based endogenous contrast agents used in molecular imaging.

Recent Advances in Metabolic Profiling And Imaging of Prostate Cancer

Cancer is a metabolic disease. Cancer cells, being highly proliferative, show significant alterations in metabolic pathways such as glycolysis, respiration, the tricarboxylic acid (TCA) cycle, oxidative phosphorylation, lipid metabolism, and amino acid metabolism. Metabolites like peptides, nucleotides, products of glycolysis, the TCA cycle, fatty acids, and steroids can be an important read out of disease when characterized in biological samples such as tissues and body fluids like urine, serum, etc. The cancer metabolome has been studied since the 1960s by analytical techniques such as mass spectrometry (MS) and nuclear magnetic resonance (NMR) spectroscopy. Current research is focused on the identification and validation of biomarkers in the cancer metabolome that can stratify high-risk patients and distinguish between benign and advanced metastatic forms of the disease. In this review, we discuss the current state of prostate cancer metabolomics, the biomarkers that show promise in distinguishing indolent from aggressive forms of the disease, the strengths and limitations of the analytical techniques being employed, and future applications of metabolomics in diagnostic imaging and personalized medicine of prostate cancer.

15N-permethylated amino acids as efficient probes for MRI-DNP applications

The synthesis, NMR properties and preliminary polarization tests on protonated and perdeuterated forms of α-trimethylglutamine (NMe3Gln), α-trimethylglutamate (NMe3Glu) and ε-trimethyllysine (NMe3Lys) are reported. The (15)N-permethylated, perdeuterated amino acids display very long (15)N-T1 values, ranging between 190 and 330 s, are well polarized by the dynamic nuclear polarization (DNP) procedure, yielding good polarization levels (10%), and appear to be well tolerated by cells and mice. The obtained results make perdeuterated amino acids excellent candidates for innovative DNP (15)N-MRI applications such as perfusion or targeting studies.

Chemistry and biochemistry of 13C hyperpolarized magnetic resonance using dynamic nuclear polarization

The study of transient chemical phenomena by conventional NMR has proved elusive, particularly for non-(1)H nuclei. For (13)C, hyperpolarization using the dynamic nuclear polarization (DNP) technique has emerged as a powerful means to improve SNR. The recent development of rapid dissolution DNP methods has facilitated previously impossible in vitro and in vivo study of small molecules. This review presents the basics of the DNP technique, identification of appropriate DNP substrates, and approaches to increase hyperpolarized signal lifetimes. Also addressed are the biochemical events to which DNP-NMR has been applied, with descriptions of several probes that have met with in vivo success.

Optimization of SABRE for polarization of the tuberculosis drugs pyrazinamide and isoniazid

Hyperpolarization produces nuclear spin polarization that is several orders of magnitude larger than that achieved at thermal equilibrium thus providing extraordinary contrast and sensitivity. As a parahydrogen induced polarization (PHIP) technique that does not require chemical modification of the substrate to polarize, Signal Amplification by Reversible Exchange (SABRE) has attracted a lot of attention. Using a prototype parahydrogen polarizer, we polarize two drugs used in the treatment of tuberculosis, namely pyrazinamide and isoniazid. We examine this approach in four solvents, methanol-d4, methanol, ethanol and DMSO and optimize the polarization transfer magnetic field strength, the temperature as well as intensity and duration of hydrogen bubbling to achieve the best overall signal enhancement and hence hyperpolarization level.

Para-hydrogen perspectives in hyperpolarized NMR

The first instance of para-hydrogen induced polarization (PHIP) in an NMR experiment was serendipitously observed in the 1980s while investigating a hydrogenation reaction (Seldler et al., 1983; Bowers and Weitekamp, 1986, 1987; Eisenschmid et al., 1987) [1-4]. Remarkably a theoretical investigation of the applicability of para-hydrogen as a hyperpolarization agent was being performed in the 1980's thereby quickly providing a theoretical basis for the PHIP-effect (Bowers and Weitekamp, 1986) [2]. The discovery of signal amplification by a non-hydrogenating interaction with para-hydrogen has recently extended the interest to exploit the PHIP effect, as it enables investigation of compounds without structural alteration while retaining the advantages of spectroscopy with hyperpolarized compounds [5]. In this article we will place more emphasis of the future applications of the method while only briefly discussing the efforts that have been made in the understanding of the phenomenon and the development of the method so far.

Improved PHIP polarization using a precision, low noise, voltage controlled current source

Existing para-hydrogen induced polarization (PHIP) instrumentation relies on magnetic fields to hyperpolarize substances. These hyperpolarized substances have enhanced magnetic resonance imaging (MRI) signals over 10,000 fold, allowing for MRI at the molecular level. Required magnetic fields are generated by energizing a solenoid coil with current produced by a voltage controlled voltage source (VCVS), also known as a power supply. A VCVS lacks the current regulation necessary to keep magnetic field fluctuations to a minimum, which results in low PHIP polarization. A voltage controlled current source (VCCS) is an electric circuit that generates a steady flow of electrons proportional to an input voltage. A low noise VCCS provides the solenoid current flow regulation necessary to generate a stable static magnetic field (Bo). We discuss the design and implementation of a low noise, high stability, VCCS for magnetic field generation with minimum variations. We show that a precision, low noise, voltage reference driving a metal oxide semiconductor field effect transistor (MOSFET) based current sink, results in the current flow control necessary for generating a low noise and high stability Bo. In addition, this work: (1) compares current stability for ideal VCVS and VCCS models using transfer functions (TF), (2) develops our VCCS design's TF, (3) measures our VCCS design's thermal & 1/f noise, and (4) measures and compares hydroxyethyl-propionate (HEP) polarization obtained using a VCVS and our VCCS. The hyperpolarization of HEP was done using a PHIP instrument developed in our lab. Using our VCCS design, HEP polarization magnitude data show a statistically significant increase in polarization over using a VCVS. Circuit schematic, bill of materials, board layout, TF derivation, and Matlab simulations code are included as supplemental files.

Utilizing a water-soluble cryptophane with fast xenon exchange rates for picomolar sensitivity NMR measurements

Hyperpolarized (129)Xe chemical exchange saturation transfer ((129)Xe Hyper-CEST) NMR is a powerful technique for the ultrasensitive, indirect detection of Xe host molecules (e.g., cryptophane-A). Irradiation at the appropriate Xe-cryptophane resonant radio frequency results in relaxation of the bound hyperpolarized (129)Xe and rapid accumulation of depolarized (129)Xe in bulk solution. The cryptophane effectively "catalyzes" this process by providing a unique molecular environment for spin depolarization to occur, while allowing xenon exchange with the bulk solution during the hyperpolarized lifetime (T(1) ≈ 1 min). Following this scheme, a triacetic acid cryptophane-A derivative (TAAC) was indirectly detected at 1.4 picomolar concentration at 320 K in aqueous solution, which is the record for a single-unit xenon host. To investigate this sensitivity enhancement, the xenon binding kinetics of TAAC in water was studied by NMR exchange lifetime measurement. At 297 K, k(on) ≈ 1.5 × 10(6) M(-1) s(-1) and k(off) = 45 s(-1), which represent the fastest Xe association and dissociation rates measured for a high-affinity, water-soluble xenon host molecule near rt. NMR line width measurements provided similar exchange rates at rt, which we assign to solvent-Xe exchange in TAAC. At 320 K, k(off) was estimated to be 1.1 × 10(3) s(-1). In Hyper-CEST NMR experiments, the rate of (129)Xe depolarization achieved by 14 pM TAAC in the presence of radio frequency (RF) pulses was calculated to be 0.17 μM·s(-1). On a per cryptophane basis, this equates to 1.2 × 10(4)(129)Xe atoms s(-1) (or 4.6 × 10(4) Xe atoms s(-1), all Xe isotopes), which is more than an order of magnitude faster than k(off), the directly measurable Xe-TAAC exchange rate. This compels us to consider multiple Xe exchange processes for cryptophane-mediated bulk (129)Xe depolarization, which provide at least 10(7)-fold sensitivity enhancements over directly detected hyperpolarized (129)Xe NMR signals.

Accessing long-lived nuclear singlet states between chemically equivalent spins without breaking symmetry

Long-lived nuclear spin states could greatly enhance the applicability of hyperpolarized nuclear magnetic resonance. Using singlet states between inequivalent spin pairs has been shown to extend the signal lifetime by more than an order of magnitude compared to the spin lattice relaxation time (T1), but they have to be prevented from evolving into other states. In the most interesting case the singlet is between chemically equivalent spins, as it can then be inherently an eigenstate. However this presents major challenges in the conversion from bulk magnetization to singlet. In the only case demonstrated so far, a reversible chemical reaction to break symmetry was required. Here we present a pulse sequence technique that interconverts between singlet spin order and bulk magnetization without breaking the symmetry of the spin system. This technique is independent of field strength and is applicable to a broad range of molecules.

Molecular body imaging: MR imaging, CT, and US. Part II. Applications

Molecular imaging is expected to have a major impact on the early diagnosis of diseases and disease monitoring in the next decade. Traditionally, nuclear imaging techniques have been the mainstay of molecular imaging in the clinical arena. However, with continued development of molecularly targeted contrast agents for nonnuclear imaging techniques such as magnetic resonance (MR), computed tomography (CT), and ultrasonography (US), the spectrum of clinical molecular imaging applications is expanding. In the second part of this review series, an overview of applications of molecular MR imaging-, CT-, and US-based imaging strategies that show promise for clinical translation is presented, and key challenges that need to be addressed to successfully translate these promising techniques in the future are discussed. © RSNA, 2012.

Molecular body imaging: MR imaging, CT, and US. part I. principles

Molecular imaging, generally defined as noninvasive imaging of cellular and subcellular events, has gained tremendous depth and breadth as a research and clinical discipline in recent years. The coalescence of major advances in engineering, molecular biology, chemistry, immunology, and genetics has fueled multi- and interdisciplinary innovations with the goal of driving clinical noninvasive imaging strategies that will ultimately allow disease identification, risk stratification, and monitoring of therapy effects with unparalleled sensitivity and specificity. Techniques that allow imaging of molecular and cellular events facilitate and go hand in hand with the development of molecular therapies, offering promise for successfully combining imaging with therapy. While traditionally nuclear medicine imaging techniques, in particular positron emission tomography (PET), PET combined with computed tomography (CT), and single photon emission computed tomography, have been the molecular imaging methods most familiar to clinicians, great advances have recently been made in developing imaging techniques that utilize magnetic resonance (MR), optical, CT, and ultrasonographic (US) imaging. In the first part of this review series, we present an overview of the principles of MR imaging-, CT-, and US-based molecular imaging strategies.

Metabolic imaging of multiple x-nucleus resonances

This study describes a technique for fast imaging of x-nuclei metabolites. Due to increased sensitivity and larger chemical shift dispersion at high magnetic fields, images of multiple metabolites can be obtained simultaneously by selective excitation of their resonances with a multifrequency selective radiofrequency pulse at any desired flip angle. This aim is achieved by combining a three-dimensional gradient echo imaging sequence with a Shinnar-LeRoux optimized excitation pulse. A proper choice of bandwidth, imaging matrix size, and field of view allows using the chemical shift dispersion of the different resonances to completely separate their images within one large field of view. The method of fast metabolic imaging is illustrated with (13)C measurements of a phantom containing a solution of (13)C labeled glucose, lactate, and sodium octanoate and by dynamic measurements of the (31)P metabolites phosphocreatine and β-adenosine triphosphate in human femoral muscle in vivo, both at 7T. With dynamic selective (31)P imaging of the larger part of the upper leg, phosphocreatine signal intensity changes of specific muscles can be studied simultaneously by analyzing the sum of phosphocreatine signals within arbitrarily shaped regions of interest following the muscles' contours. This concept of dynamic metabolic imaging can be applied to other organs and further expanded to other MR-detectable nuclei and metabolites.

Para-hydrogen induced polarization of Si-29 NMR resonances as a potentially useful tool for analytical applications

Para-hydrogen-induced polarization effects have been observed in the (29)Si NMR spectra of trimethylsilyl para-hydrogenated molecules. The high signal enhancements and the long T(1) values observed for the (29)Si hyperpolarized resonances point toward the possibility of using (29)Si for hyperpolarization applications. A method for the discrimination of multiple compounds and/or complex mixtures of hydroxylic compounds (such as steroids), consisting of the silylization of alcoholic functionalities with an unsaturated silylalkyl moiety and subsequent reaction with para-H(2), is proposed.

15N magnetic resonance hyperpolarization via the reaction of parahydrogen with 15N-propargylcholine

(15)N-Propargylcholine has been synthesized and hydrogenated with para-H(2). Through the application of a field cycling procedure, parahydrogen spin order is transferred to the (15)N resonance. Among the different isomers formed upon hydrogenation of (15)N-propargylcholine, only the nontransposed derivative contributes to the observed N-15 enhanced emission signal. The parahydrogen-induced polarization factor is about 3000. The precise identification of the isomer responsible for the observed (15)N enhancement has been attained through a retro-INEPT ((15)N-(1)H) experiment. T(1) of the hyperpolarized (15)N resonance has been estimated to be ca. 150 s, i.e., similar to that reported for the parent propargylcholine (144 s). Experimental results are accompanied by theoretical calculations that stress the role of scalar coupling constants (J(HN) and J(HH)) and of the field dependence in the formation of the observed (15)N polarized signal. Insights into the good cellular uptake of the compound have been gained.

Molecular imaging: challenges of bringing imaging of intracellular targets into common clinical use

Molecular imaging (MI) takes advantage of several new techniques to detect biomarkers or biochemical and cellular processes, with the goal of obtaining high sensitivity, specificity and signal-to-noise ratio imaging of disease. The imaging modalities bearing the most promise for MI are positron emission tomography (PET), single photon emission computer tomography (SPECT) and different optical imaging techniques with high sensitivity. Also magnetic resonance imaging (MRI) with contrast agents like ultra-small superparamagnetic iron oxide particles (USPIO), magnetic resonance spectroscopy and ultrasound imaging with contrast agents may be useful approaches. MI techniques have been used in the clinic for many years, i.e. PET imaging using (18)  F-labeled fluorodeoxyglucose. Animal studies have during the last years revealed great potential for MI also with several other agents. The focus of the present article is the challenges of clinical imaging of intracellular targets following intravenous injection of the agents. Thus, the great challenge of getting enough contrast agent into the cytosol and at the same time obtaining a low signal from tissue just outside the diseased area is discussed.

Watching tumours gasp and die with MRI: the promise of hyperpolarised 13C MR spectroscopic imaging

A better understanding of tumour biology has led to the development of "targeted therapies", in which a drug is designed to disrupt a specific biochemical pathway important for tumour cell survival or proliferation. The introduction of these drugs into the clinic has shown that patients can vary widely in their responses. Molecular imaging is likely to play an increasingly important role in predicting and detecting these responses and thus in guiding treatment in individual patients: so-called "personalised medicine". The aim of this review is to discuss how hyperpolarised (13)C MR spectroscopic imaging might be used for treatment response monitoring. This technique, which increases the sensitivity of detection of injected (13)C-labelled molecules by >10,000-fold, has allowed a new approach to metabolic imaging. The basic principles of the technique and its potential advantages over other imaging methods for detecting early evidence of treatment response will be discussed. Given that the technique is poised to translate to the clinic, I will also speculate on its likely applications.

Hyperpolarized (13)C-pyruvate magnetic resonance imaging in cancer diagnostics

INTRODUCTION

The use of hyperpolarized molecules allows one to obtain information about metabolism in both cells and animals; such a task represents a tremendous advancement with respect to the results achieved so far with in vivo NMR techniques. Pyruvate appears an excellent tumor biomarker as it allows the attainment of early diagnosis, stadiation and monitoring of response to therapy.

AREAS COVERED

As pyruvate conversion to lactate in the glycolytic pathway is highly enhanced in tumor cells, the 1-(13)C-lactate levels after administration of hyperpolarized 1-(13)C-pyruvate are markedly higher in tumor tissues and depend on the type and grade of the tumor. This review covers the most recent research results (both in vitro and in vivo) about the use of hyperpolarized 1-(13)C-pyruvate for tumor localization, stadiation and for monitoring the response to therapy. The technique may find application in clinics, especially when other imaging modalities are of difficult applicability.

EXPERT OPINION

While (13)C-pyruvate has been shown to be the candidate of choice for metabolic imaging, high expectations are present in the scientific community to see if other hyperpolarized substrates could provide more specific and sensitive biomarkers. The use of hyperpolarized molecules will have a tremendous impact in the armory of diagnostic tools.

Molecular imaging for personalized cancer care

Molecular imaging is rapidly gaining recognition as a tool with the capacity to improve every facet of cancer care. Molecular imaging in oncology can be defined as in vivo characterization and measurement of the key biomolecules and molecularly based events that are fundamental to the malignant state. This article outlines the basic principles of molecular imaging as applied in oncology with both established and emerging techniques. It provides examples of the advantages that current molecular imaging techniques offer for improving clinical cancer care as well as drug development. It also discusses the importance of molecular imaging for the emerging field of theranostics and offers a vision of how molecular imaging may one day be integrated with other diagnostic techniques to dramatically increase the efficiency and effectiveness of cancer care.

Hyperpolarized 13C MR spectroscopic imaging can be used to monitor Everolimus treatment in vivo in an orthotopic rodent model of glioblastoma

Glioblastoma (GBM) is the most common and lethal primary malignant brain tumor in humans. Because the phosphatidylinositol-3-kinase (PI3K) signaling pathway is activated in more than 88% of GBM, new drugs which target this pathway, such as the mTOR inhibitor Everolimus, are currently in clinical trials. Early tumor response to molecularly targeted treatments remains challenging to assess non-invasively, because it is often associated with tumor stasis or slower tumor growth. Innovative neuroimaging methods are therefore critically needed to provide metabolic or functional information that is indicative of targeted therapeutic action at early time points during the course of treatment. In this study, we demonstrated for the first time that hyperpolarized (HP) 13C magnetic resonance spectroscopic imaging (MRSI) can be used on a clinical MR system to monitor early metabolic response of orthotopic GBM tumors to Everolimus treatment through measurement of the HP lactate-to-pyruvate ratios. The study was performed on a highly invasive non-enhancing orthotopic GBM tumor model in rats (GS-2 tumors), which replicates many fundamental features of human GBM tumors. Seven days after initiation of treatment there was a significant drop in the HP lactate-to-pyruvate ratio from the tumor tissue in treated animals relative to day 0 (67%±27% decrease). In the control group, no significant changes in the HP lactate-to-pyruvate ratios were observed. Importantly, at the 7 day time point, conventional MR imaging (MRI) was unable to detect a significant difference in tumor size between control and treated groups. Inhibition of tumor growth by conventional MRI was observed from day 15 of treatment. This implies that the decrease in the HP lactate-to-pyruvate ratio could be detected before any treatment-induced inhibition of tumor growth. Using immunohistochemical staining to further examine tumor response to treatment, we found that the decrease in the HP lactate-to-pyruvate ratio was associated with a drop in expression of lactate dehydrogenase, the enzyme that catalyzes pyruvate to lactate conversion. Also evident was decreased staining for carbonic anhydrase IX (CA-IX), an indicator of hypoxia-inducible factor 1α (HIF-1α) activity, which, in turn, regulates expression of lactate dehydrogenase. To our knowledge, this study is the first report of the use of HP 13C MRSI at a clinical field strength to monitor GBM response to molecularly targeted treatments. It highlights the potential of HP lactate-to-pyruvate ratio as an early biomarker of response, thereby supporting further investigation of this non-invasive imaging approach for eventual clinical application.

Synthesis and testing of a p-H2 hyperpolarized 13C probe based on the pyrazolo[1,5-a]pyrimidineacetamide DPA-713, an MRI vector to target the peripheral benzodiazepine receptors

DPA-713 is the lead compound of a recently developed 2-phenylpyrazolo[1,5-a]pyrimidineacetamide series that has been shown to display a good targeting capability toward peripheral benzodiazepine receptors, recently renamed translocator protein (18 kDa) or in short TSPO. On the basis of this structure, a novel derivative bearing a [(13)C]butynoate moiety has been designed and synthesized (three steps-42% overall yield) providing, upon rapid and quantitative para-hydrogenation, the corresponding hyperpolarized [(13)C]alkene. Para-hydrogen-induced polarization effects have been detected in both (1)H and (13)C-NMR spectra. Upon applying a field cycling procedure, the spin order of para-H(2) added hydrogens is transferred on the (13)C carboxylate moiety yielding a signal enhancement of approximately 4500 times. T(1) of the carboxylate carbon atom is approximately 21.9 s (at 9.37 T). A (13)C-MR image has been acquired by using the (13)C RARE (Rapid Acquisition by Relaxation Enhancement) acquisition protocol on a 10-mM solution. The main limitation to the in vivo use of this novel para-hydrogenated [(13)C]derivative is its relatively low solubility in aqueous systems.