Rational Design of [13 C,D14 ]Tert-butylbenzene as a Scaffold Structure for Designing Long-lived Hyperpolarized 13 C Probes

Directly monitoring the dynamic in vivo metabolisms of hyperpolarized 13C-oligopeptides

Peptides play essential roles in biological phenomena, and, thus, there is a growing interest in detecting in vivo dynamics of peptide metabolisms. Dissolution-dynamic nuclear polarization (d-DNP) is a state-of-the-art technology that can markedly enhance the sensitivity of nuclear magnetic resonance (NMR), providing metabolic and physiological information in vivo. However, the hyperpolarized state exponentially decays back to the thermal equilibrium, depending on the spin-lattice relaxation time (T1). Because of the limitation in T1, peptide-based DNP NMR molecular probes applicable in vivo have been limited to amino acids or dipeptides. Here, we report the direct detection of in vivo metabolic conversions of hyperpolarized 13C-oligopeptides. Structure-based T1 relaxation analysis suggests that the C-terminal [1-13C]Gly-d2 residue affords sufficient T1 for biological uses, even in relatively large oligopeptides, and allowed us to develop 13C-β-casomorphin-5 and 13C-glutathione. It was found that the metabolic response and perfusion of the hyperpolarized 13C-glutathione in the mouse kidney were significantly altered in a model of cisplatin-induced acute kidney injury.

Quo Vadis Hyperpolarized 13C MRI?

Over the last two decades, hyperpolarized 13C MRI has gained significance in both preclinical and clinical studies, hereby relying on technologies like PHIP-SAH (ParaHydrogen-Induced Polarization-Side Arm Hydrogenation), SABRE (Signal Amplification by Reversible Exchange), and dDNP (dissolution Dynamic Nuclear Polarization), with dDNP being applied in humans. A clinical dDNP polarizer has enabled studies across 24 sites, despite challenges like high cost and slow polarization. Parahydrogen-based techniques like SABRE and PHIP offer faster, more cost-efficient alternatives but require molecule-specific optimization. The focus has been on imaging metabolism of hyperpolarized probes, which requires long T1, high polarization and rapid contrast generation. Efforts to establish novel probes, improve acquisition techniques and enhance data analysis methods including artificial intelligence are ongoing. Potential clinical value of hyperpolarized 13C MRI was demonstrated primarily for treatment response assessment in oncology, but also in cardiology, nephrology, hepatology and CNS characterization. In this review on biomedical hyperpolarized 13C MRI, we summarize important and recent advances in polarization techniques, probe development, acquisition and analysis methods as well as clinical trials. Starting from those we try to sketch a trajectory where the field of biomedical hyperpolarized 13C MRI might go.

Structure-guided design enables development of a hyperpolarized molecular probe for the detection of aminopeptidase N activity in vivo

Dynamic nuclear polarization (DNP) is a cutting-edge technique that markedly enhances the detection sensitivity of molecules using nuclear magnetic resonance (NMR)/magnetic resonance imaging (MRI). This methodology enables real-time imaging of dynamic metabolic status in vivo using MRI. To expand the targetable metabolic reactions, there is a demand for developing exogenous, i.e., artificially designed, DNP-NMR molecular probes; however, complying with the requirements of practical DNP-NMR molecular probes is challenging because of the lack of established design guidelines. Here, we report Ala-[1-¹³C]Gly-d₂-NMe₂ as a DNP-NMR molecular probe for in vivo detection of aminopeptidase N activity. We developed this probe rationally through precise structural investigation, calculation, biochemical assessment, and advanced molecular design to achieve rapid and detectable responses to enzyme activity in vivo. With the fabricated probe, we successfully detected enzymatic activity in vivo. This report presents a comprehensive approach for the development of artificially derived, practical DNP-NMR molecular probes through structure-guided molecular design.

Evaluation of enzymatic and magnetic properties of γ-glutamyl-[1-13C]glycine and its deuteration toward longer retention of the hyperpolarized state

Dynamic nuclear polarization (DNP) is an emerging cutting-edge method of acquiring metabolic and physiological information in vivo. We recently developed γ-glutamyl-[1-13C]glycine (γ-Glu-[1-13C]Gly) as a DNP nuclear magnetic resonance (NMR) molecular probe to detect γ-glutamyl transpeptidase (GGT) activity in vivo. However, the detailed enzymatic and magnetic properties of this probe remain unknown. Here, we evaluate a γ-Glu-Gly scaffold and develop a deuterated probe, γ-Glu-[1-13C]Gly-d 2, that can realize a longer lifetime of the hyperpolarized signal. We initially evaluated the GGT-mediated enzymatic conversion of γ-Glu-Gly and the magnetic properties of 13C-enriched γ-Glu-Gly (γ-Glu-[1-13C]Gly and γ-[5-13C]Glu-Gly) to support the validity of γ-Glu-[1-13C]Gly as a DNP NMR molecular probe for GGT. We then examined the spin-lattice relaxation time (T 1) of γ-Glu-[1-13C]Gly and γ-Glu-[1-13C]Gly-d 2 under various conditions (D2O, PBS, and serum) and confirmed that the T 1 of γ-Glu-[1-13C]Gly and γ-Glu-[1-13C]Gly-d 2 was maintained for 30 s (9.4 T) and 41 s (9.4 T), respectively, even in serum. Relaxation analysis of γ-Glu-[1-13C]Gly revealed a significant contribution of the dipole-dipole interaction and the chemical shift anisotropy relaxation pathway (71% of the total relaxation rate at 9.4 T), indicating the potential of deuteration and the use of a lower magnetic field for realizing a longer T 1. In fact, by using γ-Glu-[1-13C]Gly-d 2 as a DNP probe, we achieved longer retention of the hyperpolarized signal at 1.4 T.

Design of Nuclear Magnetic Resonance Molecular Probes for Hyperpolarized Bioimaging

Nuclear hyperpolarization has emerged as a method to dramatically enhance the sensitivity of NMR spectroscopy. By application of this powerful tool, small molecules with stable isotopes have been used for highly sensitive biomedical molecular imaging. The recent development of molecular probes for hyperpolarized in vivo analysis has demonstrated the ability of this technique to provide unique metabolic and physiological information. This review presents a brief introduction of hyperpolarization technology, approaches to the rational design of molecular probes for hyperpolarized analysis, and examples of molecules that have met with success in vitro or in vivo.

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.