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

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.

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.

LIGHT-SABRE Hyperpolarizes 1-13C-Pyruvate Continuously without Magnetic Field Cycling

Nuclear spin hyperpolarization enables real-time observation of metabolism and intermolecular interactions in vivo. 1-¹³C-pyruvate is the leading hyperpolarized tracer currently under evaluation in several clinical trials as a promising molecular imaging agent. Still, the quest for a simple, fast, and efficient hyperpolarization technique is ongoing. Here, we describe that continuous, weak irradiation in the audio-frequency range of the ¹³C spin at the 121 μT magnetic field (approximately twice Earth's field) enables spin order transfer from parahydrogen to ¹³C magnetization of 1-¹³C-pyruvate. These so-called LIGHT-SABRE pulses couple nuclear spin states of parahydrogen and pyruvate via the J-coupling network of reversibly exchanging Ir-complexes. Using ∼100% parahydrogen at ambient pressure, we polarized 51 mM 1-¹³C-pyruvate in the presence of 5.1 mM Ir-complex continuously and repeatedly to a polarization of 1.1% averaged over free and catalyst-bound pyruvate. The experiments were conducted at -8 °C, where almost exclusively bound pyruvate was observed, corresponding to an estimated 11% polarization on bound pyruvate. The obtained hyperpolarization levels closely match those obtained via SABRE-SHEATH under otherwise identical conditions. The creation of three different types of spin orders was observed: transverse ¹³C magnetization along the applied magnetic field, ¹³C z-magnetization along the main field B ₀, and ¹³C-¹H zz-spin order. With a superconducting quantum interference device (SQUID) for detection, we found that the generated spin orders result from ¹H-¹³C J-coupling interactions, which are not visible even with our narrow linewidth below 0.3 Hz and at -8 °C.

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.