Radical-free hyperpolarized MRI using endogenously occurring pyruvate analogues and UV-induced nonpersistent radicals.

Claudia C Zanella,Rolf Gruetter,Jessica A M Bastiaansen,Andrea Capozzi,Alice Radaelli,Lionel P Arn,Adèle L C Mackowiak,Hikari A I Yoshihara

doi:10.1002/nbm.4584

Abstract

It was recently demonstrated that nonpersistent radicals can be generated in frozen solutions of metabolites such as pyruvate by irradiation with UV light, enabling radical‐free dissolution dynamic nuclear polarization. Although pyruvate is endogenous, the presence of pyruvate may interfere with metabolic processes or the detection of pyruvate as a metabolic product, making it potentially unsuitable as a polarizing agent. Therefore, the aim of the current study was to characterize solutions containing endogenously occurring alternatives to pyruvate as UV‐induced nonpersistent radical precursors for in vivo hyperpolarized MRI. The metabolites alpha‐ketovalerate (αkV) and alpha‐ketobutyrate (αkB) are analogues of pyruvate and were chosen as potential radical precursors. Sample formulations containing αkV and αkB were studied with UV–visible spectroscopy, irradiated with UV light, and their nonpersistent radical yields were quantified with electron spin resonance and compared with pyruvate. The addition of 13C‐labeled substrates to the sample matrix altered the radical yield of the precursors. Using αkB increased the 13C‐labeled glucose liquid‐state polarization to 16.3% ± 1.3% compared with 13.3% ± 1.5% obtained with pyruvate, and 8.9% ± 2.1% with αkV. For [1–13C]butyric acid, polarization levels of 12.1% ± 1.1% for αkV, 12.9% ± 1.7% for αkB, 1.5% ± 0.2% for OX063 and 18.7% ± 0.7% for Finland trityl, were achieved. Hyperpolarized [1–13C]butyrate metabolism in the heart revealed label incorporation into [1–13C]acetylcarnitine, [1–13C]acetoacetate, [1–13C]butyrylcarnitine, [5‐13C]glutamate and [5‐13C]citrate. This study demonstrates the potential of αkV and αkB as endogenous polarizing agents for in vivo radical‐free hyperpolarized MRI. UV‐induced, nonpersistent radicals generated in endogenous metabolites enable high polarization without requiring radical filtration, thus simplifying the quality‐control tests in clinical applications.

Highlights

Hyperpolarization via dissolution dynamic nuclear polarization (DNP) can enhance the polarization of nuclear spins by several orders of magnitude.[1]
To characterize structural changes and absorption characteristics of UV-light irradiation on the metabolites αkV, αkB and pyruvic acid (PA), UV–Vis spectroscopy and Electron spin resonance (ESR) measurements were performed: UV–Vis absorption spectra showed an $ 1.7-fold higher absorbance for αkV compared with αkB or PA (Figure 1A). αkB and PA showed nearly identical absorbance maxima in the UV range of 300–400 nm (Figure 1A)
ESR performed on frozen samples prior to UV irradiation indicated the initial absence of unpaired electron spins in the matrixes of glycerolwater mixed with 5 M of αkV, αkB or PA

Summary

Introduction

Hyperpolarization via dissolution dynamic nuclear polarization (DNP) can enhance the polarization of nuclear spins by several orders of magnitude.[1]. Hyperpolarization via DNP requires the presence of polarizing agents, in the form of free radicals, which will transfer their high spin order to the surrounding nuclei upon microwave irradiation at an appropriate frequency. These free radicals are persistent and added to the sample via chemical doping. Nonpersistent radicals recombine into diamagnetic and biocompatible species at 190 K17 and are thereby eliminated instantly during the dissolution process, resulting in radical-free hyperpolarized solutions. This obviates the need for filtration of the endogenous radical precursor

Objectives

Results

Discussion

Conclusion