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

Abstract

It was demonstrated that nonpersistent radicals can be generated in frozen solutions of metabolites such as pyruvate by irradiation with ultraviolet (UV) light, enabling radical-free dissolution DNP. Although pyruvate is endogenous, an excess of additional pyruvate may perturb metabolic processes, making it potentially unsuitable as a polarizing agent when studying fatty acids or carbohydrate metabolism. Therefore, the aim of the 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 (AKV) and alpha-ketobutyrate (AKB) are analogues of pyruvate and were chosen as potential radical precursors. Sample formulations containing AKV and AKB were studied with UV-visible spectroscopy, irradiated with UV light, and their nonpersistent radical yields were quantified with ESR and compared to pyruvate. The addition of 13C labeled substrates to the sample matrix altered the radical yield of the precursors. Using AKB 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 AKV. For [1-13C]butyric acid, polarization levels of 12.1 +/- 1.1% for AKV and 12.9 +/- 1.7% for AKB 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 AKV and AKB 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.