  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.
