15N SABRE-SHEATH and NMR/DFT Characterization of Amino-Metronidazole, a Metabolic Product of the Antibiotic and Prospective Hypoxia Contrast Agent Metronidazole.

Abstract

The antibiotic metronidazole (MNZ) has gained interest as a potential MRI contrast agent for imaging hypoxia. ¹⁵N-labeled MNZ can be efficiently hyperpolarized via SABRE-SHEATH (Signal Amplification By Reversible Exchange in SHield Enables Alignment Transfer to Heteronuclei), but the envisioned MRI approach requires that MNZ rapidly undergoes structural changes in hypoxic environments with significant ¹⁵N frequency differences manifested in its downstream metabolic products. We have performed NMR studies of the anticipated metabolic product amino-MNZ (despite anticipated stability concerns) accompanied by computational density functional theory (DFT) studies to predict the ¹⁵N chemical shifts of different relevant species. Direct hyperpolarization of sparse naturally abundant ¹⁵N spins in amino-MNZ via SABRE-SHEATH (enhancement up to ∼9400 fold), along with ¹H-decoupled ¹⁵N NMR, allowed comparison with both ¹⁵N₃-MNZ and naturally abundant MNZ. The results show significant ¹⁵N shift differences that agree with the DFT predictions. Taken together, the results show that it should be possible to readily distinguish the parent MNZ from product amino-MNZ in envisioned MRI approaches at clinically relevant magnetic fields.