¹⁵N NMR Signal Amplification by Reversible Exchange of [¹⁵N₃]Ornidazole Antibiotic

Hyperpolarization techniques provide a dramatic enhancement of the sensitivity of magnetic resonance spectroscopy and imaging. Signal amplification by reversible exchange in shield enables alignment transfer to heteronuclei (SABRE-SHEATH) is an efficient technique for hyperpolarization of ¹³C and ¹⁵N nuclei in various biologically relevant compounds. Nitroimidazole antibiotics are prone to reduction under anaerobic conditions, making them prospective molecular contrast agents for the visualization of hypoxic tumors. In this work, we systematically studied ¹⁵N SABRE-SHEATH hyperpolarization of the ornidazole antibiotic at natural abundance (n.a.) of the ¹⁵N isotope and in the isotopically ¹⁵N₃-labeled form. Optimization of such experimental conditions as polarization transfer magnetic field, temperature, parahydrogen pressure, and flow rate resulted in average ¹⁵N polarization levels of 9.3 ± 0.5% for n.a. ornidazole and 5.3 ± 0.2% for [¹⁵N₃]ornidazole. T₁ polarization decay time of 7.4 min was achieved for the ¹⁵NO₂ group of [¹⁵N₃]ornidazole at a clinically relevant 1.4 T magnetic field, allowing for the detection of the ¹⁵N NMR signal more than 20 min after hyperpolarization. DFT calculations of ¹⁵N chemical shifts for ornidazole and plausible intermediates and products of its reduction under hypoxic conditions showed that it should be feasible to discriminate these compounds by using ¹⁵N NMR.