¹⁵N hyperpolarisation of the antiprotozoal drug ornidazole by Signal Amplification By Reversible Exchange in aqueous medium

Signal amplification by reversible exchange (SABRE) offers a cost-effective route to boost nuclear magnetic resonance (NMR) signal by several orders of magnitude by employing readily available para-hydrogen as a source of hyperpolarisation. Although ¹H spins have been the natural choice of SABRE hyperpolarisation since its inception due to its simplicity and accessibility, limited spin lifetimes of ¹H makes it harder to employ them in a range of time-dependent NMR experiments. Heteronuclear spins, for example, ¹³C and ¹⁵N, in general have much longer T₁ lifetimes and thereby are found to be more suitable for hyperpolarised biological applications as demonstrated previously by para-hydrogen induced polarisation (PHIP) and dynamic nuclear polarisation (DNP). In this study we demonstrate a simple procedure to enhance ¹⁵N signal of an antibiotic drug ornidazole by up to 71,000-folds with net ¹⁵N polarisation reaching ~23%. Further, the effect of co-ligand strategy is studied in conjunction with the optimum field transfer protocols and consequently achieving ¹⁵N hyperpolarised spin lifetime of >3 min at low field. Finally, we present a convenient route to harness the hyperpolarised solution in aqueous medium free from catalyst contamination leading to a strong ¹⁵N signal detection for an extended duration of time.