¹ H/ ¹⁵ N NMR and Low-Field ¹ H MRI of SABRE-Hyperpolarized Pyrazinamide─An Approved Antibiotic and Potential MRI Contrast Agent

Pyrazinamide (PZA), an FDA-approved antibiotic, was investigated for potential use as a hyperpolarized MRI contrast agent. PZA was readily hyperpolarized via parahydrogen-based NMR Signal Amplification By Reversible Exchange (SABRE) at ∼5.5 mT, allowing ¹H polarization and relaxation dynamics to be characterized. More importantly, performing SABRE hyperpolarization in the microtesla regime (i.e., SABRE in SHield Enables Alignment Transfer to Heteronuclei, SABRE-SHEATH) allowed direct detection of enhanced ¹⁵N NMR and corresponding polarization/relaxation dynamics to be characterized in this system for the first time, despite the low natural abundance of ¹⁵N. Initial experiments detected at 9.4 T following SABRE-SHEATH at 0.2 μT permitted observation of a single ¹⁵N resonance at 332.7 ppm (tentatively assigned to the ring N site <i>meta</i> to the amide group, based on the ¹H enhancement pattern, suggesting the less sterically hindered N site dominates this substrate's interaction with the Ir-based SABRE catalyst). Direct SABRE-SHEATH ¹⁵N hyperpolarization using a more optimized setup (with respect to mixing field, temperature, <i>p</i>-H₂ flow rate, etc.) that is directly coupled to a 1.4 T benchtop NMR system resulted in improved detection sensitivity, including ¹⁵N enhancements of >140,000-fold for the primary ¹⁵N resonance (corresponding to a polarization of ∼7% with substrate concentration of over 100 mM). This effort also yielded a ¹⁵N <i>T</i>₁ measurement for PZA of over 2 min at 1.4 T. Finally, the potential utility of hyperpolarized PZA as a contrast agent was also demonstrated via quantitative ¹H MRI studies performed using a low-field (64 mT) portable "point-of-care" clinical scanner.