State-of-the-art accounts of hyperpolarized 15N-labeled molecular imaging probes for magnetic resonance spectroscopy and imaging

Abstract

Hyperpolarized isotope-labeled agents have significantly advanced nuclear magnetic resonance spectroscopy and imaging (MRS/MRI) of physicochemical activities at molecular levels. An emerging advance in this area is exciting developments of ¹⁵N-labeled hyperpolarized MR agents to enable acquisition of highly valuable information that was previously inaccessible and expand the applications of MRS/MRI beyond commonly studied ¹³C nuclei. This review will present recent developments of these hyperpolarized ¹⁵N-labeled molecular imaging probes, ranging from endogenous and drug molecules, and chemical sensors, to various ¹⁵N-tagged biomolecules. Through these examples, this review will provide insights into the target selection and probe design rationale and inherent challenges of HP imaging in hopes of facilitating future developments of ¹⁵N-based biomedical imaging agents and their applications.

Fig. 1. Hyperpolarization of the nuclear-spin population to enhance NMR signals.

Fig. 2. First hyperpolarization experiments of ¹⁵N-choline. (A) Schematic conversion of ¹⁵N-choline to ¹⁵N-phosphocholine. (B) (Top): Enzymatic conversion of hyperpolarized ¹⁵N-Cho to ¹⁵N-PCho, scanned at the maximum buildup PCho (t = 114 s, Δ¹⁵N = ∼0.2 ppm), and (bottom): peak integral plotted against imaging time in seconds, with squares = ¹⁵N-Cho and circles = ¹⁵N-PCho. (C) First in vivo polarization decay graph of ¹⁵N-Cho spectra, with the ¹⁵N peak referenced to nitromethane. (B) Adapted with permission from ref. . Copyright 2008, American Chemical Society. (C) Adapted with permission from ref. . Copyright 2010, The Royal Society of Chemistry.

Fig. 3. Structures of ¹⁵N-cholines with various degrees of deuteration, showing deuteration of the methyl and methylene groups of choline elongates the T₁ lifetime.

Fig. 4. (A) Structures of perdeuteromethylated ¹⁵N glutamine, glutamate, and lysine analogs. T₁ values of all three analogs were measured at 14.1 T and 37 °C. (B) HP-MRI of [¹³C, ¹⁵N₂] urea (left) and (CD₃)₃¹⁵N⁺Gln (right) at the peak of signal accumulation. Image laid over ¹H MRI, demonstrating the localized ¹⁵N-glutamine signal in the kidneys. (C) Plot of the signal-to-noise ratio (SNR) of ¹³C-urea and ¹⁵N-glutamine signals over a time course from kidney and blood vessel regions. (B and C) Adapted with permission from ref. . Copyright 2016, John Wiley and Sons.

Fig. 5. (A) Structures of endogenous l-carnitine and its acetylated product. (B) T₁ lifetimes of L-¹⁵N-carnitine-d₉ in water and in vivo. (C) Spectral grid used for MR imaging overlaid on the ¹H anatomic image. (D) ¹⁵N spectra of each spectral grid (E) hyperpolarized ¹⁵N-carnitine signals in color overlaid on the anatomic image, illustrating the biodistribution of ¹⁵N-carnitine in the liver and kidney. (C–E) Adapted with permission from ref. . Copyright 2020, John Wiley and Sons.

Fig. 6. Structures and hyperpolarized lifetimes of (A) singly and triply labeled ¹⁵N-AZT, (B) ¹⁵N-nicotinamide, and (C) ¹⁵N-dalfampridine.

Fig. 7. (A) pH-dependent ¹⁵N chemical shifts of free-base and protonated ¹⁵N-pyridine. (B) Hyperpolarization signal decay of ¹⁵N-pyridine in rat plasma with a T₁ value of ∼11 s. (C) Determination of ¹⁵N₂-imidazole pK_a using ¹⁵N chemical shifts. (D) Chemical shifts of thermally polarized ¹⁵N₂-imidazole in water at various pH values. (A and B) Adapted with permission from ref. . Copyright 2015, Springer Nature. (C and D) Adapted with permission from ref. .

Fig. 8. (A) Scheme of H₂O₂ detection probe reaction. (B) Scans of the hyperpolarized H₂O₂ detection probe in the presence of various concentrations of H₂O₂ (in PBS, 50 s after mixing). (C) Scheme of carboxyl esterase detection probe reaction. (D) Scans of the hyperpolarized carboxyl esterase detection probe in the presence of esterase (125 units mL⁻¹ in PBS). (B and D) Adapted with permission from ref. . Copyright 2013, Springer Nature.

Fig. 9. (A) Schematic illustration of sequential nitro reduction under hypoxic conditions. (B) T₁ lifetimes of the three ¹⁵N centers in ¹⁵N-labeled metronidazole. (C) ¹⁵N-Labeled nimorazole as a hyperpolarized imaging agent of hypoxia. (D) 2D sub-second ¹⁵N MRI visualization of HP [¹⁵N₃]nimorazole in a 5 mm NMR tube (9.4 T). Axial (left) and coronal (right) projections of the first scan of ¹⁵N MRI. (D) Adapted with permission from ref. . Copyright 2020, John Wiley and Sons.

Fig. 10. (A) ¹⁵N-TMPA based Ca²⁺ detection probe and Ca²⁺ level-dependent ¹⁵N chemical shifts (measured in HEPES buffer, 40 s after mixing). (B) ¹⁵N-APTRA based Ca²⁺ detection probe and ¹⁵N NMR spectra with and without Ca²⁺. (C) [¹⁵N]TPA-d₆ based Zn²⁺ detection probe and ¹⁵N NMR spectra of hyperpolarized [¹⁵N]TPA-d₆ (1.2 mM) with various concentrations of Zn²⁺ (1–500 μM). (D) Time-dependent ¹⁵N spectra collected using intact PNT1A cells after addition of 2.8 mM of HP-[¹⁵N]TPA-d₆ (left) and its first ¹⁵N spectrum showing the detection of in vitro Zn²⁺ (right) (pH 7.4, 9.4 T). (A) Adapted with permission from ref. . Copyright 2013, Springer Nature. (B) Adapted with permission from ref. . Copyright 2015, The Royal Society of Chemistry. (C–E) Adapted with permission from ref. . Copyright 2020, Springer Nature.

Fig. 11. Selected examples of ¹⁵N₂-diazirine-tagged endogenous and drug molecules. Hyperpolarized with d-DNP and all T₁ lifetimes were measured at 1 T.

Fig. 12. Selected examples of ¹⁵N₃-azide-tagged endogenous and drug molecules. Hyperpolarized with d-DNP and all T₁ lifetimes were measured at 1 T.