Parahydrogen induced polarization of 1-(13)C-phospholactate-d(2) for biomedical imaging with >30,000,000-fold NMR signal enhancement in water

Abstract

The synthetic protocol for preparation of 1-(13)C-phosphoenolpyruvate-d2, precursor for parahydrogen-induced polarization (PHIP) of 1-(13)C-phospholactate-d2, is reported. (13)C nuclear spin polarization of 1-(13)C-phospholactate-d2 was increased by >30,000,000-fold (5.75 mT) in water. The reported (13)C polarization level approaching unity (>15.6%), long lifetime of (13)C hyperpolarized 1-(13)C-phospholactate-d2 (58 ± 4 s versus 36 ± 2 s for nondeuterated form at 47.5 mT), and large production quantities (52 μmoles in 3 mL) in aqueous medium make this compound useful as a potential contrast agent for the molecular imaging of metabolism and other applications.

Figure 1

(a) Rh(I) water-soluble catalyst preparation outside of PHIP polarizer and subsequent catalytic cycle inside the PHIP polarizer leading to ¹H polarized 1-¹³C-phospholactate-d₂ (1-¹³C-PLAC-d₂). (b) Molecular addition of parahydrogen and polarization transfer leading to ¹³C HP 1-¹³C-phospholactate (1-¹³C-PLAC). (c) Molecular addition of parahydrogen and polarization transfer leading to ¹³C HP 1-¹³C-phospholactate-d₂ (1-¹³C-PLAC-d₂).

Figure 2

Step-wise optimization of deuterium exchange of pyruvic acid, synthesis of PEP from sodium pyruvate, and preparation of 1-¹³C-phosphoenolpyruvate-d₂ (1-¹³C-PEP-d₂, 6). (a) Deuterium exchange: (a) (i) D₂O (450 mL), 100 °C, 5 h; (ii) 0.95 eq. NaHCO₃; (iii) Rec. D₂O/EtOH; sodium pyruvate-d₃ (2, 54% yield, ratio C₃D₃O₃^– to C₃D₂HO₃^– = 1:0.28). (b) Potassium phosphoenolpyruvate synthesis based on sodium pyruvate: (b) 0.95 eq. H₂SO₄, 0.95 eq. Br₂ (dry), CCl₄; (c) (i) P(OMe)₃, THF; (ii) H₂O; (iv) KOH to pH ∼ 2.7; (v) Rec. H₂O/EtOH; potassium phosphoenolpyruvate (3, 52% over two steps); (c) preparation for potassium 1-¹³C-phosphoenolpyruvate-d₃ (1-¹³C-PEP-d₃): (a) (i) D₂O (450 mL), 100 °C, 5 h; (ii) 0.95 eq. NaHCO₃; (iii) Rec. D₂O/EtOH; sodium 1-¹³C-pyruvate-d₃ (4, 75% yield, C₂¹³CD₃O₃^– to C₂¹³CD₂HO₃^– = 1:0.25). (b′) 0.95 eq. D₂SO₄, 0.95 eq. Br₂ (dry), CCl₄; (c′) (i) P(OMe)₃, THF; (ii) D₂O; (iii) KOH to pH ∼ 2.7; (iv) Rec. H₂O/EtOH (6, 43% over 2 steps, C₂¹³CH₂D₂O₆P^– to C₂¹³CH₃DO₆P^– to C₂¹³CH₄O₆P^– = 1:0.10:0.05).

Figure 3

(a) Diagram of PEP to PLAC conversion using automated PHIP polarizer. (b) pH optimization of PEP (5 mM) in phosphate buffer (25 mM) using Rh(I) catalyst (5.3 mM) at 55 ± 2 °C. (c) pH optimization of PEP (30 mM) in phosphate buffer (30 mM) using Rh(I) catalyst (5.3 mM) in the pH range of interest. Rh(I) catalyst performs several hydrogenation cycles using 30 mM PEP, Figure 1. The reaction temperatures were 68 ± 1 °C (red trace) and 55 ± 1 °C (blue trace). (d) Final pH (of solutions exiting PHIP polarizer) as a function of the starting pH (solutions entering PHIP polarizer) of solutions used in (c). All hydrogenations were performed at ∼7 atm of H₂ gas partial pressure during an ∼5 s reaction time.

Figure 4

PHIP of 1-¹³C-PEP-d₂ to HP 1-¹³C-PLAC-d₂. (a) Diagram of molecular hydrogenation of 1-¹³C-PEP-d₂, deuterium enrichment of 95% (SI, section 2.4), with parahydrogen to form HP 1-¹³C-PLAC-d₂. (b) ¹³C spectrum of PHIP HP 1-¹³C-PLAC-d₂ (52 μmoles in ∼3 mL and ∼12 μmoles of unreacted 1-¹³C-PEP-d₂ corresponding to ∼82% conversion) acquired at 5.75 mT in situ of the automated PHIP polarizer using 7 atm of parahydrogen and ∼92 °C. (c) Signal reference ¹³C spectrum of sodium 1-¹³C-acetate (165 mmol in 52 mL of D₂O) with polarization P = 4.70 × 10^–4% at acquisition. (d) T₁ decay of ¹³C hyperpolarization of HP 1-¹³C-PLAC-d₂.