Hyperpolarized [1-¹³C]-Ascorbic and Dehydroascorbic Acid: Vitamin C as a Probe for Imaging Redox Status in Vivo

Abstract

Dynamic nuclear polarization (DNP) of 13C-labeled metabolic substrates in vitro and their subsequent intravenous administration allow both the location of the hyperpolarized substrate and the dynamics of its subsequent conversion into other metabolic products to be detected in vivo. We report here the hyperpolarization of [1-13C]-ascorbic acid (AA) and [1-13C]-dehydroascorbic acid (DHA), the reduced and oxidized forms of vitamin C, respectively, and evaluate their performance as probes of tumor redox state. Solution-state polarization of 10.5 ± 1.3% was achieved for both forms at pH 3.2, whereas at pH 7.0, [1-13C]-AA retained polarization of 5.1 ± 0.6% and [1-13C]-DHA retained 8.2 ± 1.1%. The spin–lattice relaxation times (T1's) for these labeled nuclei are long at 9.4 T: 15.9 ± 0.7 s for AA and 20.5 ± 0.9 s for DHA. Extracellular oxidation of [1-13C]-AA and intracellular reduction of [1-13C]-DHA were observed in suspensions of murine lymphoma cells. The spontaneous reaction of DHA with the cellular antioxidant glutathione was monitored in vitro and was approximately 100-fold lower than the rate observed in cell suspensions, indicating enzymatic involvement in the intracellular reduction. [1-13C]-DHA reduction was also detected in lymphoma tumors in vivo. In contrast, no detectable oxidation of [1-13C]-AA was measured in the same tumors, consistent with the notion that tumors maintain a reduced microenvironment. This study demonstrates that hyperpolarized 13C-labeled vitamin C could be used as a noninvasive biomarker of redox status in vivo, which has the potential to translate to the clinic.

Watching tumours gasp and die with MRI: the promise of hyperpolarised13C MR spectroscopic imaging

“…We have demonstrated that tumour extracellular pH can be imaged from the ratio of the signal intensities of hyperpolarised H 13 CO 3 and 13 CO 2 following intravenous injection of hyperpolarised H 13 CO 3 [59] and, more recently, that the tumour redox state can be determined by monitoring the oxidation and reduction of hyperpolarised [1][2][3][4][5][6][7][8][9][10][11][12][13] C]ascorbate and [1][2][3][4][5][6][7][8][9][10][11][12][13] C]dehydroascorbate, the reduced and oxidised forms of vitamin C, respectively [60] (Figure 6). The polarised bicarbonate experiment showed, as expected, that the tumour extracellular space has a low pH, as has been observed using other MR probes in pre-clinical animal models [61].…”

Relevance of electron spin dissipative processes to dynamic nuclear polarization via thermal mixing

“…Moreover, according the observation first reported in [10,14], trace amounts (1-2 mM) of gadolinium complexes added to the solutions can further improve the DNP signal enhancement. The addition of gadolinium, now commonly exploited in standard protocols for DNP sample preparation [15][16][17], was shown to affect neither the electron linewidth nor the nuclear spin-lattice relaxation time, while it induces a significant reduction of the electron spin relaxation time T 1e [10,18,19]. In the Borghini's framework, however, such reduction would affect the nuclear polarization by less than 10 % [10], whereas the gadolinium-induced enhancement observed experimentally is up to four-fold.…”

In vivo single-shot 13C spectroscopic imaging of hyperpolarized metabolites by spatiotemporal encoding

“…Further developments are in progress to exploit additional benefits of spatiotemporal encoding, such as a built-in restricted FOV capability [24,26], multi-echo experiments and parallel imaging acquisition [34]. Representative metabolic images from a rat following injection of hyperpolarized [1][2][3][4][5][6][7][8][9][10][11][12][13] …”

“…DNP can result in signal enhancements >10,000 for 13 C; an unprecedented increase that is leading to new opportunities for imaging pathology and monitoring treatment [3][4][5][6][7]. Pyruvic acid has emerged as the principal 13 C-labeled agent in d-DNP, but there is significant on-going research to develop other compounds for dynamic imaging of metabolism [8][9][10]. Despite this unique potential, in vivo d-DNP 13 C NMR remains a challenge.…”