Multi-compound polarization by DNP allows simultaneous assessment of multiple enzymatic activities in vivo

Abstract

Methods for the simultaneous polarization of multiple 13 C-enriched metabolites were developed to probe several enzymatic pathways and other physiologic properties in vivo, using a single intravenous bolus. A new method for polarization of 13 C sodium bicarbonate suitable for use in patients was developed, and the co-polarization of 13 C sodium bicarbonate and [1-13 C]pyruvate in the same sample was achieved, resulting in high solution state polarizations (15.7% and 17.6%, respectively) and long spin-lattice relaxation times (T 1 ) (46.7s and 47.7s respectively at 3T). Consistent with chemical shift anisotropy dominating the T 1 relaxation of carbonyls, T 1 values for 13 C bicarbonate and [1-13 C]pyruvate were even longer at 3T (49.7s and 67.3s, respectively). Co-polarized 13 C bicarbonate and [1-13 C] pyruvate were injected into normal mice and a murine prostate tumor model at 3T. Rapid equilibration of injected hyperpolarized 13 C sodium bicarbonate with 13 C CO 2 allowed calculation of pH on a voxel by voxel basis, and simultaneous assessment of pyruvate metabolism with cellular uptake and conversion of [1-13 C] pyruvate to its metabolic products. Initial studies in a Transgenic Adenocarcinoma of Mouse Prostate (TRAMP) model demonstrated higher levels of hyperpolarized lactate and lower pH within tumor, relative to surrounding benign tissues and to the abdominal viscera of normal controls. There was no significant difference observed in the tumor lactate/pyruvate ratio obtained after the injection of co-polarized 13 C bicarbonate and [1-13 C] pyruvate or polarized [1-13 C]pyruvate alone. The technique was extended to polarize four 13 C labelled substrates potentially providing information on pH, metabolism, necrosis and perfusion, namely [1-13 C]pyruvic acid, 13 C sodium bicarbonate, [1,4-13 C]fumaric acid, and 13 C urea with high levels of solution polarization (17.5, 10.3, 15.6 and 11.6%, respectively) and spin-lattice relaxation values similar to those recorded for the individual metabolites. These studies demonstrated the feasibility of simultaneously measuring in vivo pH and tumor metabolism using nontoxic, endogenous species, and the potential to extend the multi-polarization approach to include up to four Publisher's Disclaimer: This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. NIH Public Access

Hyperpolarized 13 C dehydroascorbate as an endogenous redox sensor for in vivo metabolic imaging

“…The principal advantage of hyperpolarized 13 C MRI over other molecular imaging modalities is detection of 13 C metabolites by chemical shift, allowing real-time evaluation of physiologically relevant enzyme fluxes in vivo. Data obtained are potentially confounded by differential perfusion of (and transport into) tissues of interest, a problem that has been addressed by copolarization of 13 C probes with a perfusion agent, for example the intravascular agent 13 C urea (36). As for prior studies employing [1-13 C] pyruvate, we found for [1-13 C] DHA that the observed total 13 C signal (VitCþ DHA) was highest in tissues that are well perfused, including brain, liver, kidneys, and tumor.…”

Spin echo measurements of the extravasation and tumor cell uptake of hyperpolarized [1‐13C]lactate and [1‐13C]pyruvate

“…Exchange of hyperpolarized 13 C label between [1-13 C]pyruvate and endogenous lactate was analyzed using a simple two-site exchange model (8), to give the first order rate constant describing flux of label between pyruvate and lactate (k P ) and the relaxation rate constant for both species (R 1 ). In experiments with [1][2][3][4][5][6][7][8][9][10][11][12][13] C]lactate, there is minimal exchange of label with the small endogenous pyruvate pool (17) and therefore R 1 for lactate was estimated directly from its apparently monoexponential signal decay.…”

“…For studies of the effect of MCT inhibition on the echo/FID ratio following injection of [1][2][3][4][5][6][7][8][9][10][11][12][13] C]lactate (n ¼ 25), data were acquired using a surface coil and the S180 pulse sequence with an echo time of 310 ms. These acquisition conditions match those used in the heteronuclear spin-echo experiment described in our previous study with L-[U-2 H,1-…”

“…The development of dynamic nuclear polarization in combination with a rapid dissolution process that brings the polarized material to physiological temperatures (1) has allowed real-time MR spectroscopic imaging of metabolism using hyperpolarized 13 C-labeled cell metabolites (2)(3)(4). The most widely used 13 C-labeled substrate to date has been pyruvate, which has found applications in imaging metabolism, particularly in tumors (4,5) and heart (6).…”

“…Previously this has only been achieved by measuring the FID and echo signals separately after two sequential injections of pyruvate (14,15). In a recent study, in which we measured exchange of deuterium label between the C2 position of hyperpolarized L-[U-2 H, [1][2][3][4][5][6][7][8][9][10][11][12][13] C]lactate and the C2 protons in endogenous lactate, where the exchange was monitored indirectly by phase modulation of the spin-coupled hyperpolarized 13 C label in a heteronuclear spin echo experiment (TE ¼ 310 ms) (17), we observed an increase in the echo signal relative to the FID signal. The increase in this echo/FID ratio can be explained by suppression of the lactate signal in the vascular space due to flow, with the increase in the ratio reflecting redistribution of lactate between the vascular and extravascular spaces.…”