Real‐time cardiac metabolism assessed with hyperpolarized [1‐¹³C]acetate in a large‐animal model

Abstract

Dissolution-Dynamic Nuclear Polarization (dissolution-DNP) for Magnetic Resonance (MR) Spectroscopic Imaging has recently emerged as a novel technique for non invasive studies of the metabolic fate of biomolecules in vivo. Since acetate is the most abundant extra- and intra-cellular short-chain fatty acid, we focused on [1-13C]acetate as a promising candidate for a chemical probe to study myocardial metabolism of beating heart. Dissolution-DNP procedure of Na[1-13C]acetate for in vivo cardiac applications with 3T MR scanner was optimized in pigs during bolus injection of doses up to 3 mmoles. The Na[1-13C]acetate formulation was characterized by a liquid-state polarization of 14.2% and T1Eff in vivo of 17.6 ± 1.7 s. In vivo Na[1-13C]acetate kinetic displayed a bimodal shape: [1-13C]acetyl carnitine (AcC) was detected in a slice covering the cardiac volume, and the signal of 13C-acetate and 13C-AcC was modeled using the total Area Under the Curve (AUC) for kinetic analysis. A good correlation was found between the ratio AUC(AcC)/AUC(acetate) and the apparent kinetic constant of metabolic conversion kAcC/r1 from [1-13C]acetate to [1-13C]AcC. Our study proved the feasibility and limitations of administration of large doses of hyperpolarized [1-13C]acetate with dissolution DNP to study by MR spectroscopy the myocardial conversion of [1-13C]acetate in [1-13C]acetyl-carnitine generated by acetyltransferase in healthy pigs.

Direct noninvasive estimation of myocardial tricarboxylic acid cycle flux in vivo using hyperpolarized 13C magnetic resonance

“…In this study, the estimated maximum plasma concentration at the end of the bolus injection was between 3 and 15 mM, which immediately decayed after the bolus injection. In previous metabolic studies, 45 min-long acetate infusions have been used, leading to plasma concentrations up to 10 mM [16,51], and no adverse effects were reported in hyperpolarized MR studies with comparable doses of 13 C-acetate [35,36,[52][53][54]. In the present study, all monitored physiological parameters remained stable before and after injection (Table 1), and no adverse effects were observed following acetate administration.…”

“…In contrast to earlier investigations in cardiac muscle performed at 2.35 T or 3 T [36,52,54], the increased spectral resolution obtained at 9.4 T allowed for the direct observation of the [5-13 C]citrate resonance, and illustrates the benefit and need for high field. The delayed observation of citrate with respect to acetylcarnitine is consistent with the role of carnitine as a shuttle for acetyl groups across the mitochondrial membrane, and as a mitochondrial buffer for excess acetylCoA.…”

Magnetic resonance imaging with hyperpolarized agents: methods and applications

“…Combined approaches using both [1- 13 C] and [2- 13 C] pyruvate [190, 191] or dual-enriched [1,2- 13 C 2 ] pyruvate allow for simultaneous investigation of pyruvate dehydrogenase flux, TCA cycle flux, and pH [192]. Other 13 C-labeled substrates applied to cardiac functional or metabolic imaging include [U- 13 C] α-ketobutyrate [193], [1- 13 C] lactic acid [194, 195], [1- 13 C] acetate [196–199], 13 C urea [200], and [1- 13 C] butyrate [201, 202]. Recently, co-polarization of 13 C urea and [1- 13 C] pyruvate was demonstrated to simultaneously assess myocardial perfusion and metabolism [80].…”

Hyperpolarized 13C-Magnetic Resonance Spectroscopy

“…15 Other 13 C-enriched molecules (other than pyruvate) have been tested in cardiac studies as surrogates for short-chain fatty acid metabolism. 16,17 More than 95% of the ATP formed in the heart, which fuels contractile shortening and ion pumps of the sarcoplasmic reticulum and sarcolemma, is derived from mitochondrial oxidative phosphorylation. This energy comes from electrons transferred from reactions that generate NADH and FADH2, which are derived primarily from the fatty acid β-oxidation pathway, the citric acid cycle, and, to a lesser extent, pyruvate dehydrogenase reaction and glycolysis.…”

“…These limitations arise mainly from the kinetics of conversion, the low signal-to-noise ratio by the use of a volume coil, and a limited amount of 13 C (≈5-6× lower in pigs) compared with small animal studies. 16 However, hyperpolarized butyrate is characterized by an initial low level of 7% polarization. This leads to a better understanding of the multistep metabolism into ketone bodies, acetoacetate, and several intermediates of the Krebs cycle in small animals; it is characterized by low fluxes of conversion and significant spectral density, comparable with [2- 19 Hyperpolarized 13 C-MRS methods could be made available for human studies of cardiovascular disease in the near future.…”