Hyperpolarized [1,4-(13)C]-diethylsuccinate: a potential DNP substrate for in vivo metabolic imaging

Abstract

The tricarboxylic acid (TCA) cycle performs an essential role in the regulation of energy and metabolism, and deficiencies in this pathway are commonly correlated with various diseases. However, the development of non-invasive techniques for the assessment of the cycle in vivo has remained challenging. In this work, the applicability of a novel imaging agent, [1,4-(13)C]-diethylsuccinate, for hyperpolarized (13)C metabolic imaging of the TCA cycle was explored. In vivo spectroscopic studies were conducted in conjunction with in vitro analyses to determine the metabolic fate of the imaging agent. Contrary to previous reports (Zacharias NM et al. J. Am. Chem. Soc. 2012; 134: 934-943), [(13)C]-labeled diethylsuccinate was primarily metabolized to succinate-derived products not originating from TCA cycle metabolism. These results illustrate potential issues of utilizing dialkyl ester analogs of TCA cycle intermediates as molecular probes for hyperpolarized (13)C metabolic imaging.

Keywords: dynamic nuclear polarization; hyperpolarized carbon-13; magnetic resonance spectroscopy; tricarboxylic acid cycle.

Figure 1

(A) Overview of compounds related to the TCA cycle that are generally detected when employing hyperpolarized [1-¹³C]- or [2-¹³C]-pyruvate. (B) Strategy for probing TCA cycle metabolism. DNP substrate, [¹³C]-DES, enters intracellular environment and is converted into TCA cycle intermediate.

Figure 2

(A) Polarization buildup curves (3.35 T, 1.4 K) of [¹³C]-DES doped with 20 mM BDPA taken at P(+) = 94.116 GHz. (B) Liquid-state T₁ decay (3 T, 298 K) of [¹³C]-DES samples with 20 mM BDPA (T₁ = 38 s).

Figure 3

(A) Representative time-averaged spectra from 6 - 45 s obtained from heart beginning at the administration of 80 mM [¹³C]-DES. (B) Representative stackplot of spectra obtained from heart at 3 s intervals beginning at the administration of 80 mM [¹³C]-DES.

Figure 4

Synthesis of reference sample containing succinate, DES and MES. ¹³C-NMR spectrum displays the relative chemical shift of these compounds.

Figure 5

Exposure of [¹³C]-DES to rat blood. Representative spectrum (20 min) shown to display the conversion to [¹³C]-MES via endogenous esterases.