Deuteration of Hyperpolarized ¹³C‐Labeled Zymonic Acid Enables Sensitivity‐Enhanced Dynamic MRI of pH

Abstract

Aberrant pH is characteristic of many pathologies such as ischemia, inflammation or cancer. Therefore, a non-invasive and spatially resolved pH determination is valuable for disease diagnosis, characterization of response to treatment and the design of pH-sensitive drug-delivery systems. We recently introduced hyperpolarized [1,5- C ]zymonic acid (ZA) as a novel MRI probe of extracellular pH utilizing dissolution dynamic polarization (DNP) for a more than 10000-fold signal enhancement of the MRI signal. Here we present a strategy to enhance the sensitivity of this approach by deuteration of ZA yielding [1,5- C , 3,6,6,6-D ]zymonic acid (ZA ), which prolongs the liquid state spin lattice relaxation time (T ) by up to 39 % in vitro. Measurements with ZA and ZA on subcutaneous MAT B III adenocarcinoma in rats show that deuteration increases the signal-to-noise ratio (SNR) by up to 46 % in vivo. Furthermore, we demonstrate a proof of concept for real-time imaging of dynamic pH changes in vitro using ZA , potentially allowing for the characterization of rapid acidification/basification processes in vivo.

Hyperpolarized Amino Acid Derivatives as Multivalent Magnetic Resonance pH Sensor Molecules

“…So far, two promising HP 13 C-labelled chemical shift–based pH sensors for preclinical applications have been presented in the literature. 13 C-diethyl malonic acid (DEMA) exhibits a long T 1 (≈106 s, B 0 = 11.7 T) and was used in vitro to sense pH [ 25 ], while [1,5-13C]zymonic acid (ZA) has already been applied for pH in vivo imaging in rodent kidneys and tumors [ 40 , 42 ]. Serine derivatives and DAP (Δδ = 4.2 ppm) exhibit a 2.4 or 1.5-fold higher pH sensitivity than diethyl malonic acid (Δδ = 1.7 ppm) and zymonic acid (Δδ = 2.4 ppm), respectively.…”

“…Liquid state polarization of SA and DAP could be further improved, e.g., by filtration (radical and paramagnetic impurities) and the usage of magnetic transfer lines [ 37 , 44 , 45 , 46 , 47 , 48 , 49 , 50 , 51 , 52 , 53 ]. Although the pH sensitivity of SA and DAP is higher than for diethyl malonic acid and zymonic acid, their T 1 is relatively short (up to ≈18 s at B 0 = 1 T) compared to DEMA (≈106 s, B 0 = 11.7 T) [ 25 ] and ZA (≈56 s at B 0 = 1 T) [ 42 ]. Deuteration, dissolution in D 2 O [ 33 , 42 ], or the use of long-lived singlet states [ 54 , 55 , 56 ] could enhance the HP signal lifetime and will be subject for further studies.…”

Imaging tumor acidosis: a survey of the available techniques for mapping in vivo tumor pH

“…The accuracy of pH measurement (0.1 pH unit) as well as the long signal lifetime (T 1 of 17 s at 7 T in vivo) makes this new reporter valuable for further preclinical and clinical studies. Further, perdeuteration of ZA increased the T1 relaxation time significantly, prolonging hyperpolarization and increasing sensitivity [118], allowing dynamic monitoring of pH changes in vivo (Fig. 11).…”

Crystal structure of zymonic acid and a redetermination of its precursor, pyruvic acid

“…The use of 13 C-zymonic acid has enabled mapping of pH changes, independently of concentration, in mammalian organs and tumors via hyperpolarized magnetic resonance . Thus, zymonic acid is a non-invasive extracellular imaging sensor to localize and quantify pH in vivo Hundshammer et al, 2017), with many possible applications in medical diagnosis (Schilling et al, 2016). As part of the process resulting in the aforementioned invention, the detailed 1 H and 13 C NMR spectra of pure zymonic acid have been reported (Hundshammer et al, 2017).…”

“…Thus, zymonic acid is a non-invasive extracellular imaging sensor to localize and quantify pH in vivo Hundshammer et al, 2017), with many possible applications in medical diagnosis (Schilling et al, 2016). As part of the process resulting in the aforementioned invention, the detailed 1 H and 13 C NMR spectra of pure zymonic acid have been reported (Hundshammer et al, 2017). Herein, we contribute new information to characterize zymonic acid by reporting for the first time its crystal structure, along with a low-temperature redetermination of pyruvic acid.…”