Metabolic imaging in the anesthetized rat brain using hyperpolarized [1‐¹³C] pyruvate and [1‐¹³C] ethyl pyruvate

Abstract

Formulation, polarization, and dissolution conditions were developed to obtain a stable hyperpolarized solution of [1-13 C]-ethyl pyruvate. A maximum tolerated concentration and injection rate were determined, and 13 C spectroscopic imaging was used to compare the uptake of hyperpolarized [1- 13 (3,4) and response to therapy (5,6). However, for potential neurologic applications, the blood-brain transport of pyruvate may be a limiting factor. Age, anesthesia, and dietary state can all impact transport rates (7-9), and under some conditions, the 1-to 2-min window of useful hyperpolarized [1-13 C]-pyruvate lifetime in vivo may not be sufficient. The brain uptake index (a measure of metabolite in brain versus blood), as reported for pyruvate in normal anesthetized rat brain 10 sec after an arterial injection (7), suggests that even at the very highest intravascular doses possible, less than 1 mM pyruvate would be transported into brain tissue. An alternative approach is the use of ethyl pyruvate (EP), a lipophilic analog of pyruvate that is expected to have faster transport across the blood-brain barrier. EP is a food additive and has been studied as an anti-inflammatory compound with therapeutic potential (10). Biologic and therapeutic differences between EP and pyruvate are not fully understood. EP has been shown to attenuate kainic acid-induced neuronal cell death in the mouse hippocampus (11) and reduce the impact of stroke (12). Under conditions of slow intravenous infusion, EP is tolerated in large doses in both animals and humans (13). In this initial study, we explore the feasibility of polarizing a stable formulation of [1-13 C]-EP and compare spectroscopic images of uptake and metabolism in normal anesthetized rat brain relative to the uptake and metabolism of hyperpolarized [1-13 C]-pyruvate. MATERIALS AND METHODS Animal PreparationThe experiments were performed on nine healthy (160-295 g) male Wistar rats anesthetized with 1-3% isoflurane in oxygen ($1.5 L/min). Respiration, rectal temperature, heart rate, and oxygen saturation were monitored throughout the experiment. Rectal temperature was kept at 37 C by heating the animal with a temperature-controlled warm water blanket. Hyperpolarized solution was injected via tail vein catheter. For imaging studies, rats were injected two or more times at a 2-h interval with either pyruvate or EP solutions. Preparation and physiologic monitoring of the animals in the 13 C experiment followed the protocol approved by the local Institutional Animal Care and Use Committees. Maximum tolerated concentration and injection rate of EP were determined by dose escalation. Hyperpolarized [1-13 C] PyruvateHyperpolarized solutions of [1-13 C]-pyruvate were prepared as described previously (14), using a HyperSense polarizer (Oxford Instruments Molecular Biotools, Oxford, UK). The concentration of the dose used in rats H76 and H88 was 100 mM. Rats H91, H92, H110, and H111 were injected at 80 mM (see Table 1). The level of liquid state polarization was estimated from solid state d...

Hyperpolarized [1,3‐13C2]ethyl acetoacetate is a novel diagnostic metabolic marker of liver cancer

“…Smaller esters are often highly volatile (e.g., ethyl acetate) and activated esters such as ethyl pyruvate spontaneously hydrolyze in aqueous solutions. Hyperpolarized [1-13 C]ethyl pyruvate has been used as substrate for carboxyl esterases in rat brain, 23 however this compound is due to its instability difficult to envision as a clinical marker. [1,[3][4][5][6][7][8][9][10][11][12][13] C 2 ]ethyl acetoacetate is a stable compound that proved a good substrate compromise with a high affinity for CE-1 due to its beta-keto acid group and a reasonable long T 1 due to its low molecular weight.…”

Tumor imaging using hyperpolarized 13C magnetic resonance spectroscopy

“…However, where the blood-brain barrier is still intact, such as in the normal brain, there is relatively slow uptake of pyruvate. Recently, hyperpolarized [1-13 C]ethyl pyruvate, a lipophilic analog of pyruvate that can be hydrolyzed to pyruvate by esterases, was demonstrated to have rapid and preferential uptake into the brain, with subsequent formation of both labeled pyruvate and lactate (34). Such an approach represents an interesting way of bypassing a rate-limiting membrane transporter.…”

Dual Component Analysis for In Vivo T2 * Decay of Hyperpolarized 13C Metabolites

“…2e). The decay curves were first fitted using the non-negative least squares (NNLS) method (10) and two T 2 * components were consistently observed for both [1][2][3][4][5][6][7][8][9][10][11][12][13] C] pyruvate and [1][2][3][4][5][6][7][8][9][10][11][12][13] C] lactate from most of the dynamic spectra. Based on this result, decay curves were fitted using a nonlinear curve fitting algorithm (11) with the following dual exponential signal model with DC bias A:…”

“…Following injection of hyperpolarized [1][2][3][4][5][6][7][8][9][10][11][12][13] C] pyruvate, increased [1-13 C] lactate signal was observed by rapid exchange of 13 C labeling between pyruvate and lactate (3) due to the high glycolytic rate of tumor characteristics (4). Intravenously injected [1-13 C] pyruvate is assumed to be predominantly in the intravascular space while the lactate is presumed to be largely in the intracellular space where the label exchange in the reaction catalyzed by the enzyme lactate dehydrogenase occurs (5).…”