Initial investigation of glucose metabolism in mouse brain using enriched 17O-glucose and dynamic 17O-MRS
Robert Borowiak1,2, Wilfried Reichardt1,2, Dmitry Kurzhunov1, Christian Schuch3, Jochen Leupold1, Thomas Lange1, Marco Reisert1, Axel Krafft1,2, Elmar Fischer1, and Michael Bock1

1University Medical Center Freiburg, Dept. of Radiology - Medical Physics, Freiburg, Germany, 2German Cancer Consortium (DKTK), German Cancer Research Center (DKFZ), Heidelberg, Germany, 3NUKEM Isotopes Imaging GmbH, Alzenau, Germany

Synopsis

In this work, we demonstrate the feasibility of monitoring glucose uptake in mouse brain using direct 17O-MRS at 9.4 Tesla for the first time. Time-resolved 17O-MRS spectra (temporal resolution: 42 s) are acquired in vivo after injection of D-glucose with 17O-labeled hydroxyl groups. The cerebral rate of glucose metabolism CMRGlc is estimated using a pharmacokinetic model in an anesthetized (1.25% isoflurane) mouse to 0.43 ± 0.21 µmol/g/min, which is of the same order of magnitude as reported by 18F-FDG PET.

Introduction

Altered glucose uptake is associated with altered pathologies - malignant tumors, for example, gain energy by anaerobic glycolysis (Warburg effect [1]). The clinical gold standard to monitor metabolic rates of glucose is FDG-PET, which uses the radioactively labeled [18F]-fluordeoxyglucose (FDG). As a non-radioactive alternative to FDG-PET we propose to use dynamic 17O-MRS of 17O-labeled D-glucose [2-4] to assess the metabolic glucose pathway.

Material & Methods

To optimize the acquisition parameters for dynamic 17O-MRS in mouse brain, phantom experiments (Fig. 1) were conducted with 17O-labeled D-glucose (Nukem Isotopes Alzenau, Germany) on a 9.4 Tesla animal MR system (Bruker BioSpin). For RF excitation and signal reception a two-turn 2 cm diameter surface coil tuned to the 17O frequency (54.271 MHz) was built, and spectra were acquired with an FID sequence.
Animal preparation:
Each mouse was anesthetized (4% induction/1.25% continous rate of isoflurane) prior to introducing a catheter into the tail vein. During the MRS experiment the mouse was continously anesthetized with 1.25% isoflurane, breathing rate and body temperature were monitored and kept as stable as possible.
After placing the surface coil on the mouse brain, baseline spectra were acquired over 27 min. The following MRS parameters were used: spectral bandwidth 62.5 kHz, pulse length 62.5µs, acquisition delay 25µs, TE = 56µs, TR = 5.4ms, flip angle α ≈ 90, 5791 averages, Tacq = 5ms, 313 points were sampled with a dwell time of 16µs. Then, a solution was injected through the catheter which consisted of 80mg of either 17O-labeled 1-glucose, 1,6-glucose, 6-glucose, or unlabeled glucose dissolved in 200µl 0.9% NaCl, and 17O spectra were acquired over the course of up to 163 min (temporal resolution: 42 s). Glucose uptake was measured from the signal change of the H217O water peak. In the phantom, the 17O spectra (Fig.1) were quantified in the time domain using the HLSVD algorithm of JMRUI [5]. In the dynamic in vivo spectra, at first the H217O peak was separated from the short-T2 glucose peak at -11 ppm by discarding initial points from the FID emulating an echo time of TE = 216ms. H217O signal intensities were quantified from the peak height of the magnitude spectra, and converted into absolute H217O concentrations by normalizing to the natural abundance of 17O (assuming a mouse brain weight of 0.4g and water content of 73%). The dynamics of 1-glucose and 6-glucose peaks were plotted after suppressing the water peak with HLSVD (Fig. 2). Since the anomeric 1-OH position has a temperature and pH-dependent chemical exchange with unlabeled water in solution [6,7], only the time course of the 6-glucose experiment was chosen to further investigate glucose metabolism. Using a pharmacokinetic model [8], the time course of the H217O data was fitted yielding cerebral rates of glucose metabolism CMRGlc (Fig. 3). Glucose enrichment factor α in blood was estimated from separated measurement of blood sugar concentration and included as constant fit-parameter. Note that, the model was modified by considering that 1mol 6-glucose is converted into 1mol H217O.

Results & Discussion

In the phantom spectra the following line widths and chemical shifts relative to the water peak (FWHM =2 ppm) were obtained: Δf1-OH = 40±2 ppm / FWHM1-OH = 27ppm, Δf6-OH = -11 ± 2 ppm / FWHM6-OH =13ppm. The chemical shifts of the 1-OH and 6-OH position are in good agreement with literature [2], if a mixture of 36% α and 64% β glucose is assumed. In vivo line widths and chemical shifts were similar to those of the phantom measurements. In all in vivo experiments with labeled glucose a H217O signal increase of about 12-47% was observed, whereas no increase of H217O signal was seen with unlabeled glucose. From the model fit, a CMRGlc of 0.43 ± 0.21 µmol/g/min was found which is about 65% higher than literature value of 0.26 ± 0.10 µmol/g/min measured with 18F-FDG PET in mouse under 1.0% isoflurane [9]. The delay of about 13 min between bolus injection and signal increase in glucose-6 experiment is 56% longer compared to a 13C glucose bolus experiment performed in humans without anesthetic [10]. In this work, dynamic 17O-MRS spectra are acquired in vivo in mouse brain for the first time. From the concentration time courses metabolic rates were obtained which indicate that the labelled oxygen in glucose is transformed into H217O water by glycolysis. The deviations from literature values can be a result of uncertainties in the model fit, or chemical exchange processes that occur before glycolysis.These measurements are a first step towards a MR-based method for the quantification of glucose metabolism.

Acknowledgements

Authors would like to thank Prof. Dr. Dieter Leibfritz for fruitful discussion. Financial support from NUKEM Isotopes Imaging GmbH is gratefully acknowledged.

References

[1] Warburg O Science (1956) 123: 309-314 [2] Gerothanassis IP et al. JMR (1982) 48: 431-446 [3] Schulte J et al. JMR (1993) 101:95-97 [4] de Graaf R et al. JMR (2008) 193:63-67 [5] Naressi A Magma et al. (2001) 12:141-152 [6] Risley JM et al. Biochemistry (1982) 21: 6360-6365 [7] Mega TL et al. J.Org.Chem. (1990) 55: 522-528 [8] Atkinson IC et al. NeuroImage (2010) 51: 723-733 [9] Tomayo H et al. (2004) J Nucl Med. 45: 1398-1405 [10] Mason GF et al. (2003) Brain Research Protocols. 10: 181-190

Figures

Fig.1:17O-MRS of a 55 mM aqueous solution: 15 mg D-glucose 68 % enriched in the anomeric 1(68%)-OH (green) and 6(43%)-OH position (red) dissolved in phosphate-buffered saline (1.5ml). Despite the broad line width of the 6-glucose peak compared to the chemical shift difference, water quantification is feasible.

Fig.2:Time-resolved 17O spectra from mouse brain after intravenous infusion of 80 mg 1,6-17O-glucose dissolved in a 200µl 0.9% NaCl solution at t=26.6 min. Note, that the water peak was suppressed with HLSVD. Over time both glucose peaks decrease due to glycolysis, uptake in the liver and chemical exchange with water.

Fig.3:H217O concentration in the mouse brain after injection of 2.2 M glucose is shown. The highest signal increase of 47% is found for 1,6 glucose indicating that 17O at the 1-position is not only transformed by glycolysis but also undergoes chemical exchange with unlabeled water in blood.



Proc. Intl. Soc. Mag. Reson. Med. 24 (2016)
3964