Laurie J Rich1, Puneet Bagga1, Gabor Mizsei1, Mitchell D Schnall1, John A Detre2, Mohammad Haris3, and Ravinder Reddy1
1Radiology, University of Pennyslvania, Philadelphia, PA, United States, 2Neurology, University of Pennyslvania, Philadelphia, PA, United States, 3Research Branch, Qatar University, Doha, Qatar
Synopsis
A
key hallmark of malignant tissues is a metabolic shift from oxidative phosphorylation
to glycolytic metabolism, leading to increased lactate production. Probing the kinetics
of lactate production in vivo may play
a key role in studying disease mechanisms and developing biomarkers of
treatment response. Here, we developed a new approach for studying glycolytic
metabolism in glioblastoma by combining 1H MRS with infusion of deuterated
glucose. Infusion of [6,6'-2H2]glucose leads to
downstream deuterium labeling of lactate, resulting in a reduction in the 1.33
ppm lactate peak on 1H MRS and making it is possible to monitor the
metabolic turnover of lactate.
Introduction
It
is well recognized that brain tumor development is associated with significant
changes in cellular metabolism1. Specifically, tumor cells are known
to rely heavily on glycolysis instead of oxidative phosphorylation in order to
support growth, proliferation and survival2. A hallmark of this
inefficient metabolic process is the increased uptake of glucose and subsequent
conversion to lactate (Lac), with Lac production increasing significantly
compared to normal physiological conditions (Fig 1)1,2. Measuring this metabolic switch has the potential to provide
insight into tumor aggressiveness, disease mechanisms and treatment response3,4.
Currently, PET is the only clinically utilized metabolic imaging technique that
can assess glycolytic metabolism5. However, PET is also limited as it requires the
radioactive glucose analog 2-18F-fluoro-2-deoxy-d-glucose (18FDG)
and only provides information on glucose uptake and not downstream metabolism6. Recently,
2H magnetic resonance spectroscopy (MRS), also referred to as
deuterium MRS (DMRS), has been evaluated for its potential to assess neural metabolism
following administration of deuterated glucose7. Preliminary
studies have demonstrated the utility of DMRS for measuring Lac production in
human glioblastoma8. However, DMRS is also limited by its low sensitivity and
requirement for specialized coils. Here, building on the recent advances of DMRS,
we developed a new approach, exchanged
label turnover quantitative
measurement (ELOQUENT) MRS (eMRS), which
utilizes a combination of 1H MRS and deuterium labeled glucose to
measure tumor metabolism. Infusion of deuterium labeled glucose leads to downstream
labeling of Lac in tumors. As the deuterium label is invisible on 1H
MRS, accumulation of deuterium labeled Lac results in a reduction in the 1.33
ppm lactate peak, making it possible to quantify Lac production. Therefore, this
provides a straightforward and easily implementable approach for studying downstream
glycolytic metabolism, all of which can be performed with standard 1H
MRS hardware and acquisition sequences. Methods
For
experiments, three 6-8 week old female F344/NCR
rats (120-130 g) were implanted with the rat
glioblastoma cell line (F98) as described previously9. Briefly,
after sterilization of the surgical site, a hole was drilled 3 mm lateral and 3
mm posterior to the bregma, and a 10 µl suspension of 5x105 F98
cells in PBS injected into the cortex. All the MRI and MRS experiments were performed on a 9.4T, 30 cm
horizontal bore magnet interfaced to a Bruker console using commercial
radiofrequency coils. During imaging, animal body temperature was
maintained at 37ºC and respiratory rate monitored throughout the experiment. Rats were injected with [6,6′-2H2]glucose (Cambridge Isotope
Laboratories Inc, Tewksbury, MA) at a dose of 1.95 g/kg for a period up to 90
mins via an automated infusion pump and tail vein catheter using a bolus
variable infusion protocol as described previously10. eMRS spectra were acquired
from a 64 µl voxel localized in the tumor using PRESS (TR/ TE=2500/16 ms,
spectral width = 4 kHz, 90° pulse bandwidth = 5400 Hz,
180° pulse bandwidth = 2400 Hz, number of points = 4006,
VAPOR water suppression, averages=128). All displayed spectra were processed using
the Bruker TOPSPIN software package.Results and Discussion
We hypothesized that eMRS should be capable of capturing increased
Lac turnover in brain malignancies. To address this, rats were orthotopically
implanted with the F98 syngeneic rat glioma model and allowed to grow for three
weeks, after which eMRS was performed. Fig 2 displays representative eMRS spectra
from an F98 glioblastoma bearing rat acquired before and after [6,6′-2H2] glucose infusion.
Prior to infusion, a large peak can be observed at 1.33 ppm representing a combined Lac and lipid peak (Fig 2 left). After 60 min of [6,6′-2H2] glucose infusion,
a marked reduction in the Lac peak at 1.33 ppm was observed, suggesting
considerable 2H labeling of Lac (Fig 2 right). Subtraction
of post-infusion 1H MRS spectra from the pre-infusion spectra show
the time course of Lac labeling, with a clear
increase in labeling occurring only 10 min post-infusion (Fig 3). It was
also possible to calculate the percent change in the 1.33 ppm Lac peak amplitude,
providing kinetics of the Lac labeling (Fig 4). These results are consistent with previously published results using
DMRS following the infusion of [6,6′-2H2]glucose8.Conclusions
In
this work, we demonstrate the potential of eMRS to measure glycolytic activity
of glioblastoma following [6,6′-2H2]glucose infusion. Detection
and quantification of the rate of Lac production may provide crucial
information regarding tumor metabolism. Hence, eMRS is expected to open up new
opportunities to probe changes in these metabolic rates in a variety of human
diseases, including cancer and neurological disorders.Acknowledgements
Acknowledgements: The authors would thank Dr. Damodara Reddy for help with animal model preparation, Dr. Mark Elliot for his expertise on MRS acquisition and processing, and Drs. Stephen Pickup and Weixia Liu for their technical assistance in using the 9.4 T horizontal bore animal MR scanner.
Funding: This work was carried out at a US National Institutes of Health–supported resource, with funding from: the NIBIB under Grant No. P41 EB015893, National Institute of Neurological Disorders and Stroke Award Number R01NS087516 and the training grant T32EB020087-02.
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