Cancer is the second leading cause of death after heart disease (1). Although significant advancements in treatment are contributing to the reduction of the health burden of cancer, early detection and treatment monitoring are imperative for treatment efficacy and patient wellbeing. In addition, current diagnostics such as biopsies and in vitro testing often lack the sensitivity and specificity essential for making informed decisions on treatment, thus causing a significant concern for patient health. Therefore, improvements in cancer diagnostics and treatment monitoring are required to enhance positive patient health outcomes. Pivotal to current diagnostic paradigms is in vivo imaging. Imaging is the only means to internally assess cancer through visual information when direct physical assessment is not an option. Current imaging protocols can be improved through the development of better approaches of establishing metabolic contrast in a safe and noninvasive manner.
One technique to increase metabolic contrast is deuterium magnetic resonance imaging (DMRI), which generates contrast without the utilization of ionizing radiation. DMRI has already been applied to a cancer setting by de Feyter et al. 2018, in which the authors demonstrated that a human glioblastoma tumor can be imaged through the real time detection of 2H-lactate and 2H-glutamine/glutamate (glx) produced from [6,6-2H2]glucose (2). DMRI has features that suggest it as an alternative to the current gold standard of imaging, 2-[18F]fluorodeoxyglucose positron emission tomography (FDG-PET). FDG-PET employs a radioactive glucose analog that inflicts a risk for secondary cancer, and therefore is not suited for serial monitoring applications (3). Furthermore, FDG-PET is restricted by a sparse reporting of glucose uptake and retention, whereas DMRI can account for glycolysis and energy metabolism, which can present a more comprehensive and sensitive readout of tumor metabolic activity.
Although [6,6-2H2]glucose DMRI has been used to detect cancer, we hypothesized that assessing deuterated water (HDO) production from [2H7]glucose might further strengthen metabolic contrast and allow for tumor detection and treatment monitoring. A novel imaging platform was developed and tested by assessing cancer therapeutic efficacy in a mouse flank tumor model. C57BL/6J mice were injected subcutaneously with highly glycolytic Yale University mouse melanoma 1.7 (Yumm 1.7) cells, which model late stage human melanoma expressing constitutively active BRAF V600E protein (4). In vitro characterization demonstrated that treating Yumm 1.7 cells with a combination of dabrafenib (BRAF inhibitor) and trametinib (MEK inhibitor), which are known to acutely perturb glycolysis, significantly reduced MAPK signaling as well as HDO production from [2H7]glucose. For in vivo experiments, tumors were grown to ~10 mm diameter and mice were subsequently imaged to establish a baseline followed by oral gavage treatment with vehicle control or with 600 μg dabrafenib and 6 μg trametinib for 3 days after which mice were imaged again to assess the effects of treatment. For imaging sessions, 1.95 g/kg [2H7]glucose was injected and mice were imaged on an 11 T scanner with deuterium fast low angle shot (2H FLASH) and 1D spectroscopy to acquire total 2H signal and the localized contributions of HDO, [2H7]glucose, and 2H-lactate.
To our knowledge, this is the first application of [2H7]glucose DMRI for in vivo treatment monitoring. Imaging total 2H-signal and spectroscopically measuring HDO production in tumor-bearing mice clearly distinguished glucose utilization across baseline tumors, tumors treated with vehicle control or BRAFi and MEKi therapy, and contralateral healthy tissue. Compared to baseline and control tumors, treated tumors generated the least 2H-signal and HDO. Our observations show that HDO is a quantitative marker of tumor glucose utilization in vivo and that this imaging technique has translational capability in humans due to its safe classification, non-invasive administration, and suitability for routine treatment monitoring.
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(Left) Representative 3D rendering of animal set up showing the detection of [2H7]glucose metabolism which generates metabolic contrast through partially deuterated water (HDO) production. (Middle Left) Total 2H imaging after [2H7]glucose administration shows reduced metabolic activity in tumors treated with anti-glycolytic therapy. (Middle Right) Imaging quantification shows that treated tumors generated significantly less total 2H signal compared to control tumors and tumors from day 0, pre-treatment animals. Significance between baseline/day 0, control, and treatment animals was established by Student’s t-test. Significant differences are labeled as “*” if P≤0.05 and “**” if P≤0.01. (Right) All tumors were analyzed by mass spectrometry and subsequently isotopologue and metabolic flux modeling analyses which elucidated the kinetic contributions of metabolic reactions to HDO production.
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University of Florida