Telomerase reverse transcriptase induces targetable alterations in glutathione and nucleotide biosynthesis in glioblastomas

Telomerase reverse transcriptase (TERT) is essential for glioblastoma (GBM) proliferation. Delineating metabolic vulnerabilities induced by TERT can lead to novel GBM therapies. We previously showed that TERT upregulates glutathione (GSH) pool size in GBMs. Here, we show that TERT acts via the FOXO1 transcription factor to upregulate expression of the catalytic subunit of glutamate-cysteine ligase (GCLC), the rate-limiting enzyme of de novo GSH synthesis. Inhibiting GCLC using siRNA or buthionine sulfoximine (BSO) reduces synthesis of 13 C-GSH from [U- 13 C]-glutamine and inhibits clonogenicity. However, GCLC inhibition does not induce cell death, an effect that is associated with elevated [U- 13 C]-glutamine metabolism to glutamate and pyrimidine nucleotide biosynthesis. Mechanistically, GCLC inhibition activates MYC and leads to compensatory upregulation of two key glutamine-utilizing enzymes i.e., glutaminase (GLS), which generates glutamate from glutamine, and CAD (carbamoyl-phosphate synthetase 2, aspartate transcarbamoylase, dihydroorotatase), the enzyme that converts glutamine to the pyrimidine nucleotide precursor dihydroorotate. We then examined the therapeutic potential of inhibiting GLS and CAD in combination with GCLC. 6-diazo-5-oxy-L-norleucin (DON) is a potent inhibitor of glutamine-utilizing enzymes including GLS and CAD. The combination of BSO and DON suppresses GSH and pyrimidine nucleotide biosynthesis and is synergistically lethal in GBM cells. Importantly, in vivo stable isotope tracing indicates that combined treatment with JHU-083 (a brain-penetrant prodrug of DON) and BSO abrogates synthesis of GSH and pyrimidine nucleotides from [U- 13 C]-glutamine and induces tumor shrinkage in mice bearing intracranial GBM xenografts. Collectively, our studies exploit a mechanistic understanding of TERT biology to identify synthetically lethal metabolic vulnerabilities in GBMs.

SIGNIFICANCE

Using in vivo stable isotope tracing, metabolomics, and loss-of-function studies, we demonstrate that TERT expression is associated with metabolic alterations that can be synergistically targeted for therapy in glioblastomas.

Hyperpolarized δ-[1- 13C]gluconolactone imaging visualizes response to TERT or GABPB1 targeting therapy for glioblastoma

TERT promoter mutations are a hallmark of glioblastoma (GBM). Accordingly, TERT and GABPB1, a subunit of the upstream mutant TERT promoter transcription factor GABP, are being considered as promising therapeutic targets in GBM. We recently reported that the expression of TERT or GABP1 modulates flux via the pentose phosphate pathway (PPP). Here, we investigated whether 13C magnetic resonance spectroscopy (MRS) of hyperpolarized (HP) δ- [1-13C]gluconolactone can serve to image the reduction in PPP flux following TERT or GABPB1 silencing. We investigated two different human GBM cell lines stably expressing shRNAs targeting TERT or GABPB1, as well as doxycycline-inducible shTERT or shGABPB1cells. MRS studies were performed on live cells and in vivo tumors, and dynamic sets of 13C MR spectra were acquired following injection of HP δ-[1-13C]gluconolactone. HP 6-phosphogluconolactone (6PG), the product of δ-[1-13C]gluconolactone via the PPP, was significantly reduced in TERT or GABPB1-silenced cells or tumors compared to controls in all our models. Furthermore, a positive correlation between TERT expression and 6PG levels was observed. Our data indicate that HP δ-[1-13C]gluconolactone, an imaging tool with translational potential, could serve to monitor TERT expression and its silencing with therapies that target either TERT or GABPB1 in mutant TERT promoter GBM patients.

Imaging telomerase reverse transcriptase expression in oligodendrogliomas using hyperpolarized δ-[1-13C]-gluconolactone

Background

Telomere maintenance by telomerase reverse transcriptase (TERT) is essential for immortality in most cancers, including oligodendrogliomas. Agents that disrupt telomere maintenance such as the telomere uncapping agent 6-thio-2'-deoxyguanosine (6-thio-dG) are in clinical trials. We previously showed that TERT expression in oligodendrogliomas is associated with upregulation of glucose-6-phosphate dehydrogenase (G6PD), the rate-limiting enzyme of the pentose phosphate pathway (PPP). We also showed that hyperpolarized δ-[1-13C]-gluconolactone metabolism to 6-phosphogluconate (6-PG) can be used to probe the PPP in glioblastomas. The goal of this study was to determine whether hyperpolarized 13C imaging using δ-[1-13C]-gluconolactone can monitor TERT expression and response to 6-thio-dG in oligodendrogliomas.

Methods

We examined patient-derived oligodendroglioma cells and orthotopic tumors to assess the link between TERT and hyperpolarized δ-[1-13C]-gluconolactone metabolism. We performed in vivo imaging to assess the ability of hyperpolarized δ-[1-13C]-gluconolactone to report on TERT and response to 6-thio-dG in rats bearing orthotopic oligodendrogliomas in vivo.

Results

Doxycycline-inducible TERT silencing abrogated 6-PG production from hyperpolarized δ-[1-13C]-gluconolactone in oligodendroglioma cells, consistent with the loss of G6PD activity. Rescuing TERT expression by doxycycline removal restored G6PD activity and, concomitantly, 6-PG production. 6-PG production from hyperpolarized δ-[1-13C]-gluconolactone demarcated TERT-expressing tumor from surrounding TERT-negative normal brain in vivo. Importantly, 6-thio-dG abrogated 6-PG production at an early timepoint preceding MRI-detectable alterations in rats bearing orthotopic oligodendrogliomas in vivo.

Conclusions

These results indicate that hyperpolarized δ-[1-13C]-gluconolactone reports on TERT expression and early response to therapy in oligodendrogliomas. Our studies identify a novel agent for imaging tumor proliferation and treatment response in oligodendroglioma patients.

TMET-12. HYPERPOLARIZED Δ- [1-13C] GLUCONOLACTONE IMAGING VISUALIZES TERT-ASSOCIATED CHANGES IN DYNAMIC PENTOSE PHOSPHATE PATHWAY METABOLISM IN GLIOBLASTOMA

BACKGROUND

TERT promoter mutations are a hallmark of glioblastoma (GBM). We recently reported that the expression of TERT and its upstream transcriptional factor, GABPB1, are strongly correlated with redox in GBM with TERT promoter mutations. However, further imaging biomarkers which visualize redox-associated metabolism and TERT expression are needed. Here we demonstrate that 13C magnetic resonance spectroscopy (MRS) of hyperpolarized δ-[1-13C] gluconolactone metabolism is a useful imaging tool to visualize changes in dynamic pentose phosphate pathway (PPP) metabolism that reflect TERT-associated changes in redox in GBM.

METHODS

U251 human GBM cells stably expressing shRNA targeting TERT or GABPB1 were compared to controls. Doxycyclin-inducible shTERT or shGABPB1 U251 cells were also examined. For in vivo studies, cells were injected into immunodeficient rat brains and tumors confirmed by T2-weighted MRI. δ-[1-13C] gluconolactone was polarized using a Hypersense DNP polarizer and injected into live cells or tumor-bearing rats. For cells 13C-MRS was acquired using a 500MHz Agilent spectrometer and analyzed using Mnova and Matlab software. For in vivo13C-MRS studies, spectra were acquired using a 3T Bruker scanner and a spectral spatial echo-planar spectroscopic imaging sequence. Spectral signal to noise was improved using Tensor denoising. Spectra were processed using a custom-written Matlab script.

RESULTS

Hyperpolarized 6-phosphogluconolactone (6PG), the metabolic product from δ-[1-13C] gluconolactone via the PPP, was significantly reduced in TERT or GABPB1 silenced cells compared to control cells in both U251 models. The positive correlation between TERT expression and 6PG level was confirmed by linear regression analysis. Hyperpolarized 6PG production in both tumor models was also significantly reduced in TERT or GABPB1-silenced tumors compared to controls.

CONCLUSION

We successfully visualized TERT-associated changes in dynamic PPP metabolism in a GBM model in cells and in vivo. Hyperpolarized δ-[1-13C] gluconolactone is a potential tool for monitoring TERT expression in GBM with TERT promoter mutations.

NIMG-68. IMAGING PHOSPHOLIPID BIOSYNTHESIS ENABLES NON-INVASIVE ASSESSMENT OF BRAIN TUMOR RESPONSE TO THERAPY

Elevated phospholipid biosynthesis is a metabolic hallmark of cancer. Due to the high membrane turnover associated with uncontrolled proliferation, tumor cells invariably upregulate phospholipid biosynthesis. Choline is a dietary nutrient that is phosphorylated to phosphocholine (PC) and subsequently incorporated into phosphatidylcholine, which is the primary membrane phospholipid in mammalian cells. Choline kinase α (CKα) is the key enzyme in this pathway and its expression is elevated in most cancers. Identifying non-invasive methods of imaging CKα activity, can, therefore, enable assessment of tumor burden and response to therapy. Deuterium magnetic resonance spectroscopy (DMRS) following administration of 2H-labeled substrates recently emerged as a simple, robust, clinically translatable method of assessing metabolic activity in vivo. The goals of the current study were to determine whether DMRS-based assessment of PC production from [2H9]-choline tracks CKα activity and to establish the utility of [2H9]-choline for glioma imaging in vivo. First, we show that [2H9]-choline metabolism to PC can be observed in live patient-derived glioma cells (GBM6 and BT88). Silencing CKα abrogates PC production from [2H9]-choline in both GBM6 and BT88 models, thereby confirming that [2H9]-choline provides a readout of CKα activity. Next, we examined the ability of [2H9]-choline to monitor tumor burden and response to therapy in vivo. [2H9]-choline metabolism to PC can be observed in mice bearing orthotopic patient-derived GBM6 or BT257 tumors, but not in tumor-free healthy controls. Importantly, following treatment of mice bearing orthotopic BT257 tumors with temozolomide, which is standard of care for glioma patients, PC production from [2H9]-choline is reduced at early timepoints when changes in tumor volume cannot be detected by anatomical imaging. Taken together, our studies, for the first time, identify [2H9]-choline as a probe of CKα activity and highlight its ability to assess tumor burden and early response to therapy, which is a challenge in glioma imaging.

TMET-16. DIRECT DETECTION OF 2HG AND GLUTAMATE PRODUCTION USING HYPERPOLARIZED [1-13C-5-12C]-Α-KETOGLUTARATE IN CELL AND IN VIVO GLIOMA MODELS

Mutant IDH leads to 2HG production, which drives glioma development. 13C MRS monitoring of hyperpolarized [1-13C]α-ketoglutarate (αKG) metabolism to 2HG and glutamate provides a non-invasive assessment of the IDH mutation and normal metabolism, respectively. However, monitoring 2HG production in vivo has been challenging because its resonance is within 0.1 ppm of the natural abundance [5-13C]αKG signal of the [1-13C]αKG substrate. Here, we utilized [1-13C -5-12C]αKG, which eliminated the [5-13C]αKG peak. This new approach, combined with an optimized sequence, made it possible to readily monitor the production of both 2HG and glutamate in cells and in vivo in healthy rats or rats with orthotopic patient-derived glioma. Hyperpolarized [1-13C -5-12C]αKG was injected into genetically engineered NHAIDHmut cell lysates, healthy rats, and rats implanted orthotopically with BT257 cells intravenously. 1-D dynamic 13C MRS spectra from cells and in vivo slab spectroscopy data were then acquired using an 11.7 T NMR system and a 3 T pre-clinical scanner, respectively. Injection of the hyperpolarized [1-13C -5-12C]αKG into cell lysates showed clearly detectable dynamic conversion of hyperpolarized [1-13C-5-12C]αKG to 2HG and glutamate. The normal brain showed clear production of glutamate but no 2HG was detected. In tumor-bearing rats, we were able to clearly detect the dynamic production of both 2HG and glutamate. This study demonstrated the utility of hyperpolarized [1-13C-5-12C]αKG as a substrate to clearly assess 2HG production without the confounding presence of the natural abundance peak which cannot be distinguished from 2HG in vivo. Importantly, the detection of 2HG provides a clear indicator of the IDH mutation within the tumor.

EXTH-13. COMBINED TARGETING OF TELOMERASE REVERSE TRANSCRIPTASE AND METABOLISM ABROGATES GLIOBLASTOMA PROLIFERATION IN VIVO

Telomerase reverse transcriptase (TERT) is essential for tumor immortality. Since TERT is silenced in normal cells, it is an attractive therapeutic target and the TERT inhibitor 6-thio-2’-deoxyguanosine (6-thio-dG) is in clinical trials for gliomas. However, there is a lag period following TERT inhibition before tumor shrinkage is observed. Novel strategies that can be combined with TERT inhibition to enhance anti-tumor activity are sorely needed. We previously demonstrated that TERT increases expression of the alanine transporter ASCT2 in gliomas, an effect that could be non-invasively monitored using hyperpolarized 13C imaging of alanine metabolism (Viswanath et al., Nature Communications, 2021). The goals of the current study were to determine whether combined inhibition of TERT and ASCT2 inhibits GBM growth and to assess the ability of hyperpolarized [1-13C]-alanine to monitor treatment response. Our results indicated that, while both the TERT inhibitor 6-thio-dG and the ASCT2 inhibitor V-9302 individually inhibited viability, the combination of 6-thio-dG and V-9302 maximally inhibited cell viability in the patient-derived GBM1 and GBM6 models. In line with the effect on cell viability, while both compounds reduced lactate production from hyperpolarized [1-13C]-alanine, the combination of 6-thio-dG and V-9302 resulted in the highest inhibition of lactate production in both models. Importantly, treatment of rats bearing orthotopic GBM6 tumors with a combination of 6-thio-dG and V-9302 caused tumor shrinkage, an effect that could be visualized by magnetic resonance imaging by day 21 after treatment. Furthermore, lactate production from hyperpolarized [1-13C]-alanine was significantly reduced at day 7 after treatment with 6-thio-dG and V-9302, when anatomical alterations were absent. Collectively, our results indicate that simultaneously targeting TERT and ASCT2 provides a novel therapeutic opportunity for GBMs and that hyperpolarized [1-13C]-alanine serves as a companion agent for imaging early response to therapy. Our findings pave the way for precision therapy and response assessment for GBM patients.

TMET-20. TERT ACTS VIA FOXO1 TO ALTER REDOX METABOLISM IN BRAIN TUMORS

Telomere maintenance is a fundamental hallmark of cancer. Most tumors maintain telomere length via reactivation of telomerase reverse transcriptase (TERT) expression. Identifying imaging biomarkers of TERT can enable non-invasive assessment of tumor proliferation and response to therapy. Deuterium magnetic resonance spectroscopy (DMRS) following administration of 2H-labeled substrates recently emerged as a novel, clinically translatable method of monitoring metabolic activity in vivo. The goal of this study was to delineate metabolic reprogramming associated with TERT expression and to leverage this information for non-invasive imaging of tumor burden and treatment response in gliomas. Our results indicate that TERT expression is associated with elevated levels of the redox metabolite NADH in glioblastomas and oligodendrogliomas. Mechanistically, TERT expression is associated with inhibitory phosphorylation and cytosolic sequestration of FOXO1. FOXO1, in turn, negatively regulates nicotinamide phosphoribosyl transferase (NAMPT), which is the rate-limiting enzyme in NAD+ biosynthesis. As a result, TERT upregulates NAMPT, resulting in elevated steady-state pools of NAD+ and NADH. Concomitantly, FOXO1 negatively regulates the glycolytic enzyme glyceraldehyde-3-phosphate dehydrogenase, which converts NAD+ to NADH. As a result, TERT upregulates the NADH/NAD+ ratio. Since elevated NADH and NADH/NAD+ ratio drive pyruvate conversion to lactate, we then examined whether DMRS-based imaging of [U-2H]-pyruvate metabolism reports on TERT expression in gliomas. Our results indicate that doxycycline-inducible TERT silencing significantly reduces lactate production from [U-2H]-pyruvate in tumor-bearing mice. Importantly, [U-2H]-pyruvate metabolism to lactate differentiates tumor from normal brain in vivo, including at clinical field strength (3T). Furthermore, [U-2H]-pyruvate reports on early response to treatment with TERT inhibitors or with radiochemotherapy in mice bearing orthotopic patient-derived gliomas at early timepoints before radiographic alterations can be visualized by magnetic resonance imaging. Collectively, our studies integrate a mechanistic understanding of TERT biology with innovative imaging that has the potential to improve assessment of tumor burden and treatment response for glioma patients.

A 13C/31P surface coil to visualize metabolism and energetics in the rodent brain at 3 Tesla

PURPOSE

We constructed a 13C/31P surface coil at 3 T for studying cancer metabolism and bioenergetics. In a single scan session, hyperpolarized 13C-pyruvate MRS and 31P MRS was carried out for a healthy rat brain.

METHODS

All experiments were carried out at 3 Tesla. The multinuclear surface coil was designed as two coplanar loops each tuned to either the 13C or 31P operating frequency with an LCC trap on the 13C loop. A commercial volume proton coil was used for anatomical localization and B0 shimming. Single tuned coils operating at either the 13C or 31P frequency were built to evaluate the relative performance of the multinuclear coil. Coil performance metrics consisted of measuring Q factor ratio, calculating system input power using a single-pulse acquisition, and acquiring SNR and flip angle maps using 2D CSI sequences. To observe in vivo spectra, a bolus of hyperpolarized [1-13C] pyruvate was administered via tail vein. In vivo13C and endogenous 31P spectra were obtained in a single scan session using 1D slice selective acquisitions.

RESULTS

When compared with single tuned surface coils, the multinuclear coil performance showed a decrease in Q factor ratio, SNR, and transmit efficiency. Flip angle maps showed adequate flip angles within the phantom when the transmit voltage was set using an external phantom. Results show good detection of 13C labeled lactate, alanine, and bicarbonate in addition to ATP from 31P MRS.

CONCLUSIONS

The coil enables obtaining complementary information within a scan session, thus reducing the number of trials and minimizing biological variability for studies of metabolism and bioenergetics.

Deuterium Metabolic Imaging Reports on TERT Expression and Early Response to Therapy in Cancer

PURPOSE

Telomere maintenance is a hallmark of cancer. Most tumors maintain telomere length via reactivation of telomerase reverse transcriptase (TERT) expression. Identifying clinically translatable imaging biomarkers of TERT can enable noninvasive assessment of tumor proliferation and response to therapy.

EXPERIMENTAL DESIGN

We used RNAi, doxycycline-inducible expression systems, and pharmacologic inhibitors to mechanistically delineate the association between TERT and metabolism in preclinical patient-derived tumor models. Deuterium magnetic resonance spectroscopy (2H-MRS), which is a novel, translational metabolic imaging modality, was used for imaging TERT in cells and tumor-bearing mice in vivo.

RESULTS

Our results indicate that TERT expression is associated with elevated NADH in multiple cancers, including glioblastoma, oligodendroglioma, melanoma, neuroblastoma, and hepatocellular carcinoma. Mechanistically, TERT acts via the metabolic regulator FOXO1 to upregulate nicotinamide phosphoribosyl transferase, which is the key enzyme for NAD+ biosynthesis, and the glycolytic enzyme glyceraldehyde-3-phosphate dehydrogenase, which converts NAD+ to NADH. Because NADH is essential for pyruvate flux to lactate, we show that 2H-MRS-based assessment of lactate production from [U-2H]-pyruvate reports on TERT expression in preclinical tumor models in vivo, including at clinical field strength (3T). Importantly, [U-2H]-pyruvate reports on early response to therapy in mice bearing orthotopic patient-derived gliomas at early timepoints before radiographic alterations can be visualized by MRI.

CONCLUSIONS

Elevated NADH is a metabolic consequence of TERT expression in cancer. Importantly, [U-2H]-pyruvate reports on early response to therapy, prior to anatomic alterations, thereby providing clinicians with a novel tool for assessment of tumor burden and treatment response in cancer.

IMG-16. Non-invasive metabolic imaging of response to therapy in diffuse midline gliomas

Diffuse midline gliomas (DMGs) are universally lethal pediatric tumors that are defined by the presence of histone H3K27 alterations. The infiltrative nature and anatomical location of these tumors prohibit surgical resection. Radiotherapy, which is standard of care, does not significantly enhance long-term outcome. Novel therapies are sorely needed for DMG patients. Imipridone drugs ONC201 and ONC206 have demonstrated anti-tumor activity in preclinical cancer models, including DMGs and have shown promise in pilot studies in DMG patients. Successful clinical deployment of imipridones requires the identification of companion imaging biomarkers that report on response to therapy. Magnetic resonance spectroscopy (MRS) is a safe, non-radioactive, non-invasive method of imaging metabolism in vivo. 1H-MRS assesses steady-state metabolite levels and is used in clinics. 2H-MRS following administration of 2H-labeled substrates is a novel, clinically translatable method of imaging metabolic pathway activity. Our results indicate that treatment of SF7761 DMG cells with ONC206 causes a significant reduction in 1H-MRS-detectable lactate, glutamate, glutathione and phosphocholine, pointing to inhibition of glycolysis, oxidative phosphorylation, redox and phosphatidylcholine biosynthesis respectively. Examination of [6,6’-2H]-glucose metabolism using 2H-MRS indicates that lactate production from [6,6’-2H]-glucose is significantly reduced in ONC206-treated SF7761 cells relative to controls. We then investigated the effect of ONC206 on mice bearing orthotopic SF8628 DMG tumors. At day 7 following the treatment onset, at a timepoint when no change in tumor volume can be observed by anatomical imaging, in vivo1H-MRS-detectable lactate and total choline are reduced relative to day 0. Collectively, our studies indicate that imipridones induce alterations in DMG metabolism that can be leveraged for non-invasive 1H- and 2H-MRS-based imaging of response to therapy. By providing clinicians with an early readout of treatment response prior to anatomical changes, our biomarkers will enable early assessment of treatment response and, thereby, clinical translation of these promising therapeutics.

Imaging biomarkers of TERT or GABPB1 silencing in TERT-positive glioblastoma

BACKGROUND

TERT promoter mutations are observed in 80% of wild-type IDH glioblastoma (GBM). Moreover, the upstream TERT transcription factor GABPB1 was recently identified as a cancer-specific therapeutic target for tumors harboring a TERT promoter mutation. In that context, noninvasive imaging biomarkers are needed for the detection of TERT modulation.

METHODS

Multiple GBM models were investigated as cells and in vivo tumors and the impact of TERT silencing, either directly or by targeting GABPB1, was determined using 1H and hyperpolarized 13C magnetic resonance spectroscopy (MRS). Changes in associated metabolic enzymes were also investigated.

RESULTS

1H-MRS revealed that lactate and glutathione (GSH) were the most significantly altered metabolites when either TERT or GABPB1 was silenced, and lactate and GSH levels were correlated with cellular TERT expression. Consistent with the drop in lactate, 13C-MRS showed that hyperpolarized [1-13C]lactate production from [1-13C]pyruvate was also reduced when TERT was silenced. Mechanistically, the reduction in GSH was associated with a reduction in pentose phosphate pathway flux, reduced activity of glucose-6-phosphate dehydrogenase, and reduced NADPH. The drop in lactate and hyperpolarized lactate were associated with reductions in glycolytic flux, NADH, and expression/activity of GLUT1, monocarboxylate transporters, and lactate dehydrogenase A.

CONCLUSIONS

Our study indicates that MRS-detectable GSH, lactate, and lactate production could serve as metabolic biomarkers of response to emerging TERT-targeted therapies for GBM with activating TERT promoter mutations. Importantly these biomarkers are readily translatable to the clinic, and thus could ultimately improve GBM patient management.

Deuterium magnetic resonance spectroscopy enables noninvasive metabolic imaging of tumor burden and response to therapy in low-grade gliomas

BACKGROUND

The alternative lengthening of telomeres (ALT) pathway is essential for tumor proliferation in astrocytomas. The goal of this study was to identify metabolic alterations linked to the ALT pathway that can be exploited for noninvasive magnetic resonance spectroscopy (MRS)-based imaging of astrocytomas in vivo.

METHODS

Genetic and pharmacological methods were used to dissect the association between the ALT pathway and glucose metabolism in genetically engineered and patient-derived astrocytoma models. 2H-MRS was used for noninvasive imaging of ALT-linked modulation of glycolytic flux in mice bearing orthotopic astrocytomas in vivo.

RESULTS

The ALT pathway was associated with higher activity of the rate-limiting glycolytic enzyme phosphofructokinase-1 and concomitantly elevated flux of glucose to lactate in astrocytoma cells. Silencing the ALT pathway or treating with the poly(ADP-ribose) polymerase inhibitor niraparib that induces telomeric fusion in ALT-dependent astrocytoma cells abrogated glycolytic flux. Importantly, this metabolic reprogramming could be non-invasively visualized by 2H-MRS. Lactate production from [6,6'-2H]-glucose was higher in ALT-dependent astrocytoma tumors relative to the normal brain in vivo. Furthermore, treatment of orthotopic astrocytoma-bearing mice with niraparib reduced lactate production from [6,6'-2H]-glucose at early timepoints when alterations in tumor volume could not be detected by anatomical imaging, pointing to the ability of [6,6'-2H]-glucose to report on pseudoprogression in vivo.

CONCLUSIONS

We have mechanistically linked the ALT pathway to elevated glycolytic flux and demonstrated the ability of [6,6'-2H]-glucose to non-invasively assess tumor burden and response to therapy in astrocytomas. Our findings point to a novel, clinically translatable method for metabolic imaging of astrocytoma patients.

Acquisition and quantification pipeline for in vivo hyperpolarized 13 C MR spectroscopy

PURPOSE

The goal of this study was to combine a specialized acquisition method with a new quantification pipeline to accurately and efficiently probe the metabolism of hyperpolarized ¹³ C-labeled compounds in vivo. In this study, we tested our approach on [2-¹³ C]pyruvate and [1-¹³ C]α-ketoglutarate data in rat orthotopic brain tumor models at 3T.

METHODS

We used a multiband metabolite-specific radiofrequency (RF) excitation in combination with a variable flip angle scheme to minimize substrate polarization loss and measure fast metabolic processes. We then applied spectral-temporal denoising using singular value decomposition to enhance spectral quality. This was combined with LCModel-based automatic ¹³ C spectral fitting and flip angle correction to separate overlapping signals and rapidly quantify the different metabolites.

RESULTS

Denoising improved the metabolite signal-to-noise ratio (SNR) by approximately 5. It also improved the accuracy of metabolite quantification as evidenced by a significant reduction of the Cramer Rao lower bounds. Furthermore, the use of the automated and user-independent LCModel-based quantification approach could be performed rapidly, with the kinetic quantification of eight metabolite peaks in a 12-spectrum array achieved in less than 1 minute.

CONCLUSION

The specialized acquisition method combined with denoising and a new quantification pipeline using LCModel for the first time for hyperpolarized ¹³ C data enhanced our ability to monitor the metabolism of [2-¹³ C]pyruvate and [1-¹³ C]α-ketoglutarate in rat orthotopic brain tumor models in vivo. This approach could be broadly applicable to other hyperpolarized agents both preclinically and in the clinical setting.

BIOM-14. METABOLIC BIOMARKERS OF TERT-TARGETED THERAPY FOR HUMAN GLIOBLASTOMA DETECTED BY MAGNETIC RESONANCE SPECTROSCOPY

BACKGROUND

TERT promoter mutations that result in TERT expression are observed in over 80% of GBM. Moreover, the upstream transcription factor GABPB1 was recently identified as an ideal therapeutic target for tumors with TERT promoter mutations. In that context, non-invasive reliable biomarkers that can help detect TERT expression are needed. The aim of this research was to assess the value of MRS-detectable metabolic changes as biomarkers of TERT expression and TERT-targeted therapy in GBM.

METHODS

Genetically engineered GBM cells (NHARas/TERT) treated with TERT siRNA were compared to siCtrl-treated cells, and stable TERT and GABPB1 knock down GBM cells (U251, GBM1) were compared to shCtrl. 1H-MRS and 13C-MRS metabolic data was acquired from cell extracts using a Bruker 500MHz scanner. Hyperpolarized MRS studies of live cells used a HyperSense DNP polarizer and data was acquired using a Varian 500MHz scanner. Spectra were analyzed using Mnova and Matlab software. Multivariate data analysis was performed using SIMCA software.

RESULTS

Unbiased PCA analysis of 1H-MRS metabolic data showed separation of TERT or GABPB1 knock down and control cells. VIP predictive scores revealed that lactate and GSH were the top altered metabolites with a significant drop observed in both metabolites in every model following TERT silencing. Consistent with the reduction in GSH, spectrophotometric assays showed a significant drop in NADPH and NADH. 2-13C glucose flux analysis revealed that both glycolysis and PPP-related metabolites were reduced in TERT knock down cells. Hyperpolarized [1-13C]-pyruvate flux to lactate was also reduced, confirming that the glycolytic pathway was altered following TERT knock down.

CONCLUSION

1H MRS-detectable lactate and GSH, combined with hyperpolarized 13C MRS-detectable metabolic fluxes, could serve as metabolic biomarkers of TERT-targeted therapy for human GBM with TERT promoter mutations. These biomarkers could be translated to the clinical, improve the monitoring of GBM patients and advance precision medicine.

NIMG-50. DEUTERIUM METABOLIC IMAGING OF THE ALTERNATIVE LENGTHENING OF TELOMERES PATHWAY REPORTS ON TUMOR BURDEN AND PSEUDOPROGRESSION IN LOW-GRADE GLIOMAS

Glioma patient management relies heavily on magnetic resonance imaging (MRI). However, MRI is often inadequate for assessment of tumor burden and pseudoprogression. Non-invasive methods that report on molecular pathways such as telomere maintenance that drive tumor proliferation are needed. Among brain tumors, low-grade astrocytomas (LGAs) use the alternative lengthening of telomeres (ALT) pathway for telomere maintenance. The goal of this study was to identify ALT-linked metabolic alterations that can be exploited for non-invasive magnetic resonance spectroscopy (MRS)-based imaging of LGAs. We examined the patient-derived BT257 model and compared neurospheres that are ALT-dependent (BT257 ALT+) with those in which the ALT pathway has been silenced (BT257 ALT-). Our studies suggest that expression and activity of the rate-limiting glycolytic enzyme phosphofructokinase-1 are significantly higher in BT257 ALT+ neurospheres relative to ALT-, an effect that is associated with elevated glucose flux to lactate. Studies indicate that poly(ADP-ribose) polymerase inhibitors such as niraparib selectively induce telomeric fusion and cell death in ALT-dependent cells. We find that the telomeric fusion-mediated cytotoxicity of niraparib is associated with significantly reduced glycolytic flux in BT257 ALT+ neurospheres. We then examined whether 2H-MRS using [6,6’-2H]-glucose, which is a clinically translatable method of imaging glycolytic flux, can be used to monitor the ALT pathway in vivo. [6,6’-2H]-glucose flux to lactate is elevated in tumor relative to normal brain in mice bearing orthotopic BT257 tumors. Importantly, following treatment of BT257 tumor-bearing mice with niraparib, lactate production from [6,6’-2H]-glucose is significantly reduced at early timepoints when alterations in tumor volume cannot be observed by MRI, pointing to the ability of [6,6’-2H]-glucose to report on pseudoprogression in vivo. Collectively, our studies mechanistically link the ALT pathway with elevated glycolytic flux via phosphofructokinase-1 and identify deuterium metabolic imaging as a novel, non-invasive method of imaging tumor burden and treatment response in LGAs in vivo.

NIMG-51. DEUTERIUM METABOLIC IMAGING OF BRAIN TUMOR IMMORTALITY USING 2H-PYRUVATE

Telomere maintenance is essential for tumor immortality and sustained tumor proliferation. Most tumors, including high-grade glioblastomas and low-grade oligodendrogliomas achieve telomere maintenance via reactivation of the expression of telomerase reverse transcriptase (TERT), which is silenced in normal somatic cells. Due to this essential role, TERT is a therapeutic target and TERT inhibitors such as 6-thio-2’-deoxyguanosine are in clinical trials. Non-invasive methods of imaging TERT, therefore, have the potential to provide a readout of tumor proliferation and response to therapy. We previously showed that TERT expression is associated with elevated levels of NADH in gliomas. Since NADH is essential for the conversion of pyruvate to lactate, measuring pyruvate flux to lactate could be useful for imaging TERT expression. In this context, deuterium magnetic resonance spectroscopy (2H-MRS) recently emerged as a novel, clinically translatable method of monitoring metabolic fluxes. The goal of this study was to assess the potential of [U-2H]pyruvate for non-invasive imaging of TERT status in gliomas. Following intravenous injection of [U-2H]pyruvate, lactate production was significantly higher in mice bearing orthotopic oligodendroglioma (SF10417, BT88) or glioblastoma (GBM1, GBM6) tumors relative to tumor-free controls. 2D chemical shift imaging showed localization of lactate production to tumor vs. contralateral normal brain. Importantly, following treatment of mice bearing orthotopic GBM6 or BT88 tumors with the TERT inhibitor 6-thio-2’-deoxyguanosine, lactate production from [U-2H]pyruvate was significantly reduced at early timepoints when alterations in tumor volume could not be detected by anatomical imaging, pointing to the ability of [U-2H]pyruvate to report on pseudoprogression. Collectively, we have, for the first time, demonstrated the utility of [U-2H]pyruvate for metabolic imaging of brain tumor burden and treatment response in vivo. Importantly, since 2H-MRS can be implemented on clinical scanners, our results provide a novel, non-invasive method of integrating information regarding a fundamental tumor hallmark, i.e. TERT, into glioma patient management.

EXTH-46. MRS BASED BIOMARKERS OF IDH1 MUTANT GLIOMA RESPONSE TO THE IDH INHIBITOR BAY-1436032

Gliomas are the most prevalent type of brain tumor in the central nervous system. Mutations in the cytosolic enzyme isocitrate dehydrogenase 1 (IDH1) are a common feature of primary low-grade gliomas, catalyzing the conversion of α-ketoglutarate (αKG) to the oncometabolite 2-hydroxyglutarate (2HG), and mutant IDH1 is a therapeutic target for these tumors. Several mutant IDH inhibitors are currently in clinical trials, nonetheless, complementary non-invasive early biomarkers to assess drug delivery and potential therapeutic response are still needed. The goal of this study was therefore to determine the potential of 1H and hyperpolarized 13C magnetic resonance spectroscopy (MRS)-based biomarkers as indicators of mutant IDH1 low-grade glioma response to treatment with the clinically-relevant IDH1 inhibitor BAY-1436032 in cells and animal models. Immortalized human astrocytes engineered to express mutant IDH1 were treated with 500nM (IC50 value) of BAY-1436032 and BT257 tumors implanted in rats were treated with 150mg/kg of BAY-1436032. To assess steady-state metabolite levels, 1H MRS spectra were acquired on a 500 MHz MRS cancer for cells and a 3 T scanner for animal studies. To assess metabolic fluxes, we used hyperpolarized 13C MRS and probed the fate of hyperpolarized [1-13C]αKG. 1H MRS showed a significant decrease in 2HG as well as a significant increase in glutamate (Glu) and phosphocholine (PCh) following BAY-1436032 treatment in both cell and animal models compared to controls. Furthermore, hyperpolarized 13C MRS showed that hyperpolarized 2HG production from hyperpolarized [1-13C]αKG was decreased and hyperpolarized glutamate production from hyperpolarized [1-13C]αKG was increased in the BAY-1436032 treated groups compared to controls. These findings are consistent with our previous study, which investigated the MRS-detectable consequences of two other mutant IDH inhibitors: AG120 and AG881. Collectively, our work identifies translatable MRS-based metabolic biomarkers of mutant IDH1 inhibition.

BIOM-10. PRECLINICAL PLATFORM FOR THE IDENTIFICATION OF DEUTERIUM MAGNETIC RESONANCE SPECTROSCOPY-BASED BIOMARKERS OF BRAIN TUMOR METABOLISM

Metabolic reprogramming is a fundamental hallmark of cancer, which can be exploited for non-invasive tumor imaging. Deuterium magnetic resonance spectroscopy (2H-MRS) recently emerged as a novel, translational method of interrogating flux from 2H-labeled substrates to metabolic products. However, to date, preclinical studies have been performed in vivo, an endeavor which suffers from low-throughput and potential wastage of animal life, especially when considering studies of treatment response. Developing in vitro assays for monitoring metabolism of 2H-labeled substrates will enhance throughput, lead to the rapid evaluation of new 2H-based probes and enable identification of treatment response biomarkers, thereby allowing the best 2H-based probes to be translated for further in vivo assessment. The goal of this study was to develop a preclinical cell-based platform for quantifying metabolism of 2H-labeled probes in brain tumor models. Since the Warburg effect, which is characterized by elevated glycolytic production of lactate, is a metabolic phenotype of cancer, including brain tumors, we examined metabolism of 2H-glucose or 2H-pyruvate in patient-derived glioblastoma (GBM6) and oligodendroglioma (BT88) cells and compared to normal human astrocytes (NHACONTROL). Following incubation in media containing [6,6’-2H]glucose or [U-2H]pyruvate, 2H-MR spectra obtained from live cell suspensions showed elevated 2H-lactate production in GBM6 and BT88 cells relative to NHACONTROL. Importantly, 2H-lactate production from [6,6’-2H]glucose or from [U-2H]pyruvate was reduced in GBM6 or BT88 cells subjected to irradiation and temozolomide, which is standard of care for glioma patients, pointing to the utility of this method for detecting response to therapy. Collectively, we have, for the first time, demonstrated the ability to quantify metabolism of 2H-MRS probes in live cell suspensions and validated the utility of our assay for differentiating tumor from normal cells and assessing response to therapy. Our studies will expedite the identification of novel 2H-MRS probes for imaging brain tumors and potentially other types of cancer.

TAMI-40. PEDIATRIC H3K27M MUTANT GLIOMAS UNDERGO METABOLIC REPROGRAMMING THAT CAN BE LEVERAGED FOR NON-INVASIVE METABOLIC IMAGING

Diffuse midline gliomas (DMGs) are a universally lethal form of childhood cancer. The infiltrative nature of DMGs makes them difficult to visualize by conventional magnetic resonance imaging. Genomics studies indicate that DMGs are driven by unique histone H3K27M mutations that result in broad epigenetic dysregulation. Many of the resulting changes in gene expression have the potential to induce metabolic reprogramming, which has been identified as a hallmark of cancer. The goal of this study was to dissect metabolic reprogramming in preclinical DMG models in order to identify novel magnetic resonance spectroscopy (MRS)-detectable metabolic biomarkers that can be exploited for non-invasive imaging. First, we used 1H-MRS, which reports on steady-state metabolism, to examine H3K27M mutant SF7761 cells and H3 wild-type normal human astrocytes (NHA). Lactate, glutathione and phosphocholine, which are involved in glycolysis, redox and phospholipid metabolism respectively, were elevated in SF7761 cells relative to NHAs. Mechanistically, these metabolic alterations were associated with upregulation of key enzymes including hexokinase 2, glutamate cysteine ligase and choline kinase a. Importantly, in vivo 1H-MRS showed elevated lactate, glutathione and total choline (combined signal from choline, phosphocholine and glycerophosphocholine) in mice bearing orthotopic SF7761 tumors relative to tumor-free controls. We then examined alterations in dynamic metabolic pathways in our models. Using thermally-polarized 13C-MRS, we identified elevated production of [2-13C]-lactate from [2-13C]-glucose in SF7761 cells relative to NHAs. Hyperpolarized 13C-MRS is a method of enhancing the 13C-MR signal such that metabolic fluxes can be interrogated with high sensitivity. Hyperpolarized [1-13C]-pyruvate flux to [1-13C]-lactate non-invasively monitors glycolysis and is in clinical trials in adult glioma patients. Importantly, hyperpolarized [1-13C]-pyruvate metabolism to lactate was elevated in SF7761 cells relative to NHAs. Collectively, our studies suggest that H3K27M mutant DMGs undergo reprogramming of glucose, redox and phospholipid metabolism that can be leveraged for non-invasive 1H- and hyperpolarized 13C-MRS-based imaging.

Metabolic imaging detects elevated glucose flux through the pentose phosphate pathway associated with TERT expression in low-grade gliomas

BACKGROUND

Telomerase reverse transcriptase (TERT) is essential for tumor proliferation, including in low-grade oligodendrogliomas (LGOGs). Since TERT is silenced in normal cells, it is also a therapeutic target. Therefore, non-invasive methods of imaging TERT are needed. Here, we examined the link between TERT expression and metabolism in LGOGs, with the goal of leveraging this information for non-invasive magnetic resonance spectroscopy (MRS)-based metabolic imaging of LGOGs.

METHODS

Immortalized normal human astrocytes with doxycycline-inducible TERT silencing, patient-derived LGOG cells, orthotopic tumors and LGOG patient biopsies were studied to determine the mechanistic link between TERT expression and glucose metabolism. The ability of hyperpolarized [U- 13C, U- 2H]-glucose to non-invasively assess TERT expression was tested in live cells and orthotopic tumors.

RESULTS

TERT expression was associated with elevated glucose flux through the pentose phosphate pathway (PPP), elevated NADPH, which is a major product of the PPP, and elevated GSH, which is maintained in a reduced state by NADPH. Importantly, hyperpolarized [U- 13C, U- 2H]-glucose metabolism via the PPP non-invasively reported on TERT expression and response to TERT inhibition in patient-derived LGOG cells and orthotopic tumors. Mechanistically, TERT acted via the sirtuin SIRT2 to upregulate the glucose transporter GLUT1 and the rate-limiting PPP enzyme glucose-6-phosphate dehydrogenase.

CONCLUSIONS

We have, for the first time, leveraged a mechanistic understanding of TERT-associated metabolic reprogramming for non-invasive imaging of LGOGs using hyperpolarized [U- 13C, U- 2H]-glucose. Our findings provide a novel way of imaging a hallmark of tumor immortality and have the potential to improve diagnosis and treatment response assessment for LGOG patients.

Imaging 6-Phosphogluconolactonase Activity in Brain Tumors In Vivo Using Hyperpolarized δ-[1-13C]gluconolactone

INTRODUCTION

The pentose phosphate pathway (PPP) is essential for NADPH generation and redox homeostasis in cancer, including glioblastomas. However, the precise contribution to redox and tumor proliferation of the second PPP enzyme 6-phosphogluconolactonase (PGLS), which converts 6-phospho-δ-gluconolactone to 6-phosphogluconate (6PG), remains unclear. Furthermore, non-invasive methods of assessing PGLS activity are lacking. The goal of this study was to examine the role of PGLS in glioblastomas and assess the utility of probing PGLS activity using hyperpolarized δ-[1-13C]gluconolactone for non-invasive imaging.

METHODS

To interrogate the function of PGLS in redox, PGLS expression was silenced in U87, U251 and GS2 glioblastoma cells by RNA interference and levels of NADPH and reduced glutathione (GSH) measured. Clonogenicity assays were used to assess the effect of PGLS silencing on glioblastoma proliferation. Hyperpolarized δ-[1-13C]gluconolactone metabolism to 6PG was assessed in live cells treated with the chemotherapeutic agent temozolomide (TMZ) or with vehicle control. 13C 2D echo-planar spectroscopic imaging (EPSI) studies of hyperpolarized δ-[1-13C]gluconolactone metabolism were performed on rats bearing orthotopic glioblastoma tumors or tumor-free controls on a 3T spectrometer. Longitudinal 2D EPSI studies of hyperpolarized δ-[1-13C]gluconolactone metabolism and T2-weighted magnetic resonance imaging (MRI) were performed in rats bearing orthotopic U251 tumors following treatment with TMZ to examine the ability of hyperpolarized δ-[1-13C]gluconolactone to report on treatment response.

RESULTS

PGLS knockdown downregulated NADPH and GSH, elevated oxidative stress and inhibited clonogenicity in all models. Conversely, PGLS expression and activity and steady-state NADPH and GSH were higher in tumor tissues from rats bearing orthotopic glioblastoma xenografts relative to contralateral brain and tumor-free brain. Importantly, [1-13C]6PG production from hyperpolarized δ-[1-13C]gluconolactone was observed in live glioblastoma cells and was significantly reduced by treatment with TMZ. Furthermore, hyperpolarized δ-[1-13C]gluconolactone metabolism to [1-13C]6PG could differentiate tumor from contralateral normal brain in vivo. Notably, TMZ significantly reduced 6PG production from hyperpolarized δ-[1-13C]gluconolactone at an early timepoint prior to volumetric alterations as assessed by anatomical imaging.

CONCLUSIONS

Collectively, we have, for the first time, identified a role for PGLS activity in glioblastoma proliferation and validated the utility of probing PGLS activity using hyperpolarized δ-[1-13C]gluconolactone for non-invasive in vivo imaging of glioblastomas and their response to therapy.

BIMG-05. TO BE OR NOT TO BE GLYCOLYTIC: DEUTERATED GLUCOSE-BASED ASSESSMENT OF THE WARBURG EFFECT ALLOWS NON-INVASIVE IMAGING OF TUMOR BURDEN AND TREATMENT RESPONSE IN MUTANT IDH GLIOMAS IN VIVO

The Warburg effect, characterized by elevated glucose uptake and flux to lactate, is a metabolic hallmark of cancer. Recent studies have identified deuterium ²H-magnetic resonance spectroscopy (MRS) using 6,6’-²H-glucose as a novel method of imaging the Warburg effect in high-grade primary glioblastomas (GBMs). However, its utility for imaging low-grade gliomas has not been tested. The goal of this study was to determine whether 6,6’-²H-glucose can be used for imaging tumor burden and treatment response in mutant isocitrate dehydrogenase (IDHmut) low-grade gliomas in vivo. We examined mice bearing orthotopic tumors of the patient-derived BT257 astrocytoma model. ¹H-MRS, providing a readout of steady-state metabolite levels, confirmed the presence of 2-hydroxyglutarate, the product of IDHmut, in BT257 tumor tissue but not normal brain. Previous studies comparing IDHmut gliomas with GBMs suggest that IDHmut gliomas undergo lactate dehydrogenase silencing, potentially leading to a non-glycolytic phenotype. Nevertheless, our results indicated that, compared to normal brain, glucose uptake and concomitant flux to lactate were significantly higher in BT257 tumor tissue. Importantly, 6,6’-²H-glucose metabolism to lactate was observed in BT257 tumor-bearing mice, but not tumor-free mice. Furthermore, imaging studies confirmed spatial localization of lactate production to the tumor vs. contralateral normal brain. We then examined the ability of 6,6’-²H-glucose to assess treatment response. Poly-(adenosine 5′-diphosphate-ribose) polymerase inhibitors (PARPi) inhibit IDHmut glioma growth and are in clinical trials for IDHmut glioma patients. Treatment with the PARPi niraparib reduced 6,6’-²H-glucose flux to lactate in BT257 tumor-bearing mice. Importantly, this reduction was observed at early time-points when no difference in tumor volume could be detected using anatomical imaging, pointing to the ability of 6,6’-²H-glucose to assess pseudoprogression. Collectively, our results suggest that IDHmut gliomas display a glycolytic phenotype amenable to non-invasive ²H-MRS-based imaging of tumor burden and treatment response.

BIMG-02. IMAGING IMMORTALITY: TERT EXPRESSION ALTERS GLUCOSE METABOLISM IN LOW-GRADE GLIOMAS IN A MANNER THAT CAN BE LEVERAGED FOR NONINVASIVE METABOLIC IMAGING

Telomerase reverse transcriptase (TERT) is essential for tumor immortality and uncontrolled proliferation, including in low-grade oligodendrogliomas (LGOGs). Since it is silenced in somatic cells, TERT is also a therapeutic target. Non-invasive imaging of TERT can differentiate tumor from normal brain or lesions such as gliosis and allow assessment of response to therapy. The goal of this study was to identify magnetic resonance spectroscopy (MRS)-detectable metabolic alterations associated with TERT that can be leveraged for noninvasive imaging in LGOGs. We examined patient-derived BT54 neurospheres in which TERT expression was silenced by RNA interference. ¹H-MRS showed that steady-state levels of NAD(P)/H, glutathione, aspartate and AXP were elevated in BT54_TERT+ neurospheres relative to BT54_TERT-. Glucose flux through the pentose phosphate pathway (PPP) is essential for generating NADPH, which maintains glutathione homeostasis. ¹³C-MRS confirmed that [2-¹³C]-glucose flux through the PPP was elevated in BT54_TERT+ neurospheres relative to BT54_TERT-, an effect associated with higher activity of the PPP enzyme glucose-6-phosphate dehydrogenase (G6PDH). Hyperpolarized ¹³C-MRS is a method of increasing the signal to noise ratio of ¹³C-MRS such that it can monitor metabolic fluxes noninvasively in cells, animals and patients. Consistent with elevated PPP flux and G6PDH activity, hyperpolarized [U-¹³C]-glucose metabolism via the PPP to 6-phosphogluconate (6-PG) was elevated in BT54_TERT+ neurospheres relative to BT54_TERT-. Importantly, examination of an additional patient-derived LGOG model, the SF10417 model which readily forms orthotopic tumor xenografts in rats, showed that 6-PG production from hyperpolarized [U-¹³C]-glucose demarcated tumor from normal brain. Furthermore, LGOG patient biopsies had elevated NAD(P)/H, glutathione, aspartate, AXP and G6PDH activity relative to gliosis biopsies, confirming the clinical validity of our observations. Collectively, we have identified a metabolic signature of TERT expression that can be leveraged via hyperpolarized [U-¹³C]-glucose to improve diagnosis and treatment response monitoring for LGOG patients.

BIMG-08. DEUTERIUM MAGNETIC RESONANCE SPECTROSCOPY USING 2H-PYRUVATE ALLOWS NON-INVASIVE IN VIVO IMAGING OF TERT EXPRESSION IN BRAIN TUMORS

Telomere shortening constitutes a natural barrier to uncontrolled proliferation and all tumors must find a mechanism of maintaining telomere length. Most human tumors, including high-grade primary glioblastomas (GBMs) and low-grade oligodendrogliomas (LGOGs) achieve telomere maintenance via reactivation of the expression of telomerase reverse transcriptase (TERT), which is silenced in normal somatic cells. TERT expression is, therefore, a driver of tumor proliferation and, due to this essential role, TERT is also a therapeutic target. However, non-invasive methods of imaging TERT are lacking. The goal of this study was to identify magnetic resonance spectroscopy (MRS)-detectable metabolic biomarkers of TERT expression that will enable non-invasive visualization of tumor burden in LGOGs and GBMs. First, we silenced TERT expression by RNA interference in patient-derived LGOG (SF10417, BT88) and GBM (GS2) models. Our results linked TERT silencing to significant reductions in steady-state levels of NADH in all models. NADH is essential for the conversion of pyruvate to lactate, suggesting that measuring pyruvate flux to lactate could be useful for imaging TERT status. Recently, deuterium (²H)-MRS has emerged as a novel, clinically translatable method of monitoring metabolic fluxes in vivo. However, to date, studies have solely examined ²H-glucose and the use of [U-²H]pyruvate for non-invasive ²H-MRS has not been tested. Following intravenous injection of a bolus of [U-²H]pyruvate, lactate production was higher in mice bearing orthotopic LGOG (BT88 and SF10417) and GBM (GS2) tumor xenografts relative to tumor-free mice, suggesting that [U-²H]pyruvate has the potential to monitor TERT expression in vivo. In summary, our study, for the first time, shows the feasibility and utility of [U-²H]pyruvate for in vivo imaging. Importantly, since ²H-MRS can be implemented on clinical scanners, our results provide a novel, non-invasive method of integrating information regarding a fundamental cancer hallmark, i.e. TERT, into glioma patient management.

Non-invasive assessment of telomere maintenance mechanisms in brain tumors

Telomere maintenance is a universal hallmark of cancer. Most tumors including low-grade oligodendrogliomas use telomerase reverse transcriptase (TERT) expression for telomere maintenance while astrocytomas use the alternative lengthening of telomeres (ALT) pathway. Although TERT and ALT are hallmarks of tumor proliferation and attractive therapeutic targets, translational methods of imaging TERT and ALT are lacking. Here we show that TERT and ALT are associated with unique 1H-magnetic resonance spectroscopy (MRS)-detectable metabolic signatures in genetically-engineered and patient-derived glioma models and patient biopsies. Importantly, we have leveraged this information to mechanistically validate hyperpolarized [1-13C]-alanine flux to pyruvate as an imaging biomarker of ALT status and hyperpolarized [1-13C]-alanine flux to lactate as an imaging biomarker of TERT status in low-grade gliomas. Collectively, we have identified metabolic biomarkers of TERT and ALT status that provide a way of integrating critical oncogenic information into non-invasive imaging modalities that can improve tumor diagnosis and treatment response monitoring.

Glutamate Is a Noninvasive Metabolic Biomarker of IDH1-Mutant Glioma Response to Temozolomide Treatment

Although lower grade gliomas are driven by mutations in the isocitrate dehydrogenase 1 (IDH1) gene and are less aggressive than primary glioblastoma, they nonetheless generally recur. IDH1-mutant patients are increasingly being treated with temozolomide, but early detection of response remains a challenge and there is a need for complementary imaging methods to assess response to therapy prior to tumor shrinkage. The goal of this study was to determine the value of magnetic resonance spectroscopy (MRS)-based metabolic changes for detection of response to temozolomide in both genetically engineered and patient-derived mutant IDH1 models. Using 1H MRS in combination with chemometrics identified several metabolic alterations in temozolomide-treated cells, including a significant increase in steady-state glutamate levels. This was confirmed in vivo, where the observed 1H MRS increase in glutamate/glutamine occurred prior to tumor shrinkage. Cells labeled with [1-13C]glucose and [3-13C]glutamine, the principal sources of cellular glutamate, showed that flux to glutamate both from glucose via the tricarboxylic acid cycle and from glutamine were increased following temozolomide treatment. In line with these results, hyperpolarized [5-13C]glutamate produced from [2-13C]pyruvate and hyperpolarized [1-13C]glutamate produced from [1-13C]α-ketoglutarate were significantly higher in temozolomide-treated cells compared with controls. Collectively, our findings identify 1H MRS-detectable elevation of glutamate and hyperpolarized 13C MRS-detectable glutamate production from either pyruvate or α-ketoglutarate as potential translatable metabolic biomarkers of response to temozolomide treatment in mutant IDH1 glioma. SIGNIFICANCE: These findings show that glutamate can be used as a noninvasive, imageable metabolic marker for early assessment of tumor response to temozolomide, with the potential to improve treatment strategies for mutant IDH1 patients.

TAMI-08. A TALE OF TWO TELOMERE MAINTENANCE MECHANISMS: TERT EXPRESSION AND THE ALT PATHWAY INDUCE UNIQUE MRS-DETECTABLE METABOLIC REPROGRAMMING IN LOW-GRADE GLIOMAS

Telomeres are nucleoprotein structures at chromosomal ends that shorten with cell division and constitute a natural barrier to proliferation. In order to proliferate indefinitely, all tumors require a telomere maintenance mechanism (TMM). Telomerase reverse transcriptase (TERT) expression is the TMM in most tumors, including low-grade oligodendrogliomas (LGOGs). In contrast, low-grade astrocytomas (LGAs) use the alternative lengthening of telomeres (ALT) pathway as their TMM. As molecular hallmarks of tumor proliferation, TMMs are attractive tumor biomarkers and therapeutic targets. Non-invasive imaging of TMM status will, therefore, allow assessment of tumor proliferation and treatment response. However, translational methods of imaging TMM status are lacking. Here, we show that TERT expression and the ALT pathway are associated with unique magnetic resonance spectroscopy (MRS)-detectable metabolic reprogramming in LGOGs and LGAs respectively. In genetically-engineered and patient-derived LGOG models, TERT expression is linked to elevated 1H-MRS-detectable NAD(P)/H, glutathione, aspartate and AXP. In contrast, the ALT pathway in LGAs is associated with higher α-ketoglutarate, glutamate, alanine and AXP. Importantly, elevated flux of hyperpolarized [1-13C]-alanine to pyruvate, which depends on α-ketoglutarate, is a non-invasive in vivo imaging biomarker of the ALT pathway in LGAs while elevated flux of hyperpolarized [1-13C]-alanine to lactate, which depends on NADH, is an imaging biomarker of TERT expression in LGOGs. Mechanistically, the ALT pathway in LGAs is linked to higher glutaminase (GLS), a key enzyme for α-ketoglutarate biosynthesis while TERT expression in LGOGs is associated with elevated nicotinamide phosphoribosyltransferase (NAMPT), a key enzyme for NADH biosynthesis. Notably, TERT expression and the ALT pathway are linked to MRS-detectable metabolic reprogramming in LGOG and LGA patient biopsies, emphasizing the clinical validity of our observations. Collectively, we have identified unique metabolic signatures of TMM status that integrate critical oncogenic information with noninvasive imaging modalities that can improve diagnosis and treatment response monitoring for LGOG and LGA patients.

BIOM-19. METABOLIC ALTERATION INDUCED BY SELECTIVE KNOCK DOWN OF GABPB1L IN U251 CELLS

BACKGROUND

TERT promoter mutations that result in TERT expression are observed in over 80% of GBM and upstream inhibition of TERT expression by targeting GABPB1L is currently under investigation. In that context, non-invasive reliable biomarkers that can help detect TERT expression are needed. The aim of this research was to assess the value of magnetic resonance spectroscopy (MRS)-detectable metabolic changes as biomarkers of TERT expression in GBM.

METHODS

GABPB1L knock down clones (GABPB1LKD) were established by introducing Crispr Cas9 plasmid vector targeting GABPB1L into U251 cells. Two representative clones with different knock down efficiency were chosen and compared to control cells. Tumor forming capacity was evaluated by colony formation assay and magnetic resonance imaging of orthotopically implanted tumors in mice. Cells were extracted using the dual phase extraction method and 1H-MRS data of cell extracts acquired using a Bruker 500 scanner. The data was analyzed using Mnova software. Multivariate analysis was performed using the SIMCA software.

RESULTS

TERT expression was significantly reduced in GABPB1LKD compared to control cells depending on the GABPB1L knock down efficiency. Colony forming capacity was impaired in GABPB1LKD compared to control cells. In vivo MRI data showed significantly smaller tumor volumes in GABPB1LKD compared to control. Unbiased PCA analysis of 1H-MRS data showed separation of GABPB1LKD and control extracts and VIP scores derived from the OPLS-DA analysis, demonstrated that the common metabolites leading to separation of GABPB1LKD and control cells were aspartate, glutathione, glycerophosphocholine, glutamine, NAD(P)+, AXP. This data was confirmed by univariate analysis that revealed that aspartate, glutathione, glutamine, NAD(P)+, AXP level was significantly reduced in GABPB1LKD.

CONCLUSIONS

GABPB1L knock down cells that show reduced TERT expression demonstrate MRS-detectable metabolic changes. These could be translated into clinical applications, improve the monitoring of GBM patients and advance precision medicine.

MR-detectable metabolic biomarkers of response to mutant IDH inhibition in low-grade glioma

Mutations in isocitrate dehydrogenase 1 (IDH1mut) are reported in 70-90% of low-grade gliomas and secondary glioblastomas. IDH1mut catalyzes the reduction of α-ketoglutarate (α-KG) to 2-hydroxyglutarate (2-HG), an oncometabolite which drives tumorigenesis. Inhibition of IDH1mut is therefore an emerging therapeutic approach, and inhibitors such as AG-120 and AG-881 have shown promising results in phase 1 and 2 clinical studies. However, detection of response to these therapies prior to changes in tumor growth can be challenging. The goal of this study was to identify non-invasive clinically translatable metabolic imaging biomarkers of IDH1mut inhibition that can serve to assess response. Methods: IDH1mut inhibition was confirmed using an enzyme assay and 1H- and 13C- magnetic resonance spectroscopy (MRS) were used to investigate the metabolic effects of AG-120 and AG-881 on two genetically engineered IDH1mut-expressing cell lines, NHAIDH1mut and U87IDH1mut. Results:1H-MRS indicated a significant decrease in steady-state 2-HG following treatment, as expected. This was accompanied by a significant 1H-MRS-detectable increase in glutamate. However, other metabolites previously linked to 2-HG were not altered. 13C-MRS also showed that the steady-state changes in glutamate were associated with a modulation in the flux of glutamine to both glutamate and 2-HG. Finally, hyperpolarized 13C-MRS was used to show that the flux of α-KG to both glutamate and 2-HG was modulated by treatment. Conclusion: In this study, we identified potential 1H- and 13C-MRS-detectable biomarkers of response to IDH1mut inhibition in gliomas. Although further studies are needed to evaluate the utility of these biomarkers in vivo, we expect that in addition to a 1H-MRS-detectable drop in 2-HG, a 1H-MRS-detectable increase in glutamate, as well as a hyperpolarized 13C-MRS-detectable change in [1-13C] α-KG flux, could serve as metabolic imaging biomarkers of response to treatment.

CBMT-02. UP-REGULATION OF Γ-GLUTAMYL-TRANSFERASE CAN BE USED TO IMAGE GLIOBLASTOMA USING HYPERPOLARIZED Γ-GLUTAMYL-[1-13C]GLYCINE MRS

γ-glutamyl-transferase (GGT) is a key enzyme in the γ-glutamyl cycle, which regulates glutathione homeostasis. The enzyme is localized on the outer cell membrane and cleaves glutathione to glutamate and cysteinylglycine. As such, it facilitates uptake of the amino acids essential for intracellular synthesis of glutathione (GSH), which is the major thiol anti-oxidant. GGT is upregulated in glioblastoma, but remains low in normal brain. It is therefore an attractive molecular imaging target for specific detection of glioblastoma. The goal of our study was therefore to assess for the first time the value of hyperpolarized (HP) γ-glutamyl-[1-13C]glycine (γ-Glu-[1-13C]Gly) for imaging glioblastoma in orthotopic tumor-bearing rats. Athymic nude rats with U87 glioblastoma tumors or tumor-free controls were investigated. First, we confirmed that GGT expression was significantly higher in U87 tumors compared to normal rat brain tissue. GSH levels were also higher. Imaging studies were then performed by injecting HP γ-Glu-[1-13C]Gly and acquiring dynamic 13C MR spectra using a flyback spectral-spatial slab sequence on a preclinical Bruker 3T MR system. The dynamic data were analyzed by measuring the signal-to-noise (SNR) ratios of the substrate (γ-Glu-[1-13C]Gly) and the product ([1-13C]Glycine). Comparison of control and tumor-bearing rats showed no statistically significant difference in the SNR of the substrate. In contrast, the SNR of the product demonstrated a significantly higher level of HP [1-13C]Glycine in the tumor-bearing rats compared to controls. Consistent with the higher levels of HP [1-13C]Glycine, the [1-13C]Gly-to-γ-Glu-[1-13C]Glycine ratio was also significantly higher in tumor-bearing animals relative to controls (0.046±0.004 vs 0.021±0.008 respectively; p=0.03). Further studies are needed to assess the generality of our findings. Nonetheless, this study demonstrates for the first time the feasibility of using γ-Glu-[1-13C]Gly to monitor GGT activity and thus could serve as a new approach for monitoring the presence of tumor.

EXTH-20. HYPERPOLARIZED [2-13C] PYRUVATE TO [5-13C] GLUTAMATE AS BIOMARKERS OF IDH1 MUTANT GLIOMA RESPONSE TO TEMOZOLOMIDE THERAPY

Low-grade gliomas, driven by mutations in the cytosolic isocitrate dehydrogenase 1 (IDH1) gene, are less aggressive than primary glioblastoma, but nonetheless always recur and ultimately lead to patient death. The treatment of IDH1 mutant patients with Temozolomide (TMZ) improves survival, but there remains a need for complementary imaging methods to assess response to therapy at an early time point. The goal of this study was, therefore, to determine the value of magnetic resonance spectroscopy (MRS)-based metabolic imaging biomarkers for detection of response to treatment. To this end we investigated NHA and U87 cells expressing IDH1 R132H mutant gene (NHAIDHmut and U87IDHmut) and first used 1H MRS combined with chemometrics to examined the metabolic alterations that occurred following treatment with the IC50 value of TMZ. We observed a significant increase in 2-hydroxyglutarate (2-HG), glutamate, and glutamine, and metabolic pathway analysis showed tricarboxylic acid (TCA) cycle and pyruvate metabolism to be significantly altered pathways following TMZ treatment compared to DMSO control. To confirm changes in TCA cycle flux and to assess the metabolic pathways contributing to the increase in 2-HG and glutamate/glutamine, cells were then labelled with [1-13C] glucose and [3-13C] glutamine. Our data indicated that both glucose flux via the TCA to glutamate and 2HG, and the contribution of glutamine to glutamate and 2HG were increased following TMZ treatment. Finally, we used hyperpolarized 13C-MRS to dynamically probe the metabolism of hyperpolarized [2-13C] pyruvate and its conversion to hyperpolarized [5-13C] glutamate via the TCA cycle. Consistent with our previous findings, we observed that hyperpolarized [5-13C] glutamate synthesis was significantly higher in TMZ-treated cells compared to controls. Collectively, our findings identify 1H MRS-detectable elevation of 2-HG and glutamate/glutamine as well as hyperpolarized 13C-MRS-detectable [5-13C] glutamate production from [2-13C] pyruvate as potentially translatable metabolic biomarkers of response to TMZ therapy in mutant IDH1 glioma.

CBMT-08. IN VIVO EVALUATION OF PENTOSE PHOSPHATE PATHWAY ACTIVITY IN ORTHOTOPIC GLIOMA USING HYPERPOLARIZED δ-[1-13C]GLUCONOLACTONE

The pentose phosphate pathway (PPP) generates NADPH and ribose 5-phosphate, which are involved in the scavenging of reactive oxygen species and the synthesis of nucleotides. As such, the PPP is typically upregulated in cancer cells to address the metabolic needs of rapid cell proliferation. Imaging PPP upregulation could therefore be useful in tumor assessment. One intermediate of the pathway is 6-phospho-δ-gluconolactone (6P-δ-GL), which is produced by phosphorylation of δ-gluconolactone. 6P-δ-GL is further metabolized to 6-phospho-gluconate (6PG). The goal of our study was to evaluate, for the first time, whether hyperpolarized (HP) δ-[1-13C]gluconolactone can be used to assess PPP flux and detect the presence of tumor in an orthotopic glioma rat model. Athymic nude rats bearing orthotropic U87 tumors or age-matched tumor-free controls were investigated. HP studies were performed following intravenous injection of HP δ-[1-13C]gluconolactone and metabolic images using a flyback spectral-spatial echo-planar spectroscopic imaging pulse were acquired. The data were processed using in-house Matlab code. 6P-δ-GL and 6-phospho-γ-[1-13C]gluconolactone were observed in all rats ~10 seconds after HP δ-[1-13C]gluconolactone injection, followed ~5 seconds later by production of 6PG observed at 179.3ppm. These data indicate that HP δ-[1-13C]gluconolactone likely crosses the blood-brain barrier, consistent with its transport via glucose transporters, and is rapidly metabolized. Importantly, 6PG was significantly higher in tumor voxels. The ratio of 6PG-to-6P-δ-GL was comparable in normal brain and in normal-appearing contralateral brain of tumor-bearing rats at 0.43±0.09 and 0.45±0.06 respectively (p=0.85), but significant higher in the tumor regions at 0.70±0.11 (p=0.04 and p=0.02 respectively), consistent with the elevated PPP flux that typically occurs in tumor cells. Our results indicate, to our knowledge for the first time, that metabolism of HP δ-[1-13C]gluconolactone can be assessed in the brain and that elevated 6PG production in glioma provides a potential metabolic imaging approach to probe tumor development, recurrence and response to therapy.

CBMT-41. IMAGING A HALLMARK OF CANCER: HYPERPOLARIZED 13C-MAGNETIC RESONANCE SPECTROSCOPY CAN NON-INVASIVELY MONITOR TERT EXPRESSION IN LOW-GRADE GLIOMAS IN VIVO

Telomerase reverse transcriptase (TERT) expression is a hallmark of cancer, including in primary glioblastomas and low-grade oligodendrogliomas. Since TERT is essential for glioma proliferation and is an attractive therapeutic target, metabolic imaging of TERT status can inform on tumor progression and response to therapy. To that end, the goal of this study was to identify non-invasive, translational, hyperpolarized 13C-magnetic resonance spectroscopy-detectable metabolic imaging biomarkers of TERT in low-grade oligodendrogliomas. Unbiased metabolomic analysis of immortalized normal human astrocytes without (NHAcontrol) and with TERT (NHAtert) indicated that TERT induced unique metabolic reprogramming. Notably, TERT increased NADPH and NADH levels. Glucose flux through the pentose phosphate pathway (PPP) is a major producer of NADPH. Non-invasive imaging of PPP flux using hyperpolarized [U-13C,U-2H]-glucose indicated that production of the PPP metabolite 6-phosphogluconate (6-PG) was elevated in NHAtert cells relative to NHAcontrol. Importantly, hyperpolarized [U-13C,U-2H]-glucose flux to 6-PG clearly differentiated tumor from normal brain in orthotopic NHAtert tumor xenografts. Next, we exploited the observation that TERT expression increased NADH, which is essential for the metabolism of hyperpolarized [1-13C]-alanine to lactate. Lactate production from hyperpolarized [1-13C]-alanine was higher in NHAtert cells relative to NHAcontrol. Importantly, hyperpolarized [1-13C]-alanine imaging in orthotopic NHAtert tumors revealed pronounced differences in lactate production between tumor tissue and normal brain. Mechanistically, TERT increased expression of glucose-6-phosphate dehydrogenase (G6PDH), the rate-limiting enzyme for 6-PG and NADPH production, and of nicotinamide phosphoribosyltransferase (NAMPT), a rate-limiting enzyme for NADH biosynthesis. Silencing TERT reversed G6PDH and NAMPT expression and normalized hyperpolarized [U-13C,U-2H]-glucose and [1-13C]-alanine metabolism, validating our imaging biomarkers. Finally, hyperpolarized [U-13C,U-2H]-glucose and [1-13C]-alanine could monitor TERT status in the clinically relevant, patient-derived BT54 oligodendroglioma model. In summary, we demonstrate, for the first time, non-invasive in vivo imaging of TERT status in gliomas that can enable longitudinal analysis of tumor burden and treatment response in the clinic.

PI3K/mTOR inhibition of IDH1 mutant glioma leads to reduced 2HG production that is associated with increased survival

70-90% of low-grade gliomas and secondary glioblastomas are characterized by mutations in isocitrate dehydrogenase 1 (IDHmut). IDHmut produces the oncometabolite 2-hydroxyglutarate (2HG), which drives tumorigenesis in these tumors. The phosphoinositide-3-kinase (PI3K)/mammalian target of rapamycin (mTOR) pathway represents an attractive therapeutic target for IDHmut gliomas, but noninvasive indicators of drug target modulation are lacking. The goal of this study was therefore to identify magnetic resonance spectroscopy (MRS)-detectable metabolic biomarkers associated with IDHmut glioma response to the dual PI3K/(mTOR) inhibitor XL765. ¹H-MRS of two cell lines genetically modified to express IDHmut showed that XL765 induced a significant reduction in several intracellular metabolites including 2HG. Importantly, examination of an orthotopic IDHmut tumor model showed that enhanced animal survival following XL765 treatment was associated with a significant in vivo ¹H-MRS detectable reduction in 2HG but not with significant inhibition in tumor growth. Further validation is required, but our results indicate that 2HG could serve as a potential noninvasive MRS-detectable metabolic biomarker of IDHmut glioma response to PI3K/mTOR inhibition.

Abstract 5263: 1H and 13C MRS-based metabolic markers of IDH1 mutant glioma response to temozolomide therapy

Low-grade gliomas, driven by mutations in the isocitrate dehydrogenase 1 (IDH1) gene, are less aggressive than primary glioblastoma (GBM), but nonetheless always recur and ultimately lead to patient death. To improve patient survival, one therapeutic strategy is treatment with the alkylating chemotherapeutic agent Temozolomide (TMZ), previously reserved for the treatment of primary GBM. Though the treatment of IDH1 mutant patients with TMZ improves survival, early detection of treatment-associated effects remains challenging. The goal of this study was, therefore, to determine the value of magnetic resonance spectroscopy (MRS)-based biomarkers for detection of response to treatment. To this end, we examined the global metabolic alterations that occurred following TMZ treatment in a genetically engineered IDH1 mutant immortalized Normal Human Astrocyte (NHAIDHmut)-based cell model using 1H MRS combined with chemometrics and 13C MRS combined with labeling of cells with [1-13C]-glucose and [3-13C]-glutamine. Cells were treated either with the IC50 value of TMZ (100 μM; N=5), or with DMSO (0.2%; N=5) for 72 hours. Then, metabolites were extracted from cells using the dual-phase extraction method. 1H-MRS and proton-decoupled 13C-MRS spectra were acquired using a 500 MHz Bruker Avance spectrometer. 1H MRS data was subjected to multivariate analysis using SIMCA. Specific metabolites were also quantified using Mnova7, integrals normalized to TSP and to cell number and statistical significance of differences determined using unpaired Student’s t-test. Pathway enrichment analysis of dysregulated metabolites was performed using MetPA. principal component analysis score plot showed separation of TMZ-treated from DMSO-treated control cells. Further, improved separation between the groups was obtained by orthogonal-partial least squares discriminant analysis. Most significant metabolites contributing to class separation were identified using correlation ≥0.6 and variable importance in projection score ≥1. Glutamine, glutamate, 2-hydroxyglutarate (2-HG), pyruvate, succinate, glucose, phosphocholine, isoleucine, valine, lysine, phenylalanine, NAD(P)+ and AMP/ADP/ATP were observed to be significantly higher in TMZ-treated cells as compared to controls. Based on pathway impact score, tricarboxylic acid (TCA) cycle was identified to be the significantly altered pathway following treatment Further, there was a significant increases in glucose- and glutamine-derived glutamate and 2-HG, indicating an increase in flux to the TCA cycle following treatment. These findings demonstrate that IDH1 mutant glioma show a clear metabolomic fingerprint in response to TMZ therapy, which, combined with complementary hyperpolarized 13C MRS approaches, may help assess early response to TMZ therapy in mutant IDH1 glioma.

Note: This abstract was not presented at the meeting.

Citation Format: Elavarasan Subramani, Chloe Najac, Georgios Batsios, Pavithra Viswanath, Marina Radoul, Anne Marie Gillespie, Russell O. Pieper, Sabrina M. Ronen. 1H and 13C MRS-based metabolic markers of IDH1 mutant glioma response to temozolomide therapy [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 5263.

In vivo investigation of hyperpolarized [1,3-13C2]acetoacetate as a metabolic probe in normal brain and in glioma

Dysregulation in NAD⁺/NADH levels is associated with increased cell division and elevated levels of reactive oxygen species in rapidly proliferating cancer cells. Conversion of the ketone body acetoacetate (AcAc) to β-hydroxybutyrate (β-HB) by the mitochondrial enzyme β-hydroxybutyrate dehydrogenase (BDH) depends upon NADH availability. The β-HB-to-AcAc ratio is therefore expected to reflect mitochondrial redox. Previous studies reported the potential of hyperpolarized ¹³C-AcAc to monitor mitochondrial redox in cells, perfused organs and in vivo. However, the ability of hyperpolarized ¹³C-AcAc to cross the blood brain barrier (BBB) and its potential to monitor brain metabolism remained unknown. Our goal was to assess the value of hyperpolarized [1,3-¹³C₂]AcAc in healthy and tumor-bearing mice in vivo. Following hyperpolarized [1,3-¹³C₂]AcAc injection, production of [1,3-¹³C₂]β-HB was detected in normal and tumor-bearing mice. Significantly higher levels of [1-¹³C]AcAc and lower [1-¹³C]β-HB-to-[1-¹³C]AcAc ratios were observed in tumor-bearing mice. These results were consistent with decreased BDH activity in tumors and associated with increased total cellular NAD⁺/NADH. Our study confirmed that AcAc crosses the BBB and can be used for monitoring metabolism in the brain. It highlights the potential of AcAc for future clinical translation and its potential utility for monitoring metabolic changes associated with glioma, and other neurological disorders.

Hybrid multiband excitation multiecho acquisition for hyperpolarized (13) C spectroscopic imaging

PURPOSE

Fast dynamic imaging of hyperpolarized (13) C-labeled pyruvate and its downstream metabolites shows great potential for probing metabolic changes in the heart. Sequences that allow for fast encoding of the spectral and spatial information of the myocardial metabolism and optimal signal excitation are usually limited by gradient performance, especially at high magnetic fields. Here we propose a combination of a spectral-spatial multiband excitation and multiecho readout to overcome these limitations.

METHODS

By using a low-bandwidth, two-pulse excitation, a thinner slice compared with conventional spectral-spatial excitation is achieved, while at the same time allowing for low flip angle excitation on pyruvate and high flip angle excitation on bicarbonate and lactate, which optimizes signal-to-noise ratio (SNR) in cardiac metabolic imaging. The implementation was evaluated in 13 healthy female Sprague-Dawley rats at 9.4T.

RESULTS

Using a slice thickness of 4 mm, a mean (± standard deviation) peak SNR of 18.3 (±8.4), 15.2 (±6.6), and 8.6 (±2.0) was observed for pyruvate, lactate, and bicarbonate, respectively.

CONCLUSION

This approach provides high SNR in metabolic images while at the same time allowing for a thin slice selection even at high magnetic fields. This is crucial in metabolic imaging in small animal models.