Donghyun Hong

Alpha-ketoglutarate (aKG) is a central intermediate of cerebral energy metabolism and a precursor for glutamate synthesis in the brain. Alterations in aKG metabolism occur in pathological contexts, including isocitrate dehydrogenase (IDH) mutant astrocytomas and oligodendrogliomas, in which mutant IDH converts aKG to the oncometabolite 2-hydroxyglutarate. Given its central role in brain metabolism, non-invasive interrogation of aKG-dependent metabolic flux is needed. Hyperpolarized (HP) ¹³C MR enables real-time visualization of metabolic conversion by transiently enhancing signal intensity by several orders of magnitude. Leveraging this approach, we report the first-in-human feasibility and safety study of HP [1-¹³C]aKG MR spectroscopy in the healthy brain (n = 3). A standard operating procedure (SOP) was developed for sterile [1-¹³C]aKG dose production, achieving reproducible polarization levels averaging 30.5 ± 2.2%. Following intravenous administration, time-resolved ¹³C spectra in healthy volunteers demonstrated the detection of HP aKG resonance and a measurable downstream glutamate signal, consistent across repeat acquisitions, with a delayed temporal profile relative to aKG observed in a representative dataset. Although performed in healthy volunteers, these results establish feasibility for HP [1-¹³C]aKG metabolic imaging to open a new window into normal and pathological brain cellular metabolism.

Background: Telomerase reverse transcriptase (TERT) promoter mutations are observed in most glioblastoma (GBM) tumors, leading to TERT expression, which is crucial for tumor growth. Accordingly, inhibiting TERT or its upstream tumor-specific transcription factor GA-binding protein transcription factor subunit beta 1 (GABPB1) was shown to inhibit tumor growth. In addition, epidermal growth factor receptor (EGFR) was shown to signal upstream of TERT and GABPB1 and to control TERT expression, and EGFR inhibition also inhibits GBM growth. Methods: This study investigated the individual as well as combined effects of EGFR, TERT, and GABPB1 inhibition on cell and orthotopic rat models. We assessed cell proliferation, animal survival, tumor size, 1H magnetic resonance spectroscopy (MRS)-detectable steady-state lactate, and 13C MRS-detectable hyperpolarized (HP) lactate production. Results: When TERT or GABPB1 were inhibited simultaneously with EGFR, the combination treatment resulted in enhanced inhibition of cell and tumor growth as well as animal survival compared not only to controls but also to any of the single treatments. Our study also found that steady-state 1H MRS-detectable lactate and HP 13C MRS-detectable lactate production dropped following every treatment, and the drop was significantly greater following combination treatments. Furthermore, the metabolic changes occurred prior to changes in tumor size, and a reversal of these metabolic biomarkers was associated with tumor recurrence. Conclusion: Our study points to the value of steady-state 1H MRS-detectable lactate and HP 13C MRS-detectable lactate as potential biomarkers of response to combination EGFR/TERT inhibition.

Mutant isocitrate dehydrogenase (IDHmut) catalyzes 2-hydroxyglutarate (2HG) production and is considered a therapeutic target for IDHmut tumors. However, response is mostly associated with inhibition of tumor growth. Response assessment via anatomic imaging is therefore challenging. Our goal was to directly detect IDHmut inhibition using a new hyperpolarized (HP) ¹³C magnetic resonance spectroscopy-based approach to noninvasively assess α-ketoglutarate (αKG) metabolism to 2HG and glutamate. We studied IDHmut-expressing normal human astrocyte (NHAIDH1mut) cells and rats with BT257 tumors, and assessed response to the IDHmut inhibitor BAY-1436032 (n ≥ 4). We developed a new ¹³C Echo Planar Spectroscopic Imaging sequence with an optimized RF pulse to monitor the fate of HP [1-¹³C]αKG and [5-¹²C,1-¹³C]αKG with a 2.5 × 2.5 × 8 mm³ spatial resolution. Cell studies confirmed that BAY-1436032-treatment leads to a drop in HP 2HG and an increase in HP glutamate detectable with both HP substrates. Data using HP [5-¹²C,1-¹³C]αKG also demonstrated that its conversion to 2HG is detectable without the proximal 1.1% natural abundance [5-¹³C]αKG signal. In vivo studies showed that glutamate is produced in normal brains but no 2HG is detectable. In tumor-bearing rats, we detected the production of both 2HG and glutamate, and BAY-1436032-treatment led to a drop in 2HG and an increase in glutamate. Using HP [5-¹²C,1-¹³C]αKG we detected metabolism with an signal-to-noise ratio of 23 for 2HG and 17 for glutamate. Our findings point to the clinical potential of HP αKG, which recently received FDA investigational new drug approval for research, for noninvasive localized imaging of IDHmut status.

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 ¹³C‐labeled compounds in vivo. In this study, we tested our approach on [2‐¹³C]pyruvate and [1‐¹³C]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 ¹³C 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 ¹³C data enhanced our ability to monitor the metabolism of [2‐¹³C]pyruvate and [1‐¹³C]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.

Approximately 80% of low-grade glioma (LGGs) harbor mutant isocitrate dehydrogenase 1/2 (IDH1/2) driver mutations leading to accumulation of the oncometabolite 2-hydroxyglutarate (2-HG). Thus, inhibition of mutant IDH is considered a potential therapeutic target. Several mutant IDH inhibitors are currently in clinical trials, including AG-881 and BAY-1436032. However, to date, early detection of response remains a challenge. In this study we used high resolution ¹H magnetic resonance spectroscopy (¹H-MRS) to identify early noninvasive MR (Magnetic Resonance)-detectable metabolic biomarkers of response to mutant IDH inhibition. In vivo ¹H-MRS was performed on mice orthotopically-implanted with either genetically engineered (U87IDHmut) or patient-derived (BT257 and SF10417) mutant IDH1 cells. Treatment with either AG-881 or BAY-1436032 induced a significant reduction in 2-HG. Moreover, both inhibitors led to a significant early and sustained increase in glutamate and the sum of glutamate and glutamine (GLX) in all three models. A transient early increase in N-acetylaspartate (NAA) was also observed. Importantly, all models demonstrated enhanced animal survival following both treatments and the metabolic alterations were observed prior to any detectable differences in tumor volume between control and treated tumors. Our study therefore identifies potential translatable early metabolic biomarkers of drug delivery, mutant IDH inhibition and glioma response to treatment with emerging clinically relevant therapies.

γ-Aminobutyric acid (GABA), the major inhibitory neurotransmitter, is challenging to measure using proton spectroscopy due to its relatively low concentration, J-coupling and overlapping signals from other metabolites. Currently, the prevalent methods for detecting GABA at ultrahigh field strengths (≥ 7 T) are GABA-editing and model fitting of non-editing single voxel spectra. These two acquisition approaches have their own advantages: the GABA editing approach directly measures the GABA resonance at 3 ppm, whereas the fitting approach on the non-editing spectrum allows the detection of multiple metabolites, and has an SNR advantage over longer echo time (TE) acquisitions. This study aims to compare these approaches for estimating GABA at 7 T. We use an interleaved sequence of semi-LASER (sLASER: TE = 38 ms) and MEGA-sLASER (TE = 80 ms). This simultaneous interleaved acquisition minimizes the differential effect of extraneous factors, and enables an accurate comparison of the two acquisition methods. Spectra were acquired with an 8 ml isotropic voxel at six different brain regions: anterior-cingulate cortex, dorsolateral-prefrontal cortex, motor cortex, occipital cortex, posterior cingulate cortex, and precuneus. Spectral fitting with LCModel quantified the GABA to total Cr (tCr: Creatine + Phosphocreatine) concentration ratio. After correcting the T₂ relaxation time variation, GABA/tCr ratios were similar between the two acquisition approaches. GABA editing showed smaller spectral fitting error according to Cramér–Rao lower bound than the sLASER approach for all regions examined. We conclude that both acquisition methods show similar accuracy but the precision of the MEGA-editing approach is higher for GABA measurement. In addition, the 2.28 ppm GABA resonance was found to be important for estimating GABA concentration without macromolecule contamination in the GABA-edited acquisition, when utilizing spectral fitting with LCModel.

B₁ inhomogeneity and chemical shift displacement error (CSDE) increase with the main magnetic field strength and are therefore deleterious for magnetic resonance spectroscopy (MRS) at ultrahigh field. A solution is to use adiabatic pulses which operate over a broad range of B₁ and thus are insensitive to B₁ inhomogeneity. Moreover, adiabatic pulses usually have a relatively higher bandwidth, which makes CSDE low to negligible. The use of exclusively adiabatic pulses for single‐voxel spectroscopy (SVS) typically brings the disadvantage of a long echo time (TE), but the advantage of a low and matched CSDE. Herein, we took advantage of short‐duration, low‐power, matched‐phase adiabatic spin echo (MASE) pulses to implement a matched CSDE semi‐localized by adiabatic selective refocusing (sLASER) sequence capable of attaining short TEs, while CSDE is matched and still comparatively low. We also demonstrate here the feasibility of the direct measurement of the γ‐aminobutyric acid (GABA) resonance at 2.28 ppm well separated from the neighboring glutamate resonance at 7 T using the implemented MASE‐sLASER sequence at TEs of 68 and 136 ms. The shorter duration of MASE pulses also made it possible to implement a Mescher–Garwood‐semi‐localized by adiabatic selective refocusing (MEGA‐sLASER) (with MASE) sequence with TE = 68 ms for editing GABA at 7 T, the results for which are also shown.