Metabolic imaging of brain tumors using (13)C Magnetic Resonance Spectroscopy (MRS) of hyperpolarized [1-(13)C] pyruvate is a promising neuroimaging strategy which, after a decade of preclinical success in glioblastoma (GBM) models, is now entering clinical trials in multiple centers. Typically, the presence of GBM has been associated with elevated hyperpolarized [1-(13)C] lactate produced from [1-(13)C] pyruvate, and response to therapy has been associated with a drop in hyperpolarized [1-(13)C] lactate. However, to date, lower grade gliomas had not been investigated using this approach. The most prevalent mutation in lower grade gliomas is the isocitrate dehydrogenase 1 (IDH1) mutation, which, in addition to initiating tumor development, also induces metabolic reprogramming. In particular, mutant IDH1 gliomas are associated with low levels of lactate dehydrogenase A (LDHA) and monocarboxylate transporters 1 and 4 (MCT1, MCT4), three proteins involved in pyruvate metabolism to lactate. We therefore investigated the potential of (13)C MRS of hyperpolarized [1-(13)C] pyruvate for detection of mutant IDH1 gliomas and for monitoring of their therapeutic response. We studied patient-derived mutant IDH1 glioma cells that underexpress LDHA, MCT1 and MCT4, and wild-type IDH1 GBM cells that express high levels of these proteins. Mutant IDH1 cells and tumors produced significantly less hyperpolarized [1-(13)C] lactate compared to GBM, consistent with their metabolic reprogramming. Furthermore, hyperpolarized [1-(13)C] lactate production was not affected by chemotherapeutic treatment with temozolomide (TMZ) in mutant IDH1 tumors, in contrast to previous reports in GBM. Our results demonstrate the unusual metabolic imaging profile of mutant IDH1 gliomas, which, when combined with other clinically available imaging methods, could be used to detect the presence of the IDH1 mutation in vivo.

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.

The alkylating agent temozolomide (TMZ), previously reserved for treatment of glioblastoma, is now being considered for the treatment of low-grade glioma that are driven by mutations in the isocitrate dehydrogenase 1 (IDH1) gene. Though the treatment of IDH1 mutant patients with TMZ improves survival, there is a need for metabolic imaging to help in assessing early response to therapy. 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 address this, we examined the global metabolic alterations that occurred following TMZ treatment in a genetically engineered IDH1 mutant immortalized normal human astrocyte cell line (NHAIDHmut) using 1H and 13C MRS combined with chemometrics. Cells were treated either with the IC50 of TMZ (100 μM; N=5), or with DMSO (0.2%; N=5) for 72 hours. Then, metabolites were extracted from cells and 1H spectra acquired. Data were analyzed using SIMCA and the most significant metabolites contributing to class separation were identified using multivariate and univariate analyses. Alternatively, live cells were exposed to hyperpolarized 2-13C-pyruvate and dynamic sets of 13C-MRS spectra recorded to monitor the production of 5-13C-glutamate over time. 1H MRS showed that glutamine, glutamate, pyruvate, succinate, glucose, phosphocholine, isoleucine, valine, lysine, phenylalanine, NAD+/NADP+ and ATP/ADP/AMP were significantly higher in TMZ-treated cells as compared to controls. Accordingly, the tricarboxylic acid (TCA) cycle was identified as a significantly altered pathway following TMZ treatment. Consistent with this finding, dynamically probing the metabolism of hyperpolarized 2-13C-pyruvate revealed that build-up of 5-13C-glutamate, which is associated with flux to the TCA cycle, was significantly higher in TMZ-treated cells as compared to controls. Further studies are warranted for validation of our findings in other mutant IDH1 models. Nonetheless, our findings identify potential MRS-detectable early biomarkers of response to TMZ therapy in mutant IDH1 glioma.