Monitoring response to a clinically relevant IDH inhibitor in glioma-Hyperpolarized 13C magnetic resonance spectroscopy approaches

Abstract

Background: 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.

Methods: 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.

Results: 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.

Conclusions: 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.

Keywords: IDH inhibitor; hyperpolarized 13C MRS; imaging biomarker; mutant IDH glioma; therapeutic response.

Figure 1.

(A). Multiband excitation pulse newly designed for this study. (B). Illustration of matrix orientations for the slab (blue) and EPSI (green) acquisitions on typical T2-weighted axial (left) and sagittal (right) tumor images.

Figure 2.

(A) ¹H MRS spectra from control and treated NHAIDH1mut cells with 2HG and glutamate highlighted in red and blue, respectively. (B) Comparison of 2HG and glutamate levels prior to and following BAY-1436032 treatment as quantified from the ¹H MRS spectra of control and treated NHAIDH1mut cells. (C) Summed ¹³C MRS spectrum over time post-HP [1-¹³C]αKG injection in control cells. Insert panel shows the spectrum at 48 s illustrating 2HG at 183.9 ppm and the natural abundance peak of [5-¹³C]αKG at 184 ppm. (D) Spectra around 184 ppm following injection of the 2 HP substrates: [1-¹³C]αKG (left) and [5-¹²C,1-¹³C]αKG (right) and illustrating 2HG production without the [5-¹³C]αKG peak for [5-¹²C,1-¹³C]αKG. (E) Quantification of HP 2HG and glutamate levels in control and treated cells following [1-¹³C]αKG injection. (F) Quantification of HP 2HG and glutamate levels in control and treated cells following [5-¹²C,1-¹³C]αKG injection. An unpaired Student’s t-test was used to compare the control (n = 4) and treatment (n = 4) groups. A P-value ≤ .05 was considered statistically significant. * signifies P < .05, **P < .01, ***P < 0.001, and ns = not significant.

Figure 3.

(A) Temporal evolution of average tumor volume in control and treated groups using an unpaired Student’s t-test for comparison of groups. (B) Kaplan–Meier survival probability of BT257 tumor-bearing rats comparing control and treated animals. (C) T₂-weighted BT257 tumor image (pink) illustrating the single voxel tumor localization (yellow) for ¹H spectroscopy. ¹H single voxel spectra with LCModel fitting of 2HG and glutamate prior to (D) and following (E) treatment. (F) Comparison of 2HG and glutamate levels prior to (n = 6) and following (n = 4) treatment based on ¹H spectra. Statistical significance was estimated with a paired t-test. A P-value ≤ .05 was considered statistically significant. *Signifies P < .05, **P < .01, ***P < .001, and ns = not significant.

Figure 4.

Summed ¹³C spectra of HP [1-¹³C]α-ketoglutarate metabolism over time in (A) a healthy rat and (B) pre- and (C) post-BAY-1436032 treatment BT257 tumor-bearing rats. Dynamic curves of each metabolite from corresponding models are shown in (D), (E), and (F). (G) Quantified AUC of the dynamic curves from (D, n = 5), (E, n = 6), and (F, n = 4) with comparisons using an unpaired t-test. A P-value ≤ .05 was considered statistically significant. *Signifies P < .05, **P < .01, ***P < .001, and ns = not significant.

Figure 5.

Summed spectra of HP [5-¹²C,1-¹³C]α-ketoglutarate metabolism over time in (A) a healthy rat and (B) pretreatment and (C) posttreatment BT257 tumor-bearing rats. Dynamic curves of each metabolite from corresponding models are shown in (D), (E), and (F). (G) Quantified AUC of dynamic curves from (D, n = 5), (E, n = 6), and (F, n = 4) with comparisons using an unpaired t-test. A P-value ≤ .05 was considered statistically significant. *Signifies P < .05, **P < .01, ***P < .001, and ns = not significant.

Figure 6.

Dynamic heatmaps of 2HG and glutamate obtained from the HP ¹³C EPSI acquisition following [5-¹²C,1-¹³C]α-ketoglutarate injection into rats prior to (A) and following (B) treatment. Inserts illustrate sum spectra over time from tumor and contralateral voxels (red and orange inserts respectively) (C) Quantification of metabolites in tumor and contralateral brain pretreatment and posttreatment (Tx). A P-value ≤ .05 was considered statistically significant. *Signifies P < .05, **P < .01, ***P < .001, and ns = not significant.