2HG Optimized PRESS MRS for Metabolic Profiling of IDH-mutant Gliomas in Patient-Derived Orthotropic Mouse Model of Human Brain Tumor
Vivek Tiwari1, Tomoyuki Mashimo2, Sandeep Kumar Ganji1, Zhongxu An1, Keith Hulsey3, Vamsidhara Vemireddy2, Shanrong Zhang1, Elizabeth Maher2, Robert Bachoo2, and Changho Choi1

1Advanced Imaging Research Center, UT Southwestern Medical Center, Dallas, TX, United States, 2Department of Internal Medicine, UT Southwestern Medical Center, Dallas, TX, United States, 3Department of Radiology, UT Southwestern Medical Center, Dallas, TX, United States

Synopsis

2-Hydroxyglutarate (2HG) is elevated in gliomas harboring IDH1 or IDH2 mutations, and can be a biomarker for the diagnosis of IDH-mutant gliomas. The present study was undertaken to develop PRESS MRS Methodology for 2HG-detection with enhanced precision, and studying 2HG kinetics together with metabolic profiling in Patient-Derived Xenograft Orthotropic Mouse model. In-vivo PRESS MRS was performed at an optimized long-TE of 96ms. A large inverted-signal of 2HG at 2.25-ppm was obtained from IDH-mutant gliomas, well separated from neighboring signals of positive-polarity, with minimal interference from macromolecules. The method was successfully employed to detect 2HG and brain metabolism with tumor progression.

Purpose

2-Hydroxyglutarate (2HG) is elevated in gliomas harboring IDH1/2-mutations, suggesting the potential of 2HG as a biomarker for the diagnosis of IDH-mutant gliomas1-2. A recent development of 1H MRS for in-vivo detection of 2HG has provided a major breakthrough in cancer research3-4. Development of therapeutics to tackle 2HG production would require a preclinical model system. Thus development of in-vivo MRS methodologies for precise measurement of 2HG in IDH-mutant mouse model of gliomas, and further application of 2HG optimized MRS method to evaluate the efficacy of 2HG or IDH-inhibiting drugs would provide a clinical advancement for therapeutic strategies. In-vivo measurement of 2HG is of high significance, but accurate evaluation in-vivo remains challenging, especially when 2HG elevation is moderate. Hence the present study was undertaken to develop PRESS MRS Method for 2HG detection with enhanced precision in Patient-Derived Xenograft Orthotropic Mouse model (PDX). Methodology was further extended to detect 2HG and metabolic profile with tumor-progression in PDX-mice.

Methods

Volume-localized density-matrix simulations were performed to optimize echo time (TE1 and TE2) of PRESS for 2HG detection, and to create the model spectra of metabolites for LCModel spectral fitting. Several PDX-mouse lines were generated from the inoculum of tumor cells from different IDH-mutant patients. In-vivo MRS data was obtained from PDX mice (n=7), and wild-type (n=3) at 9.4T horizontal-bore animal MR-scanner (Agilent) using a home-built 15mm-diameter single-loop transmit/receive surface-coil. The voxel localization was achieved with a 550μs 90°-RF pulse (bandwidth=10.7 kHz) and two 3.0ms 180°-RF pulses (bandwidth=5.4 kHz), at an RF-field intensity of 60μT. In-vivo PRESS MRS was acquired at short-TE (TE1=8ms, TE2=6.7ms), and long-TE (TE1=19ms, TE2=77ms) with signal-averages=128 and 512 respectively (TR=3s). 2HG MRS with tumor-progression was conducted at intervals of ~10-12 days over a period of 35-days after tumor establishment. Spectral-fitting was performed with LCModel software5, using in-house basis spectra calculated upon incorporating the volume-localizing RF and gradient pulses of PRESS. Following the normalization of LCModel estimates to water signal, metabolite concentration was estimated with reference to total creatine at 8 mM.

Results and Discussion

Volume-localized numerical simulations indicated that strongly-coupled 2HG signal at 2.25-ppm depends on PRESS sub-echo times, TE1 and TE2 (Fig.1A(1)). The signal had positive polarity at short-TE (=TE1+TE2< 40ms) but inverted at TE>60ms and <110ms, for which TE1 is shorter than TE2. Maximum amplitude was obtained for TE=96ms (TE1=19ms and TE2=77ms). Numerical calculation at TE1=8ms, TE2=6.7ms (Fig.1A(2)) for signal strength and pattern for 2HG, glutamate (Glu), GABA, and glycine showed that 2HG signal is masked at short-TE while Fig.1A(3) depicts that at TE=96ms, the 2HG signal is clearly discernible as it shows negative-polarity, while neighboring signals have positive-polarity. In-vivo spectra obtained with 2HG-optimized PRESS TE=96ms from IDH-mutant mice glioma showed a large inverted peak at 2.25-ppm, which was well reproduced by the LCModel fit generated using basis set with 2HG, and was estimated to be 26.1mM (CRLB=1%) (Fig.1B(1)). 2HG was detected in all of the 7 IDH-mutant gliomas using optimized PRESS, the level ranging from 1.6±0.25 to 26.1±0.26 mM. IDH-mutation was confirmed with IDH-immunohistochemistry (Fig.1B(2)). tCho which was found to be 2.8±0.25 mM in wild-type mouse brain was increased to 11±4 mM in the IDH-mutant gliomas. In-vivo PRESS spectrum obtained from a low-2HG PDX tumor line at long-TE resulted in a residual at 2.25-ppm when fitted to basis set without-2HG (Fig.2A(2), marked with arrow), indicating that inverted-signal at 2.25-ppm was primarily attributed to 2HG H4-resonances. However, when short-TE in-vivo spectrum was fitted with and without-2HG, the residuals obtained were about the same in both cases, indicating that the 2HG signal detection at short-TE is not very selective. Moreover, absence of 2HG in the basis set altered the estimation of neighboring metabolites (Fig.2A-B). Application of 2HG optimized long-TE PRESS-method at different stages of glioma growth showed that amount of 2HG became precisely detectable (2.4mM, CRLB=7%) in smaller tumor volume after 20-days of its establishment, and rose to 4mM (CRLB=4%) after 35-days (Fig.3), indicating the potential of the method to pinpoint the exact stage for application of 2HG-inhibiting drugs based upon 2HG kinetics. Mean 2HG and tCho level in low-2HG cohort of mice rose from 1.7±1 to 3.3±0.6 mM, and 2.1±0.4 to 8.3±2.0 mM respectively with tumor growth, while NAA (11.7±0.5 to 5.2±1.6 mM, p=0.01), and Glu (7.3±0.5 to 4.5±0.6 mM, p=0.007) showed significant reduction suggestive of altered metabolic-homeostasis.

Conclusion

At 9.4T, PRESS TE=96ms gives an inverted 2HG signal at 2.25-ppm with minimal interference from neighboring resonances, thus provides selective and improved detection of 2HG compared to short-TE MRS, and can be effectively used for in-vivo evaluation of drugs targeted to treat IDH-mutant gliomas.

Acknowledgements

This study was supported by CPRIT RP14021-P04.

References

1. Dang L, White DW, Gross S, et al. Cancer-associated IDH1 mutations produce 2-hydroxyglutarate. Nature 2009; 462:739-744.

2. Ward PS, Patel J, Wise DR, et al. The common feature of leukemia-associated IDH1 and IDH2 mutations is a neomorphic enzyme activity converting alpha-ketoglutarate to 2-hydroxyglutarate. Cancer Cell. 2010; 17:225-34.

3. Choi C, Ganji SK, DeBerardinis J, et al. 2-hydroxyglutarate detection by magnetic resonance spectroscopy in subjects with IDH-mutated gliomas. Nat Med. 2012; 18(4):624-629.

4. Pope WB, Prins RM, Albert Thomas M, et al. Non-invasive detection of 2-hydroxyglutarate and other metabolites in IDH1 mutant glioma patients using magnetic resonance spectroscopy. J Neurooncol. 2012;107:197-205.

5. Provencher SW, et al. Estimation of metabolite concentrations from localized in vivo proton NMR spectra. Magn Res Med 1993;30:672-679.

Figures

(A1) Volume-localized density-matrix simulation plot of 2HG H4 resonance for PRESS-TE optimization. Calculated PRESS-spectra of 2HG, GABA, Glu at TE =14.7ms (A2) and 96ms (A3). (B1) In-vivo spectrum from IDH-mutant mice brain at TE=96ms with LCModel-fit, and 2HG-estimate. T2-weighted images were obtained for voxel-localization. (B2) Immunohistochemistry for IDH, and H&E-staining.

In-vivo PRESS spectrum from an IDH1-mutated mice brain tumor at TE=96ms (upper-panel) and TE=14.7ms (lower-panel), together with LCModel fits, and residuals from fitting with 2HG (left), or without 2HG (right) in the basis set. Estimated concentrations (mM), and CRLBs in parenthesis are reported.

Long-TE 2HG-PRESS MRS with tumor-growth in IDH-mutant mice brain at 12, 24 and 35 days after tumor establishment. T2-weighted axial images were acquired to identify the voxel of interest. The voxel size was 3.0×3.0×3.0 mm3. LCModel fits, baseline and residuals with estimated concentrations (mM), and CRLBs are shown.



Proc. Intl. Soc. Mag. Reson. Med. 24 (2016)
4341