MR Studies of Glioblastoma Models Treated with Dual PI3K/mTOR Inhibitor and Temozolomide:Metabolic Changes Are Associated with Enhanced Survival

Abstract

The current standard of care for glioblastoma (GBM) is surgical resection, radiotherapy, and treatment with temozolomide (TMZ). However, resistance to current therapies and recurrence are common. To improve survival, agents that target the PI3K signaling pathway, which is activated in approximately 88% of GBM, are currently in clinical trials. A challenge with such therapies is that tumor shrinkage is not always observed. New imaging methods are therefore needed to monitor response to therapy and predict survival. The goal of this study was to determine whether hyperpolarized (13)C magnetic resonance spectroscopic imaging (MRSI) and (1)H magnetic resonance spectroscopy (MRS) can be used to monitor response to the second-generation dual PI3K/mTOR inhibitor voxtalisib (XL765, SAR245409), alone or in combination with TMZ. We investigated GS-2 and U87-MG GBM orthotopic tumors in mice, and used MRI, hyperpolarized (13)C MRSI, and (1)H MRS to monitor the effects of treatment. In our study, (1)H MRS could not predict tumor response to therapy. However, in both our models, we observed a significantly lower hyperpolarized lactate-to-pyruvate ratio in animals treated with voxtalisib, TMZ, or combination therapy, when compared with controls. This metabolic alteration was observed prior to MRI-detectable changes in tumor size, was consistent with drug action, and was associated with enhanced animal survival. Our findings confirm the potential translational value of the hyperpolarized lactate-to-pyruvate ratio as a biomarker for noninvasively assessing the effects of emerging therapies for patients with GBM. Mol Cancer Ther; 15(5); 1113-22. ©2016 AACR.

Figure 1

GS-2 (left) and U87-MG models (right): (A and B) T2-weighted images and corresponding hyperpolarized ¹³C MR spectra from tumor voxels. (C and D) Temporal evolution of hyperpolarized [1-¹³C]-lactate in the tumor and in normal brain voxels. (E and F) Average hyperpolarized Lac/Pyr detected in tumor voxel in GS-2 tumors treated with Everolimus at D9±2 and in U87-MG tumors treated with TMZ at D6±1. *: P<0.05

Figure 2

GS-2 model: (A) Axial T2-weighted images and corresponding hyperpolarized ¹³C MRSI spectra from the tumor voxel at D0 (top) and D9±2 of treatment (bottom). The tumor regions are contoured with white dashed line. (B) Temporal evolution of average tumor size as a percentage of pre-treatment value. (C) Average tumor size detected at D9±2. (D) Average Lac/Pyr detected at D9±2. (E) Kaplan-Meier survival curves comparing control and treatment groups. *: P<0.05

Figure 3

GS-2 model: IHC showing the expression of CA-IX, LDH-A, phospho-S6 and phospho-Chk1 for each treatment group at the end of study. Bar indicates 20µm.

Figure 4

GS-2 model: (A) Axial T2-weighted images and corresponding ¹H spectra of the tumor voxel from each treatment group. (B) Average tCho levels at D9±2. (C) Representative ex vivo ¹H HR-MAS spectrum of a biopsy from control tumor. The inserts, zoomed on choline-containing metabolites, illustrate each treatment group. (D) Comparison of tCho levels and individual choline-containing metabolites. Error bars correspond to the standard deviation of tCho.

Figure 5

U87-MG model: (A) Anatomical axial T2-weighted images and corresponding hyperpolarized ¹³C MRSI spectra from the tumor voxel at D0 (top) and D6±1 of treatment (bottom). Tumor regions are contoured in white dashed line. (B) Temporal evolution of average tumor size. (C) Average tumor size detected at D6±1 of treatment. (D) Average hyperpolarized Lac/Pyr detected at D6±1 of treatment. (E) Kaplan-Meier survival curves comparing survival between control and treatment groups.

Figure 6

U87-MG tumor model: (A) Representative ex vivo ¹H HR-MAS spectra from of a biopsy of control tumor. Inserts exhibit spectra zoomed on the choline region. (B) tCho levels and individual choline-containing metabolites. The error bars correspond to the standard deviation of the tCho.